The Case for a 500 GeV Linear Collider

1. P. Grannis HEPAP subpanel meeting SLAC, May 23, 2001 The Case for a 500 GeV Linear Collider • Understanding the source of electroweak symmetry breaking is the most likely question on which fundamental progress is expected in the next 10 – 20 years. • 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 should discover new phenomena, make precision measurements that illuminate the nature of EWSB, and point the way toward higher energy phenomena. • The e+ e- linear colliders are well developed technically and it is very likely that we will make decisions a linear collider within the next few years.

2. 22 Linear ee Colliders TESLA JLC-C NLC/JLC-X * Ldesign (1034)3.4 5.80.432.2 3.4 ECM (GeV)500 800 500500 1000 Eff. Gradient (MV/m) 23.4 3534 70 RF freq. (GHz) 1.35.711.4 Dtbunch (ns)337 1762.81.4 #bunch/train2820 4886 72190 Beamsstrahlung (%) 3.2 4.44.6 8.8 * US and Japanese X-band R&D cooperation, but machine parameters may differ L= 1x1034 cm-2s-1 for 107 sec. year gives 100 fb-1 per year While initial operation will be e+e-, the possibility for gg, eg, e-e- collisions would benefit some physics topics. Multi-TeV, 30 GHz, 150 MV/m gradient with high current, low energy drive beam power source; in R&D phase

3. 21 There are two fundamental questions before experimental high energy physics at present: • 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?

4. 20 Main themes for the Linear Collider physics program : Experiments in the past decade (LEP, SLC, Tevatron, n scattering) have made precision measurements that clearly indicate the need for something like the Higgs boson below a few 100 GeV. LEP has indication of ~115 GeV state (2.9s ). Study the `Higgs boson’ (or its surrogate) and measure its characteristics. The SM Higgs mechanism is unstable; vacuum polarization contributions from the known particles should drive its mass, and the gauge boson masses, 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 at relatively low mass (2.6s ). Find and explore this new physics sector. (Also a rich secondary program – top physics, QCD, precision EW measurements, etc.)

5. What is the Higgs ?? (The Higgs is what the measurements tell us it is! ) 19 (SM) Mhiggs < 190 GeV at 90% CL. LEP2 limit Mhiggs > 113.5 GeV. Tevatron can discover up to 180 GeV W mass ( 34 MeV) and top mass ( 5 GeV) agree with precision measures and indicate low SM Higgs mass LEP Higgs search – Maximum Likelihood for Higgs signal at mH = 115.0 GeV with overall significance (4 experiments) = 2.9s If LEP indication is correct, there must be physics beyond the SM before the GUT scale Higgs self-coupling diverges Higgs potential has 2nd min.

6. 18 Program for Higgs Study Find a Higgs boson candidate, and measure its mass (or masses of added Higgs states in SM extensions) LHC (or Tevatron) should discover Higgs, and measure the mass adequately (unless Higgs decays dominantly in invisible modes - then the LC finds it). Measure total width, and couplings to all fermion pairs and gauge bosons. Are the couplings proportional to mass? Do they conform to the simple SM? to Susy models? Do they saturate the full gHVV (are there more Higgs?) LHC will not do GTOT (or do rather poorly). Ratios of somecouplings only to ~20%. Linear Collider can measure G and couplings to ~5%; these are the crucial measurements for establishing the nature of the higgs. Measure the quantum numbers of the Higgs states: for the SM, expect JPC = 0++ ; for Susy, both 0++ and 0-+. LHC will not do; Linear Collider will do easily Explore the Higgs potential. The self-couplings lead to multiple Higgs production. LHC will not do; LC can do, with sufficient luminosity LHC should discover a Higgs candidate; LC should discover what it really is.

