Nuclear Parton Distribution Functions Research Articles

Isolated photon measurements in pp and PbPb collisions with CMS

Abstract We report new measurements using isolated photons based on the high statistics 2.76 TeV pp reference data collected in 2013. The nuclear modification factors of reconstructed isolated photons, obtained by the comparison of heavy ion collisions and pp data, are presented and provide useful constraints to the nuclear parton distribution functions. For events containing a high p T isolated photon with an associated jet, the photon + jet p T imbalance is studied as a function of collision centrality and compared to pp data and model calculations. These measurements allow an unbiased characterization of the in-medium parton energy loss.

Centrality and rapidity dependence of inclusive jet production in sNN=5.02 TeV p+Pb collisions with the ATLAS detector

Abstract Measurements of reconstructed jets in high-energy proton–nucleus collisions over a wide rapidity and transverse momentum range are a fundamental probe of the partonic structure of nuclei. Inclusive jet production is sensitive to the modification of parton distribution functions (PDF) in the high-density nuclear environment. Furthermore, any modification of jet production in p + A collisions has implications for understanding the strong suppression seen in central A + A collisions. The latest results on inclusive jet production in 29/nb of proton–lead collisions at 5.02 TeV with the ATLAS detector at the LHC are presented. The centrality of p + Pb events is characterized through the sum of the transverse energy in the Pb-going forward calorimeter. In minimum bias p + Pb collisions, the jet yields are seen to be consistent with calculations incorporating nuclear PDF effects. However, the jet yields in central and peripheral p + Pb collisions are found to be strongly suppressed and enhanced, respectively, relative to geometric expectations. Furthermore, the modifications at forward rapidities are seen to be consistent with a scaling in the total jet energy, suggesting that the modifications may depend on the initial kinematics of the hard parton–parton scattering.

Probing the small-x nuclear gluon distributions with isolated photons at forward rapidities in p+Pb collisions at the LHC

Inclusive direct photon production in p+Pb collisions at the LHC is studied within the NLO perturbative QCD. Our aim is to quantify the dominant $x$ regions probed at different rapidities and to identify the best conditions for testing the nuclear gluon parton distribution functions (nPDFs) at small $x$. A comparison to the inclusive pion production reveals that from these two processes the photons carry more sensitivity to the small-$x$ partons and that this sensitivity can be further increased by imposing an isolation cut for the photon events. The details of the isolation criteria, however, seem to make only a small difference to the studied $x$ sensitivity and have practically no effect on the expected nuclear modifications. We consider also the yield asymmetry between forward and backward rapidities which can be used to probe the nPDFs irrespectively of whether an accurate p+p baseline is available.

Spectra of identified particles, geometry categorization and bias and global observables in d+Au collisions

Abstract Geometry selection in d + Au / p + Pb collisions is crucial for understanding the physics underlying modified nuclear parton distribution functions, gluon saturation or shadowing, initial state energy loss, and possible hydrodynamic flow in these small systems. The PHENIX Collaboration tests for auto-correlation biases in the geometry determination in small collision systems. These biases are well understood and an order of magnitude smaller at RHIC as compared to the LHC. As a result, auto-correlation biases are unable to describe the suppression of high transverse momentum ( p T ) π 0 's seen in the ratio of central-to-peripheral d + Au collisions. The centrality dependent d + Au pion, kaon and proton yields relative to binary collision-scaled p + p yields are also reported, including the high p T π 0 and K S 0 . At intermediate p T , between 2 and 5 GeV / c , baryons are enhanced in central d + Au collisions. The baryon enhancement is present in d + Au and Au + Au collisions and increases with centrality. We compare identified particle yields in peripheral Au + Au collisions to central d + Au collisions that have a comparable number of participants and binary collisions. The p T dependence of this ratio is strikingly similar for mesons and baryons.

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.

