Global polarization of Λ and Λ hyperons in Pb Pb collisions at s NN = 2.76 TeV

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1 Global polarization of and hyperons in Pb Pb collisions at s NN = 2.76 TeV Maxim Konyushikhin Wayne State University On behalf of the ALICE collaboration March 29, 27 QCD Chirality Workshop 27

2 Outline Motivation direct probe of the vorticity and the B-field expectations at the LHC energies Observable to measure measurement strategy the ALICE Detector Event plane reconstruction with neutron ZDCs event plane resolution expected sensitivity of the measurement Signal extraction Results in centrality, p T and rapidity Feed-down correction (all values and plots shown on the slides are not corrected for it) Summary and perspectives 2 / 2

3 Motivation: non-central collisions The system created in non-central nucleus nucleus collisions possesses large orbital angular momentum. Things to study: Vorticity ω = 2 v = 2 curl v, v velocity field: How much is transferred from the initial state? Effects of viscosity? Strong magnetic field B: How strong and for how long? Theoretical predictions vary by orders of magnitude. Effects of viscosity? 3 / 2

4 Motivation: non-central collisions The system created in non-central nucleus nucleus collisions possesses large orbital angular momentum. Things to study: Vorticity ω = 2 v = 2 curl v, v velocity field: How much is transferred from the initial state? Effects of viscosity? Strong magnetic field B: How strong and for how long? Theoretical predictions vary by orders of magnitude. Effects of viscosity? Anomalous transport phenomena in a chiral medium (CME, CVE, etc) 3 / 2

5 Motivation: non-central collisions The system created in non-central nucleus nucleus collisions possesses large orbital angular momentum. Things to study: Vorticity ω = 2 v = 2 curl v, v velocity field: How much is transferred from the initial state? Effects of viscosity? Strong magnetic field B: How strong and for how long? Theoretical predictions vary by orders of magnitude. Effects of viscosity? Non-chiral phenomena (with possible chiral contributions): directed flow v is a direct probe of the vorticity global hyperon polarization is a direct probe of the vorticity and the B-field 3 / 2

6 Motivation: global hyperon polarization Produced particles become globally polarized along the direction of the system s angular momentum. Vorticity global polarization: spin-orbit coupling: P H ω acquired and global polarizations are of same magnitudes and point in the same direction (parallel to ω) Magnetic field global polarization: magnetic field coupling: P H µ H B acquired and polarizations are of same magnitudes, but point in opposite directions (parallel to B) 4 / 2

7 Motivation: global hyperon polarization Produced particles become globally polarized along the direction of the system s angular momentum. Vorticity global polarization: spin-orbit coupling: P H ω acquired and global polarizations are of same magnitudes and point in the same direction (parallel to ω) Magnetic field global polarization: magnetic field coupling: P H µ H B acquired and polarizations are of same magnitudes, but point in opposite directions (parallel to B) On average ω B and both are perpendicular to the reaction plane. The reaction plane is spanned by the collision axis and the direction of the impact parameter. P ± P disentangle the vorticity contribution from the magnetic field contribution. 4 / 2

8 Motivation: extend STAR results to the LHC energies Dependence on the collision energy? (%) P H Au+Au 2 5% arxiv: [nucl ex] arxiv: [nucl ex] PRC (27) PRC (27) arxiv:6.477 [nucl th] Talk by M. Lisa at SQM6 Talk by I. Upsal at QM7 Recent STAR submission: arxiv: [nucl-ex] 2??? I. Karpenko, F. Becattini, arxiv:6.477 [nucl-th] 2 3 (GeV) s NN What to expect at the LHC energies? 5 / 2

9 What to expect at the LHC energies? Assumptions: the slope of directed flow.5(v π+ + v π ) closely follows the vorticity value hyperon polarization.5(p + P ) closely follows the vorticity value same polarization feed-down correction at all collision energies P H 4% Au Au: or y= dν dy % Pb Pb: 2 5% Au Au: dν /dy of π +, ICPPA 2 dν /dy of π, ICPPA 2 dν /dy of ch, PRL (23).5(P + P ), arxiv: PRC (27) P, arxiv:6.477 [nucl th] Polarization-to-v -slope ratio:.78 ±.9 Expectations: 2 GeV: (.26 ±.3)% T. Niida (.8 ±.8)% P H dν /dy blue.5.5(red + black) green.5 2.5(red + black) fit:.78 ±.9 3 s NN (GeV) 2.76 TeV: (.7 ±.)% 6 / 2

