14 Search Results

Differential two-particle normalized cumulants (R₂) and transverse momentum correlations (P₂) are measured as a function of the relative pseudorapidity and azimuthal angle difference (ΔηΔφ) of charged particle pairs in minimum bias pp collisions at $$\sqrt{s}$$ = 13 TeV. The measurements use charged hadrons in the pseudorapidity region of |η| < 0.8 and the transverse momentum range 0.2 < p_T < 2.0 in order to focus on soft multiparticle interactions and to complement prior measurements of these correlation functions in p–Pb and Pb–Pb collisions. The correlation functions are reported for both unlike-sign and like-sign pairs and their charge-independent and charge-dependent combinations.more » Both the R₂ and P₂ measured in pp collisions exhibit features qualitatively similar to those observed in p–Pb and Pb–Pb collisions. The Δη and Δφ root mean square widths of the near-side peak of the correlation functions are evaluated and compared with those observed in p–Pb and Pb–Pb collisions and show smooth evolution with the multiplicity of charged particles produced in the collision. The comparison of the measured correlation functions with predictions from PYTHIA8 shows that this model qualitatively captures their basic structure and characteristics but feature important differences. In addition, the R$$^{CD}_{2}$$ is used to determine the charge balance function of hadrons produced within the detector acceptance of the measurements. The integral of the balance function is found to be compatible with those reported by a previous measurement in Pb–Pb collisions.« less

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

Inclusive J/ψ production is studied in minimum-bias proton-proton collisions at a centre-of-mass energy of √s = 5.02 TeV by ALICE at the CERN LHC. The measurement is performed at mid-rapidity (|y| < 0.9) in the dielectron decay channel down to zero transverse momentum p_T, using a data sample corresponding to an integrated luminosity of L_int = 19.4 ± 0.4 nb^-1. The measured p_T-integrated inclusive J/ψ production cross sec- tion is dσ/dy = 5.64 ± 0.22(stat.) ± 0.33(syst.) ± 0.12(lumi.) μb. The p_T-differential cross section d²σ/dp_Tdy is measured in the p_T range 0–10 GeV/c and compared with state-of- the-art QCD calculations.more » The J/ψ 〈pT〉 and 〈B²_T〉 are extracted and compared with results obtained at other collision energies.« 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 elliptic (v₂), triangular (v₃), and quadrangular (v₄) flow coefficients of π^±, K^±, p+$$ \overline{p}\ $$,Λ+$$ \overline{Λ}\ $$,K_S⁰, and the Φ-meson are measured in Pb-Pb collisions at $$ {\sqrt{s}}_{\mathrm{NN}}=5.02 $$ TeV. Results obtained with the scalar product method are reported for the rapidity range |y| < 0.5 as a function of transverse momentum, p_T, at different collision centrality intervals between 0–70%, including ultra-central (0–1%) collisions for π^±, K^±, and p + $$ \overline{p}\ $$. For p_T< 3 GeV/c, the flow coefficients exhibit a particle mass dependence. At intermediate transverse momenta (3 < p_T < 8–10 GeV/c), particles show an approximatemore » grouping according to their type (i.e., mesons and baryons). The Φ-meson v₂, which tests both particle mass dependence and type scaling, follows p + $$ \overline{p}\ $$ v₂ at low p_T and π^± v₂ at intermediate p_T. The evolution of the shape of v_n(p_T) as a function of centrality and harmonic number n is studied for the various particle species. Flow coefficients of π^±, K^±, and p + $$ \overline{p}\ $$ for p_T < 3 GeV/c are compared to iEBE-VISHNU and MUSIC hydrodynamical calculations coupled to a hadronic cascade model (UrQMD). The iEBE-VISHNU calculations describe the results fairly well for p_T < 2.5 GeV/c, while MUSIC calculations reproduce the measurements for p_T < 1 GeV/c. A comparison to v_n coefficients measured in Pb-Pb collisions at $$ {\sqrt{s}}_{\mathrm{NN}}=2.76 $$ TeV is also provided. [Figure not available: see fulltext.].« less

Neutral pion and η meson invariant differential yields were measured in non-single diffractive p–Pb collisions at $$ \sqrt{s}=5.02 $$ TeV with the ALICE experiment at the CERN LHC. The analysis combines results from three complementary photon measurements, utilizing the PHOS and EMCal calorimeters and the Photon Conversion Method. The invariant differential yields of π0 and η meson inclusive production are measured near mid-rapidity in a broad transverse momentum range of 0.3 4 GeV / c at 0.483 ± 0. 015 stat± 0. 015 sys. A deviation from mT scaling is observed for pT< 2 GeV / c. The measured η/more » π⁰ ratio is consistent with previous measurements from proton-nucleus and pp collisions over the full pT range. The measured η/ π⁰ ratio at high pT also agrees within uncertainties with measurements from nucleus–nucleus collisions. The π⁰ and η yields in p–Pb relative to the scaled pp interpolated reference, RpPb, are presented for 0.3 < pT< 20 GeV / c and 0.7 < pT< 20 GeV / c, respectively. The results are compared with theoretical model calculations. The values of RpPb are consistent with unity for transverse momenta above 2 GeV / c. These results support the interpretation that the suppressed yield of neutral mesons measured in Pb–Pb collisions at LHC energies is due to parton energy loss in the hot QCD medium.« less