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the-higgs-boson-and-beyond.org
Summer Science Exhibition30th June 2014 to 6th July 2014
THE ROYAL SOCIETY 6 Carlton House Terrace, London SW1Y 5AG
Exhibit coordinators: Wahid Bhimji, Cristina Lazzeroni, Konstantinos NikolopoulosBooklet editor: Karl Harrison
Artwork and design: Rebecca Pitt
The Higgs boson and beyond was made possible through help and supportfrom the participating institutes (left), and from:
CERN: European Laboratory for Particle PhysicsGridPP: UK Computing for Particle Physics
Higgs Centre for Theoretical PhysicsSTFC: Science and Technology Facilities Council
SEPnet: South East Physics Network
1st edition - June 2014 2nd edition - September 2014
Brunel UniversityJo Cole
Peter HobsonAkram KhanPaul KyberdDawn Leslie
CERNQuentin King
Imperial College LondonLouie Corpe
Paul DaunceyAdinda de WitPatrick DunneRebecca LaneRobyn Lucas
Sasha NikitenkoMonica Vazquez Acosta
Lancaster UniversityHarald Fox
Kathryn GrimmRoger Jones
Queen Mary, University of London
Cristiano AlpigianiLucio CerritoTeppei Katori
Ruth SandbachGiacomo Snidero
Tom Whyntie
Royal Holloway, University of London
Tracey BerryVeronique Boisvert
Ian ConnellyMichele Faucci-Giannelli
Russell KirkPedro Teixeira-Dias
Joshuha Thomas-Wilsker
Rutherford Appleton LaboratoryJohn Baines
Alastair DewhurstJens Dopke
Stephen HaywoodJelena IlicJulie Kirk
Stephen McMahonDavid Sankey
Monika Wielers
University College LondonJonathan Butterworth
Rebecca ChislettBen Cooper
Gavin HeskethAndreas KornJosh McFayden
Ines OchoaTim Scanlon
David Wardrope
University of BirminghamLudovica Aperio Bella
Matthew BacaAndrew Chisholm
Simon HeadCristina LazzeroniTom McLaughlan
Richard MuddJavier Murillo
Konstantinos Nikolopoulos
University of BristolRobin Aggleton
Paolo BaessoEuan Cowie
Maarten van DijkSudarshan Paramesvaran
Daniel Saunders
University of CambridgeMiguel Arratia-Munoz
Giovanna Cottin-BuracchioSteve Green
Karl HarrisonSteven Kaneti
Thibaut MuellerRebecca Pitt
Stephen WottonBoruo Xu
University of EdinburghSahra BhimjiWahid Bhimji
Flavia DiasVictoria Martin
Benjamin Wynne
University of GlasgowDavid Britton
Thomas Doherty Tony Doyle
University of LiverpoolJohn Anders
Carl GwilliamMatthew Jackson
Max KleinPaul LaycockAllan Lehan
Monica d’Onofrio Joe Price
Joost Voosebeld
University of ManchesterIain HaugthonSteve Marsden
Clara NellistJames Robinson
Michaela Queitsch-MaitlandSabah Salih
Stefan Söldner-RemboldTerry Wyatt
University of OxfordAlan Barr
Kathryn Boast Daniela Bortoletto
Philip BurrowsAlexandru Dafinca
Claire GwenlanChris HaysDavid Hall
James HendersonMalcolm JohnCraig Sawyer
University of SheffieldChristos Anastopoulos
Ian DawsonGary Fletcher
Dan Tovey
University of SussexCarlos Chavez Barajas
Antonella DeSantoDaniel GibbonDorothy Lamb
Fabrizio SalvatoreNicky Santoyo-Castillo
Yusufu ShehuKerim Suruliz
Iacopo Vivarelli
University of WarwickSteve Boyd
Sinead FarringtonMichal KrepsTim Martin Bill Murray
Elisabetta Pianori
Exhibit teamThe Higgs boson and beyond was organised, with help from a few friends, by researchers from the UK Particle-Physics
groups that collaborate on the ATLAS and CMS experiments, at the Large Hadron Collider, near Geneva.
The institutes and people involved with the exhibit were as follows:
Highlighting the signature in white, with blue text -- for a darker back-ground. I’ve boosted the cyan component here, to reduce the ‘muddy’ effect.
An alternative: reversing the colours.
One alternative is to use a monochrome two-tone or full-white version
Suggested variants on the final Higgs Centre logoPR 17 Apr 2013
2 3
The discovery of the Higgs boson was a momentous occasion, but what lies beyond
could be even more exciting.
What we know Konstantinos Nikolopoulos
The observation of a new particle is always a major event, but what do we really know about the particle announced in July 2012? The detailed work to map out this particle's basic properties began immediately after the discovery.
Thanks to the excellent performance of the Large Hadron Collider, and of the ATLAS and
CMS experiments, we have been able to establish that the new particle closely resembles the Higgs boson predicted by the Standard Model, the set of theories that describes the physics of elementary
particles.
Previously detected elementary particles behave as if they're spinning. Measurements indicate
that the new particle has no spin, matching the theoretical expectations for the Higgs boson.
The new particle decays rapidly, to pairs of force carriers (W bosons, Z bosons or photons),
or to a matter particle and corresponding anti-matter particle. Within the current
experimental precision, of about twenty to thirty per cent, the measured decay rates are
in agreement with calculations in the framework of the Standard Model.
Studies to date set the stage for even more detailed investigations in the future, with
upgraded accelerator and detectors. We will be looking for deviations from the theoretical
predictions, which would signal physics beyond the Standard Model, a really exciting possibility!
