1,143 research outputs found
Dielectron production at midrapidity at low transverse momentum in peripheral and semi-peripheral Pb–Pb collisions at √sNN = 5.02 TeV
No full textThe first measurement of the e+e− pair production at low lepton pair transverse momentum (p T,ee) and low invariant mass (m ee) in non-central Pb–Pb collisions at sNN = 5.02 TeV at the LHC is presented. The dielectron production is studied with the ALICE detector at midrapidity (|η e | < 0.8) as a function of invariant mass (0.4 ≤ m ee < 2.7 GeV/c 2) in the 50–70% and 70–90% centrality classes for p T,ee < 0.1 GeV/c, and as a function of p T,ee in three m ee intervals in the most peripheral Pb–Pb collisions. Below a p T,ee of 0.1 GeV/c, a clear excess of e+e− pairs is found compared to the expectations from known hadronic sources and predictions of thermal radiation from the medium. The m ee excess spectra are reproduced, within uncertainties, by different predictions of the photon–photon production of dielectrons, where the photons originate from the extremely strong electromagnetic fields generated by the highly Lorentz-contracted Pb nuclei. Lowest-order quantum electrodynamic (QED) calculations, as well as a model that takes into account the impact-parameter dependence of the average transverse momentum of the photons, also provide a good description of the p T,ee spectra. The measured 〈pT,ee2〉 of the excess p T,ee spectrum in peripheral Pb–Pb collisions is found to be comparable to the values observed previously at RHIC in a similar phase-space region. [Figure not available: see fulltext.]
Publisher Correction: Unveiling the strong interaction among hadrons at the LHC
No full textCorrection to: Nature https://doi.org/10.1038/s41586-020-3001-6Published online 09 December 2020
In Fig. 1c of this Article, owing to an error during the production process, the equation incorrectly began ‘C(k*, r*) = …’ instead of ‘C(k*) = …’. In addition, in affiliation 71 ‘Dipartimento di Fisica dell’Università degli studi di Bari Aldo Moro’ has been corrected to read ‘Dipartimento di Fisica dell’Università degli studi di Cagliari’. The original Article has been corrected online
Upgrade of the ALICE central barrel tracking detectors: ITS and TPC
Get PDFThe ALICE Collaboration will undertake a major upgrade of the detector
apparatus during the second LHC Long Shutdown LS2 (2019-2020) in view of
the Runs 3 and 4 (2021-2029). The objective of the upgrade is two-fold:
i) an improvement of the tracking precision and efficiency, in
particular in the low-momentum range; ii) an improvement of the readout
capabilities of the experiment, in order to fully exploit the luminosity
for heavy ions envisaged after LS2. The first goal will be achieved by
replacing the Inner Tracking System with a new tracker, composed of
seven layers of silicon pixel detectors. The new tracker will be made up
of about 25000 Monolithic Active Pixel Sensors with fast readout,
resulting in a material thickness reduced to 0.3\% (inner layers) -1\%
(outer layers) of the radiation length and a granularity of 28 x28 mu
m(2). The second goal will be achieved, among other measures, by
replacing the readout chambers of the 90 m(3) Time Projection Chamber
with Micro Pattern Gaseous Detectors. In particular, the new readout
chambers will consist of stacks of 4 Gas Electron Multiplier foils
combining different hole pitches. The upgraded detector will operate
continuously without the use of a triggered gating grid. It will thus be
able to record all Pb-Pb collisions at the LHC interaction rate of 50
kHz
One-dimensional pion, kaon, and proton femtoscopy in Pb-Pb collisions at root(NN)-N-S=2.76 TeV
Get PDFThe size of the particle emission region in high-energy collisions can be deduced using the femtoscopic correlations of particle pairs at low relative momentum. Such correlations arise due to quantum statistics and Coulomb and strong final state interactions. In this paper, results are presented from femtoscopic analyses of pi(+/-) pi(+/-), K-+/- K-+/-, K-S(0) K-S(0), pp, and (pp) over bar correlations from Pb-Pb collisions at root s(NN) = 2.76 TeV by the ALICE experiment at the LHC. One-dimensional radii of the system are extracted from correlation functions in terms of the invariant momentum difference of the pair. The comparison of the measured radii with the predictions from a hydrokinetic model is discussed. The pion and kaon source radii display a monotonic decrease with increasing average pair transverse mass m(T) which is consistent with hydrodynamic model predictions for central collisions. The kaon and proton source sizes can be reasonably described by approximate m(T) scaling.Peer reviewe
