Excited Hyperons Produced in Proton-Proton Collisions with Anke@COSY
Izabella Zychor
备注：
Talk given at the STORI'08 Conference, Lanzhou, China, September 14-18, 2008, 7 pages, to be published as a special issue of the International Journal of Modern Physics E
Excited neutral hyperons Y0* produced in the pp -> p K+ Y0* reaction with a COSY beam momentum of 3.65 MeV/c have masses below 1540 MeV/c2. The ANKE spectrometer allows the simultaneous observation of different decay modes: Y0* -> pion0Sigma0, pion+-Sigma-+, pion0Lambda, K-p by measuring kaons and pions of either charge in coincidence with protons. We have found indications for a neutral excited hyperon resonance Y0* with a mass of (1480 +- 15) MeV/c2 and a width of (60 +- 15) MeV/c2. The cross section for Y0* is of the order of few hundred nanobarns. It can be either a Sigma0 or a Lambda hyperon and on the basis of existing data no conclusion could be made whether it is a three-quark baryon or an exotic state. Missing- and invariant-mass techniques have been used to identify the Lambda(1405) resonance decaying via Sigma0pion0. The cross section for Lambda(1405) production is equal to (4.5 +- 0.9(stat) +- 1.8(syst)) microbarn. The shape and position of the Lambda(1405) distribution are similar to those found from other decay modes, so no support is given to the two-pole model.
Machine studies for the development of storage cells at the ANKE facility of COSY
本文测量了注入能量为45 MeV的COSY质子束在ANKE靶相互作用点处的横向强度分布，并在2.65 GeV加速后进行了测量。在2.65 GeV时，也确定了机器的接受度。从强度分布的光束的大小是确定的，并与测量的机器接受，存储单元的尺寸为双偏振实验的偏振内部气体目标在ANKE光谱仪指定。用于ANKE实验的最佳存储单元的尺寸应该是15 mm x 20 mm x 390 mm（垂直x水平x纵向），这样在COSY中存储10^10个粒子的束流时，可以达到约2.5*10^29 cm^-2*s^-1的亮度。
The roles of the COSY-TOF and COSY-ANKE spectrometers in the measurement of strange meson production are briefly reviewed, mainly in connection with new results on the pp -> K^+ p Lambda, pp -> K^+ p Sigma^0 and pp -> K^+ n Sigma^+ reactions.
A value for the mass of the eta meson has been determined at the COSY-ANKE facility through the measurement of a set of deuteron laboratory beam momenta and associated 3He center-of-mass momenta in the d+p -> 3He+X reaction. The eta was then identified by the missing-mass peak and the production threshold determined. The individual beam momenta were fixed with a relative precision of 3 x 10^-5 for values around 3 GeV/c by using a polarized deuteron beam and inducing an artificial depolarizing spin resonance, which occurs at a well-defined frequency. The final-state momenta in the two-body d+p …
Some of the important results from the COSY-Juelich spin programme are summarised. These include the measurement of the deuteron beam momentum through the excitation of a depolarising resonance, which allowed the mass of the eta-meson to be determined to high precision. The charge exchange of polarised deuterons on hydrogen gave rise to a detailed study of the spin dependence of large angle neutron-proton elastic scattering amplitudes. The measurements of the cross section and analysing powers for pion production in both pp and pn collisions at 353 MeV could be described very successfully in t…
A polarized atomic beam source was developed for the polarized internal storage-cell gas target at the magnet spectrometer ANKE of COSY-Jülich. The intensities of the beams injected into the storage cell, measured with a compression tube, are $7.5\cdot 10^{16}$ hydrogen atoms/s (two hyperfine states) and $3.9\cdot 10^{16}$ deuterium atoms/s (three hyperfine states). For the hydrogen beam the achieved vector polarizations are $p_{\rm z}\approx\pm0.92$. For the deuterium beam, the obtained combinations of vector and tensor ($p_{\rm zz}$) polarizations are $p_{\rm z}\approx\pm 0.90$ (with a const…
