Exhibit team

The and beyond was organised, with help from a few friends, by researchers from the UK Particle- groups that collaborate on the ATLAS and CMS experiments, at the , near Geneva.

The institutes and people involved with the exhibit were as follows:

Brunel University Rutherford Appleton University of Cambridge Jo Cole Laboratory Miguel Arratia-Munoz Alan Barr Peter Hobson John Baines Giovanna Cottin-Buracchio Kathryn Boast Akram Khan Alastair Dewhurst Steve Green Daniela Bortoletto Paul Kyberd Jens Dopke Karl Harrison Philip Burrows Dawn Leslie Stephen Haywood Steven Kaneti Alexandru Dafinca Jelena Ilic Thibaut Mueller Claire Gwenlan CERN Julie Kirk Rebecca Pitt Chris Hays Quentin King Stephen McMahon Stephen Wotton David Hall David Sankey Boruo Xu James Henderson Imperial College London Monika Wielers Malcolm John Louie Corpe Craig Sawyer Paul Dauncey University College London Sahra Bhimji Adinda de Wit Jonathan Butterworth Wahid Bhimji University of Sheffield Patrick Dunne Rebecca Chislett Flavia Dias Christos Anastopoulos Rebecca Lane Ben Cooper Victoria Martin Ian Dawson Robyn Lucas Gavin Hesketh Benjamin Wynne Gary Fletcher Sasha Nikitenko Andreas Korn Dan Tovey Monica Vazquez Acosta Josh McFayden Ines Ochoa David Britton University of Sussex Lancaster University Tim Scanlon Thomas Doherty Carlos Chavez Barajas Harald Fox David Wardrope Tony Doyle Antonella DeSanto Kathryn Grimm Daniel Gibbon Roger Jones University of Birmingham Dorothy Lamb Ludovica Aperio Bella John Anders Fabrizio Salvatore Queen Mary, University of Matthew Baca Carl Gwilliam Nicky Santoyo-Castillo London Andrew Chisholm Matthew Jackson Yusufu Shehu Cristiano Alpigiani Simon Head Max Klein Kerim Suruliz Lucio Cerrito Cristina Lazzeroni Paul Laycock Iacopo Vivarelli Teppei Katori Tom McLaughlan Allan Lehan Ruth Sandbach Richard Mudd Monica d’Onofrio Giacomo Snidero Javier Murillo Joe Price Steve Boyd Tom Whyntie Konstantinos Nikolopoulos Joost Voosebeld Sinead Farrington Michal Kreps Royal Holloway, University University of Bristol University of Manchester Tim Martin of London Robin Aggleton Iain Haugthon Bill Murray Tracey Berry Paolo Baesso Steve Marsden Elisabetta Pianori Veronique Boisvert Euan Cowie Clara Nellist Ian Connelly Maarten van Dijk James Robinson Michele Faucci-Giannelli Sudarshan Paramesvaran Michaela Queitsch-Maitland Russell Kirk Daniel Saunders Sabah Salih Pedro Teixeira-Dias Stefan Söldner-Rembold Joshuha Thomas-Wilsker Terry Wyatt

Suggested variants on the final Higgs Centre logo PR 17 Apr 2013

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 Summer Science Exhibition 30th June 2014 to 6th July 2014 THE ROYAL SOCIETY 6 Carlton House Terrace, London SW1Y 5AG

Exhibit coordinators: Wahid Bhimji, Cristina Lazzeroni, Konstantinos Nikolopoulos Booklet editor: Karl Harrison Artwork and design: Rebecca Pitt

The Higgs boson and beyond was made possible through help and support from the participating institutes (left), and from:

CERN: European Laboratory for GridPP: UK Computing for Particle Physics Higgs Centre for Theoretical Physics STFC: Science and Technology Facilities Council SEPnet: South East Physics Network

1st edition - June 2014

the-higgs-boson-and-beyond.org The Higgs boson and beyond

The discovery of the Higgs boson was a momentous occasion, but what lies beyond could be even more exciting.

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 Why it matters Geneva, was a scientific milestone. Subsequent Cristina Lazzeroni studies, with more data, have consolidated the discovery, and prompted the award of Putting together a science exhibit is hard work, the 2013 Nobel Prize in Physics to two of the but one thing I really like is that it makes me think key contributors to the original theoretical about what I do, and why it's relevant. So when developments: François Englert and Peter Higgs. a new particle, the Higgs boson, was discovered, in 2012, I jumped at the opportunity of telling In August 2013, when it was looking likely everybody all about it. This led to the staging of that the year's Nobel Prize in Physics would be Understanding the Higgs boson at the 2013 Royal awarded for work relating to the Higgs boson, Society Summer Science Exhibition. 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 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 What comes next theoretical expectations for the Higgs boson. Wahid Bhimji

The new particle decays rapidly, to pairs of People often ask me if the discovery of the Higgs force carriers (W bosons, Z bosons or photons), Boson means that our work is done. Actually, this or to a matter particle and corresponding discovery doesn't come close to answering all of anti-matter particle. Within the current our questions about the Universe. For one thing, experimental precision, of about twenty to the fact that the Higgs particle isn't as heavy as thirty per cent, the measured decay rates are theory suggests, is a strong hint there's something in agreement with calculations in the still to be discovered, for example the new particles framework of the Standard Model. introduced by the theory of supersymmetry. Also, astronomical measurements indicate that most Studies to date set the stage for even more of the mass of the Universe is in the form of dark detailed investigations in the future, with matter, and we don't know what this is. Ultimately, upgraded accelerator and detectors. We will we want to create a Theory of Everything, which be looking for deviations from the theoretical brings together our understanding of elementary predictions, which would signal physics beyond particles, and our understanding of gravity, which the Standard Model, a really exciting possibility! 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 All everyday objects seem to be made from just three types of basic world is building block – the up quark, the built down quark, and the electron.

