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  • richardmitnick 2:03 pm on September 16, 2016 Permalink | Reply
    Tags: , Supersymmetry, , ,   

    From Symmetry: “The secret lives of long-lived particles” 

    Symmetry Mag

    Symmetry

    09/16/16
    Sarah Charley

    A theoretical species of particle might answer nearly every question about our cosmos—if scientists can find it.

    1
    ATLAS collaboration

    The universe is unbalanced.

    Gravity is tremendously weak. But the weak force, which allows particles to interact and transform, is enormously strong. The mass of the Higgs boson is suspiciously petite. And the catalog of the makeup of the cosmos? Ninety-six percent incomplete.

    Almost every observation of the subatomic universe can be explained by the Standard Model of particle physics—a robust theoretical framework bursting with verifiable predictions.

    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 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.

    But because of these unsolved puzzles, the math is awkward, incomplete and filled with restrictions.

    A few more particles would solve almost all of these frustrations. Supersymmetry (nicknamed SUSY for short) is a colossal model that introduces new particles into the Standard Model’s equations.

    Standard model of Supersymmetry DESY
    Standard model of Supersymmetry DESY

    It rounds out the math and ties up loose ends. The only problem is that after decades of searching, physicists have found none of these new friends.

    But maybe the reason physicists haven’t found SUSY (or other physics beyond the Standard Model) is because they’ve been looking through the wrong lens.

    “Beautiful sets of models keep getting ruled out,” says Jessie Shelton, a theorist at the University of Illinois, “so we’ve had to take a step back and consider a whole new dimension in our searches, which is the lifetime of these particles.”

    In the past, physicists assumed that new particles produced in particle collisions would decay immediately, almost precisely at their points of origin. Scientists can catch particles that behave this way—for example, Higgs bosons—in particle detectors built around particle collision points. But what if new particles had long lifetimes and traveled centimeters—even kilometers—before transforming into something physicists could detect?

    This is not unprecedented. Bottom quarks, for instance, can travel a few tenths of a millimeter before decaying into more stable particles. And muons can travel several kilometers (with the help of special relativity) before transforming into electrons and neutrinos. Many theorists are now predicting that there may be clandestine species of particles that behave in a similar fashion. The only catch is that these long-lived particles must rarely interact with ordinary matter, thus explaining why they’ve escaped detection for so long. One possible explanation for this aloof behavior is that long live particles dwell in a hidden sector of physics.

    “Hidden-sector particles are separated from ordinary matter by a quantum mechanical energy barrier—like two villages separated by a mountain range,” says Henry Lubatti from the University of Washington. “They can be right next to each other, but without a huge boost in energy to get over the peak, they’ll never be able to interact with each other.”

    High-energy collisions generated by the Large Hadron Collider could kick these hidden-sector particles over this energy barrier into our own regime. And if the LHC can produce them, scientists should be able to see the fingerprints of long-lived particles imprinted in their data.

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

    Long-lived particles jolted into our world by the LHC would most likely fly at close to the speed of light for between a few micrometers and a few hundred thousand kilometers before transforming into ordinary and measurable matter. This incredibly generous range makes it difficult for scientists to pin down where and how to look for them.

    But the lifetime of a subatomic particle is much like that of any living creature. Each type of particle has an average lifespan, but the exact lifetime of an individual particle varies. If these long-lived particles can travel thousands of kilometers before decaying, scientists are hoping that they’ll still be able to catch a few of the unlucky early-transformers before they leave the detector. Lubatti and his collaborators have also proposed a new LHC surface detector, which would extend their search range by many orders of magnitude.

    Because these long-lived particles themselves don’t interact with the detector, their signal would look like a stream of ordinary matter spontaneously appearing out of nowhere.

    “For instance, if a long lived particle decayed into quarks while inside the muon detector, it would mimic the appearance of several muons closely clustered together,” Lubatti says. “We are triggering on events like this in the ATLAS experiment.” After recording the events, scientists use custom algorithms to reconstruct the origins of these clustered particles to see if they could be the offspring of an invisible long-lived parent.

    If discovered, this new breed of matter could help answer several lingering questions in physics.

    “Long-lived particles are not a prediction of a single new theory, but rather a phenomenon that could fit into almost all of our frameworks for beyond-the-Standard-Model physics,” Shelton says.

