Elementary Particle Physics
- Lecture Notes
- Weekly Exercises
- Examination Papers
- Particle Properties
- Web Links
- Higgs Challenge
Elementary Particle Physics (EPP | PHY306)
Year: 3 | Semester: B | Level: 6 | Units: 1 | Credits: 15Prerequisites: PHY-215 or equivalent introductory course in quantum physics
Lectures: 30 | Lec: 114 310 312 (notation)
Exam: 2.5 hour written paper (80%), coursework (20%)
Practical work: none | Ancillary teaching: exercises
- An introduction to the standard model of particle physics - the strong and electroweak interactions between the basic constituents of the world, quarks and leptons, via the exchange of gluons, photons and W and Z particles. Recent results on CP violation and neutrino mixing. The search for the Higgs particle. Beyond the standard model - Grand unified theories and supersymmetry.
- The main aim of the Elementary Particle Physics course is to teach the fundamentals of the Standard Model of Particle Physics.
- By the end of the course, the successful student is expected to: be able to describe the basic constituents of the Standard Model, the quarks and leptons and the interactions between them and to be able to use Feynman diagrams to classify and illustrate these interactions; demonstrate the conservation rules, quantum numbers and basic quark parton model upon which the Standard Model is built; be able to describe the basics of electroweak interactions, the Higgs mechanism and CP violation; describe the experimental observation of neutrino mixing and explain its implications for neutrino masses; appreciate the limitations of the Standard Model and describe how some of these limitations are overcome in other models.
Alessandro Bettini Introduction to Elementary Particle Physics Cambridge (2008) ISBN 978-0-521-88021-3 Martin, B.R. & Shaw, G. Particle Physics Wiley (1997) ISBN 0-471-97285-1
Welcome to the Elementary Particle Physics (PHY-300) Home PageFrom Wikipedia, the free encyclopedia:
Particle physics is a branch of physics that studies the existence and interactions of particles that are the constituents of what is usually referred to as matter or radiation. In current understanding, particles are excitations of quantum fields and interact following their dynamics. Most of the interest in this area is in fundamental fields, each of which cannot be described as a bound state of other fields. The current set of fundamental fields and their dynamics are summarized in a theory called the Standard Model, therefore particle physics is largely the study of the Standard Model's particle content and its possible extensions.
The ATLAS Experiment under construction.
TextbooksThe following books contain information relevant to this course.
Introduction to Elementary Particle PhysicsAlessandro Bettini, Cambridge.
Chapter 1 deals with preliminary notions, Chapter 2 with Nucleons, leptons and boson, Chapter 3 with Symmetries,
Chapter 4 with Hadrons, Chapter 5 with Quantum Electrodynamics, Chapter 6 with Chromodynamics, Chapter 7 with Weak Interactions, Chapter 8 with the neutral K and B mesons and CP violation, Chapter 9 with
the Standar Model and Chapter 10 beyond the Standard Model including neutrino mixing. There are also useful
appendices. If you intend to buy a book, buy this one.
Particle PhysicsB.R.Martin and G.Shaw, Wiley. (Second Edition)
This book covers most of course at about the right level but is somewhat dated now and doesn't include new results on CP violation or anything on neutrino oscillations. There are plenty of problems with solutions.
Chapter 1 and 2 deal with Basics, Chapter 3 with Accelerators and Detectors,
Chapter 4 and 5 with Quantum Numbers, Chapter 6 with Quarks, Chapter 7 with the
Strong Interaction, Chapter 8 with Weak Interactions, Chapter 9 with
Electroweak unification and the Higgs mechanism, Chapter 10 with Parity and CP
violation and the neutral kaon system and
Chapter 11 with
Neutrinos, beyond the Standard Model and Dark Matter. There are also useful
Elementary Particle Physics in a NutshellChristopher G Tully, Princeton
This book covers most of course but at a slightly too advanced level for this course. It might be interesting for students who would like to explore the mathematics a bit more. There are plenty of problems with solutions.
Chapter 1 is a brief overview. Chapter 2 deals with the Dirac Equation leading to QED. Chapter 3 introduces the gauge principle. Chapter 4 discusses hadrons and QCD. Chapter 5 introduces detectors and detector techniques. Chapter 6 discusses neutrino interactions and CKM measurements. Chapter 7 reviews electron-positron collider physics and Chapter 8 hadron collider physics. Chapter 9 discusses the Higgs mechanism.
