Transverse Polarization in STAR Au-Au Collisions Polarization definition and models E896 results STAR analysis STAR results Significance of results Future analysis R. Bellwied for M. Castro
Dec 21, 2015
Transverse Polarization in STAR Au-Au Collisions
Polarization definition and models E896 results STAR analysis STAR results Significance of results Future analysis
R. Bellwied for M. Castro R. Bellwied for M. Castro
What is transverse polarization ?
Preferred spin direction of perpendicular to reaction plane Measure angular distribution of decay proton in rest frame of
dN/dcos = A(cos(1+Pcos) = 0.65 (-decay asymmetry
parameter = s- and p-wave interference term)
A(cos) = detector acceptance P = polarization + or - cos = spin up or down
R. Bellwied, STAR meeting 6/01
R. Bellwied, STAR meeting 6/01
Models on the Quark Level
Parton Recombination Model polarization due to Thomas precession in quark recombination process valence di-quark recombines with sea s-quark slow sea s-quark has transverse momentum component strong push in longitudinal direction from recombination spin of s-quark interacts with Thomas precession vector
Lund Model color field between di-quark and collision region color field materializes into s-sbar pair generate pair has angular momentum perpendicular to string which
has to be compensated by spin of pair
R. Bellwied, STAR meeting 6/01
R. Bellwied, STAR meeting 6/01
Models on the Quark Level
Quark Fragmentation Model polarization due fragmentation of a color field s-quark (D. Boer et al.)
Regge Model polarization due to intermediate baryon dissociation or
electromagnetic decay of intermediate SU(3) baryon one pion exchange model polarization can be introduced in final state interaction
R. Bellwied, STAR meeting 6/01R. Bellwied, STAR meeting 6/01
Heavy Ion Models
Disappearance of polarization due to QGP color field in QGP possesses momentum color field provides transverse momentum to s-sbar pair no correlation between inherent transverse momentum and spin
of s-quark
Simple rescattering any final state interaction of the Lambda itself or the decay proton
can potentially destroy the preferred spin direction measurements at high transverse momentum and high
Feynman-x should reduce rescattering probability
R. Bellwied, STAR meeting 6/01
R. Bellwied, STAR meeting 6/01
Key measurements in pp and pA
pt-dependence xf-dependence
R. Bellwied, STAR meeting 6/01
R. Bellwied, STAR meeting 6/01
Summary of HEP results
is negatively polarized with respect to production plane for pt < 1 GeV/c the polarization is linearly increasing with p t
with increasing slope as a function of xf
for pt > 1 GeV/c the polarization is constant with pt but still increases linear as a function of xf
effects due to incident energy and target size (in pA reactions) are small (P = -1.1 A -0.15)
other hyperons: neg.P and decrease with xf, pos.P and constant with xf, Anti- and Anti-: P=0, Anti- shows polarization
R. Bellwied, STAR meeting 6/01
R. Bellwied, STAR meeting 6/01
E896-SDDA polarization results
cos in 5 xf bins
xf = 0.1-0.2 , <pt>=0.57 GeV/c
xf = 0.2-0.3, <pt>=0.79 GeV/c
xf = 0.3-0.4, <pt>=1.23 GeV/c
xf = 0.4-0.5, <pt>=1.61 GeV/c
xf > 0.5, <pt>=1.98 GeV/c
R. Bellwied, STAR meeting 6/01
R. Bellwied, STAR meeting 6/01
E896-SDDA polarization results
R. Bellwied, STAR meeting 6/01
R. Bellwied, STAR meeting 6/01
R a n g e P in %
xf = 0.1 – 0.2 -0.3 +- 6.0 %
xf = 0.2 – 0.3 -2.1 +- 3.2 %
xf = 0.3 – 0.4 -4.5 +- 3.9 %
xf = 0.4 – 0.5 -14.1 +- 5.9 %
xf > 0.5 -25.7 +- 7.6 %
Can we measure polarization in STAR ?
R. Bellwied, STAR meeting 6/01
R. Bellwied, STAR meeting 6/01
detector is perfectly symmetric
beam direction for generated Lambda is unknown, choose beam always in +z direction
if Lambda is polarized the polarization would cancel out
solution: do an ‘asymmetric’ analysis
STAR data analysis
540,000 central Au-Au events
apply Matt’s cuts
around 70,000
R. Bellwied, STAR meeting 6/01
R. Bellwied, STAR meeting 6/01
STAR cos () distribution (uncorrected)
R. Bellwied, STAR meeting 6/01
R. Bellwied, STAR meeting 6/01
Cause for detector response function
simulation input (y=+-0.5) 1 hit on positive + negative
1 hit on positive only MC – RC associated, no cuts
Detector response function summary
symmetric shape of response function due to loss of one of the two daughters even if the is within y=+-0.5
additional asymmetric component due to magnetic field and cuts in V0 reconstruction
R. Bellwied, STAR meeting 6/01
R. Bellwied, STAR meeting 6/01
STAR polarization: P = -1.5% +- 1.8%
All in central events integrated over pt and xf
and corrected for detector response function
STAR polarization results
cos in 5 pt bins xf = 0.01 – 0.04
pt = 0 - 0.5 GeV/c
pt = 0.5 – 1.0 GeV/c
pt = 1.0 – 1.5 GeV/c
pt = 1.5 – 2.0 GeV/c
pt = 2.0 – 5.0 GeV/c
STAR pt dependent polarization
apparent polarization effect at low momentum (statistically significant) (9.9%+-2.5% for pt =0.5-1.0 GeV/c)
fake? systematic? due to low pt sweep?
has to be wrong because of symmetry argument
R. Bellwied, STAR meeting 6/01
R. Bellwied, STAR meeting 6/01
How to measure polarization in
STAR
R. Bellwied, STAR meeting 6/01
R. Bellwied, STAR meeting 6/01
do ‘asymmetric analysis’ by cutting on either rapidity or xf (if xf = p / pb instead of xf = |p| / |pb|)
we cut on rapidity ( from 0.2 to 0.5 and –0.2 to –0.5)
if polarization, then it should change sign (but not magnitude) for the two rapidity bins