Showing papers on "Strangeness published in 2012"

Fluctuations and Correlations of net baryon number, electric charge, and strangeness: A comparison of lattice QCD results with the hadron resonance gas model

Abstract: We calculate the quadratic fluctuations of net baryon number, electric charge and strangeness as well as correlations among these conserved charges in ($2+1$)-flavor lattice QCD at zero chemical potential. Results are obtained using calculations with tree-level improved gauge and the highly improved staggered quark actions with almost physical light and strange quark masses at three different values of the lattice cutoff. Our choice of parameters corresponds to a value of 160 MeV for the lightest pseudoscalar Goldstone mass and a physical value of the kaon mass. The three diagonal charge susceptibilities and the correlations among conserved charges have been extrapolated to the continuum limit in the temperature interval $150\text{ }\text{ }\mathrm{MeV}\ensuremath{\le}T\ensuremath{\le}250\text{ }\text{ }\mathrm{MeV}$. We compare our results with the hadron resonance gas (HRG) model calculations and find agreement with HRG model results only for temperatures $T\ensuremath{\lesssim}150\text{ }\text{ }\mathrm{MeV}$. We observe significant deviations in the temperature range $160\text{ }\text{ }\mathrm{MeV}\ensuremath{\lesssim}T\ensuremath{\lesssim}170\text{ }\text{ }\mathrm{MeV}$ and qualitative differences in the behavior of the three conserved charge sectors. At $T\ensuremath{\simeq}160\text{ }\text{ }\mathrm{MeV}$ quadratic net baryon number fluctuations in QCD agree with HRG model calculations, while the net electric charge fluctuations in QCD are about 10% smaller and net strangeness fluctuations are about 20% larger. These findings are relevant to the discussion of freeze-out conditions in relativistic heavy ion collisions.

Freeze-Out Conditions in Heavy Ion Collisions from QCD Thermodynamics

Abstract: We present a determination of chemical freeze-out conditions in heavy ion collisions based on ratios of cumulants of net electric charge fluctuations. These ratios can reliably be calculated in lattice QCD for a wide range of chemical potential values by using a next-to-leading order Taylor series expansion around the limit of vanishing baryon, electric charge and strangeness chemical potentials. From a computation of up to fourth order cumulants and charge correlations we first determine the strangeness and electric charge chemical potentials that characterize freeze-out conditions in

The strangeness content of the nucleon from effective field theory and phenomenology

Abstract: We revisit the classical relation between the strangeness content of the nucleon, the pion-nucleon sigma term and the $SU(3)_F$ breaking of the baryon masses in the context of Lorentz covariant chiral perturbation theory with explicit decuplet-baryon resonance fields. We find that a value of the pion-nucleon sigma term of $\sim$60 MeV is not necessarily at odds with a small strangeness content of the nucleon, in line with the fulfillment of the OZI rule. Moreover, this value is indeed favored by our next-to-leading order calculation. We compare our results with earlier ones and discuss the convergence of the chiral series as well as the uncertainties of chiral approaches to the determination of the sigma terms.

Sigma term and strangeness content of octet baryons

Abstract: By using lattice QCD computations we determine the sigma terms and strangeness content of all octet baryons by means of an application of the Hellmann-Feynman theorem. In addition to polynomial and rational expressions for the quark-mass dependence of octet members, we use SU(3) covariant baryon chiral perturbation theory to perform the extrapolation to the physical up and down quark masses. Our Nf = 2 + 1 lattice ensembles include pion masses

Strangeness Contribution to the Proton Spin from Lattice QCD

Abstract: We compute the strangeness and light-quark contributions Δs, Δu, and Δd to the proton spin in n(f)=2 lattice QCD at a pion mass of about 285 MeV and at a lattice spacing a≈0.073 fm, using the nonperturbatively improved Sheikholeslami-Wohlert Wilson action. We carry out the renormalization of these matrix elements, which involves mixing between contributions from different quark flavors. Our main result is the small negative value Δs(MS)(√(7.4) GeV)=-0.020(10)(4) of the strangeness contribution to the nucleon spin. The second error is an estimate of the uncertainty, due to the missing extrapolation to the physical point.

Hadron-Quark Crossover and Massive Hybrid Stars with Strangeness

Abstract: Using the idea of a smooth crossover from the hadronic matter with hyperons to quark matter with strangeness, we show that the maximum mass of neutron stars with quark matter core can be larger than those without quark matter core. This is in contrast to the conventional softening of equation of state due to exotic components at high density. Essential conditions to reach our conclusion are (i) the crossover takes place at relatively low densities, around 3 times the normal nuclear density, and (ii) the quark matter is strongly interacting in the crossover region. By these, the pressure of the system can be greater than that of purely hadronic matter in the crossover region and leads to the maximum mass greater than 2 solar mass. This conclusion is insensitive to the different choice of the hadronic equation of state with hyperons. Several implications of this result to the nuclear incompressibility, the hyperon mixing, and the neutrino cooling are also remarked.

