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Search for dark forces

Physics program > Standard Model tests

The Heavy Photon Search (HPS) is an experiment proposed for Jefferson Laboratory to search for new heavy vector boson(s), aka "heavy photons" or "dark photons" or "hidden sector photons", in the mass range of $20$ MeV/c$^2$ to $1000$ MeV/c$^2$. Such particles will arise if there are additional U(1) gauge bosons in nature, and they will couple, albeit weakly, to electric charge through kinetic mixing. Many Beyond-the-Standard-Model theories predict the existence of additional U(1)'s, and recent observations of high energy electrons and positrons  in the cosmic rays may be the result of primordial dark matter annihilating into heavy photons. HPS searches for electro-produced heavy photons that decay in di-lepton pairs using both invariant mass and separated decay vertex signatures. With good mass resolution, heavy photons will appear as sharp resonances above the QED continuum. For  suitable values of mass and coupling, heavy photons will have long lifetimes, resulting in discernible secondary decay vertex. The HPS employs a large acceptance forward magnetic spectrometer with precise momentum measurement and vertexing capability, followed by a highly segmented lead-tungstate crystal electromagnetic calorimeter for fast  triggering and electron identification. Tracks position is detected by silicon microstrips while the state-of-the-art data acquisition system based on 250MHz fADCs allows for high ratetriggering to fully exploit CEBAF CW, high intensity, electron beam.

A muon identification system, just downstream of the ECal, significantly boosts the experimental reach for heavy photon masses above the di-muon threshold and provides an independent trigger. HPSprobes a unique region of the mass-coupling parameter space where the heavy photon signal would be lost in the trident background without the vertex signature, and it simultaneously accesses a region at higher coupling strength by relying on bump hunting alone. HPS is sensitive to a region of parameter space favored by accounting for the discrepancy between measured and calculated values for the muon's g-2 with the existence of a heavy photon, and probes an extensive region suggested by parameters which could account for dark matter annihilations into heavy photons. In broader terms, HPS searches for heavy photons in a region suggested on very general theoretical grounds. With the final approval from JLab management, and US-DOE approval and funding, the HPS  Collaboration can design, build and commission the experiment and begin in earnest its search for spectacular new physics at the Intensity Frontier.

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