240 Search Results

The ATLAS experiment at the Large Hadron Collider explores the use of modern neural networks for a multi-dimensional calibration of its calorimeter signal defined by clusters of topologically connected cells (topo-clusters). The Bayesian neural network (BNN) approach not only yields a continuous and smooth calibration function that improves performance relative to the standard calibration but also provides uncertainties on the calibrated energies for each topo-cluster. The results obtained by using a trained BNN are compared to the standard local hadronic calibration and to a calibration provided by training a deep neural network. The uncertainties predicted by the BNN are interpretedmore » in the context of a fractional contribution to the systematic uncertainties of the trained calibration. They are also compared to uncertainty predictions obtained from an alternative estimator employing repulsive ensembles.« less

The Precision Proton Spectrometer (PPS) of the CMS and TOTEM experiments collected 107.7 fb$$^{-1}$$ in proton-proton (pp) collisions at the LHC at 13 TeV (Run 2). This paper describes the key features of the PPS alignment and optics calibrations, the proton reconstruction procedure, as well as the detector efficiency and the performance of the PPS simulation. The reconstruction and simulation are validated using a sample of (semi)exclusive dilepton events. The performance of PPS has proven the feasibility of continuously operating a near-beam proton spectrometer at a high luminosity hadron collider.

A search for nonresonant Higgs boson ($$H$$) pair production via gluon and vector boson ($$V$$) fusion is performed in the four-bottom-quark final state, using proton-proton collision data at 13 TeV corresponding to 138 fb$$^{−1}$$ collected by the CMS experiment at the LHC. The analysis targets Lorentz-boosted $$H$$ pairs identified using a graph neural network. It constrains the strengths relative to the standard model of the $$H$$ self-coupling and the quartic VVHH couplings, $$κ_{2V}$$, excluding $$κ_{2V} = 0$$ for the first time, with a significance of 6.3 standard deviations when other H couplings are fixed to their standard model values.

A search for pairs of dijet resonances with the same mass is conducted in final states with at least four jets. Results are presented separately for the case where the four jet production proceeds via an intermediate resonant state and for nonresonant production. The search uses a data sample corresponding to an integrated luminosity of 138 fb$$^{−1}$$ collected by the CMS detector in proton-proton collisions at $$ \sqrt{s} $$ = 13 TeV. Model-independent limits, at 95% confidence level, are reported on the production cross section of four-jet and dijet resonances. These first LHC limits on resonant pair production of dijetmore » resonances via high mass intermediate states are applied to a signal model of diquarks that decay into pairs of vector-like quarks, excluding diquark masses below 7.6 TeV for a particular model scenario. There are two events in the tails of the distributions, each with a four-jet mass of 8 TeV and an average dijet mass of 2 TeV, resulting in local and global significances of 3.9 and 1.6 standard deviations, respectively, if interpreted as a signal. The nonresonant search excludes pair production of top squarks with masses between 0.50 TeV to 0.77 TeV, with the exception of a small interval between 0.52 and 0.58 TeV, for supersymmetric R-parity-violating decays to quark pairs, significantly extending previous limits. Here, the most significant excess above the predicted background occurs at an average dijet mass of 0.95 TeV, for which the local and global significances are 3.6 and 2.5 standard deviations, respectively.[graphic not available: see fulltext]« less

The first measurements of the Fourier coefficients ( $V_{n\mathrm{\Delta }}$ ) of the azimuthal distributions of charged hadrons emitted from photon-proton (γp) interactions are presented. The data are extracted from 68.8 nb⁻¹ of ultra-peripheral proton-lead (pPb) collisions at $\sqrt{s_{{}_{\mathrm{NN}}}}=8.16\text{TeV}$ using the CMS detector. The high energy lead ions produce a flux of photons that can interact with the oncoming proton. This γp system provides a set of uniquemore » initial conditions with multiplicity lower than in photon-lead collisions but comparable to recent electron-positron and electron-proton data. The $V_{n\mathrm{\Delta }}$ coefficients are presented in ranges of event multiplicity and transverse momentum ( $p_{\mathrm{T}}$ ) and are compared to corresponding hadronic minimum bias pPb results. For a given multiplicity range, the mean $p_{\mathrm{T}}$ of charged particles is smaller in γp than in pPb collisions. For both the γp and pPb samples, $V_{1\mathrm{\Delta }}$ is negative, $V_{2\mathrm{\Delta }}$ is positive, and $V_{3\mathrm{\Delta }}$ consistent with 0. For each multiplicity and $p_{\mathrm{T}}$ range, $V_{2\mathrm{\Delta }}$ is larger for γp events. The γp data are consistent with model predictions that have no collective effects.« less

A search for the exotic decay of the Higgs boson to a pair of light pseudoscalars, each of which subsequently decays into a pair of photons, is presented. The search uses data from proton-proton collisions at $$ \sqrt{s} $$ = 13 TeV recorded with the CMS detector at the LHC that corresponds to an integrated luminosity of 132 fb$$^{−1}$$. The analysis probes pseudoscalar bosons with masses in the range 15–62 GeV, coming from the Higgs boson decay, which leads to four well-isolated photons in the final state. No significant deviation from the background-only hypothesis is observed. Upper limits are setmore » on the product of the Higgs boson production cross section and branching fraction into four photons. The observed (expected) limits range from 0.80 (1.00) fb for a pseudoscalar boson mass of 15 GeV to 0.26 (0.24) fb for a mass of 62 GeV at 95% confidence level.[graphic not available: see fulltext]« less