Z and W boson production in pPb collisions with CMS

Abstract The electroweak boson production is an important benchmark measurement in ultra-relativistic heavy-ion collisions which can provide constraints on the nuclear parton distribution functions. In this paper the first results from the proton–lead collision data taken in early 2013 are presented. The Z boson production cross section is measured in the muon decay channel in bins of transverse momentum and rapidity together with the forward–backward ratio. The W production is studied in the muon and electron decay channels and the differential cross sections, lepton-charge and forward–backward asymmetries are computed as a function of the lepton pseudorapidity. All results are compared with theory predictions with and without nuclear modification of the parton distribution functions showing hints of nuclear effects.

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.

Dijet measurements in pPb collisions

Abstract Results on dijet production in pPb collisions at 5.02 TeV are discussed with a focus on dijet pseudorapidity, η dijet , and centrality classification biases on it. The normalized η dijet distributions from data and from calculations with CT10 proton parton distribution function (pPDF) and with EPS09 nuclear parton distribution function (nPDF) are compared. Despite the agreement between data and theoretical calculations for an inclusive centrality selection, as the raw transverse energy deposited in the forward calorimeter (forward activity) increases, η dijet distribution shifts towards lead beam direction an amount that is larger than what would be expected from the impact parameter dependence of nPDFs. To understand the driving mechanism of this shift an additional measurement of 〈 η dijet 〉 as a function of forward activity in the lead direction is carried out in differential bins of forward activity in the proton direction. The shift is reduced by keeping forward activity in the proton direction in a narrow interval.

Probing nuclear dynamics in jet production with a global event shape

We study single jet production in electron-nucleus collisions ${e}^{\ensuremath{-}}+{N}_{A}\ensuremath{\rightarrow}J+X$, using the 1-jettiness (${\ensuremath{\tau}}_{1}$) global event shape. It inclusively quantifies the pattern of radiation in the final state, gives enhanced sensitivity to soft radiation at wide angles from the nuclear beam and final-state jet, and facilitates the resummation of large Sudakov logarithms associated with the veto on additional jets. Through their effect on the observed pattern of radiation, 1-jettiness can be a useful probe of nuclear parton distribution functions and power corrections from dynamical effects in the nuclear medium. This formalism allows for the standard jet shape analysis while simultaneously providing sensitivity to soft radiation at wide angles from the jet. We use a factorization framework for cross-sections differential in ${\ensuremath{\tau}}_{1}$ and the transverse momentum (${P}_{{J}_{T}}$) and rapidity ($y$) of the jet, in the region ${\ensuremath{\tau}}_{1}\ensuremath{\ll}{P}_{{J}_{T}}$. The restriction ${\ensuremath{\tau}}_{1}\ensuremath{\ll}{P}_{{J}_{T}}$ allows only soft radiation between the nuclear beam and jet directions, thereby acting as a veto on additional jets. This region is also insensitive to the details of the jet algorithm, allowing for better theoretical control over resummation, while providing enhanced sensitivity to nuclear medium effects. We give numerical results at leading twist, with resummation at the next-to-next-to-leading logarithmic level of accuracy, for a variety of nuclear targets. Such studies would be ideal for the electron-ion collider and the LHeC proposals for a future electron-ion collider, where a range of nuclear targets are planned.

Electroweak bosons in heavy-ion collisions with the CMS detector at =2.76 TeV

Electroweak gauge bosons W and Z, do not interact strongly, and thus constitute clean probes of the initial state of nucleus-nucleus collisions. The comparison of their production cross-sections in pp and in nuclear collisions provides an estimate of the nuclear parton distribution functions. Despite the low production cross section of weak bosons compared to other nuclear processes, the relatively clean signal of their leptonic decay channel allows their detection. This paper reports measurements of Z and W bosons, produced in PbPb and pp collisions both at nucleon-nucleon center of mass energy =2.76 TeV with the CMS detector. The Z boson yield and the nuclear modification factor (RAA) corresponding to the integrated luminosity of 150 μb−1 for PbPb collisions are presented. The search for W bosons has been performed in the muon plus neutrino channel, using the data sample with integrated luminosity of 7.2 μb−1 for PbPb collisions. Event centrality an muon pseudorapidity dependencies are studied for the complete W candidate sample as well as samples separated by charge (W+ and W−).

