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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 function 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 predictions of the quark fragmentation model of Feynman and Field.

The study o f inclusive resonance production in hadron and charged lepton collisions has been the subject o f 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 leptoproduct ion, 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 o f 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 o f 2289 charged-current 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 momentum of ~ 1 GeV/c. We have checked that the mis- assignment o f protons as pions does not produce any peak at the p0 mass.

The production o f 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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of a number o f 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 distr ibutions. The resolut ion in the 7r+n - effective mass is typical ly bet ter than +20 MeV. A clear O 0 sig- nal is evident in the neutral combina t ion of fig. l a , whereas the doubly charged mass spectrum of fig. l b shows no structure and is well fit by a quadrat ic form over the range 0 . 5 - 1 . 2 GeV. To determine the amoun t of inclusive p0 product ion , we have fit the n+n - mass spectrum to an incoherent sum of a quadratic background and a P-wave Bre i t -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 hadronic 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 specific 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-

4 0 0

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 o f 518 -+ 80 p0's wi th a ×2/DF of 1.2. This n u m b e r corre- sponds to (0.21 -+ 0.03) p° /event or to (0.12 -+ 0.02) pO/7 r - *3, where 7r- in the denomina to r 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 interact ions [1 ], suggesting a certain universali ty o f hadronic product ion *4

The W and Q2 dependences of the pO produc t ion have been studied. Tables 1 and 2 list the product ion rates for three ranges o f these variables. The first line of the tables gives the number o f p0,s per event, whereas the second line gives p ° / n - averaged over all events wi thin the W 2 or Q2 selections. As a funct ion of Q2,

bo th p° /event and pO/rr- are constant wi th in errors. A similar result is found for inelastic m u o n - n u c l e o n scattering [2]. In contrast , the p0 product ion rate increases with W, al though the pO/n- ratio is consistent wi th 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 mul t ip l ic i ty is essentially independent o f Q2, bu t increases with W.

In hadronic experiments , it has been found that the p0 transverse m o m e n t u m dis t r ibut ion is flatter than that o f pions. Table 3 lists the results o f f i t t ing 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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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 determine the shape of the background, the results are consistent 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 product ion as a function o f z . 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 product ion as a function of z o f 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 produced 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- t ion region) more often than pions which do not origi- nate from p0 decay.

Finally, we have looked at the c.m. rapidity distri- but ion 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 kinematic 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. I f we assume that the central contr ibution 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. In this comparison, we used the W value of each event and generated jets in the center-of-mass system according to the model 's prescription. We also assumed that the (ud) diquark system fragments in the same fashion as the single d quark * s. Baryons are not included in the fragmenta-

• s The overall features of single-quark and diquark jets are remarkably similar. The different quantum numbers of a (ud) quark pair from a single d quark will also effect the nature of leading particles. We have no quantitative prescription for these effects [6].

z < 0 . 2 0 . 2 < z < 0 . 4 0 . 4 < z < 0 . 6 0 . 6 < z < 0 . 8

pO/event 0.04 -+ 0.01 0.08 +- 0.02 0.04 -+ 0.02 0.05 -+ 0.03 pO/rr- 0.04 + 0.01 0.19 -+ 0.04 0.24 + 0.11 0.70 +- 0.40

Table 5 p0 production rates versus YR.

-1.0 < YR < -0.5 -0.5 < YR < 0.0 0.0 < YR < 0.5 0.5 < YR < 1.0

pO/event 0.02 -+ 0.01 0.04 + 0.02 0.05 + 0.02 0.11 + 0.02 p°br- 0.16 +- 0.06 0.12 -+ 0.05 0.13 -+ 0.04 0.31 -+ 0.04

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tion process so that the model is not expected to repro- duce quantitatively the data in the target fragmentation region. In the model, the mesons are produced using a recursive procedure. The first meson results from com- bining the scattered quark with an antiquark (?:11) from the pair ql?:ll . The next particle is a combination of ql and ~12, where c12 comes from a second q~l pair. The model 's prescription is to keep only those mesons which are produced in the same hemisphere as that of the scattered quark. The model, which assumes equal production o f pseudoscalar and vector mesons, predicts 480 p0,s in good agreement with our results of 518 + 12. As a function o f W, we find that (2 + 2)%, (26 -+ 6)%, and (72 + 12)% of the p° ' s are produced in the three W ranges of table 1. The model predicts values of 17%, 27%, and 56%. Thus, except for the lowest W range, where nucleon resonance production is known to be large, the model and data are consistent with one an- other. The model is also in good agreement with the observed Q2 dependence.

Field and Feynman also use a PT distribution o f primary mesons that is gaussian with (pT) = 0.439 GeV. This compares quite well with our result o f (pT)po = 0.423 -+ 0.080 GeV.

Since we assume that the diquark fragments in the same way as a single quark, the model predicts an equal number of p0 mesons for YR > 0 and YR < 0. However, baryon production is an important , but in the model neglected channel for YR < 0 and since the average mult ipl ici ty is low in our experiment, the inclu- sion of baryons will result in fewer p0 being produced for YR < 0 than for YR > 0. Of the 518 p0's observed, 380 -+ 60 are produced in the current direction YR > 0, whereas the model predicts 245 p0 mesons in this region. The main disagreement occurs for 0.5 < YR < 1.0 where we find (69 -+ 12)% of the forward p0 's as com- pared to 31% predicted by the model.

In discussing the z-dependence, we restrict the comparison to the region z > 0.2, as the effects from the baryon are expected to be less in this region. Normal- izing to the data with z > 0.2, the percentages o f p° ' s in the ranges 0.2 < z < 0.4, 0.4 < z < 0.6, 0.6 < z < 0.8 are, respectively, (47 + 12)%, (24 -+ 12)%, and (29 -+ 18)% for the data sample, while the model gives 48%, 32%, and 19%. In terms ofpO/n -, the values are 0.19 + 0.04, 0.24 -+ 0.11, and 0.70 -+ 0.40 for the data and 0.18, 0.32, and 0.40 for the model. Thus the shape o f the z distribution agrees with the model within errors.

This experiment was made possible by the support of the Neutrino Department at Fermilab and by the operating crew of the 15 ft bubble chamber. We also wish to thank our scanners for their effort. The research was supported by the US Department o f Energy.

References

[1 ] Results on inclusive 0 o production in hadronic experiments can be found in: K. B6ckman, Inclusive vector meson production and hadron structure, talk at the Symposium on Hadron structure and multiparticle production (Kazimierz, Poland, 1977); P.D. Higgins et al., Phys. Rev. D19 (1979) 65, R. Raja et al., Phys. Rev. D16 (1977) 2733.

[2] For results on inclusive p0 in muon production, see: C. del Papa et al., Phys. Rev. Lett. 40 (1978) 90.

[3] R.D. Field and R.P. Feynman, Nucl. Phys. B136 (1978) 1. [4] M. Derrick et al., Phys. Rev. D17 (1978) 1. [5] S.J. Barish et al., Phys. Rev. D18 (1978) 2205;

E. Fernandez, A. Garf'mkel and A.T. Laasanen, Purdue Univ. Report PU79-480, unpublished.

[6] M. Derrick et al., Phys. Lett. 88B (1979) 177; R.D. Field, private communication.

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