Elsevier

Physics Letters B

Volume 698, Issue 3, 11 April 2011, Pages 219-225
Physics Letters B

Hypernuclear spectroscopy with K at rest on 7Li, 9Be, 13C and 16O

https://doi.org/10.1016/j.physletb.2011.02.060Get rights and content

Abstract

The FINUDA experiment collected data to study the production of hypernuclei on different nuclear targets. The hypernucleus formation occurred through the strangeness-exchange reaction Kstop+ZAZΛA+π. From the analysis of the momentum of the emerging π, binding energies and formation probabilities of LiΛ7, BeΛ9, CΛ13 and OΛ16 have been measured and are here presented. The behavior of the formation probability as a function of the atomic mass number A is also discussed.

Introduction

FINUDA stands for Fisica Nucleare a DAΦNE, that is Nuclear Physics at DAΦNE, the e+e collider of the INFN “Laboratori Nazionali di Frascati”, close to Rome. One of the main aims of the experiment was the study of production and decay of Λ-hypernuclei. The creation of a hypernucleus [1], that is a nucleus in which a nucleon is replaced by a hyperon (for example a neutron is substituted by a Λ), requires the injection of strangeness into the nucleus. This is possible in different ways (see [2] and references therein for details), mainly using a π+ or a K beam on fixed targets; recently also electron beams have been used [3], [4]. The use of meson beams (π+ and K) usually requires relatively thick targets (few g/cm2) in order to allow the particle a reasonable interaction rate, reducing thus the instrumental resolution. The original idea of FINUDA [5] was to use a particular feature of the DAΦNE machine, where the e+e beams circulate with an energy of 510 MeV in order to produce the Φ(1020) meson in the head-on collisions. This particle decays, with a branching ratio of 49.2%, in two back-to-back kaons (ΦK+K) with kinetic energy as low as ∼16 MeV. In this way an unconventional monochromatic source of very low energy K was available for the experiment, allowing the use of much thinner targets, 0.1–0.2 g/cm2 compared to some g/cm2 of previous hypernuclear fixed-target experiments. In FINUDA the Kʼs were slowed down to rest in the targets leading to the production of Λ hypernuclei through the strangeness-exchange reaction:Kstop+ZAZΛA+π where ZA indicates the target nucleus and ZΛA the Λ hypernucleus in which a Λ particle replaced a neutron. By precisely measuring the momentum of the outgoing pions, it is possible to determine the energy levels of the produced hypernucleus, and by counting them the hypernucleus formation probability can be deduced. Up to now only few measurements of formation probability have been performed. Following the first experiment on a 12C stopping target [6], measurement on some other nuclei were subsequently performed [7]. A low statistics measurement on the (Kstop,πo) reaction on 12C was recently published [8]. A first measurement by FINUDA on a 12C target was also previously reported [9]. Other information about hypernuclear state formation can be found in recent FINUDA publications on mesonic and non-mesonic hypernuclei decay [10], [11]. Theoretical calculations [12], [13], [14], [15], [16], [17] have mostly reported formation rates which are substantially lower than those reported in previous measurements, and this holds true also for the recent [18] when compared with the formation rates reported in the present work. In the following, after a brief description of the FINUDA experimental apparatus, details on the data analysis and the results on hypernucleus formation probabilities and binding energies for LiΛ7, BeΛ9, CΛ13 and OΛ16 are presented.

Section snippets

The FINUDA experiment

Differently from previous hypernuclear fixed target experiments, FINUDA had an unconventional geometry, typical of collider experiments. The whole apparatus was contained inside a superconducting solenoid which provided a homogeneous magnetic field of 1.0 T over a cylindrical volume of 146 cm in radius and 211 cm in length and it had been designed to obtain a large acceptance of about 2π sr around the beam interaction region. More detailed descriptions of the detector may be found in [9], [10],

Data analysis

The kaons coming from the decay of the Φ meson, when flying outwards, are slowed down in the beam pipe, in the TOFINO scintillators and in the ISIM silicon detectors before encountering one of the 8 targets. The target thickness, of the order of some mm depending on the material density (2 mm for 9Be, 3 mm for D2O, 4 mm for 7Li and 1 cm for 13C), has been chosen so as to stop the kaons inside the target itself close to the external surface. The stopping point of both K+ and K is calculated

Results

In the following, results about the formation probability and the binding energy will be presented target by target, as a function of A: 7Li, 9Be, 13C and 16O. The number of Gaussians used to perform each of the following fits was chosen taking into account the number of clear signals in the histograms, previous experimental data, theoretical predictions and the χ2 of the fit.

Discussion of the results and conclusions

As discussed in the previous section, Ref. [7] reported measurements of formation probability with stopped K for three types of target elements, 4He, 12C and 16O. Probabilities for the ground state formation have been found to be (17.9±1.5)×103 for HeΛ4, (0.98±0.12)×103 for CΛ12 and (0.13±0.04)×103 for OΛ16. In 2005 FINUDA [9] reported a probability of (1.01±0.11±0.10)×103 for the ground state of CΛ12. Ref. [8] also measured the hypernuclei formation probability in the (Kstop,πo) reaction

Acknowledgements

The authors would like to thank Avraham Gal for the fruitful discussions about the interpretation of the data presented in this Letter.

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