Order Perturbative QCD Research Articles

Joint resummation for gaugino pair production at hadron colliders

We calculate direct gaugino pair production at hadron colliders at next-to-leading order of perturbative QCD, resumming simultaneously large logarithms in the small transverse-momentum and threshold regions to next-to-leading logarithmic accuracy. Numerical predictions are presented for transverse momentum and invariant mass spectra as well as for total cross sections and compared to results obtained at fixed order and with pure transverse-momentum and threshold resummation. We find that our new results are in general in good agreement with the previous ones, but often even more precise.

Measurement of the inclusive isolated prompt photon cross section inppcollisions ats=7 TeVwith the ATLAS detector

A measurement of the cross section for the inclusive production of isolated prompt photons in pp collisions at a centre-of-mass energy sqrt(s) = 7TeV is presented. The measurement covers the pseudorapidity ranges |eta|<1.37 and 1.52<|eta|<1.81 in the transverse energy range 15 < E_T <100 GeV. The results are based on an integrated luminosity of 880 nb-1, collected with the ATLAS detector at the Large Hadron Collider. Photon candidates are identified by combining information from the calorimeters and from the inner tracker. Residual background in the selected sample is estimated from data based on the observed distribution of the transverse isolation energy in a narrow cone around the photon candidate. The results are compared to predictions from next-to-leading order perturbative QCD calculations.

DETERMINATION OF THE STRONG COUPLING CONSTANT FROM NLO QCD ANALYSIS OF PROTON STRUCTURE FUNCTION

In this article we present a determination of the strong coupling constant and parton distribution functions (PDFs) based on a next-to-leading order (NLO) perturbative QCD analysis of proton structure function. More precisely, we extract [Formula: see text] and PDFs by fitting perturbative QCD predictions to the data from the measurements of the proton structure function [Formula: see text] in deep inelastic scattering, which are based on perturbative QCD calculations up to NLO. We obtain at NLO [Formula: see text] in the variable-flavor number scheme.

W and Z Production in the Forward Region at LHC

Results are presented on the production of Z \rightarrow \mu \mu and W \rightarrow \mu \nu in the forward region at LHCb. The analysis is based on an integrated luminosity of 16.5 ±1.7 pb-1. The production cross-section for Z with muons in the pseudorapidity range 2<\eta \mu <4.5 and 81 Z <101 GeV is 74±2±3±7 pb and is also measured differentially in rapidity of the Z. The total cross-section for W production is \sigma W+(2<\eta <4.5) = 1007±48±101 pb and \sigma W-(2<\eta <4.5) = 680±40±68 pb in the lepton pseudorapidity range 2 < \eta < 4.5. The charge asymmetry is measured as a function of the lepton pseudorapidity. All results are in agreement with next to leading order perturbative QCD predictions.

HATHOR – HAdronic Top and Heavy quarks crOss section calculatoR

We present a program to calculate the total cross section for top-quark pair production in hadronic collisions. The program takes into account recent theoretical developments such as approximate next-to-next-to-leading order perturbative QCD corrections and it allows for studies of the theoretical uncertainty by separate variations of the factorization and renormalization scales. In addition it offers the possibility to obtain the cross section as a function of the running top-quark mass. The program can also be applied to a hypothetical fourth quark family provided the QCD couplings are standard. Program summary Program title: Hathor Catalogue identifier: AEID_v1_0 Program summary URL: http://cpc.cs.qub.ac.uk/summaries/AEID_v1_0.html Program obtainable from: CPC Program Library, Queen's University, Belfast, N. Ireland Licensing provisions: GNU GPL 3 No. of lines in distributed program, including test data, etc.: 5405 No. of bytes in distributed program, including test data, etc.: 327 718 Distribution format: tar.gz Programming language: C++, Fortran, Java Computer: Standard PCs (x86, x86_64 processors) Operating system: Linux RAM: 256 MB Classification: 11.1 External routines: Interface to LHAPDF for the user's choice of parton distribution functions, see http://projects.hepforge.org/lhapdf/ Nature of problem: Computation of total cross section in perturbative QCD. Solution method: Numerical integration of hard parton cross section convoluted with parton distribution functions. Running time: A few seconds to a few minutes on standard desktop PCs or notebooks, depending on the chosen options.

