Order Perturbative QCD Calculations Research Articles

Dimuons from neutrino-nucleus collisions in the semi-inclusive DIS approach

We present a next-to-leading order perturbative QCD calculation of dimuon production in neutrino-nucleus collisions. This process is typically calculated by assuming it to be proportional to inclusive charm production, which requires an effective acceptance correction to take the experimental cuts on the decay-muon kinematics into account. Here, we instead compute the dimuon production cross section directly as a convolution of semi-inclusive deep inelastic scattering to produce charmed hadrons, and a decay function fitted to e+e− data to produce a muon from the charmed hadrons. The presented approach is in a good agreement with available experimental data and will serve as a starting point for higher-order QCD calculations without an external acceptance correction. The uncertainties arising from the decay function and scale dependence are sizeably smaller than those from the nuclear parton distribution functions. We also calculate the effective acceptances within our approach and compare them to those usually used in global fits of parton distribution functions, finding differences of the order of 10 %, depending on the kinematics, perturbative order, and applied parton distributions.

Production of light-flavor hadrons in pp collisions at sqrt{s}~=~7text { and }sqrt{s} = 13 , text { TeV}

The production of pi ^{pm }, mathrm{K}^{pm }, mathrm{K}^{0}_{S}, mathrm{K}^{*}(892)^{0}, mathrm{p}, phi (1020), Lambda , Xi ^{-}, Omega ^{-}, and their antiparticles was measured in inelastic proton–proton (pp) collisions at a center-of-mass energy of sqrt{s} = 13 TeV at midrapidity (|y|<0.5) as a function of transverse momentum (p_{mathrm{T}}) using the ALICE detector at the CERN LHC. Furthermore, the single-particle p_{mathrm{T}} distributions of mathrm{K}^{0}_{S}, Lambda , and overline{Lambda } in inelastic pp collisions at sqrt{s} = 7 TeV are reported here for the first time. The p_{mathrm{T}} distributions are studied at midrapidity within the transverse momentum range 0le p_{mathrm{T}}le 20 GeV/c, depending on the particle species. The p_{mathrm{T}} spectra, integrated yields, and particle yield ratios are discussed as a function of collision energy and compared with measurements at lower sqrt{s} and with results from various general-purpose QCD-inspired Monte Carlo models. A hardening of the spectra at high p_{mathrm{T}} with increasing collision energy is observed, which is similar for all particle species under study. The transverse mass and x_{mathrm{T}}equiv 2p_{mathrm{T}}/sqrt{s} scaling properties of hadron production are also studied. As the collision energy increases from sqrt{s} = 7–13 TeV, the yields of non- and single-strange hadrons normalized to the pion yields remain approximately constant as a function of sqrt{s}, while ratios for multi-strange hadrons indicate enhancements. The p_mathrm{{T}}-differential cross sections of pi ^{pm }, mathrm {K}^{pm } and mathrm {p} (overline{mathrm{p}}) are compared with next-to-leading order perturbative QCD calculations, which are found to overestimate the cross sections for pi ^{pm } and mathrm{p} (overline{mathrm{p}}) at high p_mathrm{{T}}.

Measurement of the inclusive isolated photon production cross section in text{ p }text{ p } collisions at sqrt{s}=7 TeV

The production cross section of inclusive isolated photons has been measured by the ALICE experiment at the CERN LHC in pp collisions at a centre-of-momentum energy of sqrt{s}= 7 TeV. The measurement is performed with the electromagnetic calorimeter EMCal and the central tracking detectors, covering a range of |eta |<0.27 in pseudorapidity and a transverse momentum range of 10< p_mathrm {T}^{gamma }< 60~mathrm {GeV}/c. The result extends the p_mathrm {T} coverage of previously published results of the ATLAS and CMS experiments at the same collision energy to smaller p_mathrm {T}. The measurement is compared to next-to-leading order perturbative QCD calculations and to the results from the ATLAS and CMS experiments. All measurements and theory predictions are in agreement with each other.

