Drell-Yan Production of New Particles at Fixed-Target Experiments: Heavy Neutral Lepton as a Case Study

TL;DR

The paper investigates how Drell–Yan production from deep inelastic scattering can illuminate light BSM physics at fixed-target experiments, using heavy neutral leptons (HNLs) produced via a light vector mediator $Z'$ with $m_{Z'}\in[2,20]$ GeV. Employing PDFs and a narrow-width resonance framework, the authors compute $pp\to Z'\to NN$ cross sections and project detectable yields for SBND, DarkQuest, DUNE ND, and SHiP, focusing on HNL decays $N\to \nu\pi^0$ and $N\to \nu e^+e^-$. They demonstrate that the Drell–Yan channel yields more energetic final states than meson-decay production, enabling robust background rejection, and they report 90% CL sensitivities on the HNL mixing $|U_\tau|$ and $U(1)_X$ gauge couplings $g_X$ across $U(1)_{B-L}$, $U(1)_{B-3L_τ}$, and $U(1)_B$ models, including reach to the Type-I seesaw band for DUNE ND and SHiP. The results show substantial improvement over current bounds and indicate that fixed-target experiments can comprehensively probe light dark sector scenarios, with potential extensions to other light mediators and dark matter models.

Abstract

We demonstrate the sensitivity of Drell-Yan production processes from deep inelastic scattering in searches for beyond-the-Standard Model (BSM) physics at fixed-target or beam-bump experiments. We take heavy neutral leptons (HNLs) as a case study, produced from the decay of a light vector boson mediator with mass in the range of $2-20$ GeV, which itself is generated via the Drell-Yan process. The produced HNLs subsequently decay into Standard Model final states. We consider several current and future experiments, including SBND, DarkQuest, DUNE Near Detector (ND), and SHiP. Utilizing $νπ^0$ and $νe^+e^-$ final states from HNL decays, we find that the Drell-Yan mechanism provides important contributions and significantly enhances the HNL search sensitivity, owing to the production of energetic final-state particles that are more readily detectable over the expected backgrounds. We find that at $90\%$ C.L. sensitivity, for gauge couplings $g_{X} \sim 10^{-2}\ (10^{-3})$ and kinematically accessible mass range, SBND and DarkQuest can probe the HNL flavor mixing $|U_{\ell}| \sim 3\times 10^{-4}\ (10^{-3})$, whereas DUNE ND and SHiP may extend the sensitivity down to the Type-I Seesaw prediction of $|U_{\ell}| \sim 10^{-5}$. Finally, for our chosen benchmark $|U_{\ell}| = 10^{-3}$ outside of the current experimental constraints, with a fixed mass ratio $m_{Z'}/m_N = 2.1$, and working within the $U(1)_{B-L}$, $U(1)_{B-3L_τ}$, and $U(1)_{B}$ parameter spaces, we find that both SBND and DarkQuest can probe $g_{X} \sim 10^{-3}$, DUNE ND can reach $g_{X} \sim 10^{-4}$, and SHiP can probe down to $g_{X}\sim 5\times 10^{-6}$. Our approach provides a powerful new technique to study HNL production at future fixed-target experiments and can readily be extended to other light BSM particle production within a broader class of dark sector models.

Figures (8)

• Figure 1: The decay of $Z'$ in the $U(1)_{B-L}$ model versus its mass branching fractions (left panel), and total decay width and decay length (right panel). The sharp peaks near $m_{Z'} = 0.75$ GeV and $1$ GeV are due to the $\rho/\omega$, and $\phi$ meson resonances, respectively. In the left panel, we have fixed $m_{Z'}/m_N=2.1$, whereas in the right panel, we also show the $m_{Z'}/m_N=5$ case.
• Figure 2: HNL decay branching fractions to various SM final states with (a) equal mixing angles with the three lepton generations (top panel), and (b) only $U_{\tau}$ mixing (bottom panel).
• Figure 3: Iso-cross section contours for the process $pp \to Z'\to NN$ normalized to unit coupling $g_{B-L}$ with $m_N \ll m_{Z'}$. The minimum scale is fixed at $\sqrt{\hat{s}}_{\rm min} = 2$ GeV. In the gray-shaded region, $m_{Z'}>\sqrt s$, on-shell production of $Z'$ is kinematically forbidden.
• Figure 4: (Left) Quark parton luminosity for on-shell light vector boson production at various laboratory-frame energies for this study. (Right) The number of HNLs produced within the decay volume in each benchmark experiment, where we have fixed $m_{Z'}/{m_N} = 2.1$.
• Figure 5: Kinematic distributions for $N$ energy at various experiments.