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TORCH: a novel detector combining TOF and RICH

TORCH: a novel detector combining TOF and RICH. Roger Forty (CERN) on behalf of the LHCb RICH group. TORCH ( T ime O f internally R eflected CH erenkov light) is a possible solution for low-momentum particle ID under study for the upgrade of the LHCb experiment

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TORCH: a novel detector combining TOF and RICH

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  1. TORCH: a novel detector combining TOF and RICH Roger Forty (CERN) on behalf of the LHCb RICH group TORCH (Time Of internally Reflected CHerenkov light)is a possible solution for low-momentum particle ID under study for the upgrade of the LHCb experiment Closely related concept to the TOP of Belle II [Toru Iijima] — a new generation of PID devices profiting from fast photodetectorsTORCH is at an earlier stage, but is aiming for higher resolution • The LHCb upgrade • TORCH concept • TORCH R&D Int. Workshop on Probing Strangeness in Hard Processes, Frascati, 18–21 October 2010

  2. 1. The LHCb Upgrade • LHCb is one of the four major experiments at the LHC, dedicated to the search for new physics in CP violation and rare decays of heavy flavours • It is a forward spectrometer (10–300 mrad) operating in pp collider modeParticle identification provided by two RICH detectors [Clara Matteuzzi]Currently with three radiators: silica aerogel, C4F10 and CF4 gas RICH-1 RICH-2 The TORCH detector concept

  3. LHC luminosity • LHCb was commissioned ready for the LHC startup in 2008After some teething trouble the LHC is now performing excellently • Peak luminosity 1032 cm-2s-1 achievedIntegrated L ~ 20 pb-1 recorded so farTarget for next year: 1 fb-1 • The nominal luminosity for LHCb is only few  1032 cm-2s-1, to maximize events with single pp interactions • First phase of LHCb will run for next ~ five years, integrating 5–10 fb-1 • Plan to then upgrade the experimentas doubling time would get too longAim to increase luminosity by 10(will already be available from machine) Exponential increase 1.5 per week! The TORCH detector concept

  4. Current performance • Detector is performing superbly: clean b-hadron signals accumulating rapidly Applying particle ID cuts with the RICH system, can select a cleansignal for Bs→ K+K- (first mass peak for this mode) → Excellent K-p separation at high p B → hh signal Monte Carlo B → hh data 3 pb-1(without PID cut) The TORCH detector concept

  5. Low-momentum PID Upgrade 1st Phase • Flavour tagging (distinguishing B from B) is one of the primary requirements for low-momentum particle ID in LHCb (2–10 GeV) currently provided by aerogel • Can now be studied in data using B0–B0 oscillations • Monte Carlo studies of high-luminosity running indicate that aerogel will be less effective, due to its low photon yield (< 10 p.e./saturated track) and the high occupancy environment The TORCH detector concept

  6. Upgrade plan • Need to prepare for upgrade even though experiment has only just begun to accumulate data, due to long lead time (R&D + construction + installation) • Aim for installation of upgrade in 2016, during a planned LHC shutdown • Main focus is on trigger, which must be upgraded to handle higher luminosity Current bottleneck is hardware level that reduces 40 MHz bunch crossing rate to 1 MHz for readout into HLT → read out complete experiment at 40 MHz into the CPU farm, fully software trigger • RICH system will be kept for PID with photodetectors replaced Propose to replace the aerogel with time-of-flight based detector • First muon station will be removed → space available for new device The TORCH detector concept

  7. 2. TORCH concept • Want positive identification of kaons in region below their threshold for producing light in the C4F10 gas of RICH-1, i.e. p < 10 GeV • Difficult to achieve with a RICH system (aerogel was the best choice of radiator for this region), so possibility of time-of-flight investigated DTOF (p-K) = 35ps at 10GeVover a distance of ~ 10 m→ aim for 15ps resolution per track • Difficult to achieve with scintillator or other traditional TOF • Cherenkov light production is prompt→ use quartz as source of fast signal • Large-area fast photodetectors under development by the Picosecond timinggroup (http://psec.uchicago.edu/) but unlikely to be available in time for our application (and we would need ~ 30 m2!) The TORCH detector concept

  8. DIRC-like detector • Consider instead a first (naïve) design based on quartz bars, à la DIRC of BaBar:Cherenkov photons produced in the quartz transported to the end of the bar by total internal reflection, where their arrival would be timed • 1cm thickness of quartz is enough to produce ~ 50 detected photons/track (assuming a reasonable quantum efficiency of the photon detector) → ~ 70 ps resolution required per detected photon • However, spread of arrival times is much greater than this, due to different paths taken by photons in the bar 3 m Photon arrival time 25 ns The TORCH detector concept

  9. Planar detector • Need to measure angles of photons, so their path length can be reconstructed: ~ 1 mrad precision required on the angles in both transverse planes • This would be prohibitive for a set of quartz bars, but borrow nice idea from the end-cap DIRC of PANDA [Matthias Hoek]: use a plane of quartz → coarse segmentation (~ 1cm) is sufficient for the transverse direction (qx) ~ 1 cm The TORCH detector concept

