c

Volume 74B, number 3

PHYSICS LETTI'RS

10 April 1978

BACKWARD ENHANCEMENT IN THE A 1 REGION AND A 2 PRODUCTION IN THE REACTION n - p ~ pfn + n - ,t- AT 9 AND 12 GeV/c A. FERRER, B. BOUQUET, B. D'ALMAGNE, C. DANG VU, A. JACHOLKOWSKI 1 H. NGUYEN, P. P E T R O F F , F. RICHARD, P. ROUDEAU and J. SIX Laboratoire de l'Accflerateur L#t~;aire, 91405 Orsay, France D. TREILLE CERN, Geneva, Switzerland P. RIVET and A. VOLTE Coll~ge de France, 75005 Paris, France and P. BENKHEIRI, B. EISENSTEIN 2 p. FLEURY, G. DE ROSNY, A. ROUGE, H. YOSHIDA 3 LPNltE, Ecoh" Polytechnique. 91128 Palaiseau, France Received 9 December 1977 Revised manuscript received 8 February 1978

Results on backward (3rr)- system produced in rr-p -~ pfrr+rr-rr- reaction at 9 and 12 GeV/c are given. The p°rr- mass spectra show two clear signals at 1050 MeV (A 1 region) and 1303 McV (A2). The width of the enhancement in the A 1 region (195 -+ 32 MeV) is narrower than found in diffractive experiments. Total backward cross sections for those signals are of the same order of magnitude (~ 0.5 ub).

Many partial wave analyses of (37r) + systems produced in the forward direction in the reactions 7r+p -+ (3rr)-+p have shown, apart from the clear production of the well-established meson A 2 (1310), that the (3rr) system is predominantly produced in a series of JP-unnatural states, characterized by rather broad Breit-Wigner shapes, but with smoothly varying phases relative to other background waves. This last observation has been at the origin of the interpretation ofprr(J P = 1+, ~ 1 1 0 0 MeV) and frr(2-, ~ 1 6 8 0 MeV) enhancenqtents (the controversial A 1 and A3), as threshold kinematical bumps that could be ex1 Now at Institute of Experimental Physics, Warsaw University, Poland. Now at University of Illinois at Urbana Champaign, USA. 3 Now at University of Fukui, Japan.

plained by reggeized Deck type effects [1,2]. Two recent results tend, on the contrary, to favour the resonant nature of the diffractive A 1bump: one is the reanalysis of C E R N - I H E P rr-p -+ (37r)-p data using fully analytic, unitary functions which concludes that the data do not completely rule out the resonant nature of the A 1 [3] ; the other comes from a partial wave analysis of (37r)- data produced coherently on nuclear targets and reports variations of 1+S phase relative to 0 - P phase across the A 1 peak of about 90 degrees, and establishes the resonant behaviour of the 2 - state in the A 3 region [4]. No evidence has been found, however, from the search of A10 -+ (3rr) 0 state in either charge [5] or hypercharge [6] exchange experiments. Concerning the backward production of the A 1 , 287

Volume 74B, number 3

PttYS1CS LI-TTFRS

10 April 1978

in baryon exchange experiments, some results are available from inclusive [7] and exclusive [8] rr-p reactions. Recently, good evidence for backward production of A~" has been reported from the analysis of the K - p -+ ~ rr+Tr+Tr- reaction where a clear 1+,p0~r+ enhancement at 1040 MeV has been established [9]. In this letter we present new results concerning the backward production of the (37r)- system in the reaction rr-p ~ pfrr+rr-Tr - ,

(1) -0.5

at 9 GeV/c and 12 GeV/c incident rr- momenta (Pinc)- The data come from an experiment done with the CERN ~ spectrometer, using a fast proton trigger device whose detailed description can be found elsewhere [10,11]. This experiment was designed to carry on a systematic study of baryon exchange reactions, characterized by a small four-momentum transfer squared u from incident w- to outgoing fast proton Pf. Let us briefly recall that the geometrical acceptance of our fast proton detectors allow a good efficiency (~75%) for detecting forward emitted proton (Ohb <~ 150 mrad) with momentum greater than 0.5Pinc. To keep the acceptance roughly the same at 9 and 12 GeV/c the g2 magnetic field intensity was adquately reduced in the 9 GeV/c run. The kinematical limits accepted with our trigger were then : lu l < 1.2 GeV 2 and lul< 1.6 GeV 2 at 9 and 12 GeV/c, respectively, for proton recoil mass less than ~2.0 GeV. The total number of triggers recorded in this experiment were 1.6 X 106 and 1.1 X 106 at 9 and 12 GeV/c, equivalent to a nominal sensitivity of 9.4 and 5,8 events/nanobarn, respectively. To select candidates to reaction (1) we retained 4-prong charge-balanced events and 3-prong events where one of the "slow" charged particles is undetected in the ~2 optical spark chambers or non-reconstructed by our analysis programs. Each topology represents ~20% of the data. We reduced the 3-prong data by applying the missing mass squared criteria: ( - 0 . 3 5 < MM 2 < 0.07 GeV 2) to the 7r-p --+ pflr+-Tr-(MM 2) hypothesis, ensuring a good signal/ background ratio to reaction (1) as can be seen in fig. 1. We then tried kinematical 4-C fits to 4-prong candidates and chose the events with a fit probability 288

