(beginner) git rm

Hi,

I am learning git and there is something I don't get : why a file deleted in a branch gets also deleted in my master? (For the moment I'm only working locally, no remote repository)

I have a master branch containing 4 text files called a.txt , b.txt , c.txt , d.txt . (in HEAD)

I create a new branch from there (masters HEAD) and go to this new branch. $ git branch new $ git checkout new

I delete one file in the new branch $ git rm d.txt $ ls a.txt b.txt c.txt

Then I go back to the master branch and list the files there $ git checkout master $ls a.txt b.txt c.txt

I have not modified the master branch, why is d.txt deleted there also?

There must be something I did not understand in git behaviour (something to do with the index?) Thank you for your help.