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DOE OSTI2020

Measurement of the mass difference and the binding energy of the hypertriton and antihypertriton

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. In this work, 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 detector at the Relativistic Heavy Ion Collider, we measure the Λ hyperon binding energy B Λ 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. 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.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗

DOE OSTI2020

Photoproduction of the Λ(1520) hyperon with a 9 GeV photon beam at GlueX

The GlueX experiment is located at the Thomas Jefferson National Accelerator Facility (JLab) in Newport News, VA, USA. It features a hermetic 4π detector with excellent tracking and calorimetry capabilities. Its 9 GeV linearly polarized photon beam is produced from the 12 GeV electron beam, delivered by JLab’s Continuous Electron Beam Accelerator Facility (CEBAF), via bremsstrahlung on a thin diamond and is incident on a LH2 target. GlueX recently finished its first data taking period and published first results. The main goal of GlueX is to measure gluonic excitations of mesons. These so-called hybrid or exotic mesons are predicted by Quantum Chromodynamics (QCD) but haven’t been experimentally confirmed yet. They can have quantum numbers not accessible by ordinary quark-antiquark pairs which helps in identifying them using partial wave analysis techniques. The search for exotic mesons requires a very good understanding of photoproduction processes in a wide range of final states, one of them being pK + K - which contains many meson and baryon reactions. The Λ(1520) is a prominent hyperon resonance in this final state and is the subject of this presentation. This talk will give an introduction to the GlueX experiment and show preliminary results for the photoproduction of the Λ(1520) hyperon. The measurement of important observables like the photon beam asymmetry and spin-density matrix elements will be discussed and an outlook to possible measurements of further hyperon states in the pK + K - final state will be given.

Pauli, Peter↗

DOE OSTI2020

Excluded-volume model for quarkyonic matter. II. Three-flavor shell-like distribution of baryons in phase space

We extend the excluded-volume model of isospin symmetric two-flavor dense quarkyonic matter [Phys. Rev. C 101, 035201 (2020)] including strange particles and address its implications for neutron stars. The effective sizes of baryons are defined from the diverging hard-core potentials in the short interdistance regime. Around the hard-core density, the repulsive core between baryons at short distances leads to a saturation in the number density of baryons and generates perturbative quarks from the lower phase space, which leads to the shell-like distribution of baryons by the Pauli exclusion principle. The strange-quark Fermi sea always appears at high densities but the Λ hyperon shell only appears when the effective size of the Λ hyperon is smaller than the effective size of nucleons. We find that the pressure of strange quarkyonic matter can be large enough to support neutron stars with two times solar mass and can have a large sound speed, $c^2_s ≃ 0.7$. Finally, the fraction of the baryon number carried by perturbative quarks is about 30% at the inner core of most massive neutron stars.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗

DOE OSTI2023

Search for CP violation and measurement of branching fractions and decay asymmetry parameters for Λ$^+_c$ → Λ h + and Λ$^+_c$ → Σ 0 h + ( h = K, π )

