Hypernuclei production with stopped K- at DAΦNE

Nuclear Physics A$$3 (1993) 851c--856c North-Holland, Amsterdam

NUCLEAR PHYSICS A

Hypernuclei production with stopped K" at DAC~NE M. Agnelloa, G.C. Bonazzolab, E. Bottab, T. Bressanib, D. Calvob, P. Camerinic, S. Costab, F. D'Isepb, A. Donzellad, A. Feliciello b, A. Filippib, P. Gianotti b, N. Grionc, C. Guaraldoe, F. lazzia, A. Lanaroe, E. Lodi Rizzinid, V. Lucherinie, S. Marcellob, B. Minettia, V. Paticchiof, E. Rossettob, R. Ruic, A. Sharmab, S. Tessaroc, L. Venturellid and A. Zenonig aDipartimento di Fisica del Politecnico di Torino and INFN, Sezione di Torino, 1-10125 Torino, Italy bDipartimento di Fisica Sperimentale, Universit~ di Torino and INFN, Sezione di Torino, 1-10125 Torino, Italy CDipartimento di Fisica, Universit~ di Trieste and INFN, Sezione di Trieste, 1-34127 Trieste, Italy dDipartimento di Elettronica per I'Automazione, Universith di Brescia and INFN, Sezione di Torino, 1-25060 Brescia, Italy elstituto Nazionale di Fisica Nucleate, Laboratori Nazionali di Frascati, 1-(10044Frascati, Italy fDipartimento di Fisica, Universit~ di Bari and INFN, Sezione di Bali, 1-70126 Bari, Italy gDipartimento di Fisica Nucleare e Teorica, Universifft di Pavia and INFN, Sezione di Pavia, 1-27100 Pavia, Italy

Abstract The low-energy, background free and tagged K- following the ~(1020) decay produced at the DAONE ~-faetory in Frascati represent a unique opportunity to carry out a dedicated program of hypernuclear physics with K- at rest. In this paper we give a survey of this challenging field and we describe the performances of a magnetic spectrometer designed for precise measurements of hypemuclei energy levels and of their decays modes. The DA~NE (Double Annular ~-factory for Nice Experiments) project was approved by INFN in June 1990. It foresees the construction of a two ring machine for (e+e-) colliding beams, provided by a 510 MeV e+/e - Linae injector. The planned initial luminosity is £, = 1032 cm-2s-l at the ~(1020) c.m. energy. Studies and engineering designs were started at the beginning of 1991; construction and commissioning are scheduled for the end of 1995. At least one year will be then necessary in order to reach the designed luminosity top value .f_,= 1033 em-2s -1. A complete description of the machine complex can be found in Ref. I 1]. DA~NE will provide, at L = 1033 cm-2s-I, - 4 x 103 ~ s-1. The ~ decays with a branching

0375-9474/93/$06.00 © 1993 - Elsevier Science Publishers B.V. All rights reserved.

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M. Agnello et al. / Hypernuclei prothtction with stopped K" at DAq~NE

ratio of ~ 50% in K+K-, of ~ 35% in K°K ° and of ~ 15% in other channels (mostly 3n): a ~factnry can therefore be regarded as an intense source of low-energy, background free KK pairs. Due to the above circumstances, the most natural physics case for a ~factory is certainly a precise measurement of the CP-violating parameters 12-3], in particular of the e'/e ratio [4]. On the other hand, as it was noticed by Bressani [5], the unique properties of DAONE may be also exploited for a very interesting nuclear physics program [6]. In particular we are planning to perform an extensive program of high resolution hypemuelear spectroscopy with K- at rest. First of all with K" at rest it is possible to populate many different hypemuclear levels in contrast to what happens with K- in flight around the "magic" momentum. The energy spectrum of the hypemuclear levels can be determined by accurate measurements of the energy of the xen,fitted in the two-body reaction: Kstop + AXz ---)~Xz + n"

