The 5th Workshop on Chirality, Vorticity and Magnetic Field in Heavy Ion Collisions Status of experimental searches for the chiral magnetic effect Fuqiang Wang （王福强） Purdue University, Huzhou University
Outline • Why is CME important? • Background issue • New progresses • Summary Fuqiang Wang The 5th Chirality Workshop, Tsinghua University, April 8-12, 2019 2 / 27
Why is CME important? • QCD vacuum fluctuations – Non-zero topological charge gluon configuration – Chirality imbalance, quantum anomaly – Local P, CP violations Derek Leinweber – Strong CP problem, matter-antimatter asymmetry Diakonov, PPNP 51 • Chira symmetry restoration – Current quark degrees of freedom • Strong magnetic field Kharzeev, et al. NPA 803 (2008) 227 Fuqiang Wang The 5th Chirality Workshop, Tsinghua University, April 8-12, 2019 3 / 27
Why is CME important? • QCD vacuum fluctuations – Non-zero topological charge gluon configuration – Chirality imbalance, quantum anomaly – Local P, CP violations Derek Leinweber – Strong CP problem, matter-antimatter asymmetry Diakonov, PPNP 51 • Chira symmetry restoration – Current quark degrees of freedom • Strong magnetic field Kharzeev, et al. NPA 803 (2008) 227 Message #1: Extraordinary claims require extraordinary evidence. Can you make your signal go away? Fuqiang Wang The 5th Chirality Workshop, Tsinghua University, April 8-12, 2019 4 / 27
To measure CME: the g observable p + p + RP p - p - Voloshin, PRC 70 (2004) 057901 𝛿 𝛽𝛾 = cos(𝜒 𝛽 + 𝜒 𝛾 − 2𝜔 𝑆𝑄 ) 𝛿 +− >0 ; 𝛿 ++ −− <0 Fuqiang Wang The 5th Chirality Workshop, Tsinghua University, April 8-12, 2019 5
The Background Issue: It’s big! CME CME π + + momentum conserv. CME + momentum conserv. OS π + OS + charge conserv. ρ 𝛿 +− >0 OS π - 𝛿 ++ −− <0 SS π - Signal is ambiguous Use OS-SS SS SS (magnitude underestimated by x10-100) Voloshin, PRC 70 (2004) 057901 𝛿 𝛽𝛾 = cos(𝜒 𝛽 + 𝜒 𝛾 − 2𝜔 𝑆𝑄 ) = cos[(𝜒 𝛽 + 𝜒 𝛾 − 2𝜒 𝑑𝑚𝑣𝑡𝑢. ) + 2(𝜒 𝑑𝑚𝑣𝑡𝑢. − 𝜔 𝑆𝑄 )] = 𝑂 𝑑𝑚𝑣𝑡𝑢. cos(𝜒 𝛽 + 𝜒 𝛾 − 2𝜒 𝑑𝑚𝑣𝑡𝑢. ) 𝑤 2,𝑑𝑚𝑣𝑡𝑢. 2 𝑂 𝜌 Background: nonflow coupled with flow v 2, / N Fuqiang Wang The 5th Chirality Workshop, Tsinghua University, April 8-12, 2019 6
The Background Issue: It’s big! CME CME π + + momentum conserv. CME + momentum conserv. OS π + OS + charge conserv. ρ 𝛿 +− >0 OS π - 𝛿 ++ −− <0 SS π - Signal is ambiguous Use OS-SS SS SS (magnitude underestimated by x10-100) Voloshin, PRC 70 (2004) 057901 𝛿 𝛽𝛾 = cos(𝜒 𝛽 + 𝜒 𝛾 − 2𝜔 𝑆𝑄 ) = cos[(𝜒 𝛽 + 𝜒 𝛾 − 2𝜒 𝑑𝑚𝑣𝑡𝑢. ) + 2(𝜒 𝑑𝑚𝑣𝑡𝑢. − 𝜔 𝑆𝑄 )] = 𝑂 𝑑𝑚𝑣𝑡𝑢. cos(𝜒 𝛽 + 𝜒 𝛾 − 2𝜒 𝑑𝑚𝑣𝑡𝑢. ) 𝑤 2,𝑑𝑚𝑣𝑡𝑢. 2 𝑂 𝜌 Background: nonflow coupled with flow v 2, / N FW, Jie Zhao, PRC 95 (2017) 051901 FW 2009, Bzdak, Koch, Liao 2010, Pratt, Schlichting 2010, … 𝑂 𝑑𝑚𝑣𝑡𝑢. cos(𝜒 𝛽 + 𝜒 𝛾 − 2𝜒 𝑑𝑚𝑣𝑡𝑢. ) 𝑤 2,𝑑𝑚𝑣𝑡𝑢. 2 𝑂 𝜌 20/100 2 *0.65 ~ 10 -3 0.1 Pratt, Schlichting Fuqiang Wang The 5th Chirality Workshop, Tsinghua University, April 8-12, 2019 7
