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  • richardmitnick 12:56 pm on September 29, 2017 Permalink | Reply
    Tags: , , , CERN Open Data Portal, Jets,   

    From MIT: “First open-access data from large collider confirm subatomic particle patterns” 

    MIT News

    MIT Widget

    MIT News

    September 29, 2017
    Jennifer Chu

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    The Compact Muon Solenoid is a general-purpose detector at the Large Hadron Collider. Image courtesy of CERN

    LHC

    CERN/LHC Map

    CERN LHC Tunnel

    CERN LHC particles

    In November of 2014, in a first, unexpected move for the field of particle physics, the Compact Muon Solenoid (CMS) experiment — one of the main detectors in the world’s largest particle accelerator, the Large Hadron Collider — released to the public an immense amount of data, through a website called the CERN Open Data Portal.

    The data, recorded and processed throughout the year 2010, amounted to about 29 terabytes of information, yielded from 300 million individual collisions of high-energy protons within the CMS detector. The sharing of these data marked the first time any major particle collider experiment had released such an information cache to the general public.

    A new study by Jesse Thaler, an associate professor of physics at MIT and a long-time advocate for open access in particle physics, and his colleagues now demonstrates the scientific value of this move. In a paper published in Physical Review Letters, the researchers used the CMS data to reveal, for the first time, a universal feature within jets of subatomic particles, which are produced when high-energy protons collide. Their effort represents the first independent, published analysis of the CMS open data.

    “In our field of particle physics, there isn’t the tradition of making data public,” says Thaler. “To actually get data publicly with no other restrictions — that’s unprecedented.”

    Part of the reason groups at the Large Hadron Collider and other particle accelerators have kept proprietary hold over their data is the concern that such data could be misinterpreted by people who may not have a complete understanding of the physical detectors and how their various complex properties may influence the data produced.

    “The worry was, if you made the data public, then you would have people claiming evidence for new physics when actually it was just a glitch in how the detector was operating,” Thaler says. “I think it was believed that no one could come from the outside and do those corrections properly, and that some rogue analyst could claim existence of something that wasn’t really there.”

    “This is a resource that we now have, which is new in our field,” Thaler adds. “I think there was a reluctance to try to dig into it, because it was hard. But our work here shows that we can understand in general how to use this open data, that it has scientific value, and that this can be a stepping stone to future analysis of more exotic possibilities.”

    Thaler’s co-authors are Andrew Larkoski of Reed College, Simone Marzani of the State University of New York at Buffalo, and Aashish Tripathee and Wei Xue of MIT’s Center for Theoretical Physics and Laboratory for Nuclear Science.

    Seeing fractals in jets

    When the CMS collaboration publicly released its data in 2014, Thaler sought to apply new theoretical ideas to analyze the information. His goal was to use novel methods to study jets produced from the high-energy collision of protons.

    Protons are essentially accumulations of even smaller subatomic particles called quarks and gluons, which are bound together by interactions known in physics parlance as the strong force. One feature of the strong force that has been known to physicists since the 1970s describes the way in which quarks and gluons repeatedly split and divide in the aftermath of a high-energy collision.

    This feature can be used to predict the energy imparted to each particle as it cleaves from a mother quark or gluon. In particular, physicists can use an equation, known as an evolution equation or splitting function, to predict the pattern of particles that spray out from an initial collision, and therefore the overall structure of the jet produced.

    “It’s this fractal-like process that describes how jets are formed,” Thaler says. “But when you look at a jet in reality, it’s really messy. How do you go from this messy, chaotic jet you’re seeing to the fundamental governing rule or equation that generated that jet? It’s a universal feature, and yet it has never directly been seen in the jet that’s measured.”

    Collider legacy

    In 2014, the CMS released a preprocessed form of the detector’s 2010 raw data that contained an exhaustive listing of “particle flow candidates,” or the types of subatomic particles that are most likely to have been released, given the energies measured in the detector after a collision.

    The following year, Thaler published a theoretical paper with Larkoski and Marzani, proposing a strategy to more fully understand a complicated jet in a way that revealed the fundamental evolution equation governing its structure.

