-
Search of the Chiral Magnetic Wavewith Anisotropic Flow of Identified Particles
at RHIC
Qi-Ye Shou (for the STAR Collaboration)
Shanghai Institute of Applied PhysicsBrookhaven National Laboratory
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CMW measurement at RHIC-STAR (Qi-Ye Shou) Quark Matter 2018, Venice, Italy 2
Chiral Magnetic Wave
~1015 T
Y. Burnier, D. E. Kharzeev, J. Liao, and H.-U. Yee, Phys. Rev. Lett. 107, 52303 (2011)
monopole, non-zero net charge density
quadrupole, CMW contribution
Asymmetry in the azimuthal distributions of h+ and h-
Ach
AchAch
Ach
Observables: Δv2, Ach
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CMW measurement at RHIC-STAR (Qi-Ye Shou) Quark Matter 2018, Venice, Italy 3
Chiral Magnetic Wave
Y. Burnier, D. E. Kharzeev, J. Liao, and H.-U. Yee, Phys. Rev. Lett. 107, 52303 (2011)
Possible best probe: negative and positive pions due to the small difference in the absorption cross sections
For negative and positive kaons and antiprotons and protons, the large differences in the absorption cross sections could mask or reverse the potential signal
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CMW measurement at RHIC-STAR (Qi-Ye Shou) Quark Matter 2018, Venice, Italy 4
Brief history of CMW
First CMW prediction paper published
Measurement at STAR published
Measurement at ALICE published
Measurement in p(Pb)-Pb 5 TeV at CMS presented
Measurement in p(d)-Au at STAR presented
Hydrodynamics theory proposed
LCC theory proposed
20122011 2013 2014 2015 2016 2017
First measurement at STAR presented
Measurement with 3-particle correlation at ALICE presented
Y. Burnier, D. E. Kharzeev, J. Liao, and H.-U. Yee, Phys. Rev. Lett. 107, 52303 (2011)
STAR Collaboration, PRL 114, 252302 (2015)
ALICE Collaboration, PRC 93, 044903 (2016)
CMS Collaboration, arXiv:1708.08901
A. Bzdak, P. BożekPhysics Letters B 726 (2013) 239–243
Y. Hatta et al. Nuclear Physics A 947 (2016) 155–160
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CMW measurement at RHIC-STAR (Qi-Ye Shou) Quark Matter 2018, Venice, Italy 5
Brief history of CMW
First CMW prediction paper published
Measurement at STAR published
Measurement at ALICE published
Measurement in p(Pb)-Pb 5 TeV at CMS presented
Measurement in p(d)-Au at STAR presented
Hydrodynamics theory proposed
LCC theory proposed
20122011 2013 2014 2015 2016 2017
First measurement at STAR presented
Measurement with 3-particle correlation at ALICE presented
Y. Burnier, D. E. Kharzeev, J. Liao, and H.-U. Yee, Phys. Rev. Lett. 107, 52303 (2011).
STAR Collaboration, PRL 114, 252302 (2015)
ALICE Collaboration, PRC 93, 044903 (2016)
CMS Collaboration, arXiv:1708.08901
A. Bzdak, P. BożekPhysics Letters B 726 (2013) 239–243
Y. Hatta et al. Nuclear Physics A 947 (2016) 155–160
CMS
STAR
ALICE
CMS results show the consistency between p+Pb and Pb+Pb, and between v2 and v3,what about RHIC?
