Reweighting simulated events using machine-learning techniques in the CMS experiment

Data analyses in particle physics rely on an accurate simulation of particle collisions and a detailed simulation of detector effects to extract physics knowledge from the recorded data. Event generators together with a geant-based simulation of the detectors are used to produce large samples of simulated events for analysis by the LHC experiments. These simulations come at a high computational cost, where the detector simulation and reconstruction algorithms have the largest CPU demands. This article describes how machine-learning (ML) techniques are used to reweight simulated samples obtained with a given set of parameters to samples with different parameters or samples obtained from entirely different simulation programs. The ML reweighting method avoids the need for simulating the detector response multiple times by incorporating the relevant information in a single sample through event weights. Results are presented for reweighting to model variations and higher-order calculations in simulated top quark pair production at the LHC. This ML-based reweighting is an important element of the future computing model of the CMS experiment and will facilitate precision measurements at the High-Luminosity LHC

Search for pair production of heavy particles decaying to a top quark and a gluon in the lepton+jets final state in proton–proton collisions at s=13 TeV\sqrt{s}=13\,\text {Te}\hspace{-.08em}\text {V}

A search is presented for the pair production of new heavy resonances, each decaying into a top quark (t) or antiquark and a gluon (g). The analysis uses data recorded with the CMS detector from proton-proton collisions at a center-of-mass energy of 13 TeV at the LHC, corresponding to an integrated luminosity of 138 fb$^{-1}$. Events with one muon or electron, multiple jets, and missing transverse momentum are selected. After using a deep neural network to enrich the data sample with signal-like events, distributions in the scalar sum of the transverse momenta of all reconstructed objects are analyzed in the search for a signal. No significant deviations from the standard model prediction are found. Upper limits at 95% confidence level are set on the product of cross section and branching fraction squared for the pair production of excited top quarks in the t$^∗$ → tg decay channel. The upper limits range from 120 to 0.8 fb for a t$^∗$ with spin-1/2 and from 15 to 1.0 fb for a t∗ with spin-3/2. These correspond to mass exclusion limits up to 1050 and 1700 GeV for spin-1/2 and spin-3/2 t$^∗$ particles, respectively. These are the most stringent limits to date on the existence of t$^∗$ → tg resonances

Observation of Λ Hyperon Local Polarization in p-Pb Collisions at sNN\sqrt{s_{NN}}=8.16 TeV

The polarization of the Λ and ¯Λ hyperons along the beam direction has been measured in proton-lead (p-Pb) collisions at a center-of-mass energy per nucleon pair of 8.16 TeV. The data were obtained with the CMS detector at the LHC and correspond to an integrated luminosity of 186.0 $\pm$ 6.5 nb$^{−1}$. A significant azimuthal dependence of the hyperon polarization, characterized by the second-order Fourier sine coefficient P$_{z;s2}$, is observed. The P$_{z;s2}$ values decrease as a function of charged particle multiplicity, but increase with transverse momentum. A hydrodynamic model that describes the observed P$_{z;s2}$ values in nucleus-nucleus collisions by introducing vorticity effects does not reproduce either the sign or the magnitude of the p-Pb results. These observations pose a challenge to the current theoretical implementation of spin polarization in heavy ion collisions and offer new insights into the origin of spin polarization in hadronic collisions at LHC energies

Search for heavy neutral resonances decaying to tau lepton pairs in proton-proton collisions at s=13 TeV

A search for heavy neutral gauge bosons ((Formula presented)) decaying into a pair of tau leptons is performed in proton-proton collisions at (Formula presented) at the CERN LHC. The data were collected with the CMS detector and correspond to an integrated luminosity of (Formula presented). The observations are found to be in agreement with the expectation from standard model processes. Limits at 95% confidence level are set on the product of the (Formula presented) production cross section and its branching fraction to tau lepton pairs for a range of (Formula presented) boson masses. For a narrow resonance in the sequential standard model scenario, a (Formula presented) boson with a mass below 3.5 TeV is excluded. This is the most stringent limit to date from this type of search

Search for heavy long-lived charged particles with large ionization energy loss in proton-proton collisions at s\sqrt{s} = 13 TeV

