The NA60 experiment at the CERN SPS “Production of open charm and prompt dimuons in collisions of proton and heavy ion beams on nuclear targets” Results and perspectives Detector concept and overview. Dimuon production in proton-nucleus collisions: mass resolution, phase space coverage. Vertex position resolution in Pb-Pb collisions. Perspectives. Johann M. Heuser RIKEN - The Institute of Physical and Chemical Research Wako, Saitama 351-0198, Japan For the NA60 Collaboration

Questions left open by previous SPS experiments What is the origin of the intermediate mass dimuon excess? Thermal dimuons produced from a QGP phase? Is the open charm yield enhanced in nucleus-nucleus collisions ? How does it compare to the suppression pattern of charmonium states? Which physics variable drives the onset of ’, c and J/ suppression ? Energy density ? Cluster density? What is the normal nuclear absorption pattern of the c? NA50 NA50 melting of cc? New and better measurements are needed ! melting of directly produced J/?

NA60 detector concept: an “eye” in the vertex region muon spectrometer muon filter beam m p, K  m D  m { offset vertex vertex spectrometer Track matching through the muon filter: Improved dimuon mass resolution. Improved signal / background ratio (rejection of p and K decays). Muon track offset measurement: Separate charm from prompt (thermal ?) dimuons.

Dimuon mass resolution: simulation The use of a silicon vertex telescope clearly improves the mass resolution (from 70 to ~ 20 MeV at the w mass). with pixels without pixels J/ ’  Vertex spectrometer

Separating charm decays from prompt dimuons muon track offset resolution better than 35 mm for p  15 GeV/c Background Signal 0 100 200 300 400 500 600 700 offset (mm) D+ : ct = 317 mm D0 : ct = 124 mm prompt dimuons offset  90 mm open charm Background Signal 90  offset  800 mm and muons away from each other  180 mm in the transverse plane at Zvertex

Silicon Micro-Strip Detectors Silicon Pixel Detectors Overview of the NA60 detector: the reality Beam Tracker Interaction Counter Target box Muon Spectrometer 2.5 T dipole magnet beam vertex tracker ZDC Quartz Blade Silicon Micro-Strip Detectors Silicon Pixel Detectors

Cryogenic Silicon Beam Tracker Two stations of back-to-back mounted micro-strip sensors. Operated at 130 K  radiation hard. 24 strips of 50 mm pitch per sensor. 20 mm resolution on the transverse coordinates of the interaction point. timing with 1.7 ns accuracy 20 GeV/nucleon Pb beam profiles (very broad beam)

The NA60 Silicon Micro-Strip Detectors 8 double tracking planes of 300 m silicon sensors. 1536 strips of pitch from 60 to 227 mm  occupancy < 3%. Tilted lines to improve momentum and angles resolution. 90 mm diameter to cover dimuon acceptance at 40 cm from target. Beam hole through sensor wafer. SCTA read-out chips (from ATLAS) operated at 40 MHz.  track & vertex reconstruction in low multiplicity environment.

The NA60 Silicon Pixel Detectors 16 tracking planes in two sizes (4 or 8 chips). ~100 ALICE1LHCb pixel detector assemblies. Matrix of 32 x 256 pixels per assembly. Pixel size 50 x 425 mm2 . Ion. radiation hardness: up to ~30 Mrad. Operation at 10 MHz r/o clock.  accurate track and vertex reconstruction in a very high multiplicity environment. Pixel efficiency Convolution of pixel and tracking resolution (including alignment) using only two tracking planes. Full telescope: < 10 mm expected. X residual (cm) preliminary Spatial resolution x = 14.4 m preliminary X (cm) Int. hit map from Pb-Pb collisions

Results from the June 2002 proton run Z-vertex resolution ~ 900 μm Obtained with the micro-strip detector telescope. Present mass resolution : ~ 25 MeV at the w and ~ 30 MeV at the f (it was around 80 MeV in NA50). A good measurement of the nuclear dependence of w and f production should be feasible. 400 GeV protons target box window NA60 dimuon mass resolution Dimuon mass distribution for each target sw ~ 25 MeV Dimuon mass spectrum from muon spectrometer NA50-like resolution sf ~ 30 MeV p-Be J/y vertex identification muon track matching between vertex telescope and muon spectrometer

Dimuon phase space coverage NA60 low mass dimuon data extends down to much lower transverse momentum values than previous measurements (NA38, NA50, Helios-3). Much improved comparisons with CERES dielectrons and with NA49 f  KK results. f mid-rapidity pT (GeV/c) pT (GeV/c) w

Xvertex from beam tracker (cm) Xvertex from pixel telescope (cm) October 2002 Pb-Pb data: 20/30 GeV/nucl. beams average event (36 tracks) beam tracking pixel detector telescope partly installed Resolution in the determination of the interaction vertex σZ ~ 190 mm σX ~ 20 mm Pb targets Beam tracker vs. pixel telescope Xvertex from beam tracker (cm) beam tracker sensor target box windows Correlation width ~ 30 mm Xvertex from pixel telescope (cm)

Summary and Outlook NA60 new detectors are almost fully developed. Completion of the Pixel Detector Telescope for Fall 2003 ongoing. Data collected in 2002 confirms feasibility of the experiment:  dimuon mass resolution at the w and φ peaks ~ 25–30 MeV.  phase space coverage extends down to low pT and masses.  resolution in transverse vertex coordinates ~ 20 mm. In September-October 2003 NA60 will study In-In collisions:  J/y and y’ suppression patterns.  r, w and φ production.  open charm production.  thermal dimuon production.

Overview of Silicon Tracking Telescope for next runs Eight small (4 chips) and eight large (8 chips) pixel planes. Last tracking plane at 32 cm from the center of the target. In-In Mixed setup : 8 strip tracking stations complemented by pixel planes. Strip planes : faster read-out, less material and bigger angular coverage. Pixel planes : much better granularity and signal to noise ratio. p-A

Ongoing completion of the pixel detector vertex spectrometer 5” sensor wafer (layout). 8” ALICE1/LHCb chip wafer prepared for bump-bonding to yield the pixel detectors. Three pixel planes, used in Pb-Pb run 10/2002. Two more planes assembled. Material for the full telescope: 20 µm solder bump bond (VTT, Finland). Ceramic hybrids. Readout electronics. Cooling structure on a module. Printed circuit boards. Quality check: Pixel detector assemblies on probe station.

The NA60 Collaboration ~50 people, 12 institutes, 7 countries R. Arnaldi, K. Banicz, K. Borer, J. Buytaert, J. Castor, B. Chaurand, W. Chen, B. Cheynis, C. Cicalò, A. Colla, P. Cortese, A. David, A. Devaux, A. Drees, L. Ducroux, H. En’yo, A. de Falco, A. Ferretti, M. Floris, P. Force, A. Grigorian, J.Y. Grossiord, N. Guettet, A. Guichard, H. Gulkanian, J. Heuser, M. Keil, L. Kluberg, Z. Li, C. Lourenço, J. Lozano, F. Manso, N. de Marco, A. Masoni, A. Neves, H. Ohnishi, C. Oppedisano, P. Parracho, G. Puddu, E. Radermacher, P. Rosinský, E. Scomparin, J. Seixas, S. Serci, R. Shahoyan, P. Sonderegger, R. Tieulent, G. Usai, H. Vardanyan, R. Veenhof and H. Wöhri ~50 people, 12 institutes, 7 countries