Probing the Randall-Sundrum geometric origin of flavor with lepton flavor violation

Abstract

The "anarchic" Randall-Sundrum model of flavor is a low energy solution to both the electroweak hierarchy and flavor problems. Such models have a warped, compact extra dimension with the standard model fermions and gauge bosons living in the bulk, and the Higgs living on or near the TeV brane. In this paper we consider bounds on these models set by lepton flavor violation constraints. We find that loop-induced decays of the form l → l ′ γ are ultraviolet sensitive and uncalculable when the Higgs field is localized on a four-dimensional brane; this drawback does not occur when the Higgs field propagates in the full five-dimensional space-time. We find constraints at the few TeV level throughout the natural range of parameters, arising from µ − e conversion in the presence of nuclei, rare µ decays, and rare τ decays. A "tension" exists between loopinduced dipole decays such as µ → eγ and tree-level processes such as µ − e conversion; they have opposite dependences on the five-dimensional Yukawa couplings, making it difficult to decouple flavor-violating effects. We emphasize the importance of the future experiments MEG and PRIME. These experiments will definitively test the Randall-Sundrum geometric origin of hierarchies in the lepton sector at the TeV-scale.

Combining direct & indirect kaon CP violation to constrain the warped KK scale

“…This is expected since the 5D theory is non-renormalizable with negative mass dimension operators. For instance, when the Higgs is localized on the IR brane, the above one-loop is divergent, and a counter-term in the form of a higher dimensional IR operator is included [3,15,16]. We can derive a bound on the corresponding cutoff of the theory required to satisfy the constraints from b → sγ and / K (similarly to the µ → eγ case [16]).…”

“…where i, j are flavour indices and m, n are the KK levels of the fermions in the loop, and we only consider the first KK state, since at one-loop the above contribution is finite (or at most log-divergent) [3,15,16]. Following the analysis in [9] (allowing 20% departure from the SM value of B(B → X s γ)) and using the values of the quark Yukawa couplings at 5 TeV [14], we obtain for β = 1 by requiring…”

Neutrino mixing from Wilson lines in warped space

“…While most of the flavor literature in WED has been devoted to the quark sector, the lepton sector has also been extensively explored [12][13][14][15][16][17][18][19][20][21][22][23][24][25][26][27][28][29][30]32]. In fact the lepton sector is qualitatively different from the quark sector as the charged lepton spectrum is hierarchical while the neutrino spectrum and PMNS mixing angles are mostly anarchical [33].…”

A model of lepton masses from a warped extra dimension

“…Generically there are two types of LFV sources: tree-level LFV via the exchange of heavy neutral particles (like Z') or off-diagonal Z couplings, or loop induced rare decays via charged current interactions. Tree-level LFV operators have been considered in [11], and it was found that the lepton flavor scale is at least 5 TeV, and larger for smaller brane Yukawa couplings. We will first show here that the structure of the A 4 symmetry used here is such that all tree-level sources of lepton flavor violation are absent in this theory.…”

“…While these flavor violations are suppressed by the so-called RS-GIM mechanism [6,8], the KK mass scale still has to be as large as 20 TeV in order to safely suppress FCNC's in the quark sector [9]. In the lepton sector the anarchic approach [10] imposes a bound of order 10 TeV on the KK mass scale [11]. These bounds imply that the theory is out of reach for the LHC and unlikely to be useful for solving the (little) hierarchy problem.…”