Open and Hidden Strangeness Production in Nucleon-Nucleon Collisions
Radhey Shyam
备注：
12 pages, 8 figures, Lecture presented at the 4th DAE-BRNS workshop on Hadron Physics, Aligarh Muslim University, India, Feb. 18-23, 2008, Revised version, some typos corrected
We present an overview of the description of K and eta meson productions in nucleon-nucleon collisions within an effective Lagrangian model where meson production proceeds via excitation, propagation and subsequent decay of intermediate baryonic resonant states. The $K$ meson contains a strange quark ($s$) or antiquark ($\bar s$) while the $η$ meson has hidden strangeness as it contains some component of the $s{\bar s}$ pair. Strange meson production is expected to provide information on the manifestation of quantum chromodynamics in the non-perturbative regime of energies larger than that of the low energy pion physics. We discuss specific examples where proper understanding of the experimental data for these reactions is still lacking.
Observables from parity violation in elastic electron-nucleon scattering and neutral current quasi-elastic neutrino-nucleus scattering are employed as tools to improve the current knowledge on the strangeness content in the nucleon.
We review a recent theoretical determination of the strange quark content of the electromagnetic form factors of the nucleon. These are compared with a global analysis of current experimental measurements in parity-violating electron scattering.
Strange quark contributions to neutrino(antineutrino) scattering are investigated on the nucleon level in the quasi-elastic region. The incident energy range between 500 MeV and 1.0 GeV is used for the scattering. All of the physical observable by the scattering are investigated within available experimental and theoretical results for the strangeness form factors of the nucleon. In specific, a newly combined data of parity violating electron scattering and neutrino scattering is exploited. Feasible quantities to be explored for the strangeness contents are discussed for the application to neu…
The calculation of the strangeness content of the nucleon and its experimental verification is a fundamental step in establishing non-perturbative QCD as the correct theory describing the structure of hadrons. It holds a role in QCD analogous to the correct calculation of the Lamb shift in QED. We review the latest developments in the vector and scalar matrix elements of the strange quarks in the proton, where there has recently been considerable progress.
We use the meson cloud model, including the kaon and the $K^*$ contributions, to estimate the electric and magnetic strange form factors of the nucleon. We compare our results with the recent measurements of the strange quark contribution to parity-violating asymmetries in the forward G0 electron-proton scattering experiment. We conclude that it is not possible to explain the data using this model.
We present a systematic study of the sensitivity of quasielastic neutrino-nucleus cross sections at intermediate energies to the strange quark sea of the nucleon. To this end, we investigate the impact of the weak strangeness form factors on the ratio of proton-to-neutron knockout, the ratio of neutral-to-charged current cross sections, on the Paschos-Wolfenstein relation, and on the longitudinal helicity asymmetry. The influence of axial as well as vector strangeness effects is discussed. For the latter, we introduce strangeness parameters from various hadron models and from a recent fit to d…
Isobar models for the electromagnetic production of kaons are discussed with emphasis on the K^+ photoproduction at very small kaon angles and K^0 photoproduction on deuteron. Distorted-wave impuls approximation calculations of the cross sections for the electroproduction of hypernuclei are presented on the case of the ^{12}B_Λproduction.
The parity-violating nucleon-nucleon interaction is the key to understanding the strangeness-conserving hadronic weak interaction at low energies. In this brief talk, I review the past accomplishments in and current status of this subject, and outline a new joint effort between experiment and theory that tries to address the potential problems in the past by focusing on parity violation in few-nucleon systems and using the language of effective field theory.
The polarization observables in phi photoproduction are suggested for probing the nucleon strangeness. Based on models for phi photoproduction, some double polarization observables are shown to be very sensitive to the strangeness content of the proton because of the different spin structures of the amplitudes associated with different mechanisms.
