Hunting for the Higgs Boson

An introduction to modern dayelementary particle physics

Dr Jeff ForshawUniversity of Manchester

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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Quasars10 billion light years

Quarks: pointlike?

Andromeda2 million light years

Crab nebula1000 light years

Sun, radius1 million km

Manchester1 km

Proton1 trillionth mm

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 physicsElementary particle physics

What is matter made of?What is matter made of? How does matter behave at the How does matter behave at the

smallest distances?smallest distances? Today we know that the Universe is Today we know that the Universe is

made up of just a few made up of just a few elementary elementary particlesparticles..

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 force:The STRONG force.

Forces are mediated by Forces are mediated by particlesparticles

Photons Photons mediate electric and mediate electric and magnetic forces. magnetic forces. (Faraday and Amp(Faraday and Ampèère re demonstrated that electric and magnetic forces were demonstrated that electric and magnetic forces were different manifestations of the same “electromagnetic” different manifestations of the same “electromagnetic” force.)force.)

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Forces are mediated by Forces are mediated by particlesparticles

Photons Photons mediate electric and mediate electric and magnetic forces. magnetic forces. (Faraday and Amp(Faraday and Ampèère re demonstrated that electric and magnetic forces were demonstrated that electric and magnetic forces were different manifestations of the same “electromagnetic” different manifestations of the same “electromagnetic” force.)force.)

Gluons Gluons mediate the strong force.mediate the strong force.q

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There is also the There is also the weakweak force force

It is responsible for the process by which two It is responsible for the process by which two protons “fuse” together in the core of the sun.protons “fuse” together in the core of the sun.

It is “carried” by the W and Z particles.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 Gravity is the only other force.force.

It so weak as to be negligible in It so weak as to be negligible in particle physics experiments.particle physics experiments.

Einstein’s “General Theory of Einstein’s “General Theory of Relativity” superseded Newton’s Relativity” superseded Newton’s Theory of Gravity in 1915.Theory of Gravity in 1915.

An “ultimate” theory should explain An “ultimate” theory should explain how how gravitons gravitons mediate gravity…….?mediate gravity…….?

The Standard ModelThe Standard Model

The weak and electromagnetic forces The weak and electromagnetic forces were were unified unified by Glashow, Weinberg &by Glashow, Weinberg &Salam. Salam. Electroweak forceElectroweak force

GWS also explained how to incorporateGWS also explained how to incorporateQCD, the model of the strong force.QCD, the model of the strong force.

Their model defines the laws for Their model defines the laws for allall knownknowninteractions except gravity.interactions except gravity.

Electric Magnetic

ElectromagneticWeak

StrongElectroweak

Gravity

Theory of Everything?

Standard Model

Ampere, Faraday, Maxwell

Glashow, Salam, Weinberg

What is the Standard What is the Standard Model?Model?

A single and very elegant theoretical A single and very elegant theoretical framework.framework.

Can describe “everything except Can describe “everything except gravity” in terms of about 20 gravity” in terms of about 20 parameters.parameters.

Has been tested to astonishing Has been tested to astonishing precision.precision. 0000061.00359895.1371

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Lepton & quark kinetic energies and their interactions via W, Z, g,

W, Z and Higgs masses and couplings.

Lepton and quark masses and coupling to Higgs.

W, Z, g, kinetic energies & self interactions.

Building the “Master Building the “Master Equation”Equation”

The Standard Model is built on the The Standard Model is built on the two pillars of modern physics…….two pillars of modern physics…….

Einstein’s Theory of RelativityEinstein’s Theory of Relativity Quantum TheoryQuantum Theory

RelativityRelativity

The speed of light is a universalThe speed of light is a universalconstant.constant.

Which means that you can never Which means that you can never catch up with a beam of light.catch up with a beam of light.

Quantum TheoryQuantum Theory

Particles act like waves?!Particles act like waves?! The best we can do is predict theThe best we can do is predict the

probability that something will probability that something will happen.happen.

The WavefunctionThe Wavefunction

Elementary particles are described Elementary particles are described by a quantum wavefunction, by a quantum wavefunction, ΨΨ..

