Search for a light dark sector particle at LHCb

TL;DR

This paper reports a search for a hypothetical light dark sector particle in B meson decays at LHCb, setting new constraints on theories predicting low-mass dark bosons using collision data at 7 and 8 TeV.

Contribution

It presents the first search for a hidden-sector boson in B decays at LHCb, providing the most stringent limits to date on low-mass dark bosons.

Findings

No significant signal was observed.

Upper limits were set on the branching fractions.

Constraints on dark sector theories were improved.

Abstract

A search is presented for a hidden-sector boson, $\chi$, produced in the decay $B^0 \rightarrow K^* (892)^0 \chi$, with $K^* (892)^0 \rightarrow K^+ \pi^-$ and $\chi \rightarrow \mu^+ \mu^-$ . The search is performed using a $pp$-collision data sample collected at $\sqrt{s}=7$ and 8 TeV with the LHCb detector, corresponding to integrated luminosities of 1 and 2 fb$^{-1}$ respectively. No significant signal is observed in the mass range $214 \le m_\chi \le 4350$ MeV, and upper limits are placed on the branching fraction product $\mathcal{B}(B^0 \rightarrow K^* (892)^0 \chi) \times \mathcal{B}(\chi \rightarrow \mu^+ \mu^- )$ as a function of the mass and lifetime of the $\chi$ boson. These limits place the most stringent constraints to date on many theories that predict the existence of additional low-mass dark bosons.