Prompt photon measurements with PHENIX's MPC-EX detector

The MPC-EX detector is a Si-W preshower extension to the existing Muon Piston Calorimeter (MPC). The MPC-EX consists of eight layers of alternating W absorber and Si mini-pad sensors. Located at forward rapidity, 3.1 < |η| < 3.8, the MPC and MPC-EX will access low-x partons in the Au nucleus in p+Au collisions and high-x partons in the projectile in polarized p+p collisions. With the addition of the MPC-EX, the neutral pion reconstruction energy range extends to the luminosity limit, energies > 80 GeV, a factor of four improvement over current capabilities. Not only will the MPC-EX strengthen PHENIX's existing forward π0 and jet measurements, it will provide sufficient prompt photon and π0 separation to make a prompt photon measurement possible. Prompt photon yields at high pT, pT > 3 GeV/c, can be statistically extracted using the double ratio method. In transversely polarized p+p collisions, the measurement of the prompt photon single spin asymmetry, AN, will resolve the sign discrepancy between the Sivers and twist-3 extractions of AN. In p+Au collisions, the prompt photon RpAu will quantify the level of gluon saturation in the Au nucleus at low-x, x ~ 10−3, with a projected systematic error band a factor of four smaller than EPS09's current allowable range. The MPC-EX detector will expand our understanding of the gluon nuclear parton distribution functions, providing important information about the initial state of heavy ion collisions, and clarify how the valence parton's transverse momentum and spin correlates to the proton spin.

Reconstructed Jet Results in p + p, d + Au and Cu + Cu collisions at 200 GeV from PHENIX

Abstract Jet reconstruction in heavy ion collisions at RHIC and the LHC is becoming a popular tool to explore medium effects including the energy loss and modified fragmentation of hard-scattered partons. In p + A and d + A collisions, reconstructed jets are important for evaluating cold nuclear matter effects such as the impact parameter dependence of nuclear parton distribution functions and initial state energy loss. We present current PHENIX results from p + p, d + Au, and Cu + Cu collisions at 200 GeV using the Gaussian filter and anti- k T algorithms. The systematic study of direct jet reconstruction across a variety of collisions systems at PHENIX will help to tell a coherent story of jet physics at RHIC.

Global analysis of nuclear PDFs – latest developments

In this review talk I discuss the latest developments in the DGLAP-based global analysis of nuclear parton distribution functions (nPDFs), focusing on the recent EPS09, nCTEQ and DSSZ global fits. I also briefly review the recent analysis for assigning a spatial dependence to the globally analysed nPDFs, resulting in the new sets EPS09s and EKS98s (“s” for spatial). With these, one can now compute nuclear hard-process cross sections and estimate their nPDF-originating uncertainties in different centrality classes for the first time consistently with the global nPDF fits.

Cold Nuclear Matter Effects in d + Au Collisions at PHENIX

To interpret the measurements in heavy-ion collisions at the Relativistic Heavy-Ion Collider (RHIC) it is crucial to understand the initial state of the colliding gold (Au) nuclei. The parton distribution in Au nuclei is modified compared to protons, and their isospin composition is different due to the presence of neutrons. d+Au collisions at RHIC at the same collision energies are an important tool to study initial state modifications. PHENIX has measured pi0, eta, and reconstructed jets at high transverse momentum. These data are compared to predictions from nuclear parton distribution functions. Furthermore, single electrons from heavy-flavor decays have been measured by PHENIX.

Agreement of Neutrino Deep Inelastic Scattering Data with Global Fits of Parton Distributions

The compatibility of neutrino-nucleus deep inelastic scattering data within the universal, factorizable nuclear parton distribution functions has been studied independently by several groups in the past few years. The conclusions are contradictory, ranging from a violation of the universality up to a good agreement, most of the controversy originating from the use of the neutrino-nucleus data from the NuTeV Collaboration. Here, we pay attention to non-negligible differences in the absolute normalization between different neutrino data sets. We find that such variations are large enough to prevent a tensionless fit to all data simultaneously and could therefore misleadingly point towards nonuniversal nuclear effects. We propose a concrete method to deal with the absolute normalization and show that an agreement between independent neutrino data sets is established.

