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  • richardmitnick 10:20 am on November 14, 2014 Permalink | Reply
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    From FNAL- “Frontier Science Result: CMS Origin of the smallest masses” 


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

    Friday, Nov. 14, 2014
    Jim Pivarski

    Since the discovery of the Higgs boson two years ago, about 80 analyses have helped to pin down its properties. Today, we know that it does not spin, that it is mirror-symmetric, and that it decays into pairs of W bosons, pairs of Z bosons, pairs of tau leptons, and pairs of photons (through a pair of short-lived top quarks). There are even weak hints at a fifth decay mode: decays into pairs of b quarks. All of these results are in agreement with expectations for a Standard Model Higgs boson, but they are still coarse measurements with significant uncertainties.

    sm
    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.

    To say that this boson is a Standard Model Higgs is to say that it is exactly the particle that was predicted in 1964. That leaves a lot of room for surprises. Without interference from new phenomena, the rate that this boson decays into particle-antiparticle pairs would be proportional to the square of the mass of the particle-antiparticle pairs. The best way to check the proportionality of something is to look at it on an extreme range. Since the Higgs is believed to give mass to everything from 0.0005-GeV electrons to 173-GeV top quarks, there’s plenty of room to check.

    dots
    Muons (red) are 18 times lighter than tau leptons (blue), so we expect Higgs decays to muon pairs to be about 300 times less common than Higgs decays to tau pairs.

    The highest decay rates are easiest to detect, so only the heaviest particle-antiparticle pairs have been tested so far. The lightest particle-antiparticle decay that has been observed is Higgs to pairs of tau leptons, which are 1.8 GeV each. The next-lighter final state that could be observed is Higgs to pairs of muons, which are 0.1 GeV each. By the expected scaling, Higgs to muon pairs should be 300 times less common. However, muons are easy to detect and clearly identify, so they make a good target.

    Even if you combine all the LHC data collected so far, it would not be enough to see evidence of this decay mode. However, the LHC is scheduled to restart next spring at almost twice its former energy. Higher energy and more intense beams would produce more Higgs bosons, making a future detection of Higgs to muon pairs possible.

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

    To prepare for such a discovery and find potential problems early, CMS scientists searched for Higgs to muon pairs in the current data set. They didn’t find any, but they did establish that no more than 0.16 percent of Higgs bosons decay into muons, only a factor of 7 from the expected number, and then they used these results to project sensitivity in future LHC data. Incidentally, the Higgs boson is the first particle known to decay into tau lepton pairs much more (6.3 percent) than muon pairs (0.023 percent). All other particles decay into taus and muons almost equally.

    CERN CMS New
    CMS in the LHC at CERN

    They also searched for Higgs decays into electrons, the lighter cousin of muons and tau leptons. Since electrons are 200 times lighter than muons, Higgs to electron pairs is expected only 0.00000051 percent of the time. None were found, though an observation would been an exciting surprise!

    See the full article here.

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  • richardmitnick 10:31 am on October 31, 2014 Permalink | Reply
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    From FNAL- “Frontier Science Result: CMS Boosted W’s” 


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

    Friday, Oct. 31, 2014

    FNAL Don Lincoln
    Don Lincoln

    Today’s article covers an interesting topic. It’s interesting not because it explores new physics, but because of how it reveals some of the mundane aspects of research at the LHC. It also shows how the high energy of the LHC makes certain topics harder to study than they were during the good old days at lower-energy accelerators.

