Backward Rapidity Research Articles

ϕ production at forward rapidity in pp, p–Pb and Pb–Pb collisions with ALICE

The ALICE experiment at the LHC measured vector meson production in p-Pb collisions at sNN=5.02 TeV, pp and Pb-Pb collisions at sNN=2.76 TeV. In pp collisions, the ϕ differential cross section as a function of pT was measured in the range 1<pT<5 GeV/c and compared with the calculations from PHOJET and PYTHIA. In p-Pb collisions, measurements of the ϕ yield and the nuclear modification factor in the rapidity ranges 2.03<y<3.53 (p-going direction) and −4.46<y<−2.96 (Pb-going direction) are shown. An asymmetry between the cross section at forward and backward rapidity is observed. Results are compared with the predictions provided by commonly used event generators. In Pb-Pb collisions, the ϕ yield and the nuclear modification factor are obtained as a function of centrality in the intermediate pT region (2<pT<5 GeV/c) and for 2.5<y<4. Differences are observed between these results and those measured in the same pT range at midrapidity in the KK channel.

Low mass dimuon measurements in pp, p–Pb and Pb–Pb collisions with ALICE

We present results on the ϕ analysis in pp, p–Pb and Pb–Pb collisions at forward rapidity 2.5<y<4 with the ALICE muon spectrometer. The production cross-section of ϕ mesons is studied differentially, as a function of the transverse momentum (pT) in pp collisions at s=2.76 TeV and as a function of both pT and rapidity in p–Pb collisions sNN=5.02 TeV. Additionally, in p–Pb collisions we discuss the nuclear modification factor RpPb at both forward and backward rapidity. We also show the BRσϕ/(BRσρ+BRσω) ratio and the ϕ nuclear modification factor RAA measured in Pb–Pb collisions at sNN=2.76 TeV in the intermediate pT region 2<pT<5 GeV/c as a function of the collision centrality. Remarkable differences are observed in the comparison between the RAA at forward rapidity and the one measured in the same pT range at midrapidity in the hadronic decay channel ϕ→KK.

Centrality dependence of ψ(2S) suppression in p-Pb collisions at s N N = 5.02 $$ {\sqrt{s}}_{\mathrm{NN}}=5.02 $$ TeV

The inclusive production of the $\psi$(2S) charmonium state was studied as a function of centrality in p-Pb collisions at the nucleon-nucleon center of mass energy $\sqrt{s_{\rm NN}}$ = 5.02 TeV at the CERN LHC. The measurement was performed with the ALICE detector in the center of mass rapidity ranges $-4.46<y_{\rm cms}<-2.96$ and $2.03<y_{\rm cms}<3.53$, down to zero transverse momentum, by reconstructing the $\psi$(2S) decay to a muon pair. The $\psi$(2S) production cross section $\sigma_{\psi(\rm 2S)}$ is presented as a function of the collision centrality, which is estimated through the energy deposited in forward rapidity calorimeters. The relative strength of nuclear effects on the $\psi$(2S) and on the corresponding 1S charmonium state J/$\psi$ is then studied by means of the double ratio of cross sections $[\sigma_{\psi(\rm 2S)}/\sigma_{\rm J/\psi}]_{\rm pPb}/[\sigma_{\psi(\rm 2S)}/\sigma_{\rm J/\psi}]_{\rm pp}$ between p-Pb and pp collisions, and by the values of the nuclear modification factors for the two charmonium states. The results show a large suppression of $\psi$(2S) production relative to the J/$\psi$ at backward (negative) rapidity, corresponding to the flight direction of the Pb-nucleus, while at forward (positive) rapidity the suppressions of the two states are comparable. Finally, comparisons to results from lower energy experiments and to available theoretical models are presented.