O)

n- from (1) cony a momentum ranging from ~ 250 to ~ 280 MeV/c, depending on the target nucleus and on the hypemuclear final state. A production mechanism similar to that of (1) holds for ,V-hypemuclei, noticing that the ~:-hypemn has three states of charge and therefore a n+ can be emitted in the case of Y--hyperauelei production. ~ following z-hypernuclei production would have typical momenta in the range 180 ÷ 200 MeV/c, again depending on the target and on the produced final states. However we recall that experiments at KEK [7] using stopped Khave reported no clear evidence of the existence of narrow ~:-hypernuclei bound states, previously claimed by experiments with K- in flight. So it may be very challenging to try to solve this intriguing puzzle. Last but not least, the decay of hypernuclei plays a crucial role in understanding the bchaviour of elementary particles embedded in a nuclear medium. A free A-hyperon decays mostly into a nucleon and a pinn via the weak non leptonic decay ^ ~ N + x, with a lifetime of 2.63 x 10-1os, the energy release being 37 MeV. The situation changes completely when the A is embedded in the nuclear medium: in the nucleus, in fact, it is bound by up to 20 MeV (for heavy hypernuelei), so the phase space for the mesonic decay of the free A is greatly reduced. An even further suppression of this decay is due to the fact that the produced final-state nucleon has a very low momentum (< 100 MeV/c) and it is consequently Pauli blocked. In other words, the nuclear medium affects the weak decay mode of the A-hyperon, by introducing a new nonmesonic decay mode A + N ~ N + N. The corresponding energy release is approximately 176 MeV, leaving each of the final nucleons with a momentum of 417 MeV/c. This channel has a much larger phase space (relatively to the mesonic one) and the outgoing nucleons are no more Pauli blocked, hence it dominates largely the weak decay process in medium or heavy hypemuclei. However very poor experimental data exist on this subject, due to the difficulty of producing hypemuclei in their ground state and of detecting the (n, p) and (n, n) pairs from non-mesonic decays. A recent analysis of data on light hypernuclei indicated, at a 2o level, a violation of the 4I = I/2 rule for non-mesonic decays of hypernuclei [8]. It is of paramount importance to confirm possibly this result The energy resolution on the hypemuclear levels and the counting rates in experiments with stopped K- were up to now limited by K- "beams" characteristics (~ contamination, energy spread). These problems will be completely overcome thanks to the monoehromaticity (14.7 MeV) and the cleanliness of the K from the ~ decay. Then the possibility of performing fine spectroscopy measurements depends in a crucial way on the capability of the magnetic

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M. Agnello et al. I Hypernuclei production with stopped K" at DAq~NE

spectrometer as for as it concerns the momentum resolution for the charged particles. The characteristics of the K" following the # decay allow to minimize the worsening in the momentum resolution due to the incertitude on the interaction point in the stopping target: in fact they have a range of the order of I g/cm2 (depending on the material) and the straggling on the range is of the order of 50 mg/cm2. The uncertainty on the interaction point leads typically to a momentum spread, for the outgoing x-, of ~ 1O0 KeV;c. The target design and assembly must take into account the fact that the K- are emitted over a solid angle of 4x, with an angular distribution proportional to sin20, 0 being the emission angle. The interaction/targut region, schematically represented by Figure 1, has a cylindrical geometry and consists of four different layers: the inner beam pipe ( ~ = 3.5 cm), a first set of scintillators arranged as a barrel, a microstrip gas chamber [9-10] and, finally, the stopping target. The beam pipe must, obvinus!y, be kept as thin as possible, compatibly with the safety requirements: beryllium (200 inn thick) would probably be the best choice for the material. The scintillator barrel (2 m m thick), I',esides the usual timing purposes, will provide the selection of K- by AE/Ax (see Figure 2). > 0

12000

microstrip gas chamber

c

K +

__.

"n"

+ + ..... /z.I-

d beam pipe

_

L I

8000

0 I! I#

4000

L~ stopping target

scintillator barrel

o

o

10

&E in i n t e r n o l s c i n t i l ! o t o r [MeVl

Figure 1. Schematic cross section of the interaction region (not in scale).