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CMW measurement at RHIC-STAR (Qi-Ye Shou) Quark Matter 2018, Venice, Italy 6
Possible background — Hydrodynamics with isospin chemical potential (μI)
v2(π+) < v2(π-)v2(K+) > v2(K-)
“… the STAR results can be understood within the standard viscous hydrodynamics without invoking the CMW…”
Δv2 ∝ - μI; Ach ∝ - μI (assumed); -> Δv2 ∝ Ach“… the slope r for the kaons should be negative, in contrast to the pion case, and the magnitude is expected to be larger… (in wider pT coverage)”
Y. Hatta et al. Nuclear Physics A 947 (2016) 155–160
STAR Collaboration, Phys. Rev. Lett. 110, 142301 (2013)
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CMW measurement at RHIC-STAR (Qi-Ye Shou) Quark Matter 2018, Venice, Italy 7
Possible background — Local Charge Conservation
A. Bzdak, P. Bożek, Physics Letters B 726 (2013) 239–243
Low pT
High pT
Low pT clusters -> larger opening angles in the lab-> more likely to miss one particle
If such a lost particle is:positive: Ach decreases; mean pT(-) < mean pT(+); v2(-) < v2(+)negative: Ach increases; mean pT(-) > mean pT(+); v2(-) > v2(+)
Same relationship is also valid for v3
Multi-particle emission from "clusters" (resonance decays, strongly flowing fluid elements)
TPC Detector
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CMW measurement at RHIC-STAR (Qi-Ye Shou) Quark Matter 2018, Venice, Italy 8
The STAR experiment at RHIC and analysis method
P
Kπ
Particle identificationPrimary tracks with DCA < 1 cmπ: |nσπ| < 2, 0
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CMW measurement at RHIC-STAR (Qi-Ye Shou) Quark Matter 2018, Venice, Italy
Ach
9
Dependence of Δ⟨pT⟩ and Δv2 on Ach for pions in different kinematic windows
⟨pT⟩ and v2 differences of π+ and π- are tested as functions of AchThe relative variation of ⟨pT⟩ (∼0.1%) is typically smaller than the relative variation of v2 (∼1%) by an order of magnitude.
A wider pT range enhances particle yields -> important for analyses involving K and p.
η coverage is reduced to half∆ v2(Ach) slope does not display a significant variation, suggesting the smallness of the LCC effect in the data.
STAR Preliminary STAR Preliminary
Ach
STAR Preliminary
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CMW measurement at RHIC-STAR (Qi-Ye Shou) Quark Matter 2018, Venice, Italy 10
Dependence of ⟨pT⟩ and v2 on Ach for pions in different kinematic windows
The slope parameters obtained with different phase space selections show similar trends and values
% Most Central0 20 40 60 80
Slop
e (%
)
4−
2−
0
2
4
2 v∆ πAu+Au 200 GeV,
|
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CMW measurement at RHIC-STAR (Qi-Ye Shou) Quark Matter 2018, Venice, Italy 11
Dependence of the ∆v2(Ach) slope on centrality and collision energy for K
∆v2(Ach) slope for K is positive and close to the π slope
Contradicts the prediction of the viscous hydrodynamics model with μI(Note that the intercept for kaons is negative)
Centrality dependence of slopes for K behave similarly to that of πNo significant absorption effect (see slide 4)Hydrodynamics with μI cannot be the dominant mechanism
0.04− 0.02− 0 0.02 0.04
(%)
2v
6.2
6.3
6.4
6.5
6.6K+K-
Au+Au 200 GeV, 30-40%
< 1 GeV/cT
0.15 < p
a
chA0.04− 0.02− 0 0.02 0.04
(%)
2 v
∆0.2−
0.1−
0
0.1
Kπ
b
STAR Preliminary
STAR Preliminary
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CMW measurement at RHIC-STAR (Qi-Ye Shou) Quark Matter 2018, Venice, Italy
% Most Central0 20 40 60 80
Slop
e (%
)
2−
0
2
4
Au+Au 200 GeV < 0.5 GeV/c
T: 0.15 < pπ
< 2 GeV/cT
p: 0.4 < p
12
Dependence of the ∆v2(Ach) slope on centrality and collision energy for p
0.05− 0 0.05
(%)
2v
9.6
9.7
9.8
9.9
10p+p-
Au+Au 200 GeV, 30-40%
< 2 GeV/cT
0.4 < p
a
chA0.05− 0 0.05
(%)
2 v
∆0.2−
0.1−
0
0.1pπ
b
∆v2(Ach) slopes for (anti-)protons are typically much smaller than those for π and K
The proton slopes are close to zero except for the positive values in 40 − 70% collisions.