Spectrum of Hadrons with Strangeness

Abstract: We describe a calculation of the spectrum of strange and nonstrange hadrons that simul- taneously correlates the dressed-quark-core masses of meson and baryon ground- and excited-states within a single framework. The foundation for this analysis is a symmetry-preserving Dyson-Schwinger equation treatment of a vector×vector contact interaction. Our results exemplify and highlight the deep impact of dynamical chiral symmetry breaking on the hadron spectrum: an accurate description of the meson spectrum entails a similarly successful prediction of the spectrum of baryons, including those with strangeness. The analysis also provides numerous insights into baryon structure. For exam- ple, that baryon structure is largely flavour-blind, the first radial excitation of ground-state baryons is constituted almost entirely from axial-vector diquark correlations, and DCSB is the foundation for

Strangeness nuclear physics: a critical review on selected topics

Abstract: Selected topics in strangeness nuclear physics are critically reviewed. This includes production, structure and weak decay of Λ-Hypernuclei, the $$\bar K$$ nuclear interaction and the possible existence of $$\bar K$$ bound states in nuclei. Perspectives for future studies on these issues are also outlined.

Nonextensive statistical effects in the quark-gluon plasma formation at relativistic heavy-ion collisions energies

Abstract: We investigate the relativistic equation of state of hadronic matter and quark-gluon plasma at finite temperature and baryon density in the framework of the non-extensive statistical mechanics, characterized by power-law quantum distributions. We impose the Gibbs conditions on the global conservation of baryon number, electric charge and strangeness number. For the hadronic phase, we study an extended relativistic mean-field theoretical model with the inclusion of strange particles (hyperons and mesons). For the quark sector, we employ an extended MIT-Bag model. In this context we focus on the relevance of non-extensive effects in the presence of strange matter.

Hyperon-nucleon interactions from quantum chromodynamics and the composition of dense nuclear matter.

Abstract: eld theory. The interactions determined from QCD are consistent with those extracted from hyperon-nucleon experimental data within uncertainties, and strengthen theoretical arguments that the strange quark is a crucial component of dense nuclear matter. The interactions between hyperons and nucleons are important for understanding the composition of dense nuclear matter. In high-density baryonic systems, the large values of the Fermi energies may make it energetically advantageous for some of the nucleons to transform into hyperons via the weak interactions, with the increase in rest mass being more than compensated for by the decrease in combined Fermi energy of the baryon-lepton system. This is speculated to occur in the interior of neutron stars, but a quantitative understanding of this phenomenon depends on knowledge of the hyperon-nucleon (YN) interactions in the medium. In this letter we use n scattering phase shifts in the 1 S0 and 3 S1 spinchannels calculated with Lattice QCD (LQCD) to quantify the energy shift of the hyperon in dense neutron matter, as might occur in the interior of a neutron star. Our results strongly suggest an important role for strangeness in such environments. Precise nucleon-nucleon (NN) interactions constrained by experiment and chiral symmetry, together with numerically small but important three-nucleon interactions, have served as input to rened many-body techniques for

The Influence of Hyperons and Strong Magnetic Field in Neutron Star Properties

Abstract: Neutron stars are among of the most exotic objects in the universe and constitute a unique laboratory to study nuclear matter above the nuclear saturation density. In this work, we study the equation of state (EoS) of the nuclear matter within a relativistic model subject to a strong magnetic field. We then apply this EoS to study and describe some of the physical characteristics of neutron stars, especially the mass–radius relation and chemical compositions. To study the influence of the magnetic field and the hyperons in the stellar interior, we consider altogether four solutions: two different magnetic field to obtain a weak and a strong influence; and two configurations: a family of neutron stars formed only by protons, electrons, and neutrons and a family formed by protons, electrons, neutrons, muons, and hyperons. The limit and the validity of the results found are discussed with some care. In all cases, the particles that constitute the neutron star are in β equilibrium and zero total net charge. Our work indicates that the effect of a strong magnetic field has to be taken into account in the description of magnetars, mainly if we believe that there are hyperons in their interior, in which case the influence of the magnetic field can increase the mass by more than 10 %. We have also seen that although a magnetar can reach 2.48 M ⊙, a natural explanation of why we do not know pulsars with masses above 2.0 M ⊙ arises. We also discuss how the magnetic field affects the strangeness fraction in some standard neutron star masses, and to conclude our paper, we revisit the direct Urca process related to the cooling of the neutron stars and show how it is affected by the hyperons and the magnetic field.