    At the LHC, quarks or gluons are scattered out of the collision. It’s the most common thing that happens at the LHC. Regular readers of this column know that it is impossible to see isolated quarks and gluons and that these particles convert into jets as they exit the collision. Jets are collimated streams of particles that have more or less the same energy as the parent quark or gluon. Interactions that produce jets are governed by the strong force.

    map
    In the green region, we show what a W boson looks like before it decays. Moving left to right, the boson is created with more and more momentum. In the yellow region, we repeat the exercise, this time looking at the same W boson after it decays into quarks, which have then turned into jets. Finally in the pink region, we look at a jet originating from a quark or gluon. This looks much like a high-momentum W boson decaying into quarks. Because ordinary jets are so much more common, this highlights the difficulty inherent in finding high-momentum W bosons that decay into jets.
    No image credit

    Things get more interesting when a W boson is produced. One reason for this is that making a W boson requires the involvement of the electroweak force, which is needed for the decay of heavy quarks. Thus studies of W bosons are important for subjects such as the production of the top quark, which is the heaviest quark. W bosons are also found in some decays of the Higgs boson.

    A W boson most often decays into two light quarks, and when it decays, it flings the light quarks into two different directions, which can be seen as two jets.

    But there’s a complication in this scenario at the LHC, where the W bosons are produced with so much momentum that it affects the spatial distribution of particles in those two jets. As the momentum of the W boson increases, the two jets get closer together and eventually merge into a single jet.

    As mentioned earlier, individual jets are much more commonly made using the strong force. So when one sees a jet, it is very hard to identify it as coming from a W boson, which involves the electroweak force. Since identifying the existence of W bosons is very important for certain discoveries, CMS scientists needed to figure out how to tell quark- or gluon-initiated jets from the W-boson-initiated jets. So they devised algorithms that could identify when a jet contained two lumps of energy rather than one. If there were two lumps, the jet was more likely to come from the decay of a W boson.

    CERN CMS New
    CMS

    In today’s paper, CMS scientists explored algorithms and studied variables one can extract from the data to identify single jets that originated from the decay of W bosons. The data agreed reasonably well with calculations, and the techniques they devised will be very helpful for future analyses involving W bosons. In addition, the same basic technique can be extended to other interesting signatures, such as the decay of Z and Higgs bosons.

    See the full article here.

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  • richardmitnick 2:05 pm on October 17, 2014 Permalink | Reply
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    From FNAL- “Frontier Science Result: CMS Off the beaten path” 


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

    Friday, Oct. 17, 2014
    Jim Pivarski

    The main concern for most searches for rare phenomena is to control the backgrounds. Backgrounds are observations that resemble the one of interest, yet aren’t. For instance, fool’s gold is a background for gold prospectors. The main reason that the Higgs boson was hard to find is that most Higgs decays resemble b quark pair production, which is a million times more common. You not only have to find the one-in-a-million event picture, you have to identify some feature of it to prove that it is not an ordinary event.

    This is particularly hard to do in proton collisions because protons break apart in messy ways — the quarks from the proton that missed each other generate a spray of particles that fly off just about everywhere. Look through a billion or a trillion of these splatter events and you can find one that resembles the pattern of new physics that you’re looking for. Physicists have many techniques for filtering out these backgrounds — requiring missing momentum from an invisible particle, high energy perpendicular to the beam, a resonance at a single energy, and the presence of electrons and muons are just a few.

    nu
    Most particles produced by proton collisions originate in the point where the beams cross. Those that do not are due to intermediate particles that travel some distance before they decay

    A less common yet powerful technique for eliminating backgrounds is to look for displaced particle trajectories, meaning trajectories that don’t intersect the collision point. Particles that are directly created by the proton collision or are created by short-lived intermediates always emerge from this point. Those that emerge from some other point in space must be due to a long-lived intermediate.

    A common example of this is the b quark, which can live as long as a trillionth of a second before decaying into visible particles. That might not sound like very long, but the quark is traveling so quickly that it covers several millimeters in that trillionth of a second, which is a measurable difference.

    In a recent analysis, CMS scientists searched for displaced electrons and muons. Displaced tracks are rare, and electrons and muons are also rare, so displaced electrons and muons should be extremely rare. The only problem with this logic is that b quarks sometimes produce electrons and muons, so one other feature is needed to disambiguate. A b quark almost always produces a jet of particles, so this search for new physics also required that the electrons and muons were not close to jets.

