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  • richardmitnick 5:16 pm on June 26, 2016 Permalink | Reply
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    From particle bites: “Can’t Stop Won’t Stop: The Continuing Search for SUSY” 

    particlebites bloc


    June 19, 2016
    Julia Gonski


    Title: “Search for top squarks in final states with one isolated lepton, jets, and missing transverse momentum in √s = 13 TeV pp collisions with the ATLAS detector
    Author: The ATLAS Collaboration
    Publication: Submitted 13 June 2016, arXiv 1606.03903

    Things at the LHC are going great. Run II of the Large Hadron Collider is well underway, delivering higher energies and more luminosity than ever before. ATLAS and CMS also have an exciting lead to chase down– the diphoton excess that was first announced in December 2015. So what does lots of new data and a mysterious new excess have in common? They mean that we might finally get a hint at the elusive theory that keeps refusing our invitations to show up: supersymmetry.

    Standard model of Supersymmetry DESY
    Standard model of Supersymmetry DESY

    Figure 1: Feynman diagram of stop decay from proton-proton collisions.

    People like supersymmetry because it fixes a host of things in the Standard Model. But most notably, it generates an extra Feynman diagram that cancels the quadratic divergence of the Higgs mass due to the top quark contribution. This extra diagram comes from the stop quark. So a natural SUSY solution would have a light stop mass, ideally somewhere close to the top mass of 175 GeV. This expected low mass due to “naturalness” makes the stop a great place to start looking for SUSY. But according to the newest results from the ATLAS Collaboration, we’re not going to be so lucky.

    Using the full 2015 dataset (about 3.2 fb-1), ATLAS conducted a search for pair-produced stops, each decaying to a top quark and a neutralino, in this case playing the role of the lightest supersymmetric particle. The top then decays as tops do, to a W boson and a b quark. The W usually can do what it wants, but in this case the group chose to select for one W decaying leptonically and one decaying to jets (leptons are easier to reconstruct, but have a lower branching ratio from the W, so it’s a trade off.) This whole process is shown in Figure 1. So that gives a lepton from one W, jets from the others, and missing energy from the neutrino for a complete final state.

    Figure 2: Transverse mass distribution in one of the signal regions.

    The paper does report an excess in the data, with a significance around 2.3 sigma. In Figure 2, you can see this excess overlaid with all the known background predictions, and two possible signal models for various gluino and stop masses. This signal in the 700-800 GeV mass range is right around the current limit for the stop, so it’s not entirely inconsistent. While these sorts of excesses come and go a lot in particle physics, it’s certainly an exciting reason to keep looking.

    Figure 3 shows our status with the stop and neutralino, using 8 TeV data. All the shaded regions here are mass points for the stop and neutralino that physicists have excluded at 95% confidence. So where do we go from here? You can see a sliver of white space on this plot that hasn’t been excluded yet— that part is tough to probe because the mass splitting is so small, the neutralino emerges almost at rest, making it very hard to notice. It would be great to check out that parameter space, and there’s an effort underway to do just that. But at the end of the day, only more time (and more data) can tell.

    (P.S. This paper also reports a gluino search—too much to cover in one post, but check it out if you’re interested!)

    Figure 3: Limit curves for stop and neutralino masses, with 8 TeV ATLAS dataset.

    See the full article here .

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    What is ParticleBites?

    ParticleBites is an online particle physics journal club written by graduate students and postdocs. Each post presents an interesting paper in a brief format that is accessible to undergraduate students in the physical sciences who are interested in active research.

    The papers are accessible on the arXiv preprint server. Most of our posts are based on papers from hep-ph (high energy phenomenology) and hep-ex (high energy experiment).

    Why read ParticleBites?

    Reading a technical paper from an unfamiliar subfield is intimidating. It may not be obvious how the techniques used by the researchers really work or what role the new research plays in answering the bigger questions motivating that field, not to mention the obscure jargon! For most people, it takes years for scientific papers to become meaningful.

    Our goal is to solve this problem, one paper at a time. With each brief ParticleBite, you should not only learn about one interesting piece of current work, but also get a peek at the broader picture of research in particle physics.

    Who writes ParticleBites?

    ParticleBites is written and edited by graduate students and postdocs working in high energy physics. Feel free to contact us if you’re interested in applying to write for ParticleBites.

    ParticleBites was founded in 2013 by Flip Tanedo following the Communicating Science (ComSciCon) 2013 workshop.

    Flip Tanedo UCI Chancellor’s ADVANCE postdoctoral scholar in theoretical physics. As of July 2016, I will be an assistant professor of physics at the University of California, Riverside

    It is now organized and directed by Flip and Julia Gonski, with ongoing guidance from Nathan Sanders.

  • richardmitnick 5:35 pm on March 18, 2016 Permalink | Reply
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    From CERN: “CMS hunts for supersymmetry in uncharted territory” 

    Cern New Bloc

    Cern New Particle Event

    CERN New Masthead


    Mar 18, 2016

    The CMS collaboration is continuing its hunt for signs of supersymmetry (SUSY), a popular extension to the Standard Model that could provide a weakly interacting massive-particle candidate for dark matter, if the lightest supersymmetric particle (LSP) is stable.

    Standard model of Supersymmetry Illustration: CERN & IES de SAR
    The Standard Model of Supersymmetry Illustration: CERN & IES de SAR

    With the increase in the LHC centre-of-mass energy from 8 to 13 TeV, the production cross-section for hypothetical SUSY partners rises; the first searches to benefit are those looking for the strongly coupled SUSY partners of the gluon (gluino) and quarks (squarks) that had the most stringent mass limits from Run 1 of the LHC. By decaying to a stable LSP, which does not interact in the detector and instead escapes, SUSY particles can leave a characteristic experimental signature of a large imbalance in transverse momentum.

