Minijet and transverse energy production in the BFKL regime

Abstract

Minijet production in hadronic and nuclear collisions through a BFKL pomeron ladder is studied for the energies of the future LHC heavy-ion collisions. We use unintegrated gluon densities compatible with the small-x increase of parton distributions observed at HERA. We show that at LHC energies the BFKL minijet and transverse energy production is at most of the same order of magnitude as that in the collinear factorization approach. CERN-TH/96-124 June 19961 On leave of absence from Laboratory of High Energy Physics, Department of Physics, University of Helsinki, Finland 2 Alexander von Humboldt Fellow, on leave from Lebedev Physics Institute, 117924 Leninski pr. 53, Moscow, Russia Semihard parton scatterings with transverse momenta p T ∼ few GeV have been suggested to explain the rapid growth of the inelastic and total cross sections in high energy pp(p) collisions at √ s > 20 GeV [1,2]. Especially in heavy-ion collisions at very high energies the semihard processes are expected to be abundant and dominate the transverse energy production in the central rapidity region [3]. Event generators emphasizing the importance of semihard processes in ultrarelativistic heavy-ion collisions at √ s ≥ 200 AGeV have also been actively developed during recent years [4,5].Perturbative processes take place at the very early stages of the time evolution of an ultrarelativistic heavy-ion collision [6]. In the central rapidity region, these semihard processes occur at time scales τ ∼ 1/p T , i.e. within the first fractions of fm/c, while the particle production from soft processes is expected to take longer, ∼ 1 fm/c. Therefore, semihard particle production and associated transverse energy production give initial conditions for further space-time evolution of the formed partonic system, which eventually may lead to a thermalized quark-gluon plasma if the produced system is dense enough. The key feature of the semihard parton production is that it can be computed by using perturbative QCD [3,6].At central rapidities the semihard scatterings with momentum exchanges p T ∼ 2 GeV probe typically parton distributions at x ∼ 2p T / √ s. Especially in nuclear collisions at the CERN Large Hadron Collider (LHC) with √ s = 5.5 TeV, these x-values fall dominantly in the region of a rapid increase of the structure function F p 2 (x, Q 2 ) as observed in deep inelastic ep collisions at HERA [7]. This increase persists down to Q 2 = 1.5 GeV 2 [8], thus strongly enhancing production of semihard partons as discussed in [9]. Nuclear effects in the parton distributions, in particular the observed nuclear shadowing of F A 2 (x, Q 2 ) at small x [10], are expected to be important, although the nuclear gluon distributions are not well known at the moment [11,12].In the above-mentioned studies, the production mechanism of semihard partons, minijets, is based on multiple independent 2 → 2 scatterings of partons. Collinear factorization is assumed to hold, enabling separation of the perturbatively calculable hard processes from the parton distribu...

Inclusive production ofJ/ψmeson in proton-proton collisions at BNL RHIC

“…Here in the practical applications we shall use a simple parametrization [27] for the numerical solution [28] and use acronym BFKL. Another distribution closely related to the BFKL approach was constructed by Blümlein [29].…”

Yang-Mills radiation in ultrarelativistic nuclear collisions

“…[5] via (66). This implies that the results quoted for the BFKL contribution to minijets in [5] taking K N = 1 are approximately a factor 5 too small. With the value in eq.…”

“…Figure 1 shows how the parametrization of the gluon structure compares to preliminary "data" at Q 2 = 7 GeV 2 from HERA [17,18] obtained via a DGLAP analysis of the scaling violations from F 2 (x, Q 2 ). Also shown is the BFKL-like parametrization of the gluon structure used in [5] for comparison. Both the GRV95 and the BFKL parametrizations significantly overestimate the moderate Q 2 data of interest here at x < 10 −3 .…”

“…[3] within the Low-Nussinov approximation. Another quantal distribution, based on the Gribov-Levin-Ryzkin ladder approximation [4], was recently applied by Eskola et al [5]. Finally, there has been considerable recent effort to compute moderate p ⊥ (mini-jet) distributions based on the conventional collinear factorized pQCD approach [6,7,8].…”

New phenomenological model for hadron production in high energy particle collisions

“…In QCD this mechanism is described by the BFKL Pomeron exchange [3]. The radiated partons from this Pomeron have presumably a typical for the pQCD power law spectrum [4]. Schematically figure 1 shows these two sources of particles produced in high energy baryonic collisions.…”