Charge-dependent two- and three-particle correlations measured in Xe-Xe collisions at $\sqrt{s_{\mathrm{NN}}} = 5.44$ TeV are presented. Results are obtained for charged particles in the pseudorapidity range $|\eta|<0.8$ and transverse momentum interval $0.2 \le p_{\rm T}<5.0$ GeV/$c$ for different collision centralities. The three-particle correlator ${\gamma_{\alpha\beta}} \equiv {\langle \cos(\varphi_\alpha + \varphi_\beta - 2{\Psi_{\rm 2}}) \rangle}$, calculated for different combinations of charge sign $\alpha$ and $\beta$, is expected to be sensitive to the presence of the Chiral Magnetic Effect (CME). Its magnitude is similar to the one observed in Pb-Pb collisions in contrast to a smaller CME signal in Xe-Xe collisions than in Pb-Pb collisions predicted by Monte Carlo (MC) calculations including a magnetic field induced by the spectator protons. These observations point to a large non-CME contribution to the correlator. Furthermore, the charge dependence of ${\gamma_{\alpha\beta}}$ can be described by a blast wave model calculation that incorporates background effects and by the Anomalous Viscous Fluid Dynamics model with values of the CME signal consistent with zero. The Xe-Xe and Pb-Pb results are combined with the expected CME signal dependence on the system size from the MC calculations including a magnetic field to obtain the fraction of CME contribution in ${\gamma_{\alpha\beta}}$, $f_{\rm CME}$. The CME fraction is compatible with zero for the 30% most central events in both systems and then becomes positive; averaging over the 0-70% centrality interval yields an upper limit of 2% (3%) and 25% (32%) at 95% (99.7%) confidence level for the CME signal contribution to ${\gamma_{\alpha\beta}}$ in Xe-Xe and Pb-Pb collisions, respectively.
$\langle \cos(\varphi_{\alpha} - \varphi_{\beta}) \rangle$ (opposite charge pairs) as a function of centrality in Xe-Xe collisions at $\sqrt{s_{\rm NN}} = 5.44$ TeV.
$\langle \cos(\varphi_{\alpha} - \varphi_{\beta}) \rangle$ (same charge pairs) as a function of centrality in Xe-Xe collisions at $\sqrt{s_{\rm NN}} = 5.44$ TeV.
$\langle \cos(\varphi_{\alpha} + \varphi_{\beta} - 2\Psi_{2}) \rangle$ (opposite charge pairs) as a function of centrality in Xe-Xe collisions at $\sqrt{s_{\rm NN}} = 5.44$ TeV.
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.
The ALICE Collaboration reports the measurement of semi-inclusive distributions of charged-particle jets recoiling from a high transverse momentum (high $p_{\rm T}$) hadron trigger in proton$-$proton and central Pb$-$Pb collisions at $\sqrt{s_{\rm NN}} = 5.02$ TeV. A data-driven statistical method is used to mitigate the large uncorrelated background in central Pb$-$Pb collisions. Recoil jet distributions are reported for jet resolution parameter $R=0.2$, 0.4, and 0.5 in the range $7 < p_{\rm T,jet} < 140$ GeV$/c$ and trigger$-$recoil jet azimuthal separation $\pi/2 < \Delta\varphi < \pi$. The measurements exhibit a marked medium-induced jet yield enhancement at low $p_{\rm T}$ and at large azimuthal deviation from $\Delta\varphi\sim\pi$. The enhancement is characterized by its dependence on $\Delta\varphi$, which has a slope that differs from zero by 4.7$\sigma$. Comparisons to model calculations incorporating different formulations of jet quenching are reported. These comparisons indicate that the observed yield enhancement arises from the response of the QGP medium to jet propagation.
Corrected $\Delta_\mathrm{recoil} (p_\mathrm{T,ch jet})$ distributions measured for $R=$ 0.2, 0.4, and 0.5 in pp collisions at $\sqrt{s}=5.02$ TeV.
Corrected $\Delta_\mathrm{recoil} (p_\mathrm{T,ch jet})$ distributions measured for $R=$ 0.2, 0.4, and 0.5 in Pb--Pb collisions at $\sqrt{s}=5.02$ TeV.
$I_\mathrm{AA} (p_\mathrm{T,ch jet})$ measured for $R=$ 0.2, 0.4, and 0.5 in Pb--Pb collisions at $\sqrt{s}=5.02$ TeV.
