2. Drain beans, put them in a pot and add the chicken stock, carrots, celery and enough water to cover the beans by an inch or so.
3. Bring to a simmer uncovered over medium-high heat. Do not boil. Skim foam that comes to the surface. (Do not add salt because it will prevent the beans from becoming tender.)
4. When the beans are tender, about 1 hour, remove from heat. Drain and discard the carrots and celery. Set beans aside. (The cannellini beans can be cooked ahead of time, covered and refrigerated up to 3 days.)
5. In a large skillet, heat olive oil over medium heat. Add the pancetta and cook for 2 minutes. Add the garlic and cook 1 minute more.
6. Add the beans, season with salt and pepper, and heat thoroughly.
7. Turn off the heat and gently fold in the arugula leaves. Serve warm or at room temperature.