Identifying Exclusive Displaced Hadronic Signatures in the Forward Region of the LHC

The LHCb detector provides accurate vertex reconstruction and hadronic particle identification, which make the experiment an ideal place to look for light long-lived particles (LLP) decaying into Standard Model (SM) hadrons. In contrast with the typical search strategy relying on energetic jets and a high multiplicity of tracks from the LLP decay, LHCb can identify LLPs in exclusive, specific hadronic final states. To illustrate the idea, we study the sensitivity of LHCb to an exotic Higgs decay $h\to SS$, followed by the displaced decay of GeV-scale scalars into charged kaons $S\to K^+K^-$. We show that the reconstruction of kaon vertices in narrow invariant mass windows can efficiently eliminate the combinatorial backgrounds from $B$-meson decays. While the same signal is extremely difficult to probe in the existing displaced jet searches at ATLAS/CMS, the LHCb search we propose can probe the branching ratio BR$(h\to SS)$ down to $0.1\%$ ($0.02\%$) level with $15$ ($300$) fb$^{-1}$ of data. We also apply this projected bound to two scenarios with Higgs portal couplings, where the scalar mediator $S$ either couples to a) the SM quarks only, or b) to both quarks and leptons in the minimal flavor violation paradigm. In both scenarios we compare the reach of our proposed search with the expected constraints from ATLAS and CMS on the invisible Higgs width and with the constraints from rare B-decays studies at LHCb. We find that for 1 GeV $< m_S < $ 2 GeV and $0.5~{\rm mm} \lesssim c \tau \lesssim 10$ mm our proposed search will be competitive with the ATLAS and CMS projections, while at the same time providing crucial information of the hadronic interactions of $S$, which can not be obtained from the {\it indirect} measurement of the Higgs invisible width.