Charge dependent azimuthal correlations with the ALICE detector at the LHC

1. Charge dependent azimuthal correlations with the ALICE detector at the LHC Panos Christakoglou1, for the ALICE Collaboration 1Nikhef Panos.Christakoglou@nikhef.nl - P and CP odd effects in hot and dense matter, BNL

2. Motivation • Suggestions that heavy-ion collisions may form domains where the parity symmetry in strong interaction is locally violated • In non-central collisions, these domains may manifest themselves by a separation of charge, above and below the reaction plane. • The resulting charge separation is a consequence of two factors • the difference in numbers of quarks with positive and negative chiralities due to a non-zero topological charge of the region, • the interaction of these particles with the extremely strong and short lived magnetic field produced in such a collision (the Chiral Magnetic Effect-CME). • The existence of the CME, is directly related to the Chiral Symmetry restoration and to extreme B field values • ~1018 Gaus, stronger than on the surface of a neutron star • D. Kharzeev, Phys. Lett. B633, 260 (2006). • D. Kharzeev and A. Zhitnitsky, Nucl. Phys. A797, 67 (2007). • D. E. Kharzeev, L. D. McLerran and H. J. Warringa, Nucl. Phys. A803, 227 (2008). • K. Fukushima, D. E. Kharzeev and H. J. Warringa, Phys. Rev. D78, 074033 (2008). Panos.Christakoglou@nikhef.nl - P and CP odd effects in hot and dense matter, BNL

3. Proposed tools: azimuthal correlations S. Voloshin, Phys. Rev. C70, 057901 (2004) 3–particle correlator 2–particle correlator correlations in-plane correlations out-of-plane Panos.Christakoglou@nikhef.nl - P and CP odd effects in hot and dense matter, BNL

4. ALICE: Experimental setup Not shown: ZDC ~116m from I.P. Panos.Christakoglou@nikhef.nl - P and CP odd effects in hot and dense matter, BNL

5. Studies in ALICE: Analysis details • The centrality is selected using the magnitude of the VZERO signal (~multiplicity) as the default estimator • Centrality bins: 0-5%, 5-10%, 10-20%,…,60-70% • Different centrality estimators (TPC tracks, SPD clusters) investigated • Results used for the systematic uncertainty • Analysis of the Pb-Pb events recorded in November/December 2010 during the first LHC heavy-ion run • Event sample split in two sets having different magnetic field polarities (results used for the systematic uncertainties) • Trigger conditions: • SPD, VZERO-A, VZERO-C (2 out of 3) • VZERO-A && VZERO-C • Due to the small magnitude of the potential signal, we need to have the acceptance corrections under control: • The TPC tracks provide a uniform acceptance with minimal corrections • Disadvantage: contamination from secondaries • Investigated by varying the cut on the distance of closest approach (results used for the systematic uncertainty). Panos.Christakoglou@nikhef.nl - P and CP odd effects in hot and dense matter, BNL

6. Centrality dependence: Charge combinations Correlations measured with the cumulant technique • Clear charge asymmetry observed • Results for (++) and (--) consistent (combined later as “Same charge”) • The magnitude of the correlations between the same charged pairs is larger than the one of the opposite charges (excluding the most peripheral collisions  due to large non-flow?) Panos.Christakoglou@nikhef.nl - P and CP odd effects in hot and dense matter, BNL

7. Different methods: event plane estimate from detectors Event plane from charged particles at forward rapidity Event plane from charged particles at mid-rapidity Event plane from the neutron spectators Investigation with four independent methods Panos.Christakoglou@nikhef.nl - P and CP odd effects in hot and dense matter, BNL

8. Centrality dependence: Comparison of methods Very good agreement between the four methods Charge asymmetry due to correlation wrt the reaction plane Panos.Christakoglou@nikhef.nl - P and CP odd effects in hot and dense matter, BNL

9. 3-particle correlator: LHC vs RHIC • Magnitude of the effect seems to be similar to what is reported by STAR. • Some models predict a much lower effect at LHC energies (see next slide) • Signal and background should both scale with the inverse of the square of the multiplicity • The effect can be similar depending on the t0 of the magnetic field • D. Kharzeev et al., Nucl. Phys. A803, (227) 2008 • A. R. Zhitnitsky, arXiv:1201.2665 [hep-ph]. Stat. error: error bars Syst. error: shaded area STAR Collaboration: Phys. Rev. Lett. 81, 251601 (2009) STAR Collaboration: Phys. Rev. C81, 054908 (2010) Panos.Christakoglou@nikhef.nl - P and CP odd effects in hot and dense matter, BNL

