AA->HH Study AA->4b analysis & (preliminary) Summary Shin-ichi Kawada (AdSM, Hiroshima University) 22nd General KEK (2011/9/3)1

Outline Introduction Previous results Today’s topic – AA->4b BG analysis – analysis of BG suppression (AA->WW, ZZ, 4b) Summary & Next plan 22nd General KEK (2011/9/3)2

Introduction Photon Linear Collider (PLC) has been proposed as one of the options of ILC. It is expected that we can observe the Higgs self-coupling in PLC. Feasibility study of Higgs pair creation in PLC is needed. 22nd General KEK (2011/9/3)3

About PLC 22nd General KEK (2011/9/3)4 Electrons Backward Compton scattering ILC : e+ e- collision PLC : gamma-gamma collision gamma-gamma collision

Reaction in gamma-gamma collision Signal : AA->HH->4b Many backgrounds – AA->WW, ZZ, 4b, etc… Analysis of background reduction is necessary. – We assumed that Higgs mass equals to 120GeV in this study. 22nd General KEK (2011/9/3)5 4b = b b-bar b b-bar

Previous result (1) Main backgrounds – AA->WW (~11.6pb, 1.5*10 7 events/year) – AA->ZZ (~9.42fb, 1.2*10 4 events/year) Analysis with Neural Network – We can observe Higgs pair creation if we can develop good b-tagging method and jet clustering. 22nd General KEK (2011/9/3)6 BGusualcheat AA->WW AA->ZZ

Previous result (2) Other background --- AA->4b – cross-section = 46.6fb (with luminosity & (2-jet invariant mass) < 15 GeV were cut) – ~ 6*10 4 events/year – need the detector simulation & analysis 22nd General KEK (2011/9/3)7 γ b b-bar optimistic event γ γ b b b-bar pessimistic event : color string γ b b-bar

Topic (1) : AA->4b BG analysis 22nd General KEK (2011/9/3)8 at the 21st General Meeting 4b(opt) : 1.81σ 4b(pes) : 1.47σ not bad, but not enough need more effective cuts/NN analysis use other useful other parameters cosθ of jet (cosθ of 2-jet) event shape (thrust, sphericity, Y-value, etc...) jet pairing jet pair of least χ H 2 -> χ H 2, cosθ of 2-jet jet pair of least χ A 2 -> χ A 2, cosθ of 2-jet

Analysis procedure (1) MC samples – remained events after W filter & Z filter 22nd General KEK (2011/9/3)9 real clustering signal4b(opt)4b(pes) generate pre-selection W filter Z filter cheat clustering signal (cheat) 4b(opt)4b(pes) generate pre-selection W filter (cheat) Z filter (cheat) input these events to NN

Analysis procedure (2) 17 Input parameters to NN – no relation to χ 2 (11 parameters) visible energy, # of tracks, ycut value of jet clustering, thrust, sphericity, Y-value, cosθ of jet (4), max|cosθ of jet| – jet pairing of least χ H 2 (3 parameters) χ H 2, cosθ of 2-jet (2) – jet pairing of least χ A 2 (3 parameters) χ A 2, cosθ of 2-jet (2) 22nd General KEK (2011/9/3)10

Result 22nd General KEK (2011/9/3)11 real clusteringsignal4b(opt) remained events significance (evaluated signal & 4b BG only) 1.92σ real clusteringsignal4b(pes) remained events σ cheat clusteringsignal (cheat) 4b(opt) remained events σ cheat clusteringsignal (cheat) 4b(pes) remained events σ Good suppression!!

