Order Perturbative QCD Research Articles

On Higgs-exchange DIS, physical evolution kernels and fourth-order splitting functions at large x

We present the coefficient functions for deep-inelastic scattering (DIS) via the exchange of a scalar ϕ directly coupling only to gluons, such as the Higgs boson in the limit of a very heavy top quark and n f effectively massless light flavours, to the third order in perturbative QCD. The two-loop results are employed to construct the next-to-next-to-leading order physical evolution kernels for the system ( F 2 , F ϕ ) of flavour-singlet structure functions. The practical relevance of these kernels as an alternative to MS ¯ factorization is bedevilled by artificial double logarithms at small values of the scaling variable x, where the large top-mass limit ceases to be appropriate. However, they show an only single-logarithmic enhancement at large x. Conjecturing that this feature persists to the next order also in the present singlet case, the three-loop coefficient functions facilitate exact predictions (backed up by their particular colour structure) of the double-logarithmic contributions to the fourth-order singlet splitting functions, i.e., of the terms ( 1 − x ) a ln k ( 1 − x ) with k = 4 , 5 , 6 and k = 3 , 4 , 5 , respectively, for the off-diagonal and diagonal quantities to all powers a in ( 1 − x ) .

Higgs boson decay into heavy quarks and heavy leptons: higher order corrections

Theoretical predictions for the decay width of Standard Model Higgs boson into bottom quarks and tau-leptons, in the case when M_H< 2M_W, are briefly reviewed. The effects of higher order perturbative QCD (up to alpha_s^4-level) and QED corrections are considered. The uncertainties of the decay width of Higgs boson into bb and tau+tau- are discussed.

Longitudinal double-spin asymmetry and cross section for inclusive neutral pion production at midrapidity in polarized proton collisions ats=200 GeV

We report a measurement of the longitudinal double-spin asymmetry A_LL and the differential cross section for inclusive Pi0 production at midrapidity in polarized proton collisions at sqrt(s) = 200 GeV. The cross section was measured over a transverse momentum range of 1 < p_T < 17 GeV/c and found to be in good agreement with a next-to-leading order perturbative QCD calculation. The longitudinal double-spin asymmetry was measured in the range of 3.7 < p_T < 11 GeV/c and excludes a maximal positive gluon polarization in the proton. The mean transverse momentum fraction of Pi0's in their parent jets was found to be around 0.7 for electromagnetically triggered events.

Measurement of the inclusive isolated prompt photon cross section inpp¯collisions ats=1.96 TeVusing the CDF detector

A measurement of the cross section for the inclusive production of isolated photons by the CDF experiment at the Fermilab Tevatron collider is presented. The measurement covers the pseudorapidity region |eta^gamma|<1.0 and the transverse energy range E_T^gamma>30 GeV and is based on 2.5/fb of integrated luminosity. The sample is almost a factor of seven larger than those used for recent published results and extends the E_T^gamma coverage by 100 GeV. The result agrees with next-to-leading order perturbative QCD calculations within uncertainties over the range 50<E_Tgamma<400 GeV, though the energy spectrum in the data shows a steeper slope at lower E_T^gamma.

Gamma-Jet Tomography of Quark-Gluon Plasma in High-Energy Nuclear Collisions

Within the next-to-leading order (NLO) perturbative QCD (pQCD) parton model, suppression of away-side hadron spectra associated with a high p T photon due to parton energy loss is shown to provide a complete tomographic picture of the dense matter formed in high-energy heavy-ion collisions at RHIC. Dictated by the shape of the γ-triggered jet spectrum in NLO pQCD, hadron spectra at large z T = p T h / p T γ ≳ 1 are more susceptible to parton energy loss and therefore are dominated by surface emission of γ-triggered jets, whereas small z T hadrons mainly come from fragmentation of jets with reduced energy from volume emission. These lead to different centrality dependence of the hadron suppression in different regions of z T .

Tomography of High-Energy Nuclear Collisions with Photon-Hadron Correlations

Within the next-to-leading order (NLO) perturbative QCD (PQCD) parton model, suppression of away-side hadron spectra associated with a high pT photon due to parton energy loss is studied in high-energy heavy-ion collisions. Because of the sharp falloff of the gamma-jet spectrum in momentum imbalance pTjet-pTgamma>0 in NLO PQCD, hadron spectra at large zT=pTh/pTgamma greater than approximately 1 are more susceptible to parton energy loss and therefore are dominated by surface emission of gamma-associated jets with almost no energy loss, whereas small zT hadrons mainly come from the volume emission of jets with reduced energy. These lead to different centrality dependence of the gamma-hadron suppression for different values of zT. Therefore, a complete measurement of the suppression of gamma-triggered hadron spectra allows a true tomographic study of the quark-gluon plasma in high-energy heavy-ion collisions.

