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CMS Collaboration

Publications and source records attributed to CMS Collaboration.

At least 19 recordsLinked to original sources
arXiv2026

Spin-dependent azimuthal asymmetry of coherent J/$\psi$ photoproduction in hadronic PbPb collisions at $\sqrt{s_\mathrm{NN}}$ = 5.36 TeV

We report a measurement of the azimuthal asymmetry of muons from the decay of coherently photoproduced J/$\psi$ mesons with respect to the second-order event plane in hadronic lead-lead (PbPb) collisions. The analysis is based on data collected by the CMS experiment at the LHC at a nucleon-nucleon center-of-mass energy of 5.36 TeV, corresponding to an integrated luminosity of 1.26 nb$^{-1}$. The asymmetry is quantified using the amplitude $A_{2}$ of the second-order harmonic modulation of the decay-muon azimuthal distribution. A negative $A_{2}$ = $-$0.194 $\pm$ 0.022 (stat.) $\pm$ 0.018 (syst.) is observed with a significance greater than five standard deviations for the first time. The $A_{2}$ values show no significant dependence on the J/$\psi$ meson rapidity or the degree of overlap of the colliding ions. A theoretical model in which the coherently photoproduced J/$\psi$ meson inherits the linear polarization of the incident photon is consistent with the data. When considered together with recent measurements of coherent $\rho^0$ $\to$ $\pi^+\pi^-$ photoproduction in ultraperipheral collisions, these results provide a direct observation of spin dependence in the azimuthal asymmetry of decay products from coherently photoproduced vector mesons. In addition, the observed correlation with the event plane may enable a novel approach to reaction plane determination in the future.

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arXiv2026

Search for the rare Higgs boson decay H $\to$ Z$\gamma$ in proton-proton collisions at $\sqrt{s}$ = 13 and 13.6 TeV

A search is presented for the rare, loop-induced Higgs boson decay H $\to$ Z$\gamma$, where Z $\to$ $\ell^+\ell^-$ and $\ell$ = e or $\mu$. The search is performed using a sample of proton-proton (pp) collision data at the center-of-mass energies of 13 and 13.6 TeV, recorded by the CMS experiment at the LHC and corresponding to a total integrated luminosity of 200 fb$^{-1}$. The analysis design separately considers and optimizes sensitivity to Higgs boson production via gluon-gluon fusion, vector boson fusion, and associated-production processes. The signal is extracted from a simultaneous fit to the invariant mass distributions of the $\ell^+\ell^-\gamma$ system in the various production channels and event categories. The observed (expected) signal strength $\mu$, defined as the ratio of the measured product the of cross section and decay branching fraction value $\sigma$(pp $\to$ H)$\mathcal{B}$(H $\to$ Z$\gamma$) to the corresponding value predicted in the standard model, is found to be $\mu$ = 1.10$^{+0.52}_{-0.61}$ (1.00$^{+0.49}_{-0.46}$) for a Higgs boson mass of 125.38 GeV. The signal has an observed (expected) significance of 1.9 (2.3) standard deviations.

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arXiv2026

Constraint on the Higgs boson width in the diphoton final state using signal-background interference in proton-proton collisions at $\sqrt{s}$ = 13 TeV

The standard model Higgs boson, with a mass of 125 GeV, has a predicted decay width, $\Gamma_\mathrm{H}$, of 4.1 MeV. This Letter presents the first constraint on $\Gamma_\mathrm{H}$ obtained through the interference between the resonant H $\to$ $\gamma\gamma$ process and the continuum $\gamma\gamma$ production, both produced via gluon-gluon fusion. The analysis is performed using proton-proton collision data at $\sqrt{s}$ = 13 TeV, collected by the CMS experiment between 2016 and 2018, corresponding to an integrated luminosity of 138 fb$^{-1}$. The observed (expected) limit on the Higgs boson width is $\Gamma_\mathrm{H}$ $\lt$ 92 (138) MeV at 95% confidence level. This is the most stringent constraint on $\Gamma_\mathrm{H}$ from on-shell Higgs boson measurements to date.