10. 3-particle correlator: Comparison with models V.D. Toneev and V. Voronyuk, arXiv:1012.1508v1 [nucl-th] • HIJING results between pairs of same and opposite charge are consistent  combined into one point • HIJING points consistent with the (+-) data points • HIJING points scaled with the square of the multiplicity, consistent with the idea of having the correlations originating from emerging clusters (jets, resonances) • The only available quantitative prediction for LHC energies (@4.5 TeV) • According to the authors the magnitude should roughly scale with 1/√s • Applied in the figure to convert the prediction to √sNN = 2.76 TeV Panos.Christakoglou@nikhef.nl - P and CP odd effects in hot and dense matter, BNL

11. 2–particle correlations: Centrality dependence • (++) and (--) combined into one set of points (“Same charge”). • Similarity to STAR: the magnitude of the opposite charged pairs which is larger than the same charged ones. • Difference with STAR: • Sign of the same charged correlations • Strength of the correlations • Correlations between opposite charges are positive and large • Correlations of same charged pairs are also positive and have a smaller magnitude • Results between (++) and (--) are consistent Panos.Christakoglou@nikhef.nl - P and CP odd effects in hot and dense matter, BNL

12. Decomposition • Similar magnitude for the cos terms for same and opposite charged pairs • Higher magnitude for the sin terms for same than opposite charged pairs Panos.Christakoglou@nikhef.nl - P and CP odd effects in hot and dense matter, BNL

13. Background effects: flow fluctuations • The orientation angle of the dipole asymmetry shows a preference out-of-plane. • This results in a net v1 out of plave with a small magnitude • The magnitude of the correlations depending on the freeze-out conditions can give a potentially significant contribution • The hydrodynamic calculation though does not describe the charge separation! • Baseline shift in our measurement? D. Teaney and L. Yan, arXiv:1010.1876v1 [nucl-th] Panos.Christakoglou@nikhef.nl - P and CP odd effects in hot and dense matter, BNL

14. Background effects: initial state fluctuations (cont.) Charge independent correlator P. Christakoglou (for the ALICE Collaboration), Phys. G G38, (2011) 124165 Paper at the last stage of the Collaboration review (will released soon after the workshop): 2- and 3-particle integrated correlator + differential analysis (3-particle correlator) Panos.Christakoglou@nikhef.nl - P and CP odd effects in hot and dense matter, BNL

15. Summary • The possibility of observing parity odd domains was investigated by using both a 2-particle and a 3-particle P-even correlator. • The results from the 2-particle correlator studies show that the sign of the correlations is the same regardless of the charge combination, contrary to what was observed in STAR • Need to take into account the different non-flow contributions • The centrality dependence of the 3-particle correlator illustrates a remarkable agreement in both the magnitude and the behavior with the results reported by STAR in Au-Au collisions at √sNN = 0.2 TeV • Hydro calculations indicate that the dipole asymmetry’s preferential out-of-plane orientation might result into a v1 contribution out-of-plane, but the charge asymmetry is not explained. • Baseline shift from the fluctuations of the initial geometry? • Theory was not clear about the possible energy dependence of the effect • Significant need for quantitative (realistic) calculations of the CME effects for both RHIC and LHC energies Charge asymmetry is seen experimentally with a similar magnitude as at the highest RHIC energy  Theory is challenged by the latest findings! Panos.Christakoglou@nikhef.nl - P and CP odd effects in hot and dense matter, BNL

16. Outlook (towards QM…and beyond) • Charge conservation coupled to elliptic flow seems to describe the difference of the 3-particle correlator for same and opposite charged pairs at RHIC • Look at the balance function wrt Ψ • Look at other correlators (e.g. double harmonics) • Correlations between identified particles • Chiral vortical effect studies S. Schlichting and S. Pratt, Phys. Rev. C83, 014913 (2011). S. Pratt, S. Schlichting and S. Gavin, Phys. Rev. C84, 024909 (2011) S. Voloshin, arXiv:1111.7241 [nucl-ex] D. Kharzeev Phys. Rev. Lett. 106 (2011) 062301 Panos.Christakoglou@nikhef.nl - P and CP odd effects in hot and dense matter, BNL

17. BACKUP Panos.Christakoglou@nikhef.nl - P and CP odd effects in hot and dense matter, BNL