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Paper proposal for kaon anti-flow

Updated on Thu, 2025-03-13 23:36. Originally created by liuzw on 2023-09-27 06:48.

Paper Title: Measurement of Kaon Directed Flow in Au+Au Collisions in the High Baryon Density Region

Target journal: PRL

PAs: Xin Dong, Chitrasen Jena,  Li-Ke Liu, Zuowen Liu, Kishora Nayak, Sooraj Radhakrishnan, Sharang Rav Sharma, Shusu Shi, Guoping Wang, Xing Wu, Nu Xu
PA representative: Zuowen Liu

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Abstract:

Rapidity-odd directed flow $v_1$ measurements are presented for $K^{\pm}$ and $K^0_S$ in Au + Au collisions at $\sqrt{s_{NN}}$ = 3.0, 3.2, 3.5, and 3.9 GeV with STAR experiment. For comparison, $v_1$ of $\pi^{\pm}$, protons, and $\Lambda$s from the same collisions are also discussed. The mid-rapidity $v_1$ slope $dv_1/dy|_{y=0}$ for protons and $\Lambda$s are all positive in those collisions. On the other hand, a strong transverse momentum dependence of the $v_1$ slope of kaons is observed: negative $v_1$ slopes are observed at low $p_T$ region, $p_T < 0.6$~GeV/$c$, while positive slopes are seen at high $p_T$ region. A similar $p_T$ dependence is also evident for the $v_1$ slope of charged pions. Compared to the spectator-removed calculations in Au+Au collisions at 3.0–3.9 GeV, the JAM model demonstrates a pronounced shift of the $v_1$ slopes of mesons towards the negative direction. This supports that the observed kaon anti-flow at low $p_T$ arises from the shadowing effect of the spectators in the non-central collisions in the high baryon density region.


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Paper figures:



Fig. 1. The efficiency uncorrected density distribution in transverse momentum ($p_T$) and particle rapidity ($y$) for $\pi^+$, $K^+$, $K^-$, and $K_S^0$ measured with STAR detectors TPC and TOF in Au + Au collisions at $\sqrt{s_{NN}}$ = 3.0, 3.2, 3.5, and 3.9 GeV. Note that the red arrows show the target rapidity in the center-of-mass frame.


Fig. 2. Directed flow ($v_1$) of $\pi^{+}$ (solid square), $\pi^{-}$ (open square), $K^{+}$ (solid diamond), $K^{-}$ (open diamond), $K^{0}_S$ (solid star), protons (solid circle), and $\Lambda$s (open circle) as a function of rapidity in Au + Au 10-40\% collisions at $\sqrt{s_{NN}}$ = 3.0, 3.2, 3.5, and 3.9 GeV. Statistical and systematic uncertainties are shown as bars and gray bands, respectively. Data points of $K^{+}$ are shifted horizontally to improve visibility. The JAM calculations for $K^0$ and protons are represented by red and blue lines, with the dash and solid lines representing cascade and baryonic mean-field (BMF) modes, respectively.

Fig. 3. Collision energy dependence of the $p_T$ integrated mid-rapidity $v_1$ slope ($dv_1/dy|_{y=0}$) for $\pi^{\pm}$, $K^{\pm}$, $K^0_S$, protons, and $\Lambda$s in Au + Au 10-40\% collisions. Statistical and systematic uncertainties are shown as bars and gray bands, respectively. Data points are staggered by $\pm$ 0.5 GeV horizontally to improve visibility. The JAM calculations with baryon mean field for protons and $\Lambda$s shown as solid and dashed lines, respectively. Without the mean field option, the predicted $v_1$ slopes are about a factor of four smaller for both baryons. The $p_T$ ranges for pions, kaons, and protons$/\Lambda$s are $0.2 < p_T < 1.6$ GeV/$c$, $0.4 < p_T < 1.6$ GeV/$c$, and $0.4 < p_T < 2.0$ GeV/$c$, respectively.