The Higgs boson and beyond
The existence of the Higgs boson was first suggested in 1964, as part of a theory that
explains how elementary particles, the point-like building blocks of nature, can have non-zero
mass. Its observation by the ATLAS and CMS experiments, at the Large Hadron Collider, in
Geneva, was a scientific milestone. Subsequent studies, with more data, have consolidated the discovery, and prompted the award of
the 2013 Nobel Prize in Physics to two of the key contributors to the original theoretical
developments: François Englert and Peter Higgs.
In August 2013, when it was looking likely that the year's Nobel Prize in Physics would be awarded for work relating to the Higgs boson,
Kostas, Wahid and I had discussions with other particle physicists about how we could highlight the science behind the prize. It quickly became
clear that what we wanted to show was how the discovery of the Higgs marks the end of one journey, but the start of an exciting new journey, to unknown lands. The result is The Higgs boson
and beyond. Like its predecessor, this has involved researchers from all of the eighteen UK institutes participating in the ATLAS and CMS experiments.
Why, then, is the Higgs boson so relevant? Well, without it our world would be very different.
Elementary particles would all have zero mass, with dramatic consequences. For example,
electrons would be unable to enter into orbits around protons and neutrons, and so atoms
couldn't form. The physics of elementary particles ultimately underlies everything we know and
experience, and this is why I find it so fascinating. I hope that you too will find fascinating The Higgs
boson and beyond.
What comes next Wahid Bhimji
People often ask me if the discovery of the Higgs Boson means that our work is done. Actually, this discovery doesn't come close to answering all of our questions about the Universe. For one thing, the fact that the Higgs particle isn't as heavy as
theory suggests, is a strong hint there's something still to be discovered, for example the new particles
introduced by the theory of supersymmetry. Also, astronomical measurements indicate that most of the mass of the Universe is in the form of dark
matter, and we don't know what this is. Ultimately, we want to create a Theory of Everything, which
brings together our understanding of elementary particles, and our understanding of gravity, which
is key for describing the motion of galaxies and planets. So there's still a lot to do, and we'll need to create even higher energies, and even bigger
machines, to do it!
Why it matters Cristina Lazzeroni
Putting together a science exhibit is hard work, but one thing I really like is that it makes me think
about what I do, and why it's relevant. So when a new particle, the Higgs boson, was discovered,
in 2012, I jumped at the opportunity of telling everybody all about it. This led to the staging of Understanding the Higgs boson at the 2013 Royal
Society Summer Science Exhibition.
4 5
How the world is
built
10-15 m to 10-14 m Protons and neutrons
combine in atomic nuclei
10-10 m Protons and neutrons, in atomic nuclei, combine with electrons to form atoms (electric charge: 0)
The diameter of an atom is more than
10,000 times the diameter of its nucleus.
<10-18 m Elementary
particles – no detected structure
-1/3
down quark
electron
-1
10-15 m Quarks combine as protons and
neutrons
neutron (0)
proton (+1)
oxygen nucleus oxygen atom
IonsAn atom that loses or gains electrons, and so becomes
electrically charged, is an ion.
All everyday objects seem to be made from just three types of basic building block – the up quark, the
down quark, and the electron.
+
+
+
+
+
+
++
++
+
++
+++
The up quark, down quark and electron are the basic
building blocks of everyday objects.
The electron neutrino is involved in radioactive
decays, and in processes that power the stars.
Elements and isotopes (collections of atoms)Atoms grouped together form an element if they all have the same number of protons. If they also all have the same number of neutrons, they form a particular isotope of the
element. A single element can have several naturally occurring isotopes.
Lead atom 82 protons
24.1%124 neutrons
1.4%122 neutrons
52.4%126 neutrons
22.1%125 neutrons
Naturally occurring
isotopes of lead
up quark
+2/3
relative electric charge
electron neutrino
0
A human cell contains 46 DNA double helixes
and around2 x 1014 atoms.
10-10 m to 10-8 m Atoms combine to
form molecules
10-7 m to 10-4 m Atoms and
molecules combine to form microscopic
structures
10-3 m Small-scale structures,
visible to the eye
50%sodium
50%chlorine
1018 atoms
51%oxygen
3%other
14%silicon
15%magnesium
17%iron
1050 atoms
Powers Of TenPhysics deals both with very big numbers and with very
small numbers. These numbers are often shown as multiples of 10 to some power, written as a superscript.
DNAPairs of deoxyribonucleic acid (DNA) molecules
form a double-helix structure that encodes genetic information in living organisms.
In humans, each structure contains 2 x 109 to 8 x 109 atoms
of just 5 different types: hydrogen, oxygen, carbon, nitrogen, and phosphorus.
7 x 1027
atoms
62%hydrogen
24%oxygen
12%carbon
2%other
Human Planet Earth
Grain of Salt
water molecule
hydrogen atoms
oxygen atom
100
1
102
100
104
10000
106
1000000
10–4
0.0001
10–2
0.0110–6
0.000001
106 m to 108 m Planets
10-2 m to 103 m Macroscopic structures
6 7
FERMIONS
H~+
H~-
H~0 h
~0 A~0
W~ +
W~ -
Z~0γ~0 G
~
g~
g~
g~
AN
TI-
LEPT
ON
SLE
PTO
NS
d s b
QU
AR
KS
d– s– b–
AN
TI-
QU
AR
KS
AN
TI-
PAR
TIC
LES
PAR
TIC
LES
Winos and Zino
photino
gluinos
Force carriers for gravity
gravitino
GAUGINOS
HIGGSINOS
e+ µ+ τ+
positron anti-muon anti-tau
e µ τ
anti-tauneutrino
anti-muonneutrino
anti-electronneutrino
e µ τ
tauneutrino
muonneutrino
electronneutrino
e- µ- τ -
electron muon tau
tcu
topcharmup
down strange bottom
anti-down anti-strange anti-bottom
u– c– t–
anti-up anti-charm anti-top
Particles produced at accelerators.
