the complete spectrum of heavy quarkonia (J/Ψ, Ψ′, ϒ, ϒ′, ϒ′′) via their decay in the μ+μ– channel. Heavy quarkonium states, provide an essential tool to study the early and hot stage of heavy-ion collisions. In particular they are expected to be sensitive to Quark-Gluon Plasma formation. In the presence of a deconfined medium (i.e. QGP) with high enough energy density, quarkonium states are dissociated because of colour screening. This leads to a suppression of their production rates. At the high LHC collision energy, both the charmonium states (J/Ψ and Ψ′) as well as the bottomonium states (ϒ, ϒ′ and ϒ′′) can be studied. The Dimuon spectrometer is optimized for the detection of these heavy quark resonances. Muons may be identified using the just described technique by using the fact that they are the only charged particles able to pass almost undisturbed through any material. This behaviour is connected to the fact that muons with momenta below a few hundred GeV/c do not suffer from radiative energy losses and so do not produce electromagnetic showers. Also, because they are leptons, they are not subject to strong interactions with the nuclei of the material they traverse. This behaviour is exploited in muon spectrometers in high-energy physics experiments by installing muon detectors behind the calorimeter systems or behind thick absorber materials. All charged particles other than muons are completely stopped, producing electromagnetic (and hadronic) showers. The muon spectrometer in the forward region of ALICE features a very thick and complex front absorber and an additional muon filter consisting of an iron wall 1.2 m thick. Muon candidates selected from tracks penetrating these absorbers are measured precisely in a dedicated set of tracking detectors. Pairs of muons are used to collect the spectrum of heavy-quark vector-meson resonances (J/Psi). Their production rates can be analysed as a function of transverse momentum and collision centrality in order to investigate dissociation due to colour screening. The acceptance of the ALICE Muon Spectrometer covers the pseudorapidity interval 2.5 ≤ η ≤ 4 and the resonances can be detected down to zero transverse momentum. Characterization of the collision Finally, we need to know how powerful the collision was: this is done by measuring the remnants of the colliding nuclei in detectors made of high density materials located about 110 meters on both sides of ALICE (the ZDCs) and by measuring with the FMD, V0 and T0 the number