STAR Preliminary
STAR Preliminary
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CMW measurement at RHIC-STAR (Qi-Ye Shou) Quark Matter 2018, Venice, Italy 13
Dependence of the ∆v3(Ach) slope on centrality for π in Au+Au collisions
In contrast with CMS results, π v3 at RHIC depends weakly on Ach, and the ∆v3(Ach) slope is much smaller than the ∆v2(Ach) slope.
0.04− 0.02− 0 0.02 0.04
(%)
3v
0.65
0.7
0.75
0.8
0.85 +π-π
Au+Au 200 GeV, 30-40%
< 0.5 GeV/cT
0.15 < p
a
chA0.04− 0.02− 0 0.02 0.04
(%)
3 v
∆
0.1−
0
0.1
0.23v2v
b
% Most Central0 20 40 60 80
1−
0
1
2 v∆3 v∆
Au+Au 200 GeV
< 2 GeV/cT
p
c
Nor
m. S
lope
(%)
2−
1−
0
1
< 1 GeV/cT
p
b
4−
2−
0
< 0.5 GeV/cT
p
a
0.15 < pT < 0.5 GeV/c,
the norm. ∆v3(Ach) slopes are lower than or consistent with zero for all centrality intervals pT upper bound is increased to 1 or 2 GeV/c,
the norm. ∆v3(Ach) slopes gradually approach the Norm. ∆v2(Ach) slopes.
STAR Preliminary
STAR Preliminary
CMS 5.02 TeVCMS Collaboration, arXiv:1708.08901
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CMW measurement at RHIC-STAR (Qi-Ye Shou) Quark Matter 2018, Venice, Italy 14
The ∆v2(Ach) slopes for π in p+Au, d+Au and U+U collisions
The CMW signals are expected to disappear in the small systems The orientation decoupling between the magnetic field and the 2nd-order event plane
The ∆v2(Ach) slopes in both p+Au and d+Au (analyzed with the 2nd-order event plane from TPC) are consistent with zeroDemonstrates the smallness of the possible background in small systems.
The ∆v2(Ach) slopes in U+U collisions are systematically higher than the results in Au+Au collisions. A uranium nucleus has more protons than a gold nucleus, leading to a stronger magnetic field?partN10
210
Slop
e (%
)
4−
2−
0
2
4
p + Au 200 GeVd + Au 200 GeVAu + Au 200 GeVU + U 193 GeV
< 0.5 GeV/cT
0.15 < p
STAR Preliminary
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CMW measurement at RHIC-STAR (Qi-Ye Shou) Quark Matter 2018, Venice, Italy
The ⟨pT⟩ dependence on Ach exists but is insignificant. However, one should still try to keep the pT upper limit as low as possible.
The similarity between pion and kaon slopes suggests that the hydrodynamics is not the dominant contribution to the pion or kaon slopes. The isospin effect, however, remains a potential contributor to the proton slopes in Au+Au collisions at √sNN = 200 GeV.
The difference between the normalized v2 and v3 slopes for pions at various pT, centrality intervals suggests that the CMW picture remains a viable interpretation at RHIC.
The measured slopes are consistent with zero in p+Au and d+Au collisions demonstrating the smallness of the possible background in the small systems.
The difference in the pion Δv2(Ach) slope between Au+Au and U+U is consistent with the expectation from the CMW picture.
Conclusion
15
Thanks for your attention!
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Backup
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CMW measurement at RHIC-STAR (Qi-Ye Shou) Quark Matter 2018, Venice, Italy 17
Previous experimental results from RHIC and LHC
ALICE Collaboration, PRC 93, 044903 (2016)STAR Collaboration, PRL 114, 252302 (2015)
The linear dependence between (Δ)v2 and Ach is observed at RHIC-STAR and LHC-ALICE. The extracted slopes are within the expectation of CMW theory.