    Searches for new physics based on final states with jets (a bundle of particles) and large transverse-momentum imbalance are sensitive to broad classes of new-physics models, including supersymmetry. CMS has searched for SUSY in this final state using a variable called the “stransverse mass”, MT2, to measure the transverse-momentum imbalance, which strongly suppresses fake contributions due to potential hadronic-jet mismeasurement. This allows us to control the background from copiously produced QCD multi-jet events. The remaining background comes from Standard Model processes such as W, Z and top-quark pair production with decays to neutrinos, which also produce a transverse-momentum imbalance. We estimate our backgrounds from orthogonal control samples in data targeted to each. To cover a wide variety of signatures, we categorise our signal events according to the number of jets, the number of jets arising from bottom quarks, the sum of the transverse momenta of hadronic jets (HT), and MT2. Some SUSY scenarios predict spectacular signatures, such as four top quarks and two LSPs, which would give large values for all of these quantities, while others with small mass splittings produce much softer signatures.

    Unfortunately, we did not observe any evidence for SUSY in the 2015 data set. Instead, we are able to significantly extend the constraints on the masses of SUSY partners beyond those from the LHC Run 1. The gluino has the largest production cross-section and many potential decay modes. If the gluino decays to the LSP and a pair of quarks, we exclude gluino masses up to 1550–1750 GeV, depending on the quark flavour, extending our Run 1 limits by more than 300 GeV. We are also sensitive to squarks, with our constraints summarised in figure 1. We set limits on bottom-squark masses up to 880 GeV, top squarks up to 800 GeV, and light-flavour squarks up to 600–1260 GeV, depending on how many states are degenerate in mass.

    Even though SUSY was not waiting for us around the corner at 13 TeV, we look forward to the 2016 run, where a large increase in luminosity gives us another chance at discovery.

    See the full article here.

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  • richardmitnick 3:00 pm on July 30, 2015 Permalink | Reply
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    From Symmetry: “One Higgs is the loneliest number” 


    July 30, 2015.
    Katie Elyce Jones

    Physicists discovered one type of Higgs boson in 2012. Now they’re looking for more.


    When physicists discovered the Higgs boson in 2012, they declared the Standard Model of particle physics complete; they had finally found the missing piece of the particle puzzle.

    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.

    And yet, many questions remain about the basic components of the universe, including: Did we find the one and only type of Higgs boson? Or are there more?

    A problem of mass

    The Higgs mechanism gives mass to some fundamental particles, but not others. It interacts strongly with W and Z bosons, making them massive. But it does not interact with particles of light, leaving them massless.

    These interactions don’t just affect the mass of other particles, they also affect the mass of the Higgs. The Higgs can briefly fluctuate into virtual pairs of the particles with which it interacts.

    Scientists calculate the mass of the Higgs by multiplying a huge number—related to the maximum energy for which the Standard Model applies—with a number related to those fluctuations. The second number is determined by starting with the effects of fluctuations to force-carrying particles like the W and Z bosons, and subtracting the effects of fluctuations to matter particles like quarks.

    While the second number cannot be zero because the Higgs must have some mass, almost anything it adds up to, even at very small numbers, makes the mass of the Higgs gigantic.

    But it isn’t. It weighs about 125 billion electronvolts; it’s not even the heaviest fundamental particle.

    “Having the Higgs boson at 125 GeV is like putting an ice cube into a hot oven and it not melting,” says Flip Tanedo, a theoretical physicist and postdoctoral researcher at the University of California, Irvine.

    A lightweight Higgs, though it makes the Standard Model work, doesn’t necessarily make sense for the big picture. If there are multiple Higgses—much heavier ones—the math determining their masses becomes more flexible.

    “There’s no reason to rule out multiple Higgs particles,” says Tim Tait, a theoretical physicist and professor at UCI. “There’s nothing in the theory that says there shouldn’t be more than one.”

    The two primary theories that predict multiple Higgs particles are Supersymmetry and compositeness.

    Supersymmetry standard model
    Standard Model of Supersymmetry


    Popular in particle physics circles for tying together all the messy bits of the Standard Model, Supersymmetry predicts a heavier (and whimsically named) partner particle, or “sparticle,” for each of the known fundamental particles. Quarks have squarks and Higgs have Higgsinos.

    “When the math is re-done, the effects of the particles and their partner particles on the mass of the Higgs cancel each other out and the improbability we see in the Standard Model shrinks and maybe even vanishes,” says Don Lincoln, a physicist at Fermi National Accelerator Laboratory.

    The Minimal Supersymmetric Standard Model—the supersymmetric model that most closely aligns with the current Standard Model—predicts four new Higgs particles in addition to the Higgs sparticle, the Higgsino.

    While Supersymmetry is maybe the most popular theory for exploring physics beyond the Standard Model, physicists at the LHC haven’t seen any evidence of it yet. If Supersymmetry exists, scientists will need to produce more massive particles to observe it.

    “Scientists started looking for Supersymmetry five years ago in the LHC,” says Tanedo. “But we don’t really know where they will find it: 10 TeV? 100 TeV?”


    The other popular theory that predicts multiple Higgs bosons is compositeness. The composite Higgs theory proposes that the Higgs boson is not a fundamental particle but is instead made of smaller particles that have not yet been discovered.

    “You can think of this like the study of the atom,” says Bogdan Dobrescu, a theoretical physicist at Fermi National Accelerator Laboratory. “As people looked closer and closer, they found the proton and neutron. They looked closer again and found the ‘up’ and ‘down’ quarks that make up the proton and neutron.”

    Composite Higgs theories predict that if there are more fundamental parts to the Higgs, it may assume a combination of masses based on the properties of these smaller particles.

    The search for composite Higgs bosons has been limited by the scale at which scientists can study given the current energy levels at the LHC.

    On the lookout

    Physicists will continue their Higgs search with the current run of the LHC.

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

    At 60 percent higher energy, the LHC will produce Higgs bosons more frequently this time around. It will also produce more top quarks, the heaviest particles of the Standard Model. Top quarks interact energetically with the Higgs, making them a favored place to start picking at new physics.

