11 Search Results

The coherent photoproduction of J/ψ and ψ' mesons was measured in ultra-peripheral Pb–Pb collisions at a center-of-mass energy $$\sqrt{s_{\mathrm {NN}}}~=~5.02$$ TeV with the ALICE detector. Charmonia are detected in the central rapidity region for events where the hadronic interactions are strongly suppressed. The J/ψ is reconstructed using the dilepton (l⁺l^-) and proton–antiproton decay channels, while for the ψ' the dilepton and the l⁺l^-π⁺π^- decay channels are studied. The analysis is based on an event sample corresponding to an integrated luminosity of about 233 μb^-1. The results are compared with theoretical models for coherent J/ψ and ψ' photoproduction. The coherent cross sectionmore » is found to be in a good agreement with models incorporating moderate nuclear gluon shadowing of about 0.64 at a Bjorken-x of around 6×10^-4, such as the EPS09 parametrization, however none of the models is able to fully describe the rapidity dependence of the coherent J/ψ cross section including ALICE measurements at forward rapidity. The ratio of ψ' to J/ψ coherent photoproduction cross sections was also measured and found to be consistent with the one for photoproduction off protons.« less

The measurement of the azimuthal-correlation function of prompt D mesons with charged particles in pp collisions at \(\sqrt{s} =5.02\ \hbox {TeV}\) and p–Pb collisions at \(\sqrt{s_{\mathrm{NN}}} = 5.02\ \hbox {TeV}\) with the ALICE detector at the LHC is reported. The \(\mathrm{D}^{0}\), \(\mathrm{D}^{+} \), and \(\mathrm{D}^{*+} \) mesons, together with their charge conjugates, were reconstructed at midrapidity in the transverse momentum interval \(3< p_\mathrm{T} < 24\ \hbox {GeV}/c\) and correlated with charged particles having \(p_\mathrm{T} > 0.3\ \hbox {GeV}/c\) and pseudorapidity \(|\eta | < 0.8\). The properties of the correlation peaks appearing in the near- and away-side regions (for \(\Delta \varphimore » \approx 0\) and \(\Delta \varphi \approx \pi \), respectively) were extracted via a fit to the azimuthal correlation functions. The shape of the correlation functions and the near- and away-side peak features are found to be consistent in pp and p–Pb collisions, showing no modifications due to nuclear effects within uncertainties. The results are compared with predictions from Monte Carlo simulations performed with the PYTHIA, POWHEG+PYTHIA, HERWIG, and EPOS 3 event generators.« less

Here, we present a study of the inclusive charged-particle transverse momentum (p_T) spectra as a function of charged-particle multiplicity density at mid-pseudorapidity, dN_ch/dη, in pp collisions at $$ \sqrt{s}$$ = 5.02 and 13 TeV covering the kinematic range |η| < 0.8 and 0.15 < p_T < 20 GeV/c. The results are presented for events with at least one charged particle in |η| < 1 (INEL>0). The p_T spectra are reported for two multiplicity estimators covering different pseudorapidity regions. The p_T spectra normalized to that for INEL>0 show little energy dependence. Moreover, the high-p_T yields of charged particles increase faster than themore » charged-particle multiplicity density. The average p_T as a function of multiplicity and transverse spherocity is reported for pp collisions at $$ \sqrt{s}$$=13 TeV. For low- (high-) spherocity events, corresponding to jet-like (isotropic) events, the average p_T is higher (smaller) than that measured in INEL>0 pp collisions. Within uncertainties, the functional form of ⟨p_T⟩ (N_ch) is not affected by the spherocity selection. While EPOS LHC gives a good description of many features of data, PYTHIA overestimates the average p_T in jet-like events.« 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

We measured two-particle angular correlations in pp collisions at √s=7 TeV for pions, kaons, protons, and lambdas, for all particle/anti-particle combinations in the pair. Data for mesons exhibit an expected peak dominated by effects associated with mini-jets and are well reproduced by general purpose Monte Carlo generators. However, for baryon–baryon and anti-baryon–anti-baryon pairs, where both particles have the same baryon number, a near-side anti-correlation structure is observed instead of a peak. This effect is interpreted in the context of baryon production mechanisms in the fragmentation process. It currently presents a challenge to Monte Carlo models and its origin remains an openmore » question.« less

The invariant differential cross sections for inclusive π 0 and η mesons at midrapidity were measured in pp collisions at √s=2.76 TeV for transverse momenta 0.4 < p _T < 40 GeV/c and 0.6 < p _T < 20 GeV/c, respectively, using the ALICE detector. This large range in p _T was achieved by combining various analysis techniques and different triggers involving the electromagnetic calorimeter (EMCal). Particularly, a new single-cluster, shower-shape based method was developed for the identification of high-p _T neutral pions, which exploits that the showers originating from their decay photons overlap in the EMCal. Above 4 GeV/c, the measured crossmore » sections are found to exhibit a similar power-law behavior with an exponent of about 6.3. Next-to-leading-order perturbative QCD calculations differ from the measured cross sections by about 30% for the π ⁰ , and between 30–50% for the η meson, while generator-level simulations with PYTHIA 8.2 describe the data to better than 10–30%, except at p _T < 1 GeV/c. The new data can therefore be used to further improve the theoretical description of π ⁰ and η meson production.« less

The azimuthal correlations of D mesons with charged particles were measured with the ALICE apparatus in pp collisions at $\sqrt{s}=7\mathrm{T}\mathrm{e}\mathrm{V}$ and p–Pb collisions at $\sqrt{s_{\mathrm{N}\mathrm{N}}}=5.02\mathrm{T}\mathrm{e}\mathrm{V}$ at the Large Hadron Collider. ${\mathrm{D}}^0$, ${\mathrm{D}}^{more »}$ + , and ${\mathrm{D}}^{\ast +}$ mesons and their charge conjugates with transverse momentum $3<p_{\mathrm{T}}<16\mathrm{G}\mathrm{e}\mathrm{V}/c$ and rapidity in the nucleon-nucleon centre-of-mass system $|y_{\mathrm{c}\mathrm{m}\mathrm{s}}|<0.5$ (pp collisions) and $-0.96<y_{\mathrm{c}\mathrm{m}\mathrm{s}}<0.04$ (p–Pb collisions) were correlated to charged particles with $p_{\mathrm{T}}>0.3\mathrm{G}\mathrm{e}\mathrm{V}/c$. The yield of charged particles in the correlation peak induced by the jet containing the D meson and the peak width are compatible within uncertainties in the two collision systems. The data are described within uncertainties by Monte-Carlo simulations based on PYTHIA, POWHEG, and EPOS 3 event generators.« less

A detailed study of pseudorapidity densities and multiplicity distributions of primary charged particles produced in proton–proton collisions, at s= 0.9, 2.36, 2.76, 7 and 8 TeV, in the pseudorapidity range | η| < 2 , was carried out using the ALICE detector. Measurements were obtained for three event classes: inelastic, non-single diffractive and events with at least one charged particle in the pseudorapidity interval | η| < 1. The use of an improved track-counting algorithm combined with ALICE’s measurements of diffractive processes allows a higher precision compared to our previous publications. A KNO scaling study was performed in the pseudorapiditymore » intervals | η| < 0.5, 1.0 and 1.5. The data are compared to other experimental results and to models as implemented in Monte Carlo event generators PHOJET and recent tunes of PYTHIA6, PYTHIA8 and EPOS.« less