10-10 m Protons and neutrons, in atomic nuclei, combine -15 -15 -14 10 m 10 m to 10 m with electrons to form Quarks combine Protons and neutrons atoms (electric charge: 0) as protons and combine in atomic nuclei neutrons <10-18 m Elementary particles – no detected structure

+ + + + + + relative + + + + + electric + +2/3 + + charge proton + + up quark

oxygen atom 0 oxygen nucleus The diameter of electron neutrino an atom is more than neutron 10,000 times -1/3 the diameter of its nucleus.

down quark Ions An atom that loses or gains -1 electrons, and so becomes electrically charged, is an ion. electron 22.1% 24.1% 125 neutrons 124 neutrons The up quark, down quark and electron are the basic building blocks of everyday Naturally objects. occurring 1.4% The electron neutrino is isotopes of lead 122 neutrons involved in radioactive Lead atom decays, and in processes 82 protons that power the stars. 52.4% 126 neutrons

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. Powers Of Ten Physics 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.

6 DNA 4 10 2 10 1000000 Pairs of deoxyribonucleic acid (DNA) molecules 0 10 10000 –2 10 100 form a double-helix structure that encodes –4 10 1 –6 10 0.01 genetic information in living organisms. Each 10 0.0001 structure contains around 0.000001 2 x 109 atoms of just 5 different types: hydrogen, oxygen, carbon and phosphorus.

106 m to 108 m Planets

10-2 m to 103 m 10-10 m to 10-8 m Macroscopic Atoms combine to structures form molecules 10-3 m 10-7 m to 10-4 m Small-scale structures, Atoms and visible to the eye molecules combine to form microscopic structures

hydrogen atoms

oxygen atom A human cell contains water molecule 46 DNA double helixes and around 1 x 1012 atoms.

50% 24% 51% sodium oxygen oxygen 62% 3% hydrogen other

Grain Human Planet of Salt 14% Earth 12% 7 x 1027 silicon 1018 atoms 1050 atoms carbon atoms

2% 50% other 15% 17% chlorine magnesium iron FERMIONS

MATTER PARTICLES Buiilding blocks of everyday objects. 1st 2nd 3rd generation generation generation

Particles produced at accelerators.

e µ τ

electron muon tau neutrino neutrino neutrino Particles of the LEPTONS e- µ- τ- Standard Model Only these particles have been electron muon tau GAUGINOS detected experimentally.

~g u c t PARTICLES ~ up charm top g

QUARKS ~ ~ ~ Force carriers d s b g W+ H+ for gravity down strange bottom ~ ~ ~ ~ ~0 ~ ~ ~ g g γ0 Z 0 H h0 A0 G photino gravitino – – – ~ ~ ~ d s b g W- H-

anti-down anti-strange anti-bottom Winos and ~g Zino HIGGSINOS – – – u c t ANTI-QUARKS ~ anti-up anti-charm anti-top g

gluinos

e+ µ+ τ+ positron anti-muon tau Particle ANTI-PARTICLES e µ τ Universe ANTI-LEPTONS anti-electron anti-muon anti-tau neutrino neutrino neutrino

Elementary particles, objects with no detected structure, include quarks, leptons, force carriers, and Anything made of the Higgs boson. quarks and leptons is matter Standard Model of Particle Physics Anything made of The Standard Model describes how particles behave under three anti-quarks and types of force: strong force, weak force, and electromagnetic force. Each force has an associated charge, and only particles anti-leptons is antimatter that carry this charge feel the effect of the force. Forces that affect particles Particles outside of the Standard Model subject to subject to subject to None of these particles have been detected electromagnetic force weak force strong force experimentally. The graviton is introduced by theories of gravity. Other particles are added by supersymmetry. BOSONS

~ ~ ~ Vτ Vµ Ve SLEPTONS stau smuon selectron neutrino neutrino neutrino PARTICLES

~- ~- ~- FORCE CARRIERS τ µ e (GAUGE BOSONS) stau smuon selectron

g ~ ~ ~ Higgs boson

t c u SQUARKS of the Carrier of electromagnetic Standard g stop scharm sup Model force

~ ~ ~ H+ W+ g b s d sbottom sstrange sdown G A0 h0 H0 Z0 γ g g graviton photon

~– ~– ~– ANTI-SQUARKS H- W- g b s d

W and Z anti-sbottom anti-sstrange anti-sdown ANTI-PARTICLES bosons HIGGS g PARTICLES ~– ~– ~– Carriers of weak force t c u g anti-stop anti-scharm anti-sup

gluons

Carriers of ~+ ~+ ~+ ANTI-SLEPTONS strong force τ µ e

anti-stau anti-smuon anti-spositron

~– ~– ~– Particles and Anti-Particles Vτ Vµ Ve Each particle has an anti-particle, with identical mass but anti-stau anti-smuon anti-spositron neutrino neutrino neutrino oppositely signed charge values.

