Transcript Search for effects related to Chiral Magnetic Wave at STAR Gang Wang (UCLA) for STAR Collaboration.

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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