c

26 May 1975

PHYSICS LETTERS

Volume 56B, number 5

DIFFERENTIAL CROSS SECTIONS FOR BACKWARD r- p --f nr” FROM 2.6 to 8 GeVlc C. DeMARZO, L. GUERRIERO,

C. NICOLINI*'

, F. POSA, E. VACCARI and F. WALDNER

Istituto di Fiska*‘, Universita di Bari, and Istituto Nazionale di Fisica Nucleare, Sezione di Bari, Bari, Italy

G.T.Y. CHEN*3, C.R. FLETCHER*4, Department

R.E. LANOU Jr. and J.T. MASSIMO

of Physics*5, Brown University, Providence, Rhode Island 02912,

USA

and D.S. BARTON, B.A. NELSON, L. ROSENSON and R.E. THERN*6 Department of Physics and Laboratory for Nuclear Science, MassachusettsInstitute of Technology*‘, Cambridge, Mass. 02139,

USA

Received 7 April 1975 We report differential cross sections for n-p + rm” in the backward hemisphere at incident momenta of 2.6, 3.5, 4.3,6.0, and 8.0 CeV/c. We observe less pronounced structure than some previous measurements with a shallow dip displaced from that seen in n+p elastic scattering.

An important aspect of the phenomenology of backward pion-nucleon scattering comprises the position and depth of the dips observed in both II+ elastic and charge exchange scattering near u = -0.2 (GeV/c)2 [e.g. I] . Systematic differences among the various previously reported charge exchange measurements [26] have occurred in the region from urnax to the dip. We have measured the differential cross section for backward charge exchange (CEX) scattering with good momentum transfer resolution in an experiment performed at the Brookhaven A.G.S. at fourteen values of incident pion momentum from 2 to 8 GeV/c. This letter reports the results at 2.6,3.5,4.3,6.0, and 8 :O GeV/c with acceptance in momentum transfer : -2 (GeV/c)2 5 u < urnax.

*’ Present address: Temple University, Philadelphia, Pennsylvania. *2 Work supported in part by the Italy-USA Scientific Cooperation Program. *3 Present address: Harvard University Medical School, Boston, Massachusetts. *4 Present address: Lockheed Industries Co., Houston, Texas. *’ Work supported by the U.S. Atomic Energy Commission. *6 Present address: University of Pennsylvania, Philadelphia, Pennsylvania.

The equipment consisted of an array of scintillation counters and steel-plate, optical spark chambers covering the full solid angle around a liquid hydrogen target. A detailed description of the apparatus and detection method will be published elsewhere. The backward hemisphere was covered by 5 nested spark chambers containing 8 radiation lengths of steel. A charged particle veto and forward veto shower counter closed the system. In addition a steel spark chamber and scintillation counter neutron detector containing -1.7 nuclear mean free paths was located downstream of the shower counter. The detection of a neutron was not required but neutron interactions were recorded and used to sharpen the angular resolution. Measured positions and directions of showers permitted a fit for the interaction point with the resulting resolution on T-ray lab angles s + so and on neutron angle of + 0.3”. Our sample of (CEX) events is somewhat novel in that it encompasses both two y-ray events and those with only one y-ray detected. In addition to minimizing the effects of spark chamber inefficiency and low energy shower losses, the inclusion of the one y-ray events in the acceptance range of the neutron detector reduces the uncertainty due to scanning inefficiency. A partial rescan has shown that missed showers 487

Volume 56B, number 5

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PHYSICS LETTERS

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Fig. 1. Distributions of: c.m. opening angles for two "r-rayevents at (a) 2.6, (b) 3.5, and (c) 6.0 GeV/c; c.m. colinearity angles at 3.5 GeV/c, (d) lr°-n for two "r-ray events with 11°-~< 0~3, < 35° , and (e) 3,-n for one ~ray events. Vertical strokes indicate acceplance cuts: (solid) neutron events, (dashed) no-neutron events. The dashed curves indicate the background subtraction determined from higher multiplicity events, including (e) contributions for 2"t events with 030, > 35°.