Renormalization of quark bilinear operators in a momentum-subtraction scheme with a nonexceptional subtraction point

We extend the Rome-Southampton regularization independent momentum-subtraction renormalization scheme(RI/MOM) for bilinear operators to one with a nonexceptional, symmetric subtraction point. Two-point Green's functions with the insertion of quark bilinear operators are computed with scalar, pseudoscalar, vector, axial-vector and tensor operators at one-loop order in perturbative QCD. We call this new scheme RI/SMOM, where the S stands for "symmetric". Conversion factors are derived, which connect the RI/SMOM scheme and the MSbar scheme and can be used to convert results obtained in lattice calculations into the MSbar scheme. Such a symmetric subtraction point involves nonexceptional momenta implying a lattice calculation with substantially suppressed contamination from infrared effects. Further, we find that the size of the one-loop corrections for these infrared improved kinematics is substantially decreased in the case of the pseudoscalar and scalar operator, suggesting a much better behaved perturbative series. Therefore it should allow us to reduce the error in the determination of the quark mass appreciably.

Measurements of differential cross sections of [formula omitted] events in [formula omitted] collisions at [formula omitted

We present cross section measurements for Z/γ∗+jets+X production, differential in the transverse momenta of the three leading jets. The data sample was collected with the DØ detector at the Fermilab Tevatron pp¯ collider at a center-of-mass energy of 1.96 TeV and corresponds to an integrated luminosity of 1 fb−1. Leading and next-to-leading order perturbative QCD predictions are compared with the measurements, and agreement is found within the theoretical and experimental uncertainties. We also make comparisons with the predictions of four event generators. Two parton-shower-based generators show significant shape and normalization differences with respect to the data. In contrast, two generators combining tree-level matrix elements with a parton shower give a reasonable description of the shapes observed in data, but the predicted normalizations show significant differences with respect to the data, reflecting large scale uncertainties. For specific choices of scales, the normalizations for either generator can be made to agree with the measurements.

Measurement ofγ+b+Xandγ+c+XProduction Cross Sections inpp¯Collisions ats=1.96 TeV

First measurements of the differential cross sections for the inclusive production of a photon in association with a heavy quark (c, b) jet are presented, covering photon transverse momenta 30-150 GeV, photon rapidities | y_gamma| < 1.0, jet rapidities |y_jet| < 0.8, and jet transverse momenta pT_jet > 15 GeV. The results are based on an integrated luminosity of 1 fb^-1 in ppbar collisions at sqrt(s)=1.96 TeV recorded with the D0 detector at the Fermilab Tevatron Collider. The results are compared with next-to-leading order perturbative QCD predictions.

Next-to-leading order calculation of the single transverse spin asymmetry in the Drell-Yan process

We calculate the next-to-leading order perturbative QCD corrections to the transverse momentum weighted single transverse spin asymmetry in Drell-Yan lepton pair production in hadronic collisions. We identify the splitting function relevant for the scale evolution of the twist-three quark-gluon correlation function. We comment on the consequences of our results for phenomenology.

Up- and down-quark masses from finite-energy QCD sum rules to five loops

The up- and down-quark masses are determined from an optimized QCD finite-energy sum rule involving the correlator of axial-vector divergences, to five-loop order in perturbative QCD, and including leading nonperturbative QCD and higher order quark-mass corrections. This finite-energy sum rule is designed to reduce considerably the systematic uncertainties arising from the (unmeasured) hadronic resonance sector, which in this framework contributes less than 3--4% to the quark mass. This is achieved by introducing an integration kernel in the form of a second degree polynomial, restricted to vanish at the peak of the two lowest lying resonances. The driving hadronic contribution is then the pion pole, with parameters well known from experiment. The determination is done in the framework of contour improved perturbation theory, which exhibits a very good convergence, leading to a remarkably stable result in the unusually wide window ${s}_{0}=1.0--4.0\text{ }\text{ }{\mathrm{GeV}}^{2}$, where ${s}_{0}$ is the radius of the integration contour in the complex energy (squared) plane. The results are ${m}_{u}(Q=2\text{ }\text{ }\mathrm{GeV})=2.9\ifmmode\pm\else\textpm\fi{}0.2\text{ }\text{ }\mathrm{MeV}$, ${m}_{d}(Q=2\text{ }\text{ }\mathrm{GeV})=5.3\ifmmode\pm\else\textpm\fi{}0.4\text{ }\text{ }\mathrm{MeV}$, and $({m}_{u}+{m}_{d})/2=4.1\ifmmode\pm\else\textpm\fi{}0.2\text{ }\text{ }\mathrm{MeV}$ (at a scale $Q=2\text{ }\text{ }\mathrm{GeV}$).

Surface versus volume emissions in photon–hadron correlations

High-p T photon–hadron correlations are studied within the next-to-leading order (NLO) perturbative QCD parton model with modified parton-jet fragmentation functions due to jet quenching in high-energy A+A collisions. In central A+A collisions, the away-side hadrons with large z T=p T /p T are controlled mainly by the surface emission of the gamma-jet events, while a small z T region will be volume emission bias. In other words, gamma jets for a small-z T region probe the dense matter deeper than those gamma jets for a large-z T region, so the small-z T gamma jets are found to be slightly more sensitive to the properties of the dense matter than the large-z T gamma jets.

