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  • richardmitnick 12:00 pm on August 8, 2018 Permalink | Reply
    Tags: , , , Could a new type of quark fix the “unnaturalness” of the Standard Model?, , , , , Quarks,   

    From CERN ATLAS: “Could a new type of quark fix the “unnaturalness” of the Standard Model?” 

    CERN ATLAS Higgs Event


    8th August 2018
    ATLAS Collaboration

    Figure 1: One of the Feynman diagrams for T pair production at the LHC. (Image: ATLAS Collaboration © CERN 2018)

    While the discovery of the Higgs boson at the Large Hadron Collider (LHC) in 2012 confirmed many Standard Model predictions, it has raised as many questions as it has answered. For example, interactions at the quantum level between the Higgs boson and the top quark ought to lead to a huge Higgs boson mass, possibly as large as the Planck mass (>1018 GeV). So why is it only 125 GeV? Is there a mechanism at play to cancel these large quantum corrections caused by the top quark (t)? Finding a way to explain the lightness of the Higgs boson is one of the top (no pun intended) questions in particle physics.

    A wide range of solutions have been proposed and a common feature in many of them is the existence of vector-like quarks – in particular, a vector-like top quark (T). Like other quarks, vector-like quarks would be spin-½ particles that interact via the strong force. While all spin-½ particles have left- and right-handed components, the weak force only interacts with the left-handed components of Standard Model particles. However, vector-like quarks would have “ambidextrous” interactions with the weak force, giving them a bit more leeway in how they decay. While the Standard Model top quark always decays to a bottom quark (b) by emitting a W boson (t→Wb), a vector-like top can decay three different ways: T→Wb, T→Zt or T→Ht (Figure 1).

    Figure 2: Lower limit (scale on right axis) on the mass of a vector-like top as a function of the branching ratio to Wb and Ht (bottom and left axes). (Image: ATLAS Collaboration © CERN 2018)

    The ATLAS collaboration uses a custom-built programme to search for vector-like top pairs in LHC data. It utilizes data from several dedicated analyses, each of them sensitive to various experimental signatures (involving leptons, boosted objects and/or large missing transverse momentum). This allows ATLAS to look for all of possible decays, increasing the chance of discovery.

    ATLAS has now gone one step further by performing a combination of all of the individual searches. While individual analyses are designed to study a particular sets of decays, combined results provide sensitivity to all possible sets of decays. These have allowed ATLAS to search for vector-like tops with masses over 1200 GeV. It appears, however, that vector-like tops are so far nowhere to be found. On the bright side, the combination allows ATLAS to set the most stringent lower limits on the mass of a vector-like top for arbitrary sets of branching ratios to the three decay modes (Figure 2).

    Between these limits on vector-like top quarks and those on other theories that could offer a solution (like supersymmetry), the case for a naturally light Higgs boson is not looking good… but Nature probably still has a few tricks up its sleeve for us to uncover.

    The Standard Model of elementary particles (more schematic depiction), with the three generations of matter, gauge bosons in the fourth column, and the Higgs boson in the fifth.

    Standard Model of Particle Physics from Symmetry Magazine

    See the full article here .

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    CERN map

    CERN LHC Grand Tunnel
    CERN LHC particles
    LHC at CERN

    CERN Courier


    Quantum Diaries

  • richardmitnick 4:04 pm on June 30, 2017 Permalink | Reply
    Tags: , , , Quarks, , What really hapens?   

    From Symmetry: “What’s really happening during an LHC collision?” 

    Symmetry Mag


    Sarah Charley

    It’s less of a collision and more of a symphony.

    Wow!! ATLAS collaboration.

    The Large Hadron Collider is definitely large.


    CERN/LHC Map

    CERN LHC Tunnel

    CERN LHC particles

    With a 17-mile circumference, it is the biggest collider on the planet. But the latter fraction of its name is a little misleading. That’s because what collides in the LHC are the tiny pieces inside the hadrons, not the hadrons themselves.

    Hadrons are composite particles made up of quarks and gluons.

    The quark structure of the proton 16 March 2006 Arpad Horvath

    The gluons carry the strong force, which enables the quarks to stick together and binds them into a single particle.


    The main fodder for the LHC are hadrons called protons. Protons are made up of three quarks and an indefinable number of gluons. (Protons in turn make up atoms, which are the building blocks of everything around us.)

    If a proton were enlarged to the size of a basketball, it would look empty. Just like atoms, protons are mostly empty space. The individual quarks and gluons inside are known to be extremely small, less than 1/10,000th the size of the entire proton.

    “The inside of a proton would look like the atmosphere around you,” says Richard Ruiz, a theorist at Durham University. “It’s a mixture of empty space and microscopic particles that, for all intents and purposes, have no physical volume.

    “But if you put those particles inside a balloon, you’ll see the balloon expand. Even though the internal particles are microscopic, they interact with each other and exert a force on their surroundings, inevitably producing something which does have an observable volume.”

    So how do you collide two objects that are effectively empty space? You can’t. But luckily, you don’t need a classical collision to unleash a particle’s full potential.

    In particle physics, the term “collide” can mean that two protons glide through each other, and their fundamental components pass so close together that they can talk to each other. If their voices are loud enough and resonate in just the right way, they can pluck deep hidden fields that will sing their own tune in response—by producing new particles.

    “It’s a lot like music,” Ruiz says. “The entire universe is a symphony of complex harmonies which call and respond to each other. We can easily produce the mid-range tones, which would be like photons and muons, but some of these notes are so high that they require a huge amount of energy and very precise conditions to resonate.”

    Space is permeated with dormant fields that can briefly pop a particle into existence when vibrated with the right amount of energy. These fields play important roles but almost always work behind the scenes. The Higgs field, for instance, is always interacting with other particles to help them gain mass. But a Higgs particle will only appear if the field is plucked with the right resonance.

    When protons meet during an LHC collision, they break apart and the quarks and gluons come spilling out. They interact and pull more quarks and gluons out of space, eventually forming a shower of fast-moving hadrons.

    This subatomic symbiosis is facilitated by the LHC and recorded by the experiment, but it’s not restricted to the laboratory environment; particles are also accelerated by cosmic sources such as supernova remnants. “This happens everywhere in the universe,” Ruiz says. “The LHC and its experiments are not special in that sense. They’re more like a big concert hall that provides the energy to pop open and record the symphony inside each proton.”

    See the full article here .

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    Symmetry is a joint Fermilab/SLAC publication.

  • richardmitnick 10:31 am on October 9, 2015 Permalink | Reply
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    From FNAL- “Frontier Science Result: CMS Why three?” 

    FNAL II photo

    Fermilab is an enduring source of strength for the US contribution to scientific research world wide.

    Oct. 9, 2015
    FNAL Don Lincoln
    Don Lincoln

    Temp 1
    The Standard Model includes six types of quarks, arranged into three generations. Only generation I is necessary to make ordinary matter. So why are there three generations, and what message are the other four quarks telling us?