    Whether scientists find evidence for Supersymmetry or a composite Higgs (if they find either), that discovery would mean much more than just an additional Higgs.

    “For example, finding new Higgs bosons could affect our understanding of how the fundamental forces unify at higher energy,” Tait says.

    “Supersymmetry would open up a whole ‘super’ world out there to discover. And a composite Higgs might point to new rules on the fundamental level beyond what we understand today. We would have new pieces of the puzzle to look at it.”

    See the full article here.

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    Symmetry is a joint Fermilab/SLAC publication.

  • richardmitnick 10:13 am on July 28, 2015 Permalink | Reply
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    From Discovery: “LHC Keeps Bruising ‘Difficult to Kill’ Supersymmetry” 

    Discovery News
    Discovery News

    Jul 27, 2015


    In a new blow for the futuristic “supersymmetry” theory of the universe’s basic anatomy, experts reported fresh evidence Monday of subatomic activity consistent with the mainstream Standard Model of particle physics.

    New data from ultra high-speed proton collisions at Europe’s Large Hadron Collider (LHC) showed an exotic particle dubbed the “beauty quark” behaves as predicted by the Standard Model, said a paper in the journal Nature Physics.

    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.

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

    Previous attempts at measuring the beauty quark’s rare transformation into a so-called “up quark” had yielded conflicting results. That prompted scientists to propose an explanation beyond the Standard Model — possibly supersymmetry.


    But the latest observations were “entirely consistent with the Standard Model and removes the need for this hypothesis” of an alternative theory, Guy Wilkinson, leader of LHC’s “beauty experiment” told AFP.

    “It would of course have been very exciting if we could show that there was something wrong with the Standard Model — I cannot deny that would have been sensational,” he said.

    The Standard Model is the mainstream theory of all the fundamental particles that make up matter, and the forces that govern them.

    But the model has weaknesses: it doesn’t explain dark matter or dark energy, which jointly make up 95 percent of the universe. Nor is it compatible with Einstein’s theory of general relativity — the force of gravity as we know it does not seem to work at the subatomic quantum scale.

    Supersymmetry, SUSY for short, is one of the alternatives proposed for explaining these inconsistencies, postulating the existence of a heavier “sibling” for every particle in the universe.

    This may also explain dark matter and dark energy.

    ‘Many-Headed Monster’

    But no proof of supersymmetric twins has been found at the LHC, which has observed all the particles postulated by the Standard Model — including the long-sought Higgs boson, which confers mass to matter.

    Supersymmetry predicts the existence of at least five types of Higgs boson, but only one, believed to be the Standard Model Higgs, has so far been found.

    Wilkinson said it was “too soon” to write off supersymmetry.

    “It is very difficult to kill supersymmetry: it is a many-headed monster,” he said.

    But “if nothing is seen in the next couple of years, supersymmetry would be in a much harder situation. The number of true believers would drop.”

    Quarks are the most basic particles, building blocks of protons and neutrons, which in turn are found in atoms.

    There are six types of quarks — the most common are the “up” and “down” quarks, while the others are called “charm”, “strange”, “beauty” and “top.”

    The beauty quark, heavier than up and down quarks, can shift shape, and usually takes the form of a charm quark when it does.

    Much more rarely, it morphs into an up quark. Wilkinson’s team have now measured — for the first time — how often that happens.

    “We are delighted because it is the sort of measurement nobody thought was possible at the LHC,” he said. It had been thought that an even more powerful machine would be needed.

    The revamped LHC, a facility of the European Organisation for Nuclear Research (CERN), was restarted in April after a two-year revamp to boost its power from eight to 13, potentially 14, teraelectronvolts (TeV).

    “If you expect Earth-shattering news from the new run, it’s a bit early,” CERN director-general Rolf Heuer told journalists in Vienna Monday at a conference of the European Physical Society.

    “The main harvest will come in the years to come, so you have to stay tuned.”

    So far, the new run at 13 TeV has re-detected all the Standard Model particles except for the Higgs boson, but Heuer insisted: “We are sure that it is there.

    See the full article here.

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  • richardmitnick 7:24 am on July 16, 2015 Permalink | Reply
    Tags: , , Helmholtz Association, , , Supersymmetry   

    From Helmholtz via DESY: “What is supersymmetry?” 



    Kristine August

    Using huge particle accelerators, physicists are searching for supersymmetry.

    Supersymmetry standard model
    Standard Model of Supersymmetry

    Their existence could help us to understand the composition of dark matter. But is it possible for something to be more symmetrical than symmetrical? Wilfried Buchmüller from the Deutsches Elektronen-Synchrotron facility (DESY) explains:

    “We usually associate symmetry with spatial symmetry – in connection with an image or a form, for example. But in the standard model of physics, when we think about symmetries we are thinking about something else – the forces between particles. When, for example, the force between two matter particles remains the same after reversal of the electrical charges, we are referring to “a symmetry”.

    The various forces in the standard model possess a number of such symmetries. According to the standard model, it is valid that the smaller the gaps between the matter particles, the greater the similarity becomes between the mathematical formulas that describe the forces there. We would say here that the theory becomes more symmetrical.

    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.

    Expanding on this concept, the last remaining differences are likely to cancel each other out at some point. It is our goal to describe all forces – gravity as well – and all particles on the basis of one unified principle of symmetry – supersymmetry (“SUSY”).

    But the fundamental difference still exists between matter particles and the particles that transfer forces. Although there are different types of particles, the supersymmetry theory is nevertheless able to interconnect them mathematically. We suspect that every particle has an attendant partner, a hidden supersymmetrical partner, i.e. a “superpartner”; in other words, one half of all matter is completed by its mirror image. Such a superpartner, in supersymmetrical theories, comprises the cornerstone of dark matter. Whenever the different types of particles then appear together, all of the forces become more similar to one another due to the superpartners. It is our ambition that we can also finally prove the existence of “SUSY” in reality. Namely, by finding the superpartners. They would play a key role in helping us to understand the origins of our universe.”

