Nuclear Experiment·nucl-ex

Tue·Jul 28, 2026Tuesday·July 28, 2026

9 papers—2 primary·7 cross-listed

01
Light hadron production measurements with Au+Au Collisions from $s_{N N} = 3.2$ -- $4.5$ GeV with STAR
Mathias C. Labonté🇺🇸
One of the main physics goals of the Beam Energy Scan program at RHIC is to study the QCD phase diagram, specifically around the phase transition between the quark-gluon plasma and hadronic matter. Beam Energy Scan Phase-I studied Au+Au collisions from center-of-mass energy ( $s_{N N}$ ) of 7.7 to 62.4 GeV. Beam Energy Scan Phase-II extended these measurements in several important ways, one of which was the addition of a fixed-target program that pushed the collision energy down to 3.0 GeV (or baryon chemical potential, $μ_{B}$ , up to 720 MeV). Fixed-target collisions at STAR allow for a more extensive scanning of the QCD phase diagram to an important region where the QCD critical point may lie, and to a region dominated by dense baryonic matter. One key measurement in the fixed-target program is the spectrum of the lightest hadrons ( $π^{\pm}$ , $K^{\pm}$ , p) as a function of transverse momentum, rapidity, and collision centrality. From the $p_{T}$ spectra, a blast-wave model is used to study the temperature at kinetic freeze-out and the surface velocity of the expanding matter. These results provide important input to models of heavy ion collisions at these energies, and can help constrain the equation of state of QCD matter. Here, results are shown for four collision energies in the fixed-target range: $s_{N N} = 3.2$ , 3.5, 3.9, and 4.5 GeV.
nucl-exJ.Subatomic Part.Cosmol.(2026)·0 citations
02
Exploring the Effects of Color Reconnection on Net-Charge Fluctuations in $pp$ Collisions at $s = 2.76 TeV$ using PYTHIA Monash
Dibakar Dhar · Tumpa Biswas🇮🇳 · Zubayer Ahammed🇮🇳 · Prabir Kumar Haldar🇮🇳
This work explores the effects of various color reconnection (CR) configurations on the net charge fluctuations of charged particles generated in $pp$ collisions at a center-of-mass energy of $s$ = 2.76 TeV, simulated with the PYTHIA Monash using the variable $ν_{(+, -, dyn)}$ . The results are compared with experimental data from ALICE, revealing that the current PYTHIA simulation settings overestimate the net charge fluctuations relative to the experimental observations. Furthermore, $ν_{(+, -, dyn)}$ exhibits a clear dependence on the MPICR range parameter, where increasing the range leads to more negative $ν_{(+, -, dyn)}$ values, suggesting the geometric extent of CR directly modulates the strength of local charge correlation. To ensure these effects are not a trivial consequence of particle density, a comparison with a retuned NoCR baseline at fixed multiplicity was performed. This analysis confirms that CR induces a genuine dynamical shift in fluctuations that persists independently of the average charged-particle density. The study also shows that the net charge and net pion channels display the strongest suppression in $⟨ N_{ch} ⟩ ν_{(+-, dyn)}$ compared to the net kaon and net proton channels. Overall, the study offers a useful benchmark against experimental results and sheds light on the underlying dynamics of the PYTHIA Monash.
nucl-exhep-ph0 citations
03
What we talk about when we talk about nuclear structure
S. Ragnar Stroberg🇺🇸
I provide and introductory overview of the field of nuclear structure, with a focus on physical concepts. I describe some basic nuclear structure observables, followed by a qualitative description of nuclear forces. I then outline some nuclear structure models which are most widely used to interpret the experimental data in terms of interacting protons and neutrons.
↳ nucl-thnucl-ex1 citation
04
Future directions in nuclear $β$ decay at FRIB and beyond
Garrett B. King🇺🇸 · Ayala Glick-Magid🇮🇱 · Grigor Sargsyan🇺🇸 · Mark A. Caprio🇺🇸 · Kyle G. Leach🇺🇸 · John A. Behr🇨🇦 · Francesca Bonaiti🇺🇸 · Maxime Brodeur🇺🇸 · Graham Chambers-Wall🇺🇸 · Heather L. Crawford🇺🇸 · Maria Dawid🇺🇸 · Wouter Dekens🇺🇸 and 29 other authors