    CERN CMS New
    CERN CMS

    With these simple selection criteria, the experimenters found only as many events as would be expected from standard physics. Therefore, it constrains any theory that predicts displaced electrons and muons. One of these is “displaced supersymmetry,” which generalizes the usual supersymmetry scenario by allowing the longest-lived supersymmetric particle to decay on the millimeter scale that this analysis tests. Displaced supersymmetry was introduced as a way that supersymmetry might exist yet be missed by most other analyses. Experiments like this one illuminate the dark corners in which supersymmetry might be hiding.

    See the full article here.

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  • richardmitnick 7:04 pm on September 21, 2014 Permalink | Reply
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    From physicsworld: “A day in the life of CERN’s director-general” 

    physicsworld
    physicsworld.com

    Sep 16, 2014
    By Rolf-Dieter Heuer, Geneva

    There is no such thing as a typical day in the life of a CERN director-general (DG), certainly not this one in any case. In my experience, each incumbent has carved out a slightly different role for themself, shaped by the laboratory’s priorities and activities at the time of their mandate. For me, every day goes beyond science, management and administration, and I am particularly fortunate to have been DG through a remarkable period that has seen not only the successful launch of the Large Hadron Collider (LHC) and confirmation of the Brout–Englert–Higgs mechanism, but also an opening of CERN to the world – an area that I have pursued with particular vigour.

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

    dg
    All in a day’s work. (Courtesy: CERN)

    As I regularly joke, we have changed the “E” of CERN from “Europe” to “Everywhere”, and that has meant a lot of travel for the CERN DG, as we hold discussions with prospective new members of the CERN family. And when the CERN Council opened up membership to countries from beyond the European region in 2010, it seemed to me that we should also be extending our contacts in other directions as well.

    For that reason, I have taken up the CERN DG’s standing invitation to attend the World Economic Forum’s annual meeting in Davos, where I strive to get science further up the agenda, and I have actively pursued a policy of engagement with other international organizations. CERN’s host city is home to a concentration of international organizations like nowhere else on Earth, and our missions overlap in areas ranging from technology to standards to intellectual property. A typical day might see me paying a visit to the United Nations Office in Geneva, or receiving a visit from the ambassador of an existing or prospective CERN member state.

    But the role goes beyond one of diplomacy. The CERN DG has, first and foremost, a lab to run. Although I have a strong team of directors and department leaders to help me, issues ranging from liaison between experiments to delicate issues in human resources or dealings with officials from our two host states – France and Switzerland – find their way to my door. Each year is punctuated by fixed points for the meetings of advisory and governance bodies, for directorate meetings and presentations to personnel.

    With all this going on, there is no typical day, so I’ll describe the most untypical of all during my term of office: Wednesday 4 July 2012.

    I’d been told that people were so keen to have a seat in CERN’s main auditorium for that day’s Higgs-update seminar that some were prepared to camp out all night to secure their place, so I came in early to see if it were true. I expected to see a few hardy souls at 7 a.m., but not the long snaking queue, headed up by sleeping bags, that started outside the doors of the auditorium, carried on all the way along corridors and ended up down the stairs in main entrance lobby. The atmosphere was reserved, yet excited, with an air of expectation about it. I went up to my office to prepare my notes and gather my thoughts.

    We had not known until the last minute whether or not we would be announcing a discovery or just another step on the way. Yet the world was expectant. Peter Higgs and François Englert were at CERN, as were Gerry Guralnik and Carl Hagen – two of the three authors of the other pioneering paper from the 1960s that had anticipated what we now know as the Brout–Englert–Higgs mechanism. Robert Brout, unfortunately, did not live to see the confirmation of his ideas, while Tom Kibble – Guralnik and Hagen’s co-author – was at a parallel event in London. The press were also there in force, and the CERN Council’s meeting room was converted into a media centre for the day.

