Today XENON1T has seen its first light:
This is literally the first event recorded by the detector in that is is a single photon that was detected by one of the photomultipliers and recorded by the whole XENON1T data acquisition setup. What you can see from the picture is that our noise is indeed very low compared to the smallest possible signal – that of a single photon! And this is even without any fine-tuning of our electronics yet.
The detector is still empty and we are checking the photomultipliers one by one before making first background measurements. Filling with liquid xenon will happen as soon as those tests are concluded successfully.