Dark photons and resonant monophoton signatures in Higgs boson decays at the LHC

Abstract

Motivated by dark-photonγ scenarios extensively considered in the literature, we explore experimentally allowed models where the Higgs boson coupling to photon and dark photon Hγγ can be enhanced. Correspondingly, large rates for the H → γγ decay become plausible, giving rise to one monochromatic photon with E γ mH /2 (i.e., more than twice the photon energy in the rare standard-model decay H → γZ → γνν), and a similar amount of missing energy. We perform a model-independent study of this exotic resonant monophoton signature at the LHC, featuring a distinctive E γ T peak around 60 GeV, and γ + / E T transverse invariant mass ruled by mH . At parton level, we find a 5 σ sensitivity of the present LHC data set for a H → γγ branching fraction of 0.5%. Such large branching fractions can be naturally obtained in dark U (1)F models explaining the origin and hierarchy of the standard model Yukawa couplings. We urge the LHC experiments to search for this new exotic resonance in the present data set, and in future LHC runs.Introduction. Although dark matter (DM) is five times more abundant in the Universe than ordinary baryonic matter [1], its properties are yet unknown. It is plausible that the dark sector, which is weakly coupled to the standard model (SM), possesses rich internal structure and interactions. Among the most popular scenarios is the idea that the dark sector contains light or massless gauge bosons [2] that mediate long-range forces between dark particles. In cosmology the dark photons may solve the small-scale structure formation problems [3,4] and, for massless dark photons [5], predict dark discs of galaxies [6]. In astroparticle physics dark photons may induce Sommerfeld enhancement of DM annihilation cross section needed to explain the PAMELA-Fermi-AMS2 positron anomaly [7], may assist light DM annihilations to reach the phenomenologically required magnitude, and make asymmetric DM scenarios phenomenologically viable [8]. Dark/hidden photon scenarios have also been extensively considered in beyond-the-SM frameworks in particle physics [9][10][11][12][13][14][15].Recently, a new paradigm has been proposed for generating exponentially spread SM Yukawa couplings from unbroken U (1) F quantum numbers in the dark sector [16]. In this approach, nonperturbative flavor-and chiral-symmetry breaking is transferred from the dark to visible sector via heavy scalar messenger fields [16,17] In this work we show that, in the unbroken dark U (1) scenarios, the Higgs-boson two-body decay H → γγ to one photon γ and one dark photonγ can be enhanced despite existing constraints, providing a very distinctive NP signature of a single photon plus missing energy at the Higgs resonance. If this signature will be discovered at the LHC, CP invariance will imply the spin-1 nature of the missing energy, excluding axions or other ultralight scalar particles.Monophoton plus / E T signatures have been used by the LHC experiments to search for NP scenarios such as extra dimensions, supersymmetry, DM pair production [19], and SM con...

Radiative Yukawa couplings in the simplest left-right symmetric model

“…As a first example, we adopt here the setup originally proposed in [4], consisting in a set of dark massive Dirac fermions singlet under the SU (2) L ×SU (2) R ×U (1) Y group, but charged under an unbroken U (1) F gauge interaction. We then make use of a non-perturbative mechanism, based on the solution of the gap-equation via the Nambu-Jona-Lasion mechanism [24], to generate the exponential spread in the dark fermion mass splitting [23,28]. This requires the existence of a a higher derivative term in the pure U (1) F gauge sector, which can be associated to the presence of a massive Lee-Wick ghost in the spectrum [25,26].…”

“…The chiral symmetry breaking in the hidden sector, encoded in the dark-fermion masses, is finally transmitted to the SM through a set of scalar messenger fields which interact with both the dark and SM particles. Gauge invariance of such couplings forces the messenger fields to carry the same quantum numbers as squarks and sleptons of the SM supersymmetric extensions, leading to interesting phenomenological implications at the LHC and future e + e − linear colliders investigated for instance in [28]. To embed the mechanism of [4] in our framework, we separate the full Lagrangian into four sectors…”

Dark-photon searches via ZH production at e+e− colliders

“…On the contrary, dark-photon couplings to the Higgs boson can show non-decoupling properties (a typical example is when the messenger fields have the same quantum numbers as squarks and sleptons [23]). An effective gauge-invariant low-energy Hγγ interaction can indeed arise at one loop.…”

“…R is given by a product of quantum charges (see for instance Eq. (4) in [23] for notations). The scale µ is connected to the vev of a heavy singlet scalar field needed to generate effective Yukawa couplings at 1-loop [15].…”

“…(2) shows that the scale Λ ef f has a non-decoupling behavior, being proportional to the Higgs vev as Λ ef f ∼ O(v/ξ 2 ). It can then potentially lead to observable effects even in case of a heavy messenger sector [23], since in the GRFM typically one has ξ ∼ a few tens %. Furthermore, we assume that the lightest messenger mass m − satisfies the lower bound m − > ∼ 2 TeV, in order to avoid a conflict with present collider limits on the direct search of new colored particles.…”