7 Relativistic Nucleus-Nucleus Collisions and the QCD Matter Phase Diagram
443
7.9.6.2 Lambda Polarization and Fireball Vorticity
Huge global angular momenta are generated in non-central A + A collisions. In fact
they get converted to the vorticity of a QGP [352] and get transmitted to particle
polarization at the stage of hadron formation and particlization whence hadrons are
emitted from the hypersurface of a hydrodynamically expanding fireball (51, 52).
The global angular momentum can thus lead to the local polarization of hadrons.
The polarization, in turn, can be measured by and ¯
hyperon weak decay into a
proton and a pion which is “self-analyzing” since the proton is emitted preferentially
along the direction of the Lambda spin in its rest frame. The global polarization (the
net polarization of the local ones in an event which is aligned in the direction of
the event plane, i.e. along the direction of the angular momentum of the plasma)
of and ¯
has recently been measured by the RHIC STAR Collaboration [355] at
collisional energies below 62.4 GeV. At higher energies, including LHC, the falling
of the polarization with energy [355] still precludes measurement. The measurement
determines the event average of sin(( p − RP ), where p and RP are the
azimuthal angle of the proton momentum in the Lambda rest frame, and that of
the reaction plane. Its orientation cannot be directly measured but is approximated
by the event plane determined from the hadronic directed flow; this is accounted for
by a reaction plane resolution factor [355]. Alternative methods have been proposed
recently [356]. For an overview see ref. [357].
From the data one can estimate [353, 354] the local vorticity in the plasma, the
result implying that the QCD matter created in such collisions is the most vortical
fluid known as of yet. Moreover, this new observable can shed a new, independent
light on the equilibrium properties encountered (or not) in the course of (relativistic
hydro) expansion toward hadronization [358].
7.9.6.3 EOS from Neutron Star Mergers
The recent observation of gravitational waves from a neutron star merger has, in
fact, been anticipated with regard to its possible sensitivity to the EOS of dense
hadronic matter at low temperature and high baryochemical potential. This is a
topic of nuclear collision research already since the early BEVALAC studies at LBL
when it was realized that the EOS should be of importance, both, to the dynamics
of supernova explosions and to the radial structure of neutron stars, also affecting
their maximum mass with regard to black hole formation. The hope was to derive
the EOS from the data concerning hydrodynamic directed sidewards expansion,
and from Kaon production in A + A “compressive collisions” [359] at the low
BEVALAC energies, in the GeV per nucleon energy range. One of the key problems
arose from the fact that even at these energies the collisional fireball temperature
ranges up to about 100 MeV whereas the above astrophysical phenomena require the
EOS of cold, compressed matter [360]. The newly accessible Neutron star mergers
promise to open up a window to the low T EOS [361], and to crucial transport
443
7.9.6.2 Lambda Polarization and Fireball Vorticity
Huge global angular momenta are generated in non-central A + A collisions. In fact
they get converted to the vorticity of a QGP [352] and get transmitted to particle
polarization at the stage of hadron formation and particlization whence hadrons are
emitted from the hypersurface of a hydrodynamically expanding fireball (51, 52).
The global angular momentum can thus lead to the local polarization of hadrons.
The polarization, in turn, can be measured by and ¯
hyperon weak decay into a
proton and a pion which is “self-analyzing” since the proton is emitted preferentially
along the direction of the Lambda spin in its rest frame. The global polarization (the
net polarization of the local ones in an event which is aligned in the direction of
the event plane, i.e. along the direction of the angular momentum of the plasma)
of and ¯
has recently been measured by the RHIC STAR Collaboration [355] at
collisional energies below 62.4 GeV. At higher energies, including LHC, the falling
of the polarization with energy [355] still precludes measurement. The measurement
determines the event average of sin(( p − RP ), where p and RP are the
azimuthal angle of the proton momentum in the Lambda rest frame, and that of
the reaction plane. Its orientation cannot be directly measured but is approximated
by the event plane determined from the hadronic directed flow; this is accounted for
by a reaction plane resolution factor [355]. Alternative methods have been proposed
recently [356]. For an overview see ref. [357].
From the data one can estimate [353, 354] the local vorticity in the plasma, the
result implying that the QCD matter created in such collisions is the most vortical
fluid known as of yet. Moreover, this new observable can shed a new, independent
light on the equilibrium properties encountered (or not) in the course of (relativistic
hydro) expansion toward hadronization [358].
7.9.6.3 EOS from Neutron Star Mergers
The recent observation of gravitational waves from a neutron star merger has, in
fact, been anticipated with regard to its possible sensitivity to the EOS of dense
hadronic matter at low temperature and high baryochemical potential. This is a
topic of nuclear collision research already since the early BEVALAC studies at LBL
when it was realized that the EOS should be of importance, both, to the dynamics
of supernova explosions and to the radial structure of neutron stars, also affecting
their maximum mass with regard to black hole formation. The hope was to derive
the EOS from the data concerning hydrodynamic directed sidewards expansion,
and from Kaon production in A + A “compressive collisions” [359] at the low
BEVALAC energies, in the GeV per nucleon energy range. One of the key problems
arose from the fact that even at these energies the collisional fireball temperature
ranges up to about 100 MeV whereas the above astrophysical phenomena require the
EOS of cold, compressed matter [360]. The newly accessible Neutron star mergers
promise to open up a window to the low T EOS [361], and to crucial transport