Phased polaron beam was the polaron particle discharge of the extremely powerful ship-mounted directed energy weapon used by Dominion starships in the late 24th century. Jem'Hadar fighters were equipped with a phased polaron beam emitter as their primary offensive weapon.
During the early years of hostilities, from 2370 onward, phased polaron beam weaponry gave Dominion ships a decisive tactical advantage against Alpha Quadrant forces. The beam was capable of penetrating the deflector shields used by Federation, Romulan, and Cardassian starships, as if the ships weren't shielded at all. (DS9: "The Jem'Hadar", "The Die is Cast") The ablative armor of the USS Defiant proved to be an effective defense against phased polaron beams, allowing the Defiant to survive several direct hits in battle. (DS9: "The Search, Part I")
In 2372, while on a diplomatic mission to negotiate a trade agreement between the Karemma and the Ferengi, the Defiant was for the first time capable of repelling enemy fire from Jem'Hadar fighters with its shields. (DS9: "Starship Down") The Dominion did not become aware of the Federation shield upgrades until the Second Battle of Deep Space 9 in 2373, when Dominion forces were surprised to see Deep Space 9's shields holding under phased polaron beam fire. The station's shields were eventually penetrated, but only after a protracted battle that saw the polaron beams reduced in effectiveness to the level of any other energy weapon. (DS9: "Call to Arms")