Here, we report a study of Λ$^+_c$ → Λ h + and Λ$^+_c$ → Σ 0 h + ( h = K, π ) decays based on a data sample of 980 fb -1 collected with the Belle detector at the KEKB energy-asymmetric e + e - collider. The first results of direct CP asymmetry in two-body singly Cabibbo-suppressed (SCS) decays of charmed baryons are measured, A$^{dir}_{CP}(Λ^+_{c} → ΛK^{+})$ = +0.021 ± 0.026 ± 0.001 and A$^{dir}_{CP}(Λ^+_{c} → Σ^{0}K^{+})$ = +0.025 ± 0.054 ± 0.004. We also make the most precise measurement of the decay asymmetry parameters (α) for the four modes of interest and search for CP violation via the α-induced CP asymmetry (A$^α_{CP}$ ). We measure A$^α_{CP}$ ($Λ^+_{c} → ΛK^{+}$) = -0.023 ± 0.086 ± 0.071 and A$^α_{CP}$ ($Λ^+_{c} → Σ^{0}K^{+}$) = +0.08 ± 0.35 ± 0.14, which are the first A$^α_{CP}$ results for SCS decays of charmed baryons. We search for Λ -hyperon CP violation in $Λ^+_{c}$ → (Λ, Σ 0 )π + and find A$^α_{CP}$(Λ → pπ - ) = +0.013 ± 0.007 ± 0.011 . This is the first time that hyperon CP violation has been measured via Cabibbo-favored charm decays. No evidence of baryon CP violation is found. We also obtain the most precise branching fractions for two SCS Λ$^+_c$ decays, $\mathscr{B}$($Λ^+_{c} → ΛK^{+}$) = (6.57 ± 0.17 ± 0.11 ± 0.35) x 10 -4 and $\mathscr{B}$($Λ^+_{c} → Σ^{0}K^{+}$) = (3.58 ± 0.19 ±0.06 ±0.19) x 10 -4 . The first uncertainties are statistical and the second systematic, while the third uncertainties come from the uncertainties on the world average branching fractions of $Λ^+_{c}$ → (Λ, Σ 0 )π + .

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗

DOE OSTI2022

Evolution of Λ polarization in the hadronic phase of heavy-ion collisions

Using the AMPT+MUSIC+URQMD hybrid model, we study the global and local spin polarizations of Λ hyperons as functions of the freeze-out temperature of the spin degree of freedom in the hadronic phase of Au+Au collisions at $\sqrt{s_{NN}}$ = 19.6 GeV. Including contributions from both the thermal vorticity and thermal shear of the hadronic matter, we find that with the spin freeze-out temperature dropping from the hadronization temperature of 160 MeV to 110 MeV at the kinetic freeze-out, both the global and local spin polarizations of Λ hyperons due to the thermal vorticity decrease by a factor of 2, while those due to the thermal shear decrease quickly and become negligibly small at 140 MeV. Here our results thus suggest that it is important to take into account the evolution of the spin degree of freedom in the hadronic stage of relativistic heavy-ion collisions when theoretically predicted global and local Λ spin polarizations are compared with the experimental measurements.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗

DOE OSTI2024

Vortex rings in event-by-event relativistic heavy-ion collisions

We present event-by-event simulations for central asymmetric light+heavy and Au +Au collisions to investigate the formation and evolution of vortex-ring structures in the longitudinal flow velocity profile. The production-plane polarization of Λ hyperons, defined with respect to the Λ momentum and the beam, can track the “vortex-ring” feature in the event, a characteristic vortical structure generated by longitudinal flow gradients. We make comprehensive model predictions for the rapidity-dependent vortex-ring observables for different collision system sizes at √s NN = 200 and 72 GeV. Furthermore, our predictions at the latter energy can be explored in the future LHCb fixed-target experiment at the Large Hadron Collider.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗

DOE OSTI2026

Measuring spin correlation between quarks during QCD confinement

The vacuum is now understood to have a rich and complex structure, characterized by fluctuating energy fields and a condensate of virtual quark–antiquark pairs. The spontaneous breaking of the approximate chiral symmetry, signalled by the nonvanishing quark condensate $\langle$$q\bar{q}$$\rangle$, is dynamically generated through topologically nontrivial gauge configurations such as instantons. The precise mechanism linking the chiral symmetry breaking to the mass generation associated with quark confinement remains a profound open question in quantum chromodynamics (QCD)—the fundamental theory of strong interaction. High-energy proton–proton collisions could liberate virtual quark–antiquark pairs from the vacuum that subsequently undergo confinement to form hadrons, whose properties could serve as probes into QCD confinement and the quark condensate. Here we report evidence of spin correlations in $Λ\bar{Λ}$ hyperon pairs inherited from spin-correlated strange quark–antiquark virtual pairs. Measurements by the STAR experiment at the Relativistic Heavy Ion Collider (RHIC) at Brookhaven National Laboratory reveal a relative polarization signal of (18 ± 4)% that links the virtual spin-correlated quark pairs from the QCD vacuum to their final-state hadron counterparts. Crucially, this correlation vanishes when the hyperon pairs are widely separated in angle, consistent with the decoherence of the quantum system. Our findings provide a new experimental model for exploring the dynamics and interplay of quark confinement and entanglement.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗

