1,355 research outputs found
Model-independent constraints on the shape parameters of dilepton angular distributions
The coefficients determining the dilepton decay angular distribution of
vector particles obey certain positivity constraints and a rotation-invariant
identity. These relations are a direct consequence of the covariance properties
of angular momentum eigenstates and are independent of the production
mechanism. The Lam-Tung relation can be derived as a particular case, simply
recognizing that the Drell-Yan dilepton is always produced transversely
polarized with respect to one or more quantization axes. The dilepton angular
distribution continues to be characterized by a frame-independent identity also
when the Lam-Tung relation is violated. Moreover, the violation can be easily
characterized by measuring a one-dimensional distribution depending on one
shape coefficient.Comment: 7 page
DD correlations as a sensitive probe for thermalization in high-energy nuclear collisions
We propose to measure azimuthal correlations of heavy-flavor hadrons to address the status of thermalization at the partonic stage of light quarks and gluons in high-energy nuclear collisions. In particular, we show that hadronic interactions at the late stage cannot significantly disturb the initial back-to-back azimuthal correlations of DDbar pairs. Thus, a decrease or the complete absence of these initial correlations does indicate frequent interactions of heavy-flavor quarks and also light partons in the partonic stage, which are essential for the early thermalization of light partons
Open questions in quarkonium and electromagnetic probes
In my ("not a summary") talk at the Hard Probes 2006 conference, I gave "a
personal and surely biased view on only a few of the many open questions on
quarkonium and electromagnetic probes". Some of the points reported in that
talk are exposed in this paper, having in mind the most important of all the
open questions: do we have, today, from experimental data on electromagnetic
probes and quarkonium production, convincing evidence that shows, beyond
reasonable doubt, the existence of "new physics" in high-energy heavy-ion
collisions?Comment: Invited talk at the 2nd Int. Conf. on Hard and EM Probes of
High-Energy Nuclear Collisions, Asilomar, California, June 9--16, 2006. To be
published in Nuclear Physics A. Late submission to the arXi
Quarkonium production in the LHC era: a polarized perspective
Polarization measurements are usually considered as the most difficult challenge for the QCD description of quarkonium production. In fact, global data fits for the determination of the non-perturbative parameters of bound-state formation traditionally exclude polarization observables and use them as a posteriori verifications of the predictions, with perplexing results. With a change of perspective, we move polarization data to the centre of the study, advocating that they actually provide the strongest fundamental indications about the production mechanisms, even before we explicitly consider perturbative calculations.Peer Reviewe
Determination of chi_c and chi_b polarizations from dilepton angular distributions in radiative decays
The angular distributions of the decay products in the successive decays
chi_c (chi_b) to J/psi (Upsilon) gamma and J/psi (Upsilon) to l+l- are
calculated as a function of the angular momentum composition of the decaying
chi meson and of the multipole structure of the photon radiation, using a
formalism independent of production mechanisms and polarization frames. The
polarizations of the chi states produced in high energy collisions can be
derived from the dilepton decay distributions of the daughter J/psi or Upsilon
mesons, with a reduced dependence on the details of the photon reconstruction
or simulation. Moreover, this method eliminates the dependence of the
polarization measurement on the actual details of the multipole structure of
the radiative transition. Problematic points in previous calculations of the
chi_c decay angular distributions are identified and clarified.Comment: Submitted to Phys Rev
D-Dbar Correlations as a sensitive probe for thermalization in high-energy nuclear collisions
We propose to measure azimuthal correlations of heavy-flavor hadrons to
address the status of thermalization at the partonic stage of light quarks and
gluons in high-energy nuclear collisions. In particular, we show that hadronic
interactions at the late stage cannot significantly disturb the initial
back-to-back azimuthal correlations of DDbar pairs. Thus, a decrease or the
complete absence of these initial correlations does indicate frequent
interactions of heavy-flavor quarks and also light partons in the partonic
stage, which are essential for the early thermalization of light partons.Comment: 5 pages, 4 figures. Version accepted for publication in Phys.Lett.
Latest results from NA60
The NA60 experiment has measured the production of muon pairs and of charged
particles in In+In collisions at a beam energy of 158 AGeV. For invariant
dimuon masses below the phi the space-time averaged rho spectral function was
isolated by a novel procedure. It shows a strong broadening but essentially no
shift in mass. The production of J/psi was measured as a function of the
collision centrality. As in previous experiments studying Pb+Pb collisions an
anomalous supression is observed, setting in at approximately 90 participant
nucleons. Using the charged particles the reaction plane was reconstructed. The
elliptic flow of charged particles increases with pt showing a saturation for
pt > 2GeV/c. For the first time azimuthal distributions for J/psi are shown.Comment: 9 pages, 11 figures, talk given at the conference "Strangeness in
Quark Matter 2006 (SQM2006)", March 2006, Los Angeles, USA, accepted for
publication in Journal of Physics
Evidence for radial flow of thermal dileptons in high-energy nuclear collisions
The NA60 experiment at the CERN SPS has studied low-mass dimuon production in
158 AGeV In-In collisions. An excess of pairs above the known meson decays has
been reported before. We now present precision results on the associated
transverse momentum spectra. The slope parameter Teff extracted from the
spectra rises with dimuon mass up to the rho, followed by a sudden decline
above. While the initial rise is consistent with the expectations for radial
flow of a hadronic decay source, the decline signals a transition to an
emission source with much smaller flow. This may well represent the first
direct evidence for thermal radiation of partonic origin in nuclear collisions.Comment: Accepted for publication in Physical Review Letter
First Measurement of the rho Spectral Function in High-Energy Nuclear Collisions
We report on a precision measurement of low-mass muon pairs in 158 AGeV
indium-indium collisions at the CERN SPS. A significant excess of pairs is
observed above the yield expected from neutral meson decays. The unprecedented
sample size of 360 000 dimuons and the good mass resolution of about 2% allow
us to isolate the excess by subtraction of the decay sources. The shape of the
resulting mass spectrum is consistent with a dominant contribution from
pi+pi-->rho-->mu+mu- annihilation. The associated space-time averaged rho
spectral function shows a strong broadening, but essentially no shift in mass.
This may rule out theoretical models linking hadron masse
Low Mass Dimuon Production in Indium-Indium Collisions at the CERN SPS
NA60 is a fixed-target experiment at the CERN SPS which measured dimuon
production in nucleus-nucleus and proton-nucleus collisions. In this paper we
report on a precision measurement of low-mass muon pairs in 158 AGeV
indium-indium collisions. A significant excess of pairs is observed above the
yield expected from neutral meson decays. The excess can be isolated by
subtraction of expected sources, thanks to the excellent mass resolution and
large sample size.Comment: 4 pages, 3 figures, Contribution at XLIst Rencontres de Moriond, "QCD
and High Energy Hadronic Interactions
- …