Function Of Transverse Momentum Research Articles

Studies of exotic quarkonium states at CMS

Using large data samples of dimuon events in pp collisions at TeV, CMS has performed studies in the field of "exotic" quarkonium spectroscopy. The production of the X(3872) is studied measuring the ratio of the X(3872) and ψ(2S) cross sections times their branching fractions into J/ψπ+π− as a function of transverse momentum (pT). In addition, the fraction of X(3872) originating from B decays is determined. From these measurements the prompt X(3872) differential cross section times branching fraction as a function of pT is extracted. The π+π− mass spectrum of the J/ψπ+π− system in the X(3872) decays is also investigated. Preliminary studies on structures in the J/ψϕ mass spectrum of the exclusive B+ → J/ψϕK+ decays are reported.

Charmonia production in ALICE

Quarkonia states are expected to provide essential information on the properties of the high-density strongly-interacting system formed in the early stages of high-energy heavy-ion collisions. ALICE is the LHC experiment dedicated to the study of nucleus-nucleus collisions and can study charmonia at forward rapidity (2.5 < y < 4) via the mu+ mu- decay channel and at mid rapidity (|y| < 0.9) via the e+ e- decay channel. In both cases charmonia are measured down to zero transverse momentum. The inclusive J/psi production as a function of transverse momentum and rapidity in pp collisions at sqrt{s} = 2.76 and 7 TeV are presented. For pp collisions at sqrt{s} = 7 TeV, the inclusive J/psi production as a function of the charged particle multiplicity, the inclusive J/psi polarization at forward rapidity and the J/psi prompt to non-prompt fraction are discussed. Finally, the analysis of the inclusive J/psi production in the Pb-Pb data collected fall 2011 at a center of mass energy of sqrt{s_{NN}} = 2.76 TeV is presented. Results on the nuclear modification factor are then shown as a function of centrality, transverse momentum and rapidity and compared to model predictions. First results on inclusive J/psi elliptic flow are given.

Measurement of high pT isolated prompt photons in lead–lead collisions at sNN=2.76TeV with the ATLAS detector at the LHC

Prompt photons are a powerful tool to study heavy ion collisions. Their production rates provide access to the initial state parton distribution functions and also provide a means to calibrate the expected energy of jets that are produced in the medium. The ATLAS detector measures photons with its hermetic, longitudinally segmented calorimeter, which gives excellent spatial and energy resolutions, and detailed information about the shower shape of each measured photon. This provides significant rejection against the expected background from the decays of neutral pions in jets. Rejection against jet fragmentation products is further enhanced by requiring candidate photons to be isolated. First results on the spectra of isolated prompt photons from a dataset with an integrated luminosity of approximately 0.13 nb^-1 of lead-lead collisions at sqrt(s_NN)=2.76 TeV are shown as a function of transverse momentum and centrality. The measured spectra are compared to expectations from perturbative QCD calculations.

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.

Measurement with the ATLAS detector of multi-particle azimuthal correlations in [formula omitted] collisions at [formula omitted

In order to study further the long-range correlations (“ridge”) observed recently in p+Pb collisions at sNN=5.02 TeV, the second-order azimuthal anisotropy parameter of charged particles, v2, has been measured with the cumulant method using the ATLAS detector at the LHC. In a data sample corresponding to an integrated luminosity of approximately 1 μb−1, the parameter v2 has been obtained using two- and four-particle cumulants over the pseudorapidity range |η|<2.5. The results are presented as a function of transverse momentum and the event activity, defined in terms of the transverse energy summed over 3.1<η<4.9 in the direction of the Pb beam. They show features characteristic of collective anisotropic flow, similar to that observed in Pb+Pb collisions. A comparison is made to results obtained using two-particle correlation methods, and to predictions from hydrodynamic models of p+Pb collisions. Despite the small transverse spatial extent of the p+Pb collision system, the large magnitude of v2 and its similarity to hydrodynamic predictions provide additional evidence for the importance of final-state effects in p+Pb reactions.