Figure 2. Monte Carlo evaluation of the distributions of the energy lost by different particles in the inner scintillator array: the described geometry for the interaction/target region has been taken into account.

The tagging with K+, emitted back to back to the K- and with the same energy, will allow the determination of the interaction point within the target, already thin (1.5 nun in case of carbon). The microstrip gas chamber will allow the measurements of the K+/K- directions just before entering the target. In this way the 0 angle of the K- can be determined. Since its energy is

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M. Agnello o" al. , tlypcrnuclei production with stopped K" at DAONE

exactly known, the depth in the target at which the (Kstop, ~-) reaction occurs can be determined, providing the way for the necessary off-line correction on the momentum of the emitted ~-. The thickness of the stopping target must be such to allow the stopping of Kentitted from 45 ° to 135°, with respect to the beam axis. In a nearly homogeneous solenoidal magnetic field, the minimal information necessary to determine the momentum of a charged particle is the coordinates of three points in space, along its trajectory. The first point, as close as possible to the target, can be measured, with a very good precision, by means of a silicon microstrip array; further two points, at least, have to be measured by other devices. A schematic view of the apparatus actually under study is given in Figure 3. It consists of a (superconductive) solenoid with a radius of ~ ! m and a length of ~ 2 m, which will provide a maximum field intensity of 1.5 T along its axis, constant within 2%. The main designed features of this spectrometer are: cylindrical geometry, large solid angle (> 2~), large momentum acceptance (100 + 300 MeV/e), excellent momentum resolution (< 3 %0 at 270 MeV/c), possibility of detecting secondary slower charged particles (x and p from the decay of the hypernuclear states). Furthermore the apparatus should be able to detect neutrons from non-mesonic decays of hypemuclei.

He filled tank ~ comaenmtor ~ maghets ~

~

tofextemal barrel drift chambers

~

j

e" --~ ~

~--- e +

g/////////////////d b ~

Im

Figure 3. Schematic side view of the apparatus. The gap between the Si microstrip and the solenoid coils will be instrumented with a set of polygonal, low-mass drift chambers, positioned at regular intervals. In order to optimize the momentum resolution, the total thickness of the materials of the chambers, in particular the mylar windows, must be kept as thin as possible since it represenLs the main source of multiple Coulomb scattering along the particle's trajectory. A good compromise between this essential requirement and the need of a satisfactory pattern recognition efficiency, seems to be achieved by using a set of three drift chambers. For the two innermost, one-dimensional readout cham.r~*~rs,drift wires will be arranged parallel to the beam axis. The designed resolution in the

M. Agnello et al. I Hypernuclei production with stopped K" at DA~NE

85.5c

x-y plane is ~ 150 ~m FWHM. A less accurate determination of the z-coordinate can be obtained by the usual charge division method. The outermost, two-dimensional readout chamber will provide the localization of the particle's impact point with a precision of ~ 150 Inn FWHM in all directions: no matter, in fact, for its thickness because of its location at the end of the particle trajectories, inside the magnetic field. All the chambers will be filled with the usual (At, C2H6) gas mixture and will be put in a tank filled with Helium, at NTP, again in order to minimize multiple Coulomb scattering. We are also evaluating the possibility to use for the chambers an aitemative gas mixture of He and DME. The He tank has been shaped in such a way to allow an easy positioning of the compensating magnets rather close to the interaction region. The last detector, just before the solenoid coils, will be a second barrel of scintillators (10 cm thick) that will allow, together with the inner one, time of flight measurements and provide the first level trigger. Furthermore, it will detect neutrons from non-mesonic decay with ~ 10% efficiency. Two walls of scintillators, 10 cm thick, will close the cylindrical volume in order m increase to ~ 4~ the solid angle for neutron detection. The behaviour of the above described apparatus has been studied by means of the GEANT3 simulation package [11]. Very encouraging, even if preliminary, results have been reported in Ref. [12]. We have furthermore investigated how the tagging technique may help to clean the particle momentum spectra, by simulating the response of our spectrometer to monochromatic ~- of 270 MeV/c, emitted isotropicaUy from the targeL Distributions presented in Figure 4 allow to appreciate the effect of such correction: the dotted line represents the simply reconstructed ~momentum, while the solid one is the same distribution obtained taking into account the information from the K + tagging. The comparison shows an improvement, in particular as far as it concern the elimination of the low-momentum tail.

