154 research outputs found
First measurement of coherent Ï0 photoproduction in ultra-peripheral XeâXe collisions at âsNN = 5.44 TeV
The first measurement of the coherent photoproduction of Ï0 vector mesons in ultra-peripheral XeâXe collisions at sNN=5.44 TeV is presented. This result, together with previous HERA Îłp data and ÎłâPb measurements from ALICE, describes the atomic number (A) dependence of this process, which is particularly sensitive to nuclear shadowing effects and to the approach to the black-disc limit of QCD at a semi-hard scale. The cross section of the Xe+XeâÏ0+Xe+Xe process, measured at midrapidity through the decay channel Ï0âÏ+Ïâ, is found to be dÏ/dy=131.5±5.6(stat.)â16.9+17.5(syst.) mb. The ratio of the continuum to resonant contributions for the production of pion pairs is also measured. In addition, the fraction of events accompanied by electromagnetic dissociation of either one or both colliding nuclei is reported. The dependence on A of cross section for the coherent Ï0 photoproduction at a centre-of-mass energy per nucleon of the ÎłA system of WÎłA,n=65 GeV is found to be consistent with a power-law behaviour Ï(ÎłAâÏ0A)âAα with a slope α=0.96±0.02(syst.). This slope signals important shadowing effects, but it is still far from the behaviour expected in the black-disc limit.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 Î_{c}^{+} and the Î_{c}^{+}/D^{0} production ratios were measured at midrapidity with the ALICE detector in pp and p-Pb collisions at sqrt[s_{NN}]=5.02ââTeV. These new measurements show a clear decrease of the Î_{c}^{+}/D^{0} ratio with increasing transverse momentum (p_{T}) in both collision systems in the range 2<p_{T}<12ââGeV/c, exhibiting similarities with the light-flavor baryon-to-meson ratios p/Ï and Î/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
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J/Ï elliptic and triangular flow in Pb-Pb collisions at âsNN = 5.02 TeV
The inclusive J/Ï elliptic (v2) and triangular (v3) flow coefficients measured at forward rapidity (2.5 < y < 4) and the v2 measured at midrapidity (|y| < 0.9) in Pb-Pb collisions at sNN = 5.02 TeV using the ALICE detector at the LHC are reported. The entire Pb-Pb data sample collected during Run 2 is employed, amounting to an integrated luminosity of 750 ÎŒbâ1 at forward rapidity and 93 ÎŒbâ1 at midrapidity. The results are obtained using the scalar product method and are reported as a function of transverse momentum pT and collision centrality. At midrapidity, the J/Ï v2 is in agreement with the forward rapidity measurement. The centrality averaged results indicate a positive J/Ï v3 with a significance of more than 5Ï at forward rapidity in the pT range 2 < pT< 5 GeV/c. The forward rapidity v2, v3, and v3/v2 results at low and intermediate pT (pT âČ 8 GeV/c) exhibit a mass hierarchy when compared to pions and D mesons, while converging into a species-independent curve at higher pT. At low and intermediate pT, the results could be interpreted in terms of a later thermalization of charm quarks compared to light quarks, while at high pT, path-length dependent effects seem to dominate. The J/Ï v2 measurements are further compared to a microscopic transport model calculation. Using a simplified extension of the quark scaling approach involving both light and charm quark flow components, it is shown that the D-meson vn measurements can be described based on those for charged pions and J/Ï flow. [Figure not available: see fulltext.