Understanding pseudorapidity dependence of elliptic flow in heavy-ion collisions using a transport model

Author(s): Nasim, M; Esha, R; Huang, HZ | Abstract: A systematic study of the pseudorapidity dependence of elliptic flow parameter using transport models (e.g., a multiphase transport model, AMPT, and ultrarelativistic quantum molecular dynamics, UrQMD) has been presented. We have observed that while at mid-pseudorapidity the elliptic flow measured using the event-plane method differs significantly from that measured by actual reaction plane method, both the event-plane and reaction-plane methods give the same elliptic flow for far forward and backward pseudorapidity. This indicates that the magnitude of measured v2 around midrapidity strongly depends on the analysis method. Therefore, one should use the same procedure (as used in data analysis) in model calculations while comparing model results and experimental data. We find the shape of v2(η) measured by the PHOBOS experiment is not reproduced by using actual v2 (i.e., measured with respect to the reaction plane) from AMPT and UrQMD models. The shape and magnitude of measured v2(η) can be explained by the AMPT model with string-melting mode only if one uses the same procedure as used in data analysis. Magnitude of elliptic flow can be reproduced for all pseudorapidity range by taking the parton-parton interaction cross section to be 3 mb at sNN=62.4 and 200 GeV. This implies that the partonic interactions are necessary to reproduce data at sNN=62.4 and 200 GeV and the strength of partonic interactions at far forward and backward rapidity is as strong as at midrapidity. Both UrQMD and AMPT with default mode fail to explain the data.

J/ψ photoproduction in ultra-peripheral Pb-Pb and p-Pb collisions with the ALICE detector

Ultra-relativistic heavy ions generate strong electromagnetic fields which offer the possibility to study gamma-gamma, gamma-nucleus and gamma-proton processes at the LHC in ultra-peripheral Pb-Pb and p-Pb collisions (UPC). Exclusive photoproduction of J/ψ vector mesons is sensitive to the gluon distribution of the interacting target (proton or nucleus). This process is expected to be sensitive to saturation phenomena and nuclear gluon shadowing. Here we report on the ALICE measurement of J/ψ coherent photoproduction in Pb-Pb UPC at sNN=2.76 TeV at forward and central rapidities. Furthermore, we also present results on the J/ψ photoproduction in p-Pb UPC at sNN=5.02 TeV in the forward and backward rapidities, where the rapidity is measured in the laboratory frame with respect to the proton beam direction. The J/ψ mesons have been identified through their leptonic decays.

Evidence forx-dependent proton color fluctuations inpAcollisions at the CERN Large Hadron Collider

We argue that the pattern of the deviation from the Glauber approximation prediction for the centrality dependence of the rate of forward jet production observed in pA collisions at the LHC provides the first experimental evidence that parton configurations in the projectile proton containing a parton with large $x$ interact with a nuclear target with a significantly smaller than average cross section and have smaller than average size. We implement the effects of fluctuations of the interaction strength and, using the ATLAS analysis of how hadron production at backward rapidities depends on the number of wounded nucleons, make quantitative predictions for the centrality dependence of the jet production rate as a function of the $x$-dependent interaction strength $\sigma(x)$. We find that \sigma(x)\sim 0.6 ~\sigma_{tot}(pp) gives a good description of the x=0.6 data and may shed a light on the origin of the EMC effect.

Centrality dependence of inclusive J/ψ production in p-Pb collisions at s N N = 5.02 $$ \sqrt{s_{\mathrm{NN}}}=5.02 $$ TeV