DOE OSTI2024

Realtime dynamics of hyperon spin correlations from string fragmentation in a deformed four-flavor Schwinger model

Self-polarizing weak decays of Λ -hyperons provide unique insight into the role of entanglement in the fragmentation of QCD strings through measurements of the spin correlations of Λ Λ ¯ pairs produced in collider experiments. The simplest quantum field theory representing the underlying parton dynamics is the four-flavor massive Schwinger model plus an effective spin-flip term, where the flavors are mapped to light (up or down) and heavy (strange) quarks and their spins. This construction provides a novel way to explore hyperon spin correlations in 1 + 1 dimensions. We investigate the evolution of these correlations for different string configurations that are sensitive to the rich structure of the model Hamiltonian. Published by the American Physical Society 2024

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗

DOE OSTI2020

Global polarization of ΛΛ¯ hyperons in Pb-Pb collisions at √ s NN = 2.76 and 5.02 TeV

The global polarization of the Λ and Λ ¯ hyperons is measured for Pb-Pb collisions at s N N = 2.76 and 5.02 TeV recorded with the ALICE at the Large Hadron Collider (LHC). The results are reported differentially as a function of collision centrality and hyperon's transverse momentum ( p T ) for the range of centrality 5–50%, 0.5 < p T < 5 GeV/ c , and rapidity | y | < 0.5 . The hyperon global polarization averaged for Pb-Pb collisions at s N N = 2.76 and 5.02 TeV is found to be consistent with zero, ( P H ) ( % ) ≈ 0.01 ± 0.06 (stat.) ± 0.03 (syst.) in the collision centrality range 15–50%, where the largest signal is expected. The results are compatible with expectations based on an extrapolation from measurements at lower collision energies at the Relativistic Heavy Ion Collider, hydrodynamical model calculations, and empirical estimates based on collision energy dependence of directed flow, all of which predict the global polarization values at LHC energies of the order of 0.01 % .

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗

DOE OSTI2022

Extracting the $\Lambda $ Polarizing Fragmentation Function from Belle e + e − Data within the TMD Formalism

Data from the Belle Collaboration for associated production (with a light unpolarized hadron) and single-inclusive production of transversely polarized Λ-hyperons in e + e − annihilation processes allowed to extract within a TMD approach, for the first time, the Λ polarizing fragmentation function. Recent theoretical developments on the computation of cross sections for single-inclusive hadron production in e + e − annihilation processes, within a Soft Collinear Effective Theory approach, combined with the CSS formalism for the double-hadron production case, can be used to perform a renewed analysis, adopting a full TMD factorization scheme. Preliminary results and a comparison between the two analyses are presented.

CSS evolution equations↗

DOE OSTI2020

Hyperon polarization in relativistic heavy ion collisions and axial U(1) symmetry breaking at high temperature

Experiments at the Relativistic Heavy Ion Collider have measured the net polarization of $\text{Λ}$ and $\bar{Λ}$ hyperons and attributed it to a coupling between their spin and the vorticity of the fluid created in heavy ion collisions, but how the spin comes to equilibrium with vorticity is an open problem. Recently, we found that vorticity fluctuations and helicity flip of strange quarks in quark-gluon plasma through perturbative QCD processes resulted in equilibration times far too long to be relevant. Here, we consider the Nambu-Jona-Lasinio model with the inclusion of the six-quark Kobayashi-Maskawa–'t Hooft interaction which breaks axial U(1). In this work, using instanton inspired models for the temperature dependence of the axial symmetry breaking, we find that constituent strange quarks can reach spin equilibrium at temperatures below about 170 MeV, just before they hadronize to form hyperons.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗

