1,648 research outputs found
Soft-gluon resolution scale in QCD evolution equations
QCD evolution equations can be recast in terms of parton branching processes.
We present a new numerical solution of the equations. We show that this
parton-branching solution can be applied to analyze infrared contributions to
evolution, order-by-order in the strong coupling , as a function of
the soft-gluon resolution scale parameter. We examine the cases of
transverse-momentum ordering and angular ordering. We illustrate that this
approach can be used to treat distributions which depend both on longitudinal
and on transverse momenta.Comment: Latex, 8 pages, 4 figure
Collinear and TMD Quark and Gluon Densities from Parton Branching Solution of QCD Evolution Equations
We study parton-branching solutions of QCD evolution equations and present a
method to construct both collinear and transverse momentum dependent (TMD)
parton densities from this approach. We work with next-to-leading-order (NLO)
accuracy in the strong coupling. Using the unitarity picture in terms of
resolvable and non-resolvable branchings, we analyze the role of the soft-gluon
resolution scale in the evolution equations. For longitudinal momentum
distributions, we find agreement of our numerical calculations with existing
evolution programs at the level of better than 1 percent over a range of five
orders of magnitude both in evolution scale and in longitudinal momentum
fraction. We make predictions for the evolution of transverse momentum
distributions. We perform fits to the high-precision deep inelastic scattering
(DIS) structure function measurements, and we present a set of NLO TMD
distributions based on the parton branching approach.Comment: 27 pages, 8 figure
Convergence of the Linear Delta Expansion in the Critical O(N) Field Theory
The linear delta expansion is applied to the 3-dimensional O(N) scalar field
theory at its critical point in a way that is compatible with the large-N
limit. For a range of the arbitrary mass parameter, the linear delta expansion
for converges, with errors decreasing like a power of the order n in
delta. If the principal of minimal sensitivity is used to optimize the
convergence rate, the errors seem to decrease exponentially with n.Comment: 26 pages, latex, 8 figure
New developments in the statistical approach of parton distributions
We recall how parton distributions are constructed in a statistical physical
picture of the nucleon. The chiral properties of QCD lead to strong relations
between quarks and antiquarks distributions and the importance of the Pauli
exclusion principle is also emphasized. A global next-to-leading order QCD
analysis of unpolarized and polarized deep-inelastic scattering data allows to
determine a small number of free parameters. Some predictions are compared to
recent experimental results and we discuss the prospects of this physical
framework.Comment: 10 pages, 12 figures. Invited talk presented at the " Corfu Summer
Institute on EPP,CORFU2005, Corfu, Greece, September 4-26, 2005. To be
published in Journal of Physics, Conference Series (2006
Phenomenological model of elastic distortions near the spin-Peierls transition in
A phenomenological model of the Landau type forms the basis for a study of
elastic distortions near the spin-Peierls transition in . The
atomic displacements proposed by Hirota {\it et al.} [Phys. Rev. Lett. {\bf
73}, 736 (1994)] are accounted for by the model which includes linear coupling
between and distortions. displacements are seen to be responsible
for anomalies in the elastic properties {\it at} , whereas incipient
distortions give rise to temperature dependence below . A discussion of
possible critical behavior is also made.Comment: 1 figure available upon reques
The photon PDF from high-mass Drell Yan data at the LHC
Achieving the highest precision for theoretical predictions at the LHC requires the calculation of hard-scattering cross-sections that include perturbative QCD corrections up to (N)NNLO and electroweak (EW) corrections up to NLO. Parton distribution functions (PDFs) need to be provided with matching accuracy, which in the case of QED effects involves introducing the photon parton distribution of the proton, . In this work a determination of the photon PDF from fits to recent ATLAS measurements of high-mass Drell-Yan dilepton production at TeV is presented. This analysis is based on the xFitter framework, and has required improvements both in the APFEL program, to account for NLO QED effects, and in the aMCfast interface to account for the photon-initiated contributions in the EW calculations within MadGraph5_aMC@NLO. The results are compared with other recent QED fits and determinations of the photon PDF, consistent results are found
Crossover between a displacive and an order-disorder phase transition
The phase transition in a three-dimensional array of classical anharmonic oscillators with harmonic nearest-neighbor coupling (discrete
Ï
4
model) is studied by Monte Carlo (MC) simulations and by analytical methods. The model allows us to choose a single dimensionless parameter a determining completely the behavior of the system. Changing a from 0 to
+
â
allows to go continuously from the displacive to the order-disorder limit. We calculate the transition temperature
T
c
and the temperature dependence of the order parameter down to
T
=
0
for a wide range of the parameter a. The
T
c
from MC calculations shows an excellent agreement with the known asymptotic values for small and large a. The obtained MC results are further compared with predictions of the mean-field and independent-mode approximations as well as with predictions of our own approximation scheme. In this approximation, we introduce an auxiliary system, which yields approximately the same temperature behavior of the order parameter, but allows the decoupling of the phonon modes. Our approximation gives the value of
T
c
within an error of 5% and satisfactorily describes the temperature dependence of the order parameter for all values of a
The Toroid Moment of Majorana Neutrino
If neutrino is the Majorana particle it can possess only one electromagnetic
characteristic, the toroid dipole moment (anapole) in the static limit and
nothing else. We have calculated the diagonal toroid moment (form factor) of
the Majorana neutrino by the dispersion method in the one-loop approximation of
the Standard Model and found it to be different from zero in the case of
massive as well as massless neutrinos. All external particles are on the mass
shells and there are no problems with the physical interpretation of the final
result. Some manifestations of the toroid interactions of Majorana neutrinos,
induced by their toroid moments, are also remarked.Comment: 22 pages, 1 table and 3 EPS-figures included, uses prd.sty,
preprint.sty, aps.sty and epsfig.sty (RevTeX is used), major conceptual
changes of E2-96-53 are include
Lattice models and Landau theory for type II incommensurate crystals
Ground state properties and phonon dispersion curves of a classical linear
chain model describing a crystal with an incommensurate phase are studied. This
model is the DIFFOUR (discrete frustrated phi4) model with an extra
fourth-order term added to it. The incommensurability in these models may arise
if there is frustration between nearest-neighbor and next-nearest-neighbor
interactions. We discuss the effect of the additional term on the phonon
branches and phase diagram of the DIFFOUR model. We find some features not
present in the DIFFOUR model such as the renormalization of the
nearest-neighbor coupling. Furthermore the ratio between the slopes of the soft
phonon mode in the ferroelectric and paraelectric phase can take on values
different from -2. Temperature dependences of the parameters in the model are
different above and below the paraelectric transition, in contrast with the
assumptions made in Landau theory. In the continuum limit this model reduces to
the Landau free energy expansion for type II incommensurate crystals and it can
be seen as the lowest-order generalization of the simplest Lifshitz-point
model. Part of the numerical calculations have been done by an adaption of the
Effective Potential Method, orginally used for models with nearest-neighbor
interaction, to models with also next-nearest-neighbor interactions.Comment: 33 pages, 7 figures, RevTex, submitted to Phys. Rev.
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