In lieu of a typical HEP paper summary this month, I’m linking a comprehensive overview of the new results shown at this year’s Moriond conference, originally published in the CERN EP Department Newsletter. Since this includes the latest and greatest from all four experiments on the LHC ring (ATLAS, CMS, ALICE, and LHCb), you can take it as a sort of “state-of-the-field”. Here is a sneak preview:
“Every March, particle physicists around the world take two weeks to promote results, share opinions and do a bit of skiing in between. This is the Moriond tradition and the 52nd iteration of the conference took place this year in La Thuile, Italy. Each of the four main experiments on the LHC ring presented a variety of new and exciting results, providing an overview of the current state of the field, while shaping the discussion for future efforts.”
Read more in my article for the CERN EP Department Newsletter here!
The integrated luminosity of the LHC with proton-proton collisions in 2016 compared to previous years. Luminosity is a measure of a collider’s performance and is proportional to the number of collisions. The integrated luminosity achieved by the LHC in 2016 far surpassed expectations and is double that achieved at a lower energy in 2012.
March is an exciting month for high energy physicists. Every year at this time, scientists from all over the world gather for the annual Moriond Conference, where all of the latest results are shown and discussed. Now that this physics Christmas season is over, I, like many other physicists, am sifting through the proceedings, trying to get a hint of what is the new cool physics to be chasing after. My conclusions? The Higgsino search is high on this list.
Physicists chatting at the 2017 Moriond Conference. Image credit ATLAS-PHOTO-2017-009-1.
The search for Higgsinos falls under the broad and complex umbrella of searches for supersymmetry (SUSY). We’ve talked about SUSY on Particlebites in the past; see a recent post on thestop search for reference. Recall that the basic prediction of SUSY is that every boson in the Standard Model has a fermionic supersymmetric partner, and every fermion gets a bosonic partner.
So then what exactly is a Higgsino? The naming convention of SUSY would indicate that the –ino suffix means that a Higgsino is the supersymmetric partner of the Higgs boson. This is partly true, but the whole story is a bit more complicated, and requires some understanding of the Higgs mechanism.
To summarize, in our Standard Model, the photon carries the electromagnetic force, and the W and Z carry the weak force. But before electroweak symmetry breaking, these bosons did not have such distinct tasks. Rather, there were three massless bosons, the B, W, and Higgs, which together all carried the electroweak force. It is the supersymmetric partners of these three bosons that mix to form new mass eigenstates, which we call simply charginos or neutralinos, depending on their charge. When we search for new particles, we are searching for these mass eigenstates, and then interpreting our results in the context of electroweak-inos.
SUSY searches can be broken into many different analyses, each targeting a particular particle or group of particles in this new sector. Starting with the particles that are suspected to have low mass is a good idea, since we’re more likely to observe these at the current LHC collision energy. If we begin with these light particles, and add in the popular theory of naturalness, we conclude that Higgsinos will be the easiest to find of all the new SUSY particles. More specifically, the theory predicts three Higgsinos that mix into two neutralinos and a chargino, each with a mass around 200-300 GeV, but with a very small mass splitting between the three. See Figure 1 for a sample mass spectra of all these particles, where N and C indicate neutralino or chargino respectively (keep in mind this is just a possibility; in principle, any bino/wino/higgsino mass hierarchy is allowed.)
Figure 1: Sample electroweak SUSY mass spectrum. Image credit: T. Lari, INFN Milano
This is both good news and bad news. The good part is that we have reason to think that there are three Higgsinos with masses that are well within our reach at the LHC. The bad news is that this mass spectrum is very compressed, making the Higgsinos extremely difficult to detect experimentally. This is due to the fact that when C1 or N2 decays to N1 (the lightest neutralino), there is very little mass difference leftover to supply energy to the decay products. As a result, all of the final state objects (two N1s plus a W or a Z as a byproduct, see Figure 2) will have very low momentum and thus are very difficult to detect.
Figure 2: Electroweakino pair production and decay (CMS-PAS-SUS-16-048).
The CMS collaboration Higgsino analysis documented here uses a clever analysis strategy for such compressed decay scenarios. Since initial state radiation (ISR) jets occur often in proton-proton collisions, you can ask for your event to have one. This jet radiating from the collision will give the system a kick in the opposite direction, providing enough energy to those soft particles for them to be detectable. At the end of the day, the analysis team looks for events with ISR, missing transverse energy (MET), and two soft opposite sign leptons from the Z decay (to distinguish from hadronic SM-like backgrounds). Figure 3 shows a basic diagram of what these signal events would look like.
Figure 3: Signal event vector diagram. Image credit C. Botta, CERN
In order to conduct this search, several new analysis techniques were employed. Reconstruction of leptons at low pT becomes extremely important in this regime, and the standard cone isolation of the lepton and impact parameter cuts are used to ensure proper lepton identification. New discriminating variables are also added, which exploit kinematic information about the lepton and the soft particles around it, in order to distinguish “prompt” (signal) leptons from those that may have come from a jet and are thus “non prompt” (background.)
In addition, the analysis team paid special attention to the triggers that could be used to select signal events from the immense number of collisions, creating a new “compressed” trigger that uses combined information from both soft muons (pT > 5 GeV) and missing energy ( > 125 GeV).
With all of this effort, the group is able to probe down to a mass splitting between Higgsinos of 20 GeV, excluding N2 masses up to 230 GeV. This is an especially remarkable result because the current strongest limit on Higgsinos comes from the LEP experiment, a result that is over ten years old! Because the Higgsino searches are strongly limited by the low cross section of electroweak SUSY, additional data will certainly mean that these searches will proceed quickly, and more stringent bounds will be placed (or, perhaps, a discovery is in store!)
Figure 4: Figure 5: The observed exclusion contours (black) with the corresponding 1 standard deviation uncertainties. The dashed red curves present the expected limits with 1 SD experimental uncertainties (CMS-PAS-SUS-16-048).
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.
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.
