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

Top-quark charge asymmetry and polarization in ttˉW±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

Production of a Higgs pseudoscalar plus two jets in hadronic collisions

We consider the production of a Higgs pseudoscalar accompanied by two jets in hadronic collisions. We work in the limit that the top quark is much heavier than the Higgs pseudoscalar and use an effective Lagrangian for the interactions of gluons with the pseudoscalar. We compute the amplitudes involving: 1) four gluons and the pseudoscalar, 2) two quarks, two gluons and the pseudoscalar and 3) four quarks and the pseudoscalar. We find that the pseudoscalar amplitudes are nearly identical to those for the scalar case, the only differences being the overall size and the relative signs between terms. We present numerical cross sections for proton-proton collisions with center-of-mass energy 14 TeV.Comment: 12 pages, LaTeX, 4 Postscript figures, submitted to Phys. Rev.

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

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

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.

Inconstant Planck's constant

Motivated by the Dirac idea that fundamental constant are dynamical variables and by conjectures on quantum structure of spacetime at small distances, we consider the possibility that Planck constant $\hbar$ is a time depending quantity, undergoing random gaussian fluctuations around its measured constant mean value, with variance $\sigma^2$ and a typical correlation timescale $\Delta t$. We consider the case of propagation of a free particle and a one--dimensional harmonic oscillator coherent state, and show that the time evolution in both cases is different from the standard behaviour. Finally, we discuss how interferometric experiments or exploiting coherent electromagnetic fields in a cavity may put effective bounds on the value of $\tau= \sigma^2 \Delta t$.Comment: To appear on the International Journal of Modern Physics