Longer-lived mediators from charged mesons and photons at neutrino experiments

Since many of the dark-sector particles interact with Standard Model (SM) particles in multiple ways, they can appear in experimental facilities where SM particles appear in abundance. In this study, we explore a particular class of longer-lived mediators that are dominantly produced from photons and charged mesons that arise in proton-beam fixed-target-type neutrino experiments. This class of mediators encompasses light scalars that appear in theories like extended Higgs sectors, muon(electro)philic scalars, etc. We evaluate the sensitivities of these mediators at beam-based neutrino experiments such as the finished ArgoNeuT, ongoing MicroBooNE, SBND, ICARUS, and the upcoming DUNE experiment. We find that muonphilic scalars are more enhanced while produced from three-body decay of charged mesons. The above-mentioned experiments can probe unexplored regions of parameter space that can explain the current discrepancy in the anomalous magnetic moment of muons. We further find that Compton-like scattering of photons is the largest source of electrophilic scalars. By utilizing this, the DUNE Near Detector can explore new regions in the sensitivity space of electrophilic scalars. We also show that Bethe-Heitler scattering processes can be used to probe flavor-specific lepton final states even for the mediator masses below twice the lepton mass.

Figure 1

Energy spectra of muonphilic scalars at (a) DUNE ND and (b) ICARUS. Three scalar mass values are shown: $m_{\varphi }=0.01$ GeV, 0.1 GeV, and 0.16 GeV with Yukawa coupling $Y_{22}={10}^{-4}$.

Figure 2

Feynman diagrams depicting the production of scalars from charged kaons. (a) Production of both HPS and flavor-specific scalars from the charged lepton leg. (b) HPS produced from the $W$ boson leg. (c) Feynman diagram for two-body decay of $K^{+}$ to ${\pi }^{+}$ and HPS $S$ which couples to an intermediate top quark.

Figure 3

Feynman diagrams depicting the production of scalars from photons. (a) Primakoff scattering of a photon to produce HPS as well as flavor-specific scalars. (b) Compton-like scattering of a photon to produce an electrophilic scalar.

Figure 4

Variation of $I(\beta )$ with $\beta$.

Figure 5

Feynman diagrams depicting the production of scalars from electrons and positrons: (a) associated production with positrons impinging on target electrons and (b) resonance production.

Figure 6

Number of scalars produced from each source per unit coupling. Left: Higgs portal scalars; Center: muonphilic scalars; and Right: electrophilic scalars.

Figure 7

Feynman diagrams illustrate the various scattering channels that mediators can undergo once they reach the detector. (a) Inverse Primakoff process where incoming scalars ($\ell =e$, $\mu$) scatter into a single photon. (b) Bethe-Heitler splitting process. (c) Inverse Compton-like scattering of ${\varphi }_e$ into an electron and a photon.

Figure 8

Number of scalars that reach the detector as a function of mass for various couplings. Left, Higgs portal scalars; center, muonphilic scalars; and right, electrophilic scalars.

Figure 9

95% CL lines for our three benchmark scalar mediator models under the assumption of zero backgrounds. (a) Higgs portal scalars. The dotted lines are from two-body decays at other experiments studied in [31]. The 95% CL sensitivity lines for $m_S<0.21\mathrm{GeV}$ correspond to the decay of scalars to $e^{+},e^{-}$ only. Beyond 0.21 GeV, the hanging lobe includes ${\mu }^{+},{\mu }^{-}$ signals and ${\pi }^{+},{\pi }^{-}$ as well for $m_S>0.278\mathrm{GeV}$. In this plot, we include 90% CL bounds imposed by CHARM [70], LSND [71], PS191 [72], NA62 [73], E949 [74], and 95% CL bounds from $\mu \mathrm{BooNE}$ [29], and LHCb [75]. (b) Muonphilic scalar model. The pink band shows the preferred regions of the current muon $g-2\mu$ anomaly. Scalars lighter than 0.21 GeV dominantly scatter via spitting into ${\mu }^{+}{\mu }^{-}$, and decay into two photons, the latter dominating for $m_{\varphi }>0.0001\mathrm{GeV}$. For scalars greater than 0.21 GeV they decay to ${\mu }^{+}{\mu }^{-}$, which is manifested by the sharp drop in sensitivity. The 90% CL bounds from E137 [38], 95% CL bounds from B factories, $g-2$ excluded regions, and constraints from SN cooling are depicted in the shaded regions. (c) Electrophilic model. The parameters in the orange band of this model explain current electron $g-2$ uncertainty. These scalars give rise to two photons via decay and $e^{+},e^{-}$ via splitting for $m_{\varphi }<0.001\mathrm{GeV}$, and $e^{+},e^{-}$ only via decay for $m_{\varphi }>0.001\mathrm{GeV}$. The shaded regions represent 90% CL bounds from E137 [69], E141 [76], E774 [77], Orsay [78], NA64 [79], and HB stars [41] and SN cooling [80].

Figure 10

Sensitivities where experimental and detection constraints are considered. (a) Sensitivity plot for HPSs for three events as well as 100 events for DUNE ND and ICARUS. In this plot, we include 90% CL bounds imposed by CHARM [70], LSND [71], PS191 [72], NA62 [73], E949 [74], and 95% CL bounds from $\mu \mathrm{BooNE}$ [29], and LHCb [75]. (b) Parameters for 10, 100 muonphilic scalar decays at DUNE ND. The 90% CL bounds from E137 [38], 95% CL bounds from B factories, $g-2$ excluded regions, and constraints from SN cooling are depicted in the shaded regions. (c) Sensitivity plots for electrophilic scalars at DUNE ND where the minimum threshold kinetic energy ($E_t$) required to identify the decay products is 0, 10, and 30 MeV. The shaded regions represent 90% CL bounds from E137 [69], E141 [76], E774 [77], Orsay [78], NA64 [79], and HB stars [41] and SN cooling [80].

Figure 11

95% CL sensitivity plot (with no backgrounds) of the $U(1{)}_{T_{3R}}$ gauge boson with only visible decays.

Figure 12

Energy flux of photons, positrons, charged pions, and kaons produced at BNB.

Figure 13

Energy flux of photons, positrons, charged pions, and kaons produced at NuMI.