    See the full article here.

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    DESY is one of the world’s leading accelerator centres. Researchers use the large-scale facilities at DESY to explore the microcosm in all its variety – from the interactions of tiny elementary particles and the behaviour of new types of nanomaterials to biomolecular processes that are essential to life. The accelerators and detectors that DESY develops and builds are unique research tools. The facilities generate the world’s most intense X-ray light, accelerate particles to record energies and open completely new windows onto the universe. 
That makes DESY not only a magnet for more than 3000 guest researchers from over 40 countries every year, but also a coveted partner for national and international cooperations. Committed young researchers find an exciting interdisciplinary setting at DESY. The research centre offers specialized training for a large number of professions. DESY cooperates with industry and business to promote new technologies that will benefit society and encourage innovations. This also benefits the metropolitan regions of the two DESY locations, Hamburg and Zeuthen near Berlin.

  • richardmitnick 12:03 pm on April 29, 2015 Permalink | Reply
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    From Symmetry: “Natural SUSY’s last stand” 


    April 29, 2015
    Mike Ross

    Photo by Claudia Marcelloni De Oliveira, CERN

    Either Supersymmetry will be found in the next years of research at the Large Hadron Collider, or it isn’t exactly what theorists hoped it was.

    One of the big questions scientists are asking with experiments at the Large Hadron Collider is this: Does every fundamental particle we know about have a hidden partner that we have yet to meet?

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

    A popular set of theories predict that they do.

    The first run of the LHC came and went without any of these partner particles turning up. But a recent paper shows that the real test of the theories that predict their existence could happen during the next run, when particles will collide at higher energies than ever before.

    These theoretical partner particles come from the idea of Supersymmetry, or SUSY, a mathematical framework developed over the past 40 years that could answers questions such as: Are all of the forces we know just parts of a single, unified force? How is the Higgs boson so light? What is dark matter? Is the world made up of the tiny, vibrating strings described by string theory?

    A key aspect of SUSY is that each of the dozens of particles in the Standard Model of particle physics must have a partner, called a superparticle or sparticle.

    Standard Model of Particle Physics. The diagram shows the elementary particles of the Standard Model (the Higgs boson, the three generations of quarks and leptons, and the gauge bosons), including their names, masses, spins, charges, chiralities, and interactions with the strong, weak and electromagnetic forces. It also depicts the crucial role of the Higgs boson in electroweak symmetry breaking, and shows how the properties of the various particles differ in the (high-energy) symmetric phase (top) and the (low-energy) broken-symmetry phase (bottom).

    Supersymmetry standard model
    Standard Model of Supersymmetry

    Scientists think all of these sparticles must ultimately decay into a light, stable particle. If they are light enough, supersymmetric particles that interact through the strong force, such as supersymmetric quarks (squarks) or supersymmetric gluons (gluinos), could be produced at large rates at the LHC.

    There are many different manifestations of Supersymmetry, explains theorist JoAnne Hewett of SLAC National Accelerator Laboratory. A subset of them are known as “natural” theories. That is, they could answer many of the questions above. Their lightest sparticle could be the dark matter particle. The math could work out for all of the forces to have come from a single origin. They could help explain the mass of the Higgs boson.

    Data from the LHC’s first run, from 2010 to 2013, snuffed out any hope that the simplest natural version of SUSY exists.

    “But, there are millions of possible models consistent with natural Supersymmetry that have not been explored,” says Hewett’s advisee, Stanford graduate student Matthew Cahill-Rowley.

    According to a paper they worked on together with two other physicists, the second run of the LHC will investigate nearly all of them.

    Supersymmetry is enormously complex. Even its minimal form involves more than 100 independent parameters. To deal with this, theorists have over the years proposed several higher-level conditions that simplify the theory and reduce the number of parameters. These theories can predict ranges of possible masses for sparticles that might turn up at the LHC.

    Courtesy of: JoAnne Hewett, SLAC

    The figure above shows a plot of some 300,000 more complex SUSY models, identified by their squark and gluino masses on the vertical and horizontal axes, respectively. Colors indicate the fraction that have already been excluded by experiments at the LHC. Darker colors indicate a higher fraction excluded. Regions that are black have been totally ruled out.

    Any points lying below and to the left of the dashed white line represent models that, in SUSY’s most simplified version, are excluded by the LHC.

    Courtesy of: JoAnne Hewett, SLAC

    A second image shows which regions can be discovered or ruled out in the second run of the LHC. Almost every natural SUSY theory falls into that category.

    SUSY may be too complicated to ever truly rule out. But if it doesn’t turn up at the LHC in the next run, it’s not quite the SUSY scientists were looking for.

    See the full article here.

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    Symmetry is a joint Fermilab/SLAC publication.

  • richardmitnick 9:31 am on January 27, 2015 Permalink | Reply
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    From Huff Post: “The Future of Physics” 

    Huffington Post
    The Huffington Post


    Dr. Sten Odenwald, Astronomer, National Institute of Aerospace

    In another few months the Large Hadron Collider. will be powered up to explore its maximum energy range. Many physicists fervently hope we will see definite signs of “new physics,” especially a phenomenon called “supersymmetry.” In the simplest view, the Standard Model souped-up with supersymmetry will offer a massive new partner particle for every known particle (electron, quark, neutrino, etc). One of these, called the neutralino, may even explain dark matter itself!

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

    Supersymmetry standard model
    Standard Model of Supersymmetry

    But Wait, There’s More!

    Supersymmetry is the foundational cornerstone on which string theory rests. That’s why physicists call this “stringy” theory of matter “superstring theory.” If the LHC does not turn up any signs of supersymmetry during the next two or three years, not only will simple modifications to the current Standard Model be ruled out, but the most elegant forms of supersymmetry theory will fall too.

