Large Pseudorapidity Research Articles

Long-range two-particle correlations of strange hadrons with charged particles in pPb and PbPb collisions at LHC energies

Measurements of two-particle angular correlations between an identified strange hadron (KS0 or Λ/Λ‾) and a charged particle, emitted in pPb collisions, are presented over a wide range in pseudorapidity and full azimuth. The data, corresponding to an integrated luminosity of approximately 35 nb−1, were collected at a nucleon–nucleon center-of-mass energy (sNN) of 5.02 TeV with the CMS detector at the LHC. The results are compared to semi-peripheral PbPb collision data at sNN=2.76 TeV, covering similar charged-particle multiplicities in the events. The observed azimuthal correlations at large relative pseudorapidity are used to extract the second-order (v2) and third-order (v3) anisotropy harmonics of KS0 and Λ/Λ‾ particles. These quantities are studied as a function of the charged-particle multiplicity in the event and the transverse momentum of the particles. For high-multiplicity pPb events, a clear particle species dependence of v2 and v3 is observed. For pT<2 GeV, the v2 and v3 values of KS0 particles are larger than those of Λ/Λ‾ particles at the same pT. This splitting effect between two particle species is found to be stronger in pPb than in PbPb collisions in the same multiplicity range. When divided by the number of constituent quarks and compared at the same transverse kinetic energy per quark, both v2 and v3 for KS0 particles are observed to be consistent with those for Λ/Λ‾ particles at the 10% level in pPb collisions. This consistency extends over a wide range of particle transverse kinetic energy and event multiplicities.

Cross section and transverse single-spin asymmetry ofηmesons inp↑+pcollisions ats=200 GeVat forward rapidity

We present a measurement of the cross section and transverse single-spin asymmetry (AN) for η mesons at large pseudorapidity from s=200 GeV p↑+p collisions. The measured cross section for 0.5<pT<5.0 GeV/c and 3.0<|η|<3.8 is well described by a next-to-leading-order perturbative-quantum-chromodynamics calculation. The asymmetries AN have been measured as a function of Feynman-x (xF) from 0.2<|xF|<0.7, as well as transverse momentum (pT) from 1.0<pT<4.5 GeV/c. The asymmetry averaged over positive xF is ⟨AN⟩=0.061±0.014. The results are consistent with prior transverse single-spin measurements of forward η and π0 mesons at various energies in overlapping xF ranges. Comparison of different particle species can help to determine the origin of the large observed asymmetries in p↑+p collisions.4 MoreReceived 19 June 2014DOI:https://doi.org/10.1103/PhysRevD.90.072008© 2014 American Physical Society

Studies of dijet and photon-jet properties in pp, pPb, and PbPb collisions with CMS

Abstract Studies of dijet and photon-jet properties in pPb collisions are of great importance for establishing a QCD baseline for hadronic interactions with cold nuclear matter. Dijet and photon-jet production has been measured in pPb collisions at a nucleon–nucleon center-of-mass energy of 5.02 TeV. The transverse momentum balance and azimuthal angle correlations are studied in both dijet and photon-jet channels, leading to the observation that there is no significant modification, which allows these systems to be used as tools to probe the nuclear modifications of the parton distribution functions (PDFs). In the dijet system, pseudorapidity distributions are studied as a function of the transverse energy in the forward calorimeters ( E T HF ). The mean value of the dijet pseudorapidity is found to change monotonically with increasing E T HF , indicating a correlation between the energy emitted at large pseudorapidity and the longitudinal motion of the dijet frame. The pseudorapidity distribution of the dijet system is compared with next-to-leading-order perturbative QCD predictions obtained from both nucleon and nuclear PDFs, and the data more closely match the latter. In addition to the studies of initial state, the photon-jet measurements related to quenching in PbPb are updated to have a more precise pp reference based on the 2013 LHC run at 2.76 TeV.

Studies of dijet pseudorapidity distributions and transverse momentum balance in pPb collisions at [formula omitted

Dijet production has been measured in pPb collisions at a nucleon–nucleon center-of-mass energy of 5.02 TeV. A data sample corresponding to an integrated luminosity of 35 nb−1 was collected using the Compact Muon Solenoid detector at the Large Hadron Collider. The dijet transverse momentum balance and pseudorapidity distributions are studied as a function of the transverse energy in the forward calorimeters (ETHF[|η|>4]). For pPb collisions, the dijet transverse momentum ratio is comparable to the same quantities obtained from a simulated pp reference and insensitive to ETHF[|η|>4]. In contrast, the distribution of the dijet pseudorapidity is changing with increasing ETHF[|η|>4], indicating a correlation between the energy emitted at large pseudorapidity and the longitudinal motion of the dijet frame. The observed pseudorapidity distribution of the dijet system in minimum bias pPb collisions is consistent with next-to-leading-order perturbative QCD predictions from nuclear parton distribution functions.

