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SUMMARY:Imprinting quantum fluctuations on hydrodynamic initial conditions
DTSTART;VALUE=DATE-TIME:20120815T154000Z
DTEND;VALUE=DATE-TIME:20120815T160000Z
DTSTAMP;VALUE=DATE-TIME:20200927T203627Z
UID:indico-contribution-308513@indico.cern.ch
DESCRIPTION:Speakers: J. Scott Moreland (Duke University)\nThe precise val
ue of the QGP kinematic shear viscosity eta/s is a question of intense top
ical interest. Viscous hydrodynamic simulations are a tool for extracting
this information from experiment. The key observables are the anisotropic
flow coefficients v_n which (i) can be measured very precisely and (ii) ar
e very sensitive to eta/s which controls the "conversion efficiency" v_n/e
cc_n for turning initial fireball eccentricities of harmonic order n into
final flows of the same harmonic order. \n\nBoth ecc_n and v_n fluctuate s
trongly from collision to collision. These event-by-event fluctuations hav
e a key influence on the measurements [1] and must be properly taken into
account when extracting eta/s. Until recently\, most initial-state models
accounted only for the shape and density fluctuations arising from the flu
ctuating positions of the nucleons in the colliding nuclei. This leads to
fluctuations in the location of the newly produced matter\, and thus of th
e initial energy density profile of the expanding fireball and its eccentr
icities ecc_n\, but does not account for additional quantum fluctuations i
n the quark and gluon fields inside the nucleons that lead to fluctuating
numbers of secondary particles per nucleon-nucleon interaction. Several re
cent papers have addressed the implementation of these quantum fluctuation
s in the hydrodynamic initial conditions for the expanding collision fireb
all. \n\nStarting from the Monte Carlo Kharzeev-Levin-Nardi (MC-KLN) model
for generating fluctuating initial profiles for the gluon saturation mome
ntum Q_sat(x_T) in the transverse plane\, we have developed a Monte Carlo
algorithm that uses a Gaussian Random Field (GRF) generator [2] to generat
e a distribution of gluonic energy densities centered at the value corresp
onding to the field Q_sat(x_T)\, but fluctuating around this profile with
the two-point covariance function derived in [3] from the Glasma model. To
ensure that the energy density is everywhere positive the GRF is mapped t
o an appropriate negative binomial distribution (NBD) with the same varian
ce. NBD fluctuations have been recently shown to arise naturally from the
Glasma model and to describe the measured multiplicity distributions in pp
collisions at the LHC. The resulting density profile features ``hot spots
" as in the MC-KLN model overlaid with a fluctuating field texture charact
erized by an intrinsic length scale 1/Q_sat(x_T).\n \nWe show that inclusi
on of these additional gluonic quantum field fluctuations leads to only a
small (few percent) increase of the initial eccentricities ecc_n in centra
l collisions and to almost negligible effects at larger impact parameters.
These findings disagree with some of the results reported in [4]\, and we
will discuss possible origins for this discrepancy. Our results imply tha
t an earlier extraction of the QGP shear viscosity from a combined analysi
s of elliptic and triangular flow data from Pb-Pb collisions at the LHC [5
] is robust.\n\nReferences\n \n[1] Zhi Qiu and U. Heinz\, ``Event-by-eve
nt shape and flow fluctuations of relativistic heavy-ion collision firebal
ls\,'' Phys. Rev. C84\, 024911 (2011).\n\n[2] X. Emery and C. Lantuejoul\,
``TBSIM: A computer program for conditional simulation of 3-dimensional G
aussian random fields via the turning bands method\," Computers and Geosci
ences 32\, 1615 (2006).\n\n[3] B. Muller and A. Schafer\, ``Transverse ene
rgy density fluctuations in the Color Glass Condensate Model\,'' arXiv:111
1.3347 [hep-ph].\n\n[4] B. Schenke\, P. Tribedy and R. Venugopalan\, ``Flu
ctuating Glasma initial conditions and flow in heavy ion collisions\,'' ar
Xiv:1202.6646 [nucl-th]\; F. Gelis\, T. Lappi and L. McLerran\, ``Glitteri
ng Glasmas\,'' Nucl. Phys. A 828\, 149 (2009)\; A. Dumitru and Y. Nara\, `
`KNO scaling of fluctuations in pp and pA\, and eccentricities in heavy-io
n collisions\,'' Phys. Rev. C85\, 034907 (2012).\n\n[5] Z. Qiu\, C. Shen a
nd U. Heinz\, ``Hydrodynamic elliptic and triangular flow in Pb-Pb collisi
ons at sqrt(s)=2.76ATeV\,'' Phys. Lett. B707\, 151 (2012).\n\nhttps://indi
co.cern.ch/event/181055/contributions/308513/
LOCATION: Empire
URL:https://indico.cern.ch/event/181055/contributions/308513/
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