Britney Spears is collaborating with a top R'n'B producer on her new album.The 'Toxic' singer has been in Las Vegas since Tuesday working with Lukasz Gottwald, who has previously collaborated with Missy Elliot, Mos Def and Kelis. A source told People magazine: "Britney is really pulling out the big guns on this album. She's been partying a lot lately but she's been working in studios in Miami and Los Angeles too."She's rehearsing material, working hard and having fun all at the same time."The 25-year-old is staying in the Hugh Hefner Sky Villa of the Palm's hotel Fantasy Tower with her two sons, 16-month-old Sean Preston and four-month-old Jayden James. Britney recently spent the night with model Isaac Cohen in the same $25,000 suite, which comes complete with Jacuzzi, rotating bed and Infiniti pool.The singer will share joint custody of her two sons with estranged husband Kevin Federline for the month of February, after the temporary agreement for January was extended. Britney and Kevin are both seeking sole custody of the children.