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Volume 91B, number 2 PHYSICS LETTERS 7 April 1980

INCLUSIVE ,o 0 PRODUCTION IN Pp CHARGED-CURRENT INTERACTIONS

M. DERRICK, P. GREGORY ~, F. LOPINTO, B. MUSGRAVE, J. SCHLERETH, P. SCHREINER and R. SINGER Argonne National Laboratory, Argonne, IL 60439, USA

S.J. BARISH, R. BROCK, A. ENGLER, T. KIKUCHI, R.W. KRAEMER, F. MESSING, B.J. STACEY and M. TABAK Carnegie-Mellon University, Pittsburgh, PA 15213, USA

and

V.E. BARNES, D.D. CARMONY, E. FERNANDEZ :, A.F. GARFINKEL and A.T. LAASANEN Purdue University, W. Lafayette, 1N47907, USA

Received 23 November 1979 Revised manuscript received 7 February 1980

Using data from the Fermilab 15 ft hydrogen bubble chamber, we have studied inclusive po production in antineutrino- proton charged-current interactions. We measure (0.21 +- 0.03) p/event, corresponding to pO/n- = 0.12 +- 0.02. As a func- tion of Q2 and for hadronic masses above a threshold region, the pO/~r- ratio shows little variation. At least 50% of the o's are consistent with coming from the current fragmentation region. The results agree reasonably well with the pre- dictions of the quark fragmentation model of Feynman and Field.

The study of inclusive resonance production in hadron and charged lepton collisions has been the subject of much work in the last few years [1,2]. These studies may be used to determine the fraction of the final state pions produced directly as opposed to those coming from the decay of resonances. In deep inelastic leptoproduction, the model of Field and Feynman [3] has been quite successful in ex- plaining the gross features of the hadronic system. This model incorporates a specific algorithm for the fragmentation of a quark into the observed final state particles. One basic, and so far untested, ingre- dient in this model is the amount and characteristics of resonance production. In this paper, we report the first measurements of inclusive pO production in high energy 9p interactions.

1 Present address: CERN, EP Division, Geneva 23, Switzerland. 2 Present address: Argonne National Laboratory, Argonne,

IL 60439, USA.

The data were obtained from exposures of the Fermilab 15 ft hydrogen bubble chamber to a wide- band antineutrino beam [4], with almost all of the data coming from a run using a 400 GeV incident proton beam. The ~ charged-current events are sepa- rated from the neutral-current and neutrino-induced background events using a kinematic method [5]. The final data sample consists of 2289 charged-cur- rent antineutrino events selected with E~ > 5 GeV ,1

All particles are assigned a pion mass unless other- wise identified by a physicist editing of the f'flm. Pro- tons can be identified by ionization below a momen- tum of ~ 1 GeV/c. We have checked that the mis- assignment of protons as pions does not produce any peak at the p0 mass.

The production of p0 can be studied as a function

:~1 One-prong events were not included in the analysis. Events for which y = (E u - Elz)/E u is greater than 0.8 were also rejected.

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Volume 91B, number 2 PHYSICS LETTERS 7 April 1980

of a number of variables +2. For our inclusive sample of the data, the mean values are (E~) = 31 GeV, (W) = 3.4 GeV, and (Q2) = 2.7 (GeV/c) 2.

Fig. 1 shows the inclusive n+n - and lr +- 7r mass distributions. The resolution in the 7r+n - effective mass is typically better than +20 MeV. A clear O 0 sig- nal is evident in the neutral combinat ion of fig. la, whereas the doubly charged mass spectrum of fig. lb shows no structure and is well fit by a quadratic form over the range 0 .5 -1 .2 GeV. To determine the amount of inclusive p0 production, we have fit the n+n - mass spectrum to an incoherent sum of a quadratic back- ground and a P-wave Breit-Wigner formula allowing

*2 The variables we consider are: (1) Q2, the four-momentum transfer between the ~ and ta +, (2) W, the mass of the ha- dronic system, (3) PT, the transverse momentum with re- spect to the momentum transfer direction, (4) z = Eh/(Ef~ - Ep+), the fraction of the hadronic energy taken by a spe- cific particle or combination of particles, and (5) rapidity,

1 YR = ~ln[(E +PII)/(E - PII)], where P I is a component O f momentum along the momentum transfer direction and E is the particle's energy, both measured in the current-proton center-of-mass system.