0.25

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Fig. 1. The distribution of the missing mass squared MM2 to the reaction rr-p-~ pfrr+-n-(MM2) for all 3-prong candidates. The two vertical lines define the retained events for the kinematical fit to the reaction rr-p --* pfTr+a-Tr-. The curve is the background parametrization. P{X2) > 1%. In addition, 3-prong candidates with good I-C fits (P(x 2) > 5%) were also chosen. This procedure left us with 29060 and 11 800 events at 9 and 12 GeV/c, with ~ 1/3 of these events being 4-C fits. The loss of 3-prong events, due to the above MM2 cut was estimated as 25%, and the residual background after kinematical fits is about 10% in 1-C fit sample and less than 4% in 4-C fit sample. Careful acceptance calculations were performed to correct for event losses [11 ]. The efficiency of the fast proton trigger was found to be ~0.63 and that of the kinematic programs ~0,85. Furthermore a Monte-Carlo method was developed to correct for slow-particle detection and reconstruction losses. The mean probability for an event of reaction (1) to be detected as either 3-prong or 4-prong is ~80%. The most important feature of reaction (1)we found is the dominant production of quasi-two-body processes of the type rr-p-+ N*0p 0 and N*0f 0, where N *0 stands for A0(1232), N0(1520) and N0(1688). These quasi-two-body reactions are discussed elsewhere [11,12]. To avoid the reflection of these dominant processes on the (3rr) system we reject events with a mass rn(pfTr-) < 1800 MeV in any of the two combinations. This cut eliminates 70% of the data. For the remaining data, we show in fig. 2 the Chew - L o w plot for the (3a) system. We observe a large accumulation of events at small u and small 3rr masses, particularly in the A 1 and A 2 region (high 3rr masses are suppressed by the m ( p f r r - ) cut).

V o l u m e 74B, n u m b e r 3

PHYSICS

LETTERS

10 April 1978

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- events w i t h m (pfrr-) > 1 . 8 G e V , - . . . . same, but w i t h u ' < 0 . 5 G e V 2.

To increase the signal/background ratio in the A 1 A 2 region, we further restrict our samples by choosing only p 0 n - events. This is justified by the fact that the n+n- spectra (not shown) give a strong p0 peak [11 ], and a very weak f0(1270) signal, precluding any f07r- study. We define the p0 meson by 600 < m0r+Tr - ) < 900 MeV on any of the two combinations. Fig. 3 shows the unweighted p°Tr- mass spectra, where a bump appears near 1050 MeV. This bump is enhanced after selecting the most backward p07revents, e.g. u' < 0.5 GeV 2, (with u' = Ureax - u ) as can be seen in that same figure. A clear signal of the production of the A2(1310) can also be observed. We verified with Monte-Carlo generated events that none of the experimental cuts could generate enhancements or holes on the (37r)- mass. We also verified that the acceptance was not at the origin of the double peak

structure in the p°rr- mass spectra. A fit to the p07r- mass spectra with only an A 2 Breit-Wigner resonance and a smooth background cannot reproduce the bump in the A 1 region. This A 1 signal corresponds to ~ 4 standard deviation effect for the total samples (9 + 12 GeV/c) with u' < 0.5 GeV 2, as can be seen in fig. 4 (dotted curve ). On the contrary, a good fit is obtained with two Breit-Wigner functions, describing respectively the A 1 enhancement and the A 2 (fig. 4, full curve). We were led therefore to include the two BreitWigner functions in our fits of the separate mass spectra. The masses and widths resulting from the best fits are given in table 1. In order to learn on the production mechanism of these events we have fitted the p0rr- mass spectra, after correction for acceptance losses, as a function of

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289

Volume 7413, number 3

PHYSICS LETTERS

theless some crude checks have been perforined: the angular p0 decay distribution in the 3~r rest mass system for events lying in the A 1interval is found compatible with isotropy, as expected for the S-wave pOTr- decay of the AI-. Moreover, when the higher p°~r- mass interval is selected (for the A2-)the deviation from isotropy is significant suggesting that our A2- signal is not incompatible with the expected D-wave p07r- decay distribution. We determined also the density matrix elements for the events lying in the A 1 and A 2 intervals from the angular decay distribution of the normal to the (370 decay plane in the Jackson frame, using the known 1+ and 2 + angular decay distributions. Our resuits show that all the allowed spin projections are equally populated. Finally we computed the corresponding cross section for the backward production of the A 1 enhancement and of the A 2 as seen in our experiment. Correcting for all acceptance and analysis cut losses and taking into account the partial decay modes [ 15] (A 1 -+ p°Tr-)/(A 1 -+ all) = 0.5 and (A 2 ~ pOrt-)/ (A 2 ~ all) = 0.355, we found the cross sections reported in table 1. We point out that these cross sections are in fair agreement with the extrapolated theoretical cross sections predicted for backward A 1 production [16]. From the quoted cross sections we obtain an energy dependence, o ~ s - 1.7, for the A2, in qualitative agree-

