chiral magnetic effect Fuqiang Wang Purdue University, Huzhou - - PowerPoint PPT Presentation

Status of experimental searches for the chiral magnetic effect

Fuqiang Wang（王福强） Purdue University, Huzhou University

The 5th Workshop on Chirality, Vorticity and Magnetic Field in Heavy Ion Collisions

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 – Strong CP problem, matter-antimatter asymmetry

• Chira symmetry restoration

– Current quark degrees of freedom

• Strong magnetic field

Fuqiang Wang 3 / 27 The 5th Chirality Workshop, Tsinghua University, April 8-12, 2019

Kharzeev, et al. NPA 803 (2008) 227 Derek Leinweber

Diakonov, PPNP 51

Why is CME important?

• QCD vacuum fluctuations

– Non-zero topological charge gluon configuration – Chirality imbalance, quantum anomaly – Local P, CP violations – Strong CP problem, matter-antimatter asymmetry

• Chira symmetry restoration

– Current quark degrees of freedom

• Strong magnetic field

Fuqiang Wang 4 / 27 The 5th Chirality Workshop, Tsinghua University, April 8-12, 2019

Kharzeev, et al. NPA 803 (2008) 227 Derek Leinweber

Diakonov, PPNP 51

Message #1:

Extraordinary claims require extraordinary evidence. Can you make your signal go away?

To measure CME: the g observable

𝛿𝛽𝛾 = cos(𝜒𝛽 + 𝜒𝛾 − 2𝜔𝑆𝑄) 𝛿+−>0 ; 𝛿++

−−<0

Fuqiang Wang The 5th Chirality Workshop, Tsinghua University, April 8-12, 2019

p+ p- p+ p-

RP

5

Voloshin, PRC 70 (2004) 057901

The Background Issue: It’s big!

CME OS SS OS SS CME + momentum conserv. OS SS CME + momentum conserv. + charge conserv. Use OS-SS ρ π- π+ π+ π- Signal is ambiguous

Fuqiang Wang The 5th Chirality Workshop, Tsinghua University, April 8-12, 2019

𝛿+−>0 𝛿++

−−<0

6

Voloshin, PRC 70 (2004) 057901

𝛿𝛽𝛾 = cos(𝜒𝛽 + 𝜒𝛾 − 2𝜔𝑆𝑄) = cos[(𝜒𝛽 + 𝜒𝛾 − 2𝜒𝑑𝑚𝑣𝑡𝑢.) + 2(𝜒𝑑𝑚𝑣𝑡𝑢. − 𝜔𝑆𝑄)] = 𝑂𝑑𝑚𝑣𝑡𝑢. 𝑂𝜌

2

cos(𝜒𝛽 + 𝜒𝛾 − 2𝜒𝑑𝑚𝑣𝑡𝑢.) 𝑤2,𝑑𝑚𝑣𝑡𝑢.

(magnitude underestimated by x10-100)

2, /

v N





Background: nonflow coupled with flow

The Background Issue: It’s big!

CME OS SS OS SS CME + momentum conserv. OS SS CME + momentum conserv. + charge conserv. Use OS-SS ρ π- π+ π+ π- Signal is ambiguous

FW 2009, Bzdak, Koch, Liao 2010, Pratt, Schlichting 2010, …

Fuqiang Wang The 5th Chirality Workshop, Tsinghua University, April 8-12, 2019

𝛿+−>0 𝛿++

−−<0

7

2, /

v N





Background: nonflow coupled with flow

Voloshin, PRC 70 (2004) 057901

𝛿𝛽𝛾 = cos(𝜒𝛽 + 𝜒𝛾 − 2𝜔𝑆𝑄) = cos[(𝜒𝛽 + 𝜒𝛾 − 2𝜒𝑑𝑚𝑣𝑡𝑢.) + 2(𝜒𝑑𝑚𝑣𝑡𝑢. − 𝜔𝑆𝑄)] = 𝑂𝑑𝑚𝑣𝑡𝑢. 𝑂𝜌

2

cos(𝜒𝛽 + 𝜒𝛾 − 2𝜒𝑑𝑚𝑣𝑡𝑢.) 𝑤2,𝑑𝑚𝑣𝑡𝑢.

20/1002*0.65 ~ 10-3

𝑂𝑑𝑚𝑣𝑡𝑢. 𝑂𝜌

2

cos(𝜒𝛽 + 𝜒𝛾 − 2𝜒𝑑𝑚𝑣𝑡𝑢.) 𝑤2,𝑑𝑚𝑣𝑡𝑢.

0.1

FW, Jie Zhao, PRC 95 (2017) 051901

(magnitude underestimated by x10-100)

Pratt, Schlichting

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 9 / 27 The 5th Chirality Workshop, Tsinghua University, April 8-12, 2019

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 10 / 27 The 5th Chirality Workshop, Tsinghua University, April 8-12, 2019

Message #2:

Be cautious, and be persistent!

The infamous k parameter

• k=1 is a misconception
• Background size assumed!
• No new info beyond the original

Dg measurement. Over-sold.