Elliptic and hexadecapole flow of charged hadrons in viscous hydrodynamics with Glauber and color glass condensate initial conditions for Pb–Pb collision at = 2.76 TeV

The experimentally measured elliptic (v2) and hexadecapole (v4) flow of charged particles as a function of transverse momentum (pT) at midrapidity in Pb–Pb collisions at = 2.76 TeV is compared with the relativistic viscous hydrodynamic model simulations. The simulations are carried out for two different initial energy density profiles obtained from (i) the Glauber model, and (ii) the color glass condensate (CGC) model. A comparison to experimental data for 10–20% to 40–50% centrality shows that a centrality dependent shear viscosity to entropy density (η/s) ratio with values ranging between 0.0 to 0.12 is needed to explain the v2 data for simulations with the Glauber based initial condition, whereas for the CGC based initial conditions a slightly higher value of η/s is preferred, around 0.08 to 0.16. From the comparison of the v4 simulated results to the corresponding experimental measurements we observe that for the centralities 20–30% to 40–50% the η/s values lie between 0.0 to 0.12 for both the initial conditions studied. The η/s values obtained from our studies for Pb–Pb collisions at = 2.76 TeV are compared to other studies which use both transport and hydrodynamic approaches.

Event shape engineering with ALICE

The strong fluctuations in the initial energy density of heavy-ion collisions allow an efficient selection of events corresponding to a specific initial geometry. For such "shape engineered events", the elliptic flow coefficient, $v_2$, of unidentified charged particles, pions and (anti-)protons in Pb-Pb collisions at $\snn = 2.76$ TeV is measured by the ALICE collaboration. $v_2$ obtained with the event plane method at mid-rapidity, $|\eta|<0.8$, is reported for different collision centralities as a function of transverse momentum, $\pt$, out to $\pt=20$ GeV/$c$. The measured $v_2$ for the shape engineered events is significantly larger or smaller than the average which demonstrates the ability to experimentally select events with the desired shape of the initial spatial asymmetry.

Nuclear modification factor of muons from heavy-flavour decays and muon elliptic flow in Pb–Pb collisions at sNN=2.76 TeV

Results from the ALICE experiment on the production of muons from heavy-flavour decays, at forward rapidity, in Pb–Pb collisions at s NN = 2.76 TeV are reported. A particular emphasis is placed on the measurement of the nuclear modification factor as a function of transverse momentum and centrality, and on the comparison to model predictions. First results on the inclusive muon elliptic flow are presented.

Measurement of Isolated Direct Photons in Lead–Lead Collisions at sNN=2.76TeV with the ATLAS Detector

Direct photons are a powerful tool to study heavy ion collisions. Their production rates provide access to the parton distribution functions, which are expected to be modified by nuclear effects. They also provide a mean to calibrate the energy of jets that are produced in the medium, and thus are a tool to probe the physics of jet quenching more precisely both through jet rates and fragmentation properties. The ATLAS detector measures photons with its hermetic, longitudinally segmented calorimeter, which gives excellent spatial and energy resolution, and detailed information about the shower shape of each measured photon. This gives powerful rejection against backgrounds from neutral pions produced in jets. Rejection against jet fragmentation products is further enhanced by isolation criteria, which can be based on energy flow in the calorimeter. First results on the rates of isolated direct photons from approximately 0.13 nb − 1 of lead–lead data will be shown, as a function of transverse momentum, pseudorapidity and centrality.

Jet-Medium Interactions in Pb–Pb Collisions

Abstract Previous experimental measurements from nuclear collisions have indicated modifications of jets by interaction with the medium created in the collision. Observables from particle correlations in the ALICE detector continue to provide access to key properties of the hot deconfined nuclear matter. New results from two- and three-particle number and transverse momentum correlations are discussed. Specifically, correlation function properties are characterized as a function of transverse momentum and centrality and for different charge combinations. Fourier decompositions are performed, identified particle ratios are studied in the jet-like peak and in the bulk, and the away-side shape is looked at in three-particle correlations.