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Richard Feynman figured out how to translate the content of the master equation into diagrams…..

The recipeThe recipe

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

1.1. Specify the Specify the particlesparticles we want to describe. we want to describe.2.2. Relativity & Quantum TheoryRelativity & Quantum Theory automatically automatically

tell us how the particles propagate without tell us how the particles propagate without interactions. interactions. Which is not very interesting or realistic.Which is not very interesting or realistic.

3.3. Insist that our model is “Insist that our model is “gauge invariantgauge invariant”. ”.

SymmetrySymmetry

Symmetry is abundant in Nature.Symmetry is abundant in Nature. Some symmetries relate to shapes in Some symmetries relate to shapes in

space whilst others are more space whilst others are more abstract.abstract.

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

Gauge InvarianceGauge Invariance

Is a symmetry of the Master Is a symmetry of the Master Equation, i.e. the Master Equation Equation, i.e. the Master Equation does not change when we change does not change when we change the wavefunction of each particle by the wavefunction of each particle by a “gauge transform” a “gauge transform” Just like the equilateral triangle does Just like the equilateral triangle does

not change when we change it by a “flip transform”. not change when we change it by a “flip transform”.

It is quite an abstract symmetry… It is quite an abstract symmetry… It It

corresponds to changing the phase of the wavefunction by an arbitrary amount for corresponds to changing the phase of the wavefunction by an arbitrary amount for each point in space.each point in space.

But, it can only be a symmetry if we But, it can only be a symmetry if we introduce a introduce a newnew particle for each type of particle for each type of original particle. original particle.

The new particles are the The new particles are the force-carriersforce-carriers, , i.e. photon, gluon, W and Z.i.e. photon, gluon, W and Z.

The particles now The particles now interactinteract with each other with each other as embodied in the “Master Equation”.as embodied in the “Master Equation”.

Almost “for free” Almost “for free” gauge symmetry has gauge symmetry has turned a boring model without turned a boring model without interactions into a powerful model of interactions into a powerful model of nature!nature!

We do NOT yet know the origin of Gauge Symmetry

The problem of massThe 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’ mechanismHiggs’ mechanism

Higgs proposed that empty space Higgs proposed that empty space (vacuum) is not really empty.(vacuum) is not really empty.

Some particles move around Some particles move around unhindered (massless) whilst others unhindered (massless) whilst others are dragged back by the vacuum are dragged back by the vacuum (massive).(massive).

In this way the gauge symmetry is In this way the gauge symmetry is more “hidden” rather than “broken”.more “hidden” rather than “broken”.

Broken versus Hidden Broken versus Hidden symmetrysymmetry

A block of ferromagnetic material is A block of ferromagnetic material is unmagnetised at high temperature: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.

Broken versus Hidden Broken versus Hidden symmetrysymmetry

A block of ferromagnetic material is A block of ferromagnetic material is magnetised at low temperature:magnetised at low temperature:

At low temperature the magnetsall line up so the netmagnetisation is not zero.

Broken versus Hidden Broken versus Hidden symmetrysymmetry

A block of ferromagnetic material is A block of ferromagnetic material is magnetised at low temperature:magnetised at low temperature:

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

The basic laws which govern the The basic laws which govern the ferromagnet have a ferromagnet have a rotational symmetryrotational symmetry..Since there is no preferred direction in space.Since there is no preferred direction in space.

But at low temperatures the “ground But at low temperatures the “ground state” of the ferromagnet is NOT state” of the ferromagnet is NOT rotationally symmetric. rotationally symmetric. Imagine being tiny and living inside a ferromagnet.Imagine being tiny and living inside a ferromagnet.

The symmetry is said to be The symmetry is said to be hiddenhidden.. The Higgs mechanism is analogous: an The Higgs mechanism is analogous: an

“invisible” field “invisible” field (analogous to the magnetic field of the ferromagnet) (analogous to the magnetic field of the ferromagnet)

permeates all space, selectively hindering permeates all space, selectively hindering certain particles.certain particles.

As a result the gauge symmetry is notAs a result the gauge symmetry is notreally broken at all….really broken at all….

And particles can therefore be And particles can therefore be massive.massive.

