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

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