The electroweak form factors of the nucleon as obtained within a three flavor pseudoscalar vector meson soliton model are employed to predict the ratio of the proton and neutron yields from $^{12}C$, which are induced by quasi-elastic neutrino reactions. These predictions are found to vary only moderately in the parameter space allowed by the model. The antineutrino flux of the up-coming experiment determining this ratio was previously overestimated. The corresponding correction is shown to have only a small effect on the predicted ratio. However, it is found that the experimental result for t…
Strangeness (K$^{0,+}$, $Λ$ and $Σ^{-,0,+}$) production in heavy-ion collisions near threshold energies has been investigated within the Lanzhou quantum molecular dynamics (LQMD) transport model. The kaon (anti-kaon)-nucleon and hyperon-nucleon potentials in dense nuclear matter are implemented in the model, which are computed from the chiral perturbation approach and the relativistic mean-field model, respectively. It is found that the in-medium potentials change the structure of transverse flow, and also affect the rapidity distributions and the inclusive spectra for strangeness production. …
We have used the event generator LUCIAE to analyse NA35 data of the $\bar p$ and $\barΛ$ yields, the ratio $\barΛ/\bar p$, and the transverse mass distributions of $\bar p$ and $\barΛ$ in pp and central sulphur-nucleus collisions at 200A GeV. The NA35 data could be reproduced reasonably if one assumes that the s quark pair suppression factor and concerns in nucleus-nucleus collisions are larger than the nucleon-nucleon collision. It seems to indicate that NA35 data might imply the reduction of strangeness suppression in ultrarelativistic nucleus-nucleus collisions comparing to the nucleon-nucl…
Dispersion relations provide a powerful tool to describe the low-energy structure of hadrons. We review the status of the strange vector form factors of the nucleon in dispersion theory. We also comment on open questions and the relation to chiral perturbation theory.
Parity violation in elastic electron-nucleon scattering is studied with the basic goal of improving the understanding of electroweak hadronic structure with special emphasis on the strangeness content in the nucleon. Models for the parity-violating (PV) asymmetry are provided and compared with the world data measured at very different kinematics. The effects introduced in the PV asymmetry due to alternative descriptions of the hadronic structure are analyzed in detail. In particular, a wide selection of prescriptions for dealing with the electromagnetic and neutral current weak interaction nuc…
Global strangeness production in relativistic heavy ion collisions at SPS and RHIC is reviewed. Special emphasis is put on the comparison with the statistical model and the canonical suppression mechanism. It is shown that recent RHIC data on strange particle production as a function of centrality can be explained by a superposition of a fully equilibrated hadron gas and particle emission from single independent nucleon-nucleon collisions in the outer corona.
We report on a novel ab initio approach for nuclear few- and many-body systems with strangeness. Recently, we developed a relevant no-core shell model technique which we successfully applied in first calculations of lightest $Λ$ hypernuclei. The use of a translationally invariant finite harmonic oscillator basis allows us to employ large model spaces, compared to traditional shell model calculations, and use realistic nucleon-nucleon and nucleon-hyperon interactions (such as those derived from EFT). We discuss formal aspects of the methodology, show first demonstrative results for ${}_Λ^3$H, $…
We investigate $ϕ$ meson electroproduction to probe the hidden strangeness content of the nucleon. We found that even a small amount of the $s\bar{s}$ admixture in the nucleon wavefunction can lead to a significant change in several double polarization asymmetries in $ϕ$ electroproduction, which can be tested experimentally at current electron facilities.
