Barron Hilton -- Goes Radio Silent with Police Investigating Attack

Barron Hilton has logged on to zipit.com ... refusing to get back to police officers investigating the beatdown he caught at the hands of Lindsay Lohan's pal -- and it's preventing them from making an arrest ... TMZ has learned.

As TMZ first reported ... Miami Beach PD are looking to arrest the puncher -- LiLo's friend Ray LeMoine -- however ... law enforcement sources now tell us they haven't heard back from Paris Hilton's little bro.

We're told police need to speak with him to button up the investigation -- but Barron has fallen off the radar since initially speaking to cops on Friday morning, right after the attack.

As we told you ... multiple sources confirm there was a ton of drugs, including cocaine, inside the party.

We got Barron's folks, Rick and Kathy Hilton, out last night ... and Rick said he believes LeMoine nailed his son with brass knuckles.

As for LeMoine, he went on an online rant, claiming Barron started the fight by pushing him ... then claiming BH would not have been cut except for his "stupid glasses." He also calls Barron "Conrad Hilton's spoiled sperm experiment," and says when the confrontation began, Barron proclaimed, "Do you know who my sisters are?"