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Volume 110B, number 3,4 PHYSICS LETTERS 1 April 1982

~b MESON INCLUSIVE PRODUCTION BY 70 GeV/c PROTONS

Yu.M. ANTIPOV, V.A. BESSUBOV, N.P. BUDANOV, S.P. DENISOV, Yu.P. GORIN,

V.G. KARTASHEVA, S.V. KLIMENKO, A.A. LEBEDEV, A.I. PETRUKHIN,

S.A. POLOVNIKOV, V.N. ROINISHVILI ~, D.A. STOYANOVA and F.A. YOTCH Institute for High Energy Physics, Serpukhov, USSR

Received 14 September 1981

Results of the experimental study of inclusive ~ meson production off berillium nuclei by 70 GeV/c protons in the beam fragmentation region x > 0.41 are presented. Fitting of the measured differential cross sections to the forms do/dx ~ (1 - x)n/x and do/dp~ ~ exp( -bp~) gives the parameters n = 6.0 • 1.7 and b = (2.4 + 0.7) (GeV/c) -2. In the framework of a quark -pa r ton model the measured ~ meson x-distribution suggests that in the region of x > 0.41 nearly 90% of the 0 mesons are produced via a strange sea quark fusion. The ratio of the OZI-forbidden and OZI- allowed transition coupling constants has been found to be ten times smaller than the same ratio calculated from the partial decay widths. The total inclusive cross section is estimated to be o(pN --, ~X) = (0.7_+ 0.2) × 10-27 cm 2.

To continue our study of pure neutral vector- meson inclusive production with the "Sigma" spec- trometer [1-3] we have investigated q5 meson production in the reaction

p + Be -~ ~b + anything

L p+p- at 70 GeV/c beam momentum. In this experiment the ¢'s were produced in the kinematical region x = 2p[ /x /~ >~ 0.4 and pt 2 < 1.0 (GeV/c) 2. The sensitivity of the exposure was 18.4 × 10 -33 cm2/ weighted event/Be nucleus.

The detected dimuon events have been weighted for geometric acceptance. To calculate it the 4)decay angular distribution was assumed to be isotropic. If instead we used (1 + cos 0~) distributions, then all the cross sections would change by -+20%, how- ever, the changes in the shapes of the differential spectra would be negligible.

Fig. 1 shows the weighted p + p - mass spectrum for all the events. A clear ~ signal is seen with the statistical significance of more than 5 standard devia-

: Institute of Physics, Tbilisi, USSR.

tions. The smooth curve in fig. 1 represents the fit with a gaussian of 70 + 4 MeV FWHM and central value 1021 +- 7 MeV for the ~b meson together with

500C

m 4000

U3

Z ~J > 3000

W

2000

,7,

1000

p . B e ~ u.'ja- • X 70 GeV/e

XF > 0.43

J

600

900 1000 1100

M~, MeV

500 700 900 1100 1300

DIMUON MASS (MeV)

Fig. 1. Weighted dimuon mass spectrum,

326 0 031-9163/82/0000-0000/$02.75 © 1982 Noah-Holland


10

(D

. . . . 1 . . . . F '

p ÷ B e - - ¢ . X

0.5 1.0 p2,(GeV/c) 2

Fig. 2. The ~ meson p~ distribution.

an incoherent sum of the sophist icated B r e i t -

Wigner formulae for the p0 + co peak (for details see ref. [3]) and a second-order po lynomia l for a nonresonan t background. The measured ¢ meson produc t ion inclusive cross section for x > 0.41 and pt 2 < 1 (GeV/c) 2 was found to be (15 -+ 3) X 10 - 3 0

cm2/Be nucleus where the error is statistical only. To determine the differential d is t r ibut ion of

meson with respect to pt 2 or x variable, the data were first divided in to suitable bins in p2 or x, and

the ¢ signals were then obta ined by fi t t ing the corre- sponding/2+/2 - mass spectra. The posi t ion and the apparent width of the ¢ peak were kept fixed during these fits. The procedure used makes the background subtract ion from a neighbour ing mass region pleonastic.

The p2 dis t r ibut ion of 4~ mesons is presented in t

fig. 2 and also in table 1. I t is f i t ted by the expo- nent ia l func t ion e x p ( - b p 2) with b = (2.4 + 0.7) (GeV/c) - 2 . This value of b is compatible with those from other exper iments on ¢ meson p roduc t ion by non-strange hadrons [4,5].