Fig. 4. Transverse momentum ($p_T$) dependence of the mid-rapidity $v_1$ slopes ($dv_1/dy|_{y=0}$) for $\pi^{+}$, $K^{+}$, $K^{-}$, and $K^{0}_{S}$ in Au + Au 10-40\% collisions at $\sqrt{s_{NN}}$ = 3.0, 3.2, 3.5, and 3.9 GeV. Statistical and systematic uncertainties are shown as bars and gray bands, respectively. Data points of $\pi^{+}$ and $K^{0}_{S}$ are shifted $\pm$ 0.05 GeV/$c$ horizontally for better visibility. The JAM model calculations of $dv_1/dy|_{y=0}$ for $K^{0}$: with and without the spectators are shown as blue and red solid lines, respectively. Note that $dv_1/dy|_{y=0}$ of $K^{\pm}$ at $p_T < 0.4$ GeV/$c$  are not measured for 3.0 and 3.2 GeV due to the limited of acceptance.
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Conclusions:

In summary, we report measurements of $v_1$ for $\pi^{\pm}$, $K^{\pm}$, $K_S^0$, protons, and $\Lambda$s in mid-central (10-40\%) Au + Au collisions at $\sqrt{s_{NN}}$ = 3.0, 3.2, 3.5, and 3.9 GeV. The $v_1$ slopes of all above mentioned particles decrease in magnitude as collision energy increases. A strong $p_T$ dependence is observed in the slopes of $v_1$ ($dv_1/dy|_{y=0}$) for both pions and kaons, where the slopes are negative at low $p_T$ ($p_T \le 0.6$ GeV/$c$) in all of these collisions. As mentioned in the introduction, the E895 experiment reported kaon anti-flow (quantified by measurements of sideward flow $\langle p_x \rangle$) in a similar low $p_T$ region, which was attributed to a repulsive kaon potential. We applied the same $p_T$ cut to measure sideward flow $\langle p_x \rangle$ of kaons and protons as was used in the E895 experiment. The $\langle p_x \rangle$ flow of protons at $\sqrt{s_{NN}}$ = 3.9 GeV shows consistency between the STAR and E895 experiments. However, the anti-flow of $K^0_S$ measured by STAR is a factor of eight lower than that observed by E895. Additionally, the STAR measurements show negative mid-rapidity $v_1$ slopes at low $p_T$ for the different kaon species.

In the hadronic transport model JAM, a baryonic mean field is necessary to reproduce the observed mid-rapidity $v_1$ of protons and $\Lambda$s. Furthermore, the JAM model calculations, encompassing both scenarios with and without spectators, demonstrate that the presence of spectators induces a shift in the $v_1$ slope of mesons towards the negative direction, even in the absence of a kaon potential. This suggests that the anti-flow of kaons could be due to the shadowing effect of spectators rather than being exclusively caused by the kaon potential. Our findings indicate that spectators contribute significantly to the observed anti-flow of kaons at low $p_T$ in the high baryon density region of heavy-ion collisions.

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Presentations: 

Paper proposal at FCV meeting:
https://drupal.star.bnl.gov/STAR/system/files/Kaon_antiflow_paperProposal_ver5.pdf
https://drupal.star.bnl.gov/STAR/system/files/Kaon_antiflow_paperProposal_ver4.pdf
https://drupal.star.bnl.gov/STAR/system/files/Kaon_antiflow_paperProposal.pdf

Oral talk at QM 23:
https://drupal.star.bnl.gov/STAR/system/files/QM23_AnisotropicFlow_Zuowen.pdf
Talks at STAR Collaboration Meeting:
Spring 2023: https://drupal.star.bnl.gov/STAR/system/files/lightHadrons_v1v2_FXT.pdf
Autumn 2022: https://drupal.star.bnl.gov/STAR/system/files/CollMeeting_FXT_FlowAnalysis_0.pdf

Presentations of preliminary request at FCV meeting:
3.0 GeV(Zuowen):
https://drupal.star.bnl.gov/STAR/system/files/3GeV_RequestPreliminary.pdf
https://drupal.star.bnl.gov/STAR/system/files/3GeV_piKp_K0sLam_sysmaticUncertainty.pdf
3.2 GeV (Li-ke):
https://drupal.star.bnl.gov/STAR/system/files/3p2_Official_FCVPWG_0607.pdf
https://drupal.star.bnl.gov/STAR/system/files/3p2_K0SFlow_request_preliminary.pdf
3.5 GeV(Zuowen):
https://drupal.star.bnl.gov/STAR/system/files/3p5GeV_RequestPreliminary.pdf
https://drupal.star.bnl.gov/STAR/system/files/Update_kaonAntiflow_chargedKaonProton_3and3p5GeV.pdf
3.9 GeV(Xing and Guoping):
https://drupal.star.bnl.gov/STAR/system/files/pion_flow_3.9GeV.pdf
https://drupal.star.bnl.gov/STAR/system/files/3p9GeV_ks0_lambda_request_preliminary_PWG.pdf
https://drupal.star.bnl.gov/STAR/system/files/3p9GeV_LamK0S_Systematic_uncertainties_update.pdf

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