After many years of planning, designing and building, we on COMET are finally going to turn our experiment on, and receive a beam of protons from the J-PARC accelerator facility. Now, this is still a commissioning run, which we are calling Phase-α, as opposed to Phase I and II which will follow; those are when we will take data that could lead to new discoveries about the Universe. Even then, this is a major milestone, and Phase-I is expected to follow rapidly once Phase-α has concluded successfully.

The proton beam that has been producing neutrinos for T2K for 13 years will arrive this week at the COMET experimental hall for the first time. This will be at a lower energy of 8 GeV compared to 30 GeV, and it will be "slow-extracted" from the Main Ring synchrotron round which the protons are accelerated, meaning that every millionth of a second, a small fraction of the protons will be sent our way, instead of the entire beam being "fast-extracted", as is done when it slams into the graphite beam target in one go for T2K. So even though it is the same beam, everything is quite different and new.

We will intentionally run with the lowest intensity of beam possible, and try and understand it the best we can first. One reason is that if we run with the higher intensities of Phase-I and Phase-II, the beam line area will become radioactive; for now we need to start with baby steps, so we can be confident for when we ramp up to Phase-I.

Underground in the COMET Experimental Hall, explaining how the experiment will work to the Minister for Science, George Freeman, and Susie Kitchens of BEIS and other members of the UK delegation. "Swarms of muons will soon emerge from the superconducting solenoid on your right to form muonic atoms just about where your heads are!"

In December, the area was still more easily accessible, and I was able to have the pleasure of showing round a delegation from the UK and the UK Embassy in Japan, but now it is all closed up, ready for beam, with particle detectors in place where we were standing just weeks before.

The "Straw Tube Tracker" particle detector, installed at the end of the superconducting solenoid for COMET Phase-α. This detector will be used for Phases I and II as well, so it is fantastic to see it in place, about to see real COMET beam particles for the first time. More detectors have now been placed in front of this now, ready for beam (Photo: Hajime Nishiguchi)

Next week we will run for just five days to focus on getting the beam right and in March, we will run again in earnest, with a range of detectors observing the particles with various different detector and beam line configurations to help us understand everything. As part of this, here at Imperial, we built a new beam "masking" system, which allows us to alter the kinematics of the beam of pions and muons as it enters the main superconducting solenoid. Last September, I visited J-PARC while my colleagues Oliver and Kevin who designed and built this rather complicated contraption, installed and tested it together with our local colleagues from J-PARC, led by Shun—and took photos of this work....

The variable beam mask system, being worked on by Imperial engineers Kevin and Oliver, and then being lifted into position, just in front of the superconducting solenoid, at the other end from the views in the preceding photographs.

Our colleague Koh has been on-site since last year and will be until the end of Phase-α, having been the person who has led the whole project since we first went to the lab to receive approval for it. We will have a whole team descending on J-PARC over the next couple of months, including other senior researchers and PhD students from collaborating  institutions around the world. In particular, we will be working closely with our colleagues from several universities in Malaysia; one of the amazing things in recent years has been our work together, supported by the Newton Ungku-Omar fund, and that is about to culminate in our joint work on-site for data-taking with Phase-α.

I will try and persuade Koh to report back with more photos and news. In the meantime, I can point you in the direction of a talk I gave at the Physics Advisory Panel for the laboratory in January, which has more detailed status updates and photos, although it is admittedly a bit technical....