Charm-quark mass from weighted finite energy QCD sum rules

The running charm-quark mass in the $\overline{\mathrm{MS}}$ scheme is determined from weighted finite energy QCD sum rules involving the vector current correlator. Only the short distance expansion of this correlator is used, together with integration kernels (weights) involving positive powers of $s$, the squared energy. The optimal kernels are found to be a simple pinched kernel and polynomials of the Legendre type. The former kernel reduces potential duality violations near the real axis in the complex $s$ plane, and the latter allows us to extend the analysis to energy regions beyond the end point of the data. These kernels, together with the high energy expansion of the correlator, weigh the experimental and theoretical information differently from e.g. inverse moments finite energy sum rules. Current, state of the art results for the vector correlator up to four-loop order in perturbative QCD are used in the finite energy sum rules, together with the latest experimental data. The integration in the complex $s$ plane is performed using three different methods: fixed order perturbation theory, contour improved perturbation theory, and a fixed renormalization scale $\ensuremath{\mu}$. The final result is ${\overline{m}}_{c}(3\text{ }\text{ }\mathrm{GeV})=1008\ifmmode\pm\else\textpm\fi{}26\text{ }\text{ }\mathrm{MeV}$, in a wide region of stability against changes in the integration radius ${s}_{0}$ in the complex $s$ plane.

QCDNUM: Fast QCD evolution and convolution

The qcdnum program numerically solves the evolution equations for parton densities and fragmentation functions in perturbative QCD. Un-polarised parton densities can be evolved up to next-to-next-to-leading order in powers of the strong coupling constant, while polarised densities or fragmentation functions can be evolved up to next-to-leading order. Other types of evolution can be accessed by feeding alternative sets of evolution kernels into the program. A versatile convolution engine provides tools to compute parton luminosities, cross-sections in hadron–hadron scattering, and deep inelastic structure functions in the zero-mass scheme or in generalised mass schemes. Input to these calculations are either the qcdnum evolved densities, or those read in from an external parton density repository. Included in the software distribution are packages to calculate zero-mass structure functions in un-polarised deep inelastic scattering, and heavy flavour contributions to these structure functions in the fixed flavour number scheme. Program summary Program title: QCDNUM version: 17.00 Catalogue identifier: AEHV_v1_0 Program summary URL: http://cpc.cs.qub.ac.uk/summaries/AEHV_v1_0.html Program obtainable from: CPC Program Library, Queen's University, Belfast, N. Ireland Licensing provisions: GNU Public Licence No. of lines in distributed program, including test data, etc.: 45 736 No. of bytes in distributed program, including test data, etc.: 911 569 Distribution format: tar.gz Programming language: Fortran-77 Computer: All Operating system: All RAM: Typically 3 Mbytes Classification: 11.5 Nature of problem: Evolution of the strong coupling constant and parton densities, up to next-to-next-to-leading order in perturbative QCD. Computation of observable quantities by Mellin convolution of the evolved densities with partonic cross-sections. Solution method: Parametrisation of the parton densities as linear or quadratic splines on a discrete grid, and evolution of the spline coefficients by solving (coupled) triangular matrix equations with a forward substitution algorithm. Fast computation of convolution integrals as weighted sums of spline coefficients, with weights derived from user-given convolution kernels. Restrictions: Accuracy and speed are determined by the density of the evolution grid. Running time: Less than 10 ms on a 2 GHz Intel Core 2 Duo processor to evolve the gluon density and 12 quark densities at next-to-next-to-leading order over a large kinematic range.

Gell-Mann-Oakes-Renner Relation: Chiral Corrections from Sum Rules

The next to leading order chiral corrections to the S U ( 2 ) × S U ( 2 ) Gell-Mann-Oakes-Renner (GMOR) relation are obtained using the light-quark pseudoscalar correlator to five-loop order in perturbative QCD, together with new finite energy sum rules (FESR) incorporating polynomial, Legendre type, integration kernels. These kernels are designed to suppress hadronic contributions in the region where they are least known. We obtain for the corrections to the GMOR relation, δ π , the value δ π = ( 6.2 ± 1.6 ) % . As a byproduct, the chiral perturbation theory (unphysical) low energy constant H 2 r is determined to be H 2 r ( ν χ = M ρ ) = − ( 5.1 ± 1.8 ) × 10 − 3 .

Formula omitted] + jet production at the Tevatron

Vector boson plus jet production is interesting for Higgs search, beyond the Standard Model physics and provides standard candles for calibration. This is complementary to inclusive jet production measurements which provide precision tests of perturbative QCD. A multitude of W/Z plus heavy and light flavour jet measurements in ppbar collisions at a centre of mass energy of sqrt{s}=1.96TeV is discussed. Next-to-Leading order perturbative QCD predictions and various models are compared to the measurements.

Measurement of the dijet invariant mass cross section in [formula omitted] collisions at [formula omitted

The inclusive dijet production double differential cross section as a function of the dijet invariant mass and of the largest absolute rapidity of the two jets with the largest transverse momentum in an event is measured in pp¯ collisions at s=1.96 TeV using 0.7 fb−1 of integrated luminosity collected with the D0 detector at the Fermilab Tevatron Collider. The measurement is performed in six rapidity regions up to a maximum rapidity of 2.4. Next-to-leading order perturbative QCD predictions are found to be in agreement with the data.

