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

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

4. ALFA background H.Stenzel, 16.03.07 4 elastic scattering

5. ALFA background H.Stenzel, 16.03.07 5 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) y*y* y*y* parallel-to-point focusing y det IP L eff special optics: high ß*

6. ALFA background 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 σ x,y beam divergence σ ’ x,y momentum dispersion beam transport MadX tracking IP1  RP high β * optics V6.5 including apertures ALFA simulation track reconstruction t-spectrum luminosity determination later: GEANT4 simulation

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

8. ALFA background H.Stenzel, 16.03.07 8 luminosity determination inputfit Stat. error L8.10 10 26 8.151 10 26 1.77 % σ tot 101.5 mb101.14 mb0.9% B18 Gev -2 17.93 Gev -2 0.3% ρ0.150.1434.3% Simulating 10 M events, running 100 hrs fit range 0.00055-0.055

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

10. ALFA background 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 ●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

11. ALFA background H.Stenzel, 16.03.07 11 beam halo Calculations are carried out for the high β*-optics with ε N =1μrad m and at L=10 27 cm -2 s -1 ●beam halo from collimation inefficiencies betatron cleaning momentum cleaning ●halo beam-gas interactions elastic and quasi-elastic p-N interactions Calculations are carried out for the high β*-optics with ε N =1μrad m and at L=10 27 cm -2 s -1 ●beam halo from collimation inefficiencies betatron cleaning momentum cleaning ●halo beam-gas interactions elastic and quasi-elastic p-N interactions

12. ALFA background 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 ●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 ●accidental coincidence rate inside detector acceptance of about 9 Hz (elastic: 27 Hz) ●potentially dangerous since all at small t ●accidental coincidence rate inside detector acceptance of about 9 Hz (elastic: 27 Hz) ●potentially dangerous since all at small t single rates

13. ALFA background 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

14. ALFA background H.Stenzel, 16.03.07 14 background calculation RP IP signal & background in asymmetric configuration 240m RP 240m 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 ●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

15. ALFA background 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 % ●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 %

16. ALFA background 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. The comparison of the rate of distant and local beam-gas background shows that the latter contribution can be neglected.

17. ALFA background 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... ●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...

18. ALFA background H.Stenzel, 16.03.07 18 from Vincent Hedberg

19. ALFA background 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)? ●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)?