Experimental results from CMS

We present some of the latest results of the CMS experiment, covering analyses in QCD multijet, top, bottom, Higgs, forward/small-x QCD, heavyion, exotic, and supersymmetry physics utilizing LHC integrated luminosity up to ∼80 fb−1.


Introduction
CMS, as a major general-purpose detector at the LHC, provides an abundance of high-quality data that enable the performance of analyses on a plethora of physics topics. We present some of the legacy and most recent results with some emphasis on Quantum Chromodynamics (QCD) relevance.

QCD multijet physics
Double-differential inclusive jet production cross sections have been measured with 8 TeV [1] and 13-TeV [2] data, corresponding to luminosity of 19.7 fb −1 and 71 pb −1 respectively. Particle-flow [3] jets are reconstructed with the anti-k t algorithm [4] with R distance parameter of 0.4 and 0.7. Results demonstrate excellent understanding of QCD next-to-leading order (NLO) fixed-order calculations corrected for non-perturbative (NP) effects from parton showering, hadronization and multiparton interactions. As shown in Figs. 1 and 2, the Standard Model (SM) agrees with the observed distributions for a cross-section range of 18 orders of magnitude, up to 2 TeV in jet-transverse momentum p T and in seven absolute rapidity y bins up to 4.7, for jets reconstructed with both R=0. 4 and R=0.7.
The triple-differential dijet cross section, measured as a function of p T , the difference, and sum of jet rapidities with 19.7-fb −1 , 8-TeV data [6], is not described sufficiently by NLO QCD calculations with NP and electroweak corrections. This fact is used for constraining the proton parton-distribution functions (PDF). The inclusion of these CMS results in the PDF fits changes the gluon PDF shape at lower x values and reduces the systematic uncertainty effect at higher x, compared to deep-inelastic-scattering-only fits, as shown in Fig. 3.
A measurement of the ratio of inclusive 3-jet to 2-jet cross section with R=0.7 jets with 19.7-fb −1 data from 8-TeV collisions provides an accurate measurement of the strong coupling constant a s [7], due to canceling common systematic uncertainties on the numerator       and denominator quantities. The analysis is done in four average leading-two-jet p T bins, resulting in measurement of a S for four values of energy scale Q, as shown in Fig. 4.
The production of a photon in association with jets is an excellent test of QCD. Inclusivephoton double-differential and photon+jet triple-differential measurements (shown in Fig. 5) with 2.26-fb −1 , 13-TeV data agree well with the NLO-in-QCD JetPhox generator [8].
The same luminosity was used for the measurement of the inclusive dijet cross section as a function of groomed and ungroomed jet mass, for 11 p T bins of R=0.8 jets, as a test of soft QCD effects [9]. Good agreement with Pythia8 is observed, especially for groomed masses below 200 GeV, and comparisons with theoretical calculations are made, as shown in Fig. 6.
The measurement of the jet charge, defined as the sum of jet-constituent charge with three different variations of momentum weights, has been measured in 8 TeV dijet data that correspond to luminosity of 19.7 fb −1 [10].

Top physics
A 13-TeV, 35.8-fb −1 analysis looks for tt events in final states with lepton+jets [11]. Along with the lepton, at least 4 jets are required, at least two of which should be b-tagged. The  presence of extra jets is studied as a QCD test and for the determination of backgrounds in new-physics searches. Main background comes from QCD multijet events, bosons+jets and other tt channels. Powheg, with Pythia8 showering and hadronization, describes the jet multiplicity, as shown in Fig. 7, but some kinematics are only consistent with prediction after the inclusion of theoretical uncertainties in the unfolding.
The corresponding decays of the top pair to dileptons were also recently analyzed with the same luminosity [12] by requiring two leptons and at least two jets, one of which b-tagged. The analysis was performed in separate lepton-flavor channels which were eventually combined. Main backgrounds come from top+W, W/Z+jet, diboson and tt+W. Good agreement in jet-multiplicities compared to Powheg+Pythia8 were observed, as shown in Fig. 8, and the top chromomagnetic dipole moment and its y-charge asymmetry were determined.