    There are a lot of mysteries in particle physics, but one of the most curious is called the flavour problem. Physicists use the word flavour to mean the different varieties of particles. As readers of this column will recall, there are six types of quarks: up and down, charm and strange, and top and bottom. Up and down quarks are found in ordinary matter and are called generation I. The other two generations are very similar in their properties compared to the first generation, but they have higher mass.

    So why are there these two other copies? Scientists have many ideas. One is that the quarks contain smaller particles within them. While these smaller particles have never been found, they already have a name — preons. Another idea is that there may be only one generation, but this generation exists in a higher dimensional space. In the same way that a football viewed end on looks like a circle while looking like, well, a football from the side, perhaps the different generations are just a single particle seen from different angles.

    Nobody knows the answer to the flavour problem, and, to share a personal note, this particular physics problem is the one that keeps me awake at night. The existence of these extra generations would tell us something profound about the universe if we just had the wits to understand what it is saying.

    Another interesting possibility is that there are other generations, say generation IV. We already have names for the quarks of that generation — t’ and b’, pronounced t prime and b prime. However, no evidence for the existence of this fourth generation has been found. Indeed, a measurement from experiments from back in the 1990s using the LEP accelerator at CERN suggests that there are only three generations.

    LEP at CERN

    There are possible loopholes in that measurement, but it might well be that there are but three generations.

    Yet another idea, generally based on the preon concept, is that the hypothetical constituents of the quarks can be given energy. Just as a slumbering beehive is different from one that was recently kicked, maybe enough energy can stir up the quarks’ constituents.

    In the simplest incarnation, this principle should be true of all quarks, but one could also imagine adding energy only to the constituents of the third-generation quarks. Scientists call such a configuration an excited quark and denote such a quark with an asterisk. So CMS scientists went looking for a b*.

    CERN CMS Detector
    CMS at CERN

    The idea is that the b* would be a very massive particle and could decay into a W boson and a top quark. No evidence was found for a b*, and physicists ruled out the existence of a b* with a mass below about 1,500 GeV. A particular peculiarity of this particle is that it wouldn’t get its mass from the Higgs field, but from the energy that excited the quark.

    A negative measurement isn’t as much fun as a positive one, but at least we know what’s not the answer for the flavor problem, and that’s progress. But it doesn’t help my insomnia, and so we keep looking, hoping to solve this thorniest of problems.

    See the full article here .

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    Fermilab Campus

    Fermi National Accelerator Laboratory (Fermilab), located just outside Batavia, Illinois, near Chicago, is a US Department of Energy national laboratory specializing in high-energy particle physics. Fermilab is America’s premier laboratory for particle physics and accelerator research, funded by the U.S. Department of Energy. Thousands of scientists from universities and laboratories around the world
    collaborate at Fermilab on experiments at the frontiers of discovery.

  • richardmitnick 5:35 am on March 17, 2015 Permalink | Reply
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    From New Scientist: “Quark stars: How can a supernova explode twice?” 2013 But Well Worth the Read 


    New Scientist

    09 December 2013
    Anil Ananthaswamy

    What do you get when you melt a neutron star? An unimaginably dense lump of strange matter and a whole new celestial beast

    (Image: Matt Murphy)

    ON 22 September last year, the website of The Astronomer’s Telegram alerted researchers to a supernova explosion in a spiral galaxy about 84 million light years away. There was just one problem. The same object, SN 2009ip, had blown up in a similarly spectacular fashion just weeks earlier. Such stars shouldn’t go supernova twice, let alone in quick succession. The thing was, it wasn’t the only one, the next year another supernova, SN 2010mc, did the same.

    One of the few people not to be bamboozled was Rachid Ouyed. “When I looked at those explosions, they were talking to me right away,” he says. Ouyed, an astrophysicist at the University of Calgary in Alberta, Canada, thinks that these double explosions are not the signature of a supernova, but something stranger. They may mark the violent birth of a quark star, a cosmic oddity that has only existed so far in the imaginations and equations of a few physicists. If so they would be the strongest hints yet that these celestial creatures exist in the cosmic wild.

    The implications would be enormous. These stars would take pride of place alongside the other heavenly heavyweights: neutron stars and black holes. They could help solve some puzzling mysteries related to gamma-ray bursts [GRBs] and the formation of the heftiest elements in the universe. Back on Earth, quark stars would help us better understand the fundamental building blocks of matter in ways that even machines like the Large Hadron Collider cannot.

    CERN LHC Map
    CERN LHC Grand Tunnel
    CERN LHC particles
    LHC at CERN

    Astrophysicists can thank string theorist Edward Witten for quark stars. In 1984, he hypothesised that protons and neutrons may not be the most stable forms of matter.

    Both are made of two types of smaller entities, known as quarks: protons are comprised of two “up” quarks and one “down” quark, whereas neutrons are made of two downs and one up. Up and down are the lightest of six distinct “flavours” of quark. Add the third lightest to the mix and you get something called strange quark matter. Witten argued that this kind of matter may have lower net energy and hence be more stable than nuclear matter made of protons and neutrons.

    Quark nova

    If so, we might all start decaying into strange matter. But don’t fret. You either need to wait around longer than the age of the universe for the stuff to form spontaneously, or find somewhere with the right conditions to start the process. One place this could happen is inside neutron stars, the dense remnants of certain types of supernovae.

    When a star many times more massive than the sun runs out of fuel, its inner core implodes. The outer layers are cast off in a spectacular explosion. What’s left behind is a rapidly spinning neutron star, which as the name implies is made mainly of neutrons, with a crust of iron. Whirling up to 1000 times per second, a neutron star is constantly shedding magnetic fields. Over time, this loss of energy causes the star to spin slower and slower. As it spins down, the centrifugal forces that kept gravity at bay start weakening, allowing gravity to squish the star still further.

    In what is a blink of an eye in cosmic time, the neutrons can be converted to strange quark matter, which is a soup of up, down and strange quarks. In theory, this unusual change happens when the density inside the neutron star starts increasing. New particles called hyperons begin forming that contain at least one strange quark bound to others.

    However, the appearance of hyperons marks the beginning of the end of the neutron star. “Once you start to form hyperons, then you can start the nucleation of the first droplet of strange quark matter,” says Giuseppe Pagliara of the University of Ferrara in Italy. As the density in the core continues to increase, the star’s innards “melt”, freeing quarks from their bound state. In fact, a single droplet of strange matter is enough to trigger a runaway process that converts all the neutrons. What was a neutron star turns into a quark star.

    Of course, this assumes that Witten is correct about strange quark matter being more stable than neutrons. No one has yet proved him wrong, but it is a tough idea for some to swallow. “More conservative thinkers are just not open to the idea that free quarks exist in neutron stars,” says Fridolin Weber, an astrophysicist at the San Diego State University in California.

