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. This hypernucleus is the lightest one yet discovered and consists of a proton, a neutron and a Λ hyperon. With data recorded by the STAR detector1–3 at the Relativistic Heavy Ion Collider, we measure the Λ hyperon binding energy BΛ for the hypertriton, and find that it differs from the widely used value4 and from predictions5–8, where the hypertriton is treated as a weakly bound system. Our results place stringent constraints on the hyperon–nucleon interaction9,10 and have implications for understanding neutron star interiors, where strange matter may be present11. A precise comparison of the masses of the hypertriton and the antihypertriton allows us to test CPT symmetry in a nucleus with strangeness, and we observe no deviation from the expected exact symmetry.The STAR collaboration reports a measurement of the mass difference and binding energy of the hypertriton and its antiparticle. This work constrains the hyperon–nucleon interaction and allows us to test the CPT theorem in a nucleus with strangeness.
A
bstract
We report on the measurement of the Central Exclusive Production of charged particle pairs
h
+
h
−
(
h
=
π, K, p
) with the STAR detector at RHIC in proton-proton collisions at
s
= 200 GeV. ...The charged particle pairs produced in the reaction
pp → p
′ +
h
+
h
−
+
p
′ are reconstructed from the tracks in the central detector and identified using the specific energy loss and the time of flight method, while the forward-scattered protons are measured in the Roman Pot system. Exclusivity of the event is guaranteed by requiring the transverse momentum balance of all four final-state particles. Differential cross sections are measured as functions of observables related to the central hadronic final state and to the forward-scattered protons. They are measured in a fiducial region corresponding to the acceptance of the STAR detector and determined by the central particles’ transverse momenta and pseudorapidities as well as by the forward-scattered protons’ momenta. This fiducial region roughly corresponds to the square of the four-momentum transfers at the proton vertices in the range 0
.
04 GeV
2
< −t
1
, −t
2
<
0
.
2 GeV
2
, invariant masses of the charged particle pairs up to a few GeV and pseudorapidities of the centrally-produced hadrons in the range
|η| <
0
.
7. The measured cross sections are compared to phenomenological predictions based on the Double Pomeron Exchange (DPE) model. Structures observed in the mass spectra of
π
+
π
−
and
K
+
K
−
pairs are consistent with the DPE model, while angular distributions of pions suggest a dominant spin-0 contribution to
π
+
π
−
production. For
π
+
π
−
production, the fiducial cross section is extrapolated to the Lorentz-invariant region, which allows decomposition of the invariant mass spectrum into continuum and resonant contributions. The extrapolated cross section is well described by the continuum production and at least three resonances, the
f
0
(980),
f
2
(1270) and
f
0
(1500), with a possible small contribution from the
f
0
(1370). Fits to the extrapolated differential cross section as a function of
t
1
and
t
2
enable extraction of the exponential slope parameters in several bins of the invariant mass of
π
+
π
−
pairs. These parameters are sensitive to the size of the interaction region.
Quark interactions with topological gluon configurations can induce local chirality imbalance and parity violation in quantum chromodynamics, which can lead to the chiral magnetic effect (CME) -- an ...electric charge separation along the strong magnetic field in relativistic heavy-ion collisions. The CME-sensitive azimuthal correlator observable (\(\Delta\gamma\)) is contaminated by background arising, in part, from resonance decays coupled with elliptic anisotropy (\(v_{2}\)). We report here differential measurements of the correlator as a function of the pair invariant mass (\(m_{\rm inv}\)) in 20-50\% centrality Au+Au collisions at \(\sqrt{s_{_{\rm NN}}}\)= 200 GeV by the STAR experiment at RHIC. Strong resonance background contributions to \(\Delta\gamma\) are observed. At large \(m_{\rm inv}\) where this background is significantly reduced, the \(\Delta\gamma\) value is found to be significantly smaller. An event-shape-engineering technique is deployed to determine the \(v_{2}\) background shape as a function of \(m_{\rm inv}\). We extract a \(v_2\)-independent and \(m_{\rm inv}\)-averaged signal \(\Delta\gamma_{\rm sig}\) = (0.03 \(\pm\) 0.06 \(\pm\) 0.08) \(\times10^{-4}\), or \((2\pm4\pm5)\%\) of the inclusive \(\Delta\gamma(m_{\rm inv}>0.4\) GeV/\(c^2\))\( =(1.58 \pm 0.02 \pm 0.02) \times10^{-4}\), within pion \(p_{T}\) = 0.2 - 0.8~\gevc and averaged over pseudorapidity ranges of \(-1 < \eta < -0.05\) and \(0.05 < \eta < 1\). This represents an upper limit of \(0.23\times10^{-4}\), or \(15\%\) of the inclusive result, at \(95\%\) confidence level for the \(m_{\rm inv}\)-integrated CME contribution.
