Hyperon Production Research Articles

Absorption of Λ(1520) hyperons in photon-nucleus collisions

In the framework of the nuclear spectral function approach for incoherent primary photon-nucleon and secondary pion-nucleon production processes we study the inclusive Λ(1520)-hyperon production in the interaction of 2-GeV photons with nuclei. In particular, the A and momentum dependences of the absolute and relative Λ(1520)-hyperon yields are investigated in two scenarios for its in-medium width. Our model calculations show that the pion-nucleon production channel contributes appreciably to the Λ(1520) creation at intermediate momenta both in light and heavy nuclei in the chosen kinematics and, hence, has to be taken into consideration on close examination of the dependences of the Λ(1520)-hyperon yields on the target mass number with the aim to get information on its width in the medium. They also demonstrate that the A and momentum dependences of the absolute and relative Λ(1520)-hyperon production cross sections at incident energy of interest are markedly sensitive to the Λ(1520) in-medium width, which means that these observables may be an important tool to determine the above width.

System size dependence of particle production at the SPS

Recent results on the system size dependence of net-baryon and hyperon production as measured at the CERN SPS are discussed. The observed Npart dependences of yields, but also of dynamical properties, such as average transverse momenta, can be described in the context of the core corona approach. Other observables, such as antiproton yields and net-protons at forward rapidities, do not follow the predictions of this model. Possible implications for a search for a critical point in the QCD phase diagram are discussed. Event-by-event fluctuations of the relative core to corona source contributions might influence fluctuation observables (e.g. multiplicity fluctuations). The magnitude of this effect is investigated.

Kaon and hyperon production in antiproton-induced reactions on nuclei

We study the strangeness production in antiproton-nucleus collisions at the beam momenta from 200 MeV/c to 15 GeV/c within the Giessen Botzmann-Uehling-Uhlenbeck (GiBUU) transport model. The GiBUU model contains a very detailed description of the underlying antinucleon-nucleon cross sections, in-particular, of the strangeness production channels. We compare our calculations with the bubble chamber data on \Lambda- and K^0_S-production in antiproton-nucleus collisions and with the earlier intranuclear cascade calculations. The contributions of the various partial channels to the hyperon production are reported and systematic differences with experiment are discussed. The possible formation of the bound \Lambda- and \Lambda\Lambda-nucleus systems is also considered. Finally, the results on the \Xi-hyperon production are presented in view of the forthcoming experiments with antiproton beams at FAIR.

Experimental plan ofΣpscatterings at J-PARC

In order to test theoretical frameworks of the baryon-baryon interactions and to confirm the effect between quarks for the first time, we propose an experiment to measure low-energy hyperon proton scattering cross sections in the following channels with high statistics, 1. Σ − p elastic scattering, 2. Σ − p → Λn inelastic scattering, 3. Σ + p elastic scattering. According to theoretical models based on quark-gluon picture for the short range part of the baryon-baryon interactions, the Σ + p channel is expected to have an extremely repulsive core due to the Pauli effect between quarks, which leads a Σ + p cross section twice as large as that predicted by conventional meson exchange models with a phenomenologically treated short range repulsive core. In addition, measurement of the Σ − p channel where the quark Pauli effect is not effective is also necessary to test the present theoretical models based on meson exchange picture with the flavor SU(3) symmetry. Thus this experiment will provide essential data to test the frameworks of the theoretical models of the baryon-baryon interactions and to investigate the nature of the repulsive core which has not been understood yet. In order to overcome the experimental difficulties in measuring low-energy hyperon pro- ton scattering, we will use a new experimental technique in which a liquid H2 target is used as hyperon production and hyperon scattering targets with a detector system surrounding the LH2 target for detection of a scattered proton and a decay product from a hyperon. The hyperon scattering event is kinematically identified. Because imaging detectors used in past experiments are not employed, high intensity π beam can be used, allowing us to take high statistics data of 100 times more than the previous experiments. We have proposed an experiment of Σ p scattering at the K1.8 beam line by utilizing the K1.8 beam line spectrometer and the SKS spectrometer. A high intensity π beams of 2×10 7 /spill at 1.32 GeV/c and 1.42 GeV/c for the Σ − and Σ + productions, respectively, are used to produce as many hyperon beam as possible. With 16×10 6 Σ − beam and 55×10 6 Σ + beam around 500 MeV/c which are tagged by the spectrometers, we will detect ∼10,000 Σ − p and Σ + p scattering events and ∼6,000 Σ − p → Λn inelastic reaction events in 60 days beam time in total. In this proceedings, we will present the experimental plan of the scattering experiment and results of the detailed simulation studies.