Charged jet cross section and fragmentation in proton-proton collisions at s=7 TeV

We report the differential charged jet cross section and jet fragmentation distributions measured with the ALICE detector in proton-proton collisions at a centre-of-mass energy $\sqrt{s}=$ 7 TeV. Jets with pseudo-rapidity $\left| \eta \right| < {\rm 0.5}$ are reconstructed from charged particles using the anti-$k_{\rm T}$ jet finding algorithm with a resolution parameter $R$ = 0.4. The jet cross section is measured in the transverse momentum interval 5 $\leq p_{\rm T}^{\rm ch \; jet} <$ 100 GeV/$c$. Jet fragmentation is studied measuring the scaled transverse momentum spectra of the charged constituents of jets in four intervals of jet transverse momentum between 5 GeV/$c$ and 30 GeV/$c$. The measurements are compared to calculations from the PYTHIA model as well as next-to-leading order perturbative QCD calculations with POWHEG + PYTHIA8. The charged jet cross section is described by POWHEG for the entire measured range of $p_{\rm T}^{\rm ch \; jet}$. For $p_{\rm T}^{\rm ch \; jet}$ $>$ 40 GeV/$c$, the PYTHIA calculations also agree with the measured charged jet cross section. PYTHIA6 simulations describe the fragmentation distributions to 15%. Larger discrepancies are observed for PYTHIA8.

Moli\xe8re scattering in quark-gluon plasma: finding point-like scatterers in a liquid

By finding rare (but not exponentially rare) large-angle deflections of partons within a jet produced in a heavy ion collision, or of such a jet itself, experimentalists can find the weakly coupled short-distance quark and gluon particles (scatterers) within the strongly coupled liquid quark-gluon plasma (QGP) produced in heavy ion collisions. This is the closest one can come to probing QGP via a scattering experiment and hence is the best available path toward learning how a strongly coupled liquid emerges from an asymptotically free gauge theory. The short-distance, particulate, structure of liquid QGP can be revealed in events in which a jet parton resolves, and scatters off, a parton from the droplet of QGP. The probability for picking up significant transverse momentum via a single scattering was calculated previously, but only in the limit of infinite parton energy which means zero angle scattering. Here, we provide a leading order perturbative QCD calculation of the Molière scattering probability for incident partons with finite energy, scattering at a large angle. We set up a thought experiment in which an incident parton with a finite energy scatters off a parton constituent within a “brick” of QGP, which we treat as if it were weakly coupled, as appropriate for scattering with large momentum transfer, and compute the probability for a parton to show up at a nonzero angle with some energy. We include all relevant channels, including those in which the parton that shows up at a large angle was kicked out of the medium as well as the Rutherford-like channel in which what is seen is the scattered incident parton. The results that we obtain will serve as inputs to future jet Monte Carlo calculations and can provide qualitative guidance for how to use future precise, high statistics, suitably differential measurements of jet modification in heavy ion collisions to find the scatterers within the QGP liquid.

Pi ^{0} and eta meson production in proton-proton collisions at sqrt{s}=8 TeV

An invariant differential cross section measurement of inclusive pi ^{0} and eta meson production at mid-rapidity in pp collisions at sqrt{s}=8 TeV was carried out by the ALICE experiment at the LHC. The spectra of pi ^{0} and eta mesons were measured in transverse momentum ranges of 0.3<p_{ text{ T }} <35 text{ GeV/c } and 0.5<p_{ text{ T }} <35 text{ GeV/c }, respectively. Next-to-leading order perturbative QCD calculations using fragmentation functions DSS14 for the pi ^{0} and AESSS for the eta overestimate the cross sections of both neutral mesons, although such calculations agree with the measured eta /pi ^0 ratio within uncertainties. The results were also compared with PYTHIA 8.2 predictions for which the Monash 2013 tune yields the best agreement with the measured neutral meson spectra. The measurements confirm a universal behavior of the eta /pi ^0 ratio seen for NA27, PHENIX and ALICE data for pp collisions from sqrt{s}=27.5 GeV to sqrt{s}=8 TeV within experimental uncertainties. A relation between the pi ^{0} and eta production cross sections for pp collisions at sqrt{s}=8 TeV is given by m_{ text{ T }} scaling for p_{ text{ T }} >3.5 text{ GeV/c }. However, a deviation from this empirical scaling rule is observed for transverse momenta below p_{ text{ T }} <3.5 text{ GeV/c } in the eta /pi ^0 ratio with a significance of 6.2sigma .

