Search for Invisible Decays of a Dark Photon Produced in e+e- Collisions at BaBar (1702.03327v2)

Abstract: We search for single-photon events in 53 fb^-1 of e+e- collision data collected with the BaBar detector at the PEP-II B-factory. We look for events with a single high-energy photon and a large missing momentum and energy, consistent with production of a spin-1 particle A' through the process e+e->gamma A', A'->invisible. Such particles, referred to as "dark photons", are motivated by theories applying a U(1) gauge symmetry to dark matter. We find no evidence for such processes and set 90% confidence level upper limits on the coupling strength of A' to e+e- in the mass range m_A'<=8 GeV. In particular, our limits exclude the values of the A' coupling suggested by the dark-photon interpretation of the muon (g-2) anomaly, as well as a broad range of parameters for the dark-sector models.

• The paper presents a search for dark photons via single high-energy photon events in e+e⁻ collisions, targeting invisible decay signatures.
• It employs a multivariate Boosted Decision Tree with 12 discriminating variables to separate signal from background in 514 fb⁻¹ of data.
• The study establishes 90% confidence level upper limits on the dark photon coupling, excluding viable parameter space for the muon g-2 anomaly.

Search for Invisible Decays of a Dark Photon Produced in $e^+e^-$ Collisions at BABAR

The paper conducted by the BABAR Collaboration investigates the potential presence of dark photons, represented as $A'$, through the detection of single-photon events in electron-positron ($e^+e^-$) collisions. This research, conducted at the BABAR detector located at the SLAC National Accelerator Laboratory, explores a theoretical extension to the Standard Model positing a $U(1)$ gauge symmetry applied to the dark sector. Such extensions have been considered in efforts to resolve discrepancies such as the muon $g-2$ anomaly and offer novel explanations for certain astrophysical phenomena.

Methodology

To identify the presence of the elusive dark photons, the research utilizes a dataset of $\mathcal{L}$ corresponding to $L=514 \text{fb}^{-1}$ of $e^+e^-$ collision data. The experimental approach approximates the presence of $A'$ by observing the process $$e^+e^-\rightarrow\gamma A'; A'\rightarrow\text{invisible}$$, manifesting as a single high-energy photon event with missing momentum and energy – haLLMarks of an undetected $A'$ particle.

Disparate signal and background events are discriminated through a multivariate Boosted Decision Tree (BDT) algorithm leveraging 12 discriminating variables designed to optimize the expected upper limits on the $A'$ cross section $\sigma_{A'}$. The detector’s specifics, the $BABAR$ trigger systems, and the data processing protocols have been meticulously crafted to reduce significant background interference from dominant processes such as $e^+e^-\rightarrow\gamma\gamma$ and low-angle radiative Bhabha scattering.

Results and Constraints

Observational analyses yielded no positive signals for the existence of $A'$. The paper establishes 90% confidence level upper limits on the possible coupling strength $\varepsilon$ of the dark photon to electrons across the mass range $m_{A'}\le8$ GeV. These constraints significantly intersect with and exclude parameter spaces previously suggested as viable explanations for the muon $g-2$ anomaly, including $\varepsilon\sim10^{-3}$.

Implications and Future Directions

The implications of these findings are substantial for both phenomenological models and experimental approaches in dark matter and particle physics. The exclusion of specific mass and coupling parameters for $A'$ narrows the possibilities for dark matter-related processes consistent with observable discrepancies in the Standard Model. Moreover, the methodology and framework established here can serve as a pivotal reference for future experimental searches at higher energies and luminosities, or utilizing different collision configurations.

Future research may refine or extend upon these constraints through collaborative efforts, wherein other experimental setups or improvements in detector sensitivities may re-evaluate the assumptions and outcomes of this analysis. Theoretical advancements may similarly guide further experimental directions, exploiting novel interpretations or theoretical models extending beyond current dark photon frameworks.

Thus, this paper represents a crucial step in advancing the discourse within dark matter research, offering stringent experimental constraints and thoughtful insights to propel both theoretical and experimental explorations of the dark sector onward.