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Charge dependent azimuthal correlations in Pb–Pb collisions at √s NN = 2.76TeV

Charge dependent azimuthal correlations in Pb–Pb collisions at √s NN = 2.76TeV. Panos Christakoglou 1,2 , for the ALICE Collaboration 1 NIKHEF 2 Utrecht University. Motivation. Suggestions that heavy-ion collisions may form domains where the parity symmetry is locally violated

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Charge dependent azimuthal correlations in Pb–Pb collisions at √s NN = 2.76TeV

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  1. Charge dependent azimuthal correlations in Pb–Pb collisions at √sNN = 2.76TeV PanosChristakoglou1,2, for the ALICE Collaboration 1NIKHEF 2Utrecht University Panos.Christakoglou@cern.ch - Quark Matter 2011, Annecy-France

  2. Motivation • Suggestions that heavy-ion collisions may form domains where the parity symmetry 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). • The existence of the CME, is directly related to the Chiral Symmetry restoration and to weird B field effects • 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). See also Dima’s talk on Monday at this session Panos.Christakoglou@cern.ch - Quark Matter 2011, Annecy-France

  3. Proposed tools: correlation analysis 2–particle correlator 3–particle correlator • Affected by the signal and the correlations both in and out-of-plane. • Sensitive also to detector effects • Background: Difference between the correlations projected onto an axis in the reaction plane and the ones projected onto an axis perpendicular to the reaction plane • Measuring both correlators allows us to get an idea about the potential parity signal but also about the background contributions • Correlations in and out of plane Panos.Christakoglou@cern.ch - Quark Matter 2011, Annecy-France

  4. Studies in ALICE: Analysis details • The centrality is selected using the VZERO magnitude as the default estimator • Centrality bins: 0-5%, 5-10%, 10-20%,…,70-80% • Different centrality estimators (TPC tracks, SPD clusters) investigates • 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) • The trigger consists of the following criteria (at least two out of three): • two pixel chips hit in the outer layer of the SPD, • signal in VZERO-A detector, • signal in VZERO-C detector. • 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). For a description of the centrality determination, check Alberica Toia’s talk For a description of the experimental setup, check Jurgen Schukraft‘s talk Panos.Christakoglou@cern.ch - Quark Matter 2011, Annecy-France

  5. ALICE 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@cern.ch - Quark Matter 2011, Annecy-France

  6. 3-particle correlations Panos.Christakoglou@cern.ch - Quark Matter 2011, Annecy-France

  7. Three-particle correlators: Differential analysis in Δη • Charge separation starts to develop when moving away from the most central bins • Correlations between opposite charges are smaller than those between same charges • Correlation width Δη = |ηα - ηβ|~1 Panos.Christakoglou@cern.ch - Quark Matter 2011, Annecy-France

  8. Three-particle correlators: Differential analysis in ΔpT • Correlations not localized in small values of ΔpT • Contribution from short range correlations of same/opposite charges limited? Panos.Christakoglou@cern.ch - Quark Matter 2011, Annecy-France

  9. Three-particle correlators: Differential analysis in sum pT • Correlations of same charges have larger signal with increasing transverse momentum of the pair contrary to the expectation from theory (i.e. signal localized at the low pT region) D. Kharzeev et al., Nucl. Phys. A803, 227 (2008) Panos.Christakoglou@cern.ch - Quark Matter 2011, Annecy-France

  10. Centrality dependence: Charge combinations • Clear charge separation observed • Results for (++) and (--) consistent (combined in the next plots into one “Same charge” point) • The magnitude of the correlations between the same charged pairs is larger than the one of the opposite charges (excluding the last bin) Panos.Christakoglou@cern.ch - Quark Matter 2011, Annecy-France

  11. Different methods: event plane estimation VZERO TPC ZDC Investigation with four independent methods For further details on the ZDC and the VZERO check the talk of Ilya Selyuzhenkov Panos.Christakoglou@cern.ch - Quark Matter 2011, Annecy-France

  12. Centrality dependence: Comparison of methods Very good agreement between the four methods Panos.Christakoglou@cern.ch - Quark Matter 2011, Annecy-France