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l-ig. 4. The p°rr-invariant mass spectrum for (9+12) GeV/c data, with u'rr-,p f < 0.5 GeV2 . The curves are the results of the fits described in the text. u'. These distributions were fitted with an exponential of the form do/du' = A e x p ( - B u ' ) and the slopes B obtained in the A 1 region (900 < m(pOrr- ) < 1150 MeV) and in the A 2 region (1200 < m(pOir -) < 1400 MeV) are reported in table I. We have no indication that the background under those enhancements affects in a sensitive manner the quoted results. Our data give then evidence that the backward slopes shrink with energy for both reported signals. It is worthwhile to emphasize that the parameters listed in table 1 are in good agreement with previous values repotted in backward .1 production experiments [7,9]. The background remaining under the A 1 and A 2 signals (~60%), as well as the bias introduced by our selection criteria on m ( p f i r - ) , do not allow us to determine the spin and parity of these objects. Never-

41 Some authors [ 13,141 try to explain the backward A] production by a double baryon exchange reggeized model, and land for the p%r- mass distribution an approximate BreitWigner shape centered at ~ 1040 MeV but with a width varying from ~450 MeV [13] to ~230 MeV [14]. No production cross sections have been computed for this effect.

Table 1 Mass (M), width (l'), u' production slope (B) and cross section o of backward produced enhancement in the A1 region and A~. The backward cross sections are corrected for all decay modes, and the errors do not include an overall normalization uncertainty of 12c/<. Incident re- nlomentum

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Volume 74B, number 3

PHYSICS LETTERS

ment with a simple/x 6 Regge exchange mechanism. This " s m o o t h " energy d e p e n d e n c e is to be c o m p a r e d with a~s -3, the energy dependence o f the cross section we obtain for the d o m i n a n t channels lr- p ~ N * 0 p 0 and N * 0 f 0, which in turn are found to be d o m i n a t e d by the nucleon exchange mechanism [111. In conclusion, from the analysis o f reaction (1) obtained in the fast p r o t o n triggered e x p e r i m e n t at the C E R N ~2 spectrometer, we observe the backward production o f the A~-(1310) meson, and find strong evidence for the backward p r o d u c t i o n o f the controversial meson Ai- , with the properties listed in table 1 and with backward cross sections o f the same order as for the A ~ p r o d u c t i o n . The b u m p reminiscent o f the A 1 has a definite narrower w i d t h (195 -+ 32 MeV) than generally obtained in forward p r o d u c t i o n experiments (>~ 300 MeV).

References [1 ] [2] [3] [4]

E. L. Berger, Phys. Rev. 166 (1968) 1525. G. Ascoli et al., Phys. Rev. D8 (1973) 3894. R. L. Schult and H. W. Wyld, Phys. Rev. D16 (1977) 62. J. Pernegr et al., Evidence for resonance behaviour of A1

10 April 1978

and A 3 mesons coherently produced on Nuclei, CERN preprint 30.9.77, submitted to Nucl. Phys. B. [51 F. Wagner et al., Phys. Lett. 58B (1975) 201; M. J. Emms et al., Phys. Lett. 60B (1975) 109. [6] M. Cerrada et al., Nucl. Phys. B126 (1977) 241. [7] E. W. Anderson et al., Phys. Rev. Lett. 22 (1969) 1390. [8] A. Abashian et al., Phys. Rev. Lett. 34 (1975) 691; Phys. Rev. D13 (1976) 5; D. L. Scharre et al., Meson production by Delta exchange in rr-p interactions at 4 GeV/c, preprint LBL6150 (1977). [9] Ph. Gavillet et al., Phys. Lett. 69B (1977) 119. [10] J. Boucrot et al., Nucl. Phys. BI21 (1977) 251; A. Jacholkowski et al., Nucl. Phys. B126 (1977) I; T. Hofmokl et al., Nucl. Phys. B129 (1977) 19. [11 ] A. Ferret, Th~se Doctorat d'Etat, LAE report 1295 (Orsay, 1977). [12] P. Benkheiri et al., Baryon exchange in ~r- induced twobody reactions, preprint LPNHE/X/77, submitted to the Europ. Conf. on Particle physics (Budapest, Hungary, 1977). [13] C. C. Shih and B. L. Young, Phys. Rev. D1 (1970) 2631. [14] J. C. Anjos et aI., Double Regge model for non-diffractive A t production, preprint, Centro Brasileiro de Pesquisas Fisicas, Rio de Janeiro, Brasil (1977). [15 ] Particle Data Group, Review of Particle Properties, Rev. Mod. Phys. 48 (1976) 1. [16] H. E. Haber and G. L. Kane, Nucl. Phys. B129 (1977) 429.

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