Abstract
We searched for the μ+μ− decay of a light vector gauge boson, also known as dark photon, in the e+e−→μ+μ−γISR process by means of the Initial State Radiation (ISR) method. We used 1.93 fb−1 of data collected by the KLOE experiment at the DAΦNE ϕ-factory. No structures have been observed over the irreducible μ+μ− background. A 90% CL limit on the ratio ε2=α′/α between the dark coupling constant and the fine structure constant of 3×10−6–2×10−7 has been set in the dark photon mass region between 519 MeV and 973 MeV. This new limit has been combined with the published result obtained investigating the hypothesis of the dark photon decaying into hadrons in e+e−→π+π−γISR events. The combined 90% CL limit increases the sensitivity especially in the ρ–ω interference region and excludes ε2 greater than (13−2)×10−7. For dark photon masses greater than 600 MeV the combined limit is lower than 8 ×10−7 resulting more stringent than present constraints from other experiments.
Highlights
Many gravitational anomalies observed since the first decades of the twentieth century, as well as large-scale structure formation in the early Universe, can be explained by the existence of a non-baryonic matter known as dark matter (DM) [1]
The minimal extension of the SM consists of just one additional abelian gauge symmetry UD(1) with associated a light vector gauge boson, the dark photon – known as U boson, γ or A – as mediator of the new force, called for this reason dark force
The process is responsible for both production and decay of the dark photon in SM interactions resulting in an ε2 suppression
Summary
Many gravitational anomalies observed since the first decades of the twentieth century, as well as large-scale structure formation in the early Universe, can be explained by the existence of a non-baryonic matter known as dark matter (DM) [1]. In the simplest scenario [2], the coupling with SM particles arises from a vector portal known as kinetic mixing consisting in loops of heavy dark particles charged under both the electromagnetic and the dark force. The new search confirms no U-boson signal in the dimuon invariant mass spectrum: a new 90% CL exclusion limit in ε2 is estimated. This limit is of comparable magnitude with respect to the previous ones, a combined search of dark photon decays into both muon and pion pairs would increase the sensitivity of the single channel searches, it is more effective in the region of the ρ − ω interference where the search for U → μ+μ− loses sensitivity
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