Search for a new gauge boson in the $A'$ Experiment (APEX) (1108.2750v2)

Abstract: We present a search at Jefferson Laboratory for new forces mediated by sub-GeV vector bosons with weak coupling $\alpha'$ to electrons. Such a particle $A'$ can be produced in electron-nucleus fixed-target scattering and then decay to an $e^+e^-$ pair, producing a narrow resonance in the QED trident spectrum. Using APEX test run data, we searched in the mass range 175--250 MeV, found no evidence for an $A'\to e^+e^-$ reaction, and set an upper limit of $\alpha'/\alpha \simeq 10^{-6}$. Our findings demonstrate that fixed-target searches can explore a new, wide, and important range of masses and couplings for sub-GeV forces.

• The paper demonstrates a fixed-target electron scattering experiment to search for the A' gauge boson, establishing an upper limit on the coupling at approximately 10⁻⁶.
• Researchers used high-resolution spectrometers and a tantalum target to accurately detect e⁺e⁻ pairs, effectively isolating potential signals from QED background processes.
• The results narrow the parameter space for sub-GeV gauge bosons, providing crucial constraints for extensions to the Standard Model and related dark matter theories.

Overview of the APEX Search for a New Gauge Boson

The paper under review details the findings and methodology of the A' Experiment (APEX) conducted at Jefferson Laboratory. The primary focus of the experiment was the search for new forces mediated by sub-GeV vector gauge bosons, specifically an abelian vector boson designated as $A'$. The context of this research situates itself within the broader investigation of potential forces that could have eluded detection within the current framework of the Standard Model, particularly if such forces are mediated by particles with weak coupling to known fermions. This paper leverages electron-nucleus fixed-target scattering experiments to explore this uncharted territory.

Experimental Framework and Technique

The APEX collaboration designed an experimental setup optimized to detect $A'$ decays via $e^+e^-$ in the invariant mass range of 175–250 MeV. A fixed-target experiment was conducted using the Continuous Electron Beam Accelerator Facility at Jefferson Laboratory, utilizing a tantalum target and observing the resultant $e^+e^-$ pairs. High-resolution spectrometers (HRSs) were employed to precisely measure these pairs' trajectories and energies, essential to identify the narrow resonances indicative of $A'$ presence over prevailing quantum electrodynamic backgrounds.

The experiment capitalized on the high luminosity and favorable kinematics intrinsic to fixed-target setups to probe weak coupling parameters and vector boson masses below one GeV. Importantly, APEX implemented techniques to minimize background interference, primarily stemming from quantum electrodynamics (QED) trident processes. By calibrating their set-up with precision, they were able to isolate potential $A'$ signals from these backgrounds with high accuracy.

Results and Implications

The results of the test run did not reveal a statistically significant signal for $A'$ mediated forces within the explored mass range, setting an upper limit on the coupling $\alpha'/\alpha \approx 10^{-6}$ at a 90% confidence level. This constraint is pivotal as it extends the parameter space exploration for sub-GeV gauge bosons, especially in the context of addressing anomalies related to dark matter and the muon’s anomalous magnetic moment, which have previously implicated the possible existence of such bosons.

APEX’s contribution to understanding potential new forces is significant in its ability to narrow down the regions where these forces could manifest, thereby refining the focus for future investigations. The experiment’s robust design and methodology offer a scalable framework that can be expanded in terms of energy range and data collection, promising further insights into this paradigm of particle physics.

Future Perspectives

As a precursor to more extensive investigations, this test run forms the foundation for subsequent experiments that aim to enhance sensitivity across a more extensive range of masses and coupling strengths. Future iterations of APEX and similar ventures are crucial to deepen the search for deviations from Standard Model predictions potentially caused by weakly interacting vector bosons. Such refinements could eventually illuminate the potential connections between these particles and larger cosmological questions, particularly regarding dark matter. Furthermore, these experiments pave the way for potentially incorporating other decay channels and exploring the existence of more complex hidden sector dynamics.

In conclusion, the APEX experiment has made significant strides in expanding our understanding of potential new forces through sophisticated experimental techniques and careful analyses. The paper underlines the viability of fixed-target electron scattering as an instrumental approach in progressing the search for less constrained, theoretically motivated gauge bosons, thereby facilitating the continued exploration of theories extending beyond the Standard Model.