21 Search Results

A search for new physics in events featuring a single photon and missing transverse momentum is presented, using proton-proton √𝑠 =13 TeV collision data corresponding to an integrated luminosity of 101 fb⁻¹ collected by the CMS experiment at the CERN LHC between 2017 and 2018. This analysis, combined with a previous study of 36 fb⁻¹ of 2016 data (totaling 137 fb⁻¹), reveals no significant deviations from standard model expectations. The results are then used to establish 95% confidence level limits on parameters in theoretical models involving dark matter and large extra dimensions. Compared to the 2016-only analysis, this search achievesmore » up to a 14% improvement in exclusion reach for mediator masses in simplified dark matter models, along with 11% and 10% enhancements in the limits on the effective field theory suppression scale and the fundamental Planck scale, respectively. These results are the most stringent constraints on these parameters to date.« less

A hot and dense state of nuclear matter, known as the quark-gluon plasma, is created in collisions of ultrarelativistic heavy nuclei. Highly energetic quarks and gluons, collectively referred to as partons, lose energy as they travel through this matter, leading to suppressed production of particles with large transverse momenta (𝑝_T). Conversely, high-𝑝_T particle suppression has not been seen in proton-lead collisions, raising questions regarding the minimum system size required to observe parton energy loss. Oxygen-oxygen (OO) collisions examine a region of effective system size that lies between these two extreme cases. The CMS detector at the CERN LHC has beenmore » used to quantify charged-particle production in inclusive OO collisions for the first time via measurements of the nuclear modification factor (𝑅_AA). The 𝑅_AA is derived by comparing particle production to expectations based on proton-proton (𝑝⁢𝑝) data and has a value of unity in the absence of nuclear effects. The data for OO and 𝑝⁢𝑝 collisions at a nucleon-nucleon center-of-mass energy $$\sqrt{s_{NN}}$$ = 5.36  TeV correspond to integrated luminosities of 6.1  nb⁻¹ and 1.02  pb⁻¹, respectively. The 𝑅_AA is below unity with a minimum of 0.69 ± 0.04 around 𝑝_T = 6  GeV. The data exhibit better agreement with theoretical models incorporating parton energy loss as compared to baseline models without energy loss.« less

At hadron colliders, the net transverse momentum of particles that do not interact with the detector (missing transverse momentum, $$^→_𝑝$$$$^{miss}_{T}$$) is a crucial observable in many analyses. In the standard model, $$^→_𝑝$$$$^{miss}_{T}$$ originates from neutrinos. Many beyond-the-standard-model particles, such as dark matter candidates, are also expected to leave the experimental apparatus undetected. This paper presents a novel deep neural network based $$^→_𝑝$$$$^{miss}_{T}$$ estimator, DeepMET, developed by the CMS Collaboration at the LHC. The DeepMET algorithm produces a weight for each reconstructed particle based on its properties. The estimator is based on the negative vector sum of the weighted transverse momentamore » of all reconstructed particles in an event. Compared with other estimators currently employed by CMS, DeepMET improves the $$^→_𝑝$$$$^{miss}_{T}$$ resolution by 10%–30%, shows improvement for a wide range of final states, is easier to train, and is more resilient against the effects of additional proton-proton interactions accompanying the collision of interest.« less

A search is presented for narrow resonances, with a mass between 0.6 and 1.8 TeV, decaying to pairs of jets, in proton-proton collisions at $$\sqrt{s}=13$$ TeV. The search is performed using dijets that are reconstructed, selected, and recorded in a compact form by the high-level trigger in a technique referred to as “data scouting”, from data collected in 2016–2018 corresponding to an integrated luminosity of 117 fb⁻¹. The dijet mass spectra are well described by a smooth parameterization, and no significant evidence for the production of new particles is observed. Model-independent upper limits are presented on the product of themore » cross section, branching fraction, and acceptance for the individual cases of narrow quark-quark, quark-gluon, and gluon-gluon resonances, and are compared to the predictions from a variety of models of narrow dijet resonance production. The upper limit on the coupling of a dark matter mediator to quarks is presented as a function of the mediator mass. The sensitivity of this search goes beyond what is expected from statistical scaling with the integrated luminosity alone, as a consequence of the use of fewer parameters in the background function within a more robust statistical procedure.« less

Inclusive and differential cross section measurements of top quark pair $$(\textrm{t}\overline{\textrm{t}})$$ production in association with a photon (γ) are performed as a function of lepton, photon, top quark, and $$\textrm{t}\overline{\textrm{t}}$$ kinematic observables, using data from proton-proton collisions at $$\sqrt{s}=13$$ TeV, corresponding to an integrated luminosity of 138 fb⁻¹, collected at the CERN LHC with the CMS detector. Events containing two leptons (electrons or muons) and a photon in the final state are considered. The fiducial cross section of $$\textrm{t}\overline{\textrm{t}}$$γ is measured to be 137 ± 8 fb, in a phase space including events with a high momentum, isolated photon. Themore » fiducial cross section of $$\textrm{t}\overline{\textrm{t}}$$γ is also measured to be 56 ± 5 fb when considering only events where the photon is emitted in the production part of the process. Both measurements are in agreement with the theoretical predictions, of 126 ± 19 fb and 57 ± 5 fb, respectively. Differential measurements are performed at the particle and parton levels. Additionally, inclusive and differential ratios between the cross sections of $$\textrm{t}\overline{\textrm{t}}$$γ and $$\textrm{t}\overline{\textrm{t}}$$ production are measured. The inclusive ratio is found to be 0.0133 ± 0.0005, in agreement with the standard model prediction of 0.0127 ± 0.0008. The top quark charge asymmetry in $$\textrm{t}\overline{\textrm{t}}$$γ production is also measured to be −0.012 ± 0.042, compatible with both the standard model prediction and with no asymmetry.« less