The polarization observables in phi meson photoproduction is studied to probe the strangeness content of the nucleon. In addition to the dominant diffractive production and the one-pion-exchange process, we take into account the direct knockout mechanism that arises from the possible hidden strangeness content of the nucleon. We find that some double polarization observables are very sensitive to the strangeness content of the proton because of the different spin structures of the amplitudes associated with different mechanisms. This suggests that such measurements could be very useful in prob…
The systematics of strangeness enhancement is calculated using the HIJING and VENUS models and compared to recent data on $\,pp\,$, $\,pA\,$ and $\,AA\,$ collisions at CERN/SPS energies ($200A\,\, GeV\,$). The HIJING model is used to perform a {\em linear} extrapolation from $pp$ to $AA$. VENUS is used to estimate the effects of final state cascading and possible non-conventional production mechanisms. This comparison shows that the large enhancement of strangeness observed in $S+Au$ collisions, interpreted previously as possible evidence for quark-gluon plasma formation, has its origins in no…
The nucleon's strange-quark vector current form factors are studied from the perspective of chiral symmetry. It is argued that chiral perturbation theory cannot yield a prediction for the strangeness radius and magnetic moment. Arrival at definite predictions requires the introduction of additional, model-dependent assumptions which go beyond the framework of chiral perturbation theory. A variety of such model predictions is surveyed, and the credibility of each is evaluated. The most plausible prediction appears in a model where the unknown chiral counterterms are identified with $t$-channel …
In these lectures we discuss the investigation of the strange meson production in proton-proton ($pp$) and in proton-nucleus ($pA$) reactions within an effective Lagrangian model. The kaon production proceeds mainly via the excitations of $N^*$(1650), $N^*$(1710), and $N^*$(1720) resonant intermediate nucleonic states, in the collision of two initial state nucleons. Therefore, the strangeness production is expected to provide information about the resonances lying at higher excitation energies. For beam energies very close to the kaon production threshold the hyperon-proton final state interac…
A crucial input for recent meson hyperon cloud model estimates of the nucleon matrix element of the strangeness current are the nucleon-hyperon-K* (NYK*) form factors which regularize some of the arising loops. Prompted by new and forthcoming information on these form factors from hyperon-nucleon potential models, we analyze the dependence of the loop model results for the strange-quark observables on the NYK* form factors and couplings. We find, in particular, that the now generally favored soft N-Lambda-K* form factors can reduce the magnitude of the K* contributions in such models by more t…
We calculate the instanton contribution to the proton strangeness in the MIT bag enriched by the presence of a dilute instanton liquid. The evaluation is based on expressing the nucleon matrix elements of bilinear strange quark operators in terms of a model valence nucleon state and interactions producing quark-antiquark fluctuations on top of that valence state. Our method combines the usage of the evolution operator containing a strangeness source, and the Feynman-Hellmann theorem. It enables one to evaluate the strangeness in different Lorentz channels in, essentially, the same way. Only th…
Three-body forces acting on a $Λ$ hyperon in a nuclear medium are investigated, with special focus on the so-called hyperon puzzle in neutron stars. The hyperon-nucleon two-body interaction deduced from SU(3) chiral effective field theory is employed at next-to-leading order. Hyperon-nucleon three-body forces are approximated using saturation by decuplet baryons and are transcribed to density-dependent effective two-body interactions. These together are taken as input in a Brueckner-Bethe-Goldstone equation with explicit treatment of the $ΛN\leftrightarrowΣN$ and $ΛNN\leftrightarrowΣNN$ couple…
The nucleon strange and anti-strange distribution asymmetry is an important issue in the study of the nucleon structure. In this work, we show that the heavy quark recombination processes from a perturbative QCD picture can give a sizable influence on the measurement of the nucleon strangeness asymmetry from charged-current charm production processes, such as the CCFR and NuTeV dimuon measurements. When the influence of heavy quark recombination is considered, a positive effective $δS^-_{\mathrm{HR}}$ should be added to the initially extracted strangeness asymmetry $S^-\equiv\int dx[s(x)-\bar …
I review recent studies of strangeness content in the nucleon pertaining to the flavor-singlet $g_A^0$, the $\bar{s}s$ matrix element and the strangeness electric and magnetic form factors $G_E^s(q^2)$ and $G_M^s(q^2)$, based on lattice QCD calculations. I shall also discuss the relevance of incorporating the strangeness content in nuclei in regard to strange baryon-antibaryon productions from proton-nucleus and nucleus-nucleus collisions at SPS and RHIC energies.