Building blocks of everyday objects.
g~
g~
g~
g~
g~
Particles of the Standard Model
Only these particles have been detected experimentally.
MATTER PARTICLES
1st generation
2nd generation
3rdgeneration
Particle Universe
Elementary particles, objects with no detected structure, include
quarks, leptons, force carriers, and the Higgs boson.
Anything made of anti-quarks and
anti-leptons is antimatter
Anything made of quarks and leptons
is matterStandard Model
of Particle PhysicsThe Standard Model describes how particles behave under three
types of force: strong force, weak force, and electromagnetic force. Each force has an associated charge, and only particles
that carry this charge feel the effect of the force.
BOSONS
H-
H+
t~
c~
u~
b~
s~
d~
b–~
s–~
d–~
t–~ c–
~u–~
e~+τ~+ µ~+
V–~
µV–~τ V
–~e
V~
τ
e~-τ~- µ~-
W+
W-
Z0 γG
SLEPTO
NS
SQU
AR
KS
AN
TI-SQ
UA
RK
SA
NT
I-SLEPTO
NS
g
g
A0 h0 H0
selectronneutrino
smuonneutrino
stauneutrino
stau smuon selectron
supscharmstop
sbottom sstrange sdown
anti-sbottom anti-sstrange anti-sdown
anti-stop anti-scharm anti-sup
anti-stau anti-smuon anti-spositron
anti-spositronneutrino
anti-smuonneutrino
anti-stauneutrino
FORCE CARRIERS (GAUGE BOSONS)
W and Z bosons
photongraviton
gluons
g
g
g
g
Carriers of strong force
Carriers of weak force
Carrier of electromagnetic
force
V~
µ V~
e
gg
Higgs boson of the
Standard Model
Particles outside of the Standard Model
None of these particles have been detected experimentally. The graviton is introduced by theories of gravity. Other particles are added
by supersymmetry.
AN
TI-PA
RT
ICLE
SPA
RT
ICLE
S
HadronsQuarks and anti-quarks aren't detected as free particles, but
are bound together in composite objects, as one of three types of hadron. These are baryons (three quarks), anti-baryons (three anti-quarks) and mesons (quark and anti-quark).
Supersymmetry Theories for improving on the Standard Model have been
developed around a concept called supersymmetry. This adds additional Higgs bosons, and makes the boson world a mirror image of the fermion world. The simplest formulation is the Minimal Supersymmetric Model. As they haven't so far been
detected, supersymmetry particles, if they exist, must be heavier than their partners in the Standard Model.
Particles and Anti-ParticlesEach particle has an anti-particle, with identical mass but oppositely signed charge values. In some cases, particle
and anti-particle are the same.
HIGGS PARTICLES
subject to electromagnetic force
subject to weak force
subject to strong force
Forces that affect particles
FERMIONS BOSONS
spin = , , ... 1_2
3_2
5_2
spin = 0 spin = 1, 2, 3,...
Fermions and Bosons Particles behave as if they're spinning, and are identified as fermions or bosons depending on the amount of spin. Spin
values are integer or half-integer multiples of a quantity known as the Dirac constant. Fermions can always be
told apart, for example in terms of energy or direction of spin, but bosons may be identical. The whole of chemistry
depends on the fact that electrons are fermions.
8 9
For an interaction or decay to be fully described by a diagram, the particles involved need to be indicated.
Each line and vertex is shorthand for a lengthy mathematical expression. Multiplying together the expressions for all parts of the diagram gives a measure of how often the process represented occurs.
Interactions and decays Particles announce their presence in two types of processes:
interactions and decays.
+ +
Interaction Decay
Z0
µ-
µ+
Particle dynamics
Feynman diagramsParticle interactions and decays are represented pictorially in Feynman diagrams. These use different types of line
to identify different types of particle, and show what happens as time advances from left to right.
The theories of particle intreactions and decays
bring together many of the big ideas of
twentieth-century physics.
A point where lines meet is called a vertex, and represents an interaction. The Standard Model allows only a few types of vertex.
carrier of strong forcematter particle Higgs boson carrier of electroweak force
In an interaction, two particles may scatter off one another, or may give up their energies for the creation of new particles.
In a decay, a particle of higher mass disintegrates,
leaving behind two, or more, new particles, of
lower mass.
gluon
gluont
t
H0
photon
photon
Production of Higgs boson, followed by
decay to two photons
These are specific types of Relativistic quantum field theory.
They're the basis of the Standard Model.
Quantum chromodynamics
Deals with: Strong force
Big ideas: The strong force is carried by gluons.
Particles that feel the strong force possess a type of charge called colour charge.
Electroweak theoryDeals with:
Electromagnetic and weak forces, combined as electroweak forces
Big ideas: The weak force is carried by W± and Z0 bosons.
Different forces can be described in a single mathematical framework (unification).
Particles and anti-particles gain mass by interacting with an energy field, present everywhere in the Universe.
Particles and anti-particles can behave differently.
Quantum electrodynamics
Deals with: Electromagnetic force – interaction of light and matter
Big ideas: The electromagnetic force is carried by photons.
Each particle has an anti-particle, with identical mass but
opposite charge.
Relativistic quantum field theory
Deals with: The very small and very fast
Big ideas: Particle interactions and decays involve emission and absorption of force carriers (gauge bosons).
Descriptions of interactions and decays are unchanged by certain mathematical operations (gauge symmetry).
Special relativityDeals with: The very fast
Big ideas: At high speeds, lengths shorten, and time advances more slowly.
The speed of light, c, is a universal constant.
Energy, E, and mass, m, are equivalent: E = mc2.
Quantum mechanicsDeals with: The very small
Big ideas: Energy comes in packages (quanta).
Particles behave sometimes like point objects and sometimes like waves (complementarity).
