Global polarization of Λ hyperons in Au+Au collision …...Becattini, Karpenko, Lisa, Upsal, and...
Transcript of Global polarization of Λ hyperons in Au+Au collision …...Becattini, Karpenko, Lisa, Upsal, and...
![Page 1: Global polarization of Λ hyperons in Au+Au collision …...Becattini, Karpenko, Lisa, Upsal, and Voloshin, PRC95.054902 (2017) ω = (P Λ +P Λ)k B T/ℏ, ∼ 0.6−2.7×1022s−1](https://reader033.fdocuments.net/reader033/viewer/2022052518/5f0a294a7e708231d42a4eba/html5/thumbnails/1.jpg)
Global polarization of Λ hyperons in Au+Au collision
at STAR
Kosuke Okubo University of Tsukuba
Heavy Ion Cafe June 22-23 @Sophia University
�1
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Important features in non-central heavy-ion collision
participant
spectator
B L
Strong magnetic field
B ∼ 1013 TD.Kharzeev,L.McLerran,andH.Warringa, Nucl.Phys.A803,227(2008) Mcerran and Skokov,Nucl.Phys.A929,184(2014)
Orbital angular momentum
L ∼ 105ℏZ.-T. Liang and X.-N. Wang, PRL94, 102301 (2005)
!2
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Vorticity in heavy-ion collision
!3
beam direction
impact parameter
PRC77,024906(2008)
In non-central collision, The initial collective longitudinal flow velocity dependent on x.
ωy =12
(∇ × v)y ≈ −12
∂vz
∂x
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Global polarization
!4
particle antiparticle
-Z-T.Liang and X.-N. Wang, PRL94, 102301 (2005)
✔Non-zero angular momentum transfers to the spin of freedom
-S.Voloshin, nucl-th/0410089(2004)
・Polarization due to spin-orbit coupling
- Particle and anti-particle’s spin are aligned with angular momentum L
・Spin alignment by B-field
-Particle and antiparticle’s spins are aligned oppositely along B due to the opposite sign of magnetic moment
![Page 5: Global polarization of Λ hyperons in Au+Au collision …...Becattini, Karpenko, Lisa, Upsal, and Voloshin, PRC95.054902 (2017) ω = (P Λ +P Λ)k B T/ℏ, ∼ 0.6−2.7×1022s−1](https://reader033.fdocuments.net/reader033/viewer/2022052518/5f0a294a7e708231d42a4eba/html5/thumbnails/5.jpg)
How to measure the polarization?
!5
parity-violating decaydaughter proton preferentially decays into the direction of Λ’s spin (opposite for anti-Λ)
PH =8
παH
⟨sin(Ψ1 − ϕ*p )⟩Res(Ψ1)
STAR,PRC76,024915
ϕ*p :φ of daughter proton in Λ rest frameΨ1:first order event planeαH :decay parameter
Λ → p + π−(63.9%)
Λ → p + π+
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Solenoidal Tracker At RHIC
!6
![Page 7: Global polarization of Λ hyperons in Au+Au collision …...Becattini, Karpenko, Lisa, Upsal, and Voloshin, PRC95.054902 (2017) ω = (P Λ +P Λ)k B T/ℏ, ∼ 0.6−2.7×1022s−1](https://reader033.fdocuments.net/reader033/viewer/2022052518/5f0a294a7e708231d42a4eba/html5/thumbnails/7.jpg)
Lambda reconstruction
!7
・identify daughter (p,π) with TPC and TOF・use the information on decay topology to reduce the contribution background
TPC
TOF
![Page 8: Global polarization of Λ hyperons in Au+Au collision …...Becattini, Karpenko, Lisa, Upsal, and Voloshin, PRC95.054902 (2017) ω = (P Λ +P Λ)k B T/ℏ, ∼ 0.6−2.7×1022s−1](https://reader033.fdocuments.net/reader033/viewer/2022052518/5f0a294a7e708231d42a4eba/html5/thumbnails/8.jpg)
Signal extraction with Λ hyperons
!8
STAR, PRC98, 014910 (2018)
⟨sin(Ψ1 − ϕ*p )⟩obs = (1 − fBg(Minv)⟨sin(Ψ1 − ϕ*p )⟩Sg
+fBg(Minv)⟨sin(Ψ1 − ϕ*p )⟩Bg
PH =8
παH
⟨sin(Ψ1 − ϕ*p )⟩Res(Ψ1)
Λ’s signal is opposite to anti-Λ’s
Λ and anti-Λ are polarized in the same direction
(parity-violating decay)
STAR, PRC98, 014910 (2018)
![Page 9: Global polarization of Λ hyperons in Au+Au collision …...Becattini, Karpenko, Lisa, Upsal, and Voloshin, PRC95.054902 (2017) ω = (P Λ +P Λ)k B T/ℏ, ∼ 0.6−2.7×1022s−1](https://reader033.fdocuments.net/reader033/viewer/2022052518/5f0a294a7e708231d42a4eba/html5/thumbnails/9.jpg)
Energy dependence of polarization
!9
10 210 [GeV] NNs
0
1
2
3
[%]
HP
STAR Au+Au 20%-50%
Nature548.62 (2017)
Λ Λ
PRC76.024915 (2007)
Λ Λ
Phys. Rev. C 98 (2018)
Λ Λ
ΛUrQMD+vHLLE, primary primary+feed-down
ΛAMPT, primary primary+feed-down
AMPT:H.Li et al.,Phys,Rev.C96,054908(2017)
UrQMD+vHLLE: I. Karpenko and F. Becattini, EPJC(2017)77:213
✔Positive polarization Signal!