7. Higgs studies -- Discovery & Mass 17 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 well. LC can produce 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, mm, qq modes). ~30K evts/yr at 350 GeV; ~15K evts/yr at 500 GeV (m=120 GeV ) dM/M ~ 1.2x10-3 ZH brems Higgs Z/W couplings WW/ZZ fusion dominates at high mass, energy -- measures VV couplings. Measuring the lightest Higgs coupling in ZH production tests whether there are additional higher mass Higgs. In MSSM or 2 doublet models:S g2(h ZZ)i = (MZgEW / cosqW)2 GTOT to few% for mass < 150 GeV. Total width BR(H WW*) = GWW / GTOTGWW from sZH or WW fusion 100 150

8. 16 Higgs Couplings We need to determine experimentally that Higgs couplings are indeed proportional to mass. In Susy, 2 complex Higgs doublets f1and f2. After mass generation for W, Z get : h0, H0 (CP even; mh < mH ); A0 (CP odd); H For Susy mass scale < 1 TeV, Mh < 130 GeV . Higgs sector in MSSM controlled by tan b = <f1> / <f2>and mA . At large mA, h0 becomes SM-like and H0,A0,H become massive and nearly degenerate. (decoupling limit) (Mh = 120 GeV) 500 fb-1 for 300 GeV LC H bb 2.4% H cc 8.3% H gg 5.5 % H t t 6.0% H WW 5.4% From likelihood using vertex, jet mass, shape information BR’s approx. errors 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)

9. 15 Higgs spin parity q =cm production angle; f=fermion decay angle in Z frame • JP = 0+ JP = 0- • ds/dcosq sin2q(1 - sin2q) • ds/dcosf sin2f(1 +/- cosf)2 • and angular dependences near threshold permits unambiguous determination of spin-parity Can produce CP even and odd states separately using polarized gg collisions. gg H or A(can reach higher masses than e+e-) Higgs self couplings Measures Higgs potential l 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. Dl/l error 36% 18%

10. Physics beyond the Standard Model 14 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 framework. Supersymmetry Susy models come in many variants with different 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. (Technicolor/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. Something different? LC must be able to sort out which is at work. Can imagine cases where LHC sees new phenomena, but misunderstands the source

11. Supersymmetry 13 If Susy is to stabilize the Higgs & gauge boson masses (and give grand unification) it is ‘natural’ to believe that some Supersymmetric particles will appear at a 500 GeV LC. The main goal is to measure the underlying model parameters and deduce the character of the supersymmetry, energy scale for Susy breaking. There are ~ 105 unknown MSSM parameters, all of which should 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 stateswith similar quantum numbers -- the 2 stops, sbottoms, staus, and the 2 chargino and 4 neutralino states (partners of the g/Z/W and supersymmetric Higgs states). Susy may well be the next frontier for flavor physics – FCNC, CP violation in the sparticles, generational patterns, etc. The LHC will discover Susy if it exists. But disentangling the information on the full mass spectrum, 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- )

12. Supersymmetry studies at the Linear Collider 12 An example: production of selectron pairs -- have two diagrams; typically the t-channel dominates and allows measurement of neutralino couplings (gaugino vs. higgsino) to lepton/slepton. ~ e+ e+ e+ ~ e+ g,Z c0 ~ e- e- ~ e- e- ~ e e c10 Upper & lower edges of decay electron energy distribution from gives masses of left and right handed selectrons and neutralino. ~ eL,R e c10 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.

13. Chargino studies 11 e+ c+ g, Z Masses are determined from end points in reactions like e+ e- c1+ c1- , with decays: c1+ c10W+ / c10l n / c10q’ q as for previous cases (few %). The mass values of c1+ , c2+constrain the mass mixing parameters: M2(c1+) +M2(c2+) = M22 + 2 M2(W) + m2 M(c1+) xM(c2+) = mM2 - M2(W) sin(2b) e- c- c+ e+ ~ n c- e- Polarization is again crucial: eR- e+ c1+ c1-removes the t-channel diagram; cross section and AFB give the higgsino/wino content of c1+. Tests of Susy relations are possible (measure MW to ~ 23 MeV from purely Susy quantities.) eL- e+ c1+ c1-allows test of SUSY coupling relation g(c+ ne) = g(W+ne ) ~ eL- e+ c1+ c1- has strong s & t channel interference, sensitive to m( n) to about 2 ECM. ~ ~ ~ ~ Similar studies for neutralino, t, t, m, production lead to independent measures of similar parameters and should enable a constrained fit to Susy model.