  13. Integrated 3-particle correlator: LHC vs RHIC STAR Collaboration: Phys. Rev. Lett. 81, 251601 (2009) STAR Collaboration: Phys. Rev. C81, 054908 (2010) Stat. error: error bars Syst. error: shaded area • Magnitude of the effect seems to be similar to what is reported by STAR. • Most theories predict a much lower effect at LHC energies. • Signal and background should both scale with 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) Panos.Christakoglou@cern.ch - Quark Matter 2011, Annecy-France

  14. Comparison of 2– and 3–particle correlators • STAR’s 2-particle correlations for same charged pairs have the same magnitude as the points coming from the 3-particle correlation analysis. • Larger magnitude of the correlations in than out of plane? • ALICE data demonstrate a larger magnitude of the 2-particle correlations but also a change in sign • Differences in the correlations vs reaction plane between energies? • Larger magnitude of the correlations out of than in plane? STAR Collaboration: Phys. Rev. C81, 054908 (2010) Panos.Christakoglou@cern.ch - Quark Matter 2011, Annecy-France

  15. Integrated 3-particle correlator: Comparison with models S. A. Voloshin, Phys. Rev. C 70, 057901 (2004). V.D. Toneev and V. Voronyuk, arXiv:1012.1508v1 [nucl-th] • HIJING points consistent with the (+-) data points • HIJING w/o flow consistent with each other • 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 published 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@cern.ch - Quark Matter 2011, Annecy-France

  16. Summary and outlook • 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 • Change in the correlation pattern vs reaction plane? • The results of the 3-particle correlator indicate that: • the signal has a hadronic width of one unit in η, • doesn’t have any obvious contribution from short range correlations (i.e. HBT), • increases with increasing pair pt. • The centrality dependence of the integrated 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 • The majority of models predict a smaller signal at LHC energies. Theory is challenged by the latest findings; looking forward to the feedback from the theory community!!! Panos.Christakoglou@cern.ch - Quark Matter 2011, Annecy-France

  17. BACKUP Panos.Christakoglou@cern.ch - Quark Matter 2011, Annecy-France

  18. What we know so far…(STAR) • Correlations between opposite charges are positive and large • Correlations of same charged pairs are negative and have a significantly smaller magnitude STAR Collaboration: Phys. Rev. C81, 054908 (2010) STAR Collaboration: Phys. Rev. Lett. 81, 251601 (2009) • Differential analysis of the 3-particle correlator indicates that the signal: • increases with increasing pt of the pair, • has a hadronic width of one unit in η, • demonstrates a weak dependence on the pair’s pt difference • Differences in the correlations of same and opposite charged pairs. • The sign follows the one of the 2-particle correlator • The magnitude of the correlations of same charged pairs is larger than the one of opposite charged particles • None of the studied models describe the data Panos.Christakoglou@cern.ch - Quark Matter 2011, Annecy-France

  19. ALICE 2–particle correlations: Differential analysis • Correlations have the same behavior regardless of the charge combination. • Change of sign @ ~2 GeV/c • Change of physics @ ~5 GeV/c in ΔpT • Correlations localized in η • Different charge combinations have the same correlations in sign but not in magnitude. Panos.Christakoglou@cern.ch - Quark Matter 2011, Annecy-France

  20. Summary and outlook • 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: • there is a change in physics at Δpt~5GeV/c, which is already seen by other analyses (flow, RAA), • the sign of the correlations is the same regardless of the charge combination, contrary to what was observed in STAR • Change in the correlation pattern vs reaction plane? • The results of the 3-particle correlator indicate that: • the signal has a hadronic width of one unit in η, • doesn’t have any obvious contribution from short range correlations (i.e. HBT), • increases with increasing pair pt. • The centrality dependence of the integrated 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 • The majority of models predict a smaller signal at LHC energies. Theory is challenged by the latest findings; looking forward to the feedback from the theory community!!! Panos.Christakoglou@cern.ch - Quark Matter 2011, Annecy-France

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