Discovering New Light States at Neutrino Experiments

TL;DR

This work analyzes how high-luminosity neutrino experiments can probe very weakly coupled light exotics, focusing on pseudo-Nambu-Goldstone bosons (PNGBs) and kinetically mixed gauge bosons ($A'$). By modeling PNGB and $A'$ production in proton-nucleus and muon interactions and reinterpreting LSND and modern beamline data, the authors derive new constraints and highlight regions of parameter space accessible to current and planned facilities, including COMPASS and Project $X$. They show LSND already constrains PNGBs below $2m_\mu$ and that modern experiments like MINOS/MINERvA and MiniBooNE can extend the reach, while muon-fixed-target experiments offer a complementary path to test leptophilic PNGBs and address gaps with $(F,m_a)$ and $(m_{A'},\epsilon)$. The results underscore the practical significance of dedicated analyses of neutrino-detector data for discovering or constraining new weakly coupled sectors, with clear guidance for future experimental strategies.

Abstract

Experiments designed to measure neutrino oscillations also provide major opportunities for discovering very weakly coupled states. In order to produce neutrinos, experiments such as LSND collide thousands of Coulombs of protons into fixed targets, while MINOS and MiniBooNE also focus and then dump beams of muons. The neutrino detectors beyond these beam dumps are therefore an excellent arena in which to look for long-lived pseudoscalars or for vector bosons that kinetically mix with the photon. We show that these experiments have significant sensitivity beyond previous beam dumps, and are able to partially close the gap between laboratory experiments and supernovae constraints on pseudoscalars. Future upgrades to the NuMI beamline and Project X will lead to even greater opportunities for discovery. We also discuss thin target experiments with muon beams, such as those available in COMPASS, and show that they constitute a powerful probe for leptophilic PNGBs.

Figures (5)

• Figure 1: Left: The rest-frame lifetime of a pseudo-Nambu-Goldstone boson (PNGB) as a function of its mass $m_a$ and decay constant $F$. Right: The lifetime of a dark photon $A'$ as a function of its mass $m_{A'}$ and $\epsilon$, the strength of its mixing with the Standard Model hypercharge gauge boson. In both plots, the black lines correspond to different decay lengths ($c\tau$): $10~\mu m$ (solid), 1 cm (dot-dashed), 1 m (dashed), and 100 m (dotted). In the blue, purple, red, green, and white shaded regions the decays are prompt ($<10~\mu$m), displaced with $<1$ cm, displaced with $>1$ cm, "invisible" with $>100$ cm, or "invisible" with $> 100$m, respectively. In fixed-target or beam dump experiments the particles typically get a large boost that increases their decay length by $E_{\rm beam}/$mass. The feature in the left plot at 2$m_\mu$ occurs since PNGB's coupling to a Standard Model particle is proportional to that particle's mass, and at this point decays to two muons are allowed. The dip in the right plot near 0.7 GeV is due to the $\rho$-resonance. The lifetime for both the PNGB and the $A'$ is calculated assuming decays directly into Standard Model particles.
• Figure 2: Left: Constraints on pseudo-Nambu-Goldstone bosons as a function of their decay constants $F$ and their mass $m_a$ from various meson decays: $K^+\to {\rm anything}+e^+e^-$ (green), $K^+\to \pi^++$ invisible (blue), $B^+\to K^+ \ell^+\ell^-$ (yellow) ($\ell = e, \mu$), and $B^+\to K^+ +$ invisible (red). Constraints from $\Upsilon(1S)$ or $\Upsilon(3S) \to \gamma a \to \gamma +$ invisible and $K^+\to \pi^+ \ell^+\ell^-$ decays are weaker than those from $B^+\to K^+ +$ invisible and $B^+\to K^+ \ell^+\ell^-$, respectively, and thus not shown. Details are in § \ref{['sec:mesons']}. Right: Gray shaded background region is the combined exclusion region from meson decays in the left figure. In the green exclusion region, the proton beam dump experiment CHARM at CERN would have seen at least five events (this exclusion region agrees roughly with that in Bergsma:1985qz) -- see § \ref{['sec:modern']}. Here the PNGB is produced directly in the proton dump by a small mixing with the pion. For $m_a < 2 m_\mu$, the PNGB decays to an electron pair, while in the "bubble" for $m_a>2 m_\mu$ the PNGB decays predominantly to a muon pair. The blue region is the limit from the supernova SN 1987a (see § \ref{['sec:SN']}). The light red region is the constraint from the muon anomalous magnetic moment and fills the gap for low $m_a$ and $F$ left by the meson constraints (see § \ref{['sec:amu']}). The region excluded by the Fermilab E137 dump lies mostly within the CHARM excluded region and is not shown (it is instead shown in Fig. \ref{['fig:comboLepto']}).
• Figure 3: Sensitivity of various neutrino experiments to pseudo-Nambu-Goldstone bosons as a function of their decay constants $F$ and their mass $m_a$. The thick (thin) black solid line corresponds to 10 (1000) events in LSND, the thick (thin) dashed blue line corresponds to 3 (1000) events in MiniBooNE, and the thick (thin) dot-dashed red line corresponds to 3 (1000) events in MINOS/MINERvA (in each case the inner regions correspond to more events than indicated by the line). Here the PNGB is produced directly in the proton dump by a small mixing with the pion. For $m_a < 2 m_\mu$, the PNGB decays to an electron pair, while in the "bubbles" for $m_a>2 m_\mu$ the PNGB decays predominantly to a muon pair. The gray shaded regions are the combined existing constraints from other beam dump experiments, meson decays, anomalous muon magnetic moment, and SN 1987a shown in the right plot of Fig. \ref{['fig:constraints']}.
• Figure 4: The sensitivity of various beam dump, collider, and astrophysical probes of light vector bosons that kinetically mix with the standard model hypercharge, as a function of the kinetic mixing parameter $\epsilon$ and the vector boson mass $m_{A'}$ (from Bjorken:2009mm). The thick (thin) solid black line corresponds to 10 (1000) signal events in LSND. The region enclosed by the thick line can be viewed as a very rough exclusion limit from the LSND experiment. A re-analysis of the LSND data by the LSND collaboration could further extend the sensitivity of that experiment Batell:2009di. Further details are described in the text.
• Figure 5: Sensitivity and constraints of various experiments to leptophilic pseudo-Nambu-Goldstone bosons as a function of their decay constants $F$ and their mass $m_a$. Here the PNGB is produced by bremsstrahlung off an incident muon or electron beam. Thick (thin) lines show rough sensitivity regions and correspond to 3 (1000) displaced $e^+e^-$ pairs in MINOS/MINERvA (red dot-dashed lines), MiniBooNE (blue dashed lines), and in a thin target experiment using the COMPASS muon beam (green dotted line, see § \ref{['sec:thin']}). The thick (thin) dotted black lines correspond to $S/\sqrt{B} = 3~(10)$, where $S$ ($B$) are the number of prompt $\mu^+\mu^-$ signal (radiative background) events in COMPASS (see § \ref{['sec:thin']}) (we have ignored the Bethe-Heitler background and the finite acceptance, so these lines should not be viewed as real significance lines but only as very rough estimates of what could be probed). Inside the gray shaded region, E137 would have seen at least one event -- since they saw none, this region gives their approximate constraint. Details are described in the text. The light red region is the constraint from the muon anomalous magnetic moment (see § \ref{['sec:amu']}).