Studies of dijet transverse momentum balance and pseudorapidity distributions in pPb collisions at $$\sqrt{s_{\mathrm{NN}}} = 5.02$$ s NN = 5.02 $$\,\text {TeV}$$ TeV

Dijet production has been measured in $$\mathrm {p}\mathrm {Pb}$$ collisions at a nucleon–nucleon centre-of-mass energy of 5.02 $$\,\text {TeV}$$ . A data sample corresponding to an integrated luminosity of 35 $${{\,\text {nb}^{{-1}}}} $$ was collected using the Compact Muon Solenoid detector at the Large Hadron Collider. The dijet transverse momentum balance, azimuthal angle correlations, and pseudorapidity distributions are studied as a function of the transverse energy in the forward calorimeters ( $$E_\mathrm {T}^{4<|\eta |<\mathrm {5.2}} $$ ). For $$\mathrm {p}\mathrm {Pb}$$ collisions, the dijet transverse momentum ratio and the width of the distribution of dijet azimuthal angle difference are comparable to the same quantities obtained from a simulated $$\mathrm {p}\mathrm {p}$$ reference and insensitive to $$E_\mathrm {T}^{4<|\eta |<\mathrm {5.2}} $$ . In contrast, the mean value of the dijet pseudorapidity is found to change monotonically with increasing $$E_\mathrm {T}^{4<|\eta |<\mathrm {5.2}} $$ , indicating a correlation between the energy emitted at large pseudorapidity and the longitudinal motion of the dijet frame. The pseudorapidity distribution of the dijet system in minimum bias $$\mathrm {p}\mathrm {Pb}$$ collisions is compared with next-to-leading-order perturbative QCD predictions obtained from both nucleon and nuclear parton distribution functions, and the data more closely match the latter.

Measurement of Event Plane Correlations in Pb–Pb Collisions at sNN=2.76TeV with the ATLAS Detector

A measurement of correlations between event-plane angles Φn is presented as a function of centrality for Pb–Pb collisions ant sNN=2.76TeV. These correlations are estimated from observed event-plane angles Ψn obtained from charged particle or transverse energy flow measured over a large pseudorapidity range |η|<4.8, followed by a resolution correction that accounts for the dispersion of Ψn relative to Φn. Various correlators involving two or three event planes with acceptable resolution are measured. Significant positive correlations are observed for 4(Φ2−Φ4),6(Φ2−Φ6),6(Φ3−Φ6),2Φ2+3Φ3−5Φ5,2Φ2+4Φ4−6Φ6 and −10Φ2+4Φ4+6Φ6. However, the measured correlations for 2Φ2+6Φ3+4Φ4 are negative. These results may shed light on the patterns of the fluctuation of the created matter in the initial state as well as the subsequent hydrodynamic evolution.

Jets and Jet-like correlations in STAR

The propagation and modification of hard-scattered partons in the QGP can be studied using various types of jet and jet-like correlation measurements. The STAR detector with its full azimuthal and large pseudorapidity acceptance, as well as its wide transverse momentum (pT) coverage, is well-suited for these measurements. At mid-rapidity, azimuthal correlations of charged hadrons with the axis of a reconstructed trigger jet are used to study the modification of jet shapes and associated hadron yields from p+p to Au+Au. Dihadron correlations with back-to-back high-pT hadron pairs are used to investigate dijets and fragmentation biases. STAR's increased particle identification capabilities due to the Time-Of-Flight detector are utilized to investigate the differences between jet-related and bulk-related particle production. Dihadron correlations with identified trigger particles provide experimental tests of simple recombination theories. The comprehensive set of STAR jet-quenching measurements can be used to further constrain theories of parton energy loss at RHIC.