We report on the first measurement of charm-strange meson \(D_s^{\pm}\) production at midrapidity in Au+Au collisions at \(\sqrt{s_{_{\rm NN}}}\) = 200 GeV from the STAR experiment. The yield ratio ...between strange (\(D_{s}^{\pm}\)) and non-strange (\(D^{0}\)) open-charm mesons is presented and compared to model calculations. A significant enhancement, relative to a PYTHIA simulation of \(p\)+\(p\) collisions, is observed in the \(D_{s}^{\pm}/D^0\) yield ratio in Au+Au collisions over a large range of collision centralities. Model calculations incorporating abundant strange-quark production in the quark-gluon plasma (QGP) and coalescence hadronization qualitatively reproduce the data. The transverse-momentum integrated yield ratio of \(D_{s}^{\pm}/D^0\) at midrapidity is consistent with a prediction from a statistical hadronization model with the parameters constrained by the yields of light and strange hadrons measured at the same collision energy. These results suggest that the coalescence of charm quarks with strange quarks in the QGP plays an important role in \(D_{s}^{\pm}\) meson production in heavy-ion collisions.
The STAR Collaboration reports measurements of the transverse single-spin asymmetry (TSSA) of inclusive \(\pi^0\) at center-of-mass energies (\(\sqrt s\)) of 200 GeV and 500 GeV in transversely ...polarized proton-proton collisions in the pseudo-rapidity region 2.7 to 4.0. The results at the two different energies show a continuous increase of the TSSA with Feynman-\(x\), and, when compared to previous measurements, no dependence on \(\sqrt s\) from 19.4 GeV to 500 GeV is found. To investigate the underlying physics leading to this large TSSA, different topologies have been studied. \(\pi^0\) with no nearby particles tend to have a higher TSSA than inclusive \(\pi^0\). The TSSA for inclusive electromagnetic jets, sensitive to the Sivers effect in the initial state, is substantially smaller, but shows the same behavior as the inclusive \(\pi^0\) asymmetry as a function of Feynman-\(x\). To investigate final-state effects, the Collins asymmetry of \(\pi^0\) inside electromagnetic jets has been measured. The Collins asymmetry is analyzed for its dependence on the \(\pi^0\) momentum transverse to the jet thrust axis and its dependence on the fraction of jet energy carried by the \(\pi^0\). The asymmetry was found to be small in each case for both center-of-mass energies. All the measurements are compared to QCD-based theoretical calculations for transverse-momentum-dependent parton distribution functions and fragmentation functions. Some discrepancies are found, which indicates new mechanisms might be involved.