✔polarization looks to increase in lower energies
✔anti-Λ is systematically larger than Λ
imply magnetic couplingneed more events
BES-Ⅱ
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Observation of fluid vortices in HIC
!10
STAR, Nature 548, 62 (2017)
PΛ ≃12
ωT
+μΛB
TPΛ ≃
12
ωT
−μΛB
T
Becattini, Karpenko, Lisa, Upsal, and Voloshin, PRC95.054902 (2017)
ω = (PΛ + PΛ)kBT/ℏ
,
∼ 0.6 − 2.7 × 1022s−1
μΛ :Λ magnetic moment
T:temperature at Thermal equilibrium
The most vortical fluid ever observed
![Page 11: Global polarization of Λ hyperons in Au+Au collision …...Becattini, Karpenko, Lisa, Upsal, and Voloshin, PRC95.054902 (2017) ω = (P Λ +P Λ)k B T/ℏ, ∼ 0.6−2.7×1022s−1](https://reader033.fdocuments.net/reader033/viewer/2022052518/5f0a294a7e708231d42a4eba/html5/thumbnails/11.jpg)
Possible probe of magnetic field
!11
PΛ ≃12
ωT
+μΛB
T
PΛ ≃12
ωT
−μΛB
T
Becattini, Karpenko, Lisa, Upsal, and Voloshin, PRC95.054902(2017)
B = (PΛ + PΛ)kBT/μN
∼ 5.0 × 1013 [Tesla]
nuclear magneton μN = − 0.613μΛ
✔Extracted B-field is close to our expectation.
QM2017, I. Upsal (STAR)
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Centrality dependence of polarization
!12
STAR PRC 98, 014910 (2018)
→
In most central collision
no initial angular momentum
the polarization decrease in more central collision
Y.Jiang et al.,PRC94,044910(2016)
AMPT model
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dependence of polarization
!13
pT
STAR PRC 98, 014910 (2018)
✔No significant pT dependence
✔One might expect....
1.The polarization decrease in low pT
smearing effect
2.The polarization decrease in high pT
jet fragmentation
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η dependence of polarization
!14
STAR PRC 98, 014910 (2018)
W.-T.Deng and X.-G Huang:C93,064907
✔The vorticity is expected decrease at large rapidities
✔The data do not show significant η dependence
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Azimuthal angle dependence of polarization
!15
IMSD 2018 Biao
I.Karpenko and F.Becattini, EPJC(2017)77:213
・Larger polarization in in-plane than out-of-plane
・Opposite to hydrodynamic model
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Λ polarization vs charged asymmetry
!16
STAR PRC 98, 014910 (2018)
Chiral Separation Effect
STAR PRC 98, 014910 (2018)
・Use charge asymmetry Ach instead of μv
μv /T ∝⟨N+ − N−⟩⟨N+ + N−⟩
= Ach
✔Slopes of Λ and anti-Λ seem to be different
✔Possibly a contribution from axial current?
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Polarization along the beam direction
!17
✔Stronger flow in in-plane than in out-of-plane cloud make local polarization along beam axis
S.Volshin,SQM2017 F.Becattini and I. Karpenko, PRL120.012302(2018)
αH :decay parameter(if�perfect�detector)
θ*p :θof daughter proton in Λ rest frame
✔Longitudinal component, Pz, can be expressed with ⟨cos θ*p ⟩
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Polarization along the beam direction
!18
in-plane
out-of-plane
Hydrodynamic model
✔Opposite sign to hydrodynamics model and a transport model(AMPT)
✔Sin structure as expected from the elliptic flow
- AMPT model: X. Xia, H. Li, Z. Tang, Q. Wang, arXiv:1803.0086- Hydro model: F. Becattini and I. Karpenko, PRL.120.012302 (2018)
AMPT
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Centrality dependence of
!19
PZ
✔Strong centrality dependence
✔Similar magnitude to the global polarization
⟨pT⟩ of Λ ~1.4 GeV/c✔~5 times smaller magnitude than the Hydro and AMPT
(0.5<pT<6 GeV/c)
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dependence of local polarization
!20
pT
✔No significant pT dependence for pT > 1.0 GeV
✔can not conclusion about low pT dependence
✔Opposite sign to hydrodynamics model
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Summary
!21
✔Observation of positive Λ polarization at √sNN = 7.7-200GeV ・Indicating the thermal vorticity of system in HIC ω~ .1022s−1
・Polarization decrease at higher energies.
・Larger signal in more peripheral collision but no significant dependence on pt and η
・Larger signal in in-plane than in out-of-plane
Disagree with hydrodynamic model・Charged asymmetry dependence (~2σ level) in the polarization
A possible relation to the axial current induced by B-filed
✔Λ polarization along the beam direction at √sNN = 200 GeV・Quadrupole structure relative to the 2nd-order event plane, as expected from the elliptic flow・Strong centrality dependence as in the elliptic flow but no significant dependence on pt
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Back up
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Contribution to Pz in hydro
!23
sμ ∝ ϵμρστωρσ pτ = ϵμρστ(∂ρβσ)pτ
= ϵμρστpτ∂ρ ( 1T ) uσ +
1T
2[ωμ(u ⋅ p) − uμ(ω ⋅ p)] + ϵμρστpτAσuρ
gradT “NR�vorticity” accelerationtemperature gradient kinematic vorticity relativistic term
I.Karpenko,QM2018
✔Pz dominated by temperature gradient and relativistic term, but not by kinematic vorticity based on the hydro model.
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Blast-wave model parameterization
✔Hydro-inspired model parameterized with freeze-out condition assuming the longitudinal boost invariance
-Freeze-out temperature Tf
-Radial flow rapidity and its modulation ρ0 ρ2
-Source size Rx and Ry
✔Calculate vorticity at the freeze-out using the parameters extracted From spectra, v2, and HBT fit
u : local flow velocity, Ln, Kn: modified Bessel function