Annalisa Mastroserio - INFN, University of Bari
What was your motivation to study physics and move in the field of heavy-ion physics?
I was in the physics lab during the secondary school and didn't know the phenomenon of electromagnetic induction.My physics professor moved a magnet in the air and electric current started flowing in a cable in front of him and us, but the cable was not connected to any plug. It was very intriguing and I was so curious to know more and that's how I decided to study physics.
During my undergraduate studies I decided that I wanted to become a particle physicist while studying for the elementary particle physics exam. The spontaneous symmetry breaking, the mixing matrices, how the gluon was discovered and many other things were so charming to me. I decided to study heavy ion physics while working on pp collision data.
When did you start working in the ALICE experiment?
I started working with ALICE in 2004 studying the performance of the ALICE detector based on the Cherenkov effect, namely the HMPID.
What were the difficulties you faced when the machine started running?
It was like working for the first time with a sophisticated prototype. Theory was precisely describing every detail and the single pieces had been tested many times, but once the accelerator starts running many unexpected things could happen and problems must be tackled very quickly. I belonged to the ALICE First Physics group and I was the data quality responsible of the Silicon Pixel Detector. This was the key detector for the very first ALICE measurement at all the energies and colliding systems : the charged particle multiplicity. It was tough but also very exciting.
What was your feeling at that time?
It is not easy to find words to describe the feelings and the emotions of a physicist waiting for several years for such moments. Every time there was a step forward in energy or LHC moved from pp to PbPb collisions I was in the ALICE control room. As soon as the first tracks apperared in the event display I stopped breathing, watched the event display for a moment, then clapped the hands and congratulated with colleagues and...champagne!. Of course it lasted few seconds only, since I had to monitor the SPD data flow and the quality of such data.
Could you describe the inner tracking system of ALICE?
The Inner Tracking System (ITS) is the closest detector to the beam pipe, where collisions take place, and it is constituted of three subsystems : the Silicon Pixel Detector(SPD), the Silicon Drift Detector (SDD), the Silicon Strip Detector (SSD) . The ITS is one of the main tracking detectors of the ALICE experiment and it is unique in tracking particles at low momenta. I have been working for the SPD since the end of 2008. The pixel detector is the first detector seen by the particles produced in the collisions; it detects the passage of a charged particle without interfering too much with it. Furthermore it is crucial to determine if a particle comes from the interaction point (namely the primary vertex) or from a secondary vertex. It happens, in fact, that some particles produced in the collisions are short lived and at some point they decay into other particles. The point where the decay occurs is called "secondary vertex". Particles constituted of charm or bottom quarks are an example of short lived particles and their secondary vertex is roughly 0.1-0.3 millimeters away from the interaction vertex. Such particles play a fundamental role in understanding the dynamics of the Quark Gluon Plasma, therefore the ITS contribution to distinguish two close vertices is crucial for the ALICE physics program.
The Inner Tracking System installed - ALICE
The ITS is also able to identify particles in a regime not reachable by other detectors and its contribution to the commissioning phase and the data acquisition of the very first collisions at all the energies was very important. The Inner Tracking System is unique and its good performance is also the result of the effort of a great
What is so particular about the silicon detectors?
The silicon detectors are very reliable and versatile. They are fundamental in tracking, vertexing, some technologies are suitable for particle identification and ALICE has also exploited the possibility of using them as trigger detectors. Silicon detectors are present also outside particle or nuclear physics, in particular, they are used for medical applications.
Which are your current plans in terms of instrumentation?
New developments in the silicon detector instrumentation are under studies for the upgrade of the ALICE ITS. My contribution to the ITS upgrade was more on the software side, in particular for the simulation of its future performance.
The Silicon Pixel detector of ALICE
Were you involved in data analysis for ALICE?
At the beginning of my studies I worked on simulated data. I checked the performance of the ALICE Cherekov detector and the feasibility of a resonance identification at high momenta in two different QGP scenarios; such a resonance was constituted of a quark s and anti-quark s.
From the end of 2008 I worked in the SPD team and my work started with the calibration procedures. As soon as the first collision data arrived I worked on the detector response quality and data correction at all energies (0.9 TeV, 2.76 TeV, 7 TeV) both for pp and PbPb collisions.
Recently I started new activities more focused on data analysis.
And how does the study of these phenomena allow us to get a better understanding of QGP?
The heavy quark production mechanism is very important to understand the physics of the quark gluon plasma. The quark c and b are formed just in the plasma where they live and interact in a peculiar way. The study of the properties of particles containing such quarks reveals important information on the medium they passed through. Unfortunately they are very rare signals and in less than one mm they decay into other particles. Their seconday vertices are very close to the primary one and thousand other particle types produced in the collisions may mimic the decay products. It is crucial then to disentangle if the daughter-like particle are from the secondary vertex and not from the primary one.
The ITS plays a fundamental role in this respect so it helps the understanding of the QGP medium properties.
Finally, I would like to ask you about your involvement with the offline group of ALICE and particular the GRID. How does ALICE benefit from the Grid?
ALICE benefits a lot from the grid because of its big data load and processing time needed expecially for central PbPb collisions. All the ALICE data are reconstructed and analysed in such a distributed system. Everything is reproducible and available to all the people of the collaboration and in any place of the world. This is very important in a big and international collaboration.
My main contribution in the offline team was to provide tools to be used on the grid, the most famous one is the Correction Framework, I was one of the pioneers of this framework.
How does a young researcher decide which direction to take?
From my experience when a student comes to CERN he/she has an idea based on textbooks. Particle physics also involves many other aspects. Every experiment is a technological channel, a search of a hidden part of Nature. A new theory needs a lot of discussions with several groups and experimentalists. The world behind a physics result is not on textbooks and it is hard to be explained in words or images. CERN is a unique laboratory, it has so many facilities that only spending some time in there a student may have an idea. In my opinion only the experience in the lab and the collaboration with one or more groups makes a young physicist understand his/her favorite subject to work on and the direction to take.