    Although just a few days earlier I didn’t know what message I’d be bringing to the expectant crowd, at 7 a.m. that day I had what I needed to announce a discovery. Over the preceding weeks and days, Fabiola Gianotti and Joe Incandela had each kept me up to date with the status of the analyses from the ATLAS and CMS experiments of which they were the spokespersons, and by the Friday before the seminar, I’d seen enough. Although by that time neither experiment was sure they’d be able to announce the required 5σ significance needed to claim discovery, I’d seen both experiment’s results, and that was enough for me to know that taken together the 5σ would be reached.

    CERN ATLAS New
    ATLAS

    CERN CMS New
    CMS

    rdh
    Wednesday 4 July 2012 was an extraordinary day. (Courtesy: CERN)

    By the time I went back down to the auditorium, the doors had been opened and people had taken their seats, yet the crowd outside seemed even bigger than before. Inside the room, the mood was an unusual mixture of party and scientific seminar. We were being watched around the world: nearly half a million people tuned in to the webcast, I’m told, and we had a room full of physicists in Melbourne assembled there on the eve of that year’s major particle-physics conference, beamed to a screen above my head. It culminated in joyous scenes as the experiments announced their results: as it turned out, they didn’t need me to announce the discovery. Peter Higgs, sitting next to François Englert whom he’d met for the first time that day, had a smile on his face that said it all.

    The seminar was over, but for me the day was just beginning. Fabiola, Joe and I were ushered into the media centre for a press conference, in which the theorists were given a front-row seat. Once the media scrum has subsided and Peter Higgs had graciously led the theorists in saying that this was a day for the experiments and there’d be time to talk to him later, the three of us recounted the story all over again before spending the day giving interview after interview.

    Eventually, the cameras stopped clicking, the microphones were put back in their bags, and it was time to head off for the airport to catch my flight to Melbourne for the conference. It was only when I got on the plane and ordered a glass of champagne that the enormity of the day sunk in. It had been an incredible day, full of emotion, leaving me happy not only with the result, but also with that fact that it had so strongly captured the world’s imagination.

    A day in the life of the CERN DG? Always challenging, sometimes exhausting, frequently frustrating but always rewarding. And although 4 July 2012 may not have been a typical day, it is for me one of the most memorable of all.

    See the full article here.

    PhysicsWorld is a publication of the Institute of Physics. The Institute of Physics is a leading scientific society. We are a charitable organisation with a worldwide membership of more than 50,000, working together to advance physics education, research and application.

    We engage with policymakers and the general public to develop awareness and understanding of the value of physics and, through IOP Publishing, we are world leaders in professional scientific communications.
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  • richardmitnick 12:34 pm on September 19, 2014 Permalink | Reply
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    From Fermilab- “Frontier Science Result: CMS Three ways to be invisible” 


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

    Friday, Sept. 19, 2014
    Jim Pivarski

    There is a common misconception that the LHC was built only to search for the Higgs boson. It is intended to answer many different questions about subatomic particles and the nature of our universe, so the collision data are reused by thousands of scientists, each studying their own favorite questions. Usually, a single analysis only answers one question, but recently, one CMS analysis addressed three different new physics: dark matter, extra dimensions and unparticles.

    CERN CMS New
    CMS

    CERN LHC Grand Tunnel
    CERN LHC Map
    CERN LHC particles
    LHC

    The study focused on proton collisions that resulted in a single jet of particles and nothing else. This can only happen if some of the collision products are invisible — for instance, one proton may emit a jet before collision and the collision itself produces only invisible particles. The jet is needed to be sure that a collision took place, but the real interest is in the invisible part.

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

    Sometimes, the reason that nothing else was seen in the detector is mundane. Particles may be lost because their trajectories missed the active area of the detector or a component of the detector was malfunctioning during the event. More often, the reason is due to known physics: 20 percent of Z bosons decay into invisible neutrinos. If there were an excess of invisible events, more than predicted by the Standard Model, these extra events would be evidence of new phenomena.