Global polarization of \(\Xi\) and \(\Omega\) hyperons has been measured for the first time in Au+Au collisions at \(\sqrt{s_{_{NN}}}\) = 200 GeV. The measurements of the \(\Xi^-\) and ...\(\bar{\Xi}^+\) hyperon polarization have been performed by two independent methods, via analysis of the angular distribution of the daughter particles in the parity violating weak decay \(\Xi\rightarrow\Lambda+\pi\), as well as by measuring the polarization of the daughter \(\Lambda\)-hyperon, polarized via polarization transfer from its parent. The polarization, obtained by combining the results from the two methods and averaged over \(\Xi^-\) and \(\bar{\Xi}^+\), is measured to be \(\langle P_\Xi \rangle = 0.47\pm0.10~({\rm stat.})\pm0.23~({\rm syst.})\,\%\) for the collision centrality 20\%-80\%. The \(\langle P_\Xi \rangle\) is found to be slightly larger than the inclusive \(\Lambda\) polarization and in reasonable agreement with a multi-phase transport model (AMPT). The \(\langle P_\Xi \rangle\) is found to follow the centrality dependence of the vorticity predicted in the model, increasing toward more peripheral collisions. The global polarization of \(\Omega\), \(\langle P_\Omega \rangle = 1.11\pm0.87~({\rm stat.})\pm1.97~({\rm syst.})\,\%\) was obtained by measuring the polarization of daughter \(\Lambda\) in the decay \(\Omega \rightarrow \Lambda + K\), assuming the polarization transfer factor \(C_{\Omega\Lambda}=1\).
The Beam Energy Scan (BES) program at the Relativistic Heavy Ion Collider (RHIC) was extended to energies below \(\sqrt{\textit{s}_{NN}}\) = 7.7 GeV in 2015 by successful implementation of the ...fixed-target mode of operation in the STAR (Solenoidal Track At RHIC) experiment. In the fixed-target mode, ions circulate in one ring of the collider and interact with a stationary target at the entrance of the STAR Time Projection Chamber. The first results for Au+Au collisions at \(\sqrt{\textit{s}_{NN}}\) = 4.5 GeV are presented, including directed and elliptic flow of identified hadrons, and radii from pion femtoscopy. The proton flow and pion femtoscopy results agree quantitatively with earlier measurements by Alternating Gradient Synchrotron experiments at similar energies. This validates running the STAR experiment in the fixed-target configuration. Pion directed and elliptic flow are presented for the first time at this beam energy. Pion and proton elliptic flow show behavior which hints at constituent quark scaling, but large error bars preclude reliable conclusions. The ongoing second phase of BES (BES-II) will provide fixed-target data sets with 100 times more events at each of several energies down to \(\sqrt{\textit{s}_{NN}}\) = 3.0 GeV.
The STAR Collaboration reports a measurement of the transverse single-spin asymmetries, \(A_{N}\), for neutral pions produced in polarized proton collisions with protons (\(pp\)), with aluminum ...nuclei (\(p\rm{Al}\)) and with gold nuclei (\(p\rm{Au}\)) at a nucleon-nucleon center-of-mass energy of 200 GeV. Neutral pions are observed in the forward direction relative to the transversely polarized proton beam, in the pseudo-rapidity region \(2.7<\eta<3.8\). Results are presented for \(\pi^0\)s observed in the STAR FMS electromagnetic calorimeter in narrow Feynman x (\(x_F\)) and transverse momentum (\(p_T\)) bins, spanning the range \(0.17<x_F<0.81\) and \(1.7<p_{T}<6.0\) GeV/\(c\). For fixed \(x_F<0.47\), the asymmetries are found to rise with increasing transverse momentum. For larger \(x_F\), the asymmetry flattens or falls as \({p_T}\) increases. Parametrizing the ratio \(r(A) \equiv A_N(pA)/A_N(pp)=A^P\) over the kinematic range, the ratio \(r(A)\) is found to depend only weakly on \(A\), with \({\langle}P{\rangle} = -0.027 \pm 0.005\). No significant difference in \(P\) is observed between the low-\(p_T\) region, \(p_T<2.5\) GeV/\(c\), where gluon saturation effects may play a role, and the high-\(p_T\) region, \(p_T>2.5\) GeV/\(c\). It is further observed that the value of \(A_N\) is significantly larger for events with a large-\(p_T\) isolated \(\pi^0\) than for events with a non-isolated \(\pi^0\) accompanied by additional jet-like fragments. The nuclear dependence \(r(A)\) is similar for isolated and non-isolated \(\pi^0\) events.
