The polarization of the $Λ$ and $\overlineΛ$ hyperons along the beam direction has been measured in proton-lead (pPb) 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 pPb 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.
The second-order Fourier sine coefficients of $\Lambda$, $\bar{\Lambda}$ and $\Lambda+\bar{\Lambda}$ polarizations along the beam direction as functions of $N_\mathrm{trk}^\mathrm{offline}$ in pPb collisions at 8.16 TeV.
The second-order Fourier sine coefficients of $\Lambda+\bar{\Lambda}$ polarization along the beam direction as functions of $p_{T}$ in pPb collisions at 8.16 TeV.
The second-order Fourier sine coefficients of $K_{S}^{0}$ polarization along the beam direction as functions of $N_\mathrm{trk}^\mathrm{offline}$ in pPb collisions at 8.16 TeV.
Measurements of light-by-light scattering (LbL, $γγ$$\to$$γγ$) and the Breit-Wheeler process (BW, $γγ$$\to$$\mathrm{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 produced $γγ$ or $\mathrm{e^+e^-}$ pair with invariant masses $m^{γγ,\mathrm{ee}}$$>$ 5 GeV, along with other fiducial criteria, are selected. The measured BW fiducial production cross section, $σ_\text{fid}$($γγ$$\to$$\mathrm{e^+e^-}$) = 263.5 $\pm$ 1.8 (stat) $\pm$ 17.8 (syst) $μ$b, as well as the differential distributions for various kinematic observables, are in agreement with leading-order quantum electrodynamics predictions complemented with final-state photon radiation. The measured differential BW cross sections allow discriminating between different theoretical descriptions of the photon flux of the lead ion. In the LbL final state, 26 exclusive diphoton candidate events are observed compared with 12.0 $\pm$ 2.9 expected for the background. Combined with previous results, the observed significance of the LbL signal with respect to the background-only hypothesis is above five standard deviations. The measured fiducial LbL scattering cross section, $σ_\text{fid} (γγ$$\to$$γγ)$ = 107 $\pm$ 24 (stat) $\pm$ 13 (syst) nb, is in agreement with next-to-leading-order predictions. Limits on the production of axion-like particles coupled to photons are set over the mass range 5-100 GeV, including the most stringent limits to date in the range of 5-10 GeV.
Probability for different neutron multiplicity classes (0n, 1n, and Xn with X $\ge$1) measured on each ZDC side for the exclusive $e^{+}e^{-}$ events passing the fiducial phase space of Table 1. The measured ratios are compared with SUPERCHIC 4.2, STARLIGHT 3.13, and gamma-UPC 1.6 predictions.
Differential cross sections for exclusive dielectron production, in the fiducial phase space of Table 1, as functions of the pair $p_T$. Data are compared with SUPERCHIC + FSR(PHOTOS++), STARLIGHT + FSR(PY8), and gamma-UPC + FSR(PY8) predictions.
Differential cross sections for exclusive dielectron production, in the fiducial phase space of Table 1, as functions of the pair rapidity. Data are compared with SUPERCHIC + FSR(PHOTOS++), STARLIGHT + FSR(PY8), and gamma-UPC + FSR(PY8) predictions.
The production cross sections of B$^0_\mathrm{s}$ and B$^+$ mesons are reported in proton-proton (pp) collisions recorded by the CMS experiment at the CERN LHC with a center-of-mass energy of 5.02 TeV. The data sample corresponds to an integrated luminosity of 302 pb$^{-1}$. The cross sections are based on measurements of the B$^0_\mathrm{s}$$\to$ J/$ψ(μ^+μ^-)ϕ$(1020) (K$^+$K$^-$) and B$^+$$\to$ J/$ψ(μ^+μ^-)$K$^+$ decay channels. Results are presented in the transverse momentum ($p_\mathrm{T}$) range 7-50 GeV/$c$ and the rapidity interval $\lvert y \rvert$$\lt$ 2.4 for the B mesons. The measured $p_\mathrm{T}$-differential cross sections of B$^+$ and B$^0_\mathrm{s}$ in pp collisions are well described by fixed-order plus next-to-leading logarithm perturbative quantum chromodynamics calculations. Using previous PbPb collision measurements at the same nucleon-nucleon center-of-mass energy, the nuclear modification factors, $R_\mathrm{AA}$, of the B mesons are determined. For $p_\mathrm{T}$$\lt$ 10 GeV/$c$, both mesons are found to be suppressed in PbPb collisions (with $R_\mathrm{AA}$ values significantly below unity), with less suppression observed for the B$^0_\mathrm{s}$ mesons. In this $p_\mathrm{T}$ range, the $R_\mathrm{AA}$ values for the B$^+$ mesons are consistent with those for inclusive charged hadrons and D$^0$ mesons. Below 10 GeV/$c$, both B$^+$ and B$^0_\mathrm{s}$ are found to be less suppressed than either inclusive charged hadrons or D$^0$ mesons, with the B$^0_\mathrm{s}$$R_\mathrm{AA}$ value consistent with unity. The $R_\mathrm{AA}$ values found for the B$^+$ and B$^0_\mathrm{s}$ are compared to theoretical calculations, providing constraints on the mechanism of bottom quark energy loss and hadronization in the quark-gluon plasma, the hot and dense matter created in ultrarelativistic heavy ion collisions.
