|
In March 2020, the STAR Collaboration published "Measurement of the mass difference and the binding energy of the
hypertriton and antihypertriton" in
Nature Physics.
In this paper, we present two measurements from gold-gold collisions at a center-of-mass energy per nucleon pair of
$\sqrt{s_{NN}} = 200$ GeV: the relative mass difference between $\rm^3_\Lambda H$ (the hypertriton) and $\rm^3_{\bar{\Lambda}}\overline{H}$
(the antihypertriton) (see Fig. 1), as well as the $\Lambda$ hyperon binding energy for $\rm^3_\Lambda H$ and $\rm^3_{\bar{\Lambda}}\overline{H}$
(see Fig. 2). The hypernucleus $\rm^3_{\Lambda}H$ is reconstructed through its mesonic decay channels
$\rm^3_{\Lambda}H \rightarrow {^3}He + \pi^-$ (2-body decay) and $\rm^3_{\Lambda}H \rightarrow$ $ d + p + \pi^-$ (3-body decay).
The significance $S/ \sqrt{S+B}$, where $S$ is signal counts and $B$ is background counts in the invariant mass window $2.986 - 2.996$ GeV$/c^{2}$, is 11.4 for $^3_\Lambda$H and 6.4 for $\rm^3_{\bar{\Lambda}}\overline{H}$.
The signal counts from 2-body/3-body decay channels are about 121/35 for $^3_\Lambda$H and 36/21 for $\rm^3_{\bar{\Lambda}}\overline{H}$, respectively.
According to the CPT theorem, which states that the combined operation of charge conjugation, parity transformation and time reversal must be
conserved, particles and their antiparticles should have the same mass and lifetime but opposite charge and magnetic moment. Here, we test CPT
symmetry in a nucleus containing a strange quark, more specifically in the hypertriton. A comparison of the masses of the hypertriton and the
antihypertriton allows us to test CPT symmetry in a nucleus with strangeness for the first time, and we observe no deviation from the expected
exact symmetry with precision of 10$^{-4}$.
This hypernucleus is the lightest one yet discovered and consists of a proton, a neutron, and a $\Lambda$ hyperon. We measure the
$\Lambda$ hyperon binding energy $B_{\Lambda}$ for the hypertriton, and find that it differs from the widely used value and from
predictions, where the hypertriton is treated as a weakly bound system. Our results place stringent constraints on the hyperon-nucleon
interaction, and have implications for understanding neutron star interiors, where strange matter may be present.
|
|
As shown in Fig. 1, the mass difference between $^3_\Lambda$H and $\rm^3_{\bar{\Lambda}}\overline{H}$ observed in the present
data is consistent with zero, and the precision is an order of magnitude improved over prior data with same mass number, published by the
ALICE collaboration. The current measurement extends the validation of CPT invariance to a nucleus containing a strange quark. We can place
a new constraint on the relative mass difference between $\rm^{3}He$ and $\rm^{3}\overline{He}$ based on the current STAR measurement,
namely $\rm\Delta m_{{\rm^{3}He}}/m_{{\rm^{3}He}}$ = [-1.5 $\pm$ 2.6 (stat.) $\pm$ 1.2 (syst.)]$\times 10^{-4}$.
|
|
The $\Lambda$ binding energy $B_\Lambda$ for $^3_\Lambda$H and $\rm^3_{\bar{\Lambda}}\overline{H}$ is $B_\Lambda =0.41\pm 0.12 {\rm(stat.)}\pm 0.11{\rm (syst.)~MeV}$.
This binding energy is presented in Fig. 2 (left panel) along with earlier measurements from nuclear emulsion and
helium bubble chamber experiments. The current STAR result differs from zero with a statistical significance of 3.4$\sigma$ and the
central value of the current STAR measurement is larger than the commonly used measurement from 1973. Theoretical calculations
of $B_\Lambda$ for $^3_\Lambda$H are also available (see right panel of Fig. 2). For example, Dalitz reported the
calculation $B_\Lambda = 0.10$ MeV in 1972. In recent calculations, $B_\Lambda = 0.262$ MeV was obtained through SU(6) quark model baryon-baryon
interactions, and $B_\Lambda$ was calculated to be 0.23 MeV using auxiliary field diffusion Monte Carlo (AFDMC). A span of values ranging
from 0.046 MeV to 0.135 MeV was obtained in SU(3) chiral effective field theory. The divergence of results among different calculations
emphasizes the need for a precise determination of $B_\Lambda$ from experiment. In a model based on effective field theory is used to
extract a scattering length of $13.80^{+3.75}_{-2.03}$ fm from the earlier average value of $\rm 0.13\pm 0.05(stat.)\,MeV$; when applied to
our value of $\rm 0.41\pm 0.12(stat.)\,MeV$ it yields a significantly smaller value of $7.90^{+1.71}_{-0.93}$ fm. The larger $B_\Lambda$ and
shorter effective scattering length suggest a stronger $YN$ interaction between the $\Lambda$ and the relatively low-density nuclear core of
the $^3_\Lambda$H. This, in certain models, requires SU(3) symmetry breaking and a more repulsive $YN$ interaction at high density, consistent
with implications from the range of masses observed for neutron stars.
|
