φ meson inclusive production by 70 GeVc protons

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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-parton 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 pro- duction 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)de- cay 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 . Be~ 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

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

    10

    (D

    . . . . 1 . . . . F '

    pBe- - .X

    0.5 1.0 p2,(GeV/c) 2

    Fig. 2. The ~ meson p~ distribution.

    an incoherent sum of the sophisticated Bre i t - Wigner formulae for the p0 + co peak (for details see ref. [3]) and a second-order polynomial for a nonresonant background. The measured meson production inclusive cross section for x > 0.41 and pt 2 < 1 (GeV/c) 2 was found to be (15 -+ 3) X 10 -30 cm2/Be nucleus where the error is statistical only.

    To determine the differential distribution of meson with respect to pt 2 or x variable, the data were first divided into suitable bins in p2 or x, and the signals were then obtained by fitting the corre- sponding/2+/2 - mass spectra. The position and the apparent width of the peak were kept fixed during these fits. The procedure used makes the back- ground subtraction from a neighbouring mass region pleonastic.

    The p2 distribution of 4~ mesons is presented in t

    fig. 2 and also in table 1. It is fitted by the expo- nential funct ion exp( -bp 2) with b = (2.4 + 0.7) (GeV/c) -2 . This value of b is compatible with those from other experiments on meson product 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- tion is presented in fig. 3 and also in table 1. Fitt ing this do/dx distribution to the forms (1 - x)n/x and (1 - x) n' we obtained the values o fn = 6.0 -+ 1.7 (the solid curve in fig. 3) and n' = 7.0 -+ 1.8 for x > 0.41. These values of the exponent are comparable

    p* Be~.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.

    327

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

    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 quark-parton model to fit the do/dx dis- tributions. 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 lS 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 ) cor- responds to the OZI-allowed strange sea quark fusion, F2(x ) describes the contribution 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