    The classic scenario involving invisible particles is dark matter. Dark matter has been observed through its gravitational effects on galaxies and the expansion of the universe, but it has never been detected in the laboratory. Speculations about the nature of dark matter abound, but it will remain mysterious until its properties can be studied experimentally.

    Another way to get invisible particles is through extra dimensions. If our universe has more than three spatial dimensions (with only femtometers of “breathing room” in the other dimensions), then the LHC could produce gravitons that spin around the extra dimensions. Gravitons interact very weakly with ordinary matter, so they would appear to be invisible.

    A third possibility is that there is a new form of matter that isn’t made of indivisible particles. These so-called unparticles can be produced in batches of 1½ , 2¾ , or any other amount. Unparticles, if they exist, would also interact weakly with matter.

    All three scenarios produce something invisible, so if the CMS data had revealed an excess of invisible events, any one of the scenarios could have been responsible. Follow-up studies would have been needed to determine which one it was. As it turned out, however, there was no excess of invisible events, so the measurement constrains all three models at once. Three down in one blow!

    LHC scientists are eager to see what the higher collision energy of Run 2 will deliver.

    See the full article here.

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  • richardmitnick 9:14 am on August 8, 2014 Permalink | Reply
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    From Fermilab- “Frontier Science Result: CMS An ambidextrous W boson?” 


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

    Friday, Aug. 8, 2014

    Fermilab Don Lincoln
    Don Lincoln

    Of the four known subatomic forces, the weak force has what seems to be a particularly bizarre behavior. It’s a lefty. What exactly does that mean in the subatomic realm? To understand, you need to remember that fundamental particles spin and that they spin in a peculiar way.

    Suppose you’re playing catch with someone using a football. If you throw the football properly, the ball spins around the long axis of the ball. As you see the ball coming towards you, you see the ball spin in either a clockwise or counterclockwise direction. Scientists call the clockwise rotation left-handed, because if you take your left hand and aim your thumb in the direction the ball is moving (toward you), the fingers of your left hand naturally wrap in the clockwise direction.

    The strong and electromagnetic forces, with their gluons and photons, don’t care which way the particle is spinning. But the weak force, with its W boson, will interact with only a left-handed particle. W bosons just don’t interact with counterclockwise-spinning particles. And, while it seems weird, the theory of the weak force has long accommodated this fact.

    On the other hand, given the symmetry of the other forces, maybe the problem is that we just haven’t found the kind of W boson that interacts with right-handed particles. This isn’t an unreasonable conjecture — maybe the right-handed W bosons are just much heavier than the familiar left-handed versions. If that’s the case, then the LHC, with its high-energy beams, would be a perfect place for finding right-handed W bosons.

    While symmetry and aesthetic considerations might be enough reason to look for heavy right-handed W bosons, some related theoretical ideas have been proposed. One such thought involves the neutrino, which is the only particle that feels only the weak force. Neutrinos are incredibly light, much lighter than other particles that have mass. Given this discrepancy, maybe the neutrinos don’t get their mass from the Higgs boson as other particles do. It could be an entirely different mechanism. Neutrinos are also left-handed particles, and scientists have proposed that perhaps there exists a heavier and right-handed version. Under this theory, the masses of the left-handed and right-handed neutrinos are connected in that if one goes up, the other goes down. This connection is called the seesaw mechanism.

    Given the interesting possibilities, CMS searched for the two inextricably linked theoretical particles: heavy, right-handed W bosons and neutrinos. No evidence was found for either, although the new measurement is the most precise one thus far.

    See the full article here.