Results are presented from a search for CP violation in top quark pair production, using proton-proton collisions at a center-of-mass energy of 13 TeV. The data used for this analysis consist of final states with two charged leptons collected by the CMS experiment, and correspond to an integrated luminosity of 35.9 fb$$^{−1}$$. The search uses two observables, $$ \mathcal{O} _{1}$$ and $$ \mathcal{O} _{3}$$, which are Lorentz scalars. The observable $$ \mathcal{O} _{1}$$ is constructed from the four-momenta of the charged leptons and the reconstructed top quarks, while $$ \mathcal{O} _{3}$$ consists of the four-momenta of the charged leptons andmore » the b quarks originating from the top quarks. Asymmetries in these observables are sensitive to CP violation, and their measurement is used to determine the chromoelectric dipole moment of the top quark. The results are consistent with the expectation from the standard model.[graphic not available: see fulltext]« less

Results are presented from a search for the Higgs boson decay H → Zγ, where Z → ℓ$$^{+}$$ℓ$$^{−}$$ with ℓ = e or μ. The search is performed using a sample of proton-proton (pp) collision data at a center-of-mass energy of 13 TeV, recorded by the CMS experiment at the LHC, corresponding to an integrated luminosity of 138 fb$$^{−1}$$. Events are assigned to mutually exclusive categories, which exploit differences in both event topology and kinematics of distinct Higgs production mechanisms to enhance signal sensitivity. The signal strength μ, defined as the product of the cross section and the branching fractionmore » $$ \left[\sigma \left(\textrm{pp}\to \textrm{H}\right)\mathcal{B}\left(\textrm{H}\to \textrm{Z}\upgamma \right)\right] $$ relative to the standard model prediction, is extracted from a simultaneous fit to the ℓ$$^{+}$$ℓ$$^{−}$$γ invariant mass distributions in all categories and is measured to be μ = 2.4 ± 0.9 for a Higgs boson mass of 125.38 GeV. The statistical significance of the observed excess of events is 2.7 standard deviations. This measurement corresponds to $$ \left[\sigma \left(\textrm{pp}\to \textrm{H}\right)\mathcal{B}\left(\textrm{H}\to \textrm{Z}\upgamma \right)\right]=0.21\pm 0.08 $$ pb. The observed (expected) upper limit at 95% confidence level on μ is 4.1 (1.8), where the expected limit is calculated under the background-only hypothesis. The ratio of branching fractions $$ \mathcal{B}\left(\textrm{H}\to \textrm{Z}\upgamma \right)/\mathcal{B}\left(\textrm{H}\to \upgamma \upgamma \right) $$ is measured to be $$ {1.5}_{-0.6}^{+0.7} $$, which agrees with the standard model prediction of 0.69 ± 0.04 at the 1.5 standard deviation level.[graphic not available: see fulltext]« less

An inclusive search for long-lived exotic particles decaying to a pair of muons is presented. The search uses data collected by the CMS experiment at the CERN LHC in proton-proton collisions at $$ \sqrt{s} $$ = 13 TeV in 2016 and 2018 and corresponding to an integrated luminosity of 97.6 fb$$^{−1}$$. The experimental signature is a pair of oppositely charged muons originating from a common secondary vertex spatially separated from the pp interaction point by distances ranging from several hundred μm to several meters. The results are interpreted in the frameworks of the hidden Abelian Higgs model, in which themore » Higgs boson decays to a pair of long-lived dark photons Z$$_{D}$$, and of a simplified model, in which long-lived particles are produced in decays of an exotic heavy neutral scalar boson. For the hidden Abelian Higgs model with m(Z$$_{D}$$) greater than 20 GeV and less than half the mass of the Higgs boson, they provide the best limits to date on the branching fraction of the Higgs boson to dark photons for cτ(Z$$_{D}$$) (varying with m(Z$$_{D}$$)) between 0.03 and ≈0.5 mm, and above ≈0.5 m. Our results also yield the best constraints on long-lived particles with masses larger than 10 GeV produced in decays of an exotic scalar boson heavier than the Higgs boson and decaying to a pair of muons.[graphic not available: see fulltext]« less

A precise measurement of the invisible width of the Z boson produced in proton-proton collisions at a center-of-mass energy of 13 TeV is presented using data recorded by the CMS experiment at the LHC, corresponding to an integrated luminosity of 36.3 ${\text{fb}}^{-1}$ . The result is obtained from a simultaneous fit to kinematic distributions for two data samples of Z boson plus jets: one dominated by Z boson decays to invisible particles and the other by Z boson decays to muon and electron pairs. The invisible width is measured to be $more »$ 523±3(stat)±16(syst)MeV . This result is the first precise measurement of the invisible width of the Z boson at a hadron collider, and is the single most precise direct measurement to date, competitive with the combined result of the direct measurements from the LEP experiments.« less