Exploring cold nuclear matter effects in d + Au with high-pT reconstructed jets at PHENIX

Proton–nucleus (p + A) collisions can be used to investigate cold nuclear matter effects on hard-scattered partons and serve as an important baseline for heavy–ion collisions. In particular, p + A collisions at different centrality selections can probe the impact parameter dependence of nuclear parton distribution functions, initial state energy loss and final state parton interactions in the cold nucleus. Jet reconstruction can better determine the initial parton kinematics and recent improvements in analysis techniques allow the exploration of these effects over a wide p T range. We present the latest jet reconstruction measurements performed with the PHENIX detector at RHIC in deuteron–gold ( d + Au) collisions at 200 GeV using the Gaussian filter algorithm and discuss the possible implications on descriptions of cold nuclear matter.

Cold Nuclear Matter Effects on Isolated Prompt Photon and Isolated Prompt Photon+Jet Productions in Relativistic Heavy-Ion Collisions

We investigate the cold nuclear matter (CNM) effects on isolated prompt photon and isolated prompt photon associated jet productions in nuclear collisions at the NLO accuracy by using the EPS09 NLO nuclear parton distribution functions and their error sets. Nuclear modification factors of isolated prompt photon and isolated prompt photon+jet productions due to CNM effects in p+A and A+A reactions at the RHIC and the LHC are provided with varying rapidity and transverse momentum of the final state photon. It is shown that the CNM effects on isolated prompt photon and photon+jet are modest, which give a small enhancement at low pT region and a more obvious suppression at large pT at central rapidity. At forward rapidity a pronounced suppression of γ as well as γ+jet is always observed.

N-jet event shapes as probes of nuclear dynamics

We propose the use of $N$-jettiness (${\ensuremath{\tau}}_{N}$), a global event shape, as a probe of nuclear dynamics in lepton-nucleus collisions. It characterizes the amount of soft radiation between the jet and nuclear beam directions. We give a new factorization formula for the 1-jettiness (${\ensuremath{\tau}}_{1}$) distribution for the production of a single hard jet ($J$) in lepton-nucleus collisions: $\ensuremath{\ell}+A(P)\ensuremath{\rightarrow}J({P}_{J})+X$. Each nuclear target gives rise to a unique pattern of radiation, affected by nuclear dynamics, that can be quantified by the ${\ensuremath{\tau}}_{1}$ distribution. Up to power corrections, the ${\ensuremath{\tau}}_{1}$ distribution allows for a direct measurement of the nuclear parton distribution functions. Additional nuclear-dependent effects will be dominated through power corrections of size $\ensuremath{\sim}{Q}_{s}^{2}(A)/({\ensuremath{\tau}}_{1}{P}_{JT})$, where ${Q}_{s}(A)$ is a dynamical nuclear scale and ${P}_{JT}$ is the transverse momentum of the jet. Such nuclear-dependent effects can be studied through a dedicated program to measure ${\ensuremath{\tau}}_{1}$ distributions for a range of nuclei and kinematics. We give numerical results for the 1-jettiness distribution for the simplest case of a proton target at next-to-leading-log accuracy.

Reconstructed Jet Results in p+p, d+Au and Cu+Cu collisions at 200 GeV from PHENIX

Jet reconstruction in heavy ion collisions at RHIC and the LHC is becoming a popular tool for exploring medium effects including the energy loss and modified fragmentation of hard-scattered partons. In p+A and d+A collisions, reconstructed jets are important for evaluating cold nuclear matter effects such as the impact parameter dependence of nuclear parton distribution functions and initial state energy loss. We present current PHENIX results from p+p, d+Au, and Cu+Cu collisions at 200 GeV using the Gaussian filter and anti-kT algorithms. The systematic study of direct jet reconstruction across a variety of collisions systems at PHENIX will help to tell a coherent story of jet physics at RHIC.