Can’t Stop Won’t Stop: The Continuing Search for SUSY

Title: “Search for top squarks in final states with one isolated lepton, jets, and missing transverse momentum in √s = 13 TeV pp collisions with the ATLAS detector”
Author: The ATLAS Collaboration
Publication: Submitted 13 June 2016, arXiv 1606.03903

Things at the LHC are going great. Run II of the Large Hadron Collider is well underway, delivering higher energies and more luminosity than ever before. ATLAS and CMS also have an exciting lead to chase down– the diphoton excess that was first announced in December 2015. So what does lots of new data and a mysterious new excess have in common? They mean that we might finally get a hint at the elusive theory that keeps refusing our invitations to show up: supersymmetry.

Feynman diagram of stop decay from proton-proton collisions.
Figure 1: Feynman diagram of stop decay from proton-proton collisions.

People like supersymmetry because it fixes a host of things in the Standard Model. But most notably, it generates an extra Feynman diagram that cancels the quadratic divergence of the Higgs mass due to the top quark contribution. This extra diagram comes from the stop quark. So a natural SUSY solution would have a light stop mass, ideally somewhere close to the top mass of 175 GeV. This expected low mass due to “naturalness” makes the stop a great place to start looking for SUSY. But according to the newest results from the ATLAS Collaboration, we’re not going to be so lucky.

Using the full 2015 dataset (about 3.2 fb-1), ATLAS conducted a search for pair-produced stops, each decaying to a top quark and a neutralino, in this case playing the role of the lightest supersymmetric particle. The top then decays as tops do, to a W boson and a b quark. The W usually can do what it wants, but in this case the group chose to select for one W decaying leptonically and one decaying to jets (leptons are easier to reconstruct, but have a lower branching ratio from the W, so it’s a trade off.) This whole process is shown in Figure 1. So that gives a lepton from one W, jets from the others, and missing energy from the neutrino for a complete final state.

Transverse mass distribution in one of the signal regions.
Figure 2: Transverse mass distribution in one of the signal regions.

The paper does report an excess in the data, with a significance around 2.3 sigma. In Figure 2, you can see this excess overlaid with all the known background predictions, and two possible signal models for various gluino and stop masses. This signal in the 700-800 GeV mass range is right around the current limit for the stop, so it’s not entirely inconsistent. While these sorts of excesses come and go a lot in particle physics, it’s certainly an exciting reason to keep looking.

Figure 3 shows our status with the stop and neutralino, using 8 TeV data. All the shaded regions here are mass points for the stop and neutralino that physicists have excluded at 95% confidence. So where do we go from here? You can see a sliver of white space on this plot that hasn’t been excluded yet— that part is tough to probe because the mass splitting is so small, the neutralino emerges almost at rest, making it very hard to notice. It would be great to check out that parameter space, and there’s an effort underway to do just that. But at the end of the day, only more time (and more data) can tell.

(P.S. This paper also reports a gluino search—too much to cover in one post, but check it out if you’re interested!)

Limit curves for stop and neutralino masses, with 8 TeV ATLAS dataset.
Figure 3: Limit curves for stop and neutralino masses, with 8 TeV ATLAS dataset.

References & Further Reading

  1. “Supersymmetry, Part I (Theory), PDG Review
  2. “Supersymmetry, Part II (Experiment), PDG Review
  3. ATLAS Supersymmetry Public Results Twiki
  4. “Opening up the compressed region of stop searches at 13 TeV LHC”, arXiv hep-ph 1506.00653


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Julia Gonski

Julia is a Ph.D. student and NSF Graduate Research Fellow in the high energy experimental group at Harvard. A New Jersey native, she graduated summa cum laude from Rutgers University, with a thesis on the search for supersymmetry at CMS. Currently, her work involves the study of micromega chambers as an upgrade to the ATLAS muon detector.

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