Transcript What is the Higgs? - University of Manchester

Hunting for the Higgs Boson
An introduction to modern day
elementary particle physics
Dr Jeff Forshaw
University of Manchester
The goal of theoretical physics
is to figure out the laws that
underpin all natural phenomena.
From the very largest (galaxies) all the way to
the very smallest (quarks and leptons).
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Quasars
10 billion light years
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metres
Andromeda
2 million light years
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Crab nebula
1000 light years
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Sun, radius
1 million km
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Manchester
1 km
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1 light year = 10 m
Proton
1 trillionth mm
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Quarks: pointlike?
This doesn’t mean we can
understand everything!
Some systems are very complex and
knowing the basic rules doesn’t help
much, e.g. humans.
Elementary particle physics
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What is matter made of?
How does matter behave at the smallest
distances?
Today we know that the Universe is made
up of just a few elementary particles.
Protons and neutrons are made up
of quarks bound together by gluons.
Like charges repel, so why does
the positive charge within a
proton not cause the proton to
explode?
The (Coulomb) repulsion is
defeated by a new force:
The STRONG force.
Forces are mediated by particles
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Photons mediate electric and magnetic
forces. (Faraday and Ampère demonstrated that electric and
magnetic forces were different manifestations of the same
“electromagnetic” force.)
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Forces are mediated by particles
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Photons mediate electric and magnetic
forces. (Faraday and Ampère demonstrated that electric and
magnetic forces were different manifestations of the same
“electromagnetic” force.)
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Gluons mediate the strong force.
q
q
q
g
q
There is also the weak force
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It is responsible for the process by which two
protons “fuse” together in the core of the sun.
p  p  p  n  e   e
It is “carried” by the W and Z particles.
Neutrons transform to protons via
beta decay. It is a result of the weak
force.
Gravity is the only other force.
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It so weak as to be negligible in particle
physics experiments.
Einstein’s “General Theory of Relativity”
superseded Newton’s Theory of Gravity in
1915.
An “ultimate” theory should explain how
gravitons mediate gravity…….?
The Standard Model
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The weak and electromagnetic forces
were unified by Glashow, Weinberg &
Salam. Electroweak force
GWS also explained how to incorporate
QCD, the model of the strong force.
Their model defines the laws for all known
interactions except gravity.
Theory of Everything?
Standard Model
Gravity
Electroweak
Strong
Glashow, Salam, Weinberg
Weak
Electromagnetic
Ampere, Faraday, Maxwell
Electric
Magnetic
What is the Standard Model?
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A single and very elegant theoretical
framework.
Can describe “everything except gravity”
in terms of about 20 parameters.
Has been tested to astonishing precision.
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 137.0359895  0.0000061
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L   W W  B  B 
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W, Z, g,  kinetic energies &
self interactions.
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 L   i   g  W  g  B  L
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 R   i   g  B  R
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Lepton & quark kinetic
energies and their
interactions via W, Z, g, 
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  i   g  W  g  B     V  
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W, Z and Higgs masses
and couplings.
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 G1 LR  G 2 L c R  hermitian conjugate
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Lepton and quark masses
and coupling to Higgs.
Building the “Master Equation”
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The Standard Model is built on the two
pillars of modern physics…….
Einstein’s Theory of Relativity
Quantum Theory
Relativity
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The speed of light is a universal
constant.
Which means that you can never catch up
with a beam of light.
Quantum Theory
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Particles act like waves?!
The best we can do is predict the
probability that something will happen.
The Wavefunction
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Elementary particles are described by a
quantum wavefunction, Ψ.
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Wavefunction
biggest
Richard Feynman figured out how to
translate the content of the master
equation into diagrams…..
W
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d
u
The recipe
How did GSW know to write down the Master
Equation?
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Specify the particles we want to describe.
Relativity & Quantum Theory automatically tell
us how the particles propagate without
interactions. Which is not very interesting or realistic.
Insist that our model is “gauge invariant”.
Symmetry
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Symmetry is abundant in Nature.
Some symmetries relate to shapes in
space whilst others are more abstract.
e.g. Triangle
e.g. Average A Level score
is same for females as
for males. Not an exact symmetry.
Gauge Invariance
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Is a symmetry of the Master Equation, i.e.
the Master Equation does not change
when we change the wavefunction of each
particle by a “gauge transform” Just like the equilateral
triangle does not change when we change it by a “flip transform”.
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It is quite an abstract symmetry… It corresponds to
changing the phase of the wavefunction by an arbitrary amount for each point in space.
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But, it can only be a symmetry if we
introduce a new particle for each type of
original particle.
The new particles are the force-carriers,
i.e. photon, gluon, W and Z.
The particles now interact with each other
as embodied in the “Master Equation”.
Almost “for free” gauge symmetry has
turned a boring model without interactions
into a powerful model of nature!
We do NOT yet know the origin of Gauge Symmetry
The problem of mass
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That’s almost the whole story….
But the gauge symmetry of the Standard Model forbids
particles from having mass since a mass term in the
Master Equation “breaks” gauge invariance.