14. Linear Collider Supersymmetry 10 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. Thresholds for selected sparticle pair productions -- at LHC mSUGRA model points. Point 1 2 3 4 5 6 GeV GeV GeV GeV GeV GeV reaction c10 c10 336 336 90160 244 92 c10 c20 494 489 142 228 355 233 c1+ c1- 650 642192294 464 304 c1+ c2- 1089 858 368462 750459 e e/ m m 920 9224221620396 470 t t 860 850412 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 1276364 q q 1882 1896 630 1828 1352 1010 RED: Accessible at 500 GeV GREEN:added at 1 TeV ~ ~ ~ ~ ~ ~ ~ ~ Operation in eg mode can increase mass reach: e-g e c10 (g-2) result suggests relatively light sfermions or charginos, if Susy is at work. ~ ~ It is likely that, in the case that supersymmetry exists, one will want upgrades of energy to at least 1 TeV.

15. Susy breaking mechanism 9 The LC complements the LHC. LHC will see those particles coupling to color, some Higgs, lighter gauginos if present in cascade decays of squarks and gluinos. LC will do sleptons, sneutrinos, gauginos well. 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 mass parameters, couplings reveal the model class without assumptions. This study for ~ 1000 fb-1 LC operation and LHC meas. of gluinos and squarks show dramatically distinct mass parameter patterns for mSUGRA and GMSB.

16. Precision studies constrain ANY any new physics generating the Higgs mechanism 8 S & T measure effect on W/Z vacuum polarization amplitudes. S for weak isoscalar and T for isotriplet All EW observables are linear functions of S & T and are presently measured to 0.01 to be in agreement with SM. sin2 qW Giga-Z samples at LC (20 fb-1) would improve sin2qW by x10 (requires e+ polarization), WW threshold run improves dMW to 6 MeV, etc. Factor 8 improvement on S,T. LC Will measure top mass to 200 MeV. 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 DT or negative DS. Several classes of models to do this, but it is difficult to evade observable consequences at LC. Present 68% S,T limits 68% S,T limits at LC; location of precision ellipse gives model info. The precision measurement of S&T at a linear collider could be crucial to understand the nature of the new physics, if no Susy.

17. Strong Coupling Gauge Models see JoAnne Hewitt talk 7 For many, fundamental scalars are unnatural. We have a theory (QCD) in which pseudoscalars (pions) arise as bound states of fundamental fermions. A new Strong Interaction could provide the Higgs mechanism and generate EWSB. At LC, strong coupling composite ‘higgs’ should be constrained to < 500 GeV through precision measurements. Observables in Strong coupling models: Bound states of new fermions should occur on the TeV scale. Since the longitudinal components of W/Z are primordial higgs particles, WW (ZZ) scattering is modified: seen at LHC or higher energy LC. Expect observable modifications to WWg coupling. 500 GeV LC is complementary to LHC and in many cases better. For Dkg,Z , 500 GeV LC precision is 10-20 times better than LHC and able to see effects expected in Strong Coupling models. Anomalous top couplings to Z,g expected, only observable at LC. Large Extra Dimensions see JoAnne Hewitt talk Fields propagating in extra spatial dimensions give observable consequences -- Kaluza Klein excitations, missing ET signatures, modifications to cross sections. 500 GeV LC generally has comparable reach to LHC, and is often complementary: e.g. if supersymmetry in the bulk, towers of Gravitinos modify polarized e+e- e e cross sections for higher dimension Planck scale up to 10 TeV. Polarized g g WW process has large sensitivity to graviton exchange. Sensitivity to KK gauge boson from TeV scale extra dimensions with 500 GeV LC exceeds LHC. ~ ~