The B+ meson $p_{\rm{T}}$-dependent production cross section in pp collisions. The measurment was carried out inside a fiducial region respecting ($p_{\rm{T}}$<10 & 1.5<|y|<2.4) and ($p_{\rm{T}}$>10 & |y|<2.4). The luminosity is 302.3 pb^{-1}.
The Bs meson $p_{\rm{T}}$-dependent production cross section in pp collisions. The measurment was carried out inside a fiducial region respecting (pT<10 & 1.5<|y|<2.4) and ($p_{\rm{T}}$>10 & |y|<2.4). The luminosity is 302.3 pb^{-1}.
The B+ meson $p_{\rm{T}}$-dependent RAA in PpPp. The measurment was carried out inside a fiducial region respecting ($p_{\rm{T}}$<10 & 1.5<|y|<2.4) and ($p_{\rm{T}}$>10 & |y|<2.4).
The pseudorapidity ($\eta$) distributions of charged hadrons are measured using data collected at the highest ever nucleon-nucleon center-of-mass energy of $\sqrt{s_\mathrm{NN}}$ = 5.36 TeV for collisions of lead-lead ions. The data were recorded by the CMS experiment at the LHC in 2022 and correspond to an integrated luminosity of 0.30 $\pm$ 0.03 $\mu$b$^{-1}$. Using the CMS silicon pixel detector, the yields of primary charged hadrons produced in the range $\vert\eta\vert$$\lt$ 2.6 are reported. The evolution of the midrapidity particle density as a function of collision centrality is also reported. In the 5% most central collisions, the charged-hadron $\eta$ density in the range $\vert\eta\vert$ $\lt$ 0.5 is found to be 2032 $\pm$ 91 (syst), with negligible statistical uncertainty. This result is consistent with an extrapolation from nucleus-nucleus collision data at lower center-of-mass energies. Comparisons are made to various Monte Carlo event generators and to previous measurements of lead-lead and xenon-xenon collisions at similar collision energies. These new data detail the dependence of particle production on the collision energy, initial collision geometry, and the size of the colliding nuclei.
Charged-hadron $\mathrm{d}N_{\mathrm{ch}}/\mathrm{d}\eta$ distributions in PbPb collisions at 5.36 TeV for events in the 0--80%, 0--5% and 50--55% centrality class.
Charged-hadron $\mathrm{d}N_{\mathrm{ch}}/\mathrm{d}\eta$ in PbPb collisions at 5.36 TeV at midrapidity as a function of event centrality.
Charged-hadron $\mathrm{d}N_{\mathrm{ch}}/\mathrm{d}\eta/2A$ in PbPb collisions at 5.36 TeV at midrapidity as a function of event centrality.
Nuclear medium effects on B$^+$ meson production are studied using the binary-collision scaled cross section ratio between events of different charged-particle multiplicities from proton-lead collisions. Data, collected by the CMS experiment in 2016 at a nucleon-nucleon center-of-mass energy of $\sqrt{s_\mathrm{NN}}$ = 8.16 TeV, corresponding to an integrated luminosity of 175 nb$^{-1}$, were used. The scaling factors in the ratio are determined using a novel approach based on the Z $\to$$\mu^+\mu^-$ cross sections measured in the same events. The scaled ratio for B$^+$ is consistent with unity for all event multiplicities, putting stringent constraints on nuclear modification for heavy flavor.