Dielectron production in proton-proton collisions at root s=7 TeV
Get PDFCNPQ - CONSELHO NACIONAL DE DESENVOLVIMENTO CIENTÍFICO E TECNOLÓGICOFINEP - FINANCIADORA DE ESTUDOS E PROJETOSFAPESP - FUNDAÇÃO DE AMPARO À PESQUISA DO ESTADO DE SÃO PAULOThe first measurement of e(+)e(-) pair production at mid-rapidity (vertical bar eta vertical bar(e) < 0.8) in pp collisions at root s = 7TeV with ALICE at the LHC is presented. The dielectron production is studied as a function of the invariant mass (m(ee )< 3.3 GeV/c(2)), the pair transverse momentum (p(T,ee) < 8 GeV/c), and the pair transverse impact parameter (DCA(ee)), i.e., the average distance of closest approach of the reconstructed electron and positron tracks to the collision vertex, normalised to its resolution. The results are compared with the expectations from a cocktail of known hadronic sources and are well described when PYTHIA is used to generate the heavy-flavour contributions. In the low-mass region (0.14 < m(ee) < 1.1 GeV/c(2)), prompt and non-prompt e(+)e(-) sources can be separated via the DCA(ee). In the intermediate-mass region (1.1 < m(ee) < 2.7 GeV/c(2)), a double-differential fit to the data in m(ee) and p(T,ee) and a fit of the DCA(ee) distribution allow the total cc and bb cross sections to be extracted. Two different event generators, PYTHIA and POWHEG, can reproduce the shape of the two-dimensional m(ee) and p(T,ee) spectra, as well as the shape , of the DCA(ee) distribution, reasonably well. However, differences in the c (c) over bar and b (b) over bar cross sections are observed when using the generators to extrapolate to full phase space. Finally, the ratio of inclusive to decay photons is studied via the measurement of virtual direct photons in the transverse-momentum range 1 < p(T) < 8 GeV/c. This is found to be unity within the statistical and systematic uncertainties and consistent with expectations from next-to-leading order perturbative quantum chromodynamic calculations.9147CNPQ - CONSELHO NACIONAL DE DESENVOLVIMENTO CIENTÍFICO E TECNOLÓGICOFINEP - FINANCIADORA DE ESTUDOS E PROJETOSFAPESP - FUNDAÇÃO DE AMPARO À PESQUISA DO ESTADO DE SÃO PAULOCNPQ - CONSELHO NACIONAL DE DESENVOLVIMENTO CIENTÍFICO E TECNOLÓGICOFINEP - FINANCIADORA DE ESTUDOS E PROJETOSFAPESP - FUNDAÇÃO DE AMPARO À PESQUISA DO ESTADO DE SÃO PAULOSem informaçãoSem informaçãoSem informaçãoAgências de fomento estrangeiras apoiaram essa pesquisa, mais informações acesse artig
First measurement of the vertical bar t vertical bar-dependence of coherent J/psi photonuclear production
Get PDFThe first measurement of the cross section for coherent J/Psi photoproduction as a function of vertical bar t vertical bar, the square of the momentum transferred between the incoming and outgoing target nucleus, is presented. The data were measured with the ALICE detector in ultra-peripheral Pb-Pbcollisions at a centre-of-mass energy per nucleon pair root s(NN) = 5.02 TeV with the J/Psi produced in the central rapidity region vertical bar y vertical bar < 0.8, which corresponds to the small Bjorken-xrange (0.3 - 1.4) x 10(-3). The measured vertical bar t vertical bar-dependence is not described by computations based only on the Pb nuclear form factor, while the photonuclear cross section is better reproduced by models including shadowing according to the leading-twist approximation, or gluon-saturation effects from the impact-parameter dependent Balitsky-Kovchegov equation. These new results are therefore a valid tool to constrain the relevant model parameters and to investigate the transverse gluonic structure at very low Bjorken- x. (C) 2021 The Author(s). Published by Elsevier B.V
Measurement of strange baryon–antibaryon interactions with femtoscopic correlations
