Quantum Diaries

I am back on shift again, and after a few minor tests, hiccups and changes, some ALICE detectors are now taking data with LHC beam dumps and all is relatively quiet, so I finally have 5 minutes here to bring you some news.

A few days ago, the first physics paper to use LHC collision data – an ALICE study using 284 events from the LHC’s first 900 GeV collisions of 23rd November – was approved by CERN, and only yesterday (1st December) it was accepted by European Physics Journal C. Because of this, I feel confident telling you about it at last 🙂 The paper, which can be found here, describes the distribution of particles in the collisions, and was submitted on 28th November – just days after the collisions took place. I was lucky enough to be working in the group that deals with ALICE’s early studies, so for the past few weeks  my colleagues and I have been very busy!

The story began on 23rd November, when our Silicon Pixel Detector (SPD), a cylindrical detector surrounding the beam pipe,  selected the collision events (looking for hits in its two layers indicating particles, whenever bunches of protons were passing through our detector from both sides). Once an event was triggered, the Central Trigger Processor (CTP) instructed the SPD, and the other detectors in our central “Inner Tracking System” (ITS), the Silicon “Drift” and “Strip” detectors, to record what they saw.

Also observing these events were our two “V0” detectors – scintillator counters placed further along the beam (what we call “forward” as apposed to “central”), either side of the ITS. The timing of the signals from these detectors could determine whether the event they came from was a collision within the ITS or an interaction with gas further down the beam pipe.

The High Level Trigger (HLT) system used information from the detectors to reconstruct the particles’ paths and locate the “vertex” or interaction point of the events, which, for such preliminary collisions, was surprisingly centered. This helped us get rid of the last of the background, including one or two cosmic ray events that had timed themselves perfectly with the proton bunch crossings! Finally, after careful analysis, and thanks to injection tests and cosmic ray data taking leading up to collisions that helped us to better understand our detectors, the “pseudorapidity density of charged primary particles in the central region” was measured.

So what does this mean? Roughly speaking, a pseudorapidity distribution is the angular distribution of particles in collisions (angular with respect to the beam, that is). In this picture, pseudorapidity density is a measure of the number of particles in a given “angular” range – in this case, looking in the very central region – particles coming out at right angles to the beam, straight into the SPD. Later, when more of our detectors are available (in particular, our three Forward Multiplicity Detectors, which again are positioned on either side of the ITS, further along the beam pipe) and we have more statistics, we will be able to see this picture more clearly – how particles distribute themselves in more “forward” regions.

We will also be able to make more interesting physics observations as time goes on – with our Time Projection Chamber and magnetic field, and eventually many more detectors, we will be able to reconstruct a large number of tracks, including particle decays, in collisions of even higher energy, and identify the particles causing the tracks in the first place. For now though, it is baby steps, and to see simply that this result is consistent compared with p-antip collisions of the same energy is a nice cross-check that the world still makes sense.

The atmosphere has really changed over the past month, and not just for ALICE. People are more on edge. Coffee is being consumed at a higher rate and the jumpiness it causes is evident. Sleep feels like a weakness. Tiny, insignificant details seem like potential disasters. Big decisions are needed at short notice. As a result, big changes to plan are occurring at short notice.  People are prone to panic. (As a person who has always been somewhat prone to panic, I have found the experience strangely helpful – I am, for the first time, finding myself able to put on a calm face to myself as well as to others.) Competition between rival experiments is rearing its head. (Expression ranging from jovial to bitter. With the healthy middle ground of constructive.) It has all become very real. The fact that collisions are scheduled in the next week doesn’t seem like a crazy dream. People are talking about “data”. Tiny, insignificant steps are breaking brand new ground. Meetings have become twice, even thrice daily. (Significant updates are still reported in each one.) Poorly timed obstacles such as having children to look after, being stranded in another country with failing communication technology, having been awake for days on end or being confined to a hospital bed with no visiting rights are no longer enough to hold people back. Seeing what can be achieved during this exciting, stressful time has really impressed me. I hope this head-bursting, stomach-flipping, maddening and satisfying atmosphere doesn’t fade too soon, because we are thriving on it.