    In addition to rounding out the Standard Model’s mathematics, inert long-lived particles could be cousins of dark matter—an invisible form of matter that only interacts with the visible cosmos through gravity. They could also help explain the origin of matter after the Big Bang.

    “So many of us have spent a lifetime studying such a tiny fraction of the universe,” Lubatti says. “We’ve understood a lot, but there’s still a lot we don’t understand—an enormous amount we don’t understand. This gives me and my colleagues pause.”

    See the full article here .

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    Symmetry is a joint Fermilab/SLAC publication.


     
  • richardmitnick 7:24 am on August 27, 2016 Permalink | Reply
    Tags: Supersymmetry, ,   

    From Symmetry: “Winners declared in SUSY bet” 

    Symmetry Mag

    Symmetry

    08/26/16
    Kathryn Jepsen

    1
    Peter Munch Andersen

    Physicists exchanged cognac in Copenhagen at the conclusion of a bet about supersymmetry and the LHC.

    As a general rule, theorist Nima Arkani-Hamed does not get involved in physics bets.

    “Theoretical physicists like to take bets on all kinds of things,” he says. “I’ve always taken the moral high ground… Nature decides. We’re all in pursuit of the truth. We’re all on the same side.”

    But sometimes you’re in Copenhagen for a conference, and you’re sitting in a delightfully unusual restaurant—one that sort of reminds you of a cave—and a fellow physicist gives you the opportunity to get in on a decade-old wager about supersymmetry and the Large Hadron Collider. Sometimes then, you decide to bend your rule. “It was just such a jovial atmosphere, I figured, why not?”

    That’s how Arkani-Hamed found himself back in Copenhagen this week, passing a 1000-Krone bottle of cognac to one of the winners of the bet, Director of the Niels Bohr International Academy Poul Damgaard.

    Arkani-Hamed had wagered that experiments at the LHC would find evidence of supersymmetry by the arbitrary date of June 16, 2016. Supersymmetry, SUSY for short, is a theory that predicts the existence of partner particles for the members of the Standard Model of particle physics.

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

    Standard model of Supersymmetry DESY
    Standard model of Supersymmetry DESY

    The deadline was not met. But in a talk at the Niels Bohr Institute, Arkani-Hamed pointed out that the end of the gamble does not equal the end of the theory.

    “I was not a good student in school,” Arkani-Hamed explained. “One of my big problems was not getting homework done on time. It was a constant battle with my teachers… Just give me another week! It’s kind of like the bet.”

    He pointed out that so far the LHC has gathered just 1 percent of the total amount of data it aims to collect.

    With that data, scientists can indeed rule out the most vanilla form of supersymmetry. But that’s not the version of supersymmetry Arkani-Hamed would expect the LHC to find anyway, he said.

    It is still possible LHC experiments will find evidence of other SUSY models—including the one Arkani-Hamed prefers, called split SUSY, which adds superpartners to just half of the Standard Model’s particles. And if LHC scientists don’t find evidence of SUSY, Arkani-Hamed pointed out, the theoretical problems it aimed to solve will remain an exciting challenge for the next generation of theorists to figure out.

    “I think Winston Churchill said that in victory you should be magnanimous,” Damgaard said after Arkani-Hamed’s talk. “I know also he said that in defeat you should be defiant. And that’s certainly Nima.”

    Arkani-Hamed shrugged. But it turned out he was not the only optimist in the room. Panelist Yonit Hochberg of the University of California, Berkeley conducted an informal poll of attendees. She found that the majority still think that in the next 20 years, as data continues to accumulate, experiments at the LHC will discover something new.

    See the full article here .

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    Symmetry is a joint Fermilab/SLAC publication.


     
  • richardmitnick 10:30 am on August 4, 2016 Permalink | Reply
    Tags: , , Supersymmetry, , Ghostly particles could be haunting our universe   

    From New Scientist: “Ghost particles may explain why gravity is so surprisingly weak” 

    NewScientist

    New Scientist

    3 August 2016
    No writer credit found

    1
    Spot the strange particles. ESO/T.Preibisch

    Ghostly particles could be haunting our universe. A new theory claims that the cosmos is full of unseen particle families that don’t interact with each other. If true, the model could explain why gravity is so puzzlingly weak.

    The idea is an alternative to supersymmetry, a theory in which every known particle has a heavier partner.

