I think that scientist can adapt some alien technologies to improve human techs and make anti-alien techs
Examples:
Antialien:
PBW and Plasma weapons both fire charged projectiles, so they can be deflected with electrical field. Particle beams can be dispersed with special "smoke"
Missiles with proton containers. If small amount of protons get's to UFOs antimatter storage HUGE explosion will destroy UFO. The BIG problem si delivering them to this storage
Hybrid:
Particle pseudo-conductor for bolter rifle -- conductor rails in bolter rifle are replaced with "tubes" where particles move. Fast movement of particles means that current is very big so Ampère's force (which pushes bolt) is very-very big. This tech requires "magazine" from PBW.
Theoreticaly this is less effective than pure particle beams but bolts can not be deflected with electrical field shilds (if UFOs or aliens have them).
Also mass of bolt is bigger than mass of particles so with same energies
momentum of bolt will be bigger that momentum of particles sqrt(m2/m1) times where m2 is mass of bolt and m1 is mass of particles. So momentum will be millions times bigger.
But not all energy from particles goes to energy of bolt. This energy depends on speed of particles and length of acceleration path. But even small percents of this energy'll lead to tremendous momentum
Alien computer adapting, reverse-engineering etc. Even "low-end" alien navigation computers are much more faster than ours. If we could understand how to reprogramm it, we could use them in our reserches.
Plasma Engines. Plasma can be used for reactive force for high-speed flights in upper layers of atmosphere
P.S. What energy source in plasma weapons is used to heat and accelerate plasma? If there is some great power source it can be used for laser weapons