The differential and total cross-sections for the dp \to 3He ηreaction have been measured in a high precision high statistics COSY-ANKE experiment near threshold using a continuous beam energy ramp up to an excess energy Q of 11.3 MeV with essentially 100% acceptance. The kinematics allowed the mean value of Q to be determined to about 9 keV. Evidence is found for the effects of higher partial waves for Q > 4 MeV. The very rapid rise of the total cross-section to its maximum value within 0.5 MeV of threshold implies a very large η3He scattering length and hence the presence of a quasi-bound st…
The proton analysing power in $\vec{p}p$ elastic scattering has been measured at small angles at COSY-ANKE at 796 MeV and five other beam energies between 1.6 and 2.4 GeV using a polarised proton beam. The asymmetries obtained by detecting the fast proton in the ANKE forward detector or the slow recoil proton in a silicon tracking telescope are completely consistent. Although the analysing power results agree well with the many published data at 796 MeV, and also with the most recent partial wave solution at this energy, the ANKE data at the higher energies lie well above the predictions of th…
We study the non-resonant (non-$ϕ$) production of $K^+K^-$ pairs by protons of 2.83 GeV kinetic energy on C, Cu, Ag, and Au targets within the collision model, based on the nuclear spectral function, for incoherent primary proton--nucleon and secondary pion--nucleon creation processes. The model takes into account the initial proton and final kaon absorption, target nucleon binding and Fermi motion as well as nuclear mean-field potential effects on these processes. We calculate the antikaon momentum dependences of the exclusive absolute and relative $K^+K^-$ pair yields in the acceptance windo…
Energy-dependent and single-energy fits to the existing nucleon-nucleon database have been updated to incorporate recent measurements. The fits cover a region from threshold to 3 GeV, in the laboratory kinetic energy, for proton-proton scattering, with an upper limit of 1.3 GeV for neutron-proton scattering. Experiments carried out at the COSY-WASA and COSY-ANKE facilities have had a significant impact on the partial-wave solutions. Results are discussed in terms of both partial-wave and direct reconstruction amplitudes.
Observation of an ABC effect in proton-proton collisions
在COSY-ANKE实验中，我们测量了pp->ppX反应中包含性多介子产生的截面，在最终pp系统中，当双质子准粒子处于1 S 0态时，在低激发能条件下，测量了四种束流能量（0.8，1.1，1.4和2.0 GeV）下的截面。在三个较高的能量，丢失的质量Mx谱显示出在低Mx的强烈增强，对应于ABC效应，随着能量的增加，该效应稳定地移动到更大的值。尽管在0.8 GeV时丢失的质量结构看起来非常不同，但Mx和光束能量的变化与通过激发两个Delta（1232）等压线介导的双π介子产生是一致的，耦合到初始pp系统的S-和D-态。
The analyzing powers in proton-deuteron elastic and proton-neutron quasi-elastic scattering have been measured at small angles using a polarized proton beam at the COSY storage ring incident on an unpolarized deuterium target. The data were taken at 796MeV and five higher energies from 1600MeV to 2400MeV. The analyzing power in pd elastic scattering was studied by detecting the low energy recoil deuteron in telescopes placed symmetrically in the COSY plane to the left and right of the beam whereas for pn quasi-elastic scattering a low energy proton was registered in one of the telescopes in co…
Hyperon production in near threshold nucleon-nucleon collisions
为了理解COSY-11合作组最近公布的pp --> p λ K+和pp --> p σ 0 K+反应的新数据，我们在有效拉格朗日模型下，研究了核子-核子碰撞中在更大的阈能范围内产生λ-k介子和σ-k介子的机制.在这个理论中，超子的产生是通过N*（1650）、N*（1710）和N*（1720）重子共振的激发进行的。各种共振的相对贡献的相互作用的截面，讨论了作为一个功能的光束能量在一个更大的近阈值能量域。给出了pp --> p Sigma+ K ~ 0，pp --> n Sigma+K ~+和pn --> n Lambda K ~+反应总截面的预测.
