159 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

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

The combination of measurements of the W boson polarization in top quark decays performed by the ATLAS and CMS collaborations is presented. The measurements are based on proton-proton collision data produced at the LHC at a centre-of-mass energy of 8 TeV, and corresponding to an integrated luminosity of about 20 fb$$^{−1}$$ for each experiment. The measurements used events containing one lepton and having different jet multiplicities in the final state. The results are quoted as fractions of W bosons with longitudinal (F$$_{0}$$), left-handed (F$$_{L}$$), or right-handed (F$$_{R}$$) polarizations. The resulting combined measurements of the polarization fractions are F$$_{0}$$ = 0.693more » ± 0.014 and F$$_{L}$$ = 0.315 ± 0.011. The fraction F$$_{R}$$ is calculated from the unitarity constraint to be F$$_{R}$$ = −0.008 ± 0.007. These results are in agreement with the standard model predictions at next-to-next-to-leading order in perturbative quantum chromodynamics and represent an improvement in precision of 25 (29)% for F$$_{0}$$ (F$$_{L}$$) with respect to the most precise single measurement. A limit on anomalous right-handed vector (V$$_{R}$$), and left- and right-handed tensor (g$$_{L}$$, g$$_{R}$$) tWb couplings is set while fixing all others to their standard model values. The allowed regions are [−0.11, 0.16] for V$$_{R}$$, [−0.08, 0.05] for g$$_{L}$$, and [−0.04, 0.02] for g$$_{R}$$, at 95% confidence level. Limits on the corresponding Wilson coefficients are also derived.[graphic not available: see fulltext]« less

This paper presents the combinations of single-top-quark production cross-section measurements by the ATLAS and CMS Collaborations, using data from LHC proton-proton collisions at $$ \sqrt{s} $$ = 7 and 8 TeV corresponding to integrated luminosities of 1.17 to 5.1 fb$$^{−1}$$ at $$ \sqrt{s} $$ = 7 TeV and 12.2 to 20.3 fb$$^{−1}$$ at $$ \sqrt{s} $$ = 8 TeV. These combinations are performed per centre-of-mass energy and for each production mode: t-channel, tW, and s-channel. The combined t-channel cross-sections are 67.5 ± 5.7 pb and 87.7 ± 5.8 pb at $$ \sqrt{s} $$ = 7 and 8 TeV respectively. Themore » combined tW cross-sections are 16.3 ± 4.1 pb and 23.1 ± 3.6 pb at $$ \sqrt{s} $$ = 7 and 8 TeV respectively. For the s-channel cross-section, the combination yields 4.9 ± 1.4 pb at $$ \sqrt{s} $$ = 8 TeV. The square of the magnitude of the CKM matrix element V$$_{tb}$$ multiplied by a form factor f$$_{LV}$$ is determined for each production mode and centre-of-mass energy, using the ratio of the measured cross-section to its theoretical prediction. It is assumed that the top-quark-related CKM matrix elements obey the relation |V$$_{td}$$|, |V$$_{ts}$$| ≪ |V$$_{tb}$$|. All the |f$$_{LV}$$V$$_{tb}$$|$$^{2}$$ determinations, extracted from individual ratios at $$ \sqrt{s} $$ = 7 and 8 TeV, are combined, resulting in |f$$_{LV}$$V$$_{tb}$$| = 1.02 ± 0.04 (meas.) ± 0.02 (theo.). All combined measurements are consistent with their corresponding Standard Model predictions.« less

The exclusive photoproduction of $$\mathrm {\Upsilon }\mathrm {(nS)} $$ meson states from protons, $$\gamma \mathrm {p} \rightarrow \mathrm {\Upsilon }\mathrm {(nS)} \,\mathrm {p}$$ (with $$\mathrm {n}=1,2,3$$ ), is studied in ultraperipheral $$\mathrm {p}$$ Pb collisions at a centre-of-mass energy per nucleon pair of $$\sqrt{\smash [b]{s_{_{\mathrm {NN}}}}} = 5.02\,\text {TeV} $$ . The measurement is performed using the $$\mathrm {\Upsilon }\mathrm {(nS)} \rightarrow \mu ^+\mu ^-$$ decay mode, with data collected by the CMS experiment corresponding to an integrated luminosity of 32.6 $$\,\text {nb}^{-1}$$ . Differential cross sections as functions of the $$\mathrm {\Upsilon }\mathrm {(nS)} $$ transverse momentum squared $$p_{\mathrmmore » {T}} ^2$$ , and rapidity y, are presented. The $$\mathrm {\Upsilon (1S)}$$ photoproduction cross section is extracted in the rapidity range $|y |< 2.2$ , which corresponds to photon–proton centre-of-mass energies in the range $91« less