We present a measurement of inclusive J/$\psi$ production in p-Pb collisions at $\sqrt{s_{\rm NN}}$ = 5.02 TeV as a function of the centrality of the collision, as estimated from the energy deposited in the Zero Degree Calorimeters. The measurement is performed with the ALICE detector down to zero transverse momentum, $p_{\rm T}$, in the backward ($-4.46 < y_{\rm cms} < -2.96$) and forward ($2.03 < y_{\rm cms} < 3.53$) rapidity intervals in the dimuon decay channel and in the mid-rapidity region ($-1.37 < y_{\rm cms} < 0.43$) in the dielectron decay channel. The backward and forward rapidity intervals correspond to the Pb-going and p-going direction, respectively. The $p_{\rm T}$-differential J/$\psi$ production cross section at backward and forward rapidity is measured for several centrality classes, together with the corresponding average $p_{\rm T}$ and $p^2_{\rm T}$ values. The nuclear modification factor, $Q_{\rm pPb}$, is presented as a function of centrality for the three rapidity intervals, and, additionally, at backward and forward rapidity, as a function of $p_{\rm T}$ for several centrality classes. At mid- and forward rapidity, the J/$\psi$ yield is suppressed up to 40% compared to that in pp interactions scaled by the number of binary collisions. The degree of suppression increases towards central p-Pb collisions at forward rapidity, and with decreasing $p_{\rm T}$ of the J/$\psi$. At backward rapidity, the $Q_{\rm pPb}$ is compatible with unity within the total uncertainties, with an increasing trend from peripheral to central p-Pb collisions.

Rapidity and transverse-momentum dependence of the inclusive J/ψ nuclear modification factor in p-Pb collisions at s N N $$ \sqrt{s_{N\ N}} $$ = 5.02 TeV

We have studied the transverse-momentum ($p_{\rm T}$) dependence of the inclusive J/$\psi$ production in p-Pb collisions at $\sqrt{s_{\rm NN}} = 5.02$ TeV, in three center-of-mass rapidity ($y_{\rm cms}$) regions, down to zero $p_{\rm T}$. Results in the forward and backward rapidity ranges ($2.03 < y_{\rm cms} < 3.53$ and $-4.46 <y_{\rm cms}< -2.96$) are obtained by studying the J/$\psi$ decay to $\mu^+\mu^-$, while the mid-rapidity region ($-1.37 < y_{\rm cms} < 0.43$) is investigated by measuring the ${\rm e}^+{\rm e}^-$ decay channel. The $p_{\rm T}$ dependence of the J/$\psi$ production cross section and nuclear modification factor are presented for each of the rapidity intervals, as well as the J/$\psi$ mean $p_{\rm T}$ values. Forward and mid-rapidity results show a suppression of the J/$\psi$ yield, with respect to pp collisions, which decreases with increasing $p_{\rm T}$. At backward rapidity no significant J/$\psi$ suppression is observed. Theoretical models including a combination of cold nuclear matter effects such as shadowing and partonic energy loss, are in fair agreement with the data, except at forward rapidity and low transverse momentum. The implications of the p-Pb results for the evaluation of cold nuclear matter effects on J/$\psi$ production in Pb-Pb collisions are also discussed.

Low mass dimuon production in pp, p–Pb and Pb–Pb collisions with the ALICE muon spectrometer

We present results on the low mass dimuon analysis in pp, p-Pb and Pb-Pb collisions.In pp collisions at = 2.76 TeV the ϕ differential cross section as a function of the transverse momentum has been measured, while the ϕ yield and the nuclear modification factor RpPb at forward and backward rapidity have been measured in p-Pb collisions at = 5.02 TeV. The . ratio and the 0 nuclear modification factor RAA have been measured in Pb-Pb collisions at = 2.76 TeV in the intermediate pT region 2 < pT < 5 GeV/c, as a function of the number of participating nucleons. Remarkable differences are observed in the comparison between these results and the ones measured in the same pT range at midrapidity in the hadronic decay channel ϕ → KK.

Measurement of W-boson production in p–Pb collisions at = 5.02 TeV with ALICE at the LHC

In hadronic collisions, electroweak bosons are produced in initial hard scattering processes and they are not affected by the strong interaction. In proton-proton collisions, they have been suggested as standard candles for luminosity monitoring and their measurement can improve the evaluation of detector performances. In nucleus-nucleus and proton-nucleus collisions, W-bosons allow one to check at first order the validity of binary collision scaling, while small deviations allow to study the nuclear modifications of parton distribution functions. The W-boson production in p-Pb collisions at = 5.02 TeV is measured via the contribution of W-boson decays to the inclusive pT-differential muon yield reconstructed with the ALICE muon spectrometer at forward (2.03 < yμcms < 3.53) and backward (-4.46 < yμcms < -2.96) rapidity. This paper reports the production cross section of muons from W-boson decays for pμT > 10 GeV/c and the yields normalised to the average number of binary nucleon-nucleon collisions as a function of the event activity.

