72 research outputs found
Direct photons at forward rapidities in high-energy pp collisions
We investigate direct photon production in pp collisions at the energies of
RHIC, CDF and LHC, at different rapidities employing various color-dipole
models. The cross section peaks at forward rapidities due to the abelian
dynamics of photon radiation. This opens new opportunities for measurement of
direct photons at forward rapidities, where the background from radiative
hadronic decays is strongly suppressed. Our model calculations show that photon
production is sensitive to the gluon saturation effects, and strongly depends
on the value of the anomalous dimension.Comment: 7 pages, 8 figures, minor clarifications added. The version to appear
in PL
Non-linear QCD dynamics in two-photon interactions at high energies
Perturbative QCD predicts that the growth of the gluon density at high
energies should saturate, forming a Color Glass Condensate (CGC), which is
described in mean field approximation by the Balitsky-Kovchegov (BK) equation.
In this paper we study the interactions at high energies and
estimate the main observables which will be probed at future linear colliders
using the color dipole picture. We discuss in detail the dipole - dipole cross
section and propose a new relation between this quantity and the dipole
scattering amplitude. The total ,
cross-sections and the real photon structure function are
calculated using the recent solution of the BK equation with running coupling
constant and the predictions are compared with those obtained using
phenomenological models for the dipole-dipole cross section and scattering
amplitude. We demonstrate that these models are able to describe the LEP data
at high energies, but predict a very different behavior for the observables at
higher energies. Therefore we conclude that the study of
interactions can be useful to constrain the QCD dynamics.Comment: 11 pages, 5 figures. Version to be published in European Physical
Journal
Energy dependence of the saturation scale and the charged multiplicity in pp and AA collisions
A natural framework to understand the energy dependence of bulk observables
from lower energy experiments to the LHC is provided by the Color Glass
Condensate, which leads to a "geometrical scaling" in terms of an energy
dependent saturation scale Q_s. The measured charged multiplicity, however,
seems to grow faster (~\sqrt{s}^0.3) in nucleus-nucleus collisions than it does
for protons (~\sqrt{s}^0.2), violating the expectation from geometric scaling.
We argue that this difference between pp and AA collisions can be understood
from the effect of DGLAP evolution on the value of the saturation scale, and is
consistent with gluon saturation observations at HERA.Comment: RevTeX, 8 pages, 4 figures. V2: modified discussion of fragmentation,
published in EPJ
Nuclear DVCS at small-x using the color dipole phenomenology
Using the high energy color dipole formalism, we study the coherent and
incoherent nuclear DVCS process in the small-x regime. We consider simple
models for the elementary dipole-hadron scattering amplitude that captures main
features of the dependence on atomic number A, on energy and on momentum
transfer t. Using the obtained amplitudes we make predictions for the nuclear
DVCS cross section at photon level in the collider kinematics.Comment: 6 pages, 5 figures. Version to be published in European Physical
Journal
Heavy-quark mass dependence in global PDF analyses and 3- and 4-flavour parton distributions
We study the sensitivity of our recent MSTW 2008 NLO and NNLO PDF analyses to
the values of the charm- and bottom-quark masses, and we provide additional
public PDF sets for a wide range of these heavy-quark masses. We quantify the
impact of varying m_c and m_b on the cross sections for W, Z and Higgs
production at the Tevatron and the LHC. We generate 3- and 4-flavour versions
of the (5-flavour) MSTW 2008 PDFs by evolving the input PDFs and alpha_S
determined from fits in the 5-flavour scheme, including the eigenvector PDF
sets necessary for calculation of PDF uncertainties. As an example of their
use, we study the difference in the Z total cross sections at the Tevatron and
LHC in the 4- and 5-flavour schemes. Significant differences are found,
illustrating the need to resum large logarithms in Q^2/m_b^2 by using the
5-flavour scheme. The 4-flavour scheme is still necessary, however, if cuts are
imposed on associated (massive) b-quarks, as is the case for the experimental
measurement of Z b bbar production and similar processes.Comment: 40 pages, 11 figures. Grids can be found at
http://projects.hepforge.org/mstwpdf/ and in LHAPDF V5.8.4. v2: version
published in EPJ
Genome-Wide Association Study in BRCA1 Mutation Carriers Identifies Novel Loci Associated with Breast and Ovarian Cancer Risk
BRCA1-associated breast and ovarian cancer risks can be modified by common genetic variants. To identify further cancer risk-modifying loci, we performed a multi-stage GWAS of 11,705 BRCA1 carriers (of whom 5,920 were diagnosed with breast and 1,839 were diagnosed with ovarian cancer), with a further replication in an additional sample of 2,646 BRCA1 carriers. We identified a novel breast cancer risk modifier locus at 1q32 for BRCA1 carriers (rs2290854, P = 2.7×10-8, HR = 1.14, 95% CI: 1.09-1.20). In addition, we identified two novel ovarian cancer risk modifier loci: 17q21.31 (rs17631303, P = 1.4×10-8, HR = 1.27, 95% CI: 1.17-1.38) and 4q32.3 (rs4691139, P = 3.4×10-8, HR = 1.20, 95% CI: 1.17-1.38). The 4q32.3 locus was not associated with ovarian cancer risk in the general population or BRCA2 carriers, suggesting a BRCA1-specific associat
A candidate locus for variation in dispersal rate in a butterfly metapopulation
Frequent extinctions of local populations in metapopulations create opportunities for migrant females to establish new populations. In a metapopulation of the Glanville fritillary butterfly (Melitaea cinxia), more mobile individuals are more likely to establish new populations, especially in habitat patches that are poorly connected to existing populations. Here we show that flight metabolic rate and the frequency of a specific allele of the metabolic enzyme phosphoglucose isomerase (pgi) were both highest in newly established, isolated populations. Furthermore, genotypes with this pgi allele had elevated flight metabolic rates. These results suggest that genetic variation in pgi or a closely linked locus has a direct effect on flight metabolism, dispersal rate, and thereby on metapopulation dynamics in this species. These results also contribute to an emerging understanding of the mechanisms by which population turnover in heterogeneous landscapes may maintain genetic and phenotypic variation across populations
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