Transcript H - Agenda INFN

Riunione CSN4 di autunno
Rome, 22 September 2009
The LHC and the problem of
electroweak breaking
G.F. Giudice
1
Our knowledge of matter and forces
General
relativity
Yang-Mills
theory
EW breaking
sector
gravity
strong & EW
quarks & leptons
particle
masses
2
Our knowledge of matter and forces
General
relativity
Yang-Mills
theory
EW breaking
sector
gravity
strong & EW
quarks & leptons
particle
masses
• Elegance & simplicity:
ruled by gauge symmetry
principle
• Few free parameters
• Fare marvelously when
compared with data
3
Our knowledge of matter and forces
General
relativity
Yang-Mills
theory
EW breaking
sector
gravity
strong & EW
quarks & leptons
particle
masses
• Elegance & simplicity:
ruled by gauge symmetry
principle
• Arbitrary structure not
determined by symmetry
principle
• Few free parameters
• Many free parameters in flavor
sector
• Fare marvelously when
compared with data
• Quantum stability of EW scale
4
Our knowledge of matter and forces
General
relativity
Yang-Mills
theory
EW breaking
sector
gravity
strong & EW
quarks & leptons
particle
masses
• Elegance & simplicity:
ruled by gauge symmetry
principle
• Arbitrary structure not
determined by symmetry
principle
• Few free parameters
• Many free parameters in flavor
sector
• Fare marvelously when
compared with data
• Quantum stability of EW scale
The least satisfying sector is also
largely unexplored by experiments
5
The LHC plays a crucial role in the quest to
expand our understanding of matter and forces
The LHC is a discovery machine aimed at the
exploration of uncharted territories
Not just generic motivations, but solid reasons
to expect new discoveries
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1) EW breaking problem:
Scattering of longitudinal
modes grows with E
 nonsense at E > TeV
E
E
New degrees of freedom or new dynamics must exist
below TeV  solution within reach of the LHC
Higgs mechanism is the most plausible solution
7
2) Quantum stability of the EW scale:
V H    H   H
2
H
2
4
H2 very sensitive to high-energy corrections

H
t
H
 2H  
3GF
2
2
2
m



0.2



t
2
2 2
Goodreason to speculate the
existence of new physics below TeV
 spectacular discoveries at the LHC
8
3) Dark Matter as thermal relic:
Our understanding is limited to 4% of
the energy content of the universe
Good reason to believe that there is
more than the SM
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0.22  m 



k TeV 
2
DM
k
=
128 m 2
H0, T, MP
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
If not a coincidence
 DM at the LHC

