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SUMMARY:Shear viscosity and resonance lifetimes in the hadron gas
DTSTART;VALUE=DATE-TIME:20180515T132000Z
DTEND;VALUE=DATE-TIME:20180515T134000Z
DTSTAMP;VALUE=DATE-TIME:20201124T043237Z
UID:indico-contribution-2870021@indico.cern.ch
DESCRIPTION:Speakers: Jean-Bernard Rose (Frankfurt Institute for Advanced
Studies)\nPrevious calculations of the shear viscosity to entropy density
ratio in the hadron gas have failed to reach a consensus\, with $\\eta/s$
predictions differing by almost an order of magnitude. This work addresses
and solves this discrepancy by providing an independent extraction of thi
s coefficient using the newly-developed SMASH (Simulating Many Accelerated
Strongly interacting Hadrons) transport code and the Green-Kubo formalism
. We compare the results from SMASH with numerical solutions of the Boltzm
ann equation for various systems using the Chapman-Enskog expansion as wel
l as previous results in the literature. Substantial deviations of the coe
fficient are found between transport approaches mainly based on resonance
propagation with finite lifetime (such as SMASH) and other (semi-analytica
l) approaches with energy-dependent cross-sections\, where interactions do
not introduce a timescale other than the inverse scattering rate. Our con
clusion is that long-lived resonances strongly affect the transport proper
ties of the system\, resulting in significant differences in η/s with res
pect to other approaches where binary collisions dominate. We argue that t
he relaxation time of the system —which characterizes the shear viscosit
y— is determined by the interplay between the mean-free time and the lif
etime of resonances. We finally show how an artificial shortening of the r
esonance lifetimes or the addition of a background elastic cross section n
icely interpolate between the two discrepant results. To turn this around\
, we finally note that the temperature dependence of $\\eta/s$ can be used
to constrain the properties of the hadron gas.\n\nhttps://indico.cern.ch/
event/656452/contributions/2870021/
LOCATION:Palazzo del Casinò Sala Volpi\, 1st Floor
URL:https://indico.cern.ch/event/656452/contributions/2870021/
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