Low mass dilepton production in pp, p–Pb and Pb–Pb collisions with ALICE

Low mass vector meson ( ρ, ω, ϕ ) production provides key information on the hot and dense state of strongly interacting matter produced in high-energy heavy-ion collisions (called Quark-Gluon Plasma, QGP). Strangeness enhancement is one of the possible signatures of the Quark-Gluon Plasma formation and can be accessed through the measurement of ϕ meson production with respect to ρ and ω mesons. The production dynamics and hadronization process in relativistic heavy-ion collisions can be probed through the measurement of the ϕ nuclear modification factor.We present results on the low mass dimuon analysis in pp, p–Pb and Pb–Pb collisions. In pp collisions at √ s = 2.76 TeV the ϕ differential cross section as a function of the transverse momentum has been measured, serving as a baseline for Pb–Pb data.The ϕ yield and the nuclear modification factor R pPb at forward and backward rapidity have been measured for p–Pb collisions at √ s = 5.02 TeV. At forward rapidity, R pPb increases as a function of p T , saturating for p T > 3 GeV/ c at R pPb ~ 1. At backward rapidity, R pPb shows an increase as a function of the transverse momentum up to a factor of 1.6 for p T ~ 3-4 GeV/ c , followed by a decrease at higher p T .The ϕ yield and nuclear modification factor R AA have been measured for Pb–Pb collisions at √ s NN = 2.76 TeV in the intermediate p T region 2 p T c , as a function of the number of participating nucleons. Remarkable differences are observed in the comparison between the results at forward rapidity in the dimuon channel and the ones measured in the same p T range at midrapidity in the hadronic channel ϕ → KK .In the dielectron channel at midrapidity, the invariant mass distributions in the range 0 m ee c 2 are compared with the expected hadronic sources for pp collisions at √ s = 7 TeV, and for p–Pb collisions at √ s NN = 5.02 TeV. Latest results of the analysis of Pb–Pb collisions at √ s NN = 2.76 TeV are presented.

Feasibility Studies for Quarkonium Production at a Fixed-Target Experiment Using the LHC Proton and Lead Beams (AFTER@LHC)

Being used in the fixed-target mode, the multi-TeV LHC proton and lead beams allow for studies of heavy-flavour hadroproduction with unprecedented precision at backward rapidities, far negative Feynman-x, using conventional detection techniques. At the nominal LHC energies, quarkonia can be studied in detail inp+p,p+d, andp+Acollisions atsNN≃115 GeV and in Pb +pand Pb +Acollisions atsNN≃72 GeV with luminosities roughly equivalent to that of the collider mode that is up to 20 fb−1 yr−1inp+pandp+dcollisions, up to 0.6 fb−1 yr−1inp+Acollisions, and up to 10 nb−1 yr−1in Pb +Acollisions. In this paper, we assess the feasibility of such studies by performing fast simulations using the performance of a LHCb-like detector.

Nuclear matter effects onJ/ψproduction in asymmetric Cu + Au collisions atsNN=200GeV

We report on $J/\psi$ production from asymmetric Cu+Au heavy-ion collisions at $\sqrt{s_{_{NN}}}$=200 GeV at the Relativistic Heavy Ion Collider at both forward (Cu-going direction) and backward (Au-going direction) rapidities. The nuclear modification of $J/\psi$ yields in Cu$+$Au collisions in the Au-going direction is found to be comparable to that in Au$+$Au collisions when plotted as a function of the number of participating nucleons. In the Cu-going direction, $J/\psi$ production shows a stronger suppression. This difference is comparable in magnitude and has the same sign as the difference expected from shadowing effects due to stronger low-$x$ gluon suppression in the larger Au nucleus. The relative suppression is opposite to that expected from hot nuclear matter dissociation, since a higher energy density is expected in the Au-going direction.