Neutron Stars with Hyperons subject to Strong Magnetic Field

Abstract: Neutron stars are one of the most exotic objects in the universe and a unique laboratory to study the nuclear matter above the nuclear saturation density. In this work, we study the equation of state of the nuclear matter within a relativistic model subjected to a strong magnetic field. We then apply this EoS to study and describe some of the physical characteristics of neutron star, especially the mass-radius relation and chemical compositions. To study the influence of a the magnetic field and the hyperons in the stellar interior, we consider altogether four solutions: two different values of magnetic field to obtain a weak and a strong influence, and two configurations: a family of neutron stars formed only by protons, electrons and neutrons and a family formed by protons, electrons, neutrons, muons and hyperons. The limit and the validity of the results found are discussed with some care. In all cases the particles that constitute the neutron star are in $\beta$ equilibrium and zero total net charge. Our work indicates that the effect of a strong magnetic field has to be taken into account in the description of magnetars, mainly if we believe that there are hyperons in their interior, in which case, the influence of the magnetic field can increase the mass by more than 10%. We have also seen that although a magnetar can reach 2.48$M_{\odot}$, a natural explanation of why we do not know pulsars with masses above 2.0$M_{\odot}$ arises. We also discuss how the magnetic field affects the strangeness fraction in some standard neutron star masses and, to conclude our paper, we revisit the direct URCA process related to the cooling of the neutron stars and show how it is affected by the hyperons and the magnetic field.

Sigma terms and strangeness content of the nucleon with $N_f=2+1+1$ twisted mass fermions

Abstract: We study the nucleon matrix elements of the quark scalar-density operator using maximally twisted mass fermions with dynamical light ($u$,$d$), strange and charm degrees of freedom. We demonstrate that in this setup the nucleon matrix elements of the light and strange quark densities can be obtained with good statistical accuracy, while for the charm quark counterpart only a bound can be provided. The present calculation which is performed at only one value of the lattice spacing and pion mass serves as a benchmark for a future more systematic computation of the scalar quark content of the nucleon.

S -wave scattering of strangeness − 3 baryons

Abstract: We explore the interactions of two strangeness -3 baryons in multiple spin channels with lattice QCD. This system provides an ideal laboratory for exploring the interactions of multi-baryon systems with minimal dependence on light quark masses. Model calculations of the two-$\Omega^-$ system in two previous works have obtained conflicting results, which can be resolved by lattice QCD. The lattice calculations are performed using two different volumes with $L\sim2.5$ and 3.9 fm on anisotropic clover lattices at $m_\pi \sim 390$ MeV with a lattice spacing of $a_s \sim 0.123$ fm in the spatial direction and $a_t\sim{a}_s/3.5$ in the temporal direction. Using multiple interpolating operators from a non-displaced source, we present scattering information for two ground state $\Omega^-$ baryons in both the S=0 and S=2 channels. For S=0, $k\cot\delta$ is extracted at two volumes, which lead to an extrapolated scattering length of $a^{\Omega\Omega}_{S=0}=0.16 \pm 0.22 \ \text{fm}$, indicating a weakly repulsive interaction. Additionally, for S=2, two separate highly repulsive states are observed. We also present results on the interactions of the excited strangeness -3, spin-1/2 states with the ground spin-3/2 states for the spin-1 and spin-2 channels. Results for these interactions are consistent with attractive behavior.

Sigma terms and strangeness content of the nucleon with Nf=2+1+1 twisted mass fermions

Abstract: 7 pages, 6 figures. Talk given at the XXX International Symposium on Lattice Field Theory - Lattice 2012, June 24-29, 2012 Cairns, Australia

Strange quark content of the nucleon and dark matter searches

Abstract: The strange quark scalar content plays an important role in both the description of nucleon structure and in the determination of dark matter direct detection cross sections. As a measure of the strange-quark contribution to the nucleon mass, the strange-quark sigma term (\sigma_s) provides important insight into the nature of mass generation in QCD. The phenomenological determination of \sigma_s exhibits a wide range of variation, with values suggesting that the strange quark contributes anywhere between 0 and more than 30% of the nucleon mass. In the context of dark matter searches, coupled with relatively large Higgs coupling to strangeness, this variation dominates the uncertainty in predicted cross sections for a large class of dark matter models. Here we report on the recent results in lattice QCD, which are now giving a far more precise determination of \sigma_s than can be inferred from phenomenology. As a consequence, the lattice determinations of \sigma_s can now dramatically reduce the uncertainty in dark matter cross sections associated with the hadronic matrix elements.