The role of spin in fragmentation production [formula omitted] and [formula omitted

In this work we calculate the process-independent fragmentation functions for a heavy quark Q to fragment into vector Q Q ¯ ( S 1 3 ) and pseudoscalar Q Q ¯ ( S 0 1 ) heavy quarkonium to leading order perturbative QCD. We investigate the role of the spin orientation of the initial and final heavy quarks on the fragmentation functions. In the special case, we calculate fragmentation functions by using average over spin of initial state heavy quark and sum over spin of final states. The universal fragmentation probabilities for the c → j / ψ ( 1 S ) and b → ϒ ( 1 S ) are about 10 − 4 and 10 − 5 , respectively. Moreover, the spin dependent observables such as V / ( V + P ) and V / P ratios are estimated.

Shadowing effects in direct photon and hadron production in heavy-ion collisions

Nuclear shadowing effects are checked in the production of direct photons and neutral pions in heavy-ion collisions at RHIC/LHC energy within a next-to-leading order perturbative QCD parton model. The transverse momentum dependence of the nuclear modification factors is presented in p + A and A + A collisions to distinguish different parameterizations for nuclear shadowing effects. Our numerical results show that in central A + A collisions the nuclear modification factor for direct photons is more sensitive to shadowing effects than the factor for pion hadrons, and it is a powerful observable to discriminate different parameterizations for parton distributions.

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.

NLO QCD corrections to top quark pair production in association with one hard jet at hadron colliders

We compute the QCD corrections to the production of a t t ¯ pair in association with one hard jet at the Tevatron and the LHC, using the method of generalized D-dimensional unitarity. Top quark decays are included at leading order in perturbative QCD. We present kinematic distributions of top quark decay products in lepton plus jets and dilepton final states at the Tevatron and the LHC, using realistic selection cuts. We confirm a strong reduction of the top quark forward–backward asymmetry in p p ¯ → t t ¯ j at next-to-leading order, first observed by Dittmaier, Uwer and Weinzierl. We argue that there is a natural way to understand this reduction and that it does not imply a breakdown of the perturbative expansion for the asymmetry.

Bulk matter evolution and extraction of jet transport parameters in heavy-ion collisions at energies available at the BNL Relativistic Heavy Ion Collider (RHIC)

Within the picture of jet quenching induced by multiple parton scattering and gluon bremsstrahlung, medium modification of parton fragmentation functions and therefore the suppression of large transverse-momentum hadron spectra are controlled by both the value and the space-time profile of the jet transport parameter along the jet propagation path. Experimental data on single-hadron suppression in high-energy heavy-ion collisions at the Relativistic Heavy Ion Collider energy are analyzed within the higher-twist (HT) approach to the medium-modified fragmentation functions and the next-to-leading order perturbative QCD parton model. Assuming that the jet transport parameter $\mathrm{q\ifmmode \hat{}\else \^{}\fi{}}$ is proportional to the particle number density in both quark gluon plasma (QGP) and hadronic phase, experimental data on jet quenching in deeply inelastic scattering off nuclear targets can provide guidance on ${\mathrm{q\ifmmode \hat{}\else \^{}\fi{}}}_{h}$ in the hot hadronic matter. One can then study the dependence of the extracted initial value of jet-quenching parameter ${\mathrm{q\ifmmode \hat{}\else \^{}\fi{}}}_{0}$ at initial time ${\ensuremath{\tau}}_{0}$ on the bulk medium evolution. Effects of transverse expansion, radial flow, phase transition, and nonequilibrium evolution are examined. The extracted values are found to vary from ${\mathrm{q\ifmmode \hat{}\else \^{}\fi{}}}_{0}{\ensuremath{\tau}}_{0}=0.54$ GeV${}^{2}$ in the ($1+3$)d ideal hydrodynamic model to 0.96 GeV${}^{2}$ in a cascade model, with the main differences coming from the initial nonequilibrium evolution and the later hadronic evolution. The overall contribution to jet quenching from the hadronic phase, about $22%--44$$%$, is found to be significant. Therefore, a realistic description of the early nonequilibrium parton evolution and later hadronic interaction will be critical for accurate extraction of the jet transport parameter in the strongly interacting QGP phase in high-energy heavy-ion collisions.

