Dijet production as a centrality trigger forppcollisions at CERN LHC

Abstract

We demonstrate that a trigger on hard dijet production at small rapidities allows us to establish a quantitative distinction between central and peripheral collisions in $\overline{p}p$ and pp collisions at Tevatron and LHC energies. Such a trigger strongly reduces the effective impact parameters as compared to minimum bias events. This happens because the transverse spatial distribution of hard partons $(x\ensuremath{\gtrsim}{10}^{\ensuremath{-}2})$ in the proton is considerably narrower than that of soft partons, whose collisions dominate the total cross section. In the central collisions selected by the trigger, most of the partons with $x\ensuremath{\gtrsim}{10}^{\ensuremath{-}2}$ interact with a gluon field whose strength rapidly increases with energy. At LHC (and to some extent already at Tevatron) energies the strength of this interaction approaches the unitarity (``blackbody'') limit. This leads to specific modifications of the final state, such as a higher probability of multijet events at small rapidities, a strong increase of the transverse momenta and depletion of the longitudinal momenta at large rapidities, and the appearance of long-range correlations in rapidity between the forward or backward fragmentation regions. The same pattern is expected for events with production of new heavy particles (Higgs boson, SUSY). Studies of these phenomena would be feasible with the CMS-TOTEM detector setup, and would have considerable impact on the exploration of the physics of strong gluon fields in QCD, as well as the search for new particles at LHC.