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  • richardmitnick 10:59 am on July 11, 2014 Permalink | Reply
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    From Fermilab- “Frontier Science Result: CMS Excited quarks” 


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

    Friday, July 11, 2014

    Fermilab Don Lincoln
    This article was written by Dr. Don Lincoln

    It is well known that the Standard Model of particle physics is incomplete and that it is an approximation of a deeper and more fundamental theory.

    sm
    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.

    One idea that might lead the way to a better understanding of the universe is that the quarks and leptons, particles now treated as point-like, are actually composite and made of even smaller particles. Given that the quarks and leptons have electrical charge, it follows that at least some of these hypothetical components also have electrical charge. And where electrical charge exists, the photon must follow. This is because photons are emitted by electrically charged particles.

    quark
    A proton, composed of two up quarks and one down quark. (The color assignment of individual quarks is not important, only that all three colors be present.)

    lepton
    Leptons are involved in several processes such as beta decay.

    If quarks and leptons contain smaller objects, these objects are bound together with some sort of force. Further, since we know that the quarks and leptons act very much like point-like particles, this force must be very strong. Still, it is possible that if you hit a quark or lepton hard enough, you might be able to add energy to the constituents. Physicists call these energy-added particles “excited” quarks and leptons. The hypothetical constituents would somehow radiate that added energy and return to the quiescent states that are the familiar quarks and leptons. This is kind of like hitting a hornet’s nest, which will cause a swarm of hornets to fly around before the insects return to a quiet state.

    If the constituent particles radiate energy, the quark might emit photons. The resulting signature of this excited quark would be a regular quark emitted with a lot of energy accompanied by the emitted photon. Since quarks turn into jets, the experimental signature is a jet and a photon. CMS scientists looked for this signature to try to find excited quarks. While no evidence was found, researchers were able to set limits on the energy scale at which excited quarks can be made. With the resumption of operations of the LHC in the spring of 2015, scientists will be able to look for even more massive excited quarks.

    CERN CMS Detector
    CERN/CMS

    See the full article here.

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  • richardmitnick 11:34 am on July 3, 2014 Permalink | Reply
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    From CMS at CERN: “CMS closes major chapter of Higgs measurements” 

    CERN New Masthead
    CERN

    2014-07-03
    Tiziano Camporesi

    The data reveal that the particle discovered at CERN continues to behave just like the Standard Model predicts

    sm
    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.

    Since the discovery of a Higgs boson by the CMS and ATLAS Collaborations in 2012, physicists at the LHC have been making intense efforts to measure this new particle’s properties. The Standard Model Higgs boson is the particle associated with an all-pervading field that is believed to impart mass to fundamental particles via the Brout-Englert-Higgs mechanism [Robert Brout June 14, 1928 – May 3, 2011 deceased, so no Nobel]. Awaited for decades, the 2012 observation was a historical milestone for the LHC and led to the award of the 2013 Nobel Prize in Physics to Peter Higgs and François Englert. An open question arising from the discovery is whether the new particle is the one of the Standard Model – or a different one, perhaps just one of many types of Higgs bosons waiting to be found. Since the particle’s discovery, physicists at the LHC have been making intense efforts to answer this question.

    This week, at the 37th International Conference on High Energy Physics, a bi-annual major stage for particle physics, which in 2014 is held in Valencia, Spain, the CMS Collaboration is presenting a broad set of results from new studies of the Higgs boson. The new results are based on the full Run 1 data from pp collisions at centre-of-mass energies of 7 and 8 TeV. The analysis includes the final calibration and alignment constants and contains about 25 fb−1 of data.

    Decay to two photons

    The Higgs boson is an ephemeral particle. It decays into pairs of lighter particles almost immediately after it is produced in LHC collisions. One such “decay channel” is the one in which the Higgs transforms into two photons. The latest CMS results in this decay channel [1] show a peak in the data with a significance of 5σ; the probability that random fluctuations would give a peak this significant at this mass in the absence of a new boson is less than one in 3,000,000. Figure 1 shows the clear signal of the Higgs over the background in the data. CMS has also measured the mass of the Higgs boson with a precision of a few parts per thousand, with the systematic uncertainty of the measurement four times smaller than the previous preliminary value.

