336 Search Results

Anomaly detection methods used in a recent search for new phenomena by CMS at the CERN LHC are presented. The methods use machine learning to detect anomalous jets produced in the decay of new massive particles without depending on a specific theory model. The effectiveness of these approaches in enhancing sensitivity to various simulated signal samples is studied and compared using data collected in proton–proton collisions at a center-of-mass energy of 13$${\,\textrm{TeV}}$$. In an example analysis, the capabilities of anomaly detection methods are further demonstrated by identifying large-radius jets consistent with Lorentz-boosted hadronically decaying top quarks in a model-agnostic framework.

Production fraction ratios of $B^{+}$ , $B^0$ , and $B_s^0$ mesons are measured in proton-proton collisions at $\sqrt{s}=13\mathrm{TeV}$ using a special dataset recorded in 2018 with high-rate triggers designed to collect an unbiased sample of $10^{10}$ $b$ hadrons with the CMS experiment at the LHC. These data allow the study of the open-charm decays of $B$ mesons ( $B_{\left(s\right)}\to \pi D_{more »}$ ( s ) ) where the $D$ meson decays into fully hadronic final states. By utilizing known branching fractions and precise theoretical calculations, production fraction ratios as functions of $B$ meson transverse momentum $\left(p_{\mathrm{T}}\right)$ and rapidity ( $y$ ) are measured using open-charm decays in the kinematic range of $8<p_{\mathrm{T}}<60\mathrm{GeV}$ and $\left|y\right|<2.25$ . In addition, the same dataset is used to measure the relative production fraction ratios with the charmonium decay channels [ $B_{\left(s\right)}\to XJ/\psi$ with $X$ indicating a $K^{+}$ , $K^{*}(892{)}^0$ , or $\varphi \left(1020\right)$ meson] where the $J/\psi$ meson decays into a pair of muons. The open-charm results are used to normalize the relative production fraction ratios obtained from the charmonium samples. Measurements of the ratios of branching fractions of $B$ meson decays to charmonium and open-charm final states are also reported, which will improve the world-average values of these ratios. Finally, we test isospin invariance in $B$ meson production in proton-proton collisions and observe that it holds within the experimental precision.« less

This analysis presents an observation of the photon-fusion production of W boson pairs using the CMS detector at the LHC. The total cross section of the W$$^{+}$$W$$^{−}$$ production in photon fusion is measured using proton-proton collision data with an integrated luminosity of 138 fb$$^{−1}$$ collected with the CMS detector in 2016–2018 at a center-of-mass energy of $$ \sqrt{s}=13 $$ TeV. Events are selected in the final state with one isolated electron and one isolated muon, and no additional tracks associated with the electron-muon production vertex. The total and fiducial production cross sections are $$ {643}_{-78}^{+82} $$ fb and $$ {3.96}_{-0.51}^{+0.53}more » $$ fb, respectively, in agreement with the standard model predictions of 631 ± 126 fb and 3.87 ± 0.77 fb. This agreement enables stringent constraints on anomalous quartic gauge couplings within a dimension-8 effective field theory framework.[graphic not available: see fulltext]« less

A search is performed for dark matter produced in association with bottom quarks and a pair of electrons or muons in data collected with the CMS detector at the LHC, corresponding to 138 fb$$^{−1}$$ of integrated luminosity of proton-proton collisions at a center-of-mass energy of 13 TeV. For the first time at the LHC, the associated production of a bottom quark-antiquark pair and a new heavy neutral Higgs boson (H) that subsequently decays into a leptonically decaying Z boson and a pseudoscalar (a) is explored. The latter acts as a dark matter mediator in the context of the two Higgsmore » doublet model plus a pseudoscalar (2HDM+a). Multivariate techniques that target a wide range of mass configurations for the H and a particles are used. The observations are consistent with the expectations from standard model processes. Upper limits at 95% confidence level are set on the product of cross section and branching fraction of the new particles, ranging from 10$$^{−2}$$ pb for an H mass of 400 GeV to 10$$^{−3}$$ pb for an H mass of 2000 GeV. Constraints on the parameter space of a benchmark 2HDM+a model are derived and compared with expectations in the context of cosmological predictions.[graphic not available: see fulltext]« less

