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  • richardmitnick 5:45 pm on June 17, 2016 Permalink | Reply
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    From Pauline Gagnon in Quantum Diaries: “Enough data to explore the unknown” 

    6.17.16

    Pauline Gagnon
    Pauline Gagnon

    The Large Hadron Collider (LHC) at CERN has already delivered more high energy data than it had in 2015. To put this in numbers, the LHC has produced 4.8 fb-1, compared to 4.2 fb-1 last year, where fb-1 represents one inverse femtobarn, the unit used to evaluate the data sample size. This was achieved in just one and a half month compared to five months of operation last year.

    With this data at hand, and the projected 20-30 fb-1 until November, both the ATLAS and CMS experiments can now explore new territories and, among other things, cross-check on the intriguing events they reported having found at the end of 2015. If this particular effect is confirmed, it would reveal the presence of a new particle with a mass of 750 GeV, six times the mass of the Higgs boson. Unfortunately, there was not enough data in 2015 to get a clear answer. The LHC had a slow restart last year following two years of major improvements to raise its energy reach. But if the current performance continues, the discovery potential will increase tremendously. All this to say that everyone is keeping their fingers crossed.

    If any new particle were found, it would open the doors to bright new horizons in particle physics. Unlike the discovery of the Higgs boson in 2012, if the LHC experiments discover a anomaly or a new particle, it would bring a new understanding of the basic constituents of matter and how they interact. The Higgs boson was the last missing piece of the current theoretical model, called the Standard Model. This model can no longer accommodate new particles. However, it has been known for decades that this model is flawed, but so far, theorists have been unable to predict which theory should replace it and experimentalists have failed to find the slightest concrete signs from a broader theory. We need new experimental evidence to move forward.

    Although the new data is already being reconstructed and calibrated, it will remain “blinded” until a few days prior to August 3, the opening date of the International Conference on High Energy Physics. This means that until then, the region where this new particle could be remains masked to prevent biasing the data reconstruction process. The same selection criteria that were used for last year data will then be applied to the new data. If a similar excess is still observed at 750 GeV in the 2016 data, the presence of a new particle will make no doubt.

    Even if this particular excess turns out to be just a statistical fluctuation, the bane of physicists’ existence, there will still be enough data to explore a wealth of possibilities. Meanwhile, you can follow the LHC activities live or watch CMS and ATLAS data samples grow. I will not be available to report on the news from the conference in August due to hiking duties, but if anything new is announced, even I expect to hear its echo reverberating in the Alps.

    See the full article here .

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  • richardmitnick 8:11 pm on December 16, 2015 Permalink | Reply
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    From Pauline Gagnon at Quantum Diaries: “If, and really only if…” 

    12.16.15

    Pauline Gagnon
    Pauline Gagnon

    On December 15, at the End-of-the-Year seminar, the CMS and ATLAS experiments from CERN presented their first results using the brand new data accumulated in 2015 since the restart of the Large Hadron Collider (LHC) at 13 TeV, the highest operating energy so far.

    CERN CMS Detector
    CMS

    CERN ATLAS New
    ATLAS

    CERN LHC Map
    CERN LHC Grand Tunnel
    CERN LHC particles
    LHC

    Although the data sample is still only one tenth of what was available at lower energy (namely 4 fb-1 for ATLAS and 2.8-1 fb for CMS collected at 13 TeV compared to 25 fb-1 at 8 TeV for each experiment), it has put hypothetical massive particles within reach. If the LHC were a ladder and particles, boxes hidden on shelves, operating the LHC at higher energy is like having a longer ladder giving us access to higher shelves, a place never checked before. ATLAS and CMS just had their first glimpse at it.

    Both experiments showed how well their detectors performed after several major improvements, including collecting data at twice the rate used in 2012. The two groups made several checks on how known particles behave at higher energy, finding no anomalies. But it is in searches for new, heavier particles that every one hopes to see something exciting. Both groups explored dozens of different possibilities, sifting through billions of events.

