Search for narrow four-baryon states

Nuclear Physics B182 (1981) 3 3 7 - 3 4 2 O North-Holland Pubhshing C o m p a n y

SEARCH FOR N A R R O W F O U R - B A R Y O N STATES B. B A D E L E K ,

Insntute of Experimental Phystcs, University of Warsaw, Warsaw, Poland A. H A L L G R E N 1 and S. K U L L A N D E R

Gustaf Werner Institute, University of Uppsala, Uppsala, Sweden J.P N A S S A L S K I

Instttute for Nuclear Research, Warsaw, Poland Received 16 June 1980 (Revised 10 N o v e m b e r 1980) Highly excited (4.10 ~
A search for structures having quark content other than (qq) or (qqq) is a natural consequence of successful attempts to understand the properties of hadrons in the framework of the quark model. In the string (cluster) or bag models, the properties of several colourless N - q u a r k (e.g. N = 3, 6, 9 , . . . ) states are predicted. These states are single hadrons, N quarks in one bag systems. For N = 12 (four-baryon state) the simplified, spherically symmetric, s-wave bag model predicts the existence of a state having the quantum numbers of the helium nucleus and a mass around 4.9 G e V / c 2 [1 ], and the quark-cluster model predicts the existence of a twelve-quark system with a mass of about 4.8 G e V / c 2 [2]. More such states might be predicted if the possibility of changeable bag shapes or vibrations of the string were included in the models. However, the models give practically no quantitative predictions concerning the stability, width or production cross section of these multiquark systems. The following reaction has been studied at 5 G e V / c : 1r- + 4He -->zr- + R + anything,

(1)

where R stands for the recoiling nuclear fragment: p, 2H or 3H. This experiment has been primarily designed to study the isobar contribution to the 4He wave function [3]. However, at the same time it gives a possibility to search for highly excited 1 Now at C E R N . 337

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B Badetek et al. / Search for narrow four-baryon states

four-baryon states (4He*) produced and decaying in a particular kinematical configuration, i.e.: produced in two-body reactions at small four-momentum transfer (0.005 < It,ll < 0.15 (GeV/c) 2) and high energy transfer (0.0 < (zaE)~ < 2.0 GeV): ¢r- + 4He -~ It- + 4He* ;

(2a)

decaying via the emission of a high momentum (see table 1) nuclear fragment: 4He~

2H + anything.

13H

(2b)

In the production process (2a), the pion may either bring a pre-existing virtual four-baryon state to the mass-shell or create such a system during the interaction. In the first case, the production cross section is proportional to the transition probability of the 4He nucleus into a twelve-quark state, roughly estimated to be 6 . 1 0 .2 [4]. In the second case, its upper limit can be estimated from the quadruple scattering contributions in 7r- 4He elastic interactions in our t-range, giving a probability of the order of 1 0 - 4 - 1 0 - 6 . A detailed description of the experimental set up has already been published [5]. In the experiment, the beam particle measured in a counter hodoscope interacted in a high-pressure thin-walled helium gas target and the momentum of an outgoing pion and also the momentum and the energy loss of a recoiling nuclear fragment were measured in the fast particle and recoil spectrometers, respectively (fig. 1). The fast particle spectrometer consisted of two sets of multiwire proportional chambers placed before and after the magnet. Only negatively charged particles scattered to the left of the beam with scattering angle smaller than 5 ° and momentum bigger than 3 G e V / c were measured. We did not identify the fast, negatively charged, forward produced particle, because at our beam energy there is ~ 1% chance that such a particle is not a ~--. In our analysis, this background, due to production of K-, has been neglected. The recoil spectrometer consisted of MWPC's, solid-state detectors and scintillators arranged in two symmetric arms placed to the left and to the right of TABLE 1 Ranges of variables Variable t,,~ (GeV/c) 2 p,, (GeV/c) ~bR~ (degrees) cos OR Pa (GeV/c)

Range 0 005 ~ l t ~ [ ~ < 0 15 p~ > 2,8 + 10.3It, ll, p~>3.0 I ¢ a ~ [ < 1 8 ° or I¢R~--180°1 ~<18 ° --0.5
B. Badetek et al. / Search for narrow [our-baryon states

339

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Fig 1 T h e experimental setup. B's and V l , 2 a r e b e a m defining counters, V3 is a b e a m veto counter, H's are scmtdlator hodoscopes, S's are counters used to define trigger, PC's are fast particle M W P C ' s

