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Acharya, S.,Adamová,, D.,Adhya, S.P.,Adler, A.,Adolfsson, J.,Aggarwal, M.M.,Aglieri Rinella, G.,Agnello, M.,Agrawal, N.,Ahammed, Z.,Ahmad, S.,Ahn, S.U.,Aiola, S.,Akindinov, A.,Al-Turany, M.,Alam North-Holland Pub. Co 2019 Physics letters. Section B Vol.798 No.-
<P><B>Abstract</B></P> <P>The ALICE collaboration performed the first rapidity-differential measurement of coherent J / ψ photoproduction in ultra-peripheral Pb–Pb collisions at a center-of-mass energy <SUB> s NN </SUB> = 5.02 TeV. The J / ψ is detected via its dimuon decay in the forward rapidity region ( − 4.0 < y < − 2.5 ) for events where the hadronic activity is required to be minimal. The analysis is based on an event sample corresponding to an integrated luminosity of about 750 μb<SUP>−1</SUP>. The cross section for coherent J / ψ production is presented in six rapidity bins. The results are compared with theoretical models for coherent J / ψ photoproduction. These comparisons indicate that gluon shadowing effects play a role in the photoproduction process. The ratio of <SUP> ψ ′ </SUP> to J / ψ coherent photoproduction cross sections was measured and found to be consistent with that measured for photoproduction off protons.</P>
Flow Dominance and Factorization of Transverse Momentum Correlations in Pb-Pb Collisions at the LHC
Adam, J.,Adamová,, D.,Aggarwal, M. M.,Aglieri Rinella, G.,Agnello, M.,Agrawal, N.,Ahammed, Z.,Ahmad, S.,Ahn, S. U.,Aiola, S.,Akindinov, A.,Alam, S. N.,Albuquerque, D. S. D.,Aleksandrov, D.,Aless American Physical Society 2017 Physical review letters Vol.118 No.16
<P>We present the first measurement of the two-particle transverse momentum differential correlation function, P-2 = <Delta pT Delta p(T)>/ < p(T)>(2), in Pb-Pb collisions at root s(NN) = 2.76 TeV. Results for P-2 are reported as a function of the relative pseudorapidity (Delta eta) and azimuthal angle (Delta phi) between two particles for different collision centralities. The Delta phi dependence is found to be largely independent of Delta eta for broken vertical bar Delta eta broken vertical bar >= 0.9. In the 5% most central Pb-Pb collisions, the two-particle transverse momentum correlation function exhibits a clear double-hump structure around Delta phi=pi (i. e., on the away side), which is not observed in number correlations in the same centrality range, and thus provides an indication of the dominance of triangular flow in this collision centrality. Fourier decompositions of P-2, studied as a function of the collision centrality, show that correlations at broken vertical bar Delta eta broken vertical bar >= 0.9 can be well reproduced by a flow ansatz based on the notion that measured transverse momentum correlations are strictly determined by the collective motion of the system.</P>
Acharya, S.,Adamová,, D.,Adhya, S.P.,Adler, A.,Adolfsson, J.,Aggarwal, M.M.,Aglieri Rinella, G.,Agnello, M.,Agrawal, N.,Ahammed, Z.,Ahmad, S.,Ahn, S.U.,Akindinov, A.,Al-Turany, M.,Alam, S.N.,Alb Elsevier 2019 Physics letters: B Vol.797 No.-
<P><B>Abstract</B></P> <P>An improved value for the lifetime of the (anti-)hypertriton has been obtained using the data sample of Pb–Pb collisions at <SUB> s NN </SUB> = 5.02 TeV collected by the ALICE experiment at the LHC. The (anti-)hypertriton has been reconstructed via its charged two-body mesonic decay channel and the lifetime has been determined from an exponential fit to the d<I>N</I>/d(<I>ct</I>) spectrum. The measured value, τ = 242 − 38 + 34 (stat.) ± 17 (syst.) ps, is compatible with representative theoretical predictions, thus contributing to the solution of the longstanding hypertriton lifetime puzzle.</P>
Neutral pion and η meson production at midrapidity in Pb-Pb collisions at sNN=2.76 TeV
Acharya, S.,Acosta, F. T.-.,Adamová,, D.,Adolfsson, J.,Aggarwal, M. M.,Aglieri Rinella, G.,Agnello, M.,Agrawal, N.,Ahammed, Z.,Ahn, S. U.,Aiola, S.,Akindinov, A.,Al-Turany, M.,Alam, S. N.,Albuqu American Physical Society 2018 Physical review. C Vol.98 No.4
