Hooman Davoudiasl

Bulk gauge fields in the Randall-Sundrum model

Abstract We explore the consequences of placing the Standard Model gauge fields in the bulk of the recently proposed localized gravity model of Randall and Sundrum. We find that the Kaluza–Klein excitations of these fields are necessarily strongly coupled and we demonstrate that current precision electroweak data constrain the lowest states to lie above ≃23 TeV. Taking the weak scale to be ∼1 TeV, the resulting implications on the model parameters force the bulk curvature, R5, to be larger than the higher dimensional Planck scale, M, violating the consistency of the theory. In turn, to preserve |R5|≲M2, the weak scale must be pushed to ≳100 TeV. Hence we conclude that it is disfavored to place the Standard Model gauge fields in the bulk of this model as it is presently formulated.

Phenomenology of the randall-sundrum gauge hierarchy model

We explore the phenomenology associated with the recently proposed localized gravity model of Randall and Sundrum where gravity propagates in a 5-dimensional non-factorizable geometry and generates the 4-dimensional weak-Planck scale hierarchy by an exponential function of the compactification radius, called a warp factor. The Kaluza-Klein tower of gravitons which emerge in this scenario have strikingly different properties than in the factorizable case with large extra dimensions. We derive the form of the graviton tower interactions with the Standard Model fields and examine their direct production in Drell-Yan and dijet events at the Tevatron and LHC as well as the KK spectrum line-shape at high-energy linear e + e − colliders. In the case where the first KK excitation is observed, we outline the procedure to uniquely determine the parameters of this scenario. We also investigate the effect of KK tower exchanges in contact interaction searches. We find that present experiments can place meaningful constraints on the parameters of this model.

Brane localized curvature for warped gravitons

We study the effects of including brane localized curvature terms in the Randall-Sundrum (RS) model of the hierarchy. This leads to the existence of brane localized kinetic terms for the graviton. Such terms can be induced by brane and bulk quantum effects as well as Higgs-curvature mixing on the brane. We derive the modified spectrum of Kaluza-Klein (KK) gravitons and their couplings to 4-dimensional fields in the presence of these terms. We find that the masses and couplings of the KK gravitons have considerable dependence on the size of the brane localized terms; the weak-scale phenomenology of the model is consequently modified. In particular, the weak-scale spin-2 graviton resonances which generically appear in the RS model may be significantly lighter than previously assumed. However, they may avoid detection as their widths may be too narrow to be observable at colliders. In the contact interaction limit, for a certain range of parameters, the experimental reach for the scale of the theory is independent of the size of the boundary terms.

The g-2 of the Muon in Localized Gravity Models

Abstract The ( g −2) of the muon is well known to be an important model building constraint on theories beyond the Standard Model. In this Letter, we examine the contributions to ( g −2) μ arising in the Randall–Sundrum model of localized gravity for the case where the Standard Model gauge fields and fermions are both in the bulk. Using the current experimental world average measurement for ( g −2) μ , we find that strong constraints can be placed on the mass of the lightest gauge Kaluza–Klein excitation for a narrow part of the allowed range of the assumed universal 5-dimensional fermion mass parameter, ν . However, employing both perturbativity and fine-tuning constraints we find that we can further restrict the allowed range of the parameter ν to only one fourth of its previous size. The scenario with the SM in the RS bulk is thus tightly constrained, being viable for only a small region of the parameter space.

Experimental Probes of the Randall-Sundrum Infinite Extra Dimension

The phenomenological possibilities of the Randall-Sundrum noncompact extra dimension scenario with the AdS horizon increased to approximately a millimeter length, corresponding to an effective brane tension of (TeV){sup 4}, are investigated. The corrections to the Newtonian potential are found to be the only observationally accessible probe of this scenario, as previously suggested in the literature. In particular, the presence of the continuum of Kaluza-Klein modes does not lead to any observable collider signatures. The extent to which experimental tests of Newtonian gravity can distinguish this scenario from the scenario of Arkani-Hamed, Dimopoulos, and Dvali, with 1- and 2-mm size extra dimensions, is explicitly demonstrated.