Third harmonic flow of charged particles in Au+Au collisions atsNN=200 GeV

We report measurements of the third harmonic coefficient of the azimuthal anisotropy, v_3, known as triangular flow. The analysis is for charged particles in Au+Au collisions at sqrtsNN = 200 GeV, based on data from the STAR experiment at the Relativistic Heavy Ion Collider. Two-particle correlations as a function of their pseudorapidity separation are fit with narrow and wide Gaussians. Measurements of triangular flow are extracted from the wide Gaussian, from two-particle cumulants with a pseudorapidity gap, and also from event plane analysis methods with a large pseudorapidity gap between the particles and the event plane. These results are reported as a function of transverse momentum and centrality. A large dependence on the pseudorapidity gap is found. Results are compared with other experiments and model calculations.

Multiplicity distributions in the forward rapidity region in proton-proton collisions at the Large Hadron Collider

Measured multiplicity distributions of primary charged particles produced in the forward rapidity region of the proton-proton ($pp$) collisions at the center-of-mass energy, $\sqrt{s}=7\text{ }\text{ }\mathrm{TeV}$, at the LHC have been analyzed in terms of the negative binomial distribution function. Like the multiplicity distributions in the midrapidity region for the $pp$ collisions at $\sqrt{s}=7\text{ }\text{ }\mathrm{TeV}$, the distributions for the minimum bias events in the forward region are also better described with the superposition of two negative binomail distributions, as proposed by a two-component model of particle production from two processes, the soft and the hard. However, the multiplicity distribution for the ``hard-QCD'' events in a large pseudorapidity window does not oblige the two-component model.

Studies of the nuclear stopping power in PbPb collisions at 2.76 TeV with CMS

Abstract The energy density, d E / d η , in PbPb collisions at 2.76 A TeV nucleus-nucleus center-of-mass energy is measured as a function of pseudorapidity, η , and collision centrality using the CMS detector at the LHC. The very large pseudorapidity coverage of this measurement is used as a powerful constraint for various hadronic interaction models. The CASTOR calorimeter extends the η coverage of CMS up to | η | = 6.6 , which is only 1.4 units away from the beam rapidity for s NN = 2.76 TeV . This is comparable to the most forward measurements at RHIC. A comparison of the centrality dependence of the very forward energy density to that at smaller pseudorapidities is presented and compared to event generator predictions of models for nuclear collisions. The very wide pseudorapidity range of the presented measurement is unique at this center-of-mass energy and the average energy weighted pseudorapidity can be estimated which is compared with results at lower center-of-mass energy. The comparison shows the same energy dependence of the average energy weighted pseudorapidity as seen by RHIC.

Results from ALICE

ALICE, the dedicated heavy-ion experiment at the LHC, has also a rich pp program benefiting from its low material budget, low magnetic field, and its extensive particle identification capabilities. The talk presents recent measurements of relevance for cosmic-ray physics. In particular, ALICE measured the inelastic, single- and doublediffractive cross-sections in pp collisions at √ s = 0.9, 2.76, and 7 TeV [1]. Further, the charged-particle pseudorapidity distribution ( dN ch / dη ) in Pb-Pb collisions at √ s NN = 2.76 TeV was measured over a large pseudorapidity range (-5.0 < η < 5.5) using collisions from displaced vertices [2]. Potential direct contributions of ALICE to cosmic-ray physics are also reviewed [3].

Inclusive charged hadron elliptic flow in Au+Au collisions atsNN=7.7–39 GeV

A systematic study is presented for centrality, transverse momentum ($p_T$) and pseudorapidity ($\eta$) dependence of the inclusive charged hadron elliptic flow ($v_2$) at midrapidity($|\eta| < 1.0$) in Au+Au collisions at $\sqrt{s_{NN}}$ = 7.7, 11.5, 19.6, 27 and 39 GeV. The results obtained with different methods, including correlations with the event plane reconstructed in a region separated by a large pseudorapidity gap and 4-particle cumulants ($v_2{4}$), are presented in order to investigate non-flow correlations and $v_2$ fluctuations. We observe that the difference between $v_2{2}$ and $v_2{4}$ is smaller at the lower collision energies. Values of $v_2$, scaled by the initial coordinate space eccentricity, $v_{2}/\varepsilon$, as a function of $p_T$ are larger in more central collisions, suggesting stronger collective flow develops in more central collisions, similar to the results at higher collision energies. These results are compared to measurements at higher energies at the Relativistic Heavy Ion Collider ($\sqrt{s_{NN}}$ = 62.4 and 200 GeV) and at the Large Hadron Collider (Pb + Pb collisions at $\sqrt{s_{NN}}$ = 2.76 TeV). The $v_2(p_T)$ values for fixed $p_T$ rise with increasing collision energy within the $p_T$ range studied ($< 2 {\rm GeV}/c$). A comparison to viscous hydrodynamic simulations is made to potentially help understand the energy dependence of $v_{2}(p_{T})$. We also compare the $v_2$ results to UrQMD and AMPT transport model calculations, and physics implications on the dominance of partonic versus hadronic phases in the system created at Beam Energy Scan (BES) energies are discussed.

