The QCD phase diagram in the region of moderate temperature and high baryon density: study of dimuon production in the 20-160 AGeV interval at the CERN SPS

May 19, 2014, 5:50 PM
helium (darmstadtium)



Schlossgraben 1 64283 Darmstadt Germany
Contributed Talk Electromagnetic Probes Electromagnetic probes


Gianluca Usai (Universita e INFN (IT))


The structure of the QCD phase diagram in the region of moderate temperature and high baryon density is still almost unexplored. In this regime, the QGP and hadronic phases should be separated by a first order transition region. On the other hand, for sufficiently low baryonic chemical potential a simple cross-over is expected. The end point of the first order transition region is the so called critical point. Measurements of the ratio $K^+/\pi^+$ vs energy performed at the CERN SPS by the NA49 collaboration showed that the early stage produced in central Pb-Pb collisions at $E_{lab}$=30-40 AGeV may have reached the transition line - marking the onset of deconfinement. However, this interpretation is still controversial. Dilepton measurements with a rich set of independent observables offer a completely independent way to shed light on the onset of deconfinement and at the same time on the issue of chiral restoration. The CERN SPS is unique for systematic investigations along these lines, due to its wide beam energy range from 20-160 AGeV coupled to very high luminosities over the full range. The objective of this talk is to present a new proposal for measuring dimuon production in a comprehensive energy scan at the SPS - specifically both below and above the maximum of the $K^+/\pi^+$ ratio. To advance the field with measurements which could provide quantitative insight, a further significant increase in the precision and in the collected statistics with respect to the past reference experiment NA60 is needed. To this end, we propose a novel NA60-like apparatus, with improved performance, based on the coupling of a muon spectrometer to a a silicon pixel spectrometer in the vertex region before the absorber. The first key element is a high-precision measurement of the temperature $T$ of the thermal dimuon continuum in the mass range $1.1\lt M\lt2.5$ GeV (IMR) vs. beam energy. The temperature is accessible through the spectral shape of the mass spectra, and since mass is a Lorentz-invariant, $T$ is immune to any motion of the emitting sources and thus purely thermal, in contrast to the slope parameters of dilepton $m_T$ spectra or photon $p_T$ spectra. At top SPS energies, values of about 200 MeV were found, indicating dominantly partonic emission sources. For decreasing beam energies one should expect that the partonic contribution will also decrease, becoming negligible at the onset of deconfinement. Thus the onset of deconfinement might be tagged by a precision measurement of $T$ in the IMR. The second element is related to the transverse momentum spectra, which encode - besides temperature - also radial flow, another key property of the fireball. At topmost SPS energies the effective temperature extracted from the $m_T$ spectra vs mass shows an increase up to $M\sim1$ GeV corresponding to $\rho$ production which is maximally coupled to radial flow through pions. At 1 GeV a sudden drop occurs and the temperature is constant at 180-200 MeV - a sign of production of thermal dimuons from the partonic medium without any flow (at the SPS). The evolution of the pattern of $T_{eff}$ vs. $M$ towards lower energies, in particular the possible decrease or disappearence of the drop, will be most revealing: thermal radiation from multi-pion processes should exhibit a monotonic increase of $T_{eff}$ vs $M$ so that around the onset of deconfinement the drop should vanish. The new experiment should reach a sensitivity at the MeV level in the measurements of $T$ and $T_{eff}$ vs $M$. This will be possible integrating luminosities at least an order of magnitude larger than in case of NA60 or so while retaining a good signal to background ratio - well above than 1/100 in the IMR even in Pb-Pb central collisions. For masses below 1 GeV (LMR), at high energies, hadronic many body models describe $\rho$ production and the notion that the total baryon density drives the broadening is now well accepted. At lower energies, the baryon density gets maximal, thus the effects of $\rho$ broadening can be measured with utmost precision. In this measurement the mass resolution is a key factor and we propose to improve it over NA60 by a factor 2-3, reaching $\sim$10 MeV at the $\omega$ mass. The experimental apparatus must have also a good $p_T$-$y$ coverage down to the lowest beam energies. In the talk the detector perfomance in terms of acceptances at different energies and mass resolution will be discussed in detail. The physics performance will be presented on the basis of simulations of Pb-Pb collisions at 20 and 40 AGeV including a background estimate and a modeling of the involved physics processes - in medium $\rho$ in the LMR, multi-pion processes and partonic radiation in the IMR. An overview of the possible detector technologies together with a cost estimate will be also presented. Finally, an experimental program consisting of measurements at different energies and/or different collision systems will be discussed in terms of beam time needed to collect the required event statistics.
On behalf of collaboration: None

Primary author

Gianluca Usai (Universita e INFN (IT))

Presentation materials