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The ATLAS Luminosity System Per Grafstrom CERN

The ATLAS Luminosity System Per Grafstrom CERN. Luminosity Measurement – WHY ?. Cross sections for “Standard “ processes t-tbar production W/Z production ……. Theoretically known to better than 10% ……will improve in the future

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The ATLAS Luminosity System Per Grafstrom CERN

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  1. The ATLAS Luminosity System Per Grafstrom CERN Manchester 13.12.04

  2. Luminosity Measurement – WHY ? • Cross sections for “Standard “ processes • t-tbar production • W/Z production • ……. Theoretically known to better than 10% ……will improve in the future • New physics manifesting in deviation of  x BR relative the Standard Model predictions • Important precision measurements • Higgs production  x BR • tan measurement for MSSM Higgs • ……. Manchester 13.12.04

  3. Relative precision on the measurement of HBR for various channels, as function of mH, at Ldt = 300 fb–1. The dominant uncertainty is from Luminosity: 10% (open symbols), 5% (solid symbols). (ATLAS-TDR-15, May 1999) Luminosity Measurement – WHY ? (cont.) Examples Higgs coupling tan measurement Systematic error dominated by luminosity (ATLAS Physics TDR ) Manchester 13.12.04

  4. Luminosity Measurement Options • Relative luminosity a DEDICATED luminosity monitor is needed See Jim Pinfolds talk on LUCID • Absolute luminosity • Goal: • measure L with ≲ 2-3% accuracy • How: • LHC Machine parameters • Use ZDC in heavy ion runs to understand machine parameters • rates of well-calculable processes:e.g. QED, QCD • optical theorem: forward elastic rate + total inelastic rate: Use Roman Pots • needs ~full |η| coverage-ATLAS coverage limited • Use totmeasured by others (TOTEM) • Combine machine luminosity with optical theorem • luminosity from Coulomb Scattering Use Roman Pots ATLAS pursuing all options Manchester 13.12.04

  5. L from LHC Machine Parameters • Luminosity depends exclusively on beam parameters: • Luminosity accuracy limited by: • extrapolation of x, y (or , x*, y*) from measurements of beam profiles elsewhere to IP; knowledge of optics, … • Precision in the measurement of the the bunch current • beam-beam effects at IP, effect of crossing angle at IP, … Manchester 13.12.04

  6. Use ZDC in heavy ion runs to calibrate machine instrumentation (Sebastian White) Manchester 13.12.04

  7. IP1&IP5 absorbers TAN@140m Exploded view Luminosity Monitoring Manchester 13.12.04

  8. Luminosity from other Physics Signals • QED: pp  (p+*)+(p+*)p+()+p • signal: (μμ)-pair with |η(μ)|<2.5, pT(μ)≳ 5-6 GeV, pT(μμ) ≃ 0 • small rate ~1pb (~0.01 Hz at L=1034) • clean: backgrounds from DY, b, c- decays can be handled by appropriate offline cuts • uncertainties: μ trigger acceptance & efficiency, … • ( A.Shamov & V.Telnov, hep-ex/0207095) • QCD: W/Z  leptons • high rate: Wℓν : ~60 Hz at L=1034 (ε = 20%) • current “theory” systematics: PDF and parton cross sections  4% • gives relevant parton luminosity directly… • detection systematics: • trigger/acceptance/identification efficiency/ backgrounds • detailed study for ATLAS detector needed • Both processes will be used Manchester 13.12.04

  9. Manchester 13.12.04

  10. Roman Pots Manchester 13.12.04

  11. 10 8 9 5 0 1 2 4 3 6 7 The ATLAS Detector Calorimetry: Tracking: R -chambers Barrel Diffraction/Proton Tagging Region EndCap RP Tracking ZDC/TAN FCAL TAS LUCID y Manchester 13.12.04

  12. Manchester 13.12.04

  13. Extension of ATLAS- A two stage process • Short time scale Forward detector for Luminosity measurement Elastic scattering in the Coulomb region • Longer time scale Gain experience in working close to the beam  propose a diffractive physics program using additional detectors Manchester 13.12.04

  14. Elastic scattering in the Coulomb region • From the fit we will get • tot ,ρ, b and L Manchester 13.12.04

  15.  =2.2 (best fit) ) totvs sand fit to (lns)  =1.0 The total cross section Manchester 13.12.04

  16. The ρ parameter • ρ = Re F(0)/Im F(0) linked to the total cross section via dispersion relations • ρ is sensitive to the total cross section beyond the energy at which ρ is measured  predictions of tot beyond LHC energies is possible • Inversely :Are dispersion relations still valid at LHC energies? Manchester 13.12.04