Introduction to High Energy PhysicsD.H.Perkins, Addison Wesley (Third Edition)
Chapter 1 is an Introduction and Overview. Chapter 2 deals with Detectors and
Accelerators, Chapter 3 with Invariance Principles and Conservation Laws,
Chapter 4 with Hadron Hadron Interactions, Chapter 5 with the Quark Model,
Chapter 6 with Electromagnetic Interactions, Chapter 7 with Weak Interactions,
Chapter 8 with Quark Quark Interactions and Chapter 9 with Electroweak
Unification and Other Interactions.
Quarks and LeptonsF.Halzen and A.D.Martin, Wiley
Chapter 1 which summarises Particle Physics and Chapter 2 which deals with the
quark model should however be accessible to good undergraduates.
QED - The Strange Theory of Light and MatterR.P.Feynman, Penguin
The New PhysicsEdited by P.Davis, Cambridge
Chapter 5 deals
with Quantum Gravity, Chapter 14 with the Quark Structure of Matter, Chapter 15
with Grand Unified Theories, Chapter 16 with Quantum Field Theories, Chapter 17
with Gauge Theories in Particle Physics and Chapter 20 provides an overview of
A Brief History of TimeS.W.Hawking, Bantam Press
Lecture NotesThe main topics of the lectures is given below. Since the amount of material varies from topic to topic, the actual lectures will not correspond exactly to each of these. A week by week guide is also given but this will not be strictly adhered to. There is one week set aside for reading week and one for catch up if necessary. In parenthesis, the correspondings sections of the textbooks by Alessandro Bettini (AB) and B.R.Martin and G.Shaw (MS) are shown. Please note that there are some differences between the slides and what discussed in the textbooks. Homeworks and exam papers are actually based on what discussed in the slides.
There are pdf versions of the slides suitable for printing (print), lower resolution (low) and for slideshows (show). The latter maintains the animations on the slides. In some cases there are some additional notes (extra).
[If you have trouble with fonts try installing this font.]
|2||Brief History of Particle Physics||low||show||AB: 2.1, 2.2, 2.4|
|3||Basic Concepts||low||show||AB: 1.5, 1.7, 1.8 MS: 1.1, 1.4, 1.5|
|4||Anti-particles||low||show||AB: 2.5 MS: 1.2|
|5||Feynman Diagrams||Wk 2||low||show||extra||AB: 5.5 MS: 1.3|
|6||Kinematics||low||show||AB: 1.1-4, 1.6 MS: 5.3, A, B|
|7||Particle Accelerators||Wk 3||low||show||AB: 1.10 MS: 3.1|
|8||Particle Detectors||low||show||AB: 1.11 MS: 3.3|
|9||Quantum Numbers||Wk 4||low||show||AB: 3 MS: 4.3, 4.4, 5.1, 5.2|
|Hadrons and the Strong Interaction|
|10||Hadron Hadron Interactions||low||show||AB: 4 MS: 5.3, 5.4|
|11||The Quark Parton Model||low||show||AB: 3.9, 4.6-8 MS: 6.2, 7.5|
|12||Colour and QCD||Wk 5||low||show||AB: 6.3-6 MS: 6.3, 7.1-3|
|13||Heavy Quarks||low||show||AB: 4.9-10, 6.1, 9.9 MS: 3.4, 6.1|
|14||Electromagnetic Interactions||Wk 6||low||show||AB: 5.3, 5.8|
|15||Weak Interactions||low||show||AB: 7 MS: 8, 10.1|
|16||Electroweak Theory||Wk 8||low||show||MS: 9.1-3|
|17||The Higgs Particle||low||show||AB: 9.11 MS: 9.2|
|18||K and B Meson Mixing||Wk 9||low||show||AB: 8.1-3 MS: 10.2|
|19||CP Violation||low||show||AB: 8.4-6 MS: 10.2|
|20||Solar Neutrinos||low||show||AB: 10.3-4 MS: 11.1.2|
|21||Neutrino Oscillations||Wk 10||low||show||AB: 10.1-2 MS: 11.1|
|22||Neutrino Masses||low||show||AB: 10.5-6|
|Beyond the Standard Model|
|23||Beyond the Standard Model||low||show||
|24||Dark Matter and Dark Energy||low||show||AB: 10.6 MS: 11.4|
|25||Grand Unified Theories||Wk 11||low||show||MS: 11.2|
|26||Composite Models and SUSY||low||show||AB: 10.6 MS: 11.3|
|27||Quantum Gravity and Superstrings||low||show||
|Exercise Sheet 1||Hand in January 16|
|Exercise Sheet 2||Hand in January 23|
|Exercise Sheet 3||Hand in January 30|
|Exercise Sheet 4||Hand in February 6|
|Exercise Sheet 5||Hand in February 13|
|Exercise Sheet 6||Hand in February 27|
|Exercise Sheet 7||Hand in March 6|
|Exercise Sheet 8||Hand in March 13|
|Exercise Sheet 9||Hand in March 20|
Examination PapersThese are some previous year's examination papers. Note that the course (and lecturer) changed in 2009-10. The answers are indicative of what is expected rather than complete solutions.