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arXiv2026

Evidence for the dead-cone effect in bottom quark initiated jets produced in proton-proton collisions at $\sqrt{s}$ = 13 TeV

The suppression of collinear gluon emissions from a massive quark, or the ``dead-cone effect,'' is the primary manifestation of quark mass effects in parton showers. A proton-proton collision data set recorded by the CMS experiment at $\sqrt{s}$ = 13 TeV with an integrated luminosity of 59.8 fb$^{-1}$ is analyzed. A differential measurement is presented of the emission density of bottom quark jets, studied as a function of the angular separation between iteratively declustered subjets. The measurement is performed in bottom quark initiated jets with transverse momenta between 40 and 200 GeV originating from top quark pair production. Compared to data, a parton shower Monte Carlo model that includes the dead-cone effect is favored, while a model without the dead-cone effect is disfavored with a significance of 4.3 standard deviations.

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arXiv2026

Search for scalar leptoquarks produced via muon-quark scattering in proton-proton collisions at $\sqrt{s}$ = 13 TeV

The first search for TeV-scale scalar leptoquarks (LQs) produced in muon-quark ($\mu$q) interactions is presented. It is based on proton-proton collision data recorded at a center-of-mass energy of 13 TeV with the CMS detector at the LHC, corresponding to an integrated luminosity of 138 fb$^{-1}$. Quantum fluctuations inside the proton generate charged-lepton components, enabling the study of lepton-induced processes. In particular, the interaction of a muon from one colliding proton with a quark from the other proton enables the study of resonant production of a single LQ and its subsequent decay. The final state includes one or two muons and a jet that can come from the hadronization of either a light quark (u) or a b quark. The primary observable is the invariant mass of the muon-jet system, whose distribution peaks at the LQ mass. The data are found to be in agreement with the background predictions. Upper limits are set on the product of the LQ production cross section and its decay branching fraction to the $\mu$q final state. The results exclude broad regions in the parameter plane of the LQ mass and the LQ-$\mu$q coupling, improving upon previous CMS searches by extending the coverage for LQs with masses between 1.5 and 3.6 TeV, and couplings between 0.2 and 0.6 for the LQ(u$\mu$) scenario; and masses above 1.8 TeV, and couplings above 1 for the LQ(b$\mu$) scenario.

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arXiv2026

Observation of an excess at the top quark pair production threshold in the single-lepton channel

A search is presented for top quark-antiquark ($\mathrm{t\bar{t}}$) quasi-bound ``toponium'' states, near the kinematic $\mathrm{t\bar{t}}$ production threshold in final states with a single electron or muon and jets. The study uses proton-proton collision data at $\sqrt{s}$ = 13 TeV, collected by the CMS experiment at the CERN LHC, corresponding to an integrated luminosity of 138 fb$^{-1}$. The analysis examines the relative velocity between the top quark and antiquark, along with two angular observables sensitive to the parity and spin of the $\mathrm{t\bar{t}}$ system. A significant excess of events is observed relative to the standard model prediction for $\mathrm{t\bar{t}}$ production calculated at next-to-next-to-leading order in perturbative quantum chromodynamics (QCD). The excess corresponds to an observed total cross section of 5.1$\pm$0.9 pb and is consistent with a simplified model of a color singlet pseudoscalar toponium motivated by nonrelativistic QCD, with a resulting observed significance of 6.1 standard deviations. The result provides an independent confirmation of the excess reported in the dilepton channel.

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arXiv2026

Search for top quark flavor-changing neutral currents in multilepton final states using an effective field theory approach in proton--proton collisions at $\sqrt{s}$ = 13 TeV

We present a search for flavor-changing neutral current (FCNC) interactions involving the top quark in production and decay. This analysis establishes the first global search in the top quark FCNC sector within the standard model effective field theory where all relevant operators are constrained simultaneously in a single fit to data, providing a framework for future measurements and combinations at the LHC. The analysis uses proton$-$proton collision data collected at $\sqrt{s}$ = 13 TeV by the CMS detector, corresponding to an integrated luminosity of 138 fb$^{-1}$. Signal contributions are probed in final states with two same-sign charged leptons or three leptons. Multivariate discriminants are used to separate signal from background, and their distributions enter a likelihood fit to determine 95% confidence intervals for 24 Wilson coefficients. No evidence for FCNC interactions is observed. These results provide the first simultaneous constraints on the complete operator set and place the most stringent limits to date on FCNC top quark decay branching fractions via two-quark$-$two-lepton interactions.