Strangeness production in antiproton-nucleus annihilation

The results of the microscopic transport calculations of -nucleus interactions within a GiBUU model are presented. The dominating mechanism of hyperon production is the strangeness exchange processes → γπ and → ΞK. The calculated rapidity spectra of Ξ hyperons are significantly shifted to forward rapidities with respect to the spectra of S = −1 hyperons. We argue that this shift should be a sensitive test for the possible exotic mechanisms of -nucleus annihilation. The production of the double Λ-hypernuclei by Ξ− interaction with a secondary target is calculated.

Study of heavy hyperons production in DIS at COMPASS

The yields of heavy hyperons and antihyperons have been studied in deep inelastic scattering at the COMPASS experiment at CERN. The data were collected in 2003-2004 using a 160 GeV polarised muon beam scattered off a large polarised 6LiD target. Signals from Σ+(1385), Σ−(1385), Ξ−(1321) and their antiparticles were reconstructed. The ratios of Σ±/Λ, Σ±/ Λ, Ξ−/Λ and Ξ+/ Λ were determined. These ratios were used to tune parameters of the LEPTO generator. Comparison with the yields of the heavy hyperons at low Q2 - region was done.

Final State Interactions and Polarization Observables in the Reactionpp→pKΛ

Due to the lack of high quality hyperon beams, final state interactions in hyperon production reactions are a compelling tool to study hyperon-nucleon interactions. The COSY-TOF experiment has recently been upgraded in order to reconstruct the pK+ Λ final state with sufficient precision to determine the spin triplet pΛ scattering length with a polarized proton beam. We find an unexpected behavior of the K + analyzing power which prevents the extraction method to be used with the available statistics. A theoretical explanation is pending. Furthermore, the polarized beam together with the self analyzing decay of the Λ allows us to determine the Λ depolarization. This is especially sensitive to K + and π exchange in the production mechanism. Our finding verifies, to a large extent, the result from DISTO [2] that has so far been the only measurement close to the production threshold.

Preliminary results on prompt \(\Xi^+/\Xi^-\) production ratio in pp collisions at\(\sqrt{s} = 7\) TeV

The ratio of prompt \(\Xi^+/\Xi^-\) production has been investigated from a limitedsample of data registered by the LHCb experiment at the CERN Large Hadron Colliderin 2010. This ratio has been determined in the rapidity region \(1.9 \leq y \leq 4.9\) covered bythe LHCb detector. The determination of anti-hyperons to hyperons production ratio willallow to study the transport of baryon number from \(pp\) collisions to final state hadrons andto constrain and tune Monte Carlo models.

Strangeness production in antiproton-nucleus collisions

Antiproton annihilations on nuclei provide a very interesting way to study the behaviour of strange particles in the nuclear medium. In low energy \(\bar p\) annihilations, the hyperons are produced mostly by strangeness exchange mechanisms. Thus, hyperon production in \(\bar p A\) interactions is very sensitive to the properties of the antikaon-nucleon interaction in nuclear medium. Within the Giessen Boltzmann–Uehling–Uhlenbeck transport model (GiBUU), we analyse the experimental data on Λ and \(K^{0}_S\) production in \(\bar p A\) collisions at p lab = 0.2 − 4 GeV/c. A satisfactory overall agreement is reached, except for the \(K^{0}_S\) production in \(\bar p+{^{20}}\)Ne collisions at p lab = 608 MeV/c, where we obtain substantially larger \(K^{0}_S\) production rate. We also study the Ξ hyperon production, important in view of the forthcoming experiments at FAIR and J-PARC.

Production of a cascade hyperon in theK−-proton interaction

We investigate the production of a cascade hyperon (Xi) through the K^- + p --> K^+ (K^0) + Xi^- (Xi^0) reactions, within an effective Lagrangian model where these reactions proceed via excitations of Lambda and Sigma resonance intermediate states in s- and u-channels. The coupling constants at the various vertices are taken from previous studies and SU(3) symmetry considerations. The calculated total cross sections of these reactions, which are in good agreement with the available data, are dominated by the contributions from the Lambda(1520) intermediate resonant state. However, the K^+ meson angular distributions show selectivity to other resonant states in different angular regions and interference among these states leads to their strong backward peaking.

