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  • richardmitnick 10:31 am on October 9, 2015 Permalink | Reply
    Tags: , , Particle Physics,   

    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 6:43 am on October 6, 2015 Permalink | Reply
    Tags: , Kavli IPMU, , Particle Physics   

    From Kavli IPMU: “2015 Nobel Prize in Physics awarded to Takaaki Kajita” 


    The Kavli Foundation

    Kavli IPMU
    Kavli IMPU


    Takaaki Kajita

    The 2015 Nobel Prize in Physics has been awarded to Takaaki Kajita, Director of the University of Tokyo Institute for Cosmic Ray Research and Kavli IPMU Principal Investigator. Congratulations! This year’s prize recognizes the work carried out on atmospheric neutrino oscillations at the Kamiokande and Super Kamiokande/a> detectors in Kamioka, central Japan, which produced definite proof that neutrinos have mass.

    Super-Kamiokande experiment Japan
    Super-Kamiokande neutrino detector

    Kavli IPMU Director Hitoshi Murayama’s comment on today’s Nobel Prize:

    “I’ve always believed that Kajita’s discovery in 1998 should be awarded Nobel Prize in Physics. All Nobel Prizes in particle physics so far were given to achievements that led to the establishment of the theory called ‘Standard Model‘.

    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.

    On the other hand, Kajita, and the joint awardee Art McDonald,


    have shown for the first time in history that the Standard Model cannot explain everything in the Universe. Their work is historic in that they have shown that the Standard Model is not the ultimate goal, but rather needs to be expanded to a yet bigger framework. Actually there is a long-standing problem “Why do we exist in the Universe? Universe created matter and anti-matter one to one, but somehow the balance was tilted towards matter at the level of one part in billion, so that matter and anti-matter did” not completely annihilate each other and a small amount of matter remained to date. How was the balance changed?” This is literally a matter of life and death for us. Now that they discovered that the neutrinos have tiny amount of mass, there is a very strong anticipation in the community that neutrino is our ‘father’ who protected us from the complete annihilation, by tilting the balance between matter and anti-matter. This is a theory put forward by Fukugita and Yanagida at Kavli IPMU, but it became very plausible after Kajita’s discovery. As a matter of fact, particle physics in the US puts research in this area as its first priority. Clearly Kajita’s work changed the direction of research in particle physics.

    Useful link

    Atmospheric Neutrinos and Neutrino Oscillations (written by Takaaki Kajita): http://www.ipmu.jp/webfm_send/557

    See the full article here .

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    Kavli IPMU (Kavli Institute for the Physics and Mathematics of the Universe) is an international research institute with English as its official language. The goal of the institute is to discover the fundamental laws of nature and to understand the Universe from the synergistic perspectives of mathematics, astronomy, and theoretical and experimental physics. The Institute for the Physics and Mathematics of the Universe (IPMU) was established in October 2007 under the World Premier International Research Center Initiative (WPI) of the Ministry of Education, Sports, Science and Technology in Japan with the University of Tokyo as the host institution. IPMU was designated as the first research institute within the University of Tokyo Institutes for Advanced Study (UTIAS) in January 2011. It received an endowment from The Kavli Foundation and was renamed the “Kavli Institute for the Physics and Mathematics of the Universe” in April 2012. Kavli IPMU is located on the Kashiwa campus of the University of Tokyo, and more than half of its full-time scientific members come from outside Japan. http://www.ipmu.jp/
    Stem Education Coalition
    The Kavli Foundation, based in Oxnard, California, is dedicated to the goals of advancing science for the benefit of humanity and promoting increased public understanding and support for scientists and their work.

    The Foundation’s mission is implemented through an international program of research institutes, professorships, and symposia in the fields of astrophysics, nanoscience, neuroscience, and theoretical physics as well as prizes in the fields of astrophysics, nanoscience, and neuroscience.

  • richardmitnick 5:26 pm on September 29, 2015 Permalink | Reply
    Tags: , , , , , , Particle Physics   

    From SMU: “Top Quark: New precise particle measurement improves subatomic tool for probing mysteries of universe” 


    SMU Research

    September 28, 2015
    Margaret Allen

    In post-Big Bang world, nature’s top quark — a key component of matter — is a highly sensitive probe that physicists use to evaluate competing theories about quantum interactions

    Physicists at Southern Methodist University, Dallas, have achieved a new precise measurement of a key subatomic particle, opening the door to better understanding some of the deepest mysteries of our universe.

    The researchers calculated the new measurement for a critical characteristic — mass — of the top quark.

    A collision event involving top quarks

    Quarks make up the protons and neutrons that comprise almost all visible matter. Physicists have known the top quark’s mass was large, but encountered great difficulty trying to clearly determine it.

    The newly calculated measurement of the top quark will help guide physicists in formulating new theories, said Robert Kehoe, a professor in SMU’s Department of Physics. Kehoe leads the SMU group that performed the measurement.

    Top quark’s mass matters ultimately because the particle is a highly sensitive probe and key tool to evaluate competing theories about the nature of matter and the fate of the universe.

    Physicists for two decades have worked to improve measurement of the top quark’s mass and narrow its value.

    “Top” bears on newest fundamental particle, the Higgs boson

    The new value from SMU confirms the validity of recent measurements by other physicists, said Kehoe.

    But it also adds growing uncertainty about aspects of physics’ Standard Model.

    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.

    The Standard Model is the collection of theories physicists have derived — and continually revise — to explain the universe and how the tiniest building blocks of our universe interact with one another. Problems with the Standard Model remain to be solved. For example, gravity has not yet been successfully integrated into the framework.

    The Standard Model holds that the top quark — known familiarly as “top” — is central in two of the four fundamental forces in our universe — the electroweak force, by which particles gain mass, and the strong force, which governs how quarks interact. The electroweak force governs common phenomena like light, electricity and magnetism. The strong force governs atomic nuclei and their structure, in addition to the particles that quarks comprise, like protons and neutrons in the nucleus.

