No Arabic abstract
A new heavy-ion experiment on fixed target, NA60+, has been proposed at the CERN SPS for data taking in the next years. Its main goals will be focused on precision studies of thermal dimuons, heavy quark and strangeness production in Pb-Pb collisions at center-of-mass energies ranging from 5 to 17 GeV, which will provide a unique opportunity to investigate the region of the QCD phase diagram at high baryochemical potential ($mu_B sim 200-400$~MeV). The key points of the NA60+ very broad and ambitious physics program will be described.
Since 1986, several heavy ion experiments have studied some signatures of the formation of the quark-gluon plasma and a few exciting results have been found. However, some important questions are still unanswered and require new measurements. The NA60 experiment, with a new detector concept that vastly improves dimuon detection in proton-nucleus and heavy-ion collisions, studies several of those open questions, including the production of open charm. This paper presents the experiment and some first results from data collected in 2002.
Precise measurements of the branching ratios for the $Ktopi ubar{ u}$ decays can provide unique constraints on CKM unitarity and, potentially, evidence for new physics. It is important to measure both decay modes, $K^+topi^+ ubar{ u}$ and $K_Ltopi^0 ubar{ u}$, since different new physics models affect the rates for each channel differently. We are investigating the feasibility of performing a measurement of BR($K_Ltopi^0 ubar{ u}$) using a high-energy secondary neutral beam at the CERN SPS in a successor experiment to NA62. The planned experiment would reuse some of the NA62 infrastructure, including possibly the NA48 liquid-krypton calorimeter. The mean momentum of $K_L$ mesons decaying in the fiducial volume is 70 GeV; the decay products are boosted forward, so that less demanding performance is required from the large-angle photon veto detectors. On the other hand, the layout poses particular challenges for the design of the small-angle vetoes, which must reject photons from $K_L$ decays escaping through the beam pipe amidst an intense background from soft photons and neutrons in the beam. We present some preliminary conclusions from our feasibility studies, summarizing the design challenges faced and the sensitivity obtainable for the measurement of BR($K_Ltopi^0 ubar{ u}$).
The NA60 experiment is a fixed-target experiment at the CERN SPS. It has measured the dimuon yield in Indium--Indium collisions with an In beam of 158 AGeV/c and in p-A collisions with a proton beam of 400 and 158 AGeV/c. The results allow to address three important physics topics, namely the study of the rho spectral function in nuclear collisions, the clarification of the origin of the dimuon excess measured by NA50 in the intermediate mass range, and the J/psi suppression pattern in a collision system different from Pb-Pb. An overview of these results will be given in this paper.
The exploration of the phase diagram of Quantum ChromoDynamics (QCD) is carried out by studying ultrarelativistic heavy-ion collisions. The energy range covered by the CERN SPS ($sqrt{s_{rm scriptscriptstyle{NN}}} sim$ 6-17 GeV) is ideal for the investigation of the region of the phase diagram corresponding to finite baryochemical potential ($mu_{rm B}$), and has been little explored up to now. We propose in this document a new experiment, NA60+, that would address several observables which are fundamental for the understanding of the phase transition from hadronic matter towards a Quark-Gluon Plasma (QGP) at SPS energies. In particular, we propose to study, as a function of the collision energy, the production of thermal dimuons from the created system, from which one would obtain a caloric curve of the QCD phase diagram that is sensitive to the order of the phase transition. In addition, the measurement of a $rho$-a$_1$ mixing contribution would provide conclusive insights into the restoration of the chiral symmetry of QCD. In parallel, studies of heavy quark and quarkonium production would also be carried out, addressing the measurement of transport properties of the QGP and the investigation of the onset of the deconfinement transition. The document also defines an experimental set-up which couples a vertex telescope based on monolithic active pixel sensors (MAPS) to a muon spectrometer with tracking (GEM) and triggering (RPC) detectors within a large acceptance toroidal magnet. Results of physics performance studies for most observables accessible to NA60+ are discussed, showing that the results of the experiment would lead to a significant advance of our understanding of strong interaction physics. The document has been submitted as an input to the European Particle Physics Strategy Update 2018-2020 (http://europeanstrategyupdate.web.cern.ch/).
NA60 is a fixed-target experiment at the CERN SPS which measured dimuon production in nucleus-nucleus and proton-nucleus collisions. In this paper we report on a precision measurement of low-mass muon pairs in 158 AGeV indium-indium collisions. A significant excess of pairs is observed above the yield expected from neutral meson decays. The excess can be isolated by subtraction of expected sources, thanks to the excellent mass resolution and large sample size.