Transcript Document
Smashing the Standard Model:
Physics at the CERN LHC
Kenneth Johns
University of Arizona
Outline
Opening remarks – 5 min
Destroyed magnets, black hole video
Standard model and Higgs – 12 min
CERN and LHC accelerator – 8 min
ATLAS detector – 5 min
October disaster – 5 min
Higgs – 12 min
Production
Decay
Discovery potential
Other LHC physics and conclusions – 5 min
Total
UA contributions
2
First Beam in the LHC
Sept 10, 2008 in the ATLAS control room
3
First Beam in the LHC
No black hole or stranglet production
4
First Beam in the LHC
No black hole or stranglet production
5
First Malfunction at the LHC
Sept 19, 2008 in the LHC tunnel
6
Physics at the LHC
“There are known knowns. These are
things we know that we know. There
are known unknowns. That is to say,
there are things that we know we don't
know. But there are also unknown
unknowns. There are things we don't
know we don't know.”
Donald Rumsfeld
7
Fundamental Forces
8
Fundamental Particles
9
Fundamental Particles
Or just another pattern to unravel?
10
Standard Model
The Standard Model unifies the strong, weak,
and electromagnetic interactions in the sense
that they all arise from a local symmetry
principle
Local gauge invariance
A minor problem is that the symmetries of the
Standard Model do not allow for massive gauge
bosons
There are no experimental contradictions to
the predictions of the Standard Model, which
is complete in that its mathematical structure
allows calculations to be carried out
Tested to a high precision (1 part in 1000)
11
Standard Model
Local gauge invariance
T hefree particleDirac Lagrangian is given by
L iψ γ μ μψ mψ ψ
We first ask is the theory (L) invariant under
global gauge transformations?
x x ei x
We next ask is the theory (L) invariant under
local gauge transformations?
x x ei x x
12
Standard Model
We can make the theory locally gauge invariant
by introducing a gauge covariant derivative
that includes a gauge field
D ieA
1
where A A A x
e
Now our Lagrangian does remain invariant
under local gauge transformations
Using this derivative leads directly to QED!
And tells us that the photon is massless!
L iψ γμ ψ mψ ψ e A
μ
13
Standard Model
We could apply the same idea to the
weak interaction Lagrangian (SU(2))
We’d find the need for three gauge
covariant derivatives containing three
gauge bosons
We’d like to identify them as the W+, W-,
and Z except they too are massless
14
Standard Model
Spontaneous Symmetry Breaking (SSB) occurs
when a Lagrangian is invariant under some
symmetry but the ground state (vacuum) is not
Pencil falling
Heisenberg ferromagnet
15
2008 Nobel Prize to Nambu for discovering SSB
Standard Model
Higgs mechanism
We have SSB when a Lagrangian is
invariant under some symmetry but the
ground state (vacuum) is not
If the broken symmetry is a continuous
symmetry, then there necessarily exists
one or more massless spin 0 particles
(Goldstone bosons)
If the broken symmetry is a local gauge
symmetry, then the Goldstone bosons get
absorbed (eaten) by the massless gauge
bosons thereby acquiring mass
16
Standard Model
Consider a charged self-interacting complex
scalar field (the Higgs field)
Require the Lagrangian to be locally gauge invariant
1 2
2
2
* 2
L D
2
For 2 > 0 we have QED of charged scalars
For 2 < 0 we have SSB and a continuum of
degenerate vacuum states
2 v2
0
2
2
2
17
Standard Model
The Lagrangian for small perturbations about
the ground state
For v i / 2
And after using a specificgauge transformation
1
qv
2 2
L 2
A A interact.
2
2
A massive scalar (Higgs)
with m2 2 2
2 2
A massive gauge boson
2
2 2
with m A q v
And no massless Goldstone boson
18
Standard Model
Summary
Massive
Higgs
Boson
Higgs
Mechanism
Local
Gauge
Invariance
Massive
Gauge
Bosons
19
Standard Model
An often used
analogy for mass
generation
20
Standard Model Successes
Tested from 10-17 to 1022 cm
No significant deviations (including
quantum corrections) at the 10-3 level
Predicted weak neutral currents –
discovered
Required the existence of W, Z –
discovered
Necessitated charm and top –
discovered
Predicts only 3 neutrino families
21
Standard Model Successes
There are no
experimental
discrepancies with
Standard Model
predictions
But no Higgs boson
observation either
22
Standard Model Parameters
On the other hand, the Standard Model
does contain a lot of parameters
23
Seeking the underlying patterns of matter
The basic constituents of matter are
the 6 quarks and the 6 leptons, and
the 4 carriers of the fundamental
forces. The three quark and lepton
generations have very similar
properties.
All the particles we know of (protons,
neutrons, nuclei, atoms are made
from these simple building blocks.
As far as we know, there are no
smaller units than quarks and leptons.
24
Fundamental Forces
Interactions arise from
Fields (classical field theory)
Exchanged quanta (quantum field theory)
25
Fundamental Fermions
There are three families of leptons
and quarks
26
Fundamental Particles
Or just another pattern to unravel?
e
e
e
e
u u u
d d d
u u u
L
d d d
Z
0
W
L
c c c
s s s
c c c
t t t
b b b
t t t
s s s
t t t
W
g
27
So what is this thing called the
Standard Model we are trying to smash
– and why
Let’s start with the fundamental
particles and their interactions
You’ve seen this many times so I won’t
linger here
28
One of the goals of physics is to
understand the common elements of
these forces and particles
Perhaps they can be unified in the
sense that electricity and magnetism
are unified as electromagnetism
And in fact, in the 1960’s it was shown
that the electromagnetic force and
weak force had a common origin
29