A new composition-sensitive parameter for ultra-high energy cosmic rays

► A new parameter, Sb, is proposed for composition studies in cosmic ray surface arrays. ► The discrimination capability (η) of Sb between proton and iron maximizes for b ∼ 3. ► If muon contribution is enhanced, optimal b is still 3 and η increases. ► A major advantage is that S3 is almost independent on zenith angle. ► Composition accuracy using S3 is of the same order than that from Xmax.