Note that ALICE data show weak centrality dependence comparing to STAR data, indicating the possible difference (magnetic field, collectivity…) between two collision energies.
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CMW measurement at RHIC-STAR (Qi-Ye Shou) Quark Matter 2018, Venice, Italy 18
Previous experimental results from RHIC and LHC
CMS Collaboration, arXiv:1708.08901
Significant and similar linear relationships are observed for v2 and v3 at LHC-CMS, which cannot be explained by CMW but is consistent with predictions based on Local Charge Conservation. Similar linear dependences are also found in pPb and PbPb system.
CMS results challenge CMW, and are in favor of Local Charge Conservation.
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CMW measurement at RHIC-STAR (Qi-Ye Shou) Quark Matter 2018, Venice, Italy 19
Dependence of ⟨pT⟩ and v2 on Ach for pions in different kinematic windows
0.05− 0 0.05
> T<
p
0.338
0.34
0.342
+π-π
Au+Au 200 GeV, 30-40%
< 0.5 GeV/cT
0.15 < p
a
0.05− 0 0.05
> T<
p
0.474
0.476
0.478
+π-π
< 1 GeV/cT
0.15 < p
b
0.05− 0 0.05
> T<
p
0.524
0.526
0.528
+π-π
< 2 GeV/cT
0.15 < p
c
chA0.05− 0 0.05
> T <
p∆
0.004−
0.002−
0
0.002
< 0.5 GeV/cT
0.15 < p < 1 GeV/c
T0.15 < p
< 2 GeV/cT
0.15 < p
< 0.5 GeV/cT
0.15 < p < 1 GeV/c
T0.15 < p
< 2 GeV/cT
0.15 < p
d
0.04− 0.02− 0 0.02 0.04
(%)
2v
5.7
5.8
5.9
+π-π
Au+Au 200 GeV, 30-40%
| < 1η < 1 GeV/c, |T
0.15 < p
a
0.04− 0.02− 0 0.02 0.04
(%)
2 v
∆
0.1−
0
0.1
0.2 < 1 GeV/cT0.15 < p < 0.5 GeV/c
T0.15 < p
b
0.05− 0 0.05
(%)
2v
5.8
5.9
6
6.1 +π-π < 1 GeV/cT| < 0.5, pη|
c
chA0.05− 0 0.05
(%)
2 v
∆
0.2−
0.1−
0
0.1
0.2 | < 0.5η|| < 1η|
d
Over the same Ach range, the relative variation of ⟨pT⟩ (∼0.1%) is typically smaller than the relative variation of v2 (∼1%) by an order of magnitude.
A wider pT range enhances particle yields, which is important for analyses involving K and p. (Some statistical uncertainties are invisible on the current scale)
When the η coverage is reduced to half, the ∆ v2(Ach) slope, does not display a significant variation, suggesting the smallness of the LCC effect in these data.
STAR Preliminary STAR Preliminary
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CMW measurement at RHIC-STAR (Qi-Ye Shou) Quark Matter 2018, Venice, Italy 20
Possible background — Local Charge Conservation
A. Bzdak, P. Bożek, Physics Letters B 726 (2013) 239–243
Ach decreasemean pT(-) < mean pT(+)v2(-) < v2(+)
+-
+-
Low pT
High pT
-
-
+
+Low pT
High pT
Ach increasemean pT(-) > mean pT(+)v2(-) > v2(+)
clusters (resonances, fluid elements)
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CMW measurement at RHIC-STAR (Qi-Ye Shou) Quark Matter 2018, Venice, Italy 21
Possible background — Local Charge Conservation, another mechanism
Ach decreasev2(-) < v2(+)
Ach increasev2(-) > v2(+)
clusters (resonances, fluid elements)
+-
+-*small ηlarge v2
large ηsmall v2
-+
+-small ηlarge v2
large ηsmall v2
*PHOBOS data
A. Bzdak, P. Bożek, Physics Letters B 726 (2013) 239–243
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CMW measurement at RHIC-STAR (Qi-Ye Shou) Quark Matter 2018, Venice, Italy 22
Intercept(K) in √sNN 200 GeV
Does this observation conflict with our knowledge of (anti-)particle flow? No, since the intercepts are negative.