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We report on the inclusive J / ψ nuclear modification factor in p–Pb collisions at s NN = 5.02 TeV as a function of rapidity y and transverse momentum p T . The experimental coverage extends down to p T = 0 GeV / c in the three rapidity ranges accessible by ALICE ( − 4.46 < y cms < − 2.96 , − 1.37 < y cms < 0.46 , 2.03 < y cms < 3.53 ). The obtained results as a function of rapidity are in agreement with theory predictions based only on shadowing or on coherent energy loss. At forward and backward rapidity, the ψ ( 2 S ) measurement complements the J / ψ results. The ratio between the ψ ( 2 S ) and J / ψ cross section is significantly smaller in p–Pb than in pp collisions in both rapidity regions.

Production of inclusive ϒ(1S) and ϒ(2S) in p–Pb collisions at [formula omitted

We report on the production of inclusive ϒ(1S) and ϒ(2S) in p–Pb collisions at sNN=5.02 TeV at the LHC. The measurement is performed with the ALICE detector at backward (−4.46<ycms<−2.96) and forward (2.03<ycms<3.53) rapidity down to zero transverse momentum. The production cross sections of the ϒ(1S) and ϒ(2S) are presented, as well as the nuclear modification factor and the ratio of the forward to backward yields of ϒ(1S). A suppression of the inclusive ϒ(1S) yield in p–Pb collisions with respect to the yield from pp collisions scaled by the number of binary nucleon–nucleon collisions is observed at forward rapidity but not at backward rapidity. The results are compared to theoretical model calculations including nuclear shadowing or partonic energy loss effects.

Multiple scattering effects on heavy meson production in p+A collisions at backward rapidity

We study the incoherent multiple scattering effects on heavy meson production in the backward rapidity region of p+A collisions within the generalized high-twist factorization formalism. We calculate explicitly the double scattering contributions to the heavy meson differential cross sections by taking into account both initial-state and final-state interactions, and find that these corrections are positive. We further evaluate the nuclear modification factor for muons that come form the semi-leptonic decays of heavy flavor mesons. Phenomenological applications in d+Au collisions at a center-of-mass energy s=200 GeV at RHIC and in p+Pb collisions at s=5.02 TeV at the LHC are presented. We find that incoherent multiple scattering can describe rather well the observed nuclear enhancement in the intermediate pT region for such reactions.

Measurements of long-range angular correlation and anisotropy in d+Au collisions at sNN=200 GeV from PHENIX

We present azimuthal angular correlations measured in central d+Au collisions at sNN=200GeV between charged hadrons and energy deposited in calorimeter towers. The charged hadrons are measured at mid-rapidity |η|<0.35, and the energy deposits are measured at forward and backward rapidities (−3.7<η<−3.1, Au-going direction and 3.1<η<3.9, d-going direction). An enhanced near-side angular correlation across |Δη|> 2.75 is observed in d+Au collisions. We also present the correlation between energy deposited in the d-going and Au-going direction. Similar to previous case, an enhancement is also observed in the near-side pairs correlations in most central d+Au collisions which across |Δη|>6.2. Using the event-plane method applied to the Au-going energy distribution, we extract the anisotropy strength v2 for inclusive charged hadrons at mid-rapidity up to pT=4.5GeV/c. We also present the measurement of v2 for identified π± and (anti)protons in central d+Au collisions, and observe a mass-ordering pattern similar to that seen in A+A heavy ion collisions. These results are compared with measurements from p+Pb at sNN=5.02TeV.

