  MAXI J1820+070 is a low-mass X-ray binary with a black hole as a compact
object. This binary underwent an exceptionally bright X-ray outburst from March
to October 2018, showing evidence of a non-thermal particle population through
its radio emission during this whole period. The combined results of 59.5 hours
of observations of the MAXI J1820+070 outburst with the H.E.S.S., MAGIC and
VERITAS experiments at energies above 200 GeV are presented, together with
Fermi-LAT data between 0.1 and 500 GeV, and multiwavelength observations from
radio to X-rays. Gamma-ray emission is not detected from MAXI J1820+070, but
the obtained upper limits and the multiwavelength data allow us to put
meaningful constraints on the source properties under reasonable assumptions
regarding the non-thermal particle population and the jet synchrotron spectrum.
In particular, it is possible to show that, if a high-energy gamma-ray emitting
region is present during the hard state of the source, its predicted flux
should be at most a factor of 20 below the obtained Fermi-LAT upper limits, and
closer to them for magnetic fields significantly below equipartition. During
the state transitions, under the plausible assumption that electrons are
accelerated up to ~ 500 GeV, the multiwavelength data and the gamma-ray upper
limits lead consistently to the conclusion that a potential high-energy and
very-high-energy gamma-ray emitting region should be located at a distance from
the black hole ranging between 10^11 and 10^13 cm. Similar outbursts from
low-mass X-ray binaries might be detectable in the near future with upcoming
instruments such as CTA.
