506 Search Results

This paper describes the experience with the calibration,reconstruction and evaluation of the timing capabilities of the CMSHGCAL prototype in the beam tests in 2018. The calibrationprocedure includes multiple steps and corrections ranging from tensof nanoseconds to a few hundred picoseconds. The timing performanceis studied using signals from positron beam particles with energiesbetween 20 GeV and 300 GeV. The performance is studied as afunction of particle energy against an external timing reference aswell as standalone by comparing the two different halves of theprototype. The timing resolution is found to be 60 ps forsingle-channel measurements and better than 20 ps for fullmore » showersat the highest energies, setting excellent perspectives for theHGCAL calorimeter performance at the HL-LHC.« less

A search for quantum black holes in electron + jet and muon + jet invariant mass spectra is performed with 140 fb^-1 of data collected by the ATLAS detector in proton-proton collisions at $$\sqrt{s}$$ =13 TeV at the Large Hadron Collider. The observed invariant mass spectrum of lepton + jet pairs is consistent with Standard Model expectations. Upper limits are set at 95% confidence level on the production cross section times branching fractions for quantum black holes decaying into a lepton and a quark in a search region with invariant mass above 2.0 TeV. The resulting quantum black hole lowermore » mass threshold limit is 9.2 TeV in the Arkani-Hamed-Dimopoulos-Dvali model, and 6.8 TeV in the Randall-Sundrum model.« less

A search is presented for single production of a vector-like B quark decaying into a Standard Model b-quark and a Standard Model Higgs boson, which decays into a $$b\overline{b}$$ pair. The search is carried out in 139 fb^-1 of $$\sqrt{s}$$ = 13 TeV proton-proton collision data collected by the ATLAS detector at the LHC between 2015 and 2018. No significant deviation from the Standard Model background prediction is observed, and mass-dependent exclusion limits at the 95% confidence level are set on the resonance production cross-section in several theoretical scenarios determined by the couplings c_W, c_Z and c_H between the Bmore » quark and the Standard Model W, Z and Higgs bosons, respectively. For a vector-like B occurring as an isospin singlet, the search excludes values of c_W greater than 0.45 for a B resonance mass (m_B) between 1.0 and 1.2 TeV. For 1.2 TeV < m_B < 2.0 TeV, c_W values larger than 0.50–0.65 are excluded. If the B occurs as part of a (B, Y) doublet, the smallest excluded c_Z coupling values range between 0.3 and 0.5 across the investigated resonance mass range 1.0 TeV < m_B < 2.0 TeV.« less

As part of its HL-LHC upgrade program, the CMS collaboration is replacing its existing endcap calorimeters with a high-granularity calorimeter (CE). The new calorimeter is a sampling calorimeter with unprecedented transverse and longitudinal readout for both electromagnetic and hadronic compartments. Due to its compactness, intrinsic time resolution, and radiation hardness, silicon has been chosen as active material for the regions exposed to higher radiation levels. The silicon sensors are fabricated as 20 cm (8”) wide hexagonal wafers and are segmented into several hundred pads which are read out individually. As part of the sensor qualification strategy, 8” sensor irradiation withmore » neutrons has been conducted at the Rhode Island Nuclear Science Center (RINSC) and followed by their electrical characterisation in 2020-21. The completion of this important milestone in the CE's R&D program is documented in this paper and it provides detailed account of the associated infrastructure and procedures.The results on the electrical properties of the irradiated CE silicon sensors are presented.« less

The upgrade of the CMS experiment for the high luminosity operation of the LHC comprises the replacement of the current endcap calorimeter by a high granularity sampling calorimeter (HGCAL). The electromagnetic section of the HGCAL is based on silicon sensors interspersed between lead and copper (or copper tungsten) absorbers. The hadronic section uses layers of stainless steel as an absorbing medium and silicon sensors as an active medium in the regions of high radiation exposure, and scintillator tiles directly read out by silicon photomultipliers in the remaining regions. As part of the development of the detector and its readout electronicmore » components, a section of a silicon-based HGCAL prototype detector along with a section of the CALICE AHCAL prototype was exposed to muons, electrons and charged pions in beam test experiments at the H2 beamline at the CERN SPS in October 2018. The AHCAL uses the same technology as foreseen for the HGCAL but with much finer longitudinal segmentation. The performance of the calorimeters in terms of energy response and resolution, longitudinal and transverse shower profiles is studied using negatively charged pions, and is compared to GEANT4 predictions. This is the first report summarizing results of hadronic showers measured by the HGCAL prototype using beam test data.« less

A measurement of the top quark pair-production cross section in the lepton + jets decay channel is presented. It is based on 4.6 fb^-1of $$\sqrt{s}$$ = 7 TeV pp collision data collected during 2011 by the ATLAS experiment at the CERN Large Hadron Collider. A three-class, multidimensional event classifier based on support vector machines is used to differentiate $$t\bar{t}$$ events from backgrounds. The $$t\bar{t}$$ production cross section is found to be σ_$$t\bar{t}$$ = 168.5 ± 0.7(stat)$$^{+6.2}_{-5.9}$$(syst)$$^{+ 3.4}_{-3.2}$$(lumi) pb. In conclusion, the result is consistent with the Standard Model prediction based on QCD calculations at next-to-next-to-leading order.