    As an astronomer I am not too worried. The verified Standard Model is now fully capable of accounting for everything we see in the universe since a trillion-trillion-trillionth of a second after the Big Bang to the present time, once you include gravity, and don’t worry too much about dark matter and dark energy.

    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.

    But we need some explanation for dark energy and dark matter to complete our understanding of the cosmos, and for that we still need our physicist friends to show the way. Currently their only answers involve supersymmetry theory. If this idea falls, astronomers will be completely stumped to explain what governs our universe on the largest scales.

    Beyond supersymmetry, we also have the huge investment of talent that has pursued superstring theory since the early 1980s. Without supersymmetry, the “super” part of string theory also falls, and you end up with a non-super string theory that is clunky, inelegant and pretty dismal in accounting for the finer details of our physical world, often termed “quantum gravity.” A big part of this is the idea that our universe inhabits more than four dimensions — perhaps as many as 11!

    On April 26, 2006, I had the following exchanges with Stanford theoretical physicist Leonard Susskind, who is widely regarded as one of the fathers of string theory, along with other provocative and foundational ideas such as the “holographic universe.” His comments are still relevant seven years later.


    Odenwald: Why is it so important for physicists to consider the universe having more than four dimensions, as the mathematics of superstring theory seems to require?

    Susskind: Almost all working high-energy theoretical physicists are convinced some sort of extra dimensions are needed to explain the complexity of elementary particles. That particles move in extra dimensions is another way of talking about the fact that they have more complex properties than just position and velocity. It is hard to find a serious paper about particle phenomenology that doesn’t in some way use the tools of superstring theory. Furthermore, we all agree that the origin of elementary particles is most likely at the planck scale and cannot be understood without a good theory of quantum gravity.

    Odenwald: So if superstring theory were found to be an incorrect model for our particular universe, is that like turning the clock back to circa 1978 in physics?

    Susskind: I agree that going back to the ’70s would be turning the clock back in the sense that we would be ignoring the vast amount of mathematical knowledge that has been gained over the subsequent years, mostly from string theory. That is just not going to happen. The changes in our theoretical understanding of quantum field theory, gravity, black holes, are completely irreversible. [String theory mathematics] has even worked its way into nuclear physics and heavy ion collisions as well as into condensed matter physics.

    Odenwald: Kind of a hard place for modern theoreticians to revisit, but for astronomy and cosmology the 1970s seem not such a bad place. Without superstring theory, we would still have cosmological inflation. Without superstring theory, what happened at the instant of the Big Bang would remain unknown, logically indescribable, and still a great puzzle… as it always has.

    Susskind: [Not quite.] Recent cosmology has been completely dominated by studying the cosmic microwave background [CMB] and inflationary theory.

    Cosmic Background Radiation Planck
    CMB per ESA/Planck

    ESA Planck

    Gravitational Wave Background
    Gravitational waves from BICEP2 in support of Inflation theory not yet accepted

    BICEP 2

    The CMB fluctuation spectrum is widely understood as a quantum effect. Inflation is a gravitational effect. Is there any question that quantum gravity (quantum plus gravity) will be the framework for understanding the early universe? No, there is not. Also I might add that the old inflation that you are referring to was a disaster. It didn’t work. Many inflationary cosmologists like Linde, Vilenkin, and Guth are looking to string theory for possible answers to the puzzles of inflation.

    Odenwald: Is it fair to say that superstring theory is “too big to fail”? I am reminded of the alledged quotation by Einstein as he was awaiting news about a major test of relativity in 1919. A reporter is said to have asked, “Well, what would it mean if your theory is wrong?” to which Einstein allegedly replied, “Then I would feel sorry for the good Lord; the theory is correct!” Is the physics community in the same predicament because superstring theory is a “beautiful” theory that seems to explain so much, and its mathematics is so impeccable that it is used in other theoretical settings in physics?

    Susskind: Exactly right. However, it is fair to say that while theorists were developing powerful tools, they mostly had wrong expectations for what the theory was indicating. Most theorists hoped that string theory would lead to an absolutely unique set of particles, coupling constants, with exactly vanishing cosmological constant. What we have learned from the theory itself is that it is a theory of tremendous diversity. Unexpectedly, string theory is most comfortable with a huge multiverse of tremendous variety instead of the small “knowable” and unique universe we once imagined.

    Odenwald: So what would our theoretical explanations for our universe look like without added dimensions, quantum gravity or string theory?

    Susskind: Without these things the world as we know it couldn’t exist. Giving up quantum gravity means giving up either the ideas of quantum [mechanics] or of gravity [and general relativity]. In a cosmological context quantum gravity is responsible for the primordial density fluctuations [we directly observe in the CMB] that ultimately condensed to form stars, galaxies, planets, etc. Without string theory we should not have the diversity of possibilites that allow pocket universes [Alan Guth’s term] with the ultra-fine tuning needed to insure conditions for our kind of life.

    Odenwald: If string theory loses its experimental support at the LHC, wouldn’t it be far worse than merely going back to cosmology circa 1975 or even 1965? We would have to question the very mathematical tools we have been using for the last 50 years!

    Susskind: I agree with your analysis, except that I would add: Expect the unexpected. Unforseen surprises are the rule in science, not the exception. Remember: Stuff happens.

    Odenwald: If superstring theory falls, are there any competing theories out there that could hold out some hope?

    Susskind: Not as far as I know.

    • * * * *

    So there you have it. The Large Hadron Collider absolutely has to find some clue about supersymmetry, or superstring theory is compromised, we will have no good idea about dark matter, and we will definitely be in a bad place until the super-LHC is built in the 2030s.

    Patience, however, is a still a necessary virtue. Physicists were in this same quandary before the Higgs boson was finally discovered after 50 years of increasingly panicked searching.

    See the full article here.