Measurement of the azimuthal anisotropy for charged particle production insNN=2.76TeV lead-lead collisions with the ATLAS detector

Differential measurements of charged particle azimuthal anisotropy are presented for lead-lead collisions at sqrt(s_NN) = 2.76 TeV with the ATLAS detector at the LHC, based on an integrated luminosity of approximately 8 mb^-1. This anisotropy is characterized via a Fourier expansion of the distribution of charged particles in azimuthal angle (phi), with the coefficients v_n denoting the magnitude of the anisotropy. Significant v_2-v_6 values are obtained as a function of transverse momentum (0.5<pT<20 GeV), pseudorapidity (|eta|<2.5) and centrality using an event plane method. The v_n values for n>=3 are found to vary weakly with both eta and centrality, and their pT dependencies are found to follow an approximate scaling relation, v_n^{1/n}(pT) \propto v_2^{1/2}(pT). A Fourier analysis of the charged particle pair distribution in relative azimuthal angle (Dphi=phi_a-phi_b) is performed to extract the coefficients v_{n,n}=<cos (n Dphi)>. For pairs of charged particles with a large pseudorapidity gap (|Deta=eta_a-eta_b|>2) and one particle with pT<3 GeV, the v_{2,2}-v_{6,6} values are found to factorize as v_{n,n}(pT^a,pT^b) ~ v_n(pT^a)v_n(pT^b) in central and mid-central events. Such factorization suggests that these values of v_{2,2}-v_{6,6} are primarily due to the response of the created matter to the fluctuations in the geometry of the initial state. A detailed study shows that the v_{1,1}(pT^a,pT^b) data are consistent with the combined contributions from a rapidity-even v_1 and global momentum conservation. A two-component fit is used to extract the v_1 contribution. The extracted v_1 is observed to cross zero at pT\sim1.0 GeV, reaches a maximum at 4-5 GeV with a value comparable to that for v_3, and decreases at higher pT.

Higher-order anisotropic flows and dihadron correlations in Pb-Pb collisions atsNN=2.76TeV in a multiphase transport model

Using a multiphase transport model that includes both initial partonic and final hadronic scatterings, we have studied higher-order anisotropic flows as well as dihadron correlations as functions of pseudorapidity and azimuthal angular differences between trigger and associated particles in Pb-Pb collisions at $\sqrt{s_{NN}}=2.76$ TeV. With parameters in the model determined previously from fitting the measured multiplicity density of mid-pseudorapidity charged particles in central collisions and their elliptic flow in mid-central collisions, the calculated higher-order anisotropic flows from the two-particle cumulant method reproduce approximately those measured by the ALICE Collaboration, except at small centralities where they are slightly overestimated. Similar to experimental results, the two-dimensional dihadron correlations at most central collisions show a ridge structure at the near side and a broad structure at the away side. The short- and long-range dihadron azimuthal correlations, corresponding to small and large pseudorapidity differences, respectively, are studied for triggering particles with different transverse momenta and are found to be qualitatively consistent with experimental results from the CMS Collaboration. The relation between the short-range and long-range dihadron correlations with that induced by back-to-back jet pairs produced from initial hard collisions is also discussed.

Probing New Top Physics at the LHCb Experiment

We suggest that top quark physics can be studied at the LHCb experiment and that top quark production could be observed. Since LHCb covers a large pseudorapidity region in the forward direction, it has unique abilities to probe new physics in the top quark sector. Furthermore, we demonstrate that LHCb may be able to measure a t ̄t production rate asymmetry and, thus, indirectly probe an anomalous forward-backward t ̄t asymmetry in the forward region, a possibility suggested by the enhanced forward-backward asymmetry reported by the CDF experiment.

Collective flow and long-range correlations in relativistic heavy ion collisions

Making use of recently released data on dihadron correlations by the STAR Collaboration, I analyze the long-range (“ridge-like”) part of these data and show that the dependence on both transverse momentum as well as orientation with respect to the event plane are consistent with correlations expected from only collective flow. In combination with previously analyzed centrality-dependent data, they provide strong evidence that only collective flow effects are present at large relative pseudorapidity. In contrast, by analyzing a “background subtracted” signal, the authors presenting the new data concluded that the ridge-like part of the measured correlation could not in fact be entirely generated from collective flow of the medium. I explain the discrepancy and illustrate some pitfalls of using the ZYAM prescription to remove flow background.