DOE OSTI2021

Global Polarization of Ξ and Ω Hyperons in Au + Au Collisions at s N N = 200 GeV

Global polarization of Ξ and Ω hyperons has been measured for the first time in Au+Au collisions at $\sqrt{s_{NN}}$=200 GeV. The measurements of the $Ξ^{-}$ and $\overline{Ξ}^{+}$ 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 Ξ→Λ+π, as well as by measuring the polarization of the daughter Λ hyperon, polarized via polarization transfer from its parent. The polarization, obtained by combining the results from the two methods and averaged over $Ξ^{-}$ and $\overline{Ξ}^{+}$, is measured to be $\langle{P_{Ξ}}\rangle$=0.47±0.10(stat)±0.23(syst)% for the collision centrality 20%-80%. The $\langle{P_{Ξ}}\rangle$ is found to be slightly larger than the inclusive Λ polarization and in reasonable agreement with a multiphase transport model. The $\langle{P_{Ξ}}\rangle$ is found to follow the centrality dependence of the vorticity predicted in the model, increasing toward more peripheral collisions. Finally, the global polarization of Ω, $\langle{P_{Ω}}\rangle$=1.11±0.87(stat)±1.97(syst)% was obtained by measuring the polarization of daughter Λ in the decay Ω→Λ+K, assuming the polarization transfer factor $C_{ΩΛ}=1$.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗

DOE OSTI2024

$$\Lambda $$ polarization in very high energy heavy ion collisions as a probe of the quark–gluon plasma formation and properties

Abstract We have studied the spin polarization of $$\Lambda $$ Λ hyperons in heavy ion collisions at center-of-mass energies $$\sqrt{s_\textrm{NN}}= 200$$ s NN = 200 GeV and $$\sqrt{s_\textrm{NN}}= 5.02$$ s NN = 5.02 TeV carried out at RHIC and LHC colliders. We have calculated the mean spin vector at local thermodynamic equilibrium, including all known first-order terms in the gradients of the thermo-hydrodynamic fields, assuming that the hadronization hypersurface has a uniform temperature. We have also included the feed-down contributions to the polarization of $$\Lambda $$ Λ stemming from the decays of polarized $$\Sigma ^*$$ Σ ∗ and $$\Sigma ^0$$ Σ 0 hyperons. The obtained results are in good agreement with the data. In general, the component of the spin vector along the global angular momentum, orthogonal to the reaction plane, shows strong sensitivity to the initial longitudinal flow velocity. Furthermore, the longitudinal component of the spin vector turns out to be very sensitive to the bulk viscosity of the plasma at the highest LHC energy. Therefore, the azimuthal dependence of spin polarization can effectively constrain the initial hydrodynamic conditions and the transport coefficients of the quark gluon plasma.

Physics↗

DOE OSTI2024

Transverse Λ polarization in unpolarized $pp$ → jet Λ ↑ X

In this Letter, we investigate the spontaneous transverse polarization of Λ hyperons produced in unpolarized pp collisions inside a jet, by adopting a TMD approach where transverse momentum effects are included only in the fragmentation process. We will present predictions based on the parametrizations of the Λ polarizing fragmentation function as extracted from fits to Belle e + e – data. These estimates will be compared against preliminary STAR data. We will then be able to explore the universality properties of the quark polarizing fragmentation function and, for the first time, the role of its gluon counterpart.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗

DOE OSTI2024

Cross-Section Measurements of Lambda(1405) Photoproduction from CLAS G12 Data

The nature of the Λ(1405) state remains an unsolved puzzle despite being predicted in 1959 and experimentally confirmed soon thereafter in 1961. It has an unusual line shape that deviates from a Breit-Wigner form, typically associated with resonances. Theoretical explanations of this first, excited Λ hyperon range from it being a three-quark state (uds) to being a dynamically generated, quasi-bound KN¯ molecule to consisting of a two-pole (two resonances) structure. None of these descriptions of Λ(1405) are universally accepted. Thus, there is a need for additional data on this notoriously cryptic state to help guide theoretical models. Measurements such as cross sections can assist to elucidate our understanding of resonances and how they’re produced. In this analysis, data from the CLAS g12 experiment were studied and differential cross-section measurements for Λ(1405) photoproduction were extracted. The results contained within this work are in relatively good agreement with prior photoproduction measurements (CLAS g11). Moreover, the energy range over which the existing measurements extend is expanded to higher values with these new results. These higher energy cross-section measurements should prove to be beneficial for subsequent analyses of Λ(1405).