The differential and total cross sections for the d+p->3He+eta reaction have been measured at COSY-ANKE at excess energies of 19.5, 39.4, and 59.4 MeV over the full angular range. The results are in line with trends apparent from the detailed near-threshold studies and also agree with those from CELSIUS, though the present data have higher precision. While at 19.5 MeV the results can be described in terms of s- and p-wave production, by 59.4 MeV higher partial waves are required. Including the 19.5 MeV point together with the near-threshold data in a global s- and p-wave fit gives a poorer ove…
The pp -> ppK+K- reaction was measured below the phi threshold at a beam energy of 2.568 GeV using the COSY-ANKE magnetic spectrometer. By assuming that the four-body phase space is distorted only by the product of two-body final state interactions, fits to a variety of one-dimensional distributions permit the evaluation of differential and total cross sections. The shapes of the distributions in the Kp and Kpp invariant masses are reproduced only if the K-p interaction is even stronger than that found at higher energy. The cusp effect in the K+K- distribution at the K0K0bar threshold is much …
The momentum spectra of K+ produced at small angles in proton-proton and proton-deuteron collisions have been measured at four beam energies, 1.826, 1.920, 2.020, and 2.650 GeV, using the ANKE spectrometer at COSY-Juelich. After making corrections for Fermi motion and shadowing, the data indicate that K+ production near threshold is stronger in pp- than in pn-induced reactions. However, most of this difference could be made up by the unobserved K0 production in the pn case.
Hyperon production in the threshold region was studied in the reaction pp -> K+Lp using the time-of-flight spectrometer COSY-TOF. Exclusive data, covering the full phase-space, were taken at the three different beam momenta of p_beam=2.95, 3.20 and 3.30 GeV/c, corresponding to excess energies of epsilon=204, 285 and 316 MeV, respectively. Total cross-sections were deduced for the three beam momenta to be 23.9+/-0.8 +/-2.0 ub, 28.4+/-1.3 +/-2.2 ub and 35.0+/-1.3 +/-3.0 ub. Differential observables including Dalitz plots were obtained. The analysis of the Dalitz plots reveals a strong influence …
We report on a theoretical study of the pp->p Lambda K+ and pp->p Sigma0 K+ reactions near threshold using a chiral dynamical approach. The production process is described by single-pion and single-kaon exchange. The final state interactions of nucleon-hyperon, K-hyperon and K-nucleon systems are also taken into account. We show that our model leads to a fair description of the experimental data on the total cross section of the pp->p Lambda K+ and pp->p Sigma0 K+ reactions. We find that the experimental observed strong suppression of Sigma0 production compared to Lambda production at the same…
The production of non-phi K+K- pairs by protons of 2.83 GeV kinetic energy on C, Cu, Ag, and Au targets has been investigated using the COSY-ANKE magnetic spectrometer. The K- momentum dependence of the differential cross section has been measured at small angles over the 0.2--0.9 GeV/c range. The comparison of the data with detailed model calculations indicates an attractive K- -nucleus potential of about -60 MeV at normal nuclear matter density at a mean momentum of 0.5 GeV/c. However, this approach has difficulty in reproducing the smallness of the observed cross sections at low K- momenta.
Hard bremsstrahlung production in proton-proton collisions has been studied with the ANKE spectrometer at COSY-Juelich in the energy range of 353-800 MeV by detecting the final proton pair {pp}_s from the pp -> {pp}_s reaction with very low excitation energy. Differential cross sections were measured at small diproton c.m. angles from 0 to 20 degrees and the average over this angular interval reveals a broad peak at a beam energy around 650 MeV with a FWHM of about 220 MeV, suggesting the influence of Delta(1232)N intermediate states. Comparison with deuteron photodisintegration shows that the…
The production of phi mesons in the collisions of 2.83 GeV protons with C, Cu, Ag, and Au at forward angles has been measured via the phi -> K+K- decay using the COSY-ANKE magnetic spectrometer. The phi meson production cross section follows a target mass dependence of A^0.56+/-0.02 in the momentum region of 0.6-1.6 GeV/c. The comparison of the data with model calculations suggests that the in-medium phi width is about an order of magnitude larger than its free value.