Note that these files are only accessible to Queen Mary staff and students.
Particle PropertiesFull gory details of all imaginable particles can be found at the Particle Data Group.
GlossaryNote: Most of this glossary is copied from the excellent The Particle Adventure at the Lawrence Berkeley Laboratory.
AcceleratorA machine used to accelerate particles to high speeds (and thus high energy compared to their rest-mass energy).
AnnihilationA process in which a particle meets its corresponding antiparticle and both disappear. The energy appears in some other form, perhaps as a different particle and its antiparticle (and their energy), perhaps as many mesons, perhaps as a single neutral boson. The produced particles may be any combination allowed by conservation of energy and momentum and of all the charge types.
AntimatterMaterial made from antifermions. We define the fermions that are common in our universe as matter and their antiparticles as antimatter. In the particle theory there is no a priori distinction between matter and antimatter. The asymmetry of the universe between these two classes of particles is a deep puzzle for which we are not yet completely sure of an explanation.
AntiparticleFor every fermion type there is another fermion type that has exactly the same mass but the opposite value of all other charges (quantum numbers). This is called the antiparticle. For example, the antiparticle of an electron is a particle of positive electric charge called the positron. Bosons also have antiparticles except for those that have zero value for all charges, for example a photon or a composite boson made from a quark and its corresponding antiquark. In this case there is no distinction between the particle and the antiparticle, they are the same object.
AntiquarkThe antiparticle of a quark.
AstrophysicsThe physics of astronomical objects such as stars and galaxies.
B-factoryAn accelerator designed to maximize the production of B mesons. The properties of the B mesons are then studied with special detectors.
BaryonA hadron made from three quarks. The proton (uud) and the neutron (udd) are both baryons. They may also contain additional quark-antiquark pairs.
BeamThe particle stream produced by an accelerator usually clustered in bunches.
Big Bang TheoryThe theory of an expanding universe that begins as an infinitely dense and hot medium. The initial instant is called the Big Bang.
BosonA particle that has integer intrinsic angular momentum (spin) measured in units of h-bar (spin =0, 1, 2, ...). All particles are eitherfermions or bosons. The particles associated with all the fundamental interactions (forces) are bosons. Composite particles with even numbers of fermion constituents (quarks) are also bosons.
Bottom quark (b)The fifth flavour of quark (in order of increasing mass), with electric charge of -1/3.
CERNThe major European international accelerator laboratory located near Geneva, Switzerland.
ChargeA quantum number carried by a particle. Determines whether the particle can participate in an interaction process. A particle with electric charge has electrical interactions; one with h2 charge has h2 interactions, etc.
Charge ConservationThe observation that electric charge is conserved in any process of transformation of one group of particles into another.
Charm Quark (c)The fourth quark (in order of increasing mass), with electric charge +2/3.
ColliderAn accelerator in which two beams traveling in opposite directions are steered together to provide high-energy collisions between the particles in one beam and those in the other.
Colour ChargeThe quantum number that determines participation in h2 interactions. Quarks and gluons carry nonzero colour charges.
ConfinementThe property of the h2 interaction that quarks or gluons are never found separately but only inside colour-neutral composite objects.
ConservationWhen a quantity (e.g. electric charge, energy, or momentum) is conserved, it is the same after a reaction between particles as it was before.
CosmologyThe study of the history of the universe.
Dark MatterMatter that is in space but is not visible to us because it emits no radiation by which to observe it. The motion of stars around the centers of their galaxies implies that about 90% of the matter in a typical galaxy is dark. Physicists speculate that there is also dark matter between the galaxies but this is harder to verify.
DecayA process in which a particle disappears and in its place different particles appear. The sum of the masses of the produced particles is always less than the mass of the original particle.