(a ) 7r* ~r-

400

300 ~' \~k

200 \ \

~- I00 E)

~- 0 , I , I L I z I ' I ' I o_ 300

- - ZSO

o 200

150

I00

50

, I q [ , I n, ~0.2 0.6 I.O 1.4

M~rT (GeV)

Fig. 1. Inclusive (a) 7r%r- and (b) 7r-+lr mass distributions. The solid curve is the result of the fit to an incoherent sum of a quadratic background and a P-wave Breit-Wigner formula for the pO. The dashed curve is the background given by the fit.

the mass and width of the pO to vary. The fit, repre- sented by the curves in fig. la, gives a total of 518 -+ 80 p0's with a 2/DF of 1.2. This number corre- sponds to (0.21 -+ 0.03) p/event or to (0.12 -+ 0.02) pO/7 r- *3, where 7r- in the denominator refers to all 7r- including those from the pO decay. This pO/rr- ratio is the same as that found in medium-to-high- energy pp and ~p interactions [1 ], suggesting a certain universality of hadronic production *4

The W and Q2 dependences of the pO production have been studied. Tables 1 and 2 list the production rates for three ranges of these variables. The first line of the tables gives the number of p0,s per event, where- as the second line gives p /n - averaged over all events within the W 2 or Q2 selections. As a function of Q2, both p/event and pO/rr- are constant within errors. A similar result is found for inelastic muon-nuc leon scattering [2]. In contrast, the p0 production rate increases with W, although the pO/n- ratio is consis- tent with being constant for W > 2.7 GeV and with being zero for W < 2.7 GeV. This behavior is connected with the fact that the charge multipl icity is essentially independent of Q2, but increases with W.

In hadronic experiments, it has been found that the p0 transverse momentum distribution is flatter than that of pions. Table 3 lists the results of fitting the

+3 The fit gives a pO mass and width (0.755 0.005) GeV and (0.150 +-- 0.010) GeV, respectively. In all other fits discussed in this paper, the po mass and width were fixed at these values.

*4 For n-+p collisions in the energy range 10-50 GeV, the ratio pOhr- is about 0.20, while for energies above 100 GeV, the ratio falls.

Table 1 pO production rates versus W.

W (GeV) < 2.7 2.7-3.7 > 3.7

p/event 0.02 -+ 0.02 0.17 0.04 0.37 -+ 0.06 p/Tr- 0.02 0.02 0.12 0.03 0.17 +- 0.03

Table 2 pO production rates versus Q2.

Q2 (GeV/c)2

< 1.5 1.5-4.5 > 4.5

pO/event 0.21 0.07 0.17 + 0.06 0.21 0.06 pOfir- 0.13 + 0.04 0.11 + 0.04 0.13 + 0.04

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Volume 91B, number 2 PHYSICS LETTERS 7 April 1980

Table 3 po production rates versus PT.

PT (GeV/c)

< 0.25 0.25-0.5 0.5-0.75

p/event 0.07 -+ 0.02 0.06 :t 0.02 0.06 -+ 0.02 p/n- 0.09 -+ 0.03 0.09 -+ 0.03 0.33 -+ 0.09

7r+Tr - mass distribution for various ranges of p~+~r-. For PT > 0.75 GeV/c, where it is difficult to deter- mine the shape of the background, the results are con- sistent with pO/rr- = 0.5. Thus we also see that there is a trend for the ratio pO/zr- to increase with PT"

Table 4 gives results on p0 production as a function ofz . We note the increase in pO/rc- with z, an effect also observed in a muon scattering experiment [2]. However, it is found that zr+Tr - pairs from side bands around the p0 region have a z distribution similar in shape to that of the pO's. This implies that the increase of pO/Tr- with z is probably kinematic rather than dy- namic in origin.

It is also interesting to study p0 production as a function of z of the decay pions. We have thus analyzed the 7r+n - mass spectrum for three selections on z,r and zTr-: (1) zTr < 0.2 and z~r- < 0.2, (2) zTr+ > 0.2 and z~r- > 0.2, and (3) Z~r+ < 0.2 and zTr- > 0.2 or z~r+ > 0.2 and zn- < 0.2. We find that (30 -+ 7)% of the p's are pro- duced when both pions have z < 0.2, while for cases (2) and (3), the percentages are (15 +- 7)% and (55 -+ 8)%, respectively. For rr+rr - pairs in side bands around the p0 region, the equivalent numbers are (52 -+ 1)%, (12 -+ 1)%, and (36 + 1)%. Thus pions from p0 decay tend

Table 4 pO production rates versus z.

to have larger z values (occur in the current fragmenta- tion region) more often than pions which do not origi- nate from p0 decay.

Finally, we have looked at the c.m. rapidity distri- bution of the p0 mesons and find that essentially all of the p0's are produced in the range -1 .0 < YR < 1.0, with 70% having YR > 0.0. From table 5, we see that over the range -1 .0 < YR < 0.5, the p0/event ratio is constant or increases slowly as YR increases, while the ratio pO/n- is constant. Then in the range 0.5 < YR < 1.0, the rate suddenly increases. This is not a kine- matic effect as the ratio of all n+zr - pairs to 7r- peaks near YR -- 0 and, in addition , only (22 -+ 1)% of all zr+Tr - pairs are in the region 0.5 < YR < 1.0 as com- pared to (52 +- 10)% of the p0's. Thus there seem to be two contributions to pO production: a central part which cannot be unequivocally associated with either current or target fragmentation and a part resulting from current fragmentation. If we assume that the cen- tral contribution is symmetric about YR = 0, then we estimate that at least 50% of the produced p0,s result from current fragmentation.

We now compare our results with the predictions of the Field and Feynman model.