First measurement of the |t|-dependence of coherent J/ψ photonuclear production

The first measurement of the cross section for coherent J/ψ photoproduction as a function of |t|, the square of the momentum transferred between the incoming and outgoing target nucleus, is presented. The data were measured with the ALICE detector in ultra-peripheral Pb–Pb collisions at a centre-of-mass energy per nucleon pair sNN=5.02TeV with the J/ψ produced in the central rapidity region |y|<0.8, which corresponds to the small Bjorken-x range (0.3−1.4)×10−3. The measured |t|-dependence is not described by computations based only on the Pb nuclear form factor, while the photonuclear cross section is better reproduced by models including shadowing according to the leading-twist approximation, or gluon-saturation effects from the impact-parameter dependent Balitsky–Kovchegov equation. These new results are therefore a valid tool to constrain the relevant model parameters and to investigate the transverse gluonic structure at very low Bjorken-x.publishedVersio

A(c)(+) Production and Baryon-to-Meson Ratios in pp and p-Pb Collisions at root S-NN=5.02 TeV at the LHC

The prompt production of the charm baryon \u39bc+ and the \u39bc+/D0 production ratios were measured at midrapidity with the ALICE detector in pp and p-Pb collisions at sNN=5.02 TeV. These new measurements show a clear decrease of the \u39bc+/D0 ratio with increasing transverse momentum (pT) in both collision systems in the range 2<12 GeV/c, exhibiting similarities with the light-flavor baryon-to-meson ratios p/\u3c0 and \u39b/KS0. At low pT, predictions that include additional color-reconnection mechanisms beyond the leading-color approximation, assume the existence of additional higher-mass charm-baryon states, or include hadronization via coalescence can describe the data, while predictions driven by charm-quark fragmentation processes measured in e+e- and e-p collisions significantly underestimate the data. The results presented in this Letter provide significant evidence that the established assumption of universality (colliding-system independence) of parton-to-hadron fragmentation is not sufficient to describe charm-baryon production in hadronic collisions at LHC energies

A(c)(+) Production and Baryon-to-Meson Ratios in pp and p-Pb Collisions at root S-NN=5.02 TeV at the LHC

The prompt production of the charm baryon Λ_{c}^{+} and the Λ_{c}^{+}/D^{0} production ratios were measured at midrapidity with the ALICE detector in pp and p-Pb collisions at sqrt[s_{NN}]=5.02  TeV. These new measurements show a clear decrease of the Λ_{c}^{+}/D^{0} ratio with increasing transverse momentum (p_{T}) in both collision systems in the range 2<p_{T}<12  GeV/c, exhibiting similarities with the light-flavor baryon-to-meson ratios p/π and Λ/K_{S}^{0}. At low p_{T}, predictions that include additional color-reconnection mechanisms beyond the leading-color approximation, assume the existence of additional higher-mass charm-baryon states, or include hadronization via coalescence can describe the data, while predictions driven by charm-quark fragmentation processes measured in e^{+}e^{-} and e^{-}p collisions significantly underestimate the data. The results presented in this Letter provide significant evidence that the established assumption of universality (colliding-system independence) of parton-to-hadron fragmentation is not sufficient to describe charm-baryon production in hadronic collisions at LHC energies

Suppression in Pb-Pb Collisions at the LHC.

The production of the ψ(2S) charmonium state was measured with ALICE in Pb-Pb collisions at sqrt[s_{NN}]=5.02  TeV, in the dimuon decay channel. A significant signal was observed for the first time at LHC energies down to zero transverse momentum, at forward rapidity (2.5<y<4). The measurement of the ratio of the inclusive production cross sections of the ψ(2S) and J/ψ resonances is reported as a function of the centrality of the collisions and of transverse momentum, in the region p_{T}<12  GeV/c. The results are compared with the corresponding measurements in pp collisions, by forming the double ratio [σ^{ψ(2S)}/σ^{J/ψ}]_{Pb-Pb}/[σ^{ψ(2S)}/σ^{J/ψ}]_{pp}. It is found that in Pb-Pb collisions the ψ(2S) is suppressed by a factor of ∼2 with respect to the J/ψ. The ψ(2S) nuclear modification factor R_{AA} was also obtained as a function of both centrality and p_{T}. The results show that the ψ(2S) resonance yield is strongly suppressed in Pb-Pb collisions, by a factor of up to ∼3 with respect to pp. Comparisons of cross section ratios with previous Super Proton Synchrotron findings by the NA50 experiment and of R_{AA} with higher-p_{T} results at LHC energy are also reported. These results and the corresponding comparisons with calculations of transport and statistical models address questions on the presence and properties of charmonium states in the quark-gluon plasma formed in nuclear collisions at the LHC

Constraining the Chiral Magnetic Effect with charge-dependent azimuthal correlations in Pb-Pb collisions at root s(NN)=2.76 and 5.02 TeV

