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Abstract Despite the f₀(980) hadron having been discovered half a century ago, the question about its quark content has not been settled: it might be an ordinary quark-antiquark ($${{\rm{q}}}\overline{{{\rm{q}}}}$$ $q\overline{q}$ ) meson, a tetraquark ($${{\rm{q}}}\overline{{{\rm{q}}}}{{\rm{q}}}\overline{{{\rm{q}}}}$$ $q\overline{q}q\overline{q}$ ) exotic state, a kaon-antikaon ($${{\rm{K}}}\overline{{{\rm{K}}}}$$ $K\overline{K}$ ) molecule, or a quark-antiquark-gluon ($${{\rm{q}}}\overline{{{\rm{q}}}}{{\rm{g}}}$$ $q\overline{q}g$ ) hybrid. This paper reports strong evidence that the f₀(980) state is an ordinary$${{\rm{q}}}\overline{{{\rm{q}}}}$$ $q\overline{q}$ meson, inferred from the scaling of elliptic anisotropies (v₂) with the number of constituent quarks (n_q), as empirically established using conventional hadrons in relativistic heavy ion collisions. The f₀(980) state is reconstructed via its dominant decay channel f₀(980) →π⁺π⁻, in proton-lead collisions recorded by the CMS experiment at the LHC, and itsv₂is measured as a function of transverse momentum (p_T). It is found that then_q= 2 ($${{\rm{q}}}\overline{{{\rm{q}}}}$$ $q\overline{q}$ state) hypothesis is favored overn_q= 4 ($${{\rm{q}}}\overline{{{\rm{q}}}}{{\rm{q}}}\overline{{{\rm{q}}}}$$ $q\overline{q}q\overline{q}$ or$${{\rm{K}}}\overline{{{\rm{K}}}}$$ $K\overline{K}$ states) by 7.7, 6.3, or 3.1 standard deviations in thep_T< 10, 8, or 6 GeV/cranges, respectively, and overn_q= 3 ($${{\rm{q}}}\overline{{{\rm{q}}}}{{\rm{g}}}$$ $q\overline{q}g$ hybrid state) by 3.5 standard deviations in thep_T< 8 GeV/crange. This result represents the first determination of the quark content of the f₀(980) state, made possible by using a novel approach, and paves the way for similar studies of other exotic hadron candidates. 

A first search for beyond the standard model physics in jet multiplicity patterns of multilepton events is presented, using a data sample corresponding to an integrated luminosity of $138{\mathrm{fb}}^{-1}$ of 13 TeV proton-proton collisions recorded by the CMS detector at the LHC. The search uses observed jet multiplicity distributions in one-, two-, and four-lepton events to explore possible enhancements in jet production rate in three-lepton events with and without bottom quarks. The data are found to be consistent with the standard model expectation. The results are interpreted in terms of supersymmetric production of electroweak chargino-neutralino superpartners with cascade decays terminating in prompt hadronic $R$ -parity violating interactions. 

Abstract Measurements in the highly Lorentz-boosted regime provoke increased interest in probing the Higgs boson properties and in searching for particles beyond the standard model at the LHC. In the CMS Collaboration, various boosted-object tagging algorithms, designed to identify hadronic jets originating from a massive particle decaying to bb̅ or cc̅, have been developed and deployed across a range of physics analyses. This paper highlights their performance on simulated events, and summarizes novel calibration techniques using proton-proton collision data collected at √(s) = 13 TeV during the 2016–2018 LHC data-taking period. Three dedicated methods are used for the calibration in multijet events, leveraging either machine learning techniques, the presence of muons within energetic boosted jets, or the reconstruction of hadronically decaying high-energy Z bosons. The calibration results, obtained through a combination of these approaches, are presented and discussed. 

A<sc>bstract</sc> A first study is presented of the cross section for the scattering of same-sign W boson pairs via the detection of aτlepton. The data from proton-proton collisions at the center-of-mass energy of 13 TeV were collected by the CMS detector at the LHC, and correspond to an integrated luminosity of 138 fb⁻¹. Events were selected that contain two jets with large pseudorapidity and large invariant mass, oneτlepton, one light lepton (e orμ), and significant missing transverse momentum. The measured cross section for electroweak same-sign WW scattering is$${1.44}_{-0.56}^{+0.63}$$times the standard model prediction. In addition, a search is presented for the indirect effects of processes beyond the standard model via the effective field theory framework, in terms of dimension-6 and dimension-8 operators. 

A<sc>bstract</sc> Measurements of light-by-light scattering (LbL,γγ → γγ) and the Breit-Wheeler process (BW,γγ →e⁺e⁻) are reported in ultraperipheral PbPb collisions at a centre-of-mass energy per nucleon pair of 5.02 TeV. The data sample, corresponding to an integrated luminosity of 1.7 nb⁻¹, was collected by the CMS experiment at the CERN LHC in 2018. Events with an exclusively producedγγore⁺e⁻pair with invariant massesm^γγ,ee>5 GeV, along with other fiducial criteria, are selected. The measured BW fiducial production cross section,σ_fid(γγ → e⁺e⁻) = 263.5±1.8(stat)±17.8(syst)μb, as well as the differential distributions for various kinematic observables, are in agreement with leading-order quantum electrodynamics predictions complemented with final-state photon radiation. The measured differential BW cross sections allow discrimination between different theoretical descriptions of the photon flux of the lead ion. In the LbL final state, 26 exclusive diphoton candidate events are observed compared with 12.0 ± 2.9 expected for the background. Combined with previous results, the observed significance of the LbL signal with respect to the background-only hypothesis is above five standard deviations. The measured fiducial LbL scattering cross section,σ_fid(γγ→γγ) = 107 ± 24(stat) ± 13(syst) nb, is in agreement with next- to-leading-order predictions. Limits on the production of axion-like particles coupled to photons are set over the mass range 5–100 GeV, including the most stringent limits to date in the range of 5–10 GeV. 

The first search for a heavy neutral spin-1 gauge boson ( $Z^{\prime }$ ) 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 $\sqrt{s}=13\mathrm{TeV}$ , collected from 2016 to 2018 with the CMS experiment and corresponding to an integrated luminosity of $138{\mathrm{fb}}^{-1}$ . The data are consistent with the standard model predictions. Upper limits are set on the product of the cross section for production of the $Z^{\prime }$ boson and its branching fraction to $\tau \tau$ or $WW$ . The presence of a $Z^{\prime }$ boson decaying to ${\tau }^{+}{\tau }^{-}$ ( $W^{+}{W}^{-}$ ) is excluded for masses up to 2.45(1.60) TeV, depending on the $Z^{\prime }$ boson coupling to standard model weak bosons, and assuming a $Z^{\prime }\to {\tau }^{+}{\tau }^{-}$ ( $W^{+}{W}^{-}$ ) branching fraction of 50%.