The kinetic mixing of the vector boson of hypercharge with the vector boson(s) associated with particle sectors beyond the Standard Model is one of the best motivated windows to new physics. The resulting phenomenology depends on whether the new vect
or boson is massive or massless. The phenomenology associated with the massive phase has received considerable attention in recent years with many theoretical explorations and new experimental efforts, while the massless phase is linked to the phenomenology of milli-charged particles. In this paper we introduce the more general case where the kinetic mixing is with a vector boson that is a linear combination of both a massive and a massless state (as hypercharge is in the Standard Model). We demonstrate that the general phase is only weakly constrained when the mass scale associated with it is above about 100 MeV. Finally, we show that a new dedicated experiment at the LHC, proposed recently in Ref. [1], can explore large parts of the parameter space in the mass range between 100 MeV and 100 GeV. In particular, it is uniquely sensitive to a new signature that only arises in the general phase.
The development of techniques for identifying hadronic signals from the overwhelming multi-jet backgrounds is an important part of the Large Hadron Collider (LHC) program. Of prime importance are resonances decaying into a pair of partons, such as th
e Higgs and $rm W$/$rm Z$ bosons, as well as hypothetical new particles. We present a simple observable to help discriminate a dijet resonance from background that is effective even when the decaying resonance is not strongly boosted. We find consistent performance of the observable over a variety of processes and degree of boosts, and show that it leads to a reduction of the background by a factor of $3-5$ relative to signal at the price of $10-20%$ signal efficiency. This approach represents a significant increase in sensitivity for Standard Model (SM) measurements and searches for new physics that are dominated by systematic uncertainties, which is true of many analyses involving jets - particularly in the high-luminosity running of the LHC.
We show that the existence of new, light gauge interactions coupled to Standard Model (SM) neutrinos give rise to an abundance of sterile neutrinos through the sterile neutrinos mixing with the SM. Specifically, in the mass range of MeV-GeV and coupl
ing of $g sim 10^{-6} - 10^{-2}$, the decay of this new vector boson in the early universe produces a sufficient quantity of sterile neutrinos to account for the observed dark matter abundance. Interestingly, this can be achieved within a natural extension of the SM gauge group, such as a gauged $L_mu-L_tau$ number, without any tree-level coupling between the new vector boson and the sterile neutrino states. Such new leptonic interactions might also be at the origin of the well-known discrepancy associated with the anomalous magnetic moment of the muon.
Models that seek to produce a line at ~130 GeV as possibly present in the Fermi data face a number of phenomenological hurdles, not the least of which is achieving the high cross section into gamma gamma required. A simple explanation is a fermionic
dark matter particle that couples to photons through loops of charged messengers. We study the size of the dimension 5 dipole (for a pseudo-Dirac state) and dimension 7 Rayleigh operators in such a model, including all higher order corrections in 1/M_{mess}. Such corrections tend to enhance the annihilation rates beyond the naive effective operators. We find that while freezeout is generally dominated by the dipole, the present day gamma-ray signatures are dominated by the Rayleigh operator, except at the most strongly coupled points, motivating a hybrid approach. With this, the Magnetic inelastic Dark Matter scenario provides a successful explanation of the lines at only moderately strong coupling. We also consider the pure Majorana WIMP, where both freezeout and the Fermi lines can be explained, but only at very strong coupling with light (~200 - 300 GeV) messengers. In both cases there is no conflict with non-observation of continuum photons.
We address two closely related problems associated with the singlet scalars potential that are often present in supersymmetric U(1) models, especially those which maintain the gauge unification of the MSSM in a simple way. The first is the possibilit
y of an accidental global symmetry which results in a light Goldstone boson. The second is the problem of generating a vacuum expectation value for more than one field without reintroducing the $mu$ problem. We give sufficient conditions for addressing both issues and provide a concrete example to generate them.
