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Machine induced background in ALFA The ALFA detector elastic scattering and luminosity

Machine induced background in ALFA The ALFA detector elastic scattering and luminosity background generation, rejection and subtraction impact on luminosity determination Conclusion & open issues. Hasko Stenzel Background WG meeting. Forward Roman Pots for ATLAS. ATLAS. 240 m. ALFA.

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Machine induced background in ALFA The ALFA detector elastic scattering and luminosity

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  1. Machine induced background in ALFA • The ALFA detector • elastic scattering and luminosity • background generation, rejection and subtraction • impact on luminosity determination • Conclusion & open issues Hasko Stenzel Background WG meeting

  2. Forward Roman Pots for ATLAS ATLAS 240 m ALFA H.Stenzel, 16.03.07

  3. The ALFA detector RP RP RP RP 240m 240m IP RP RP RP RP MAPMTs FE electronics & shield PMT baseplate optical connectors scintillating fibre detectors glued on ceramic supports 10 U/V planes overlap&trigger Roman Pot Unit Roman Pot H.Stenzel, 16.03.07

  4. elastic scattering H.Stenzel, 16.03.07

  5. special optics: high ß* parallel-to-point focusing ydet y* y* IP Leff • Transversal displacement of • particles in the ring away from • the IP: • Special optics with high * and parallel-to-point focusing: • independent of the vertex position properties at the roman pot (240m) H.Stenzel, 16.03.07

  6. Simulation set-up elastic generator PYTHIA6.4 with coulomb- and ρ-term SD+DD non-elastic background, no DPE beam properties at IP1 size of the beam spot σx,y beam divergence σ’x,y momentum dispersion ALFA simulation track reconstruction t-spectrum luminosity determination later: GEANT4 simulation beam transport MadX tracking IP1RP high β* optics V6.5 including apertures H.Stenzel, 16.03.07

  7. Simulation of elastic scattering hit pattern for 10 M elastic events simulated with PYTHIA + MADX for the beam transport t reconstruction: • special optics • parallel-to-point focusing • high β* H.Stenzel, 16.03.07

  8. luminosity determination Simulating 10 M events, running 100 hrs fit range 0.00055-0.055 H.Stenzel, 16.03.07

  9. Performance estimation: systematic uncertainties Recent work obtained for the ALFA TDR (in review) Background contribution H.Stenzel, 16.03.07

  10. background considerations • physics background: single diffraction • can be rejected by means of vertex and acollinearity cuts • is reduced to a negligible level • machine background • beam halo originating from cleaning inefficiencies and distant quasi-elastic beam gas interactions, calculations were provided by Igor Bayshev, IHEP • local inelastic beam-gas interactions (showers), calculations were provided by Igor Azhgirey, IHEP H.Stenzel, 16.03.07

  11. beam halo Calculations are carried out for the high β*-optics with εN =1μrad m and at L=1027cm-2s-1 • beam halo from collimation inefficiencies • betatron cleaning • momentum cleaning • halo beam-gas interactions • elastic and quasi-elastic p-N interactions H.Stenzel, 16.03.07

  12. beam halo background • distributions of halo impacts in the transversal plane at the detector • normalized per proton hitting a collimator/interacting with beam gas • This can be turned into single and accidental coincidence rates by • main question: what is the lifetime contribution for beam gas? • 100 hrs for MC & BC • 1000 hrs for beam gas single rates • accidental coincidence rate inside detector acceptance of about 9 Hz (elastic: 27 Hz) • potentially dangerous since all at small t H.Stenzel, 16.03.07

  13. beam halo rejection cuts Exploit back-to-back signature of elastic events and vertex reconstruction after vertex and acollinearity cuts still 140 k events survive! (compared to 6.6 M elastic signal) irreducible background at small t in the luminosity region! must be subtracted H.Stenzel, 16.03.07

  14. background calculation RP RP RP RP 240m 240m IP RP RP RP RP signal & background in asymmetric configuration pure background • signal and irreducible background appear in asymmetric configurations: +/- and -/+ • pure background is also present in symmetric configurations +/+ and -/- • from this the irreducible background can be calculated by inverting randomly (left/right) the vertical sign of the hits • halo asymmetries can be corrected for using data • free of MC, good systematics H.Stenzel, 16.03.07

  15. systematic uncertainty of background • In principle the method is free of syst. uncertainties, since all is determined from the data itself • However, the calculated background sample is subject to statistical fluctuations, i.e. the subtraction not exact. • this effect is estimated by generating a large number of background sample with equal statistics and applying the subtraction procedure. In the end the RMS of the fitted luminosity results is quoted as syst. error. • Result: ΔL/L = 1.1-1.5 % • Total systematic error: 2.2-2.6 % • Total error : 2.8-3.2 % H.Stenzel, 16.03.07

  16. local inelastic beam-gas background The comparison of the rate of distant and local beam-gas background shows that the latter contribution can be neglected. H.Stenzel, 16.03.07

  17. conclusion • ATLAS proposes to determine the absolute luminosity using elastic scattering in the Coulomb-Nuclear interference region measured with the ALFA subdetector • The success of this measurement depend crucially on the beam conditions • The background calculations provided by IHEP Protvino constitute an essential element in the performance estimation • A precision of about 3% for the luminosity is within reach • Other methods for the luminosity determination (W/Z counting, optical theorem, ..) are in parallel pursued • Open issues : beam-gas background for LUCID ... H.Stenzel, 16.03.07

  18. from Vincent Hedberg H.Stenzel, 16.03.07

  19. open issue: beam-gas background for LUCID • The beam-gas background entering LUCID from the back has been estimated to be at a small level • The beam gas entering LUCID from the front is presumably rather small (length ratio) but could be dangerous, since it is pointing to LUCID • Can we get a background calculation for this contribution at a scoring plane of the LUCID front face (~17m)? H.Stenzel, 16.03.07

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