Heavy hyperons production in DIS at COMPASS

The yields of heavy hyperons and antihyperons were measured in deep inelastic scattering at the COMPASS experiment at CERN. The data were collected in 2003-2004 using 160 GeV polarised muon beam scattered off a large polarized 6 LiD target. Clear signals from Σ + ( 1385 ) , Σ − ( 1385 ) , Ξ − ( 1321 ) hyperons and their antiparticles were observed. The ratios of yields of various hyperons ( Σ ± / Λ , Σ ¯ ± / Λ ¯ , Ξ − / Λ and Ξ ¯ + / Λ ¯ ) were measured. These ratios were used to tune parameters of the LEPTO generator. The comparison of these yields with the corresponding ones obtained for the low– Q 2 region is also presented.

Design of a very thin target for hypernuclear production using antiprotons at FAIR

The future PANDA experiment is dedicated to hadron physics. In particular a new idea to produce doubly strange systems from Ξ− hyperons obtained from antiprotons is proposed. The developments of the hypernuclear as well as of the exotic hyperatom physics requires the production of statistically significant amounts of doubly strange Ξ− hyperons. The FAIR project will supply intense beam of antiprotons from the HESR ring. A target should be put inside the beam pipe and its features will play a crucial role in producing high rates of double hypernuclei. A new set-up with a novel idea for a hyperon production target, together with the results of some preliminary tests, will be described in this paper.

Hyperon production in Ar + KCl collisions at 1.76A GeV

We present transverse momentum spectra, rapidity distribution and multiplicity of Lambda-hyperons measured with the HADES spectrometer in the reaction Ar(1.76A GeV)+KCl. The yield of Xi- is calculated from our previously reported Xi-/(Lambda+Sigma0) ratio and compared to other strange particle multiplicities. Employing a strangeness balance equation the multiplicities of the yet unmeasured charged Sigma hyperons can be estimated. Finally a statistical hadronization model is used to fit the yields of pi-, K+, K0s, K-, phi, Lambda and Xi-. The resulting chemical freeze-out temperature of T=(76+-2) MeV is compared to the measured slope parameters obtained from fits to the transverse mass distributions of the particles.

P -wave excited baryons from pion- and photo-induced hyperon production

We report evidence for $N(1710)P_{11}$, $N(1875)P_{11}$, $N(1900)P_{13}$, $\Delta(1600)P_{33}$, $\Delta(1910)P_{31}$, and $\Delta(1920)P_{33}$, and find indications that $N(1900)P_{13}$ might have a companion state at 1970\,MeV. The controversial $\Delta(1750)P_{31}$ is not seen. The evidence is derived from a study of data on pion- and photo-induced hyperon production, but other data are included as well. Most of the resonances reported here were found in the Karlsruhe-Helsinki (KH84) and the Carnegie-Mellon (CM) analyses but were challenged recently by the Data Analysis Center at GWU. Our analysis is constrained by the energy independent $\pi N$ scattering amplitudes from either KH84 or GWU. The two $\pi N$ amplitudes from KH84 or GWU, respectively, lead to slightly different $\pi N$ branching ratios of contributing resonances but the debated resonances are required in both series of fits.

Production and polarization of the Λ and [formula omitted] hyperons in DIS at COMPASS

Λ and Λ ¯ hyperons were produced at the COMPASS experiment at CERN, using Deep-Inelastic Scattering (DIS) of 160 GeV/c polarized muons on a longitudinally polarized target. The study of Λ and Λ ¯ hyperon in DIS is important for the understanding of the nucleon structure, the mechanisms of hyperon production and the hyperon spin structure. In particular, it may provide valuable information on the unpolarized strange quark distributions s ( x ) and s ¯ ( x ) in the nucleon. The data sample contains about 70 000 Λ and 42 000 Λ ¯ . Large and comparable statistics on both Λ and Λ ¯ hyperons is a distinct feature of the COMPASS experiment. Preliminary results on polarization of Λ and Λ ¯ hyperons and yields of heavy hyperons in DIS are presented.