    The top plays a role with the newest fundamental particle in physics, the Higgs boson, in seeing if the electroweak theory holds water.

    Some scientists think the top quark may be special because its mass can verify or jeopardize the electroweak theory. If jeopardized, that opens the door to what physicists refer to as “new physics” — theories about particles and our universe that go beyond the Standard Model.

    Other scientists theorize the top quark might also be key to the unification of the electromagnetic and weak interactions of protons, neutrons and quarks.

    In addition, as the only quark that can be observed directly, the top quark tests the Standard Model’s strong force theory.

    “So the top quark is really pushing both theories,” Kehoe said. “The top mass is particularly interesting because its measurement is getting to the point now where we are pushing even beyond the level that the theorists understand.”

    He added, “Our experimental errors, or uncertainties, are so small, that it really forces theorists to try hard to understand the impact of the quark’s mass. We need to observe the Higgs interacting with the top directly and we need to measure both particles more precisely.”

    The new measurement results were presented in August and September at the Third Annual Conference on Large Hadron Collider Physics, St. Petersburg, Russia, and at the 8th International Workshop on Top Quark Physics, Ischia, Italy.

    “The public perception, with discovery of the Higgs, is ‘Ok, it’s done,’” Kehoe said. “But it’s not done. This is really just the beginning and the top quark is a key tool for figuring out the missing pieces of the puzzle.”

    The results were made public by DZero, a collaborative experiment of more than 500 physicists from around the world. The measurement is described in Precise measurement of the top quark mass in dilepton decays with optimized neutrino weighting and is available online at arxiv.org/abs/1508.03322.

    SMU measurement achieves surprising level of precision

    To narrow the top quark measurement, SMU doctoral researcher Huanzhao Liu took a standard methodology for measuring the top quark and improved the accuracy of some parameters. He also improved calibration of an analysis of top quark data.

    “Liu achieved a surprising level of precision,” Kehoe said. “And his new method for optimizing analysis is also applicable to analyses of other particle data besides the top quark, making the methodology useful within the field of particle physics as a whole.”

    The SMU optimization could be used to more precisely understand the Higgs boson, which explains why matter has mass, said Liu.

    The Higgs was observed for the first time in 2012, and physicists keenly want to understand its nature.

    “This methodology has its advantages — including understanding Higgs interactions with other particles — and we hope that others use it,” said Liu. “With it we achieved 20-percent improvement in the measurement. Here’s how I think of it myself — everybody likes a $199 iPhone with contract. If someday Apple tells us they will reduce the price by 20 percent, how would we all feel to get the lower price?”

    Another optimization employed by Liu improved the calibration precision by four times, Kehoe said.

    Shower of Top quarks post Big Bang

    Top quarks, which rarely occur now, were much more common right after the Big Bang 13.8 billion years ago. However, top is the only quark, of six different kinds, that can be observed directly. For that reason, experimental physicists focus on the characteristics of top quarks to better understand the quarks in everyday matter.

    To study the top, physicists generate them in particle accelerators, such as the Tevatron, a powerful U.S. Department of Energy particle accelerator operated by Fermi National Laboratory in Illinois, or the Large Hadron Collider in Switzerland, a project of the European Organization for Nuclear Research, CERN.

    FNAL Tevatron
    FNAL DZero
    Tevatron, DZero, CDF

    CERN LHC Map
    CERN LHC Grand Tunnel
    CERN LHC particles
    CERN CMS Detector

    SMU’s measurement draws on top quark data gathered by DZero that was produced from proton-antiproton collisions at the Tevatron, which Fermilab shut down in 2011.

    The new measurement is the most precise of its kind from the Tevatron, and is competitive with comparable measurements from the Large Hadron Collider. The top quark mass has been precisely measured more recently, but there is some divergence of the measurements. The SMU result favors the current world average value more than the current world record holder measurement, also from Fermilab. The apparent discrepancy must be addressed, Kehoe said.

    Critical question: Universe isn’t necessarily stable at all energies

    “The ability to measure the top quark mass precisely is fortuitous because it, together with the Higgs boson mass, tells us whether the universe is stable or not,” Kehoe said. “That has emerged as one of today’s most important questions.”

    A stable universe is one in a low energy state where particles and forces interact and behave according to theoretical predictions forever. That’s in contrast to metastable, or unstable, meaning a higher energy state in which things eventually change, or change suddenly and unpredictably, and that could result in the universe collapsing. The Higgs and top quark are the two most important parameters for determining an answer to that question, Kehoe said.

    Recent measurements of the Higgs and top quark indicate they describe a universe that is not necessarily stable at all energies.

    “We want a theory — Standard Model or otherwise — that can predict physical processes at all energies,” Kehoe said. “But the measurements now are such that it looks like we may be over the border of a stable universe. We’re metastable, meaning there’s a gray area, that it’s stable in some energies, but not in others.”

    Are we facing imminent doom? Will the universe collapse?

    That disparity between theory and observation indicates the Standard Model theory has been outpaced by new measurements of the Higgs and top quark.

    “It’s going to take some work for theorists to explain this,” Kehoe said, adding it’s a challenge physicists relish, as evidenced by their preoccupation with “new physics” and the possibilities the Higgs and Top quark create.

    “I attended two conferences recently,” Kehoe said, “and there’s argument about exactly what it means, so that could be interesting.”

    So are we in trouble?

    “Not immediately,” Kehoe said. “The energies at which metastability would kick in are so high that particle interactions in our universe almost never reach that level. In any case, a metastable universe would likely not change for many billions of years.”

    Top quark — a window into other quarks

    As the only quark that can be observed, the top quark pops in and out of existence fleetingly in protons, making it possible for physicists to test and define its properties directly.

    “To me it’s like fireworks,” Liu said. “They shoot into the sky and explode into smaller pieces, and those smaller pieces continue exploding. That sort of describes how the top quark decays into other particles.”

