{"id":3865,"date":"2016-07-26T18:29:03","date_gmt":"2016-07-26T18:29:03","guid":{"rendered":"http:\/\/www.particlebites.com\/?p=3865"},"modified":"2017-02-19T01:10:27","modified_gmt":"2017-02-19T01:10:27","slug":"jets-more-than-riff-tony-and-a-rumble","status":"publish","type":"post","link":"https:\/\/www.particlebites.com\/?p=3865","title":{"rendered":"Jets: More than Riff, Tony, and a rumble"},"content":{"rendered":"
Review Bite<\/strong>: Jet Physics
\n(This is the first in a series of posts on jet physics by Reggie Bain.<\/em>)<\/div>\n

Ubiquitous in the LHC\u2019s ultra-high energy collisions are collimated sprays of particles called <\/span>jets<\/b>. 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\u2019re totally new to the idea of jets…this <\/span>bite from July 18th, 2016 by Julia Gonski<\/span><\/a> is a nice experimental introduction to the importance of jets. In this bite, we\u2019ll look at the basic ideas of jet physics from a more theoretical perspective. Let\u2019
\ns address a few basic questions:<\/span><\/p>\n

    \n
  1. What is\u00a0a jet?<\/b> Jets are highly\u00a0collimated collections of particles that are frequently observed in detectors. In visualizations of collisions in the ATLAS detector, one can often identify jets by <\/span>eye.<\/span><\/b><\/b><\/b><\/li>\n<\/ol>\n
    \"A<\/a>
    A nicely colored visualization of a multi-jet event in the ATLAS detector. Reason #172 that I\u2019m not an experimentalist…actually sifting out useful information from the detector (or even making a graphic like this) is insanely hard.<\/figcaption><\/figure>\n

    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\/\u03a8, \u03a5, \u03c7c<\/sub> and many others. This process is often referred to as <\/span>hadronization<\/span><\/i>. <\/span><\/p>\n

      \n
    1. Why do jets exist?<\/b> Jets are a fundamental prediction of Quantum Field Theories like Quantum Chromodynamics (QCD). \u00a0One common process studied in field theory textbooks is electron–positron annihilation into a pair of quarks, e+<\/sup>e–<\/sup> \u2192 q\u00a0<\/em>q<\/em>. In order to calculate the
      \n      cross-section<\/span><\/a> of this process, it turns out that one has to consider the possibility that additional gluons are produced along with the q<\/em>q<\/em>. Since no detector has infinite resolution, it\u2019s always possible that there are gluons that go unobserved by your detector.<\/span> This could be because they are incredibly soft (low energy) or because they travel almost exactly collinear to the q<\/em> or q<\/em> itself. In this region of momenta, the cross-section gets very large and the process favors the creation of this extra radiation. <\/span>Since these gluons carry color\/anti-color, they begin to hadronize and decay so as to become stable, colorless states.<\/span><\/a> When the q<\/em>, q<\/em> 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<\/em>,q<\/em> 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 <\/span>fragmentation<\/span><\/i> in the literature—a hot topic in QCD research.<\/span><\/span><\/span><\/li>\n<\/ol>\n

       <\/p>\n

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      1. How do we measure\/study jets?<\/b> 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 <\/span>jet algorithm.<\/span><\/i> One of the first concepts of such jet definitions was introduced by <\/span>Geroge Sterman and Steven Weinberg in 1977<\/span><\/a>. There they defined a jet using two parameters \u03b8, E<\/em>. These restricted the angle and energy of particles that are in or out of a jet. \u00a0Today, we have a variety of jet algorithms that fall into two categories: <\/span><\/li>\n<\/ol>\n