are primarily short, and that there is a negligible loss into the zero -/-ray category. Two 7-ray events were selected on the basis of the characteristic, peaked distribution of opening angles between -/-ray pairs in the production center-of-mass system [7]. The distributions of opening angles for all two -/-ray events at 2.6, 3.5, and 6.0 GeV/c are given in fig. l(a, b, c). For events without a detected neutron a tight cut was made on the opening angle in order that the angle bisector might be used as a measure of the rr° direction. For events with a measured neutron interaction, a wide opening angle cut was followed by a cut on the colinearity of the neutron and the closer of the two rr° solutions predicted from the -/-ray directions. This calculation must necessarily be carried out assuming CEX kinematics. A modified version of this colinearity test can be applied to the 2 0 - 2 5 percent of the events with a detected neutron, in which only one -/ray was visible. The lost -/-ray is almost always that with the lower c.m. energy, leaving the remaining 3, ray, which carried most of the Ir° momentum, well correlated with the neutron direction. The distributions of colinearity angle used in the selection of the two and one -/-ray CEX events at 3.5 GeV/c are shown in fig. 1 (d, e). Background subtractions have been applied to the distributions obtained from higher multiplicity events by discarding small showers. These subtractions, normalized for ff < 165 ° (see fig. 1), were found to have less structure than the raw charge exchange sample, and in particular to have less backward peaking. The 488

subtraction applied to the angular distribution of two -/-ray events without a measured neutron was also determined from higher multiplicity data, since no other true two "/-ray events produce opening angles in the 7r° region. The normalization was taken from the region just above the lr° opening angle cut. The experimental angular distributions have been corrected by Monte Carlo simulation for geometric detection inefficiency, veto attenuation o f - / r a y s and neutrons, and for losses due to kinematic cuts. In the backward hemisphere in the laboratory, where the spark chamber geometry gives the most sensitivity to low energy losses, the inclusion of the one -/-ray events ensures a smooth and straightforward correction. In the region outside the neutron detector acceptance, higher -/-ray energies in the laboratory reduce this sensitivity. In this region, our results based only on the two -/-ray events join smoothly with the more backward data. The cross section calculations have also included corrections for beam contamination, internal conversion of -/-rays, and conversions of 7 rays in the target assembly. Radiative effects in pion-nucleon scattering have been calculated by Sogard [8], and are experiment dependent. An additional -/ray may accompany a CEX event which causes a veto in the shower counter or is visually rejected on multiplicity grounds. The radiative corrections for our geometry are nearly independent of u but vary with incident momentum from 7% at 2.6 GeV[c to 12% at 8 GeV/c. The corrected differential cross sections do/du are

Volume 56B, number 5

PHYSICS LETTERS

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26 May 1975

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Fig. 2. Differential cross sections for rr-p - nrrO, including data from ref. [2-6] with typical errors. The errors shown are statistical; normalization uncertainty is ~14%. shown in fig. 2*. The value o f the momentum transfer u was calculated from the bisector of the c.m. opening angle for the two 3'-ray events without a neutron measurement, and from an appropriately weighted average of the neutron and rr° (or single 3'-ray) directions for the events with a detected neutron. The width o f the data bins is. typically more than twice the rms resolution, so that resolution unfolding has produced only very slight changes from the experimental angular distributions. We have checked numerous experimental distributions which are sensitive to the resolution with Monte Carlo simulations with excellent results. Various other measures o f u (e.g., only bisector, only neutron, etc.) yield angular distributions after resolution unfolding which are consistent, within statistical errors, with each other and with the results presented here. The data all exhibit a backward peak, a shallow dip in the vicinity of u ~ - 0 . 3 (GeV/c) 2 and a shoulder or secondary maximum around u ~ - 0 . 7 (GeV/c) 2. The results o f exponential fits to the data between Umax and u ~ - 0 . 2 (GeV/c) 2 are given in table 1. * Results in tabular form are available upon request (DSB).