    By measuring the particles to which the top quark decays, scientists capture a measure of the top quark, Liu explained

    But study of the top is still an exotic field, Kehoe said. “For years top quarks were treated as a construct and not a real thing. Now they are real and still fairly new — and it’s really important we understand their properties fully.” — Margaret Allen

    See the full article here .

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    SMU Campus

    A nationally ranked private university with seven degree-granting schools, SMU is a distinguished center for teaching and research located near the heart of Dallas. SMU’s 11,000 students benefit from small classes, leadership opportunities, international study and innovative programs.

    SMU is celebrating the centennial of its founding in 1911 and its opening in 1915. As SMU enters a second century of achievement, it is recognized as a university of increasing national prominence.

    SMU prepares students for leadership in their professions and in their communities. The University’s location near the heart of Dallas – a thriving center of commerce and culture – offers students enriching experiences on campus and beyond. Relationships in the Dallas area provide a platform for launching careers throughout the world.

    The University offers a strong foundation in the humanities and sciences and undergraduate, graduate and professional degree programs through seven schools. The learning environment includes opportunities for research, community service, internships, mentoring and study abroad.

  • richardmitnick 7:22 pm on September 28, 2015 Permalink | Reply
    Tags: Amplituhedrons, , Geometry, Particle Physics, , Quantum field theory   

    From Quanta: “A Jewel at the Heart of Quantum Physics” 

    Quanta Magazine
    Quanta Magazine

    September 17, 2013
    Natalie Wolchover

    Artist’s rendering of the amplituhedron, a newly discovered mathematical object resembling a multifaceted jewel in higher dimensions. Encoded in its volume are the most basic features of reality that can be calculated — the probabilities of outcomes of particle interactions. Illustration by Andy Gilmore

    Physicists have discovered a jewel-like geometric object that dramatically simplifies calculations of particle interactions and challenges the notion that space and time are fundamental components of reality.

    “This is completely new and very much simpler than anything that has been done before,” said Andrew Hodges, a mathematical physicist at Oxford University who has been following the work.

    The revelation that particle interactions, the most basic events in nature, may be consequences of geometry significantly advances a decades-long effort to reformulate quantum field theory, the body of laws describing elementary particles and their interactions. Interactions that were previously calculated with mathematical formulas thousands of terms long can now be described by computing the volume of the corresponding jewel-like amplituhedron, which yields an equivalent one-term expression.

    “The degree of efficiency is mind-boggling,” said Jacob Bourjaily, a theoretical physicist at Harvard University and one of the researchers who developed the new idea. “You can easily do, on paper, computations that were infeasible even with a computer before.”

    The new geometric version of quantum field theory could also facilitate the search for a theory of quantum gravity that would seamlessly connect the large- and small-scale pictures of the universe. Attempts thus far to incorporate gravity into the laws of physics at the quantum scale have run up against nonsensical infinities and deep paradoxes. The amplituhedron, or a similar geometric object, could help by removing two deeply rooted principles of physics: locality and unitarity.

    “Both are hard-wired in the usual way we think about things,” said Nima Arkani-Hamed, a professor of physics at the Institute for Advanced Study in Princeton, N.J., and the lead author of the new work, which he is presenting in talks and in a forthcoming paper. “Both are suspect.”

    Nima Arkani-Hamed

    Locality is the notion that particles can interact only from adjoining positions in space and time. And unitarity holds that the probabilities of all possible outcomes of a quantum mechanical interaction must add up to one. The concepts are the central pillars of quantum field theory in its original form, but in certain situations involving gravity, both break down, suggesting neither is a fundamental aspect of nature.

    In keeping with this idea, the new geometric approach to particle interactions removes locality and unitarity from its starting assumptions. The amplituhedron is not built out of space-time and probabilities; these properties merely arise as consequences of the jewel’s geometry. The usual picture of space and time, and particles moving around in them, is a construct.

    “It’s a better formulation that makes you think about everything in a completely different way,” said David Skinner, a theoretical physicist at Cambridge University.

    The amplituhedron itself does not describe gravity. But Arkani-Hamed and his collaborators think there might be a related geometric object that does. Its properties would make it clear why particles appear to exist, and why they appear to move in three dimensions of space and to change over time.

    Because “we know that ultimately, we need to find a theory that doesn’t have” unitarity and locality, Bourjaily said, “it’s a starting point to ultimately describing a quantum theory of gravity.”

    Clunky Machinery

    The amplituhedron looks like an intricate, multifaceted jewel in higher dimensions. Encoded in its volume are the most basic features of reality that can be calculated, “scattering amplitudes,” which represent the likelihood that a certain set of particles will turn into certain other particles upon colliding. These numbers are what particle physicists calculate and test to high precision at particle accelerators like the Large Hadron Collider in Switzerland.

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

    The iconic 20th century physicist Richard Feynman invented a method for calculating probabilities of particle interactions using depictions of all the different ways an interaction could occur. Examples of “Feynman diagrams” were included on a 2005 postage stamp honoring Feynman. United States Postal Service

    The 60-year-old method for calculating scattering amplitudes — a major innovation at the time — was pioneered by the Nobel Prize-winning physicist Richard Feynman. He sketched line drawings of all the ways a scattering process could occur and then summed the likelihoods of the different drawings. The simplest Feynman diagrams look like trees: The particles involved in a collision come together like roots, and the particles that result shoot out like branches. More complicated diagrams have loops, where colliding particles turn into unobservable “virtual particles” that interact with each other before branching out as real final products. There are diagrams with one loop, two loops, three loops and so on — increasingly baroque iterations of the scattering process that contribute progressively less to its total amplitude. Virtual particles are never observed in nature, but they were considered mathematically necessary for unitarity — the requirement that probabilities sum to one.