    “This idea had not existed before,” Thaler says. “That you could distill the messiness of the jet into a pattern, and that pattern would match beautifully onto that equation — this is what we found when we applied this method to the CMS data.”

    To apply his theoretical idea, Thaler examined 750,000 individual jets that were produced from proton collisions within the CMS open data. He looked to see whether the pattern of particles in those jets matched with what the evolution equation predicted, given the energies released from their respective collisions.

    Taking each collision one by one, his team looked at the most prominent jet produced and used previously developed algorithms to trace back and disentangle the energies emitted as particles cleaved again and again. The primary analysis work was carried out by Tripathee, as part of his MIT bachelor’s thesis, and by Xue.

    “We wanted to see how this jet came from smaller pieces,” Thaler says. “The equation is telling you how energy is shared when things split, and we found when you look at a jet and measure how much energy is shared when they split, they’re the same thing.”

    The team was able to reveal the splitting function, or evolution equation, by combining information from all 750,000 jets they studied, showing that the equation — a fundamental feature of the strong force — can indeed predict the overall structure of a jet and the energies of particles produced from the collision of two protons.

    While this may not generally be a surprise to most physicists, the study represents the first time this equation has been seen so clearly in experimental data.

    “No one doubts this equation, but we were able to expose it in a new way,” Thaler says. “This is a clean verification that things behave the way you’d expect. And it gives us confidence that we can use this kind of open data for future analyses.”

    Thaler hopes his and others’ analysis of the CMS open data will spur other large particle physics experiments to release similar information, in part to preserve their legacies.

    “Colliders are big endeavors,” Thaler says. “These are unique datasets, and we need to make sure there’s a mechanism to archive that information in order to potentially make discoveries down the line using old data, because our theoretical understanding changes over time. Public access is a stepping stone to making sure this data is available for future use.”

    This research was supported, in part, by the MIT Charles E. Reed Faculty Initiatives Fund, the MIT Undergraduate Research Opportunities Program, the U.S. Department of Energy, and the National Science Foundation.

    See the full article here .

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  • richardmitnick 4:05 pm on August 14, 2016 Permalink | Reply
    Tags: , Jets, ,   

    From particlebites: “Jets: More than Riff, Tony, and a rumble” 

    particlebites bloc

    particlebites

    July 26, 2016 [Just today in social media.]
    Reggie Bain

    Ubiquitous in the LHC’s ultra-high energy collisions are collimated sprays of particles called jets. The study of jet physics is a rapidly growing field where experimentalists and theorists work together to unravel the complex geometry of the final state particles at LHC experiments. If you’re totally new to the idea of jets…this bite from July 18th, 2016 by Julia Gonski is a nice experimental introduction to the importance of jets. In this bite, we’ll look at the basic ideas of jet physics from a more theoretical perspective. Let’s address a few basic questions:

    1. What is a jet? Jets are highly collimated collections of particles that are frequently observed in detectors. In visualizations of collisions in the ATLAS detector, one can often identify jets by eye.

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    A nicely colored visualization of a multi-jet event in the ATLAS detector. Reason #172 that I’m not an experimentalist…actually sifting out useful information from the detector (or even making a graphic like this) is insanely hard.

    Jets are formed in the final state of a collision when a particle showers off radiation in such a way as to form a focused cone of particles. The most commonly studied jets are formed by quarks and gluons that fragment into hadrons like pions, kaons, and sometimes more exotic particles like the $latex J/Ψ, Υ, χc and many others. This process is often referred to as hadronization.

