Constraints on sub-GeV hidden sector gauge bosons from a search for heavy neutrino decays

Abstract

Several models of dark matter motivate the concept of hidden sectors consisting of SU(3)_C x SU(2)_L x U(1)_Y singlet fields. The interaction between our and hidden matter could be transmitted by new abelian U'(1) gauge bosons A' mixing with ordinary photons. If such A's with the mass in the sub-GeV range exist, they would be produced through mixing with photons emitted in two photon decays of η,η' neutral mesons generated by the high energy proton beam in a neutrino target. The A's would then penetrate the downstream shielding and be observed in a neutrino detector via their A'-> e+e- decays. Using bounds from the CHARM neutrino experiment at CERN that searched for an excess of e+e- pairs from heavy neutrino decays, the area excluding the γ- A' mixing range 10^{-7} < ε< 10^{-4} for the A' mass region 1 < M_A' <500 MeV is derived. The obtained results are also used to constrain models, where a new gauge boson X interacts with quarks and leptons. New upper limits on the branching ratio as small as Br(η-> γX) < 10^{-14} and Br(η' -> γX) < 10^{-12} are obtained, which are several orders of magnitude more restrictive than the previous bounds from the Crystal Barrel experiment.

Figures (5)

• Figure 1: Schematic illustration of a proton beam dump experiment on search for $P \to \gamma A', ~�A' \to e^+e^-$ decay chain: neutral mesons $P$ generated by the proton beam in the target produce a flux of high energy $A'$'s through the $\gamma-A'$ mixing in the decay $P\to \gamma \gamma$, which penetrate the downstream shielding and decay into $e^+e^-$ pair in a neutrino detector. The same setup can be used to search for the process $P \to \gamma X, ~�X \to e^+e^-$. See text.
• Figure 2: Invariant cross section for $\eta$ production in pp collisions at $\sqrt{s}=$ 27.4 GeV vs $p_T$ and rapidity $y_{c.m.}$ over the range $-3.0 < y_{c.m.}< 3.0$.
• Figure 3: Invariant cross section for $\eta$ mesons production in pp collisions at $\sqrt{s}=27.4$ GeV calculated as a function of $p_T$ in the low-$p_T$ region for the B-G parameterization of Eq.(4) (solid curve), and from Eq.(\ref{['lowpt']})( dashed curve).
• Figure 4: Exclusion region in the ($M_{A'}; \epsilon$) plane obtained in the present work from the results of the CHARM experiment charm-1charm-2. The areas excluded from the (g-2) considerations, the results of the Nomad sngx and electron beam dump experiments E137 jdbe137, E141 e141, E774 e774, and from the searches by APEX apex, KLOEkloe, BaBarbabar and MAMI mami are also shown for comparison.
• Figure 5: The 90$\%~C.L.$ upper limits on the branching ratio $Br(\eta(\eta') \to \gamma X) Br(X \to e^+e^-)$ versus $\tau_{X}$ obtained from the CHARM experiment for $\eta$ (solid curves) and $\eta'$ (dashed curves) decays. The numbers near the curves indicate the corresponding values of $M_{X}$.