{"id":3775,"date":"2016-07-20T16:54:57","date_gmt":"2016-07-20T16:54:57","guid":{"rendered":"http:\/\/www.particlebites.com\/?p=3775"},"modified":"2017-02-19T01:12:34","modified_gmt":"2017-02-19T01:12:34","slug":"probing-the-standard-model-with-muons-new-results-from-meg","status":"publish","type":"post","link":"https:\/\/www.particlebites.com\/?p=3775","title":{"rendered":"Probing the Standard Model with muons: new results from MEG"},"content":{"rendered":"
Article: <\/strong> Search for the lepton flavor violating decay \u03bc+<\/sup> \u2192 e+<\/sup>\u03b3 with the full dataset of the MEG experiment
\nAuthors:<\/strong> MEG Collaboration
\nReference:<\/strong>\u00a0arXiv:1605.05081<\/a><\/div>\n

I work on the Muon g-2 experiment<\/a>, which is housed inside a brand new building at Fermilab.\u00a0 Next door, another experiment hall is under construction. It will be the home of the Mu2e experiment<\/a>, which is slated to use Fermilab\u2019s muon beam as soon as Muon g-2 wraps up in a few years. Mu2e will search for evidence of an extremely rare process \u2014 namely, the conversion of a muon to an electron in the vicinity of a nucleus. You can read more about muon-to-electron conversion in a previous post<\/a> by Flip.<\/p>\n

Today, though, I bring you news of a different muon experiment, located at the Paul Scherrer Institute in Switzerland. The MEG experiment was operational from 2008-2013, and they recently released their final result.<\/p>\n

Context of\u00a0the MEG experiment<\/strong><\/p>\n

<\/a>
Figure 1: Almost 100% of the time, a muon will decay into an electron and two neutrinos.<\/figcaption><\/figure>\n

MEG (short for \u201cmu to e gamma\u201d) and Mu2e are part of the same family of experiments. They each focus on a particular example of charged lepton flavor violation (CLFV). Normally, a muon decays into an electron and two neutrinos. The neutrinos ensure that lepton flavor is conserved; the overall amounts of \u201cmuon-ness\u201d and \u201celectron-ness\u201d do not change.<\/p>\n

Figure 2 lists some possible CLFV muon processes. In each case, the muon transforms into an electron without<\/i> producing any neutrinos \u2014 so lepton flavor is not conserved! These processes are allowed by the standard model, but with such minuscule probabilities that we couldn\u2019t possibly measure them. If that were the end of the story, no one would bother doing experiments like MEG and Mu2e \u2014 but of course that\u2019s not the end of the story. It turns out that many new physics models predict CLFV at levels that are within range of the next generation of experiments. If an experiment finds evidence for one of these CLFV processes, it will be a clear indication of beyond-the-standard-model physics.<\/p>\n

<\/a>
Figure 2: Some examples of muon processes that do not conserve lepton flavor. Also listed are the current\/upcoming experiments that aim to measure the probabilities of these never-before-observed processes.<\/figcaption><\/figure>\n

Results from MEG<\/strong><\/p>\n

The goal of the MEG experiment was to do one of two things:<\/p>\n

    \n
  1. Measure the branching ratio of the \u03bc+<\/sup>\u00a0\u2192 e+<\/sup>\u03b3 decay, or<\/li>\n
  2. Establish a new upper limit<\/li>\n<\/ol>\n

    Outcome #1 is only possible if the branching ratio is high enough to produce a clear signal. Otherwise, all the experimenters can do is say \u201cthe branching ratio must<\/i> be smaller than such-and-such, because otherwise we would have seen a signal\u201d (i.e., outcome #2).<\/p>\n

    MEG saw no evidence of \u03bc+<\/sup>\u00a0\u2192 e+<\/sup>\u03b3 decays. Instead, they determined that the branching ratio is less than 4.2 \u00d7 10^-13 (90% confidence level). Roughly speaking, that means if you had a pair of magic goggles that let you peer directly into the subatomic world, you could stand around and watch 2 \u00d7 10^12 muons decay without seeing anything unusual. Because real experiments are messier and less direct than magic goggles, the MEG result is actually based on data from 7.5 \u00d7 10^14 muons.<\/p>\n

    Before MEG, the previous experiment to search for \u03bc+<\/sup>\u00a0\u2192 e+<\/sup>\u03b3 was the MEGA experiment at Los Alamos; they collected data from 1993-1995, and published their final result<\/a> in 1999. They found an upper limit for the branching ratio of 1.2 \u00d7 10^-11. Thus, MEG achieved a factor of 30 improvement in sensitivity over the previous result.<\/p>\n

    How the experiment works<\/strong><\/p>\n

    <\/a>
    Figure 3: The MEG signal consists of a back-to-back positron and gamma, each carrying half the rest energy of the parent muon.<\/figcaption><\/figure>\n

    A continuous beam of positive muons enters a large magnet and hits a thin plastic target. By interacting with the material, about 80% of the muons lose their kinetic energy and come to rest inside the target. Because the muons decay from rest, the MEG signal is simple. Energy and momentum must be conserved, so the positron and gamma emerge from the target in opposite directions, each with an energy of 52.83 MeV (half the rest energy of the muon).1<\/sup>\u00a0 The experiment is specifically designed to catch and measure these events. It consists of three detectors: a drift chamber to measure the positron trajectory and momentum, a timing counter to measure the positron time, and a liquid xenon detector to measure the photon time, position, and energy. Data from all three detectors must be combined to get a complete picture of each muon decay, and determine whether it fits the profile of a MEG signal event.<\/p>\n

    <\/a>
    Figure 4: Layout of the MEG experiment. Source: arXiv:1605.05081<\/a>.<\/figcaption><\/figure>\n
    <\/div>\n

    In principle, it sounds pretty simple\u2026.to search for MEG events, you look at each chunk of data and go through a checklist:<\/p>\n