The STAR Collaboration at the Relativistic Heavy Ion Collider reports the first measurement of inclusive jet production in peripheral and central Au+Au collisions at \(\sqrt{s_{NN}}\)=200 GeV. Jets ...are reconstructed with the anti-k\(_{T}\) algorithm using charged tracks with pseudorapidity \(|\eta|<1.0\) and transverse momentum \(0.2<p_{T,jet}^{ch}<30\) GeV/\(c\), with jet resolution parameter \(R\)=0.2, 0.3, and 0.4. The large background yield uncorrelated with the jet signal is observed to be dominated by statistical phase space, consistent with a previous coincidence measurement. This background is suppressed by requiring a high-transverse-momentum (high-\(p_T\)) leading hadron in accepted jet candidates. The bias imposed by this requirement is assessed, and the \(p_T\) region in which the bias is small is identified. Inclusive charged-particle jet distributions are reported in peripheral and central Au+Au collisions for \(5<p_{T,jet}^{ch}<25\) GeV/\(c\) and \(5<p_{T,jet}^{ch}<30\) GeV/\(c\), respectively. The charged-particle jet inclusive yield is suppressed for central Au+Au collisions, compared to both the peripheral Au+Au yield from this measurement and to the \(pp\) yield calculated using the PYTHIA event generator. The magnitude of the suppression is consistent with that of inclusive hadron production at high \(p_T\), and that of semi-inclusive recoil jet yield when expressed in terms of energy loss due to medium-induced energy transport. Comparison of inclusive charged-particle jet yields for different values of \(R\) exhibits no significant evidence for medium-induced broadening of the transverse jet profile for \(R<0.4\) in central Au+Au collisions. The measured distributions are consistent with theoretical model calculations that incorporate jet quenching.
In this letter, measurements of the shared momentum fraction (\(z_{\rm{g}}\)) and the groomed jet radius (\(R_{\rm{g}}\)), as defined in the SoftDrop algorihm, are reported in \pp collisions at ...\(\sqrt{s} = 200\) GeV collected by the STAR experiment. These substructure observables are differentially measured for jets of varying resolution parameters from \(R = 0.2 - 0.6\) in the transverse momentum range \(15 < p_{\rm{T, jet}} < 60\) GeV\(/c\). These studies show that, in the \(p_{\rm{T, jet}}\) range accessible at \(\sqrt{s} = 200\) GeV and with increasing jet resolution parameter and jet transverse momentum, the \(z_{\rm{g}}\) distribution asymptotically converges to the DGLAP splitting kernel for a quark radiating a gluon. The groomed jet radius measurements reflect a momentum-dependent narrowing of the jet structure for jets of a given resolution parameter, i.e., the larger the \(p_{\rm{T, jet}}\), the narrower the first splitting. For the first time, these fully corrected measurements are compared to Monte Carlo generators with leading order QCD matrix elements and leading log in the parton shower, and to state-of-the-art theoretical calculations at next-to-leading-log accuracy. We observe that PYTHIA 6 with parameters tuned to reproduce RHIC measurements is able to quantitatively describe data, whereas PYTHIA 8 and HERWIG 7, tuned to reproduce LHC data, are unable to provide a simultaneous description of both \(z_{\rm{g}}\) and \(R_{\rm{g}}\), resulting in opportunities for fine parameter tuning of these models for \pp collisions at RHIC energies. We also find that the theoretical calculations without non-perturbative corrections are able to qualitatively describe the trend in data for jets of large resolution parameters at high \(p_{\rm{T, jet}}\), but fail at small jet resolution parameters and low jet transverse momenta.
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