A measurement of the cross section of the reaction pp→n ++(1232) at ISR energies

Volume 62B, number 3

PHYSICS LETTERS

A MEASUREMENT

7 June 1976

OF THE CROSS SECTION OF

T H E R E A C T I O N p p --,n

++(1 2 3 2 ) A T I S R E N E R G I E S

N. KWAK 1 , E. NAGY 2 , M. REGLER 3 , W. SCHMIDT-PARZEFALL, K.R. SCHUBERT 4 and K. WINTER CERN, Geneva, Swttzerland A. BRANDT, H. DIBON, G. FLi]GGE s , F. NIEBERGALL and P.E. SCHUMACHER H Instttut far Expenmentalphyslk, Hamburg, Germany J.J. AUBERT 6, C. BROLL 6, G. COIGNET 6 , J. FAVIER 6, L. MASSONET 6 and M. VIVARGENT 6 Instttut de Phystque Nuclbatre, Orsay, France W. BARTL, H. EICHINGER, Ch. GOTTFRIED and G. NEUHOFER Instltut ftir Hochenergwphysik, Wien. Austrta Recelved 6 April 1976 A measurement of the cross section of the charge-exchange reaction pp --, A~'(1232)n at x/s-= 23, 31 and 45 GeV at the CERN-ISR is reported. The energy dependence continues to follow a power law p/anbwith n = 1.94 -+0.03 indicating dominance of one-pion exchange at the lowest ISR energy; there Is some evidence for deviation from this at the higher ISR energies.

It has been found that the total cross section of the charge-exchange reaction p + p ~ A++ (1232) + n

(1)

decreases approximately hke p~a2 in the low energy accelerator region [1 ]. The reaction is fairly well understood in terms of a single-(modified) plon exchange model [2]. A question still to be answered is whether one-plon exchange would dominate the reaction at ISR energies. Contributions of O and A2-exchange would manifest themselves by a less rapid energy dependence of their cross section, in the form 1 Visitor from the Umverslty of Kansas, Lawrence, Kansas, U.S.A. 2 Visitor from the Central Research Institute for Physics, Budapest, Hungary. a Now at Instltut fhr Hochenenerglephyslk, Wien, Austria. 4 Now at InsUtut ftir Hochenenerglephyslk, Heidelberg, Germany. s Now at DESY, Hamburg, Germany 6 Now at L.A.P.P., Annecy, France

ofPlab1 • We report here new results on the reaction pp ~ (pn+)n, obtained at x/'S-= 23, 31 and 45 GeV using the Split Field Magnet detector (SFM) [3] at the CERN Intersecting Storage Rings (ISR), and discuss the s-dependence of the cross section. The present experiment utihzes the forward telescope with proportional wire chambers, and two neutron detectors [4], consisting of a carbon converter plate (40 × 40 cm 2) of I0 cm thickness and multiwire proportional chambers to measure the neutron impact point. The neutron detectors are positioned at 9 m from the beam intersection; they detect neutrons within an angular range of 0 to 23 mrad with an efficiency varying from 4% to 6% with the neutron momentum and impact point. The SFM magnetic field ranges from 0.5 to 1.0 T, resulting in an average mom e n t u m resolution of about -+ 7% for the mean proton m o m e n t u m of reaction (1) and a mass resolution of AM = 20 MeV/c 2 at the A-mass. Events are selected in two steps; a fast trigger re359

Volume 62B, number 3

PHYSICS LETTERS

V-~ = 31 GeV

V% = 23 GeV 60

7 June 1976

26.+9

l

V-~ = 45 GeV

23+.7

20+.6

m

~0

20

100

200

300

MeV/c

100

200

300

MeV/c

100

200

300 MeV/c

Fig. 1. Observed distributions in a transverse component of missing m o m e n t u m , I~pz I. Events have invarlant mass In the range 1155 < M(n+p) < 1325 MeV/c 2 and I~Pxl< 100 MeV/c. The observed number of events attributed to reaction (1)is given.

