Measurements of JPsi polarization in Ru+Ru and Zr+Zr collisions at √(S_NN ) 200GeV at STAR

PAs: Dandan Shen, Qian Yang, Rongrong Ma, Zebo Tang, Li Yi, Wangmei Zha

Target Journal: RLB 

Abstract:

The first inclusive J/ψ polarization is measured in Ru+Ru and Zr+Zr collisions at√(S_NN )= 200 GeV using the STAR detector. The J/ψ is reconstructed through its di-electron decay channel at mid-rapidity (|y| < 0.8). The polarization parameters (λ_θ, λ_ϕ) are measured in both the Helicity frame and Collins-Soper frame. They are found to be consistent with in zero in the J/ψ transverse momentum range of 0.2 to 10 GeV/c for centrality range of 0-80% in both frames, with no significant dependence on centrality or p_T. These results represent the most precise JPsi polarization measurements at the RHIC energy range, with a measurement precision of 0.05 in cosθ in the Helicity frame. The results are compared with those from pp collisions at √(S_NN )= 200 GeV in a similar kinematic region at STAR experiments.

Figure:

 

     

 

Figure 1. The invariant mass distribution of the ee  pair is shown before(blue solid circles) and after(black solid circles) subtracting the mix-event unlike-sign background (black open circles). The green curve represents a fit to the mass spectrum of J/ψ candidates. The red line indicates the residual background, crystal-ball function (green shade) used to describe the J/ψ signals. The error bars represent the statistical uncertainties.

 

 

Figure 2. Upper: The raw J/ψ yield and A×ϵ from the final iteration as a function of e  cosθ (left) and ϕ (right) in the Helicity frame for 0.2<pT<10 GeV/c. Lower: The J/ψ yields corrected for acceptance and efficiency, along with the simultaneous fit.The counts are after arbitrary normalization.

 

 

Figure 3. The inclusive J/ψ polarization parameters (from top to bottom: λθ, λϕ, λinv) as a function of p_T in the Helicity frame (left) and Collins-Soper frame (right). The error bars show statistical uncertainties, while the boxes represent systematic uncertainties.

 

 

Figure 4. The inclusive J/ψ polarization parameters (from top to bottom: λ_θ, λ_ϕ, λ_inv) as a function of centrality in the Helicity frame (left) and Collins-Soper frame (right). The error bars show statistical uncertainties, while the boxes represent systematic uncertainties.

Table:

 

 

Table 1: The inclusive J/ψ polarization parameters in the HX and CS frames in different p_T  bins measured through the di-electron channel within |y|<0.8. The first uncertainty is statistical and the second systematic.

 

 

 

 

Table 2: The inclusive J/ψ polarization parameters in the HX and CS frames in different centrality bins measured through the di-electron channel within |y|<0.8. The first uncertainty is statistical and the second systematic.

Summer:

    We have presented the first measurement of inclusive JPsi polarization in heavy-ion collisions at RHIC.  The JPsi polarization parameters are consistent with zero in the pT range of 0.2 to 10 GeV.c and centrality range of 0-80% for both the HX and CS frames. No significant centrality or pT dependence is observed.

Related presentations:

    Here is a summary of links about related presentations on pwg meeting.
https://drupal.star.bnl.gov/STAR/system/files/Request_preliminary_Jpsi_polarization_0.pdf
https://drupal.star.bnl.gov/STAR/system/files/Request_preliminary_Jpsi_polarization.pdf
https://drupal.star.bnl.gov/STAR/system/files/HP20230810_0.pdf
https://drupal.star.bnl.gov/STAR/system/files/HP_Rawdata2023.4.6.pdf