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Elastic scattering, total cross section and luminosity measurements with ATLAS

Elastic scattering, total cross section and luminosity measurements with ATLAS. C. Sbarra INFN and University of Bologna On behalf of the ATLAS Luminosity and Forward Physics Working Group. Introduction Atlas strategy Experimental techniques Current status.

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Elastic scattering, total cross section and luminosity measurements with ATLAS

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  1. Elastic scattering, total cross section and luminosity measurements with ATLAS C. Sbarra INFN and University of Bologna On behalf of the ATLAS Luminosity and Forward Physics Working Group • Introduction • Atlas strategy • Experimental techniques • Current status DESY, Hamburg, 21-25 May 2007

  2. Elastic Scattering = 14 TeV prediction of BSW model momentum transfer -t ~ (pq)2 q = beam scattering angle p = beam momentum ds/dt (mb/GeV2) L,stot,b,and rfrom FIT in CNI region (UA4) CNI region: |fC| ~ |fN|  @ LHC: -t~6.5 10-4 GeV2; qmin~3.4mrad (qmin~120 mrad @ SPS) DESY, Hamburg, 21-25 May 2007 C. Sbarra

  3. Total Cross-Section Luminosity-independent measurement via optical-theorem  simultaneous evaluation of forward elastic and inelastic rate (TOTEM) • elastic rate down to |t|=10-3 GeV2 to keep extrapolation error small (1-2%) • Sufficient h coverage to access Nel+Ninel stot (LHC) ~ 110 mb (g=2; best-fit) stot(LHC) ~ 95 mb (g=1) Inversely: (stot + dN/dt|t=0) (DL/L > ~ 2 Dstot/stot) (L + dN/dt|t=0) (Dstot/stot > ~ ½ DL/L) DESY, Hamburg, 21-25 May 2007 C. Sbarra

  4. The LHC Luminosity Nxi = number of protons in bunch i of beam x; f=revolution frequency; sx,sy=transverse beam dimensions at the IP; Kb = number of bunches; b*=bfunction at IP; eN=s*xs*yg/b* normalized emittance; g=E/mp (~7460) Accuracy limited by • Precision in measurement of bunch currents • Extrapolation of sxsyfrom measureament point to IP • Beam-beam effects at IP, beam crossing angle, ... Typical accuracy from machine 5-10% DESY, Hamburg, 21-25 May 2007 C. Sbarra

  5. Physics Interest in L Relates the cross section s of a given process to its event rate N=sL overall normalization of physics analysis; monitor of LHC performances Higgs coupling tanb measurement DL/L=10% DL/L=10% DL/L=5% Systematic error dominated by luminosity (ATLAS TDR ) DESY, Hamburg, 21-25 May 2007 C. Sbarra

  6. ATLAS Strategy Goal precision on L ~ 2-3% Elastic scattering in CNI region to get L, b, randstotatL ~1027 cm-2s-1(optical theorem as a back-up solution) (ALPHA detector in Roman Pots) Luminosity monitor calibrated at low lumi but working up to L ~1034cm-2s-1(LUCID) + • Absolute L from QED (pp ppmm) and QCD (Wln, Zll) processes (need to control PDF) • Improve Luminosity from machine with ZDC • Further luminosity/beam monitoring with BCM, MBTS... DESY, Hamburg, 21-25 May 2007 C. Sbarra

  7. RP RP RP RP 240m 240m IP RP RP RP RP parallel-to-point focusing ydet y* y* IP Leff CNI region challenge Intrinsic Beam divergence during physics run ~ 30 mrad Details in dedicated talk Experimental technique: • Largeb* optics beam divergence ~ 0.2 mrad, low luminosity • independence of vertex position: parallel to point focusing • ALFA detector(scintillating fibers) at 10-15sbeam in Roman Pots @ 240m (after 2009) b* = 2625 m L~1027 cm-2s-1 DESY, Hamburg, 21-25 May 2007 C. Sbarra

  8. ALPHA performance (H. Stenzel ATL-LUM-PUB-2007-001) Fit to simulated dN/dt data corresponding to ~1 week (10M events) of running at L = 1027 cm-2s-1 Systematics on L • beam divergence and optics • detector acceptance, resolution & alignment • background from halo (beam-gas, off-momentum, betatron oscillations) • Background from non-elastic interactions D L/L ~ 3% - Available after 2009 DESY, Hamburg, 21-25 May 2007 C. Sbarra