We have included in fig. 2 other angular distribution data at 6.0 and 8.0 GeV/c. It is clear that the data of Schneider et al. [2] and of Boright et al. [4] have steeper backward peaks than those of Chase et al. [3] and of this experiment. An exponential fit to the backward points of ref. [3] gives a slope parameter b = 6.4 -+ 1.1 (GeV/c) - 2 compared to b = 11.3 -+ 1.2 (GeV/c) - 2 reported in ref. [4]. The experiments of Schneider et al. and Boright et al. both utilize only the two 3' rays in the analysis, whereas this experiment and Chase et al. have also detected and used the neutron direction with an improvement in momentum transfer resolution by almost a factor of three (Au 0.03 for u ~ --0.2 at 6 GeV/c). Resolution unfolding cannot fully explain the discrepancy between the various measurements. The significantly smaller integrated backward cross section in the previous data (from 180 ° to the dip) might be due in part to sensitivity to Monte Carlo estimates of 3'-ray losses. Our sample of one 3'-ray events has made the largest fractional contribution just in this angular region (~21% at 6 GeV/c). The trend o f the data o f ref. [2] toward steeper peaks at lower energies could be explained by such sensitivity near u = - 0 . 1 where their detection efficiency was only 5-10%. The data of Boright et al. also display a steeper falloff at large lul than the resuits of this experiment, of Chase et al., and of Brocke4t et al. [6]. Fig. 2. Also includes backward points measured by Kistiakowsky et al. [5] at 2.6, 3.5, and 6.0 GeV/c. Our data are in generally good agreement, but since none of the previous authors have applied radiative corrections appropriate for their experiments, comparisons principally of normalization may be mislead-

ing. In fig. 3 we have plotted values of do/du at various fixed values o f u versus s on a logarithmic scale. The 489

Volume 56B, number 5 I00

PHYSICS LETTERS

only slightly shifted from that in 7r+p [1]. Over the range o f energies covered by this experiment the shal. low dip seen in our data does not occur at the same values o f u as for 7r+p, nor does it approach the same value o f u ' = u - Umax [11].

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We gratefully acknowledge the assistance o f the Brookhaven A.G.S. Staff, the technical support o f Mr. A. Distante, Mr. E. Druek, Mr. G. Krey, Mr. A. Libertini, and Mr. T. Lyons, and the careful work o f the three scanning groups. In addition we wish to thank Dr. M. Sogard for his calculation o f radiative corrections appropriate to this experiment.

References

three higher energy points at each value o f u have been fitted to a power law dependence, A s n. In terms o f a simple Regge model where n = 2,v(u) - 2 we obtain or(0) = - 0 . 4 8 + 0.06 which is consistent with, but somewhat lower than, the intercept customarily accepted for the N a trajectory, viz., a ~ - 0 . 4 + 0.9u. The dominance o f the Na trajectory in IrN backward scattering in the energy region 5 - 2 0 GeV has been advanced as an explanation o f the deep dip observed in 7r+p scattering near u = - 0 . 1 5 at all energies above a few GeV [9, 10]. In models o f backward charge exchange b o t h with an N~ nonsense wrong signature zero or with cuts, a dip occurs that should be

490

[1] E.L. Bergcr and G.C. Fox, Nucl. Phys. B26 (1971) 1; V. Barger and D. £1ine, Phenomenological theories of high energy scattering (Benjamin, New York, 1969). [2] J. Schneider et al., Phys. Rev. Lett. 23 (1969) 1068. [3] R.C. (~ase et al., Phys. Rev. D2 (1970) 2588. [4] J.P. Bofight et al., Phys. Lett. 33B (1970) 615. [5] V. Kistiakowsky et al., Phys. Rev. D6 (1972) 1882. [6] W.S. Brockett et al., Phys. Lett. 51B (1974) 390. [7] F. Bulos et al., Phys. Rev. 187 (1969) 1827. [8] M.R. Sogard, Phys. Rev. D9 (1974) 1486 and private communication. [9] C.B. Chiu and J.D. Stack, Phys. Rev. 153 (1967) 1575. [10] V. Barger et al., Nucl. Phys. 1349 (1972) 206. [11] V. Barger et al., Nuel. Phys. B57 (1973) 401.