Higgs physics
The current CMS combined RunII measured Higgs boson cross section divided by the SM prediction (signal strength µ) is 1.17 ± 0.1 [14]. Improvements in the precision of the ggH production rate of around 50%, compared with RunI, has been achieved due to higher luminosity and collider energy. Fig. 9 shows the good agreement of measured cross section with SM prediction for all Higgs production and decay channels.
The combination of 13-TeV data with the 7-and 8-TeV data allowed for the first observation of the Higgs boson production in association with a top-quark pair [15]. This is a 5.2-σ discovery (expected 4.2-σ). All Higgs-decay channels signal strength agree with expectation, as shown in Fig. 10.
Latest results include the decay H → ZZ →  The production of Higgs in association with a vector boson and the subsequent Higgs decay to two tau leptons was recently completed, at 13 TeV with 35.9 fb −1 [17]. At least one of the tau hadrons is required to decay hadronically. A 2.3-sigma excess in the signal strength was observed (µ = 2.54 +1.35 −1.26 ). The results are combined with earlier H → ττ analyses.

Heavy-ion physics
The CMS maintains a very active and productive heavy-ion collisions program. One of the latest released results measures the center-of-mass pseudorapidity distribution of the muon from W ± boson decays in 8.16-TeV pPb collisions [18]. This analysis probes the nuclear PDFs of u and d quarks. A shown in Fig. 11, modified PDFs are needed to describe the pseudorapidity distributions. A recent study of anisotropic flow in XeXe collisions at 5.44 TeV and comparison with the respective PbPb results probes the system-size dependence and the role of initial state effects [19].

B physics
Bottomonia help us study perturbation and lattice calculations of non-relativistic potentials. An analysis of 13-TeV data using 80 fb −1 of luminosity [21] led to the first observation of the resolved χ b1 (3P) and χ b2 (3P) resonances. Decays to (Y(3S ) → µµ)(γ → ee) were  considered. The separation was achieved after a photon-energy scale calibration that utilizes χ b (nP) decays to Y(nS )γ leading to a measured Y(3S )γ invariant mass resolution of 2.2 MeV, as shown in Fig. 13(a). The invariant mass is shown in Fig. 13(b).

Standard Model overview
An overview of the measurement of production cross sections of SM processes with a great variety of final-state objects [23] shows good agreement with predictions for 7-, 8-, and 13-TeV data.

Exotic and supersymmetry physics
CMS runs a diverse program of analyses dedicated to direct discovery of new physics, either model-independent or described by existing supersymmetric (SUSY) or exotic models.
A 13-TeV, 35.9-fb −1 analysis looks for pair-produced resonances decaying to diquark pairs [24]. The four jets are either reconstructed as boosted dijets, with use of appropriate grooming, for resonances below 400 GeV or they are fully resolved above 400 GeV. A version of the analysis also requires two of the jets to be b-tagged. The results are consistent with expectation and limits are set on R-parity violating SUSY production pair of scalar top quarks. Fig. 14(a) shows the limit for the sub-analysis that does not require b-tagged jets.
Using multijet events in 13-TeV, 38.5-fb −1 data , an analysis searched for potential production and decay of long-lived particles through the detection of displaced vertices in the tracker [25]. The results are interpreted in R-parity violating SUSY as pair production of pairs of scalar-top quarks or gluinos or neutralinos. Cross section upper limits are set as a function of the mass and lifetime of the particles. For masses between 800 and 2600 GeV and mean decay lengths between 1 and 40 mm, cross sections above 0.3 fb are excluded at 95% confidence level.
A search for heavy, narrow resonances decaying to a Higgs boson and a photon has been completed with 13-TeV, 35.9-fb −1 data [26]. Investigating masses above 700 GeV leads to boosted Higgs bosons which decay to bb pairs reconstructed as single "fat" jet. Anti-k t jets A 13-TeV, 16.1-fb −1 analysis [27] looks for dark quarks, which are the decay products of heavy mediators in the BSSW dark-QCD model. The signature consists of 4 high-p T jets two from ordinary quarks and two from dark quarks, which result in emerging jets with many displaced vertices from the decay of dark hadrons to the SM charged particles. Mediator particles with masses between 400 and 1250 GeV are excluded for dark hadron decay lengths between 5 and 225 mm. Fig. 14(b) shows exclusion limits for models with a 1-GeV dark pion.
A 13-TeV, 35.9-fb −1 analysis looks for same-sign dimuons with at least two jets [28]. Results are consistent with SM expectation and limits are set in R-parity-violating single chargino or neutralino production in association with a muon, as shown in Fig. 14(c).