  17. The b-parameter for lt l< .1 GeV2 “Old” language : shrinkage of the forward peak b(s)  2 ’ log s ; ’ the slope of the Pomeron trajectory ; ’  0.25 GeV2 Not simple exponential - t-dependence of local slope Structure of small oscillations? The b-parameter or the forward peak Manchester 13.12.04

  18. What else can we do? Coulomb region extremely challenging. All aspects of design optimized for this. • Medium (0.1-1.0 GeV**2) elastic scattering needs medium beta* optics, low Lumi, short runs) • Large t (1-10 GeV**2) elastic scattering needs high Lumi, standard optics, and continuous runs. Studies needed • Proton tagging to identify a diffractive interaction must be possible at some level . BUT t and  acceptance and t and  resolution need to be understood. Simulation and optics investigations needed to see if there is any physics reach for single and central diffraction using proton tagging. Signal and background rates have to be studied. Trigger set up? Many open questions Manchester 13.12.04

  19. Requirements to reach the Coulomb region • Required reach in t : • Requires: • small intrinsic beam angular spread at IP • insensitive to transverse vertex smearing • large effective lever arm Leff • detectors close to the beam, at large distance from IP Parallel–to–point focusing Manchester 13.12.04

  20. parallel-to-point focusing ydet y* y* IP Leff Experimental Technique • Independence of vertex position: • Limit on minimum |t|min: The main potential difficulties are all derived from the above • Leff,y large detectors must be far away form the IP  potential interference with machine hardware • small tmin • * largespecial optics • small emittance • small nσ halo under control and the detector must be close to the beam Manchester 13.12.04

  21. Roman Pot Locations One Roman Pot Station per side on left and right from IP1 Each RP station consists of two Roman Pot Units separated by 3.4 m, centered at 240.0 m from IP1 Manchester 13.12.04

  22. β [m] D [m] Very high b* (2625 m) optics • Solution with following characteristics • At the IP  = 2625 m * = 610 m , * = 0.23 rad • At the detector y,d = 119 m, y,d = 126 m x,d = 88 m, x,d = 109 m (for N =1 m rad) • Detector at 1.5 mm or 12 tmin =0.0004 GeV2 • Smooth path to injection optics exists • All Quads are within limits • Q4 is inverted w.r.t. standard optics! Endorsed by LTC Compatible with TOTEM optics see LEMIC minutes 9/12/2003 Manchester 13.12.04

  23. Emittance • Emittance of ~1×10–6 mrad needed to reach Coulomb region • Nominal LHC emittance: 3.75×10–6 mrad • Emittances achieved during MD’s in SPS: • Vertical plane 1.1×10–6 mrad and Horizontal plane 0.9×10–6 mrad for 7x1010 protons per bunch • 0.6-0.7×10–6 mrad obtained for bunch intensities of 0.5×1010 protons per bunch • However • Preserve emittance into LHC means that injection errors must be controlled (synchrotron radiation damping might help us at LHC energy) • emittance εN, number of protons/bunch Np, and collimator opening nσ,coll(in units of σ) are related via a resistive (collimator) wall instability limit criterion: thus: εN ≥ 1.5×10–6 m for Np = 1010, nσ,coll= 6  Best parameter space from beam tuning sessions Manchester 13.12.04

  24. Beam halo is a serious concern for Roman Pot operation it determines the distance of closest approach dmin of (sensitive part of) detector: nσ = dmin/σbeam:9 ≤ nσ≤ 15 Expected halo rate(43 bunches, Np=1010, εN = 1.0 μm rad, nσ=10): 6 kHz (RA LHC MAC 13/3/03) nσ-reach? Beam Halo: limit on nσ Manchester 13.12.04

  25. Requirements for Roman Pot Detectors • “Dead space”d0 at detector’s edge near the beam : d0≲ 100m (full/flat efficiency away from edge) • Detector resolution: d = 30 m • Same d = 10 m relative position accuracy between opposite detectors (e.g. partially overlapping detectors, …) • Radiation hardness: 100 Gy/yr • Operate with the induced EM pulse from circulating bunches (shielding, …) • Rate capability: O(MHz) (40 MHz); time resolution t = O(1 ns) • Readout and trigger compatible with the experiment DAQ • Other: • simplicity, cost • extent of R&D needed, time scale, manpower, … • issues of LHC safety and controls Manchester 13.12.04