Measurement of the inclusive isolated prompt photon cross section in pp collisions at s = 8 $$ \sqrt{s}=8 $$ TeV with the ATLAS detector

A measurement of the cross section for the inclusive production of isolated prompt photons in proton-proton collisions at a centre-of-mass energy of $\sqrt{s} = 8$ TeV is presented. The measurement covers the pseudo rapidity ranges $|\eta^{\gamma}| < 1.37$ and $1.56 \leq |\eta^{\gamma}| < 2.37$ in the transverse energy range $25 < E_{\rm T}^{\gamma} < 1500$ GeV. The results are based on an integrated luminosity of 20.2 fb$^{-1}$, recorded by the ATLAS detector at the LHC. Photon candidates are identified by combining information from the calorimeters and the inner tracker. The background is subtracted using a data-driven technique, based on the observed calorimeter shower-shape variables and the deposition of hadronic energy in a narrow cone around the photon candidate. The measured cross sections are compared with leading-order and next-to-leading order perturbative QCD calculations and are found to be in a good agreement over ten orders of magnitude.

Measurement of inclusive jet production and nuclear modifications in pPb collisions at [Formula: see text

Inclusive jet production in pPb collisions at a nucleon–nucleon (NN) center-of-mass energy of sqrt{s_{_mathrm {NN}}} =5.02,mathrm{TeV} is studied with the CMS detector at the LHC. A data sample corresponding to an integrated luminosity of 30.1 nb^{-1} is analyzed. The jet transverse momentum spectra are studied in seven pseudorapidity intervals covering the range -2.0<eta _mathrm {CM}< 1.5 in the NN center-of-mass frame. The jet production yields at forward and backward pseudorapidity are compared and no significant asymmetry about eta _mathrm {CM} = 0 is observed in the measured kinematic range. The measurements in the pPb system are compared to reference jet spectra obtained by extrapolation from previous measurements in pp collisions at sqrt{s}=7,mathrm{TeV} . In all pseudorapidity ranges, nuclear modifications in inclusive jet production are found to be small, as predicted by next-to-leading order perturbative QCD calculations that incorporate nuclear effects in the parton distribution functions.

Centrality, rapidity, and transverse momentum dependence of isolated prompt photon production in lead-lead collisions atsNN=2.76TeV measured with the ATLAS detector

Prompt photon production in $\sqrt{s_{\mathrm{NN}}} = 2.76$ TeV Pb+Pb collisions has been measured by the ATLAS experiment at the Large Hadron Collider using data collected in 2011 with an integrated luminosity of 0.14 nb$^{-1}$. Inclusive photon yields, scaled by the mean nuclear thickness function, are presented as a function of collision centrality and transverse momentum in two pseudorapidity intervals, $|\eta| < 1.37$ and $1.52 < |\eta| < 2.37$. The scaled yields in the two pseudorapidity intervals, as well as the ratios of the forward yields to those at midrapidity, are compared to the expectations from next-to-leading order perturbative QCD calculations from JETPHOX. The measured cross sections agree well with the predictions for proton-proton collisions within statistical and systematic uncertainties. Both the yields and ratios are also compared to two other pQCD calculations, one which uses the isospin content appropriate to colliding lead nuclei, and another which includes the EPS09 nuclear modifications to the nucleon parton distribution functions.

Z 0 / γ ∗ +Jet via electron decay mode at s = $\boldsymbol {\sqrt {s}=}$ 7 TeVin CMS@LHC

The area-normalized angular distributions in events containing a Z 0 boson and a jet, using the electron decay mode, are presented. The data samples correspond to 5 fb −1 of proton–proton collisions at $\sqrt {s}=7$ TeV, collected by the CMS detector. Events in which there is a Z boson and at least one jet, with a jet transverse momentum threshold of 30 GeV /c and absolute jet rapidity less than 2.4, are selected for this analysis. We compare our measurements with a next-to-leading order perturbative QCD calculation and two generators that combine tree-level matrix element calculations with parton showers.

Measurement of the differential [formula omitted]-jet cross section and the ratio [formula omitted] in [formula omitted] collisions at [formula omitted

We present the first measurements of the differential cross section dσ/dpTγ for the production of an isolated photon in association with at least two b-quark jets. The measurements consider photons with rapidities |yγ|<1.0 and transverse momenta 30<pTγ<200 GeV. The b-quark jets are required to have pTjet>15 GeV and |yjet|<1.5. The ratio of differential production cross sections for γ+2b-jets to γ+b-jet as a function of pTγ is also presented. The results are based on the proton–antiproton collision data at s=1.96 TeV collected with the D0 detector at the Fermilab Tevatron Collider. The measured cross sections and their ratios are compared to the next-to-leading order perturbative QCD calculations as well as predictions based on the kT-factorization approach and those from the sherpa and pythia Monte Carlo event generators.