The ALICE Collaboration reports measurements of the semi-inclusive distribution of charged-particle jets recoiling from a high transverse momentum (high $p_{\rm T}$) charged hadron, in pp and central Pb$-$Pb collisions at center of mass energy per nucleon$-$nucleon collision $\sqrt{s_{\rm NN}}=5.02$ TeV. The large uncorrelated background in central Pb$-$Pb collisions is corrected using a data-driven statistical approach, which enables precise measurement of recoil jet distributions over a broad range in $p_{\rm T,ch\,jet}$ and jet resolution parameter $R$. Recoil jet yields are reported for $R=0.2$, 0.4, and 0.5 in the range $7 < p_{\rm T,ch\, jet} < 140$ GeV$/c$ and $\pi/2<\Delta\varphi<\pi$, where $\Delta\varphi$ is the azimuthal angular separation between hadron trigger and recoil jet. The low $p_{\rm T,ch\,jet}$ reach of the measurement explores unique phase space for studying jet quenching, the interaction of jets with the quark-gluonnplasma generated in high-energy nuclear collisions. Comparison of $p_{\rm T,ch\,jet}$ distributions from pp and central Pb$-$Pb collisions probes medium-induced jet energy loss and intra-jet broadening, while comparison of their acoplanarity distributions explores in-medium jet scattering and medium response. The measurements are compared to theoretical calculations incorporating jet quenching.
Corrected $\Delta_\mathrm{recoil} (p_\mathrm{T,ch jet})$ distributions measured for $R=$ 0.2, 0.4, and 0.5 in pp collisions at $\sqrt{s}=5.02$ TeV.
Corrected $\Delta_\mathrm{recoil} (p_\mathrm{T,ch jet})$ distributions measured for $R=$ 0.2, 0.4, and 0.5 in Pb--Pb collisions at $\sqrt{s}=5.02$ TeV.
$I_\mathrm{AA} (p_\mathrm{T,ch jet})$ measured for $R=$ 0.2, 0.4, and 0.5 in Pb--Pb collisions at $\sqrt{s}=5.02$ TeV.
Multiplicity ($N_{\rm ch}$) distributions and transverse momentum ($p_{\rm T}$) spectra of inclusive primary charged particles in the kinematic range of $|\eta| < 0.8$ and 0.15 GeV/$c$$< p_{T} <$ 10 GeV/$c$ are reported for pp, p-Pb, Xe-Xe and Pb-Pb collisions at centre-of-mass energies per nucleon pair ranging from $\sqrt{s_{\rm NN}} = 2.76$ TeV up to $13$ TeV. A sequential two-dimensional unfolding procedure is used to extract the correlation between the transverse momentum of primary charged particles and the charged-particle multiplicity of the corresponding collision. This correlation sharply characterises important features of the final state of a collision and, therefore, can be used as a stringent test of theoretical models. The multiplicity distributions as well as the mean and standard deviation derived from the $p_{\rm T}$ spectra are compared to state-of-the-art model predictions. Providing these fundamental observables of bulk particle production consistently across a wide range of collision energies and system sizes can serve as an important input for tuning Monte Carlo event generators.
Charged-particle multiplicity distribution for pp collisions at 2.76 TeV.
Charged-particle multiplicity distribution for pp collisions at 2.76 TeV.
Koba-Nielsen-Olesen scaled charged-particle multiplicity distribution for pp collisions at 2.76 TeV.
K$^{+}$K$^{-}$ pairs may be produced in photonuclear collisions, either from the decays of photoproduced $\phi (1020)$ mesons, or directly as non-resonant K$^{+}$K$^{-}$ pairs. Measurements of K$^{+}$K$^{-}$ photoproduction probe the couplings between the $\phi (1020)$ and charged kaons with photons and nuclear targets. The kaon$-$proton scattering occurs at energies far above those available elsewhere. We present the first measurement of coherent photoproduction of K$^{+}$K$^{-}$ pairs on lead ions in ultra-peripheral collisions using the ALICE detector, including the first investigation of direct K$^{+}$K$^{-}$ production. There is significant K$^{+}$K$^{-}$ production at low transverse momentum, consistent with coherent photoproduction on lead targets. In the mass range $1.1 < M_{\rm{KK}} < 1.4$ GeV/$c^2$ above the $\phi (1020)$ resonance, for rapidity $|y_{\rm{KK}}|<0.8$ and $p_{\rm T,KK} < 0.1$ GeV/$c$, the measured coherent photoproduction cross section is $\mathrm{d}\sigma/\mathrm{d}y$ = 3.37 $\pm\ 0.61$ (stat.) $\pm\ 0.15 $ (syst.) mb. The center-of-mass energy per nucleon of the photon-nucleus (Pb) system $W_{\gamma \mathrm{Pb, n}}$ ranges from 33 to 188 GeV, far higher than previous measurements on heavy-nucleus targets. The cross section is larger than expected for $\phi (1020)$ photoproduction alone. The mass spectrum is fit to a cocktail consisting of $\phi (1020)$ decays, direct K$^{+}$K$^{-}$ photoproduction, and interference between the two. The confidence regions for the amplitude and relative phase angle for direct K$^{+}$K$^{-}$ photoproduction are presented.
