5,905 research outputs found

### Momentum spectra of charmonium produced in a quark-gluon plasma

We calculate rapidity and transverse momentum distributions of charmonium
formed in high energy heavy ion collsions from incoherent recombination of
charm quarks. The results are very sensitive to the corresponding distributions
of the charm quarks, and thus can serve as a probe of the state of matter
produced in the heavy ion collision. At one extreme we generate a set of charm
pair momenta directly from pQCD amplitudes, which are appropriate if one can
neglect interaction of the quarks with the medium. At the other extreme we
generate momenta of charm quarks in thermal equilibrium with the expanding
medium, appropriate for an extremely strong interaction. Explicit predictions
are made for J/Psi formation in Au-Au interactions at RHIC. We find that for
the case in which charm quark momenta are unchanged from the pQCD production
calculation, both the rapidity and transverse momentum spectra of the formed
J/Psi are substantially narrower than would be anticipated in scenarios which
do not include the in-medium formation. In particular, the average transverse
momentum of the J/Psi will exhibit a non-monotonic behavior in the progression
from p-p to p-A to A-A interactions.Comment: Final published version, clarifying remarks adde

### QCD tools for the LHC

This contribution provides a pedagogical introduction to and review of the
current status and ongoing progress in the development of Monte Carlo tools for
the calculation and simulation of high-Q^2 processes in hadronic collisions.Comment: Talk at the International Workshop on QCD: QCD@Work 2003 - Conversano
(Italy) 14-18 June 2003 (eConf C030614), 9 page

### LHC physics: the first one--two year(s)

We discuss the strategy to commission the LHC experiments and understand
standard physics at sqrt{s}=14TeV before data taking starts and in the early
phases of the LHC operation. In particular, we review the various steps needed
to understand and calibrate the ATLAS and CMS detectors, from construction
quality checks, to beam tests, to cosmics runs, to first collisions. We also
review the preparation and tuning of Monte Carlo tools, and present a few
examples of physics goals for integrated luminosities of up to a few inverse
fb.Comment: Proceedings of the 2nd Italian Workshop on the physics of Atlas and
CM

### QCD in e+e- collisions at 2 TeV

We discuss some topics in QCD studies at 2 TeV. Particular emphasis is given
to the separation of pure QCD events from the WW and the t-tbar backgroundsComment: 10 pages, Latex, epsfig, 7 figures To appear in the Proceedings of
the 1995 "Workshop on Physics with e+e- Linear Colliders", Annecy-Gran
Sasso-DES

### The Resummation of Soft Gluon in Hadronic Collisions

We compute the effects of soft gluon resummation for the production of high
mass systems in hadronic collisions. We carefully analyse the growth of the
perturbative expansion coefficients of the resummation formula. We propose an
expression consistent with the known leading and next-to-leading resummation
results, in which the coefficients grow much less than factorially. We apply
our formula to Drell--Yan pair production, heavy flavour production, and the
production of high invariant mass jet pairs in hadronic collisions. We find
that, with our formula, resummation effects become important only fairly close
to the threshold region. In the case of heavy flavour production we find that
resummation effects are small in the experimental configurations of practical
interest.Comment: 45 pages, Latex, epsfig, 10 figures. Minor corrections to text,
notation and references. The previously quoted HERAb NLO bottom production
cross section was wrong, and it has been fixed. Accepted for Publication on
Nucl. Phys.

### Top-quark charge asymmetry and polarization in $t\bar{t}W^\pm$ production at the LHC

We study the charge asymmetry between the $t$ and $\bar t$ quark at the LHC,
when they are produced in association with a $W$ boson. Though sizably reducing
the cross section with respect to the inclusive production, requiring a $W$
boson in the final state has two important implications. First, at leading
order in QCD, $t \bar t W^{\pm}$ production can only occur via $q \bar q$
annihilation. As a result, the asymmetry between the $t$ and $\bar t$ generated
at NLO in QCD is significantly larger than that of inclusive $t \bar t$
production, which is dominated by gluon fusion. Second, the top quarks tend to
inherit the polarization of the initial-state quarks as induced by the
$W$-boson emission. Hence, the decay products of the top quarks display a
sizable asymmetry already at the leading order in QCD. We study the relevant
distributions and their uncertainties in the standard model, compare them to
those obtained in a simple axigluon model and discuss prospects for
measurements at the LHC and beyond.Comment: 11 pages (with figures), version accepted by PLB for publicatio

### Top-Antitop-Quark Production and Decay Properties at the Tevatron

At the Tevatron, the collider experiments CDF and DO have data sets at their
disposal that comprise a few thousand reconstructed top-antitop-quark pairs and
allow for precision measurements of the cross section as well as production and
decay properties. Besides comparing the measurements to standard model
predictions, these data sets open a window to physics beyond the standard
model. Dedicated analyses look for new heavy gauge bosons, fourth generation
quarks, and flavor-changing neutral currents. In this mini-review the current
status of these measurements is summarized.Comment: Mini-review to be submitted to Mod. Phys. Lett. A, was derived from
the proceedings of the 21st Rencontres de Blois: Windows on the Universe,
Blois, France, 21. - 27. June 2009. 19 pages. 2nd revision: correct a few
minor mistakes, update references

### Topics in Jet Physics

We review two subjects in the theoretical study of jet production at the
Tevatron collider: the uncertainties in the determination of the partonic
densities inside the proton and the uncertainties in the calculation of
higher-order corrections to the QCD matrix elements.Comment: 12 pages, Latex, epsfig, 7 figure

### Implication of the proton-deuteron radiative capture for Big Bang Nucleosynthesis

The astrophysical $S$-factor for the radiative capture $d(p,\gamma)^3$He in
the energy-range of interest for Big Bang Nucleosynthesis (BBN) is calculated
using an {\it ab-initio} approach. The nuclear Hamiltonian retains both two-
and three-nucleon interactions - the Argonne $v_{18}$ and the Urbana IX,
respectively. Both one- and many-body contributions to the nuclear current
operator are included. The former retain for the first time, besides the $1/m$
leading order contribution ($m$ is the nucleon mass), also the next-to-leading
order term, proportional to $1/m^3$. The many-body currents are constructed in
order to satisfy the current conservation relation with the adopted Hamiltonian
model. The hyperspherical harmonics technique is applied to solve the $A=3$
bound and scattering states. A particular attention is used in this second case
in order to obtain, in the energy range of BBN, an uncertainty on the
astrophysical $S$-factor of the order or below $\sim$1 %. Then, in this energy
range, the $S$-factor is found to be $\sim$10 % larger than the currently
adopted values.Part of this increase (1-3 %) is due to the $1/m^3$ one-body
operator, while the remaining is due to the new more accurate scattering wave
functions. We have studied the implication of this new determination for the
$d(p,\gamma)^3$He $S$-factor on deuterium primordial abundance. We find that
the predicted theoretical value for $^2$H/H is in excellent agreement with its
experimental determination, using the most recent determination of baryon
density of Planck experiment, and with a standard number of relativistic
degrees of freedom $N_{\rm eff}=3.046$ during primordial nucleosynthesis.Comment: 5 pages, 2 figures, submitted to Phys. Rev. Let

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