10 Observable to measure Hyperon decay channels: p + π and p + π +. Probability density of daughter (anti-)proton in ( ) rest frame: dw d sin θ p dφ p = ( ) + α 4π, P H cos θp α = α =.642 ±.3 hyperon decay parameter θ p polar angle of p or p in hyperon rest frame φ p azimuthal angle of p or p in hyperon rest frame 7 / 2

11 Observable to measure Hyperon decay channels: p + π and p + π +. Probability density of daughter (anti-)proton in ( ) rest frame: dw d sin θ p dφ p = ( ) + α 4π, P H cos θp α = α =.642 ±.3 hyperon decay parameter θ p polar angle of p or p in hyperon rest frame φ p azimuthal angle of p or p in hyperon rest frame P H component RP, averaged over events: P, = 8 sin(φ p ψ RP ) πα, ψ RP reaction plane angle 7 / 2

12 Measurement strategy Polarization is measured using well-established anisotropic flow techniques: sin(φ p ψ () EP ) P = 8, πα, R () EP ψ () EP, R() EP first-order event plane angle and its resolution ψ () EP is evaluated with neutron ZDC detectors (anti-)proton and pion tracks are reconstructed in ITS+TPC background contribution in the numerator is subtracted by considering the numerator as a function of p + π inv.mass Statistical uncertainty of the measurement: 8 (2R () EP #hyperons) πα, 8 / 2

13 The ALICE Detector (anti-)proton and pion tracks ( η <.9) are reconstructed in ITS+TPC () ψep is evaluated with neutron ZDC detectors ( η & 8.8) event centrality is evaluated with V detectors ( 3.7 < η <.7 and 2.8 < η < 5.) 9 / 2

14 Event plane reconstruction with neutron ZDCs The two neutron Zero Degree Calorimeters are: located 4 m away from the IP on both sides (A and C) measure energy deposits E i of spectator neutrons 2 2-tower (azimuthal angles φ i = 45, 35, 225, 35 ) ψ () EP is estimated separately by each ZDC: ψ ZDCA,C = atan2(y A,C, X A,C ) the raw flow vectors X raw = 4 E i cos φ i i= Y raw = 4 E i i= are re-centered to satisfy X = Y = 4 E i sin φ i i= 4 E i i= / 2

15 Event plane reconstruction with neutron ZDCs The two neutron Zero Degree Calorimeters are: located 4 m away from the IP on both sides (A and C) measure energy deposits E i of spectator neutrons 2 2-tower (azimuthal angles φ i = 45, 35, 225, 35 ) ψ () EP is estimated separately by each ZDC ZDCC )> ψ ZDCA <cos(ψ ALICE Preliminary Pb Pb 2 s NN = 2.76 TeV Centrality (%) ALI PREL 9757 Assuming same reaction plane resolution on both sides: R () EP cos(ψ ZDCA ψ ZDCC ) R () EP.39 at the peak / 2

16 What to expect from ALICE Pb TeV data Statistical uncertainty of the measurement: 8 πα, (2R () EP #hyperons) ZDCC )> ψ ZDCA <cos(ψ ALICE Preliminary Pb Pb 2 s NN = 2.76 TeV Centrality (%) ALI PREL 9757 Pb Pb@2.76 TeV data: taken in 2 and 2 on average, about three per event at centrality 3% (similar yield for ) 2M events in 5-5% 2M events in 5-55% 2 / 2

17 What to expect from ALICE Pb TeV data Statistical uncertainty of the measurement: 8 πα, (2R () EP #hyperons) Expected statistical uncertainty of the measurement:.%. Expected polarization value (.7 ±.)%. ZDCC )> ψ ZDCA <cos(ψ ALICE Preliminary Pb Pb 2 s NN = 2.76 TeV Centrality (%) ALI PREL 9757 Pb Pb@2.76 TeV data: taken in 2 and 2 on average, about three per event at centrality 3% (similar yield for ) 2M events in 5-5% 2M events in 5-55% 2 / 2

18 * Extraction of signal sin(φ p ψ ZDCA,C ) loose and selection criteria were applied for more / yield f (M) background fraction is evaluated as a function of the invariant mass M pπ Entries Pb Pb 2 s NN = 2.76 TeV data background fit ALICE Preliminary p >.5 GeV/c T y <.5 bgr fraction (scaled) Centrality 2 3% ZDCA )> ψ p <sin(φ ALICE Preliminary data fit Pb Pb 2 s NN = 2.76 TeV.8 < p <. GeV/c T y < M (GeV/c 2 pπ + ) ALI PREL M (GeV/c 2 pπ ) ALI PREL 9748 Fit for signal extraction: ( f (M)) p H + f (M) [linear function] p H the polarization signal evaluated in slices of event centrality, particle s transverse momentum or particle s rapidity 3 / 2