All properties of a particle can't be exactly measured at the same time
(uncertainty principle).
A field is a region in time and space, where the value of a quantity is defined at
every point. Relativistic quantum field theory
describes fields of force and energy.
10 11
Mass and the Higgs
boson
Each generation seems to be a replica of the first, but at higher mass. Only the first generation is needed to explain everyday matter (atoms). The reason for there
being three generations is unknown.
Particle masses are expressed as their energy equivalent. An electronvolt is the energy gained by an electron in pasing through 1 volt, and corresponds to a mass of
1.78 x 10-36 kg .
Inertial mass measures resistance to change in speed or
direction:
F = ma
Gravitational mass measures ability to create,
and be influenced by, gravitational forces:
Mmr2
F = G
MassIn everyday language, mass and weight are often used
interchangeably. Technically, the weight of an object is the force on the object due to gravity. Weighing scales measure
this force, and convert to a mass value.
Mass-energy equivalenceAn object's energy content, E, and mass, m, are related by
the speed of light, c:
E = mc2
m mass
a acceleration
F force
m
mass
r distance
M mass
universal gravitational
constant
Elementary particles acquire mass through interactions with an energy field, where
the Higgs boson acts as energy carrier.
105 106 107 108 109 1010 1011 1012
MASS (electronvolts)
e-u
d
s
µ-
b
c
τ- t
Z0
H0
29
29 34
Cu (Copper)
1
1
H (Hydrogen)
Au (Gold)
79
79 118
8
8 8
O (Oxygen)
electrons
protons
neutrons
FIR
STG
ENER
AT
ION
SEC
ON
DG
ENER
AT
ION
TH
IRD
GEN
ERA
TIO
N
gau
ge
boso
ns
atom
s
charged leptonsdown-type quarks
up-type quarks
com
bin
e to
form
ato
m
s
Particle Masses
qu
arks
& c
har
ged
lep
ton
s
W±
The mass of a hydrogen atom is much higher than the combined masses of the electron and quarks from which it is built. Most of the mass of an atom comes from the energy of holding quarks together inside protons and neutrons
EE
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E
EE
EE
EE
EE
EE
EE
E
EE
EE
EE
EE
EE
EE
E
EE
EE
EE
EE
EE
EE
E
EE
EE
EE
particle with no mass Higgs field
An energy field, known as the Higgs field, is present everywhere
in the Universe
The Higgs boson is a short-lived particle that transfers energy between the Higgs field
and other particles: it is an energy carrier
The energy that a particle gains from the Higgs field is detected as the particle's mass.
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EE
EE
E
EE
EE
EE
EE
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EE
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EE
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E
H
H
H
E
HE
The theory of the Higgs field is able to explain how elementary particles
can have non-zero mas. Why different particles have different
masses remains a mystery.?1 Theory of electroweak
force, unifying electromagnetic and
weak forces, developed by Sheldon Glashow.
This theory introduced the W± and Z0 as carriers
of the weak force. The formulation required that
these have zero mass, inconsistent with the
force's short range.
2Mass mechanism, explaining how the carriers of the weak force can have non-zero mass, proposed by six physicists,
in three groups.
The physicists involved were: Robert Brout and François Englert; Peter Higgs; Gerald Guralnik, Carl Hagen and Tom Kibble. The proposed explanation required a new type of
particle: the Higgs boson.
3 Mass machanism shown
by Abdus Salam and Steven Weinberg to
be able to account also for the masses of quarks
and leptons.
4Mathematical consistency
of electroweak theory, including the mass
mechanism, formally proven by Gerard 't Hooft
and Martinus Veltman
5W± and Z0 first detected,
by UA1 and UA2 experiments.
6 Higgs boson first detected,
by ATLAS and CMS experiments.
1961 1964 19671972
1983
2012
The Higgs boson and the origin of mass
12 13
Colliding particles
The European Laboratory for Particle Physics (CERN) hosts the world's highest-energy
particle accelerator: the Large Hadron Collider.
Particle sourcesElectrons can be obtained by heating a metal wire
(thermionic emission), for example by passing an electric current through it.
Protons can be obtained from hydrogen gas, using a device called a duoplasmatron. This generates electric fields that break down the hydrogen molecules, separating them into
protons and electrons.
Creating particle beamsCharged particles can be accelerated to higher energies using electric fields, and can be steered using electric and magnetic fields. The particles can then be focused
into beams.
Highest energies are achieved using radiofrequency cavities. These are metal chambers, containing an electric
field with peaks and troughs that switch many times a second. As a particle moves through a chamber, it's pushed along in much the way that a surfer is pushed by waves on the sea. The peaks and troughs cause particles in a beam
to group together in bunches.
Particle energies are measured in multiples of the electronvolt (eV), the energy gained by an electron
accelerated through 1 volt.
kiloelectronvolt (keV) = 1,000 eV
megaelectronvolt (MeV) = 1,000,000 eV
gigaelectronvolt (GeV) = 1,000,000,000 eV
teraelectronvolt (TeV) = 1,000,000,000,000 eV
Particle acceleratorsTwo designs of particle accelerator
commonly used are the linear accelerator and the synchrotron. Particles are sent
once along a straight-line path in a linear accelerator, and move many times around
a circular path in a synchrotron. Both types of accelerator may be operated as colliders, where beams of particles,
travelling in opposite directions are made to cross. At each crossing, some fraction of
the particles may interact.
Collider parametersWhen two particles collide, their energies can be used in the creation of new particles. At a collision energy E, the probability of creating a particle of type X is represented by a quantity known as the cross section, σ(X,E). This is
conventionally measured in multiples of a unit called the barn (b), where 1 b = 10-28 m2. The collision rate is expressed
in terms of a luminosity, L, measured in units of inverse barn per second (b-1 s-1).
electric field
Luminosity, L ≈ n2
A × t
Bunch of n particles
Time separation, t
Beam area, A
Crossing point
The rate, N(X), of collisions where a particle of type X is created is the product of cross section and luminosity:
N(X) = σ(X,E) × L.