    The precision of the new mass measurement testifies to the inspired design and meticulous construction of the CMS detector, its efficient operation and calibration throughout Run 1 of the LHC, and the tireless efforts of the analysis teams in understanding all aspects of the detector performance.

    Combining decay channels, production modes

    The two-photon analysis completes the set of Run 1 measurements with final calibration and alignment, covering the five primary decay modes of the Higgs boson [2,3,4,5]. This paves the way for a preliminary combination of all the decay channels observed thus far [6], to extract the maximum possible information on the properties of the new boson, including its couplings to the fundamental particles. The combined best-fit ratio of the signal strength observed to that expected in the Standard Model, is found to be 1.00 ± 0.13, in square agreement with state-of-the-art Standard Model calculations. Furthermore, when the data are dissected into the separate production and decay properties of the Higgs boson, no significant deviations from the expectations for the Standard Model are found. In addition to the coupling results, the preliminary combination includes a combined measurement of the Higgs boson mass from the two-photon and ZZ→4ℓ channels: mH = 125.03 ± 0.30 GeV. Taken together, the results represent an impressive tour de force, the culmination of four years of painstaking effort that began with the first CMS searches for the Higgs boson in 2010.

    “After half a century of searching, it is exhilarating to piece together the Higgs puzzle, standing on the shoulders of giants, both those who built the experiments and those who carried out the Standard Model calculations,” says Prof Jim Olsen, who is currently convening the Higgs Analysis Group in CMS.

    Spin-parity of the particle

    Finally, the spin structure of the Higgs boson has been probed with unprecedented detail in a new set of CMS results searching for anomalous couplings to vector bosons. If the new particle is indeed a Higgs boson it should be a scalar, a particle with zero spin and positive parity. The analyses include separate investigations of the WW→2ℓ2ν [7] and ZZ→4ℓ [8] decay channels to test alternative spin-parity assignments against the expected scalar nature of the Standard Model Higgs boson. For the first time, the possibility that the particle is an admixture of different parity states is also investigated. Results are combined for the two channels and all alternative hypotheses studied are found to be significantly disfavoured with respect to the Standard Model hypothesis.

    Along with the recent CMS publication in Nature Physics demonstrating strong evidence for the Higgs boson decay to fermions [9], the new results presented in Valencia provide further striking signs of its Standard Model nature. With the wrapping up of Run 1 results, the CMS experiment is now intensely focused on preparations for Run 2, where the centre-of-mass energy of the LHC will be raised to up to 13 TeV and the luminosity will be much increased. With a more powerful accelerator and the upgraded CMS detector, the collaboration looks forward to the promise of new and exciting results on the Higgs boson in Run 2.

    Andre Tinoco Mendes, a researcher at CERN who is reporting the Higgs results from CMS at the conference, stressed, “It will take more data and better calculations to sharpen the picture further and exploit the full potential of the LHC.”

    See the full article, with references, here.

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  • richardmitnick 3:30 pm on May 27, 2014 Permalink | Reply
    Tags: , , CERN CMS, , ,   

    From Fermilab: “Frontier Science Result – CMS Half-life of the Higgs boson” 


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

    Tuesday, May 27, 2014
    Jim Pivarski

    It is almost two years since physicists from the CMS and ATLAS collaborations announced the discovery of a Higgs-like boson. Today, the evidence has strengthened to the point that they no longer qualify it as “Higgs-like.” The signal is now much clearer, the particle is spinless (as a Higgs boson must be), more decay modes have been observed, and the proportions of decays into those modes are about right (with 15 percent uncertainties). What more could you want?

    graph
    This plot shows how well the Higgs half-life (λ) is known: Less than 20 yoctoseconds is ruled out with 95 percent confidence, but greater than 20 yoctoseconds is still possible. The standard prediction is that the half-life is 100 yoctoseconds.

