Role of a Linear Collider after the LHC Findings

1. Role of a Linear Collider after the LHC Findings Sreerup Raychaudhuri Indian Institute of Technology, Kanpur ACFA-8 Daegu, Korea July 11, 2005

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3. LHC will start operating only in 2008… Linear Collider will not be ready before 2014…

4. The LHC will explore an unknown energy regime… What kind of new physics can we expect? It is somewhat like trying to predict the geography of a country which we are just planning to set out to explore…

9. To predict the role of LC after this is a bit like writing a travel diary about this country which we have never visited… • Combination of • information • guesswork • imagination

10. e.g. Baron Münchausen's Narrative of his Marvellous Travels(1795)

11. Q. Why are we so sure that the LHC will discover something new? After all, LEP and the Tevatron discovered nothing but the top quark and the tau neutrino, which were expected anyway… Despite its successes… …the Standard Model is incomplete… …and inconsistent… Ergo, there must be new physics!

12. The second half of the 20th century has seen the overwhelming triumph of gauge theories─ simple, elegant, stable and self-consistent • However, pure gauge theories cannot have massive particles • The Standard Model constructs particle masses by postulating a non-gauge interaction ─ the self- and Yukawa interactions of the Higgs field  electroweak symmetry-breaking

13. Standard Electroweak Model has been verified to great precision… • Z factories • LEP1 and SLC • W measurements at colliders • LEP2 and Tevatron MZ = 91.1875  .0021 GeV MW = 80.451  .033 GeV

14. Standard Model works… to the 1% level

15. Precision measurements predicted the top quark mass just where CDF/DO found it…

16. What is wrong with the Standard Model? • The non-gauge interaction seems to be simple and elegant, but it is not stable and self-consistent when we consider a quantum theory, i.e. loop effects •  hierarchy problem • To make it stable and self-consistent, we need new physics…

17. This is not just a piece of theoretical fussiness…. • The Standard Model is a quantum (field) theory • Even tree-level results are just the lowest order in perturbation theory • One-loop predictions are also tested to great accuracy at LEP etc. • It is meaningless to consider only tree-level results, unless we can prove that higher orders give small contributions • Higher order corrections to Higgs boson mass are very large…

18. Q. How can we protect the Higgs boson mass from these large quantum corrections? • Only two ways: • bring down the cutoff  to the TeV scale • composite models • brane-worlds • introduce some symmetry into the theory • supersymmetry • little Higgs models  new physics at a TeV  symmetry must be broken around TeV…

19. Question of unification of forces: • Electric + magnetic = electromagnetic • Electromagnetic + weak = electroweak • Electroweak + strong = grand unification • GUT + gravity = super-unification Running coupling constants

20. SU(5)-based one-step grand unification

21. Neutrinos… • …have always been a slight embarrassment in the Standard Model • Earlier they were thought to be massless  accommodated in the Standard Model by assuming there is no right-handed neutrino • All that is special about a right-handed neutrino is that it is a gauge singlet • There is as much reason to suppose that gauge singlet fermions exist as there is to suppose that they do not exist • Hence the huge number of models for neutrino mass(es) constructed in the 1980s

22. SuperK has changed the scene  since neutrinos undergo flavour oscillations they must have nonzero masses But the masses are very very small…. Q. How to explain such unnaturally tiny masses?

23. There is an elegant explanation… The Seesaw mechanism: Diagonalise: M ~ 100 TeV

24. Many variations of the simplest seesaw mechanism exist  many of them proposed to explain the large mixing angle found by SuperK many of them require the right-handed neutrino to have some special properties… All require a heavy mass scale  new physics at scales of TeV or higher…

25. Further Hints of New Physics: • CP-Violation: baryon asymmetry • Cold dark matter: what could it be? • Cosmological constant:  > 0 • Belanger’s talk • Heavily dependent on prejudices • Do not indicate the TeV scale per se

26. The main purpose of building high-energy machines like the LHC and LC is TO DISCOVER THE PHYSICS OF THE ELECTROWEAK SYMMETRY-BREAKING SECTOR OF THE STANDARD MODEL, ESPECIALLY THE NEW PHYSICS

27. Complementary methods of discovery • Brute force…. • Increase the energy of the experiment(s) and directly produce the new particles • Indirect ways… • Make precision measurements of particle properties where new physics shows up through quantum effects Complete understanding of the physics requires both approaches to be carried out simultaneously/successively…