Handling background • When background is small – Can be a bit sloppy in background estimation. Imprecision can be afforded by systematic uncertainty. – Can be somewhat model-dependent (theo. syst. uncertainties) • When background is large – Have to cleanly remove background – Extreme care should be taken. Small error in background can result in big mistake in signal. – Should not rely on theory/model (unless theory is very precise) – Better be data-driven, often involving new observables and methods. Fuqiang Wang The 5th Chirality Workshop, Tsinghua University, April 8-12, 2019 9 / 27
Handling background • When background is small – Can be a bit sloppy in background estimation. Imprecision can be afforded by systematic uncertainty. – Can be somewhat model-dependent (theo. syst. uncertainties) • When background is large – Have to cleanly remove background – Extreme care should be taken. Small error in background can result in big mistake in signal. – Should not rely on theory/model (unless theory is very precise) Message #2: – Better be data-driven, often involving new observables and Be cautious, and be persistent! methods. Fuqiang Wang The 5th Chirality Workshop, Tsinghua University, April 8-12, 2019 10 / 27
The infamous k parameter * k STAR, PRL 113 (2014) 052302 D H Dg - k v 2 d k =1 is a misconception • • Background size assumed! • No new info beyond the original Dg measurement. Over-sold. Fuqiang Wang The 5th Chirality Workshop, Tsinghua University, April 8-12, 2019 11 / 27
The infamous k parameter * k STAR, PRL 113 (2014) 052302 D H Dg - k v 2 d k =1 is a misconception • • Background size assumed! • No new info beyond the original Dg measurement. Over-sold. Message #3: Be courageous, but be rigorous! Fuqiang Wang The 5th Chirality Workshop, Tsinghua University, April 8-12, 2019 12 / 27
CMS clarification * k 𝑂 𝑑𝑚𝑣𝑡𝑢. 𝛿 = cos[(𝜒 𝛽 + 𝜒 𝛾 − 2𝜒 𝑑𝑚𝑣𝑡𝑢. ) + 2(𝜒 𝑑𝑚𝑣𝑡𝑢. − 𝜔 𝑆𝑄 )] = cos(𝜒 𝛽 + 𝜒 𝛾 − 2𝜒 𝑑𝑚𝑣𝑡𝑢. ) 𝑤 2,𝑑𝑚𝑣𝑡𝑢. 2 𝑂 𝜌 k 2 = k 3 accidental? Fuqiang Wang The 5th Chirality Workshop, Tsinghua University, April 8-12, 2019 13 / 27
To eliminate background D g Background N + - cos( 2 ) v clus 2, clus N N Make v 2 zero… STAR Fuqiang Wang The 5th Chirality Workshop, Tsinghua University, April 8-12, 2019 14 / 27
Make v 2 “zero” – EbyE technique Event-by-event v 2 technique cos( +-2y ) correlator Still has residual background, Here D is similar to STAR PRC 89 (2014)044908 because background ~ v 2, not v 2,p FW, Jie Zhao, PRC 95 (2017) 051901(R) STAR PRC 89 (2014) 044908 Fuqiang Wang The 5th Chirality Workshop, Tsinghua University, April 8-12, 2019 15 / 27
