What can an e+ e- Linear Collider teach us ?

P. Grannis/PHENO 2001 May 7, 2001

Understanding the source of electroweak symmetry breaking is the most pressing issue in high energy physics for the coming decade.

The LHC (or the Tevatron) seems assured of discovering new phenomena related to EWSB but will leave critical questions unanswered.

An e+e- linear collider at ~500 GeV can discover new phenomena and make precision measurements that illuminate the nature of EWSB, and point the way toward higher energy phenomena. There is likely to be a need for evolution of the collider.

The e+ e- linear colliders are well developed technically and it is likely that we will make decisions a linear collider within the next few years. It is imperative that we understand the physics case as clearly as possible.

Linear ee Colliders

TESLA JLC-C NLC/JLC-X *

Ldesign (1034) 3.4 5.8 0.43 2.2 3.4

ECM (GeV) 500 800 500 500 1000

Eff. Gradient (MV/m) 23.4 35 34 70

RF freq. (GHz) 1.3 5.7 11.4

tbunch (ns) 337 176 2.8 1.4

#bunch/train 2820 4886 72 190

Beamsstrahlung (%) 3.2 4.4 4.6 8.8

TESLA : Design report March 2001; German Science Council recommendation mid-2002

NLC : aim complete R&D for Design Rept 2003

JLC : set milestones end 2000: Design Rept ~2003

CLIC : multi-TeV, 30 GHz, 150 MV/m gradient with drive beam power source; in R&D phase

* US and Japanese X-band R&D cooperation, but machine parameters may differL = 1x1034 cm-2s-1 for 107 sec.

year gives 100 fb-1 per year

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NLC baseline 2001: 26 km site (2 in CA, 2 in IL). Two 10 km linacs sized for 1 TeV; final focus, injector for 1.5 TeV. Two IRs; ‘Hi E’ IR with no bend (crossing angle 20 mrad) can work at multi-TeV; ‘Lo E IR requires bend; maximum energy 500 GeV ( 1 TeV?)

NLC - 2001

Recent work:

Improved klystrons and SS modulators give x3-4 efficiency gain.

New compact final focus region.

Optimum cost for gradient 70 MV/m but deterioration of accelerating structure surfaces seen (at high group velocity). Active R&D this year to understand. If need to reduce to 50 MV/m, cost penalty is 5-10%.

Cost reduction to date: from $5.1B in Lehman review ($FY00, no escalation, contingency, detectors) to $3.7B (30% reduction). Another 10 – 15% from possible scope reductions.

Injectors: 19%; Linacs: 39%; beam delivery: 11%; global costs: 17%; management/business: 14%

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TESLA

TESLA site length = 33 km (15 km linacs). Operates with superconducting RF cavities; design for 500 GeV is 22 MV/m. Bunches are separated by 337 ns, allowing for head-on collisions without satellite crossings.

spec

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Cost: $3.16B (using 0.93$/Euro). Includes 1 IR, 1 detector ($233M). xFEL added cost is $495M.

Cost in 2000 prices; no contingency (HERA was on budget); no escalation; no second detector/IR; exclusive of manpower at collaborating institutes (6933 man-yrs estimated: ~$700M)

• What is the origin of the symmetry breaking observed in the electroweak interaction?

What gives the W/Z (and fermions) their mass?

Is there unification of forces, and if so, at what scale? Can gravity be incorporated?

Are there new phenomena or new particles associated with the physics responsible for EWSB?

• What is the origin of flavors?

Why three generations and the peculiar fermion mass patterns?

Why is there CP violation, and why is it insufficient to give the matter/antimatter asymmetry in the universe?

Does flavor physics (neutrino mass) imply something about physics at the GUT scale?

There are two fundamental questions before experimental high energy physics at present:

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Main themes for the Linear Collider physics program :

Experiments in the past decade (LEP, SLC, Tevatron, scattering) have made precision measurements that clearly indicate the need for something like the Higgs boson. LEP has indication of ~115 GeV state (2.9).

The SM Higgs mechanism is unstable; vacuum polarization contributions from the known particles should drive its mass to the force unification scale. We expect some new physics entering at the TeV scale. BNL (g-2 ) experiment comparison with theory suggests new physics (2.6).

Study the `Higgs boson’ (or its surrogate) and measure its characteristics.

Find and explore this new physics sector.

