1. 13 July 2005 ACFA8 Gamma Finding procedure for Realistic PFA T.Fujikawa(Tohoku Univ.), M-C. Chang(Tohoku Univ.), K.Fujii(KEK), A.Miyamoto(KEK), S.Yamashita(ICEPP), T.Yoshioka(ICEPP) On behalf of ACFA-SIM-J Group

2. 13 July 2005 ACFA8 Motivation To get high energy resolution in multi jet event, we use the tracker information for charged particles and calorimeter hit information for neutral particles. (tracker resolution is much better than calorimeter’s.) Particle Flow Algorithm (PFA) is studied to achieve an event reconstruction. We studied part of PFA, “gamma finding method” for GLD detector, using full simulator (Jupiter).

3. 13 July 2005 ACFA8 1.Small Clustering (collect fired cells with its neighbors) → Define thrust and broadening for each small cluster. 2. Gamma Finding (Separate gamma/e from hadron) → Small Cluster based. 3. Cluster-Track Matching (Separate charged to neutral) → Calorimeter Cell based. Note: → Cheating method is used for charged hadrons in the Endcap region. → Assuming the remaining clusters be neutral hadrons. Flow to make PFObject - Current Scheme of the PFA Clustering flow for GLD detector

4. 13 July 2005 ACFA8 Detector configuration 40 cm 270 cm 210 cm Full One Tower EM + HD 27 X 0 6.1λ Y.Yamaguchi (Tsukuba Univ.) In current Jupiter geometry, calorimeter has 4cmx4cm/12cmx12cm scintillator tile size in ECAL/HCAL as a default setting. We used this configuration in our study.

5. 13 July 2005 ACFA8 Why good gamma finding is necessary? Expected jet-event in the future ILC, it has about 20% gamma contribution. Also, if we can separate gamma (and also electron) well, it make the charged hadron clustering easier. good gamma/charged hadron separation is necessary! (Z->qqbar @ Ecm = 91.2 GeV)

6. 13 July 2005 ACFA8 Flow of the gamma finding As a first step of the clustering, we collect hit cells with its neighbors (with ~50MeV energy deposit upper limit). We call it “small cluster”. Our gamma finding method is performed based on the small clusters. 1.Find “large” small cluster within ECAL region (we call it “mother small cluster”). 2.Apply “Gamma-Fitter” chi-square /ndf cut. (cut 1) 3.Apply “the distance from the nearest track” cut for mother small cluster. (cut 2) 4.Collect small clusters within a tube. The flow of Gamma Finding

7. 13 July 2005 ACFA8 Gamma finding method 1.Gamma Fitter (chi-square cut of fit) Find large small cluster within ECAL region and make a small tube along its thrust axis. Take an average of energy deposit for each 3-layers within a tube and fit the total energy deposit by Gamma distribution function. The result of chi-square/ndf is used to separate gamma/electron from hadrons.

8. 13 July 2005 ACFA8 Energy deposit in three layers are merged to reduce fluctuation. Choose large small cluster (mother small cluster) within ECAL region and make a small tube (R = 5cm) along to its thrust axis. Fit the longitudinal shower profile of the energy deposition within a tube as an electromagnetic cascade (gamma distribution function). Gamma Fitter: algorithm HCAL projection axis take an average energy deposit for each 3-layers mother small cluster ECAL

9. 13 July 2005 ACFA8 Gamma Fitter: single particle test The result of fitting for a single particle case (gamma at 3GeV)

10. 13 July 2005 ACFA8 3 GeV 10 GeV5 GeV 1 GeV Yellow: gamma Red : pion We can separate gamma/electron from pion using chi2/ndf cut. Comparison of chi2/ndf (single particle)

11. 13 July 2005 ACFA8 Blue : efficiency Red : purity Gamma Fitter: efficiency and purity (Z->qqbar @ Ecm = 91.2 GeV) The “Gamma fitter” works good for z->qqbar event, too. But it doesn’t have high efficiency for electron/gamma finding. (maximum ~ 90%)