Attenuation of Λ(1520) Hyperons in near-threshold pA reactions

In the framework of the nuclear spectral function approach for incoherent primary proton-nucleon and secondary pion-nucleon production processes we study the inclusive Λ(1520)-hyperon production in the interaction of 2.83-GeV protons with nuclei. In particular, the A and momentum dependences of the absolute and relative Λ(1520)-hyperon yields are investigated within the different scenarios for their in-medium width. Our model calculations show that the pion-nucleon production channel contributes distinctly to the “low-momentum” Λ(1520) creation both in light and heavy nuclei in the chosen kinematics and, hence, has to be taken into consideration on close examination of the dependences of the Λ(1520)-hyperon yields on the target mass number with the aim to get information on their width in the medium. They also demonstrate that both the A dependence of the relative Λ(1520)-hyperon production cross section and momentum dependence of the absolute Λ(1520)-hyperon yield at incident energy of interest are appreciably sensitive to the Λ(1520) in-medium width, which means that these observables may be an important tool to determine the above width.

Strangeness production at SPS energies in a (3+1)-dimensional Boltzmann+hydrodynamics approach

Results on strange particle production at energies from –200 A GeV from a (3+1)-dimensional Boltzmann+hydrodynamics hybrid model are presented and compared to results from the ultra-relativistic quantum molecular dynamics transport approach and existing experimental data. The hybrid model with an intermediate hydrodynamical evolution for the hot and dense stage of the collision allows us to explore the centrality and system size dependence of (multi-strange) hyperon production in A+A collisions. The transition from a system which is not fully thermalized, such as in C+C or Si+Si collisions, to supposedly fully thermalized systems, e.g. created in Pb+Pb reactions, is investigated. The hyperon yields are enhanced in the hybrid approach due to the local equilibrium assumption and therefore in line with the experimental data. The excitation functions of strange to non-strange particle yields (e.g. K+/π+) calculated in the hybrid approach are able to reproduce the famous ‘horn’ structure at SPS energies with a purely hadronic equation of state in contrast to the pure transport approach, which failed to explain the experimental data.

Testing the Λ(1520) hyperon in-medium width in near-threshold proton–nucleus reactions

In the framework of the nuclear spectral function approach for incoherent primary proton–nucleon and secondary pion–nucleon production processes, we study the inclusive Λ(1520) hyperon production in the interaction of 2.83 GeV protons with nuclei. In particular, the A and momentum dependences of the absolute and relative Λ(1520) hyperon yields are investigated in two scenarios for its in-medium width. Our model calculations show that the pion–nucleon production channel contributes distinctly to the ‘low-momentum’ Λ(1520) creation both in light and heavy nuclei in the chosen kinematics and, hence, has to be taken into consideration on close examination of the dependences of the Λ(1520) hyperon yields on the target mass number with the aim of getting information on its width in the medium. They also demonstrate that both the A dependence of the relative Λ(1520) hyperon production cross-section and momentum dependence of the absolute Λ(1520) hyperon yield at incident energy of interest are appreciably sensitive to the Λ(1520) in-medium width, which means that these observables may be an important tool to determine the above width.

Cross sections of the [formula omitted] reaction close to threshold

We have measured inclusive data on K+-meson production in pp collisions at COSY Jülich close to the hyperon production threshold and determined the hyperon–nucleon invariant mass spectra. The spectra were decomposed into three parts: Λp, Σ0p and Σ+n. The cross section for the Σ+n channel was found to be much smaller than a previous measurement in that excess energy region. The data together with previous results at higher energies are compatible with a phase space dependence.

Influence of [formula omitted]-resonances on hyperon production in the channel [formula omitted] at 2.95, 3.20 and 3.30 GeV/c beam momentum

Hyperon production in the threshold region was studied in the reaction pp→K+Λp using the time-of-flight spectrometer COSY-TOF. Exclusive data, covering the full phase-space, were taken at the three different beam momenta of pbeam=2.95, 3.20 and 3.30 GeV/c, corresponding to excess energies of ε=204, 285 and 316 MeV, respectively. Total cross-sections were deduced for the three beam momenta to be 23.9±0.8±2.0 μb, 28.4±1.3±2.2 μb and 35.0±1.3±3.0 μb. Differential observables including Dalitz plots were obtained. The analysis of the Dalitz plots reveals a strong influence of the N(1650)-resonance at pbeam=2.95 GeV/c, whereas for the higher momenta an increasing relative contribution of the N(1710)- and/or of the N(1720)-resonance was observed. In addition, the pΛ-final-state interaction turned out to have a significant influence on the Dalitz plot distribution.