9
What do we learn from measuring the Higgs?
LEP: MH > 114.4 GeV (95% CL)
Tevatron: 160 GeV < MH < 170 GeV
(excluded 95% CL)
EW: MH < 153 GeV (95% CL)
10
The two best measurements of
sin2W do not agree
A 0,b
fb
 m H  (230  800)GeV
A (SLD)  m H  (13  65)GeV
This makes the argument for a
light Higgs less compelling
LEPEWWG 07
The possibility of a heavy Higgs or no Higgs is still open
11
Actually, both the lower and upper
limits on mH can be violated for
certain modifications of the SM
Evading mH upper bound from EWPT?
Requires new physics with
T=0.20.3 and S small
S  H W  H B 
T  H  D H H  D H
It can be obtained with a second
Higgs doublet with no vev and
no coupling to fermions or with a
colour-octet Higgs doublet
12
• Evading mH lower bound from LEP?
Controversial signal at 115 GeV
2.3 excess at 98 GeV
ghZZ 
Excess at 100 GeV requires  (SM) 
ghZZ 
2
Bh  bb   0.2
New decay modes (new particles)
Change Higgs nature (mixing with other states)
Limits on unconventional decay modes
h  
mh  117GeV
h  2 jets
mh  113GeV
h  invisible
mh  114 GeV
h  bb bb
mh  110GeV
h      
mh  87GeV
h  anything
mh  82GeV
OPAL
13
The Higgs mass carries
important information on
the underlying theory
14
In the metastable regions, the Higgs mass carries information
relevant for cosmology, since large primordial Higgs
fluctuations are excluded
2-
1-
• Limits on TRH from the absence of thermal fluctuations
• Limits on HI from the absence of quantum fluctuations
during inflation
15
Understanding the mechanism of EW breaking is
crucial for expanding our knowledge of the particle
world
Its discovery has important intellectual implications in
the understanding of the principles of nature
Discovering the Higgs is measuring the quantum
excitations associated with a substance filling the
vacuum
The LHC addresses the properties of the vacuum in
nature
16
Understanding “nothing” has a long history
Western thought was influenced by Artistotle’s proof that
vacuum cannot exist in nature
“Any motion will come to a standstill unless
some external force keeps driving it”
In empty space, why should a body stop in
one place rather than another?
Absurd!
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“Natural motions (without external agent)
make heavy elements (earth, water) fall
down and light elements (air, fire) go up”
How could natural motions exist in empty
space?
The principle of Horror Vacui: Nature will work
in a way to prevent the appearance of vacuum
17
Experimental proof of the
Horror Vacui principle
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Empedocles’
Hydra
a dna ™em i Tkciu Q
rosserpmoced )desserpmocnU ( F FI T
.e rutc ip s iht ees o t dedeen era
18
Experimental proof of the
Horror Vacui principle
Empedocles’
Hydra
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a dna ™em i Tkciu Q
rosserpmoced )desserpmocnU ( F FI T
.e rutc ip s iht ees o t dedeen era
a dna ™em i Tkciu Q
rosserpmoced )desserpmocnU ( F FI T
.e rutc ip s iht ees o t dedeen era
19
Experimental proof of the
Horror Vacui principle
a dna ™em i Tkciu Q
rosserpmoced )desserpmocnU ( F FI T
.e rutc ip s iht ees o t dedeen era
a dna ™em i Tkciu Q
rosserpmoced )desserpmocnU ( F FI T
.e rutc ip s iht ees o t dedeen era
a dna ™em i Tkciu Q
rosserpmoced )desserpmocnU ( F FI T
.e rutc ip s iht ees o t dedeen era
Empedocles’
Hydra
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Experimental proof of the
Horror Vacui principle
a dna ™em i Tkciu Q
rosserpmoced )desserpmocnU ( F FI T
.e rutc ip s iht ees o t dedeen era
a dna ™em i Tkciu Q
rosserpmoced )desserpmocnU ( F FI T
.e rutc ip s iht ees o t dedeen era
a dna ™em i Tkciu Q
rosserpmoced )desserpmocnU ( F FI T
.e rutc ip s iht ees o t dedeen era
a dna ™em i Tkciu Q
rosserpmoced )desserpmocnU ( F FI T
.e rutc ip s iht ees o t dedeen era
Empedocles’
Hydra
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21
It took Torricelli and Pascal to understand
that the effect is due to air pressure
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Evangelista Torricelli’s
experiment (1644)
air
Blaise Pascal’s
vide dans le vide
22
Does nature abhor void after all?
Even if we remove all matter from space, we do not get “nothing”
The walls of the container emit blackbody radiation
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Because of Heisenberg’s
principle, virtual particles are
produced out of nothing
Physical measurable effect: Casimir force
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LHC:
Nature chooses a vacuum made of “something”, rather
than “nothing”, because she saves energy this way
The LHC may discover that nature abhors
“nothingness” and desires to fill “emptiness”
Paradoxically, the LHC will show that horror vacui was
basically right, after all
24
Is the Higgs elementary or composite?
Determine the nature of the force that breaks EW
Elementary
SM (with mH < 190 GeV)
SUSY (H,Q,L are all chiral superfields)
Composite: Higgs is a light
remnant of a strong force
pseudoGoldstone
Little Higgs
Gauge-Higgs unification
Holographic Higgs
….
Signatures at LHC? New resonances, W’,Z’,t’, KK excitations
Distinctive model-independent features of compositeness at the
25
LHC in Higgs interactions
General structure of a composite Higgs
quarks, leptons
& gauge bosons
strong
sector
Communicate via gauge (ga)
and (proto)-Yukawa (i)
In the limit I, ga =0, strong sector contains
Higgs as Goldstone bosons
Ex. H = SU(3)/SU(2)U(1) or H = SO(5)/SO(4)
Higgs boson described by -model of strong EW
dynamics, distorted by gauge and Yukawa interactions
26
Deviations of order v2/f2, where 4f is the
compositeness scale 
10 TeV 2
 10%
The effects in h BRh are of the order of 
  
v2 1

f2 4

Deviations from SM
Higgs couplings can
test v2 / f2 up to
LHC
SLHC
2040 %
10 %
ILC
1 %  4f =
30 TeV
27
Genuine signal of Higgs compositeness
at high energies
In spite of light Higgs, longitudinal gauge-boson
scattering amplitude violates unitarity at high energies
WL
WL
h
Modified coupling
WL
WL
v4
 pp  VLVLX   4  pp  VLVLX H
f
VLVL scattering is an important channel, even for light Higgs

28
Higgs is viewed as pseudoGoldstone boson:
its properties are related to those of the exact
(eaten) Goldstones: O(4) symmetry
Strong gauge-boson scattering
 strong Higgs production
Can bbbb at high invariant mass be separated from background?
h  WW  leptons is more promising
Sum rule (with cuts ||and s>M2):
29
CONCLUSIONS
• The LHC addresses questions of broad and universal
intellectual value
• Discovering the Higgs is not just finding a new particle:
it is unveiling the phenomenon that gives rise to a
fundamental scale in physics
• The discovery of the mechanism of EW breaking will
open new questions, some of which will be addressed
by the LHC
30