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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.780 No.-
<P><B>Abstract</B></P> <P>The production of <SUP> Z 0 </SUP> bosons at large rapidities in Pb–Pb collisions at <SUB> s NN </SUB> = 5.02 TeV is reported. <SUP> Z 0 </SUP> candidates are reconstructed in the dimuon decay channel ( <SUP> Z 0 </SUP> → <SUP> μ + </SUP> <SUP> μ − </SUP> ), based on muons selected with pseudo-rapidity − 4.0 < η < − 2.5 and <SUB> p T </SUB> > 20 GeV/ c . The invariant yield and the nuclear modification factor, <SUB> R AA </SUB> , are presented as a function of rapidity and collision centrality. The value of <SUB> R AA </SUB> for the 0–20% central Pb–Pb collisions is 0.67 ± 0.11 (stat.) ± 0.03 (syst.) ± 0.06 (corr. syst.) , exhibiting a deviation of 2.6<I>σ</I> from unity. The results are well-described by calculations that include nuclear modifications of the parton distribution functions, while the predictions using vacuum PDFs deviate from data by 2.3<I>σ</I> in the 0–90% centrality class and by 3<I>σ</I> in the 0–20% central collisions.</P>
Acharya, S.,Adam, J.,Adamová,, D.,Adolfsson, J.,Aggarwal, M.M.,Aglieri Rinella, G.,Agnello, M.,Agrawal, N.,Ahammed, Z.,Ahmad, N.,Ahn, S.U.,Aiola, S.,Akindinov, A.,Al-Turany, M.,Alam, S.N.,Alba, Elsevier 2018 Physics letters: B Vol.781 No.-
<P><B>Abstract</B></P> <P>First results on the longitudinal asymmetry and its effect on the pseudorapidity distributions in Pb–Pb collisions at <SUB> s NN </SUB> = 2.76 TeV at the Large Hadron Collider are obtained with the ALICE detector. The longitudinal asymmetry arises because of an unequal number of participating nucleons from the two colliding nuclei, and is estimated for each event by measuring the energy in the forward neutron-Zero-Degree-Calorimeters (ZNs). The effect of the longitudinal asymmetry is measured on the pseudorapidity distributions of charged particles in the regions | η | < 0.9 , 2.8 < η < 5.1 and − 3.7 < η < − 1.7 by taking the ratio of the pseudorapidity distributions from events corresponding to different regions of asymmetry. The coefficients of a polynomial fit to the ratio characterise the effect of the asymmetry. A Monte Carlo simulation using a Glauber model for the colliding nuclei is tuned to reproduce the spectrum in the ZNs and provides a relation between the measurable longitudinal asymmetry and the shift in the rapidity ( <SUB> y 0 </SUB> ) of the participant zone formed by the unequal number of participating nucleons. The dependence of the coefficient of the linear term in the polynomial expansion, <SUB> c 1 </SUB> , on the mean value of <SUB> y 0 </SUB> is investigated.</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>
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.,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>
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>
Acharya, S.,Adam, J.,Adamová,, D.,Adolfsson, J.,Aggarwal, M.M.,Aglieri Rinella, G.,Agnello, M.,Agrawal, N.,Ahammed, Z.,Ahmad, N.,Ahn, S.U.,Aiola, S.,Akindinov, A.,Al-Turany, M.,Alam, S.N.,Albuqu North-Holland Pub. Co 2018 Physics letters. Section B Vol.777 No.-
<P><B>Abstract</B></P> <P>In ultrarelativistic heavy-ion collisions, the event-by-event variation of the elliptic flow <SUB> v 2 </SUB> reflects fluctuations in the shape of the initial state of the system. This allows to select events with the same centrality but different initial geometry. This selection technique, Event Shape Engineering, has been used in the analysis of charge-dependent two- and three-particle correlations in Pb–Pb collisions at <SUB> s NN </SUB> = 2.76 TeV. The two-particle correlator 〈 cos ( <SUB> φ α </SUB> − <SUB> φ β </SUB> ) 〉 , calculated for different combinations of charges <I>α</I> and <I>β</I>, is almost independent of <SUB> v 2 </SUB> (for a given centrality), while the three-particle correlator 〈 cos ( <SUB> φ α </SUB> + <SUB> φ β </SUB> − 2 <SUB> Ψ 2 </SUB> ) 〉 scales almost linearly both with the event <SUB> v 2 </SUB> and charged-particle pseudorapidity density. The charge dependence of the three-particle correlator is often interpreted as evidence for the Chiral Magnetic Effect (CME), a parity violating effect of the strong interaction. However, its measured dependence on <SUB> v 2 </SUB> points to a large non-CME contribution to the correlator. Comparing the results with Monte Carlo calculations including a magnetic field due to the spectators, the upper limit of the CME signal contribution to the three-particle correlator in the 10–50% centrality interval is found to be 26–33% at 95% confidence level.</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>
Inclusive J/<i>ψ</i> production in Xe–Xe collisions at <sub> s NN </sub> = 5.44 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.,Albuquerqu North-Holland Pub. Co 2018 Physics letters. Section B Vol.785 No.-
<P><B>Abstract</B></P> <P>Inclusive J/<I>ψ</I> production is studied in Xe–Xe interactions at a centre-of-mass energy per nucleon pair of <SUB> s NN </SUB> = 5.44 TeV, using the ALICE detector at the CERN LHC. The J/<I>ψ</I> 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 <SUB> R AA </SUB> for inclusive J/<I>ψ</I>, measured in the centrality range 0–90% as well as in the centrality intervals 0–20% and 20–90% are presented. The <SUB> R AA </SUB> values are compared to previously published results for Pb–Pb collisions at <SUB> s NN </SUB> = 5.02 TeV and to the calculation of a transport model. A good agreement is found between Xe–Xe and Pb–Pb results as well as between data and the model.</P>