<P>Neutral pion and eta meson production in the transverse momentum range 1 < p(T) < 20 GeV/c have been measured at midrapidity by the ALICE experiment at the Large Hadron Collider (LHC) in central and semicentral Pb-Pb collisions at root S-NN = 2.76 TeV. These results were obtained using the photon conversion method as well as the Photon Spectrometer (PHOS) and Electromagnetic Calorimeter detectors. The results extend the upper p(T) reach of the previous ALICE pi(0) measurements from 12 to 20 GeV/c and present the first measurement of eta meson production in heavy-ion collisions at the LHC. The eta/pi(0) ratio is similar for the two centralities and reaches at high p(T) a plateau value of 0.457 +/- 0.013(stat) +/- 0.018(syst) A suppression of similar magnitude for pi(0) and eta meson production is observed in Pb-Pb collisions with respect to their production in pp collisions scaled by the number of binary nucleon-nucleon collisions. We discuss the results in terms of Next to Leading Order (NLO) pQCD predictions and hydrodynamic models. The measurements show a stronger suppression than observed at lower center-of-mass energies in the p T range 6 < p(T) < 10 GeV/c. For p(T) < 3 GeV/c, hadronization models describe the pi(0) results while for the eta some tension is observed.</P>
Acharya, S.,Acosta, F.T.,Adamová,, D.,Adolfsson, J.,Aggarwal, M.M.,Aglieri Rinella, G.,Agnello, M.,Agrawal, N.,Ahammed, Z.,Ahn, S.U.,Aiola, S.,Akindinov, A.,Al-Turany, M.,Alam, S.N.,Albuquerque, Elsevier 2018 Physics letters: B Vol.785 No.-
<P><B>Abstract</B></P> <P>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 <SUB> k T </SUB> for different collision centralities in Pb–Pb collisions at <SUB> s NN </SUB> = 2.76 TeV . We find that the <SUB> R side </SUB> and <SUB> R out </SUB> 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.</P>
Acharya, S.,Adamová,, D.,Adolfsson, J.,Aggarwal, M.M.,Aglieri Rinella, G.,Agnello, M.,Agrawal, N.,Ahammed, Z.,Ahn, S.U.,Aiola, S.,Akindinov, A.,Al-Turany, M.,Alam, S.N.,Albuquerque, D.S.D.,Aleks North-Holland Pub. Co 2018 Physics letters. Section B Vol.783 No.-
<P><B>Abstract</B></P> <P>The ALICE Collaboration reports the measurement of semi-inclusive distributions of charged-particle jets recoiling from a high-transverse momentum trigger hadron in p–Pb collisions at <SUB> s NN </SUB> = 5.02 TeV. Jets are reconstructed from charged-particle tracks using the anti- <SUB> k T </SUB> 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 < p T , jet ch < 50 GeV / c and are compared in various intervals of p–Pb 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–Pb 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–Pb collisions with high event activity, the average medium-induced out-of-cone energy transport for jets with R = 0.4 and 15 < p T , jet ch < 50 GeV / c is 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–Pb collisions at <SUB> s NN </SUB> = 2.76 TeV . Comparison is made to theoretical calculations of jet quenching in small systems, and to inclusive jet measurements in p–Pb collisions selected by event activity at the LHC and in d–Au collisions at RHIC.</P>
Adam, J.,Adamová,, D.,Aggarwal, M. M.,Aglieri Rinella, G.,Agnello, M.,Agrawal, N.,Ahammed, Z.,Ahmad, S.,Ahn, S. U.,Aiola, S.,Akindinov, A.,Alam, S. N.,Albuquerque, D. S. D.,Aleksandrov, D.,Aless Springer Berlin Heidelberg 2017 The European physical journal. C, Particles and fi Vol.77 No.4
<P>The azimuthal correlations of D mesons with charged particles were measured with the ALICE apparatus in pp collisions at [FORMULA OMISSION] and p–Pb collisions at [FORMULA OMISSION] at the Large Hadron Collider. [FORMULA OMISSION], [FORMULA OMISSION], and [FORMULA OMISSION] mesons and their charge conjugates with transverse momentum [FORMULA OMISSION] and rapidity in the nucleon-nucleon centre-of-mass system [FORMULA OMISSION] (pp collisions) and [FORMULA OMISSION] (p–Pb collisions) were correlated to charged particles with [FORMULA OMISSION]. The yield of charged particles in the correlation peak induced by the jet containing the D meson and the peak width are compatible within uncertainties in the two collision systems. The data are described within uncertainties by Monte-Carlo simulations based on PYTHIA, POWHEG, and EPOS 3 event generators.</P>
ϒ suppression at forward rapidity in Pb–Pb collisions at <sub> s NN </sub> = 5.02 TeV
Acharya, S.,Acosta, F.T.,Adamová,, D.,Adolfsson, J.,Aggarwal, M.M.,Aglieri Rinella, G.,Agnello, M.,Agrawal, N.,Ahammed, Z.,Ahn, S.U.,Aiola, S.,Akindinov, A.,Al-Turany, M.,Alam, S.N.,Albuquerque, Elsevier 2019 Physics letters: B Vol.790 No.-
