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An indirect measurement of the antiproton flux in cosmic rays is possible asthe particles undergo deflection by the geomagnetic field. This effect can bemeasured by studying the deficit in the flux, or shadow, created by the Moon asit absorbs cosmic rays that are headed towards the Earth. The shadow isdisplaced from the actual position of the Moon due to geomagnetic deflection,which is a function of the energy and charge of the cosmic rays. Thedisplacement provides a natural tool for momentum/charge discrimination thatcan be used to study the composition of cosmic rays. Using 33 months of datacomprising more than 80 billion cosmic rays measured by the High Altitude WaterCherenkov (HAWC) observatory, we have analyzed the Moon shadow to search forTeV antiprotons in cosmic rays. We present our first upper limits on the$\bar{p}/p$ fraction, which in the absence of any direct measurements, providethe tightest available constraints of $\sim1\%$ on the antiproton fraction forenergies between 1 and 10 TeV.

Constraining the $\bar{p}/p$ Ratio in TeV Cosmic Rays with Observations of the Moon Shadow by HAWC