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The ALICE Dimuon Spectrometer

Hard, penetrating probes, such as 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.

The ALICE forward muon spectrometer will study the complete spectrum of heavy quarkonia (J/Ψ, Ψ′, ϒ, ϒ′, ϒ′′) via their decay in the μ+μ– channel. The spectrometer acceptance covers the pseudorapidity interval 2.5 ≤ η ≤ 4 and the resonances can be detected down to zero transverse momentum. The invariant mass resolution is of the order of 70 MeV in the J/Ψ region and about 100 MeV close to the ϒ. These values allow to resolve and measure individually all five resonance states.

The main components of the spectrometer are shown on the following sketch.

Basic principle of the Dimuon spectrometer: an absorber to filter the background, a set of tracking chambers before, inside and after the magnet and a set of trigger chambers.

The front absorber suppresses all particles except muons coming from the interaction vertex. It is made of carbon and concrete in order to limit the multiple scattering and the energy loss of the muons. The inner beam shield protects the chambers from background originating from particles at small angles. It is made of tungsten, lead and stainless steel to minimize the background arising from primary particles emitted in the collision and from their showers produced in the beam pipe and in the shield itself.

The front absorber (credits: Antonio Saba)

The tracking system is made of 10 cathode pad/strip chambers arranged in 5 stations of 2 chambers each. The spatial resolution should be better than 100 mm and all the chambers are made of composite material (< 3% X0 per chamber) to minimize the scattering of the muons in order to obtain the required resolution. To limit the occupation rate to a maximum of 5% the full set of chambers has more than 1 million channels.

The small chambers (Credit: Aurelien Muller)

The large chambers (credit: Aurelien Muller)

The trigger system is designed to select heavy quark resonance decays. The selection is made on the pt of the two individual muons. The 4 planes of RPCs (Resistive Plate Chambers) arranged in 2 stations and positioned behind a passive muon filter provide the transverse momentum of each μ. The spatial resolution should be better than 1 cm. Special front-end electronics have been designed to obtain the time resolution of 2 ns necessary for the identification of the bunch crossing.

The trigger chambers (credit: Aurelien Muller)

The dipole magnet is positioned at about 7 m from the interaction vertex and it is one of the biggest warm dipoles in the world (free gap between poles ≈ 3 m, height of the yoke ~ 9 m). The magnetic field (Bnom = 0.7 T, 3 Tm field integral) is defined by the requirements on the mass resolution.

Video of the assembly of the dipole magnet

Dipole Magnet in its final position with the muon wall

Physics program

Heavy quarkonia

The production of all the quarkonia from the Ψ and ϒ families will be studied:
• as a function of the centrality, to identify suppression/enhancement patterns;
• as a function of pt, to disentangle QGP models;
• with respect to the reaction plane (determined with other ALICE detectors, e.g. the PMD) to unravel Glauber and comover absorption;
• for different colliding nuclei (e.g. Pb-Pb and Ar-Ar) to investigate the dependence of quarkonia yields as a function of the system size;
• for p-p and p-nucleus (or d-nucleus or α-nucleus) to establish a reference for nucleus-nucleus.

Open Charm and Beauty

In the dimuon invariant mass spectrum, the quarkonia signals will be sitting on top of a continuum mainly coming from open charm and bottom decay. This opens the possibility of studying the production of open charm and beauty in parallel with the one of heavy quarkonia. It is relevant because:
• open charm and beauty represent the most natural normalization of the quarkonia signals;
• measurements of open flavours cross-sections are expected to shed light on production mechanisms for heavy quarkonia other than direct hard-scattering.

Run conditions and statistics

The following data taking scenario is at present foreseen in the first LHC years:
• p-p runs at √s = 14 TeV at L = 3 1030 cm–2 s–1
• Pb-Pb runs at L = 1027 cm–2 s–1
• Ar-Ar runs at L = 1027 cm–2 s–1
• p-A or d-A or α-A runs at L = 1031 cm–2 s–1

In addition to this program, later options will be considered depending on the outcome of the first years. With the luminosities quoted above, the number of J/Ψ (ϒ) expected to be recorded in 106 s of data taking (i.e. about one month) is expected to be of the order of several hundred thousands (several thousands) both for Pb-Pb and Ca-Ca central collisions as well as for p-p collisions. For the ϒ, an excellent signal to background ratio (typically of the order of 10) is expected. Dipole Magnet Resistive Plate Chamber for the trigger system Quadrant of the small chambers Slats of the large chambers Geometry Monitoring System Basic principle of the Dimuon spectrometer: an absorber to filter the background, a set of tracking chambers before, inside and after the magnet and a set of trigger chambers. Front-end electronic board designed for the trigger chambers.

More information about the muon spectrometer