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

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

The discovery of the Higgs boson was a momentous occasion, but what lies beyond

could be even more exciting.

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.

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.

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!

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!

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

up quark

+2/3

10-15 m Quarks combine as protons and

neutrons

neutron

proton

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

relative electric charge

electron neutrino

0

A human cell contains 46 DNA double helixes

and around1 x 1012 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. Each

structure contains around 2 x 109 atoms

of just 5 different types: hydrogen, oxygen, carbon 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

FERMIONS

H~+

H~-

H~0 h

~0 A~0

W~ +

W~ -

Z~0!~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+ "+

positron anti-muon tau

e µ "

anti-tauneutrino

anti-muonneutrino

anti-electronneutrino

e µ "tau

neutrinomuon

neutrinoelectronneutrino

e- µ- " -

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.

Buiilding 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~+"~+ µ~+

V–~

µV–~" V

–~e

V~

"

e~-"~- µ~-

W+

W-

Z0 !G

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

ICLES

PAR

TIC

LES

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.

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.

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.

Different types of line identify different types of particle:

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 antiparticles 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

Application: Physics of 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 the fields of force and energy

Mass and the Higgs

boson

Each generation seems to be a replica of the first, but at higher mass. Only the fist 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

"- t

Z0

H0

29

29 34

Cu (Copper)

1

1

H (Hydrogen)

Au (Gold)

79

79 118

8

8 8

O (Oxygen)

electrons

neutrons

protons

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

bine

to fo

rm a

tom

s

Particle Masses

quar

ks &

ch

arge

d le

pton

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

EE

EE

EE

EE

EE

E

EE

EE

EE

EE

EE

EE

E

EE

EE

EE

EE

EE

EE

E

EE

EE

EE

EE

EE

EE

E

particle with no mass Higgs field

An energy field, known as the Higgs field, is present everywhere

in the Universe

EE

EE

EE

EE

EE

EE

EE

EE

EE

EE

EE

EE

EE

EE

EE

EE

EE

EE

E

EE

EE

EE

EE

EE

EE

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.

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

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, (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 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) = (X,E) ! L.

0.5%X-ray

synchrotrons

More than 30,000

particle accelerators worldwide

6%Biomedical

research

0.5%Physics

research 44%Radiotherapy

9%Industrial processing

40% Ion

implantation

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)

Circu

mference: 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

4 TeV

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 travels in here

Collisions Proton beams cross in the middle of the detector. Particles produced in resulting

collisions travel through the different detector layers

Proton beam 2travels in 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.

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.

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).

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 115 GeV < mH < 127 GeV

600 GeV < mH < 1000 GeV

measurements and their uncertainty

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

deca

ys

1

10-1

10-2

10-3

ZZ

WW

gg

Z!!!

""

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_

!

!

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.

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

"+ "-H0

Higgs decay to two tau leptons

Higgs decay to two photons

H0! !

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

tsModulus 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

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

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 through

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 either 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

Undergratuate 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.

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.

the-higgs-boson-and-beyond.org