<P><B>Abstract</B></P> <P>Inclusive ϒ(1S) and ϒ(2S) production have been measured in Pb–Pb collisions at the centre-of-mass energy per nucleon–nucleon pair <SUB> s NN </SUB> = 5.02 TeV, using the ALICE detector at the CERN LHC. The ϒ mesons are reconstructed in the centre-of-mass rapidity interval 2.5 < y < 4 and in the transverse-momentum range <SUB> p T </SUB> < 15 GeV/<I>c</I>, via their decays to muon pairs. In this Letter, we present results on the inclusive ϒ(1S) nuclear modification factor <SUB> R AA </SUB> as a function of collision centrality, transverse momentum and rapidity. The ϒ(1S) and ϒ(2S) <SUB> R AA </SUB> , integrated over the centrality range 0–90%, are 0.37 ± 0.02 ( stat ) ± 0.03 ( syst ) and 0.10 ± 0.04 ( stat ) ± 0.02 ( syst ) , respectively, leading to a ratio R AA ϒ ( 2S ) / R AA ϒ ( 1S ) of 0.28 ± 0.12 (stat) ± 0.06 (syst) . The observed ϒ(1S) suppression increases with the centrality of the collision and no significant variation is observed as a function of transverse momentum and rapidity.</P>
Analysis of the apparent nuclear modification in peripheral Pb–Pb collisions at 5.02 TeV
Acharya, S.,Acosta, F.T.,Adamová,, D.,Adolfsson, J.,Aggarwal, M.M.,Aglieri Rinella, G.,Agnello, M.,Agrawal, N.,Ahammed, Z.,Ahn, S.U.,Aiola, S.,Akindinov, A.,Al-Turany, M.,Alam, S.N.,Albuquerque, Elsevier 2019 Physics letters: B Vol.793 No.-
<P><B>Abstract</B></P> <P>Charged-particle spectra at midrapidity are measured in Pb–Pb collisions at the centre-of-mass energy per nucleon–nucleon pair <SUB> s NN </SUB> = 5.02 TeV and presented in centrality classes ranging from most central (0–5%) to most peripheral (95–100%) collisions. Possible medium effects are quantified using the nuclear modification factor ( <SUB> R AA </SUB> ) by comparing the measured spectra with those from proton–proton collisions, scaled by the number of independent nucleon–nucleon collisions obtained from a Glauber model. At large transverse momenta ( 8 < <SUB> p T </SUB> < 20 GeV / c ), the average <SUB> R AA </SUB> is found to increase from about 0.15 in 0–5% central to a maximum value of about 0.8 in 75–85% peripheral collisions, beyond which it falls off strongly to below 0.2 for the most peripheral collisions. Furthermore, <SUB> R AA </SUB> initially exhibits a positive slope as a function of <SUB> p T </SUB> in the 8–20 GeV / c interval, while for collisions beyond the 80% class the slope is negative. To reduce uncertainties related to event selection and normalization, we also provide the ratio of <SUB> R AA </SUB> in adjacent centrality intervals. Our results in peripheral collisions are consistent with a PYTHIA-based model without nuclear modification, demonstrating that biases caused by the event selection and collision geometry can lead to the apparent suppression in peripheral collisions. This explains the unintuitive observation that <SUB> R AA </SUB> is below unity in peripheral Pb–Pb, but equal to unity in minimum-bias p–Pb collisions despite similar charged-particle multiplicities.</P>
Measurement of an Excess in the Yield ofJ/ψat Very LowpTin Pb-Pb Collisions atsNN=2.76 TeV
Adam, J.,Adamová,, D.,Aggarwal, M. M.,Aglieri Rinella, G.,Agnello, M.,Agrawal, N.,Ahammed, Z.,Ahn, S. U.,Aiola, S.,Akindinov, A.,Alam, S. N.,Aleksandrov, D.,Alessandro, B.,Alexandre, D.,Alfaro M American Physical Society 2016 Physical review letters Vol.116 No.22
<P>We report on the first measurement of an excess in the yield of J/psi at very low transverse momentum (pT < 0.3 GeV/c) in peripheral hadronic Pb-Pb collisions at root s(NN) = 2.76 TeV, performed by ALICE at the CERN LHC. Remarkably, the measured nuclear modification factor of J/psi in the rapidity range 2.5 < y < 4 reaches about 7 (2) in the p(T) range 0-0.3 GeV/c in the 70%-90% (50%-70%) centrality class. The J/psi production cross section associated with the observed excess is obtained under the hypothesis that coherent photoproduction of J/psi is the underlying physics mechanism. If confirmed, the observation of J/psi coherent photoproduction in Pb-Pb collisions at impact parameters smaller than twice the nuclear radius opens new theoretical and experimental challenges and opportunities. In particular, coherent photoproduction accompanying hadronic collisions may provide insight into the dynamics of photoproduction and nuclear reactions, as well as become a novel probe of the quark-gluon plasma.</P>