Detecting Hidden Particles with MATHUSLA

TL;DR

The paper investigates ultra-long-lived hidden-sector particles produced in rare meson decays and exotic Higgs decays, focusing on a minimal Higgs-mixed scalar as a benchmark. It quantifies MATHUSLA’s potential reach relative to SHiP using Pythia8 simulations for $b\bar{b}$ and kaon production and derives scalar phenomenology from a Lagrangian with a mixing angle $\theta$, including $\Gamma_s = \sin^2\theta\,\Gamma_h(m_s)$ and $BR(B\to sX_s) \sim 6.2\,(1 - m_s^2/m_B^2)^2\sin^2\theta$. The analysis shows MATHUSLA can exceed SHiP for very long lifetimes and, for certain kaon-decay regimes, rival SHiP at low energies, while Higgs decays to scalars extend sensitivity beyond meson decays. The work highlights MATHUSLA’s unique capability to explore unexplored regions of hidden-sector parameter space and discusses design modifications to enhance low-energy sensitivity and background discrimination, emphasizing the importance of further cost-benefit studies.

Abstract

A hidden sector containing light long-lived particles provides a well-motivated place to find new physics. The recently proposed MATHUSLA experiment has the potential to be extremely sensitive to light particles originating from rare meson decays in the very long lifetime region. In this work, we illustrate this strength with the specific example of a light scalar mixed with the standard model-like Higgs boson, a model where MATHUSLA can further probe unexplored parameter space from exotic Higgs decays. Design augmentations should be considered in order to maximize the ability of MATHUSLA to discover very light hidden sector particles.

Figures (4)

• Figure 1: The ratio of the total number of long-lifetime hidden sector particles $X$ ($m_X=100$ MeV) above a minimum energy threshold $E_X$ delivered to MATHUSLA in 3 ab$^{-1}$ of LHC data over the total delivered to SHiP (of any energy) for $2\times 10^{20}$ protons on target for the two-body decays of $B$-mesons (blue, solid), $K_L$ (green, dashed), and $K^\pm$ (red, dot-dashed).
• Figure 2: Scalar branching ratios in the light hadron region used in this work. For masses below $\sim 1.4$ GeV, we use the results of Donoghue:1990xh. We use a smooth extrapolation up to $2m_D$. The muon branching ratio is shown with the green, dashed curve.
• Figure 3: Current and projected constraints on light Higgs-mixed scalars. The projected limits from $B$-meson decays at MATHUSLA are shown by the solid red curve assuming the experiment is background-free. The projected limits from the SHiP experiment Anelli:2015pba are shown by the orange, dashed line. Other current limits Acciarri:1996umBuskulic:1993giBergsma:1985qzAaij:2015tnaAaij:2016qsmArtamonov:2009sz are described in the text.
• Figure 4: Purple contours show the projected sensitivity of MATHUSLA to exotic Higgs decays. Contours are shown for branching ratios of 10% to 0.001% in steps of factors of 10. Model restrictions on the maximum allowed branching ratio (Eq. \ref{['eq:BRmax']}) influence the shape of the contours at higher masses. The meson decay and other constraints are as in figure \ref{['fig:MATHNoExo']}.