P. Grannis – Snowmass Workshop July 1, 2001

1. P. Grannis – Snowmass Workshop July 1, 2001 Linear Collider Physics Studies Any major new accelerator facility should address the big questions of particle physics research for the next several decades: • What is the origin of symmetry breaking in the electroweak interaction ? W/Z fermion masses, new physics at the TeV scale, force unification? • Where do the quark and lepton flavors come from ? why 3 generations? CP violation? Fermion mass disparities and mixing patterns? • What are the unseen elements of the universe ?dark matter, dark energy, cosmological constant = 0 Snowmass studies should address how the Linear Collider can shed fundamental insights on these overarching questions

2. 2 Linear ee Colliders TESLAaJLC-C NLCb/JLC-XgLdesign (1034)3.4 5.80.432.0 3.4 ECM (GeV)500 800 500500 1000 Gradient (MV/m) 23.4 3534 70 RF freq. (GHz) 1.35.711.4 Dtbunch (ns)337 1762.81.4 a: TESLA TDR DESY 2001-011 (March 2001) b:NLC, http://www-project.slac.stanford.edu/lc/wkshp/snowmass2001/g: US and Japanese X-band R&D cooperation; machine parameters may differ electron polarization ~ 80%; may have positron polarization ~60% Optional gg, e- g, e-e- collisions at reduced luminosity for special physics L= 1x1034 cm-2s-1 for 107 sec. year gives 100 fb-1/year For example: ECM = 500 GeV, 1 year @ 1x1034cm-2s-1 For 220 GeV charginos, 6000 c+c- pairs; 70,000 top pairs For 120 GeV Higgs 6000 Higgs from HZ Higgstrahlung 8000 Higgs from WW fusion s(fb) 106 Sqq ECM =500 GeV s(fb) WW Hnn tt E=500 GeV 800 GeV 103 HZ Zh ~ ~ c+c- ~ ~ 1 HA Hee mR+mR- MH ECM

3. 3 Main themes for the Linear Collider physics program : Precision measurements in the past decade (LEP, SLC, Tevatron, n scattering) indicate the need for something like the Higgs boson below a few 100 GeV. 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 at the TeV scale. Find and explore this new physics sector. (e.g. if Supersymmetry, a whole set of new sfermions with which to explore Flavor Physics, seek dark matter, etc.) (Also a rich additional program – top physics, QCD studies, precision EW measurements, etc.) Recent comprehensive physics studies: TESLA TDR, DESY 2001 – 011, (Part III – Physics at an e+e- Linear Collider) LC Physics : Resource Book for Snowmass 2001, (hep-ex0106055, 056, 057, 058 including: “The Case for a 500 GeV e+e- Linear Collider”, hep-ex/0007022 )

4. 4 W/top Z lineshape Z asym n scat Z BR’s What is the Higgs ?? (Only measurements will tell us ! ) Measurements tell us Higgs mass is low: (SM) Mhiggs < 190 GeV at 90% CL. LEP2 limit Mhiggs > 113.5 GeV. Tevatron can discover up to 180 GeV LEP Higgs search – Maximum Likelihood for Higgs signal at mH = 115.0 GeV with 2.9s significance (4 experiments) Measurements from past 10 years are telling us that there should be a light Higgs, or something that mimics it. Nature would seem particularly malign to point so clearly to something that is not there !! Higgs self-coupling diverges If LEP indication is correct, there must be physics beyond the SM before the GUT scale Higgs potential has 2nd min.

5. Program for Higgs Study 5 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 well (unless Higgs decays dominantly in invisible modes - then the LC finds it). Measure total width, and couplings to all available 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 coupling ? (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 trilinear coupling, with sufficient luminosity LHC should discover a Higgs candidate; LC should discover what it really is. We will likely need both !

6. Higgs studies -- Discovery & Mass 6 Fitted recoil mass from Z ZH WW WW fusion 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). Fitted dijet mass ZH brems Z recoil mass Higgs Z/W couplings fitted Higgs mass WW/ZZ fusion dominates at mass, energy -- measures VV couplings. Can distinguish from ZH using jet tags and missing mass. Missing mass Measuring the lightest Higgs coupling in ZH production tests whether there are additional higher mass Higgs. Total width GTOT to few% for M > 110 GeV. BR(H WW*) = GWW / GTOTGWW from sZH or WW fusion. Test for unexpected Higgs decays 100 150

7. 7 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 < 135 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) 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% (Mh = 120 GeV) BR From likelihood fn’s using vertex, jet mass, shape information BR’s approx. errors MH (lose the ff BRs for MH > 180 GeV, except tt) Measurement of BR’s is powerful indicator of new physics, and senses MA well above ECM. MSSM Higgs BR’s must agree with MSSM parameters from many other measurements. SM value (decoupling limit)

8. 8 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 permit 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 shape l, independent of Higgs mass meas. 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%

9. 9 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 Can’t explain baryon asymmetry in universe … Many possible new theories are 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.