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arXiv2026

Search for anomalous couplings in WW and WZ production with single-lepton final states in proton-proton collisions at $\sqrt{s}$ = 13 TeV

A search for deviations from the standard model using an effective field theory approach is carried out using the proton-proton collision data set recorded by the CMS experiment at the LHC at a center-of-mass energy of 13 TeV, corresponding to an integrated luminosity of 138 fb$^{-1}$. In this study Wilson coefficients corresponding to dimension-six effective field theory operators that would lead to anomalous gauge boson self-couplings and modified couplings between vector bosons and quarks are constrained. Diboson (WW and WZ) production processes with one W boson decaying to a lepton plus antineutrino or charge conjugate and the second W or Z boson decaying hadronically are considered. Since the contribution from anomalous couplings is expected to be most visible at high energy scales, the focus is on final states where the hadronic decay products of a W or Z boson are merged into a single large-radius jet. A dedicated classifier based on machine learning is employed to separate hadronic W and Z boson decays from background processes. The most stringent constraints to date on the Wilson coefficients of operators corresponding to anomalous triple gauge boson couplings are reported. The bounds set on the couplings between vector bosons and quarks are competitive with those from previous inclusive jet measurements.

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arXiv2026

Observation of enhanced baryon production using strange hadrons in oxygen-oxygen collisions

A hot deconfined medium known as the quark-gluon plasma (QGP) is created in collisions of ultrarelativistic heavy nuclei. In these systems, hadron production is strongly modified relative to proton-proton (pp) interactions. In the hadron transverse momentum region 2 $\lt$ $p_\mathrm{T}$ $\lt$ 5 GeV, collective radial flow and quark coalescence are important mechanisms that can produce such modifications. While these effects are well-established for collisions of large ions, such as lead, it is unknown if they are present in systems produced using smaller ions. To address this question, the $p_\mathrm{T}$ spectra of two strange hadrons, K$^0_\mathrm{S}$ and $\Lambda$, are measured in oxygen-oxygen (OO) and pp collisions at a center-of-mass energy per nucleon pair of 5.36 TeV. The measurements are performed using the CMS detector at the LHC, and the OO and pp datasets correspond to integrated luminosities of 6.8 nb$^{-1}$ and 1.1 pb$^{-1}$, respectively. The spectral modification of K$^0_\mathrm{S}$ mesons in OO collisions relative to pp collisions is found to be similar to that of inclusive charged hadrons. Conversely, the $\Lambda$-to-K$^0_\mathrm{S}$ cross section ratio exhibits a pronounced enhancement in OO collisions when compared to pp collisions. The results provide the first observation of a baryon-to-meson enhancement in OO collisions and suggest the formation of a deconfined medium, offering insight into the roles of strangeness, radial flow, and quark coalescence in QGP.

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arXiv2026

Performance of heavy-flavour jet identification in the CMS high-level trigger in proton-proton collisions at $\sqrt{s}$ = 13.6 TeV

The CMS trigger system plays a crucial role during data taking, reducing the large collision rate delivered by the LHC to a few kHz for data storage and subsequent offline analysis. The system aims to maintain a high selection efficiency for a wide range of processes, including those involving jets originating from heavy-flavour quarks (b and c), which provide a distinctive signature in many physics analyses. To achieve this while maintaining a sustainable trigger output rate, dedicated jet flavour identification methods are developed and optimized for use in the high-level trigger (HLT). This paper presents the design, commissioning, and performance of deep-learning-based jet identification algorithms deployed in the HLT during 2022$-$2024, for proton-proton collisions at $\sqrt{s}$ = 13.6 TeV. The new algorithms enabled significant improvements in signal efficiency for a variety of key physics processes, including the non-resonant production of Higgs boson pairs decaying to four b quarks, as well as Higgs boson production via both vector boson fusion and in association with a $\mathrm{t\bar{t}}$ pair, in the H $\to$ $\mathrm{b\bar{b}}$ and H $\to$ $\mathrm{c\bar{c}}$ decay channels.

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arXiv2026

Search for a resonance in events with four top quarks decaying into two leptons and jets in proton-proton collisions

A search for a heavy resonance in events containing four top quarks is presented and interpreted in models featuring a new vector, scalar, or pseudoscalar boson that interacts exclusively with top quarks. Proton-proton collision data collected by the CMS experiment at the CERN LHC at centre-of-mass energies of 13 and 13.6 TeV are analysed, corresponding to integrated luminosities of 138 and 35 fb$^{-1}$, respectively. Events with two leptons (electrons or muons) and jets are selected. The mass of the heavy resonance is reconstructed from a pair of jets that are formed using a variable-radius jet clustering algorithm. A machine-learning approach is used to identify whether these jets originate from top quarks that decay fully hadronically. The dominant background from misidentified jets is estimated from data as a function of the dijet invariant mass. No significant excess is observed in data. Upper limits at 95% confidence level are set on the resonance production cross section for resonance masses between 500 GeV and 4 TeV, and relative widths ranging from 4 to 50%. In the vector-mediator benchmark, resonances with 50% relative width are excluded up to masses of 850 GeV, with an expected exclusion limit of 1000 GeV. This is the first dedicated search for these models in the two lepton channel.