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Where is the Higgs ??

(SM) Mhiggs < 190 GeV at 90% CL. LEP2 limit Mhiggs > 113.5 GeV. Tevatron can discover up to 180 GeV

(The Higgs is what the measurements tell us it is! )

All Expts: Bknd Signal exp. Evnts 4 jet 0.93 1.60 3 missing ET 0.30 0.46 1 leptons 0.35 0.68 0 taus 0.14 0.29 0 ALL chan. 1.72 3.03 4

Higgs self-coupling diverges

Higgs potential has 2nd min.

LEP Higgs search – Maximum Likelihood for Higgs signal at mH = 115.0 GeV with overall significance (4 exp’ts) = 2.9

If LEP indication correct, there must be physics beyond the SM before the GUT scale

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LHC (Tevatron) will discover > 1 Higgs; LHC will get mass accurately; total width, couplings poorly; likely not the quantum numbers; will not do self couplings; and may well not see heavy Susy Higgs.

Higgs studies -- Mass

The key discovery question for LC is What is the nature of the `Higgs’ ? -- revealed by its quantum no’s, couplings, total width. The LHC is unlikely to do these. LC produces Higgs in association with Z allowing study of its decays without bias -- even invisible decays of Higgs are possible using the recoil Z (in ee, , qq modes).

For 120 GeV Higgs, ZH production mode:

~30K evts/yr at 350 GeV; ~15K evts/yr at 500 GeV M/M ~ 1.2x10-3 TOT : Measuring the lightest Higgs

coupling tests whether there are additional higher mass Higgs. In MSSM or 2 doublet models: g2

(h ZZ)i = (MZ

gEW / cos W)2

TOT to few% at LC for mass <

150 GeV, using measured WW

from WW fusion or ZH , & BR(H

WW). LHC can do TOT to 10-20%

in this range.

Width

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Higgs spin parity

= cm production angle; = fermion decay angle in Z frame

JP = 0+ JP = 0 d/dcos sin2 (1 - sin2 )

d/dcossin2 (1 +/- cos )2

and angular dependence near threshold permits unambiguous determination of spin-parity

Can produce CP even and odd states separately using polarized collisions. H or A(can reach higher masses than e+e-)

Higgs Couplings

Use vertex meas., jet mass, topology in likelihood to get BRs for 500 fb-1 , 300 GeV LC

H bb 2.4% H cc 8.3% H gg 5.5 % H 6.0% H WW 5.4%

(Mh = 120 GeV)

Measurement of BR’s is powerful indicator of new physics (e.g. in MSSM, these differ from the SM in a characteristic way. Higgs BR must agree with MSSM parameters from many other measurements.)

SM value (decoupling limit)

approx. errors

We need to determine experimentally that Higgs couplings are indeed proportional to mass.

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Higgs self couplings

Study ZHH production and decay to 6 jets (4 b’s). Cross section is small; premium on very good jet energy resolution. Can enhance XS with positron polarization. ~ 20% with 1000 fb-1.

Physics beyond the Standard

Model The defects of the SM are widely known:

No gauge interaction unification occurs

Higgs mass is unstable to loop corrections

Many possible new theories proposed to cure these ills and embed the SM in a larger frameworkSupersymmetry -- fermion/boson partners, extending the Poincare group to include fermionic dimensions.

Susy models come in many variants with different mechanisms and scales of Susy breaking (supergravity, gauge mediation, anomaly mediation …) Each has a different spectrum of particles, underlying parameters.A new gauge interaction like QCD with `mesons’ at larger masses. (Techicolor/topcolor) These interactions avoid introducing a fundamental scalar. `technipions’ play the role of Higgs; new particles to be observed, and modifications to WW scattering.

String-inspired models with some extra dimensions compactified at millimeter to femtometer scales. These yield anomalous mono-photon or mono-jet production, heavy Z/W states, modification to ee production LC must be able to sort out which is at work, and make precision measurements. Examples exist where LHC sees new phenomena, but mis-understands the source

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Supersymmetry

Fermion/boson symmetry stabilizes the Higgs mass -- scale of new Susy particles is O (1 TeV). Lightest higgs state < 130 GeV.

The main issue is to measure the underlying model parameters and deduce the character of the supersymmetry, energy scale for supersymmetry breaking. There are ~ 105 unknown parameters, all of which need to be measured, and used to fix models.