The Large Hadron ColliderThe Large Hadron Collider (LHC) is the highest-energy machine
in an arrangement of interlinked particle accelerators. Four main experiments are positioned around the collider ring: ATLAS, ALICE,
CMS, and LHCb. During its first period of operation (November 2009 to February 2013), the LHC collided protons at energies of
up to 8 TeV (4 TeV per beam). Following an upgrade, the collision energy will be increased to the design value of 14 TeV.
ATLAS2
3
4
PROTON SYNCHROTRON
BOOSTER (1972)
PROTON SYNCHROTRON
(1959)
SUPER PROTON SYNCHROTRON
(1976)
5
LARGE HADRON COLLIDER (2008)
Proton 1
Proton 2
LHCbALICE
CMS
1
LINEAR ACCELERATOR (First beams: 1978)
Circ
umference: 628 m
157 m
CERN's current linear accelerator replaced an earlier machine, operated 1959 to 1992
6.911 km
26.659 km
25 GeV
1.4 GeV
450 GeV
50 MeV
33m90
keV
Up to 7 TeV
0.5%X-ray
synchrotrons
More than 30,000
particle accelerators worldwide
6%Biomedical
research
0.5%Physics
research44%
Radiotherapy
9%Industrial processing
40% Ion
implantation
1514
Detecting particles
The ATLAS and CMS experiments use giant detectors
for precise measurements of particle collisions.
Basics of particle detectionWhen a particle passes through a piece of material, it
may interact with the electrons and quarks from which the material is built. The particle then transfers energy to the material at points along its path. A particle detector
is designed to measure how much energy is deposited by particles that cross it, where, and when. A detector can be
characterised by how well it measures energy and position, by time response, and by the types of particle it detects.
The retina of the eye and the sensor of a digital camera are both examples of particle detectors. They measure energy deposited by photons, at different points, over a short time
interval. They then generate electrical signals to identify points where this energy is above a threshold.
Bunches of protons cross up to 40,000,000 times a second in experiments at the Large Hadron Collider. The experiment detectors must be reset after each crossing.
Proton beam 1 arrives here
Collisions Proton beams cross in the middle of the detector. Particles produced in resulting
collisions travel through the different detector layers
Proton beam 2arrives here
ATLAS and CMSThe detectors of ATLAS and CMS, the two general-purpose experiments at the Large Hadron Collider, have a similar
basic design. Each is roughly cylindrical in shape, and built up in layers. The two detectors differ in the technology
used. Detector sizes reflect the distances over which particles must be measured for accurate determination of
paths, momentum and energy.
46 m25 m
mass: 7,000,000 kg
magnetic field: 2 tesla21 m
15 m
mass: 12,500,000
kgmagnetic field: 4 tesla
muon neutrino
protonneutron
photonelectron
CalorimetersCalorimeters are detectors
optimised for measuring particle energies, and include large blocks of dense material.
After a particle enters a calorimeter, its interactions can produce new particles,
which can themselves interact. This results in a shower of particles, which
extinguishes as all of the original particle's energy is lost to the calorimeter material.
The detector records a signal for each block, dependent on the amount of energy
absorbed.
Thinner devices, called electromagnetic calorimeters, are used to
measure the energies of photons, electrons and positrons. Thicker devices, called
hadronic calorimeters, are used to measure the energies of charged and neutral
hadrons.
Muon detectorsA muon detector is a large-volume tracking detector that's
shielded by material, often in the form of calorimeters. It allows identification of muons, as the only charged particles able to
pass through the material.
Tracking detectorsTracking detectors are optimised for measurement of position, and allow reconstruction of the flight paths of charged particles. These detectors are
intended to have little effect on a particle's speed and direction, and so must contain minimal material.
Tracking detectors are usually placed in a magnetic field. A charged particle follows a curved path in the field, and the curvature gives the particle's
momentum (product of mass and velocity).
High-precision tracking detectors are often built from thin layers of silicon, each divided into pixels or strips. The technology is like that used in
camera sensors, but satisfying requirements for large area, fast response, and radiation resistance. A detector layer records a hit in each pixel or strip
that is crossed by a charged particle, and where the particle interacts.
Larger tracking detectors are typically built from chambers, or other containers, filled with gas. A charged particle crossing the gas results in a
hit being recorded in a nearby sensor wire.
Particle paths are reconstructed by fitting curves to hits in different detector layers. Paths can then be combined to identify particles that have
a common origin, for example coming from a decay.
16 17
Discovery of the Higgs
boson
On 4th July 2012, a discovery was
cautiously announced: "CERN experiments
observe particle consistent with long-sought
Higgs boson".
Results from CDF collaboration
155 160 165 170 175 180 185 190 195
80.5
80.45
80.4
80.35
80.3
Mas
s of
W b
oson
(GeV
)ranges of values
for the mass, mH, of the Higgs boson
mH in range
115 GeV to 127 GeV
mH in range 600
GeV to 1000 GeV
experimental measurements for masses of W boson
and top quark
Mass of top quark (GeV)
Before the discoveryThe mass of the Higgs boson
can't be directly calculated in the Standard Model, but can be linked to
other quantities. By early 2012, it had been significantly constrained
by measurements of the masses of the top quark and W boson.
These were consistent with the Higgs boson having a mass in the
range 115 GeV to 127 GeV, and were incompatible with higher values.