    “The number of Feynman diagrams is so explosively large that even computations of really simple processes weren’t done until the age of computers,” Bourjaily said. A seemingly simple event, such as two subatomic particles called gluons colliding to produce four less energetic gluons (which happens billions of times a second during collisions at the Large Hadron Collider), involves 220 diagrams, which collectively contribute thousands of terms to the calculation of the scattering amplitude.

    In 1986, it became apparent that Feynman’s apparatus was a Rube Goldberg machine.

    To prepare for the construction of the Superconducting Super Collider [SSC] in Texas (a project that was later canceled), theorists wanted to calculate the scattering amplitudes of known particle interactions to establish a background against which interesting or exotic signals would stand out. But even 2-gluon to 4-gluon processes were so complex, a group of physicists had written two years earlier, “that they may not be evaluated in the foreseeable future.”

    Ill fated SSC, killed by a myopic (Democratic)US Congress theat saw “no immediate economic benefit” in the finished project.

    Stephen Parke and Tomasz Taylor, theorists at Fermi National Accelerator Laboratory in Illinois, took that statement as a challenge. Using a few mathematical tricks, they managed to simplify the 2-gluon to 4-gluon amplitude calculation from several billion terms to a 9-page-long formula, which a 1980s supercomputer could handle. Then, based on a pattern they observed in the scattering amplitudes of other gluon interactions, Parke and Taylor guessed a simple one-term expression for the amplitude. It was, the computer verified, equivalent to the 9-page formula. In other words, the traditional machinery of quantum field theory, involving hundreds of Feynman diagrams worth thousands of mathematical terms, was obfuscating something much simpler. As Bourjaily put it: “Why are you summing up millions of things when the answer is just one function?”

    “We knew at the time that we had an important result,” Parke said. “We knew it instantly. But what to do with it?”

    The Amplituhedron

    The message of Parke and Taylor’s single-term result took decades to interpret. “That one-term, beautiful little function was like a beacon for the next 30 years,” Bourjaily said. It “really started this revolution.”

    Twistor diagrams depicting an interaction between six gluons, in the cases where two (left) and four (right) of the particles have negative helicity, a property similar to spin. The diagrams can be used to derive a simple formula for the 6-gluon scattering amplitude. Arkani-Hamed et al.

    In the mid-2000s, more patterns emerged in the scattering amplitudes of particle interactions, repeatedly hinting at an underlying, coherent mathematical structure behind quantum field theory. Most important was a set of formulas called the BCFW recursion relations, named for Ruth Britto, Freddy Cachazo, Bo Feng and Edward Witten. Instead of describing scattering processes in terms of familiar variables like position and time and depicting them in thousands of Feynman diagrams, the BCFW relations are best couched in terms of strange variables called “twistors,” and particle interactions can be captured in a handful of associated twistor diagrams. The relations gained rapid adoption as tools for computing scattering amplitudes relevant to experiments, such as collisions at the Large Hadron Collider. But their simplicity was mysterious.

    “The terms in these BCFW relations were coming from a different world, and we wanted to understand what that world was,” Arkani-Hamed said. “That’s what drew me into the subject five years ago.”

    With the help of leading mathematicians such as Pierre Deligne, Arkani-Hamed and his collaborators discovered that the recursion relations and associated twistor diagrams corresponded to a well-known geometric object. In fact, as detailed in a paper posted to arXiv.org in December by Arkani-Hamed, Bourjaily, Cachazo, Alexander Goncharov, Alexander Postnikov and Jaroslav Trnka, the twistor diagrams gave instructions for calculating the volume of pieces of this object, called the positive Grassmannian.

    Named for Hermann Grassmann, a 19th-century German linguist and mathematician who studied its properties, “the positive Grassmannian is the slightly more grown-up cousin of the inside of a triangle,” Arkani-Hamed explained. Just as the inside of a triangle is a region in a two-dimensional space bounded by intersecting lines, the simplest case of the positive Grassmannian is a region in an N-dimensional space bounded by intersecting planes. (N is the number of particles involved in a scattering process.)

    It was a geometric representation of real particle data, such as the likelihood that two colliding gluons will turn into four gluons. But something was still missing.

    The physicists hoped that the amplitude of a scattering process would emerge purely and inevitably from geometry, but locality and unitarity were dictating which pieces of the positive Grassmannian to add together to get it. They wondered whether the amplitude was “the answer to some particular mathematical question,” said Trnka, a post-doctoral researcher at the California Institute of Technology. “And it is,” he said.

    A sketch of the amplituhedron representing an 8-gluon particle interaction. Using Feynman diagrams, the same calculation would take roughly 500 pages of algebra. Nima Arkani-Hamed

    Arkani-Hamed and Trnka discovered that the scattering amplitude equals the volume of a brand-new mathematical object — the amplituhedron. The details of a particular scattering process dictate the dimensionality and facets of the corresponding amplituhedron. The pieces of the positive Grassmannian that were being calculated with twistor diagrams and then added together by hand were building blocks that fit together inside this jewel, just as triangles fit together to form a polygon.

    Like the twistor diagrams, the Feynman diagrams are another way of computing the volume of the amplituhedron piece by piece, but they are much less efficient. “They are local and unitary in space-time, but they are not necessarily very convenient or well-adapted to the shape of this jewel itself,” Skinner said. “Using Feynman diagrams is like taking a Ming vase and smashing it on the floor.”

    Arkani-Hamed and Trnka have been able to calculate the volume of the amplituhedron directly in some cases, without using twistor diagrams to compute the volumes of its pieces. They have also found a “master amplituhedron” with an infinite number of facets, analogous to a circle in 2-D, which has an infinite number of sides. Its volume represents, in theory, the total amplitude of all physical processes. Lower-dimensional amplituhedra, which correspond to interactions between finite numbers of particles, live on the faces of this master structure.

    “They are very powerful calculational techniques, but they are also incredibly suggestive,” Skinner said. “They suggest that thinking in terms of space-time was not the right way of going about this.”