quires at least one charged particle In one o f the telescopes, using fast signals from the wire chambers, and a neutron conversion in the oppos:te hemisphere. A slow trigger selects events using memory levels of groups of wires [5] with a decision Ume of about 2/as. It reqmres two charged pamcles m one telescope and no charged particle in the opposite, neutron-side, telescope. The charged particles are not :dentified. The events are passed off-line through a chain of three analysis programs: A track recogmtion program selects events w:th two posmve part:cles and a reconstructed neutron :mpact. A geometrical fit in the magnetic field gwes directxons and momenta o f the posltwe particles and the:r common vertex from which the neutron direct:on :s calculated. A kmematlcal fit with 3 constraints :s performed assuming that the h:gher momentum positive parhcle is the proton and the other one is the p:on of reacUon (1). $ In order to separate genume events of reaction (1) from the large background we have examined the distribut:on of events in the transverse components of m:ssing momentum, calculated by :mposmg energy conservation. The missing longitudinal momentum $ This choice is always correct for (p~r÷) masses below 1340 MeV/c2 . 360

can be very large due to xts poorer measurement and hence does not discriminate between events of reaction (1) and background. We observe an accumulation of events around zero m:ssmg momentum and invariant mass M(pTr +) =,1232 MeV/c 2 which we attribute to reaction (1), and a broader d:stnbution due to background. We select events satisfying the following crlter:a 0) invar:ant mass 1155 MeV/c 2 0 as the energy V~- is increased, whereas m

Volume 62B, number 3

PHYSICS LETTERS

7 lune 1976

Table 1 ~/s

equivalent Plab

[GeV] 23 31 45

observed number of events

[GeV 2 ]

overall acceptance [%]

0 - 0 07 0-0.13 0 - 0 27

0.099 0.136 0.050

26+_9 20+6 23-+7

t-range

[GeVl

integrated luminosity [106/mb]

280 510 1090

16 4 8.87 90.42

total cross-section

Oexpected (a)

[ub]

Iubl

1.60 +- 0.68

1.4 053 0 12

166-+065 0.50-+020

(a) Estimate of the cross-secnon from a fit to low energy data, ref [ 1]. the backward hemisphere, cos ®j < 0, it Is increasing. The broad background distribution in fig. 1 can be extrapolated hnearly under the peak and subtracted; the number of genuine events thus obtained is given m table 1. We note that the background distribution is broadening as V~-is increased, thus reducing the uncertainty due to its subtraction. At W/~ = 23 GeV the signal to background ratio is acceptable over the whole phase space of reaction (1), after subtract:on o f background in Apz and correction for the relative geometrical acceptance for cos O j < 0 and cos O j > 0 we find equal numbers of events of reaction (1) m both hemispheres. The invariant (pn +) mass spectrum is observed to peak at 1230 MeV/c 2 at all three ISR energies. Non-resonant contributions to the selected mass range are small and have not been subtracted. ~ The observed number o f events must be corrected for the geometrical acceptance o f the detector, for the neutron detection effictency, for loss of events due to absorption and scattering m the vacuum tube and in the frames of the wire chambers, and normalized by the integrated luminosity o f the ISR to obtain cross sections. The geometrical acceptance of the detector has been calculated using Monte Carlo methods. Events were generated using a parametrization of the four-fold differential cross section which fully descnbes react:on (1): (i) the dependence in four-momentum transfer t by exp (12 t), compatible with the data at all three energies; 01) a Brelt-Wigner distribution o f lnvarlant mass around 1232 MeV/c 2 with a w:dth [6] of 110 MeV/c2; (ii0 a decay angular distribuUon in the Jackson

¢ The observed distribution above 1340 MeV/c 2 is distorted by wrong proton-pion assignment and can therefore not be extrapolated underneath the peak to perform a subtraction.