Inclusive ψ(2S) production in p–Pb collisions with ALICE

The ALICE Collaboration has studied the inclusive $\psi(\rm 2S)$ production in p-Pb collisions at $\sqrt{s_{\rm NN}} = 5.02$ TeV at the CERN LHC. Measurements are performed, in the $\mu^{+}\mu^{-}$ decay channel, in a forward (2.03$<y_{\rm cms}<$3.53) and in a backward (-4.46$<y_{\rm cms}<$-2.96) centre of mass rapidity ranges, as a function of transverse momentum or event activity. The $\psi(\rm 2S)$ production is compared to the $\rm J/\psi$ one through the double ratio $[\sigma_{\psi(\rm 2S)}/\sigma_{\rm J/\psi}]_{\rm pPb}/[\sigma_{\psi(\rm 2S)}/\sigma_{\rm J/\psi}]_{\rm pp}$ between the cross sections evaluated in p-Pb and pp collisions and by calculating the $\rm J/\psi$ and $\psi(\rm 2S)$ nuclear modification factors. Results indicate that the $\psi(\rm 2S)$ production is suppressed with respect to pp and, in particular in the backward rapidity region, the suppression is stronger than the $\rm J/\psi$ one. This unexpected difference between the $\rm J/\psi$ and $\psi(\rm 2S)$ behaviour, not accounted for by theoretical models based on nuclear parton shadowing and coherent energy loss mechanisms, indicates the presence of sizeable final state effects on the $\psi(\rm 2S)$ production.

Quarkonia results from PHENIX

Abstract Measuring the production of multiple quarkonia states in different collision systems and over a range of energies is necessary to disentangle the effects of production in a nuclear target from those due to a hot medium. Measurements by PHENIX of J / ψ production in A + A collisions show similar levels of modification over a range of collision systems as well as energies, which might be explained by an interplay between suppression and regeneration in hot nuclear matter. Measurements of J / ψ and ψ ′ production in d + Au collisions at s NN = 200 GeV have presented a number of interesting puzzles. The p T dependence of J / ψ suppression is found to be different at backward (Au-going) rapidities compared to mid and forward (d-going) rapidities. While this is unexplained by models, comparisons to the modification of heavy flavor muons show a similar discrepancy between backward and forward rapidities. The increased suppression of ψ ′ production compared to J / ψ production in central events is incompatible with either nPDF modification or nuclear breakup. However, an approximate scaling of the relative modification of the ψ ′ to the J / ψ with charged particle multiplicity between SPS p + A , PHENIX d + Au and SPS A + A is also observed, perhaps indicating that interactions with final state partons or hadrons may be important.

Influence of temperature-dependent shear viscosity on elliptic flow at backward and forward rapidities in ultrarelativistic heavy-ion collisions

We explore the influence of a temperature-dependent shear viscosity over entropy density ratio ${\ensuremath{\eta}}_{s}/s$ on the azimuthal anisotropies ${v}_{2}$ and ${v}_{4}$ of hadrons at various rapidities. We find that in $\mathrm{Au}+\mathrm{Au}$ collisions at full Relativistic Heavy Ion Collider energy, $\sqrt{{s}_{\mathrm{NN}}}=200\phantom{\rule{0.28em}{0ex}}\mathrm{GeV}$, the flow anisotropies are dominated by hadronic viscosity at all rapidities, whereas in $\mathrm{Pb}+\mathrm{Pb}$ collisions at the Large Hadron Collider energy, $\sqrt{{s}_{\mathrm{NN}}}=2760\phantom{\rule{0.28em}{0ex}}\mathrm{GeV}$, the flow coefficients are affected by the viscosity in both the plasma and hadronic phases at midrapidity, but the further away from midrapidity, the more dominant the hadronic viscosity becomes. We find that the centrality and rapidity dependence of the elliptic and quadrangular flows can help to distinguish different parametrizations of $({\ensuremath{\eta}}_{s}/s)(T)$. We also find that at midrapidity the flow harmonics are almost independent of the decoupling criterion, but they show some sensitivity to the criterion at backward and forward rapidities.