Proton Parton Distribution Functions Research Articles

Proton PDFs constraints from measurements using the ATLAS experiment

High-precision measurements of Standard Model (SM) processes provide information on different aspects contributing to the process, such as parton distribution functions (PDFs), and comparisons with the current precision reached theoretically on the calculations of the cross sections of such processes. This document describes ATLAS measurements, performed at different centre-of-mass energies, of vector boson (W and Z) cross sections and cross-section ratios. It also discusses measurements of ratios of Z-boson and top-quark pair production cross sections, which provide important information on the proton PDFs. Finally, a measurement involving the di-lepton decay channel of top-quark pairs at 8 TeV is presented, because of its direct sensitivity to the gluon PDF.

Measurement of double-differential cross sections for top quark pair production in pp collisions at sqrt{s} = 8,text {TeV} and impact on parton distribution functions

Normalized double-differential cross sections for top quark pair (mathrm{t}overline{mathrm{t}}) production are measured in pp collisions at a centre-of-mass energy of 8,text {TeV} with the CMS experiment at the LHC. The analyzed data correspond to an integrated luminosity of 19.7,text {fb}^{-1}. The measurement is performed in the dilepton mathrm {e}^{pm }mu ^{mp } final state. The mathrm{t}overline{mathrm{t}} cross section is determined as a function of various pairs of observables characterizing the kinematics of the top quark and mathrm{t}overline{mathrm{t}} system. The data are compared to calculations using perturbative quantum chromodynamics at next-to-leading and approximate next-to-next-to-leading orders. They are also compared to predictions of Monte Carlo event generators that complement fixed-order computations with parton showers, hadronization, and multiple-parton interactions. Overall agreement is observed with the predictions, which is improved when the latest global sets of proton parton distribution functions are used. The inclusion of the measured mathrm{t}overline{mathrm{t}} cross sections in a fit of parametrized parton distribution functions is shown to have significant impact on the gluon distribution.

Dynamical parton distributions from DGLAP equations with nonlinear corrections**Supported by National Basic Research Program (973 Program 2014CB845406) and Century Program of Chinese Academy of Sciences (Y101020BR0)

Determination of proton parton distribution functions is presented under the dynamical parton model assumption by applying DGLAP equations with GLR-MQ-ZRS corrections. We provide two data sets, referred to as IMParton16, which are from two different nonperturbative inputs. One is the naive input of three valence quarks and the other is the input of three valence quarks with flavor-asymmetric sea components. Basically, both data sets are compatible with the experimental measurements at high scale (Q2 > 2 GeV2). Furthermore, our analysis shows that the input with flavor-asymmetric sea components better reproduces the structure functions at high Q2. Generally, the parton distribution functions obtained, especially the gluon distribution function, are good options for inputs to simulations of high energy scattering processes. The analysis is performed under the fixed-flavor number scheme for nf = 3, 4, 5. Both data sets start from very low scales, around 0.07 GeV2, where the nonperturbative input is directly connected to the simple picture of the quark model. These results may shed some lights on the origin of the parton distributions observed at high Q2.

Uncertainty of parton distribution functions due to physical observables in a global analysis

The recent measurement of the differential γ + c-jet cross section, performed at the Tevatron collider in Run II by the D0 collaboration, is studied in a next-to-leading order (NLO) global QCD analysis to assess its impact on the proton parton distribution functions (PDFs). We show that these data lead to a significant change in the gluon and charm quark distributions. We demonstrate also that there is an inconsistency between the new high precision HERA I+II combined data and Tevatron measurement. Moreover, in this study we investigate the impact of older EMC measurements of charm structure function on the PDFs and compare the results with those from the analysis of Tevatron data. We show that both of them have the same impact on the PDFs, and thus can be recognized as the same evidence for the inefficiency of perturbative QCD in dealing with charm production in some kinematic regions.

