26,406 research outputs found
Survival probability in diffractive dijet photoproduction
We confront the latest H1 and ZEUS data on diffractive dijet photoproduction
with next-to-leading order QCD predictions in order to determine whether a
rapidity gap survival probability of less than one is supported by the data. We
find evidence for this hypothesis when assuming global factorization breaking
for both the direct and resolved photon contributions, in which case the
survival probability would have to be E_T^jet-dependent, and for the resolved
or in addition the related direct initial-state singular contribution only,
where it would be independent of E_T^jet.Comment: 8 pages, 7 figures, to appear in the proceedings of the 2008
CERN-DESY workshop on "HERA and the LHC
Factorization Breaking in Diffractive Photoproduction of Dijets
We have calculated the diffractive dijet cross section in low-Q^2 ep
scattering in the HERA regime. The results of the calculation in LO and NLO are
compared to recent experimental data of the H1 collaboration. We find that in
LO the calculated cross sections are in reasonable agreement with the
experimental results. In NLO, however, some of the cross sections disagree,
showing that factorization breaking occurs in that order. By suppressing the
resolved contribution by a factor of approximately three, good agreement with
all the data is found. The size of the factorization breaking effects in
diffractive dijet photoproduction agrees well with absorptive model
predictions.Comment: 13 pages, 9 figures. Talk presented at the 12th International
Workshop on Deep Inelastic Scattering (DIS 2004), Strbske Pleso, Slovakia,
14-18 April 2004. To appear in the proceeding
Survival probability for diffractive dijet production in p-pbar collisions from next-to-leading order calculations
We perform next-to-leading order calculations of the single-diffractive and
non-diffractive cross sections for dijet production in proton-antiproton
collisions at the Tevatron. By comparing their ratio to the data published by
the CDF collaboration for two different center-of-mass energies, we deduce the
rapidity-gap survival probability as a function of the momentum fraction of the
parton in the antiproton. Assuming Regge factorization, this probability can be
interpreted as a suppression factor for the diffractive structure function
measured in deep-inelastic scattering at HERA. In contrast to the observations
for photoproduction, the suppression factor in proton-antiproton collisions
depends on the momentum fraction of the parton in the Pomeron even at
next-to-leading order.Comment: 26 pages, 11 figure
NNLO contributions to jet photoproduction and determination of \alpha_s
We present the first calculation of inclusive jet photoproduction with
next-to-next-to-leading order (NNLO) contributions, obtained from a unified
threshold resummation formalism. The leading coefficients for direct
photoproduction are computed analytically. Together with the coefficients
pertinent to parton-parton scattering, they are shown to agree with those
appearing in our full next-to-leading order calculations. For hadron-hadron
scattering, numerical agreement is found with a previous calculation of jet
production at the Tevatron. We show that the direct and resolved NNLO
contributions considerably improve the description of final ZEUS data on jet
photoproduction and that the error on the determination of the strong coupling
constant is significantly reduced.Comment: 4 pages, 3 figure
NNLO contributions to inclusive-jet production in DIS and determination of \alpha_s
We present the first calculation of inclusive jet production in
deep-inelastic scattering with approximate next-to-next-to-leading order
(aNNLO) contributions, obtained from a unified threshold resummation formalism.
The leading coefficients are computed analytically. We show that the aNNLO
contributions reduce the theoretical prediction for jet production in
deep-inelastic scattering, improve the description of the final HERA data in
particular at high photon virtuality Q^2 and increase the central fit value of
the strong coupling constant.Comment: 5 pages, 2 figures. arXiv admin note: text overlap with
arXiv:1310.172
Large-p_T Photoproduction of D^*+- Mesons in ep Collisions
The cross section for the inclusive photoproduction of large-p_T D^*+- mesons
is calculated at next-to-leading order, adopting different approaches to
describe the fragmentation of charm quarks into D^*+- mesons. We treat the
charm quark according to the massless factorization scheme, where it is assumed
to be one of the active flavours inside the proton and the photon. We present
inclusive single-particle distributions in transverse momentum and rapidity,
including the contributions due to both direct and resolved photons. We compare
and assess the various implementations of fragmentation. We argue that, in the
high-p_T regime, a particularly realistic description can be obtained by
convoluting the Altarelli-Parisi-evolved fragmentation functions of Peterson et
al. with the hard-scattering cross sections of massless partons where the
factorization of the collinear singularities associated with final-state charm
quarks is converted to the massive-charm scheme. The predictions thus obtained
agree well with recent experimental data by the H1 and ZEUS Collaborations at
DESY HERA.Comment: 31 pages (Latex), 13 figures (Postscript). This version of the
manuscript is identical with the one being printed in Z. Phys.
