Why a Linear Collider Now S Dawson BNL

Why a Linear Collider Now? S. Dawson, BNL October, 2002 q. Asian, European, and American communities all agree q. High Energy Linear Collider is next large accelerator q 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, electronpositron linear collider, wherever it is built in the world

Linear Collider Basics • Initial design, e+e- at s=500 Ge. V • Luminosity 1034 cm 2/sec 300 fb-1/yr • 80% e- polarization • Energy upgrade to. 8 -1. 2 Te. V in future • Physics in 2012

NLC High Power Klystron • The international accelerator community believes that a Te. V-scale linear collider can be successfully built JLC Accelerator Test Facility TESLA Superconducting Cavity

Preliminary designs for Linear Colliders TESLA 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, Matter Bagger/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 antimatter 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

Why is Mass a Problem? • Lagrangian for gauge field (spin 1): L=-¼ F F F = A - A • L is invariant under transformation: A (x) A (x)- (x) • Gauge invariance is guiding principle • Mass term for gauge boson ½ m 2 A A • 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 Ge. V GF=1. 16639(1) x 10 -5 Ge. V-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 100200 Ge. V range • ASSUMES no new physics

Where is the Higgs boson? • Higgs couplings of fixed Precision measurements: • Production rates at LEP, Tevatron, LHC fixed in terms of mass • Direct search limit from LEP: • Higgs contributions to precision measurements calculable G. Mylett, Moriond 02

Tantalizingly close…. . Direct limit: Mh>114. 1 Ge. V Indirect limit: Mh<193 Ge. V ØNew Physics is just around the corner! ØFits assume Standard Model…. if Standard Model incorrect, even more exciting new physics….

Higgs mass and scale of new physics correlated…. . 130 < Mh < 170 Ge. V Sensible theory here

Fermilab Tevatron • at s=2 Te. V • May discover Higgs if very lucky • Requires light Higgs and high luminosity • Physics in 2002 -2008

Upgraded Detectors for Run. II CDF Enhanced capabilities for b tagging aid Higgs search D 0

CERN Large Hadron Collider (LHC) • pp interactions at s =14 Te. V • LHC will discover Higgs boson if it exists • Sensitive to Mh from 1001000 Ge. V • 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

Coupling Constant Measurements • LHC measures combinations of coupling constants • Typical accuracy, 10 -20% • Only some subset of couplings • Assumptions necessary to get couplings L=200 fb-1 Zeppenfeld, hep-ph/0203123

Linear Collider is Higgs Factory! • e+e- Zh produces 40, 000 Higgs/year • Clean initial state gives precision Higgs mass measurement Mh 2=s-2 s. EZ+MZ 2 • Model independent Higgs branching ratios WWh vertex ZZH vertex

Higgs mass measurements LC @ 350 Gev • LC: • LHC: Direct reconstruction of Conway, hep-ph/0203206

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! Standard Model • Couplings to fermions very different in SUSY models • LC can distinguish SM from SUSY up to MA=600 Ge. V

Higgs spin/parity in e+e- Zh Threshold behavior measures spin [20 fb-1 /point] Miller, hep-ph/0102023 • Angular correlations of decay products distinguish scalar/pseudoscalar

Measuring Higgs Self Couplings • ghhh, ghhhh completely predicted by Higgs mass • Must measure e+e- Zhh • Small rate (. 2 fb for Mh=120 Ge. V), large background • Large effects in SUSY Lafaye, hep-ph/0002238

Problem with this picture… • Fundamental Higgs is not natural • Quantum corrections to Mh are quadratically divergent Mh 2 2 • So enormous fine-tuning needed to keep Higgs light Mh 2Mh 2 MW 2Mpl 2 10 -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 • M 0 limits extraction of other masses 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

Find all the Higgs Bosons LHC Tevatron Carena, hep-ph/9907422

Into the wedge with a LC s>2 MH e+e- H+H-, H 0 A 0 observable to MH=460 Ge. V at s=1 Te. V • s<2 MH e+e- H+, H+ tb L=1000 fb-1, s=500 Ge. V, 3 signal for MH 250 Ge. V •

LC can step through Energy Thresholds Run-time Scenario for L=1000 fb-1 Year 1 L (fb-1) 10 • • • 2 4 5 6 7 40 150 200 250 SUSY masses to. 2 -. 5 Ge. V from sparticle threshold scans M 0/M 0 7% (Combine with LHC data) 445 fb-1 at s=450 -500 Ge. V 180 fb-1 at s=320 -350 Ge. V (Optimal for Higgs BRs) Higgs mass and couplings measured, gbbh 1. 5% Top mass and width measured, Mt 150 Me. V Battaglia, hep-ph/0201177

How do we know it’s SUSY? • Need to measure masses, couplings • Observe SUSY partners, eg • Discovery is straightforward • e energies measure masses • Polarization can help separate states me 1 Ge. V L=50 fb-1 LC Study, hep-ex/0106056

SUSY Couplings: • Compare rates at NLO: • • Masses from endpoints • Assume • Lowest order, • Super-oblique corrections sensitive to higher scales • Tests coupling to 1% with 20 fb-1

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? LSP is dark matter Mh=115 Ge. V M 1/2 • 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 g-2 M 0 (Ge. V) Drees, hep-ph/0210142

Standard Model Needs Top Quark • Top quark completes 3 rd generation – Why are there 3 generations, anyways? • Theory inconsistent without top

Top Quark discovery at Fermilab in 1995 Why is Mt(=175 Ge. V)>>Mb(=5 Gev)? ? D 0 top event CDF top event

Understanding the Top Quark • Why is ? • Kinematic reconstruction of tt threshold gives pole mass at LC 2 Mt (Ge. V) Groote , Yakovlov, hep-ph/0012237 • Compare LHC QCD effects well understood NNLO ~20% scale uncertainty

Top Yukawa coupling tests models • tth coupling sensitive to strong dynamics • Above tth threshold e+e tth • Theoretically clean • s=700 Ge. V, L=1000 fb-1 Baer, Dawson, Reina, hep-ph/9906419 Juste, Merino, hep-ph/9910301 • Large scale dependence in tth rate at LHC Reina, Dawson, Orr, Wackeroth Beenacker, hep-ph/0107081 • L=300 fb-1

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