We study contributions to the nucleon strange quark vector current form factors from intermediate states containing K^* mesons. We show how these contributions may be comparable in magnitude to those made by K mesons, using methods complementary to those employed in quark model studies. We also analyze the degree of theoretical uncertainty associated with K^* contributions.
We suggest that breaking of SU(3) flavor symmetry mainly resides in the baryon wave--functions while the charge operators have no (or only small) explicit symmetry breaking components. We utilize the collective coordinate approach to chiral soliton models to support this picture. In particular we compute the $g_A/g_V$ ratios for hyperon beta--decay and the strangeness contribution to the nucleon axial current matrix elements and analyze their variation with increasing flavor symmetry breaking.
An effective Lagrangian model in a coupled channels framework is applied to extract nucleon resonance parameters. In the K-matrix approximation, we simultaneously analyze all the available data for the transitions from pi N to five possible meson-baryon final states, pi N, pipi N, eta N, K Lambda, and KSigma, in the energy range from pi N threshold up to W = 2 GeV. In this work, we focus our efforts on the K Sigma channel. In particular, we include a set of Delta resonances around 1900 MeV: the S31(1900), P31(1910), and P33(1920) states. In most cases the parameters of nucleon resonances deter…
Strange matrix elements of the nucleon are calculated within the light-cone formulation of the meson cloud model. The $Q^2$ dependence of the strange vector and axial vector form factors is computed, and the strangeness radius and magnetic moment extracted, both of which are found to be very small and slightly negative. Within the same framework one finds a small but non-zero excess of the antistrange distribution over the strange at large $x$. Kaon loops are unlikely, however, to be the source of a large polarized strange quark distribution.
Kaon properties are studied within the framework of a fully covariant transport approach. The kaon-nucleon potential is evaluated in two schemes, a chiral perturbative approach and an effective One-Boson-Exchange model. Isospin effects are explicitly accounted for in both models. The transport calculations indicate a significant sensitivity of momentum distributions and total yields of $K^{0,+}$ isospin states on the choice of the kaon-nucleon interaction. Furthermore, isospin effects are rather moderate on absolute kaon yields, but appear on strangeness ratios. This is an important issue in d…
We discuss two descriptions of the nucleon's strange vector form factors in the framework of vector meson dominance. The first, an updated and extended version of Jaffe's dispersion analysis, approximates the spectral functions of the form factors as a sum of vector meson poles, whereas the second combines vector meson dominance in the $ω$ and $ϕ$ meson sector with an intrinsic strangeness distribution from a kaon cloud. (Plenary talk given at the International School for Nuclear Physics, 17th Course: Quarks and Hadrons in Nuclei, Erice, Sicily, Sept 19-27, 1995.)
The recent Investigations in the chiral quark model show that kaon productions of nucleon play an important role in understanding the structure of baryons. The evidences of a third $S_{11}$ resonance in the second resonance region and two narrow states around 2 GeV suggest a set of the molecular type baryons with hidden strangeness. Confirming these states requires further theoretical and experimental studies of the strangeness production.
We study the possibility to constrain the hidden strangeness content of the nucleon by means of the polarization observables in phi meson electroproduction. We consider the OZI evading direct knockout mechanism that arises from the non-vanishing s\bar{s} sea quark admixture of the nucleon as well as the background of the dominant diffractive and the one-boson-exchange processes. Large sensitivity on the nucleon strangeness are found in several beam-target and beam-recoil double polarization observables. The small \sqrt{s} and W region, which is accesible at some of the current high-energy elec…
The properties of the nucleons in nuclear medium have been investigated in the context of the flux tube model incorporating strangeness $(s\bar{s})$ contribution to proton structure in conformity with the experimental indication. Proton is described as a pentaquark system with strange quark contribution whereas neutron is described in three quark configuration. The Quasi particle model of diquark is used to describe the structures of the nucleons. Modifications of the properties like swelling, mass, incompressibility, ratio of the structure functions ($\frac{F_{2}^{n}(x)}{F_{2}^{p}(x)}$), Gott…
Effective quark magnetic moments are extracted from experimental measurements as a function of the strangeness magnetic moment of the nucleon. Assumptions made in even the most general quark model analyses are ruled out by this investigation. Ab initio QCD calculations demand a non-trivial role for strange quarks in the nucleon. The effective moments from QCD calculations are reproduced for a strangeness magnetic moment contribution to the proton of 0.11 $μ_N$, which corresponds to $F_2^s(0) = -0.33\ μ_N$.