    Quest for Quantum Gravity

    The seemingly irreconcilable conflict between gravity and quantum field theory enters crisis mode in black holes. Black holes pack a huge amount of mass into an extremely small space, making gravity a major player at the quantum scale, where it can usually be ignored. Inevitably, either locality or unitarity is the source of the conflict.


    Puzzling Thoughts

    Locality and unitarity are the central pillars of quantum field theory, but as the following thought experiments show, both break down in certain situations involving gravity. This suggests physics should be formulated without either principle.

    Locality says that particles interact at points in space-time. But suppose you want to inspect space-time very closely. Probing smaller and smaller distance scales requires ever higher energies, but at a certain scale, called the Planck length, the picture gets blurry: So much energy must be concentrated into such a small region that the energy collapses the region into a black hole, making it impossible to inspect. “There’s no way of measuring space and time separations once they are smaller than the Planck length,” said Arkani-Hamed. “So we imagine space-time is a continuous thing, but because it’s impossible to talk sharply about that thing, then that suggests it must not be fundamental — it must be emergent.”

    Unitarity says the quantum mechanical probabilities of all possible outcomes of a particle interaction must sum to one. To prove it, one would have to observe the same interaction over and over and count the frequencies of the different outcomes. Doing this to perfect accuracy would require an infinite number of observations using an infinitely large measuring apparatus, but the latter would again cause gravitational collapse into a black hole. In finite regions of the universe, unitarity can therefore only be approximately known.

    “We have indications that both ideas have got to go,” Arkani-Hamed said. “They can’t be fundamental features of the next description,” such as a theory of quantum gravity.

    String theory, a framework that treats particles as invisibly small, vibrating strings, is one candidate for a theory of quantum gravity that seems to hold up in black hole situations, but its relationship to reality is unproven — or at least confusing. Recently, a strange duality has been found between string theory and quantum field theory, indicating that the former (which includes gravity) is mathematically equivalent to the latter (which does not) when the two theories describe the same event as if it is taking place in different numbers of dimensions. No one knows quite what to make of this discovery. But the new amplituhedron research suggests space-time, and therefore dimensions, may be illusory anyway.

    “We can’t rely on the usual familiar quantum mechanical space-time pictures of describing physics,” Arkani-Hamed said. “We have to learn new ways of talking about it. This work is a baby step in that direction.”

    Even without unitarity and locality, the amplituhedron formulation of quantum field theory does not yet incorporate gravity. But researchers are working on it. They say scattering processes that include gravity particles may be possible to describe with the amplituhedron, or with a similar geometric object. “It might be closely related but slightly different and harder to find,” Skinner said.

    Nima Arkani-Hamed, a professor at the Institute for Advanced Study, and his former student and co-author Jaroslav Trnka, who finished his Ph.D. at Princeton University in July and is now a post-doctoral researcher at the California Institute of Technology. Courtesy of Jaroslav Trnka

    Physicists must also prove that the new geometric formulation applies to the exact particles that are known to exist in the universe, rather than to the idealized quantum field theory they used to develop it, called maximally supersymmetric Yang-Mills theory. This model, which includes a “superpartner” particle for every known particle and treats space-time as flat, “just happens to be the simplest test case for these new tools,” Bourjaily said. “The way to generalize these new tools to [other] theories is understood.”

    Beyond making calculations easier or possibly leading the way to quantum gravity, the discovery of the amplituhedron could cause an even more profound shift, Arkani-Hamed said. That is, giving up space and time as fundamental constituents of nature and figuring out how the Big Bang and cosmological evolution of the universe arose out of pure geometry.

    “In a sense, we would see that change arises from the structure of the object,” he said. “But it’s not from the object changing. The object is basically timeless.”

    While more work is needed, many theoretical physicists are paying close attention to the new ideas.

    The work is “very unexpected from several points of view,” said Witten, a theoretical physicist at the Institute for Advanced Study. “The field is still developing very fast, and it is difficult to guess what will happen or what the lessons will turn out to be.”

    See the full article here .

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    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.

  • richardmitnick 2:30 pm on September 26, 2015 Permalink | Reply
    Tags: , , , , , Leptoquarks, , Particle Physics   

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

    FNAL II photo

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

    Sept. 25, 2015
    FNAL Don Lincoln
    This article is written by Don Lincoln

    The gryphon is a mythical beast with the head of an eagle and the hindquarters of a lion. Physicists look for a proposed particle hybrid of a quark and a lepton. This theoretical particle is called a leptoquark.

    Mythology is replete with creatures that are exotic blends of more familiar animals, for example gryphons, mermaids and centaurs. Finding ordinary animals is commonplace, but discovering one of these blended ones would be a true triumph of science.

    There are similarities in particle physics. For instance, the Standard Model contains the very familiar quarks and leptons.

    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.

    These two classes of particles have very different properties. Quarks feel all of the known subatomic forces and are found in the center of atoms. Leptons feel only two of the three known subatomic forces (they do not react via the strong nuclear force), and the most familiar lepton, the electron, orbits far from the atomic nucleus. Further, a single quark cannot convert into a single lepton, and vice versa. These really are quite different beasties.

    However, the goal of particle physics is unification. We hope one day to generate a single, overlapping theory that contains but one type of particle and one type of force. We are very far from that goal and will need to somehow account for the existence of the very different quarks and leptons.

    One possibility is that a quark and lepton can fuse to make a hybrid particle called a leptoquark. Leptoquarks would contain all the properties of quarks and leptons and would be a step on the path to building a unified theory.

    Leptoquarks are speculative particles, and they pop up in many proposed theories. And, like any good researchers, CMS scientists studied their data to see if they could find evidence that supported the particle’s existence.

    CMS in the LHC at CERN

    After considerable effort, the CMS experiment submitted for publication not one, but two papers reporting on a leptoquark search. One paper looked for leptoquarks produced individually, while the other looked for leptoquarks produced in pairs.