PHENIX Measurements of Higher-order Flow Harmonics for Identified Charged Hadrons in Au + Au collisions at sNN=39–200mboxGeV

The azimuthal anisotropy coefficients v 2 , 3 , 4 , characterizing collective flow in Au + Au collisions, are presented for identified particle species as a function of transverse momentum ( p T ) , centrality (cent) and beam collision energy ( s N N = 0.039 – 0.20 TeV ) . The v n values for each particle species, show little, if any, change over the measured beam energy range, and v n / ( n q ) n / 2 vs. K E T / n q scales to a single curve (constituent quark number ( n q ) scaling) for each n , over a broad range of transverse kinetic energies ( K E T ) . A comparison of v 2 ( p T ) for individual particle species obtained in Au + Au collisions at s N N = 0.20 TeV (RHIC) and Pb + Pb collisions at s N N = 2.76 TeV (LHC), indicate stronger collective flow at the LHC. These flow measurements and their scaling patterns, can provide important additional constraints for extraction of the specific viscosity η / s .

Observation of Associated Near-Side and Away-Side Long-Range Correlations insNN=5.02 TeVProton-Lead Collisions with the ATLAS Detector

Two-particle correlations in relative azimuthal angle (Δø) and pseudorapidity (Δη) are measured in sqrt[s(NN)] = 5.02 TeV p+Pb collisions using the ATLAS detector at the LHC. The measurements are performed using approximately 1 μb(-1) of data as a function of transverse momentum (p(T)) and the transverse energy (ΣE(T)(Pb)) summed over 3.1 < η < 4.9 in the direction of the Pb beam. The correlation function, constructed from charged particles, exhibits a long-range (2 < |Δ η | < 5) "near-side" (Δø ~ 0) correlation that grows rapidly with increasing ΣE(T)(Pb). A long-range "away-side" (Δø ~ π) correlation, obtained by subtracting the expected contributions from recoiling dijets and other sources estimated using events with small ΣE(T)(Pb), is found to match the near-side correlation in magnitude, shape (in Δη and Δø) and ΣE(T)(Pb) dependence. The resultant Δø correlation is approximately symmetric about π/2, and is consistent with a dominant cos2Δø modulation for all ΣE(T)(Pb) ranges and particle p(T).

The elliptic flow of di-electrons GeV Au+Au collisions at STAR

In minimum bias (0-80%) Au+Au collisions at GeV, the elliptic flow (v2) measurements for di-electrons from the low to intermediate mass region are presented. The differential elliptic flow as a function of transverse momentum in different mass regions are also reported. The simulated and measured v2 of di-electrons are compared in the mass regions of Mee < 0.14 GeV/c2 and 0.14 < Mee < 0.3 GeV/c2.

Measurement of the elliptic anisotropy of charged particles produced in PbPb collisions atsNN=2.76TeV

The anisotropy of the azimuthal distributions of charged particles produced in PbPb collisions with a nucleon-nucleon center-of-mass energy of 2.76 TeV is studied with the CMS experiment at the LHC. The elliptic anisotropy parameter defined as the second coefficient in a Fourier expansion of the particle invariant yields, is extracted using the event-plane method, two- and four-particle cumulants, and Lee--Yang zeros. The anisotropy is presented as a function of transverse momentum (pt), pseudorapidity (eta) over a broad kinematic range: 0.3 < pt < 20 GeV, abs(eta) < 2.4, and in 12 classes of collision centrality from 0 to 80%. The results are compared to those obtained at lower center-of-mass energies, and various scaling behaviors are examined. When scaled by the geometric eccentricity of the collision zone, the elliptic anisotropy is found to obey a universal scaling with the transverse particle density for different collision systems and center-of-mass energies.

