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Inferring nuclear structure from heavy isobar collisions using Trajectum
by Govert Nijs, Wilke van der Schee
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Authors (as registered SciPost users):  Wilke van der Schee 
Submission information  

Preprint Link:  https://arxiv.org/abs/2112.13771v1 (pdf) 
Date submitted:  20220925 21:35 
Submitted by:  van der Schee, Wilke 
Submitted to:  SciPost Physics 
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Academic field:  Physics 
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Approach:  Theoretical 
Abstract
Nuclei with equal number of baryons but varying proton number (isobars) have many commonalities, but differ in both electric charge and nuclear structure. Relativistic collisions of such isobars provide unique opportunities to study the variation of the magnetic field, provided the nuclear structure is well understood. In this Letter we simulate collisions using several stateoftheart parametrizations of the $^{96}_{40}$Zr and $^{96}_{44}$Ru isobars and show that a comparison with the exciting STAR measurement arXiv:2109.00131 of ultrarelativistic collisions can uniquely identify the structure of both isobars. This not only provides an urgently needed understanding of the structure of the Zirconium and Ruthenium isobars, but also paves the way for more detailed studies of nuclear structure using relativistic heavy ion collisions.
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Reports on this Submission
Anonymous Report 2 on 2023112 (Invited Report)
 Cite as: Anonymous, Report on arXiv:2112.13771v1, delivered 20230112, doi: 10.21468/SciPost.Report.6515
Report
In this paper, the authors use a comprehensive hydrodynamical model
(their publicly available Trajectum code) to study the particle distributions, and flow coefficients in the collisions of isobars (Ruthenium and Zirconium) at the highest RHIC energies. These isobar collisions are of particular interest from the promise that given the same baseline of equal numbers of nucleons they would allow for the isolation of a novel Chiral Magnetic Effect (CME) that is potentially sensitive to the different numbers of protons. A surprise from the STAR experiments was that there were unanticipated differences in the multiplicities and flow patterns between the two systems.
The authors undertake a systematic study of the two systems with Trajectum employing a variety of initial conditions including nuclear structure features such as the quadrupole and octopole deformation parameters, and the skin depth, that are poorly known. The compararisons of this hydrodynamic model to data can therefore in turn quantify these nuclear structure properties of the isobars. The authors also compare their results to other models, notably the AMPT code which also successfully describes data. A full global analysis of parameters and including magnetic fields in their code, which they plan to do will greatly improve the quantitative accuracy of the model.
This is a solid piece of work which will prove invaluable for current and future isobar studies, and towards a novel program of constraining low energy nuclear structure parameters with high energy heavyion data. I recommend that this work be published in SciPost.
Anonymous Report 1 on 202312 (Invited Report)
 Cite as: Anonymous, Report on arXiv:2112.13771v1, delivered 20230102, doi: 10.21468/SciPost.Report.6425
Strengths
This paper explored the new connections between nuclear structure and highenergy relativistic heavyion collisions.
Report
This manuscript studied how to use measurements in ultrarelativistic heavyion collisions to probe the nuclear structure of the colliding nuclei. The authors performed high statistics numerical simulations for Ru+Ru and Zr+Zr collisions at the top RHIC energy with the Trajectum framework. They studied how particle yield, mean transverse momentum, and anisotropic flow coefficients depend on different nuclear structure configurations parameterized by five sets of WoodsSaxon parameters. The paper was written clearly and contained important physics insights for the RHIC isobar program. This study also builds connections between lowenergy nuclear structures and highenergy relativistic heavyion collisions. I would recommend it for publication once the authors clarify the following questions.
To build a connection between the structure of nuclei and highenergy heavyion collisions, the authors should explain the underlying assumptions for how the produced initialstate energy density profile in the heavyion collision is related to the nucleus' structure. For example, will different energy deposition models weaken the sensitivity of the Woods Saxon deformation parameters on heavyion observables?
The WoodsSaxon parameters listed in Table 1 assumed the nucleon were pointlike objects. However, in the Trento initial condition model, the nucleons are assumed to have finite sizes. Did the authors correct the WoodsSaxon parameters for finite nucleon sizes, as discussed in Phys. Rev. C 79, 064904 (2009)?
Did the authors consider the shortrange hardcore repulsion between nucleons in their nuclear configurations? Would these shortrange correlations affect the observable ratios between the two isobar collisions?
Author: Wilke van der Schee on 20230309 [id 3460]
(in reply to Report 1 on 20230102)We wish to thank the referee for their careful reading of the manuscript. We believe we have addressed their questions below. In addition to the referee's questions, we found that in the left panels of Fig.~6, the ratio was incorrectly labelled as ZrZr/RuRu. We have corrected this to RuRu/ZrZr.
The referee writes:
Our response:
In the ratio between Ru and Zr, dependence on model parameters usually cancels to a large degree. We show an explicit example of this in Fig.~9, where we show that changing the viscosities changes $v_2$ and $v_3$ for both Ru and Zr, but in the ratio this dependence cancels to within statistical uncertainties. Given that computing isobars is statistically demanding, we did not check explicitly whether varying parameters related to the initial energy deposition has an effect on the sensitivity of the heavyion observables on WoodsSaxon deformation parameters, but any such effects are similarly expected to cancel out when taking the ratio between Ru and Zr.
We have added the sentence ``More generally, it is expected that the dependence of observables on other model dependencies such as $d_{\rm min}$ in the initial state or other prehydrodynamic parameters mostly cancel when taking a ratio of observables from the two isobars.'' on page 8 to make this clear from the text.
The referee writes:
Our response:
Indeed one can make WoodsSaxon parameters which either describe the charge or baryon number density, or describe the point density of the nucleons. As the referee points these WoodsSaxon parameters are only approximately equal. In principle, our calculation requires the parameters for the positions of the nucleons. The parameters for case 1 and 2, however, come from relatively old references that likely do not include the effects described in Phys.~Rev.~C 79, 064904 (2009). Cases 3 till 5 are more modern and we think that they describe the point densities of the nucleons.
However, similar to the point made above the small difference in the WoodsSaxon parameters affects Ru and Zr equally and hence in the ratio these differences cancel. Since we were quite clear that our study is not a precision attempt at describing Ru and Zr separately we decided not to further comment on the charge versus point density in this paper. If the referee is interested we have a more specific discussion in 2206.13522 about this, but we did not feel it relevant enough for isobars to expand on this in the current work.
The referee writes:
Our response:
The Trento model incorporates a minimal distance requirement for the placement of the nucleons inside the nucleus, where we require nucleons to be at least $d_\text{min}$ apart. In Bayesian analyses we generally find little dependence on $d_\text{min}$, and it has little effect on observables. As mentioned in our reply to the referee's first question, especially in the isobar ratio any dependence is expected to largely cancel out.
We have added the following on page 3 to make this clear in the text: ``As in [26], Trento also includes a hardcore repulsion implemented through a minimal internucleon distance $d_\text{min}$.''