Status Report particle identification with the RICH detector

1. Status Report particle identification with the RICH detector • general overview • focus on ring radius/ Cherenkov angle resolution • Boris Polichtchouk: results from simulation Claudia Höhne - GSI Darmstadt, Germany

2. particle identification with RICH • ring finding • ring finder: Hough Transform, Elastic Net • to be implemented in framework • → efficiencies ... • determination of center and radius of ring/ Cherenkov angle • matching of rings with tracks • → tracking (momentum and position resolution), information from other detectors • pid by combining ring radius and momentum information •  detailed knowledge of resolution necessary!

3. main contribution ! ring radius resolution • Cherenkov angle/ ring radius resolution limited by: • multiple scattering • magnetic stray field in RICH • emission point: particle trajectories do not pass through the center of curvature of the mirror  smeared projection in dependence on (,) • mirror surface: enlargement of focal spot in focal plane due to a deviation of the mirror surface from the ideal spherical curvature • pixel size: resolution limited due to finite granularity of photodetector • chromatic dispersion N2 radiator investigate single rings! resolution for overlapping rings different!

4. 2% of qcmax ssingle = 2.5mrad single photon – ring resolution • distinguish between Cherenkov angle resolution for single photons sq • and the resolution for a ring sR consisting of N measured photons • table 1 from RICH-TDR of LHCb: • aim at doing a similar study

5. [T. Ypsilantis, J.Seguinot, NIM A343 (1994) 30, P. Glaessel, NIM A 433 (1999) 17] multiple scattering momentum dependent error of form with typical value for a N2 radiator of 1m and p=1GeV: → for 2.5 m N2

6. [T. Ypsilantis, J.Seguinot, NIM A343 (1994) 30, P. Glaessel, NIM A 433 (1999) 17] magnetic stray field momentum dependent error of form with being the particle angle relative to the magnetic field direction →

7. asymmetric field gaussian shaped field magnetic stray field (II) RICH front wall z=170cm → magnetic stray field (By) of order 10mT length L=2.5m at maximum By [T] By [T] z [cm] z [cm]

8. L2.5m, B=10mT → → p=1GeV [T. Ypsilantis, J.Seguinot, NIM A343 (1994) 30, P. Glaessel, NIM A 433 (1999) 17] magnetic stray field (III) momentum dependent error of form with being the particle angle relative to the magnetic field direction →

9. one quarter of mirror/ photodetector: f = 80o 60o 40o q = 5o 10o 15o 20o 25o 30o 35o 20o emission point • rings(q,f) - q polar angle, • f azimuth angle • no diffusion at reflection • no magnetic field, no multiple scattering • to do: • quantify and correct for distortions at large q,f • improve focussing/ position of focal plane • correct for remaining distortions → restrict investigation of resolution to "good" area in central region and wait for optimized setup

10. mirror surface • Be-mirror prototype: • optical surface roughness sh = 1.6nm (after glass polishing, Al covering and SiO2 coating) • → diffuse reflection of only 12% of total for l = 150nm • image diameter of a point source D0 = 0.4mm (contains 95% of reflected light) • → angular deviation from nominal curvature sR = 0.03mrad resulting radius resolution to be determined Ph.D. thesis of G. Hering (2002), CERES: ring center resolution of same order as local mirror deviation → sq < 0.1mrad

11. pixel size • finite granularity of photodetector restricts resolution • simple estimate can be made from comparing padsize d and ring radius R / Cherenkov angle q d=0.6cm padsize R=5.5cm ring radius L=225cm radiator length → Boris Polichtchouk

12. however, dN/dl also increases in UV region and • 4mrad • sq~2mrad (~0.4cm) [Landolt Boernstein Series, 6th Edition, volume II/8 Ph.D. thesis of Annick Bideau-Mehu (1982)] chromatic dispersion • strong increase of n(l) in UV region N2: l [nm] q [mrad] 600 24.42 200 26.15 150 28 100 36.75 N2 → Boris Polichtchouk

13. total resolution (I) • multiple scattering sq ~ 1 mrad (p=1 GeV) • magnetic stray field sq < 1 mrad (p=1 GeV) • emission point sq small because of corrections, optimization • angular deviation of mirror sq < 0.1 mrad • chromatic dispersion sq > 1 mrad (strongly dependent on lmin) • pixel size sq ~ 1-2 mrad •  couple of mrad contributions, independent errors qc=24.4 mrad  sq ~2-3% of qc

14. 1% 2% 3% 4% 5% total resolution (II) gaussian distributed Cherenkov angles/ radii → calculate separation power for e and p in terms of sq for different sq

15. R ring – track matching • matching: combine track and ring with closest distance • ~ 2/3 of all rings from secondary interactions, very often not reconstructed • difficulty due to high particle mutiplicities  match ring to track (e.g. ) which is nearby additional source for p-misidentification

16. pid versus R • ideal tracking: with of R –distribution due to method for ring center determination • finite tracking: distribution widened • cut on R important for efficiency and purity! 1% momentum resolution 1.3 mrad resolution in azimuth angle 0.8 mrad resolution in deep angle 200 m position resolution in mirror

17. efficiency, misidentification - misidentification in dependence on a cut in R 35 AGeV: 827  / event   0.3/827 = 410-4 -misid. (R = 0.8 cm)  = 610-4 -misid. (60% acc.) efficiency of e-identification in dependence on a cut on R R = 0.8cm: > 95% ideal tracking finite resolution ideal tracking finite resolution

18. summary/ outlook • particle identification with the RICH detector • aim: momentum dependent pid efficiency and purity • efficiency: ring finders to come • purity: started with detailed analysis of ring radius resolution •  for s=3% of qc we have 3s separation between e and p at 13.5 GeV/c • impact on detector layout: granularity of photodetector maximum wavelength range for photodetection • purity: extend tracking algorithms for extrapolation of tracks to photodetector plane • combine with information from other detectors