Jet-associated deuteron production in pp collisions at root s=13 TeV

Deuteron production in high-energy collisions is sensitive to the space-time evolution of the collision system, and is typically described by a coalescence mechanism. For the first time, we present results on jet-associated deuteron production in pp collisions at root s = 13 TeV, providing an opportunity to test the established picture for deuteron production in events with a hard scattering. Using a trigger particle with high transverse-momentum (p(T) > 5 GeV/c) as a proxy for the presence of a jet at midrapidity, we observe a measurable population of deuterons being produced around the jet proxy. The associated deuteron yield measured in a narrow angular range around the trigger particle differs by 2.4-4.8 standard deviations from the uncorrelated background. The data are described by PYTHIA model calculations featuring baryon coalescence. (C) 2021 The Author. Published by Elsevier B.V.Peer reviewe

Upsilon production and nuclear modification at forward rapidity in Pb-Pb collisions at root S-NN=5.02 TeV

The production of Upsilon mesons in Pb-Pb collisions at a centre-of-mass energy per nucleon pair root S-NN = 5.02 TeV is measured with the muon spectrometer of the ALICE detector at the LHC. The yields as well as the nuclear modification factors are determined in the forward rapidity region 2.5 < y < 4.0, as a function of rapidity, transverse momentum and collision centrality. The results show that the production of the Upsilon(1S) meson is suppressed by a factor of about three with respect to the production in proton-proton collisions. For the first time, a significant signal for the Upsilon(2S) meson is observed at forward rapidity, indicating a suppression stronger by about a factor 2-3 with respect to the ground state. The measurements are compared with transport, hydrodynamic, comover and statistical hadronisation model calculations. (C) 2021 European Organization for Nuclear Research, ALICE. Published by Elsevier B.V.Peer reviewe

Jet fragmentation transverse momentum distributions in pp and p-Pb collisions at root s, root s(NN)=5.02 TeV

Jet fragmentation transverse momentum (j(T)) distributions are measured in proton-proton (pp) and proton-lead (p-Pb) collisions at root s(NN) = 5.02 TeV with the ALICE experiment at the LHC. Jets are reconstructed with the ALICE tracking detectors and electromagnetic calorimeter using the anti-k(T) algorithm with resolution parameter R = 0.4 in the pseudorapidity range vertical bar eta vertical bar < 0.25. The j(T) values are calculated for charged particles inside a fixed cone with a radius R = 0.4 around the reconstructed jet axis. The measured j(T) distributions are compared with a variety of parton-shower models. Herwig and PYTHIA 8 based models describe the data well for the higher j(T) region, while they underestimate the lower j(T) region. The j(T) distributions are further characterised by fitting them with a function composed of an inverse gamma function for higher j(T) values (called the "wide component"), related to the perturbative component of the fragmentation process, and with a Gaussian for lower j(T) values (called the "narrow component"), predominantly connected to the hadronisation process. The width of the Gaussian has only a weak dependence on jet transverse momentum, while that of the inverse gamma function increases with increasing jet transverse momentum. For the narrow component, the measured trends are successfully described by all models except for Herwig. For the wide component, Herwig and PYTHIA 8 based models slightly underestimate the data for the higher jet transverse momentum region. These measurements set constraints on models of jet fragmentation and hadronisation.Peer reviewe

The upgrade of the ALICE TPC with GEMs and continuous readout

The upgrade of the ALICE TPC will allow the experiment to cope with the high interaction rates foreseen for the forthcoming Run 3 and Run 4 at the CERN LHC. In this article, we describe the design of new readout chambers and front-end electronics, which are driven by the goals of the experiment. Gas Electron Multiplier (GEM) detectors arranged in stacks containing four GEMs each, and continuous readout electronics based on the SAMPA chip, an ALICE development, are replacing the previous elements. The construction of these new elements, together with their associated quality control procedures, is explained in detail. Finally, the readout chamber and front-end electronics cards replacement, together with the commissioning of the detector prior to installation in the experimental cavern, are presented. After a nine-year period of R&D, construction, and assembly, the upgrade of the TPC was completed in 2020.publishedVersio

Elliptic flow of charged particles at midrapidity relative to the spectator plane in Pb–Pb and Xe–Xe collisions

Measurements of the elliptic flow coefficient relative to the collision plane defined by the spectator neutrons v2{ SP} in collisions of Pb ions at center-of-mass energy per nucleon–nucleon pair √ 2.76 TeV and Xe ions at √ sNN = sNN =5.44 TeV are reported. The results are presented for charged particles produced at midrapidity as a function of centrality and transverse momentum for the 5–70% and 0.2–6 GeV/c ranges, respectively. The ratio between v2{ SP} and the elliptic flow coefficient relative to the participant plane v2{4}, estimated using four-particle correlations, deviates by up to 20% from unity depending on centrality. This observation differs strongly from the magnitude of the corresponding eccentricity ratios predicted by the TRENTo and the elliptic power models of initial state fluctuations that are tuned to describe the participant plane anisotropies. The differences can be interpreted as a decorrelation of the neutron spectator plane and the reaction plane because of fragmentation of the remnants from the colliding nuclei, which points to an incompleteness of current models describing the initial state fluctuations. A significant transverse momentum dependence of the ratio v2{ SP}/v2{4} is observed in all but the most central collisions, which may help to understand whether momentum anisotropies at low and intermediate transverse momentum have a common origin in initial state f luctuations. The ratios of v2{ SP} and v2{4} to the corresponding initial state eccentricities for Xe–Xe and Pb–Pb collisions at similar initial entropy density show a difference of (7.0 ±0.9)%with an additional variation of +1.8% when including RHIC data in the TRENTo parameter extraction. These observations provide new experimental constraints for viscous effects in the hydrodynamic modeling of the expanding quark–gluon plasma produced in heavy-ion collisions at the LHC

