From particle bites: “Can’t Stop Won’t Stop: The Continuing Search for SUSY”

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June 19, 2016
Julia Gonski


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.

Standard model of Supersymmetry DESY
Standard model of Supersymmetry DESY

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.

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!)

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

See the full article here .

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ParticleBites is written and edited by graduate students and postdocs working in high energy physics. Feel free to contact us if you’re interested in applying to write for ParticleBites.

ParticleBites was founded in 2013 by Flip Tanedo following the Communicating Science (ComSciCon) 2013 workshop.

Flip Tanedo UCI Chancellor’s ADVANCE postdoctoral scholar in theoretical physics. As of July 2016, I will be an assistant professor of physics at the University of California, Riverside

It is now organized and directed by Flip and Julia Gonski, with ongoing guidance from Nathan Sanders.