We knowv2(π-) > v2(π+)v2(K-) < v2(K+)
so both
v2(π-) - v2(π+) = v2π(base) + rAch > 0v2(K-) - v2(K+) = v2K(base) + rAch < 0
are valid Intercept
Slope (>0)
STAR Preliminary
error bars are only statistical
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CMW measurement at RHIC-STAR (Qi-Ye Shou) Quark Matter 2018, Venice, Italy 23
Dependence of the ∆v3(Ach) slope on centrality for π in Au+Au collisions
v3 for π± as functions of Ach in 9 centralities in Au+Au collisions at √sNN = 200 GeV
0.02− 0.01− 0 0.01 0.02
0.2−
0.1−
0
0.1
0-5%Au+Au 200 GeV
0.02− 0 0.02
0.2−
0.1−
0
0.1
5-10% < 0.5 GeV/c
T0.15 < p
0.02− 0 0.02
0.1−
0
0.1
10-20%
0.04− 0.02− 0 0.02 0.04
(%)
n v
∆
0.1−
0
0.1
20-30%2v3v
0.05− 0 0.050.1−
0
0.1
30-40%
0.05− 0 0.05
0.1−
0
0.1
40-50%
0.05− 0 0.050.2−
0.1−
0
0.1
0.2
50-60%
chA0.1− 0 0.1
0.4−
0.2−
0
0.2
60-70%
0.1− 0 0.12−
1−
0
1
2
70-80%
0.01− 0 0.01 0.020.6
0.8
10-5%
+π-π
0.01− 0 0.01 0.020.6
0.7
0.8
0.9
1 5-10% < 0.5 GeV/c
T0.15 < p
0.02− 0 0.02
0.7
0.8
0.910-20%
Au+Au 200 GeV
0.02− 0 0.02
(%)
3v 0.8
0.9
20-30%
0.02− 0 0.02 0.04
0.8
0.9
1 30-40%
0.05− 0 0.050.8
0.9
1 40-50%
0.05− 0 0.050.7
0.8
0.9
1
1.150-60%
chA0.1− 0.05− 0 0.05 0.1
0.4
0.6
0.8
1
1.2 60-70%
0.1− 0 0.1
0
1
2 70-80%
Comparison of ∆v3 and ∆v2 for pions as functions of Ach in 9 centralities in Au+Au collisions at √sNN = 200 GeV
STAR Preliminary STAR Preliminary
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CMW measurement at RHIC-STAR (Qi-Ye Shou) Quark Matter 2018, Venice, Italy
Centrality 0-5% 5-10% 10-20% 20-30% 30-40% 40-50% 50-60% 60-70% 70-80%
pT < 0.5GeV/c 4.2 4.1 3.9 2.4 0.7 1.3 1.9 1.4 2.2
pT < 1 GeV/c 5.9 4.6 3.7 2.2 0.2 -0.4 0.8 1.2 2.6
pT < 2 GeV/c 4.7 4.4 3.3 1.8 0.003 -0.6 0.2 0.7 2.3
24
Dependence of the ∆v3(Ach) slope on centrality for π in Au+Au collisions
ε - combined error of ∆v2 and ∆v3 in quadrature
STAR Preliminary
These σNorm. values suggest that the STAR measurements of the ∆v2(Ach) slopes are different with the CMS measurements.
It is unlikely that a common background such as LCC could alone explain the data. There could be multiple reasons, particularly at most central and peripheral collisions. CMW picture still remains as a viable interpretation at RHIC.