I discuss the current status of efforts to constrain the strangeness-conserving weak hadronic interaction, which can be isolated in nuclear systems because of the associated parity violation.
The strange-quark vector current form factors of the nucleon are analyzed within the framework of dispersion relations. Particular attention is paid to contributions made by $K\bar{K}$ intermediate states to the form factor spectral functions. It is shown that, when the $K\bar{K}\to N\bar{N}$ amplitude is evaluated in the Born approximation, the $K\bar{K}$ contributions are identical to those arising from a one-loop calculation and entail a serious violation of unitarity. The mean square strangeness radius and magnetic moment are evaluated by imposing unitarity bounds on the kaon-nucleon parti…
We estimate the 3 πcontinuum contribution to the nucleon strange quark vector current form factors, including the effect of a 3 π<----> ρπresonance. We find the magnitude of this OZI-violating contribution to be comparable to that of typical OZI-allowed contributions. We also study the isoscalar electromagnetic form factors, and find that the presence of a ρπresonance in the multi-pion continuum may generate an appreciable contribution.
Motivated by the successes of covariant baryon chiral perturbation theory in one-baryon systems and in heavy-light systems, we study relevance of relativistic effects in hyperon-nucleon interactions with strangeness $S=-1$. In this exploratory work, we follow the covariant framework developed by Epelbaum and Gegelia to calculate the $YN$ scattering amplitude at leading order. By fitting the five low-energy constants to the experimental data, we find that the cutoff dependence is mitigated, compared with the heavy-baryon approach. Nevertheless, the description of the experimental data remains q…
The leading non-zero electric moment of the nucleon strange-quark vector current is the mean square strangeness radius, $<r_s^2>$. We evaluate the lightest OZI-allowed contribution to $<r_s^2>$, arising from the kaon cloud, using dispersion relations. Drawing upon unitarity constraints as well as $K^{+}N$ scattering and $e^+e^-\to K\bar{K}$ cross section data, we find the structure of this contribution differs significantly from that suggested by a variety of QCD-inspired model calculations. In particular, we find evidence for a strong $ϕ$-meson resonance which may enhance the scale of kaon cl…
In this talk we summarize our latest results on the study of three-body systems with explicit/hidden charm and with strangeness. In particular, we focus on the $K D D $ and $K D\bar D^*$ systems, where a charm $+2$, isospin $1/2$ and strangeness $+1$ state is found with a mass around 4140 MeV in the former and a $K^*$ state, with hidden charm, at a mass around 4307 MeV is obtained in the latter. Both states are predictions of our model and the experimental confirmation of them would be important to understand the properties of the strong interaction in the presence of heavy quarks.
