Search for a dark photon in e+e- collisions at BABAR (1406.2980v2)

Abstract: Dark sectors charged under a new Abelian force have recently received much attention in the context of dark matter models. These models introduce a light new mediator, the so-called dark photon (A'), connecting the dark sector to the Standard Model. We present a search for a dark photon in the reaction e+e- -> gamma A', A'-> e+e-, mu+mu- using 514 fb-1 of data collected with the BABAR detector. We do not observe a significant signal and we set 90% confidence level upper limits on the mixing strength between the photon and dark photon at the level of 10^-4 - 10^-3 for dark photon masses in the range 0.02 - 10.2 GeV. We further constrain the range of the parameter space favored by interpretations of the discrepancy between the calculated and measured anomalous magnetic moment of the muon.

• The paper presents a comprehensive search for dark photons in e+e- collisions, setting 90% confidence level limits on the mixing parameter between 0.02 and 10.2 GeV.
• It employs advanced event selection techniques and neural network-based cuts to suppress backgrounds and calibrate Monte Carlo simulations effectively.
• The results improve constraints on dark photon models and advance experimental efforts to resolve discrepancies like the muon’s anomalous magnetic moment.

Search for a Dark Photon in Electron-Positron Collisions at BABAR

The paper presents a meticulous search for dark photons in electron-positron collisions utilizing data from the BABAR detector. This experimental search probes the existence of a dark photon, denoted as $A'$, which arises from theories extending the Standard Model (SM) to include new gauge interactions within dark sectors. These sectors feature particles interacting weakly with SM particles, potentially mediated by a light gauge boson—the dark photon—coupled to the SM via kinetic mixing.

Experimental Approach

The search was conducted using a dataset of 514 $\text{fb}^{-1}$ collected by the BABAR detector, mainly at the $\Upsilon(4S)$ resonance, with additional data near $\Upsilon(3S)$ and $\Upsilon(2S)$. The focus lies on the reaction $$e^+e^- \rightarrow \gamma A', A' \rightarrow e^+e^-$$ and $A' \rightarrow \mu^+\mu^-$, exploring dark photon masses in the range of 0.02–10.2 GeV.

A comprehensive selection was applied to identify events characteristic of dark photon production, involving cuts and neural network approaches to suppress dominant backgrounds, such as radiative Bhabha processes and photon conversions. Event reconstruction necessitated careful correction and calibration against Monte Carlo simulations, ensuring efficient background discrimination.

Results and Constraints

The experimental analysis did not observe any significant deviation from the Standard Model expectations, culminating in setting 90% confidence level upper limits on the mixing parameter $\epsilon$ between the dark photon and the SM photon. The limits reached the sensitivity of $10^{-4} - 10^{-3}$ across the investigated dark photon mass range. These constraints markedly enhance existing limits from other experiments like KLOE, WASA-at-COSY, and HADES, offering a more stringent exclusion of the parameter space which aims to address the anomaly in the muon’s anomalous magnetic moment ($g-2$).

Implications and Further Research

The results reaffirm the BABAR experiment's prowess in probing new physics within low-mass regimes, highlighting the experimental challenges of isolating potential new particles with feeble interactions. The implications are significant—constraints on the dark photon bolster the accuracy of dark matter models, particularly on interactions that might contribute to cosmic ray anomalies and potential resolutions to SM discrepancies, such as the muon’s $g-2$.

Looking forward, the paper sets a precedent for future explorations involving low-mass dark mediators. Planned experiments are expected to probe the remaining regions of allowed parameters, especially in the low-mass sector under 50 MeV, refining our understanding of dark sectors and their interactions with the visible universe. Continued efforts in this direction are critical to uncovering potential hidden interactions contributing to the broader knowledge of particle physics and cosmology.