Measurement of J/ψ polarization in the di-electron channel in p+p collisions at √s = 200 GeV with the STAR experiment

PAs: Siwei Luo, Bingchu Huang, Zhenyu Ye

Target Journal: Physics Review D

 
Abstract:
We present a new measurement of J/ψ polarization via the di-electron decay channel in p+p collisions at √s = 200 GeV with the STAR experiment. The polarization parameters λθ, λφ, λθφ and λinv are extracted in both the J/ψ helicity and Collins-Soper frames using a maximum likelihood method. The results does not show strong dependence on the J/ψ transverse momentum in the range of 1-10 GeV/c. The results are compared with theoretical model predictions.
Key Words: J/ψ cross section, polarization.
 

 

Fig.1
Figure 1. Invariant mass spectra of di-electron pairs in different pT bins (from left to right: pT = 0-2, 2-4, 4-6, 6-8, 8-14 GeV/c). The black markers (blue filled histograms) are the spectra from unlike-sign (like-sign) charge pairs, while the red marker represent those obtained by subtracting the like-sign spectra from the unlike-sign ones. The latter are fit to Double Crystal functions represented by the red solid curves.

 

Fig.1
Figure 2. Two-dimensional distributions of J/ψ candidates in the plane (φ,cosθ) in different pT bins (from left to right: pT = 0-2, 2-4, 4-6, 6-8, 8-14 GeV/c). The top (bottom) row shows the distributions in the Helicity (Collins-Soper) frame.

 

Fig.3
Figure 3. J/ψ reconstruction efficiency as a function of (φ,cosθ) in different pT bins (from left to right: pT = 0-2, 2-4, 4-6, 6-8, 8-14 GeV/c). The top (bottom) row shows the efficiencies in the Helicity (Collins-Soper) frame.

 

Fig.6
Figure 4. Likelihood contours and fit results for J/ψ polarization parameters λθ and λφ in different pT bins (from left to right: pT = 0-2, 2-4, 4-6, 6-8, 8-14 GeV/c). The top (bottom) row shows the results in the Helicity (Collins-Soper) frame. The filled areas are the 1-σ contours of the likelihood function, representing the 68% confidence intervals that are evaluated with the λθφ values fixed by the maximum likelihood fit. The red points indicate the results of the maximum likelihood fits.

 

Fig.6
Figure 5. Same as Figure 4 but for λθ and λθφ.

 

Fig.6
Figure 6. Same as Figure 4 but for λφ and λθφ.

 

Fig.6
Figure 7. Distributions of J/ψ candidates as a function of φ in different pT bins (from left to right: pT = 0-2, 2-4, 4-6, 6-8, 8-14 GeV/c). The points show the φ distributions in the Helicity (top) and Collins-Soper (bottom) frames. The solid histograms correspond to the expected φ distributions based on J/ψ polarization paramters extracted from the data. The dashed red and blue histograms are the expected φ distributions with λφ = ±1, respectively.

 

Fig.6
Figure 8. Same as Figure 7 but for the cosθ distributions.

 

Fig.7
Figure 9. The J/ψ polarization parameters (from left to right: λθ, λφ, λθφ) as a function of pT in the Helicity (top) and Collins-Soper (bottom) frames. The red points are the results from this measurement with the statistical (systematic) uncertainties shown as the vertical bars (red shaded areas), the black (blue) points are the PHENIX (STAR) previously published results. The theoretical predictions based on Color-Singlet and Color-Octet models are shown as the grey filled area and blue filled area, respectively.

 

Fig.7
Figure 10. The J/ψ polarization parameters λinv. The red (blue) points represent the results in the Helicity (Collins-Soper) frame with the statistical (systematic) uncertainties shown as the vertical bars (shaded areas).
Conclusion:

1. The J/ψ polarization parameters have been extracted in both the helicity and Collins-Soper frames. These are the first measurement of λφ, λθφ and λinv in the Helicity frame, and the first J/ψ polarization measurement in the Collins-Soper frame for p+p collisions at √s = 200 GeV.

2. The results for λθ in the helicity frame are consistent with the previously published PHENIX and STAR results in the overlapping region, and are compatible with theoretical predictions from the Color-Singlet and Color-Octet models. The λinv in helicity frame and Collins-Soper frame are consistent within uncertainties.