Polarized parton densities and spin dependent structure function of the nucleon

We study the polarized parton distribution functions (PPDFs) from recent experimental data at the next-to-leading order (NLO) approximation in the fixed-flavor number scheme. In this analysis, we can compare our results with experimental data such as very recent COMPASS data. It would be very worth to study the PPDFs parametrization form when we want to use this new COMPASS data in QCD analysis on polarized proton structure and parton distribution functions.

Exploring the Standard Model at the LHC

The ATLAS and CMS collaborations have performed studies of a wide range of Standard Model processes using data collected at the Large Hadron Collider at center-of-mass energies of 7, 8 and 13 TeV. These measurements are used to explore the Standard Model in a new kinematic regime, perform precision tests of the model, determine some of its fundamental parameters, constrain the proton parton distribution functions, and study new rare processes observed for the first time. Examples of recent Standard Model measurements performed by the ATLAS and CMS collaborations are summarized in this report. The measurements presented span a wide range of event final states including jets, photons, W/Z bosons, top quarks, and Higgs bosons.

Simultaneous measurements of the [formula omitted], W+W−, and Z/γ⁎ → ττ production cross-sections in pp collisions at [formula omitted] with the ATLAS detector

Simultaneous measurements of the tt‾, W+W−, and Z/γ⁎→ττ production cross-sections using an integrated luminosity of 4.6 fb-1 of pp collisions at s=7 TeV collected by the ATLAS detector at the LHC are presented. Events are selected with two high transverse momentum leptons consisting of an oppositely charged electron and muon pair. The three processes are separated using the distributions of the missing transverse momentum of events with zero and greater than zero jet multiplicities. Measurements of the fiducial cross-section are presented along with results that quantify for the first time the underlying correlations in the predicted and measured cross-sections due to proton parton distribution functions. These results indicate that the correlated NLO predictions for tt‾ and Z/γ⁎→ττ significantly underestimate the data, while those at NNLO generally describe the data well. The full cross-sections are measured to be σ(tt‾)=181.2±2.8−9.5+9.7±3.3±3.3 pb, σ(W+W−)=53.3±2.7−8.0+7.3 and σ(Z/γ⁎→ττ)=1174±24−87+72±21±9 pb, where the cited uncertainties are due to statistics, systematic effects, luminosity and the LHC beam energy measurement, respectively.

QCD analysis of W- and Z-boson production at Tevatron

Recent measurements of the $W$-boson charge asymmetry and of the $Z$-boson production cross sections, performed at the Tevatron collider in Run II by the D0 and CDF collaborations, are studied using the HERAFitter framework to assess their impact on the proton parton distribution functions (PDFs). The Tevatron measurements, together with deep-inelastic scattering data from HERA, are included in a QCD analysis performed at next-to-leading order, and compared to the predictions obtained using other PDF sets from different groups. Good agreement between measurements and theoretical predictions is observed. The Tevatron data provide significant constraints on the $d$-valence quark distribution.

Measurements of differential and double-differential Drell-Yan cross sections in proton-proton collisions at [Formula: see text][Formula: see text

Measurements of the differential and double-differential Drell–Yan cross sections in the dielectron and dimuon channels are presented. They are based on proton–proton collision data at sqrt{s} = 8,text {TeV} recorded with the CMS detector at the LHC and corresponding to an integrated luminosity of 19.7,text {fb}^{-1}. The measured inclusive cross section in the mathrm{Z} peak region (60–120,text {GeV}), obtained from the combination of the dielectron and dimuon channels, is 1138 pm 8,text {(exp)} pm 25,text {(theo)} pm 30,text {(lumi)} text {,pb} , where the statistical uncertainty is negligible. The differential cross section mathrm{d}sigma /mathrm{d}{}m in the dilepton mass range 15–2000,text {GeV} is measured and corrected to the full phase space. The double-differential cross section mathrm{d}^2sigma /mathrm{d}{}m,mathrm{d}|y | is also measured over the mass range 20 to 1500,text {GeV} and absolute dilepton rapidity from 0 to 2.4. In addition, the ratios of the normalized differential cross sections measured at sqrt{s} = 7 and 8,text {TeV} are presented. These measurements are compared to the predictions of perturbative QCD at next-to-leading and next-to-next-to-leading (NNLO) orders using various sets of parton distribution functions (PDFs). The results agree with the NNLO theoretical predictions computed with fewz 3.1 using the CT10 NNLO and NNPDF2.1 NNLO PDFs. The measured double-differential cross section and ratio of normalized differential cross sections are sufficiently precise to constrain the proton PDFs.

