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Chiral Magnet Effect, where are we?
1. Measure Charge Separation
Zhangbu Xu
For the STAR Collaboration
QCD Topology Charge
2. Signal vs background study
(final-stage v2, initial colliding systems, rapidity, PID)
3. Dissect the necessary conditions
•
•
Chiral Symmetry Restoration
Strong Magnetic Field
4. Future Plans
Rencontres de Moriond: QCD and High Energy Interactions
LA THUILE, March 25- April 1, 2017
Particle Identification at STAR
TPC
TP
C K
π
p
TOF
TPC
d
e, μ
TOF
Log10(p)
Charged
hadrons
Hyperons &
Hyper-nuclei
EM particles
MTD
HFT
EMC
Jets
Jets &
Correlations
High pT
muons
Heavy-flavor
hadrons
Forward
protons
Forward
photons
Multiple-fold correlations for identified particles!
2
Observing Topological Charge Transitions
To observe in the lab
- add massless fermions
- apply a magnetic field
Paul Sorensen: QM2017
CME task force report:
arXiv: 1608.00982
PRC 81 (2010) 54908
PRL 103 (2009) 251601
A required set of Extraordinary Phenomena:
QCD Topological Charge
+ Chiral Symmetry Restoration
+ Strong Magnetic Field
Observable:
Chirally restored quarks separated
along magnetic field
Experimental strategy:
Measure 2 particle correlations (++,--,+-)
WRT reaction plane
Derek Leinweber, University of Adelaide
3
3
P. Tribedy, QM2017
4
(gOS-gSS) ´ 10
Obs
0.3
0.25
Charge separation depends on final-stage shape v•2 Str
0.2
0.15
P. Tribedy, UCLA Chirality Workshop 2016
0.1
0
(gOS-gSS) ´ 104
0.05
0.025
0.5
ce
STAR U+U 193 GeV 0-10%
Dh>0.025
Multiplicity
binning
Spectator binning
0.035
0.045
STAR
Au+Au
200
GeV
v2 {2}
Year
2004 (0909.1717)
Year 2007 (1302.3802)
Year 2011 (0-1%)
STAR
Dominancepreliminary
of
fluctuations of particip
0
Number of participants
Azimuthal anisotropy (v2)
contributes to background (could
be very large); PRC89(2014)
 magnetic field which drives the
signal, Qualitatively have similar
centrality dependence.
Most comparisons and disentangle
tools have to be quantitative.

0
0.01
0.02
0.03
v2 {2}
0.04
14
0.05
U+U and Au+Au central data:
different dependence on v2;
Not just driven by final-stage
background correlations?
P.Tribedy, QCD chirality
5
Charge Separation depends on initial systems
Same-sign
Peripheral A+A
p+Au and d+Au
qualitatively similar
0.002
same-sign 30-40% U+U 193 GeV
0
ácos(f1 + f2 - 2f3)ñ ´ Npart
magnitude of charge
separation
dependence on
correlation
conditions
(rapidity gaps)
Qualitatively different
rapidity distribution
from central to
peripheral A+A (p+A)
-0.002
STAR preliminary
70-80% U+U 193 GeV
0.004
0.002
0
-0.002
fit
Short-range-positive
Residual
0.004
0.002
0
-0.002
0-100% p+Au 200 GeV
0.4
0.8
1.2
1.6
Dh12
6
P.Tribedy,
Separation appears in many forms
PRL113(2014)
peak between 10-200GeV
Has a predicted dependence
on Global charge excess:
Chiral Magnetic Wave
7
Strangeness (PID) distinguish models
STAR Preliminary
“… We demonstrate that the STAR results can be
understood within the standard viscous
hydrodynamics without invoking the CMW…”
“… the slope r for the kaons should be negative,
in contrast to the pion case, and the magnitude is
expected to be larger… Note that in these
predictions are integrated over 0 < pT < ∞. In order
to properly test them, a wider pT coverage is
necessary…”
— Y. Hatta et al. Nuclear Physics A 947 (2016) 155
Measured kaon slope is positive:
contradict the conventional model
prediction without CMW
8
Chiral Symmetry & Magnetic Field
Two other Extraordinary
phenomena to make this
possible (QCD topology
reflects in charge separation)

Chiral Symmetry
Restoration


Disentangle and assess
necessary conditions
low-mass dilepton excess (change of
vector meson r spectral function)
Strong Magnetic Field


