Study of strange particle inclusive reactions in π - Springer Link

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Z. Phys. C - Particles and Fields 26, 359-372 (1984)

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9 Springer-Verlag1984

Study of Strange Particle Inclusive Reactions in n-p Interactions at 3.95 Gev/c B. Adeva 1, M. Aguilar-Benitez, J.A. Rubio Junta de Energia Nuclear, Madrid, Spain J.A. Garz6n, C. Pajares Facultad de Fisicas, Universidad de Santiago, Spain Received 6 January 1984

Abstract. Results are presented for the inclusive reactions

n-p--,A+X n-p~K~ n-p -->S(1385) + X n- p ~ K(890) + X at 3.95 GeV/c incident momentum, using data from a high statistics bubble chamber experiment. The total and differential inclusive cross sections are presented and compared with previous measurements. The forward backward asymmetries in A and S(1385) production are studied in the context of triple Regge theory. A phenomenological analysis of inclusive A production including the A polarization is presented.

1. Introduction A large amount of data have been collected on inclusive production of various hadrons in hadronnucleon collisions [1-10]. However, such data for the production of strange hadrons from an initial state with no strange quark content are still rather meagre [11-18]. It is of interest to examine the hypercharge exchange process since it differs dinamically from other hadron-nucleon reactions and gives the possibility to study new effects. In this paper we present a study on the inclusive

1 Present address: DESY, Hamburg, FRG

productions of A,K~ and K-+(890) in n-p reactions at an incident momentum of 3.95 GeV/c, which corresponds to a center of mass energy of _ 2.9 GeV. The data come from a large statistics bubble chamber experiment having a sensitivity of 90 events//~ barn. A brief description of the experimental data is given in Sect. 2. The total and differential inclusive cross sections (in the x and p~ variables) are presented in Sect. 3. The analysis of the forward-backward asymmetry in A and S(1385) production is given in Sect. 4. A determination of the effective trajectories exchanged in the inclusive production process is presented in Sect. 5. A phenomenological analysis of the observed A polarization appears in Sect. 6. Finally, Sect. 7 is left for a brief summary and conclusions. 2. Experimental Details The data come from an analysis of approximately 1.7.106 exposures of the CERN 2m hydrogen bubble chamber to n- beams of 3.95 GeV/c average momentum. The total sensitivity of the experiment is around 90 events/# barn. The general details of the experiment are given in [19]. We recall here that in order to ensure good scanning and measurability conditions, a limited fiducial region was imposed. To take into account scanning biases only V~ with a projected decay length of more than 3 mm were kept for further analysis. This loss and that due to decays outside the fiducial volume were compensated for with the standard weighting procedure. The average weights were 1.13 for K~ and 1.13 for A's.

B. A d e v a et ak : Strange Particle Inclusive R e a c t i o n s

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Fig. 2a and b. t c ~ + K~ - effective m a s s d i s t r i b u t i o n s for the inclusive r e a c t i o n s n - p - - , K ~ n• X. The curves are the results of the m a s s fits discussed in the text

Fig. l a and 5. A n +, A n - effective m a s s d i s t r i b u t i o n s for the inclusive r e a c t i o n s n - p - - , A n • X. T h e curves are the results of the m a s s fits discussed in the text Table

We have reactions

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inclusive

rc-p~ A + X rc-p ~ K ~ + X rc- p ~ Z+- (1385) + X

(1)

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(4)

(2) (3)

from events with a visible A or K ~ decay. After the kinematical analysis and the use of bubble density information the A - K ~ ambiguity was of the order of ___1.4~ and did not affect our conclusions on reactions (1) and (2). For reactions (3) and (4) we considered all the hypothesis containing a Arc -+ or K~ -+ combination. The non negligible level of ambiguities does not affect our results which are based on fits to the Arc +- and K~ +- effective mass distributions. Figures 1 and 2 show the Arc-+ and K~ -+ effective mass distributions. Clear signals of the E+-(1385) and K-+(890) resonances are observed. To extract the number of events of reactions (3) and (4) the Arc -+ and K~ +- effective mass distributions were

