Laure has been a member of ALICE’s community since 2008 when as a PhD student she worked on the forward muon tracker, for the upgrade of ALICE and on the analysis of the low mass resonances in the dimuon decay channel. She is currently a postdoc at SUBATECH where she will be spending one more year doing data analysis for ALICE and working on the J/psi elliptic flow. It should be noted that it is the first time that this measurement is done at the LHC. We met her during her shift in the ALICE Control Room at Point 2 and discussed on the importance of the muon forward tracking system for the ALICE experiment and what is really new about it.
A.M: What is it that motivated you to become a physicist and particularly move to high energy physics?
L.M: When I was in my first year of high school, my physics teacher was really motivating and good at teaching science. She brought me the passion for physics, at the period when I had interests in literature, languages, economy and not only in sciences. It is thanks to her, that I evolve now into that field. Since I was young I had an interest in the universe and understanding what we are made of. I then came naturally to particle physics and high energy physics. When I was doing my master degree in Lyon, I had the opportunity to make a 6 months work experience with the ALICE group of Lyon, and it is since that time I joined the Heavy Ion community.
Panos CharitosLaure Massacrier in the ALICE control room
A.M: What is a “forward muon tracker” and why is this important to the ALICE experiment?
L M : The main idea is to add a tracker in front of the hadron absorber in order to improve the vertex determination and be able to know the origin of our muons with respect to the main vertex. It is also an apparatus that will help in the low mass resonance analysis because it will improve the current resolution of our Muon spectrometer, especially at low masses. The forward muon tracker will help us to deal with the multiple scatterings induced on the muon tracks by the hadron absorber by getting a better resolution of the vertex region. This small silicon pixel detector will permit to measure muon tracks before they enter into the absorber. By associating them with those measured after the absorber, we are gaining enough pointing accuracy that allows a reliable measurement of their offset with respect to the primary vertex. I think that the addition of the silicon muon forward tracker should boost the possibilities of doing muon physics with ALICE.
Laure MassacrierGeneral view of the The Forward Muon Tracker planes
A.M. Is this the first time that a “forward muon tracker” is being used in an experiment in high energy physics? Has it been used in other detectors?
L.M. Such kind of apparatus has already been used in previous experiments. For example it was the upgrade of NA50 to NA60 and also PHENIX and to some extent STAR that used this type of detectors. It was mostly part of an upgrade process for these experiments.
The main difference in ALICE, with respect to these experiments which already used such kind of apparatus is that we are evolving in a new energy domain and we reach high multiplicity of charged particles in Heavy Ion collisions. The correct association of the tracks reconstructed before and after the absorber is thus something really challenging but the simulations we performed on that aspect are encouraging.
A.M. And what is particularly new in the muon forward tracker proposed for the ALICE experiment?
L.M. The Muon Forward Tracker is a proposal that is part of the upgrade plans of 2017. It is mainly motivated by the possibility of overcoming the intrinsic limitations of the Muon Spectrometer and the possibility to perform new measurements of general interests for the whole ALICE physics. The MFT, thanks to its tracking capability will also allow the improvement of mass resolution as I have already mentioned; it also help us to reject a large fraction of muons coming from ? and K decays, improving signal over background ratio. Finally, it can also lead to direct multiplicity measurement, event by event, of charged particles within the acceptance of the spectrometer. It really opens a new window on the way we see muons with ALICE.
The MFT planes in a view including the beam pipe design. The Beryllium section of the pipe is shown in red, while the blue parts are the Aluminium sections connected by the Aluminium bellows (brown)
It is important to note that this muon forward tracker is also a technical challenge from the detector point of view. It will be composed of really thin and small pixels sensors in order to reach the expected physics performances. The setup of the actual beam pipe was completely rethought and new proposal was made in order to provide all the services needed for the tracker