Formula omitted] and charm at STAR

Gluon saturation effects and Color Glass Condensate models become important at a scale Q S 2 ( x ) . If the saturation scale can be raised to above the charm-quark mass, saturation effects can influence charm production and it is expected, based on CGC models, that charm particle production will follow similar patterns to those found in light quarks. This scale can potentially be reached in the charm sector by going to forward rapidities at RHIC and using nuclear “targets”. The large acceptance and pseudorapidity coverage ( 2.5 < η < 4 ) of the Forward Meson Spectrometer (FMS) at STAR provide excellent geometric efficiency to measure high- x F J / ψ production in p+p and d+Au collisions. A significant observation of high- x F J / ψ in p+p collisions has been obtained. Measurements of forward open charm production also look to be feasible based on initial simulation studies. By comparing d+Au measurements to p+p measurements in both charmonium and open charm productions, the FMS at STAR has the potential to test several predictions of CGC/Saturation models in the charm sector.

Erratum: Collision-geometry fluctuations and triangular flow in heavy-ion collisions [Phys. Rev. C81, 054905 (2010)

We introduce the concepts of participant triangularity and triangular flow in heavy-ion collisions, analogous to the definitions of participant eccentricity and elliptic flow. The participant triangularity characterizes the triangular anisotropy of the initial nuclear overlap geometry and arises from event-by-event fluctuations in the participant-nucleon collision points. In studies using a multi-phase transport model (AMPT), a triangular flow signal is observed that is proportional to the participant triangularity and corresponds to a large third Fourier coefficient in two-particle azimuthal correlation functions. Using two-particle azimuthal correlations at large pseudorapidity separations measured by the PHOBOS and STAR experiments, we show that this Fourier component is also present in data. Ratios of the second and third Fourier coefficients in data exhibit similar trends as a function of centrality and transverse momentum as in AMPT calculations. These findings suggest a significant contribution of triangular flow to the ridge and broad away-side features observed in data. Triangular flow provides a new handle on the initial collision geometry and collective expansion dynamics in heavy-ion collisions.

Collision-geometry fluctuations and triangular flow in heavy-ion collisions

We introduce the concepts of participant triangularity and triangular flow in heavy-ion collisions, analogous to the definitions of participant eccentricity and elliptic flow. The participant triangularity characterizes the triangular anisotropy of the initial nuclear overlap geometry and arises from event-by-event fluctuations in the participant-nucleon collision points. In studies using a multiphase transport model (AMPT), a triangular flow signal is observed that is proportional to the participant triangularity and corresponds to a large third Fourier coefficient in two-particle azimuthal correlation functions. Using two-particle azimuthal correlations at large pseudorapidity separations measured by the PHOBOS and STAR experiments, we show that this Fourier component is also present in data. Ratios of the second and third Fourier coefficients in data exhibit similar trends as a function of centrality and transverse momentum as in AMPT calculations. These findings suggest a significant contribution of triangular flow to the ridge and broad away-side features observed in data. Triangular flow provides a new handle on the initial collision geometry and collective expansion dynamics in heavy-ion collisions.

Prompt photon photoproduction at HERA within the framework of the quark Reggeization hypothesis

We study the inclusive production of isolated prompt photons within the framework of the quasi-multi-Regge-kinematic approach, applying the quark Reggeization hypothesis. We describe accurately and without free parameters the transverse momentum and pseudorapidity spectra of prompt photons in the inclusive photoproduction at the HERA Collider. It is shown that the main mechanism of the inclusive prompt photon production in the $\ensuremath{\gamma}p$ collisions is the fusion of a Reggeized quark (antiquark) from the proton and a collinear antiquark (quark) from the photon into a photon, via the effective Reggeon-quark-gamma vertex. The fragmentation of the quark, which is produced via the gamma-Reggeon-quark and quark-Reggeon-quark vertices, into a photon is strongly suppressed by the isolation cone condition and it gives a significant contribution in the region of a large negative pseudorapidity only. At the stage of numerical calculations we use the Kimber-Martin-Ryskin prescription for unintegrated quark and gluon distribution functions, with the following collinear parton densities as input: Martin-Roberts-Stirling-Thorne for a proton and Gl\"uck-Reya-Vogt for a photon.