h 1 → a 1 a 1 → µ + µ - τ + τ - ATLAS Experiment: Search of Higgs Boson decaying into a 1 in

1. h1 → a1a1 → µ+µ-τ+τ- ATLAS Experiment: Search of Higgs Boson decaying into a1 in Next to Minimal SuperSymmetric Model By: Hou Keong(Tim) Lou Rutgers University With: Catherine Laflamme Advisor: Chris Potter McGill University

2. SuperSymmetry How do I lose some weight naturally? • A symmetry relating Bosons to Fermions • Higgs mass too low, requires unnatural cancellations • SuperSymmetry solves the Hierarchy problem (explains why actual Higgs mass ⋘ bare mass) • Allows unification of coupling parameters ~Λ2 Higgs The Planck scale Obesity is a major problem, specially for the Higgs Boson

3. (N)MSSM • By adding a minimal number of fields, one gets Minimal SuperSymmetric Model (physics on a diet) • µ-problem: scale of a superpotential is unnatural • Promote µ term to a superfield, one gets Next to MSSM

4. Higgs Sector in NMSSM • Different Higgs Particles: • 3 CP-even, 2 CP-odd, 2 charged Higgs • Lightest CP-even Higgs – h1 behaves like the Standard Model Higgs except its decay channels. • If a1 is light ~a few GeV (lightest CP-odd Higgs) • h1 → ZZ*, WW*, bbar, τ+τ-may be strongly suppressed. • h1 → a1a1 may become dominant • If the ideal Higgs version of NMSSM* is correct, we will not find the Higgs through all the standard discovery channels! * The ideal NMSSM scenario, see arXiv:0811.3537v1, by R. Dermisek and J. Gunion for details

5. Phenomenology of a1 • a1 decays primarily into the heaviest particles it can decay to (and gluons) Branching Ratio of a1 decaying into two tau leptons Branching Ratio of a1 decaying into two gluons * figures obtained from arXiv:0811.3537v1, by R. Dermisek and J. Gunion

6. Event Simulation • Official Athena 14.2.25.10 Atlas production version + scripts • PYTHIA → Detector Sim → Digitization → Reconstruction → D3PD ntuple → Analysis • a1 mass = 5 GeV, h1 mass = 100 GeV • Demand a1a1 → µµ + hadronic τ • Produced 7588 events

7. Phenomenology of a1 • a1 can decay into 2 gluons, 2 leptons, 2 quarks • For 200 pb-1data* (a1mass = 5 GeV and h1 mass = 100 GeV) • 4µ 0.26 events • 2µ 2g (2c) 5.12 events • 2µ 2τ 82 events (our focus) • 4τ 6560 events • 2 µ low mass resonance (between 2τ and 2b, 4~8 GeV) * a1 decay branching ratio obtained from arXiv:0811.3537v1, by R. Dermisek and J. Gunion ** h1 production x-section from Standard Model gluon fusion

8. Tau decay modes • τ neutrino production due to τ lepton number conservation • τ decay modes: • 35% leptonically, 2 neutrinos • Catherine Laflamme is currently studying leptonic tau decay channels • 65% hadronically with 1 neutrino • My primary focus: Both τ decaying hadronically • 50% - 1 prongs (1 charged pion) → τ jet • 15% - 3 prongs (3 charged pions) → τ jet

9. Signal vs. Background • µ+ µ- low mass resonance • 2 oppositely charged τ jets • No quarks/gluon jets Making sure the background doesn't eat away the signal

10. Background • W+jets and Z events (both negligible) • QCD + ttbar (main background) • QCD: • Huge cross sections • Expect ~103 M events in 200 pb-1 data • Impossible to produce enough simulation data • Event selection: • Exactly two oppositely charged muons • At least two oppositely charged tau jets

11. Huge QCD background after scaling, must eliminate at all costs By demanding at least two oppositely charged tau jets, exactly two oppositely charged muons and no electron, the mass of the muon pair is plotted. The result is scaled to 200 pb-1

12. Natural Selection After By demanding the number of jets to be less than 2 in all events, QCD + ttbar backgrounds are removed.

13. Efficiency • Muon cuts: exactly two oppositely charged muons, PT > 10 GeV, |η|<2.5 • Tau Jet cut: at least two oppositely charged tau jets, PT > 10 GeV, |η|<2.5 • µµ mass cut: muon pair mass between 3-10 GeV • # of jets cut: at most two jets are present (PT > 15 GeV, |η|<2.5)

14. Survival of the Fittest After The uneaten signal remains After demanding at most two jets exist in an event (PT > 15 GeV), all QCD and ttbar backgrounds are gone.

15. Higgs Transverse Mass • mT2 = m2 + pz2 By adding the four vectors of the two tau jets, two muons, and missing transverse momentum in the xy plane, the transverse mass mT is plotted. mT is always greater than the actual mass (100 GeV in this case), resulting in an edge.

16. Conclusion • Realistic NMSSM model • Background is negligible • Expect 1 signal event in 200 pb-1 • For τ decaying leptonically, also expect 1 signal event • This decay channel has not been studied previously by ATLAS/CMS

17. Questions? Comments?Concerns?Remarks?Ideas?