    Standard model of Supersymmetry DESY
    Standard model of Supersymmetry DESY

    Such superparticles would explain why the mass of our familiar set of particles is low enough to account for gravity’s weakness. But the particle-smashing Large Hadron Collider, near Geneva, Switzerland, still hasn’t seen any superparticles, despite years of searching.

    Maybe that’s because we need not just one set of partner particles, but many, says Nima Arkani-Hamed at Princeton University.

    “The idea is a little wild,” admits Tim Cohen at the University of Oregon in Eugene, who teamed up with Arkani-Hamed and others to work on the new theory. “The absence of new physics at the LHC has motivated us to – instead of introducing a few new particles – introduce 10^16 new particles.”

    The group began by tweaking the story of what happened in the big bang. After its birth, the universe inflated until it was cold and flat. Arkani-Hamed’s team believes that at this point, all the energy was locked up in the form of a single kind of particle, which they call the reheaton.

    Then, because particles always want to decay into something less energetic, the reheatons broke down into smaller particles – the ones that structure the universe today.

    Next, the physicists took our familiar standard model of particles, and introduced many copies of it into the story. These families live side-by-side, but scarcely interact with each other. Each family has the same kinds of particles, but a slightly different mass for its Higgs boson – the famous “missing piece” of the standard model, which was found to have a surprisingly light mass by the LHC in 2012.

    CERN CMS Higgs Event
    CERN CMS Higgs Event

    Because the Higgs gives mass to other particles, varying it leads to big differences in how much the particles in a group pull on each other.

    Extreme families

    If the universe was full of particles from families with very heavy or zero-mass Higgs bosons, then gravity between particles would be too strong or too weak for atoms to form. Only one family would have a Higgs boson just light enough to let planets and people develop – the Goldilocks amount of gravity. So how did our universe come to have more of the right sort of particles?

    The team says that for families with a Higgs mass just above zero, the reheatons would have decayed into one Higgs boson and another particle of the remaining energy. This wouldn’t have been the case for other families. Particles can only decay into things lighter than themselves, so for families with high Higgs boson masses, the reheatons would have decayed into two smaller entities – instead of the Higgs – plus the remainder particle. Particles prefer to decay into fewer entities, so most of the big bang’s energy would be funnelled into light-Higgs families, and only a bit to those with slightly heavier Higgs bosons.

    As a result, our familiar standard model of particles – which has the lightest possible non-zero-mass Higgs – reigns supreme.

    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 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.

    That’s one step towards solving the weak-gravity problem. The next step comes from the team’s observation that the equation for gravity between particles can be fine-tuned by adjusting the number of particle sectors.

    The team calculated that about 10^16 particle families results in a small value for gravity’s strength. If you combine the effect of large numbers of particles on the gravity equation and the reheatons favouring light Higgs bosons, you get gravity just weak enough for playing basketball instead of collapsing into a singularity, says Cohen.

    Occam’s razor

    Some physicists are sceptical of this first incarnation of the new theory. Assuming that there are huge numbers of particle classes isn’t an appealing solution, says Peter Woit at Columbia University in New York. “It’s a huge violation of Occam’s razor,” he says.

    “This is not a bad idea – in fact, it’s a pretty clever idea in terms of how things might work,” says Matt Strassler at Harvard University. However, the mathematics used to flesh out the idea is pretty contrived, he says. Hopefully now that the idea is out there, the model can be improved, he adds.

    Paddy Fox at the Fermi National Accelerator Laboratory near Batavia, Illinois, says there’s a way to test this theory within the next 10 years. A few particles from the sets with slightly heavier Higgses probably got produced. We could see these ghostly particle families haunting the universe via their effects on the cosmic microwave background – leftover radiation from the universe’s birth.

    “There’s an exciting possibility that you could see such an imprint in the next round of experiments,” says Fox. “This is not something we have to wait around and one day hope to see.”

    This means the search for new particles could move out of underground colliders and into astronomy, says Cohen. So don’t call the Ghostbusters until the astronomy results are in.

    Journal reference: arxiv.org/abs/1607.06821

    See the full article here .

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  • richardmitnick 9:43 am on July 29, 2016 Permalink | Reply
    Tags: , Supersymmetry,   

    From Yale: “Yale leads research collaboration to explore origins of the universe” 

    Yale University bloc

    Yale University

    July 27, 2016
    No writer credit found

    1
    A deformation in the shape of a nucleus along the direction of its spin axis can arise only due to the presence of new, exotic particles. To search for such a deformation, thallium (Tl) nuclei are exposed to the strong electric field inside a polar molecule (thallium fluoride, TlF) that has been polarized by an external electric field. The deformation leads to a torque on the nucleus, which causes its axis to rotate around the electric field, in the same way that the axis of a gyroscope rotates around the direction of gravity.