The properties of strangeness neutron star are studied within relativistic mean-field (RMF) model via including a logarithmic interaction as a function of scalar meson field. This logarithmic interaction, named as the $σ$-cut potential, can largely reduce the attractive contributions of scalar meson field at high density without any influence on nuclear structure around normal saturation density. In this work, the TM1 parameter set is chosen as the RMF interaction, while the strengths of logarithmic interaction are constrained by the properties of finite nuclei so that we can obtain a reasonab…
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 con…
After the EMC and subsequent experiments at CERN, SLAC and DESY on the deep inelastic scattering of polarized leptons on polarized nucleons, it is now established that the Q^2=0 value of the axial strange form factor of the nucleon, a quantity which is connected with the spin of the proton and is quite relevant from the theoretical point of view, is relatively large. In this review we consider different methods and observables that allow to obtain information on the strange axial and vector form factors of the nucleon at different values of Q^2. These methods are based on the investigation of …
The calculation of the nucleon strangeness form factors from N_f=2+1 clover fermion lattice QCD is presented. Disconnected insertions are evaluated using the Z(4) stochastic method, along with unbiased subtractions from the hopping parameter expansion. We find that increasing the number of nucleon sources for each configuration improves the signal significantly. We obtain G_M^s(0) = -0.017(25)(07), which is consistent with experimental values, and has an order of magnitude smaller error. Preliminary results for the strangeness contribution to the second moment of the parton distribution functi…
The structures of $NΩ_{(2,1/2)}$ and $ΔΩ_{(3,3/2)}$ with strangeness $s=-3$ are dynamically studied in both the chiral SU(3) quark model and the extended chiral SU(3) quark model by solving a resonating group method (RGM) equation. The first model parameters are taken from our previous work, which gave a satisfactory description of the energies of the baryon ground states, the binding energy of the deuteron, the nucleon-nucleon(NN) scattering phase shifts, and the hyperon-nucleon (YN) cross sections. The effect from the vector meson fields is very similar to that from the one-gluon exchange in…
We update Jaffe's estimate of the strange isoscalar radius and magnetic moment of the nucleon. We make use of a recent dispersion--theoretical fit to the nucleon electromagnetic form factors and an improved description of symmetry breaking in the vector nonet. We find $μ_s = -0.24 \pm 0.03$~n.m. and $r_s^2 = 0.21 \pm 0.03$~fm$^2$. The strange formfactor $F_2^s (t)$ follows a dipole with a cut--off mass of 1.46~GeV, $F_2^s (t)= μ_s (1-t/2.14 \, {\rm GeV}^2 )^{-2}$. These numbers should be considered as upper limits on the strange vector current matrix--elements in the nucleon.
Using a relativistic transport model for heavy ion collisions at energies that are below the threshold for kaon and antikaon production in nucleon-nucleon collisions, we study how their abundances approach the canonical equilibrium during the collisions. We find that kaons are far from chemical equilibrium at the initial and high density stage, and they approach equilibrium only during the expansion stage of the collisions when their production rate is small and becomes comparable to their annihilation rate. In contrast, antikaons approach chemical equilibrium much earlier but eventually fall …
If instantons are introduced into the MIT bag model in such a way that bag radii are allowed to vary, the MIT bag interior can accommodate instanton density which is by an order of magnitude larger than in the case when the radii are fixed (although it is still significantly smaller than in the nonperturbative QCD vacuum). The instanton contribution to baryon mass shifts is also correspondingly larger. The instanton-induced part of the scalar strangeness of the nucleon MIT bag is an order of magnitude larger than found previously, within the linearized approximation. The decrease of the model …
The weak production of Lambdas in nucleon-nucleon scattering is studied in a meson-exchange framework. The weak transition operator for the $NN \to N Λ$ reaction is identical to a previously developed weak strangeness-changing transition potential $ΛN \to NN$ that describes the nonmesonic decay of hypernuclei. The initial $NN$ and final $YN$ state interaction has been included by using realistic baryon-baryon forces that describe the available elastic scattering data. The total and differential cross sections as well as the parity-violating asymmetry are studied for the reaction $pn \to pΛ$. T…