Table 1 Differential cross sections for the reaction pBe ~ 4~X at 70 GeV/c.

p~-interval do/dp~ x-interval do/dx (GeV/c) 2 izb(GeV/e)2/ ~ub/Be nucleus

Be nucleus

0.00-0.05 43 _+ 22 0.41-0.43 179 -+ 89 0,00-0.08 27 -+ 14 0.43-0.45 166 -+ 79 0.05-0.15 24 -+ 14 0.45-0.47 156 -+ 58 0,08-0.15 38 ,+ 16 0.47-0.50 71 -+ 40 0.15-0.25 26 ,+ 13 0.50-0.52 72 ,+ 41 0.25-0.35 25 ,+ 11 0.50-0.55 60 ,+ 25 0,35-0.45 16 ,+ 12 0.52-0.55 45 -+ 26 0.45-0.55 15 -+ 9 0.52-0.60 26 ,+ 19 0.55-0.75 7.7 -+ 4.6 0.55-0.60 15 ,+ 12 0.75-1.00 4.3 _+ 3.5 0.60-0.80 4.4 ,+ 3.7

0.60-1.00 2.1 ,+ 1.7 0.80-1.00 1.4 ,+ 1.5

The differential cross section do/dx for ¢ produc- t ion is presented in fig. 3 and also in table 1. Fi t t ing this do/dx dis t r ibut ion to the forms (1 - x)n/x and (1 - x) n' we obta ined the values o f n = 6.0 -+ 1.7 (the solid curve in fig. 3) and n ' = 7.0 -+ 1.8 for x > 0.41. These values of the exponen t are comparable

p* B e ~ ¢ . X 70 GeV]c p2 < 0.75 (GeV/c )2

¢,

c

~3c :I,

x7 F ' x . \

015 0!6 0!7

Fig. 3, The C-meson x = 2PL/x/s distribution. Smooth curves are different kinds of fits described in the text.

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to those of the experiment [4] at 150 GeV/c (n = 4.1 -+ 0.4) but there is a contradiction with the mea- surements from ref. [6] at ~/s-= 63 GeV (the dash- ed curve in fig. 3 corresponds to their n = 2.4 --- 0.9 and is normalized to our number of events).

To understand the mechanism of the ~ meson production in the non-strange hadron collisions we used the q u a r k - p a r t o n model to fit the do/dx distributions. According to ref. [7] for small p2 one has

x R da/dx ~ F l (X ) + ~F2(x ) ,

whe re:

x R = (x 2 + 4M2/s)l/2, Fl (X ) = 2X2SlS2 ,

F2(X ) = u 1 s2 + SlU 2 + l ( d l S 2 + Sld2) + 2SlS 2 ,

Ul, u 2, d l , d 2 are the valence quark distributions, Sl, s2, the sea quark distributions, X s = 0.43, the strange sea suppression factor. The term Fl (X ) corresponds to the OZI-allowed strange sea quark fusion, F2(x ) describes the contr ibution of the OZI-forbidden non-strange valence and sea quark annihilation, the free parameter a = g2/g2 represents the coupling con-

stant ratio. Using the quark distribution parametrization from

ref. [8] and our de /dx data we estimated a<~ 10 -3 , which should be compared with a "~ 0.02, calculated from the ~b -+ 37r and the w -+ 37r partial width ratio [9] (the da shed -do t t ed curve in fig. 3 corresponds to a = 0.02). Thus the Zweig-suppression factor in the ~ product ion is an order of magnitude lower than that in its decay.

After integration of F 1 (x) and F2(x ) over x for x > 0.41 we have found that only 10% of the ~ mesons in this region o f x are produced via the OZI- forbidden process. This result is in qualitative agree- ment with the experiment [10] where the associated

production of strange particles and q~ mesons have been observed. Different kinematical regions in our experiment and in ref. [10] do not allow further comparison.

To obtain the integrated cross section per nucleon and for all x we used the q u a r k - p a t t o n model x- distribution with a = 0 and assumed the A-dependence of the cross section in the form a A ~ A 0.7, as was measured by us for K - A interactions [2]. The result is o(pN -+ ~bX) = (0.7 -+ 0.2) × 10 -27 cm 2 with an additional systematic error of 30%.

To conclude, the above results provide further information on the ~b production mechanism. In par- ticular, the ratio of the OZl-forbidden and OZI- allowed transition coupling constants has been found to be ten times smaller than the same ratio calculated from the partial decay widths.

We thank V.V. Ezhela, A.K. Likhoded, V.F. Obrastsov and S.R. Slabospitsky for useful discus- sions.

References

[1] Yu.M. Antipov et al., Phys. Lett. 60B (1976) 309; 72B (1977) 278; 76B (1978) 235; preprint IHEP 80-97 (Serpukhov, 1980).

[2] Yu.M. Antipov et al., Yad. Fiz. 28 (1978) 1299. [3] Yu.M. Antipov et al., Yad. Fiz. 32 (1980) 676. [4] K.J. Anderson et al., Phys. Rev. Lett. 37 (1976) 799. [5] B. Ghidini et al., Phys. Lett. 68B (1977) 186. [6] M.M. Block et al., preprint CERN/EP 79-82 (Geneva,

1979). [7] V.V. Kniazev et al., preprint IHEP 77-106 (Serpukhov,

.1977). [8] R.Mc Elhaney and S.F. Tuan, Phys. Rev. D8 (1973)

2267. [9] R.P. Feynman, Photon-hadron interactions (Benjamin,

New York, 1972). [10] C. Daum et aL, Phys. Lett. 98B (1981) 313.

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