Nonextensive statistical effects and strangeness production in hot and dense nuclear matter

Abstract: By means of an effective relativistic nuclear equation of state in the framework of the nonextensive statistical mechanics, characterized by power-law quantum distributions, we study the phase transition from hadronic matter to quark–gluon plasma at finite temperature and baryon density. The analysis is performed by requiring the Gibbs conditions on the global conservation of baryon number, electric charge fraction and zero net strangeness. We show that nonextensive statistical effects strongly influence the strangeness production during the pure hadronic phase and the hadron–quark–gluon mixed phase transition, also for small deviations from the standard Boltzmann–Gibbs statistics.

Strangeness in the nucleon: what have we learned?

Abstract: We review the state of our knowledge concerning the contribution of strange quarks to various nucleon properties. In the case of the electric and magnetic form factors, the level of agreement between theory and experiment is very satisfactory and gives us considerable confidence in our capacity to make reliable calculations within non-perturbative QCD. In view of the importance of the scalar form factors to the detection of dark matter candidates such as neutralinos, we place a particular emphasis on the determination of the pi-N and strange quark sigma commutators.

Phase transition toward strange matter

Abstract: The phase diagram of a system constituted of neutrons and $\Lambda$-hyperons in thermal equilibrium is evaluated in the mean-field approximation. It is shown that this simple system exhibits a complex phase diagram with first and second order phase transitions. Due to the generic presence of attractive and repulsive couplings, the existence of phase transitions involving strangeness appears independent of the specific interaction model. In addition we will show under which conditions a phase transition towards strange matter at high density exists, which is expected to persist even within a complete treatment including all the different strange and non- strange baryon states. The impact of this transition on the composition of matter in the inner core of neutron stars is discussed.

Strangeness balance in HADES experiments and the Ξ − enhancement

Abstract: The High Acceptance Di-Electron Spectrometer (HADES) data on strangeness production in Ar + KCl collisions at 1.76$A$ GeV are analyzed within a minimal statistical model. In the model the total negative strangeness content is fixed by the observed ${K}^{+}$ multiplicities. Particles with negative strangeness are assumed to remain in chemical equilibrium with themselves and in thermal equilibrium with the environment until a common freeze-out. Exact strangeness conservation in each collision event is explicitly preserved. This implies that $\ensuremath{\Xi}$ baryons can be released only in events where two or more kaons are produced. An increase of the fireball volume due to application of a centrality trigger in HADES experiments is taken into account. We find that experimental ratios of ${K}^{\ensuremath{-}}/{K}^{+}$, $\ensuremath{\Lambda}/{K}^{+}$, and $\ensuremath{\Sigma}/{K}^{+}$ can be satisfactorily described provided in-medium potentials are taken into account. However, the calculated ${\ensuremath{\Xi}}^{\ensuremath{-}}/\ensuremath{\Lambda}/{K}^{+}$ ratio proves to be significantly smaller compared to the measured value (eight times less than the experimental median value and three times less than the lower error bar). Various scenarios to explain observed $\ensuremath{\Xi}$ enhancement are discussed. Arguments are given in favor of the $\ensuremath{\Xi}$ production in direct reactions. The rates of the possible production processes are estimated and compared.

Muon production and string percolation effects in cosmic rays at the highest energies

Abstract: Ultra High Energy Cosmic Rays with energies above ~ 10^18 eV provide an unique window to study hadronic interactions at energies well above those achieved in the largest man-made accelerators. We argue that at those energies string percolation may occur and play an important role on the description of the induced Extensive Air Showers by enhancing strangeness and baryon production. This leads to a significant increase of the muon content of the cascade in agreement with recent data collected at UHECR experiments. In this work, the effects of string percolation in hadronic interactions are implemented in an EAS code and their impact on several shower observables is evaluated and discussed.

Strangeness in the nucleon: what have we learned?

Abstract: We review the state of our knowledge concerning the contribution of strange quarks to various nucleon properties. In the case of the electric and magnetic form factors, the level of agreement between theory and experiment is very satisfactory and gives us considerable confidence in our capacity to make reliable calculations within non-perturbative QCD. In view of the importance of the scalar form factors to the detection of dark matter candidates such as neutralinos, we place a particular emphasis on the determination of the pi-N and strange quark sigma commutators.

On the strangeness content of the nucleon

Abstract: We revisit the classical relation between the strangeness content of the nucleon, the pion-nucleon sigma term and the $SU(3)_F$ breaking of the baryon masses in the context of Lorentz covariant chiral perturbation theory with explicit decuplet-baryon resonance fields. We find that a value of the pion-nucleon sigma term of $\sim$60 MeV is not necessarily at odds with a small strangeness content of the nucleon, in line with the fulfillment of the OZI rule. Moreover, this value is indeed favored by our next-to-leading order calculation. We compare our results with earlier ones and discuss the convergence of the chiral series as well as the uncertainties of chiral approaches to the determination of the sigma terms.