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arXiv2026

Search for resonant production of lepton-enriched semivisible jets in proton-proton collisions at $\sqrt{s}$ = 13 TeV

This search targets the resonant production of lepton-enriched semivisible jets (SVJs) from a strongly coupled dark sector, using 138 fb$^{-1}$ of proton-proton collision data collected with the CMS detector at the CERN LHC at $\sqrt{s}$ = 13 TeV. Two scenarios are investigated: jets enriched in all lepton flavors (SVJ$\ell$ signature) and jets predominantly enriched in tau leptons (SVJ$\tau$ signature). The analysis focuses on final states in which the missing transverse momentum is aligned with jets containing nonisolated leptons. A dual machine-learning strategy is employed, using a graph neural network for jet identification and a fully connected neural network that combines jet- and event-level information to enhance signal sensitivity and background estimation. The signal models assume a heavy Z' mediator with a benchmark coupling of 0.25 to standard model quarks, together with prompt decays of unstable dark hadrons. In the SVJ$\ell$ scenario, mediator masses up to 4.7 TeV are excluded at 95% confidence level, while masses between 1.8 and 3.5 TeV are excluded in the SVJ$\tau$ scenario. These results provide the first experimental constraints on lepton-enriched semivisible jets.

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arXiv2026

Search for a top-philic Z' boson decaying into a $\mathrm{t\bar{t}}$ pair in a final state with jets and an electron or muon in proton-proton collisions at $\sqrt{s}$ = 13.6 TeV

A search for a top-philic Z' boson in a final state with jets and an electron or muon is presented. The search is based on a sample of proton-proton collision data collected at $\sqrt{s}$ = 13 TeV by the CMS experiment at the CERN LHC during 2016$-$2018, corresponding to an integrated luminosity of 138 fb$^{-1}$. The top-philic Z' boson is produced in association with a top-antitop quark pair ($\mathrm{t\bar{t}}$) and decays into a $\mathrm{t\bar{t}}$ pair, as it couples exclusively to top quarks. The analysis aims to identify a heavy Z' boson that produces Lorentz-boosted top quarks, whose hadronic decay products are merged into large-radius jets. A machine-learning algorithm is employed to identify such jets. The distribution of the invariant mass of the two top quark candidates with the highest transverse momentum is used as the discriminant variable in a Z' boson mass range of 0.5$-$3 TeV, with intrinsic widths of 4, 10, 20, and 50% relative to its mass. The results obtained are found to be in agreement with the standard model background prediction. Upper limits at 95% confidence level are set on the production cross section of the Z' boson, for each of the decay widths as a function of its mass. These results represent the most stringent constraints to date on the existence of a top-philic Z' boson.

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arXiv2026

Combination of vector boson scattering measurements in leptonic final states in proton-proton collisions at $\sqrt{s}$ = 13 TeV

A statistical combination of vector boson scattering measurements in proton-proton collisions at $\sqrt{s}$ = 13 TeV is presented. The analysis is based on a dataset corresponding to an integrated luminosity of 138 fb$^{-1}$, collected with the CMS detector in 2016$-$2018. Events are selected to contain two vector bosons, with at least one decaying leptonically, and at least two additional jets with a high invariant mass and a large pseudorapidity separation. Signal rates are extracted for same-sign WW, opposite-sign WW, as well as for WZ and ZZ channels, and are compared with the standard model predictions. The combination provides a global view of vector boson scattering by simultaneously constraining all channels in a coherent framework, improving sensitivity beyond individual inputs, with all results in agreement with the standard model predictions. In addition, a charge-separated measurement is performed in which W bosons are distinguished according to their electric charge, allowing sensitivity to the production charge asymmetry.