Down Quark (d)The second flavour of quark (in order of increasing mass), with electric charge -1/3.
Electric ChargeThe quantum number that determines participation in electromagnetic interactions.
Electromagnetic InteractionThe interaction due to electric charge; this includes magnetic interactions.
Electron (e)The least massive electrically-charged particle, hence absolutely stable. It is the most common lepton, with electric charge -1.
Electroweak InteractionIn the Standard Model, electromagnetic and weak interactions are related (unified); physicists use the term electroweak to encompass both of them.
EventWhat occurs when two particles collide or a single particle decays. Particle theories predict the probabilities of various possible events occurring when many similar collisions or decays are studied. They cannot predict the outcome for any single event.
FermilabFermi National Accelerator Laboratory in Batavia, Illinois (near Chicago). Named after particle physics pioneer Enrico Fermi.
FermionAny particle that has odd-half-integer (1/2, 3/2, ...) intrinsic angular momentum (spin), measured in units of h-bar. As a consequence of this peculiar angular momentum, fermions obey a rule called the Pauli Exclusion Principle, which states that no two fermions can exist in the same state at the same time. Many of the properties of ordinary matter arise because of this rule. Electrons, protons, and neutrons are all fermions, as are all the fundamental matter particles, both quarks and leptons.
Fixed-Target ExperimentAn experiment in which the beam of particles from an accelerator is directed at a stationary (or nearly stationary) target. The target may be a solid, a tank containing liquid or gas, or a gas jet.
FlavourThe name used for the different quarks types (up, down, strange, charm, bottom, top) and for the different lepton types (electron, muon, tau). For each charged lepton flavour there is a corresponding neutrino flavour. In other words, flavour is the quantum number that distinguishes the different quark/lepton types. Each flavour of quark and charged lepton has a different mass. For neutrinos we do not yet know if they have a mass or what the masses are.
Fundamental InteractionIn the Standard Model the fundamental interactions are the electromagnetic, weak, strong and gravitational interactions. There is at least one more fundamental interaction in the theory that is responsible for fundamental particle masses. Five interaction types are all that are needed to explain all observed physical phenomena.
Fundamental ParticleA particle with no internal substructure. In the Standard Model the quarks, leptons, photons, gluons, W+ and W- bosons, and the Z bosons are fundamental. All other objects are made from these.
GenerationA set of one of each charge type of quark and lepton, grouped by mass. The first generation contains the up and down quarks, the electron and the electron neutrino.
Gluon (g)The carrier particle of the strong interactions.
Gravitational InteractionThe interaction of particles due to their mass/energy.
GravitonThe carrier particle of the gravitational interactions; not yet directly observed.
HadronA particle made of strongly-interacting constituents (quarks and/or gluons). These include the mesons and baryons. Such particles participate in residual strong interactions.
InteractionA process in which a particle decays or it responds to a force due to the presence of another particle (as in a collision). Also used to mean the underlying property of the theory that causes such effects.
Kaon (K)A meson containing a strange quark and an anti-up (or anti-down) quark, or an anti-strange quark and an up (or down) quark.
LEPThe Large Electron Positron Collider at the CERN laboratory in Geneva, Switzerland.
LeptonA fundamental fermion that does not participate in strong interactions. The electrically-charged leptons are the electron, the muon, the tau, and their antiparticles. Electrically-neutral leptons are called neutrinos.
LHCThe Large Hadron Collider at the CERN laboratory in Geneva, Switzerland. LHC will collide protons into protons at a center-of-mass energy of about 14 TeV. When completed in the year 2009, it will be the most powerful particle accelerator in the world. It is hoped that it will unlock many of the secrets of particle physics.
LinacsAn abbreviation for linear accelerator, that is, an accelerator that has no bends in it.
MassThe rest mass of a particle is the mass defined by the energy of the isolated (free) particle at rest, divided by the speed of light squared. When particle physicists use the word "mass" they always mean the "rest mass" of the object in question.
MesonA hadron made from an even number of quark constituents The basic structure of most mesons is one quark and one antiquark.
MuonThe second flavour of charged leptons (in order of increasing mass), with electric charge -1.
Muon ChamberThe outer layers of a particle detector capable of registering tracks of charged particles. Except for the chargeless neutrinos, only muons reach this layer from the collision point.
NeutralHaving a net charge equal to zero. Unless otherwise specified, it usually refers to electric charge.