10. Supersymmetry 10 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, CP phases -- 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 for sparticles, generational patterns, etc. Susy can provide a dark matter candidate. 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 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- )

11. 11 Supersymmetry studies at the Linear Collider An example: production of selectron pairs -- have two diagrams; typically the t-channel c0exchangedominates and allows measurement of neutralino couplings (gaugino vs. higgsino) to lepton/slepton. s-channel g/Z process only for eL+ eL- and eR+ eR- . Bkgnd WW suppressed for beam eR- . ~ ~ ~ ~ ~ e+ e+ e+ ~ g,Z e+ c0 ~ ~ e- e- e- e- Decay: ~ e e c10 e distributions for both e- polarizations Upper & lower end points of decay electron energy distribution from gives masses of left and right handed selectrons and neutralino. ~ eL,R e c10 Angular distributions 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.

12. 12 Chargino studies e+ c+ g, Z Masses are again determined from end points in reactions like e+ e- c1+ c1- , with decays: c1+ c10W+ / c10l n / c10q’ q as for previous case (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- 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 (e.g. 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.

13. 13 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. Thresholds for selected sparticle pair productions -- at LHC mSUGRA model points. RED: Accessible at 500 GeV BLUE:added at 1 TeV 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- 1089858368462750 459 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 1482756 1724 1276364 q q 1882 1896 630 1828 1352 1010 ~ ~ ~ ~ ~ ~ These points need updating. New data rules some pts out, but message is still similar ~ ~ 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.

14. 14 Susy breaking mechanism The LC complements the LHC. LHC will see those particles coupling to color, some Higgs & sleptons, lighter gauginos only if present in cascade decays of squarks and gluinos. LC will do sleptons, sneutrinos, gauginos well. Electron polarization 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. Detailed patterns of mass spectra give 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.

15. 15 Precision studies constrain ANY new physics 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 with a light Higgs. 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.

16. 16 Mr = 1240 GeV ; =2500 GeV significance 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 DT dMW=30 MeV, dMTop=2 GeV 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 with Giga Z. MH Strong coupling Observables at LC: 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: a broad resonance is seen at LHC; LC sees modification to ee WW cross section. Technirho relative signal significance for LHC and LC at 500,1000, 1500 GeV

17. 17 Kg lg Extra Dimensions KZ lZ Strong Coupling Gauge Models Expect observable modifications to WWg coupling. For Dkg,Z , LC at 500 GeV has precision 10-20 times better than LHC – in the range expected in Strong Coupling models. gg WW gives orthogonal information of comparable precision. 10-2 Errors on WWg / WWZ coupling for LHC and LC at 500 , 1000 , 1500 GeV error 10-4 Discovery reach for Z’ at LC500 is better or comparable toLHC for different models; better forLC1000 by factor ~2. 10-2 error 10-4 Anomalous top couplings to Z,g are expected, only observable at LC. Fields propagating in extra spatial dimensions give observable consequences – e.g. Kaluza Klein excitations, missing ET signatures, modifications to cross sections Many possible phenomenologies to distinguish, depending on size of extra dimensions and fields propagating in the bulk.

18. 18 Extra Dimensions Large Extra Dimensions: gravity propagates in 4+d dimensions. Modify ee g/Z + unseen GKK or angular distributions in ee ff . LC500 and LHC are comparable in reach for fundamental Planck scale M*; ECM dependence at LC gives d . s(ee gGn) d=6 5 4 3 3 d=2 400 800 600 ECM If supersymmetry in the bulk, KK tower of gravitinos modifies ee ee , sensitive to M* = 12 TeV for d = 6 at LC500 using polarized e-. ~ ~ Polarized g g WW process has larger sensitivity to graviton exchange for large ED than e+e- or LHC. Warped ED/localized gravity: Sensitivity to KK resonances at LC500 is comparable to LHC; LC1000 exceeds LHC. There are many phenomenological models of Extra Dimensions; LC500 sensitivity is roughly comparable to LHC, but gives complementary information needed to unscramble the character of the model.

19. 19 Snowmass studies • LC Resource Book gives several topics for further study: • Scenarios: For different scenarios of physics after 1st years of LHC, what is the optimum LC program ? • Options: What is the need for gg, eg, e-e- operation for different physics scenarios? Benefit from positron polarization? • IR configuration: NLC has low-energy and extendible high energy IR in baseline. TESLA has optional second HE IR. What is the optimal IR strategy, and how is it physics dependent? • Detector issues: Detector cost driver is energy flow calorimetry; quantify the benefit, and seek optimizations. What benefits from fully silicon strip tracking? • Fixed target: What unique physics can done with fixed target experiments using LC beam ? and … • Physics questions: For each sector of LC physics, studies remain to be done (see Resource Book). A few examples: • Fully simulated study of Higgs branching ratios vs. mass • How well can we profile a 200 GeV Higgs ? • Measure top Yukawa coupling at tt threshold ? • How measure the full chargino /neutralino mixing matrices? Can one determine all 105 MSSM parameters? • How well can CP violation in Susy be measured? What limits can be set on lepton flavor violation? • How can LC measurements illuminate Susy breaking mechanism? • How well can one characterize Strong Coupling models ? • Role of gg, eg, e-e- for extra dimensions studies ? • Cases where LC unscrambles LHC confusion on new physics?

20. Summary A Linear Collider, starting at 500 GeV and expanding to higher energy, will bring crucial understanding of the main questions before our field, significantly beyond that obtained at LHC. This workshop offers a timely opportunity for us to assess the capability of the LC, and to guide the future of our field. LC can only be achieved through international cooperation and requires global consensus. I believe that the US community should embrace a LC proposal in the US, but prepare to strongly engage in the LC wherever it can be built.