    Not so the daring ones. Ouyed, for instance, has been trying to convince his fellow astrophysicists of the existence of quark stars for more than a decade. Not only do these intrepid few think that quarks can exist freely inside neutron stars, they have even thought about what comes next. “We all agree that if quark stars exist, then the conversion of normal, ordinary matter into a quark star will be a very exothermic process, a lot of energy will be released,” says Pagliara. “How this energy is released is a matter of debate.”

    On one hand, Pagliara and his colleagues have done extensive simulations to show that this conversion will happen in a matter of milliseconds. In what he calls a “strong deflagration”, the neutron star burns up as it turns into a quark star. There is no explosion.

    Ouyed, on the other hand, begs to differ. His team’s simulations show that the conversion is most likely to be an extremely violent process. The seed of strange quark matter spreads until it reaches the outer crust of the neutron star. As the part of the star that has been turned into quark matter separates from the iron-rich crust, it collapses. The collapse halts when the inner core becomes incredibly dense and rebounds, creating a shock wave. Much as in a supernova, the iron-rich crust and leftover neutrons are ejected in another spectacular explosion – a “quark nova”.

    Hurtling through space, the quark nova ejecta then slam into the earlier supernova remnants, causing them to light up again, as they did after the explosion of the original, conventional star. What’s left behind is a quark star. “It was very hard to find solutions where the entire neutron star turned into a quark star, in just a puff with no explosion,” says Ouyed.

    Double explosion

    Depending on the mass of the star before its first explosion, the second blast could occur anywhere from seconds to years after the original supernova. Too soon, and the two explosions would merge, appearing as one blast, smeared out in time. Too late, and the supernova ejecta would have dispersed long before the detonation of the quark nova, and there would be no re-brightening.

    But if the timing is just right, the outcome should be observable. In 2009, Ouyed’s team predicted that if the quark nova goes off days or weeks after the supernova, there should be two peaks in energy: the first being the supernova explosion itself, and the second being the reheating of the supernova ejecta. The objects SN 2009ip and SN 2010mc matched predictions in ways almost too good to be true.

    SN 2009ip had its first major explosion in early August 2012, and 40 days later flared up again. SN 2010mc was eerily similar in its outbursts, showing a double explosion in which the peaks were about 40 days apart. While other researchers continue trying to explain these unusual observations using their tried-and-tested models of supernovae, Ouyed is convinced that we have witnessed quark stars being born.

    Oddly, it is the first peak in both events that convinces him. If these have all the characteristics of a regular supernova, it makes the second boom harder to explain using conventional arguments. “When you look at the first ejecta, it looks like a duck and walks like a duck: it’s a supernova,” says Ouyed. “Then what’s the second one?”

    He points the finger at quark novae. “We just applied our model of the dual shock quark nova, and it was actually easy to fit,” he says. “That’s the beauty of it.”

    While Pagliara and Ouyed’s teams disagree on whether the transition from a neutron star to a quark star is explosive, they do agree that space should be littered with quark stars. How should we look for them?

    We might be mistaking some of them for neutron stars, says Pagliara. Most neutron stars weigh as much as 1.4 suns or slightly more. The best studied examples, orbiting each other in systems called Hulse-Taylor binary pulsars, certainly follow this pattern. Both neutron stars involved weigh in at 1.4 solar masses. However, Pagliara is bothered by two discoveries of neutron stars that tip the scales at 2 solar masses each. “It’s difficult to reach this mass with normal particle components like neutrons, protons and hyperons,” he says.

    This has to do with a property of matter called its equation of state. Equations of state describe how matter behaves under changes in physical conditions, such as pressure and temperature. Hyperons, which are precursors to strange quark matter, have a “soft” equation of state. Their existence in the dense core of a neutron star makes the star more compressible, causing it to shrink in size.

    Astronomers estimate that neutron stars are about 10 kilometres across – but squeezing 2 solar masses into an object of such size would end up creating a black hole. Pagliara says that compact stars weighing 2 solar masses or more have to be bigger, or put another way, the matter has to be “stiffer” so that gravity cannot compress it as much. There is one candidate with a stiffer equation of state: strange quark matter.

    Pagliara and his colleague Alessandro Drago and others claim that the compact stars we have spotted come from two families. The smaller ones must be the run-of-the-mill neutron stars. The larger ones must be quark stars. The only way to verify this claim is to measure their masses and also measure their radii to the nearest kilometre. A proposed European satellite mission called the Large Observatory for X-ray Timing could do just that. Its aim is to measure the equation of state for compact objects – and thus differentiate between neutron stars and quark stars.


    Meanwhile, Ouyed’s team is concentrating on predictions based on their quark nova model. One prediction has to do with the creation of heavy elements in the universe. Once a massive star goes supernova, weighty elements are synthesised in a matter of milliseconds, when neutrons are absorbed into iron nuclei. These neutron-rich nuclei are unstable and decay into elements further up in the periodic table when neutrons get converted to protons. “But the challenge with supernovae has always been to go to really heavy elements,” says Ouyed. The iron and neutrons needed for the process are in short supply because most of them are left behind in the remnant neutron star.

    The quark nova solves that problem. Its ejecta are a potent mix of neutrons and iron from the neutron star’s crust, providing just the laboratory for synthesising the heaviest elements. Ouyed is urging astronomers to study double explosions carefully. His team predicts that the second blast should show the presence of elements heavier than atomic mass 130, elements which should be missing from the first explosion.

    The conversion of a neutron star to a quark star could also solve another problem plaguing astrophysics: the source of some long-duration gamma-ray bursts, which are among the brightest events in the universe. On 9 July 2011, NASA’s SWIFT gamma-ray satellite saw a burst with two spectacular peaks of emission, spaced 11 minutes apart.

    NASA SWIFT Telescope

    And the second was the stronger of the two. The traditional “collapsar” model of gamma-ray bursts relies on a star collapsing to a black hole. As the last remnants of the doomed star fall in, it is thought to result in such an emission. But 11 minutes is an eternity for a black hole – it’s hard to make sense of the second peak.

    Pagliara thinks his team has the answer. According to their model, a neutron star converts to a quark star without an explosion. Yet there is still a tremendous release of energy, which Pagliara suspects goes into gamma rays. This could explain the second peak. “At the moment, and it’s speculation, we think that this second event could be related to quark stars,” he says. “If you want to see a possible signature of formation of quark matter, you should probably look at those gamma-ray bursts that have an activity long after the main event.”

    Quark world

    Confirming the existence of quark stars and verifying their properties could have a huge impact on particle physics. Colliders like the LHC and the Relativistic Heavy Ion Collider (RHIC) in Brookhaven National Laboratory [BNL]in New York have been smashing heavy ions head-on to create a state of matter called a quark-gluon plasma, where quarks are essentially free.

    BNL RHIC Campus
    RHIC at BNL

    The best way to study this phase of matter is using a method called lattice quantum chromodynamics [QCD]. But physicists have only been able to solve the equations of lattice QCD for high temperatures and low density – the conditions created at the LHC and the RHIC. The equations are intractable for other conditions. For instance, it is impossible to calculate the density at which protons and neutrons can melt into their constituent quarks at low temperatures.