the beam. Each arm covered a polar angle interval of 45°-135 ° and an azimuthal angle interval of + 11 °. A detailed description of the selection criteria for the recoil particle in our set up is given elsewhere [5, 3]. This method has been checked with Monte Carlo simulated events. For generated events, in the worst case of separating high m o m e n t u m deuterons and tritons, 2.5% of deuterons were identified as tritons and 7.5% of tritons were identified as deuterons [3]. The final sample consisted of the events selected by imposing limits on the ranges of measured variables. The following five variables describing reaction (1) were subject to cuts: t ~ = squared four-momentum transfer to the pion; p~ = momentum of the scattered pion; ~ a ~ r = relative azimuthal angle between the recoiling nuclear fragment R and the scattered pion; OR = polar angle of the recoil R with respect to the incident pion direction; PR momentum of the recoil R. The cuts are listed in table 1. The cuts were made for two reasons: to avoid low acceptance regions and in order to make the ranges of variables as mutually independent as possible, thus making the kinematical conditions easily interpretable. The cuts made for the second reason were very severe and r e m o v e d - 4 0 % of the events. The analysed sample consisted of 16 281 events with the recoil proton, 14 571 events with the deuteron and 7464 with the triton, out of which 3037, 1819 and 1056 respectively had 4He* mass bigger than 4.1 G e V / c 2. The sensitivity of the experiment was 7 . 0 1 . 1 0 -4 ~ b / e v e n t . The 4He* mass, rn*, is measured up to 5.35 G e V / c 2 and for m * > 4 G e V / c 2 is practically determined only by the scattered pion momentum, as demonstrated in fig. 2, where the kinematics of the reaction (2a) is displayed. The m* resolution varies with the scattered pion momentum from 23 for small to 16 M e V / c 2 for large masses =

FF/~ .

The range of the measured recoil variables puts restrictions on detectable decay modes of 4He* (2b); in particular our sample practically does not contain products of

340

/ Search for narrow four-baryon states

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341

B. Badetek et al; / Search for narrow four-baryon states

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Fig. 4. Distribution of 4He* masses defined by reaction (2a). Solid lines are results of a smooth line fit to the experimental distributions for masses greater than 4.1 GeV/c2; (a) all the recoils, (b) proton recoils, (c) deuteron recoils, (d) triton recoils. The X2 per degree of freedom for the fits is (a) 0.87, (b) 0.84, (c) 0.69 and (d) 1.62. The dashed line in (a) shows the acceptance corrections.

its two-body decays. In fig. 3 this is demonstrated for 4He* ~ 3H + p decay with the relative azimuthal angle between the triton and the scattered pion equal to 0 °. Already for m * > 4 . 5 G e V / c 2 the momentum of the decay triton exceeds its measured upper limit. None of the other recoils from other two-body decays get accepted for the above range of m*. The 4He* mass spectrum calculated for reaction (2a) for all the recoils together is presented in fig. 4a. The error bars on the figure are statistical only. The absolute normalisation is known to + 15 %. The average corrections applied to our sample and resulting from the limited detection probability of the apparatus are also shown in the figure. No significant narrow structure is seen in the spectrum. A wide enhancement seen at 3.77 G e V / c 2 corresponds to 4He break up.,This is also true when 4He* mass distributions are made for each of the decay channels (2a) separately (figs. 4b, c and d)*. A function of the form ( A I x 2 + A E X + A 3 ) e x p (A4x), where x = m * - 4 . 1 , was used to fit the shape of the m* > 4.1 G e V / c 2 part of the spectrum. Taking the fitted * A three standard deviation maximum at 4.75 GeV/c2in fig. 4d disappears in a larger sample of events where only low-acceptance cuts were made.

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B Badetek et al. / Search for narrow four-baryon states

curve as a background, we can estimate the upper limit of the (cross section) × (decay branching ratio) for the production of 4He* of width F <~20 M e V / c 2. At 4.9 G e V / c 2 a three standard deviation enhancement would correspond to a (cross section)× (decay (2b) branching ratio) of 110 nb. It should be stressed again that this upper limit of the cross section refers to a particular production (2a) and decay (2b) reaction and also to a particular region of the available phase space (table 1). It corresponds to the frequency of - 1 . 5 • 10 -3 relative to the total sample in fig. l a but for the reason given above this frequency should not be used as the upper limit for the probability of a narrow 4He* state without making large and arbitrary corrections. In order to verify the existence of six-quark and nine-quark high-mass colour systems we have also studied the missing-mass spectra for reaction (1) in ~ ' - + 2H + MM and ~r- + p + MM channels. No narrow (F ~<30 M e V / c 2) resonance signal was seen in our missing-mass regions (m(3H*)<4.4GeV/c 2 and m(2H*)< 3.3 GeV/c2). We should mention our friend Stefan Jonsson whose invaluable contribution and tremendous effort in the analysis of the experimental results continued up to the last days of his short life. Our thanks are due to J. Stepaniak for the stimulus to search for 4He* and for numerous discussions, and also to M. Chevallier and M. Lambert for their support at the beginning of this work. References [1] A.Th.M. Aerts, P.J.G. Mulders and J.J. de Swart, Phys. Rev. D17 (1978) 260, V.A Matveev and P. Sorba, Nuovo Cim. 45A (1978) 257 [2] D.B. Lichtenberg, Nuovo Clm. Lett. 23 (1978) 339; D.B Lichtenberg and R.J. Johnson, Indiana University report IUHET-38 (1979), unpubhshed [3] S. Jonsson et al., Phys. Rev. C21 (1980) 306; M. Chevallier et al., Nucl. Phys A343 (1980) 449 [4] V A. Matveev, Multiquark systems theory and experimental consequences, Lecture at l l t h Int. School of High-energy and relativistic nuclear physics, Homel, 1977, Dubna preprint P2-12080 (1978) (in Russian) [5] B. Badetek et al., Nucl. Instr. 155 (1978) 61