Direct photon production as a probe of quarks chromoelectric and chromomagnetic dipole moments at the LHC

In this paper we show that the $\gamma+$jet invariant mass distribution in proton-proton collisions at the LHC is significantly sensitive to the quarks chromoelectric ($\kappa$) and chromomagnetic ($\tilde{\kappa}$) dipole moments. It is shown that the presence of $\kappa$ or $\tilde{\kappa}$ leads to an increment of the cross section of $\gamma+$jet process in particular in the tail of $\gamma+$jet invariant mass distribution. Using the measured $\gamma+$jet invariant mass distribution by the CMS experiment at the center-of-mass energy of 8 TeV, we derive bounds on the quarks chromoelectric and chromomagnetic dipole moments. In extraction of the limits, we consider both theoretical and systematic uncertainties. The uncertainties originating from variation of the renomalization/factorization scales and the choice of proton parton distribution functions are taken into account as a function of the $\gamma+$jet invariant mass. We exclude $\kappa$ or $\tilde{\kappa}$ above $10^{-5}$ at 95$\%$ confidence level. This is the most stringent direct upper limit on $\kappa$ or $\tilde{\kappa}$.

Simultaneous measurements of thett¯,W+W−, andZ/γ*→ττproduction cross-sections inppcollisions ats=7 TeVwith the ATLAS detector

Simultaneous measurements of the $t\bar{t}$, $W^+W^-$, and $Z/\gamma^{*}\rightarrow\tau\tau$ production cross-sections using an integrated luminosity of $4.6\,\mathrm{fb}^{-1}$ of $pp$ collisions at $\sqrt{s} = 7\,\mathrm{TeV}$ collected by the ATLAS detector at the LHC are presented. Events are selected with two high transverse momentum leptons consisting of an oppositely charged electron and muon pair. The three processes are separated using the distributions of the missing transverse momentum of events with zero and greater than zero jet multiplicities. Measurements of the fiducial cross-section are presented along with results that quantify for the first time the underlying correlations in the predicted and measured cross-sections due to proton parton distribution functions. These results indicate that the correlated NLO predictions for $t\bar{t}$ and $Z/\gamma^{*}\rightarrow\tau\tau$ underestimate the data, while those at NNLO generally describe the data well. The full cross-sections are measured to be $\sigma(t\bar{t}) = 181.2 \pm 2.8^{+9.7}_{-9.5} \pm 3.3 \pm 3.3\,\mathrm{pb}$, $\sigma(W^+W^-) = 53.3 \pm 2.7^{+7.3}_{-8.0} \pm 1.0 \pm 0.5\,\mathrm{pb}$, and $\sigma(Z/\gamma^{*}\rightarrow\tau\tau) = 1174 \pm 24^{+72}_{-87} \pm 21 \pm 9\,\mathrm{pb}$, where the cited uncertainties are due to statistics, systematic effects, luminosity and the LHC beam energy measurement, respectively. The $W^+W^-$ measurement includes the small contribution from Higgs boson decays, $H\rightarrow W^+W^-$.

