Phenomenology of Bulk Scalar Production at the LHC

We examine the sensitivity of the ATLAS detector to extra-dimensional scalars in scenarios having the extra-dimensional Planck scale in the TeV range and n = 2 large extra dimensions. Such scalars appear as partners of the graviton in higher-dimensional supersymmetric theories. Using first the scalar's lowest-dimensional effective couplings to quarks and gluons, we compute the rate of production of a hard jet together with missing energy. We find a nontrivial range of bulk scalar couplings for which ATLAS could observe a signal, and in particular, higher sensitivity to couplings to gluons than to quarks. Bulk scalar emission increases the missing-energy signal by adding to graviton production, and so complicates the inference of the extra-dimensional Planck scale from the observed rate of jet + EmissT . Because bulk scalar differential cross sections resemble those for gravitons, it is unlikely that these can be experimentally distinguished should a missing energy signal be observed. However, given, for example, the Supersyminetric Large Extra Dimension (SLED) scenario, which can provide a framework for a solution to the cosmological constant problem, the D-dimensional Planck scale is approximately fixed and so is the graviton production rate. An excess of events would then be due to graviton superpartners like bulk scalars. Next, we identify the lowest-dimension interaction which is possible between Standard Model brane fields and bulk scalars. The lowest-dimension interaction is unique and involves a trilinear coupling between the Standard Model Higgs and a bulk scalar. We compute its influence on Higgs physics at ATLAS and identify how large a coupling can be detected at the LHC. Besides providing a potentially interesting signal in Higgs searches, such couplings provide a major observational constraint on 6D large-extra-dimensional models with scalars in the bulk. Finally we consider the Universal Extra Dimensions scenario in which all the SM fields propagate. Tree-level KK number conservation dictates that the associated KK excitations cannot be singly produced. We recalculate the cross sections obtained by Macesanu et al. for the direct production of KK excitations of the gluon, g*, and two distinct towers of quarks, q• and q°, in proton-proton collisions at the LHC. According to the SLED scenario, these KK states will quickly decay in a parton plus a graviton. We find that the LHC mass bound for KK quark and gluon final states is 2675 GeV at its nominal luminosity of 100 fb-1.