Inclusiveπ0,η, and direct photon production at high transverse momentum inp+pandd+Aucollisions atsNN=200GeV

We report a measurement of high-p_T inclusive pi^0, eta, and direct photon production in p+p and d+Au collisions at sqrt(s_NN) = 200 GeV at midrapidity (0 < eta < 1). Photons from the decay pi^0 -> gamma gamma were detected in the Barrel Electromagnetic Calorimeter of the STAR experiment at the Relativistic Heavy Ion Collider. The eta -> gamma gamma decay was also observed and constituted the first eta measurement by STAR. The first direct photon cross section measurement by STAR is also presented, the signal was extracted statistically by subtracting the pi^0, eta, and omega(782) decay background from the inclusive photon distribution observed in the calorimeter. The analysis is described in detail, and the results are found to be in good agreement with earlier measurements and with next-to-leading order perturbative QCD calculations.

Single-inclusive production of large-p T charged particles in hadronic collisions at TeV energies and perturbative QCD predictions

The single inclusive spectrum of charged particles with transverse momenta pT=3-150 GeV/c measured at midrapidity by the CDF experiment in proton-antiproton (p-pbar) collisions at sqrt(s)=1.96 TeV is compared to next-to-leading order (NLO) perturbative QCD calculations using the most recent parametrizations of the parton distributions and parton-to-hadron fragmentation functions. Above pT~20 GeV/c, there is a very sizeable disagreement of the Tevatron data compared to the NLO predictions and to xT-scaling expectations, suggesting a problem in the experimental data. We also present the predictions for the pT-differential charged hadron spectra and the associated theoretical uncertainties for proton-proton (p-p) collisions at LHC energies (sqrt(s)=0.9-14 TeV). Two procedures to estimate the charged hadron spectra at LHC heavy-ion collision energies (sqrt(s)=2.76,5.5 TeV) from p-p measurements are suggested.

Charged current neutrino-nucleon total cross section at high energies

We evaluate the charged current neutrino-nucleon total cross section up to neutrino energies of ${10}^{12}\text{ }\text{ }\mathrm{GeV}$ in the leading order and next-to-leading order of perturbative QCD within the framework of two different factorization schemes. The numerical implications of some inconsistent QCD calculations are illustrated.

Chiral corrections to the SU(2) × SU(2) Gell-Mann-Oakes-Renner relation

The next to leading order chiral corrections to the $SU(2)\times SU(2)$ Gell-Mann-Oakes-Renner (GMOR) relation are obtained using the pseudoscalar correlator to five-loop order in perturbative QCD, together with new finite energy sum rules (FESR) incorporating polynomial, Legendre type, integration kernels. The purpose of these kernels is to suppress hadronic contributions in the region where they are least known. This reduces considerably the systematic uncertainties arising from the lack of direct experimental information on the hadronic resonance spectral function. Three different methods are used to compute the FESR contour integral in the complex energy (squared) s-plane, i.e. Fixed Order Perturbation Theory, Contour Improved Perturbation Theory, and a fixed renormalization scale scheme. We obtain for the corrections to the GMOR relation, $\delta_\pi$, the value $\delta_\pi = (6.2, \pm 1.6)%$. This result is substantially more accurate than previous determinations based on QCD sum rules; it is also more reliable as it is basically free of systematic uncertainties. It implies a light quark condensate $<0|\bar{u} u|0> \simeq <0|\bar{d} d|0> \equiv <0|\bar{q} q|0>|_{2\,\mathrm{GeV}} = (- 267 \pm 5 MeV)^3$. As a byproduct, the chiral perturbation theory (unphysical) low energy constant $H^r_2$ is predicted to be $H^r_2 (\nu_\chi = M_\rho) = - (5.1 \pm 1.8)\times 10^{-3}$, or $H^r_2 (\nu_\chi = M_\eta) = - (5.7 \pm 2.0)\times 10^{-3}$.

Hadronisation effects in event-shape moments

We study the moments of hadronic event shapes in $e^+e^-$ annihilation within the context of next-to-next-to-leading order (NNLO) perturbative QCD predictions combined with non-perturbative power corrections in the dispersive model. This model is extended to match upon the NNLO perturbative prediction. The resulting theoretical expression has been compared to experimental data from JADE and OPAL, and a new value for $\alpha_s(M_Z)$ has been determined, as well as of the average coupling $\alpha_0$ in the non-perturbative region below $\mu_I=2$ GeV within the dispersive model: \alpha_s(M_Z)&=0.1153\pm0.0017(\mathrm{exp})\pm0.0023(\mathrm{th}),\alpha_0&=0.5132\pm0.0115(\mathrm{exp})\pm0.0381(\mathrm{th}), The precision of the $\alpha_s(M_Z)$ value has been improved in comparison to the previously available next-to-leading order analysis. We observe that the resulting power corrections are considerably larger than those estimated from hadronization models in multi-purpose event generator programs.