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The NA60 Silicon Vertex Telescopes

This study presents the use of high-precision silicon vertex telescopes for dimuon measurements in various runs, including ions in 2003 and protons in 2004. The telescopes offer improved mass resolution and reduced background for accurate muon origin determination.

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The NA60 Silicon Vertex Telescopes

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  1. The NA60 Silicon Vertex Telescopes Dimuon measurements Vertex telescope used in: Ions runs (2003) Protons runs (2004) P. Martins - CFTP-IST

  2. Measuring dimuons • Signals within reach: • Vector mesons ( η, ρ, ω, ϕ, J/ψ and ψ’ ) • Open charm simultaneous semi-leptonic decays • Drell-Yan quark anti-quark annihilation • Background • Simultaneous π and K decays P. Martins - CFTP-IST

  3. muon trigger and tracking target beam or hadron absorber ? Muon Other Measuring dimuons magnetic field Energy loss Multiple scattering • Degraded dimuon mass resolution • Large combinatorial background • Cannot distinguish prompt muons from decay muons P. Martins - CFTP-IST

  4. muon trigger and tracking hadron absorber magnetic field or ! Muon Other Measuring dimuons in NA60 2.5 T dipole magnet beam tracker vertex telescope targets Matching in coordinate and momentum space • Improved dimuon mass resolution • Reduced combinatorial background • Origin of muons can be accurately determined P. Martins - CFTP-IST

  5. A vertex telescope for heavy-ion running High precision High-energy (nuclear) collisions Dimuonproduction High statistics As a function of centrality Smallcross-section High multiplicityenvironment High luminosity Fixed-target Selectivedimuon trigger Requirements on the telescope Radiation-hardzero degree calorimeter Radiation tolerance Granularity Fast read-out Segmented target P. Martins - CFTP-IST

  6. Silicon sensor 300 µm Read-out chip 750 µm Silicon pixel detectors Pixel sensor • 12.8  13.6 mm2 active area • 256  32 cell matrix • 50  425 µm2 cell size + 5” sensor wafer (layout) 8” read-out chip wafer = ALICE1LHCB read-out chip • Operated at 10 MHz • Radiation tolerant up to ~ 30 Mrad • 32 columns parallel read-out 25 µm solder bump P. Martins - CFTP-IST

  7. The silicon pixel vertex telescope • 8 “small” 4-chip planes • 8 “big” 8-chip planes • ~ 2 – 3 % X0 per plane • 12 tracking points with good acceptance • 9 X (bending plane, |B| = 2.5 T) • 3 Y (non-bending plane) • 800’000 channels, 50 425 µm2 pixel size targets beam tracker P. Martins - CFTP-IST

  8. Z-vertexdeterminationfrom pixel telescope σz ~ 200 µm σ(µm) Dispersion between BT track and VT vertex 30 20 10 VT Vertex resolution (for σBT = 20 µm) 0 Vertexing performance • Data collected in October 2003 with 158 GeV Indium beam Target box windows In targets Beam tracker sensors Transverse coordinatesmeasuredby the vertex telescopeand the beam tracker σbt ~ 20 µm → σvt ~ 12 µm P. Martins - CFTP-IST

  9. Radiation tolerance • 158 GeV SPS Indium beam in October 2003 • ~ 4  1012ions delivered over 5 weeks • Performance was closely monitored • Sensor type inversion was expected, seen and understood • The detector was re-usedin 2004 for proton running Leakage current vs. time Hit efficiency vs. bias voltage • strong temperature dependence • increase with integrated dose (and luminosity) • inner region needs increased bias voltage after 4 weeks running P. Martins - CFTP-IST

  10. Type inversion Active volume p+ 150 V n n+ “Brand new” pixel sensor Hit maps from a bias voltage scan after 4 weeks P. Martins - CFTP-IST

