220 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 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

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

Production cross sections of Image 1, Image 2, and Image 3 states decaying into Image 4 in proton-lead ( $\mathrm{p}\mathrm{Pb}$ ) collisions are reported using data collected by the CMS experiment at $\sqrt{s_{{}_{\mathrm{NN}}}}=5.02\text{TeV}$ . A comparison is made with corresponding cross sections obtained with $\mathrm{p}\mathrm{p}$ data measured at the same collision energy and scaled by the Pb nucleus mass number. The nuclear modification factor for Image 1 ismore » found to be Image 5. Similar results for the excited states indicate a sequential suppression pattern, such that Image 6. The suppression of all states is much less pronounced in $\mathrm{p}\mathrm{Pb}$ than in PbPb collisions, and independent of transverse momentum Image 7 and center-of-mass rapidity Image 8 of the individual Image 9 state in the studied range Image 10 and Image 11. Models that incorporate final-state effects of bottomonia in pPb collisions are in better agreement with the data than those which only assume initial-state modifications.« less

This paper presents updated Monte Carlo configurations used to model the production of single electroweak vector bosons (W, Z/γ$$^{∗}$$) in association with jets in proton-proton collisions for the ATLAS experiment at the Large Hadron Collider. Improvements pertaining to the electroweak input scheme, parton-shower splitting kernels and scale-setting scheme are shown for multi-jet merged configurations accurate to next-to-leading order in the strong and electroweak couplings. The computational resources required for these set-ups are assessed, and approximations are introduced resulting in a factor three reduction of the per-event CPU time without affecting the physics modelling performance. Continuous statistical enhancement techniques are introducedmore » by ATLAS in order to populate low cross-section regions of phase space and are shown to match or exceed the generated effective luminosity. This, together with the lower per-event CPU time, results in a 50% reduction in the required computing resources compared to a legacy set-up previously used by the ATLAS collaboration. The set-ups described in this paper will be used for future ATLAS analyses and lay the foundation for the next generation of Monte Carlo predictions for single vector-boson plus jets production.[graphic not available: see fulltext]« less

The semiconductor tracker (SCT) is one of the tracking systems for charged particles in the ATLAS detector. It consists of 4088 silicon strip sensor modules. During Run 2 (2015–2018) the Large Hadron Collider delivered an integrated luminosity of 156 fb^₋1 to the ATLAS experiment at a centre-of-mass proton-proton collision energy of 13 TeV. The instantaneous luminosity and pile-up conditions were far in excess of those assumed in the original design of the SCT detector. Due to improvements to the data acquisition system, the SCT operated stably throughout Run 2. It was available for 99.9% of the integrated luminosity and achievedmore » a data-quality efficiency of 99.85%. Detailed studies have been made of the leakage current in SCT modules and the evolution of the full depletion voltage, which are used to study the impact of radiation damage to the modules.« less

Production cross sections of the Higgs boson are measured in the H $$\to$$ ZZ $$\to$$ $$4\ell$$ ($$\ell$$ $$=$$ e, $$\mu$$) decay channel. A data sample of proton-proton collisions at a center-of-mass energy of 13 TeV, collected by the CMS detector at the LHC and corresponding to an integrated luminosity of 137 fb$$^{-1}$$ is used. The signal strength modifier $$\mu$$, defined as the ratio of the Higgs boson production rate in the $$4\ell$$ channel to the standard model (SM) expectation, is measured to be $$\mu$$ $$=$$ 0.94 $$\pm$$ 0.07 (stat) $${}^{+0.09}_{-0.08}$$ (syst) at a fixed value of $$m_H$$ = 125.38 GeV.more » The signal strength modifiers for the individual Higgs boson production modes are also reported. The inclusive fiducial cross section for the H $$\to$$ $$4\ell$$ process is measured to be 2.84 $$^{+0.23}_{-0.22}$$ (stat) $${}^{+0.26}_{-0.21}$$ (syst) fb, which is compatible with the SM prediction of 2.84 $$\pm$$ 0.15 fb for the same fiducial region. Differential cross sections as a function of the transverse momentum and rapidity of the Higgs boson, the number of associated jets, and the transverse momentum of the leading associated jet are measured. A new set of cross section measurements in mutually exclusive categories targeted to identify production mechanisms and kinematical features of the events is presented. The results are in agreement with the SM predictions.« less

Jet production in lead-lead (PbPb) and proton-proton (pp) collisions at a nucleon-nucleon center-of-mass energy of 5.02 TeV is studied with the CMS detector at the LHC, using PbPb and pp data samples corresponding to integrated luminosities of 404 μb$$^{−1}$$ and 27.4 pb$$^{−1}$$, respectively. Jets with different areas are reconstructed using the anti-k$$_{T}$$ algorithm by varying the distance parameter R. The measurements are performed using jets with transverse momenta (p$$_{T}$$) greater than 200 GeV and in a pseudorapidity range of |η| < 2. To reveal the medium modification of the jet spectra in PbPb collisions, the properly normalized ratio of spectramore » from PbPb and pp data is used to extract jet nuclear modification factors as functions of the PbPb collision centrality, p$$_{T}$$ and, for the first time, as a function of R up to 1.0. For the most central collisions, a strong suppression is observed for high-p$$_{T}$$ jets reconstructed with all distance parameters, implying that a significant amount of jet energy is scattered to large angles. The dependence of jet suppression on R is expected to be sensitive to both the jet energy loss mechanism and the medium response, and so the data are compared to several modern event generators and analytic calculations. The models considered do not fully reproduce the data.[graphic not available: see fulltext]« less

Measurements of the differential cross sections of Z + jets and γ + jets production, and their ratio, are presented as a function of the boson transverse momentum. Measurements are also presented of the angular distribution between the Z boson and the closest jet. The analysis is based on pp collisions at a center-of-mass energy of 13 TeV corresponding to an integrated luminosity of 35.9 fb$$^{−1}$$ recorded by the CMS experiment at the LHC. The results, corrected for detector effects, are compared with various theoretical predictions. In general, the predictions at higher orders in perturbation theory show better agreement withmore » the measurements. This work provides the first measurement of the ratio of the differential cross sections of Z + jets and γ + jets production at 13 TeV, as well as the first direct measurement of Z bosons emitted collinearly with a jet.[graphic not available: see fulltext]« less

Measurements of the total and differential fiducial cross sections for the Z boson decaying into two neutrinos are presented at the LHC in proton-proton collisions at a center-of-mass energy of 13 TeV. The data were collected by the CMS detector in 2016 and correspond to an integrated luminosity of 35.9 fb$$^{-1}$$. In these measurements, events are selected containing an imbalance in transverse momentum and one or more energetic jets. The fiducial differential cross section is measured as a function of the Z boson transverse momentum. The results are combined with a previous measurement of charged-lepton decays of the Z boson.more » The measured total fiducial cross section for events with Z boson transverse momentum greater than 200 GeV is $$ {3000}_{-170}^{+180} $$ fb.[graphic not available: see fulltext]« less