LHC signal of the Little Higgs modelwith T-parity, focusing on thetop-partner production at the LHCwith a realistic simulation

Little Higgs Mechanism1. Higgs boson is regarded as Pseudo NG boson2. Pattern of the breaking is specially arranged (Collective symmetry breaking) in order to cancel quadratic divergent contributions to mh at 1-loop level. can be 10 TeV without the fine turning!!

Little Hierarchy Problem LEP experiment requires > 10 TeV ( = new physics scale) On the other hand, fine tuning problem on the Higgs mass mh requires < 1 TeV

Can we investigate the LittleHiggs Mechanism at the LHC?

Solution

Confirmation

Problem

I. Motivation

LH: Due to the Little Higgs mechanism, the Little Higgs partners are introdced.

II. The model　 The Littlest Higgs model with T-parity(described based on the non-linear sigma model under SU(5)/SO(5) breaking. The subgroup [SU(2)×U(1)]2 in SU(5) is gauged, and broken into the SM gauge under the breaking.

, W±, Z, h

[Arkani-Hamed, Cohen, Katz and Nelson (2002)]

H, WH±, ZH, Φ

t T

LH

Particle contents

LH

Gauge-Higgs sector Top sector

LH: Due to the Little Higgs mechanism, the Little Higgs partners are introdced. T: LH partners in the gauge Higgs sector can be T-odd partners at the same time. T-odd partners are introduced in the top sector Thanks to the T-parity, the Lightest T-oddparticle (AH) is stable, can be dark matter!!

II. The model　 The Littlest Higgs model with T-parity(described based on the non-linear sigma model under SU(5)/SO(5) breaking. The subgroup [SU(2)×U(1)]2 in SU(5) is gauged, and broken into the SM gauge under the breaking.

, W±, Z, h

[Arkani-Hamed, Cohen, Katz and Nelson (2002)]

H, WH±, ZH, Φ

t T

t– T–

TLHT

Particle contents

LH

Gauge-Higgs sector Top sectorT

III T– pair production at the LHC

T– quark is the lightest colored T-odd particle, and SU(2) singlet. Thus it decays into top quark and dark matter at almost 100% branching ratio.

Proton Proton

T-

7 TeV 7 TeV

T-– t

AH

t

AH

–

From WMAP: mAH = 80 – 400 GeVFrom EW test: mT- > 600 GeV mT- – mAH > 500 GeV Energetic top quarks with large missing energy

Constraints from WMAP and EW Precision test

WMAP

EW testWMAP

III T– pair production at the LHC

Four representative points

Detector effects AcerDET code

Event generation CompHep and HERWIG codes

T– quark is the lightest colored T-odd particle, and SU(2) singlet. Thus it decays into top quark and dark matter at almost 100% branching ratio.

Proton Proton

T-

7 TeV 7 TeV

T-– t

AH

t

AH

–

IV. Event Reconstruction

The distribution peaks at the　 top mass, clearly showing that the analysis can group the jets from correctly

Three jetsinvariant mass

Two separate jet system is expected in the sigma events.

Two separate jet system is expected in the sigma events.By observing m(jjj) in a hemisphere, t is reconstructed.

pp

T–

T–

b-jet

b-jet

tt

DM(AH)DM(AH)

W

W

jetjet

jet

jet

V. Background reduction

m(jjj) < 200 GeV in at least one of the hemisphere

1. kinematic cut

2mT- – 0.2(3) TeV < Meff < 2mT- for Sample I, II(III, IV)

The events have no isolated leptons

Background: pp tt + X(102 – 103 larger than the signal)

2. Top cut

3. Lepton veto

–

Point III

Point III

Each distribution shows the clear excess of events over the background!!

VI. Results (50 fb-1)

BGSignal + BG

Signal

I II

III IV

SummarSummaryy• Littlest Higgs model with T-parity is one of interesting model for physics beyond the Standard model.• The T- quark is the key particle at the LHC. • From WMAP and electroweak precision m

easurements, it turns out that mT- > 600 GeV, and mT- - mAH > 500 GeV. aaa• T- quark can be discovered using the hemi-sphere analysis at the LHC!

Discussion

• The study has been performed through

the tree-level calculation. Thus, the result

should be regarded as an order of

magnitude estimate.• In addition to the 1-loop level calculations,

more realistic detector simulations are needed in order to estimate the reliable significance.