  11. Type inversion 100 V Hit maps from a bias voltage scan after 4 weeks P. Martins - CFTP-IST

  12. Type inversion 80 V Hit maps from a bias voltage scan after 4 weeks P. Martins - CFTP-IST

  13. Type inversion p+ Pixels disconnected from the active volumedo not work 60 V n n+ irradiation p+ p n+ Active volume p n+ Less irradiated zones are still connectedfor the same bias voltage Hit maps from a bias voltage scan after 4 weeks P. Martins - CFTP-IST

  14. Type inversion 50 V Pixels at small radiireceive more fluence : • Lowering the bias voltage leaves an ever larger area not fully depleted (i.e. very inefficient) Hit maps from a bias voltage scan after 4 weeks P. Martins - CFTP-IST

  15. Type inversion 40 V Pixels at small radiireceive more fluence : • Lowering the bias voltage leaves an ever larger area not fully depleted (i.e. very inefficient) Hit maps from a bias voltage scan after 4 weeks P. Martins - CFTP-IST

  16. Type inversion 30 V Pixels at small radiireceive more fluence : • Lowering the bias voltage leaves an ever larger area not fully depleted (i.e. very inefficient) Hit maps from a bias voltage scan after 4 weeks P. Martins - CFTP-IST

  17. Very good mass resolution ω and ϕ perfectly resolved η→µ+µ- seen for the first time in nuclear collisions Ability to trace muon offsets Prompt dimuons (resonances) peak at low values Displaced D meson decays have a long tail Prompt + Charm Data Weighted offset Physics from the Indium run P. Martins - CFTP-IST

  18. Challenges in proton-nucleus running Measurecc production incc → J/ψ+gdecays High luminosity g→ e+e- measurement Thin detector Radiation damage Pile-up rejection Minimizes electrons’ multiple scattering and energy loss Fluences similar to the 2003 Indium run 40 MHz average* interaction rate Challenges met by… Use silicon micro-strips at the start of the telescope Operate the vertex telescope at negative temperatures More accurate timing Use ATLAS silicon pixel detectors Reduced material budget(no readout chips, hybrids or cooling rings on the way of the particles) Lower leakage current and depletion voltages Measure peaking time in micro-strips Measure single muon production time differences * Poisson statistics : 26 % of the time there are >1 interactions in 25ns P. Martins - CFTP-IST

  19. ATLAS pixel planes 50 ns readout gate(4x shorter than the ALICE pixel planes) From idea to running experiment in 8 months Two planes fully cover the angular acceptance Bad chips outside of acceptance From all charged tracks… …we only keep those with a hit in the ATLAS planes P. Martins - CFTP-IST

  20. Silicon micro-strip timing Analogue pulse shape reconstruction provides timing and amplitude. Measured the phase between the 40 MHz clock and the (random) trigger arrival time. Reconstructed pulse shape Hits in time with trigger Nominal intensity Low intensity Dispersion of the reconstructed peaking time at low intensity Hits from pile-up interactions P. Martins - CFTP-IST

  21. p-A @ 158 GeV Beam of ~ 4 × 109 p/burst ~ 30’000 J/ψ events 3 days’ data p beam158 GeV Al W Cu 3 x Be Pb U In (before muontrack matching) z-vertex (cm) 2004 proton-nucleus run 7 nuclear targets at two proton beam energies: 400 and 158 GeV Collected ~ 300’000 J/ψ events in p-A collisions at 400 GeV At 158 GeV we should be able to determine α(J/ψ) with an error better than 1% P. Martins - CFTP-IST

  22. Merry Christmas and Happy New Year Summary and conclusions • NA60 built and operated in 2003the first radiation-tolerant silicon pixel detector in the world • Unprecedented precision measurements in heavy-ion collisions • Very big impact in many physics topics in dimuon production • Integrated radiation doses comparable to years of LHC operation • Detector still operational and even used in the following year • In 2004 we further upgraded this telescope with faster detectors • Successfully operated at interaction rates in excess of the LHC Questions? P. Martins - CFTP-IST

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