d$^2\sigma$/d$y$/d$p_T^2$ in bins of $p_{T,KK}^2$ for $K^+K^-$ photoproduction in ultra-peripheral Pb$-$Pb collisions at $\sqrt{s_\mathrm{NN}}$ = 5.02 TeV .
d$^2\sigma$/d$y$/d$p_T^2$ in bins of $M_{KK}$ for $K^+K^-$ photoproduction in ultra-peripheral Pb$-$Pb collisions at $\sqrt{s_\mathrm{NN}}$ = 5.02 TeV .
This Letter presents the most precise measurement to date of the matter/antimatter imbalance at midrapidity in Pb-Pb collisions at a center-of-mass energy per nucleon pair $\sqrt{s_{\rm NN}} = 5.02$ TeV. Using the Statistical Hadronization framework, it is possible to obtain the value of the electric charge and baryon chemical potentials, $\mu_Q=-0.18\pm0.90$ MeV and $\mu_B=0.71\pm0.45$ MeV, with unprecedented precision. A centrality-differential study of the antiparticle-to-particle yield ratios of charged pions, protons, $\Omega$-baryons, and light (hyper)nuclei is performed. These results indicate that the system created in Pb-Pb collisions at the LHC is on average baryon-free and electrically neutral at midrapidity.
Antiparticle-to-particle yield ratio, 0-5% centrality
Antiparticle-to-particle yield ratio, 5-10% centrality
Antiparticle-to-particle yield ratio, 10-30% centrality
The dependence of $\mathrm{f}_{0}$(980) production on the final-state charged-particle multiplicity in p$-$Pb collisions at $\sqrt{s_{\mathrm{NN}}} = 5.02$ TeV is reported. The production of $\mathrm{f}_{0}$(980) is measured with the ALICE detector via the $\mathrm{f}_0 (980) \rightarrow \pi^{+}\pi^{-}$ decay channel in a midrapidity region of $-0.5<y<0$. Particle yield ratios of $\mathrm{f}_{0}$(980) to $\pi$ and $\mathrm{K}^{*}$(892)$^{0}$ are found to be decreasing with increasing charged-particle multiplicity. The magnitude of the suppression of the $\mathrm{f}_{0}$(980)/$\pi$ and $\mathrm{f}_{0}$(980)/$\mathrm{K}^{*}$(892)$^{0}$ yield ratios is found to be dependent on the transverse momentum $p_{\mathrm{T}}$, suggesting different mechanisms responsible for the measured effects. Furthermore, the nuclear modification factor $Q_{\mathrm{pPb}}$ of $\mathrm{f}_{0}$(980) is measured in various multiplicity ranges. The $Q_{\mathrm{pPb}}$ shows a strong suppression of the $\mathrm{f}_{0}$(980) production in the $p_{\mathrm{T}}$ region up to about 4 GeV/$c$. The results on the particle yield ratios and $Q_{\mathrm{pPb}}$ for $\mathrm{f}_{0}$(980) may help to understand the late hadronic phase in p$-$Pb collisions and the nature of the internal structure of $\mathrm{f}_{0}$(980) particle.
Transverse momentum spectra in different multiplicity classes. Each spectrum is corrected for the branching ratio of (46 $\pm$ 6)% based on [Phys. Rev. Lett. 111 no. 6, (2013) 062001].