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  • richardmitnick 11:09 am on September 21, 2014 Permalink | Reply
    Tags: , , , , , , Supersymmetry   

    From The Daily Galaxy: “”Hidden Supersymmetry?” –Debate Over a New Physics Intensifies (The Weekend Feature)” 

    Daily Galaxy
    The Daily Galaxy

    Theoretical physicists have theorized a possible solution to a longstanding mystery bolstered by the recent discovery of the Higgs boson – a way to preserve the theory of supersymmetry. It was a breakthrough with profound implications for the world as we know it: the Higgs boson, the elementary particle that gives all other particles their mass, discovered at the Large Hadron Collider in 2012. But, for many scientists, it’s only the beginning. When the LHC fires up again in 2015 at its highest-ever collision energy, theorists will be watching with intense interest.
    Earlier this year in Physical Review Letters, Csaba Csaki, Cornell professor of physics, and colleagues theorized a possible solution to a longstanding mystery bolstered by the recent discovery of the Higgs – a way to preserve the theory of supersymmetry, a popular, but experimentally unproven, extension of the Standard Model of particle physics.

    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.

    Depiction of Higgs Boson

    CERN LHC Map
    CERN LHC Grand Tunnel
    CERN LHC particles


    Supersymmetry could help explain the unusual properties of the Higgs boson, why the strong and weak interactions of subatomic particles appear to be so different, as well as the origin of dark matter, which makes up a quarter of the universe.

    The Standard Model deals with three of nature’s fundamental forces: strong, weak and electromagnetic, which govern the relationships between all the known subatomic particles. Supersymmetry extends the Standard Model by introducing new particles, called “superpartners”; every observed particle would have a corresponding superpartner, with similar properties to those of the observed particles, except heavier and with different spin values.

    Some scientists think supersymmetry ought to be abandoned after the LHC failed to detect any of these superpartners; some of them, like the top quark’s superpartner, the “stop,” is predicted to be so light that the LHC should already have seen it.

    In their paper, Csaki and colleagues counter that the particles may be hidden by the noise of other particles formed during the LHC’s unprecedented energy of proton-proton collisions.

    Their idea has to do with a concept called R-parity. All observed particles are assumed to have positive R-parity, while the unobserved superpartners would be negative, implying that the superpartners cannot decay to ordinary particles exclusively.

    Searching for the superpartners at the LHC, Csaki explained, has largely operated under the assumption that this R-parity is always exactly conserved. Csaki and colleagues pose a scenario in which R-parity is violated, and would result in a series of interactions giving rise to particle decays that would be nearly impossible to detect by the LHC’s current parameters.

    “The upshot is that there are ways to hide supersymmetry at the LHC,” Csaki said. “If the signal isn’t very different from the background, it’s very hard to find them. That’s the problem.”

    The LHC, when back online next year, is scheduled to run at a collision energy of 14 TeV (teraelectron volts) – about double the energy of previous runs. It could lead to ultimate proof of the theory of supersymmetry, which Csaki deems the “most beautiful” of the Standard Model extensions offered today – but science must make room for all possibilities.

    “It’s very possible that supersymmetry is not the right theory, and that’s OK,” he said. “The important thing is to understand the way science works, to try and make the best guesses you can, and the experimentalists go and check it. … We have to make sure we are exploring every corner, and we shouldn’t leave some potentially reasonable theory out where things could be hiding.”

    The image at the top of the page is an artist’s simulation of dark matter halo around the Milky Way galaxy.’s own backyard. Credit: NASA, ESA, and T. Brown and J. Tumlinson (STScI)

    See the full article here.

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  • richardmitnick 2:21 pm on August 19, 2014 Permalink | Reply
    Tags: , , , , Supersymmetry   

    From Quanta: “At Multiverse Impasse, a New Theory of Scale” 

    Quanta Magazine
    Quanta Magazine

    August 18, 2014
    Natalie Wolchover

    Mass and length may not be fundamental properties of nature, according to new ideas bubbling out of the multiverse.

    Though galaxies look larger than atoms and elephants appear to outweigh ants, some physicists have begun to suspect that size differences are illusory. Perhaps the fundamental description of the universe does not include the concepts of “mass” and “length,” implying that at its core, nature lacks a sense of scale.

    This little-explored idea, known as scale symmetry, constitutes a radical departure from long-standing assumptions about how elementary particles acquire their properties. But it has recently emerged as a common theme of numerous talks and papers by respected particle physicists. With their field stuck at a nasty impasse, the researchers have returned to the master equations that describe the known particles and their interactions, and are asking: What happens when you erase the terms in the equations having to do with mass and length?

    Nature, at the deepest level, may not differentiate between scales. With scale symmetry, physicists start with a basic equation that sets forth a massless collection of particles, each a unique confluence of characteristics such as whether it is matter or antimatter and has positive or negative electric charge. As these particles attract and repel one another and the effects of their interactions cascade like dominoes through the calculations, scale symmetry “breaks,” and masses and lengths spontaneously arise.

    Similar dynamical effects generate 99 percent of the mass in the visible universe. Protons and neutrons are amalgams — each one a trio of lightweight elementary particles called quarks. The energy used to hold these quarks together gives them a combined mass that is around 100 times more than the sum of the parts. “Most of the mass that we see is generated in this way, so we are interested in seeing if it’s possible to generate all mass in this way,” said Alberto Salvio, a particle physicist at the Autonomous University of Madrid and the co-author of a recent paper on a scale-symmetric theory of nature.

    In the equations of the “Standard Model of particle physics”, only a particle discovered in 2012, called the Higgs boson, comes equipped with mass from the get-go. According to a theory developed 50 years ago by the British physicist Peter Higgs and associates, it doles out mass to other elementary particles through its interactions with them. Electrons, W and Z bosons, individual quarks and so on: All their masses are believed to derive from the Higgs boson — and, in a feedback effect, they simultaneously dial the Higgs mass up or down, too.

    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.

    The new scale symmetry approach rewrites the beginning of that story.