    Enter quark stars. First, if their existence is confirmed, it proves that quarks can exist freely at high densities and low temperatures, rather than bound up in hadrons – the catch-all name given to any particle made of quarks. Second, for the explosive quark nova model Ouyed’s team has shown that the density at which quarks get freed is intimately linked to the time lag between the supernova and the quark nova. Measure the timing of the double explosion and you will glean important clues about conditions at the transition. “The quark nova is a very beautiful bridge that straddles the hadronic world and the quark world,” says Ouyed. “It’d be a very nice tool to use for physics and astrophysics.”

    Weber agrees that quark stars, if they exist, would be a unique astrophysical laboratory. They would help us probe properties of matter in ways that we cannot do with the best colliders on Earth – in the domain of high densities and low temperatures. “This is a regime that is only accessible to stars, and only stars can tell us what will happen.”

    See the full article here.

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  • richardmitnick 12:19 pm on December 29, 2014 Permalink | Reply
    Tags: , , , Quarks   

    From livescience: “7 Strange Facts About Quarks” 


    May 05, 2014
    Elizabeth Howell

    Teensy Particles


    A proton, composed of two up quarks, one down quark and the gluons “binding” them together. The color assignment of individual quarks is not important, only that all three colors be present.

    Quarks are particles that are not only hard to see, but pretty much impossible to measure. These teensy-tiny particles are the basis of subatomic particles called hadrons. With every discovery in this field of particle physics in the past 50 years, however, more questions arise about how quarks influence the universe’s growth and ultimate fate. Here are seven strange facts about quarks.

    Emerged just after Big Bang


    NASA WMAP satellite
    NASA/ WMAP spacecraft


    The first quarks appeared about 10^minus 12 seconds after the universe was formed, in the same era where the weak force (which today is the basis for some radioactivity) separated from the electromagnetic force. The antiparticles of quarks appeared around the same time.

    Discovered in an atom smasher

    Credit: Brookhaven National Lab

    A mystery arose in the 1960s when researchers using the Stanford Linear Accelerator Center found that the electrons were scattering from each other more widely than calculations suggested. More research found that there were at least three locations where electrons scattered more than expected within the nucleon or heart of these atoms, meaning something was causing that scattering. That was the basis for our understanding of quarks today.

    Mentioned by James Joyce

    Credit: Cornell Joyce Collection, Public Domain

    Murray Gell-Mann, the co-proposer for the quark model in the 1960s, drew inspiration for the spelling from the 1939 James Joyce book Finnegan’s Wake, which read: “Three quarks for Muster Mark! / Sure he has not got much of a bark / And sure any he has it’s all beside the mark.” (The book came out well before quarks were discovered and so their name has always been spelled in this way.)

    Come in flavors

    Credit: MichaelTaylor | Shutterstock

    Physicists refer to the different types of quark as flavors: up, down, strange, charm, bottom, and top. The biggest differentiation between the flavors is their mass, but some also differ by charge and by spin. For instance, while all quarks have the same spin of 1/2, three of them (up, charm and top) have charge 2/3, and the other three (down, strange and bottom) have charge minus 1/3. And just because a quark starts out as a flavor doesn’t mean it will stay that way; down quarks can easily transform into up quarks, and charm quarks can change into strange quarks.

    Tricky to measure


    Quarks can’t be measured, because the energy required produces an antimatter equivalent (called an antiquark) before they can be observed separately, among other reasons, according to a primer from Georgia State University. The mass of quarks is best determined by techniques such as using a supercomputer to simulate the interactions between quarks and gluons, with gluons being the particles that glue quarks together.

    Teach us about matter

    Credit: Chukman So

    In 2014, researchers published the first observation of a charm quark decaying into its antiparticle, providing more information about how matter behaves. Because particles and antiparticles should destroy each other, one would think the universe should just have photons and other elementary particles. Yet antiphotons and antiparticles still exist, leading to the mystery of why the universe is made mostly of matter and not antimatter.

    May set the universe’s fate


    Nailing down the mass of the top quark could reveal to researchers one of two ghastly scenarios: that the universe could end in 10 billion years, or that people could materialize out of nowhere. If the top quark is heavier than expected, energy carried through the vacuum of space could collapse. If it’s lower than expected, an unlikely scenario called “Boltzmann brain” could see self-aware entities come out of random collections of atoms. (While this isn’t a part of the Standard Model, the theory – framed as a paradox – goes that it would be more likely to see organized groups of atoms as the random ones observed in the universe.)

    The Standard Model of elementary particles, with the three generations of matter, gauge bosons in the fourth column, and the Higgs boson in the fifth.

    See the full article here.

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  • richardmitnick 12:04 pm on December 4, 2014 Permalink | Reply
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    From FNAL- “Frontier Science Result: CDF Wading through the swamp to measure top quark mass” 

    Fermilab is an enduring source of strength for the US contribution to scientific research world wide.

    Thursday, Dec. 4, 2014
    edited by Andy Beretvas

    Even after the discovery of the Higgs boson, the top quark is still a focus of attention because of its peculiar position of being the heaviest quark in the Standard Model and for its possible role in physics beyond the Standard Model.

    The Standard Model of elementary particles, with the three generations of matter, gauge bosons in the fourth column, and the Higgs boson in the fifth.

    If the Standard Model is correct, the stability of the vacuum strongly depends on the mass of the Higgs boson and the top quark mass. In this context, scientists favor the scenario that the universe is in a metastable state. A precision measurement of the top quark mass helps to better determine the relative stability of that state in this scenario.

    At the Tevatron, top quarks were produced, mostly in pairs, only once in about 10 billion collisions. They decayed right away into a W boson and a b quark. In the most abundant and yet most challenging scenario, the final state contains six collimated sprays of particles, called jets, two of which likely originated from the b quarks, with peculiar, identifiable characteristics (allowing them to be “b-tagged”). This decay mode is usually called the all-hadronic channel, for which the signal is swamped by a background associated to the production of uninteresting multijet events, which were about a factor of 1,000 more abundant than the signal events.

    FNAL Tevatron

    This new analysis uses the full CDF Run II data set.
The set contains nearly twice the number of top quark pairs as seen in our previous measurement. The analysis uses an improved simulation and relies on there being at least one b-tagged jet. An important part of the analysis is to minimize the uncertainty in our measurement of jet scale energies. Exploiting the expected behavior of top-antitop signal events, the huge background can be tamed through finely tuned requirements, yielding about 4,000 events, where about one event out of three is expected from the signal. The all-hadronic final state can then be fully reconstructed using the energies of the six jets, and the mass of the top quark can be derived comparing the data to simulations produced for different input values of the top quark mass (see the [below] figure).

