Contact Interaction Searches at the Linear Collider : Energy , Luminosity and Positron Polarization Dependencies

It is generally expected that new physics beyond the Standard Model(SM) will manifest itself at future colliders that probe the TeV scale such as the LHC and the Linear Collider(LC). This new physics(NP) may appear either directly, as in the case of new particle production, e.g., SUSY or Kaluza-Klein resonances, or indirectly through deviations from the predictions of the SM. In the case of indirect discovery the effects may be subtle and many different NP scenarios may lead to the same or very similar experimental signatures. Perhaps the most well known example of this indirect scenario in a collider context would be the observation of deviations in, e.g., various ee cross sections due to apparent contact interactions. There are many very different NP scenarios that predict new particle exchanges which can lead to contact interactions below direct production threshold; a partial list of known candidates is: compositeness[1], a Z ′ from an extended electroweak gauge model[2, 3], scalar or vector leptoquarks[2, 4], R-parity violating sneutrino(ν̃) exchange[5], scalar or vector bileptons[6], graviton Kaluza-Klein(KK) towers[7, 8] in extra dimensional models[9, 10], gauge boson KK towers[8, 11], and even string excitations[12]. Of course, there may be many other sources of contact interactions from NP models as yet undiscovered, as was the low-scale gravity scenario only a few years ago. The purpose of this paper is to overview how contact interaction search reaches are influenced by changes in the LC center of mass energy, integrated luminosity and positron polarization[13]. To be specific we will limit our discussion to the processes ee → f̄f and to four of the scenarios listed above: new Z ’s, gauge KK towers in the 5-dimensional version of the SM(5DSM), graviton exchange in the ADD model and compositeness. We will at first consider the following center of mass energies: √ s = 0.5, 0.8, 1.0, 1.2 and 1.5 TeV and luminosities in the range 0.1 ≤ L ≤ 3 ab and then generalize so that we may interpolate among these cases. Assuming an e polarization of 80% we initially consider only two possible polarizations for positrons: P+ = 0, 60% and later generalize to a continuum of values. In calculating errors, statistical uncertainties and those systematics arising from both polarization and luminosity uncertainties, δP/P = 0.003 and δL/L = 0.0025 are employed. Initial state radiation but no beamstrahlung has been included and a symmetric low angle cut θmin = 100 mrad has been imposed. Finite efficiencies for flavor tagging the final state leptons and quarks, f = e, μ, τ, c, b, t, are also included in the calculations. In performing fits we employ the following observables: the unpolarized total cross sections, σf , the unpolarized angular distributions, 1/σf dσf/d cos θ, the left-right polarization asymmetries, AfLR(cos θ) and the polarization of taus in the final state, Pτ , including the effects of a finite efficiency. Comparisons between the predictions of the new physics models to those of the SM are determined by the χ of the fit which is controlled by a single parameter, a mass scale, in each case. The resulting 95% CL bounds we obtain are consistent with those found in earlier analyses[13]. However here it is not so much the bounds themselves that we are interested in but their variation as we change the values of √ s, L and P+. For impatient readers the punchline of this analysis can be found in Section V and specifically in Figs. 8 and 9. Sections II-IV contain the justification for these later results and conclusions.