Transcript Search for effects related to Chiral Magnetic Wave at STAR

Search for effects related to
Chiral Magnetic Wave at STAR
Gang Wang (UCLA)
for STAR Collaboration
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Motivation
CSE + CME Chiral Magnetic Wave:
• gapless collective excitation
• signature of Chiral Symmetry Restoration
Peak magnetic field ~
1015 Tesla !
(Kharzeev et al. NPA 803 (2008)
227)
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Observables
Y. Burnier, D. E. Kharzeev, J. Liao and H-U Yee,
Phys. Rev. Lett. 107, 052303 (2011)
Formation of electric quadrupole:
,
where charge asymmetry is defined as
.
Then π- v2 should have a positive slope,
and π+ v2 should have a negative slope with the same magnitude.
The integrated v2 of π- is not necessarily bigger than π+: (other physics)
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only the A± dependency matters for CMW testing.
Observables
CMW + Parity-odd domain,
=> charge separation across RP

dN 
 1  2a  sin    RP
d

This charge separation effect needs to be beyond statistical fluctuation
and conventional physics background.
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Kharzeev, PLB633:260 (2006)
Kharzeev, McLerran, Warringa, NPA803:227 (2008)
STAR
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STAR: excellent PID and tracking
The correlation measurements at
STAR are accurate to relative 0.1%.
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Observed charge asymmetry
• N+ (N-) is the number of
positive (negative) particles
within |η|<1.
• The distribution was
divided into 5 bins, with
roughly equal counts.
• Tracking efficiency was
corrected later.
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Charge asymmetry dependency
• v2 was measured with the
Q-cumulant method.
• Clear A± dependency
• v2(A±) slopes for π±:
• opposite sign
• similar magnitude
• v2 difference vs A± may
have a non-zero intercept:
other physics?
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Correction for tracking efficiency
• Fit with a straight line to
extract the slope
.
• Do the same for all centralities
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Slope vs centrality
• Statistical errors only
• Smooth trend
• Positive and significant for
most centrality bins
• Systematic uncertainty:
• weak decays (Λ et al.)
• tracking efficiency
• <A±> bin center effect
• different v2 methods
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Theoretical calculations
Y. Burnier, D. E. Kharzeev,
J. Liao and H-U Yee,
private communication.
• Very similar trend between data and theoretical calculations
• To compare the magnitude, the acceptance effects need investigation.
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UrQMD
Hongwei Ke
The slopes from UrQMD are consistent with 0 using the same approach.
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Outlook
• Further studies of the weak decay contributions
• DCA (Distance of Closest Approach) cut on particles
• simulation such as UrQMD and AMPT
• Beam energy scan (39 GeV, 27 GeV and 19.6 GeV)
• different charge asymmetry distribution
• different magnetic field?
• Kaon v2
• opposite v2 ordering than pion (v2(K+) > v2(K-))
• what about A± dependency?
• Handle on the magnetic field
• trigger on spectators with Zero Degree Calorimeters
• is the signal still there when we turn off B?
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CMW + Local Parity Violation

dN 
 1  2a  sin    RP
d

A direct measurement of the P-odd
quantity “a” should yield zero.
S. Voloshin, PRC 70 (2004) 057901
Non-flow/non-parity effects:
largely cancel out
Directed flow: vanishes
if measured in a symmetric rapidity range
P-even quantity:
still sensitive to 14
charge separation
Results with different EPs
STAR Preliminary
Lost in the medium?
The correlators using TPC/ZDC event planes are consistent with
each other.
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Dilution effect
Non-zero Bin  Bout
Radial flow?
Weaker B field
STAR Preliminary
Momentum conservation?
The factor Npart is used to compensate for dilution effect.16
(OS - SS)*Npart
If Nspec is a measure for B, the signal (OS-SS)*Npart is roughly
proportional to the magnetic field in central/mid-central collisions.
Experimentally, we may trigger on Nspec to control B.
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Possible physics background
Physics background for LPV:
charge conservation/cluster + v2
+ -
Phys. Rev. C72 (2005) 014904
+ -
Qualitatively similar trend!
Need to trigger on very small Nspec.
ΨRP
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Beam energy scan
As we lower the beam energy, changes start to show from the peripheral collisions.
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27 GeV and 19.6 GeV coming soon...
If we consider OS-SS to be signal...
The signal seems to be disappearing at 7.7 GeV, but the statistical errors are large.
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Outlook
• Further studies of identified particles
• Kπ correlation
• pπ, Λπ...
• ΛΛ correlation: vorticity
See also Kent Riley's talk!
• Beam energy scan (27 GeV and 19.6 GeV)
• Is there a smooth transition from 200 GeV to 7.7 GeV?
• different magnetic field?
• Handle on the magnetic field
• trigger on spectators with ZDCs
• is there still signal when we turn off B?
• U+U collisions
• different v2 trend from Au+Au?
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Phys.Rev.Lett. 105 (2010) 172301
Backup slides
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Multi-component Coalescence (MCC) +
Quark Transport
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John Campbell & Mike Lisa, preliminary study, publication in preparation.
Dilution effect
In the quark-gluon medium, there could
be multiple P-odd domains.
The net effect is like a
random walk, but
one-dimensional.
What do we know about the position Rn after n steps?
Rn follows a Gaussian distribution: mean = 0, and rms = n
Our measurement of PV is like Rn2, expected to be n.
Compared with going in one fixed direction, where Rn2 = n2,
the "random-walk" measurement is diluted by a factor ~ n ~ Nch.
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Possible physics background
charge conservation/cluster + v2
Phys.Rev.C83:014913,2011
Phys. Rev. C72 (2005) 014904
Qualitatively similar trend!
Need to trigger on very small Nspec.
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Phys.Rev.C83:014913,2011
Balance function
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