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

2.       

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. Combination of information guesswork imagination
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. Baron Münchausen's Narrative of his Marvellous Travels (1795)
e.g.
Baron Münchausen's Narrative of his Marvellous Travels (1795)

11. Despite its successes…
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 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 =  GeV MW =  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. bring down the cutoff  to the TeV scale
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. But the masses are very very small….
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. At the LHC we are almost sure to find a light Higgs boson…if it exists…
Produce: gg  H
H gg
GeV
H WW
GeV

31. Can affect both production and decay…
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. We must understand why it is so light…
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. Linear Collider is a Higgs Factory!
e+e-Zh can produce 40k Higgs/yr
No chance of missing it…
Clean initial state gives precision Higgs mass measurement
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
Battaglia & Desch, hep-ph/

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

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. many of them form mixed states
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. To measure their quantum numbers
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. LHC will find surely(?!) find sparticles if they lie within a TeV
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
Catania, CMS

48. Can study one sparticle at a time… decay chains much simpler…
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!

51. Would be very exciting, if true…
This miracle can be achieved if there are compact extra space-like dimensions
&
Our experiments are confined within a wafer-thin slice of the extra dimensions
Would be very exciting, if true…
Concept of spacetime would (again) change
Quantum gravity at the doorstep…

52. LHC would see all of these
Experimentally
There could be missing energy from gravitons flying off into extra dimensions…
Alternatively there could be multiple graviton resonances…
Or there might be black holes produced in the laboratory…
LHC would see all of these

53. Gravitons (graviscalars) could lie beyond the kinematic reach
Q. What role will a linear collider play?
Measurement of spin-2 nature of gravitons
Differentiation between LED and warped gravity models
Differentiation between gravity and other interactions, e,g. extra Z bosons
Determination of number of extra dimensions
Studies are still under way
Gravitons (graviscalars) could lie beyond the kinematic reach

54. POSSIBLE SCENARIOS
Bagger et al
hep-ex/

55. Only the future will tell…
…once we have the machine(s)