Q. So where does mass come from?
A. From the non-trivial action of
the vacuum.
Peter Higgs
Gerardus ‘t Hooft
Higgs’ mechanism
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Higgs proposed that empty space
(vacuum) is not really empty.
Some particles move around unhindered
(massless) whilst others are dragged back
by the vacuum (massive).
In this way the gauge symmetry is more
“hidden” rather than “broken”.
Broken versus Hidden symmetry
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A block of ferromagnetic material is
unmagnetised at high temperature:
A lump of ferromagnetic material
is made of a myriad of tiny magnets
(one for each atom).
At high temperature the magnets
point randomly so the net
magnetisation is zero.
Broken versus Hidden symmetry
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A block of ferromagnetic material is
magnetised at low temperature:
At low temperature the magnets
all line up so the net
magnetisation is not zero.
Broken versus Hidden symmetry
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A block of ferromagnetic material is
magnetised at low temperature:
After heating the magnet and
then cooling it again the
magnetisation points in a
different direction.
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The basic laws which govern the ferromagnet
have a rotational symmetry.
Since there is no preferred direction in space.
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But at low temperatures the “ground state” of
the ferromagnet is NOT rotationally symmetric.
Imagine being tiny and living inside a ferromagnet.
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The symmetry is said to be hidden.
The Higgs mechanism is analogous: an
“invisible” field (analogous to the magnetic field of the ferromagnet)
permeates all space, selectively hindering certain
particles.
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As a result the gauge symmetry is not
really broken at all….
And particles can therefore be massive.
There is a consequence: There ought to
be a new particle: the Higgs Boson.
The Higgs boson is the “footprint” of the pervasive field which permeates the
vacuum.
Hunting the Higgs
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Modern day particle physics experiments
are busy searching for the higgs particle.
CERN (Geneva)
Fermilab (Chicago)
CERN
Collided electrons with
Positrons until the end
of 2000.
Will collide protons with
protons starting around
2006.
Fermilab
Collides protons with antiprotons
Particle Accelerators
They are quite like huge
cathode ray tubes!
Particle Detectors
What do the detectors see?
A real event seen by the
H1 detector at the HERA
electron-proton collider in
Hamburg.
Why do we need to collide particles
at high energies?
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Basic idea is to use Einstein’s famous
2
relation E  mc
to convert energy into mass.
If we want to produce massive particles
then we need sufficient incoming energy.
E.g. At Fermilab the collision of a single proton and antiproton is sufficiently energetic
to produce over 2000 protons.
At CERN, the electron and positron collided with sufficient energy to produce over
200 protons (electrons are more than 1000 times lighter than a proton!)
Back to searching for the
Higgs…
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LEP at CERN has seen a handful of
possible higgs events.
They hint that there might be a higgs
boson with mass about 120 times that
of the proton.
e
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Z
e
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Z
Plenty of media attention…..
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But the evidence is not compelling and
the search continues at Fermilab…..
Higgs search at Fermilab
Watch this space….
Large Hadron Collider
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If Fermilab does not find the higgs boson
(e.g. because it is too heavy) then the
baton will pass to CERN’s LHC.
The collision energy is around 10 times
that at Fermilab.
Simulation of a
Higgs particle
decaying into a
pair of Z particles
which in turn
decay into an
electron-positron
pair and a quarkantiquark pair.
Beyond the Standard Model
Despite all its successes the
Standard Model can never hope
to explain some things.
There must be something more…..
• There are lots (more than 20) free
parameters whose values are not explained.
• What is the origin of gauge symmetry?
• How does confinement work?
• Why are there 3 generations?
• Are the particles fundamental?
Is the Higgs particle there?
Maybe it’s not!
A 5th force?
In any case, something must show up
when we start to collide particles with
energies attainable at the LHC.
Beyond Particles: String Theory
& Quantum Gravity
Since Einstein, a dream of particle physicists
has been to find a single theory that explains
all natural phenomena, including gravity.
Over the years string theory has emerged as
the undisputed leader in the pursuit for a
Theory of Everything.
Rather than particles, tiny pieces of
“string” are proposed to be the basic
constituents of matter.
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So tiny (10 cm) that they look like
point particles in our experiments.
What does string theory do for us?
• Gravity & gauge symmetry for free!
• Universe has extra dimensions!
• Not a shred of evidence yet!
Supersymmetry
For string theory to make sense the
Universe must be “supersymmetric”
Lots of new particles may well be
created at the LHC….
Sparticle searches….
Duality
Around 1995, string theorists led by Ed
Witten at Princeton discovered that all the
seemingly different string theories are in fact
different aspects of the same theory!
To date, nobody has managed to write down
the underlying theory. Although it has been
given a name: M-Theory.
11-dimensional
supergravity
Type
IIA
Type
IIB
M-Theory
E8xE8
heterotic
SO(32)
heterotic
Type I
For more information….
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http://www.fnal.gov/pub/ferminews/FermiNews98-0123.pdf Excellent article on higgs bosons…..
http://theory.ph.man.ac.uk/~forshaw/home.html
This talk….