{"id":1025,"date":"2014-08-20T12:29:34","date_gmt":"2014-08-20T12:29:34","guid":{"rendered":"http:\/\/www.particlebites.com\/?p=1025"},"modified":"2017-02-19T01:36:10","modified_gmt":"2017-02-19T01:36:10","slug":"cms-evidence-of-a-possible-susy-decay-chain","status":"publish","type":"post","link":"https:\/\/www.particlebites.com\/?p=1025","title":{"rendered":"CMS evidence of a possible SUSY decay chain"},"content":{"rendered":"
\nTitle:<\/strong> \u201cSearch for physics beyond the standard model in events with two leptons, jets, and missing transverse energy in pp collisions at sqrt(s)=8 TeV.\u201d<\/ br>
\nAuthor: <\/strong>CMS Collaboration<\/ br>
\nPublished:<\/strong> CMS Public: Physics Results SUS12019\n<\/div>\n

The CMS Collaboration, one of the two main groups working on multipurpose experiments at the Large Hadron Collider, has recently reported an excess of events with an estimated significance of 2.6\u03c3. As a reminder, discoveries in particle physics are typically declared at 5\u03c3. While this excess is small enough that it may not be related to new physics at all, it is also large enough to generate some discussion.<\/p>\n

The excess occurs at an invariant mass of 20 \u2013 70 GeV in dilepton + missing transverse energy (MET<\/sub>) decays. Some theorists claim that this may\u00a0be a signature of supersymmetry. The analysis was completed using kinematic \u2018edges\u2019, an example of which can be seen in Figure 1.\u00a0These shapes are typical of the decays of new particles predicted by supersymmetry.\u00a0<\/span><\/p>\n

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\"edgeDiagram\"<\/a>
Figure 1: Diagram of kinematic \u2018edge\u2019 effects in decay chains, from “Search for an ‘edge’ with CMS”. On the left, A, B, C, and D represent particles decaying. On the right, the invariant mass of final state particles C and D is shown, where the y axis represents the number of events.<\/figcaption><\/figure>\n

The edge shape comes from the reconstructed invariant mass of the two leptons; in the diagram, these correspond to particles C and D. In models that conserve R-parity, which is the quantum number that distinguishes SUSY particles from Standard Model particles, a SUSY particle decays by emitting an SM particle and a lighter SUSY particle. In this case, two leptons are emitted in the chain. Reconstructing the invariant mass of the event is impossible\u00a0because of the invisible massive particle.\u00a0However,\u00a0the total mass of the lepton pair can have any value, provided it is less than the\u00a0maximum difference in mass between the initial and final state, as enforced by energy conservation. This maximum mass difference gives a hard cutoff, or ‘edge’, in the invariant mass distribution, as shown in the right side of Figure 1. Since the location of this cutoff is dependent on the mass of the original superparticle, these features can be very useful in obtaining information about such decays.<\/p>\n

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Figure 2 shows generated Monte Carlo for a new particle decaying to a\u00a0two lepton final state. The red and blue lines show sources of background, while the green is the simulated\u00a0signal. If the model was a good estimate of data, these three colored lines would sum to the distribution observed in data. Figure 3 shows the actual data distribution, with the relative significance of the excess around 20 \u2013 70 GeV.<\/p>\n

\"newSUSYMC\"<\/a>
Figure 2: Monte Carlo invariant mass distribution of paired electrons or muons; signal shown in green with characteristic edge.<\/figcaption><\/figure>\n
\"excessPlot\"<\/a>
Figure 3: Invariant mass data distribution for paired leptons; excess between 20 and 70 GeV constitutes an estimated 2.6\u03c3 significance.\u00a0<\/figcaption><\/figure>\n

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This excess is encouraging for physicists hoping to find stronger evidence for supersymmetry (or more generally, new physics) in Run II.\u00a0<\/span>However, 2.6\u03c3 is not especially high, and historically these excesses come and go all the time. Both CMS and ATLAS will certainly be watching this resonance in the 2015 13 TeV data, to see whether it grows into something more significant or simply fades into the background.<\/p>\n

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Further reading:<\/strong><\/p>\n