Dedicated Searches for Millicharged Particles at Intensity-Frontier Facilities: SpinQuest and SHiP

TL;DR

This work addresses millicharged particle searches at intensity-frontier fixed-target facilities by projecting sensitivity for SpinQuest and SHiP using three production channels: meson decays, Drell–Yan, and proton bremsstrahlung. The authors develop a QRA-based, gauge-invariant treatment of proton bremsstrahlung, incorporate detailed detector and background modeling, and show that forward proton bremsstrahlung dominates the low-mass region ($m_χ o$ sub-GeV) and significantly enhances discovery potential. Their projections yield 95% CL limits that improve the millicharge reach to $ε$ values as small as ~9×10^−5 around $m_χ oughly 200$ MeV, with up to 2 orders of magnitude improvement in the 1–10 GeV range when including bremsstrahlung. The results highlight the importance of intensity-frontier beams for mCP searches, demonstrate the complementarity of SpinQuest and SHiP, and provide a framework that can be extended to other facilities and detector concepts.

Abstract

We conduct a dedicated study of searches for millicharged particles (mCPs) utilizing scintillator-based detectors at high-intensity fixed-target experiments, with particular focus on the SpinQuest and forthcoming Search for Hidden Particles experiment (SHiP) facilities. The analysis incorporates the three primary production mechanisms: meson decays, Drell-Yan processes, and proton bremsstrahlung. In particular, our updated analysis reveals that proton bremsstrahlung dominates the production rate in the sub-GeV mass regime. Detailed detector simulations and background evaluations are performed to obtain realistic sensitivity estimates. Our results demonstrate that future experiments located in the SpinQuest and SHiP facilities can achieve substantial improvements in discovery potential, enhancing sensitivity to the mCP charge parameter $ε=q_χ/e$ (with $q_χ$ denoting the mCP electric charge) by up to two orders of magnitude relative to existing limits.

Figures (8)

• Figure 1: Schematic of the SpinQuest detector, modified from Ref. Apyan:2022tsd, showing the placement of a proposed 3-layer $18 \times 18$ scintillator bar mCP detector located approximately $40$ m downstream of the target.
• Figure 2: Schematic layout of the SHiP, adapted from Ref. Albanese:2878604, with the proposed three-layer $18 \times 18$ scintillator bar mCP detector setup located approximately $100$ m downstream of the target.
• Figure 3: Pair production of $\chi\bar{\chi}$ via initial state radiation in a generic non–diffractive scattering event.
• Figure 4: Distributions of mCP angle ($\theta_\chi$) relative to the beam axis and momentum ($p_\chi$) for $m_\chi = 10\,\mathrm{MeV}$, shown for the two main production mechanisms: proton bremsstrahlung (left) and $\rho$ meson decay (right).
• Figure 5: The mCP production projection for SpinQuest and SHiP from meson decays, Drell-Yan, and proton bremsstrahlung (sum of single tracks and double tracks hitting the same bar, with the Dirac electromagnetic form factor only). The uncertainty band corresponds to varying the associated cut-off scale $\Lambda_p \in [1,2]$ GeV, with the central value set to 1.5 GeV. The detector used in the simulation consists of $18\times 18$ scintillator bars/layer, located 40 m (a) and 100 m (b) downstream of the target. Proton bremsstrahlung dominates over all meson channels for $m_{\chi} < 0.5$ GeV.