Get PDFTwo-particle correlation functions were measured for p (p) over bar, p (Lambda) over bar, (p) over bar Lambda, and Lambda(Lambda) over bar pairs in Pb-Pb collisions at root sNN = 2.76 TeV and root sNN = 5.02 TeV recorded by the ALICE detector. From a simultaneous fit to all obtained correlation functions, real and imaginary components of the scattering lengths, as well as the effective ranges, were extracted for combined p (Lambda) over bar and (p) over bar Lambda pairs and, for the first time, for Lambda(Lambda) over bar pairs. Effective averaged scattering parameters for heavier baryon-antibaryon pairs, not measured directly, are also provided. The results reveal similarly strong interaction between measured baryon-antibaryon pairs, suggesting that they all annihilate in the same manner at the same pair relative momentum k*. Moreover, the reported significant non-zero imaginary part and negative real part of the scattering length provide motivation for future baryon-antibaryon bound state searches. (C) 2020 The Author(s). Published by Elsevier B.V
Energy dependence and fluctuations of anisotropic flow in Pb-Pb collisions at √sNN=5.02 and 2.76 TeV
Get PDFMeasurements of anisotropic flow coefficients with two- and multi-particle cumulants for inclusive charged particles in Pb-Pb collisions at sNN=5.02 and 2.76 TeV are reported in the pseudorapidity range |η| < 0.8 and transverse momentum 0.2 < p T < 50 GeV/c. The full data sample collected by the ALICE detector in 2015 (2010), corresponding to an integrated luminosity of 12.7 (2.0) μb −1 in the centrality range 0-80%, is analysed. Flow coefficients up to the sixth flow harmonic (v 6 ) are reported and a detailed comparison among results at the two energies is carried out. The p T 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 v n (p T ) ∼ p T n/3 is observed for all flow harmonics at low p T (0.2 < p T < 3 GeV/c). At the same time, the ratios v n /v m n/ m are observed to be essentially independent of p T for most centralities up to about p T = 10 GeV/c. Analysing the differences among higher-order cumulants of elliptic flow (v 2 ), which have different sensitivities to flow fluctuations, a measurement of the standardised skewness of the event-by-event v 2 distribution P(v 2 ) is reported and constraints on its higher moments are provided. The Elliptic Power distribution is used to parametrise P(v 2 ), 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.[Figure not available: see fulltext.]
Femtoscopic study of the proton-proton and proton-deuteron systems in heavy-ion collisions at the LHC
No full textThis work reports femtoscopic correlations of p-−p(p ̅-−p ̅) and p−−d(p ̅-−d ̅) pairs measured in Pb–Pb collisions at center-of-mass energy per nucleon sNN = 5.02 TeV in the ALICE Collaboration. A fit to the measured proton-proton correlation functions allows one to extract the dependence of the nucleon femtoscopic radius of the particle-emitting source on the pair transverse mass (mT) and on the average charge particle multiplicity 〈dNch/dη〉1/3 for three centrality intervals (0–10%,10−−30%,30−−50%). In both cases, the expected power-law and linear scalings are observed, respectively. The measured p–d correlations can be described by both two- and three-body calculations, indicating that the femtoscopy observable is not sensitive to the short-distance features of the dynamics of the p-(p-n) system, due to the large inter-particle distances in Pb–Pb collisions at the LHC. Indeed, in this study, the minimum measured femtoscopic source sizes for protons and deuterons have a minimum value at 2.73−0.05+0.05 and 3.10−0.86+1.04 fm, respectively, for the 30–50% centrality collisions. Moreover, the m T-scaling obtained for the p–p and p–d systems is compatible within 1 σ of the uncertainties. These findings provide new input for fundamental studies on the production of light (anti)nuclei under extreme conditions
Measurement of the production of high-p(T) electrons from heavy-flavour hadron decays in Pb-Pb collisions at root s(NN)=2.76 TeV
No full textElectrons from heavy-flavour hadron decays (charm and beauty) were measured with the ALICE detector in Pb-Pb collisions at a centre-of-mass of energy root s(NN) = 2.76 TeV. The transverse momentum (pT) differential production yields at mid-rapidity were used to calculate the nuclear modification factor R-AA in the interval 3 < p(T) < 18 GeV/c. The R-AA shows a strong suppression compared to binary scaling of pp collisions at the same energy (up to a factor of 4) in the 10% most central Pb-Pb collisions. There is a centrality trend of suppression, and a weaker suppression (down to a factor of 2) in semi-peripheral (50-80%) collisions is observed. The suppression of electrons in this broad p(T) interval indicates that both charm and beauty quarks lose energy when they traverse the hot medium formed in Pb-Pb collisions at LHC. (C) 2017 The Author. Published by Elsevier B.V
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