Speakers
Description
A mechanism accommodating baryonic Dark Matter (DM) has been proposed recently [arXiv:2101.02706,arXiv:1810.00880]. This has the advantage of explaining matter-anti-matter imbalance and DM at once, without requiring the proton to be unstable. The DM particles, carrying baryon number, would be produced in $b$-hadron decays, through a t-channel mediator. In this model, $b$-hadron branching fractions as high as few per mille are needed both by theoretical and experimental constraints.
The DM particles produced in this model, $\Psi_{(DS)}$ carry baryon number, and have a mass in the range $0.94 < m_{\Psi_{(DS)}} < 4.34 $ GeV/c$^2$, being therefore harder to detect in general missing transverse energy (MET) searches at ATLAS or CMS. However, knowing the position of the production vertex (PV) and the decay vertex of the $b$-hadron allows a partial reconstruction of the final state and can provide a proxy to MET, hereby referred to as MPT$_{B}$, opening a window for detection at LHCb. This technique has already proven successful in the analysis of $\Lambda_b \rightarrow p \mu \nu_\mu$ decays [arXiv:1504.01568]. Therefore, the study of DM in $b$ decays at LHCb provides unique sensitivity at the LHC, covering the entire parameter space of the model. Sharp kinematic end points are also distinctive signatures of these decays.
In this contribution, we will discuss the LHCb sensitivity to these searches, covering different topologies and giving prospects for Runs 3 and 4 of the LHC. We will give examples of $B^0$, $B^+$ and $\Lambda_b$ decays as well as discussing the advantages and disadvantages compared to other experiments, such as Belle. Notably, the use of $\Lambda_b$ baryons allows to test the entire mass range for $\Psi_{(DS)}$. For the analysis, the signals and backgrounds will be generated with Pythia [arXiv:1410.3012] and experimental effects will be emulated through a fast simulation [arXiv:2012.02692].
We show here two baseline examples, with topologies that are convenient at LHCb: $B^0 \rightarrow \Lambda^\ast + \Psi_{(DS)},\Lambda^\ast \rightarrow pK$, and $B^+ \rightarrow \Lambda_c^+(2595) \Psi_{(DS)},\Lambda_c^+(2595) \rightarrow \pi^+\pi^- \Lambda_c^+,\Lambda_c^+ \rightarrow p K \pi$. In both cases, $\Psi_{(DS)}$ is the dark particle, charged under baryon number. These decay chains are fully reconstructible through charged tracks at LHCb, with the obvious exception of the dark particle, and give access to the $b-$hadron decay points, allowing the determination of MPT$_{B}$. A simple select and count experiment of the signals and main backgrounds indicates that, in both decays, branching fractions in the $10^{-3}-10^{-5}$ range should be at reach at LHCb. This result will be tuned once experimental effects are accounted for. Figure 1 shows the distribution of MPT$_{B}$ for the signal and background in both topologies.
Figure caption: Expected MPT$_{B}$ distributions at LHCb for the $B^0 \rightarrow \Lambda^\ast + \Psi_{(DS)}$ (left) and $B^+ \rightarrow \Lambda_c^+(2595) \Psi_{(DS)}$ (right) decays and corresponding backgrounds. The yields are normalized to 1 fb$^{-1}$ of data and assume $pp$ collisions with $\sqrt{s}=14$ TeV and signal branching fractions of $10^{-3}$ and $m(\Psi_{(DS)})=2$ GeV/c$^2$. The distributions are at Pythia level and do not include experimental effects yet, but these will be included by the time of the conference.
