413 research outputs found
Evidence for non-hadronic interactions of charm degrees of freedom in heavy-ion collisions at relativistic energies
Within the Hadron-String Dynamics (HSD) transport approach we study the
suppression pattern of charmonia at RHIC with respect to centrality and
rapidity employing various model concepts such as variants of the 'comover
absorption' model or the 'charmonium melting' scenario. We find that especially
the ratio of the forward to mid-rapidity nuclear modification factors of J/Psi
(R_AA (forward) / R_AA (mid)) cannot be explained by the interactions with
'formed' comoving mesons or by the 'color screening mechanism' alone. Only when
incorporating interactions of the c or c-bar quark with a pre-hadronic medium
satisfactory results are obtained. A detailed comparison to the PHENIX data
demonstrates that non-hadronic interactions are mandatory to describe the
narrowing of the J/Psi rapidity distribution from p+p to central Au+Au
collisions. The Psi' to J/Psi ratio is found to be crucial in disentangling the
different charmonium absorption scenarios especially in the RHIC energy range.
Furthermore, a comparison of the transport calculations to the statistical
model of Gorenstein and Gazdzicki as well as the statistical hadronization
model of Andronic et al. shows differences in the energy dependence as well as
centrality dependence of the J/Psi to pion ratio which may be exploited
experimentally to disentangle different concepts. We find additionally that the
collective flow of charm in the HSD transport appears compatible with the data
at SPS energies but substantially underestimates the data at top RHIC energies
such that the large elliptic flow v_2 of charm seen experimentally has to be
attributed to early interactions of non-hadronic degrees of freedom.Comment: 35 pages, 16 Figs, v2: additional figure and corresponding changes to
the tex
Robust model-based indicators of regional differences in food-web structure in the Southern Ocean
Efforts to model marine food-webs are generally undertaken by small teams working separately on specific regions (<106 km2) and making independent decisions about how to deal with data gaps and uncertainties. Differences in these largely arbitrary decisions (which we call ‘model personality’) can potentially obscure true differences between regional food-webs or lead to spurious differences. Here we explore the influence of model personality on a comparison of four Southern Ocean regional food-web models. We construct alternative model versions which sequentially remove aspects of personality (alternative model ‘currencies’, schemes for aggregating organisms into functional groups, and energetic parameter values). These alternative versions preserve regional differences in biomass and feeding relationships. Variation in a set of model metrics that are insensitive to absolute biomass and production identifies multiple regional contrasts, a subset of which are robust to differences in model personality. These contrasts imply real differences in ecosystem structure which, in conjunction with differences in primary production and consumer biomass (spanning two and four orders of magnitude respectively), underpin differences in function. Existing regional models are therefore a useful resource for comparing ecosystem structure, function and response to change if comparative studies assess and report the influence of model personality
J/Psi suppression in colliding nuclei: statistical model analysis
We consider the suppression at a high energy heavy ion collision. An
ideal gas of massive hadrons in thermal and chemical equilibrium is formed in
the central region. The finite-size gas expands longitudinally in accordance
with Bjorken law. The transverse expansion in a form of the rarefaction wave is
taken into account. We show that suppression in such an environment,
when combined with the disintegration in nuclear matter, gives correct
evaluation of NA38 and NA50 data in a broad range of initial energy densities.Comment: 14 pages, 13 figures. Accepted for publication in Phys. Rev.