The HERA Proton

The almost 1 fb −1 of ep data collected by the H1 and ZEUS collider exper- iments at HERA allows for a precise determination of the proton's parton distribution functions (PDFs). Measurements used to constrain the PDFs — inclusive and jet cross sections, charm contribution to the F2 proton structure function, F c¯ c 2 — are presented herein. The measurement process itself includes cataloguing the sensitivity of the cross sections to the various sources of correlated systematic uncertainties. In the jet measure- ment, correlations of a statistical nature are also quantified and catalogued. These corre- lations provide a basis to combine measurements of the same physical observable across different time periods, experiments and measurement methodology. The subsequent PDF fitting procedure also takes into account such correlations. The resulting HERAPDF1.5 set based on inclusive data as well as PDF sets derived from inclusive plus charm data are presented togeteher with their predictions for pp cross sections at the LHC.

HERAFitter - an open source QCD fit framework

The parton distribution functions (PDFs) of the proton due to their non-perturbative nature are determined empirically by fitting experimental observables. The HERAFitter project aims at providing a framework for QCD analyses related to proton PDFs. The framework includes various modules and interfaces enabling a large number of theoretical and methodological options as well as study the impact of the new experimental data on the PDFs from ep, pp and pp¯ processes.

Jet and photon measurements from ATLAS

Di erential cross-section measurements of inclusive-jet and di-jet production provide stringent tests of perturbative QCD predictions and provide inputs for determination of parton density functions. Ratios of jet multiplicities are sensitive to s and have reduced theoretical uncertainties. Measurements of the inclusive isolated-photon and di-photon cross-sections provide a direct probe of short-distance physics, complementary to that from measurements of jets or vector-bosons and are sensitive to the gluon density in the proton. The measurements are compared to next-to-leading-order or higher-order QCD calculations. 1 Introduction ATLAS is a general purpose detector operating within the LHC experiment. The main goals of ATLAS are searches of the Higgs boson, study of the properties of the top quark and search for new physics. High-precision measurements to make novel tests of perturbative QCD (pQCD) will also be possible due to the higher centre-of-mass energy ( p s) available at LHC than in previous accelerators. In this pa- per, measurements of jets and photons at ATLAS are pre- sented. These measurements are one of the tools that can be used at LHC to increase the understanding of QCD in- teractions. In particular, they allow the testing of pQCD down to the shortest accessible distances. Their sensitivity to the proton parton distribution functions (PDFs) make them adequate to measure the strong coupling constant s and to the improvement of the determination of the PDFs. Photons are colorless probes to QCD and can be used to constrain the proton PDFs. Di-photons processes are in ad- dition an important background for the identification of the Higgs boson in the H ! channel. Comparison of data and next-to-leading-order (NLO) and next-to-next-leading- order (NNLO) QCD theoretical calculations are the main focus of this paper.

Jet physics in electron–proton scattering

Hadronic jets in electron–proton collisions at HERA have been used for some considerable time as a tool for tests of the theory of strong interactions, quantum chromodynamics. Using jet final states, basic concepts like the factorisation ansatz for cross-section calculations, the perturbative approach to the cross section and the universality of the proton parton distribution functions can be examined. More concretely, jet measurements provide ready access to the strong coupling of QCD, α s , and to the parton distributions. In this report, an overview of jet results from the HERA experiments H1 and ZEUS and their interpretation is given together with a description of the theoretical foundations of jet physics in electron–proton collisions and of the experimental environment at HERA. Special emphasis is put on extractions of α s values and on the influence of jet data on fits of the proton parton distribution functions. Where useful, the HERA results are also discussed in the light of results from other colliders like LEP, the Tevatron or the LHC. The central message from these studies is that QCD does not only describe most of the measurements very well, but that QCD at HERA has achieved the status of a precision theory. On the other hand it is shown that further understanding of problematic issues relies critically on theoretical progress in the form of improved models or of increased precision in analytical calculations.