  9. A detector able to count the number m of inelastic interactions per BX by measuring <M> = mean number of charged particles per BX. Luminosity Monitor Calibrated at low luminosity where the average number of particle per detected interaction <N> is measured (small probability of more than 1 interaction per BX) If e is the efficiency to detect one interaction: Calculated e and measured stot only used for consistency cross checks “Luminosity independent” calibration constant A (determined by simultaneous absolute L measurement  same precision as L ) Needed dinamic range in m (bunch by bunch L) @ LHC: 2.5 10-6 - 25 (S. Ask – ATL-LUM-PUB-2006-001) DESY, Hamburg, 21-25 May 2007 C. Sbarra

  10. LUCID LUminosity monitor using Cerenkov Integrating Detector Array of polished aluminum tubes in C4F10 Cherenkov radiator (P=1bar) light emitted at3° and read-out after ~ 3 reflections directely (or via optical fiber) by PMT • Cherenkov threshold (10 MeV for e, 2.8 GeV for p) to limit background • Pointing geometry (limit back.) • No landau fluctuations (counting particles) • Good time resolution (2-3 ns)  bunch by bunch & on-line luminosity • Light, rad-hard Installation in summer 2007 DESY, Hamburg, 21-25 May 2007 C. Sbarra

  11. LUCID location Beam pipe Phase I Pseudorapidity coverage 5.4<||<6.1 (5.6<||<6.0) front of tubes at ~ 17 m from IP • 6-7 MRad/y at L=1034 cm-2s-1 • .5-.7 MRad/y at low luminosity Radiation test with gammas  no problems with PMT up to 20 MRad DESY, Hamburg, 21-25 May 2007 C. Sbarra

  12. LUCID Phases Phase 1-low lumi L < ~ 1033 cm-2 s-1(up to 2009) Approved Feb. 2007 Calibration initially from LHC (10%), then W/Zln/ll+QED (5-10%) Phase 2–high lumi L ~ 1034 cm-2 s-1(after 2009) Calibration with ALPHA goal precision on L 2-3% Outer layer R=114.7 mm; Inner layer R=96.3 mm Tube diameter=15mm m~23 m<~7 16 tubes per side directely read-outby PMT; 4 tubes per side with fibers 168 tubes & winston cones per side read-out via optical fibers to MaPMT DESY, Hamburg, 21-25 May 2007 C. Sbarra

  13. Phase I Measurement Methods: Main method For phase 1 • Collision (zero) Counting - fine in in phase I • Hit Counting - no saturation in phase I • Particle Counting - linear by construction - sensitive to gain fuctuations (Side coincidence / single side) MC expectation Systematics (under study) • Opticsdifferences between calibration and run <~ 1% CLC/CDF experience: DL/L~ 2% + 4% + 4% ~ 6% acceptance sinel DESY, Hamburg, 21-25 May 2007 C. Sbarra

  14. Zero Degree Calorimeter housed in the shielding unit that protects the S.C magnets from radiation (TAN) LOI presented in January 2007 (CERN-LHC-2007-001) Tungsten-quartz fiber calorimeters at ~140 m from the IP Primary goal: to measure spectator neutrons in heavy ion interactions (centrality) Installation of hadronic module in fall 2007 DESY, Hamburg, 21-25 May 2007 C. Sbarra

  15. ZDC as a beam monitor ZDC at RHIC as an accelerator tool (in pp) • ZDC (lower curve) bkg free over 4 orders of magnitude • Van derMeer scan (ZDC coincidence rate vs. relative beam position ) • also measures beam displacement (red points) • Useful for crossing angle commissioning DESY, Hamburg, 21-25 May 2007 C. Sbarra

  16. Summary & Conclusions Three detectors in forward region • LUCID at 17m(summer 2007) dedicated luminosity monitor • ZDC at 140m(fall 2007) LHC parameter calibration/beam monitor • ALFA in Roman Pot at 240m(after 2009) absolute L, stot, r, b LUCID Calibration • LHC luminosity at the start-up~10% • Rate of known QED/QCD processes at mid-term~5% if PDF under control) • ALFA detector after 2009~2-3% Eager to get first beam DESY, Hamburg, 21-25 May 2007 C. Sbarra

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