Inclusive π 0 and direct-photon cross sections in the kinematic range 3.5< p T <12 GeV/c with central rapidities (y cm ) are presented for 530 and 800 GeV/c proton beams and a 515 GeV/c π − beam incident on Be targets. Current Next-to-Leading-Order perturbative QCD calculations fail to adequately describe the data for conventional choices of scales. Kinematic distributions from these hard scattering events provide evidence that the interacting partons carry significant initial-state parton transverse momentum (k T ). Incorporating these k T effects phenomenologically greatly improves the agreement between calculations and the measured cross sections.

Measurements of the production of high transverse momentum direct photons by a 515 GeV=c ÿ beam and 530 and 800 GeV=c proton beams in interactions with beryllium and hydrogen targets are presented. The data span the kinematic ranges of 3:5 < p T < 12 GeV=c in transverse momentum and 1.5 units in rapidity. The inclusive direct-photon cross sections are compared with next-to-leading-order perturbative QCD calculations and expectations based on a phenomenological parton-k T model.

We report on x c1 and x c2 production in the Feynman-x range 0.1 , x F , 0.8 in 515 GeV͞c p 2 Be collisions. The x c states are observed via their radiative decays into J͞c's. The resulting photons are detected either as showers in the electromagnetic calorimeter or after conversion in the target as e 1 e 2 pairs in the tracking system. The fraction of J͞c production due to x c1 and x c2 decays is 0.443 6 0.041 6 0.035. The ratio of the x c1 to x c2 cross section is 0.57 6 0.18 6 0.06. Our results on J͞c, c͑2S͒, and x c production indicate that 0.454 6 0.044 6 0.042 of J͞c's are produced directly. [S0031-9007(96)01608-0] PACS numbers: 13.85.Ni, 14.40.Gx, 25.80.Ls Studies of charmonium production in hadron collisions provide important information on both perturbative and nonperturbative QCD. Recent advances in the understanding of quarkonium production have been stimulated by the unexpectedly large cross sections for direct J͞c and c͑2S͒ production at large p T measured at the Fermilab Tevatron [1]. Three types of models have been used to describe charmonium formation [2]: the color-evaporation model [3,4], the color-singlet model [5,6], and the coloroctet model [7,8]. In the color-evaporation model, the directly produced charmonium meson is not constrained to the same J P C state as the cc pair produced in the hard scatter because of the emission of soft gluons during the meson's formation. In the color-singlet model, the charmonium meson retains the quantum numbers of the produced cc pair, and thus each J P C state can only be directly produced via the corresponding hard scattering color-singlet subprocesses. The color-octet mecha-nism extends the color-singlet approach by taking into account the production of cc pairs in a color-octet configuration accompanied by a gluon. The color-octet state evolves into a color-singlet state via emission of a soft gluon. These models of charmonium formation lead to different expectations for the production rates of the charmonium states. This Letter presents data on the production of these states in p 2 nucleon collisions at a significantly higher p s than previously available. The experiment was performed in the Fermilab Meson West beam line using a large-aperture, open-geometry spectrometer [9,10]. A 515 GeV͞c beam was incident on Be and Cu targets. Only p 2 Be interactions, which represent 85% of the dimuon triggers and an integrated luminosity of 7.5 pb 21 per nucleon, are discussed in this Letter. The m 1 m 2 mass distribution is shown in Fig. 1. A fit yields 7750 6 90 6 60 J͞c's with a FWHM mass resolution of 130 MeV͞c 2 [11] and a mean J͞c mass 4294 0031-9007͞96͞77(21)͞4294(4)$10.00

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