Fuqiang Wang 11 / 27 The 5th Chirality Workshop, Tsinghua University, April 8-12, 2019

STAR, PRL 113 (2014) 052302

DH  Dg - kv2d

*k

The infamous k parameter

• k=1 is a misconception
• Background size assumed!
• No new info beyond the original

Dg measurement. Over-sold.

Fuqiang Wang 12 / 27 The 5th Chirality Workshop, Tsinghua University, April 8-12, 2019

STAR, PRL 113 (2014) 052302

Message #3:

Be courageous, but be rigorous!

DH  Dg - kv2d

*k

CMS clarification

k2 = k3 accidental?

Fuqiang Wang 13 / 27 The 5th Chirality Workshop, Tsinghua University, April 8-12, 2019

𝛿 = cos[(𝜒𝛽 + 𝜒𝛾 − 2𝜒𝑑𝑚𝑣𝑡𝑢.) + 2(𝜒𝑑𝑚𝑣𝑡𝑢. − 𝜔𝑆𝑄)] =

𝑂𝑑𝑚𝑣𝑡𝑢. 𝑂𝜌

2

cos(𝜒𝛽 + 𝜒𝛾 − 2𝜒𝑑𝑚𝑣𝑡𝑢.) 𝑤2,𝑑𝑚𝑣𝑡𝑢.

*k

To eliminate background

2,

Background cos( 2 )

clus clus

N v N N

    

g    D  +

• Make v2 zero…

STAR

Fuqiang Wang The 5th Chirality Workshop, Tsinghua University, April 8-12, 2019 14 / 27

Make v2 “zero” – EbyE technique

STAR PRC 89(2014)044908 Event-by-event v2 technique

Here D is similar to cos(+-2y) correlator

Still has residual background, because background ~ v2, not v2,p

FW, Jie Zhao, PRC 95 (2017) 051901(R)

Fuqiang Wang The 5th Chirality Workshop, Tsinghua University, April 8-12, 2019

STAR PRC 89 (2014) 044908

15 / 27

v2, v2,p

final state

Make v2 “zero” – EbyE technique

STAR PRC 89(2014)044908 Event-by-event v2 technique

Here D is similar to cos(+-2y) correlator

Fuqiang Wang The 5th Chirality Workshop, Tsinghua University, April 8-12, 2019

STAR PRC 89 (2014) 044908

16 / 27

Still has residual background, because background ~ v2, not v2,p

FW, Jie Zhao, PRC 95 (2017) 051901(R)

v2, v2,p

final state

Make v2 “zero” – EbyE technique

STAR PRC 89(2014)044908 Event-by-event v2 technique

Here D is similar to cos(+-2y) correlator

Fuqiang Wang The 5th Chirality Workshop, Tsinghua University, April 8-12, 2019

STAR PRC 89 (2014) 044908

17 / 27

Still has residual background, because background ~ v2, not v2,p

FW, Jie Zhao, PRC 95 (2017) 051901(R)

v2, v2,p

final state

Make v2 “zero” – EbyE technique

STAR PRC 89(2014)044908 Event-by-event v2 technique

Here D is similar to cos(+-2y) correlator

Fuqiang Wang The 5th Chirality Workshop, Tsinghua University, April 8-12, 2019

STAR PRC 89 (2014) 044908

18 / 27

Still has residual background, because background ~ v2, not v2,p

FW, Jie Zhao, PRC 95 (2017) 051901(R)

Message #4:

Be self-critical! Sciences often progress thru self-denials.

Fuqiang Wang 19 The 5th Chirality Workshop, Tsinghua University, April 8-12, 2019

Intercept = (7.51 ± 0.75)*10-3 Intercept = (8.32 ± 1.92)*10-3

Event-by-event q2 technique

Similar to event-by-event v2 technique Wen, Bryon, Wen, Wang, CPC 42 (2018) 014001

v2 v3

Fuqiang Wang 20 The 5th Chirality Workshop, Tsinghua University, April 8-12, 2019

Intercept = (7.51 ± 0.75)*10-3 Intercept = (8.32 ± 1.92)*10-3

Event-by-event q2 technique

Similar to event-by-event v2 technique Wen, Bryon, Wen, Wang, CPC 42 (2018) 014001

v2 v3

Message #5:

Be conservative. Make claims only after having exhausted all mundane physics.