$\mathrm{B}^+$ differential cross section and FONLL calculations, scaled by the number of binary collisions, vs $p_{\mathrm{T}}$. Global uncertainty (not included in the plot) is 4.7%, which comprises of the uncertainties in the integrated luminosity measurement and the $\mathrm{B}^+$ meson branching fraction.
$\mathrm{B}^+$ differential cross section in $p_{\mathrm{T}}$ bins divided into classes of multiplicity. For better visibility, data points are scaled by a factor of 2 (blue), 4 (magenta), or 8 (red). Vertical bars represent total uncertainties.
The $R_{\mathrm{HL}}$ for $\mathrm{B}^+$ in $p_{\mathrm{T}}$ bins for the highest and lowest multiplicity classes. The error bars correspond to the statistical uncertainty, and the boxes represent the sum in quadrature of systematic uncertainties.
The first observation of the concurrent production of two J/$\psi$ mesons in proton-nucleus collisions is presented. The analysis is based on a proton-lead (pPb) data sample recorded at a nucleon-nucleon center-of-mass energy of 8.16 TeV by the CMS experiment at the CERN LHC and corresponding to an integrated luminosity of 174.6 nb$^{-1}$. The two J/$\psi$ mesons are reconstructed in their $\mu^+\mu^-$ decay channels with transverse momenta $p_\mathrm{T}$$\gt$ 6.5 GeV and rapidity $\lvert y \rvert$$\lt$ 2.4. Events where one of the J/$\psi$ mesons is reconstructed in the dielectron channel are also considered in the search. The pPb $\to$ J/$\psi$J/$\psi$ + X process is observed with a significance of 5.3 standard deviations. The measured inclusive fiducial cross section, using the four-muon channel alone, is $\sigma$(pPb $\to$ J/$\psi$J/$\psi$ + X) = 22.0 $\pm$ 8.9 (stat) $\pm$ 1.5 (syst) nb. A fit of the data to the expected rapidity separation for pairs of J/$\psi$ mesons produced in single (SPS) and double (DPS) parton scatterings yields $\sigma^{\mathrm{pPb} \to \mathrm{J}/\psi\mathrm{J}/\psi +\mathrm{X}}_\text{SPS}$ = 16.5 $\pm$ 10.8 (stat) $\pm$ 0.1 (syst) nb and $\sigma^{\mathrm{pPb} \to \mathrm{J}/\psi\mathrm{J}/\psi + \mathrm{X}}_\text{DPS}$ = 5.4 $\pm$ 6.2 (stat) $\pm$ 0.4 (syst) nb, respectively. This latter result can be transformed into a lower bound on the effective DPS cross section, closely related to the squared average interparton transverse separation in the collision, of $\sigma_\text{eff}$$\gt$ 1.0 mb at 95% confidence level.
$\mathrm{pPb}\to\mathrm{J}\mspace{-2mu}/\mspace{-2mu}\psi\,\mathrm{J}\mspace{-2mu}/\mspace{-2mu}\psi+X$
$\mathrm{pPb}\to\mathrm{J}\mspace{-2mu}/\mspace{-2mu}\psi\,\mathrm{J}\mspace{-2mu}/\mspace{-2mu}\psi+X$
$\mathrm{pPb}\to\mathrm{J}\mspace{-2mu}/\mspace{-2mu}\psi\,\mathrm{J}\mspace{-2mu}/\mspace{-2mu}\psi+X$
This Letter presents the first measurements of the groomed jet radius $R_\mathrm{g}$ and the jet girth $g$ in events with an isolated photon recoiling against a jet in lead-lead (PbPb) and proton-proton (pp) collisions at the LHC at a nucleon-nucleon center-of-mass energy of 5.02 TeV. The observables $R_\mathrm{g}$ and $g$ provide a quantitative measure of how narrow or broad a jet is. The analysis uses PbPb and pp data samples with integrated luminosities of 1.7 nb$^{-1}$ and 301 pb$^{-1}$, respectively, collected with the CMS experiment in 2018 and 2017. Events are required to have a photon with transverse momentum $p_\mathrm{T}^\gamma$$>$ 100 GeV and at least one jet back-to-back in azimuth with respect to the photon and with transverse momentum $p_\mathrm{T}^\text{jet}$ such that $p_\mathrm{T}^\text{jet} / p_\mathrm{T}^\gamma$$>$ 0.4. The measured $R_\mathrm{g}$ and $g$ distributions are unfolded to the particle level, which facilitates the comparison between the PbPb and pp results and with theoretical predictions. It is found that jets with $p_\mathrm{T}^\text{jet} / p_\mathrm{T}^\gamma$$>$ 0.8, i.e., those that closely balance the photon $p_\mathrm{T}^\gamma$, are narrower in PbPb than in pp collisions. Relaxing the selection to include jets with $p_\mathrm{T}^\text{jet} / p_\mathrm{T}^\gamma$$>$ 0.4 reduces the narrowing of the angular structure of jets in PbPb relative to the pp reference. This shows that selection bias effects associated with jet energy loss play an important role in the interpretation of jet substructure measurements.
