Study of charged-particle multiplicities with ALICE

The multiplicity measurements include the pseudorapidity density, $\rm{d}N_{\rm ch}/\rm{d}{\eta}$, and the probability distribution as a function of the number of charged particles, $\rm{P}(N_{\rm ch})$. ALICE has measured the multiplicities for three collision systems, for proton-proton, proton-lead and lead-lead collisions at Run 1 and 2 at the Large Hadron Collider. A selection of these results will be presented in these proceedings, concluding with an overview of new measurements planned.Comment: 3 pages, 4 figures. Proceedings of IFAE - Incontri di Fisica delle Alte Energie Conferenc

Multiplicity and Underlying Event in ALICE: as measurements and as tools to probe QCD

With the high collision energies at the LHC, the contributions to particle production from hard-QCD processes increase, but it remains dominated by soft-QCD processes. Such processes challenge the theoretical models, since they are described by non-perturbative phenomenology. A selection of the most recent ALICE measurements of charged-particle multiplicities and the Underlying Event will be presented, focusing on model comparisons. A summary of the current understanding of soft-QCD processes will be discussed, evaluating possible ways to further constrain theory.Comment: 4 pages, 3 figures, Rencontres de Moriond - QCD and high-energy interaction

Particle production as a function of underlying-event activity measured with ALICE at the LHC

ALICE has performed several measurements aimed at understanding the collective-like effects observed in small collision systems. New approaches may be needed to clarify particle-production mechanisms in high-multiplicity pp collisions. Transverse momentum ($p_{\rm T}$) spectra as a function of charged-particle multiplicity show intriguing features. For example, data exhibit a stronger-than-linear increase of the self-normalised high-$p_{\rm T}$ particle yields versus multiplicity. In order to understand the role of auto-correlations on these effects, it has been proposed to use the underlying event as a multiplicity estimator to factorise the hardest and the softest components of the events. This approach can also be used to study collective effects in events with exceptionally large activity in the underlying-event region with respect to the event-averaged mean. In these proceedings, $p_{\rm T}$ spectra as a function of underlying-event activity in pp collisions measured with the ALICE detector are presented. Results are compared with PYTHIA 8.2 event generator.Comment: 4 pages, 2 figures, Proceedings of the Strangeness in Quark Matter (SQM 2019)conference, 10-15 June 2018, Bari (Italy). Figure 2 updated after the conference to account for a change in the normalisation factor of the $R_{\rm T}$-integrated $p_{\rm T}$ spectrum for the transverse regio

Improved Cloud resource allocation: how INDIGO-Datacloud is overcoming the current limitations in Cloud schedulers

Trabajo presentado a: 22nd International Conference on Computing in High Energy and Nuclear Physics (CHEP2016) 10–14 October 2016, San Francisco.Performing efficient resource provisioning is a fundamental aspect for any resource provider. Local Resource Management Systems (LRMS) have been used in data centers for decades in order to obtain the best usage of the resources, providing their fair usage and partitioning for the users. In contrast, current cloud schedulers are normally based on the immediate allocation of resources on a first-come, first-served basis, meaning that a request will fail if there are no resources (e.g. OpenStack) or it will be trivially queued ordered by entry time (e.g. OpenNebula). Moreover, these scheduling strategies are based on a static partitioning of the resources, meaning that existing quotas cannot be exceeded, even if there are idle resources allocated to other projects. This is a consequence of the fact that cloud instances are not associated with a maximum execution time and leads to a situation where the resources are under-utilized. These facts have been identified by the INDIGO-DataCloud project as being too simplistic for accommodating scientific workloads in an efficient way, leading to an underutilization of the resources, a non desirable situation in scientific data centers. In this work, we will present the work done in the scheduling area during the first year of the INDIGO project and the foreseen evolutions.The authors want to acknowledge the support of the INDIGO-DataCloud (grant number 653549) project, funded by the European Commission’s Horizon 2020 Framework Programme.Peer Reviewe

Energy dependence and fluctuations of anisotropic flow in Pb-Pb collisions at 1asNN = 5.02 and 2.76 TeV

