New articles on Nuclear Experiment

[1] 2407.14697

Single-proton removal reaction in the IQMD+GEMINI model benchmarked by elemental fragmentation cross sections of $^{29-33}\mathrm{Si}$ on carbon at $\sim$230~MeV/nucleon

We report on the first measurement of the elemental fragmentation cross sections (EFCSs) of $^{29-33}\mathrm{Si}$ on a carbon target at $\sim$230~MeV/nucleon. The experimental data covering charge changes of $\Delta Z$ = 1-4 are reproduced well by the isospin-dependent quantum molecular dynamics (IQMD) coupled with the evaporation GEMINI (IQMD+GEMINI) model. We further explore the mechanisms underlying the single-proton removal reaction in this model framework. We conclude that the cross sections from direct proton knockout exhibit a overall weak dependence on the mass number of $\mathrm{Si}$ projectiles. The proton evaporation induced after the projectile excitation significantly affects the cross sections for neutron-deficient $\mathrm{Si}$ isotopes, while neutron evaporation plays a crucial role in the reactions of neutron-rich $\mathrm{Si}$ isotopes. It is presented that the relative magnitude of one-proton and one-neutron separation energies is an essential factor that influences evaporation processes.

[2] 2407.14821

Energy Dependence of Polarized $\mathbf{γγ\rightarrow e^{+}e^{-}}$ in Peripheral Au+Au Collisions at RHIC

We report the differential yields at mid-rapidity of the Breit-Wheeler process ($\gamma\gamma\rightarrow e^{+}e^{-}$) in peripheral Au+Au collisions at $\sqrt{s_{_{\rm{NN}}}} = $ 54.4 GeV and 200 GeV with the STAR experiment at RHIC, as a function of energy $\sqrt{s_{_{\rm{NN}}}}$, $e^{+}e^{-}$ transverse momentum $p_{\rm T}$, $p_{\rm T}^{2}$, invariant mass $M_{ee}$ and azimuthal angle. In the invariant mass range of 0.4 $<$ $M_{ee}$ $<$ 2.6 GeV/$c^{2}$ at low transverse momentum ($p_{\rm T}$ $ < $0.15 GeV/$c$), the yields increase while the pair $\sqrt{\langle p_{\rm T}^{2} \rangle}$ decreases with increasing $\sqrt{s_{_{\rm{NN}}}}$, a feature is correctly predicted by the QED calculation. The energy dependencies of the measured quantities are sensitive to the nuclear form factor, infrared divergence and photon polarization. The data are compiled and used to extract the charge radius of the Au nucleus.

[3] 2407.15535

First experimental test of the ratio method for nuclear-reaction analysis

Nuclear halos are very exotic quantal structures observed far from stability. Because of their short lifetime, they are mostly studied through reactions. The ratio method offers a new observable: the ratio of angular differential cross sections for breakup and scattering. It is predicted to be much more sensitive to the projectile structure than individual cross sections thanks to its independence of the reaction process. We test this new observable experimentally for the first time considering the collision of 11Be on C at 22.8 MeV/nucleon. We extend this analysis to similar data recently measured on Pb at 19.1 MeV/nucleon. Both analyses confirm the theoretical predictions, which opens the door to a new era in the study of nuclear structure near the neutron dripline. This should prove invaluable in conjunction with the start of FRIB. The ratio method could also be extended to other fields of quantum physics beyond nuclear reactions.

[4] 2407.14548

Triply charm and bottom tetraquarks in a constituent quark model

Recently, the singly, doubly and fully charmed tetraquark candidates, $T_{c\bar{s}}(2900)$, $T^+_{cc}(3875)$ and $X(6900)$ are experimentally reported by the LHCb collaboration. Hence, it is quite necessary to implement a theoretical investigation on the triply heavy tetrquarks. In this study, the S-wave triply charm and bottom tetraquarks, $\bar{Q}Q\bar{q}Q$ $(q=u,\,d,\,s;\,Q=c,\,b)$, with spin-parity $J^P=0^+$, $1^+$ and $2^+$, isospin $I=0$ and $\frac{1}{2}$ are systematically studied in a constituent quark model. Particularly, a complete S-wave tetraquark configurations, which include the meson-meson, diquark-antidiquark and K-type arrangements of quarks, along with all allowed color structures, are comprehensively considered. A high accuracy and efficient computational approach, the Gaussian expansion method (GEM), in cooperation with a powerful complex-scaling method (CSM), which is quite ingenious in dealing with the bound and resonant state of a multiquark system simultaneously, are adopted in solving the complex scaled Schr\"odinger equation. This theoretical framework has already been successfully applied in various tetra- and penta-quark systems. Bound state of triply heavy tetraquark system is unavailable in our study, nevertheless, in a fully coupled-channel calculation by the CSM, several narrow resonances are found in each $I(J^P)$ quantum states of the charm and bottom sector. In particular, triply charm and bottom tetraquark resonances are obtained in $5.59-5.94$ GeV and $15.31-15.67$ GeV, respectively. Compositeness of exotic states, such as the inner quark distance, magnetic moment and dominant component, are also analyzed. These exotic hadrons in triply heavy sector are expected to be confirmed in future high energy experiments.