Unfolded jet girth distribution in PbPb normalized to the number of jets that pass the $x_J$>0.4 selection. All systematic uncertainties are bin-to-bin fully correlated (allowing for sign-changes bin-to-bin).The covaraince matrices are provided for the statistical uncertainties from data and MC in this HepData record.
Covariance matrix of the statistical uncertainty in data for the unfolded jet girth distribution in PbPb for jets that pass the $x_J$>0.4 selection.The bin indices correspond to the bins used in the jet girth distribution.
Covariance matrix of the statistical uncertainty in MC for the unfolded jet girth distribution in PbPb for jets that pass the $x_J$>0.4 selection.The bin indices correspond to the bins used in the jet girth distribution.
Ultrarelativistic nuclear collisions create a strongly interacting state of hot and dense quark-gluon matter that exhibits a remarkable collective flow behavior with minimal viscous dissipation. To gain deeper insights into its intrinsic nature and fundamental degrees of freedom, we determine the speed of sound in an extended volume of quark-gluon plasma using lead-lead (PbPb) collisions at a center-of-mass energy per nucleon pair of 5.02 TeV. The data were recorded by the CMS experiment at the CERN LHC and correspond to an integrated luminosity of 0.607 nb$^{-1}$. The measurement is performed by studying the multiplicity dependence of the average transverse momentum of charged particles emitted in head-on PbPb collisions. Our findings reveal that the speed of sound in this matter is nearly half the speed of light, with a squared value of 0.241 $\pm$ 0.002 (stat) $\pm$ 0.016 (syst) in natural units. The effective medium temperature, estimated using the mean transverse momentum, is 219 $\pm$ 8 (syst) MeV. The measured squared speed of sound at this temperature aligns precisely with predictions from lattice quantum chromodynamic (QCD) calculations. This result provides a stringent constraint on the equation of state of the created medium and direct evidence for a deconfined QCD phase being attained in relativistic nuclear collisions.
The event fraction distribution as a function of the charged-particle multiplicity, $N_{\mathrm{ch}}$, within the kinematic range of $|\eta|<0.5$ and extrapolated to the full $p_{\mathrm{T}}$ range, in PbPb collisions at $\sqrt{s_{_{\mathrm{NN}}}}=5.02$ TeV. The $N_{\mathrm{ch}}$ value is normalized by its value in the $0-5\%$ centrality class ($N_{\mathrm{ch}}^{0}$).
The average transverse momentum of charged particles, $\langle p_{\mathrm{T}}\rangle$, as a function of the charged-particle multiplicity, $N_{\mathrm{ch}}$, within the kinematic range of $|\eta|<0.5$ and extrapolated to the full $p_{\mathrm{T}}$ range in PbPb collisions at $\sqrt{s_{_{\mathrm{NN}}}}=5.02$ TeV. Both $\langle p_{\mathrm{T}}\rangle$ and $N_{\mathrm{ch}}$ are normalized by their values in the $0-5\%$ centrality class (${\langle p_{\mathrm{T}}\rangle}^{0}$ and $N_{\mathrm{ch}}^{0}$).
The speed of sound, $c^2_{\mathrm{s}}$, as a function of the effective temperature, $T_{\mathrm{eff}}$, with the CMS data point obtained from ultra-central PbPb collision data at $\sqrt{s_{_{\mathrm{NN}}}}=5.02$ TeV.