A status report is given for a joint project of the Budapest-Marseille-Wuppertal collaboration and the Regensburg group to study the quark mass-dependence of octet baryons in SU(3) Baryon XPT. This formulation is expected to extend to larger masses than Heavy-Baryon XPT. Its applicability is tested with 2+1 flavor data which cover three lattice spacings and pion masses down to about 190 MeV, in large volumes. Also polynomial and rational interpolations in M_π^2 and M_K^2 are used to assess the uncertainty due to the ansatz. Both frameworks are combined to explore the precision to be expected i…
The scalar form factor of the nucleon and related physical quantities are investigated in the framework of the semibosonized SU(3) Nambu-Jona-Lasinio soliton model. We take into account the rotational $1/N_c$ corrections and linear $m_s$ corrections. The strangeness content of the nucleon in the scalar form factor is discussed in detail. In particular, it is found that the $m_s$ corrections play an essential role of reducing the $\langle N | \bar{s} s | N \rangle$ arising from the leading order and rotational $1/N_c$ contributions. We obtain the $σ_{πN} (0)=40.80\;\mbox{MeV}$, $Δσ= σ_{πN} (2m^…
We investigate the associated strangeness Lambda K+ and Sigma0 K+ productions in proton-proton collisions within an effective Lagrangian model. The initial interaction between the two nucleons is modeled by the exchange of pi, rho, omega, and sigma mesons and the strangeness production proceeds via excitations of N*(1650), N*(1710), and N*(1720) baryonic resonances. The parameters of the model at the nucleon-nucleon-meson vertices are determined by fitting the elastic nucleon-nucleon scattering with an effective interaction based on the exchange of these four mesons, while those at the resonan…
We analyze the process $e p \to e p ϕ$ near the threshold within $uud - s\bar s$ cluster model as a probe of the strangeness content of proton. Our consideration is based on the relativistic harmonic oscillator quark model which takes into account the Lorentz-contraction effect of the hadron wavefunctions. We find that the knockout mechanisms are comparable to the diffractive production of vector-meson-dominance model when only (3-5)% strange quark admixture is assumed, which should be compared with (10-20)% of the nonrelativistic quark model prediction. The cross sections of the $uud$- and $s…
A two phase cascade, LUCIFER II, developed for the treatment of ultra high energy ion-ion collisions is applied to the production of strangeness at SPS energies $\sqrt{s}=17-20$. This simulation is able to simultaneously describe both hard processes such as Drell-Yan and slower, soft processes such as the production of light mesons, including strange mesons, by separating the dynamics into two steps, a fast cascade involving only nucleons in the original colliding relativistic ions followed, after an appropriate delay, by multiscattering of the resulting excited baryons and mesons produced vir…
A brief review of the associated strangeness electromagnetic production is presented. Very recent K+ Lambda photoproduction data on the proton from threshold up to E=2.6 GeV are interpreted within a chiral constituent quark formalism, which embodies all known nucleonic and hyperonic resonances. The preliminary results of this work are reported here.
Recent developments and pending issues in low-energy strong interaction physics with strangeness are summarized. Chiral SU(3) effective field theory has progressed as the appropriate theoretical framework applied to antikaon- and hyperon-nuclear systems. Topics include antikaon-nucleon interactions and the Lambda(1405), KbarNN systems, recent developments in hyperon-nucleon interactions and strangeness in dense baryonic matter with emphasis on new constraints from neutron star observations.
We present the N_f=2+1 clover fermion lattice QCD calculation of the nucleon strangeness form factors. We evaluate disconnected insertions using the Z(4) stochastic method, along with unbiased subtractions from the hopping parameter expansion. We find that increasing the number of nucleon sources for each configuration improves the signal significantly. We obtain G_M^s(0) = -0.017(25)(07), where the first error is statistical, and the second is the uncertainties in Q^2 and chiral extrapolations. This is consistent with experimental values, and has an order of magnitude smaller error.
In ultrarelativistic heavy ion collisions, the multiplicity of multi-strange baryons per participating nucleon increases with centrality in a different fashion for different systems and energies. At RHIC, for copper+copper (CuCu) collisions the increase is much steeper than for gold-gold (AuAu) collisions. We show that this system size dependence is due to a core-corona effect: the relative importance of the corona as compared to the core (thermalized matter) contribution varies and the contribution of a corona nucleon to the multiplicity differs from that of a core nucleon. $ϕ$ mesons follow …
