A few things to look at/ try.
Power button could be stuck, try lightly tapping the phone near the power button on the edge of a table.
Software glitch. See if you can get into recovery mode. Press and hold power + volume up + home till it boots into recovery.
From recovery, try wiping the cache partition first. Will not erase user data. Reboot
If still looping, in recovery, Factory Reset. Will erase user data.
If still looping, try to get it into download mode.
Press and hold power + volume down + home to boot into download mode. If you can get into download mode, you can flash the stock firmware you can get at Sammobile with Odin.
http://www.sammobile.com/firmwares/
Last, since it's a new device, return for replacement.