Discussion Significance (signal v.s. single BG) with cheat jet clustering is good! – signal (cheat) v.s. WW (cheat) : 5.85σ – signal (cheat) v.s. ZZ (cheat) : 5.85σ – signal (cheat) v.s. 4b (optimistic) : 5.59σ – signal (cheat) v.s. 4b (pessimistic) : 5.49σ try to apply the NN analyses to all events! 22nd General KEK (2011/9/3)12

Topic (2) : Analysis of BG suppression Analysis procedure 1.pre-selection 2.W filter (1st NN selection) 3.4b filter (2nd NN selection) 4.Z filter (3rd NN selection) Cross-section : WW > 4b > ZZ 22nd General KEK (2011/9/3)13

pre-selection Lorentz factor β 2j > 0.05 |cosθ of 2-jet| < 0.99 nbj >= 3 – # of b-flavor jets with nsig=3.0 & noffv=1 analysis nbtj >= 2 – # of b-flavor jets with nsig=3.0 & noffv=2 analysis 22nd General KEK (2011/9/3)14 to suppress huge WW BG

Input parameters to NN 1st NN selection parameters – χ H 2, χ Z 2, # of b-flavor jet, etc... 2nd NN selection parameters – same as page10 3rd NN selection parameters – χ H 2, χ W 2, χ Z 2, etc... 22nd General KEK (2011/9/3)15 Details are shown in page24.

Cut Summary real clustering & 4b (optimistic) 22nd General KEK (2011/9/3)16 HHWWZZ4b(opt) expected events (# of MC events) 80 (50000) ( ) ( ) ( ) pre-selection47.93 (29958) (83777) 5167 (87057) (241318) W filter12.34 (7713) (9) (3259) (1083) 4b (opt) filter7.262 (4539) (2) (1245) (25) Z filter4.823 (3018) (1) (158) (13)

Cut Summary real clustering & 4b (pessimistic) 22nd General KEK (2011/9/3)17 HHWWZZ4b(pes) expected events (# of MC events) 80 (50000) ( ) ( ) ( ) pre-selection47.93 (29958) (83777) 5167 (87057) (287776) W filter12.34 (7713) (9) (3259) (1936) 4b (pes) filter8.238 (5149) 0 (0) (1422) (45) Z filter4.994 (3121) 0 (0) (124) (20)

Cut Summary cheat clustering & 4b (optimistic) 22nd General KEK (2011/9/3)18 HH (cheat)WW (cheat)ZZ (cheat)4b(opt) expected events (# of MC events) 80 (50000) ( ) ( ) ( ) pre-selection46.64 (29152) (57287) 4172 (70292) (241318) W (cheat) filter38.58 (24115) (5) (1667) 1331 (4535) 4b (opt) filter35.56 (22223) (4) (704) (13) Z (cheat) filter33.26 (20787) (2) (85) (13)

Cut Summary cheat clustering & 4b (pessimistic) 22nd General KEK (2011/9/3)19 HH (cheat)WW (cheat)ZZ (cheat)4b(pes) expected events (# of MC events) 80 (50000) ( ) ( ) ( ) pre-selection46.64 (29152) (57287) 4172 (70292) (287776) W (cheat) filter38.58 (24115) (5) (1667) 2179 (7422) 4b (pes) filter34.50 (21562) (3) (466) (9) Z (cheat) filter33.06 (20662) (3) (100) (9)

(Preliminary) Summary We can observe the Higgs self-coupling with ~5σ if we use cheat jet clustering. It is possible to measure the Higgs self- coupling in PLC if we can develop good jet clustering algorithm. 22nd General KEK (2011/9/3)20

Next Plan Check the statistical fluctuation Check/Evaluate the NN systematic error – Now : (training file) = (test file) Write a paper 22nd General KEK (2011/9/3)21

Backup slides 22nd General KEK (2011/9/3)22

sphericity, Y-value 22nd General KEK (2011/9/3)23 normalized momentum tensor M ab p i : momentum of particle i 3*3 matrix eigenvalue

Input parameters to NN (details) 22nd General KEK (2011/9/3)24 1st parameters χ H 2, χ Z 2, visible energy, # of tracks, ycut value of jet clustering, longitudinal momentum, transverse momentum, # of b-flavor jets with nsig = 3.5 & noffv = 1 analysis, # of b-flavor jets with nsig = 3.5 & noffv = 2 analysis 2nd parameters 3rd parameters same as page10 χ H 2, χ W 2, χ Z 2, energy of 2-jet (2), visible energy, # of tracks, longitudinal momentum # of b-flavor jets with nsig = 3.5 & noffv = 1 analysis, # of b-flavor jets with nsig = 3.5 & noffv = 2 analysis