In this issue, ALICE Matters interviewed Michele Floris.
What can you tell us about yourself?
I’m a CERN fellow for the ALICE experiment. When I first came here I started working on the proton-proton first physics; I made some contributions to the event selections and basic clean up of the data - in particular removal of beam/gas background, and then after this first physics publications. Right now I am working on the first physics analysis for the heavy ion run.
What can we learn already from the first heavy ion runs?
From a detector point of view, it’s very nice to see how well things are working from the first day. The detector is behaving as we expected since the first collisions. We already have interesting results.
The first physics will take a couple of months. We are aiming at a quick first publication, which will characterise the most central events; so it must detail the collisions which produce the highest numbers of particles. Then we are aiming for more detailed studies of the multiplicities… this will take a bit longer.
What does the multiplicity tell us?
This is one of the first measurements. Usually, when you study a new system, you want to see how many particles come out - per unit of rapidity, for instance - which means you want to see what the density of produced particles is. It is one of the basic measurements you use to characterise a new system.
First of all, it provides you with a global idea of how things work, it constrains models and it is one of the basic ingredients for models which aim to describe more sophisticated things.
How do we measure the multiplicity?
We have different techniques. We have several tracking detectors in ALICE which measure the tracks left by particles. Essentially we have two big subsystems. One is the Time Projection Chamber (TPC), a gas detector. Basically it measures the tracks by detecting the ionization produced by a particle in crossing the gas. Then we have the Inner Tracking System, which is placed closest to the interaction point. In this you have a sequence of points in different layers in space and by looking at those points you are able to reconstruct the path by which the particle came through.
The TPC can also work as a stand-alone tracker, or with combined tracking; you use the TPC to look at the prolongation of tracks into the inner tracking system. This has the advantage that it is much less sensitive to combinatorial background, as you measure hundreds of space points: so there is very little chance you are fooled by a set of random points.
These are the two main methods. They have independent strengths and weaknesses. If these two methods are in agreement with each other, then this is a nice proof that the analysis is working well.
What will happen next for the experiment?
The hope is that we will collect a good sample of heavy ion data and the set up makes us think that this will be so - everything went very smoothly, the data reconstruction began immediately and we were able to iterate a couple of times, with good results.
So, we want to collect a big sample of data. We have a list of things that we will measure first. We are focusing on a study of the first collisions: these are the multiplicity, then comes a study of the pT distribution - which can give us information on the high momentum particles - which is one of the main indications that you are producing matter that is significantly different from what you produce in normal proton-proton collisions. Then there is the measurement of the flow of the particles, which gives information on whether this matter that we have created is behaving collectively and how close it is to an ideal fluid.
These are the three main efforts which are going on now, and we hope that all of them will converge in a couple of months: then we will start looking a little more differentially at probes which are more rare, and which require higher statistics and a finer understanding of the detector.