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CMS evidence of a possible SUSY decay chain

Title: “Search for physics beyond the standard model in events with two leptons, jets, and missing transverse energy in pp collisions at sqrt(s)=8 TeV.”
Author: CMS Collaboration
Published: CMS Public: Physics Results SUS12019

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 2.6σ. As a reminder, discoveries in particle physics are typically declared at 5σ. 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.

The excess occurs at an invariant mass of 20 – 70 GeV in dilepton + missing transverse energy (MET) decays. Some theorists claim that this may be a signature of supersymmetry. The analysis was completed using kinematic ‘edges’, an example of which can be seen in Figure 1. These shapes are typical of the decays of new particles predicted by supersymmetry. 

 

edgeDiagram
Figure 1: Diagram of kinematic ‘edge’ 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.

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 because of the invisible massive particle. However, the total mass of the lepton pair can have any value, provided it is less than the maximum 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.

 

Figure 2 shows generated Monte Carlo for a new particle decaying to a two lepton final state. The red and blue lines show sources of background, while the green is the simulated signal. 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 – 70 GeV.

newSUSYMC
Figure 2: Monte Carlo invariant mass distribution of paired electrons or muons; signal shown in green with characteristic edge.
excessPlot
Figure 3: Invariant mass data distribution for paired leptons; excess between 20 and 70 GeV constitutes an estimated 2.6σ significance. 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

This excess is encouraging for physicists hoping to find stronger evidence for supersymmetry (or more generally, new physics) in Run II. However, 2.6σ 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.

 

Further reading:

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New Results from the CRESST-II Dark Matter Experiment

  • Title: Results on low mass WIMPs using an upgraded CRESST-II detector
  • Author: G. Angloher, A. Bento, C. Bucci, L. Canonica, A. Erb, F. v. Feilitzsch, N. Ferreiro Iachellini, P. Gorla, A. Gütlein, D. Hauff, P. Huff, J. Jochum, M. Kiefer, C. Kister, H. Kluck, H. Kraus,  J.-C. Lanfranchi, J. Loebell, A. Münster, F. Petricca, W. Potzel, F. Pröbst, F. Reindl, S. Roth, K. Rottler, C. Sailer, K. Schäffner, J. Schieck, J. Schmaler, S. Scholl, S. Schönert, W. Seidel, M. v. Sivers, L. Stodolsky, C. Strandhagen, R. Strauss, A. Tanzke, M. Uffinger, A. Ulrich, I. Usherov, M. Wüstrich, S. Wawoczny, M. Willers, and A. Zöller
  • Published: arXiv:1407.3146 [astro-ph.CO]

CRESST-II (Cryogenic Rare Event Search with Superconducting Thermometers) is a dark matter search experiment located at the Laboratori Nazionali del Gran Sasson in Italy. It is primarily involved with the search for WIMPs, or Weakly Interacting Massive Particles, which play a key role in both particle and astrophysics as a potential candidate for dark matter. If you are not yet intrigued enough about dark matter, see the list of references at the bottom of this post for more information. As dark matter candidates, WIMPs only interact via gravitational and weak forces, making them extremely difficult to detect.

CRESST-II attempts to detect WIMPs via elastic scattering off nuclei in scintillating CaWO4 crystals. This is a process known as direct detection, where scientists search for evidence of the WIMP itself; indirect detection requires searching for WIMP decay products. There are many challenges to direct detection, including the relatively low amount of recoil energy present in such scattering. An additional issue is the extremely high background, which is dominated by beta and gamma radiation of the nuclei. Overall, the experiment expects to obtain a few tens of events per kilogram-year.

CRESST1
Figure 1: Expected number of events for background and signal in 2011 CRESST-II run; from 1109.0702v1.

 

In 2011, CREST-II reported a small excess of events outside of the predicted background levels. The statistical analysis makes use of a maximum likelihood function, which parameterizes each primary background to compute a total number of expected events. The results of this likelihood fit can be seen in Figure 1, where M1 and M2 are different mass hypotheses. From these values, CRESST-II reports a statistical significance of 4.7σ for M1, and 4.2σ for M2. Since a discovery is generally accepted to have a significance of 5σ, these numbers presented a pretty big cause for excitement.

 

 

 

In July of 2014, CRESST-II released a follow up paper: after some detector upgrades and further background reduction, these tantalizingly high significances have been revised, ruling out both mass hypotheses. The event excess was likely due to unidentified  e–/γ background, which was reduced by a factor of 2 -10 via improved CaWO4 crystals used in this run. The elimination of these high signal significances is in agreement with other dark matter searches, which have also ruled out WIMP masses on the order of 20 GeV.

Figure 2 shows the most recent exclusion curve for the WIMP mass, which gives the cross section for production as a function of possible mass. The contour reported in the 2011 paper is shown in light blue. The 90% confidence limit from the 2014 paper is given in solid red, alongside the expected sensivity from the background model in light red. All other curves are due to data from other experiments; see the paper cited for more information.

CRESST2
Figure 2: WIMP parameter space for spin-independent WIMP-nucleon scattering, from 1407.3146v1.

Though this particular excess was ultimately not confirmed, these results overall present an optimistic picture for the dark matter search. Comparison between the limits from 2011 to 2014 show an much greater sensitivity for WIMP masses below 3 GeV, which were previously un-probed by other experiments. Additional detector improvements may result in even more stringent limit setting, shaping the dark matter search for future experiments.

 

Further Reading