Search results for "quantum chromodynamics"
showing 10 items of 1027 documents
QCD condensates from tau-decay data: A functional approach
2004
We study a functional method to extract the V − A condensate of dimension 6 from a comparison of τ -decay data with the asymptotic space-like QCD prediction. Our result is in agreement within errors with that from conventional analyses based on finite energy sum rules.
Infrared finite ghost propagator in the Feynman gauge
2007
We demonstrate how to obtain from the Schwinger-Dyson equations of QCD an infrared finite ghost propagator in the Feynman gauge. The key ingredient in this construction is the longitudinal form factor of the non-perturbative gluon-ghost vertex, which, contrary to what happens in the Landau gauge, contributes non-trivially to the gap equation of the ghost. The detailed study of the corresponding vertex equation reveals that in the presence of a dynamical infrared cutoff this form factor remains finite in the limit of vanishing ghost momentum. This, in turn, allows the ghost self-energy to reach a finite value in the infrared, without having to assume any additional properties for the gluon-g…
Multiple jet production at low transverse energies inpp¯collisions ats=1.8TeV
2003
We present data on multiple production of jets with transverse energies near 20 GeV in p (p) over bar collisions at roots=1.8 TeV. QCD calculations in the parton-shower approximation of PYTHIA and HERWIG and the next-to-leading order approximation of JETRAD are compared to the data for one, two, three, and four jet inclusive production. Transverse energy spectra and multiple jet angular and summed transverse-energy distributions are adequately described by the shower approximation while next-to-leading order calculations describe the data poorly.
Measurement of the differential cross section for the production of an isolated photon with associated jet in pp¯ collisions at s=1.96 TeV
2008
The process ppbar -> photon + jet + X is studied using 1.0 fb^-1 of data collected by the D0 detector at the Fermilab Tevatron ppbar collider at a center-of-mass energy sqrt(s)=1.96 TeV. Photons are reconstructed in the central rapidity region |y_gamma|<1.0 with transverse momenta in the range 3015 GeV. The differential cross section d^3sigma/dPt_gamma dy_gamma dy_jet is measured as a function of Pt_gamma in four regions, differing by the relative orientations of the photon and the jet in rapidity. Ratios between the differential cross sections in each region are also presented. Next-to-leading order QCD predictions using different parameterizations of parton distribution functions and theo…
Constraints on jet quenching in p–Pb collisions at sNN=5.02 TeV measured by the event-activity dependence of semi-inclusive hadron-jet distributions
2018
The ALICE Collaboration reports the measurement of semi-inclusive distributions of charged-particle jets recoiling from a high-transverse momentum trigger hadron in p–Pb collisions at √sNN = 5.02 TeV. Jets are reconstructed from charged-particle tracks using the anti-kT algorithm with resolution parameter R = 0.2 and 0.4. A data-driven statistical approach is used to correct the uncorrelated background jet yield. Recoil jet distributions are reported for jet transverse momentum 15 < pch T,jet < 50 GeV/c and are compared in various intervals of p–Pb event activity, based on charged-particle multiplicity and zero-degree neutral energy in the forward (Pb-going) direction. The semi-inclusive ob…
Multiplicity dependence of inclusive J/ψ production at midrapidity in pp collisions at s=13 TeV
2020
Measurements of the inclusive J/ψ yield as a function of charged-particle pseudorapidity density dNch/dη in pp collisions at √s = 13 TeV with ALICE at the LHC are reported. The J/ψ meson yield is measured at midrapidity (|y| < 0.9) in the dielectron channel, for events selected based on the charged-particle multiplicity at midrapidity (|η| < 1) and at forward rapidity (−3.7 < η < −1.7 and 2.8 < η < 5.1); both observables are normalized to their corresponding averages in minimum bias events. The increase of the normalized J/ψ yield with normalized dNch/dη is significantly stronger than linear and dependent on the transverse momentum. The data are compared to theoretical predictions, which de…
Global baryon number conservation encoded in net-proton fluctuations measured in Pb–Pb collisions at sNN=2.76 TeV
2020
Experimental results are presented on event-by-event net-proton fluctuation measurements in Pb–Pb collisions at sNN=2.76 TeV, recorded by the ALICE detector at the CERN LHC. These measurements have as their ultimate goal an experimental test of Lattice QCD (LQCD) predictions on second and higher order cumulants of net-baryon distributions to search for critical behavior near the QCD phase boundary. Before confronting them with LQCD predictions, account has to be taken of correlations stemming from baryon number conservation as well as fluctuations of participating nucleons. Both effects influence the experimental measurements and are usually not considered in theoretical calculations. For t…
Nonperturbative comparison of QCD effective charges
2009
We study the non-perturbative behavior of two versions of the QCD effective charge, one obtained from the pinch technique gluon self-energy, and one from the ghost-gluon vertex. Despite their distinct theoretical origin, due to a fundamental identity relating various of the ingredients appearing in their respective definitions, the two effective charges are almost identical in the entire range of physical momenta, and coincide exactly in the deep infrared, where they freeze at a common finite value. Specifically, the dressing function of the ghost propagator is related to the two form factors in the Lorentz decomposition of a certain Green's function, appearing in a variety of field-theoret…
How to perform QCD analysis of DIS in Analytic Perturbation Theory
2015
We apply (Fractional) Analytic Perturbation Theory (FAPT) to the QCD analysis of the nonsinglet nucleon structure function $F_2(x,Q^2)$ in deep inelastic scattering up to the next leading order and compare the results with ones obtained within the standard perturbation QCD. Based on a popular parameterization of the corresponding parton distribution we perform the analysis within the Jacobi Polynomial formalism and under the control of the numerical inverse Mellin transform. To reveal the main features of the FAPT two-loop approach, we consider a wide range of momentum transfer from high $Q^2\sim 100 {\rm GeV}^2$ to low $Q^2\sim 0.3 {\rm GeV}^2$ where the approach still works.
First measurement of coherent ρ0 photoproduction in ultra-peripheral Xe–Xe collisions at sNN=5.44 TeV
2021
Abstract The first measurement of the coherent photoproduction of ρ 0 vector mesons in ultra-peripheral Xe–Xe collisions at s NN = 5.44 TeV is presented. This result, together with previous HERA γp data and γ–Pb measurements from ALICE, describes the atomic number (A) dependence of this process, which is particularly sensitive to nuclear shadowing effects and to the approach to the black-disc limit of QCD at a semi-hard scale. The cross section of the Xe + Xe → ρ 0 + Xe + Xe process, measured at midrapidity through the decay channel ρ 0 → π + π − , is found to be d σ / d y = 131.5 ± 5.6 ( stat . ) − 16.9 + 17.5 ( syst . ) mb. The ratio of the continuum to resonant contributions for the prod…