Global polarization of <math><mi mathvariant="normal">Λ</mi></math> and <math><mover accent="true"><mi mathvariant="normal">Λ</mi><mo>¯</mo></mover></math> hyperons in <math><mrow><mi>Au</mi><mo>+</mo><mi>Au</mi></mrow></math> collisions at <math><mrow><msqrt><msub><mi>s</mi><mrow><mi>N</mi><mi>N</mi></mrow></msub></msqrt><mo>=</mo><mn>19.6</mn></mrow></math> and 27 GeV - INSPIRE

DataBETA

Global polarization of $Λ$ and $\bar{Λ}$ hyperons in $Au + Au$ collisions at $\sqrt{s_{N N}} = 19.6$ and 27 GeV

STAR

Collaboration

May 15, 2023

9 pages

Published in:

Phys.Rev.C 108 (2023) 1, 014910

Published: Jul 27, 2023

e-Print:

2305.08705 [nucl-ex]

DOI:

10.1103/PhysRevC.108.014910 (publication)

Experiments:

BNL-RHIC-STAR

View in:

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Abstract: (APS)

In relativistic heavy-ion collisions, a global spin polarization,

P_{H}

, of

Λ

and

\bar{Λ}

hyperons along the direction of the system angular momentum was discovered and measured across a broad range of collision energies and demonstrated a trend of increasing

P_{H}

with decreasing

\sqrt{s_{N N}}

. A splitting between

Λ

and

\bar{Λ}

polarization may be possible due to their different magnetic moments in a late-stage magnetic field sustained by the quark-gluon plasma which is formed in the collision. The results presented in this study find no significant splitting at the collision energies of

\sqrt{s_{N N}} = 19.6

and 27 GeV in the BNL Relativistic Heavy Ion Collisions Beam Energy Scan Phase II using the STAR detector, with an upper limit of

P_{\bar{Λ}} - P_{Λ} < 0.24

% and

P_{\bar{Λ}} - P_{Λ} < 0.35

%, respectively, at a 95% confidence level. We derive an upper limit on the naive extraction of the late-stage magnetic field of

B < 9.4 \times 10^{12}

T and

B < 1.4 \times 10^{13}

T at

\sqrt{s_{N N}} = 19.6

and 27 GeV, respectively, although more thorough derivations are needed. Differential measurements of

P_{H}

were performed with respect to collision centrality, transverse momentum, and rapidity. With our current acceptance of

| y | < 1

and uncertainties, we observe no dependence on transverse momentum and rapidity in this analysis. These results challenge multiple existing model calculations following a variety of different assumptions which have each predicted a strong dependence on rapidity in this collision-energy range.

beam: energy
quark gluon: plasma
heavy ion: scattering
gold
Lambda: polarization
Antilambda: polarization
Lambda: magnetic moment
Antilambda: magnetic moment
spin: polarization
rapidity dependence

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