[Paper Review] Searching for a U-boson with a positron beam

B. Wojtsekhowski|ArXiv.org|Jun 29, 2009

Dark Matter and Cosmic Phenomena2 references4 citations

TL;DR

This paper proposes a high-sensitivity search for a light U-boson using a 160–330 MeV positron beam incident on a liquid hydrogen target. By detecting photons from the reaction $e^+e^- \to \gamma X$ and reconstructing missing mass, the experiment achieves a projected upper limit of $|f_{eU}|^2 = 3 \times 10^{-9}$, representing a 1500-fold improvement over existing $g-2$ constraints at 10 MeV mass.

ABSTRACT

A high sensitivity search for a light \Ub{} by means of a positron beam incident on a hydrogen target is proposed. We described a concept of the experiment and two possible realizations. The projected result of this experiment corresponds to an upper limit on the square of coupling constant $ |f_{_{eU}}|^2 = 3 imes 10^{-9}$ with a signal to noise ratio of five.

Motivation & Objective

• To search for a light U-boson with mass below 100 MeV via a high-sensitivity experiment using a positron beam.
• To overcome limitations of existing $g-2$ and invisible decay mode constraints by directly probing the electron-U-boson coupling $f_{eU}$.
• To achieve a sensitivity improvement of up to 1500× over current $g-2$-based limits for $m_U = 10$ MeV.
• To demonstrate the feasibility of using a low-energy positron beam on a stationary hydrogen target as a compact, high-luminosity 'collider' for U-boson searches.
• To provide a direct measurement of $f_{eU}$ using photon energy and angle reconstruction in the $e^+e^- \to \gamma X$ process.

Proposed method

• Use a 160–330 MeV positron beam incident on a 1 cm liquid hydrogen target to produce $e^+e^-$ pairs via electron-positron scattering.
• Detect high-energy photons at small angles (1.9°–5.5° in lab frame) using a segmented photon detector to identify $e^+e^- \to \gamma X$ events.
• Employ a sweep magnet to remove charged particles and reduce detector background, or use full data acquisition with off-line vetoing of $e^+e^-$ scattering events.
• Reconstruct the missing mass from photon energy and angle to search for a peak corresponding to a U-boson of mass $m_U \approx \sqrt{E_{\text{beam}}}$.
• Calibrate the detector response using $\gamma\gamma$ coincidence events from $e^+e^- \to \gamma\gamma$ and a white photon spectrum from a beryllium or carbon target.
• Use Monte Carlo simulations to model background rates, particularly from Bhabha scattering with photon radiation, and estimate signal-to-noise ratios.

Experimental results

Research questions

• RQ1 Can a high-luminosity positron beam on a stationary hydrogen target achieve sufficient sensitivity to detect a light U-boson with $m_U \sim 10$ MeV?
• RQ2 What is the projected sensitivity of the $e^+e^- \to \gamma X$ missing mass method for detecting a U-boson with vector coupling to electrons?
• RQ3 How does the signal-to-noise ratio in the photon energy spectrum compare to the QED background, and can it be optimized to detect a signal at $10^{-6}$ of the background level?
• RQ4 Can the coupling constant $f_{eU}$ be directly measured with better precision than current $g-2$ constraints?
• RQ5 What experimental configurations—external target or internal target in a storage ring—maximize luminosity and minimize background for this search?

Key findings

• The experiment projects a sensitivity limit of $|f_{eU}|^2 = 3 \times 10^{-9}$, corresponding to a signal five times above the statistical fluctuation of the background.
• This sensitivity represents a 1500-fold improvement over the current upper limit from electron $g-2$ measurements at $m_U = 10$ MeV.
• The background rate in the 15%-wide energy window is estimated at 1.2% of the $e^+e^- \to \gamma\gamma$ annihilation rate, with a statistical uncertainty of $0.1 \times 10^7$ events.
• A 3-month run with $10^{34}$ Hz/cm² luminosity and a 330 MeV positron beam would accumulate $0.8 \times 10^{14}$ events, enabling the required statistics.
• The use of a segmented photon detector with 5% energy resolution and angular acceptance of 2°–6° allows effective separation of the U-boson signal from QED background.
• The method is feasible at existing facilities such as the SLAC positron damping ring or BINP's VEPP-III, using an internal target and existing beamline infrastructure.