Feasibility study of observation Higgs pair creation in gamma-gamma collider

1. Feasibility study of observation Higgs pair creation in gamma-gamma collider HiroshimaUniversity Nozomi Maeda

2. PLC in ILC Photon Linear Collider(PLC) is gamma-gamma collision option in ILC. Measurement of basic physical quantity of Higgs at PLC has been studied. e.g.)the two photon decay width of Higgs. We have studied feasibility of mesurement of rare process at PLC.

3. Higgs self-coupling Goal : Measurement of Higgs self-coupling constant l=lSM(1+dk) by gg->HH final state is 2particles　include loop diagram 1. This process occurs at low energy(than e+e-). 2. l contribution to cross section is different. →Can PLC measure Higgs self-coupling constant l? 　Our group study that feasibility.

4. Energy optimization L:Luminosityh:detection efficiency s:cross section Higgs mass=120GeV

5. Background (+,+), (-,-) main background is gg->WW (sB～90pb) cf.) Signal:s = 0.044fb →Reducing this background is required.

6. Simulation Framework CAIN (Luminosity) Signal : gg->HH BG : gg->WW theoretical calc. program (helicity amplitude) Helas (helicity amplitude) BASES/SPRING (Integration, Generation) Quick Detector Simulater pythia (parton shower，hadronization) Analysis

7. Luminosity distribution total luminosity of photon collision = 1.26×1035cm-2s-1 effective cross section(Signal) = 0.013fb → 16event/year effective cross section(BG) 　 = 11600fb → 1.467×107event/year hB<10-6 is required.

8. Analysis • Signal ： gg->HH 　H -> bB or WW* or gg … • Background : gg->WW 　　W -> ud or cs or en… choose HH->4b(its branching ratio is the highest). In this case, background is WW->4quark. Both are 4jets mode. →reconstruct particles, and distinguish them by c2 distribution. M1,M2 = reconstructed particle mass　　M = MH or MW　s2j = 2jet mass resolution

9. mass distribution black:HH->4bred:WW->All χ2 distribution s(gg->HH)×105 c2 distribution s(gg->HH)×105 Mass(GeV) leave here Cut s(gg->HH)×105 Cutting on c2 can’t achieve aim background reduction(hB<10-6). →b-tagging is required for achieve aim. Mass(GeV)

10. estimation b-tagging effect assume b-tagging performance el=0.005, ec=0.10, eb=0.7　　　　　　　　　 　　　(These values were achieved for 2jets in past experiment.) Background light quark(u,d,s) = q, charm quark = c → Background are WW->qqqq, WW->qqqc, WW->qqcc All reaction have same branching ratio. → Probability that 4jets are b-tagged is →hB = 6.89×10-8×B.R(4quark)=2.82×10-8 Signal →hSignal = 0.24×B.R(4b)=0.10 HH->4b →

11. Cutting on b-tagging We tried to simulate loose and tight b-tagging(nsig method). If 4 b-tagged jets are required, efficiency and background reduction are... tight : nsig=3, noffv=2 hBG = 1.50×10-6hSignal = 0.161 loose : nsig=3, noffv=1 hBG = 9.23×10-5hSignal = 0.324 Current b-tagging performance can’t achieve aim background reduction. Improvement of b-tagging or Cutting on b-tagging and kinematics parameters

12. Cutting Cut condition No. of loose b-tagged jets == 4 && 　　　 No. of tight b-tagged jets >= 3 && 　　　　c2(Higgs) < 18 && 　　　　c2(W) > 5 && 　　　　maximum chraged particle energy >= 2GeV →Result... Background : 10000000event -> 0event Signal : 50000event -> 6607event (hSignal=0.132) If PLC runs for 10 years... 16events/year×10years×0.132 = 21events → 4.6s

13. next plan • consider other background • gather H->WW* event

14. Summary • We study feasibility of measurement of Higgs self-coupling constant l at PLC. • If Higgs mass = 120GeV, optimized photon collision energy is 300GeV. • No. of Signal event is expected 16events/year when use PLC parameter(high luminosity). • Cutting on b-tagging and kinematics parameters can reduce almost gg->WW event. • gg->HH is observable with 4.6s