33 Search Results

A measurement of the production of prompt $$\Lambda^+_c$$ baryons in Pb–Pb collisions at $$\sqrt{s_{NN}} = 5.02$$ TeV with the ALICE detector at the LHC is reported. The $$\Lambda^+_c$$ and $$\Lambda^-_c$$ were reconstructed at midrapidity ($$\mid{y}\mid < 0.5$$) via the hadronic decay channel $$\Lambda^+_c$$ $$\rightarrow {pK^0_s}$$ (and charge conjugate) in the transverse momentum and centrality intervals GeV/c and 0–80%. The ratio, which is sensitive to the charm quark hadronisation mechanisms in the medium, is measured and found to be larger than the ratio measured in minimum-bias pp collisions at TeV and in p–Pb collisions at TeV. In particular, the values inmore » p–Pb and Pb–Pb collisions differ by about two standard deviations of the combined statistical and systematic uncertainties in the common interval covered by the measurements in the two collision systems. The ratio is also compared with model calculations including different implementations of charm quark hadronisation. The measured ratio is reproduced by models implementing a pure coalescence scenario, while adding a fragmentation contribution leads to an underestimation. The nuclear modification factor, , is also presented. The measured values of the of , and non-strange D mesons are compatible within the combined statistical and systematic uncertainties. They show, however, a hint of a hierarchy , conceivable with a contribution from coalescence mechanisms to charm hadron formation in the medium.« less

The procedure for the energy calibration of the high granularity electromagnetic calorimeter PHOS of the ALICE experiment is presented. The methods used to perform the relative gain calibration, to evaluate the geometrical alignment and the corresponding correction of the absolute energy scale, to obtain the nonlinearity correction coefficients and finally, to calculate the time-dependent calibration corrections, are discussed and illustrated by the PHOS performance in proton-proton (pp) collisions at √s = 13 TeV. After applying all corrections, the achieved mass resolutions for π⁰ and η mesons for p_T > 1.7 GeV/c are σ_m^π0 = 4.56 ± 0.03 MeV/c² and σ_m^ηmore » = 15.3 ± 1.0 MeV/c², respectively.« less

The ALICE Collaboration has measured the energy dependence of exclusive photoproduction of J / ψ vector mesons off proton targets in ultra–peripheral p–Pb collisions at a centre-of-mass energy per nucleon pair sNN=5.02 TeV. The e + e - and μ + μ - decay channels are used to measure the cross section as a function of the rapidity of the J / ψ in the range - 2.5 < y< 2.7 , corresponding to an energy in the γp centre-of-mass in the interval 40 < W γp < 550 GeV. The measurements, which are consistent with a power law dependencemore » of the exclusive J / ψ photoproduction cross section, are compared to previous results from HERA and the LHC and to several theoretical models. They are found to be compatible with previous measurements.« less

The pseudorapidity density of charged particles, d$$N$$_ch/d$$η$$, in p–Pb collisions has been measured at a centre-of-mass energy per nucleon–nucleon pair of $$\sqrt{s_{NN}}$$ = 8.16 TeV at mid-pseudorapidity for non-single-diffractive events. The results cover 3.6 units of pseudorapidity, |$$η$$| < 1.8. The d$$N$$_ch/d$$η$$ value is 19.1 ± 0.7 at |$$η$$| < 0.5. This quantity divided by (N_part)/2 is 4.73 ± 0.20 , where (N_part) is the average number of participating nucleons, is 9.5% higher than the corresponding value for p–Pb collisions at $$\sqrt{s_{NN}}$$ = 5.02 TeV. Measurements are compared with models based on different mechanisms for particle production. All models agreemore » within uncertainties with data in the Pb-going side, while HIJING overestimates, showing a symmetric behaviour, and EPOS underestimates the p-going side of the d$$N$$_ch/d$$η$$ distribution. Saturation-based models reproduce the distributions well for < -1.3. The d$$N$$_ch/d$$η$$ is also measured for different centrality estimators, based both on the charged-particle multiplicity and on the energy deposited in the Zero-Degree Calorimeters. A study of the implications of the large multiplicity fluctuations due to the small number of participants for systems like p–Pb in the centrality calculation for multiplicity-based estimators is discussed, demonstrating the advantages of determining the centrality with energy deposited near beam rapidity.« less

The transverse structure of jets was studied via jet fragmentation transverse momentum (j_T) distributions, obtained using two-particle correlations in proton-proton and proton-lead collisions, measured with the ALICE experiment at the LHC. The highest transverse momentum particle in each event is used as the trigger particle and the region 3 < pT_t < 15GeV/c is explored in this study. The measured distributions show a clear narrow Gaussian component and a wide non-Gaussian one. Based on Pythia simulations, the narrow component can be related to non-perturbative hadronization and the wide component to quantum chromodynamical splitting. The width of the narrow component showsmore » a weak dependence on the transverse momentum of the trigger particle, in agreement with the expectation of universality of the hadronization process. On the other hand, the width of the wide component shows a rising trend suggesting increased branching for higher transverse momentum. The results obtained in pp collisions at $$\sqrt{\mathrm{s}}=7$$ TeV and in p–Pb collisions at $$\sqrt{s_{\mathrm{NN}}}=5.02$$ TeV are compatible within uncertainties and hence no significant cold nuclear matter effects are observed. The results are compared to previous measurements from CCOR and PHENIX as well as to Pythia 8 and Herwig 7 simulations.« less