1. What do we hope to understand?
The Physics of the LHC
What do we hope to understand?
3 Dec. 2008
John Huth
Harvard University

2. Right now, the theorists are in the driver’s seat, but
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

3. 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
All seem
to be
related
3 Dec. 2008
John Huth
Harvard University

4. classical electro-magnetism
The problem with
classical electro-magnetism
Classical self-energy of the electron:
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

5. QED!! (quantum electrodynamics)
Quantum Field Theory!
Electromagnetism+quantum mechanics+special relativity =
QED!! (quantum electrodynamics)
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

6. Consequence: virtual photon cloud with electron-positron pairs screen the electron’s charge
Before QED: e
After QED:
Logarithmic terms can be handled through a process called “renormalization”, but not 1/r
3 Dec. 2008
John Huth
Harvard University

7. 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’s and Z are massive, photon
remains massless
3 Dec. 2008
John Huth
Harvard University

8. The W,Z and photon interact with Fermions – leptons and quarks
(3 “generations”)
Q=0
Leptons
Q=-1
Q=2/3
Quarks
Q=-1/3
1st
2nd
3rd
3 Dec. 2008
John Huth
Harvard University

9. Fundamental spin-1 objects
Photon: Massless,
Lorentz invariance
requires only transverse
polarization states
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

10. 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

11. 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

12. 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

13. 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

14. 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

15. Hgg high luminosity (L=10^34)
Discovery should be assured
by LHC operating parameters
3 Dec. 2008
John Huth
Harvard University

16. 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

17. u u d d Strong interactions – QCD (Quantum Chromodynamics) u d g
Force carrier is the massless
gluon – 3 colors, 8 gluons.
Dominates action at LHC
Quark charge is “anti-screened” u d g u u d d
3 Dec. 2008
John Huth
Harvard University

18. 3 Dec. 2008
John Huth
Harvard University

19. Fine tuning problem with the grand
unified scale – supersymmetry predicts
new particle species – “sparticles”
Before supersymmetry H
is supersymmetric cousin of the
top quark
After supersymmetry H
3 Dec. 2008
John Huth
Harvard University

20. 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

21. Convergence of force strength
Without supersymmetry
With supersymmetry
3 Dec. 2008
John Huth
Harvard University

22. Dark Side of the Universe: Dark Matter
Dark (invisible) matter!
Dark Matter
Gasesous Matter
Dark Matter appears to be weakly interacting massive particle
Lightest SUSY particle has these properties !
3 Dec. 2008
John Huth
Harvard University 22 22

23. 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 :
Then
And
Final state is
3 Dec. 2008
John Huth
Harvard University

24. 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

25. Looking for Extra Dimensions: Z’
1 fb-1
3 Dec. 2008
John Huth
Harvard University
T. Virdee, ICHEP08 25 25

26. 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