12. 13 July 2005 ACFA8 Chi2/ndf (Z->qqbar @ Ecm = 91.2 GeV) Yellow: e/gamma Red : all particles cut position We set a cut position at 1.0 for below gamma finding. (~70% efficiency and ~70% purity)

13. 13 July 2005 ACFA8 2. The distance from the nearest track Calculate the distance between mother small cluster and its nearest track. The distance is used to separate gamma from charged particles. Gamma finding method HCAL ECAL gamma track distance mother small cluster

14. 13 July 2005 ACFA8 Distance from the nearest track Blue : electron Red : all particles Yellow: Gamma (Z->qqbar @ Ecm = 91.2 GeV) This plot shows the distribution of the distance from the nearest track for all small clusters within ECAL region. (energy-weighted)

15. 13 July 2005 ACFA8 Distance distribution (Z->qqbar @ Ecm = 91.2 GeV) Blue : electron Red : all particles Yellow: Gamma After Chi2/ndf < 1.0 cut cut position We set a lower limit at 13cm. It kills the electron as well, but can reduce large amount of charged hadron contamination. We gain the benefit anyway.

16. 13 July 2005 ACFA8 The result of Gamma finding 1. efficiency Finally, we got efficiency ~ 60% using “gamma fitter cut” and “the distance cut”.

17. 13 July 2005 ACFA8 The result of Gamma finding 2. purity w/o n,KL contamination Purity ~ 93% Purity ~ 99% contamination is almost caused by n/K L

18. 13 July 2005 ACFA8 The result of energy sum Realistic gamma finderCheated gamma finder

19. 13 July 2005 ACFA8 Other idea (not used now) 1. Energy deposit comparison between ECAL/HCAL region Assume a tube region and compare the total energy deposit between ECAL/HCAL region within a tube. ECAL HCAL E1 E2 separate events using E1/E2

20. 13 July 2005 ACFA8 2. Energy waited Broadening/Thrust ratio of the cluster Calculate thrust and broadening for the cluster, then calculate these ratio. (EM shower has lower value compare to HD shower’s.) Other idea (not used now) ECAL HCAL In general, (B/T) EM > (B/T) HD cluster

21. 13 July 2005 ACFA8 3. Calculate energy deposition ratio within a tube in ECAL Set the tube and “Maximum layer” according to cluster energy, then calculate the energy ratio between upper/lower region of Maximum layer within a tube (region 1 and 2). Other idea (not used now) All of them are not effective “after” gamma fitter cut… ECAL region 1 region 2 region 1 region 2 gammahadron Maximum layer (energy dependent) Tube radius is also energy dependent

22. 13 July 2005 ACFA8 Summary Good gamma finding (clustering) method is necessary for future LC experiment. We got ~60% efficiency and ~99% purity using Gamma Fitter (chi2/ndf) + the nearest track distance requirement. Current gamma finding method has high purity but which doesn’t have enough efficiency.

23. 13 July 2005 ACFA8 Next step Combine other gamma finding method (or study another method other than “gamma fitter”) to get high efficiency/purity. Do the cut value optimization. Study other method which use transverse shower profile. Study the “relationship” with clustering method for charged hadron.

24. 13 July 2005 ACFA8 The result of other gamma finding method with gamma fitter chi-square cut Backup

25. 13 July 2005 ACFA8 HCAL/ECAL e-dep comparison After Chisquare/ndf < 1.0 cut Yellow: e/gamma Red : all particles The plot shows the energy deposit comparison between HCAL/ECAL regions after applying the chi-square cut for Z->qqbar event.

26. 13 July 2005 ACFA8 Broadening/Thrust Yellow: e/gamma Red : all particles After Chisquare/ndf < 1.0 cut Broadening/Thrust ratio for the cluster after applying the chi-square cut for Z->qqbar event.