Forward Jet Production in DIS

The F₂ measurements fail to distinguish between DGLAP and the BFKL approach to the QCD evolution. The hadronic final states are expected to give additional information. For events at low x, hadron production between the current jet and the proton remnant is expected to be sensitive to the effects of BFKL or DGLAP dynamics. A possible signature of BFKL dynamics is the behaviour of DIS events at low x which contain a jet that has a transverse momentum p_T²(j) $\approx$ Q² (so minimizing the phase space available for DGLAP evolution) and has longitudinal momentum fraction (of the proton) x_jet that is large (in order to maximise the phase space for BFKL evolution), fig 2.

In Fig. 7, recent data from H1 [13] and ZEUS [14] are compared with BFKL predictions [16] and fixed order QCD predictions as calculated with the MEPJET [15] program at NLO. The conditions p_T(j) $\simeq$ Q and x_jet $\gg$ x are satisfied in the two experiments by slightly different selection cuts. H1 selects events with a forward jet of p_T(j) > 3.5 GeV (in the angular region 7^o < $\theta$(j) < 20^o) with

$$\simeq$$ (2)

while ZEUS triggers on somewhat harder jets of E_T(j) > 5 GeV and $\eta$(j) < 2.6 with

0.5 < p_T(j)²/Q²   <  2  ,                x_jet = p_z(j)/E_proton > 0.036  . (3)

Fig. 7 shows that both experiments observe a forward jet cross section which rises steeply with decreasing x with substantially more forward jet events than expected from NLO QCD (labelled as Born in fig. 7a.) A BFKL calculation (the stars) gives a better agreement with the data. The overall normalisation in this calculation is uncertain and the agreement may be fortuitous. Indeed, it should also be noted that both experiments observe more centrally produced dijet events than predicted by the NLO QCD calculations. The ARIADNE Monte Carlo (CDM) model describes the steeply increasing jet cross section with decreasing x. The ARIADNE model does not have a strong ordering in transverse momentum in the QCD cascade, akin to BFKL type dynamics, although it does not make explicit use of the BFKL equation. Whilst those models that adhere to the DGLAP formalism (LEPTO and HERWIG) fail to predict this large growth. Further careful investigation is necessary before claiming that BFKL is the mechanism for this enhanced forward jet production.