67 research outputs found
Landau-Pomeranchuk-Migdal resummation for dilepton production
We consider the thermal emission rate of dileptons from a QCD plasma in the
small invariant mass (Q^2 \sim \gs^2 T^2) but large energy (q^0 \gsim T)
range. We derive an integral equation which resums multiple scatterings to
include the LPM effect; it is valid at leading order in the coupling. Then we
recast it as a differential equation and show a simple algorithm for its
solution. We present results for dilepton rates at phenomenologically
interesting energies and invariant masses.Comment: 19 pages, 7 postscript figures, test program available at
http://www-spht.cea.fr/articles/T02/150/libLPM
Two-loop Compton and annihilation processes in thermal QCD
We calculate the Compton and annihilation production of a soft static lepton
pair in a quark-gluon plasma in the two-loop approximation. We work in the
context of the effective perturbative expansion based on the resummation of
hard thermal loops. Double counting is avoided by subtracting appropriate
counterterms. It is found that the two-loop diagrams give contributions of the
same order as the one-loop diagram. Furthermore, these contributions are
necessary to obtain agreement with the naive perturbative expansion in the
limit of vanishing thermal masses.Comment: Latex, 24 pages, postscript figures included with the package
graphic
A simple sum rule for the thermal gluon spectral function and applications
In this paper, we derive a simple sum rule satisfied by the gluon spectral
function at finite temperature. This sum rule is useful in order to calculate
exactly some integrals that appear frequently in the photon or dilepton
production rate by a quark gluon plasma. Using this sum rule, we rederive
simply some known results and obtain some new results that would be extremely
difficult to justify otherwise. In particular, we derive an exact expression
for the collision integral that appears in the calculation of the
Landau-Pomeranchuk-Migdal effect.Comment: 24 latex pages, 2 postscript figure
KLN theorem, magnetic mass, and thermal photon production
We study the infrared singularities associated to ultra-soft transverse
gluons in the calculation of photon production by a quark-gluon plasma. Despite
the fact that the KLN theorem works in this context and provides cancellations
of infrared singularities, it does not prevent the production rate of low
invariant mass dileptons to be sensitive to the magnetic mass of gluons and
therefore the rate to be non perturbative.Comment: 9 pages Latex document, 5 postscript figures, modified figure 5 and
slightly updated section
Comments on two papers by Kapusta and Wong
We critically examine recently published results on the thermal production of
massive vector bosons in a quark-gluon plasma. We claim the production rate is
a collinear safe observable.Comment: 6 pages LATEX documen
Landau-Pomeranchuk-Migdal effect in thermal field theory
In recent studies, the production rate of photons or lepton pairs by a quark
gluon plasma has been found to be enhanced due to collinear singularities. This
enhancement pattern is very dependent on rather strict collinearity conditions
between the photon and the quark momenta. It was estimated by neglecting the
collisional width of quasi-particles. In this paper, we study the modifications
of this collinear enhancement when we take into account the possibility for the
quarks to have a finite mean free path. Assuming a mean free path of order
, we find that only low invariant mass photons are
affected. The region where collision effects are important can be interpreted
as the region where the Landau-Pomeranchuk-Migdal effect plays a role in
thermal photon production by bremsstrahlung. It is found that this effect
modifies the spectrum of very energetic photons as well. Based on these results
and on a previous work on infrared singularities, we end this paper by a
reasonable physical picture for photon production by a quark gluon plasma, that
should be useful to set directions for future technical developments.Comment: 28 pages Latex document, 9 postscript figures, typos corrected,
semantics cleanup, final version to appear in Phys. Rev.
Sensitivity to millicharged particles in future proton-proton collisions at the LHC with the milliQan detector
We report on the expected sensitivity of dedicated scintillator-based
detectors at the LHC for elementary particles with charges much smaller than
the electron charge. The dataset provided by a prototype scintillator-based
detector is used to characterise the performance of the detector and provide an
accurate background projection. Detector designs, including a novel slab
detector configuration, are considered for the data taking period of the LHC to
start in 2022 (Run 3) and for the high luminosity LHC. With the Run 3 dataset,
the existence of new particles with masses between 10 MeV and 45 GeV could be
excluded at 95% confidence level for charges between 0.003e and 0.3e, depending
on their mass. With the high luminosity LHC dataset, the expected limits would
reach between 10 MeV and 80 GeV for charges between 0.0018e and 0.3e, depending
on their mas
Measurements of the differential cross sections of the production of Z + jets and Îł + jets and of Z boson emission collinear with a jet in pp collisions at = 13 TeV
Measurements of the differential cross sections of Z + jets and Îł + jets production, and their ratio, are presented as a function of the boson transverse momentum. Measurements are also presented of the angular distribution between the Z boson and the closest jet. The analysis is based on pp collisions at a center-of-mass energy of 13 TeV corresponding to an integrated luminosity of 35.9 fbâ1 recorded by the CMS experiment at the LHC. The results, corrected for detector effects, are compared with various theoretical predictions. In general, the predictions at higher orders in perturbation theory show better agreement with the measurements. This work provides the first measurement of the ratio of the differential cross sections of Z + jets and Îł + jets production at 13 TeV, as well as the first direct measurement of Z bosons emitted collinearly with a jet
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