What is the Higgs?

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Hunting for the Higgs Boson An introduction to
modern dayelementary particle physicsDr Jeff
Forshaw University of Manchester
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The goal of theoretical physicsis to figure out
the laws thatunderpin all natural
phenomena. From the very largest (galaxies) all
the way to the very smallest (quarks and
leptons).
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metres
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Quasars 10 billion light years
Andromeda 2 million light years
Crab nebula 1000 light years
Sun, radius 1 million km
Manchester 1 km
Proton 1 trillionth mm
Quarks pointlike?
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This doesnt mean we can understand everything!
Some systems are very complex and knowing the
basic rules doesnt help much, e.g. humans.
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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.

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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
toexplode? The (Coulomb) repulsion isdefeated
by a new forceThe STRONG force.
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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.)

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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.

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There is also the weak force

• It is responsible for the process by which two
  protons fuse together in the core of the sun.
• It is carried by the W and Z particles.

Neutrons transform to protons via beta decay. It
is a result of the weakforce.
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Gravity is the only other force.

• It so weak as to be negligible in particle
  physics experiments.
• Einsteins General Theory of Relativity
  superseded Newtons Theory of Gravity in 1915.
• An ultimate theory should explain how gravitons
  mediate gravity.?

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The Standard Model

• The weak and electromagnetic forces were unified
  by Glashow, Weinberg Salam. Electroweak force
• GWS also explained how to incorporateQCD, the
  model of the strong force.
• Their model defines the laws for all
  knowninteractions except gravity.

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Theory of Everything?
Gravity
Standard Model
Strong
Electroweak
Glashow, Salam, Weinberg
Weak
Electromagnetic
Ampere, Faraday, Maxwell
Electric
Magnetic
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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.

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The most precise theory ever developed
W, Z, g, g kinetic energies self interactions.
Lepton quark kinetic energies and their
interactions via W, Z, g, g.
W, Z and Higgs masses and couplings.
Lepton and quark masses and coupling to Higgs.
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Building the Master Equation

• The Standard Model is built on the two pillars of
  modern physics.
• Einsteins Theory of Relativity
• Quantum Theory

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Relativity

• The speed of light is a universalconstant.
• Which means that you can never catch up with a
  beam of light.

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Quantum Theory

• Particles act like waves?!
• The best we can do is predict theprobability
  that something will happen.

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The Wavefunction

• Elementary particles are described by a quantum
  wavefunction, ?.

y
x
y
x
Wavefunction biggest
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Richard Feynman figured out how to translate the
content of the master equation into diagrams..
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The recipe

• How did GSW know to write down the Master
  Equation?
• 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.

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Symmetry

• Symmetry is abundant in Nature.
• Some symmetries relate to shapes in space whilst
  others are more abstract.

e.g. Average A Level scoreis same for females
asfor males. Not an exact symmetry.
e.g. Triangle
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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.

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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
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The problem of mass

• Thats 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.

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Peter Higgs
Gerardus t Hooft
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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.

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Broken versus Hidden symmetry

• A block of ferromagnetic material is unmagnetised
  at high temperature

A lump of ferromagnetic materialis made of a
myriad of tiny magnets (one for each atom). At
high temperature the magnetspoint randomly so
the netmagnetisation is zero.
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Broken versus Hidden symmetry

• A block of ferromagnetic material is magnetised
  at low temperature

At low temperature the magnetsall line up
so the netmagnetisation is not zero.
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Broken versus Hidden symmetry

• A block of ferromagnetic material is magnetised
  at low temperature

After heating the magnet andthen cooling it
again themagnetisation points in adifferent
direction.
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• 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 (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 notreally
  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.

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Hunting the Higgs

• Modern day particle physics experiments are busy
  searching for the higgs particle.
• CERN (Geneva)
• Fermilab (Chicago)

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CERN
Collided electrons with Positrons until the
endof 2000.Will collide protons withprotons
starting around2006.
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Fermilab
Collides protons with antiprotons
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Particle Accelerators
They are quite like hugecathode ray tubes!
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Particle Detectors
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What do the detectors see?
A real event seen by theH1 detector at the
HERAelectron-proton collider inHamburg.
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Why do we need to collide particles at high
energies?

• Basic idea is to use Einsteins famous
  relationto convert energy into mass.
• If we want to produce massive particlesthen 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!)

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Back to searching for the Higgs

• LEP at CERN has seen a handful of possible higgs
  events.
• They hint that there might be a higgsboson with
  mass about 120 times thatof the proton.

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Plenty of media attention..
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• But the evidence is not compelling andthe search
  continues at Fermilab..

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Higgs search at Fermilab
Watch this space.
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Large Hadron Collider

• If Fermilab does not find the higgs boson(e.g.
  because it is too heavy) then thebaton will pass
  to CERNs LHC.
• The collision energy is around 10 times that at
  Fermilab.

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Simulation of a Higgs particledecaying into
apair of Z particleswhich in turn decay into
anelectron-positronpair and a quark-antiquark
pair.
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Beyond the Standard Model
Despite all its successes the Standard Model can
never hope to explain some things. There must be
something more..
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• How does confinement work?

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Is the Higgs particle there? Maybe its not! In
any case, something must show up when we start to
collide particles with energies attainable at the
LHC.
A 5th force?
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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.
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Rather than particles, tiny pieces ofstring
are proposed to be the basic constituents of
matter.
So tiny ( ) that they look like
point particles in our experiments.
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What does string theory do for us?

• Gravity gauge symmetry for free!
• Universe has extra dimensions!
• Not a shred of evidence yet!

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Supersymmetry
For string theory to make sense the Universe must
be supersymmetricLots of new particles may
well becreated at the LHC.
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Sparticle searches.
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Duality
Around 1995, string theorists led by Ed Witten
at Princeton discovered that all the seemingly
different string theories are in factdifferent
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.
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For more information.

• http//www.fnal.gov/pub/ferminews/FermiNews98-01-2
  3.pdf Excellent article on higgs bosons..
• http//theory.ph.man.ac.uk/forshaw/home.htmlThis
  talk.