Anisotropic flow of charged hadrons, pions and (anti-)protons measured at high transverse momentum in Pb–Pb collisions at [formula omitted

The elliptic, v2, triangular, v3, and quadrangular, v4, azimuthal anisotropic flow coefficients are measured for unidentified charged particles, pions, and (anti-)protons in Pb–Pb collisions at sNN=2.76 TeV with the ALICE detector at the Large Hadron Collider. Results obtained with the event plane and four-particle cumulant methods are reported for the pseudo-rapidity range |η|<0.8 at different collision centralities and as a function of transverse momentum, pT, out to pT=20 GeV/c. The observed non-zero elliptic and triangular flow depends only weakly on transverse momentum for pT>8 GeV/c. The small pT dependence of the difference between elliptic flow results obtained from the event plane and four-particle cumulant methods suggests a common origin of flow fluctuations up to pT=8 GeV/c. The magnitude of the (anti-)proton elliptic and triangular flow is larger than that of pions out to at least pT=8 GeV/c indicating that the particle type dependence persists out to high pT.

Transverse Momentum, Centrality, and Participant Nucleon Number Dependence of Elliptic Flow

In the combined framework of a multisource and participant-spectator model, we study the elliptic flow as a function of transverse momentum and number of participating nucleons insNN=200 GeV Au-Au andsNN=2.76 TeV Pb-Pb collisions and argue that the geometry of the initial overlap region in the collisions determines the experimentally measured azimuthal asymmetries. Our results are approximately in agreement with the data from PHENIX and ALICE collaborations.

Measurements of Quarkonium Production Cross-Sections with the CMS Experiment

Using large data samples of dimuon events, CMS has performed detailed measurements of differential cross sections of J/Ψ, Ψ(2S) and ϒ(nS) states, as a function of transverse momentum in several rapidity bins. The measurements reach significantly higher transverse momentum than previously reported. The quarkonium production cross-sections are compared with recent predictions in the context of NLO nonrelativistic QCD.In this talk, we will present a new measurement on ϒ(nS) production cross sections in proton-proton collisions at s=7 TeV. We report the cross section results integrated over the transverse momentum range pT<50 GeV/c and rapidity range |y|<2.4. We also report the measurement of the ϒ(nS) differential cross sections as a function of transverse momentum and rapidity and the cross section ratios ϒ(2S)/ϒ(1S) and ϒ(3S)/ϒ(1S).Finally, we report the very recent measurement of χc2/χc1 production cross-sections, differentially in pT, extending to pT=25 GeV/c, thereby providing an important test of QCD calculations at higher pT than previously possible.

Formula omitted], [formula omitted] and [formula omitted] Production Measurement with the ATLAS Detector

The J/ψ and ϒ(1S) production cross-sections are measured in proton-proton collisions using the ATLAS detector at the LHC. Differential cross-sections are measured as a function of transverse momentum and rapidity. Results are compared to QCD predictions. A new χb state has been observed though radiative transitions to the ϒ(1S) and ϒ(2S) states.

Azimuthal Anisotropy in Charged Hadron Production in 2.76 TeV PbPb Collisions Measured by CMS

We report on the CMS measurements of charged hadron anisotropic azimuthal distributions from PbPb collisions at a center-of-mass energy of 2.76 TeV per nucleon pair and their decomposition into a Fourier series up to the 6th coefficient. The results are presented as a function of transverse momentum, centrality and pseudorapidity and cover a broad kinematic range. These results can provide constraints on the theoretical description of the early dynamics in the hot and dense medium and its transport properties.

J/ψ measurements in STAR Experiment at RHIC

During the last decade STAR experiment has studied the hot and dense nuclear matter produced in heavy ion collisions at RHIC. The study of quarkonia, such as J/ψ meson, production provides the information about properties of this nuclear matter. It is predicted that due to the Debye screening the quarkonia production is suppressed when Quark Gluon Plasma (QGP) is presented. In order to understand the properties of hot and dense nuclear matter, it is necessary to study the production of quarkonia in hadron-hadron, hadron-ion, and ion-ion collisions separately to distinguish cold nuclear matter effect, such as gluon shadowing, from the suppression due to the formation of QGP. Moreover, the J/ψ hadron azimuthal correlations allow to extract the fraction of J/ψ that originates from B-mesons. This proceedings presents recent J/ψ measurements at mid-rapidity in p+p, and Au+Au collisions at = 200 GeV at STAR, the fraction of J/ψ originates from B-mesons, and J/ψ eliptic flow v2 as a function of transverse momentum in Au+Au collisions in = 200 GeV at STAR.