Confronting current NLO parton fragmentation functions with inclusive charged-particle spectra at hadron colliders

The inclusive spectra of charged particles measured at high transverse momenta (pT≳2 GeV/c) in proton–proton and proton–antiproton collisions in the range of center-of-mass energies s=200–7000 GeV are compared with next-to-leading order perturbative QCD calculations using seven recent sets of parton-to-hadron fragmentation functions (FFs). Accounting for the uncertainties in the scale choices and in the parton distribution functions, we find that most of the theoretical predictions tend to overpredict the measured LHC and Tevatron cross sections by up to a factor of two. We identify the currently too-hard gluon-to-hadron FFs as the probable source of the problem, and justify the need to refit the FFs using the available LHC and Tevatron data in a region of transverse momenta, pT≳10 GeV/c, which is supposedly free from additional non-perturbative contributions and where the scale uncertainty is only modest.

Neutral pion cross section and spin asymmetries at intermediate pseudorapidity in polarized proton collisions ats=200 GeV

The differential cross section and spin asymmetries for neutral pions produced within the intermediate pseudorapidity range 0.8<η<2.0 in polarized proton-proton collisions at s=200 GeV are presented. Neutral pions were detected using the end cap electromagnetic calorimeter in the STAR detector at RHIC. The cross section was measured over a transverse momentum range of 5<pT<16 GeV/c and is found to agree with a next-to-leading order perturbative QCD calculation. The longitudinal double-spin asymmetry ALL is measured in the same pseudorapidity range and spans a range of Bjorken-x down to x≈0.01. The measured ALL is consistent with model predictions for varying degrees of gluon polarization. The parity-violating asymmetry AL is also measured and found to be consistent with zero. The transverse single-spin asymmetry AN is measured over a previously unexplored kinematic range in Feynman-x and pT. Such measurements may aid our understanding of the onset and kinematic dependence of the large asymmetries observed at more forward pseudorapidity (η≈3) and their underlying mechanisms. The AN results presented are consistent with a twist-3 model prediction of a small asymmetry over the present kinematic range.2 MoreReceived 6 September 2013DOI:https://doi.org/10.1103/PhysRevD.89.012001© 2014 American Physical Society

Measurement of the differential cross section of photon plus jet production inpp¯collisions ats=1.96 TeV

We study the process of associated photon and jet production, p (p) over bar -> gamma + jet + X, using 8.7 fb(-1) of integrated luminosity collected by the D0 detector at the Fermilab Tevatron Collider at a center-of-mass energy root s = 1.96 TeV. Photons are reconstructed with rapidity vertical bar y(gamma)vertical bar 20 GeV. The highest-p(T) jet is required to be in one of four rapidity regions up to vertical bar y(jet)vertical bar 0) and opposite signs (y(gamma)y(jet) <= 0) of photon and jet rapidities. We compare the measured triple-differential cross sections, d(3)sigma/dp(T)(gamma)dy(gamma)dy(jet), to next-to-leading order (NLO) perturbative QCD calculations using different sets of parton distribution functions, and to predictions from the SHERPA and PYTHIA Monte Carlo event generators. The NLO calculations are found to be in general agreement with the data but do not describe all kinematic regions.

A perturbative QCD study of dijets in p+Pb collisions at the LHC

Inspired by the recent measurements of the CMS collaboration, we report a QCD study of dijet production in proton+lead collisions at the LHC involving large-transverse-momentum jets, $p_T \gtrsim 100$ GeV. Examining the inherent uncertainties of the next-to-leading order perturbative QCD calculations and their sensitivity to the free proton parton distributions (PDFs), we observe a rather small, typically much less than 5% clearance for the shape of the dijet rapidity distribution within approximately 1.5 units around the midrapidity. Even a more stable observable is the ratio between the yields in the positive and negative dijet rapidity, for which the baseline uncertainty can be made negligible by imposing a symmetric jet rapidity acceptance. Both observables prove sensitive to the nuclear modifications of the gluon distributions, the corresponding uncertainties clearly exceeding the estimated baseline uncertainties from the free-proton PDFs and scale dependence. From a theoretical point of view, these observables are therefore very suitable for testing the validity of the collinear factorization and have a high potential to provide precision constraints for the nuclear PDFs.