    Alessandro Strumia of the University of Pisa, pictured speaking at a conference in 2013, has co-developed a scale-symmetric theory of particle physics called “agravity.” Thomas Lin/Quanta Magazine

    “The idea is that maybe even the Higgs mass is not really there,” said Alessandro Strumia, a particle physicist at the University of Pisa in Italy. “It can be understood with some dynamics.”

    The concept seems far-fetched, but it is garnering interest at a time of widespread soul-searching in the field. When the Large Hadron Collider at CERN Laboratory in Geneva closed down for upgrades in early 2013, its collisions had failed to yield any of dozens of particles that many theorists had included in their equations for more than 30 years. The grand flop suggests that researchers may have taken a wrong turn decades ago in their understanding of how to calculate the masses of particles.

    “We’re not in a position where we can afford to be particularly arrogant about our understanding of what the laws of nature must look like,” said Michael Dine, a professor of physics at the University of California, Santa Cruz, who has been following the new work on scale symmetry. “Things that I might have been skeptical about before, I’m willing to entertain.”

    The Giant Higgs Problem

    The scale symmetry approach traces back to 1995, when William Bardeen, a theoretical physicist at Fermi National Accelerator Laboratory in Batavia, Ill., showed that the mass of the Higgs boson and the other Standard Model particles could be calculated as consequences of spontaneous scale-symmetry breaking. But at the time, Bardeen’s approach failed to catch on. The delicate balance of his calculations seemed easy to spoil when researchers attempted to incorporate new, undiscovered particles, like those that have been posited to explain the mysteries of dark matter and gravity.

    Instead, researchers gravitated toward another approach called “supersymmetry” that naturally predicted dozens of new particles. One or more of these particles could account for dark matter. And supersymmetry also provided a straightforward solution to a bookkeeping problem that has bedeviled researchers since the early days of the Standard Model.

    Supersymmetry standard model
    Standard Model of Supersymmetry

    In the standard approach to doing calculations, the Higgs boson’s interactions with other particles tend to elevate its mass toward the highest scales present in the equations, dragging the other particle masses up with it. “Quantum mechanics tries to make everybody democratic,” explained theoretical physicist Joe Lykken, deputy director of Fermilab and a collaborator of Bardeen’s. “Particles will even each other out through quantum mechanical effects.”

    This democratic tendency wouldn’t matter if the Standard Model particles were the end of the story. But physicists surmise that far beyond the Standard Model, at a scale about a billion billion times heavier known as the “Planck mass,” there exist unknown giants associated with gravity. These heavyweights would be expected to fatten up the Higgs boson — a process that would pull the mass of every other elementary particle up to the Planck scale. This hasn’t happened; instead, an unnatural hierarchy seems to separate the lightweight Standard Model particles and the Planck mass.

    With his scale symmetry approach, Bardeen calculated the Standard Model masses in a novel way that did not involve them smearing toward the highest scales. From his perspective, the lightweight Higgs seemed perfectly natural. Still, it wasn’t clear how he could incorporate Planck-scale gravitational effects into his calculations.

    Meanwhile, supersymmetry used standard mathematical techniques, and dealt with the hierarchy between the Standard Model and the Planck scale directly. Supersymmetry posits the existence of a missing twin particle for every particle found in nature. If for each particle the Higgs boson encounters (such as an electron) it also meets that particle’s slightly heavier twin (the hypothetical “selectron”), the combined effects would nearly cancel out, preventing the Higgs mass from ballooning toward the highest scales. Like the physical equivalent of x + (–x) ≈ 0, supersymmetry would protect the small but non-zero mass of the Higgs boson. The theory seemed like the perfect missing ingredient to explain the masses of the Standard Model — so perfect that without it, some theorists say the universe simply doesn’t make sense.

    Yet decades after their prediction, none of the supersymmetric particles have been found. “That’s what the Large Hadron Collider has been looking for, but it hasn’t seen anything,” said Savas Dimopoulos, a professor of particle physics at Stanford University who helped develop the supersymmetry hypothesis in the early 1980s. “Somehow, the Higgs is not protected.”

    The LHC will continue probing for convoluted versions of supersymmetry when it switches back on next year, but many physicists have grown increasingly convinced that the theory has failed. Just last month at the International Conference of High-Energy Physics in Valencia, Spain, researchers analyzing the largest data set yet from the LHC found no evidence of supersymmetric particles. (The data also strongly disfavors an alternative proposal called “technicolor.”)

    The multiverse hypothesis has surged in begrudging popularity in recent years. But the argument feels like a cop-out to many, or at least a huge letdown.

    The implications are enormous. Without supersymmetry, the Higgs boson mass seems as if it is reduced not by mirror-image effects but by random and improbable cancellations between unrelated numbers — essentially, the initial mass of the Higgs seems to exactly counterbalance the huge contributions to its mass from gluons, quarks, gravitational states and all the rest. And if the universe is improbable, then many physicists argue that it must be one universe of many: just a rare bubble in an endless, foaming “multiverse.” We observe this particular bubble, the reasoning goes, not because its properties make sense, but because its peculiar Higgs boson is conducive to the formation of atoms and, thus, the rise of life. More typical bubbles, with their Planck-size Higgs bosons, are uninhabitable.

    “It’s not a very satisfying explanation, but there’s not a lot out there,” Dine said.

    As the logical conclusion of prevailing assumptions, the multiverse hypothesis has surged in begrudging popularity in recent years. But the argument feels like a cop-out to many, or at least a huge letdown. A universe shaped by chance cancellations eludes understanding, and the existence of unreachable, alien universes may be impossible to prove. “And it’s pretty unsatisfactory to use the multiverse hypothesis to explain only things we don’t understand,” said Graham Ross, an emeritus professor of theoretical physics at the University of Oxford.

    The multiverse ennui can’t last forever.

    “People are forced to adjust,” said Manfred Lindner, a professor of physics and director of the Max Planck Institute for Nuclear Physics in Heidelberg who has co-authored several new papers on the scale symmetry approach. The basic equations of particle physics need something extra to rein in the Higgs boson, and supersymmetry may not be it. Theorists like Lindner have started asking, “Is there another symmetry that could do the job, without creating this huge amount of particles we didn’t see?”

