Title:

Probing the gluonic structure in deuteron using d+Au ultra-peripheral collisions

PAs: 

Z. Tu*, J. Adam, Z. Chang, W. Schmidke, T. Ullrich, and S. Yang

Target Journal:

PRL

Abstract:

Gluon density distribution and its modification in nucleus are one of the most fundamental questions in nuclear physics. In recent years, the ultra-peripheral collisions (UPCs) at heavy-ion colliders have provided a new way of probing the gluon dynamics via diffractive vector meson photoproduction, e.g., the $J/\psi$ particle. Based on the UPC data at the LHC, it is suggested that the gluon density is strongly modified in the lead nucleus, while the underlying physics mechanism remains inconclusive. In this Letter, we report the first measurement of photoproduction of $J/\psi$ particle off the deuteron target at center-of-mass energy $\sqrt{s_{_{\rm NN}}}=200~\rm GeV$ in $\rm d+Au$ collisions. The differential cross section as a function of momentum transfer $p^{2}_{\rm T,J/\psi}$ is measured. In addition, the data with neutron tagging in the deuteron-going Zero-Degree Calorimeter is investigated for the first time, which is found to be consistent with expectation of incoherent diffractive scattering at low momentum transfer. Theoretical predictions based on the Color Glass Condense is compared with the measured data, where a good agreement has been observed, especially in favor of significant sub-nucleonic shape fluctuations. The data reported in this measurement provide a great insight into the gluonic structure in deuteron, as well as an experimental baseline for probing the short-range nuclear correlation in deuteron at the future Electron-Ion Collider.

Conclusion:

The differential cross section of photoproduction of $J/\psi$ is measured as a function of momentum transfer $p^{2}_{\rm T,J/\psi}$ in dAu ultra-peripheral collisions at $\sqrt{s_{_{\rm NN}}}=200~\rm GeV$ using the STAR detector. The data is corrected to the process of photon-deuteron system, where the final-state particles from the deuteron breakups are all included. In addition, the measured cross section with a single neutron detected in the forward Zero-Degree Calorimeter is reported. The data are compared with theoretical predictions based on the Color Glass Condense. The data is well described by the model for most of the measured $p^{2}_{\rm T,J/\psi}$ range from both Hulthen and AV18 deuteron wavefunctions; however, the sub-nucleonic shape fluctuation for describing the incoherent process is favored by the measured data. This is a first time measurement of photoproduction of $J/\psi$ off deuteron in UPC at hadronic center of mass energy of 25 GeV. The spectator tagging technique has been explored for the first time in collider facility and served as an experimental baseline for future investigation at the Electron-Ion Collider.

Figure.1. Illustration of dAu ultra-peripheral collisions with $J/\psi$ photoproduction off deuteron. 

Figure. 2. Left: invariant mass distribution and fit of $J/\psi$ particle photoproduced off the deuteron target at hadronic 
center-of-mass energy 25 GeV in dAu ultra-peripheral collisions.  Right: Zero-Degree Calorimeter (ZDC)  deposit energy (ADC) distribution
for both gold- and deuteron-going direction. 

Figure. 3
Differential cross section of photoproduction of $J/\psi$ particle as a function of $p^{2}_{T, J/\psi}$
in photon-deuteron center-of-mass system at energy of 25 GeV, shown in solid marker.
Data with neutron tagging in the deuteron-going ZDC is also shown with open marker. Theoretical predictions based on
Color Glass Condense are compared with data. The coherent contribution is predicted from the Hulthen wavefunction denoted by
the blue solid line, while incoherent contribution that has saturation scale ($Q_s$) and shape fluctuation is represented by the brown
solid line. The two dashed lines represent the total contribution (coherent and incoherent), where the coherent contribution is predicted
from the Hulthen wavefunction (blue solid line), while the black dashed line is without the sub-nucleonic $Q_s$/shape fluctuations.
Statistical uncertainty is presented by the error bars, and the systematic uncertainty is denoted by the shaded box. 
 

Table. 1. Based on different theoretical predictions as templates, the data is fitted with different combinations of templates. The coherent 
contribution as Hulthen and AV18 wavefunctions, while the incoherent contribution has w/o the fluctuations. The total cross section, coherent cross section,
incoherent cross section for $t<1.2 GeV^{2}$, total cross section at $t=0$, and the goodness of fit are shown. The first error is statistical, and the second error
is systematical. The systematic uncertainty includes both fitting systematic uncertainty, e.g., the fit range, and the systematic uncertainty from the measurement. 

Relevant PWG presentations:
06/03/2019: https://drupal.star.bnl.gov/STAR/system/files/dAu200_06_03_2019.pdf
08/05/2019: https://drupal.star.bnl.gov/STAR/event/2019/08/05/lfs-upc-pwg
11/18/2019: https://drupal.star.bnl.gov/STAR/event/2019/11/18/lfs-upc-pwg-meeting/upc-jpsi-dau-200-gev
05/18/2020: https://drupal.star.bnl.gov/STAR/event/2020/05/18/lfs-upc-pwg-meeting/update-dau-jpsi-analysis
12/14/2020: https://drupal.star.bnl.gov/STAR/event/2020/12/14/lfs-upc-pwg-meeting/photoproduction-jpsi-dau-upc

Analysis Note:
https://drupal.star.bnl.gov/STAR/blog/starkong/star-upc-dau-analysisnote

Paper Draft:
https://drupal.star.bnl.gov/STAR/blog/starkong/star-upc-dau-paperdraft

PWGC Preview Presentation:
https://drupal.star.bnl.gov/STAR/blog/starkong/star-upc-dau-pwgc-preview-presentation

Code for GPC - PSN0768
/star/u/starkong/gpfs01/STAR_dAu_GPC_CodeQA_July2

CWR material:
* Manuscript v7, https://drupal.star.bnl.gov/STAR/system/files/STAR_UPC_dAu_paperdraft-v7.pdf
* Supplementary Material v4, https://drupal.star.bnl.gov/STAR/system/files/Supplementary-Material-v4.pdf
* AN v6, https://drupal.star.bnl.gov/STAR/system/files/AN-STAR-dAu-Jpsi-v4_0.pdf

CWR response:
https://drupal.star.bnl.gov/STAR/blog/starkong/gpc-317-collaboration-review-reponse

Referee report & response:
https://drupal.star.bnl.gov/STAR/blog/starkong/GPC-317-Referee-report-and-responses-1