Heat broiler to high. Place the bell peppers on a rimmed baking sheet and broil, turning occasionally, until charred, 8 to 12 minutes; let cool, then remove the skins and seeds.
In a food processor, combine the bell peppers, walnuts, tahini, vinegar, sugar, cumin, and cayenne and puree until smooth. Season with ½ teaspoon salt and ¼ teaspoon black pepper. Serve with the pita chips.