    Each event is a snapshot of what happens when two protons collide in the LHC. The energy released by the collision materializes into some heavy and unstable particle that breaks apart mere instants later, giving rise to a mini firework. By catching, identifying and regrouping all particles that fly apart from the collision point, one can reconstruct the original particles that were produced.

    Both CMS and ATLAS found small excesses when selecting events containing two photons. In several events, the two photons seem to come from the decay of a particle having a mass around 750 GeV, that is, 750 times heavier than a proton or 6 times the mass of a Higgs boson.

    CERN ATLAS Higgs Event
    Higgs event at ATLAS

    Since the two experiments looked at dozens of different combinations, checking dozens of mass values for each combination, such small statistical fluctuations are always expected.

    2
    Top part: the combined mass given in units of GeV for all pairs of photons found in the 13 TeV data by ATLAS. The red curve shows what is expected from random sources (i.e. the background). The black dots correspond to data and the lines, the experimental errors. The small bump at 750 GeV is what is now intriguing. The bottom plot shows the difference between black dots (data) and red curve (background), clearly showing a small excess of 3.6σ or 3.6 times the experimental error. When one takes into account all possible fluctuations at all mass values, the significance is only 2.0σ

    What’s intriguing here is that both groups found the same thing at exactly the same place, without having consulted each other and using selection techniques designed not to bias the data. Nevertheless, both experimental groups are extremely cautious, stating that a statistical fluctuation is always possible until more data is available to check this with increased accuracy.

    3
    CMS has slightly less data than ATLAS at 13 TeV and hence, sees a much smaller effect. In their 13 TeV data alone, the excess at 760 GeV is about 2.6σ, 3σ when combined with the 8 TeV data. But instead of just evaluating this probability alone, experimentalists prefer take into account the fluctuations in all mass bins considered. Then the significance is only 1.2σ, nothing to write home about. This “look-elsewhere effect” takes into account that one is bound to see a fluctuation somewhere when ones look in so many places.

    Theorists show less restrain. For decades, they have known that the Standard Model, the current theoretical model of particle physics, is flawed and have been looking for a clue from experimental data to go further.

    4
    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.

    Many of them have been hard at work all night and eight new papers appeared this morning, proposing different explanations on which new particle could be there, if something ever proves to be there. Some think it could be a particle related to Dark Matter, others think it could be another type of Higgs boson predicted by Supersymmetry or even signs of extra dimensions. Others offer that it could only come from a second and heavier particle. All suggest something beyond the Standard Model.

    Two things are sure: the number of theoretical papers in the coming weeks will explode. But establishing the discovery of a new particle will require more data. With some luck, we could know more by next Summer after the LHC delivers more data. Until then, it remains pure speculation.

    This being said, let’s not forget that the Higgs boson made its entry in a similar fashion. The first signs of its existence appeared in July 2011. With more data, they became clearer in December 2011 at a similar End-of-the-Year seminar. But it was only once enough data had been collected and analysed in July 2012 that its discovery made no doubt. Opening one’s gifts before Christmas is never a good idea.

    See the full article here .

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  • richardmitnick 1:00 pm on April 4, 2013 Permalink | Reply
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    From CERN at Quantum Diaries: “Grey matter confronted to dark matter” 

    THIS QUANTUM DIARIES POST IS PRESENTED IN ITS ENTIRETY BECAUSE OF ITS IMPORTANCE.

    April 4th, 2013
    Pauline Gagnon

    Pauline Gagnon

    “After 18 years spent building the experiment and nearly two years taking data from the International Space Station, the Alpha Magnetic Spectrometer or AMS-02 collaboration showed its first results on Wednesday to a packed audience at CERN. But Prof. Sam Ting, one of the 1976 Nobel laureates and spokesperson of the experiment, only revealed part of the positron energy spectrum measured so far by AMS-02.

