Virtual - Sparticle Threshold Effects on Large - E T Jet Cross Sections
چکیده
We discuss the one-loop virtual-sparticle corrections to QCD jet cross sections at large ET and large dijet invariant masses, with reference to present Tevatron and future LHC collider experiments. We find characteristic peaks and dips in the sparticle threshold region, due to interferences with tree-level QCD diagrams. Their magnitudes may be several per cent of the parton subprocess cross sections, so they might provide a useful search tool that is complementary to the usual missing-energy signature for supersymmetry. CERN-TH/96-108 SHEP-96-10 April 1996 Recent data from the CDF collaboration on large-ET hadronic jets [1] have stimulated interest in the possibility that jet measurements at hadronic colliders may be sensitive to quantum corrections due to virtual particles, such as those appearing in supersymmetric models [2, 3]. Hadron colliders such as the CERN p̄p collider, the Tevatron or the LHC are usually thought of as exploratory machines, with precision physics left to ee colliders such as LEP or the LHC. However, ee colliders are certainly also discovery machines, and the enormous event rates at present and future hadron colliders may provide precision tests of the Standard Model and its possible extensions. The interpretation of large-ET jet cross sections inherits uncertainties from the non-perturbative parton distribution and fragmentation functions, but effects on the shape of the cross section in the neighbourhood of a new particle threshold may be discernible. It is conceivable that precision jet measurements could provide a useful new way to detect sparticles indirectly. Even if they are being produced copiously, their decays may be difficult to disentangle, e.g., if they are dominated by cascades, or if R parity is violated. Thus, the mass reach in direct searches could well be less than the accessible range of ET , as is now the case at the Tevatron, and could in the future also be the case at the LHC. The one-loop quantum corrections to jet cross sections in the Standard Model have been calculated [8], and studied extensively [9]. First estimates of the one-loop virtual corrections in the minimal supersymmetric extension of the Standard Model have also been presented recently, including the ultraviolet logarithms associated with the slower running of the strong coupling αs above the sparticle threshold [2], and sub-threshold effects [3]. However, a complete calculation which matches these contributions is not available, and the infrared logarithms associated with the separation between final states that do or do not contain sparticles have not been considered. In view of the physics interest mentioned in the previous paragraph, it is desirable to calculate exactly the one-loop sparticle effects from the threshold region on upwards. In this paper, we present the main results of such a calculation, including self-energy, vertex and relevant aspects of “box” diagram contributions to the parton subprocesses q̄q → q̄q, qq → qq, q̄q → gg and qg → qg, which are expected to dominate large-ET cross sections at the Tevatron and LHC. We present results which combine and match the behaviours below and above threshold, displaying them graphically as functions of the subprocess energy and centre-of-mass scattering angle. The numerical results we find are quite small, so that measurements with a statistical accuracy of the order of a percent in bins with widths of the order of 10% would be required to see effects in the ET or dijet mass distribution. A more detailed presentation of our calculations, together with more discussion of their observability based on convolution with sample parton distribution functions, will be presented in a subsequent paper [10]. We work in the context of the Minimal Supersymmetric extension of the Standard Model (MSSM), assuming for simplicity that the squark partners of both the leftand right-handed helicity states q̃L,R of the five lightest quark flavours are degenerate with common mass m. It has been suggested [5] that the apparent discrepancy reported by CDF [1] may be accommodated by these uncertainties, particularly in the gluon distribution: see however [6]. However, the CDF and D0 [4] data need to be reconciled before this issue can be resolved, after which one would know whether to entertain speculations about new physics [7], of which compositeness [1] is not the most conservative.