frame according to (1 + 3 cos 2 ®j), mdependent of the azimuthal angle ~0j, following a simple one-plon exchange description as observed at lower energies [1]. The experimental and the calculated dLstrlbutions have been compared in these variables; we estimate a + 15% uncertainty m the cross section due to the choice o f parametrlzat:on. All details and obstacles o f the vacuum tube and of the detectors are carefully simulated and the rehabfllty of the absorption and scattering correction has been tested for elastic proton-proton scattering m the same data runs [7]. We est:mate a remaining systematic uncertainty in the cross section o f + 14%. The neutron detection efficiency has been calibrated m a neutron beam [4], its stability was cross checked by protons wtth a resulting uncertainty m the cross sections o f + 10%. Trigger losses contribute + 10% uncertainty. Adding all uncertainties in quadrature the systematic error IS estimated to be + 25%. Values o f the overall acceptance thus evaluated and the accepted range o f four-momentum transfer t are given in table 1. The absolute normalization is obtained by collecting monitor counts simultaneously with data taking. A scintillation counter m o m t o r has been calibrated using the Van der Meer method [8]. The integrated luminosity at each energy and the observed total cross section are gwen in table 1 ; systematic and statistical errors have been added m quadrature. The total cross section as a function of equivalent beam momentum is shown in fig. 2 together with earlier results at lower energies [ 1]. We observe a continuing decrease of the cross section, approximately as p - 2 , from 2.8 GeV/c to the lowest ISR energy o f 280 GeV/e equivalent momentum. A fit to a form 36!

Volume 62B, number 3

PHYSICS LETTERS

7 June 1976

higher ISR energies we observe cross sections which are larger than expected on the basis of relation (2). However, in view of the large background subtraction and small statistics a firm conclusion would require further investigation.

10-26

10-27

We are grateful to F.W. Busser, H. de Kerret and I. Savm for their c o n t n b u t l o n s to parts of the experiment. We thank A. King, X. Mehlfuhrer, M. Moynot, J.P. Reuter and W. Wilmsen for their technical assistance. We gratefully acknowledge the support and excellent work of the SFM Detector Group. We wish to thank M. Metcalf, F. Ranjard, Z. Sekera and all members of the SFMD software group for programming assistance, and F. Blin and J. Guerin for their help m data processing. Useful discussion with G. Alexander is acknowledged. The group from the University of Hamburg wishes to acknowledge financial support from the Bundesmimsterium fur Forschung und Technologle, Bonn, Germany. The group from the InstltUt fur Hochenergiephysik was supported in part by the Osterre]chischer Forschungsrat.

10-28

j

P,;'b9'

E u v b

10-29 ISR

I0-3Q

\t I0-31

I

I

10

20

References 100

200

1000

P~ob (GeV/c)

Fig. 2. Total cross secUon for charge-exchange pp ~ nA÷÷ (1232) as a function of equivalent beam momentum. The sohd hne represents a fit to the data between 2.8 GeV/c and 280 GeV/c. a

-gt

o = Plab gives n = 1.94 -+ 0.03, suggesting that one-plon exchange is dominant in this energy interval. At the

362

(2)

[ 1] E. Bracci, J.P. Droulez, E. Flamimo, J.P. Hansen and D.R.O. Morrison, CERN/HERA 73-1; Reference to earlier work may be found hereto. [2] Zming Ma, G.A. Smith, R.J. Sprafka and G.T. Wilhamson, Phys. Rev. Lett. 24 (1970) 1031. [3] R. Boucher et al., Nucl. Instr. Methods 115 (1974) 235. [4] J.J. Aubert et at., Conf. on lnstrumentaUon for hzgh energy physics, Frascati 1973, p. 410. [5] A. Brandt et al., Nucl. Instr. Methods 126 (1975) 519. [6] Particle Data Group, Phys. Lett. 50B (1974) 11. [7] N. Kwak et al., Phys. Letters 58B (1975) 233. [8] S. Van der Meer, CERN internal report ISR-PO/68-31.