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arXiv2026

A measurement of the Higgs boson mass in the diphoton decay channel in proton-proton collisions at $\sqrt{s}$ = 13 TeV

A measurement of the Higgs boson mass in the diphoton decay channel is performed using proton-proton collision data at a center-of-mass energy of 13 TeV. The data set recorded with the CMS detector between 2016 and 2018 is used, corresponding to an integrated luminosity of 138 fb$^{-1}$. A refined detector calibration and new analysis techniques are employed to improve the precision of the results compared to earlier measurements. The Higgs boson mass is measured to be $m_\mathrm{H}$ = 125.13 $\pm$ 0.15 GeV = 125.13 $\pm$ 0.10 (stat) $\pm$ 0.12 (syst) GeV. In addition, a combination with the mass measurement at center-of-mass energies of 7 and 8 TeV in the diphoton final state is performed resulting in $m_\mathrm{H}$ = 125.06 $\pm$ 0.14 GeV = 125.06 $\pm$ 0.09 (stat) $\pm$ 0.11 (syst) GeV.

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arXiv2026

Measurement of the fragmentation properties of jets containing $\Upsilon$(nS) mesons in proton-proton collisions at $\sqrt{s}$ = 13 TeV

A measurement of the fragmentation properties of jets containing $\Upsilon$(nS) mesons using proton-proton collision data at $\sqrt{s}$ = 13 TeV, corresponding to an integrated luminosity of 138 fb$^{-1}$, is presented. The $\Upsilon$(nS) mesons associated with jets are reconstructed via their decays producing pairs of oppositely charged muons. The longitudinal and transverse projections of the momenta of the $\Upsilon$(nS) mesons along the momenta of the corresponding jets are studied. The results are compared with Monte Carlo predictions, including recent developments in the modelling of quarkonia production in parton showers. The description of the data by these Monte Carlo predictions is found to be unsatisfactory, leaving room for improvements in the modelling of such processes.

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arXiv2026

Off-shell Higgs boson measurements: Yukawa couplings, self-coupling, compositeness, and width

Measurements of Higgs boson production in the off-shell region are presented, using the four-lepton decay channel. Data from proton-proton collisions at the CERN LHC, collected by the CMS experiment and corresponding to an integrated luminosity of 138 fb$^{-1}$ at a center-of-mass energy of 13 TeV, are utilized. The first direct test of composite Higgs boson models is performed, with a lower limit on the compositeness scale $\Lambda_\mathrm{H}$ set at 870 GeV at the 95% confidence level. Tests of gluon-fusion production within the standard model effective field theory framework are performed. The first constraint on the Higgs boson self-coupling in the off-shell region is obtained. By combining on- and off-shell measurements, the analysis sets the tightest constraints to date on light-quark Yukawa couplings, while relaxing assumptions such as the bound on the Higgs boson coupling to vector bosons, thereby providing more model-independent results. Constraints on the Higgs boson width are provided while accounting for a range of beyond-the-standard-model effects, including both light and heavy particles in the gluon-fusion production loop, as well as modified couplings to vector bosons. A combined analysis of the H$\to$ZZ and PH$\to$WW channels is performed to improve sensitivity to the Higgs boson width, yielding $\Gamma_\mathrm{H}$ = 5.1$^{+2.0}_{-1.8}$ MeV. The scenario of no off-shell Higgs boson production is excluded at a confidence level exceeding 5 standard deviations.

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arXiv2026

Measurement of the $\gamma\gamma \to \tau \tau$ cross section and constraints on the anomalous magnetic moment of the $\tau$ lepton in ultraperipheral PbPb collisions at $\sqrt{s_\mathrm{NN}}$ = 5.02 TeV

The production of $\tau$ lepton pairs via photon-photon fusion, $\gamma\gamma \to \tau \tau$, is studied in ultraperipheral lead-lead collisions at a nucleon-nucleon center-of-mass energy of 5.02 TeV. The dataset, collected by the CMS experiment in 2018, corresponds to an integrated luminosity of 1.70 nb$^{-1}$. Four different $\tau^+\tau^-$ decay final states are analyzed. A simultaneous likelihood fit to the measured lepton transverse momentum ($p_\mathrm{T}$) distributions, which incorporates information from both spectral shape and normalization, is used to constrain the anomalous magnetic moment of the $\tau$ lepton, $a_\tau$, and to extract the cross section of the process. The measured 95% CL interval for $a_\tau$ is $-$0.039 $\lt$ $a_\tau$ $\lt$ 0.032. The fiducial cross section, $\sigma_{\gamma\gamma \to \tau \tau}^\text{fid}$ = 555$^{+60}_{-14}$ $\mu$b for tau leptons with $p_\mathrm{T}^\tau$ $\gt$ 1 GeV and pseudorapidity $\lvert\eta^\tau\rvert$ $\lt$ 3, is the most precise measurement for this process at the LHC to date and is in agreement with next-to-leading-order quantum electrodynamics predictions.

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