1. E x p e r i m e n t a l details of inclusive final states

Reaction

No. of entries in reaction

zc-p~ A + X zr-p ~ An + + X n-p~An+ X n - p ~ E + (1385) + X n - p ~ ,~- (1385) + X

45141 12762 ~ 31754 b 2213 __+ 123 3886 _ 160

~z- p --, n-p ~ n-p~ n-p ~ n- p ~

40774 12490 ~ 15128 a 2313 +__70 1048 + 90

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K~ + X K~ + + K~ - + K§ K-(890)

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are are are are

X X + X + X

1.016 entries per event 1.08 entries per event 1.028 entries per event 1.034 entries per event

fitted with parametrizations which included an incoherent superposition of a Breit-Wigner resonance term and a polynomial in the mass for the background. In Table 1 we present a s u m m a r y of the number of events (or combinations) used in this analysis.

B. Adeva et al. : Strange Particle Inclusive Reactions

361

P rr/Pmax)'E* is the center of mass energy of the particle

4.

and x/~ is the total energy in the center of mass is presented in Fig. 5. For comparison we show similar measurements at other energies. A large fraction of the cross section takes place at negative values of x, with a clear peak at x _~ - 1 corresponding to the exclusive reaction n - p ~ A K ~ indicating the dominance of production in the target fragmentation region. The behaviour of the inclusive cross section in this fragmentation region ( - 1 < x < - 0.2) can be discussed in terms of Regge-Mueller phenomenology. At large values of M 2 the asymptotic behaviour of the invariant cross section for the inclusive process p 22+ A is given by the expression

3.

0.2 using the previous formula in 8 bins of t in the region 0.2 < Itl < 1 GeV 2. F(t) and ~M(0)- 2~r,(t) were left as free parameters in each t bin. Figure 21 (a-h) displays the results of the fits over the mass spectra. It is clearly seen how the slope increases when the momentum transfer increases. Figure 22 shows the fitted values of the effective trajectory ~K,(t) when we assume aM(0) = 0.5. They lie between the extrapolation of the K* and K recurrences from the Chew-Frautschi plot, closer to the first ones, which correspond to positive naturality exchanged. We have also performed an overall zZ-fit in the region 0.2 < I tl < 1 GeV 2 in which F(t) was parametrized as A exp(Bt), and A, B, eM(0)- 2eK,(0 ) and e'K* were determined from the fit. The following values were obtained: %t(0) - 2%,(0) = 0.09 _+_0.08 ~'~, = 0.75 _+ 0.06 GeV-2 B = 0.22 _+ 0.17 GeV- 2 A = 126 _+ 23 #b GeV -2 If the fit is performed using the high energy approximation s ~ ( s - u)/2, the intercept eK,(0) goes down by 0.2 units, closer to the K trajectory, and the interpretation of the mass spectra is also good. The intercept c~K,(0) also shows some correlation with respect to the cut-off in 2 M Z / ( s - u), which is needed in order to avoid the region dominated by the resonances, and it changes by 0.1 units in the range 0.09 < 2 m ~ / ( s - u) < 0.12. A similar analysis has been done in n - p ~ A X at 15 GeV/c [5] with the result e M ( 0 ) - 2 % , ( 0 ) = 0.17 _+ 0.15. Our data are in agreement with it when we use the variable ( s - u)/2, thus indicating that K* exchange dominates also at our energy. However, if the variable M~/s is used, the effective trajectory lowers down closer to the K-trajectory.

5.1 p A Vertex In the small t region the triple Regge amplitude may be expressed: d 2 O" t/ 2 M 2 h~M(o)-2~,(t)

where e~,(t) = ex,(0) + e'x, t is the effective trajectory exchanged in the p A vertex. We make use of the crossing symmetric variable ( s - u)/2 instead of s, which is more adequate at low energies in the Regge approximation, e M(0) is the intercept of the exchanged trajectory in the "K* +" n - collision, which we have assumed to be 0.5, corresponding to m - f and

5.2 n A Vertex We have assumed that the observed A production in the direction of the n - proceeds through hyperon exchange. In Fig. 23 we show the energy dependence of the invariant differential cross-section integrated in the region 0.2 < x