A search is presented for massive narrow resonances decaying either into two Higgs bosons, or into a Higgs boson and a W or Z boson. The decay channels considered are $$ \mathrm{H}\mathrm{H}\to \mathrm{b}\overline{\mathrm{b}}{\tau}^{+}{\tau}^{-} $$ and $$ \mathrm{V}\mathrm{H}\to \mathrm{q}\overline{\mathrm{q}}{\tau}^{+}{\tau}^{-} $$ , where H denotes the Higgs boson, and V denotes the W or Z boson. This analysis is based on a data sample of proton-proton collisions collected at a center-of-mass energy of 13 TeV by the CMS Collaboration, corresponding to an integrated luminosity of 35.9 fb$$^{−1}$$. For the TeV-scale mass resonances considered, substructure techniques provide ways to differentiate among the hadronizationmore » products from vector boson decays to quarks, Higgs boson decays to bottom quarks, and quark- or gluon-induced jets. Reconstruction techniques are used that have been specifically optimized to select events in which the tau lepton pair is highly boosted. The observed data are consistent with standard model expectations and upper limits are set at 95% confidence level on the product of cross section and branching fraction for resonance masses between 0.9 and 4.0 TeV. Exclusion limits are set in the context of bulk radion and graviton models:spin-0 radion resonances are excluded below a mass of 2.7 TeV at 95% confidence level. In the spin-1 heavy vector triplet framework, mass-degenerate W′ and Z′ resonances with dominant couplings to the standard model gauge bosons are excluded below a mass of 2.8 TeV at 95% confidence level. These are the first limits for massive resonances at the TeV scale with these decay channels at $$ \sqrt{s}=13 $$ TeV.« less

Measurements of fragmentation functions for jets associated with an isolated photon are presented for the first time in pp and Pb-Pb collisions. The analysis uses data collected with the CMS detector at the CERN LHC at a nucleon-nucleon center-of-mass energy of 5.02 TeV. Fragmentation functions are obtained for jets with pTjet>30  GeV/c in events containing an isolated photon with pTγ>60  GeV/c, using charged tracks with transverse momentum pTtrk>1  GeV/c in a cone around the jet axis. The association with an isolated photon constrains the initial pT and azimuthal angle of the parton whose shower produced the jet. For central Pb-Pb collisions, modifications of themore » jet fragmentation functions are observed when compared to those measured in pp collisions, while no significant differences are found in the 50% most peripheral collisions. Jets in central Pb-Pb events show an excess (depletion) of low (high) pT particles, with a transition around 3  GeV/c. This measurement shows for the first time the in-medium shower modifications of partons (quark dominated) with well-defined initial kinematics. It constitutes a new well-controlled reference for testing theoretical models of the parton passage through the quark-gluon plasma.« less

Measurements of Higgs boson properties in the H → γγ decay channel are reported. The analysis is based on data collected by the CMS experiment in proton-proton collisions at $$ \sqrt{s}=13 $$ TeV during the 2016 LHC running period, corresponding to an integrated luminosity of 35.9 fb$$^{−1}$$. Allowing the Higgs mass to float, the measurement yields a signal strength relative to the standard model prediction of 1.18$$_{− 0.14}^{+ 0.17}$$  = 1.18$$_{− 0.11}^{+ 0.12}$$ (stat)$$_{− 0.07}^{+ 0.09}$$ (syst)$$_{− 0.06}^{+ 0.07}$$ (theo), which is largely insensitive to the exact Higgs mass around 125 GeV. Signal strengths associated with the different Higgs boson production mechanisms, couplings to bosons and fermions, and effectivemore » couplings to photons and gluons are also measured.« less

A search for heavy resonances, decaying into the standard model vector bosons and the standard model Higgs boson, is presented. The final states considered contain a b quark-antiquark pair from the decay of the Higgs boson, along with electrons and muons and missing transverse momentum, due to undetected neutrinos, from the decay of the vector bosons. The mass spectra are used to search for a localized excess consistent with a resonant particle. The data sample corresponds to an integrated luminosity of 35.9 fb$$^{−1}$$ collected in 2016 by the CMS experiment at the CERN LHC from proton-proton collisions at a center-of-massmore » energy of 13 TeV. The data are found to be consistent with background expectations. Exclusion limits are set in the context of spin-0 two Higgs doublet models, some of which include the presence of dark matter. In the spin-1 heavy vector triplet framework, mass-degenerate W′ and Z′ resonances with dominant couplings to the standard model gauge bosons are excluded below a mass of 2.9 TeV at 95% confidence level.« less