146 research outputs found
nPDF constraints from the Large Hadron Electron Collider
An updated analysis regarding the expected nuclear PDF constraints from the
future Large Hadron Electron Collider (LHeC) experiment is presented. The new
study is based on a more flexible small- parametrization which provides less
biased uncertainty estimates in the region where there are currently no data
constraints. The effect of the LHeC is quantified by directly including a
sample of pseudodata according to the expected precision of this planned
experiment. As a result, a significant reduction of the small- uncertainties
in sea quarks and gluons is observed.Comment: Talk presented at DIS2016 conference, April 11 - April 15, 2016,
DESY, Hamburg, German
Kinematic biases on centrality selection of jet events in pPb collisions at the LHC
Centrality selection has been observed to have a large effect on jet
observables in pPb collisions at the Large Hadron Collider, stronger than that
predicted by the nuclear modification of parton densities. We study to which
extent simple considerations of energy-momentum conservation between the hard
process and the underlying event affect jets observables in such collisions. We
develop a simplistic approach that considers first the production of jets in a
pp collision as described by PYTHIA. From each pp collision, the value of the
energy of the parton from the proton participating in the hard scattering is
extracted. Then, the underlying event is generated simulating a pPb collision
through HIJING, but with the energy of the proton decreased according to the
value extracted in the previous step, and both collisions are superimposed.
This model is able to capture the bulk of the centrality effect for central to
semicentral collisions, for the two available sets of data: dijets from the CMS
Collaboration and single jets from the ATLAS Collaboration. As expected, the
model fails for peripheral collisions where very few nucleons from Pb
participate
Medium-Induced Gluon Radiation off Massive Quarks Fills the Dead Cone
We calculate the transverse momentum dependence of the medium-induced gluon
energy distribution radiated off massive quarks in spatially extended QCD
matter. In the absence of a medium, the distribution shows a characteristic
mass-dependent depletion of the gluon radiation for angles smaller than m/E,
the so-called dead cone effect. Medium-modifications of this spectrum are
calculated as a function of quark mass, initial quark energy, in-medium
pathlength and density. Generically, medium-induced gluon radiation is found to
fill the dead cone, but it is reduced at large gluon energies compared to the
radiation off light quarks. We quantify the resulting mass-dependence for
momentum-averaged quantities (gluon energy distribution and average parton
energy loss), compare it to simple approximation schemes and discuss its
observable consequences for nucleus-nucleus collisions at RHIC and LHC. In
particular, our analysis does not favor the complete disappearance of energy
loss effects from leading open charm spectra at RHIC.Comment: 27 pages LaTeX, 15 eps-figure
Bayesian reweighting of nuclear PDFs and constraints from proton-lead collisions at the LHC
New hard-scattering measurements from the LHC proton-lead run have the
potential to provide important constraints on the nuclear parton distributions
and thus contributing to a better understanding of the initial state in heavy
ion collisions. In order to quantify these constraints, as well as to assess
the compatibility with available nuclear data from fixed target experiments and
from RHIC, the traditional strategy is to perform a global fit of nuclear PDFs.
This procedure is however time consuming and technically challenging, and
moreover can only be performed by the PDF fitters themselves. In the case of
proton PDFs, an alternative approach has been suggested that uses Bayesian
inference to propagate the effects of new data into the PDFs without the need
of refitting. In this work, we apply this reweighting procedure to study the
impact on nuclear PDFs of low-mass Drell-Yan and single-inclusive
hadroproduction pseudo-data from proton-lead collisions at the LHC as
representative examples. In the hadroproduction case, in addition we assess the
possibility of discriminating between the DGLAP and CGC production frameworks.
We find that the LHC proton-lead data could lead to a substantial reduction of
the uncertainties on nuclear PDFs, in particular for the small-x gluon PDF
where uncertainties could decrease by up to a factor two. The Monte Carlo
replicas of EPS09 used in the analysis are released as a public code for
general use. It can be directly used, in particular, by the experimental
collaborations to check, in a straightforward manner, the degree of
compatibility of the new data with the global nPDF analyses.Comment: 21 pages, 10 figure
Inclusive diffraction in future electron-proton and electron-ion colliders
We analyse the possibilities for the study of inclusive diffraction offered
by future electron--proton/nucleus colliders in the TeV regime, the Large
Hadron-electron Collider as an upgrade of the HL-LHC and the Future Circular
Collider in electron-hadron mode. Compared to collisions at HERA, we find
an extension of the available kinematic range in by a factor of order
and of the maximum by a factor of order for LHeC, while the FCC
version would extend the coverage by a further order of magnitude both in
and . This translates into a range of available momentum fraction of the
diffractive exchange with respect to the hadron (), down to
for a wide range of the momentum fraction of the parton with
respect to the diffractive exchange (). Using the same framework and
methodology employed in previous studies at HERA, considering only the
experimental uncertainties and not those stemming from the functional form of
the initial conditions or other ones of theoretical origin, and under very
conservative assumptions for the luminosities and systematic errors, we find an
improvement in the extraction of diffractive parton densities from fits to
reduced cross sections for inclusive coherent diffraction in by about an
order of magnitude. For , we also perform the simulations for the Electron
Ion Collider. We find that an extraction of the currently unmeasured nuclear
diffractive parton densities is possible with similar accuracy to that in .Comment: 24 pages, 16 figure
Central rapidity densities of charged particles at RHIC and LHC
Predictions on central rapidity densities of charged particles at energies ofthe Relativistic Heavy Ion Collider and the Large Hadron Collider, for centralcollisions between the largest nuclei that will be available at theseaccelerators, are reviewed. Differences among the results of the existingmodels are discussed in relation with their underlying physical basis and withthe possibilities to discriminate them
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