    2. Why do jets exist? Jets are a fundamental prediction of Quantum Field Theories like Quantum Chromodynamics (QCD). One common process studied in field theory textbooks is electron–positron annihilation into a pair of quarks, e+e– → q q. In order to calculate the cross-section of this process, it turns out that one has to consider the possibility that additional gluons are produced along with the qq. Since no detector has infinite resolution, it’s always possible that there are gluons that go unobserved by your detector. This could be because they are incredibly soft (low energy) or because they travel almost exactly collinear to the q or q itself. In this region of momenta, the cross-section gets very large and the process favors the creation of this extra radiation. Since these gluons carry color/anti-color, they begin to hadronize and decay so as to become stable, colorless states. When the q, q have high momenta, the zoo of particles that are formed from the hadronization all have momenta that are clustered around the direction of the original q,q and form a cone shape in the detector…thus a jet is born! The details of exactly how hadronization works is where theory can get a little hazy. At the energy and distance scales where quarks/gluons start to hadronize, perturbation theory breaks down making many of our usual calculational tools useless. This, of course, makes the realm of hadronization—often referred to as parton fragmentation in the literature—a hot topic in QCD research.

    3. How do we measure/study jets? Now comes the tricky part. As experimentalists will tell you, actually measuring jets can a messy business. By taking the signatures of the final state particles in an event (i.e. a collision), one can reconstruct a jet using a jet algorithm. One of the first concepts of such jet definitions was introduced by Geroge Sterman and Steven Weinberg in 1977. There they defined a jet using two parameters θ, E. These restricted the angle and energy of particles that are in or out of a jet. Today, we have a variety of jet algorithms that fall into two categories:

    Cone Algorithms — These algorithms identify stable cones of a given angular size. These cones are defined in such a way that if one or two nearby particles are added to or removed from the jet cone, that it won’t drastically change the cone location and energy
    Recombination Algorithms — These look pairwise at the 4-momenta of all particles in an event and combine them, according to a certain distance metric (there’s a different one for each algorithm), in such a way as to be left with distinct, well-separated jets.

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    Figure 2: From Cacciari and Salam’s original paper on the “Anti-kT” jet algorithm (See arXiv:0802.1189). The picture shows the application of 4 different jet algorithms: the kT, Cambridge/Aachen, Seedless-Infrared-Safe Cone, and anti-kT algorithms to a single set of final state particles in an event. You can see how each algorithm reconstructs a slightly different jet structure. These are among the most commonly used clustering algorithms on the market (the anti-kT being, at least in my experience, the most popular)

    4. Why are jets important? On the frontier of high energy particle physics, CERN leads the world’s charge in the search for new physics. From deepening our understanding of the Higgs to observing never before seen particles, projects like ATLAS,

    3
    An illustration of an interesting type of jet substructure observable called “N-subjettiness” from the original paper by Jesse Thaler and Ken van Tilburg (see arXiv:1011.2268). N-subjettiness aims to study how momenta within a jet are distributed by dividing them up into n sub-jets. The diagram on the left shows an example of 2-subjettiness where a jet contains two sub-jets. The diagram on the right shows a jet with 0 sub-jets.

    CMS, and LHCb promise to uncover interesting physics for years to come. As it turns out, a large amount of Standard Model background to these new physics discoveries comes in the form of jets. Understanding the origin and workings of these jets can thus help us in the search for physics beyond the Standard Model.

    Additionally, there are a number of interesting questions that remain about the Standard Model itself. From studying the production of heavy hadron production/decay in pp and heavy-ion collisions to providing precision measurements of the strong coupling, jets physics has a wide range of applicability and relevance to Standard Model problems. In recent years, the physics of jet substructure, which studies the distributions of particle momenta within a jet, has also seen increased interest. By studying the geometry of jets, a number of clever observables have been developed that can help us understand what particles they come from and how they are formed. Jet substructure studies will be the subject of many future bites!

    Going forward…With any luck, this should serve as a brief outline to the uninitiated on the basics of jet physics. In a world increasingly filled with bigger, faster, and stronger colliders, jets will continue to play a major role in particle phenomenology. In upcoming bites, I’ll discuss the wealth of new and exciting results coming from jet physics research. We’ll examine questions like:

    How do theoretical physicists tackle problems in jet physics?
    How does the process of hadronization/fragmentation of quarks and gluons really work?
    Can jets be used to answer long outstanding problems in the Standard Model?

    I’ll also bite about how physicists use theoretical smart bombs called “effective field theories” to approach these often nasty theoretical calculations. But more on that later…

    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.

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    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.

     
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