Why a Linear Collider Now?

S. Dawson, BNL

October, 2002

The Physics case Why we need both the LC and the LHC Examples: EWSB, SUSY, top quark

LC is Next• European, Asian, American communities all agree:

LC is next step • Initial design, • Luminosity

• 80% e- polarization• Physics arguments for 1 TeV energy scale

Energy upgrade a must!

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Combination of LHC/LC physics probes EWSB

Is mass due to a Higgs boson?• Higgs couplings of SM fixed

• Production rates at LEP, Tevatron, LHC fixed in terms of mass

• Direct search limit from LEP:

• Higgs contributions to precision measurements calculable

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Precision measurements:

G. Mylett, Moriond02

LHC

Higgs Discovery at Tevatron or LHC

Tevatron

ATLAS TDR

• Well determined initial state– Precision masses with

recoil technique

– Higgs mass independent of Higgs decay

– Model independent Higgs BRs

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Higgs mass measurements

• LC:

• LHC:

Direct reconstruction of

LC @ 350 Gev

Conway, hep-ph/0203206

MeVM

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Is it a Higgs?• How do we verify role in

EWSB?• Measure Yukawa couplings

• Measure spin/parity

• Reconstruct Higgs potential

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Coupling Constant Measurements

Battaglia & Desch,

hep-ph/0101165

LC

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Zeppenfeld, hep-ph/0203123

LHC

L=200 fb-1L=100 fb-1, s=350 GeV

• Angular correlations of decay products distinguish scalar/pseudoscalar

Miller, hep-ph/0102023

Threshold behavior measures spin

[20 fb-1 /point]

Higgs spin/parity in e+e-Zh

Measuring Higgs Self Couplings

• ghhh, ghhhh completely predicted by Higgs mass

• Must measure e+e- Zhh

• Small rate (.2 fb for Mh=120 GeV), large background

• Large effects in SUSY

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Lafaye, hep-ph/0002238

Light SUSY consistent with Precision Measurements

• SUSY predicts light Higgs

• For MA, SUSY Higgs sector looks like SM

• Can we tell them apart?

• Higgs BR are different in SUSY

• Find all SUSY Higgs,

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LHC

Find all the Higgs Bosons

Carena, hep-ph/9907422

Tevatron

Into the wedge

• s>2MH

e+e- H+H-, H0A0

observable to MH=460 GeV ats=1 TeV

• s<2MH

e+e- H+, H+tb

L=1000 fb-1, s=500 GeV,

3 signal for MH 250 GeV

Moretti, hep-ph/0209210

•LHC/Tevatron will find SUSY• SUSY mass differences

from cascade decays;eg

• M0 limits extraction of other masses

• Fit to SUGRA parameters

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LHC: Fits to SUSY Parameters

Bachacou, Hinchliffe, Paige, hep-ph/9907518

LC can step through Energy Thresholds Run-time Scenario for L=1000 fb-1

Year 1 2 4 5 6 7

L (fb-1) 10 40 150 200 250 250

• SUSY masses to .2-.5 GeV from sparticle threshold scans M0/M0 7% (Combine with LHC data)

• 445 fb-1 at s=450-500 GeV• 180 fb-1 at s=320-350 GeV (Optimal for Higgs BRs)

• Higgs mass and couplings measured, gbbh1.5%

• Top mass and width measured, Mt150 GeV

Battaglia, hep-ph/0201177

How do we know it’s SUSY?• Need to measure masses,

couplings

• Observe SUSY partners, eg

• Polarization can help separate states

• Discovery is straightforward

• e energies measure massesRL ee ~,~0

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LC Study, hep-ex/0106056

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SUSY Couplings:

• Compare rates at NLO:

• Lowest order,

• Super-oblique corrections sensitive to higher scales

•

• Masses from endpoints

• Assume

• Tests coupling to 1% with 20 fb-1

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(e+e-Zh) sensitive to SUSY Parameters

Dawson, Heinemeyer hep-ph/0203067

TESLA:

ZH2-3%,

L=500 fb-1

Understanding the Top Quark

• Why is ?

• Kinematic reconstruction of tt threshold gives pole mass at LC

• Compare LHC

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Groote , Yakovlov, hep-ph/0012237

QCD effects well understood

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Top Yukawa coupling tests models

• tth coupling sensitive to strong dynamics

• Above tth threshold

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• Theoretically clean s=700 GeV, L=1000 fb-1

• Large scale dependence in tth rate at LHC

• L=300 fb-1

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Baer, Dawson, Reina, hep-ph/9906419

Juste, Merino, hep-ph/9910301

Reina, Dawson, Orr, Wackeroth

Beenacker, hep-ph/0107081

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Exciting physics ahead

• LHC/Tevatron finds Higgs LC makes precision measurements of

couplings to determine underlying model• LHC finds evidence for SUSY, measures mass

differences

LC untangles spectrum, finds sleptons

LC makes precision measurements of couplings and masses