NeutrinoA lepton with no electric charge. Neutrinos participate only in weak and gravitational interactions and are therefore very difficult to detect. There are three known types of neutrinos, all of which are very light and could possibly have zero mass.
Neutron (n)A baryon with electric charge zero; it is a fermion with a basic structure of two down quarks and one up quark (held together by gluons). The neutral component of an atomic nucleus is made from neutrons. Different isotopes of the same element are distinguished by having different numbers of neutrons in their nucleus.
NucleusA collection of neutrons and protons that forms the core of an atom (plural: nuclei).
ParticleA subatomic object with a definite mass and charge.
Pauli Exclusion PrincipleFermions obey a rule called the Pauli Exclusion Principle, which states that no two fermions can exist in the same state at the same time.
PhotonThe carrier particle of electromagnetic interactions.
PionThe least massive type of meson, pions can have electric charges of +1, -1, or 0.
Positron (e+)The antiparticle of the electron.
Proton (p)The most common hadron, a baryon with electric charge +1 equal and opposite to that of the electron. Protons have a basic structure of two up quarks and one down quark (bound together by gluons). The nucleus of a hydrogen atom is a proton. A nucleus with electric charge Z contains Z protons; therefore the number of protons is what distinguishes the different chemical elements.
QuantumThe smallest discrete amount of any quantity (plural: quanta).
Quantum Chromodynamics (QCD)The quantum theory of the strong interaction.
Quantum Electrodynamics (QED)The quantum theory of the electromagnetic interaction.
Quantum MechanicsThe laws of physics that apply on very small scales. The essential feature is that electric charge, momentum, and angular momentum,as well as charges, come in discrete amounts called quanta.
Quark (q)A fundamental fermion that has strong interactions. Quarks have electric charge of either +2/3 (up, charm, top) or -1/3 (down, strange, bottom) in units where the proton charge is 1.
Residual InteractionInteraction between objects that do not carry a charge but do contain constituents that have that charge. Although some chemical substances involve electrically-charged ions, much of chemistry is due to residual electromagnetic interactions between electrically-neutral atoms. The residual strong interaction between protons and neutrons, due to the strong charges of their quark constituents, is responsible for the binding of the nucleus.
Rest MassThe rest mass of a particle is the mass defined by the energy of the isolated (free) particle at rest, divided by the speed of light squared. When particle physicists use the word "mass", they always mean the "rest mass" of the object in question.
SLACThe Stanford Linear Accelerator Center in Stanford, California.
SpinIntrinsic angular momentum.
StableDoes not decay. A particle is stable if there exist no processes in which a particle disappears and in its place different particles appear.
Standard ModelPhysicists' name for the theory of fundamental particles and their interactions. It is widely tested and is accepted as correct by particle physicists.
Strange Quark (s)The third flavour of quark (in order of increasing mass), with electric charge -1/3.
Strong interactionThe interaction responsible for binding quarks, antiquarks, and gluons to make hadrons. Residual strong interactions provide the nuclear binding force.
Subatomic ParticleAny particle that is small compared to the size of the atom.
SynchrotronA type of circular accelerator in which the particles travel in synchronized bunches at fixed radius.
TauThe third flavour of charged lepton (in order of increasing mass), with electric charge -1.
Top Quark (t)The sixth flavour of quark (in order of increasing mass), with electric charge 2/3. Its mass is much greater than any other quark or lepton.
TrackThe record of the path of a particle traversing a detector.
TrackingThe reconstruction of a "track" left in a detector by the passage of a particle through the detector.
Uncertainty PrincipleThe quantum principle, first formulated by Heisenberg, that states that is is not possible to know exactly both the position x and the momentum p of an object at the same time. The same is true with energy and time (see virtual particle).
Up Quark (u)The least massive flavour of quark, with electric charge 2/3.
Virtual ParticleA particle that exists only for an extremely brief instant in an intermediary process. Then the Heisenberg Uncertainty Principle allows an apparent violation of the conservation of energy. However, if one sees only the initial decaying particle and the final decay products, one observes that the energy is conserved.
W BosonA carrier particle of the weak interactions. It is involved in all electric-charge-changing weak processes.
Weak InteractionThe interaction responsible for all processes in which flavour changes, hence for the instability of heavy quarks and leptons, and particles that contain them. Weak interactions that do not change flavour (or charge) have also been observed.
