CMS highlights from the 2016 LHCP conference

Figure 1: An event in CMS recorded on May 7, 2016, including two high-energy photons (green towers) reconstructed in the Electromagnetic Calorimeter, and many charged particles (yellow curved lines) reconstructed in the Tracker.

Figure 2: Top quark pair production cross section in proton-antiproton and proton-proton collisions as a function of the centre-of-mass energy; the Tevatron combination at 1.96 TeV is displayed, as well as CMS results at 5, 7, 8 and 13 TeV in the dilepton channel. The measurements are compared to the next-to-next-to-leading order (NNLO) + next-to-next-to-logarithmic (NNLL) theory predictions.

The LHCP conference takes place this week in Lund, Sweden, where the CMS Collaboration will present 15 new results for the first time, including a mix of searches for new physics and standard model measurements at 13 TeV. CMS also summarizes its detector and physics object performance on recently collected 2016 data, demonstrating that the collaboration has emerged from the winter shutdown ready for discovery physics (Fig. 1).

The search for new physics in 13 TeV proton collisions continues in earnest, with six new results presented this week. A combined search for high-mass resonances decaying to the Zγ final state, with Z bosons decaying to leptons, in the 8 and 13 TeV data sets yields no significant deviation from background expectations for masses ranging from a few hundred GeV to 2 TeV (EXO-16-021). A similar search in the same channel, but with Z bosons decaying to quarks, will also be shown this week (EXO-16-020). CMS has searched for heavy Z’ bosons that decay preferentially to third-generation fermions, including decays to pairs of top quarks (B2G-15-003) and leptons (EXO-16-008). The top quark-pair analysis searches in the all-hadronic final state, where the highly boosted top quarks are reconstructed as single jets, using special techniques, while the search in the lepton channel is carried out in four final states depending on the decay mode. No significant signals are observed in either search, resulting in the exclusion of Z’ bosons up to 3.3 (3.8) TeV for widths of 10 (30)% relative to the mass in the top search, and 2.1 TeV in the τ lepton search. Another search using the τ lepton targets heavy neutrinos from righthanded W bosons and third-generation scalar leptoquarks in events containing jets and two hadronically decaying taus. This is the first such search for heavy neutrinos using τ leptons, and we find the data well described by standard model backgrounds.

CMS continues to probe for possible dark matter candidates produced in proton collisions, most recently in the final state including top quarks (EXO-16-017) or photons (EXO-16-014), and missing energy. The data are consistent with standard model backgrounds and limits are placed on model parameters associated with the dark matter and graviton hypotheses. A search for supersymmetric particles in the lepton plus jets final state is also presented for the first time this week (SUS-16-011). This analysis targets so-called ‘compressed spectra’ in which weakly interacting supersymmetric particles can have similar masses, giving rise to muons and electrons with very low transverse momentum (5-20 GeV). No significant signals are observed and limits are placed on the masses of top squarks and gluinos under various assumptions about the mass splittings of the intermediate states. Finally, a search for a heavy vector-like top quark T decaying to a standard top quark and a Higgs boson (B2G-16-005) is presented for the first time at LHCP. For T masses above 1 TeV, the top quark and Higgs boson are highly boosted and their decay products are reconstructed using similar techniques as in B2G-15-003. Here also the data are consistent with background expectations and limits are set on the product of cross section and branching fraction for T masses in the range 1.0 - 1.8 TeV.

Several new standard model measurements are shown for the first time at LHCP, including the first measurement of the top-quark cross section at 5 TeV (TOP-16-015), exploiting data collected during a special proton-proton reference run in 2015 (Fig. 2). A first measurement by CMS of the WW diboson cross section at 13 TeV is also reported (SMP-16-006), where the precision has already reached better than 10%. Finally, three new results on Higgs boson physics are presented for the first time, including the first searches at 13 TeV for vector boson fusion Higgs boson production in the bottom quark decay channel (HIG-16-003), a search for Higgs bosons produced in the context of the MSSM model that decay via the lepton channel (HIG-16-006). A first look at Higgs lepton-flavor-violating decays in the 13 TeV data (HIG-16-005), using the μτ channel, does not confirm a slight (2.4σ) excess observed in Run 1, although more data is needed to make a definitive conclusion.