We present measurements of cross sections for production of a leptonically decaying $$Z$$ boson in association with a large-radius jet in 13 TeV proton-proton collisions at the LHC, using 36 fb^–1 of data from the ATLAS detector. Integrated and differential cross sections are measured at particle level in both a flavor inclusive and a doubly $$b$$-tagged fiducial phase space. The large-radius jet mass and transverse momentum, its kinematic relationship to the Z boson, and the angular separation of $$b$$-tagged small-radius track jets within the large-radius jet are measured. This measurement constitutes an important test of perturbative quantum chromodynamics in kinematicmore » and flavor configurations relevant to several Higgs boson and beyond-Standard-Model physics analyses. The results highlight issues with modeling of additional hadronic activity in the flavor-inclusive selection, and a distinction between flavor-number schemes in the $$b$$-tagged phase space.« less

A measurement of the top-quark mass ($$m_t$$) in the $$t\overline{t}$$ → lepton+jets channel is presented, with an experimental technique which exploits semileptonic decays of $$b$$-hadrons produced in the top-quark decay chain. The distribution of the invariant mass $$m_{{\ell}µ}$$ of the lepton, $$\ell$$ (with $$\ell$$ = $e, µ$), from the $$W$$-boson decay and the muon, $$µ$$, originating from the b-hadron decay is reconstructed, and a binned-template profile likelihood fit is performed to extract $$m_t$$. The measurement is based on data corresponding to an integrated luminosity of 36.1 fb^–1 of $$\sqrt{s}$$ = 13 TeV $pp$ collisions provided by the Large Hadron Collidermore » and recorded by the ATLAS detector. The measured value of the top-quark mass is $$m_t$$ = 174.41 ± 0.39 (stat.) ± 0.66 (syst.) ± 0.25 (recoil) GeV, where the third uncertainty arises from changing the Pythia8 parton shower gluon-recoil scheme, used in top-quark decays, to a recently developed setup.« less

Electroweak symmetry breaking explains the origin of the masses of elementary particles through their interactions with the Higgs field. Besides the measurements of the Higgs boson properties, the study of the scattering of massive vector bosons with spin 1 allows the nature of electroweak symmetry breaking to be probed. Among all processes related to vector-boson scattering, the electroweak production of two jets and a $$Z$$-boson pair is a rare and important one. Here we report the observation of this process from proton–proton collision data corresponding to an integrated luminosity of 139 fb^-1 recorded at a centre-of-mass energy of 13 TeVmore » with the ATLAS detector at the Large Hadron Collider. We consider two different final states originating from the decays of the $$Z$$-boson pair: one containing four charged leptons and another containing two charged leptons and two neutrinos. The hypothesis of no electroweak production is rejected with a statistical significance of 5.7$$σ$$, and the measured cross-section for electroweak production is consistent with the Standard Model prediction. In addition, we report cross-sections for inclusive production of a $$Z$$-boson pair and two jets for the two final states.« less

Searches are performed for nonresonant and resonant di-Higgs boson production in the $$b\bar{b}γγ$$ final state. The dataset used corresponds to an integrated luminosity of 139 fb^–1 of proton–proton collisions at a center-of-mass energy of 13 TeV recorded by the ATLAS detector at the CERN Large Hadron Collider. No excess above the expected background is found and upper limits on the di-Higgs boson production cross sections are set. A 95% confidence-level upper limit of 4.2 times the cross section predicted by the Standard Model is set on pp → HH nonresonant production, where the expected limit is 5.7 times the Standardmore » Model predicted value. The expected constraints are obtained for a background hypothesis excluding pp → HH production. The observed (expected) constraints on the Higgs boson trilinear coupling modifier κ_λ are determined to be [–1.5, 6.7] ([– 2.4, 7.7]) at 95% confidence level, where the expected constraints on κ_λ are obtained excluding pp → HH production from the background hypothesis. For resonant production of a new hypothetical scalar particle X (X → HH → $$b\bar{b}γγ$$), limits on the cross section for pp → X → HH are presented in the narrow-width approximation as a function of m_X in the range 251 GeV ≤ m_X ≤ 1000 GeV. The observed (expected) limits on the cross section for pp → X → HH range from 640 fb to 44 fb (391 fb to 46 fb) over the considered mass range.« less