Analysis of Long Lived Particle Decays with the MATHUSLA Detector

TL;DR

This paper assesses how the MATHUSLA detector, a large-volume surface detector, can study neutral long-lived particles produced at the LHC, focusing on LLPs from exotic Higgs decays. Using limited MATHUSLA information, the authors show that the LLP mass and dominant decay mode can be inferred with fewer than $100$ observed decays (assuming BR$(h\to XX)=1\%$), and that production-mode and spin information can be accessed in more general cases. The work develops qualitative track-pattern classification, a robust mass-reconstruction strategy based on LLP boost, including a novel sphericity-based method for jet decays, and studies hadronization systematics and tau channels. It also demonstrates that template-based global fits across production modes can identify the underlying origin of the LLP and potentially determine its spin, highlighting MATHUSLA’s potential to reveal new hidden-sector physics.

Abstract

The MATHUSLA detector is a simple large-volume tracking detector to be located on the surface above one of the general-purpose experiments at the Large Hadron Collider. This detector was proposed in [1] to detect exotic, neutral, long-lived particles that might be produced in high-energy proton-proton collisions. In this paper, we consider the use of the limited information that MATHUSLA would provide on the decay products of the long-lived particle. For the case in which the long-lived particle is pair-produced in Higgs boson decays, we show that it is possible to measure the mass of this particle and determine the dominant decay mode with less than 100 observed events. We discuss the ability of MATHUSLA to distinguish the production mode of the long-lived particle and to determine its mass and spin in more general cases.

Figures (9)

• Figure 1: Exclusion reach of MATHUSLA, corresponding to 4 expected decays in the detector (solid curves), compared to the best-case ATLAS projection (dotted curves), for pair production of LLPs in exotic Higgs decays $h \to X X$, from Chou:2016lxi. The three curves of each set correspond to three different values of the LLP mass. Sensitivity up to the BBN lifetime limit Fradette:2017sdd is possible.
• Figure 2: We assume the MATHUSLA detector geometry shown on the left. On the right, we schematically show the patterns of charged tracks reconstructed for different final states of LLP decay.
• Figure 3: Kinematics of the two-body decay of an LLP: The left-hand figure shows the angles $\theta_1$, $\theta_2$; the right-hand figure illustrates the boost back to the LLP rest frame in which the 2 products are back-to-back. Note that $\hat{p}_i$ or $\hat{p}_i(\beta_X)$ denote momentum vectors normalized to unit length in each frame. For convenience we work in the coordinate system where $\hat{p}_X$ is along the $z$-axis and the decay products are in the $(x,z)$ plane (far right).
• Figure 4: Distribution of LLP boost $b = |\vec{p}_X|/m$ for different LLP masses. The solid histograms show the truth-level value of $b$, which is also close to the distribution of reconstructed boosts for $X$ decay to 2 charged particles. The dotted histograms show the distribution of reconstructed boosts for hadronic LLP decays using the sphericity-based method of Section \ref{['s.Xjj']} with only upwards going tracks.
• Figure 5: The expected number of LLP decays in MATHUSLA required to measure the LLP mass to relative precision $\Delta m/m$ using only upwards-traveling tracks, for LLP decay to muons (left) and jets (right). $N_\mathrm{obs}$ refers to the number of decays in the detector volume regardless of whether the event is reconstructed. For muons, $N_\mathrm{reconstructed} = \epsilon N_\mathrm{obs}$ where $\epsilon \approx 0.95$$(0.55)$ for $m_X = 15$$(55) \;\mathrm{GeV}$. For hadrons, $\epsilon \approx 1$. The plots show the statistical uncertainty only.