The ratio of transverse momentum spectrum to the NSD spectrum
The double ratio of particle yield of f0((980) to charged pions
The production of $\Upsilon$(2S) and $\Upsilon$(3S) mesons in lead-lead (PbPb) and proton-proton (pp) collisions is studied in their dimuon decay channel using the CMS detector at the LHC. The $\Upsilon$(3S) meson is observed for the first time in PbPb collisions, with a significance above five standard deviations. The ratios of yields measured in PbPb and pp collisions are reported for both the $\Upsilon$(2S) and $\Upsilon$(3S) mesons, as functions of transverse momentum and PbPb collision centrality. These ratios, when appropriately scaled, are significantly less than unity, indicating a suppression of $\Upsilon$ yields in PbPb collisions. This suppression increases from peripheral to central PbPb collisions. Furthermore, the suppression is stronger for $\Upsilon$(3S) mesons compared to $\Upsilon$(2S) mesons, extending the pattern of sequential suppression of quarkonium states in nuclear collisions previously seen for the $\psi$/J, $\psi$(2S), $\Upsilon$(1S), and $\Upsilon$(2S) mesons.
Measured $R_\text{AA}$ for the $\Upsilon(2S)$ state as functions of PbPb collision centrality, integrated over the full kinematic range $p_\text{T}$ < 30 GeV/c and |y| < 2.4. The global uncertainty "PP MB" represents the pp luminosity and PbPb $N_\text{MB}$ combined uncertainties, whereas the global uncertainty "PP 2S" corresponds to the uncertainty on the $\Upsilon(2S)$ pp yields.
Measured $R_\text{AA}$ for the $\Upsilon(2S)$ state as functions of PbPb collision centrality, integrated over the full kinematic range $p_\text{T}$ < 30 GeV/c and |y| < 2.4. The global uncertainty "PP MB" represents the pp luminosity and PbPb $N_\text{MB}$ combined uncertainties, whereas the global uncertainty "PP 2S" corresponds to the uncertainty on the $\Upsilon(2S)$ pp yields.
Measured $R_\text{AA}$ for the $\Upsilon(3S)$ state as functions of PbPb collision centrality, integrated over the full kinematic range $p_\text{T}$ < 30 GeV/c and |y| < 2.4. The global uncertainty "PP MB" represents the pp luminosity and PbPb $N_\text{MB}$ combined uncertainties, whereas the global uncertainty "PP 3S" corresponds to the uncertainty on the $\Upsilon(3S)$ pp yields.
Measurements of inclusive charged-particle jet production in pp and p-Pb collisions at center-of-mass energy per nucleon-nucleon collision $\sqrt{s_{\rm NN}} = 5.02$ TeV and the corresponding nuclear modification factor $R_{\rm pPb}^{\rm ch\,jet}$ are presented, using data collected with the ALICE detector at the LHC. Jets are reconstructed in the central rapidity region $|\eta_{\rm jet}| < 0.5$ from charged particles using the anti-$k_{\rm T}$ algorithm with resolution parameters $R = 0.2$, 0.3, and 0.4. The $p_{\rm T}$-differential inclusive production cross section of charged-particle jets, as well as the corresponding cross section ratios, are reported for pp and p-Pb collisions in the transverse momentum range $10 < p^{\rm ch}_{\rm T,jet} < 140$ GeV/$c$ and $10 < p^{\rm ch}_{\rm T,jet} < 160$ GeV/$c$, respectively, together with the nuclear modification factor $R_{\rm pPb}^{\rm ch\,jet}$ in the range $10 < p^{\rm ch}_{\rm T,jet} < 140$ GeV/$c$. The analysis extends the $p_{\rm T}$ range of the previously-reported charged-particle jet measurements by the ALICE Collaboration. The nuclear modification factor is found to be consistent with one and independent of the jet resolution parameter with the improved precision of this study, indicating that the possible influence of cold nuclear matter effects on the production cross section of charged-particle jets in p-Pb collisions at $\sqrt{s_{\rm NN}} = 5.02$ TeV is smaller than the current precision. The obtained results are in agreement with other minimum bias jet measurements available for RHIC and LHC energies, and are well reproduced by the NLO perturbative QCD POWHEG calculations with parton shower provided by PYTHIA8 as well as by JETSCAPE simulations.
$p_{\rm T}$-differential cross section of charged-particle jets with $R = 0.2$, $0.3$ and $0.4$ in pp collisions at $\sqrt{s} = 5.02$ TeV.
$p_{\rm T}$-differential cross section of charged-particle jets from model predictions in pp collisions at $\sqrt{s} = 5.02$ TeV.
$p_{\rm T}$-differential cross section of charged-particle jets with $R = 0.2$, $0.3$ and $0.4$ in p-Pb collisions at $\sqrt{s_{\rm NN}} = 5.02$ TeV.
Forum