Heat oil in a 5- to 6-quart pan over medium-high heat. Add onion, bell pepper, and chiles. Cook, stirring often, until onion is soft (about 5 minutes). Add broth, minced tarragon, and pepper; bring to a boil. Meanwhile, cut corn kernels from cobs. Add corn to boiling broth mixture. Reduce heat, cover, and simmer until corn is hot (about 5 minutes). If made ahead, let cool; then cover and refrigerate for up to 1 day. Serve hot or cool.
To serve, ladle soup into bowls; garnish with tarragon sprigs, if desired.