25 research outputs found
Pomeron physics: an update
Key issues in pomeron physics include whether the hard and soft pomerons are
distinct objects, and whether the hard pomeron is already present in amplitudes
at . It is urgent to learn how to combine perturbative and
nonperturbative concepts, and to construct a sound theory of perturbative
evolution at small . Other questions are whether screening corrections are
small, and gap survival probabilities large. Finally, do diffractive processes
present a good way to discover the Higgs?Comment: 12 pages, 23 figures embedded with epsf Summary talk at
Diffraction2000, Calabria, September 200
Heavy Quarkonium in a weakly-coupled quark-gluon plasma below the melting temperature
We calculate the heavy quarkonium energy levels and decay widths in a
quark-gluon plasma, whose temperature T and screening mass m_D satisfy the
hierarchy m alpha_s >> T >> m alpha_s^2 >> m_D (m being the heavy-quark mass),
at order m alpha_s^5. We first sequentially integrate out the scales m, m
alpha_s and T, and, next, we carry out the calculations in the resulting
effective theory using techniques of integration by regions. A collinear region
is identified, which contributes at this order. We also discuss the
implications of our results concerning heavy quarkonium suppression in heavy
ion collisions.Comment: 25 pages, 2 figure
Nonperturbative contributions to the quark form factor at high energy
The analysis of nonperturbative effects in high energy asymptotics of the
electomagnetic quark form factor is presented. It is shown that the
nonperturbative effects determine the initial value for the perturbative
evolution of the quark form factor and find their general structure with
respect to the high energy asymptotics. Within the Wilson integral formalism
which is natural for investigation of the soft, IR sensitive, part of the
factorized form factor, the structure of the instanton induced effects in the
evolution equation is discussed. It is demonstrated that the instanton
contributions result in the finite renormalization of the subleading
perturbative result and numerically are characterized by small factor
reflecting the diluteness of the QCD vacuum within the instanton liquid model.
The relevance of the IR renormalon induced effects in high energy asymptotic
behaviour is discussed. The consequences of the various analytization
procedures of the strong coupling constant in the IR domain are considered.Comment: REVTeX, 12 pages, 1 figure. Important references and discussions
added, misprints corrected, minor changes in tex
Gauge links for transverse momentum dependent correlators at tree-level
In this paper we discuss the incorporation of gauge links in hadronic matrix
elements that describe the soft hadronic physics in high energy scattering
processes. In this description the matrix elements appear in soft correlators
and they contain non-local combinations of quark and gluon fields. In our
description we go beyond the collinear approach in which case also the
dependence on transverse momenta of partons is taken into consideration. The
non-locality in the transverse direction leads to a complex gauge link
structure for the full process, in which color is entangled, even at
tree-level. We show that at tree-level in a 1-parton unintegrated (1PU)
situation, in which only the transverse momentum of one of the initial state
hadrons is relevant, one can get a factorized expression involving transverse
momentum dependent (TMD) distribution functions. We point out problems at the
level of two initial state hadrons, even for relatively simple processes such
as Drell-Yan scattering.Comment: 25 pages, corrected typos and updated reference
Multiple Parton Interactions in Z+jets production at the LHC. A comparison of factorized and non--factorized double parton distribution functions
We examine the contribution of Multiple Parton Interactions to Z+n-jets
production at the LHC, n=2,3,4, where the Z boson is assumed to decay
leptonically. We compare the results obtained with the correlated GS09 double
parton distribution function with those obtained with two instances of fully
factorized single parton distribution functions: MSTW2008LO and CTEQ6LO. It
appears quite feasible to measure the MPI contribution to Z+2/3/4 jets already
in the first phase of the LHC with a total luminosity of one inverse femtobarn
at 7 TeV. If as expected the trigger threshold for single photons is around 80
GeV, Z+2-jets production may well turn out to be more easily observable than
the gamma+3-jets channel. The MPI cross section is dominated by relatively soft
events with two jets balancing in transverse momentum.Comment: 15 pages, 3 plot
The Spin Structure of the Nucleon
We present an overview of recent experimental and theoretical advances in our
understanding of the spin structure of protons and neutrons.Comment: 84 pages, 29 figure
The asymptotic behaviour of parton distributions at small and large
It has been argued from the earliest days of quantum chromodynamics that at asymptotically small values of x the parton distribution functions (PDFs) of the proton behave as xαxα, where the values of αα can be deduced from Regge theory, while at asymptotically large values of x the PDFs behave as (1−x)β(1−x)β, where the values of ββ can be deduced from the Brodsky–Farrar quark counting rules. We critically examine these claims by extracting the exponents αα and ββ from various global fits of parton distributions, analysing their scale dependence, and comparing their values to the naive expectations. We find that for valence distributions both Regge theory and counting rules are confirmed, at least within uncertainties, while for sea quarks and gluons the results are less conclusive. We also compare results from various PDF fits for the structure function ratio Fn2/Fp2F2n/F2p at large x, and caution against unrealistic uncertainty estimates due to overconstrained parametrisations
Cuts from residues: the one-loop case
Using the multivariate residue calculus of Leray, we give a precise
definition of the notion of a cut Feynman integral in dimensional
regularization, as a residue evaluated on the variety where some of the
propagators are put on shell. These are naturally associated to Landau
singularities of the first type. Focusing on the one-loop case, we give an
explicit parametrization to compute such cut integrals, with which we study
some of their properties and list explicit results for maximal and
next-to-maximal cuts. By analyzing homology groups, we show that cut integrals
associated to Landau singularities of the second type are specific combinations
of the usual cut integrals, and we obtain linear relations among different cuts
of the same integral. We also show that all one-loop Feynman integrals and
their cuts belong to the same class of functions, which can be written as
parametric integrals.Comment: v2: fixed minor typos in the normalisation of cut integral