    The black dots plot the distribution of the reconstructed top mass for events containing one or more b-tags. The distribution is compared to the expected yield for background and signal events, normalized to the best fit.


    This procedure yields a value of 175.1 ± 1.2 (stat) ± 1.6 (sys) GeV/c2 for the top quark mass, with a 1 percent relative precision. This measurement complements the results obtained by CDF in other channels. Our measurement is consistent with the current world average (which includes our previous measurement in the all-hadronic channel), obtained from measurements by ATLAS, CDF, CMS and DZero. The top quark mass world average is 173.3 ± 0.8 GeV/c2.


    CERN CMS New

    FNAL DZero

    See the full article here.

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    Fermilab Campus

    Fermi National Accelerator Laboratory (Fermilab), located just outside Batavia, Illinois, near Chicago, is a US Department of Energy national laboratory specializing in high-energy particle physics.

  • richardmitnick 2:14 pm on November 29, 2014 Permalink | Reply
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    From Daily Galaxy: “The Hunt for Colossal “Quark” Stars –Do They Exist?” 

    Daily Galaxy
    The Daily Galaxy

    November 29, 2014

    No Writer Credit

    “We haven’t found strange stars yet,” explains Prashanth Jaikumar from the Argonne National Laboratory. “But that doesn’t mean they don’t exist. Maybe we have found them. Maybe some of these neutron stars are really strange stars. According to our theory, it would be very difficult to tell a strange star from a neutron star.”

    Recent research suggests that neutron stars may gradually transform into ‘strange’ stars – i.e. in stars made up primarily from the ‘strange’ quark. The conventional wisdom is that the electric field of a such a hypothetical strange star (made up from strange matter) at its surface would be so huge and its luminosity so big that it would be impossible to confuse it with anything else.

    However, Jaikumar and his fellow researchers from the Argonne National Laboratory, and two colleagues from Los Alamos National Laboratory in New Mexico, have challenged that. The team developed a theory about what a strange star would look like.

    One of the most interesting aspects of neutron stars is that they are not gaseous like usual stars, but they are so closely packed that they are liquid. Strange stars should also be liquid with a surface that is solid.

    However, Jaikumar and his colleagues challenge that. Strange stars are usually assumed to exhibit huge electric fields on their surface precisely because they are assumed to have a smooth surface. But according to the scientists neither neutron stars nor strange stars have such a smooth solid-like surface.

    “It’s like taking water,” Jaikumar says, “with a flat surface. Add detergent and it reduces surface tension, allowing bubbles to form. In a strange star, the bubbles are made of strange quark matter, and float in a sea of electrons. Consequently, the star’s surface may be crusty, not smooth. The effect of surface tension had been overlooked before.”

    One consequence is that a strange star wouldn’t have large electrical field at surface or be super-luminous. It also allows for a strange star to be less dense than originally thought, although such stars are definitely unusually dense compared to regular stars.

    Much of the matter in our Universe may be made of a type of dark matter called weakly interacting massive particles, also known as WIMPs, . Although some scientists predict that these hypothetical particles possess many of the necessary properties to account for dark matter, until recently scientists have not been able to make any definite predictions of their mass. In a new study, physicists have derived a limit on the WIMP mass by calculating how these dark matter particles can transform neutron stars into stars made of strange quark matter, or “strange” stars.


    WIMPs are thought to be largely located in the halos of galaxies. Although galaxy halos (image above) are not visible, they contain most of a galaxy’s mass in the form of the heavy WIMPs.

    Dr. M. Angeles Perez-Garcia from the University of Salamanca in Salamanca, Spain, along with Dr. Joseph Silk of the University of Oxford and Dr. Jirina R. Stone of the University of Oxford and the University of Tennessee showed that, when a neutron star gravitationally captures nearby WIMPs, the WIMPs may trigger the conversion of the neutron star into a strange star.

    One important issue is whether at high density ‘strange’ quark matter is more stable than regular matter (which is comprised of ‘up’ and ‘down’ quarks). Jaikumar and colleagues think that as a neutron star spins down and its core density increases, it may convert into the more stable state of strange quark matter, forming a strange star.

    Theorists cannot say with absolute certainty whether or not a neutron star gradually converts into a strange star. The conversion occurs, according to new research, as a result of the WIMPs seeding the neutron stars with long-lived lumps of strange quark matter, or strangelets. WIMPs captured in the neutron star’s core self-annihilate, releasing energy in the process.

    According to Jaikumar, making the distinction is rather tricky: “There might be a slight difference. You’d look at surface temperature and see how stars are cooling in time. If it is quark matter, the emission rates are different, so the strange star may cool a little faster.”

    It’s the astronomers’ job to discover whether strange stars exist or not. Either discovery will have important implications for the theory of Quantum Chromodynamics (QCD) — which is the fundamental theory of quarks. “Finding a strange star would improve our understanding of QCD, the fundamental theory of the nuclear force. And it would also be the first solid evidence of stable quark matter”, Jaikumar said.

    Elsewhere, Kwong-Sang Cheng of the University of Hong Kong, China, and colleagues have presented evidence that a quark star formed in a bright supernova called SN 1987A (above), which is among the nearest supernovae to have been observed.

    Observing a quark star could shed light on what happened just after the Big Bang, because at this time, the Universe was filled with a dense sea of quark matter superheated to a trillion °C. While some groups have claimed to have found candidate quark stars, no discovery has yet been confirmed.

    Now Kwong-Sang Cheng of the University of Hong Kong, China, and colleagues have presented evidence that a quark star formed in a bright supernova called SN 1987A (pictured), which is among the nearest supernovae to have been observed.

    The birth of a neutron star is known to be accompanied by a single burst of neutrinos. But when the team examined data from two neutrino detectors – Kamiokande II in Japan and Irvine-Michigan-Brookhaven in the US – they found that SN 1987A gave off two separate bursts.

    “There is a significant time delay between [the bursts recorded by] these two detectors,” says Cheng. They believe the first burst was released when a neutron star formed, while the second was triggered seconds later by its collapse into a quark star. The results appeared in The Astrophysical Journal (http://www.arxiv.org/abs/0902.0653v1).

    “This model is intriguing and reasonable,” says Yong-Feng Huang of Nanjing University, China. “It can explain many key features of SN 1987A.” However, Edward Witten of the Institute for Advanced Study in Princeton, New Jersey, is not convinced. “I hope they’re right,” he says. “My first reaction, though, is that this is a bit of a long shot.”

    High-resolution X-ray observatories, due to fly in space in the next decade, may have the final say. Neutron stars and quark stars should look very different at X-ray wavelengths, says Cheng.

    The image of SN 1987A at top of the page combines data from NASA’s orbiting Chandra X-ray Observatory and the 8-meter Gemini South infrared telescope in Chile, which is funded primarily by the National Science Foundation.