Mass of Higgs boson (GeV)
100 120 140 160 180 200
Frac
tion
of
dec
ays
1
10-1
10-2
10-3
ZZ
WW
gg
Zγγγ
ττ
cc_
bb_
Results from LHC Higgs Cross Section Working Group
The Standard Model also allowed calculations of how a Higgs boson would decay, depending on its mass. As a result of quantum effects, a decay is possible
into short-lived particles that nominally have a greater mass than their parent. For example, a Higgs boson of 125 GeV
can decay to two Z bosons, each of which has a nominal mass of 91.2 GeV. To balance energy in these cases, one of the particles from the decay must have
a mass lower than its nominal mass. Such a particle is said to be virtual, and its symbol is sometimes written with a star superscript. The example decay mentioned might then be written as:
H0 → Z0 Z0*
Signal from Higgs boson decay
to two Z bosons
Estimated background
11
10
9
8
7
6
5
4
3
2
1
0
100 110 120 130 140 150
Reconstructed mass (GeV)Result from ATLAS collaboration, July 2012
Even
ts
110 120 130 140 150
1500
1000
500
0
Experimental data
Estimated background
Signal from Higgs boson decay
to two photons
Wei
ghte
d e
ven
ts
Reconstructed mass (GeV)Result from CMS collaboration, July 2012
Higgs boson
Z0*
Z0
lepton
anti-lepton
lepton
anti-lepton
t
t
t_
photon
Higgs boson
Observation of a new particleProtons were first circulated in the Large Hadron
Collider on 10th September 2008. Nine days later, an electrical connector between two of the accelerator's helium-cooled superconducting magnets failed under
test. This resulted in a rapid release of helium gas, which caused extensive damage. The accelerator was shut down for remedial work, and not restarted until
20th November 2009. The first period of operation for physics studies was March 2010 to February 2013, with proton collisions at energies of 7 TeV and 8 TeV.
Particle collisions are often referred to as events. A direct search for a Higgs boson aims to select
events with a Higgs boson (signal) and to reject those without (background). This is a significant challenge. Fewer than 1 event in every 1,000,000,000 is a signal event, and signal and background can have similar
characteristics.
One strategy is to consider decays of the Higgs boson where all of the particles from the decay
can be measured. This is the case for decays to two photons and to two Z bosons. In the latter case, the
Z bosons can each decay to a charged lepton and its anti-particle, and it is these that are detected. Measurements of the energy and direction of the
particles from the decay allow reconstruction of the mass of the original particle. In a search, particles consistent with the signal decay are selected, but may actually come from a background process.
The plot of reconstructed masses typically has a smooth shape for the background, with a bump
corresponding to the signal, if present.
Results from ATLAS and CMS, on 4th July 2012, showed bumps in the distributions of masses reconstructed
from two photons and two Z bosons. These indicated the existence of a previously undetected particle,
with a mass of about 125 GeV. The fact that the new particle decayed into bosons meant that it must itself
be a boson. The measured mass was in the allowed range for the Higgs boson, from the constraints of the
Standard Model.
photon
18 19
Measuring the Higgs boson
Measured properties of the Higgs boson have been found to agree
with expectations from the Standard Model.
b b_
H0
Higgs decay to two bottom quarks
τ+ τ-H0
Higgs decay to two tau leptons
Higgs decay to two photons
H0γ γ
Higgs decay to two W bosons
W W*H0
Higgs decay to two Z bosons
H0Z0 Z*0
-0.5 0.0 0.5 1.0 1.5 2.0
measured rate / rate expected from Standard Model
CMS
ATLAS
Preliminary results from the ATLAS and CMS experiments, 2014
DecaysSince the discovery of the Higgs boson, many more events have been collected and analysed in the ATLAS and CMS experiments. This has
increased the signals for decays to two photons and to two Z bosons. It has also allowed observation of other decays: to two W bosons, to two
bottom quarks, and to two tau leptons. The rates at which the different decays occur have been measured. Within large uncertainties, they agree
with the rates predicted by the Standard Model.
International Linear ColliderThe International Linear Collider has been proposed for
colliding electrons and positrons at energies from 0.25 TeV to 1 TeV. Collisions of this type, between
elementary particles, are simpler to analyse than collisions between composite particles, such as protons. In this sense,
they produce better data for precision measurements. Decay rates of the Higgs boson could potentially be
measured with an accuracy at the per-cent level.
If approved, construction of the International Linear Collider could be completed for the late 2020s. The
machine would then have an operational life of twenty to thirty years.
SpinIf a particle decays, the way in
which it spins affects the angular distribution of the emitted
particles. Measurements have been made of emission angles
for photons from decays of Higgs bosons. The resulting distribution
has large uncertainties, but is consistent with a decaying
particle that has zero spin. This is as expected for the Higgs boson in
the Standard Model.
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
250
200
150
100
50
0
Even
ts
Modulus of cosine of photon emission angle
Measurement
Expected from Standard Model
Uncertainty in background subtraction
Improving measurementsMeasurements of the Higgs boson are consistent with
the Standard Model, but suffer from large uncertainties. Fuller understanding of the observed Higgs boson
requires measurements of higher precision. These require collection of more, and better, data.
Upgrades to the Large Hadron Collider
A series of upgrades is planned for the Large Hadron Collider, defining a rich physics programme up to the
early 2030s. The upgrades will raise the energy of proton collisions to 14 TeV, and will allow higher luminosity. The result will be to increase the rate at which Higgs bosons
are produced, giving more data.
A drawback of higher luminosity is that it means more collisions for every beam crossing. Work is in progress on
detector improvements to cope with the more challenging environment, and ensure high-quality data.
The upgraded Large Hadron Collider should allow rates for different decays of the Higgs boson to be measured to
better than 10%.
