What we do at our team amongst ourselves is call the person by their name in a voice that is not yelling but reasonably loud enough to be heard over music at typical volume. If they don't react after about 3 times of calling their name (with a couple of seconds pause between repetitions), then we do a gentle tap on the shoulder.
Do note that not all body contact is the same. A pat on the back is more invasive of one's personal space and specially so if that person is of the opposite gender. Consider that many of us only ever feel hands on our backs when hugging someone, so it can feel like a very close-relationship kind of contact. All you need is to get the person's attention, so the tap only needs to be as strong as to know they must have felt it, but no stronger. Gentle.
Also, don't put your palm on the shoulder; that can feel patronizing. Just 2 or 3 fingertips is good.
That said, because of your previous intrusions, I'm doubting even a tap on the shoulder is welcome. You should definitely sincerely apologize and ask her preference. She may be fine with a gentle tap on the shoulder, after you ask her if it's fine.
On the removing her headphones, other people have explained that it's rude, but to get a better intuition as to why, it might help to give examples of other situations that seem similarly rude. Consider that instead of listening on headphones:
- she's working on a laptop and you close the lid, or
- she's writing on a paper and you remove the pen from her hand, or
- she's watching TV and you take the remote and turn it off
The key rudeness part is that you're forcibly denying her of what she's doing. "Why would you do that when there are less forced ways of getting my attention?" is what I would be thinking in her shoes, but others aren't as chill.