So basically a professor pitched his project that needs research assistants at an event in my department on Monday. I talked to him after the event about 5 five mins, telling him my background and that I am really interested. I sent him my CV after the event on Monday evening. Now that Tuesday has passed. I still didn't get any reply from the professor. What should I do now? Email him again or go to his office directly (now it is the exam period so he probably doesn't have office hours anymore)?
Wait. As Buffy said, a day is virtually no time at all. The professor may be trying to give others a chance to respond, or may just be busy with other things. While your zeal is admirable, overdoing it will raise red flags. If he invited applications on Monday, I would wait at least until Friday before following up. Depending on the local culture, dropping by his office in person on Friday (or the following week) may be a good idea; otherwise, I would (again on/after Friday) just send a brief e-mail saying that you are still interested and that you hope to discuss the position with him soon.
A day is no time at all during busy academic periods. But trying to find him is probably a good bet. Some things are best done face to face. He probably won't have made any decisions yet and a visit might help him decide.
But an immediate email probably won't get you anywhere.
Perhaps someone in the department office can set you up with an appointment.
But if a week passes and you haven't made contact, then email should be fine.