  26. Roman Pot Detectors • Square scintillating fibers • Kuraray 0.5 mm× 0.5 mm fibers • 10 layers per coordinate • 50 μm offset between layers • Main reason for choice • Small dead space • EM parasites not a problem scintillator plate for triggering y-measurement detector x-measurement detector halo inter calibration planes (well away from beam) Manchester 13.12.04

  27. Scint. fiber detector expect Npe/hit ~ 4.9 empirically: ~3 is more likely Manchester 13.12.04

  28. Detector Performance Simulations First simulation results • strip positioning σfiber ≈ 20 μm • light and photo-electron yield: Npe = <dE/dx> dfiber (dnγ/dE) εAεTεCgRεQεd Manchester 13.12.04

  29. Manchester 13.12.04

  30. Fiber routing Radius of curvature min 30mm Average fiber length ~230mm Nb of fibers in X config.: 71 x 10 x 2 = 1420 Total fiber length: ~330m Square holes filled with glue (to be tested in prototype) Manchester 13.12.04

  31. Fiber housing installation Manchester 13.12.04

  32. Integration Free space PMs – baseplate: 106 + 40 – 137 = 9mm  Separation at PMs level or hole in base plate required Manchester 13.12.04

  33. Manchester 13.12.04

  34. Simulated Elastic Scattering • Reconstruct θ*: Inner ring: t = -0.0007 GeV2 Outer ring: t = -0.0010 GeV2 Manchester 13.12.04

  35. Simulated dNel/dt and simple fit Event generation: • 5 M events generated corresponding to ~90 hr at L 1027 cm-2 s-1 • NO systematics on beam optics! • Only 1 Roman Put unit/arm Simple fit • range for fitting: • 0.00056 < |t| < 0.030 GeV2 • ~4 M events “measured” for dN/dt Manchester 13.12.04

  36. Conclusion • ATLAS pursues a number of options for Absolute Luminosity Measurement • Coulomb normalization • W/Z rates • production of muon pairs via double photon exchange • elastic slope extrapolated to dN/dt|t=0 plus machine L • elastic slope extrapolated to dN/dt|t=0 plus tot from TOTEM • machine parameters alone • Cross calibration from ZDC in Ion runs • others… • The Coulomb Interference measurement is very challenging but seems within reach. • Small angle elastic scattering will address “old fashion” physics in terms of tot , ρ and b • This experience of working close to the beam will prepare us for a Forward Physics Program with ATLAS in a possible future upgrade Manchester 13.12.04

  37. Back up slides Manchester 13.12.04

  38. Summary on emittance and beam halo issues “Looks feasible but no guarantees can be given” However, if we don’t reach the Coulomb region the effort is not in vain we can still: • Use tot as measured by TOTEM/CMS and get the luminosity by measuring elastic scattering in a moderate t-range( -t=0.01 GeV2 ) and use the Optical theorem for the rest • Use the luminosity measured by machine parameters and again via the Optical theorem get tot and all other cross sections relative to tot with a factor 2 better precision than from the machine parameters Manchester 13.12.04

  39. Luminosity transfer 1027-1034 cm-2 sec-1 • Bunch to bunch resolution  we can consider luminosity / bunch  ~ 2 x10-4 interactions per bunch to 20 interactions/bunch  • Required dynamic range of the detector ~ 20 • Required background  < 2 x10-4 interactions per bunch • main background from beam-gas interactions • Dynamic vacuum difficult to estimate but at low luminosity we will be close to the static vacuum. • Assume static vacuum  beam gas ~ 10-7 interactions /bunch/m • We are in the process to perform MC calculation to see how much of this will affect LUCID Manchester 13.12.04

  40. Alternative photodetectors • Geiger Mode Avalanche Photodiodes + high gain, ~ 106- low QE ~ 15% (geometrically limited) + low bias voltage ~ 50 V - high dark count rate at R.T. ~ 106 Hz + very fast signal characteristics - not yet really commercialized + very simple electronics + small size GM-APD would become interesting if geometrical QE limitation can be overcome. QE > 30% is claimed to be in reach. This would mean Npe ~ 6-8 for our baseline configuration. Manchester 13.12.04

  41. Participating institutes: (as a subsystem, fully part of the ATLAS collaboration) University of Alberta CERN Ecole Polytechnique Institute of Physics Academy of Science, Czech Republic University of Manchester University of Montreal University of Texas University of Valencia SUNY Stony Brook Cost Estimates & Participants Manchester 13.12.04

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