    The X-ray light detected by Chandra is colored blue. The infrared light detected by Gemini South is shown as green and red, marking regions of slightly higher and lower-energy infrared, respectively. The core remains of the star that exploded in 1987 is not visible here. The ring is produced by hot gas (largely the X-ray light) and cold dust (largely the infrared light) from the exploded star interacting with the interstellar region. Credit: Gemini/NASA

    “Supernova 1987A is changing right before our eyes,” said Dr. Eli Dwek, a cosmic dust expert at NASA Goddard Space Flight Center in Greenbelt, Md. For several years Dwek has been following this supernova, named 1987A for the year it was discovered in the Large Magellanic Cloud, a neighboring dwarf galaxy. “What we are seeing now is a milestone in the evolution of a supernova.”

    See the full article here.

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  • richardmitnick 3:17 pm on November 11, 2014 Permalink | Reply
    Tags: , , Quarks, ,   

    From Symmetry: “The November Revolution” 


    November 11, 2014
    Amanda Solliday

    Forty years ago today, two different research groups announced the discovery of the same new particle and redefined how physicists view the universe.

    On November 11, 1974, members of the Cornell high-energy physics group could have spent the lulls during their lunch meeting chatting about the aftermath of Nixon’s resignation or the upcoming Big Red hockey season.

    But on that particular Monday, the most sensational topic was physics-related. One of the researchers in the audience stood up to report that two labs on opposite sides of the country were about to announce the same thing: the discovery of a new particle that heralded the birth of the Standard Model of particle physics.

    Ting and Richter

    The Standard Model of elementary particles, with the three generations of matter, gauge bosons in the fourth column, and the Higgs boson in the fifth.

    “Nobody at the meeting knew what the hell it was,” says physicist Kenneth Lane of Boston University, a former postdoctoral researcher at Cornell. Lane, among others, would spend the next few years describing the theory and consequences of this new particle.

    It isn’t often that a discovery comes along that forces everyone to reevaluate the way the world works. It’s even rarer for two groups to make such a discovery at the same time, using different methods.

    One announcement would come from a research group led by MIT physicist Sam Ting at Brookhaven National Laboratory in New York. The other was to come from a team headed by physicist Burton Richter at SLAC National Accelerator Laboratory, then called the Stanford Linear Accelerator Center, in California. Word traveled fast.

    “We started getting all sorts of inquiries and congratulations before we even finished writing the paper,” Richter says. “Somebody told a friend, and then a friend told another friend.”

    Ting called the new particle the J particle. Richter called it psi. It became known as J/psi, the discovery that sparked the November Revolution.

    Independently, the researchers at Brookhaven and SLAC had designed two complementary experiments.

    Ting and his team had made the discovery using a proton machine, shooting an intense beam of particles at a fixed target. Ting was interested in how photons, particles of light, turn into heavy photons, particles with mass, and he wanted to know how many of these types of heavy photons existed in nature. So his team—consisting of 13 scientists from MIT with help from researchers at Brookhaven—designed and built a detector that would accept a wide range of heavy photon masses.

    “The experiment was quite difficult,” Ting says. “I guess when you’re younger, you’re more courageous.”

    In early summer 1974, they started the experiment at a high mass, around 4 to 5 billion electronvolts. They saw nothing. Later, they lowered the mass and soon saw a peak near 3 billion electronvolts that indicated a high production rate of a previously unknown particle.

    At SLAC, Richter had created a new type of collider, the Stanford Positron Electron Asymmetric Rings (SPEAR). His research group used a beam of electrons produced by a linear accelerator and stored the particles in a ring of magnets. Then, they would generate positrons in a linear accelerator and inject them in the other direction. The detector was able to look at everything produced in electron-positron collisions.

    The goal was to determine the masses of known elementary particles, but the researchers saw strange effects in the summer of 1974. They looked at that particular region with finer resolution, and over the weekend of November 9-10, discovered a tall, thin energy peak around 3 billion electronvolts.

    At the time, Ting visited SLAC as part of an advisory committee. The laboratory’s director, Pief Panofsky, asked Richter to meet with him.

    “He called and said, ‘It sounds like you guys have found the same thing,’” Richter says.

    Both researchers sent their findings to the journal Physical Review Letters. Their papers were published in the same issue. Other labs quickly replicated and confirmed the results.

    At the time, the basic pieces of today’s Standard Model of particle physics were still falling into place. Just a decade before, it had resembled the periodic table of the elements, including a wide, unruly collection of different types of particles called hadrons.

    Theorists Murray Gell-Mann and George Zweig were the first to propose that all of those different types of hadrons were actually made up of the same building blocks, called quarks. This model included three types of quark: up, down and strange. Other theorists—Sheldon Lee Glashow, James Bjorken, and then also John Iliopoulos and Luciano Maiani—proposed the existence of a fourth quark.

    On the day of the J/psi announcement, the Cornell researchers talked about the findings well into the afternoon. One of the professors in the department, Ken Wilson, made a connection between the discovery and a seminar given earlier that fall by Tom Appelquist, a physicist at Harvard University. Appelquist had been working with his colleague David Politzer to describe something they called “charmonium,” a bound state of a new type of quark and antiquark.

    “Only a few of us were thinking about the idea of a fourth quark,” says Appelquist, now a professor at Yale. “Ken called me right after the discovery and urged me to get our paper out ASAP.”

    The J/psi news inspired many other theorists to pick up their chalk as well.

    “It was clear from day one that J/psi was a major discovery,” Appelquist says. “It almost completely reoriented the theoretical community. Everyone wanted to think about it.”

    Less than two weeks after the initial discovery, Richter’s group also found psi-prime, a relative of J/psi that showed even more cracks in the three-quark model.

    “There was a whole collection of possibilities of what could exist outside the current model, and people were speculating about what that may be,” Richter says. “Our experiment pruned the weeds.”

    The findings of the J/psi teams triggered additional searches for unknown elementary particles, exploration that would reveal the final shape of the Standard Model. In 1976, the two experiment leaders were awarded the Nobel Prize for their achievement.

    In 1977, scientists at Fermilab discovered the fifth quark, the bottom quark. In 1995, they discovered the sixth one, the top.

    Today, theorists and experimentalists are still driven to answer questions not explained by the current prevailing model. Does supersymmetry exist? What are dark matter and dark energy? What particles have we yet to discover?

    Supersymmetry standard model
    Standard Model of Supersymmetry

    “If the answers are found, it will take us even deeper into what we are supposed to be doing as high-energy physicists,” Lane says. “But it probably isn’t going to be this lightning flash that happens on one Monday afternoon.”

    Ting and Richter
    Courtesy of: SLAC National Accelerator Laboratory

    See the full article here.

    Symmetry is a joint Fermilab/SLAC publication.

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  • richardmitnick 12:56 pm on October 3, 2014 Permalink | Reply
    Tags: , , , , , , , , Quarks   

    From FNAL- “Frontier Science Result: CMS Subatomic hydrodynamics” 

    Fermilab is an enduring source of strength for the US contribution to scientific research world wide.

