Phenomenology of a little Higgs model with T-parity

Jay M. Hubisz

Cornell University

Institute for High Energy Phenomenology

with Patrick Meade

hep-ph/0411246

Introduction – The little Hierarchy Problem

We want the Higgs to be light

We want to avoid EWP constraints

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Tension

New physics that couples to SM shifts observables (e.g. EWPT's)

scale of new physics must be high!

Need to keep the Higgs mass light

scale of new physics must be low!

Goal: Introduce new physics in a way such that both issues are	resolved

Little Higgs and Supersymmetry

For some time, SUSY was the only weakly coupled theory	that solved the little hierarchy problem

strong coupling (technicolor) has EWP issues of its own (but concept	very attractive)

Are there other models of weakly coupled physics at ~1 TeV	that solve the (big) hierarchy problem?

Can we introduce this new physics in such a way that we	alleviate the little hierarchy?

little Higgs models are an attempt to do just this

Little Higgs models

The new concept is “collective symmetry breaking”

Higgs is a pseudo-Goldstone boson (PGB) kept light by approximate global	symmetries (old idea, new features)	Georgi, Kaplan – early 80's

Global symmetries are broken explicitly, but in a special way

Arkani-Hamed, Cohen, Georgi hep-ph/0105239

breaks some of global symmetries

breaks some others

Together, couplings break all symmetries protecting the Higgs mass,	and we generate an at two-loop order

Example – The littlest Higgs

Parametrize SU(5)/SO(5) breaking by nlm

VEV 0 breaks [SU(2)xU(1)]2 to SU(2)LxU(1)Y

Higgs is exact Goldstone under either SU(3)

 is a complex triplet under SU(2)L

New gauge bosons get mass ~ f

Arkani-Hamed, Cohen, Katz, Nelson hep-ph/0206021

Collective breaking (littlest Higgs)

SU(5)/SO(5) symmetry breaking pattern – gauge symmetries embedded	in SU(5) global symmetry

preserves SU(3) in bottom right

preserves SU(3) in upper left

Either SU(3) is enough to keep Higgs massless

- But the sum of all the gauge interactions breaks all	symmetries protecting the Higgs mass

New Symmetries = New Particles

New 'partners' of standard model fields cancel one loop	quadratic divergences

Global symmetry structure ensures relation between couplings

Partners (unlike in SUSY) have same statistics

Electroweak Precision

Simplest models still require fine tuning

Such diagrams give large contributions to custodial SU(2) violation –	large  (or T) parameter contributions

Also contributions to vertex

Summary of Littlest Higgs

Global SU(5) is explicitly broken to SU(3) by any one given	interaction

New gauge bosons and scalars near SU(5) breaking scale f

Interactions of standard model particles with new degrees of	freedom create tension with EWP (no solution to little hierarchy)

An effective theory which is good only to ~10 TeV

T-Parity and the littlest Higgs

Original models have stringent EWP bounds

New models which evade constraints exist, but involve more	complicated gauge/global symmetry structures

Csaki, Hubisz, Kribs, Meade, Terning

Cheng, Low

Just as R-parity cures ills of SUSY (proton decay, lepton flavor	violation), T-parity is a discrete symmetry that is added to	solve the tree level EWP issues in LH

Can also provide a weak scale dark matter candidate

T-Parity

(almost) all new particles introduced are odd under a discrete	symmetry (W', Z', A', )

Dangerous diagrams are forbidden (e.g. tree level mass shifts, and	4-Fermi operators)

Discrete symmetry exchanges the two gauge groups [SU(2)xU(1)]1	and [SU(2)xU(1)]2

T-parity enforces relations between couplings (i.e. g1 = g2)

Missing Energy Signal

If T-Parity is exact, lightest T-odd particle is stable (like	lightest SUSY partner is stable due to R-parity) and	neutral

Leaves the detector without depositing any energy

Reconstruction difficult

T-Parity fixes most parts of the low energy	spectrum shown here

Breaking scale f fixes the new gauge boson	masses

Higgs mass fixes the triplet mass

One free parameter in the new top sector

New degrees of freedom

Littlest Higgs spectrum is same, with the	exception of a new t_ SU(2) singlet

Many new T-odd fermions (T-parity partners of	standard model quarks and leptons)

In our analysis we decouple most particles not	involved in the little Higgs mechanism

Look at phenomenology of simpler model before including	many new degrees of freedom, and many new parameters

Spectrum (f ~ 1 TeV)

Collider Phenomenology

Expand nlm fields in powers of /f and read off	interactions

Program interactions into COMPHEP

Generate cross sections (LHC), decay widths, and	branching fractions of new particles

Event generation

Detector simulation

future work

What's new with T-Parity?

T-odd particles must be pair produced

t+ is the exception (new T-even particle needed for LH mechanism)

FAKING SUSY – difficult to determine spins at LHC, and final	states similar

Radically different collider signal compared to original little Higgs models

Previous studies do not apply

Heavy vector boson pair production

Primary production channels are WH + ZH and WH+WH

WH+AH suppressed by small mixing angle

EWSB effect is (v/f)2	suppressed

Heavy gauge boson decays

100%

100%

Either Higgs + W in the final state or two	Higgses, while rest goes into missing	energy

If T-parity is exact, then AH escapes detector

But T-Parity need not be exact! (If we give up on LH dark matter)

Violations of 10% or so still solve little hierarchy problem

Small deviations in gauge coupling relations lead to	leptonic decay of AH - Nice signal!

t'+ Production at the LHC

Gluon fusion and W-b fusion produce the T-even partner of the top

New particle does not need to be pair produced

Production cross sections are the same as those calculated for littlest Higgs w/o T-parity

Han, Logan, McElrath, Wang

Perelstein, Peskin, Pierce

t'+ Decays

t- is always lighter than t+

Sizable branching fraction to missing energy

Cannot extract total width as with	original littlest Higgs

t'- Production

T-odd, so can they can only be pair produced

100%

t- decay:

Triplet production

Symmetries give mass relation between Higgs and triplet:

Promising signal:

Standard model background is small, but signal gives only 10-100 events at f ~ 1 TeV

Conclusions

Possible to construct a LH model with no tree level EWP	constraints

Minimal spectrum similar to many other LH models

Collider signals can fake SUSY at the LHC

Future:

Discovery possibility analysis

EWP at one loop level

Analysis including additional heavy fermions

2004-12-07T14:27:16