Why a Linear Collider Now?

S. Dawson, BNL

October, 2002Asian, European, and American communities

all agreeHigh Energy Linear Collider is next large

accelerator WHY???

Where are we going?

• US high energy community just completed long range planning process

• 20 year roadmap for the future

• HEPAP subpanel:

We recommend that the highest priority of the U.S. program be a high-energy, high-luminosity, electron-positron linear collider, wherever it is built in the world

Linear Collider Basics

• Initial design, e+e- at s=500 GeV• Luminosity 1034 cm2/sec

300 fb-1/yr• 80% e- polarization• Energy upgrade to .8-1.2 TeV in future• Physics in 2012

• The international accelerator community believes that a TeV-scale linear collider can be successfully built

NLCHigh Power Klystron

TESLA Superconducting Cavity

JLCAcceleratorTest Facility

TESLA

Preliminary designs for Linear Colliders

NLC

?????• What are the big questions we want to

answer?• Why do we think we can predict where

we want to go?– What do we know now?– What do we expect to learn from the

Tevatron and LHC?– What questions will remain unanswered?

What is particle physics?

Study of Space, Time, MatterBagger/Barish report

The Big Questions?

• What is the origin of mass?

• Do protons decay?• Do forces unify at a

large scale?• Are there more than

four dimensions?• Why are there 4

forces?No unification of couplings in SM

Cosmic Connections

• What is dark matter?

• How are particle physics & cosmology connected?

• What is dark energy?

• Where did the anti-matter go?

Planning for the Future Based on Success of last 20 years….

• Model of electroweak physics verified at .1% level

• The problem of mass remains• W and Z bosons discovered at CERN

in 1983

• Masses not zero….or even small

GeVM

GeVM

Z

W

91

80

Why is Mass a Problem?• Lagrangian for gauge field (spin 1):

L=-¼ FF

F=A-A

• L is invariant under transformation:

A (x) A(x)-(x)• Gauge invariance is guiding principle• Mass term for gauge boson

½ m2 AA

• Violates gauge invariance• So we understand why photon is massless

Simplest possibility for Origin of Mass is Higgs Boson

• Higgs mechanism gives gauge invariant masses for W, Z

• Requires physical, scalar particle, H, with unknown mass

• Observables predicted in terms of:– MZ=91.1875.0021 GeV

– GF=1.16639(1) x 10-5 GeV-2

=1/137.0359895(61)

– Mh

• Higgs and top quark enter into quantum corrections, Mt

2, log(Mh)

Precision Measurements sensitive to top quark before it was discovered!

Large number of measurements fit

electroweak predictions

Indirect Indications for Light Higgs Mass

• Direct measurements of MW, Mt agree well with indirect measurements

• Prefer Higgs in 100-200 GeV range

• ASSUMES no new physics

Where is the Higgs boson?

• Higgs couplings of fixed

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

• Direct search limit from LEP:

• Higgs contributions to precision measurements calculable

WWWh

fffh

gMgv

mg

clGeVM h %95@114

clGeVM h %95@193

Precision measurements:

G. Mylett, Moriond02

Tantalizingly close…..

Direct limit: Mh>114.1 GeV

Indirect limit: Mh<193 GeV

New Physics is just around the corner!Fits assume Standard Model….if

Standard Model incorrect, even more exciting new physics….

Higgs mass and scale of new physics correlated…..

Sensible theory here

130 < Mh < 170 GeV

Fermilab Tevatron

• at s=2 TeV• May discover Higgs if

very lucky• Requires light Higgs

and high luminosity• Physics in 2002-2008

pp

bbhWhpp ,

CDF

D0

Upgraded Detectors for RunII

Enhanced capabilities for b tagging aid Higgs search

CERN Large Hadron Collider (LHC)

• pp interactions ats =14 TeV

• LHC will discover Higgs boson if it exists

• Sensitive to Mh from 100-1000 GeV

• Higgs signal in just a few channels

• Physics circa 2008

ATLAS TDR

Discovery isn’t enough….

• Is this a Higgs or something else?

• Linear Collider can answer critical questions– Does the Higgs generate mass for the W,Z

bosons?– Does the Higgs generate mass for fermions?– Does the Higgs generate its own mass?

Is it a Higgs?• How do we know what we’ve

found? • Measure couplings to

fermions & gauge bosons

• Measure spin/parity

• Measure self interactions

2

2

3)(

)(

m

m

h

bbh b

0PCJ

42

23

22

822h

v

Mh

v

Mh

MV hhh

Coupling Constant Measurements

Zeppenfeld, hep-ph/0203123

• LHC measures combinations of coupling constants

• Typical accuracy, 10-20%

• Only some subset of couplings

• Assumptions necessary to get couplings

L=200 fb-1

Linear Collider is Higgs Factory!

