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NA60 Physics motivation

Study of Prompt Dimuon and Charm Production with Proton and Heavy Ion Beams at the CERN SPS

The NA60 experiment has been proposed to study the phase transition from confined hadronic matter to deconfined partonic matter, by addressing specific questions left open by the current SPS heavy ion physics program. The present results strongly indicate that a new state of QCD matter, where the quarks and gluons are no longer confined to hadrons, is formed in head-on collisions of Pb nuclei at the top SPS energies. NA60 will have a deeper look into the presently available signals, and will probe new ones for the first time, with the aim of converting this evidence into solid proof.

The NA60 detector complements the muon spectrometer and zero degree calorimeter previously used in NA50 with two state-of-the-art silicon detectors, placed in the target region: a radiation hard beam tracker, based on a set of four silicon microstrip detectors placed on the ion beam and operated at 130 K (the "beamscope") and a silicon pixel telescope, made of 88 Alice1 pixel chips, of around 8000 channels each.

Besides studying prompt dimuon production with good mass resolution, we intend to measure the production rate of charmed mesons, through their semileptonic decay, by measuring the offset of the muon tracks extrapolated to the interaction point.

Low Mass Dimuon Production

In a few weeks of running, the experiment will be able to collect enough statistics to study the behaviour of low mass dimuons (and, in particular, of the rho, omega and phi resonances) as a function of the centrality of the collision, measured event by event through the charged particle multiplicity and, independently, through the forward energy detected in a Zero Degree Calorimeter, ZDC. The expected statistics and acceptance as a function of the dimuon transverse momentum will also enable us to make a detailed study of the low mass resonance production as a function of pt, for peripheral and central Pb-Pb collisions.

With a mass resolution of around 20 MeV at the omega mass, we will be able to separately study the production of omega and rho mesons, and search for a mass shift and broadening of the short-lived rho meson, from p-Be to Pb-Pb and from peripheral to central Pb-Pb collisions, as a function of transverse momentum.

Such measurements can be considered as a natural continuation of the dilepton physics program of the NA38/NA50, Helios-3 and CERES experiments. In particular, the CERES experiment has measured low mass di-electron production from p-Be to Pb-Au, at the SPS, finding intriguing evidence for a rather large enhancement of e+e- pairs in the mass region 0.2-0.7 GeV, motivating a very active theoretical research in recent years.

Intermediate Mass Dimuon Production

Another physics topic we intend to explore concerns the production of intermediate mass dimuons. The NA38/NA50 and Helios-3 experiments have observed an enhanced production of dimuons with mass between 1.2 and 2.5 GeV. The NA60 experiment will be able to clarify if this excess production is due to prompt dimuons or to muon pairs which originate at some distance from the event vertex. The tool is a measurement of the `offset' of the muon tracks relative to the interaction point. This measurement will be able to rule out one or the other of the two hypothetical explanations currently considered: an enhancement of charm production or the production of thermal dimuons.

Charm Production

The production of charm quarks leads mainly to correlated pairs of D and Dbar mesons. Only a few percent of the charmed quark pairs end up in the bound charmonia states presently studied by the NA50 experiment.

Knowing that the bound ccbar states are suppressed, it is natural to immediately ask what happens to the unbound charm. Charm quarks are so heavy that they can only be produced at the earlier stages of the heavy ion collision, before the eventual formation of the QGP state, except if the temperature of the plasma is much higher than currently assumed. D meson production also provides the natural reference with respect to which we should study the observed \jpsi\ suppression, since both production mechanisms depend on the same gluon distribution functions.

An enhancement of charm production in heavy ion collisions would provide a reasonable description of the mass and pt distributions of the measured intermediate mass dimuon continuum but would have direct implications on the interpretation of the charmonia data, boosting the anomaly of the measured J/psi suppression.

This situation reinforces the importance of having a direct measurement of open charm production in heavy ion collisions at the SPS, with a dedicated experiment able to cope with the high particle multiplicities reached in the most central Pb-Pb collisions and with the small D production cross section. The highly selective dimuon trigger of the NA60 experiment will enable us to take full advantage of the high intensity beam line in ECN3 (providing up to 10^7 Pb-Pb collisions per burst) to study this rare process.

J/psi Production and Suppression

The J/psi suppression pattern measured by NA50 is certainly among the most exciting results presently available from the SPS heavy ion physics program. Indeed, the pattern of J/psi suppression is the only measurement that explicitly shows a threshold-like behaviour, in Pb-Pb collisions. The current interpretation of these data is that there is a first drop in the \jpsi\ yield when the collisions reach a (local) energy density above the threshold for melting the chi_c charmonia states, and a second drop when the more tightly bound J/psi state itself starts melting.

If the J/psi suppression pattern in Pb-Pb collisions indicates that central Pb-Pb collisions produce a state of matter where colour is no longer confined, we should move on to the detailed understanding of how deconfinement sets in, and what physics variable governs the threshold behaviour of charmonia (chi_c) suppression: (local) energy density, density of wounded nucleons, density of percolation clusters, etc. This requires collecting data with smaller nuclear systems like In-In.

psi' Production and Suppression

The J/psi data collected in central Pb-Pb collisions indicate that we are already beyond the point where the phase transition takes place, but do not provide any information on the value of the critical temperature. Finite temperature lattice QCD tells us that the strongly bound J/psi ccbar state should be screened when the medium reaches temperatures 30-40 % higher than T_c, while the larger and more loosely bound psi' state should melt near T_c. The NA38 experiment has shown that the psi' is significantly suppressed when going from p-U to peripheral S-U collisions. We need to see if this suppression follows a smooth pattern or a sudden transition, within a single collision system rather than comparing p-U to S-U data.

If the psi' suppression is due to Debye screening, its suppression pattern could provide a clear measurement of T_c. However, the hadronic "comovers" produced in S-U collisions may "absorb" the psi' mesons, since its binding energy is only around 40 MeV. What mechanism is responsible for the psi' suppression? The presently existing results are not clear in what concerns the onset and pattern of the psi' suppression. A new measurement is needed, with improved mass resolution to have a cleaner separation of the psi' peak with respect to the J/psi shoulder, and which scans the energy density region from the p-U to the S-U data. In-In collisions are also well placed for this study.