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) LouRutgers University

With: Catherine LaflammeAdvisor: Chris Potter

McGill University

SuperSymmetry

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

How do I lose some weight naturally?

Higgs

Obesity is a major problem, specially for the Higgs Boson

The Planck scale

(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

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

Phenomenology of a1

• a1 decays primarily into the heaviest particles it can decay to (and gluons)

* figures obtained from arXiv:0811.3537v1, by R. Dermisek and J. Gunion

Branching Ratio of a1 decaying

into two gluons

Branching Ratio of a1 decaying

into two tau leptons

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

* a1 decay branching ratio obtained from arXiv:0811.3537v1, by R. Dermisek and J. Gunion

** h1 production x-section from Standard Model gluon fusion

a1 can decay into 2 gluons, 2 leptons, 2 quarks

For 200 pb-1 data* (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)

Phenomenology of a1

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

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

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

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

Huge QCD background after scaling, must eliminate at all costs

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

Natural Selection

EfficiencySignal QCD Hadronic top Leptonic top

No cut 34.6 8380 M 33.6 k 41.1 k

Muon cut 14.2 (0.411) 163 k (1.94e-5) 1330 (0.040) 1620 (0.04)

Tau Jet cut 3.7 (0.107) 2260 (2.7e-7) 156 (0.0046) 172 (0.0042)

µµ mass cut 3.68 (0.106) 615 (7.3e-8) 2.97 (8.84e-5) 3.02 (7.34e-5)

# of jets cut 1.3 (0.038) 0 0 0

• 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)

Aft

er

After demanding at most two jets exist in an event (PT > 15 GeV), all QCD and ttbar backgrounds

are gone.

The uneaten signal remains

Survival of the Fittest

Higgs Transverse Mass• mT

2 = 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 m

T is plotted. m

T is always greater than

the actual mass (100 GeV in this case), resulting in an edge.

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

Questions?

Comments?

Concerns?

Remarks?

Ideas?