28. Once a new particle/effect has been discovered, we immediately face some questions…. • How do we know what it is that we have found? • How do its properties match the predictions? • Does it give any hint of further new things? • How do we set about answering these? • Measure couplings to known fields • Measure its quantum numbers, e.g. spin, parity, CP, … • Measure its self interactions (if relevant)

29. Higgs bosons

30. Produce: gg H • At the LHC we are almost sure to find a light Higgs boson…if itexists… • H gg • 114-160 GeV • H WW • 160-220 GeV

31. Can we miss a light Higgs at the LHC? • Yes. If there are extra loops which cancel the H gg contributions, the decay products will not be seen… • Can affect both production and decay… • If the Higgs resonance is very broad (due to some kind of strong interactions)

32. Just finding/missing a light Higgs boson at the LHC is not enough… • If we don’t? We will have to find another equally good mechanism to generate masses for all elementary particles • If we do find it? We must understand why it is so light… Such understanding can come only from detailed and precise measurements of the Higgs-like properties, e.g. couplings

33. e+e-Zh can produce 40k Higgs/yr No chance of missing it… Clean initial state gives precision Higgs mass measurement Linear Collider is a Higgs Factory! WWh vertex ZZH vertex • Higgs branching ratio measurements are model-independent

34. Coupling measurements LHC LC e+e- LC at s=350 GeV L=500 fb-1, Mh=120 GeV Duhrssen, ATL-PHYS-2003-030 Battaglia & Desch, hep-ph/0101165

35. LC: Reconstruction of Z LHC: Direct reconstruction of LC @ 350 GeV Mass measurements Conway, hep-ph/0203206

36. Other measurements where a linear collider does better: • width measurements • spin, parity and CP measurements • trilinear and quartic couplings, i.e.reconstructing the scalar potential

37. Why, precisely, are such precise measurements needed? The Higgs sector of the Standard Model is the least known and the least explored ─ and the most speculated about… Q. Are there more Higgses? Q. Are Higgses composite? Q. Do Higgses form multiplets of higher symmetries? Q. Do Higgses break higher symmetries? Q. Do Higgses mix with more exotic fields?

38. Top quarks & Gauge bosons

39. LC will be useful in determining top quark properties too… Can we understand the large top Yukawa coupling?

40. LC will make precision measurements of W-boson mass and couplings… Should determine W self-interactions – arises from Higgs self-interactions (indirect probe) GigaZ option

41. Supersymmetry

42. Sparticle spectrum • Spin ½ quarks  spin 0 squarks (pair) • Spin ½ leptons  spin 0 sleptons (pair) • Spin 1 gauge bosons  spin ½ gauginos • Spin 0 Higgs  spin ½ Higgsino many of them form mixed states Wonderful for formal theory… makes quantum theories work Gold mine for experiments… lots of new things to discover Nightmare for phenomenology… 124 unknown parameters

43. Q. Is the LHC sure to find supersymmetry? It is possible for supersymmetry to exist in the decoupling limit, with only a light Higgs (114 – 130 GeV) demanded by the theory

44. Why, then, do we spend our time on it?

46. If sparticles do have masses in the 100 GeV to few TeV range, what are the main issues confronting the LHC and LC? • To find the sparticles • To measure their quantum numbers • To understand the supersymmetry-breaking mechanism • If possible to reduce the number of unknown parameters • mSUGRA, GMSB, AMSB, …

47. Discovery of many SUSY particles is straightforward Untangling spectrum is difficult  all particles are produced together SUSY mass differences arise due to complicated decay chains, e.g. M0 limits extraction of other masses LHC will find surely(?!) find sparticles if they lie within a TeV Catania, CMS

48. Role of the LC… Can study one sparticle at a time… decay chains much simpler… • Measurement of masses from threshold, e.g. charginos • Measurement of spin from angular distributions • Measurement of widths • Measurement of precise couplings • Use of beam polarization to determine chiral structure

49. Extra Dimensions

50. Hierarchy problem arises because Planck scale is so high… Can the Planck scale be brought down to 1 TeV ? Absurd! A speck of dust ~ 0.1 mm would weigh as much as the whole Earth!