A search for the pair production of heavy spin- $1/2$ or spin- $3/2$ resonances ( $t^{*}$ ) in proton-proton collisions at $\sqrt{s}=13\mathrm{TeV}$ is presented. Data collected with the CMS detector at the CERN LHC from 2016 to 2018 corresponding to an integrated luminosity of $138{\mathrm{fb}}^{-1}$ are used. The analysis targets benchmark signal scenarios where one $t^{*}$ decays into a top quark ( $t$ ) and a photon ( $\gamma$ ), and the other into a $t$ quarkmore » and a gluon ( $g$ ), i.e., $pp\to t^{*}\overline{t^{*}}\to tt\gamma g$ . All-hadronic final states from the $t$ pair decay chain are selected using jet substructure techniques. The signal is probed as a function of the $t^{*}$ candidate mass, which is reconstructed using the photon and a top quark candidate jet. No significant deviation from the background-only hypothesis is found. Observed (expected) upper limits on the signal cross section at 95% confidence level are set, excluding masses of spin- $1/2$ $t^{*}$ particles below 930 (930) GeV and spin- $3/2$ $t^{*}$ particles below 1330 (1390) GeV. This analysis marks the first search for heavy resonances in the $t\overline{t}\gamma g$ channel. Exploiting the high-energy photon to reduce the backgrounds, this search achieves sensitivity competitive with $pp\to t^{*}\overline{t^{*}}\to t\overline{t}gg$ searches for spin- $1/2$ $t^{*}$ despite the small expected $t^{*}\to t\gamma$ branching fraction.« less

The ratio between the B$$_\mathrm{c}^+$$$$\to$$ J/$$ψ$$$τ^+ν_τ$$ and B$$_\mathrm{c}^+$$$$\to$$ J/$$ψ$$$μ^+ν_μ$$ branching fractions is measured using a data sample of proton-proton collisions collected by CMS at a center-of-mass energy of 13 TeV in the years 2016$$-$$2018 and corresponding to an integrated luminosity of 138 fb$$^{-1}$$. The J$/ψ$ meson is identified through its J$/ψ$$$$\to$$$$μ^+μ^-$ decay and the tau lepton is reconstructed in the hadronic three-prong final state. The measured ratio of branching fractions in this tau decay mode, $$\mathcal{R}^\text{had}_{\mathrm{J}/ψ}$$ = 1.04 $$_{-0.44}^{+0.50}$$, is combined with the previous analysis based on the $τ^+$$$$\to$$$$μ^+ν_μ\barν_τ$$ leptonic decay channel, leading to $$\mathcal{R}_{\mathrm{J}/ψ}$$ = 0.49 $$\pm$$ 0.26. Asmore » this result is consistent with the standard model prediction of 0.258 $$\pm$$ 0.004, no evidence of lepton flavor universality violation is found.« less

A novel anomaly detection algorithm is presented. The Wasserstein normalized autoencoder (WNAE) is a normalized probabilistic model that minimizes the Wasserstein distance between the learned probability distribution—a Boltzmann distribution where the energy is the reconstruction error of the autoencoder (AE)—and the distribution of the training data. This algorithm has been developed and applied to the identification of semivisible jets—conical sprays of visible standard model (SM) particles and invisible dark matter states—with the CMS experiment at the CERN LHC. Trained on jets of particles from simulated SM processes, the WNAE is shown to learn the probability distribution of the input datamore » in a fully unsupervised fashion, such that it effectively identifies new physics jets as anomalies. The model exhibits stable, convergent training and recovers strong classification performance for a wide range of signals against the selected background process, for which a standard AE fails because of outlier reconstruction. In addition, the model improves upon standard normalized autoencoders while remaining fully agnostic to the signal. The WNAE directly tackles the problem of outlier reconstruction, a common failure mode of autoencoders in anomaly detection tasks.« less