To eliminate background

Three methods on the market that I think are hopeful to eliminate the backgrounds…

Fuqiang Wang The 5th Chirality Workshop, Tsinghua University, April 8-12, 2019 21 / 27

• 1. Event-shape engineering
• 2. Invariant mass
• 3. RP vs. PP comparison

1) Event-shape-engineering technique

ALICE PLB777(2018)151

Fuqiang Wang The 5th Chirality Workshop, Tsinghua University, April 8-12, 2019

STAR PRC89(2014)044908

Here D is similar to cos(+-2y) correlator

22

ESE displaced from ,  phase space

CMS PRC97(2018)044912

ALICE PLB777(2018)151 CMS PRC97(2018)044912

Fuqiang Wang The 5th Chirality Workshop, Tsinghua University, April 8-12, 2019

STAR PRC89(2014)044908

Here D is similar to cos(+-2y) correlator

23

v2, v2,p

final state ESE displaced from ,  phase space

1) Event-shape-engineering technique

2) The invariant mass method

arXiv:1705.05410

Fuqiang Wang The 5th Chirality Workshop, Tsinghua University, April 8-12, 2019 24 / 27

Get rid of resonances, or utilize them…

2,

Background cos( 2 )

clus clus

N v N N

    

g    D  +

• Identify the backgrounds by invariant mass

High mass Dg: Resonance structure in Dg as function of minv

Major background

Get away from resonances

Fuqiang Wang The 5th Chirality Workshop, Tsinghua University, April 8-12, 2019

Jie Zhao (STAR) Quark Matter 2018, arXiv:1807.09925 (4. 𝟔 ± 𝟑. 𝟔)%

26

Can CME survive to high mass?

• minv > 2 GeV/c2 contains appreciable low pT pions
• CME signal may not be limited only to low pT
• High mass should still contain CME

Shi et al., 1711.02496, Ann.Phys.394(2018)50 Kinematics

Fuqiang Wang The 5th Chirality Workshop, Tsinghua University, April 8-12, 2019 27 / 27

Indulge into the resonance region

• ESE q2 selects different v2, but does not

bias spectators or magnetic field

• DgA-DgB represents background shape
• Fit Dg = k*(Bkg shape) + CME
• Fit does not assume Dg ∝ v2, but only

dependent of v2

• Fit assumes constant CME. Fit c2/ndf

tells whether it’s a good assumption STAR Preliminary STAR Preliminary

2,

cos( 2 )

clus clus

N v N N

    

   +

• 

Fuqiang Wang The 5th Chirality Workshop, Tsinghua University, April 8-12, 2019 30

Jie Zhao (STAR), QM18, 1807.09925

Indulge into the resonance region

• ESE q2 selects different v2, but does not

bias spectators or magnetic field

• DgA-DgB represents background shape
• Fit Dg = k*(Bkg shape) + CME
• Fit does not assume Dg ∝ v2, but only

dependent of v2

• Fit assumes constant CME. Fit c2/ndf

tells whether it’s a good assumption STAR Preliminary STAR Preliminary

2,

cos( 2 )

clus clus

N v N N

    

   +

• 

Jie Zhao (STAR), QM18, 1807.09925

Fuqiang Wang The 5th Chirality Workshop, Tsinghua University, April 8-12, 2019

CME=(2 ± 4)%

31

3) RP vs PP comparison measurement

BKG CME BKG×a CME

RP

BKG CME BKG CME×a

PP

CME and BKG are entangled in AA

 

cos2

PP RP

a y y 

• SAME EVENT

input measured

• H. Xu, J. Zhao, X. Wang, H. Li, Z.-W. Lin, C. Shen, F. Wang, CPC 42 (2018) 084103, arXiv:1710.07265

Fuqiang Wang The 5th Chirality Workshop, Tsinghua University, April 8-12, 2019 32 / 27

Intra-event “CME-v2 Filter”

“CME-v2 Filter” results from STAR

Fuqiang Wang The 5th Chirality Workshop, Tsinghua University, April 8-12, 2019

BKG CME BKG×a CME

RP

BKG CME BKG CME×a

PP

33 / 27

Summary of Possible CME Signal

• Major physics backgrounds
• Possible CME signal ~ a few %, 1-2s from zero.

Fuqiang Wang The 5th Chirality Workshop, Tsinghua University, April 8-12, 2019

Jie Zhao (STAR) Quark Matter 2018 arXiv:1807.09925

34 / 27

A day-1 prediction for isobar

Multiplicity sensitive to nuclear structure

Fuqiang Wang 35 / 27 The 5th Chirality Workshop, Tsinghua University, April 8-12, 2019

Our prediction Woods-Saxon prediction

Haojie Xu et al. PRL 121 (2018) 022301 Haojie Xu et al. CPC 42 (2018) 084103 Hanlin Li et al. PRC 98 (2018) 054907

Summary

• Background dominate in Dg.

Rigorous treatment of background is critical.

• Three viable methods so far:

– Event-shape engineering – Invariant mass – RP vs. PP comparison

• Possible CME signal is small, a few % of Dg, 1-2s

from zero.

Fuqiang Wang 36 / 27 The 5th Chirality Workshop, Tsinghua University, April 8-12, 2019

Summary

• Background dominate in Dg.

Rigorous treatment of background is critical.

• Three viable methods so far:

– Event-shape engineering – Invariant mass – RP vs. PP comparison

• Possible CME signal is small, a few % of Dg, 1-2s

from zero.

Fuqiang Wang 37 / 27 The 5th Chirality Workshop, Tsinghua University, April 8-12, 2019

Disclaimer:

CME is not observed ≠ CME is nonexistent