Cross Sections for Charm Production in Collisions: Massive versus Massless Scheme
The next--to--leading order inclusive cross section for large-
photoproduction of charm quarks at HERA is calculated in two different
approaches. In the first approach the charm quarks are treated as massive
objects which are strictly external to the proton and the photon while in the
second approach the charm mass is neglected and the quark is assumed to be
one of the active flavours in the proton and photon structure functions. We
present single-inclusive distributions in transverse momentum and rapidity
including direct and resolved photons. The cross section in the massless
approach is found to be significantly larger than in the massive scheme. The
deviation originates from several contributions which are disentangled. We
argue that large- photoproduction of charm quarks at HERA will be
sensitive to the charm content of the photon structure function.Comment: 11 pages, Latex, epsfig, 6 figures appended as uuencoded file
(hardcopy can be obtained upon request from [email protected]
Surface structure of i-Al(68)Pd(23)Mn(9): An analysis based on the T*(2F) tiling decorated by Bergman polytopes
A Fibonacci-like terrace structure along a 5fold axis of i-Al(68)Pd(23)Mn(9)
monograins has been observed by T.M. Schaub et al. with scanning tunnelling
microscopy (STM). In the planes of the terraces they see patterns of dark
pentagonal holes. These holes are well oriented both within and among terraces.
In one of 11 planes Schaub et al. obtain the autocorrelation function of the
hole pattern. We interpret these experimental findings in terms of the
Katz-Gratias-de Boisseu-Elser model. Following the suggestion of Elser that the
Bergman clusters are the dominant motive of this model, we decorate the tiling
T*(2F) by the Bergman polytopes only. The tiling T*(2F) allows us to use the
powerful tools of the projection techniques. The Bergman polytopes can be
easily replaced by the Mackay polytopes as the decoration objects. We derive a
picture of ``geared'' layers of Bergman polytopes from the projection
techniques as well as from a huge patch. Under the assumption that no surface
reconstruction takes place, this picture explains the Fibonacci-sequence of the
step heights as well as the related structure in the terraces qualitatively and
to certain extent even quantitatively. Furthermore, this layer-picture requires
that the polytopes are cut in order to allow for the observed step heights. We
conclude that Bergman or Mackay clusters have to be considered as geometric
building blocks of the i-AlPdMn structure rather than as energetically stable
entities
Spherical Orbifolds for Cosmic Topology
Harmonic analysis is a tool to infer cosmic topology from the measured
astrophysical cosmic microwave background CMB radiation. For overall positive
curvature, Platonic spherical manifolds are candidates for this analysis. We
combine the specific point symmetry of the Platonic manifolds with their deck
transformations. This analysis in topology leads from manifolds to orbifolds.
We discuss the deck transformations of the orbifolds and give eigenmodes for
the harmonic analysis as linear combinations of Wigner polynomials on the
3-sphere. These provide new tools for detecting cosmic topology from the CMB
radiation.Comment: 17 pages, 9 figures. arXiv admin note: substantial text overlap with
arXiv:1011.427
Transport through a one-dimensional quantum dot
We examine the effects of long-range interactions in a quantum wire with two
impurities. We employ the bosonization technique and derive an effective action
for the system. The effect of the long-range interaction on the charging energy
and spectral properties of the island formed by the impurities and the linear
transport is discussed.Comment: 7 pages, 2 figure
- …