  The search for long-lived particles (LLP) is an exciting physics opportunity
in the upcoming runs of the Large Hadron Collider. In this paper, we focus on a
new search strategy of using the High Granularity Calorimeter (HGCAL), part of
the upgrade of the CMS detector, in such searches. In particular, we
demonstrate that the high granularity of the calorimeter with this upgrade,
which allows us to see "shower tracks" in the calorimeter, can play a crucial
role in identifying the signal and suppressing the background. We study the
potential reach of the HGCAL using a signal model in which the Standard Model
Higgs boson decays into a pair of LLPs, $h \to XX$. After carefully estimating
the Standard Model QCD and the misreconstructed fake-track backgrounds, we give
the projected reach for both a more conservative vector boson fusion trigger
and a novel displaced-track-based trigger. Our results show that the best reach
for the Higgs decay branching ratio, BR$(h \to XX)$, in the vector boson fusion
channel is about $\mathcal{O}(10^{-4})$ with lifetime $c\tau_X \sim 0.1$--$1$
meters, while for the gluon gluon fusion channel it is about
$\mathcal{O}(10^{-5}\text{--}10^{-6})$ for similar lifetimes. Alternatively,
for an LLP with $c\tau_X \sim 10^3$ meters, the HGCAL based search should be
able to probe BR$(h\to XX)$ down to a few $\times 10^{-4}$($10^{-2}$) in the
gluon gluon fusion (vector boson fusion) channels, respectively. In comparison
with these previous searches, our new search shows enhanced sensitivity in
complementary regions of the LLP parameter space.
