Title: Probing Late-Stage Hadronic Interactions at High Baryon Density via K^*0 Production in the RHIC Beam Energy Scan Program

PAs: Aswini Kumar Sahoo^[1], Md Nasim^[1] and Subhash Singha^[2]
    
             [1] Indian Institute of Science Education and Research, Berhampur, 760010, India
             [2] Institute of Modern Physics, Chinese Academy of Sciences, Lanzhou, 73000, China

Target journal: Physics Letters B

Abstract:
The precise yield measurement of K^*0 meson is reported in Au+Au collisions at √s_NN = 7.7, 11.5, 14.6, 19.6 and 27 GeV using high statistics data sample accumulated by the STAR detector at the Relativistic Heavy-Ion Collider (RHIC) from the Beam Energy Scan II (BES-II) program. The transverse momentum spectra is measured at mid rapidity (|y| < 1.0) using its hadronic decay channel (K^*0(\overline{K^*0}) → K^± π ^∓ at various centralities and center of mass energies. The p_T integrated yield ratios K^*0 /K at central collisions show significant suppression compared to peripheral collisions at a level of (1.67-3.34)σ. The thermal model over predicts the ratio at central collisions,  showing a deviation of (6.2-7.8)σ. These observations indicate a loss of the measured K^*0 signal due to rescattering of its decay products within the hadronic phase. At BES energies, the K^*0 /K ratio seems to deviate from the universal multiplicity scaling observed at top RHIC and LHC energies, which could be due to change in hadron-interaction cross section in different type of QCD medium produced at low (baryon dominated) and high (meson dominated) energy regime.

Proposed Figures:

Figure 1:  (Left panel) K^*0 signal from unlike sign pair (red marker) and background estimated from track rotation method (black marker).(Right panel) Invariant mass distribution of Kπ pairs after subtraction of background estimated from track rotation method.The blueline denotes the Breit Wigner fit and redline represents the residual background.

Figure 2: K^*0 p_T spectra at mid rapidity (|y| < 1.0) in various centralities in Au+Au collisions at √s_NN = 7.7, 11.5, 14.6, 19.6 and 27 GeV

Figure 3: K^*0/K ratio as a function of (dN_ch/dy)^1/3 in Au+Au collisions. The measurements are compared with compared with previous experimental results

Figure 4: K^*0/K ratio as a function of √s_NN in Au+Au collisions. The measurements are compared with thermal model (dashed line) and UrQMD model (blue band)

Summary:
The p_T distribution of  K^*0 meson has been measured at mid rapidity (|y| < 1.0) in Au+Au collisions at √s_NN = 7.7-27 GeV, using high statistics data collected from STAR BES-II program. The p_T- integrated yields of K^*0 relative to the charged kaons show suppression with respect to the peripheral collisions, which increases with system size, that is quantified by particle multiplicity ((dN_ch/dy)^1/3). This can be attributed to the rescattring of decay daughters of K^*0 in hadronic phase. Furthermore, the K^*0 /K ratio in central Au+Au collisions deviates from the thermal model calculations at a significance of more than 6σ, where as the ratio is consistent with that of thermal model at peripheral collisions, at various collision energies. This observation further ratify the dominance of rescattering effects within the hadronic phase created in heavy-ion collisions. The K^*0 /K ratio follows a universal multiplicity scaling at top-RHIC and LHC energies, where as at BES energies the scaling seems to break. which could be due to difference in hadronic interactions taking place within the medium created in low and high center of mass collision energies.

Supporting documents:

SQM 2022:  drupal.star.bnl.gov/STAR/system/files/SQM22_Aswini_v6.pdf

LFSUPC presentations:
drupal.star.bnl.gov/STAR/system/files/Kstar_BES-II_STAR_Collab_meet_20mar2024.pdf
drupal.star.bnl.gov/STAR/system/files/LFSUPC_11Dec2023_0.pdf
drupal.star.bnl.gov/STAR/system/files/Kstar_BES-II_STAR_Collab_meet_17oct2023.pdf
drupal.star.bnl.gov/STAR/system/files/Kstar_BES-II.pdf drupal.star.bnl.gov/STAR/system/files/LFSUPC_28feb2023.pdf
drupal.star.bnl.gov/STAR/system/files/LFSUPC_6thDec2022.pdf
drupal.star.bnl.gov/STAR/system/files/sqm_preliminary_request_LFSUPC_0.pdf
drupal.star.bnl.gov/STAR/system/files/LFSUPC_16may2022_BESII_result_0.pdf drupal.star.bnl.gov/STAR/system/files/StarCollaborationMeeting.pdf