Abstract
We explore the phenomenology of a QCD-like dark sector which confines around the GeV scale. The dark sector inherits a flavour structure from a coupling between dark quarks and SM quarks via a heavy mediator, which leads to exciting new phenomena. While stable baryonic bound states are the dark matter candidates, the phenomenology is dominated by the lightest composite mesons, the dark pions, which can have decay lengths ranging from millimetres to hundreds of meters. For masses below 1.5 GeV, their exclusive decays to SM mesons are calculated for the first time by matching both dark and visible sectors to a chiral Lagrangian. Constraints from big bang nucleosynthesis, dark matter direct detection and flavour single out a small region of allowed parameter space for dark pion masses below 5 GeV. It is best probed by the fixed target experiments NA62 and SHiP, where dark pions can be produced copiously in rare decays like B → KπD . The dominant πD → K±π∓ and πD → 3π decay modes are a smoking gun for a CP-odd, flavour violating new resonance. Heavier dark pions are best searched for at the LHC, where they decay after hadronisation to produce jets which emerge into SM states within the detector. Here the flavour structure ensures different flavours emerge on different length scales, leading to a striking new feature in the emerging jets signature.
Highlights
As was already realised in the seminal works [21, 22], the phenomenology of such models depends crucially on how they are coupled to the visible sector, i.e. on the so called mediators
We study for the first time the flavour structure that is imposed on a non-abelian dark sector by a bi-fundamental scalar mediator which is charged under both QCD and the dark SU(N ) symmetry
We identify the regions of parameter space consistent with ∆F = 2 flavour violating processes and impose constraints arising from ∆F = 1 flavour violating B and K meson decays as well as from cosmology
Summary
Where Nd is the number of dark colours. We further introduce nd dark quarks Q, which are singlets under GSM and transform in the fundamental of SU(Nd). Communication between the dark and visible sectors is established through a bifundamental scalar field X which transforms as (3, Nd) under SU(3)colour × SU(Nd) Such bi-fundamentals are required e.g. in the dark QCD model [8] and could appear in UV completions of twin Higgs models [29] or models where the dark gauge symmetry unifies with QCD at some higher scale. In the following we will assume that this is the case, meaning that the Yukawa couplings κ are the only source of dark flavour symmetry breaking. Instead of transforming under their own flavour symmetry group, the dark quarks were assigned to representations of the SM flavour group, the alignment limit would correspond to a minimally flavour violating (MFV) scenario in which the charges of the dark quarks and the κ matrix are chosen to be. This is a more restrictive flavour structure than our setup
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