Z BosonA carrier particle of the weak interactions. It is involved in all weak processes that do not change flavour.
Higgs ChallengeIn 1993, the then UK Science Minister, William Waldegrave, issued a challence to physicists to answer the questions 'What is the Higgs boson, and why do we want to find it?' on one side of a single sheet of paper.
Bottles of champagne were awarded to the five winning entries at the annual meeting of the British Association for the Advancement of Science. The winning entries taken from Physics World Volume 6 Number 9, are given below.
How Particles Acquire MassBy Mary and Ian Butterworth, Imperial College London, and Doris and Vigdor Teplitz, Southern Methodist University, Dallas, Texas, USA.
The Higgs boson is a hypothesised particle which, if it exists, would give the mechanism by which particles acquire mass.
Matter is made of molecules; molecules of atoms; atoms of a cloud of electrons about one-hundred-millionth of a centimetre and a nucleus about one-hundred-thousandth the size of the electron cloud. The nucleus is made of protons and neutrons. Each proton (or neutron) has about two thousand times the mass of an electron. We know a good deal about why the nucleus is so small. We do not know, however, how the particles get their masses. Why are the masses what they are? Why are the ratios of masses what they are? We can't be said to understand the constituents of matter if we don't have a satisfactory answer to this question.
Peter Higgs has a model in which particle masses arise in a beautiful, but complex, progression. He starts with a particle that has only mass, and no other characteristics, such as charge, that distinguish particles from empty space. We can call his particle H. H interacts with other particles; for example if H is near an electron, there is a force between the two. H is of a class of particles called "bosons". We first attempt a more precise, but non-mathematical statement of the point of the model; then we give explanatory pictures.
In the mathematics of quantum mechanics describing creation and annihilation of elementary particles, as observed at accelerators, particles at particular points arise from "fields" spread over space and time. Higgs found that parameters in the equations for the field associated with the particle H can be chosen in such a way that the lowest energy state of that field (empty space) is one with the field not zero. It is surprising that the field is not zero in empty space, but the result, not an obvious one, is: all particles that can interact with H gain mass from the interaction.
Thus mathematics links the existence of H to a contribution to the mass of all particles with which H interacts. A picture that corresponds to the mathematics is of the lowest energy state, "empty" space, having a crown of H particles with no energy of their own. Other particles get their masses by interacting with this collection of zero-energy H particles. The mass (or inertia or resistance to change in motion) of a particle comes from its being "grabbed at" by Higgs particles when we try and move it.
If particles do get their masses from interacting with the empty space Higgs field, then the Higgs particle must exist; but we can't be certain without finding the Higgs. We have other hints about the Higgs; for example, if it exists, it plays a role in "unifying" different forces. However, we believe that nature could contrive to get the results that would flow from the Higgs in other ways. In fact, proving the Higgs particle does not exist would be scientifically every bit as valuable as proving it does.
These questions, the mechanisms by which particles get their masses, and the relationship amongs different forces of nature, are major ones and so basic to having an understanding of the constituents of matter and the forces among them, that it is hard to see how we can make significant progress in our understanding of the stuff of which the earth is made without answering them.
The Need to Understand MassBy Roger Cashmore Department of Physics, University of Oxford, UK.
What determines the size of objects that we see around us or indeed even the size of ourselves? The answer is the size of the molecules and in turn the atoms that compose these molecules. But what determines the size of the atoms themselves? Quantum theory and atomic physics provide an answer. The size of the atom is determined by the paths of the electrons orbiting the nucleus. The size of those orbits, however, is determined by the mass of the electron. Were the electron's mass smaller, the orbits (and hence all atoms) would be smaller, and consequently everything we see would be smaller.* So understanding the mass of the electron is essential to understanding the size and dimensions of everything around us.
It might be hard to understand the origin of one quantity, that quantity being the mass of the electron. Fortunately nature has given us more than one elementary particle and they come with a wide variety of masses. The lightest particle is the electron and the heaviest particle is believed to be the particle called the top quark, which weighs at least 200,000 times as much as an electron. With this variety of particles and masses we should have a clue to the individual masses of the particles.
Unfortunately if you try and write down a theory of particles and their interactions then the simplist version requires all the masses of the particles to be zero. So on one hand we have a whole variety of masses and on the other a theory in which all masses should be zero. Such conundrums provide the excitement and the challenges of science.