18. Scenarios for New Physics 6 Some Scenarios for Physics after few yrs LHC: ( Remember that precision measurements now give rather different probabilities for these scenarios! ) 1. Higgs-like state<150 GeV and evidence for Susy: LHC/Tevatron discover: Linear Collider program is assured, exploring the Higgs and Susy spectra 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, Kaluza Klein towers, etc. 3. No Higgs, no Susy seen (LHC program barren): Verify that no Higgs to invisible modes. Measure anomalous W/Z couplings and top anomalous form factors to find strong coupling evidence. Return to the Z-pole, WW threshold for precision measurements. Increase the energy to seek new strong-coupling or extra dimension physics. 4. Multiple kinds of complex phenomena seen at LHC: A wealth of new physics that needs untangling -- Linear Collider is required, as LHC cannot cleanly plumb the new physics.

19. Options for beams / energies 5 • Several special operating conditions for the LC may add important physics capabilities, but create extra complexity or costs. How should we view these options? • Positron polarization: Polarized e+ required for improved precision EW measurements (S&T) with Giga-Z; provide increased XS for Higgs, useful for self-couplings; allow improved measurements of Susy couplings/mixings. Obtain polarized e+ from intense polarized g beams (TESLA requires a similar scheme to get e+ ). • gg , e-g, e-e- collisions: Larger cross sections in gg offsets lower luminosity; can separate Susy H/A, complementary triple gauge coupling info, lower threshold for selectrons in eg; e-e- allows clean environment, high polarization, only one subprocess, good probes for new physics (KK towers, anomalous WW production …) • Low energy collisions (MZ , WW threshold, ZH cross section maximum) For any new physics whose origin is not immediately understood, a 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 with extra bunch trains. Each of these needed in some physics scenarios. Retain capability for them

20. 4 How do we proceed? • 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 are 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. • Is the Linear Collider too expensive? • One hears 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 given 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. • Site is likely chosen by funding – which region will put up over half of the cost?

21. How do we proceed? 3 • Steps toward Internationalization: • Need a collective decision in US on the right sequence of next steps. Snowmass 2001 and HEPAP subpanel are critical activities. Proceed to a world view on the preferred next step based on worldwide planning exercises. • 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 the appropriate 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. With LHC, Europe takes the energy frontier; Asia and North America will need frontier facilities to remain healthy.

22. Summary: Why is a ~500 GeV Linear Collider the right first step? 2 Recent results from LEP, SLC, Tevatron, nscattering, CESR, etc. have dramatically improved the precision with which we understand the SU(2)xU(1) Electroweak interaction. 100’s of distinct measurements contribute and overconstrain the fit to the SM. These results give us markedly higher expectation for seeing a Higgs sector at intermediate mass than before. Precision exploration of the Higgs is our next frontier, comparable in importance to the recent studies of the Z and W. Phenomena beyond the SM are expected. Widely differing models have been proposed, but all have observable ingredients that will help identify the mechanisms using a 500 GeV LC, and will point us to the appropriate next step. While it remainspossible to invent models satisfying the existing precision constraints with few observable consequences at 500 GeV, we must keep in mind that a multitude of signposts point with high probability to there beingnew phenomena within reach at LHC and a 500 GeV LC. c.f. “The Case for a 500 GeV e+e- Linear Collider”, hep-ex/0007022 A new “Sourcebook for LC Physics” with discussion of physics, scenarios for operation, intersection region options, detectors etc – and questions for further study – available in early June.

23. Conclusions The physics case for the LC with a first 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. The US should propose a LC in this country to capitalize on this opportunity and preserve our engagement at the energy frontier. 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.