There is a consequence: There ought There is a consequence: There ought to be a new particle: the to be a new particle: the Higgs BosonHiggs Boson..The Higgs boson is the “footprint” of the pervasive field which permeates the The Higgs boson is the “footprint” of the pervasive field which permeates the vacuum.vacuum.

Hunting the HiggsHunting the Higgs

Modern day particle physics Modern day particle physics experiments are busy searching for experiments are busy searching for the higgs particle.the higgs particle.

CERN (Geneva)CERN (Geneva) Fermilab (Chicago)Fermilab (Chicago)

CERN

Collided electrons withPositrons until the endof 2000.

Will collide protons withprotons starting around2006.

FermilabFermilabCollides protons with antiprotons

Particle AcceleratorsParticle Accelerators

They are quite like hugecathode ray tubes!

Particle DetectorsParticle Detectors

What do the detectors see?What do the detectors see?

A real event seen by theH1 detector at the HERAelectron-proton collider inHamburg.

Why do we need to collide Why do we need to collide particles at high energies?particles at high energies?

Basic idea is to use Einstein’s famous Basic idea is to use Einstein’s famous relationrelationto convert energy into mass. to convert energy into mass.

If we want to produce massive particlesIf we want to produce massive particlesthen we need sufficient incoming then we need sufficient incoming energy.energy.E.g. At Fermilab the collision of a single proton and antiproton is sufficiently E.g. At Fermilab the collision of a single proton and antiproton is sufficiently energetic to produce over 2000 protons. energetic to produce over 2000 protons. At CERN, the electron and positron collided with sufficient energy to produce 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!)over 200 protons (electrons are more than 1000 times lighter than a proton!)

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LEP at CERN has seen a handful of LEP at CERN has seen a handful of possible higgs events.possible higgs events.

They hint that there might be a higgsThey hint that there might be a higgsboson with mass about 120 times boson with mass about 120 times thatthatof the proton.of the proton.

Back to searching for the Back to searching for the Higgs…Higgs…

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Plenty of media attention…..

But the evidence is not compelling But the evidence is not compelling andandthe search continues at Fermilab…..the search continues at Fermilab…..

Higgs search at FermilabHiggs search at Fermilab

Watch this space….

Large Hadron ColliderLarge Hadron Collider

If Fermilab does not find the higgs If Fermilab does not find the higgs bosonboson(e.g. because it is too heavy) then (e.g. because it is too heavy) then thethebaton will pass to CERN’s baton will pass to CERN’s LHCLHC..

The collision energy is around 10 The collision energy is around 10 times times that at Fermilab.that at Fermilab.

Simulation of a Higgs particledecaying into apair of Z particleswhich in turn decay into anelectron-positronpair and a quark-antiquark pair.

Beyond the Standard Model

Despite all its successes theStandard Model can never hopeto 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?

• Why are there 3 generations?

• Are the particles fundamental?

• How does confinement work?

Is the Higgs particle there?

Maybe it’s not!

In any case, something must show up when we start to collide particles with energies attainable at the LHC.

A 5th force?

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 of“string” are proposed to be the basic constituents of matter.

So tiny ( ) that they look like point particles in our experiments.

• Gravity & gauge symmetry for free!

• Universe has extra dimensions!

• Not a shred of evidence yet!

What does string theory do for us?

Supersymmetry

For string theory to make sense theUniverse must be “supersymmetric”

Lots of new particles may well becreated at the LHC….

Sparticle searches….Sparticle searches….

DualityAround 1995, string theorists led by Ed Witten at Princeton discovered that all theseemingly 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 beengiven a name: M-Theory.

M-Theory

11-dimensionalsupergravity

E8xE8heterotic

SO(32)heterotic

Type I

TypeIIB

TypeIIA

For more information….For more information….

http://www.fnal.gov/pub/ferminews/FermiNews98-http://www.fnal.gov/pub/ferminews/FermiNews98-01-23.pdf01-23.pdf Excellent article on higgs bosons….. Excellent article on higgs bosons…..

http://theory.ph.man.ac.uk/~forshaw/home.htmlhttp://theory.ph.man.ac.uk/~forshaw/home.htmlThis talk….This talk….