Reed, Trevor↗

DOE OSTI2023

First Measurement of Λ Electroproduction off Nuclei in the Current and Target Fragmentation Regions

Here, we report results of Λ hyperon production in semi-inclusive deep-inelastic scattering off deuterium, carbon, iron, and lead targets obtained with the CLAS detector and the Continuous Electron Beam Accelerator Facility 5.014 GeV electron beam. These results represent the first measurements of the Λ multiplicity ratio and transverse momentum broadening as a function of the energy fraction (z) in the current and target fragmentation regions. The multiplicity ratio exhibits a strong suppression at high z and an enhancement at low z. The measured transverse momentum broadening is an order of magnitude greater than that seen for light mesons. This indicates that the propagating entity interacts very strongly with the nuclear medium, which suggests that propagation of diquark configurations in the nuclear medium takes place at least part of the time, even at high z. The trends of these results are qualitatively described by the Giessen Boltzmann-Uehling-Uhlenbeck transport model, particularly for the multiplicity ratios. These observations will potentially open a new era of studies of the structure of the nucleon as well as of strange baryons.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗

DOE OSTI2025

Spin-Density Matrix Elements in K∗(892) Photoproduction with Recoiling Λ at GlueX

We report on the measurement of spin-density matrix elements for the K∗(892) → K+π0 photoproduction process with a recoiling Λ hyperon. The measurement used high-statistics GlueX data from photoproduction off a proton target at photon energies between 8.2 and 8.8 GeV, in a kinematic regime dominated by t-channel exchange processes. GlueX is a fixed-target experiment with a linearly polarized photon beam and a nearly 4π hermetic detector, allowing extraction of the full spin observ- ables from both the K+π0 and Λ → π−p systems. The GlueX data show clear evidence of the K∗(892) resonance in the K+π0 mass spectrum, along with additional structures at higher masses that may correspond to K∗0 or K∗2 resonances. The precise measurement of the spin-density matrix elements of the K∗(892) could serve as a standard candle for con- tinuing studies of higher-mass excited K∗ states. In addition, t-channel K∗ photoproduction differs from previous measurements of non-strange vector meson photoproduction, as it is free from Pomeron exchange and is virtually unexplored in this energy regime. Furthermore, the unprece- dented data collected by GlueX allow for investigations of correlations between the Λ polarization and the spin-density matrix elements of K∗ production, paving the way for future partial-wave analyses with full spin information involving the recoil hyperon.

Li, Hao [College of William and Mary, Williamsburg↗

DOE OSTI2021

$\mathrm {K_S}^{0}$- and (anti-)$\Lambda $-hadron correlations in $pp$ collisions at ${\sqrt{s}} = 13$ TeV

Two-particle Azimuthal correlations are measured with the ALICE apparatus in pp collisions at √s = 13 TeV to explore strangeness- and multiplicity-related effects in the fragmentation of jets and the transition regime between bulk and hard production, probed with the condition that a strange meson (KS o ) or baryon (Λ) with transverse momentum p T > 3 GeV/c is produced. Azimuthal correlations between kaons or Λ hyperons with other hadrons are presented at midrapidity for a broad range of the trigger (3 < $p$$^{trigg}_{T}$ < 20 GeV/c) and associated particle p T (1 GeV/c < $p$$^{assoc}_{T}$< $p$$^{trigg}_{T}$), for minimum-bias events and as a function of the event multiplicity. The near- and away-side peak yields are compared for the case of either K S o or Λ(Λ¯) being the trigger particle with that of inclusive hadrons (a sample dominated by pions). In addition, the measurements are compared with predictions from PYTHIA 8 and EPOS LHC event generators.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