    Positrons are the antimatter of electrons. Given we live in a world where matter dominates, it is not easy to explain where this excess of positrons comes from. There are currently two popular hypotheses: either the positrons come from pulsars or they originate from the annihilation of dark matter particles into a pair of electron and positron. To tell these two hypotheses apart, one needs to see exactly what happens at the high-energy end of the spectrum. But this is where fewer positrons are found, making it extremely difficult to achieve the needed precision. Yesterday, we learned that AMS-02 might indeed be able to reach the needed accuracy.

    graph
    The fraction of positrons (measured with respect to the sum of electrons and positrons) captured by AMS-02 as a function of their energy is shown in red. The vertical bars indicate the size of the uncertainty. The most important part of this spectrum is the high-energy part (above 100 GeV or 102) where the results of two previous experiments are also shown: Fermi in green and PAMELA in blue. Note that the AMS-02 precision exceeds the one obtained by the other experiments. The spectrum also extends to higher energy. The big question now is to see if the red curve will drop sharply at higher energy or not. More data is needed before the AMS-02 can get a definitive answer.

    Only the first part of the story was revealed yesterday. The data shown clearly demonstrated the power of AMS-02. That was the excellent news delivered at the seminar: AMS-02 will be able to measure the energy spectrum accurately enough to eventually be able to tell where the positrons come from.

    But the second part of the story, the punch line everyone was waiting for, will only be delivered at a later time. The data at very high energy will reveal if the observed excess in positrons comes from dark matter annihilation or from “simple” pulsars. How long will it take before the world gets this crucial answer from AMS-02? Prof. Ting would not tell. No matter how long, the whole scientific community will be waiting with great anticipation until the collaboration is confident their measurement is precise enough. And then we will know.

    If AMS-02 does manage to show that the positron excess has a dark matter origin, the consequences would be equivalent to discovering a whole new continent. As it stands, we observe that 26.8% of the content of the Universe comes in the form of a completely unknown type of matter called dark matter but have never been able to catch any of it. We only detect its presence through its gravitational effects. If AMS-02 can prove dark matter particles can annihilate and produce pairs of electrons and positrons, it would be a complete revolution.”

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  • richardmitnick 11:15 am on February 1, 2013 Permalink | Reply
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    From Pauline Gagnon at Quantum Diaries: "What’s coming up at CERN in 2013?" 

    Pauline Gagnon
    Pauline Gagnon

    “The Year of the Dragon (2012) came with a roar: a wonderful discovery and a greater understanding of how matter works. What might 2013, the Year of the Serpent, have in store for CERN? The serpent could very well represent the long and winding road of the many new upgrades ahead.

    On Monday 11 February at 6 am Geneva time, the Large Hadron Collider (LHC) will stop producing collisions, marking the start of a major overhaul for all accelerators at CERN. This will be the first in a series of three long-shutdowns to allow a complete refurbishing of the main accelerator, the LHC. The goal is to be able to increase its energy from the actual 8 TeV to 13 or even 14 TeV. This means an increased reach for new particles.

    acc

    This is not just to play a game of ‘my particle is bigger than yours’, but rather an attempt at finding the passageway to new theories. Since energy (E) and mass (m) are two forms of the same essence, as stated by the famous equation E = mc2, where c2 acts as a conversion factor between the two, increasing the accelerator energy will give us the possibility to create particles more massive than we have ever been able to produce before. It will also enhance the production rate of known particles – like the newly discovered boson – to better study them.

    It is foreseen that the accelerator complex will come back to life in 2014, with the LHC becoming operational again in 2015.”

    See the full article here.

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  • richardmitnick 4:19 pm on December 10, 2012 Permalink | Reply
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    From Quantum Diaries – Pauline Gagnon : “Gluino, Higgsino, bingo!” 

    Pauline Gagnon
    Dr. Pauline Gagnon

    Gluinos and Higgsinos are some of the many undiscovered particles we may find at the Large Hadron Collider (LHC) if a theory called supersymmetry, also known as SUSY, turns out to be true. This theory is built on the Standard Model, the current theoretical model of particle physics.