    Perhaps its decay rate: The Higgs boson is an unstable particle, so it decays within a characteristic length of time. Although the time for an individual particle to decay is random, each type of particle has an average lifespan. The time for roughly half of a collection to decay is called its half-life. The half-life of the Higgs boson is not known, but it is predicted to be 100 yoctoseconds (septillionths of a second), which is a rather long time for a particle of its mass.

    A measurement of the Higgs boson half-life would tell us a lot. Currently, only a few final states have been observed, which add up to about 2 percent of all predicted decays. For all we know, they might be nonstandard Higgses, and most of them might be decaying into exotic particles. Knowing the total decay rate would put an upper limit on this possibility. It would constrain even the decays that we don’t see.

    CMS scientists recently attempted to measure the Higgs boson’s half-life and determined that it is at least 20 yoctoseconds. This analysis established a technique that will be applied to larger data sets, which are needed to fully measure it.

    The technique is significant, because direct measurements of the half-life are far too insensitive. If, for instance, you tried to measure a Higgs’ lifespan from the distance it flies between its production and its decay, you’d be trying to measure a distance that is much smaller than an atom, beyond the capabilities of any microscope. Instead, you might take advantage of a fact from quantum mechanics, one that states that the half-life of a particle is inversely proportional to the uncertainty in its mass. Unfortunately, the detector’s mass resolution is a thousand times too insensitive to see that uncertainty. The physicists who performed this study used a clever trick involving the ratio of the real Higgs production rate divided by the virtual Higgs production rate and managed to constrain the half-life within a factor of six of its predicted value. No small feat!

    See the full article here.

    Fermilab Campus

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


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  • richardmitnick 9:26 am on April 25, 2014 Permalink | Reply
    Tags: , , CERN CMS, , , , ,   

    From Fermilab- “Frontier Science Result: CMS The shape of the jet” 


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

    Friday, April 25, 2014
    Jim Pivarski

    Despite the complexity of particle colliders and the instrumentation needed to analyze their results, the ultimate aim of most particle physics experiments is to understand something simple. At a fundamental level, most natural phenomena turn out to be simple in profound ways. By contrast, our macroscopic world is teeming with complexity: A bucket of water is by far more complex than an electron. The exact way that water sloshes, curdles in turbulent flow and pinches into droplets when it splashes would be difficult to simulate on the world’s biggest supercomputers, even though the basic interactions between individual atoms are pretty well understood.

    jet
    A jet of water sprayed through water loses energy and changes shape, as illustrated by this Jacuzzi jet. CMS scientists studied a similar phenomenon in an exotic liquid of quarks and gluons. No image credit.

    One part of the quantum world has this kind of complexity, however: the strong force that binds quarks. Unlike the electromagnetic force between atoms, the particles that make up the strong force are themselves attracted via the strong force, which begets more strong force. Physicists call them gluons because they make such a sticky mess. Like the bucket of water, the strong force is notoriously difficult to calculate because some of its properties are emergent — they arise from the interplay of many interactions.

    One of these emergent properties is the fact that a lone quark flying away from a collision creates gluons, which create quarks, which create gluons, and becomes a jet of particles flying in roughly the same direction. Another is that if you get enough quarks in a small space (by colliding heavy nuclei), they undergo a phase transition into a new kind of liquid ruled by strong force interactions. Recently, scientists discovered that jets are eaten by the liquid: They are absorbed into the droplet and sometimes disappear entirely.

    To get a more complete picture of this phenomenon, scientists have used the CMS experiment to study an in-between case, jets that are partially but not completely absorbed by the strong-force liquid. Like a hose sprayed through water, this results in misshapen jets. The angles among particles that make up the jet are noticeably wider than usual, and the exact amount of broadening tells us a little more about the nature of this new state of matter.

    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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