>--~800 o

700

o~ 600 500 o 400

L.I

300 200 100 ....................I . - . . . ~ " ~

265

....i L 270

rC m o m e n t u m [ M e V / c l

Figure 4. Simulation of the effect of the tagging tecnique on the ~- reconstructed momentum spectrum. ~- momentum distribution with (solid line) and without (dotted line) corrections.

In order to improve the pattern recognition efficiency for charged tracks we are actually evaluating the opportunity of using a time projection chamber [ 13-151, instead of the set of drift chambers, even if the filling gas typically adopted (a mixture containing mostly Neon) may tepresent a serious inconvenient since it contributes largely to the multiple Coulomb scattering. The counting rates we expect are very encouraging: at L = 103s cm-as-t we expect - 200 ev/hour for a single hypemuclear level at a rate of IO-s/stopped K-and with the K+ tagging, - 20 ev/hour for (n, p) non-mesonic decay and - 4 ev/hour for (n, n) mesonic decay. We plan to bave the described apparatus ready for the end of 1995, scheduled start of the DA@NE operation. R

ENCES

1 6. Vignola, Proc. Workshop on Physics and Detectors for DAONE, Frascati, Italy, April 9-12, 1991 (ed. 6. Pancheri, Servizio Documentazione dei Laboratori Naxionali di Frascati dell’INFN) quoted in the following as DA@NE ‘91, p. 11. 2 P. Franzini, in DA@NE ‘91, p. 733. 3 L. Maiani, in DAoNE Physics Handbook, (Servizio Documentazione dei Laboratori Nazionali di Frascati dell’INFN), LNF - 92/043 (P) (1992). 4 A. Aloisio et al., in KLOE A General purpose detector for DA@NE, (Servizio Documentazione dei Laboratori Nazionali di Frascati dell’INFN), LNF - 92/019 (IR) (1992). 5 T. Bressani, in DA@NE ‘91, p. 475. 6 T. Bressani, in Common Problems and Ideas of Modern Physics, (World Scientific, Singapore, 1992). p. 211. 7 T. Yamazaki et al., Nrcoro Cimetttt~ IOZA, (1989). p. 695. 8 R.A. Schumacher, Proc. of International Symposium. on Hypemuclear and Strange Particle Physics. Shimoda. Japan, December 9-12, (1991), quoted in the following as SHIMODA ‘91. to be published. 9 F. Angelini et al.. Nttcl. P/tys. B (Proc. Suppl.) 23A, (1991). p. 254. 10 R. Bouclier et al., Nttcl. Imtr. ottd Meth., A315, (1992). p. 521. 11 R. Bmn et al.. GEANT3, Data Handling Division DD/EE/84-1, September 1987. 12 M. Agnello et al., in SHIMODA ‘91. 13 ME. Mermikides. Proc. of SCRI School on Vector Computing in Experimental High Energy Physics, Tallahassee, Florida, USA, June 1-3, (1988). I4 L. Rolandi, Proc. of XIV International Winter Meeting on Fundamental Physics, Sant Feliu de Giuxols, Catalonia, Spain, March 17-22 (1986). 15 S.R. Amendolia et al., Nacl. Ittstr. mtd Merh., A252, (1986). p. 392.