Charged-particle multiplicities associated with large-transverse-momentum photons

We have measured the charged-particle multiplicities of events containing a large-transverse-momentum (p_⊥) photon produced near center-of-mass polar angles (θ) of 90^∘{}, 17.5^∘{}, and 8^∘{} in proton-proton collisions. The data were obtained at the CERN Intersecting Storage Rings at center-of-mass energies of 23, 31, 45, 53, and 62 GeV and for the transverse-momentum range 0≤p_⊥≤4.5 GeV/c. When the photons are detected near θ=90^∘, the associated multiplicity in the hemisphere away from the observed photon is found to increase with increasing photon transverse momentum. In the hemisphere containing the photon, the associated multiplicity decreases at the lowest energy and is approximately constant at the highest energies. The data at θ=17.5^∘ and 8^∘ are similar, with the exception that the particle multiplicities near the detected photon are found to decrease with increasing photon p_⊥ at all energies. No strong back-to-back structure is observed in the data; however, as the photon emission angle is changed, some shift of the multiplicity distributions in the direction opposite the photon is observed. The data at all three polar angles show a broad peak in the azimuthal angular distributions at φ=180^∘ (opposite the detected photon). Finally, the data are compared to a simplified picture for particle production in high-p_⊥ events, and to the various classes of high-p_⊥ models that have been proposed.