Exploring Portals to a Hidden Sector Through Fixed Targets (0906.5614v2)

Abstract: We discuss the sensitivity of neutrino experiments at the luminosity frontier to generic hidden sectors containing new (sub)-GeV neutral states. The weak interaction of these states with the Standard Model can be efficiently probed through all of the allowed renormalizable portals' (in the Higgs, vector, and neutrino sectors) at fixed target proton beam facilities, with complementary sensitivity to colliders. We concentrate on the kinetic-mixing vector portal, and show that certain regions of the parameter space for a new secluded U(1) gauge sector with long-lived sub-GeV mass states decaying to Standard Model leptons are already severely constrained by the datasets at LSND, MiniBooNE, and NuMI/MINOS. Furthermore, scenarios in which portals allow access to stable neutral particles, such as MeV-scale dark matter, generally predict that the neutrino beam is accompanied by adark matter beam', observable through neutral-current-like interactions in the detector. As a consequence, we show that the LSND electron recoil event sample currently provides the most stringent direct constraint on MeV-scale dark matter models.

• The paper presents a comprehensive sensitivity analysis of neutrino experiments to detect sub-GeV hidden sector particles via renormalizable Higgs, vector, and neutrino portals.
• It establishes stringent constraints on kinetic mixing (10⁻⁸ to 10⁻⁴) using data from LSND, MiniBooNE, and MINOS, challenging traditional collider approaches.
• The study indicates promising future directions for probing dark matter by leveraging fixed target setups in existing and upcoming neutrino facilities.

Sensitivity of Neutrino Experiments to Hidden Sectors via Fixed Target Probes

The subject of this overview is a detailed theoretical paper of the potential sensitivity of neutrino experiments at fixed target facilities to hidden sector physics, with a particular focus on sub-GeV neutral states. The authors, Batell, Pospelov, and Ritz, confront the limitations of traditional high-energy collider explorations by emphasizing a complementary strategy through portals available at the luminosity frontier. This is achieved particularly well in neutrino facilities such as LSND, MiniBooNE, and NuMI/MINOS, among others.

Analysis of Portals and Parameters

The investigation bases itself on exploring generic interactions through renormalizable portals—Higgs, vector, and neutrino. Particularly, the manuscript explores the kinetic-mixing vector portal and its significance in probing a hidden U(1) gauge sector. This is broadly relevant to contemporary dark matter models that hypothesize weak interactions outside the Standard Model.

The paper meticulously analyzes the potential production and subsequent detection of hidden sector particles considering decay lengths and cross-sections implicated by the kinetic-mixing and Higgs portal parameters. This involves comprehensive exploration of production mechanisms such as neutral pion or vector decays and secondary hadronic processes.

Numerical Results and Constraints

Strong constraints on the parameter space are mandated by datasets from existing experiments, especially LSND's extensive dataset, which provides a remarkable opportunity to rule out or confirm kinetic mixing at levels of $10^{-8}$ to $10^{-4}$ for vector masses lower than the pion mass. The paper indicates a significant number of predicted electron pair decay events is inconsistent with measured backgrounds, suggesting profound constraints on hidden sectors at LSND.

Moreover, prospective sensitivity in higher energy regimes is examined through MiniBooNE and MINOS, indicating that production modes capitalize on $\Delta$ decays and QCD processes, respectively. A promising future in this regard is anticipated with setups like T2K and NOvA, positing improvements in sensitivity to explore larger mass scales.

Theoretical and Practical Implications

The theoretical implications of these findings are numerous. They underscore the latent potential in utilizing existing neutrino experiments to extend searches beyond the usual paradigms of dark matter—crossing into dimensions of MeV-scale constituents and dark radiation accompanying traditional neutrino beams. These explorations elucidate substantial phenomenological extrapolation in constructing models tied to light dark matter and secluded gauge sectors.

Practically, the research suggests an extended horizon for probing electroweak-scale physics via sub-GeV contributions in fixed target settings. This echoes a methodology shift that could diversify approaches traditionally linear, setting a collaboratively intersecting frontier between cosmological observations and terrestrial accelerator physics.

Future Directions

The manuscript hints at rich possibilities for bridging constraints derived from improved luminosity measures, precise control in systematic neutrino experiments, and dissecting higher dimensional operators. The anticipated development of neutrino facilities accelerates these constraints, furthering dialogues about dark sector accessibility beyond the current quark-gluon narrative.

Importantly, the inclusion of exotic contributions such as DM beams—implying potential neutral-current-like interactions—is a fascinating direction. The LSND constraints are notably stringent, prompting an exhaustive review on this topic with a view towards updated MM-scale dark matter scenarios.

In conclusion, this paper meticulously bridges the conceptual and empirical divides, making a strong case for the expanded role of neutrino experiments in exploring and, potentially, constraining hidden sector physics, reiterating their unmatched potential in expanding the sensitivity horizon for new fundamental interactions.