This can be done through measurement of the masses, quantum numbers, branching ratios, asymmetries -- and in particular the pattern of mixing of states with similar quantum numbers -- the 2 stops, sbottoms, staus, and the 2 chargino and 4 neutralino states (partners of the /Z/W and supersymmetric Higgs states).

Susy may well be the next frontier for flavor physics – FCNC, CP violation in the sparticles, generation patterns, etc.

The LHC should discover Susy if it exists. But disentangling the

information on the full mass spectrum and particle quantum no’s/couplings and the mixings will be very difficult at LHC.

The LC can make these crucial measurements, (e.g. sparticle masses to 0.1 – few % level) benefitting from --

Polarization of electron (positron) beam

Known partonic cm energy

Known initial state (JP = 1- )

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Supersymmetry studies

at the Linear Collider

An example: production of selectron pairs -- have two diagrams; typically the t-channel dominates and allows measurement of neutralino couplings to lepton/slepton.

e-

e-

e+

e+ e+~

e+~

e-~e-~

,Z

Upper & lower edges of decay electron energy distribution from gives masses of left and right handed selectrons.

Angular distribution of decay electrons, using both polarization states of beam e-, tell us about quantum numbers, coupling of exchanged neutralino and give information on neutralino mixing, hence the underlying Susy mass parameters.

Similar studies for neutralino, chargino, stau etc. production lead to independent measures of similar parameters and enable constrained fit to Susy model.

e e ~

eL,R e ~

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Linear Collider Supersymmetry

The Linear Collider can determine the Susy model, and make progress to understand the higher energy supersymmetry breaking scale. To do this, one would like to see the full spectrum of sleptons, gaugino/higgsino states.

Point 1 2 3 4 5 6 GeV GeV GeV GeV GeV GeV

336 336 90 160 244 92

494 489 142 228 355 233

650 642 192 294 464 304

1089 858 368 462 750 459

e e/920 922 422 1620 396 470

860 850 412 1594 314 264

Z h 186 207 160 203 184 203

Z H/A 1137 828 466 950 727 248

H+ H - 2092 1482 756 1724 1276 364

q q 1882 1896 630 1828 1352 1010

~

~ ~

~

~~

reaction

~ ~

Thresholds for selected sparticle pair productions -- at LHC mSUGRA model points.

It is likely that, in the case that supersymmetry exists, one will want upgrades of energy to at least 1 TeV.

RED: Accessible at 500 GeV

GREEN: added at 1 TeV

Operation in e mode can increase mass reach: e.g. e- e 1

0

(g-2) result suggests relatively light sfermions or charginos

~~

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Susy breaking mechanism

The LC complements the LHC ( LC will do sleptons, sneutrinos, gauginos well). LHC will see those particles coupling to color, some Higgs, lighter gauginos if present in cascade decays of squarks and gluinos. Electron polarization (positron?) is essential for disentangling states and processes at LC.

We really want understand the origin of Susy -- determine the 105 soft parameters from experiment without assuming the model. (mSUGRA, GMSB, anomaly, gaugino … ) mediation. We want to understand Susy breaking, gain insight into the unification scale and illuminate string theory.

Mass spectra give some indications of the model class.

LC mass, cross sect. as input to RGE evolution of couplings reveal the model class without assumptions.

This study for ~ 1000 fb-1 LC operation and LHC meas. of gluinos and squarks show dramatically different patterns of mSUGRA and GMSB.

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Precision studies constrain ANY any new physics generating the Higgs mechanism

Standard Model (SU(2) x U(1) S=T = 0) agrees with data

sin2 W

S & T measure effect on W/Z vacuum polarization amplitudes. S for wk isoscalar and T for isotriplet

All EW observables are linear functions of S & T and are presently measured to 0.01.

Giga-Z samples at LC (20 fb-1) would improve sin2W by x10, WW threshold run improves MW to 6 MeV, etc.

Factor 8 improvement on S,T

The chevron shows the change in S & T as the Higgs mass increases from 100 to 1000 GeV, given the current top mass constraint. If the Higgs is heavy (> 200 GeV), need some compensating effects from new physics. Need a positive T or negative S. Several classes of models to do this – all have observable consequences at LC.

Present 68% S,T limits 68% S,T

limits at LC

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The precision measurement of S&T at a linear collider could crucial to understand the nature of the new physics.