    No evidence was observed for the existence of leptoquarks, which means either that the idea is wrong or that the measurement didn’t have enough energy to make them. These two papers were reported using LHC data recorded in 2012 at an energy of 8 trillion electronvolts. CMS is recording data now at a much higher energy, and researchers are refining their analyses to dig into this new possible treasure trove. The hunt for leptoquarks isn’t over yet.

    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 10:31 am on September 24, 2015 Permalink | Reply
    Tags: , , , , , Particle Physics   

    From FNAL: “Frontier Science Result: CDF More than expected” 

    FNAL II photo

    [I know that this article is not for the technically feint of heart. I cannot claim to understand it. I present it to show that the Tevatron produced data which is still being sifted today and which remains relevant, in spite of the move of HEP to the Large Hadron Collider at CERN.]

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

    Sept. 24, 2015
    Andy Beretvas

    Temp 1
    This plot shows the invariant-mass distribution of the Bc+ → J/ψμ+ candidate events using the full CDF data sample with a Monte Carlo-simulated signal sample. The calculated backgrounds are superimposed

    In 1998 CDF was the first to observe the Bc+ meson, which consists of two quarks: an antibottom quark and a charmed quark. The discovery consisted of a measurement involving approximately 20 decays in which the decay products were a J/ψ, a charged lepton (muon or electron) and an unobserved neutrino.


    Using the full Tevatron Run II data set, we now observe approximately 740 events in the muon decay mode. CDF looked for a signature of three muons, the mass of two oppositely charged muons being consistent with that of the J/ψ particle. This larger data set allows us to make the first measurement of the production cross section of the Bc+ meson.

    FNAL Tevatron
    Tevatron map

    One of the principal challenges in the analysis was the determination of the backgrounds, which are shown in the above figure. In the largest background, the J/ψ is correctly identified, but the third muon is misidentified as a pion, kaon or proton. Of the 1,370 Bc+ candidates, 630 are identified as being background.

    In order to minimize the error, we compared our measurement to that of a decay that is already well measured (B+ → J/ψ + K+). The cross section for B+ is 2.78 ± 0.24 microbarns for conditions very similar to our measurement of the Bc+. Using well-known properties of the B+ decay, we find the final cross section for Bc+ production to be 29 ± 4 nanobarns.

    Our result is higher than the theory expectation (by two standard deviations), but the theory calculation was done 10 years ago (kT factorization). Measurements at the LHC collider, where the cross sections should be many times larger, could resolve this problem in our understanding of a meson that is both beautiful and charmed.

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

    CDF scientists performed a job well done in determining the background, a difficult, interesting challenge.

    Learn more.

    This is my last Frontier Science Result for CDF. I’d like to thank my CDF colleagues for writing so many interesting and important physics papers that were the subject of this column. Finally, Leah Hesla deserves special praise for her wonderful job of editing.

    Fermilab Leah Hesla
    Leah Hesla

    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 7:55 pm on September 21, 2015 Permalink | Reply
    Tags: , , , , , Particle Physics   

    From CERN ALICE: “ALICE precisely compares light nuclei and antinuclei” 

    CERN New Masthead

    17 Aug 2015
    Cian O’Luanaigh

    The ALICE detector on CERN’s Large Hadron Collider (Image: A Saba/CERN)

    The ALICE experiment at the Large Hadron Collider (LHC) at CERN has made a precise measurement of the difference between ratios of the mass and electric charge of light nuclei and antinuclei. The result, published today in Nature Physics (10.1038/nphys3432), confirms a fundamental symmetry of nature to an unprecedented precision for light nuclei. The measurements are based on the ALICE experiment’s abilities to track and identify particles produced in high-energy heavy-ion collisions at the LHC.

    The ALICE collaboration has measured the difference between mass-to-charge ratios for deuterons (a proton, or hydrogen nucleus, with an additional neutron) and antideuterons, as well as for helium-3 nuclei (two protons plus a neutron) and antihelium-3 nuclei. Measurements at CERN, most recently by the BASE experiment, have already compared the same properties of protons and antiprotons to high precision. The study by ALICE takes this research further as it probes the possibility of subtle differences between the way that protons and neutrons bind together in nuclei compared with how their antiparticle counterparts form antinuclei.

    “The measurements by ALICE and by BASE have taken place at the highest and lowest energies available at CERN, at the LHC and the Antiproton Decelerator, respectively,” says CERN Director-General Rolf Heuer. “This is a perfect illustration of the diversity in the laboratory’s research programme.”

    The measurement by ALICE comparing the mass-to-charge ratios in deuterons/antideuterons and in helium-3/antihelium-3 confirms the fundamental symmetry known as CPT in these light nuclei. This symmetry of nature implies that all of the laws of physics are the same under the simultaneous reversal of charges (charge conjugation C), reflection of spatial coordinates (parity transformation P) and time inversion (T). The new result, which comes exactly 50 years after the discovery of the antideuteron at CERN and in the US, improves on existing measurements by a factor of 10-100.

    Measurements of energy loss in the time-projection chamber enable the ALICE experiment to identify antinuclei (upper curves on the left) and nuclei (upper curves on the right) produced in the lead-ion collisions at the LHC (Image: ALICE)

    The ALICE experiment records high-energy collisions of lead ions at the LHC, enabling it to study matter at extremely high temperatures and densities. The lead-ion collisions provide a copious source of particles and antiparticles, and nuclei and the corresponding antinuclei are produced at nearly equal rates. This allows ALICE to make a detailed comparison of the properties of the nuclei and antinuclei that are most abundantly produced. The experiment makes precise measurements of the curvature of particle tracks in the detector’s magnetic field and of the particles’ time of flight, and uses this information to determine the mass-to-charge ratios for the nuclei and antinuclei.

