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

2 Motivation Peak magnetic field ~ Tesla ! (Kharzeev et al. NPA 803 (2008) 227) CSE + CME  Chiral Magnetic Wave: gapless collective excitation signature of Chiral Symmetry Restoration

3 Observables Formation of electric quadrupole:, where charge asymmetry is defined as. Then π - v 2 should have a positive slope, and π + v 2 should have a negative slope with the same magnitude. The integrated v 2 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, (2011)

4 Observables CMW + Parity-odd domain, => charge separation across RP Kharzeev, PLB633:260 (2006) Kharzeev, McLerran, Warringa, NPA803:227 (2008) This charge separation effect needs to be beyond statistical fluctuation and conventional physics background.

5 STAR

6 STAR: excellent PID and tracking The correlation measurements at STAR are accurate to relative 0.1%.

7 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.

8 Charge asymmetry dependency v 2 was measured with the Q-cumulant method. Clear A ± dependency v 2 (A ± ) slopes for π ± : opposite sign similar magnitude v 2 difference vs A ± may have a non-zero intercept: other physics?

9 Correction for tracking efficiency Fit with a straight line to extract the slope. Do the same for all centralities

10 Slope vs centrality Statistical errors only Smooth trend Positive and significant for most centrality bins Systematic uncertainty: weak decays (Λ et al.) tracking efficiency bin center effect different v 2 methods

11 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.

12 UrQMD The slopes from UrQMD are consistent with 0 using the same approach. Hongwei Ke

13 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 v 2 opposite v 2 ordering than pion (v 2 (K + ) > v 2 (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?

14 CMW + Local Parity Violation A direct measurement of the P-odd quantity “a” should yield zero. S. Voloshin, PRC 70 (2004) Directed flow: vanishes if measured in a symmetric rapidity range Non-flow/non-parity effects: largely cancel out P-even quantity: still sensitive to charge separation

15 Results with different EPs STAR Preliminary The correlators using TPC/ZDC event planes are consistent with each other. Lost in the medium?

16 Dilution effect STAR Preliminary The factor N part is used to compensate for dilution effect. Weaker B field Non-zero Radial flow? Momentum conservation?

17 (OS - SS)*N part If N spec is a measure for B, the signal (OS-SS)*N part is roughly proportional to the magnetic field in central/mid-central collisions. Experimentally, we may trigger on N spec to control B.

18 Possible physics background -+ Ψ RP +- charge conservation/cluster + v 2 Physics background for LPV: Qualitatively similar trend! Need to trigger on very small N spec. Phys. Rev. C72 (2005)

19 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...

20 If we consider OS-SS to be signal... The signal seems to be disappearing at 7.7 GeV, but the statistical errors are large.

21 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 v 2 trend from Au+Au? Outlook See also Kent Riley's talk! Phys.Rev.Lett. 105 (2010)

22 Backup slides

23 Multi-component Coalescence (MCC) + Quark Transport John Campbell & Mike Lisa, preliminary study, publication in preparation.

24 Dilution effect What do we know about the position R n after n steps? R n follows a Gaussian distribution: mean = 0, and rms = Our measurement of PV is like R n 2, expected to be n. Compared with going in one fixed direction, where R n 2 = n 2, the "random-walk" measurement is diluted by a factor ~ n ~ N ch. In the quark-gluon medium, there could be multiple P-odd domains. The net effect is like a random walk, but one-dimensional.

25 Possible physics background charge conservation/cluster + v 2 Qualitatively similar trend! Need to trigger on very small N spec. Phys. Rev. C72 (2005) Phys.Rev.C83:014913,2011

26 Balance function Phys.Rev.C83:014913,2011