    Wrestling Ghosts

    Picking up where Bardeen left off, researchers like Salvio, Strumia and Lindner now think scale symmetry may be the best hope for explaining the small mass of the Higgs boson. “For me, doing real computations is more interesting than doing philosophy of multiverse,” said Strumia, “even if it is possible that this multiverse could be right.”

    For a scale-symmetric theory to work, it must account for both the small masses of the Standard Model and the gargantuan masses associated with gravity. In the ordinary approach to doing the calculations, both scales are put in by hand at the beginning, and when they connect in the equations, they try to even each other out. But in the new approach, both scales must arise dynamically — and separately — starting from nothing.

    “The statement that gravity might not affect the Higgs mass is very revolutionary,” Dimopoulos said.

    A theory called “agravity” (for “adimensional gravity”) developed by Salvio and Strumia may be the most concrete realization of the scale symmetry idea thus far. Agravity weaves the laws of physics at all scales into a single, cohesive picture in which the Higgs mass and the Planck mass both arise through separate dynamical effects. As detailed in June in the Journal of High-Energy Physics, agravity also offers an explanation for why the universe inflated into existence in the first place. According to the theory, scale-symmetry breaking would have caused an exponential expansion in the size of space-time during the Big Bang.

    However, the theory has what most experts consider a serious flaw: It requires the existence of strange particle-like entities called “ghosts.” Ghosts either have negative energies or negative probabilities of existing — both of which wreck havoc on the equations of the quantum world.

    “Negative probabilities rule out the probabilistic interpretation of quantum mechanics, so that’s a dreadful option,” said Kelly Stelle, a theoretical particle physicist at Imperial College, London, who first showed in 1977 that certain gravity theories give rise to ghosts. Such theories can only work, Stelle said, if the ghosts somehow decouple from the other particles and keep to themselves. “Many attempts have been made along these lines; it’s not a dead subject, just rather technical and without much joy,” he said.

    Marcela Carena, a senior scientist at Fermi National Accelerator Laboratory in Batavia, Ill.Courtesy of Marcela Carena

    Strumia and Salvio think that, given all the advantages of agravity, ghosts deserve a second chance. “When antimatter particles were first considered in equations, they seemed like negative energy,” Strumia said. “They seemed nonsense. Maybe these ghosts seem nonsense but one can find some sensible interpretation.”

    Meanwhile, other groups are crafting their own scale-symmetric theories. Lindner and colleagues have proposed a model with a new “hidden sector” of particles, while Bardeen, Lykken, Marcela Carena and Martin Bauer of Fermilab and Wolfgang Altmannshofer of the Perimeter Institute for Theoretical Physics in Waterloo, Canada, argue in an Aug. 14 paper that the scales of the Standard Model and gravity are separated as if by a phase transition. The researchers have identified a mass scale where the Higgs boson stops interacting with other particles, causing their masses to drop to zero. It is at this scale-free point that a phase change-like crossover occurs. And just as water behaves differently than ice, different sets of self-contained laws operate above and below this critical point.

    To get around the lack of scales, the new models require a calculation technique that some experts consider mathematically dubious, and in general, few will say what they really think of the whole approach. It is too different, too new. But agravity and the other scale symmetric models each predict the existence of new particles beyond the Standard Model, and so future collisions at the upgraded LHC will help test the ideas.

    In the meantime, there’s a sense of rekindling hope.

    “Maybe our mathematics is wrong,” Dine said. “If the alternative is the multiverse landscape, that is a pretty drastic step, so, sure — let’s see what else might be.

    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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  • richardmitnick 12:33 pm on June 20, 2014 Permalink | Reply
    Tags: , , , Supersymmetry   

    From Fermilab- “Frontier Science Result: CMS Slicing parameter space with a razor” 

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

    Friday, June 20, 2014
    Jim Pivarski

    Supersymmetry is hard to kill. It is more general than most theories of physics beyond the Standard Model: It is the basic idea that particles and forces are fundamentally the same thing but appear different because something creates an effective distinction between them, similar to the way that the Higgs boson creates an effective distinction between the electric and weak parts of the electroweak force. For supersymmetry, that “something” is unknown — many different models of supersymmetry breaking have been proposed and others could be thought up tomorrow. Each variant, including minor variations in numerical parameters, yields different decay patterns involving different cascades of particles.

    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.

    Supersymmetry standard model
    Standard Model for Supersymmetry

    Seekers of supersymmetry are faced with a dilemma: Pick a model of supersymmetry breaking and hope you’re lucky enough to find it, or look at a quantity that is sensitive to a broad class of models, but with less statistical sensitivity. If you pick a specific model and don’t find it, you can set a precise exclusion limit, but only on one model — supersymmetry itself remains elusive. Broad searches, on the other hand, are hard to formulate. You have to think of a signature that is shared by many supersymmetric models yet is different enough from the Standard Model to clearly claim a discovery or set a tight limit.

    One technique, dubbed “the razor,” shares aspects of both. It makes weak assumptions about the mechanism of supersymmetry breaking but also makes a sharp distinction between supersymmetric particles and the Standard Model. Assuming that supersymmetric particles are within reach of the LHC (like squarks and gluinos), and that they are produced in pairs (R-parity is not violated), and that they decay in cascading chains (as many variants of supersymmetry do), they would show up in LHC collisions as two rough bundles of particles, each with an energy that corresponds to the mass of the first supersymmetric particle in the chain. Different models predict different decay patterns within each bundle, but this technique looks only for the bundles.

    A group of CMS scientists selected events using the razor technique and found them to be consistent with the Standard Model. This cuts out a broad range of supersymmetric models, many more than a focused technique would. There are still others that might evade the razor’s weak assumptions, but the remaining space is getting thin.

    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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