Tracking the baryon quantum number with heavy-ion collisions

Updated on Thu, 2024-08-15 17:14 by ctsang. Originally created by nlewis on 2023-04-05 12:31.

Title: Tracking the baryon quantum number with heavy-ion collisions

PA's: Nicole Lewis, Chun Yuen (Tommy) Tsang, Yang Li, Daniel Brandenburg, Jia Chen, Xiaofeng Luo, Hui Liu, Wendi Lv, Rongrong Ma, Zebo Tang, Prithwish Tribedy, Zhangbu Xu

Target Journal: Science

https://drupal.star.bnl.gov/STAR/system/files/Analysis_note_baryon_stopping-04272024.pdf

https://drupal.star.bnl.gov/STAR/system/files/Photonuclear_Analysis_Note_4_27_24.pdf
Final-After-Institutions-Comments:

https://drupal.star.bnl.gov/STAR/system/files/Analysis_note_baryon_stopping.pdf

https://drupal.star.bnl.gov/STAR/system/files/Photonuclear_Analysis_Note_4_27_24.pdf (No change)
Final-After-Institutions-Comments-Round2:
https://drupal.star.bnl.gov/STAR/system/files/Analysis_note_baryon_stopping-08152024.pdf
https://drupal.star.bnl.gov/STAR/system/files/Photonuclear_Analysis_Note_4_27_24.pdf (No change)

Abstract:

Baryon number ($\boldsymbol{B}$) is a conserved quantum number that prevents protons ($\boldsymbol{B=1}$) (Phys. Rev. D 95, 012004 446 (2017), Phys. Rev. D 105, 112012 (2022)) and stable nuclei from disintegration and governs the visible Universe as it is today. The cosmological hypothesis of baryogenesis (Usp. Fiz. Nauk 161, 61–64 (1991)) postulates that the total baryon number of the Universe was generated at the very beginning of the Big Bang and has been constant ever since. It is conventionally assumed that the unit baryon number in a nucleon is distributed equally among its three valence quarks. An alternative picture was proposed at the birth of the theory of the Quantum Chromodynamics (QCD) half a century ago that the baryon number is instead traced by the baryon junction, a nonperturbative Y-shaped topology of gluons that is connected to all three valence quarks~\cite{Artru:1974zn,Rossi:1977cy}. Neither of these pictures have been experimentally verified. Because the electric charge of a baryon is carried by the valence quarks and these quarks have different interaction cross sections and distribution functions than gluon junctions, one can test which entity carries the baryon number by comparing baryon transport to charge transport. The STAR Collaboration reports here that the baryon transport is a factor of two larger than the charge transport in head-on isobar $\boldsymbol{{}^{96}_{44}\rm{Ru} + {}^{96}_{44}\rm{Ru}}$ and $\boldsymbol{{}^{96}_{40}\rm{Zr} + {}^{96}_{40}\rm{Zr}}$ collisions at a center-of-mass energy per nucleon-nucleon pair of ($\boldsymbol{\sqrt{s_{_{\rm{NN}}}}}$) = 200 GeV.

STAR also reports significant baryon transport and its rapidity asymmetry in photonuclear processes. This study was conducted using ultra-peripheral $\boldsymbol{\rm{Au}\rm{+}\rm{Au}}$ collisions at $\boldsymbol{\sqrt{s_{_{\rm{NN}}}} = 54.4}$ GeV, in which one nucleus emits a quasi-real photon that interact with the other colliding nucleus.

The rapidity dependence of the measured baryon transport is comparable to those observed in hadronic nucleus-nucleus collisions, and to the theoretical prediction based on gluonic topological distribution inside nucleons (Phys. Lett. B 378, 238–246 (1996)). These observations are incompatible with the scenario that the baryon number is carried by the valence quarks.

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Summary:

In this paper we compared charge stopping to baryon stopping using identified spectra from isobar collision data of $\rm{Ru}+\rm{Ru}$ and $\rm{Zr}+\rm{Zr}$ collisions. In central collisions, the net-baryon yield was twice as large the difference in net-charge yield between collision species. We also observed baryon stopping and rapidity asymmetry in $\gamma+\rm{Au}$-rich collisions. The net-proton $dN/dy$ slope in these collisions is qualitatively comparable to the slope measured in hadronic $\rm{Au}+\rm{Au}$ collisions for all centralities. Both of these results disfavor the assertion that valence quarks carry the baryon number.

Supporting Documents:
Delta decays: Study 1, Study 2
Consistency check with other STAR results: https://drupal.star.bnl.gov/STAR/system/files/Consistency%20check%20with%20other%20STAR%20results.pdf

Weekly meeting agenda:
https://drupal.star.bnl.gov/STAR/blog/marr/Baryon-stopping-analysis-group

PWGC Preview Presentation:
https://drupal.star.bnl.gov/STAR/system/files/PWGC_real.pdf

Preliminary Request Presentations:
https://drupal.star.bnl.gov/STAR/blog/ctsang/Baryon-stopping-charge-stopping-preliminary-figures-request
https://drupal.star.bnl.gov/STAR/blog/nlewis/Baryon-Stopping-Photonuclear-Collisions-Preliminary-Request