• e+e-Zh produces 40,000 Higgs/year

• Clean initial state gives precision Higgs mass measurement

Mh2=s-2sEZ+MZ

2

• Model independent Higgs branching ratios

WWh vertex

ZZH vertex

Higgs mass measurements

• LC:

• LHC:

Direct reconstruction of

LC @ 350 Gev

Conway, hep-ph/0203206

MeVM

fbGeVM

h

h

50

500,120 1

h

MeVM

fbGeVM

h

h

100

300,150 1

Precision Measurements of Higgs Couplings

• Dots are experimental error

• 1-2% measurement• Measure ALL Higgs

couplings• Bands are theory error

– Larger than experiment

– Largest error from mb

Battaglia & Desch, hep-ph/0101165

Higgs measurements test model!

• Couplings to fermions very different in SUSY models

• LC can distinguish SM from SUSY up to MA=600 GeV

Standard Model

• 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

%24

1000 1

hhh

hhh

g

g

fb

Lafaye, hep-ph/0002238

Problem with this picture…

• Fundamental Higgs is not natural

• Quantum corrections to Mh are quadratically divergent

Mh22

• So enormous fine-tuning needed to keep Higgs light

Mh2\Mh

2MW2\Mpl

210-32

Solution is Supersymmetry

• Quadratic contributions to Higgs mass cancel between scalars and fermions

• To make cancellation hold to all orders need symmetry

• Bose-Fermi symmetry….supersymmetry

Do the forces unify?

• Coupling constants change with energy

• Coupling constants unify in supersymmetric models

Hint for new physics?

New particles in SUSY Theory

• Spin ½ quarks spin 0 squarks• Spin ½ leptons spin 0 sleptons• Spin 1 gauge bosons spin ½ gauginos• Spin 0 Higgs spin ½ Higgsino Experimentalists dream….many particles to search for! What mass scale?Supersymmetry is broken….no scalar with mass of

electron

Supersymmetry

• Can we find it?

• Can we tell what it is?

• Masses of new particles depend on mechanism for breaking Supersymmetry

• Couplings of new particles predicted in terms of few parameters

• Simplest version has 105 new parameters

Simplifying Assumption:

• Assume masses unify at same scale as couplings

• Everything specified in terms of scalar/fermion masses at high scale and 3 parameters

• Predictive anzatz…..

•LHC/Tevatron will find SUSY• Discovery of many SUSY

particles is straightforward• Untangling spectrum is

difficult all particles

produced together• SUSY mass differences from

cascade decays;eg

• M0 limits extraction of other masses

qll

llqqL

0

1

02

~

~~~

Catania, CMS

Light SUSY consistent with Precision Measurements

• SUSY predicts light Higgs

• SUSY predicts 5 scalars

• For MA, SUSY Higgs sector looks like SM

• Can we tell them apart?• Higgs BR are different in

SUSY

HAHh ,,, 000

GeVM SUSYh 130

LHC

Find all the Higgs Bosons

Carena, hep-ph/9907422

Tevatron

Into the wedge with a LC

• s>2MH

e+e- H+H-, H0A0

observable to MH=460 GeV at s=1 TeV• s<2MH

e+e- H+, H+tb L=1000 fb-1, s=500 GeV,

3 signal for MH 250 GeV

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 MeV

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

,,

~~

~~

ee

eeee RLRL

2min,max,

min,max,22~

)( ee

eeCMe EE

EEEM

LC Study, hep-ex/0106056

me1 GeV

L=50 fb-1

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

XffffX gg ~~

gqqee

gqqee

gqqee

~~

~~

ss gg ~

m

mg

g

g ~ln

16

~

~

~ 2

2

RLRL eeee ,,~~

Bee~~

eeB~~

What is the universe made of?

• Stars and galaxies are only 0.1%

• Neutrinos are ~0.1–10%

• Electrons and protons are ~5%

• Dark Matter ~25%

• Dark Energy ~70%

H. Murayama

Supersymmetry provides understanding of dark matter?

• Lightest SUSY particle (LSP) could be dark matter candidate!

• LSP is weakly interacting, neutral, and stable

• LSP in range of LC/LHC• LC can determine LSP

mass; check dark matter predictions

LSP is dark matter

Mh=115 GeV

g-2

Drees, hep-ph/0210142

M0 (GeV)

M1/

2

Standard Model Needs Top Quark

• Top quark completes 3rd generation– Why are there 3

generations, anyways?

• Theory inconsistent without top

Top Quark discovery at Fermilab in 1995

CDF top eventD0 top event

Why is Mt(=175 GeV)>>Mb(=5 Gev)??

Understanding the Top Quark

• Why is ?

• Kinematic reconstruction of tt threshold gives pole mass at LC

• Compare LHC

2

vM t

MeVM

fb

t 200

40 1

Groote , Yakovlov, hep-ph/0012237

QCD effects well understood

NNLO ~20% scale uncertaintyGeVM

fb

t 21

50 1

2Mt (GeV)

Top Yukawa coupling tests models

• tth coupling sensitive to strong dynamics

• Above tth threshold

e+etth

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

• Large scale dependence in tth rate at LHC

• L=300 fb-1

%5.6tth

tth

g

g

Baer, Dawson, Reina, hep-ph/9906419

Juste, Merino, hep-ph/9910301

Reina, Dawson, Orr, Wackeroth

Beenacker, hep-ph/0107081

% 16 tth

tth

g

g

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• etc