31 km
Main linear accelerator for electrons
Damping Rings
length = 310 fields
not to scale
Main linear accelerator for positrons
© ILC / form one
20 21
Beyond the Standard
Model
The Standard Model successfully describes a
wealth of experimental data, but seems to explain only
5% of the Universe.
The mass of the Higgs boson On short timescales, an elementary particle can emit and
reabsorb other particles. A neutral elementary particle can also convert temporarily to a charged particle and its anti-particle. The energy associated with such processes contributes to a particle's total energy, and is known as
self-energy. This may be negative (energy lost) or positive (energy gained). A particle's measured mass can then be
regarded as the sum of a bare mass and a self-energy.
Contributions to the self-energy from conversion and emission processes down to a chosen distance can be
calculated in the Standard Model. The bare mass can be deduced as the difference between the measured mass
and the self-energy.
The Standard Model fails below about 10-33 m, where gravity needs to be taken into account. Down to this
distance, most particles have self-energies that are small. The exception is the Higgs boson, where the calculated self-energy is large and negative. The implied bare mass is around 1019 GeV, compared with the measured mass
of about 125 GeV. Although technically possible, the enormous difference between bare mass and measured
mass for the Higgs boson is considered suspect.
This difference is referred to as the hierarchy problem or fine-tuning problem.
measured mass
bare mass
0.511 MeV 0.469 MeV
ELECTRON
measured mass
bare mass
125 GeV
1019 GeV
HIGGS BOSON
COMPOSITE HIGGS
A Higgs boson that is a bound state of two fermions would have a certain
size. This would define a lower limit for evaluating self-energy contributions, potentially reducing the bare mass.
EXTRA DIMENSIONS
The mass of the Higgs boson could be distributed over more spatial
dimensions than the three experienced in everyday life. The measured mass would then be only a fraction of the
true mass, which could be close to the bare mass.
The different possibilities aren't mutually exclusive, and each can lead
to a variety of different models.
t_~
t_
H0 H0
t
+ ≈ 0H0 H0
t~
Several possibilities for eliminating the fine-tuning problem are being investigated by the ATLAS and CMS experiments.
SUPERSYMMETRY
Supersymmetry introduces a fermion partner for each boson of the Standard Model, and a boson partner for each fermion. The Higgs self-energy contributions from a Standard-Model particle and its supersymmetry partner
would tend to cancel out. The bare mass would then become similar to the measured mass.
Dark matter and dark energy
Stars in a galaxy move in orbits about the galactic centre. Orbital speeds measured for stars further from a galaxy's centre tend to be higher than calculated from the distribution of visible matter.
The most widely accepted explanation is that the galaxy contains unseen matter, which contributes to the gravitational pull. This
additional matter is called dark matter — unlike matter made of atoms, it doesn't absorb or emit light.
Astronomical observations indicate that the Universe is expanding, at a rate that's increasing with time. The
undetected energy that drives this accelerating expansion is given the name of dark energy. It's thought to be thinly
spread throughout the Universe.
The Standard Model provides no explanation of either dark matter or dark energy.
Dark matter could be accounted for by new types of particle. These would need to have non-zero mass, to be stable,
and to interact only weakly with the particles of the Standard Model.
One suggestion is that dark matter interacts with the Higgs boson to gain mass, but otherwise has no Standard-Model
interactions. Dark-matter particles would then exist in relative isolation from the particles of the Standard Model.
Models based on this idea are known as hidden-valley models, or as Higgs-portal models.
Another suggestion is that dark matter is made from the lowest-mass supersymmetry particles.
Values from Planck mission
68.3% Dark
energy
26.8% Dark matter
4.9% Matter
made of atoms
Space-based surveys are able to measure the
energy and matter content of the
Universe.
rota
tion
al v
eloc
ity
distance from centre
calculated
measured
the past
the present
the future
conversion to particle and anti-particle
photon emission
photon reabsorption
annihilation of particle and anti-particle
22 23
People who train as particle physicists develop specialist and
general skills that make them highly sought after. Some spend their working lives in particle-
physics research, some combine research with consultancy work,
some decide to change career. Particle physicists have gone on to work in areas as varied as fashion, big-data analytics, asset finance,
publishing, and Formula-1 racing.
Being a particle physicist
UK researchers on the ATLAS and CMS experiments make key
contributons to studies of the Higgs boson and beyond.
Groups from eighteen UK institutes are involved in experiments at the Large Hadron Collider.
Their work is mostly funded by the Science and Technology Facilities Council. Additional support
comes from organisations such as the Royal Society and the European Research Council, and
from industrial partnerships.
A standard route to working in experimental
particle physics is undergraduate study
in physics, followed by a doctorate in particle
physics. At undergraduate level, students take lecture courses in particle physics, and may carry out related
literature reviews and projects.
At doctoral level, students receive a thorough
grounding in all aspects of particle physics. They then specialise in one technical
area – for example detectors, electronics,
or computing – and also undertake a physics study.
As well as the standard route, people also come to work in particle physics after University study in a variety of other subjects. These include engineering, computer science, and mathematics.
First employment in particle physics, in the UK or another country, is usually on a
fixed-term contract, of one to three years. This will be at a University, or at a national or international research laboratory. Some
contracts are linked to research fellowships, which can include funding for equipment
and travel.
Subsequent employment may be through further fixed-term contracts,
or through a permanent position. Permanent University positions usually
combine teaching and research, with possible progression from Lecturer to
Reader to Professor.
Non-physics background
CAREER PATH
First job – University or
research laboratory
Undergraduate dregree
DPhil or PhD
Fixed-term contracts or permanent position
Lifelong particle-physics
devotee
Other careers
Richard Mudd PhD Student, Particle Physics Group,
University of Birmingham
What I do My research involves analysing the many proton-
proton collisions recorded by the ATLAS experiment, to understand the properties of the Higgs boson.
The best bitEvery day is different, and the variety of the work is
one of the aspects that I enjoy most.
