Download PPT - The Center for High Energy Physics

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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LHC will start operating only in 2008…
Linear Collider will not be ready before 2014…
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…
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
e.g.
Baron Münchausen's
Narrative of his Marvellous
Travels (1795)
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!
•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
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
Standard Model works…
to the 1% level
Precision measurements predicted the top quark
mass just where CDF/DO found it…
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…
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…
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
 new physics at a TeV
• introduce some symmetry into the theory
• supersymmetry
• little Higgs models
 symmetry must be
broken around TeV…
Question of unification of forces:
• Electric + magnetic = electromagnetic
• Electromagnetic + weak = electroweak
• Electroweak + strong = grand unification
• GUT + gravity = super-unification
Running
coupling
constants
SU(5)-based one-step grand unification
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
SuperK has changed the scene  since
neutrinos undergo flavour oscillations they
must have nonzero masses
But the masses are very very small….
11
mt / me ~ 10
Q. How to explain such unnaturally tiny masses?
There is an elegant explanation…
The Seesaw mechanism:
Lmass
 0 Vew  nR 
  nL N L  
   H .c.
Vew M   N R 
Diagonalise:
Vew
  n 2 N
M
Vew
m 
2
2M
M ~ 100 TeV
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…
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
The main purpose of building highenergy 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
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…
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)
Higgs bosons
• At the LHC we are almost sure to find
a light Higgs boson…if it exists…
M h  219GeV @ 95% cl
Produce:
gg  H
H gg
114-160 GeV
H WW
M h  114 GeV @ 95% cl
160-220 GeV
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)
• 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
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
•Higgs branching ratio
measurements are
model-independent
ZZH vertex
LHC
Coupling
measurements
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
Mass measurements
LC @ 350 GeV
• LC:
Reconstruction of Z
M h  120 GeV , 500 fb1
 M h  50 MeV
• LHC:
Direct reconstruction of
h  gg
M h  150 GeV , 300 fb1
 M h  100 MeV
Conway, hep-ph/0203206
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
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?
Top quarks
&
Gauge bosons
LC will be useful in determining top quark properties too…
Can we understand the large top Yukawa coupling?
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
Supersymmetry
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
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
Why, then, do we spend our time on it?
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, …
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.
~ 
0
~
~
qL   2 q  l l
0  
~
 l l q
1
• M0 limits extraction of other
masses
Catania, CMS
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
Extra
Dimensions
Hierarchy problem arises because Planck scale is
so high…
Can the Planck scale be brought down to 1 TeV ?
GN
M
2
P
 10 G
38
meas
N
Absurd! A speck of dust
~ 0.1 mm would weigh
as much as the whole
Earth!
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…
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
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
POSSIBLE SCENARIOS
Bagger et al
hep-ex/0007022
Only the future will tell…
…once we have the machine(s)