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Investigating the role of strangeness in baryonâantibaryon annihilation at the LHC
Annihilation dynamics plays a fundamental role in the baryonâantibaryon interaction (BâBâŸ) at low-energy and its strength and range are crucial in the assessment of possible baryonic bound states. Experimental data on annihilation cross sections are available for the pâp⟠system but not in the low relative momentum region. Data regarding the BâB⟠interaction with strange degrees of freedom are extremely scarce, hence the modeling of the annihilation contributions is mainly based on nucleonâantinucleon (NâNâŸ) results, when available. In this letter we present a measurement of the pâpâŸ, pâÎâŸâpâŸâÎ and ÎâÎ⟠interaction using correlation functions in the relative momentum space in high-multiplicity triggered pp collisions at s=13 TeV recorded by ALICE at the LHC. In the pâp⟠system the couplings to the mesonic channels in different partial waves are extracted by adopting a coupled-channel approach with recent ÏEFT potentials. The inclusion of these inelastic channels provides good agreement with the data, showing a significant presence of the annihilation term down to zero momentum. Predictions obtained using the LednickĂœâLyuboshits formula and scattering parameters obtained from heavy-ion collisions, hence mainly sensitive to elastic processes, are compared with the experimental pâÎâŸâpâŸâÎ and ÎâÎ⟠correlations. The model describes the ÎâÎ⟠data and underestimates the pâÎâŸâpâŸâÎ data in the region of momenta below 200 MeV/c. The observed deviation indicates a different contribution of annihilation channels to the two systems containing strange hadrons
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Dielectron production in proton-proton and proton-lead collisions at sNN=5.02TeV
The first measurements of dielectron production at midrapidity (ηe<0.8) in proton-proton and proton-lead collisions at sNN=5.02TeV at the LHC are presented. The dielectron cross section is measured with the ALICE detector as a function of the invariant mass mee and the pair transverse momentum pT,ee in the ranges mee<3.5 GeV/c2 and pT,ee<8 GeV/c, in both collision systems. In proton-proton collisions, the charm and beauty cross sections are determined at midrapidity from a fit to the data with two different event generators. This complements the existing dielectron measurements performed at s=7 and 13 TeV. The slope of the s dependence of the three measurements is described by FONLL calculations. The dielectron cross section measured in proton-lead collisions is in agreement, within the current precision, with the expected dielectron production without any nuclear matter effects for e+e- pairs from open heavy-flavor hadron decays. For the first time at LHC energies, the dielectron production in proton-lead and proton-proton collisions are directly compared at the same sNN via the dielectron nuclear modification factor RpPb. The measurements are compared to model calculations including cold nuclear matter effects, or additional sources of dielectrons from thermal radiation
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Azimuthal correlations of prompt D mesons with charged particles in pp and pâPb collisions at âsNN=5.02TeV
The measurement of the azimuthal-correlation function of prompt D mesons with charged particles in pp collisions at s=5.02TeV and pâPb collisions at sNN=5.02TeV with the ALICE detector at the LHC is reported. The D , D +, and D â + mesons, together with their charge conjugates, were reconstructed at midrapidity in the transverse momentum interval 30.3GeV/c and pseudorapidity | η| < 0.8. The properties of the correlation peaks appearing in the near- and away-side regions (for Î Ïâ 0 and Î Ïâ Ï, 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 pâPb 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
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Transverse-momentum and event-shape dependence of D-meson flow harmonics in PbâPb collisions at sNN=5.02TeV
The elliptic and triangular flow coefficients v2 and v3 of prompt D0, D+, and Dâ+ mesons were measured at midrapidity (|y|<0.8) in PbâPb collisions at the centre-of-mass energy per nucleon pair of sNN=5.02TeV with the ALICE detector at the LHC. The D mesons were reconstructed via their hadronic decays in the transverse momentum interval
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Measurement of beauty production via non-prompt D0 mesons in Pb-Pb collisions at sNN= 5.02 TeV
The production of non-prompt D0 mesons from beauty-hadron decays was measured at midrapidity (|y| < 0.5) in Pb-Pb collisions at a nucleon-nucleon center-of-mass energy of sNN = 5.02 TeV with the ALICE experiment at the LHC. Their nuclear modification factor (RAA), measured for the first time down to pT = 1 GeV/c in the 0â10% and 30â50% centrality classes, indicates a significant suppression, up to a factor of about three, for pT> 5 GeV/c in the 0â10% central Pb-Pb collisions. The data are described by models that include both collisional and radiative processes in the calculation of beauty-quark energy loss in the quark-gluon plasma, and quark recombination in addition to fragmentation as a hadronisation mechanism. The ratio of the non-prompt to prompt D0-meson RAA is larger than unity for pT> 4 GeV/c in the 0â10% central Pb-Pb collisions, as predicted by models in which beauty quarks lose less energy than charm quarks in the quark-gluon plasma because of their larger mass. [Figure not available: see fulltext.
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