Bulk Physics at a Graviton Factory

Abstract A general prediction of the 5-d Randall–Sundrum (RS) hierarchy model is the emergence of spin-2 Kaluza–Klein (KK) gravitons with weak scale masses and couplings. The lowest order effective theory of the RS model is given by 5-d Einstein gravity which uniquely fixes the self-interactions of gravitons. We demonstrate that large numbers of light KK resonances could be produced at a future lepton-collider-based “Graviton Factory”. Measuring the self-interactions of these KK gravitons will probe the accuracy of the 5-d Einstein gravity picture and, in addition, yield indirect information on the as yet untested self-coupling of the 4-d graviton. The self-interactions of the gravitons can be studied by measuring the decays of the heavier states to the lighter ones. Using the AdS/CFT picture, these can be interpreted as the decays of heavier resonances to lighter ones, in a strongly coupled 4-d CFT. We show that these decays have sufficient rates to be studied at future colliders and that they are also, in principle, sensitive to higher derivative operators, such as the Gauss–Bonnet term. In a generalized RS model, with non-universal 5-d fermion masses, FCNCs will be induced. We show that precise measurements of the rare decays of KK graviton/gauge states into flavor non-diagonal final states at a Graviton Factory can be used to map the 5-d fermion mass matrix.

γ γ → γ γ as a test of weak scale quantum gravity at the NLC

Recently, it has been proposed that the fundamental scale of quantum gravity can be close to the weak scale if there are large extra dimensions . This proposal has important phenomenological implications for processes at the TeV scale. We study the process {gamma}{gamma} {r_arrow} {gamma}{gamma}, assuming an ultraviolet cutoff M{sub S} {approximately} 1 TeV for the effective gravity theory. We find that, at center of mass energies {radical}s {approximately} 1 TeV, the contribution of gravitationally mediated scattering to the cross section is comparable to that coming from the one-loop Feynman diagrams of the Standard Model. We thus conclude that the effects of weak scale quantum gravity can be studied at the Next Linear Collider (NLC), in the photon collider mode. Our results suggest that, for typical proposed NLC energies and luminosities, the range 1 TeV {le} M{sub S} {le} 10 TeV can be probed.

{gamma}{gamma} {r_arrow} {gamma}{gamma} as a Test of Weak Scale Quantum Gravity at the NLC

Recently, it has been proposed that the fundamental scale of quantum gravity can be close to the weak scale if there are large extra dimensions . This proposal has important phenomenological implications for processes at the TeV scale. We study the process {gamma}{gamma} {r_arrow} {gamma}{gamma}, assuming an ultraviolet cutoff M{sub S} {approximately} 1 TeV for the effective gravity theory. We find that, at center of mass energies {radical}s {approximately} 1 TeV, the contribution of gravitationally mediated scattering to the cross section is comparable to that coming from the one-loop Feynman diagrams of the Standard Model. We thus conclude that the effects of weak scale quantum gravity can be studied at the Next Linear Collider (NLC), in the photon collider mode. Our results suggest that, for typical proposed NLC energies and luminosities, the range 1 TeV {le} M{sub S} {le} 10 TeV can be probed.

Gravi-burst: super-GZK cosmic rays from localized gravity☆

The flux of cosmic rays beyond the GZK cutoff ({approx} 10{sup 20} eV) may be explained through their production by ultra high energy cosmic neutrinos, annihilating on the relic neutrino background, in the vicinity of our galaxy. This process is mediated through the production of a Z boson at resonance, and is generally known as the Z-Burst mechanism. We show that a similar mechanism can also contribute to the super-GZK spectrum at even higher, ultra-GZK energies, where the particles produced at resonance are the Kaluza-Klein gravitons of weak scale mass and coupling from the Randall-Sundrum (RS) hierarchy model of localized gravity model. We call this mechanism Gravi-Burst. We discuss the parameter space of relevance to Gravi-Bursts, and comment on the possibility of its contribution to the present and future super-GZK cosmic ray data and place bounds on the RS model parameters. Under certain assumptions about the energy spectrum of the primary neutrinos we find that cosmic ray data could be potentially as powerful as the LHC in probing the RS model.

Compton scattering at the NLC and large extra dimensions

We study Compton scattering, {gamma}e {yields} {gamma}e, in the context of the recent proposal for Weak Scale Quantum Gravity (WSQG) with large extra dimensions. It is shown that, with an ultraviolet cutoff M{sub S} {approx} 1 TeV for the effective gravity theory, the cross section for this process at the Next Linear Collider (NLC) deviates from the prediction of the Standard Model significantly. Our results suggest that, for typical proposed NLC energies and luminosities, WSQG can be tested in the range 4 TeV {approx}< M{sub S} {approx}< 16 TeV, making {gamma}e {yields} {gamma}e an important test channel.