PERT AND NON-PERT QCD
EW AND SYMMETRY BREAKING
BEYOND THE SM
HEAVY FLAVOUR PHYSICS
NEUTRINO AND NON ACCEL PHYSICS
Valery Khoze, James Stirling
It was realized long ago that the overall structure of particle angular distributions in multi-jet events in hard scattering processes (the event portrait) is governed by the underlying colour dynamics at short distances. A natural idea has been proposed (see e.g. [1,2])to use this colour event portrait as a "partonometer", mapping the basic interaction short-distance processes,for recent detailed studies and references see Ref. and reviews . Colour-related phenomena in multi-jet topologies are intimately connected to QCD coherence effects in multiplicity flows.These were first well established experimentally in the early eighties in e+e- collisions in what has been termed the "string"  or "drag"  effect. The PETRA/PEP and later TRISTAN and LEP data have convincingly demonstrated that the soft wide-angle patricles do not belong to any patricular jet but have emission properties dependent on the overall structure of the jet ensemble. New impressive quantitative tests of QCD collective effects in three-jet events of e+e- annihilation based on the recent results of Refs.[7,8] have been reported by DELPHI and OPAL Collaborations at LEP. Of special interest here is DELPHI measurement of the yield of particles perpendicular to the production plane, which is very sensitive to the colour charge topology of the primary emitters.
At the Tevatron both CDF and DØ have started very successfully to examine the structure of multijet events and have demonstrated that the predicted bright colour interference effects successfully survive the hadronization stage and are clearly visible in the data. Especially distinctive colour coherence effects arise in the case of large ET production of colour singlet objects, for instance, in V+jet events (with V=photon, W or Z), see e.g. [1,2,9]. They may play the same role for hadron colliders as the celebrated string-drag effect in e+e- collisions. The detailed quantitative studies of V+jet production at the Tevatron were performed in Ref.. There are two leading-order subprocesses and each has its own distinctive antenna pattern.
In the most direct way the drag-effect signal is obseved in the recent DØ analysis of W + jet events by comparing the soft particle distributions around the W boson and opposing leading - ET jet in the same event. Radiophysics-like interference effects clearly manifest themselves as an enhancement of soft particle emission around the tagged jet in the event plane relatively to the transverse plane when compared with particle production around the W boson.
This is illustrated in Fig.1 which shows DØ data for the jet/W particle flow ratio as a function of the azimuthal angle ß together with the analytical result of Ref.. The theoretical prediction is in a good quantitative agreement with the data, thus providing a new evidence supporting the universal perturbative picture of multiparticle production in jets.
The prospective applications of the partonometry arise in DIS and photoproduction processes, see for review . For example, in dijet photoproduction the multiplicity flow pattern can provide a new way to distinguish the so-called direct and resolved mechanisms [7,10]. It looks very attractive to use the hadronic antenna pattern as a diagnostic tool to dissect the colour structure of the large-ET jet events at the p(pbar)p colliders, as a way to distinguish between conventional QCD and possible new physics production mechanisms (such as the searches for Higgs and new intermediate weak bosons or manifestations of extra dimensions).
Ref. exemplifies the diagnostic power of hadronic antenna patterns by considering the topology of particle flows corresponding to the intermediate mass Higgs production at hadron colliders. Another topical example concerns the central Higgs production in the events with double rapidity gaps pp(pbar) -> X + rapidity gap + H + rapidity gap + Y, which are caused by the colour-singlet exchanges in the t-channel , for recent discussion and the list of related references, see . Here we focus on the inclusive Higgs production with the gg->H->b bbar signature. For studying the angular distribution of the soft gluon jet relative to the large- ET final state b-jet ,it is convenient to parametrize the separation between them by Delta eta and Delta phi at the "LEGO plot". We are particularly interested in the shape of the radiation pattern as a function of these variables. For illustration we show in Fig.2 the dependence of the radiation patterns for signal RH and for the QCD-background RQCD for central b bbar jets, i.e. rapidity eta = 0.
We see that both patterns are identical close to the beam directions, and close to the directions of the b-quarks. The main difference arises from the radiation between the final-state jets. To study this further we can consider the symmetric point PC located at Delta eta = eta = 0, Delta phi = pi/2. This corresponds to the radiation perpendicular to the plane of the gg -> b bbar scattering. We find that there is approximately 4/3 more radiation between the b-jets for the Higgs production.This is due to the absence of a colour string connecting the b-quarks in the QCD background.The background process does, however, have strings connecting the initial- and final-state partons, and this leads to an enhancement of radiation between the jets in the scattering plane. The shape of the LEGO plot distribution,therefore, proves to provide a powerful discriminator between the signal and background. In  the hadronic radiation patterns were studied also for Higgs production in association with a W boson.
Words by Valery Khoze