2,266 research outputs found
Infrared divergences and non-lightlike eikonal lines in Sudakov processes
We propose a method to analyze infrared contributions to non-inclusive
processes in QCD. We use the one-loop Sudakov form factor as a working example.
Borrowing techniques from renormalization theory, we construct counterterms for
the contributions from the soft and collinear regions, and we relate them to
eikonal Wilson lines taken along non-lightlike directions.Comment: Latex, 1 postscript figure. Version 2: presentation improved and
typos corrected, results unchange
NLO corrections in MC event generator for angular distribution of Drell-Yan lepton pair production
Using a subtraction method, we derive the formulae suitable for use in
Monte-Carlo event generators to give the angular distribution for the
gluon-quark induced NLO corrections in Drell-Yan lepton pair production. We
also give the corresponding helicity density matrix for W and Z boson
production.Comment: 14 pages, 2 figure
Fragmentation Function in Non-Equilibrium QCD Using Closed-Time Path Integral Formalism
In this paper we implement Schwinger-Keldysh closed-time path integral
formalism in non-equilibrium QCD to the definition of Collins-Soper
fragmentation function. We consider a high p_T parton in QCD medium at initial
time t_0 with arbitrary non-equilibrium (non-isotropic) distribution function
f(\vec{p}) fragmenting to hadron. We formulate parton to hadron fragmentation
function in non-equilibrium QCD in the light-cone quantization formalism. It
may be possible to include final state interactions with the medium via
modification of the Wilson lines in this definition of the non-equilibrium
fragmentation function. This may be relevant to study hadron production from
quark-gluon plasma at RHIC and LHC.Comment: 15 pages latex, Accepted for Publication in European Physical Journal
Generalized parton correlation functions for a spin-1/2 hadron
The fully unintegrated, off-diagonal quark-quark correlator for a spin-1/2
hadron is parameterized in terms of so-called generalized parton correlation
functions. Such objects, in particular, can be considered as mother
distributions of generalized parton distributions on the one hand and
transverse momentum dependent parton distributions on the other. Therefore, our
study provides new, model-independent insights into the recently proposed
nontrivial relations between generalized and transverse momentum dependent
parton distributions. We find that none of these relations can be promoted to a
model-independent status. As a by-product we obtain the first complete
classification of generalized parton distributions beyond leading twist. The
present paper is a natural extension of our previous corresponding analysis for
spin-0 hadrons.Comment: 41 pages, 3 figures; v2: added referenc
Restoration of factorization for low hadron hadroproduction
We discuss the applicability of the factorization theorem to low-
hadron production in hadron-hadron collision in a simple toy model, which
involves only scalar particles and gluons. It has been shown that the
factorization for high- hadron hadroproduction is broken by soft gluons in
the Glauber region, which are exchanged among a transverse-momentum-dependent
(TMD) parton density and other subprocesses of the collision. We explain that
the contour of a loop momentum can be deformed away from the Glauber region at
low , so the above residual infrared divergence is factorized by means of
the standard eikonal approximation. The factorization is then restored in
the sense that a TMD parton density maintains its universality. Because the
resultant Glauber factor is independent of hadron flavors, experimental
constraints on its behavior are possible. The factorization can also be
restored for the transverse single-spin asymmetry in hadron-hadron collision at
low in a similar way, with the residual infrared divergence being
factorized into the same Glauber factor.Comment: 12 pages, 2 figures, version to appear in EPJ
Double transverse spin asymmetries in vector boson production
We investigate a helicity non-flip double transverse spin asymmetry in vector
boson production in hadron-hadron scattering, which was first considered by
Ralston and Soper at the tree level. It does not involve transversity functions
and in principle also arises in W-boson production for which we present the
expressions. The asymmetry requires observing the transverse momentum of the
vector boson, but it is not suppressed by explicit inverse powers of a large
energy scale. However, as we will show, inclusion of Sudakov factors causes
suppression of the asymmetry, which increases with energy. Moreover, the
asymmetry is shown to be approximately proportional to x_1 g_1(x_1) x_2 \bar