Motivated by the opportunities presented for studies relevant to nuclear structure, astrophysics, and fundamental symmetries with nuclear $β$ decay, the Facility for Rare Isotope Beams (FRIB) Theory Alliance topical program ``Future Directions in Nuclear $β$ Decays at FRIB'' was held in September of 2025. This white paper summarizes the main points of discussion over the two-week program, and it aims to provide a snapshot of the current status of the field while also highlighting important questions and opportunities for future work. We provide an overview of the experimental tools and techniques that enable modern $β$ decay studies, discuss the current state of nuclear many-body approaches used to study $β$ decays, and highlight the important science questions that can be addressed by weak decays.
↳ nucl-thnucl-ex1 citation
05
Proton Collectivity in Au+Au Collisions at $s_{NN} = 2.4 - 4.5$ ~GeV from a Unified Purely Hadronic EOS without QCD Phase Transition
Gao-Feng Wei🇨🇳 · Shuang-Jie Liu🇨🇳 · Yu-Liang Zhao🇨🇳 · Qi-Jun Zhi · Zhigang Xiao🇨🇳
The nuclear equation of state (EOS) is generally considered to soften in the density range of $2 - 5$ times the saturation density $ρ_{0}$ . Using a purely hadronic transport model, we calculate the proton directed, sideward, and elliptic flows and their excitation functions in heavy-ion collisions (HICs) at $s_{NN} = 2.4 - 4.5$ ~GeV and compare with the HADES, E895, and STAR data. We find that a momentum-dependent mean field with a unified incompressibility $K_{0} = 230$ ~MeV quantitatively reproduces the experimental proton flows up to 4.3 GeV, at which the maximum density reaches approximately $5 ρ_{0}$ . At 4.5 GeV, however, the pure hadronic model fails to reproduce the proton directed and elliptic flow data, providing circumstantial evidence for the onset of partonic degrees of freedom in HICs. Our results provide a hadronic baseline to characterize the high-density nuclear matter and to map the region of hadron-quark phase transition.
↳ nucl-thnucl-ex0 citations
06
$^{15}$ C inelastic $1/ 2^{+} \to 5/ 2^{+}$ excitation: A single-particle versus a collective process
C. Beckman🇺🇸 · M. Catacora-Rios🇺🇸 · C. Hebborn🇺🇸 · F. M. Nunes🇺🇸
Background: The excitation of one-neutron halo nucleus $^{15}$ C from the $1/ 2^{+}$ ground state to the $5/ 2^{+}$ first excited state was measured at Argonne National Laboratory by impinging $^{15}$ C on a deuterated target at $7.1 A$ MeV. This data was then analyzed in the Distorted Wave Born Approximation using a rigid rotor model for the excitation. Purpose: Being a one-neutron halo, we expect a single-particle excitation to better represent the excitation of $^{15}$ C rather than a collective process. We expect the breakup of $^{15}$ C to influence the reaction mechanisms because of the low one-neutron separation threshold, which is close in energy to $^{15}$ C's first excited state. The goal of this work is to explore various the reaction mechanisms to reinterpret the data of Ref.[1] for the inelastic excitation of $^{15}$ C. Method: We solve the scattering problem assuming a three-body model $^{14}$ C $+ n + d$ . We use the Continuum Discretized Coupled Channel method (CDCC) and compare the results with those obtained assuming 1-step DWBA with quadrupole deformation, as done in the original experimental analysis. We also use Bayesian uncertainty quantification to estimate the uncertainties in our predictions coming from the $n$ - $d$ interaction. Results: We analyze both the elastic and inelastic angular distributions for $^{15}$ C(d,d') $^{15} C^{*}$ at $7.1 A$ MeV. Our results show that $^{15}$ C breakup effects are important. Conclusions: While CDCC predicts the elastic angular distribution correctly, it is not able to fully describe the experimental inelastic angular distribution. We discuss additional effects that may be responsible for the remaining discrepancy.