A search for signatures of a dark analog to quantum chromodynamics is performed. The analysis targets long-lived dark mesons that decay into standard-model particles, with a high branching fraction of the dark mesons decaying into muons. The dark mesons are formed by the hadronisation of dark partons, which are produced by a decay of the Higgs boson. The search is performed using a data set corresponding to an integrated luminosity of 41.6 fb⁻¹, which was collected in proton-proton collisions at $$\sqrt{s}=13$$ TeV by the CMS experiment at the CERN LHC in 2018 using non-prompt muon triggers. The search is basedmore » on resonant muon pair signatures. Machine-learning techniques are employed in the analysis, utilising boosted decision trees to discriminate between signal and background. No significant excess is observed above the standard model expectation. Upper limits on the branching fraction of the Higgs boson decaying to dark partons are determined to be as low as 10−4 at 95% confidence level, surpassing and extending the existing limits on models with dark $$\tilde{ω}$$ mesons for mean proper decay lengths of less than 500 mm and for $$\tilde{ω}$$ masses down to 0.3 GeV. First limits are set for extended dark-shower models with two dark flavours that contain dark photons, probing their masses down to 0.33 GeV.« less

Measurements of the spin correlation coefficients in the beam basis are presented for top quark-antiquark $$(t\bar{t}⁢)$$ systems produced in proton-proton collisions at $$\sqrt{s}$$ = 13 TeV collected by the CMS experiment in 2016–2018, and corresponding to an integrated luminosity of 138 fb⁻¹. The $$t\bar{t}$$ system is reconstructed from final states containing an electron or muon and jets. Together with the previously reported results in the helicity basis, these measurements are used to decompose the system into the Bell and spin eigenstates in various kinematic regions. The spin correlation coefficients are also used to evaluate properties of the $$t\bar{t}$$ quantum state,more » such as the purity, von Neumann entropy, and entanglement. All results are consistent with standard model predictions.« less

An analysis of the flavour structure of dimension-6 effective field theory (EFT) operators in multilepton final states is presented, focusing on the interactions of quarks with Z bosons. For the first time, the flavour structure of these operators is disentangled by simultaneously probing the interactions with different quark generations. The analysis targets the associated production of a top quark pair and a Z boson, as well as diboson processes in final states with at least three leptons, which can be electrons or muons. The data were recorded by the CMS experiment in the years 2016–2018 in proton-proton collisions at amore » centre-of-mass energy of 13 TeV and correspond to an integrated luminosity of 138 fb⁻¹. Consistency with the standard model of particle physics is observed and limits are set on the selected Wilson coefficients, split into couplings to light- and heavy-quark generations.« less

Measurements of properties of top quark-antiquark pair production in association with a W boson in proton-proton collisions at a center-of-mass energy of 13 TeV are presented, using a data sample corresponding to an integrated luminosity of 138 fb⁻¹, recorded by the CMS experiment at the CERN LHC. Events are selected based on the presence of either two leptons with the same electric charge or three leptons, and multiple jets and b-tagged jets. We present measurements of differential production cross sections as a function of kinematic variables sensitive to different aspects of the process modeling, using a multivariate discriminator in themore » two-lepton selection region and a simple selection-based method in the three-lepton region. The normalized cross section measurements are generally consistent with the standard model expectations, while we observe larger values compared to the expectations in the absolute cross section measurements, consistent with previous inclusive cross section measurements. In addition, we measure the leptonic charge asymmetry of this process, obtaining an observed value of $${A}_c^{\ell }=-{0.19}_{-0.18}^{+0.16}$$, consistent with the expectation of −0.085 ± 0.006 predicted by next-to-leading order simulations.« less

A model-independent measurement of the differential production cross section of the Higgs boson decaying into a pair of W bosons, with a final state including two jets produced in association, is presented. In the analysis, events are selected in which the decay products of the two W bosons consist of an electron, a muon, and missing transverse momentum. The model independence of the measurement is maximized by employing a discriminating variable, developed through machine learning, that is agnostic to the signal hypothesis. The analysis is based on proton-proton collision data at $$\sqrt{s}=13$$ TeV collected with the CMS detector from 2016–2018,more » corresponding to an integrated luminosity of 138 fb⁻¹. The production cross section is measured as a function of the difference in azimuthal angle between the two jets. The differential cross section measurements are used to constrain Higgs boson couplings within the standard model effective field theory framework.« less