Measurement of differential [formula omitted] cross sections in [formula omitted] collisions at [formula omitted

We present new measurements of differential cross sections for Z/γ∗(→μμ)+jet+X production in a 1 fb−1 data sample collected with the DØ detector in pp¯ collisions at s=1.96 TeV. Results include the first measurements differential in the Z/γ∗ transverse momentum and rapidity, as well as new measurements differential in the leading jet transverse momentum and rapidity. Next-to-leading order perturbative QCD predictions are compared to the measurements, and reasonable agreement is observed, except in the region of low Z/γ∗ transverse momentum. Predictions from two event generators based on matrix elements and parton showers, and one pure parton shower event generator are also compared to the measurements. These show significant overall normalization differences to the data and have varied success in describing the shape of the distributions.

Moments of heavy quark current correlators at four-loop order in perturbative QCD

We present the result for the first moment of the scalar and axial-vector current correlator in third order of the strong coupling constant αs and give the details of a recent evaluation of the pseudo-scalar correlator. The results can be used to reduce the theoretical uncertainty due to higher order corrections for the determination of fundamental parameters of QCD in the context of lattice calculations.

Next-to-leading order perturbative QCD corrections to baryon correlators in matter

We compute the next-to-leading order (NLO) perturbative QCD corrections to the correlators of nucleon interpolating currents in relativistic nuclear matter. The main new result is the calculation of the O({alpha}{sub s}) perturbative corrections to the coefficient functions of the vector quark condensate in matter. This condensate appears in matter due to the violation of Lorentz invariance. The NLO perturbative QCD corrections turn out to be large which implies that the NLO corrections must be included in a sum rule analysis of the properties of both bound nucleons and relativistic nuclear matter.

Inclusive properties of hadronic final states at HERA

Recent results from the H1 and ZEUS collaborations using the hadronic final state in high Q2 Deep-Inelastic Scattering to investigate QCD are presented. Measurements of azimuthal asymmetries, event shapes and the charged particle scaled momentum spectra are compared with next-to-leading order perturbative QCD predictions and with Monte Carlo models.

Inclusive jet production at the tevatron using the CDF experiment

This contribution reports on measurements of the inclusive jet production cross section in pp collisions at s√s = 1.96 TeV. Two analyzes are presented: one uses the longitudinally invariant kT algorithm to reconstruct the jets, the other uses the midpoint algorithm. The data samples have been collected at CDF during the Run II of the Tevatron and correspond to integrated luminosities of 1.0 fb-1 for the kT study and 1.13 fb-1 for the midpoint one. The inclusive jet cross sections are measured as a function of the jet transverse momentum in five jet rapidity regions for jets in the ranges 54 < pTjet < 700 GeV/c and yjet| < 2.1. Nextto-leading order perturbative QCD predictions are in good agreement with the measured cross sections after including the non-perturbative corrections necessary to account for underlying event and hadronization effects.

Transverse and longitudinal dynamics at RHIC

We review results obtained by the BRAHMS experiment at the Relativistic Heavy Ion Collider (RHIC) for the systems of Au+Au and p+p colliding at and at . Rapidity-dependent and K±/π± ratios within 0 < y < 3 for Au+Au at are found to be enhanced in nucleus–nucleus collisions as compared to p+p collisions. The particle ratios are discussed in terms of their system size and rapidity dependence. From comparison of RAA for different systems and energies it is found that RAA increases with decreasing collision energy, decreasing system size and when going toward more peripheral collisions. However, RAA shows only a very weak dependence on rapidity (for 0 < y < 3.2), both for pions and protons. The ratio for p+p collisions at is below 0.1 at high pT (≈4 GeV/c) and y ≈ 3. Such a strong asymmetry in p and production cannot be described within next-to-leading order perturbative QCD utilizing any known sets of FFs. At the same rapidity but lower energy the charge asymmetry is stronger by an order of magnitude for both protons and kaons.

Measurement of Inclusive Jet Cross Sections inZ/γ*(→e+e−)+jetsProduction inpp¯Collisions ats=1.96 TeV

Inclusive jet cross sections in Z/gamma^* events, with Z/gamma^* decaying into an electron-positron pair, are measured as a function of jet transverse momentum and jet multiplicity in ppbar collisions at sqrt{s} = 1.96 TeV with the upgraded Collider Detector at Fermilab in Run II, based on an integrated luminosity of 1.7 fb^-1. The measurements cover the rapidity region | yjet | < 2.1 and the transverse momentum range ptjet > 30 GeV/c. Next-to-leading order perturbative QCD predictions are in good agreement with the measured cross sections.

Inclusive jet production using the k T algorithm at CDF

This contribution presents preliminary results on the inclusive jet production in p p ¯ collisions at s = 1.96 TeV . The measurements are carried out using the longitudinally invariant k T algorithm. The inclusive jet cross sections are measured as a function of the jet transverse momentum in five jet rapidity regions for jets in the ranges 54 p T j e t 700 GeV/c and | y j e t | 2.1 . The results are based on 1.0 fb −1 of data collected at CDF during the Run II of the Tevatron. They are in good agreement with next-to-leading order perturbative QCD predictions after including the non-perturbative corrections necessary to account for underlying event and hadronization effects.