New precise unpolarised differential cross sections of deuteron-proton elastic scattering have been measured at 16 different deuteron beam momenta between $p_d = 3120.17\;\textrm{MeV}/c$ and $p_d =3204.16\;\textrm{MeV}/c$ at the COoler SYnchrotron COSY of the Forschungszentrum Jülich. The data, which were taken using the magnetic spectrometer ANKE, cover the equivalent range in proton kinetic energies from $T_p = 882.2\;\textrm{MeV}$ to $T_p = 918.3\;\textrm{MeV}$. The experimental results are analysed theoretically using the Glauber diffraction model with accurate nucleon-nucleon input. The t…
The charge exchange of vector polarised deuterons on a polarised hydrogen target has been studied in a high statistics experiment at the COSY-ANKE facility at a deuteron beam energy of Td = 726 MeV. By selecting two fast protons at low relative energy E_{pp}, the measured analysing powers and spin correlations are sensitive to interference terms between specific neutron-proton charge-exchange amplitudes at a neutron kinetic energy of Tn ~ 1/2 Td =363 MeV. An impulse approximation calculation, which takes into account corrections due to the angular distribution in the diproton, describes reason…
Differential and total cross sections for the pp -> ppK+K- reaction have been measured at a proton beam energy of 2.83 GeV using the COSY-ANKE magnetic spectrometer. Detailed model descriptions fitted to a variety of one-dimensional distributions permit the separation of the pp -> pp phi cross section from that of non-phi production. The differential spectra show that higher partial waves represent the majority of the pp -> pp phi total cross section at an excess energy of 76 MeV, whose energy dependence would then seem to require some s-wave phi-p enhancement near threshold. The non-phi data …
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: $Λp$, $Σ^0p$ 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.
In order to establish links between p-wave pion production in nucleon-nucleon collisions and low energy three-nucleon scattering, an extensive programme of experiments on pion production is currently underway at COSY-ANKE. The final proton pair is measured at very low excitation energy, leading to an S-wave diproton, denoted here as {pp}_s. By using a deuterium target we have obtained data on the differential cross section and analysing power of the quasi-free pol{p}n -> {pp}_s pi^- reaction at 353 MeV. The spectator proton p_sp was either measured directly in silicon tracking telescopes or re…
The vector and tensor analysing powers, $A_y$ and $A_{yy}$, of the $\vec{p}d \to n\{pp\}_{s}$ charge-exchange reaction have been measured at a beam energy of 600 MeV at the COSY-ANKE facility by using an unpolarised proton beam incident on an internal storage cell target filled with polarised deuterium gas. The low energy recoiling protons were measured in a pair of silicon tracking telescopes placed on either side of the target. Putting a cut of 3 MeV on the diproton excitation energy ensured that the two protons were dominantly in the $^{1}S_{0}$ state, here denoted by $\{pp\}_{s}$. The pola…
The transverse spin correlations A_{x,x} and A_{y,y} have been measured in the pol{d} pol{p} -> p_spec {pp}_s pi- reaction at COSY-ANKE at 353 MeV per nucleon. Here {pp}_s denotes a proton-proton pair with low excitation energy, which is dominantly in the 1S0 state. By measuring three protons in the final state it was possible to extract events where there was a spectator proton p_spec so that the reaction could be interpreted in terms of quasi-free pol{n} pol{p} -> {pp}_s pi-. The proton analyzing power in this reaction was also deduced from this data set by averaging over the polarization of…