Measurements of anisotropic flow coefficients with two- and multi-particle cu- mulants for inclusive charged particles in Pb\u2013Pb collisions at 1asNN = 5.02 and 2.76 TeV are reported in the pseudorapidity range |\u3b7| < 0.8 and transverse momentum 0.2 < pT < 50 GeV/c. The full data sample collected by the ALICE detector in 2015 (2010), corre- sponding to an integrated luminosity of 12.7 (2.0) \u3bcb 121 in the centrality range 0\u201380%, is analysed. Flow coefficients up to the sixth flow harmonic (v6) are reported and a de- tailed comparison among results at the two energies is carried out. The pT dependence of anisotropic flow coefficients and its evolution with respect to centrality and harmonic number n are investigated. An approximate power-law scaling of the form vn(pT) 3c pn/3 T is observed for all flow harmonics at low pT (0.2 < pT < 3 GeV/c). At the same time, the ratios v /vn/m are observed to be essentially independent of p for most centralities up to nmT about pT = 10 GeV/c. Analysing the differences among higher-order cumulants of elliptic flow (v2), which have different sensitivities to flow fluctuations, a measurement of the stan- dardised skewness of the event-by-event v2 distribution P(v2) is reported and constraints on its higher moments are provided. The Elliptic Power distribution is used to parametrise P(v2), extracting its parameters from fits to cumulants. The measurements are compared to different model predictions in order to discriminate among initial-state models and to constrain the temperature dependence of the shear viscosity to entropy-density ratio

Inclusive J/\u3c8 production in Xe\u2013Xe collisions at 1asNN = 5.44 TeV

Inclusive J/\u3c8 production is studied in Xe\u2013Xe interactions at a centre-of-mass energy per nucleon pair of 1asNN = 5.44 TeV, using the ALICE detector at the CERN LHC. The J/\u3c8 meson is reconstructed via its decay into a muon pair, in the centre-of-mass rapidity interval 2.5 < y < 4 and down to zero transverse momentum. In this Letter, the nuclear modification factors RAA for inclusive J/\u3c8, measured in the centrality range 0\u201390% as well as in the centrality intervals 0\u201320% and 20\u201390% are presented. The RAA values are compared to previously published results for Pb\u2013Pb collisions at 1asNN = 5.02 TeV and to the calculation of a transport model. A good agreement is found between Xe\u2013Xe and Pb\u2013Pb results as well as between data and the model

Azimuthally-differential pion femtoscopy relative to the third harmonic event plane in Pb\u2013Pb collisions at 1asNN = 2.76TeV

Azimuthally-differential femtoscopic measurements, being sensitive to spatio-temporal characteristics of the source as well as to the collective velocity fields at freeze out, provide very important information on the nature and dynamics of the system evolution. While the HBT radii oscillations relative to the second harmonic event plane measured recently reflect mostly the spatial geometry of the source, model studies have shown that the HBT radii oscillations relative to the third harmonic event plane are predominantly defined by the velocity fields. In this Letter, we present the first results on azimuthally-differential pion femtoscopy relative to the third harmonic event plane as a function of the pion pair transverse momentum kT for different collision centralities in Pb\u2013Pb collisions at 1asNN = 2.76 TeV. We find that the Rside and Rout radii, which characterize the pion source size in the directions perpendicular and parallel to the pion transverse momentum, oscillate in phase relative to the third harmonic event plane, similar to the results from 3+1D hydrodynamical calculations. The observed radii oscillations unambiguously signal a collective expansion and anisotropy in the velocity fields. A comparison of the measured radii oscillations with the Blast-Wave model calculations indicate that the initial state triangularity is washed- out at freeze out

Constraints on jet quenching in p\u2013Pb collisions at 1asNN = 5.02\u2009TeV measured by the event-activity dependence of semi-inclusive hadron-jet distributions

The ALICE Collaboration reports the measurement of semi-inclusive distributions of charged-particle jets recoiling from a high-transverse momentum trigger hadron in p\u2013Pb collisions at 1asNN = 5.02 TeV. Jets are reconstructed from charged-particle tracks using the anti-kT algorithm with resolution parameter R = 0.2 and 0.4. A data-driven statistical approach is used to correct the uncorrelated background jet yield. Recoil jet distributions are reported for jet transverse momentum 15 < pch < 50 GeV/c and are compared in T,jet various intervals of p\u2013Pb event activity, based on charged-particle multiplicity and zero-degree neutral energy in the forward (Pb-going) direction. The semi-inclusive observable is self-normalized and such comparisons do not require the interpretation of p\u2013Pb event activity in terms of collision geometry, in contrast to inclusive jet observables. These measurements provide new constraints on the magnitude of jet quenching in small systems at the LHC. In p\u2013Pb collisions with high event activity, the average medium-induced out-of-cone energy transport for jets with R = 0.4 and 15 < pch < 50 GeV/c is T,jet measured to be less than 0.4 GeV/c at 90% confidence, which is over an order of magnitude smaller than a similar measurement for central Pb\u2013Pb collisions at 1asNN =2.76TeV. Comparison is made to theoretical calculations of jet quenching in small systems, and to inclusive jet measurements in p\u2013Pb collisions selected by event activity at the LHC and in d\u2013Au collisions at RHIC