There is, however, one very clever and very elegant solution to this problem, a solution first proposed by Peter Higgs. He proposed that the whole of space is permeated by a field, similar in some ways to the electromagnetic field. As particles move through space they travel through this field, and if they interact with it they acquire what appears to be mass. This is similar to the action of viscous forces felt by particles moving through any thick liquid. the larger the interaction of the particles with the field, the more mass they appear to have. Thus the existence of this field is essential in Higgs' hypothesis for the production of the mass of particles.
We know from quantum theory that fields have particles associated with them, the particle for the electromagnetic field being the photon. So there must be a particle associated with the Higgs field, and this is the Higgs boson. Finding the Higgs boson is thus the key to discovering whether the Higgs field does exist and whether our best hypothesis for the origin of mass is indeed correct.
* Note: This argument is the wrong way round - if the electron's mass were smaller, the orbits would be larger, and everything would be larger.
Politics, Solid State and the HiggsBy David Miller Department of Physics and Astronomy, University College, London, UK.
1. The Higgs MechanismImagine a cocktail party of political party workers who are uniformly distributed across the floor, all talking to their nearest neighbours. The ex-Prime Minister enters and crosses the room. All of the workers in her neighbourhood are strongly attracted to her and cluster round her. As she moves she attracts the people she comes close to, while the ones she has left return to their even spacing. Because of the knot of people always clustered around her she acquires a greater mass than normal, that is she has more momentum for the same speed of movement across the room. Once moving she is hard to stop, and once stopped she is harder to get moving again because the clustering process has to be restarted.
In three dimensions, and with the complications of relativity, this is the Higgs mechanism. In order to give particles mass, a background field is invented which becomes locally distorted whenever a particle moves through it. The distortion - the clustering of the field around the particle - generates the particle's mass. The idea comes directly from the physics of solids. instead of a field spread throughout all space a solid contains a lattice of positively charged crystal atoms. When an electron moves through the lattice the atoms are attracted to it, causing the electron's effective mass to be as much as 40 times bigger than the mass of a free electron.
The postulated Higgs field in the vacuum is a sort of hypothetical lattice which fills our Universe. We need it because otherwise we cannot explain why the Z and W particles which carry the weak interactions are so heavy while the photon which carries electromagnetic forces is massless.
2. The Higgs BosonNow consider a rumour passing through our room full of uniformly spread political workers. Those near the door hear of it first and cluster together to get the details, then they turn and move closer to their next neighbours who want to know about it too. A wave of clustering passes through the room. It may spread to all the corners or it may form a compact bunch which carries the news along a line of workers from the door to some dignitary at the other side of the room. Since the information is carried by clusters of people, and since it was clustering that gave extra mass to the ex-Prime Minister, then the rumour-carrying clusters also have mass.
The Higgs boson is predicted to be just such a clustering in the Higgs field. We will find it much easier to believe that the field exists, and that the mechanism for giving other particles is true, if we actually see the Higgs particle itself. Again, there are analogies in the physics of solids. A crystal lattice can carry waves of clustering without needing an electron to move and attract the atoms. These waves can behave as if they are particles. They are called phonons and they too are bosons.
There could be a Higgs mechanism, and a Higgs field throughout our Universe, without there being a Higgs boson. The next generation of colliders will sort this out.
Of Particles, Pencils and UnificationBy Tom Kibble Department of Physics, Imperial College, London, UK.
Theoretical physicists always aim for unification. Newton recognised that the fall of an apple, the tides and the orbits of the planets as aspects of a single phenomenon, gravity. Maxwell unified electricity, magnetism and light. Each synthesis extends our understanding and leads eventually to new applications.
In the 1960s the time was ripe for a further step. We had a marvellously accurate theory of electromagnetic forces, quantum electrodynamics, or QED, a quantum version of Maxwell's theory. In it, electromagnetic forces are seen as due to the exchange between electrically charged particles of photons, packets (or quanta) of electromagnetic waves. (The distinction between particle and wave has disappeared in quantum theory.) The "weak" forces, involved in radioactivity and in the Sun's power generation, are in many ways very similar, save for being much weaker and restricted in range. A beautiful unified theory of weak and electromagnetic forces was proposed in 1967 by Steven Weinberg and Abdus Salam (independently). The weak forces are due to the exchange of W and Z particles. Their short range, and apparent weakness at ordinary ranges, is because, unlike the photon, the W and Z are, by our standards, very massive particles, 100 times heavier than a hydrogen atom.