  10. Focusing system • To measure the angle in the longitudinal direction (qz) we use a focusing block, to convert angle of the photon into position on the photodetector • Event display illustrated for photons from 3 different tracks hitting plane The TORCH detector concept

  11. Photon detection • Micro-channel plate (MCP) photodetectors are currently the best choicefor fast timing of single photons • Anode pad structure can in principle be adjusted according to need • Test result from K.Inami et al [RICH2010] s(t) = 34.2 ± 0.4 ps ~10 mm pores in the MCP The TORCH detector concept

  12. Modular design • For the application in LHCb, transverse dimension of plane to be instrumented is ~ 5  6 m2 (at z = 10 m) • Unrealistic to cover with a single quartz plate  evolve to modular layout: • 18 identical moduleseach 250  66  1 cm3 ~ 300 litres of quartzin total (less than Babar) • Reflective lower edge  photon detectors only needed on upper edge 18  11 = 198 unitsEach with 1024 pads 200k channels total The TORCH detector concept

  13. Effect of edges • Reflection off the faces of plate is not a problem, as the photon angle in that direction (qz) is measured via the focusing system • In the other coordinate (x) position is measured rather than angle → reflection off the sides of the plate gives ambiguities in the reconstructed photon path • Only keep those solutions that give a physical Cherenkov angle → only ~ 2 ambiguities on average • Effect of the remaining ambiguities is simply to add a ~ flat background to reconstructed time distribution The TORCH detector concept

  14. TORCH module • Focusing block inquartz or plastic (should match refractive index) • Cylindrical mirror • Linear array ofphoton detectors • Dimensions have been chosen to correspond tothe Planacon MCPfrom Photonis • Plate thickness (~ 1 cm)to be optimized once p.e. yield known The TORCH detector concept

  15. Photon detector • Planacon XP85022 comes close to matching photodetector requirements for TORCHCurrently availablewith 32  32 anode pads • We require finer granularity in one direction than other, so assume an 8  128 anode pad layoutIn discussion with manufacturers to secure this development • Lifetime of MCP may also be an issue for our application (depends on gain, and hence electronics)Following recent development of longer-lived MCPs [Hamamatsu] with great interest The TORCH detector concept

  16. Resolution • Smearing of photon propagation time due to photodetector granularity ~ 40ps • Assuming an intrinsic arrival time measurement resolution per p.e. of 50psthe total resolution per detected p.e. is 40  50  70 ps, as required • For particle ID, need to correct for the strong chromatic dispersion of quartzAchieved by measuring the photon angles, and knowing path of track through quartz  determine Cherenkov emission anglecos qC = 1/ bnphase t – t0 = L ngroup /c Effectively the wavelength of the photon is determined by this construction The TORCH detector concept

  17. Performance • Different time-of-propagation for photons from p or K, but this effect adds to difference in time-of-flight  increases the sensitivity • To determine the time-of-flight, we also need a start time t0This is achieved using the other tracks in the event, from the primary vertexMost of them are pions, so the reconstruction logic is reversed, and the start time is determined from their average assuming they are all p(outliers from other particle types are removed) • Full algorithm has been studied,including pattern recognition,using a simple simulation of theTORCH detector, interfacedto the full simulation of LHCb • Excellent particle ID performanceachieved, up to 10 GeV as required The TORCH detector concept

  18. 3. TORCH R&D • R&D has been launched on the following aspects: • PhotodetectorPerformance of existing MCP devices; Development of suitable anode pad structure; Lifetime; Cost • Readout electronicsSpeed; 40 MHz rate; Gain; Noise; Cross-talk • Quartz radiatorPolishing; Required quality for total internal reflection; Cost • Simulation Detailed simulation of TORCH; tagging performance in upgrade • Two 64-channel Planacon MCPs procured from PhotonisCharacterisation with laser light source, starting with single channel electronics, then multichannel readout • First results expected in time for Letter of Intent at the end of this year The TORCH detector concept

  19. Lab setup at CERN Dark box Single channel electronics Laser light source PlanaconMCP The TORCH detector concept

  20. Readout electronics • Under development by Oxford Univ. group • Starting with 8-channel NINO chips and HPTDC, developed for the ALICE TOF • Test-beam studies foreseen for next year Electronics board support for tests The TORCH detector concept

  21. Conclusions Isolated tracks • TORCH is a novel detector concept proposed for the upgrade of LHCbIt is intended to complement the high-momentum particle ID provided by the RICH system • Based on time-of-flight, determined from Cherenkov light produced in quartz plateusing photon detectors at the periphery • Assuming a per-photon resolution of 70 ps excellent K-p separation achieved up to 10 GeV • R&D is in progress, starting with the photodetector and readout electronicsImpact of the TORCH on tagging performance in the upgraded experiment is under study with detailed simulation • Letter of Intent for the LHCb upgrade will be submitted at end of this year The TORCH detector concept

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