    Standard Model

    The Standard Model relies on the Higgs boson to hold true. But even with this boson, physicists know that this model cannot be the final answer as it has a few shortcomings. For example, it fails to provide an explanation for dark matter or why the masses of fundamental particles such as electrons and muons are so different. This theory of supersymmetry is one of the most popular and most promising ways to extend the Standard Model, but it has yet to manifest itself.

    SUSY is very popular since it brings lots of harmony in the world of sub-atomic particles. In the Standard Model, there are two types of particles: fermions and bosons. The fermions include quarks and leptons and are the building blocks of matter. These particles have “spin” values of ½. The force carriers are bosons, the other family of particles. They have integer values of spin, that is, 0 or 1.

    Supersymmetry would blend fermions and bosons together by associating partners to each particle: a fermion would be paired with a boson, and vice-versa. For example, each quark would come with a “squark,” the name given to the supersymmetric partners of quarks. The squarks would be bosons rather than fermions and would carry spin 0. The same thing goes for leptons. Likewise, the known bosons (gluons, Higgs, W, Z and photons) would come with fermion superpartners with half spin values. These would be the gluinos, Higgsinos, winos, zinos and photinos. A mixture of the force carrier superpartners (all except the gluinos) gives charginos and neutralinos, the latter being particles that would be the perfect candidates for dark matter….”

    And, now, it gets interesting. Dr Gagnon is a great communicator. Please read the full post here.

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  • richardmitnick 6:22 pm on September 28, 2012 Permalink | Reply
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    From CERN Blog at Quantum Diaries: “How is new physics discovered?” 

    IT HAS BEEN A WHILE SINCE I HAVE BEEN ABLE TO PRESENT A POST FROM QUANTUM DIARIES. MY AUDIENCE IS A MORE GENERALIST PUBLIC – INTERESTED, EDUCATED, BUT NOT PROFESSIONAL SCIENTISTS. NOW COMES A POST WHICH I BELIEVE MIGHT BE APPROACHABLE FOR MY READERS.

    pg
    Pauline Gagnon

    2012.09.28
    Pauline Gagnon

    “Finding an experimental anomaly is a great way to open the door to a new theory. It is such a good trick that many of us physicists are bending over backward trying to uncover the smallest deviation from what the current theory, the Standard Model of particle physics, predicts.

    sm
    Standard Model

    This is the approach the LHCb collaboration at CERN is pursuing when looking at very rare decays. A minute deviation can be more easily spotted for rare processes. One good place to look is in the rate of K meson decays, a particle made of one strange quark s and one anti-down quark d.

    Recently, the LHCb collaboration has turned its attention to measuring the decay rate of the short-lived kaons K0S, the only K mesons decaying fast enough to be seen with precision in their detector.”

    I hope that is enough to entice you to read further.

    LHCB
    LHCb Collaboration

    Pauline Gagnon is a very good writer. Read and enjoy the rest of her post here. While you are at it, look around at the various Quantum Diary blogs, Twitter feeds,member organization web sites.

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  • richardmitnick 2:13 pm on March 2, 2012 Permalink | Reply
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    From Pauline Gagnon at CERN via Quantum Diaries: “All on the Higgs for (nearly) everyone” 

    “Like most of my colleagues, the most frequently asked question I get from friends and family these days is: what is this Higgs boson business? Here is what I hope will help not only family members but also struggling physicists. It is not the simplest but it is complete and accurate.

    First of all, let’s clarify one point: it has nothing to do with God. This “God’s particle” business has got to go. It was just a bad joke to start with and like any joke, it gets stale fast.

    And we need to talk about three separate aspects: the Higgs mechanism*, the Higgs field and the Higgs boson.

    See Pauline’s very thorough discussion here.

    *Link is to Wikipedia with which some might object

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