Quantum Diaries

I am holding back on my Strangeness in Quark Matter conference blog, to take some time to tell you about what it’s like to do a shift for a high energy physics experiment that is weeks from start-up.  This is an experience that not too many people get in a lifetime, so it really should be documented! My shifts for ALICE started on Sunday, and I have been a shift leader for two days now.

That's me, highlighted in blue! 😀

The Birmingham group work on the Central Trigger Processor (CTP), which is a sort of nerve-centre at the centre of everything in ALICE. There are many sub-detectors that make up our experiment, and some of them can trigger on events. This means that when they see something that they like, they send a signal to the CTP saying so. The CTP then tells all of the sub-detectors that the event is interesting, tells the Data AQcuisition (DAQ), that the event will be taken, and sends it to a higher level trigger (HLT) for more a scrutinising selection process. The CTP’s job is a quite important one. Whenever the subdetectors want to run together to look for events (for now they are looking for cosmic rays passing through ALICE), a CTP shifter has to be around to set them up together and make sure the right detector is ready to trigger events. For the next few weeks, for eight hours out of 24, that shifter is me. And it’s rather fun!

My day-shift starts in the Alice Control Room (ACR), after a quick trip to the bakers at 7.30 and a brisk walk from my flat in St Genis. This is where most of the action happens. I arrive at 7.45am, catching up on the night’s progress and relieving the night shifter of their 8 hours. A run of (cosmic ray) data-taking from the night stops at around 8.30, ready for what we call “Happy hour” – a period for all subdetectors to play around individually, fixing bugs, checking their software works properly, making a few updates and so on. As a shift leader, it is part of my job to offer encouragement and/or fingerwag at people to make sure happy hour only lasts for an hour – there are many important tests that need to be done at a time like this, and “five more minutes” can easily turn into an hour and ruin the schedule if you aren’t careful! At around 9 I also need to make sure that everyone who is meant to be on shift is actually here. This has been a great way for me to get to know people on the experiment that I don’t see in my usual circles. It is especially useful for finding out who the experts are!

Beam updates, subdetector status, and important information are posted on big screens to keep everyone informed

For the past two days, there has been access to the ALICE pit (around 50 metres underground) so that various sub-detectors could be fixed, moved, have things installed, tested etc. It is also in preparation for switching on the L3 solenoid magnet (0.5 Tesla) that most of our subdetectors sit inside of. It is very important that any debris, metal or otherwise, is cleaned out and removed before switching on something as large and powerful as our detector’s solenoid and dipole magnets – you can probably imagine that our detector could be quite seriously damaged by a stray spanner or a metal pipe lying around!

As for the tests above ground, at the moment the main concern is to make sure that the subdetectors we need for early data (currently expected before the end of the year!) are able to trigger/read the events that will happen then. As well as setting up the triggering detectors for data-taking, the CTP can also send an artificial signals too, if a triggering detector isn’t available, or, like today, we want to recreate event rates similar to collisions and check that all our detectors can handle it. These are called Early Physics Running (EPR) tests.

A busy looking ALICE event, reconstructed beautifully by the HLT, thanks to Jochen Thaeder for the pic!

Another exciting occurence this week was ALICE’s involvement in some LHC injection tests. I remember these tests taking place in August last year – I still have the photos. This time around, we are a much more prepared experiment. And I am a much more prepared student. I am almost grateful for the delays! The tests themselves have nothing to do with ALICE – they involve injecting high energy protons (and at one point, ions! YAY!) from SPS into the LHC tunnel. On the way, LHC dumps the beam into beam-dumps (essentially thick concrete blocks), forcing the protons to shower in the material, losing energy. These dumps are in place for safety, so that if control of the beam is lost during running, within 3 circuits it can be ejected into one of these concrete absorbers. Luckily (or unluckily, depending on how prepared we are), we are downstream from the injection point from SPS, and two of these absorbers. Earlier this week we were able to observe the shower. Hundreds of thousands of tracks. This is orders of magnitude more than what we expect even in heavy ion collisions! However, if the beam energy is not absorbed enough it can do great damage to detectors. Tests like this are very useful for us, but take alot of preparation.

A typical cosmic ray event, this time selected using the Time of Flight (TOF) trigger

Another responsibility as a CTP shifter is to train people. Because our group is small, and we need to cover 24 hours a day 100% of the time, we are rather short on numbers. It is quite fun going through the possible situations that a shifter could need to deal with, because once you’ve dealt with it you know what they are supposed to do. When you have to deal with a brand new problem first hand, it is a different, rather scary, story. You have to think on your feet and know when to give in and call an expert.  The main part of being a shift leader is being aware of what is going on, and knowing who to call in a crisis you can’t solve. This is easy on quiet days, but can be quite confusing on busy ones! I have very much enjoyed the experience, and hope that over the coming weeks it continues to be as exciting as it has been.

The LHC ring and status updates

I have many shifts still to come, and I now predict that I will be on nights (midnight to 8am) when we first get collisions. I am crossing every digit I own that I get to see it live (which makes typing tricky!) but even if not, I will be a very real part of the first collisions we take, and I can’t wait.

Heavy Ions in ALICE!