A search for heavy, long-lived, charged particles with large ionization energy loss within the silicon tracker of the CMS experiment is presented. A data set of proton-proton collisions at a center of mass energy at $\sqrt{s}$ = 13 TeV, collected in 2017 and 2018 at the CERN LHC, corresponding to an integrated luminosity of 101 fb$^{−1}$, is used in this analysis. Two different approaches for the search are taken. A new method exploits the independence of the silicon pixel and strips measurements, while the second method improves on previous techniques using ionization to determine a mass selection. No significant excess of events above the background expectation is observed. The results are interpreted in the context of the pair production of supersymmetric particles, namely gluinos, top squarks, and tau sleptons, and of the Drell-Yan pair production of fourth generation (τ′) leptons with an electric charge equal to or twice the absolute value of the electron charge (e). An interpretation of a Z’ boson decaying to two τ′ leptons with an electric charge equal to 2e is presented for the first time. The 95% confidence upper limits on the production cross section are extracted for each of these hypothetical particles

Search for heavy neutral Higgs bosons A and H in the tt‾\overline{t}Z channel in proton-proton collisions at 13 TeV

A direct search for new heavy neutral Higgs bosons A and H in the tt Z channel is presented, targeting the process pp → A → ZH with H → t$\overline{t}$ . For the first time, the channel with decays of the Z boson to muons or electrons in association with all-hadronic decays of the t$\overline{t}$ system is targeted. The analysis uses proton-proton collision data collected at the CERN LHC with the CMS experiment at √ = 13 TeV, which correspond to an integrated luminosity of 138 fb$^{−1}$. No signal is observed. Upper limits on the product of the cross section and branching fractions are derived for narrow resonances A and H with masses up to 2100 and 2000 GeV, respectively, assuming A boson production through gluon fusion. The results are also interpreted within two-Higgs-doublet models, where A and H are CP-odd and CP-even states, respectively, complementing and substantially extending the reach of previous searches

Evidence for Similar Collectivity of High Transverse-Momentum Particles in p-Pb and Pb-Pb Collisions

Charged hadron elliptic anisotropies (v2 ) are presented over a wide transverse momentum (pT ) range for proton-lead (p⁢Pb ) and lead-lead (PbPb) collisions at nucleon-nucleon center-of-mass energies of 8.16 and 5.02 TeV, respectively. The data were recorded by the CMS experiment and correspond to integrated luminosities of 186 and 0.607 nb−1 for the p⁢Pb and PbPb systems, respectively. A four-particle cumulant analysis is performed using subevents separated in pseudorapidity to effectively suppress noncollective effects. At high pT (pT>8 GeV ), significant positive v2 values that are similar between p⁢Pb and PbPb collisions at comparable charged particle multiplicities are observed. This observation suggests a common origin for the multiparticle collectivity for high-pT particles in the two systems

Search for the Z Boson Decay to ττμμ in Proton-Proton Collisions at √s = 13 TeV

The first search for the boson decay to ⁢⁢⁢ at the CERN LHC is presented, based on data collected by the CMS experiment at the LHC in proton-proton collisions at a center-of-mass energy of 13 TeV and corresponding to an integrated luminosity of 138  fb−1. The data are compatible with the predicted background. For the first time, an upper limit at the 95% confidence level of 6.9 times the standard model expectation is placed on the ratio of the →⁢⁢⁢ to →4⁢ branching fractions. Limits are also placed on the six flavor-conserving four-lepton effective-field-theory operators involving two muons and two tau leptons, for the first time testing all such operators

Search for a Neutral Gauge Boson with Nonuniversal Fermion Couplings in Vector Boson Fusion Processes in Proton-Proton Collisions at s=13 TeV

The first search for a heavy neutral spin-1 gauge boson (Z0) with nonuniversal fermion couplings produced via vector boson fusion processes and decaying to tau leptons or W bosons is presented. The analysis is performed using LHC data at ffiffiffi s p 1⁄4 13 TeV, collected from 2016 to 2018 with the CMS experiment and corresponding to an integrated luminosity of 138 fb−1. The data are consistent with the standard model predictions. Upper limits are set on the product of the cross section for production of the Z0 boson and its branching fraction to ττ or WW. The presence of a Z0 boson decaying to τþτ− (WþW−) is excluded for masses up to 2.45(1.60) TeV, depending on the Z0 boson coupling to standard model weak bosons, and assuming a Z0 → τþτ− (WþW−) branching fraction of 50