When Scott Kettner looks at a map, he sees a direct line that connects the rivers of northeastern Brazil to the parishes of New Orleans and the streets of Brooklyn. A master percussionist, bandleader, producer and songwriter, Kettner is the guiding force behind Nation Beat, a band whose teeming, vibrant rhythms find common ground in the primal maracatu rhythm of Brazil’s northeastern region, the Big Easy’s funky, hypnotic second-line and strolling Mardi Gras Indians, and the unfettered freedom of big-city downtown jazz.