Strong Coupling Gauge Models

For many, fundamental scalars are unnatural. We have a theory (QCD) in which pseudoscalars (pions) arise as bound states of fundamental fermions (quarks).

Analogs of SU(3) color are postulated with `technicolor’ degrees of freedom, but fermions at higher mass scale. The ‘technipions’ generate the Higgs mechanism. Though inspired by QCD, the new model must differ quantitatively (slow evolution of coupling)

In ‘topcolor’, the 3rd generation SM quarks (top in particular) are singled out as being strongly coupled to the new sector. Fermion pair (tt) condensates play the role of `higgs’.Observables in Strong coupling models:

New `technicolor’ particles should occur on the TeV scale. Since the longitudinal components of W/Z are primordial higgs particles, WW (ZZ) scattering is modified. Also expect modifications to WW coupling and top V&A form factors, seen at LC (tough at LHC).

Envisioned S&T constraints suggest that composite Higgs state(s) should have mass < ~500 GeV.

Allowed regions in Higgs mass and T for W mass error of 30 MeV and top mass error of 2 GeV

Chivukula, Holbling, hep-ph/0002022

Seeing strong coupling effects may require LC energy above 500 GeV. Better S&T precision will be crucial.

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Large Extra Dimensions

String theories represent the only known avenue for incorporating gravity and the microscopic forces. Until recently, hope for any observable effects from the compactification of extra dimensions was dim.

Recent suggestions that the extra dimensions might be compactified on scales larger than Planck length lead to observable consequences for experiment – and could explain the heirarchy problem.

If the effective Planck mass ~TeV, gravity is modified at m scale, leading to anomalous production (with missing ET from gravitons), or modified cross sections for fermion pairs.

For compactification scale O (TeV), Kaluza-Klein excitations of graviton or gauge bosons should exist at TeV scale, and observable as excited states at the LHC or LC. If Susy in extra dimensions, gravitino towers modify XS’s.

For compactification at the GUT scale, new states are unobservable, but the characteristic Susy pattern of these models should remain, and the unification pattern of the couplings should provide information.

Models with SM gauge field propagation in TeV sized extra dimensions, can get scalars (Higgs) with SM properties and EWSB. MH = 165 – 230 GeV (and other scalar composites). (Arkani-Hamed, Cheng, Dobrescu, Hall hep-ph/0006238)

Large extra dimension models still being developed. If this is our world, it is likely that higher Linear Collider energies will be desired.

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Scenarios for New Physics

Although our experiments point to the Standard Model, the Linear Collider should be capable of illuminating the nature of physics beyond the SM. We believe that some manifestation of the equivalent to the Higgs mechanism should be seen at 500 GeV or less.

Some Scenarios for Physics after few yrs LHC:1. Higgs-like state<150 GeV and evidence for Susy:

LHC/Tevatron discover: Linear Collider program is assured, exploring the Higgs and Susy spectrum and determining their detailed structures.2. Higgs candidate seen but nothing else:

LC studies all aspects of the Higgs (accessible couplings, width etc.) to compare with SM. Revisiting the Z-pole to refine the precision measurements will be essential. Seek Z’ at LHC/LC, anomalous VVV couplings, strong WW scattering, etc.3. No Higgs, No Susy seen:

Verify that no Higgs to invisible modes. Measure anomalous W/Z couplings and top anomalous form factors. Increase the energy to seek new strong-coupling or extra dimension physics. Return to the Z-pole for precision S and T.

4. Multiple kinds of new phenomena seen at LHC/TeV:

A wealth of new physics that needs untangling -- Linear Collider has a field day!

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Options for beams / energies

There are several special operating conditions for the Linear Collider that may add important physics capabilities, but also create extra complexity or costs. How should we view these options?

Positron polarization:

Polarized e+ probably required for improved precision EW measurements (S&T) with Giga-Z; provide increased XS for H iggs, useful for self-couplings; allow improved measurements of Susy couplings/mixings. Obtain polarized e+ from intense polarized beams (TESLA requires these anyway).