g_1(x_2), which gives rise to additional suppression at small values of the
light cone momentum fractions. This implies that it is negligible for Z or W
production and is mainly of interest for \gamma^* at low energies. We also
compare the asymmetry with other types of double transverse spin asymmetries
and discuss how to disentangle them.Comment: 12 pages, Revtex, 2 Postscript figures, uses aps.sty, epsf.sty;
figures replaced, a few minor other correction
Drell-Yan production at small q_T, transverse parton distributions and the collinear anomaly
Using methods from effective field theory, an exact all-order expression for
the Drell-Yan cross section at small transverse momentum is derived directly in
q_T space, in which all large logarithms are resummed. The anomalous dimensions
and matching coefficients necessary for resummation at NNLL order are given
explicitly. The precise relation between our result and the
Collins-Soper-Sterman formula is discussed, and as a by-product the previously
unknown three-loop coefficient A^(3) is obtained. The naive factorization of
the cross section at small transverse momentum is broken by a collinear
anomaly, which prevents a process-independent definition of x_T-dependent
parton distribution functions. A factorization theorem is derived for the
product of two such functions, in which the dependence on the hard momentum
transfer is separated out. The remainder factors into a product of two
functions of longitudinal momentum variables and x_T^2, whose
renormalization-group evolution is derived and solved in closed form. The
matching of these functions at small x_T onto standard parton distributions is
calculated at O(alpha_s), while their anomalous dimensions are known to three
loops.Comment: 32 pages, 2 figures; version to appear in Eur. Phys. J.
Next-to-leading order QCD calculations with parton showers II: soft singularities
Programs that calculate observables in quantum chromodynamics at
next-to-leading order typically generate events that consist of partons rather
than hadrons -- and just a few partons at that. These programs would be much
more useful if the few partons were turned into parton showers, which could be
given to one of the Monte Carlo event generators to produce hadron showers. In
a previous paper, we have seen how to generate parton showers related to the
final state collinear singularities of the perturbative calculation for the
example of e+ + e- --> 3 jets. This paper discusses the treatment of the soft
singularities.Comment: 26 pages with 5 figures. This version is close to the version to be
publishe
Renormalization of a class of non-renormalizable theories
Certain power-counting non-renormalizable theories, including the most
general self-interacting scalar fields in four and three dimensions and
fermions in two dimensions, have a simplified renormalization structure. For
example, in four-dimensional scalar theories, 2n derivatives of the fields,
n>1, do not appear before the nth loop. A new kind of expansion can be defined
to treat functions of the fields (but not of their derivatives)
non-perturbatively. I study the conditions under which these theories can be
consistently renormalized with a reduced, eventually finite, set of independent
couplings. I find that in common models the number of couplings sporadically
grows together with the order of the expansion, but the growth is slow and a
reasonably small number of couplings is sufficient to make predictions up to
very high orders. Various examples are solved explicitly at one and two loops.Comment: 38 pages, 1 figure; v2: more explanatory comments and references;
appeared in JHE
Identified hadron production at high transverse momenta in p+p collisions at sqrt(NN) = 200 GeV in STAR
We report the transverse momentum (pT) distributions for identified charged
pions, protons and anti-protons using events triggered by high deposit energy
in the Barrel Electro-Magnetic Calorimeter (BEMC) from p + p collisions at psNN
= 200 GeV. The spectra are measured around mid-rapidity (|y|<0.5) over the
range of 3<pT<15 GeV/c with particle identification (PID) by the relativistic
ionization energy loss (rdE/dx) in the Time Projection Chamber (TPC) in the
Solenoidal Tracker at RHIC (STAR). The charged pion, proton and anti-proton
spectra at high pT are compared with published results from minimum bias
triggered events and the Next-Leading-Order perturbative quantum chromodynamic
(NLO pQCD) calculations (DSS, KKP and AKK 2008). In addition, we present the
particle ratios of pi-/pi+, pbar/p, p/pi+ and pbar/pi- in p + p collisions.Comment: 4 pages, 4 figures, Hot Quark 2008 proceedin
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