A search for partonic collective effects inside jets produced in proton-proton collisions is performed via correlation measurements of charged constituents using the CMS detector at the CERN LHC. The analysis uses data collected at a center-of-mass energy of $\sqrt{s}$ = 13 TeV, corresponding to an integrated luminosity of 138 fb$^{-1}$. Jets are reconstructed with the anti-$k_\mathrm{T}$ algorithm with a distance parameter of 0.8 and are required to have transverse momentum greater than 550 GeV and pseudorapidity $\lvert\eta\rvert$$\lt$ 1.6. Two-particle correlations among the charged constituents within the jets are studied as functions of the particles' azimuthal angle and pseudorapidity separations ($\Delta\phi^*$ and $\Delta\eta^*$) in a jet coordinate basis, where constituents' $\eta^*$, $\phi^*$ are defined relative to the direction of the jet. The correlation functions are studied in classes of in-jet charged-particle multiplicity up to $N_\text{ch}^\mathrm{j}$$\approx$ 100. Fourier harmonics are extracted from long-range azimuthal correlation functions to characterize azimuthal anisotropy for $\lvert\Delta\eta^*\rvert$$\gt$ 2. For low-$N_\text{ch}^\mathrm{j}$, the long-range elliptic anisotropic harmonic, $v^*_2$, is observed to decrease with $N_\text{ch}^\mathrm{j}$. This trend is well described by Monte Carlo event generators. However, a rising trend for $v^*_2$ emerges at $N_\text{ch}^\mathrm{j}$$\gtrsim$ 80, hinting at a possible onset of collective behavior, which is not reproduced by the models tested. This observation yields new insights into the dynamics of parton fragmentation processes in the vacuum.
Examples of two-particle angular correlations projected onto 1D $\Delta\phi^*$ for $\abs{\Delta\eta^*}>2$.
A continuous evolution of extracted two-particle Fourier coefficients $V^*_{N\Delta}$ as a function of $N_{ch}^{j}$.
The single-particle elliptic anisotropies $v^*_2$, as a function of $N_{ch}^{j}$.
Despite the f$_0$(980) hadron having been discovered half a century ago, the question about its quark content has not been settled: it might be an ordinary quark-antiquark ($\mathrm{q\bar{q}}$) meson, a tetraquark ($\mathrm{q\bar{q}q\bar{q}}$) exotic state, a kaon-antikaon ($\mathrm{K\bar{K}}$) molecule, or a quark-antiquark-gluon ($\mathrm{q\bar{q}g}$) hybrid. This paper reports strong evidence that the f$_0$(980) state is an ordinary $\mathrm{q\bar{q}}$ meson, inferred from the scaling of elliptic anisotropies ($v_2$) with the number of constituent quarks ($n_\mathrm{q}$), as empirically established using conventional hadrons in relativistic heavy ion collisions. The f$_0$(980) state is reconstructed via its dominant decay channel f$_0$(980) $\to$$π^+π^-$, in proton-lead collisions recorded by the CMS experiment at the LHC, and its $v_2$ is measured as a function of transverse momentum ($p_\mathrm{T}$). It is found that the $n_q$ = 2 ($\mathrm{q\bar{q}}$ state) hypothesis is favored over $n_q$ = 4 ($\mathrm{q\bar{q}q\bar{q}}$ or $\mathrm{K\bar{K}}$ states) by 7.7, 6.3, or 3.1 standard deviations in the $p_\mathrm{T}$$\lt$ 10, 8, or 6 GeV/$c$ ranges, respectively, and over $n_\mathrm{q}$ = 3 ($\mathrm{q\bar{q}g}$ hybrid state) by 3.5 standard deviations in the $p_\mathrm{T}$$\lt$ 8 GeV/$c$ range. This result represents the first determination of the quark content of the f$_0$(980) state, made possible by using a novel approach, and paves the way for similar studies of other exotic hadron candidates.
The elliptic flow, $v_{2}$, for $f_0(980)$ as a function of $p_{T}$ in pPb collision at 8.16 TeV.
The elliptic flow after nonflow subtraction, $v_{2}^{sub}$, for $f_0(980)$ as a function of $p_{T}$ in pPb collision at 8.16 TeV.
The elliptic flow after nonflow subtraction, $v_{2}^{sub}/2$, for $f_0(980)$ as a function of $<KE_{T}>/2$ in pPb collision at 8.16 TeV.
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