Muon ${g-2}$ discrepancy within D-brane string compactifications

Abstract

Very recently, the Muon g − 2 experiment at Fermilab has confirmed the E821 Brookhaven result, which hinted at a deviation of the muon anomalous magnetic moment from the Standard Model (SM) expectation. The combined results from Brookhaven and Fermilab show a difference with the SM prediction δa µ = (251 ± 59) × 10 −11 at a significance of 4.2σ, strongly indicating the presence of new physics. Motivated by this new result we reexamine the contributions to δa µ from both: (i) the ubiquitous U (1) gauge bosons of D-brane string theory constructions and (ii) the Regge excitations of the string. We show that, for a string scale O(PeV), the contribution from anomalous U (1) gauge bosons which couple to hadrons could help to reduce (though not fully eliminate) the discrepancy reported by the Muon g − 2 Collaboration. Consistency with null results from LHC searches of new heavy vector bosons imparts the dominant constraint. We demonstrate that the contribution from Regge excitations is strongly suppressed as it was previously conjectured. We also comment on contributions from Kaluza-Klein (KK) modes, which could help resolve the δa µ discrepancy. In particular, we argue that for 4-stack intersecting D-brane models, the KK excitations of the U (1) boson living on the lepton brane would not couple to hadrons and therefore can evade the LHC bounds while fully bridging the δa µ gap observed at Brookhaven and Fermilab.

Leptophilic U(1) massive vector bosons from large extra dimensions

“…Note that this is a positive correction that brings a µ closer to experimental data. The result in ( 3) is in agreement with the order of magnitude estimate provided elsewhere [17].…”

“…Moreover, the largest possible g c (M s ) also gives the most contribution to a µ . Such a Z boson gives a µ = 9.9 × 10 −11 [17], which is not enough to explain the observed discrepancy. The second anomalous U(1) should be much heavier to avoid the LHC bound and its contribution to a µ is negligible.…”

“…It turns out that this cannot be done because the corresponding U(1) gauge coupling becomes strong. Indeed, the 4 stack model thoroughly analyzed in [17], with gauge group U(3) a × Sp…”

Minimal embedding of the Standard Model into intersecting D-brane configurations with a bulk leptonic $U(1)$

“…The possibility to explain this discrepancy in the framework of low mass scale strings and large extra dimensions has been recently studied in [7], where three contributions to the muon anomalous magnetic moment have been examined : from Regge excitations of the string, from anomalous U (1) gauge bosons, as well as from Kaluza-Klein (KK) modes of a bulk vector field. While the first contribution is strongly suppressed, it has been shown how the second one can reduce, but not fully bridge, the discrepancy.…”

“…If its mass is of order the string scale, it is too heavy to accommodate the (g − 2) µ discrepancy. If it is lighter due to volume suppression, it can only partially explain the discrepancy since the zero mode is a linear combination of the various U (1) factors, coupled to both quarks and leptons and thus subject to stringent LHC bounds [7,8]. On the other hand, the KK excitations couple only to leptons to lowest order.…”