Measurement of the inclusive jet cross section inpp¯collisions ats=1.96 TeV

We present a measurement of the inclusive jet cross section using the Run II cone algorithm and data collected by the D0 experiment in p pbar collisions at a center-of-mass energy sqrt(s)=1.96 TeV, corresponding to an integrated luminosity of 0.70 fb^(-1). The jet energy calibration and the method used to extract the inclusive jet cross section are described. We discuss the main uncertainties, which are dominated by the jet energy scale uncertainty. The results cover jet transverse momenta from 50 GeV to 600 GeV with jet rapidities in the range -2.4 to 2.4 and are compared to predictions using recent proton parton distribution functions. Studies of correlations between systematic uncertainties in transverse momentum and rapidity are presented.

Impact of the parton distribution function uncertainties on the measurement of theWboson mass at the Tevatron and the LHC

We study at a quantitative level the impact of the uncertainties on the value of the W boson mass measured at hadron colliders due to: the proton parton distribution functions (PDFs), the value of the strong coupling constant alpha_S and the value of the charm mass used in the PDF determination. The value of the W boson mass is extracted, by means of a template fit technique, from the lepton-pair transverse mass distribution measured in the charged current Drell-Yan process. We study the determination of M_W at the Tevatron and at the LHC with 7 and 14 TeV of center-of-mass energy in a realistic experimental setup. The analysis has been done at the Born level using the event generator HORACE and at NLO-QCD using the event generators DYNNLO and ResBos. We consider the three global PDF sets, CTEQ6.6, MSTW2008 and NNPDF2.1. We estimate that the total PDF uncertainty on M_W is below 10 MeV both at the Tevatron and at the LHC for all energies and final states. We conclude that PDF uncertainties do not challenge a measurement of the W boson mass at the level of 10 MeV accuracy.

Nuclear Corrections in Neutrino-Nucleus Deep Inelastic Scattering and their Compatibility with Global Nuclear Parton-Distribution-Function Analyses

We perform a global χ² analysis of nuclear parton distribution functions using data from charged current neutrino-nucleus (νA) deep-inelastic scattering (DIS), charged-lepton-nucleus (ℓ(±)A) DIS, and the Drell-Yan (DY) process. We show that the nuclear corrections in νA DIS are not compatible with the predictions derived from ℓ(±)A DIS and DY data. We quantify this result using a hypothesis-testing criterion based on the χ² distribution which we apply to the total χ² as well as to the χ² of the individual data sets. We find that it is not possible to accommodate the data from νA and ℓ(±)A DIS by an acceptable combined fit. Our result has strong implications for the extraction of both nuclear and proton parton distribution functions using combined neutrino and charged-lepton data sets.

Sensitivity of isolated photon production at TeV hadron colliders to the gluon distribution in the proton

We compare the single inclusive spectra of isolated photons measured at high transverse energy in proton-antiproton collisions at sqrt(s)=1.96 TeV with next-to-leading order perturbative QCD predictions with various parametrizations of the parton distribution functions (PDFs). Within the experimental and theoretical uncertainties, the Tevatron data can be reproduced equally well by the recent CTEQ6.6, MSTW08 and NNPDF1.2 PDF sets. We present also the predictions for isolated gamma spectra in proton-proton collisions at sqrt(s)=14TeV for central (y=0) and forward (y=4) rapidities relevant for LHC experiments. Different proton PDFs result in maximum variations of order 30% in the expected E_T-differential isolated gamma cross sections. The inclusion of the isolated photon data in global PDF fits will place extra independent constraints on the gluon density.

Measurement of the Electron Charge Asymmetry inpp¯→W+X→eν+XEvents ats=1.96 TeV

We present a measurement of the electron charge asymmetry in pp[over ]-->W+X-->enu+X events at a center of mass energy of 1.96 TeV using 0.75 fb(-1) of data collected with the D0 detector at the Fermilab Tevatron Collider. The asymmetry is measured as a function of the electron transverse momentum and pseudorapidity in the interval (-3.2, 3.2) and is compared with expectations from next-to-leading order calculations in perturbative quantum chromodynamics. These measurements will allow more accurate determinations of the proton parton distribution functions.