Systematic studies of charge-dependent two- and three-particle correlations in Pb-Pb collisions at sNN = 2.76 and 5.02 TeV used to probe the Chiral Magnetic Effect (CME) are presented. These measurements are performed for charged particles in the pseudorapidity (\u3b7) and transverse momentum (pT) ranges |\u3b7| &lt; 0.8 and 0.2 &lt; pT&lt; 5 GeV/c. A significant charge-dependent signal that becomes more pronounced for peripheral collisions is reported for the CME-sensitive correlators \u3b31, 1 = \u3008cos(\u3c6\u3b1 + \u3c6\u3b2 12 2\u3a82)\u3009 and \u3b31, 12 3 = \u3008cos(\u3c6\u3b1 12 3\u3c6\u3b2 + 2\u3a82)\u3009. The results are used to estimate the contribution of background effects, associated with local charge conservation coupled to anisotropic flow modulations, to measurements of the CME. A blast-wave parametrisation that incorporates local charge conservation tuned to reproduce the centrality dependent background effects is not able to fully describe the measured \u3b31,1. Finally, the charge and centrality dependence of mixed-harmonics three-particle correlations, of the form \u3b31, 2 = \u3008cos(\u3c6\u3b1 + 2\u3c6\u3b2 12 3\u3a83)\u3009, which are insensitive to the CME signal, verify again that background contributions dominate the measurement of \u3b31,1

Exploration of jet substructure using iterative declustering in pp and Pb-Pb collisions at LHC energies

The ALICE collaboration at the CERN LHC reports novel measurements of jet substructure in pp collisions at s=7 TeV and central Pb–Pb collisions at sNN=2.76 TeV. Jet substructure of track-based jets is explored via iterative declustering and grooming techniques. We present the measurement of the momentum sharing of two-prong substructure exposed via grooming, the zg, and its dependence on the opening angle, in both pp and Pb–Pb collisions. We also present the measurement of the distribution of the number of branches obtained in the iterative declustering of the jet, which is interpreted as the number of its hard splittings. In Pb–Pb collisions, we observe a suppression of symmetric splittings at large opening angles and an enhancement of splittings at small opening angles relative to pp collisions, with no significant modification of the number of splittings. The results are compared to predictions from various Monte Carlo event generators to test the role of important concepts in the evolution of the jet in the medium such as colour coherence

Charged-particle production as a function of multiplicity and transverse spherocity in pp collisions at √s=5.02 and 13 TeV

We present a study of the inclusive charged- particle transverse momentum ( pT ) spectra as a function of charged-particle multiplicity density at mid-pseudorapidity, d Nch /dη, in pp collisions at √s = 5.02 and 13 TeV covering the kinematic range |η| 0). The pT spectra are reported for two multiplicity estimators covering different pseudo- rapidity regions. The pT spectra normalized to that for INEL> 0 show little energy dependence. Moreover, the high-pT yields of charged particles increase faster than the charged-particle multiplicity density. The average pT as a function of multiplicity and transverse spherocity is reported for pp collisions at √s = 13 TeV. For low- (high-) spherocity events, corresponding to jet-like (isotropic) events, the aver- age pT is higher (smaller) than that measured in INEL > 0 pp collisions. Within uncertainties, the functional form of ⟨ pT ⟩( Nch ) is not affected by the spherocity selection. While EPOS LHC gives a good description of many features of data, PYTHIA overestimates the average pT in jet-like events

Multiplicity dependence of charged-particle production in pp, p–Pb, Xe–Xe and Pb–Pb collisions at the LHC

Multiplicity (Nch) distributions and transverse momentum (pT) spectra of inclusive primary charged particles in the kinematic range of |η|<0.8 and 0.15 GeV/c<pT<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 sNN=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 pT 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

Production of light-flavor hadrons in pp collisions at root s=7 and root s=13 Tev

The production of pi(+/-), K-+/-, K-S(0), K*(892)(0), p, phi(1020), Lambda, Xi(-), Omega(-), and their antiparticles was measured in inelastic proton-proton (pp) collisions at a center-of-mass energy of root s = 13 TeV at midrapidity (vertical bar y vertical bar < 0.5) as a function of transverse momentum (P-T) using the ALICE detector at the CERN LHC. Furthermore, the single-particle p(T) distributions of K-S(0), Lambda, and (Lambda) over bar in inelastic pp collisions at root s = 7 TeV are reported here for the first time. The p(T) distributions are studied at midrapidity within the transverse momentum range 0 <= p(T) <= 20 GeV/c, depending on the particle species. The p(T) spectra, integrated yields, and particle yield ratios are discussed as a function of collision energy and compared with measurements at lower root s and with results from various general-purpose QCD-inspired Monte Carlo models. A hardening of the spectra at high p(T) with increasing collision energy is observed, which is similar for all particle species under study. The transverse mass and x(T) 2p(T)/root s scaling properties of hadron produc- tion are also studied. As the collision energy increases from root s = 7-13 TeV, the yields of non- and single-strange hadrons normalized to the pion yields remain approximately constant as a function of root s, while ratios for multi-strange hadrons indicate enhancements. The p(T)-differential cross sections of pi(+/-), K-+/- and p ((p) over bar) are compared with next-to-leading order perturbative QCD calculations, which are found to overestimate the cross sections for pi(+/-) and p ((p) over bar) at high p(T)