CMS highlights from the fourth LHCP conference

8 July 2016

The CMS collaboration presented 15 new results at the fourth annual Large Hadron Collider Physics (LHCP) conference on 13–18 June in Lund, Sweden. The results included a mixture of searches for new physics and Standard Model measurements at a centre-of-mass energy of 13 TeV. CMS also summarized 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.

The search for new physics in 13 TeV proton collisions continues in earnest, with six new results presented at LHCP. 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, produced a similar conclusion (EXO-16-020). CMS has also searched for heavy Z´ bosons that decay preferentially to third-generation fermions, including decays to pairs of top quarks (B2G-15-003) and τ leptons (EXO-16-008), and found no excess above the Standard Model prediction.

The top quark-pair analysis uses special techniques to search the all-hadronic final state, where the highly boosted top quarks are reconstructed as single jets, while the search in the τ 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 a mass of 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 looks for heavy neutrinos from right-handed 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 CMS finds the data well described by Standard Model backgrounds.

CMS continues to probe for possible dark-matter candidates, most recently in final states that contain top quarks (EXO-16-017) or photons (EXO-16-014) plus 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 was also presented for the first time (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. 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) was 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 the data are also consistent with background expectations, allowing CMS to set limits on the product of the cross section and branching fraction for T masses in the range 1.0–1.8 TeV.

Several new Standard Model measurements were shown for the first time at LHCP, including the first measurement of the top-quark cross section at 5 TeV (TOP-16-015) based on data collected during a special proton–proton reference run in 2015 (figure 1). A first measurement by CMS of the WW di-boson cross-section at 13 TeV was also reported (SMP-16-006), where the precision has already reached better than 10%. Finally, three new results on Higgs boson physics were presented for the first time, including the first searches at 13 TeV for vector boson fusion Higgs production in the bottom quark decay channel (HIG-16-003) and a search for Higgs bosons produced in the context of the MSSM model that decay via the τ 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.

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