19 Results, Pb TeV Main sources of systematic uncertainties (as fractions of stat. uncertainty): non-uniform acceptance of and reconstruction, admixture of higher harmonic terms into the global polarization measurement % fitting procedure, mainly after assuming zero background contribution 2-3% (5-5% cent) or 5-2% (5-5% cent) p + π pairs selection 2-3% 5-5% centrality: 5-5% centrality: P (%) =. ±.3(stat) ±.4(syst) P (%) =.9 ±.3(stat) ±.8(syst) P (%) =.8 ±.(stat) ±.4(syst) P (%) =.5 ±.(stat) ±.3(syst) Observed values are compatible with expectations 4 / 2

20 Measured P H vs centrality (%) P H.5 Pb Pb = 2.76 TeV s NN ALICE Preliminary p >.5 GeV/c T y < syst. error ± σ for P, centralities 5 5% and 5 5% ± σ for P, centralities 5 5% and 5 5% Centrality (%) ALI PREL / 2

21 Measured P H vs p T and rapidity (%) P H 3 2 Pb Pb s NN = 2.76 TeV ALICE Preliminary (%) P H 3 2 Pb Pb s NN = 2.76 TeV ALICE Preliminary y <.5 Centrality 5 5% 2 3 syst. error ± ± σ for P σ for P y <.5 Centrality 5 5% p (GeV/c) T ALI PREL syst. error ± ± σ for P σ for P p (GeV/c) T ALI PREL 9644 (%) P H 2 Pb Pb s NN = 2.76 TeV ALICE Preliminary p >.5 GeV/c T Centrality 5 5% (%) P H 2 Pb Pb s NN = 2.76 TeV ALICE Preliminary p >.5 GeV/c T Centrality 5 5% syst. error syst. error 2 ± ± σ for P σ for P 2 ± ± σ for P σ for P ALI PREL y y ALI PREL / 2

22 Feed-down correction I A considerable fraction of and are feed-down daughters of heavier particles. X +... channel Br( + X), % fraction f X 4/3 s(s + ) spin transfer t X Σ + γ.3 ±.2 /3 Σ(385) ±, + π ±, 87.3 ±.2 5 /3 Ω + K 67.8 <.7 5 /3 Ξ ±, + π ±, <.23.9 or.927 P meas measured, P true true polarization: P, meas = ( X f X )P, true + X f X t X P X,true f X = n X /n fractions of production yields t X average spin transfers thermal vorticity model: polarizations of produced particles are proportional to s(s + ), where s is particle s spin 7 / 2

23 Feed-down correction II X +... channel Br( + X), % fraction f X 4/3 s(s + ) spin transfer t X Σ + γ.3 ±.2 /3 Σ(385) ±, + π ±, 87.3 ±.2 5 /3 Ω + K 67.8 <.7 5 /3 Ξ ±, + π ±, <.23.9 or.927 ( P, true = P, meas 4 3 f Σ f Σ(385) f Ω. f Ξ Contributions from Ω and Ξ are 8% and -2%. The Σ(385) contribution is large due to the model-dependent coefficient 5. ) A conservative estimate: [ polarization scale feed-down] = ( 4/3 f Σ ) =.7 ±.5 8 / 2

24 Summary I: results Global hyperon polarization is a direct probe of the vorticity and the magnetic field. Pb Pb@2.76 TeV: All available ALICE Pb Pb@2.76 TeV data was analyzed. The measured polarizations of and hyperons are compatible with expectations and are consistent with zero within the precision of the measurement. Expected significance of the combined + result is at a one sigma level. A 3 sigma significance requires times more data. 9 / 2

25 Summary II: future Global hyperon polarization is a direct probe of the vorticity and the magnetic field. Assuming same ZDC event plane resolution and same feed-down: Polarization is expected to decrease very slowly with collision energy. The measurement becomes more feasible at higher collision energies due to a faster increase of the hyperon yield. Pb Pb@5.2 TeV: up to 2 better significance due to 2 more hyperons with similar amount of events. It is worth combining Pb Pb@2.76 TeV and Pb Pb@5.2 TeV. 2 / 2

26 Backup

27 v.5-3 (a) odd even v -2% 3-4% -6% with fit -.5 / p T x p.5 odd even p / p x T -6% with fit (b) v ALICE Pb-Pb@2.76TeV p >.5 GeV/c T odd v 3-6% with fit (c) -.5 STAR (scaled) odd v.37 2GeV GeV Au-Au 3-6% p >.5 GeV/c T η 22 / 2