    “The high precision of our time-of-flight detector, which determines the arrival time of particles and antiparticles with a resolution of 80 picoseconds, associated with the energy-loss measurement provided by our time-projection chamber, allows us to measure a clear signal for deuterons/antideuterons and helium-3/antihelium-3 over a wide range of momentum”, says ALICE spokesperson Paolo Giubellino.

    The measured differences in the mass-to-charge ratios are compatible with zero within the estimated uncertainties, in agreement with expectations for CPT symmetry. These measurements, as well as those that compare the protons with antiprotons, may further constrain theories that go beyond the existing Standard Model of particles and the forces through which they interact.

    See the full article here.

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    Meet CERN in a variety of places:

    Cern Courier



    CERN CMS New

    CERN LHCb New


    CERN LHC New
    CERN LHC Grand Tunnel

    LHC particles

    Quantum Diaries

  • richardmitnick 4:14 pm on September 15, 2015 Permalink | Reply
    Tags: , , , , , Particle Physics   

    From livescience: “Could Physics’ Reigning Model Finally Be Dethroned?” 


    September 10, 2015
    Tia Ghose

    CERN LHCb chamber
    The LHCb detector at CERN. Credit: CERN

    Trouble is brewing in the orderly world of subatomic physics.

    New evidence from the world’s largest atom smasher, the Large Hadron Collider in Geneva, Switzerland, suggests that certain tiny subatomic particles called leptons don’t behave as expected.

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

    So far, the data only hint at these misbehaving leptons. But if more data confirm their wayward behavior, the particles would represent the first cracks in the reigning physics model for subatomic particles, researchers say.

    Reigning model

    A single model, called the Standard Model, governs the bizarre world of the teensy tiny.

    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.

    It dictates the behavior of every subatomic particle, from ghostly neutrinos to the long-sought Higgs boson (discovered in 2012), which explains how other particles get their mass.

    CERN CMS Event
    Higgs at CMS

    CERN CMS Detector
    CMS at CERN

    In hundreds of experiments over four decades, physicists have confirmed over and over again that the Standard Model is an accurate predictor of reality.

    But the Standard Model isn’t the whole picture of how the universe operates. For one, physicists haven’t found a way to reconcile the microcosm of the Standard Model with [Albert] Einstein’s theory of general relativity, which describes how mass warps space-time on a larger scale. And neither theory explains the mysterious substance called dark matter, which makes up most of the universe’s matter, yet emits no light. So physicists have been on the hunt for any results that contradict the Standard Model’s basic premises, in the hopes that it could reveal new physics.

    Cracks in the foundation

    Physicists may have found one such contradiction at the Large Hadron Collider (LHC), which accelerates beams packed with protons around a 17-mile-long (27 kilometers) underground ring and smashes them into one another, creating a shower of short-lived particles.

    While sifting through the alphabet soup of short-lived particles, scientists with the LHC’s beauty experiment (LHCb) noticed a discrepancy in how often B mesons — particles with mass five times that of the proton — decayed into two other types of electron like particles, called the tau lepton and the muon.

    The LHCb scientists noticed slightly more tau leptons than they expected, which they first reported earlier this year. But that result was very preliminary. From LHCb data alone, there was a high chance — about 1 in 20 — that a statistical fluke could explain the findings.

    “This is a small hint, and you would have not been supremely excited until you see more of it,” said Hassan Jawahery, a particle physicist at the University of Maryland in College Park, who works on the LHCb experiment.

    But this same discrepancy in the tau-lepton-muon ratio has cropped up before, at Stanford University’s BaBar experiment, which tracked the fallout from electrons colliding with their antimatter partners, positrons.

    SLAC Babar
    SLAC Babar

    With both data sources combined, the odds that the tau-lepton-muon discrepancy is a byproduct of random chance drops significantly. The new results are at a certainty level of “4-sigma,” which means there is a 99.993 percent chance the discrepancy between tau leptons and muons represents a real physical phenomenon, and is not a byproduct of random chance, the researchers reported Sept. 4 in the journal Physical Review Letters. (Typically, physicists announce big discoveries, such as that of the Higgs boson, when data reaches a 5-sigma level of significance, meaning there’s a 1 in 3.5 million chance that the finding is a statistical fluke.)

    “Their values are totally in line with ours,” said Vera Luth, a physicist at Stanford University in California who worked on the BaBar experiment. “We’re obviously thrilled that it doesn’t look totally like a fluctuation. It may actually be right.”

    Strange new worlds?

    Of course, it’s still too early to say with absolute certainty that something fishy is going on in the world of the very small. But the fact that similar results have been found using completely different experimental models bolsters the LHCb findings, said Zoltan Ligeti, a theoretical physicist at Lawrence Berkeley National Laboratory in California, who was not involved in the current experiments. In addition, the B-factory at the atom-smashing KEK-B experiment in Japan has found a similar deviation, he added.

    KEK Belle detector
    KEK Belle Detctor

    If the phenomenon they’ve measured holds up with further testing, “the implications for theory and how we view the world would be extremely substantial,” Ligeti told Live Science. “It’s really a deviation from the Standard Model in a direction that most people would not have expected.”

    For instance, one of the top contenders to explain dark matter and dark energy is a class of theories known as supersymmetry, which posits that each known particle has a superpartner with slightly different characteristics. But the most popular versions of these theories cannot explain the new results, he said.

    Supersymmetry standard model
    Standard Model of Supersymmetry

    Still, the new results aren’t confirmed yet. That will have to wait until the team begins analyzing data from the newest run of the LHC, which ramped up to nearly double the energy levels in April, Jawahery said.

    “The uncertainties are still large, and we would like to do better,” Luth said. “I’m sure the LHCb will do that.”

    See the full article here .

