CMS presented new results of searches for signatures of supersymmetry (SUSY) at the LHCP conference held in Barcelona, Spain in May and at the EPSHEP conference held in Stockholm, Sweden in July. These results use either the full 20 fb–1 of data collected at a centre-of-mass energy of 8 TeV in 2012 or the 5 fb–1 of data at 7 TeV collected in 2011. Additional details about the results described here can be found in CMS SUSY results public page.
SUSY is a popular extension of the Standard Model (SM) because it addresses some important limitations in our current knowledge. The main motivation for SUSY is the ‘hierarchy problem’, which deals with the differences between the strengths of the gravitational and electroweak forces. In addition, some SUSY models provide a dark matter candidate particle with a mass compatible with expectations from astrophysics. Finally, it favours the convergence of the weak, strong and electromagnetic interactions strengths at high-energies, reinforcing the indication that these different interactions originate from a single more fundamental theory. SUSY introduces for each known particle a superpartner that differs in spin by a half-integer. CMS is actively searching for experimental evidence for the existence of these superpartners.
A particularly interesting particle is the top squark (or stop), the superpartner of the top quark. The stop mass is expected to be not too much larger than the top mass. This is a mass range that can be experimentally explored with the datasets collected at the LHC experiments. CMS performed a new dedicated search for stops [SUS-13-011], which considers the case where the stop decays to a top quark and a neutralino. The neutralino, itself a SUSY particle, is, if stable, an excellent dark-matter candidate; it wouldn’t be detected by CMS and we would have to infer its presence from missing energy in the detector. Similarly, the stop could also decay into a bottom quark and a chargino (yet another SUSY particle); the chargino then decays into a W boson and a neutralino. The same decay particles are produced in these two cases: a single lepton, jets and large missing energy. The results of CMS searches for these events are in agreement with the predictions from SM processes alone, predominantly top-quark production. The interpretation of the result of the search in these two decay modes is shown in Image 1, in terms of the masses of the stop and the neutralino. CMS has carefully combed over a significant region of parameter space but no stops have made themselves known so far.
Even though stops can be directly produced in LHC collisions, they may also be produced in the decays of other SUSY particles such as gluinos, the superpartners of the gluons, which could be produced in large quantities at the LHC. These processes can give rise to collision events with multiple W bosons and multiple bottom quarks. Past searches for these signatures have been performed in events with a single lepton and b-jets [SUS-13-007]. A new dedicated search for this signature, based on events with three leptons and b-jets, a very striking signature with low SM backgrounds, has been presented by CMS in SUS-13-008. Another channel pursued looks at events with a pair of isolated leptons of the same charge and jets [SUS-13-013]. These searches find no indication of such SUSY particles. A summary of the limits on the production of gluinos decaying to top quarks is presented in Image 2, which demonstrates that CMS searches probe gluinos up to about 1.3 TeV. These signatures can also arise in other models, for example in the decay of a sbottom, the SUSY partner of the bottom quark, allowing CMS to set exclusion limits for this scenario as well.
An update of a broad, general search for strong production of SUSY particles has been performed using events with multiple jets and large missing transverse momentum [SUS-13-012]. The results are consistent with Standard Model predictions and are used to constrain SUSY models of squarks and gluinos, where the SUSY particles decay to jets and the lightest supersymmetric particle (LSP). The results probe squarks up to masses of about 0.75 TeV and gluinos up to masses of about 1.1 TeV.
CMS has also performed searches for the direct electroweak production of SUSY particles, particularly the pair-production of charginos and neutralinos (mixtures of the SUSY partners of the gauge and Higgs bosons) and of sleptons (SUSY partners of leptons) [SUS-13-006]. These particles can decay to leptons or vector bosons, so these searches target events with multiple leptons in the final state. In particular, events with exactly three leptons, four leptons, two same-charge leptons, two opposite-charge-same-flavour leptons plus two jets, and two opposite-charge leptons inconsistent with Z boson decay, have been studied. The observed event rates are in agreement with expectations from the Standard Model, so the results are used to constrain the range of possible masses of charginos, neutralinos, and sleptons, as shown in Image 3.
The signatures discussed so far correspond to SUSY scenarios where an additional quantum number, R parity, is conserved. In such scenarios, the LSP is stable and carries away unseen energy, leading to large missing energy in the collision events. However, it is also interesting to consider scenarios where R-parity is violated (RPV). In these cases, SUSY particles are no longer stable and can decay into SM particles. A past CMS search focused on production of stops in an RPV scenario [SUS-13-003], which would have evaded detection in the search for stops presented above because the events are not characterised by large missing energy. In the scenarios considered, the LSP may decay to pairs of leptons and a neutrino or to a lepton, a top quark and a bottom quark. Null results in this search probe top squarks up to about 1.1 TeV.
The analysis SUS-13-010 considers the possibility that gluinos or squarks are pair-produced and each decays to jets and a neutralino LSP; the neutralino LSP in turn decays into two oppositely-charged electrons or muons and a neutrino. This leads to signatures with four electrons and muons, which is another striking signature with low SM backgrounds. Image 4 shows the results of this search, which probe gluinos up to masses of about 1.5 TeV and stops up to mass of about 1.0 TeV.
Finally, a search for resonant production of smuons (the SUSY partners of the muon) was performed [SUS-13-005]. While most SUSY searches focus on the pair-production of SUSY particles, this search focuses on the resonant production of a single smuon decaying to two same-charge leptons and two jets, by searching for a resonance in the mass of the decay products. Null results in this search are used to place the most stringent limits to date on the RPV coupling between an up quark, a down quark and a smuon.
Although SUSY hasn’t shown itself in these data, the next LHC run at a higher energy will significantly extend the sensitivity and may allow us to discover new physics that has so far eluded detection.
— Submitted by Verena Martinez and Benjamin Hooberman