Azimuthal angular correlations between heavy-flavour decay particles and charged hadrons in pp collisions in ALICE

Heavy quarks produced in pp and heavy ion collisions are studied using heavy-flavour decay electrons and heavy-flavour mesons. Detailed understanding of the production processes and fragmentation of heavy quarks can be obtained by studying the azimuthal angular correlation of heavy-flavour hadrons. The azimuthal angular correlations of heavy-flavour mesons and charged hadrons can be used to disentangle charm and beauty-hadrons in pp collisions. In this contribution the fraction of electrons from beauty-hadron decays in the heavy-flavour decay electron yield is shown as well as the beauty production cross section in pp collisions at = 2.76 TeV. The measurements are compared to the predictions from next-to leading order perturbative QCD calculations. We also show the results from correlation analysis of charged D* mesons and hadrons performed using pp collision data at = 7 TeV.

Measurement of the inclusive jet cross-section in pp collisions at [Formula: see text] and comparison to the inclusive jet cross-section at [Formula: see text] using the ATLAS detector.

The inclusive jet cross-section has been measured in proton–proton collisions at sqrt{s} = 2.76~mbox{TeV} in a dataset corresponding to an integrated luminosity of 0.20~mbox {pb^{-1}} collected with the ATLAS detector at the Large Hadron Collider in 2011. Jets are identified using the anti-k t algorithm with two radius parameters of 0.4 and 0.6. The inclusive jet double-differential cross-section is presented as a function of the jet transverse momentum p T and jet rapidity y, covering a range of 20≤p T<430 GeV and |y|<4.4. The ratio of the cross-section to the inclusive jet cross-section measurement at sqrt{s} = 7~mbox{TeV}, published by the ATLAS Collaboration, is calculated as a function of both transverse momentum and the dimensionless quantity x_{mathrm{T}} = 2 p_{mathrm{T}} / sqrt{s}, in bins of jet rapidity. The systematic uncertainties on the ratios are significantly reduced due to the cancellation of correlated uncertainties in the two measurements. Results are compared to the prediction from next-to-leading order perturbative QCD calculations corrected for non-perturbative effects, and next-to-leading order Monte Carlo simulation. Furthermore, the ATLAS jet cross-section measurements at sqrt{s} = 2.76~mbox{TeV} and sqrt{s}=7~mbox{TeV} are analysed within a framework of next-to-leading order perturbative QCD calculations to determine parton distribution functions of the proton, taking into account the correlations between the measurements.

W and Z in heavy ion collisions with CMS

Abstract Electroweak W and Z boson measurements in PbPb collisions at nucleon–nucleon center of mass energy s = 2.76 TeV at the LHC using the CMS detector are presented. The W analysis is based on 2010 PbPb data corresponding to an integrated luminosity of 7.3 μ b − 1 and also on 2011 pp data corresponding to an integrated luminosity of 231 n b − 1 . The Z analysis is based on 2011 PbPb data with an integrated luminosity of 150 μ b − 1 . The dependence of W and Z production as a function of PbPb collision centrality is studied and shown to scale with the number of NN binary collisions. W + and W − yields in PbPb and pp collisions are in agreement once we take into account the proton and neutron composition in Pb nuclei. The muon charge asymmetry is measured as a function of pseudo-rapidity and is in agreement with next-to-leading order perturbative QCD calculations.

Measurement of the inclusive differential jet cross section in pp collisions at [formula omitted

The ALICE Collaboration at the CERN Large Hadron Collider reports the first measurement of the inclusive differential jet cross section at mid-rapidity in pp collisions at s=2.76 TeV, with integrated luminosity of 13.6 nb−1. Jets are measured over the transverse momentum range 20 to 125 GeV/c and are corrected to the particle level. Calculations based on Next-to-Leading Order perturbative QCD are in good agreement with the measurements. The ratio of inclusive jet cross sections for jet radii R=0.2 and R=0.4 is reported, and is also well reproduced by a Next-to-Leading Order perturbative QCD calculation when hadronization effects are included.

Measurement of the differential [formula omitted]-jet cross section and the ratio of differential [formula omitted] and [formula omitted] cross sections in [formula omitted] collisions at [formula omitted

We present measurements of the differential cross section dσ/dpTγ for the associated production of a c-quark jet and an isolated photon with rapidity |yγ|<1.0 and transverse momentum 30<pTγ<300 GeV. The c-quark jets are required to have |yjet|<1.5 and pTjet>15 GeV. The ratio of differential cross sections for γ+c to γ+b production as a function of pTγ is also presented. The results are based on data corresponding to an integrated luminosity of 8.7 fb−1 recorded with the D0 detector at the Fermilab Tevatron pp¯ Collider at s=1.96 TeV. The obtained results are compared to next-to-leading order perturbative QCD calculations using various parton distribution functions, to predictions based on the kT-factorization approach, and to predictions from the sherpa and pythia Monte Carlo event generators.