Global Hyperon Polarization
Coherent photo-production of J/Ψ and
low-mass dilepton in non-central A+A
collisions
A required set of Extraordinary
Phenomena:
QCD Topological Charge
+ Chiral Symmetry Restoration
+ Strong Magnetic Field
Observable:
Chirally restored quarks separated
along magnetic field
9
QCD phase transition is a chiral phase
transition
Golden probe of chiral symmetry restoration:
change vector meson (r→e+e-) spectral function
STAR data (RHIC and SPS):
Consistent with continuous QGP radiation and
broadening of vector meson in-medium
PRL113(2014)
PLB750(2015)
10
Global Hyperon Polarization
new tool to study QGP and relativistic
Quantum fluid Vorticity in general
arXiv:1701.06657
Non-zero global angular momentum
transfer to hyperon polarization
11
QCD fluid responds to external field
• Positive Global Hyperon Polarization
indicating a spin-orbit (Vortical) coupling
• Current data not able to distinguish
Lambda/AntiLambda polarization difference,
• (potentially) Direct measure of
Magnetic Field effect
• Need >x10 more data
sum
STAR Preliminary
difference
12
Coherent photoproduction
in violent non-central A+A collisions?
10−1
• Non-central but not UPC photoproduction
• Large enhancement of dilepton and J/Ψ
production at very low pT (<150MeV)
d2N/(dtdy) ((GeV/c)-2)
Shower the nucleus with electromagnetic field
• Consistent with strong electromagnetic field
interacting with nucleus target collectively
AuAu@200 GeV, 0.4< M ee <0.76 GeV/c 2
−2/ndf: 3.07/4
Slope: 371 ± 31 (GeV/c)-2
10−2
AuAu@200 GeV, 1.2< M ee <2.6 GeV/c 2
−2 /ndf: 0.55/3
Slope: 287 ± 37 (GeV/c)-2
STAR preliminary
−3
10
10−4
Centrality: 60-80%
10−1
UU@193 GeV, 0.4< Mee <0.76 GeV/c 2
T
Slope: 506 ± 53 (GeV/c)
10−2
10
UU@193 GeV, 1.2< Mee <2.6 GeV/c 2
2
2
t = p2 −((GeV/c)
/ndf: 5.08/4 )
2
2
t = p2 −((GeV/c)
/ndf: 1.26/4 )
T
-2
Slope: 411 ± 81 (GeV/c)-2
−3
10−4
0
0.01
0.02
0.03
t = p2 ((GeV/c)2)
T
0
0.01
0.02
0.03
t = p2 ((GeV/c)2)
T
13
A decisive test with Isobars
96
40
Zr + 4096 Zr vs.
96
44
Ru + 4496 Ru
1.2B minbias events
RHIC run in 2018:
Zr and Ru same geometry and mass;
charge different by 10% (20% signal difference)
5s effect with 20% (signal)+80% (background)
•
•
•
Dilepton and J/Ψ:
Coherent photoproduction: Z2
Photon-photon fusion: Z4
Hadronic interaction: Z0
14
Summary
Observed charge separation was
examined in Au+Au, U+U, p+Au
and d+Au







Investigation of two major
necessary phenomena:
scaled with final-stage v2 in peripheral
 Chiral Symmetry Restoration:
and mid-central and close to zero with
observation of large excess of
different v2 in Central U+U and Au+Au
low-mass dilepton, consistent
with vector r in-medium
Qualitatively different rapidity distribution
from central to peripheral A+A (p+A)

Strong Magnetic Field:
Values depend on correlation conditions in
 Suggestive difference between
p+Au and d+Au
Global Hyperon (antihyperon)
polarization);
Correct kaon ”sign” in Chiral Magnetic
need more statistics
Wave
 Photoproduction in non-central
Largest at beam energies (10-200GeV)
collisions, a good probe of
Background (v2) and signal (B field)
electromagnetic field interacts
with nucleus collectively
predicted to have similar centrality
(geometry) dependence
Isobar collisions will provide a decisive
test
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backup
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1.2
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´10-3
a,b
PbPb sNN = 5.02 TeV
CMS
Cent. 60-70%
1.5
|h | < 2.4, 4.4 < |hc| < 5.0
a,b
|h | < 0.8, |hc| < 0.8
1
CMS
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´10-3
|Dh|
´10-3
PbPb sNN = 5.02 TeV
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SS OS
PbPb
offline
pPb sNN = 5.02 TeV (b)
offline
(a)
SS OS
fc(Pb-going)
fc(p-going)
Dh
185 £ Ntrk < 220
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185 £ Ntrk < 220 1
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b
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Dh
ácos(fa +f -2fc)ñ/v 2,c
b
ácos(fa+f -2fc)ñ/v 2,c (OS-SS)
ácos(fa +f -2fc)ñ/v 2,c