    Yale physics professor David DeMille has launched a pioneering investigation into the origins of the universe with support from the John Templeton Foundation and the Heising-Simons Foundation. DeMille plans to build a novel apparatus to sense the existence of never-before-seen subatomic particles thought to have a determining role in the formation of matter. Proving their existence — or absence — will provide a window into the earliest moments following the Big Bang.

    DeMille will undertake the project in partnership with collaborators David Kawall of the University of Massachusetts, Tanya Zelevinsky of Columbia University, and Steve Lamoreaux of Yale. The grants from the John Templeton Foundation and the Heising-Simons Foundation, totaling $3 million, will support project staff and laboratory equipment.

    “Our approach is a radical departure from the large particle accelerators that generally come to mind when you look for exotic particles,” DeMille said. “Building on work that has been done at Yale, we will conduct a new type of experiment to probe for new particles and forces responsible for the predominance of matter over antimatter in the universe. We are very grateful to the Templeton and Heising-Simons foundations for their support, which was critical for our team to launch this work.”

    Addressing a fundamental mystery in physics

    DeMille’s research partnership seeks to answer a persistent question in physics known as the matter-antimatter asymmetry: Why is the universe made entirely of matter, when an equal number of matter particles and antimatter particles were created just after the Big Bang? Astronomical observations show that the matter and antimatter mostly annihilated each other, turning back into energy. The antimatter was eliminated, but a tiny fraction of matter was somehow left over, forming all the objects in the universe today.

    The current model for all known fundamental forces and particles fails to explain how the excess matter survived. Recent mathematical theories seek to explain the matter-antimatter asymmetry by positing new, as-yet-undiscovered forces and particles, such as “supersymmetry particles.”

    Standard model of Supersymmetry DESY
    Standard model of Supersymmetry DESY

    In most of these theories, the new fundamental phenomena also cause a tiny, yet detectable, deformation in the distribution of electric charge in ordinary atomic nuclei, known as a Schiff moment. The nuclear Schiff moment arises only in the presence of new particles and forces with properties needed to explain the matter-antimatter asymmetry.

    A novel way to detect new particles

    Finding new subatomic particles is a notoriously difficult challenge. In the 1960s, physicists predicted the existence of the Higgs boson, an elementary particle in the Standard Model of particle physics, but it was not until 2013 that scientists at the CERN facility in Switzerland could prove its existence using the Large Hadron Collider — a facility measuring 17 miles in circumference and costing over $7.5 billion.

    CERN CMS Higgs Event
    CERN CMS Higgs Event

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

    In contrast, DeMille’s team will design and assemble an instrument, about 15 feet across, in an on-campus Yale physics laboratory. The device will be made up of roughly 100,000 custom-designed and fabricated parts; it will take a team of six postdoctoral fellows and graduate students three years to construct.

    “Our device will focus a cryogenic beam of diatomic molecules through an electric field to detect a nuclear Schiff moment,” DeMille said. “This technique will yield a 100-fold increase in sensitivity over the current state of the art, enough to say whether or not new particles with the properties posited by many theories to explain the matter-antimatter asymmetry actually exist. This determination will either validate 30 years of mainstream work in theoretical physics or send the field in another direction.”

    The experiment depends on a strange quirk of quantum mechanics, which posits that subatomic particles like electrons and protons must constantly spin out and reabsorb other particles — a phenomenon that DeMille and his colleagues have learned to observe in the laboratory. “I tell my students to imagine Pig Pen, the character from‘Peanuts,’” said DeMille. “Every proton is surrounded by an ever-cycling cloud of short-lived particles that pop in and out of existence. Theoretically, this cloud should include supersymmetry particles.”

    Direct observation of supersymmetry particles with the properties needed to explain the matter-antimatter asymmetry is beyond current technology, but DeMille believes he can record their influence on the proton itself — hence his search for the Schiff moment. “The forces associated with a supersymmetry particle should cause a small but observable deformation at the surface of the proton,” he said. “This will in turn cause a similar deformation in an atomic nucleus — the Schiff moment. We are looking for a minute dent on one side of the nucleus and a corresponding bulge on the other. If we find this deformation, we will have definitive proof that new particles exist.”

