Transcript Hunting for the Higgs Boson An introduction to modern day
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).
Quasars 10 billion light years Crab nebula 1000 light years Manchester 1 km 10 26 10 22 10 18 10 14 10 10 10 6 10 2 10 -2 10 -6 10 -10 10 -14 10 -18 metres Andromeda 2 million light years Sun, radius 1 million km Proton 1 trillionth mm Quarks: pointlike?
This doesn’t mean we can understand everything! Some systems are very knowing the basic rules doesn’t help much, e.g. humans.
complex and
Elementary particle physics
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
Photons mediate electric and magnetic forces. (Faraday and Ampère demonstrated that electric and magnetic forces were different manifestations of the same “electromagnetic” force.)
e
e
e
e
Forces are mediated by particles
Photons mediate electric and magnetic forces. (Faraday and Ampère demonstrated that electric and magnetic forces were different manifestations of the same “electromagnetic” force.) Gluons mediate the strong force.
q q g q q
There is also the weak force
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.
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Einstein’s “General Theory of Relativity” superseded Newton’s Theory of Gravity in 1915.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
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 interactions except gravity.
all known
Gravity Theory of Everything?
Standard Model Glashow, Salam, Weinberg Electroweak Weak Strong Electromagnetic Electric Ampere, Faraday, Maxwell Magnetic
What is the Standard Model?
A single and very elegant theoretical framework.
Can describe “everything except gravity” in terms of about 20 parameters.
Has been tested to astonishing precision.
1
137 .
0359895
0 .
0000061
L
1 4 W W 1 4 B B L R
i
g
1 2 W
g
Y 2 B L
i
g
Y 2 B R W, Z, g, kinetic energies & self interactions.
Lepton & quark kinetic energies and their interactions via W, Z, g,
i
g
1 2 W
g
Y 2 B 2 V W, Z and Higgs and couplings.
masses G 1 L R G 2 L
c
R hermitian conjugate Lepton and quark masses and coupling to Higgs .
Building the “Master Equation”
The Standard Model is built on the two pillars of modern physics…….
Einstein’s Theory of Relativity Quantum Theory
Relativity
The speed of light is a universal constant.
Which means that you can never catch up with a beam of light.
Quantum Theory
Particles act like waves?!
The best we can do is predict the probability that something will happen.
The Wavefunction
Elementary particles are described by a quantum wavefunction, Ψ.
2 0.3
0.2
0.1
0 y 1 -2 -1 0 x 1 2 y -1 0 -2 -2 -1 x 0 1 2 Wavefunction biggest
Richard Feynman figured out how to translate the content of the master equation into diagrams…..
W d u e -
The recipe
How did GSW know to write down the Master Equation?
1.
2.
3.
Specify the particles we want to describe.
Relativity & Quantum Theory us how the particles propagate without interactions. automatically tell Which is not very interesting or realistic.
Insist that our model is “ gauge invariant ”.
Symmetry
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
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”. It is quite an abstract symmetry… It corresponds to changing the phase of the wavefunction by an arbitrary amount for each point in space.
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
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
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
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
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
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.
The basic laws which govern the ferromagnet have a rotational symmetry .
Since there is no preferred direction in space.
But at low temperatures the “ground state” of the ferromagnet is NOT rotationally symmetric. Imagine being tiny and living inside a ferromagnet.
The symmetry is said to be hidden .
The Higgs mechanism is analogous: an “invisible” field particles.
(analogous to the magnetic field of the ferromagnet) permeates all space, selectively hindering certain
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 vacuum.
Higgs Boson .
The Higgs boson is the “footprint” of the pervasive field which permeates the
Hunting the Higgs
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?
Basic idea is to use Einstein’s famous relation
E
mc
2 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…
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 + e Z h
e + e Z h
Plenty of media attention…..
But the evidence is not compelling and the search continues at Fermilab…..
Higgs search at Fermilab
Watch this space….
Large Hadron Collider
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 quark antiquark 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.
10 33 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….
http://www.fnal.gov/pub/ferminews/FermiNews98-01 23.pdf
Excellent article on higgs bosons…..
http://theory.ph.man.ac.uk/~forshaw/home.html
This talk….