Make v 2 “zero” – EbyE technique Event-by-event v 2 technique cos( +-2y ) correlator Still has residual background, Here D is similar to STAR PRC 89 (2014)044908 because background ~ v 2, not v 2,p FW, Jie Zhao, PRC 95 (2017) 051901(R) v 2, STAR PRC 89 (2014) 044908 v 2, p final state Fuqiang Wang The 5th Chirality Workshop, Tsinghua University, April 8-12, 2019 16 / 27
Make v 2 “zero” – EbyE technique Event-by-event v 2 technique cos( +-2y ) correlator Still has residual background, Here D is similar to STAR PRC 89 (2014)044908 because background ~ v 2, not v 2,p FW, Jie Zhao, PRC 95 (2017) 051901(R) v 2, STAR PRC 89 (2014) 044908 v 2, p final state Fuqiang Wang The 5th Chirality Workshop, Tsinghua University, April 8-12, 2019 17 / 27
Make v 2 “zero” – EbyE technique Event-by-event v 2 technique cos( +-2y ) correlator Still has residual background, Here D is similar to STAR PRC 89 (2014)044908 because background ~ v 2, not v 2,p FW, Jie Zhao, PRC 95 (2017) 051901(R) v 2, STAR PRC 89 (2014) 044908 v 2, p Message #4: final state Be self-critical! Sciences often progress thru self-denials. Fuqiang Wang The 5th Chirality Workshop, Tsinghua University, April 8-12, 2019 18 / 27
Event-by-event q 2 technique Wen, Bryon, Wen, Wang, CPC 42 (2018) 014001 Similar to event-by-event v2 technique v 3 v 2 Intercept = (7.51 ± 0.75)*10 -3 Intercept = (8.32 ± 1.92)*10 -3 Fuqiang Wang The 5th Chirality Workshop, Tsinghua University, April 8-12, 2019 19
Event-by-event q 2 technique Wen, Bryon, Wen, Wang, CPC 42 (2018) 014001 Similar to event-by-event v2 technique v 3 v 2 Intercept = (7.51 ± 0.75)*10 -3 Intercept = (8.32 ± 1.92)*10 -3 Message #5: Be conservative. Make claims only after having exhausted all mundane physics. Fuqiang Wang The 5th Chirality Workshop, Tsinghua University, April 8-12, 2019 20
To eliminate background Three methods on the market that I think are hopeful to eliminate the backgrounds… 1. Event-shape engineering 2. Invariant mass 3. RP vs. PP comparison Fuqiang Wang The 5th Chirality Workshop, Tsinghua University, April 8-12, 2019 21 / 27
1) Event-shape-engineering technique cos( +-2y ) correlator STAR PRC 89 (2014)044908 Here D is similar to ALICE PLB777(2018)151 CMS PRC97(2018)044912 ESE displaced from , phase space Fuqiang Wang The 5th Chirality Workshop, Tsinghua University, April 8-12, 2019 22
1) Event-shape-engineering technique cos( +-2y ) correlator STAR PRC 89 (2014)044908 Here D is similar to ALICE PLB777(2018)151 CMS PRC97(2018)044912 v 2, ESE displaced from , phase space v 2, p final state Fuqiang Wang The 5th Chirality Workshop, Tsinghua University, April 8-12, 2019 23
2) The invariant mass method D g Background N + - cos( 2 ) v clus 2, clus N N Get rid of resonances, or utilize them… Identify the backgrounds by invariant mass arXiv:1705.05410 Fuqiang Wang The 5th Chirality Workshop, Tsinghua University, April 8-12, 2019 24 / 27
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