CSB-H4L and He4L binding energy

Updated on Fri, 2022-09-23 09:17. Originally created by shaoth on 2021-04-08 07:17.

Title :

Measurement of H4L and He4L binding ennergy in Au+Au collisions at sqrt{s_{NN}} = 3 GeV

PAs :

Tianhao Shao, Jinhui Chen, Yue-Hang Leung, Yugang Ma, Zhangbu Xu

Targeted journal :

Physics Letters B

Abstract :

Measurements of mass and $\Lambda$ binding energy of \hl and \hel in Au+Au reactions at $\sqrt{s_{_{\rm NN}}}=3$ GeV are presented, with an aim to address the charge-symmetry breaking problem in hypernuclei systems with atomic number A = 4. The $\Lambda$ binding energies are $\rm B_{\Lambda}(^4_{\Lambda}H) = 2.22\pm0.06(stat.) \pm0.14(syst.)$~MeV and $\rm B_{\Lambda}(^4_{\Lambda}He) = 2.38\pm0.13(stat.) \pm0.12(syst.)$~MeV for \hl and \held, respectively. In our results the $\Lambda$ binding-energy difference is $\rm \Delta B_{\Lambda}^4(0_{g.s.}^{+})=0.16\pm0.14(stat.)\pm0.10(syst.)$~MeV for ground states. Combined with the $\gamma$-ray transition energies, the binding-energy difference for excited states is $\rm \Delta B_{\Lambda}^4(1_{exc}^{+})=-0.16\pm0.14(stat.)\pm0.10(syst.)$~MeV, which is negative and comparable to the value of the ground states within uncertainties. Within current uncertainties, these new measurements on the $\Lambda$ binding-energy difference in A = 4 hypernuclei systems are matched by the theoretical calculations that result in $\rm \Delta B_{\Lambda}^4(1_{exc}^{+})\approx -\Delta B_{\Lambda}^4(0_{g.s.}^{+})<0$ and present a new method for the study of charge symmetry breaking using relativistic heavy-ion collision experiments.

Conclusions :

In this paper, we present measurements of the $\Lambda$ binding energies of \hl and \held.~The invariant-mass distributions of \hl and \hel are reconstructed in Au+Au collisions at $\sqrt{s_{\rm NN}}=3$~GeV taken in the fixed-target mode at STAR. The $\Lambda$ binding energies of \hl and \hel in ground states are obtained: $\rm B_{\Lambda}(^4_{\Lambda}H)~=~2.22~\pm~ 0.06(stat.)~\pm~0.14(syst.)$~MeV and $\rm B_{\Lambda}(^4_{\Lambda}He)~=~2.38~\pm~0.13(stat.)~\pm~0.12(syst.)$~MeV. Using the $\gamma$-ray transition energies of the excited states from previous measurements, the $\Lambda$ binding energies in excited states are also extracted. The $\Lambda$ binding-energy differences between the two A = 4 hypernuclei and between their corresponding excited states are calculated as $\rm \Delta B_{\Lambda}^4(0_{g.s.}^{+})~=~0.16~\pm~0.14(stat.)~\pm~0.10(syst.)$~MeV and $\rm \Delta B_{\Lambda}^4(1_{exc}^{+})~=~-0.16~\pm~0.14(stat.)~\pm~0.10(syst.)$~MeV, respectively. Within current uncertainties, our results are consistent with the theoretical calculation of the charge symmetry breaking effect in A = 4 hypernuclei with a positive $\rm \Delta B_{\Lambda}^4(0_{g.s.}^{+})$ and a negative $\rm \Delta B_{\Lambda}^4(1_{exc}^{+})$ of comparable magnitude; however, the statistical uncertainties are large. Our approach provides a new avenue to study the charge symmetry breaking in heavy-ion collision experiments. Higher-statistics data sets and more accurate measurements of $\gamma$-ray transition energies will be able to improve these measurements..

Figure 1. (a): The mean energy loss versus rigidity ($p/q$, where $p$ is the total momentum of the particle and $q$ is its electric charge) in the TPC. The dashed curves represent the expected values calculated by the Bichsel function for each species of particles. (b): The square of the ratio of mass and charge, $m^2/q^2$ versus rigidity in the TOF detector. The dashed curves represent the theoretical values for $\rm ^{3}He$ and $\rm ^{4}He$.

Figure 2. Invariant-mass distributions for \hl (a) and \hel (b) reconstructed with KFParticle and TMVA-BDT. The green histograms represent the rotated backgrounds. The blue dashed curves represent the background fits and are obtained by fitting the invariant-mass distributions outside of the signal regions with double-exponential functions. The black dashed curves are obtained by fitting these distributions across the full range of invariant mass with the background fit result and a Gaussian function. The orange dashed curves represent the signal Gaussian functions.

Figure 3. The average difference between the measured momentum and the MC input momentum as a function of the measured momentum for $\rm ^4He$. The red circles represent the energy loss without any corrections and the blue triangles are with energy-loss correction applied on the measured momentum.