The "electro-weak" theory has been convincingly verified, in particular by the discovery of the W and Z at CERN in 1983, and by many tests of the properties. However, the origin of their masses remains mysterious. Our best guess is the "Higgs mechanism" - but that aspect of the theory remains untested.
The fundamental theory exhibits a beautiful symmetry between W, Z and photon. But this is a spontaneously broken symmetry. Spontaneous symmetry breaking is a ubiquitous phenomenon. For example, a pencil balanced on its tip shows complete rotational symmetry - it looks the same from every side. - but when it falls it must do in some particular direction, breaking the symmetry. We think the masses of the W and Z (and of the electron) arise through a similar mechanism. It is thought there are "pencils" throughout space, even in vacuum. (of course, these are not real physical pencils - they represent the "Higgs field" - nor is their direction a direction in real physical space, but the analogy is fairly close.) The pencils are all coupled together, so that they all tend to fall in the same direction. Their presence in the vacuum influences waves travelling through it. The waves have of course a direction in space, but they also have a "direction" in this conceptual space. In some "directions", waves have to move the pencils too, so they are more sluggish; those waves are the W and Z quanta.
The theory can be tested, because it suggests that there should be another kind of wave, a wave in the pencils alone, where they are bouncing up and down. That wave is the Higgs particle. Finding it would confirm that we really do understand the origin of mass, and allow us to put the capstone on the electro-weak theory, filling in the few remaining gaps.
Once the theory is complete, we can hope to build further on it: a longer-term goal is a unified theory involving also the "strong" interactions that bind protons and neutrons together in atomic nuclei - and if we are really optimistic, even gravity, seemingly the hardest force to bring into the unified scheme.
There are strong hints that a "grand unified" synthesis is possible, but the details are still very vague. Finding the Higgs would give us very significant clues to the nature of that greater synthesis.
Ripples at the Heart of PhysicsBy Simon Hands Theory Division, CERN, Geneva, Switzerland.
The Higgs boson is an undiscovered elementary particle, thought to be a vital piece of the closely fitting jigsaw of particle physics. Like all particles, it has wave properties akin to those ripples on the surface of a pond which has been disturbed; indeed, only when the ripples travel as a well defined group is it sensible to speak of a particle at all. In quantum language the analogue of the water surface which carries the waves is called a field. Each type of particle has its own corresponding field.
The Higgs field is a particularly simple one - it has the same properties viewed from every direction, and in important respects is indistinguishable from empty space. Thus physicists conceive of the Higgs field being "switched on", pervading all of space and endowing it with "grain" like that of a plank of wood. The direction of the grain in undetectable, and only becomes important once the Higgs' interactions with other particles are taken into account. for instance, particles called vector bosons can travel with the grain, in which case they move easily for large distances and may be observed as photons - that is, particles of light that we can see or record using a camera; or against, in which case their effective range is much shorter, and we call them W or Z particles. These play a central role in the physics of nuclear reactions, such as those occurring in the core of the sun.
The Higgs field enables us to view these apparently unrelated phenomenon as two sides of the same coin; both may be described in terms of the properties of the same vector bosons. When particles of matter such as electrons or quarks (elementary constituents of protons and neutrons, which in turn constitute the atomic nucleus) travel through the grain, they are constantly flipped "head-over-heels". this forces them to move more slowly than their natural speed, that of light, by making them heavy. We believe the Higgs field responsible for endowing virtually all the matter we know about with mass.
Like most analogies, the wood-grain one is persuasive but flawed: we should think of the grain as not defining a direction in everyday three-dimensional space, but rather in some abstract internal space populated by various kinds of vector boson, electron and quark.
The Higgs' ability to fill space with its mysterious presence makes it a vital component in more ambitious theories of how the Universe burst into existence out of some initial quantum fluctuation, and why the Universe prefers to be filled with matter rather than anti-matter; that is, why there is something rather than nothing. To constrain these ideas more rigorously, and indeed flesh out the whole picture, it is important to find evidence for the Higgs field at first hand - in other words, find the boson. There are unanswered questions: the Higgs' very simplicity and versatility, beloved of theorists, makes it hard to pin down. How many Higgs particles are there? Might it/they be made from still more elementary components? Most crucial, how heavy is it? Our current knowledge can only put its mass roughly between that of an iron atom and three times that of a uranium atom. This is a completely new form of matter about whose nature we still have only vague hints and speculations and its discovery is the most exciting prospect in contemporary particle physics.