Abstract
A recently proposed scale invariant extension of the standard model is modified such that it includes a Dark Matter candidate which can annihilate into gamma-rays. For that a non-zero U(1) Y hypercharge Q is assigned to the fermions in a QCD-like hidden sector. The Nambu-Goldstone bosons, that arise due to dynamical chiral symmetry breaking in the hidden sector, are cold Dark Matter candidates, and the extension allows them to annihilate into two photons, producing a γ-ray line spectrum. We find that the γ-ray line energy must be between 0.7 TeV and 0.9 TeV with the velocity-averaged annihilation cross section 10−30 ~ 10−26 cm3 /s for Q = 1/3. With a non-zero hypercharge Q, the hidden sector is no longer completely dark and can be directly probed by collider experiments.
Highlights
We follow the idea that the energy scale in a classically scale invariant theory is generated by DχSB in a QCD-like hidden sector, which is transmitted via a SM singlet messenger field to the SM sector [46,47,48,49].1 So we assume that the fermions in the hidden sector are SM singlet and allow the presence of fundamental scalar fields
We find that the energy of the γ-ray line in our model lies between 0.7 TeV and 0.9 TeV. (We are not aiming to explain the recent observations of the galactic keV X-ray [58, 59] here.) The upper limits on the velocity-averaged annihilation cross section vσ given by Fermi LAT and HESS constrain the electric charge Q of the hidden fermions
We find that the vσ is 10−30 ∼ 10−26 cm3/s for Q = 1/3, which can well satisfy the experimental constraints of Fermi LAT and HESS
Summary
We consider an extension of the model studied in [46,47,48,49] which consists of a hidden QCDlike sector coupled via a real singlet scalar S to the SM. If we further impose that the matter content remains unchanged, there is a unique possibility for the extension that the hidden (Dirac) fermion carries a common U(1)Y charge Q.2 This implies that the hidden sector Lagrangian of the extended model is written as LH. Where we denote the pseudo Nambu-Goldstone boson after spontaneous chiral symmetry breaking as φa These dark pions are stable due to flavor symmetry and they serve as good DM candidates. In the self-consistent mean field approximation one splits up the NJL Lagrangian (2.3) into the sum LNJL = L0 + LI , where LI is normal ordered (i.e. 0|LI |0 = 0), and L0 contains at most fermion bilinears which are not normal ordered. Where M = σ + yS − GDσ2/8G2 is the constituent hidden sector fermion mass when all the CP-even scalar fields obtained their vacuum expectation values (VEV). The additional U(1)Y coupling does not contribute to the DM mass
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