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Search for effects related to Chiral Magnetic Wave at STAR. Gang Wang (UCLA) for STAR Collaboration. Motivation. CSE + CME Chiral Magnetic Wave : gapless collective excitation signature of Chiral Symmetry Restoration. Peak magnetic field ~ 10 15 Tesla ! - PowerPoint PPT Presentation
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Search for effects related to Chiral Magnetic Wave at STAR
Gang Wang (UCLA)for STAR Collaboration
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Motivation
Peak magnetic field ~ 1015 Tesla !
(Kharzeev et al. NPA 803 (2008) 227)
CSE + CME Chiral Magnetic Wave: • gapless collective excitation• signature of Chiral Symmetry Restoration
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Observables
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)only the A± dependency matters for CMW testing.
Y. Burnier, D. E. Kharzeev, J. Liao and H-U Yee, Phys. Rev. Lett. 107, 052303 (2011)
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Observables CMW + Parity-odd domain,
=> charge separation across RP
Kharzeev, PLB633:260 (2006)Kharzeev, McLerran, Warringa, NPA803:227 (2008)
RPad
dN
sin21
This charge separation effect needs to be beyond statistical fluctuation and conventional physics background.
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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
• Very similar trend between data and theoretical calculations
• To compare the magnitude, the acceptance effects need investigation.
Y. Burnier, D. E. Kharzeev, J. Liao and H-U Yee,
private communication.
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UrQMD
The slopes from UrQMD are consistent with 0 using the same approach.
Hongwei Ke
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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
RPad
dN
sin21
A direct measurement of the P-odd quantity “a” should yield zero.
S. Voloshin, PRC 70 (2004) 057901
Directed flow: vanishesif measured in a symmetric rapidity range
Non-flow/non-parity effects:largely cancel out P-even quantity:
still sensitive to charge separation
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Results with different EPsSTAR Preliminary
The correlators using TPC/ZDC event planes are consistent with each other.
Lost in the medium?
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Dilution effect
STAR Preliminary
The factor Npart is used to compensate for dilution effect.
Weaker B field
Non-zero Radial flow?
Momentum conservation?
outin BB
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(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
-+ ΨRP+ -
charge conservation/cluster + v2 Physics background for LPV:
Qualitatively similar trend! Need to trigger on very small Nspec.
Phys. Rev. C72 (2005) 014904
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Beam energy scan
As we lower the beam energy, changes start to show from the peripheral collisions.27 GeV and 19.6 GeV coming soon...
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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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• Further studies of identified particles• Kπ correlation• pπ, Λπ...• ΛΛ correlation: vorticity
• 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?
Outlook
See also Kent Riley's talk!
Phys.Rev.Lett. 105 (2010) 172301
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Backup slides
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Multi-component Coalescence (MCC) + Quark Transport
John Campbell & Mike Lisa, preliminary study, publication in preparation.
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Dilution effect
What do we know about the position Rn after n steps? Rn follows a Gaussian distribution: mean = 0, and rms =
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.
In the quark-gluon medium, there could be multiple P-odd domains.
The net effect is like a random walk, but one-dimensional.
n
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Possible physics backgroundcharge conservation/cluster + v2
Qualitatively similar trend! Need to trigger on very small Nspec.
Phys. Rev. C72 (2005) 014904
Phys.Rev.C83:014913,2011
26Balance function
Phys.Rev.C83:014913,2011