    Friday, Oct. 3, 2014
    This column was written by Don Lincoln
    FNAL Don Lincoln
    Dr. Don Lincoln

    It’s hard for most people to imagine what it’s like at the heart of a particle collision. Two particles speed toward one another from opposite directions and their force fields intertwine, causing some of the particles’ constituents to be ejected. Or possibly the energy embodied in the interaction might be high enough to actually create matter and antimatter. It’s no wonder the whole process seems confusing.

    The same basic equations that govern the flow of water are important for describing the collisions of lead nuclei. In today’s article, we’ll get a glimpse of how this works.

    Things get a little easier to imagine when the particles are the nuclei of atoms (note that I said easier, not easy). For collisions between two nuclei of lead, one can imagine two small spheres, each containing 208 protons and neutrons, coming together to collide. Depending on the violence of the collision, some or many of the protons and neutrons might figuratively melt, releasing their constituent quarks so they can scurry around willy-nilly. Physicists call this form of matter a quark-gluon plasma, and it acts much like a liquid.

    The quark structure of the proton. The color assignment of individual quarks is arbitrary, but all three colors must be present. Forces between quarks are mediated by gluons

    The quark structure of the neutron. The color assignment of individual quarks is arbitrary, but all three colors must be present. Forces between quarks are mediated by gluons

    Part of this liquid-like behavior is due to the fact that so many particles are involved. An LHC collision between two lead nuclei might involve thousands or tens of thousands of particles. Because these particles are quarks and gluons, they experience the strong nuclear force. So as long as they are close enough to each other, the particles interact strongly enough that they clump a bit together. The net outcome is that the flow of particles from collision between lead nuclei looks vaguely like splashes of water. In these cases, the equations of hydrodynamics apply. Mathematical descriptions like these have been used to make sense of other features we see in LHC collisions between lead nuclei.

    In Feynman diagrams, emitted gluons are represented as helices. This diagram depicts the annihilation of an electron and positron.

    However, there is more to understand. We can imagine collisions between the collective 416 protons and neutrons of lead nuclei as splashes of water, but when a pair of protons collide, the collision doesn’t yield enough particles to exhibit hydrodynamic behavior. So as the number of particles involved goes down, the “splash” behavior must slowly go away. In addition, in the first studies of lead nuclei collisions, only the grossest features of the collision were studied. This is because it is impossible to identify individual quarks and gluons.

    There are ways to dig into these sorts of questions. One way is to look at collisions in which one beam is a proton and the other is a lead nucleus. This is a halfway point between the usual LHC proton-proton collisions and the lead-lead ones. In addition, we can turn our attention to quarks that we can unambiguously identify, such as bottom, charm and strange quarks, to better understand the hydrodynamic behavior.

    In this study, physicists looked at particles containing strange quarks. Since strange quarks don’t exist in the beam protons, studying them gives a unique window into the dynamics of lead-lead collisions. By combining studies of particles with strange quarks in lead-lead and lead-proton collisions, scientists hope to better understand the complicated and liquid-like behavior that is just beginning to reveal its secrets.

    See the full article here.

    Fermilab Campus

    Fermi National Accelerator Laboratory (Fermilab), located just outside Batavia, Illinois, near Chicago, is a US Department of Energy national laboratory specializing in high-energy particle physics.

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  • richardmitnick 3:12 pm on August 27, 2014 Permalink | Reply
    Tags: , , Quarks   

    From Quanta: “Quark Quartet Fuels Quantum Feud” 

    Quanta Magazine
    Quanta Magazine

    August 27, 2014
    Natalie Wolchover

    In August 2003, an experiment at the KEKB particle accelerator in Japan found hints of an unexpected particle: A composite of elementary building blocks called quarks, it contained not two quarks like mesons or three like the protons and neutrons that constitute all visible matter, but four — a number that theoretical physicists had come to think the laws of nature did not permit. This candidate “tetraquark” disintegrated so quickly that it seemed a stretch to call it a particle at all. But as similar formations appeared in experiments around the world, they incited a fierce debate among experts about the correct picture of matter at the quantum scale.

    View of the KEKB accelerator beamlines, acceleration cavities, and steering magnets, at the intersection with one of the injection beamlines

    Most believed tetraquarks were a new kind of miniature molecule — essentially, two orbiting mesons, each made of one regular quark and one antimatter quark, or antiquark — while a smaller contingent saw them as stand-alone particles in which the two quarks and two antiquarks overlapped in the same small volume of space.

    “We hate each other,” said Antonio Polosa, a theorist at Sapienza University of Rome who takes the latter stance, chuckling about the rival factions. “We really hate each other.”

    Most critics find the molecular model hard to swallow. “It’s kind of a crystal glass in a nuclear explosion.”

    All parties remained uncertain that tetraquarks were real — until one turned up in data from the Large Hadron Collider, the 17-mile, proton-smashing ring near Geneva.

    CERN LHC Map
    CERN LHC Grand Tunnel
    LHC at CERN

    Detailed measurements reported in June in Physical Review Letters confirm that the particle, which was first detected in 2007 at the accelerator in Japan and designated Z(4430), is unambiguously a tetraquark. Now, the discovery is forcing physicists to extend their simple picture of quark interactions, or finally replace it with a more nuanced understanding.

    And, to mixed reviews, the properties of Z(4430) clearly favor the underdog “diquark model” and the hypothesis that tetraquarks are genuine particles. The existence of such states would suggest a menagerie of exotic “hadrons,” or particles made of quarks, including groupings of more than four. It would also attest to subtle quantum interactions that may shape the cores of hypothetical “quark stars,” the piping hot quark soup thought to have saturated the infant universe, and, closer to home, the proton and neutron building blocks of ordinary matter.

    The quark structure of the proton. (The color assignment of individual quarks is not important, only that all three colors are present.)

    The quark structure of the neutron. (The color assignment of individual quarks is not important, only that all three colors are present.)

    “Other interpretations are really not very tenable, to be honest, with respect to this particle,” said Polosa, one of the originators of the diquark model.

    But advocates of the rival molecular model disagree. To their minds, tetraquarks tell a novel but more conservative story of mesons engaging in chemistry below the ordinary atomic scale, without challenging the dogma that two- and three-quark particles are the only hadrons that exist. Z(4430), a tetraquark that looks unlike any combination of two types of mesons mingling as a molecule, certainly “makes it more difficult,” said Marek Karliner, a particle physicist at Tel Aviv University in Israel. But, he said, the diquark model has troubles of its own.

    With each model able to account for some of the 20-or-so candidate tetraquarks that seem perplexing from the other perspective, a third view has recently gained ground: the belief that any simple model falls short. “These models might all have some aspect of the truth,” said Eric Braaten, a theoretical physicist at Ohio State University, but like the proverbial blind men encountering different parts of an elephant, “they can’t describe the elephant globally.”