[5] 2407.15065

Anisotropic flow fluctuation as a possible signature of clustered nuclear geometry in O-O collisions at the Large Hadron Collider

Nuclei having $4n$ number of nucleons are theorized to possess clusters of $\alpha$ particles ($^4$He nucleus). The Oxygen nucleus ($^{16}$O) is a doubly magic nucleus, where the presence of an $\alpha$-clustered nuclear structure grants additional nuclear stability. In this study, we exploit the anisotropic flow coefficients to discern the effects of an $\alpha$-clustered nuclear geometry w.r.t. a Woods-Saxon nuclear distribution in O--O collisions at $\sqrt{s_{\rm NN}}=7$ TeV using a hybrid of IP-Glasma + MUSIC + iSS + UrQMD models. In addition, we use the multi-particle cumulants method to measure anisotropic flow coefficients, such as elliptic flow ($v_{2}$) and triangular flow ($v_{3}$), as a function of collision centrality. Anisotropic flow fluctuations, which are expected to be larger in small collision systems, are also studied for the first time in O--O collisions. It is found that an $\alpha$-clustered nuclear distribution gives rise to an enhanced value of $v_{2}$ and $v_3$ towards the highest multiplicity classes. Consequently, a rise in $v_3/v_2$ is also observed for the (0-10)\% centrality class. Further, for $\alpha$-clustered O--O collisions, fluctuations of $v_{2}$ are larger for the most central collisions, which decrease towards the mid-central collisions. In contrast, for a Woods-Saxon $^{16}$O nucleus, $v_{2}$ fluctuations show an opposite behavior with centrality. This study, when confronted with experimental data may reveal the importance of nuclear density profile on the discussed observables.

[6] 2407.15068

The Relative Abundance of Correlated Spin-zero Nucleon Pairs

We utilize the generalized contact formalism in conjunction with the Woods-Saxon mean-field description of the long-range part of the nuclear wave function to assess the relative prevalence of short-range correlation pairs within atomic nuclei. We validate our approach by fitting experimental charge density results and electron scattering experiments to a very good agreement. Applying our model, we calculate the spin-zero short-range correlations contact ratios. Interestingly, for nuclei with $A>50$, we observe a notable dependence on the neutron-to-proton ratio $N/Z$. Specifically, the probability per nucleon to find neutron-neutron pairs increases, while that of proton-proton pairs decreases, whereas the probability of finding neutron-proton pairs remains relatively constant. To interpret this isospin symmetry breaking effect, we employ a simple model based on generalized Levinger constants, linking it to differences in nuclear proton and neutron radii.

[7] 2407.15311

Polarized cold-neutron reflectometry at JRR-3/MINE2 for the development of ultracold-neutron spin analyzers for a neutron EDM experiment at TRIUMF

The neutron electric dipole moment (EDM) is a sensitive probe for currently undiscovered sources of charge-parity symmetry violation. As part of the \uline{T}RIUMF \uline{U}ltra\uline{c}old \uline{A}dvanced \uline{N}eutron (TUCAN) collaboration, we are developing spin analyzers for ultracold neutrons (UCNs) to be used for a next-generation experiment to measure the neutron EDM with unprecedented precision. Spin-state analysis of UCNs constitutes an essential part of the neutron EDM measurement sequence. Magnetized iron films used as spin filters of UCNs are crucial experimental components, whose performance directly influences the statistical sensitivity of the measurement. To test such iron film spin filters, we propose the use of polarized cold-neutron reflectometry, in addition to conventional UCN transmission experiments. The new method provides information on iron film samples complementary to the UCN tests and accelerates the development cycles. We developed a collaborative effort to produce iron film spin filters and test them with cold and ultracold neutrons available at JRR-3/MINE2 and J-PARC/MLF BL05. In this article, we review the methods of neutron EDM measurements, discuss the complementarity of this new approach to test UCN spin filters, provide an overview of our related activities, and present the first results of polarized cold-neutron reflectometry recently conducted at the MINE2 beamline.