The Physics of the LHC
What do we hope to understand?
3 Dec. 2008
John Huth
Harvard University
Martinus Veltman – 1980
Right now, the theorists are in the driver’s seat, but
in thirty years, to make any progress at all in particle
physics, we absolutely need input from experiments.
Context – this was when a high energy hadron
collider was envisaged as a “world machine” to
explore the energy scale of 100 GeV to 1 TeV,
the “symmetry breaking sector”.
3 Dec. 2008
John Huth
Harvard University
How did we get here?
Progress toward a unified theory of nature.
Fundamental particles
Fundamental interactions
Space, time
Quantum mechanics
The structure of the Universe
3 Dec. 2008
John Huth
Harvard University
All seem
to be
related
The problem with
classical electro-magnetism
Classical self-energy of the electron:
e

e2
ECoulomb 
re
Given the current limits on
the “size” of the electron,
some new physics has to
intervene to keep its mass
small (relative to known scales),
yet give it a finite mass.
What new physics?
3 Dec. 2008
John Huth
Harvard University
Quantum Field Theory!
Electromagnetism+quantum mechanics+special relativity =
QED!! (quantum electrodynamics)
e

e
Implication: A new form
of matter emerges called
“anti-matter”, which solves
the problem of the electron
self-energy.
How?
3 Dec. 2008
John Huth
Harvard University
Consequence: virtual photon cloud with
electron-positron pairs screen the
electron’s charge
Before QED:
e2
ECoulomb 
re
e
After QED:
1
E  e me log
me re
2
3 Dec. 2008
Logarithmic terms can be handled through
a process called “renormalization”, but not
1/r
John Huth
Harvard University
This might be the end of the story,
But…
Gravity: a relativistic quantum treatment is difficult
Relevant scale: Planck mass
1019 times the proton mass
Weak interactions: Experiment: from β decay, charged
current interaction part of an isotriplet state, where the
photon is included.
  W 
 

o 
W
Z 


3 Dec. 2008
W’s and Z are massive, photon
remains massless
John Huth
Harvard University
The W,Z and photon interact with
Fermions – leptons and quarks
(3 “generations”)
Q=0
Q=-1
Q=2/3
Q=-1/3
3 Dec. 2008
 e 
  
e 
  
  
 
  
  
 
u
 
d 
c
 
s
t 
 
b
1st
2nd
3rd
John Huth
Harvard University
Leptons
Quarks
Fundamental spin-1 objects
p
Photon: Massless,
Lorentz invariance
requires only transverse
polarization states
p
W,Z: Massive, add
longitudinal polarization
state
Issue: longitudinal polarization state grows with
momentum. What are the implications?
3 Dec. 2008
John Huth
Harvard University
ISSUE: processes like WW scattering
exceed unitarity above energy of 1 TeV
Cannot have a consistent
theory with massive spin-1
particles.
The solution? An initially massless theory,
where mass arises as a result of interactions
3 Dec. 2008
John Huth
Harvard University
One version: the Higgs boson
The Higgs boson is
a spin 0 object that
interacts with the spin 1
force carriers and gives
them mass – longitudinal
polarization states.
Quarks and leptons, too.
Shape of interaction
potential
3 Dec. 2008
John Huth
Harvard University
Peculiarities of the Higgs model
Coupling strength is proportional to mass.
Mass is inertial mass (what about gravity?)
The potential is a minimum with a non-zero field
(so-called “vacuum expectation value” – VEV),
denoted by Λ
Λ has been invoked to explain the “flatness” of
the universe – inflation. But, at a much different
scale – 1015 GeV, not 103 GeV
Likewise another value of Λ has been used to
explain dark energy – milli eV
3 Dec. 2008
John Huth
Harvard University
Data prefer light Higgs
Combination of precision
data – masses of W, Z,
top quark and other fits –
Conclude that:
Mh< 207 GeV
Direct search limit from
e+e-Zh
3 Dec. 2008
John Huth
Harvard University
Making the Higgs at the LHC
Decay modes – WW, ZZ, γγ,
pairs of b quarks, perhaps top,
if massive enough
3 Dec. 2008
John Huth
Harvard University
H
high luminosity
(L=10^34)
Discovery should be assured
by LHC operating parameters
3 Dec. 2008
John Huth
Harvard University
Possible problems with the Higgs
• Unappealing
– “The toilet of the standard model”
• Alternatives abound
– Mass generated dynamically
–
Technicolor, gravity
• Naturalness
– If unification includes the strong force,
problems arise – similar to the self-energy of the
electron
3 Dec. 2008
John Huth
Harvard University
Strong interactions – QCD
(Quantum Chromodynamics)
Force carrier is the massless
gluon – 3 colors, 8 gluons.
Dominates action at LHC
u
d
Quark charge is “anti-screened”
g
g
g
g
3 Dec. 2008
u
g
d
u
u
d
d
John Huth
Harvard University
3 Dec. 2008
John Huth
Harvard University
Fine tuning problem with the grand
unified scale – supersymmetry predicts
new particle species – “sparticles”
_
Before supersymmetry
t
ht2
EH 
rH
H
~t
t
is supersymmetric cousin of the
top quark
After supersymmetry
1
E H  h mt log
mt rH
2
t
H
3 Dec. 2008
~t
John Huth
Harvard University
Consequences of SUSY
• Preservation of “low” masses of particles
compared to the grand unified scale
• Unification of forces actually line up
• Doubling of number of particle species
– Mirrored by spin – ½ change
• Lighest supersymmetric partner consistent
with dark matter
3 Dec. 2008
John Huth
Harvard University
Convergence of force strength
Evolution of Coupling Constants in the SM
70
60
50
40
1/ a
Without supersymmetry
30
20
a3
a2
a1
10
0
0
10
5
10
10
10
Mass(GeV)
15
10
Evolution of Coupling Constants in SUSY
70
a3
a2
a1
60
50
With supersymmetry
1/ a
40
30
20
10
0
0
10
3 Dec. 2008
5
10
10
10
Mass(GeV)
15
10
20
10
John Huth
Harvard University
Dark Side of the Universe:
Dark Matter
Dark
(invisible)
matter!
Gasesous Matter
Dark Matter
3 Dec. 2008
Dark Matter appears to be weakly interacting
massive particle
Lightest
John HuthSUSY particle has these properties !
Harvard University
22
Example of a SUSY event at the LHC
Use SUSY cascades to the
stable LSP to sort out the
new spectroscopy.
Decay chain used is :

 1o 
20 




Then
And
b  20  b
g b b
Final state is
bb
3 Dec. 2008
John Huth
Harvard University



 10
Burning questions:
• Is there a Higgs? What is its mass
• Is there another symmetry breaking
mechanism?
• Is nature supersymmetric?
– If so, in what way?
• Tie ins to cosmology
• Is gravity involved (hidden spatial
dimensions)?
3 Dec. 2008
John Huth
Harvard University
Looking for Extra
Dimensions: Z’
1 fb-1
3 Dec. 2008
John Huth
Harvard
T. Virdee,University
ICHEP08
25
Summary
• The energy scale probed at the LHC offers
the answers to a large number of
questions that have perplexed physicists
for over forty years.
• Only experiment can clear up these
issues!
3 Dec. 2008
John Huth
Harvard University