First measurement of Ωc0 production in pp collisions at s=13 TeV

The inclusive production of the charm–strange baryon 0 c is measured for the first time via its hadronic √ decay into −π+ at midrapidity (|y| &lt;0.5) in proton–proton (pp) collisions at the centre-of-mass energy s =13 TeV with the ALICE detector at the LHC. The transverse momentum (pT) differential cross section multiplied by the branching ratio is presented in the interval 2 &lt; pT &lt; 12 GeV/c. The pT dependence of the 0 c-baryon production relative to the prompt D0-meson and to the prompt 0 c-baryon production is compared to various models that take different hadronisation mechanisms into consideration. In the measured pT interval, the ratio of the pT-integrated cross sections of 0 c and prompt + c baryons multiplied by the −π+ branching ratio is found to be larger by a factor of about 20 with a significance of about 4σ when compared to e+e− collisions

Production of muons from heavy-flavour hadron decays at high transverse momentum in Pb-Pb collisions at root S-NN=5.02 and 2.76 TeV

Measurements of the production of muons from heavy-flavour hadron decays in Pb-Pb collisions at root s(NN) = 5.02 and 2.76 TeV using the ALICE detector at the LHC are reported. The nuclear modification factor R-AA at root s(NN) = 5.02 TeV is measured at forward rapidity (2.5 7 GeV/c). The R-AA shows an increase of the suppression of the yields of muons from heavy-flavour hadron decays with increasing centrality. A suppression by a factor of about three is observed in the 10% most central collisions. The R-AA at root s(NN) = 5.02 TeV is similar to that at root s(NN) = 2.76 TeV. The precise R-AA measurements have the potential to distinguish between model predictions implementing different mechanisms of parton energy loss in the high-density medium formed in heavy-ion collisions. They place important constraints for the understanding of the heavy-quark interaction with the hot and dense QCD medium. (C) 2021 European Organization for Nuclear Research, ALICE. Published by Elsevier B.V.Peer reviewe

Inclusive heavy-flavour production at central and forward rapidity in Xe-Xe collisions at , root sNN=5.44 TeV

The first measurements of the production of muons and electrons from heavy-flavour hadron decays in Xe-Xe collisions at root s(NN) = 5.44 TeV, using the ALICE detector at the LHC, are reported. The measurement of the nuclear modification factor RAA is performed as a function of transverse momentum pT in several centrality classes at forward rapidity (2.5 < y < 4) and midrapidity (vertical bar y vertical bar < 0.8) for muons and electrons from heavy-flavour hadron decays, respectively. A suppression by a factor up to about 2.5 compared to the binary-scaled pp reference is observed in central collisions at both central and forward rapidities. The RAA of muons from heavy-flavour hadron decays is compared to previous measurements in Pb-Pb collisions at root sNN = 5.02 TeV. When the nuclear modification factors are compared in the centrality classes 0-10% for Xe-Xe collisions and 10-20% for Pb-Pb collisions, which have similar charged-particle multiplicity density, a similar suppression, with R-AA similar to 0.4 in the p(T) interval 4 < p(T) < 8 GeV/c, is observed. The comparison of the measured R-AA values in the two collision systems brings new insights on the properties of the quark-gluon plasma by investigating the system-size and geometry dependence of medium-induced parton energy loss. The results of muons and electrons from heavy-flavour hadron decays provide new constraints to model calculations. (C) 2021 The Author. Published by Elsevier B.V.Peer reviewe

Λ(+)c production in pp and in p-Pb collisions at root sNN=5.02 TeV

The production cross section of prompt A(c)(+) charm baryons was measured with the ALICE detector at the LHC at midrapidity in proton-proton (pp) and proton-lead (p-Pb) collisions at a center-of-mass energy per nucleon pair of root sNN = 5.02 TeV. The A(c)(+) and (A) over bar (-)(c)baryons were reconstructed in the hadronic decay channels A(c)(+)-> pK(-)st+ and A(c)(+)-> pK(S)(0) and respective charge conjugates. The measured differential cross sections as a function of transverse momentum (pT) and the pT-integrated A+c production cross section in pp and in p-Pb collisions are presented. The A(c)(+) nuclear modification factor (R-pPb), calculated from the cross sections in pp and in p-Pb collisions, is presented and compared with the R-pPb of D mesons. The A+c /D0 ratio is also presented and compared with the light-flavor baryon-to-meson ratios p/st and A/KS0, and measurements from other LHC experiments. The results are compared to predictions from model calculations and Monte Carlo event generators.Peer reviewe

Λ(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 Lambda(c+) and the A(c)(+)/D-0 production ratios were measured at midrapidity with the ALICE detector in pp and (p)-Pb collisions at root S-NN = 5.02 TeV. These new measurements show a clear decrease of the Lambda(+)(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-tomeson ratios p/pi and Lambda/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 charmquark 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.Peer reviewe