, e-, e-e- collisions:

Larger cross sections in offsets lower luminosity; can separate Susy H/A, complementary triple gauge coupling info, lower threshold for selectrons in e; e-e- allows clean environment, high polarization, only one subprocess, good probes for new physics (KK towers, some Susy states …)

Low energy collisions (MZ , WW threshold, ZH cross section maximum)

For any new physics whose origin is not immediately understood, return to the Z, WW threshold will greatly aid understanding. Operation at the maximum of the ZH cross section gives largest rates. Ideas exist to permit simultaneous operation at low (<500 GeV) and high energy for NLC.

X-ray Free Electron Laser

Structural biology, plasma physics, materials science, chemical kinetics, surface science all benefit from short pulse angstrom level sources. There can be synergy between HEP and these communities through use of LC as XFEL.

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How does the world community proceed?

(a personal point of view)

1. Linear Collider Timelines:

o Tesla design report in spring 2001; decision 2002

o Japan JLC proposal in few years

o US NLC R&D over next 3 years leading to proposal All 3 regions conducting studies of physics priorities for next ~20 years during coming year.

LC decision likely in next few years!

Other new projects (HEPAP whitepaper timelines):

Storage Ring might be ready for decision ~2010; nature of physics questions fairly clear ( matrix/CP) Next generation expts will probably teach us much.

Collider or multi-TeV ee collider > 2010;

VLHC after 2010

Physics case for the multi-TeV colliders is not yet clear to me; Higher energy than LHC/LC may not be highest priority if there is rich TeV scale physics.

Very large underground laboratories (proton decay, solar neutrinos, neutrino oscillations, supernova watch)

CERN is evaluating its future beyond LHC

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How do we proceed?

2. Should the LC be the next world HE machine?

Inevitable that the LC decision is the next that will be taken by the worldwide community. Real proposals exist; potential alternatives much further off. Not all regions may propose a LC in their region, but we will make decisions soon.

Worldwide support for the LC (somewhere) will be essential if it is to succeed. Arguing against the LC will not enhance the prospect for other projects. LC should not be the last frontier accelerator.

Particularly in the US community, we must engage the LC question, and consequence of opting for other paths. Snowmass 2001 and HEPAP subpanel affords the chance to confront these issues as a community.

3. Is the Linear Collider too expensive?

One hears, particularly in the US, that the likely cost of the LC is too large to sell to the government. But ANY future collider discussed is at least comparable cost. An endemic problem! We need to be optimistic enough to expect to succeed in arguing for a next project if it has a clear scientific justification!

Cost of the LC seen by some as the primary driver toward the initial stage at ~500 GeV. “Will such a stage address the crucial next questions? ” Physics arguments above show clear role for 500 GeV program. But expect that upgrades will be needed.

Cost is a factor, and we must press all ways to control it. But we must not lose sight of the probable need for future evolution in the design.

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How do we proceed?

4. Steps toward Internationalization:

Need a collective decision on the right sequence of next steps (for the US, Snowmass 2001 and HEPAP subpanel are critical activities). Proceed to a world view on the preferred next step.

Site is likely chosen by funding – which region will put up ~2/3(??) of the cost?

For LC (or other projects), allocate spheres of responsibility; empower all regions to take primary responsibility for major systems from leadership of design, R&D, construction, commissioning, operations. e.g. for LC, might assign injector/damping rings; rf/linacs; final focus/beam delivery/monitors to separate regions.

Need international advocacy for the project; ICFA, Physical Societies, Global Science Forum involvement. International cooperation in presenting proposals to national governments stressing the joint ownership of project, shared access and continuity of world wide effort.

Connect governmental science policy officials through forums on how to forge international science project structures.

International review – comparative cost, performance upgradability, technical risks assessment (being done for LC). International oversight of accelerator, detectors, scientific program.

The LC (or any other frontier project) should be fundamentally international. Each region needs strong involvement at the frontier to retain health of HEP and accelerator physics in that region. With LHC, Europe takes the energy frontier; Asia and North America will need frontier facilities to remain healthy.

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Conclusion

The physics case for the LC with a 1st stage at ~ 500 GeV is very strong. We need a linear collider to understand EWSB in any scenario. We know enough to make the choice now. Community engagement through Snowmass, HEPAP is essential if we are to reach a consensus.

With present lack of understanding of how EWSB is manifested, flexibility of Linear Collider design (energy, L , beam particles) is essential. The LC will be an evolving facility.

The cost will be high. Unless we internationalize so as to satisfy the needs of all regions and allow productive collaboration, we jeopardize the prospect of the LC, or any other new frontier facility anywhere.