  We report on recent progress in the design of the proposed MATHUSLA Long
Lived Particle (LLP) detector for the HL-LHC, updating the information in the
original Letter of Intent (LoI), see CDS:LHCC-I-031, arXiv:1811.00927. A
suitable site has been identified at LHC Point 5 that is closer to the CMS
Interaction Point (IP) than assumed in the LoI. The decay volume has been
increased from 20 m to 25 m in height. Engineering studies have been made in
order to locate much of the decay volume below ground, bringing the detector
even closer to the IP. With these changes, a 100 m x 100 m detector has the
same physics reach for large c$\tau$ as the 200 m x 200 m detector described in
the LoI and other studies. The performance for small c$\tau$ is improved
because of the proximity to the IP. Detector technology has also evolved while
retaining the strip-like sensor geometry in Resistive Plate Chambers (RPC)
described in the LoI. The present design uses extruded scintillator bars read
out using wavelength shifting fibers and silicon photomultipliers (SiPM).
Operations will be simpler and more robust with much lower operating voltages
and without the use of greenhouse gases. Manufacturing is straightforward and
should result in cost savings. Understanding of backgrounds has also
significantly advanced, thanks to new simulation studies and measurements taken
at the MATHUSLA test stand operating above ATLAS in 2018. We discuss next steps
for the MATHUSLA collaboration, and identify areas where new members can make
particularly important contributions.