↳ nucl-thnucl-exPRC(2026)·0 citations
07
Microscopic Spin-Parity Distributions of Fission Fragments
Guillaume Scamps🇫🇷 · Petar Marević🇭🇷 · Antonio Bjelčić🇺🇸 · Nicolas Schunck🇺🇸
Recent microscopic studies have investigated various features of the spin distributions of fission fragments, but their parity distributions remain largely unexplored. In this Letter, we provide a complete characterization of the spin--parity content of fission fragments within a time-dependent Hartree-Fock-Bogoliubov framework, performing for the first time simultaneous projections on angular momentum, particle number, and parity. Calculations are carried out for the thermal neutron-induced fission of $^{239}$ Pu using both the Gogny and Skyrme energy density functionals. We find that dynamical pair breaking during fission populates a significant fraction of unnatural-parity states and generates components with non-zero spin projections $K$ . The parity content is found to depend on the number parity of the fragments, with odd-mass nuclei exhibiting pronounced parity staggering and odd-odd nuclei favoring negative parity. These results show that the parity distribution of fragments can depart significantly from the equiprobable partition commonly assumed in statistical de-excitation models, with potential implications for the modeling of fragment decay.
↳ nucl-thnucl-ex0 citations
08
Interaction fingerprints in temperature-fluctuation cumulants from the QCD crossover to nuclear liquid-gas criticality
Debasish Mallick🇵🇱
Temperature fluctuations of the matter produced in relativistic heavy-ion collisions are related to event-by-event fluctuations of the mean transverse momentum and, through the specific heat, to the QCD equation of state. We calculate the temperature-fluctuation cumulants $c_{2}$ , $c_{3}$ , and $c_{4}$ in the ideal hadron resonance gas (HRG), in an excluded-volume HRG consistent with lattice QCD constraints on baryon repulsion, and in a van der Waals HRG that reproduces the nuclear ground state. At zero baryon density all three models agree with lattice QCD thermodynamics up to the chiral crossover and separate above it. Along the chemical freeze-out curve the models remain close for $s_{N N} ≳ 20$ GeV and separate strongly at lower energies, where baryonic interactions dominate the thermal response. In the van der Waals model the variance $c_{2}$ develops a minimum along the Widom line of the nuclear liquid-gas transition and $c_{3}$ changes sign across it. Temperature cumulants thus connect mean-transverse-momentum fluctuations to thermodynamic structures in two regions of the QCD phase diagram.
↳ nucl-thnucl-ex0 citations
09
Two-neutrino double-weak decays of $^{126}$ Xe and $^{134}$ Xe from different many-body methods
C. Brase🇩🇪 · L. Jokiniemi🇨🇦 · E. Kauppinen · B. Romeo🇺🇸 · J. Kotila🇫🇮 · J. Menéndez🇪🇸 · A. Schwenk🇩🇪
We calculate the nuclear matrix elements and corresponding half-lives for the two-neutrino double-electron capture of $^{126}$ Xe and the two-neutrino double-beta decay of $^{134}$ Xe. We use different many-body methods: the proton-neutron quasiparticle random-phase approximation, the nuclear shell model, the microscopic interacting boson model, and an effective field theory for heavy nuclei. For both nuclei, all our half-life predictions are generally consistent with each other when including theoretical uncertainties for each method. Interestingly, for all calculations the lower range of the predicted $^{134}$ Xe half-life is shorter than $T_{1/2}^{2 ν} \approx 2 \times 1 0^{24}$ \,y, which may be within the reach of next-generation experiments. For $^{126}$ Xe, our results typically predict one order of magnitude longer half-lives than those for $^{134}$ Xe.
↳ nucl-thhep-phnucl-ex0 citations

Affiliations

first authorsco-authorsvia INSPIRE