The ALICE experiment, with its state-of-the-art detection systems, produced a wealth of results during Run 1 of the LHC (2009–2013) – driving a new impetus in the field of heavy-ion collisions. While Run 2 (2015–2017) will see the consolidation and completion of the scientific programme for which the experiment was originally approved, the ALICE collaboration has already taken up the challenge to make a quantitative leap in the precision of its observations by exploiting the high luminosity anticipated for the LHC in Run 3 (2019–2022). The plan is to upgrade the detector during the LHC’s second long shutdown, just before Run 3. In September, the LHC Committee (LHCC) approved an addendum to the letter of intent for the ALICE upgrade programme concerning the project for the Muon Forward Tracker (MFT) – an assembly of silicon pixel planes serving as internal tracker, in the forward acceptance of ALICE’s muon arm.
The basic idea behind the MFT concept – measuring muons both before and after the hadron absorber, then matching the two pieces of information – is well established in the field of heavy-ion physics, having been exploited both at CERN’s Super Proton Synchrotron and more recently at Brookhaven’s Relativistic Heavy-Ion Collider. Evaluation of the expected scientific impact of such a major upgrade required the preparation of a detailed letter of intent, the first draft of which was submitted to the ALICE collaboration in December 2011. The final document received internal approval in March 2013 and the first discussions with the LHCC started two months later.
There are three main pillars of the MFT’s contribution to the ALICE physics programme: dimuon measurement of prompt charmonia states J/ψ and ψ´, to study in-medium colour-screening and hadronization mechanisms of cc pairs; measurement of charm and beauty production via single muons and J/ψ particles from B decay, allowing a tomography of the medium via study of the energy loss of heavy quarks; and low-mass dimuon measurements, to study thermal radiation from quark–gluon plasma and search for in-medium modifications of the spectral functions of light vector mesons. The technical feasibility was also demonstrated as reported in the letter of intent – from the choice of the CMOS pixel technology to aspects related to the detector mechanics and cooling. It is also worth emphasizing that ALICE is the only LHC experiment that is designed to perform precision measurements at forward rapidities in the high-multiplicity environment of heavy-ion collisions.
What will the MFT do for ALICE? Put simply, it will be like wearing a pair of glasses to correct myopia. The MFT will reveal the details of the muon tracks in the vertex region, allowing not only a powerful rejection of background muons but also access to measurements that are not feasible with the existing muon spectrometer. A prime example is the disentanglement of prompt (charm) and displaced (from beauty) J/ψ production (figure 1), which is achievable only by measuring precisely the distance between the primary vertex of the collision and the vertex in which the J/ψ is produced. Because such distances are in the order of a few hundreds of micrometres, a dedicated vertex detector is needed. Figure 2 illustrates how this measurement becomes possible only with the addition of the MFT to the current muon spectrometer set-up.
The team involved with the MFT project must now provide details on all of the technological aspects, producing a Technical Design Report in the second half of 2014 – the final effort before assembling the first pieces of the detector. Then four more years of intense work will be needed before the MFT is installed and becomes operational in the ALICE cavern in 2019.