Production of Double Hypernuclei with antiprotons at PANDA
aa r X i v : . [ h e p - e x ] O c t th International Conference on High Energy Physics, Philadelphia, 2008
Productions of Double Hypernuclei with antiprotons at PANDA
Katarzyna Szyma ´nska
Politecnico di Torino, Torino, Italy andInstituto Nazionale di Fisica Nucleare, Sezione di Torino, Italy
One of the goals of hypernuclear physics is to study the properties of baryon-baryon interaction including thestrangeness contribution. Double hypernuclei can provide information about the ΛΛ interaction in addition to thehyperon-hyperon and hyperon-nucleus one. A new technique for producing double hypernuclei using antiprotons isforeseen in the PANDA experiment at FAIR. Gamma ray spectroscopy is a way to measure the hyperon pair bindingenergy and the HPGe detectors can achieve the necessary resolution.
1. INTRODUCTION
The physics of the doubly strange hypernuclei, (Ξ − ) hypernuclei and double (ΛΛ) hypernuclei, presents somenovelties with respect to the traditional hypernuclear investigations. Ξ − atoms, whose formation is an intermediatestep toward the ΛΛ-hypernucleus, can give information about the interplay of the Coulomb and strong force in thelow nuclear density region, while the Ξ − decay inside the nucleus can shed light on the YN interaction at S=-2.It is worth to underline that the ΛΛ hypernuclei are the only possible tool to investigate the hyperon-hyperoninteraction. In fact the elementary hyperon-nucleon scattering data are scarce and the hyperon-hyperon data aretotally absent. The crucial parameter in testing the potential models is the difference between the binding energy ofthe ΛΛ hypernucleus and twice the binding energy of each Λ in the core nucleus. This parameters is experimentallyachievable through spectroscopy measurements. Very interesting aspects of the doubly strange systems are thedifferent contributions of the strange and not strange mesons to the Ξ N , ΛΛ, and Ξ N -ΛΛ coupling interactionmechanism. Moreover, the production of hyperfragments is related to the hyperon-hyperon interaction mechanismand can be a test for different models [1, 2]. Also the Λ decay processes show a peculiar aspect in the doublehypernuclei: mesonic and non-mesonic decays can occur simultaneously and, among the non-mesonic ones, thehyperon-induced mechanism could play a role in addition to the nuclear induced one.The PANDA Collaborations aims to investigate, among others, the double strangeness using the high energyantiprotons of the future FAIR facility at GSI. Measurements are planned in both nuclear spectroscopy and weakdecay fields. The required high precision in spectroscopy will be achieved using Ge crystals, whose performances inthe region of the fringing field have been already successfully tested. The weak decay products will be detected bySi- µ strips located in a suitable arrangement together with the hypernuclear target around the antiproton beam pipe.
2. PRODUCTION OF HYPERNUCLEI AT PANDA
The FAIR (Facility for Antiproton and Ion Research) complex [3, 4], located in the GSI site, will include the ringHESR (High Energy Storage Ring) to store antiprotons of energy between 0.8 to 14.4 MeV. Intense and high qualitybeams are foreseen: luminosity up to 10 cm − s − and momentum resolution up to 10 − are expected.The new technique proposed by the PANDA Collaboration [5] to produce double hypernuclei with antiprotons, isbased on the reaction: ¯ p + N → Ξ − + ¯Ξ (1)In this reaction two different targets are used, the first one (primary target) where Ξ − is produced and the secondone (secondary target) where ΛΛ hypernucleus is formed. 14 th International Conference on High Energy Physics, Philadelphia, 2008The whole process proceeds through the following steps: a) reaction (1) occurs quasi free in a nucleus, b) Ξ − re-scatters in the residual nucleus and it is strongly decelerated, c) Ξ − slows down to stop, d) is captured by anatom, e) undergoes to an atomic cascade, f) is captured into the nucleus, g) makes the conversion reaction:Ξ − + p → ΛΛ (2)Steps a) and b) occur in the primary target while the other ones in the secondary. After reaction (2) both Λ ′ scan stick to the nucleus or not: the fact that the excess of energy is 28 MeV makes the probability of both stickingrather high.The designed apparatus for Double Hypernuclei physics is sketched in Fig.1 (left) and includes the primary andsecondary targets in expanded view in Fig.1 (right). The efficiency of this target arrangement has been evaluated bysimulation [6] and the ratio R of the stopped Ξ − in the secondary target to the number of produced Ξ − in reaction(1) is reported in Table I as a function of different primary targets for a fixed C secondary target.The efficiency depends slightly on the mass number of the primary target material: for secondary targets of heaviernuclei one should expect an improvement of the efficiency, due to a faster slowing down to stop of Ξ − . Therefore anefficiency of the order of 10 − has to be combined with the rate of Ξ − produced at HESR, to get the Ξ − stoppedrate. The production rate will depend on the beam profile and on the constraints due to the radiation damage of thePANDA detectors. The designed structure of the antiproton beam [7] and a preliminary estimate of the radiationtolerance of ≈ · charged particle/s allow to expect a rate of ≈ − /s using a C primary target.Such a rate will produce in one month a number of stopped Ξ − higher than the existing statistics. Concerningthe efficiency in detecting the Ξ − , it must be remarked that also an anti-hyperon is released in reaction (1), whichannihilates mostly inside the primary target. This annihilation produces (at least) two anti-kaons and this peculiaraspect will be used as a tool for a high-level trigger. Figure 1:
Scheme of PANDA detector system with the assembled of Ge crystal [8] (left side) and the target system for doublehypernuclei production, on the right.
Detection of X-rays from hyperatoms [9, 10] and gamma-rays from double hypernuclear processes plays a crucialrole in the hypernuclear program of PANDA. To achieve the required resolution, High Purity Germanium detectors(HPGe) are planned to be installed.A common feature of the general purpose apparatuses, like PANDA, is the presence of large spectrometers withintense magnetic fields. In the PANDA set-up a set of HPGe detectors will be located upstream the target (seeFig.1, left), just in front of the upstream end of the solenoid. The set of Ge crystals will be assembled to cover24 th International Conference on High Energy Physics, Philadelphia, 2008
Table I: Rates of the stopped Ξ − inside the secondary target to the produced Ξ − in the primary target via reaction (1). Primary target C Al Ni Ag Ba Au Rate (%) 0.213 0.268 0.325 0.352 0.360 0.391 almost 2 π solid angle and they will be located in the area when the the fringing field is not negligible. Due to themagnetic field effects on the charge carrier trajectories the performances of the Ge detectors could be modified as wellas the electronic circuits could be affected. Moreover the data acquisition lasts in general several weeks or months.Therefore it is mandatory to check whether relevant changes in the performances of the HPGe detector might occurand if such changes are depending on the time spent in operating inside the magnetic field. Tests of long duration(about 1 year) have been performed on a HPGe crystal of the same sizes and features of those ones planned forPANDA [11]. The result showed that the worsening of the resolution was within 10% in the γ range from 0.08 MeVto 1.333 MeV and that such a worsening was not at all permanent. These results allow to design the relative positionof the crystals and of the primary target in order to maximize the acceptance.
3. CONCLUSIONS