How I became interested in particle physics My interest in particle physics began when I heard a talk by Frank Close at the Hay Festival, many years ago. I found it amazing how much it was possible to learn about physics on such an unimaginably small scale.
Alternative career If I weren't able to be a physicist, I'd like a job
that involved football — although I was never good enough to actually be a footballer!
Michaela Queitsch-Maitland
PhD Student, Particle Physics Group, University of Manchester
What I do I work on the ATLAS experiment, studying
Higgs decays to two photons. In a typical day, I write computer code, chat with colleagues,
drink coffee, attend meetings, write more code, stare at numbers that don't make sense, drink more coffee, make colourful plots, and
generally try to understand physics.
Fitting in a social life When you work on an experiment at CERN, most of your friends tend to be physicists.
This is far from a bad thing — but it can lead to some very geeky conversations in the pub!
How I became interested in particle physics
As an undergraduate, I took part in the CERN Summer Student Programme. This was the
start of a serious interest in particle physics, and convinced me to do a PhD in the subject.
Advice to younger self Stop procrastinating, and leaving everything
to the last minute!
Curious fact I have a black belt in karate.
Julie Kirk Research Physicist,
Rutherford Appleton Laboratory
What I do In the ATLAS experiment, we only have resources to record and analyse a few hundred of the tens of millions of particle collisions that occur each
second. I work on the electronics that decides which collisions to keep. Once a collision is discarded it's lost forever, so we need to be very sure that we're
keeping the right ones!
Fitting in a social life Working at CERN had a big effect on my family life, as I met my husband there. I'm now based in the United
Kingdom, and work part-time, so that I can devote time to both work and family.
Useful skill from particle physics Perseverance — particle physicists will always keep
going until they solve a problem.
Alternative career As a child, I always wanted to be an astronaut.
24 25
Alan BarrAssociate Professor of Experimental Particle Physics, University of Oxford
What I do My group and I are hunting for tell-tale signs that particles of dark matter are being produced in the ATLAS experiment. We haven't found them yet, but there’s still a lot
more work to do.
The best bit I love that so many people find our research to be such an inspiration.
Funny experience When the Large Hadron Collider was about to be turned on, some members of the
public were worried that it was going to create black holes, with catastrophic results. Reassurances that I gave in an interview to the Oxford Times resulted in the
front-page headline: "Dr Doomsday promises not to destroy the Earth!"
Rebecca LanePhD Student, High Energy Physics Group,
Imperial College London
What I do I work on the CMS experiment, searching for Higgs
decays to two tau leptons. My work is entirely computer based, and relies heavily on efficient
programming.
The best bit Seeing results that I've produced pop up in journal
articles, talks, and posters is very nice!
What friends and family think about my work Awe at the fact that I work at CERN, complete
incomprehension about what I actually do.
Making school physics more appealing to girls There is a clear stereotype that needs to be broken.
I think that it can help if female scientists make themselves accessible to school students, by visiting
schools and helping with outreach activities.
Where I started I studied for my undergraduate degree at the
University of Oxford. I was drawn to the place itself more than anything, with its interesting traditions
and history.
Monica Vazquez Acosta Research Associate, High Energy
Physics Group, Imperial College London
What I do I work on the CMS experiment, searching for
Higgs decays to tau leptons.
Memorable experience One of the most exciting moments in my
career was when the first proton beams were injected into the Large Hadron Collider. After years of preparation, this was the start of a
new era in particle physics.
Where I started I studied theoretical physics at the
Universidad Autónoma de Madrid. After graduating, I pursued a career in experimental
high-energy physics, as I was interested in testing theories that I'd learned as a student.
Alternative career I'm interested in advanced radiotherapy
techniques for cancer treatment.
Curious fact I come from the Canary Islands, where we
have two world-leading observatories.
Carl Gwilliam Research Associate, Particle Physics Group,
University of Liverpool
What I do I work on the ATLAS experiment, searching for a Higgs
decay that, so far, hasn't been seen.
What family and friends think about my work The strangest reaction that I've ever had is: "Physicist —
do you put the bubbles in Fizzy drinks?"
How I became interested in particle physics My interest in physics was driven by a great teacher. I remember him explaining radioactive decay with M&M
sweets. We did an A-level module on particle physics, and I was captivated by the thought of looking into the very
building blocks of nature.
Elisabetta Pianori Research Fellow, Elementary Particle Physics Group, University of Warwick
What I do I work on the real-time selection of interesting
collisions in the ATLAS experiment, and on studies of the Higgs boson. Every day is different. Some days
I'm needed at the experiment control room; some days are filled with meetings; some days I work by
myself in my office, trying to solve problems!
The best bit
What I love most is the people. Knowing and learning is great, but it's even better when done
together.
The worst bit Also the people! Trying to get hundreds of physicists to work together is very hard. We each tend to have
strong opinions.
Useful skills from particle physics I've developed strong analytical skills, and a creative
approach to problem solving.
Christos Anastopoulos Research Fellow, Particle Physics and Particle Astrophysics Group,
University of Sheffield
What I do I work on the ATLAS experiment, where I'm searching for Higgs decays to two Z0
bosons. I spend a lot of time with research students, discussing their results, and
planning analysis strategies.
The best bit Interacting with interesting, clever people.
The worst bit Long meetings.
What family and friends think about my work
They're very interested in knowing more about both the human and scientific
aspects.
Fitting in a social life Sometimes it's hard to balance my social life against the excitement of searching
for new physics. This gets easier with experience.
Making school physics more appealing
For both boys and girls, we need to make the subject exciting. Outreach activities are
very important in this respect.
Advice to younger self Relax, work and things will be fine.
Curious fact I drink a lot of coffee, at every possible time
of the day.
26 27
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