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  • richardmitnick 9:44 am on September 10, 2015 Permalink | Reply
    Tags: , , , , , Particle Physics   

    From FNAL: “Successful test of single-spoke cavity gives SSR1 team a reason to smile” 

    FNAL II photo

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

    Sept. 10, 2015
    Ali Sundermier

    Temp 1
    Donato Passarelli, Leonardo Ristori, Sergey Kazakov and Oleg Pronitchev, all of the Technical Division, stand next to the recently tested SSR1 cavity. Photo: Reidar Hahn

    Eight cavities might sound like a nightmare to the average person. But when it comes to speeding up particles, it’s an aspiration.

    In July, a team of scientists and engineers finished designing, building and testing the first of a series of eight special cavities for the planned PIP-II project.

    FNAL PIP-II Home
    PIP-II Home at FNAL

    All components of the cavity were designed at Fermilab and were built in U.S. industry. The team anticipates completing and testing all eight cavities, plus two cavities received by Indian collaborators, by summer 2016.

    The cavities will fit into the first single-spoke resonator cryomodule, SSR1, to be tested with particle beam in the next few years. Altogether the PIP-II project would require 116 cavities of five different types to propel protons to 800 MeV, or 84 percent the speed of light.

    “This milestone is exciting because it was really the last step in R&D for this type of cavity,” said Leonardo Ristori, task manager for the spoke resonator section of PIP-II. “We spent all these years designing, building and testing prototypes. Now we feel comfortable that we can produce these single-spoke cavities.”

    Not to be confused with the painful tooth decay that comes from eating too much sugar, accelerator cavities are meticulously designed metal structures, pumped with radio-frequency power, that give particles a boost using rapidly oscillating electric fields.

    SSR1 cavities are cylindrical, about the size and build of car tires. They are called “single-spoke” because individual cavities are divided by a hollow hourglass-shaped partition that resembles the spoke of a wheel. They are fashioned from pure niobium, a superconducting metal that, when kept under 9.3 Kelvin (or minus 443 degrees Fahrenheit), presents no electrical resistance when a voltage is applied.

    The most recent cavity test simulated the configuration of the full SSR1 cryomodule using the same pieces that would be used in the PIP-II superconducting linac.

    FNAL SSR1 Cryomodule
    SSR1 cryomodule

    In this integrated test, the team tested the performance of the power coupler and the frequency-tuning system, making sure they didn’t interfere or degrade the performance of the cavity. The team was interested in measurements of how large of an accelerating electric field the cavity could support, called the gradient, and how efficiently it uses the power put into it, referred to as the quality factor.

    Ristori said that the cavity, the coupler and the tuner all passed the tests, meeting and exceeding project requirements.

    One of the main challenges in designing the cavity was desensitizing it to helium pressure variations and other sources of vibration. This was for the most part achieved by developing a state-of-the-art, self-compensating behavior.

    “This is an encouraging result,” Ristori said. “Everybody did an excellent job in each portion, and it all came together. It motivates the team to move forward and push the design to the limits for the other sections of the planned accelerator.”

    Once all eight are complete, the team will assemble the SSR1 cryomodule in Lab 2, where they are currently installing cleanrooms. It will be the first spoke cryomodule ever completed in the United States.

    “It’s our first fully equipped cavity, tested at full power for PIP-II,” said Slava Yakovlev, head of the SRF Development Department. “We will use all the lessons we learned from this cavity in order to develop and build all the other cavities in the project and put them into operation.”

    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 9:18 am on September 10, 2015 Permalink | Reply
    Tags: , , , , , Particle Physics   

    From FNAL- “Frontier Science Result: DZero When barns collide” 

    FNAL II photo

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

    Sept. 10, 2015
    Leo Bellantoni

    Temp 1
    This plot shows the recent double-parton scattering result (σA σB)/ σAB from DZero, in blue, compared to previously published results, in red.

    Protons have parts that, as mentioned in previous columns, have the not terribly imaginative name of “partons.” When a proton collides with another proton, a parton from one proton can collide directly with a parton from the other proton. Cases in which the collision is nearly head-on are interesting for discovering new forms of energy and matter; that is the basic reason for the Tevatron and LHC science programs.

    FNAL Tevatron
    FNAL DZero
    FNAL: Tevatron, CDF and D0 collaborations

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

    Occasionally, two partons from the first proton can collide individually with two partons from the other proton. This double-parton scattering process does not involve the creation of new forms of energy or matter, but it can look that way; it can form a “background.” So it is important to measure the rate of double-parton processes to separate them from new physics.

    The natural thing to want to measure is the ratio of single-parton to double-parton collisions. But what exactly is it that one measures? What are the numbers that go into that ratio? This is where the barns come in.

    If two objects that go whizzing by each other are very likely to collide, they are in some sense fat, wide objects; if they are likely to pass without colliding, they are narrow objects. We say that the wide object has a large cross section; the narrow object, a small cross section. In particle physics, the nucleus of a uranium atom is huge; to hit it with another particle is no harder than to “hit the broad side of a barn,” as the saying goes. And so the unit for measuring cross sections, the barn, is an area corresponding to roughly the area that a uranium nucleus would cover on the top of your desk (assuming you could get away with having a uranium atom on top your desk, which is pretty unlikely).

    The symbol for a cross section is σ, sigma. A single-parton collision that creates some set of particles, say A, has a cross section σA; a single-parton collision that creates set B has cross section σB and the double-parton collision, σAB. That interesting ratio, the ratio of single parton to double parton collisions, is (σAσB)/σAB. The smaller this ratio is, the more double-parton collisions occur and the more background one has to new physics.

    DZero has recently measured this ratio (called σeff) in the case where one parton collision created a pair of photons and another parton collision created a pair of jets (sprays of particles all moving in the same direction). Then, having measured this ratio for this kind of collision, we compared it with the same ratio in other processes. The new result shown in the figure agrees well with previous studies and gives us confidence that the double-parton scattering backgrounds to new production are understood, so that we can allow for their contributions when looking for new physics.

    This is my last Frontier Science Result article for DZero. I’d like to thank my DZero colleagues and my Fermilab Today editor Leah Hesla for all their help in producing them.

    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.

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