Study of doubly strange systems using stored antiprotons The PANDA Collaboration

Bound nuclear systems with two units of strangeness are still poorly known despite their importance for many strong interaction phenomena. Stored antiprotons beams in the GeV range represent an unparalleled factory for various hyperon–antihyperon pairs. Their outstanding large production probability in antiproton collisions will open the floodgates for a series of new studies of systems which contain two or even more units of strangeness at the PANDA experiment at FAIR. For the first time, high resolution γ -spectroscopy of doubly strange -hypernuclei will be performed, thus complementing measurements of ground state decays of -hypernuclei at J-PARC or possible decays of particle unstable hypernuclei in heavy ion reactions. High resolution spectroscopy of multistrange −-atoms will be feasible and even the production of −-atoms will be within reach. The latter might open the door to the |S| = 3 world in strangeness nuclear physics, by the study of the hadronic −-nucleus interaction. For the first time it will be possible to study the behavior of + in nuclear systems under well controlled conditions. © 2016 Elsevier B.V. All rights reserved. * Corresponding author. E-mail address: pochodza@kph.uni-mainz.de (J. Pochodzalla). 1 Part of doctoral thesis. 2 Part of master thesis. 328 The PANDA Collaboration / Nuclear Physics A 954 (2016) 323–340

Nuclear Physics A 00 (2016) 1-15 1. Where QCD meets Gravity 1 One of the biggest challenges for physics in this century will be the unification of the four known fundamental 2 forces within a common theoretical framework. Pure, matter-free strong-field gravity can be studied when black 3 holes merge and gravitational waves are emitted [1]. Eventually, precise observations of gravitational waves will ance of hyperons at about two times nuclear density remains an unresolved mystery in neutron stars (hyperon puzzle). 13 At present, our incomplete understanding of the underlying baryon-baryon and of even more subtle multi-body in-14 teractions in baryonic systems seems to be the most probable reason for this dilemma. As an alternative solution to 15 this puzzle the role of gravity has been questioned [5][6][7]. In the future, gravitational waves from merging neutron 16 stars might help to probe gravity in this high density regime. The complemental study of the strong force in these 17 objects and the determination of the EoS remains even after many decades of research one of the biggest challenge for 18 physics. High energy nuclear reactions, radioactive beams mapping the chart of nuclear stability and precision studies 19 of nuclear few body systems contribute to this task. Strangeness nuclear physics with its many facets is an essential 20 protagonist in this big adventure.

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Bound strange systems -hypernuclei as well as hyperatoms -represent unique laboratories for multi-baryon in-22 teractions in the strangeness sector. The confirmation of the substantial charge symmetry breaking in the J=0 ground  Figure 2. Left: Production probability of Ξ − (blue dots) and Ξ − with momenta below 500 MeV/c (red triangles) predicted by GiBUU simulations for 2.9 GeV/c p interactions with three possible target materials. Right: Produced charged particles within the angular range covered by the silicon detectors of the secondary target (blue circles) and neutrons in the acceptance of the Germanium array (red triangles) normalized to the number of Ξ − with momenta less than 500 MeV/c. cited particle stable states will be explored at the PANDA experiment by performing high resolution γ-spectroscopy.

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Combining these three different methods we will have access to the complete level scheme of ΛΛ-hypernuclei.

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Complemented by hyperon-hyperon correlation studies in heavy ion collisions, these measurements will provide  the construction of the required detector components has started (see below). In the following we will present some 61 details concerning the choice of the primary target as an example of these studies.

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The main task of the primary target is the production of Ξ − hyperons which can be slowed down and finally stopped Target- Table 1. Ξ − production probability with respect to all inclusive interactions predicted by GiBUU transport calculations and stopping probability within the secondary boron absorbers for all produced Ξ − for primary targets made of 12 C, 28 Si, and 48 Ti. The fourth column gives the luminosity decrease caused by Coulomb scattering and energy straggling [24]. As a figure-of-merit (FoM) the product of these three numbers is given in the last column.

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For all four target materials this temperature is below the melting temperature indicated by the red shaded region 96 in Fig. 3. However, increasing the beam intensity by a factor of 10, the titanium target is likely to be destroyed. The half of the produced Ξ − in p 12 C reactions a Ξ 0 ( 30%) or a Ξ + ( 18%) escapes the 12 C target nucleus. These Ξ decay 139 with nearly 100% into an Λπ which will be used as an additional, rather exclusive trigger.

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Not all steps shown in the scheme in the right part of Fig. 1 can be treated by GEANT simulations as e.g. the  Fig. 7. Even at an antiproton interaction rate of 2·10 6 s −1 PANDA will be able to produce approximately 6·10 5 stopped 164 Ξ − hyperons per month in these heavy targets which is comparable to the maximum rate expected at J-PARC of about 165 7·10 5 stopped Ξ − per month [37]. Since only very little information on Ξ − production in antiproton-nucleus collisions 166 is presently available, it is clear that the design of the secondary absorber should be finalized once better experimental 167 information on the angular and momentum distributions of Ξ − will be available.

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The study of Ξ − -atoms will also serve as an initial step towards a study of Ω − -atoms. Like all composite particles 169 baryons are expected to be deformed objects. However, for spin J=0 and 1/2 hadrons, the spectroscopic quadrupole The long lifetime and its spin 3/2 makes the Ω − the only candidate to obtain direct experimental information on the   Measuring the quadrupole moment of the Ω − , or setting a limit to its value, would provide very useful constraints   [23] for ΛΛ pairs. Here we present first results for Ξ + Ξ − pairs produced in p+ 12 C interactions at 2.9 GeV/c. 208 Fig. 8 shows the GiBUU prediction for the average transverse asymmetry α T (Eq. 2) plotted as a function of the 209 longitudinal momentum asymmetry α L which is defined for each event as . ( As for ΛΛ pairs [66], the Σ − Λ pairs (left) show a remarkable sensitivity of α T on the scaling factor ξ Λ of the Λ-potential G-parity symmetry is therefore usually adopted to specify their default potentials. While this corresponds to ξ Λ = 1, a 215 value of ξ Λ ≈ = 0.2 might be a more appropriate considering antiproton data. In Ref.
[66] it was demonstrated that the 216 sensitivity of α T to the scaling factor ξ Λ is strongly related to re-scattering processes of the hyperons and antihyperons 217 within the target nucleus. For positive values of α L where the Λ is emitted backward with respect to the hyperon, the 218 statistics is too low to draw quantitative conclusions in the present simulation.

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In the right part of Fig. 8 we show the first attempt to calculate the momentum asymmetry for Ξ − Ξ + -pair production 220 in 2.9 GeV/c p-12 C interactions. In these GiBUU calculations about 79 million inclusive events were generated for interaction rate in the region of 5·10 6 s −1 . The spectroscopy of Ω − -atoms will be challenging, but seems possible.