Dark photons from charm mesons at LHCb

TL;DR

The paper presents a targeted search for dark photons $A'$ in the mass range $m_{A'}\in[2m_e,\Delta m_D]$ at LHCb using the decay $D^{*0}\to D^0A'$, with $A'\to e^+e^-$. It develops two complementary strategies—a displaced-vertex search (pre- and post-module) and a resonant search—enabled by Run 3 triggerless-readout, leveraging the large $D^{*0}$ yield and LHCb's excellent vertex/mass resolution. The analysis provides detailed signal and background modeling, reconstruction efficiencies, and sensitivity projections, showing that LHCb can probe the parameter space between prompt-$A'$ and beam-dump limits for $m_{A'}\lesssim100$ MeV and $\epsilon^2$ roughly in $[10^{-10},10^{-6}]$. The results highlight a practical, data-driven path to discovering or constraining dark photons through charm-meson decays, with clear avenues for improvements and cross-checks. Overall, the work demonstrates that LHCb Run 3 capabilities could significantly extend the accessible dark-photon parameter space in a region complementary to other experiments.

Abstract

We propose a search for dark photons $A^{\prime}$ at the LHCb experiment using the charm meson decay $D^*(2007)^0 \!\to D^0 A^{\prime}$. At nominal luminosity, $D^{*0} \!\to D^0 γ$ decays will be produced at about 700kHz within the LHCb acceptance, yielding over 5 trillion such decays during Run 3 of the LHC. Replacing the photon with a kinetically-mixed dark photon, LHCb is then sensitive to dark photons that decay as $A^{\prime}\!\to e^+e^-$. We pursue two search strategies in this paper. The displaced strategy takes advantage of the large Lorentz boost of the dark photon and the excellent vertex resolution of LHCb, yielding a nearly background-free search when the $A^{\prime}$ decay vertex is significantly displaced from the proton-proton primary vertex. The resonant strategy takes advantage of the large event rate for $D^{*0} \!\to D^0 A^{\prime}$ and the excellent invariant mass resolution of LHCb, yielding a background-limited search that nevertheless covers a significant portion of the $A^{\prime}$ parameter space. Both search strategies rely on the planned upgrade to a triggerless-readout system at LHCb in Run 3, which will permit identification of low-momentum electron-positron pairs online during data taking. For dark photon masses below about 100MeV, LHCb can explore nearly all of the dark photon parameter space between existing prompt-$A^{\prime}$ and beam-dump limits.

Figures (11)

• Figure 1: Current bounds on dark photons with visible decays to SM states, adapted and updated from Ref. Essig:2013lka. The upper bounds are from prompt-$A'$ searches while the wedge-shaped bounds are from beam-dump searches and supernova considerations. The LHCb search region in Fig. \ref{['fig:lhcbbounds']} covers most of the gap between these bounds for $m_{A^{\prime}}\xspace \lesssim100\mathrm{\: Me V}\xspace$, with a reach extending to $m_{A^{\prime}}\xspace \lesssim 140\mathrm{\: Me V}\xspace$. Anticipated limits from other planned experiments are shown in Fig. \ref{['fig:lhcbboundswithoverlay']}.
• Figure 2: Potential bounds from LHCb after Run 3, for both the displaced (pre-module, solid blue) and resonant (dashed blue) searches. Also shown is an alternative displaced search strategy (post-module, dotted blue) that looks for ${A^{\prime}}\xspace$ vertices downstream of the first tracking module.
• Figure 3: Dark photon Lorentz boost factors for $m_{A^{\prime}}\xspace = \{10,20,50,100\}~\MeV$. These factors are independent of $\epsilon^2$.
• Figure 4: Flight distance distributions for $m_{{A^{\prime}}\xspace} = \{10,20,50,100\}$$\mathrm{\: Me V}$ showing (left) $\ell \times\left(\epsilon^2/10^{-8}\right)$ and (right) $\ell_{\rm T} \times \left(\epsilon^2/10^{-8}\right)$.
• Figure 5: Distribution of $m_{e^+e^-}$ with (solid, dashed) and without (dotted) incorporating the $D^{*0}$ mass constraint for $m_{{A^{\prime}}\xspace} = \{10,20,50,100\}\mathrm{\: Me V}\xspace$. The solid curve shows better performance than the dashed one because F-type $D^0$ candidates have better momentum resolution than P-type ones.