    The diquark and molecular models are both attempts to salvage a cartoonish picture of hadrons that worked perfectly, if inexplicably, for the half-century before tetraquarks came along. This “quark model” treats the proton as if it is made of three bloated quarks, each contributing a third to its total mass. However, experiments have shown that the quarks in a proton (and other hadrons) are actually much lighter than their sum, each one mere thousandths of the proton’s mass. The rest of its mass derives from the energy involved in gluing the three quarks together, a stickiness known as the strong force that is conveyed by particles called gluons. “The thing you call the ‘quark’ might have quark-antiquark pairs and glue and all the rest built into it,” explained Thomas Cohen, a physics professor at the University of Maryland.

    The exact structure of hadrons is hidden in the folds of a 40-year-old theory of the strong force called quantum chromodynamics (QCD), an easy-to-write-down but infinitely self-referential and thus unsolvable set of equations. No one understands why QCD’s boundless complexity seems equivalent to the quark model, or in other words, why the dynamic confluence of quarks and gluons known as a proton “somehow behaves as if it’s a simple composite of three particles,” Braaten said. Up to now, all hadrons feigned such simplicity. Tetraquarks, which the renowned theorists Edward Witten and Sidney Coleman mistakenly argued in the 1970s were inconsistent with a simplified analogue of QCD, have turned out to be the first manifestations of the theory that aren’t also captured by the quark model.

    Quantum Chromodynamics

    Quarks have one of three “color charges,” which are analogous to the primary colors red, green and blue. Just as an atom strikes a balance between positive and negative electrical charges, particles made of quarks balance colors to reach a neutral state. In the color analogy, that means combining colors to make white.


    “The embarrassing fact that tetraquarks have been discovered experimentally and weren’t predicted is an indication that we don’t understand QCD as well as we thought we did,” Braaten said.

    Now, rather than abandon the quark model altogether, proponents of the molecular and diquark models hope to extend it to encompass the new discoveries.

    In the spirit of the original model, the two proposed extensions pretend that tetraquarks are foursomes of plump quarks. But they present opposite visions for how these components are arranged. According to both QCD and the quark model, quarks have a property called “color,” and they must enter collective states that are color-neutral. The colors of three quarks cancel one another out inside a proton just as, for example, combining the primary colors red, green and blue makes white. And, as with complementary colors like blue and yellow, quarks pair with antiquarks to form colorless mesons. But how do four quarks achieve color neutrality inside tetraquarks?

    Exotic Compositions

    Tetraquarks are composed of two primary color quarks and two antiquarks, each with one of the complementary colors yellow, magenta or cyan. Scientists are debating how these four particles combine into a color-neutral state.


    In the molecular model, quark-antiquark pairs form two color-neutral mesons that become weakly linked as a molecule.

    diquark DIQUARK MODEL

    In the diquark model, the particles form quark-quark and antiquark-antiquark pairs, which are forced to combine to balance their color charges.

    In the molecular picture, each of the two quarks pairs with one of the two antiquarks, forming two color-neutral mesons that rendezvous momentarily as a meson molecule before flying apart. But in the diquark model, the quarks form a “diquark” pair, as do the antiquarks; both pairs have net color, so they must fuse as a compact, color-neutral particle for an instant before switching partners and dissociating.

    The molecular model’s key supporting examples have masses very close to the sum of two separate mesons, implying that these mesons have briefly become bound by the smallest drop of glue (changing their combined molecular mass by a hair) before breaking up. The tetraquark signal found in Japan in 2003, designated X(3872) for its unknown identity and measured mass, exhibited just this mass coincidence; it and a smattering of subsequent examples elevated the molecular model to early prominence.

    “The Occam’s razor approach says try the simplest thing,” Karliner said. “If it works, then that’s what it is.”

    But with the recent discovery of Z(4430), the razor has turned against the molecular model. The tetraquark’s mass and other meticulously measured characteristics, such as a property called “spin-parity,” do not match up with those of any two mesons. To its critics, this kills the molecule idea. As for its holdouts, “the importance of the evidence against them hasn’t sunk in yet,” said diquark proponent Richard Lebed of Arizona State University. “There’s a certain resistance, if you’ve spent a long time working on a particular picture, to evidence that it’s not right.”

    Although Lebed finds it mildly perplexing that so many tetraquark candidates have almost exactly the masses of known meson combinations, other diquark supporters call these coincidences unavoidable: With no less than 12 varieties of quarks and antiquarks, some of their four-way combined masses are bound to accidentally fall close to those of meson pairs, they say.

    And, ultimately, most critics find the molecular model hard to swallow. How could a fragile construct of two mesons hold together with so little glue in the middle of a near-light-speed collision of protons at the Large Hadron Collider? “It’s kind of a crystal glass in a nuclear explosion,” Polosa said.

    The diquark picture may be pulling ahead in the aftermath of the Z(4430) discovery, but it too remains incomplete. Recently, adherents like Polosa have been working to round out the model with new symmetry rules that would explain why certain quark combinations seem to be permitted and others not, a pattern that was much better captured by the molecular model. Meanwhile, in a new paper accepted for publication in Physical Review Letters, Lebed and colleagues attempt to explain why the colorful diquark and antidiquark pairs might form in the first place and why they quickly decay. If correct, these rules will influence other predictions about hadron physics, such as whether exotic, quark-filled stars exist. They would also deepen the understanding of familiar hadrons like protons and neutrons.

    But not everyone feels satisfied with embellishing the diquark picture until it works.

    “These models have lots of knobs in them,” Cohen said. “When it gets things right, you declare victory, and when it doesn’t quite get things right, you start turning knobs.” The dispute between the two models, he said, is about figuring out “which one is the better starting point for describing the data reasonably well” — not whether either one is true.

    Physicists like Cohen and Braaten believe the full spectrum of tetraquarks can only be predicted by better approximating the unending equations of QCD, an effort that would also elucidate other unknown features of quarks and gluons, such as their behavior milliseconds after the Big Bang. In a recent paper in Physical Review D, Braaten and colleagues suggested an approximation scheme operable on a supercomputer that they think could work for predicting tetraquarks.

    The advantages of such an approach might be marginal, however. In 40 years, researchers have failed to build abridgments of QCD that fit the data much better than the naive quark model. Many theorists prefer to extend this simple and effective understanding rather than enter a QCD quagmire.

    So the question remains: In future diagrams of tetraquarks, will the quarks be shown hand-in-hand with quarks or with antiquarks?

    “There’s going to be a picture that’s substantially more successful than others once all the data is known,” Lebed said. “But I would not at all be surprised if there were still a few nagging mysteries.”

    See the full article here.

    Formerly known as Simons Science News, Quanta Magazine is an editorially independent online publication launched by the Simons Foundation to enhance public understanding of science. Why Quanta? Albert Einstein called photons “quanta of light.” Our goal is to “illuminate science.” At Quanta Magazine, scientific accuracy is every bit as important as telling a good story. All of our articles are meticulously researched, reported, edited, copy-edited and fact-checked.

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