Differential top quark pair production cross sections are measured in the dilepton final states e$$^{+}$$e$$^{−}$$, μ$$^{+}$$μ$$^{−}$$, and e$$^{±}$$μ$$^{∓}$$, as a function of kinematic variables of the two-neutrino system: the transverse momentum $$ {p}_{\textrm{T}}^{\nu \nu} $$ of the dineutrino system, the minimum distance in azimuthal angle between $$ {\overrightarrow{p}}_{\textrm{T}}^{\nu \nu} $$ and leptons, and in two dimensions in bins of both observables. The measurements are performed using CERN LHC proton-proton collisions at $$ \sqrt{s}=13 $$ TeV, recorded by the CMS detector between 2016 and 2018, corresponding to an integrated luminosity of 138 fb$$^{−1}$$. The measured cross sections are unfolded to themore » particle level using an unregularized least squares method. Results are compared with predictions by the standard model of particle physics, and found to be in agreement with theoretical calculations as well as Monte Carlo simulations.[graphic not available: see fulltext]« less

The results of a search for the production of two scalar bosons in final states with two photons and two tau leptons are presented. The search considers both nonresonant production of a Higgs boson pair, HH, and resonant production via a new boson X which decays either to HH or to H and a new scalar Y. The analysis uses up to 138 fb$$^{−1}$$ of proton-proton collision data, recorded between 2016 and 2018 by the CMS experiment at the LHC at a center-of-mass energy of 13 TeV. No evidence for signal is found in the data. For the nonresonant production,more » the observed (expected) upper limit at 95% confidence level (CL) on the HH production cross section is set at 930 (740) fb, corresponding to 33 (26) times the standard model prediction. At 95% CL, HH production is observed (expected) to be excluded for values of κ$$_{λ}$$ outside the range between −12 (−9.4) and 17 (15). Observed (expected) upper limits at 95% CL for the X → HH cross section are found to be within 160 to 2200 (200 to 1800) fb, depending on the mass of X. In the X → Y(ττ)H(γγ) search, the observed (expected) upper limits on the product of the production cross section and decay branching fractions vary between 0.059–1.2 fb (0.087–0.68 fb). For the X → Y(γγ)H(ττ) search the observed (expected) upper limits on the product of the production cross section and Y → γγ branching fraction vary between 0.69–15 fb (0.73–8.3 fb) in the low Y mass search, tightening constraints on the next-to-minimal supersymmetric standard model, and between 0.64–10 fb (0.70–7.6 fb) in the high Y mass search.[graphic not available: see fulltext]« less

A measurement of the angular structure of inclusive jets and those containing a prompt D$$^{0}$$ meson in proton-proton collisions at the LHC at a center-of-mass energy of 5.02 TeV is presented. The data corresponding to an integrated luminosity of 301 pb$$^{−1}$$ were collected by the CMS experiment in 2017. Two jet grooming algorithms, late-k$$_{T}$$ and soft drop, are used to study the intrajet radiation pattern using iterative Cambridge-Aachen declustering. The splitting-angle distributions of jets with transverse momentum (p$$_{T}$$) of around 100 GeV, obtained with these two algorithms, show that there is a shift of the distribution for jets containing amore » prompt D$$^{0}$$ meson with respect to inclusive jets. The suppression of emissions at small angles observed in the late-k$$_{T}$$ grooming approach is consistent with the dead-cone effect, whereas the similar suppression for splittings selected with the soft-drop algorithm appears to be induced by gluon splitting to charm quark-antiquark pairs at large angles. The measured distributions are corrected to the particle level and can be used to constrain model predictions for the substructure of high-p$$_{T}$$ charm quark jets.[graphic not available: see fulltext]« less