    The precision of this experiment will be unprecedented, notes DeMille. For comparison, if the proton were scaled up to the size of the Earth, the dent in its surface would be 1/30th the width of a human hair.
    Revising our understanding of the universe

    Steven Girvin, deputy provost for research and the Eugene Higgins Professor of Physics, has hailed this project as groundbreaking. “David DeMille, David Kawall, Tanya Zelevinsky, and Steve Lamoreaux are on the very cutting edge of physics,” he said. “Their experiment promises to detect particles 10 times more massive than what one might see in the Large Hadron Collider. If they succeed, this will be an extraordinary accomplishment, and one that revises our understanding of the universe. I am personally grateful to the Heising-Simons and Templeton foundations for their decision to work together to fund the different aspects of this large and complex project.”
    John Templeton Foundation

    Founded by Sir John Templeton, a 1934 graduate of Yale College, the John Templeton Foundation serves as a philanthropic catalyst for discoveries relating to the Big Questions of human purpose and ultimate reality. The foundation supports research on subjects ranging from complexity, evolution, and infinity to creativity, forgiveness, love, and free will, and encourages civil, informed dialogue among scientists, philosophers, theologians, and the public. More information is available at http://www.templeton.org.
    Heising-Simons Foundation

    The Heising-Simons Foundation is a family foundation located in Los Altos, California dedicated to advancing sustainable solutions in climate and clean energy, enabling groundbreaking research in science, enhancing the education of our youngest learners, and supporting human rights for all people. Learn more at http://www.heisingsimons.org.

    See the full article here .

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    Yale University Campus

    Yale University comprises three major academic components: Yale College (the undergraduate program), the Graduate School of Arts and Sciences, and the professional schools. In addition, Yale encompasses a wide array of centers and programs, libraries, museums, and administrative support offices. Approximately 11,250 students attend Yale.

     
  • richardmitnick 5:16 pm on June 26, 2016 Permalink | Reply
    Tags: , , Supersymmetry,   

    From particle bites: “Can’t Stop Won’t Stop: The Continuing Search for SUSY” 

    particlebites bloc

    particlebites

    June 19, 2016
    Julia Gonski

    Presenting:

    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

    1
    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.

    2
    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!)

    3
    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.

    2
    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
    Tags: , , , , , , Supersymmetry   

    From CERN: “CMS hunts for supersymmetry in uncharted territory” 

    Cern New Bloc

    Cern New Particle Event

    CERN New Masthead

    CERN

    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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    Meet CERN in a variety of places:

    Cern Courier

    THE FOUR MAJOR PROJECT COLLABORATIONS

    ATLAS
    CERN ATLAS New

    ALICE
    CERN ALICE New

    CMS
    CERN CMS New

    LHCb
    CERN LHCb New II

    LHC

    CERN LHC Map
    CERN LHC Grand Tunnel

    CERN LHC particles

    Quantum Diaries

     
  • richardmitnick 3:00 pm on July 30, 2015 Permalink | Reply
    Tags: , , , , , , Supersymmetry,   

    From Symmetry: “One Higgs is the loneliest number” 

    Symmetry

    July 30, 2015.
    Katie Elyce Jones

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

    1

    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.

    2
    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

    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?”

    Compositeness

    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.

    Please help promote STEM in your local schools.

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    Symmetry is a joint Fermilab/SLAC publication.


     
  • richardmitnick 10:13 am on July 28, 2015 Permalink | Reply
    Tags: , , , , , , Supersymmetry   

    From Discovery: “LHC Keeps Bruising ‘Difficult to Kill’ Supersymmetry” 

    Discovery News
    Discovery News

    Jul 27, 2015
    AFP

    1

    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.

    2
    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.

    2

    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.

    Please help promote STEM in your local schools.

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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?” 

    DESY
    DESY

    1

    28.04.2015
    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.

    2
    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.

    Please help promote STEM in your local schools.

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    desi

    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
    Tags: , , Supersymmetry,   

    From Symmetry: “Natural SUSY’s last stand” 

    Symmetry

    April 29, 2015
    Mike Ross

    1
    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.

    3
    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.

    2
    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.

    3
    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.

    Please help promote STEM in your local schools.

    STEM Icon

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    Symmetry is a joint Fermilab/SLAC publication.


     
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