Charmonium dynamics in nucleus-nucleus collisions at SPS and FAIR energies
Charmonium production and suppression in In+In and Pb+Pb reactions at SPS
energies is investigated with the HSD transport approach within the 'hadronic
comover model' as well as the 'QGP threshold scenario'. The results of the
transport calculations for J/Psi suppression and the Psi prime to J/Psi ratio
are compared with the recent data of the NA50 and NA60 Collaborations. We find
that the comover absorption model - with a single parameter |M_0|^2 for the
matrix element squared for charmonium-meson dissociation - performs best with
respect to all data sets. The 'threshold scenario' - within different
assumptions for the melting energy densities - yields a reasonable suppression
for J/Psi but fails in reproducing the Psi prime to J/Psi ratio for Pb+Pb at
158 A GeV. Predictions for Au+Au reactions are presented for a bombarding
energy of 25 A GeV in the different scenarios which will allow for a clear
distinction between the models from the experimental side at the future FAIR
facility.Comment: 22 pages, 10 figures. v2: comments added according to referee
suggestions; references updated; Nucl. Phys. A, in pres
Excitation functions of hadronic observables from SIS to RHIC energies
We calculate excitation functions for various dynamical quantities as well as
experimental observables from SIS to RHIC energies within the HSD transport
approach which is based on string, quark, diquark () and hadronic degrees of freedom without including any explicit
phase transition to a quark-gluon plasma (QGP). It is argued that the failure
of this more 'conventional' approach in comparison to experimental data should
indicate the presence of a different phase which might be either attributed to
space-time regions of vanishing scalar quark condensate ( = 0) or
to the presence of a QGP phase with strongly interacting partons. We study the
ratio, the low mass dilepton enhancement in the invariant mass regime
from 0.2 -- 1.2 GeV as well as charmonium suppression for central Au + Au
collisions as a function of the bombarding energy and present predictions for
these observables as well as hadron rapiditiy distributions at RHIC energies.
Whereas all observables studied within HSD show smooth increasing/decreasing
excitation functions, the experimental ratio indicates a maximum at
11 AGeV (or above) which is interpreted as a signature for a chirally
restored phase in the course of the reaction.Comment: 34 pages, LaTeX, including 14 postscript figures (high quality color
versions of figs. 3,4 are available from
http://theorie.physik.uni-giessen.de/~brat/own.html), Nucl. Phys.
Measurement of the polarisation of W bosons produced with large transverse momentum in pp collisions at sqrt(s) = 7 TeV with the ATLAS experiment
This paper describes an analysis of the angular distribution of W->enu and
W->munu decays, using data from pp collisions at sqrt(s) = 7 TeV recorded with
the ATLAS detector at the LHC in 2010, corresponding to an integrated
luminosity of about 35 pb^-1. Using the decay lepton transverse momentum and
the missing transverse energy, the W decay angular distribution projected onto
the transverse plane is obtained and analysed in terms of helicity fractions
f0, fL and fR over two ranges of W transverse momentum (ptw): 35 < ptw < 50 GeV
and ptw > 50 GeV. Good agreement is found with theoretical predictions. For ptw
> 50 GeV, the values of f0 and fL-fR, averaged over charge and lepton flavour,
are measured to be : f0 = 0.127 +/- 0.030 +/- 0.108 and fL-fR = 0.252 +/- 0.017
+/- 0.030, where the first uncertainties are statistical, and the second
include all systematic effects.Comment: 19 pages plus author list (34 pages total), 9 figures, 11 tables,
revised author list, matches European Journal of Physics C versio
Observation of a new chi_b state in radiative transitions to Upsilon(1S) and Upsilon(2S) at ATLAS
The chi_b(nP) quarkonium states are produced in proton-proton collisions at
the Large Hadron Collider (LHC) at sqrt(s) = 7 TeV and recorded by the ATLAS
detector. Using a data sample corresponding to an integrated luminosity of 4.4
fb^-1, these states are reconstructed through their radiative decays to
Upsilon(1S,2S) with Upsilon->mu+mu-. In addition to the mass peaks
corresponding to the decay modes chi_b(1P,2P)->Upsilon(1S)gamma, a new
structure centered at a mass of 10.530+/-0.005 (stat.)+/-0.009 (syst.) GeV is
also observed, in both the Upsilon(1S)gamma and Upsilon(2S)gamma decay modes.
This is interpreted as the chi_b(3P) system.Comment: 5 pages plus author list (18 pages total), 2 figures, 1 table,
corrected author list, matches final version in Physical Review Letter
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