Fermions and Bosons Hadrons Particles behave as if they're spinning and are identified as Quarks and anti-quarks aren't detected as free particles, but fermions or bosons depending on the amount of spin. Spin are bound together in composite objects, as one of three types values are integer or half-integer multiples of a quantity of hadron. These are baryons (three quarks), anti-baryons known as the Dirac constant. Fermions can always be (three anti-quarks) and mesons (quark and anti-quark). 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. Supersymmetry Theories for improving on the Standard Model have been FERMIONS BOSONS 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 spin = 1_ , 3_, ...5_ spin = 0 spin = 1, 2, 3,... heavier than their partners in the Standard Model. 2 2 2 Interactions and decays Particles announce their presence in two types of processes: Particle interactions and decays.

dynamics - µ

The theories of particle + + Z0 intreactions and decays bring together many of the big ideas of µ+ twentieth-century physics.

Interaction Decay In an interaction, two In a decay, a particle of particles may scatter off higher mass disintegrates, one another, or may give leaving behind two, or up their energies for the more, new particles, of creation of new particles. lower mass.

Feynman diagrams

Particle interactions and decays are represented pictorially in Feynman diagrams. Different types of line identify different types of particle:

matter particle Higgs boson carrier of electroweak force carrier of strong force

A point where lines meet is called a vertex, and represents an interaction. The Standard Model allows only a few types of vertex.

For an interaction or decay to be fully described by a diagram, the particles involved need to be indicated.

photon gluon

t Production of Higgs boson, followed by H0 decay to two photons gluon t photon

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. A field is a region in time and space, where the value of a quantity is defined at every point. Relativistic Relativistic quantum quantum field theory describes the fields of force field theory and energy 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).

Quantum mechanics Deals with: Special relativity The very small Deals with: Big ideas: The very fast Energy comes in packages (quanta). Big ideas: Particles behave sometimes like point objects At high speeds, lengths shorten, and sometimes like waves (complementarity). and time advances more slowly. All properties of a particle can't be exactly The speed of light, c, measured at the same time is a universal constant. (uncertainty principle). Energy, E, and mass, m, are equivalent: E = mc2.

Electroweak theory Deals with: Electromagnetic and weak forces, combined as electroweak forces These are specific types of Big ideas: Relativistic quantum field theory. The weak force is carried by W± and Z0 bosons. They're the basis of the Standard Model. 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 Quantum electrodynamics chromodynamics Deals with: Electromagnetic force – interaction of light and matter Deals with: Strong force Big ideas: The electromagnetic force is carried by photons. Big ideas: The strong force is carried by gluons. Each particle has an anti-particle, with identical mass, but Particles that feel the strong force possess a opposite charge. type of charge called colour charge. a acceleration r distance F m force mass m M mass mass Mass and

Inertial mass Gravitational mass measures resistance measures ability to create, the Higgs to change in speed or and be influenced by, direction: gravitational forces: F = ma F=G Mm boson universal r2 gravitational constant

Mass Elementary particles acquire In everyday language, mass and weight are often used mass through interactions interchangeably. Technically, the weight of an object is the force on the object due to gravity. Weighing scales measure with an energy field, where this force, and convert to a mass value. the Higgs boson acts as energy carrier.

Mass-energy equivalence An object's energy content, E, and mass, m, are related by the speed of light, c: The mass of a hydrogen atom is much higher E = mc2 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

Particle Masses 1 electrons 8 29 79 1 neutrons 8 8 29 34 79 118 protons

atoms H O Cu Au (Hydrogen) (Oxygen) (Copper) (Gold)

0 s Z m to a ± W

gauge m 0 bosons r o H f o t

e n i b down-type quarks m - o charged leptons t c τ

THIRD b up-type quarks GENERATION

s c -

SECOND µ GENERATION

quarks & charged leptons quarks & charged u e- FIRST d GENERATION

MASS (electronvolts) 105 106 107 108 109 1010 1011 1012

Each generation seems to be a replica of the first, but Particle masses are expressed as their energy equivalent. at higher mass. Only the fist generation is needed to An electronvolt is the energy gained by an electron in explain everyday matter (atoms). The reason for there pasing through 1 volt, and corresponds to a mass of being three generations is unknown. 1.78 x 10-36 kg . An energy field, known as the The Higgs boson is a short-lived particle Higgs field, is present everywhere that transfers energy between the Higgs field in the Universe and other particles: it is an energy carrier

E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E EH E H E HE E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E H E E E E E E E E E E E E E E E E E

particle with no mass Higgs field The energy that a particle gains from the Higgs field is detected as the particle's mass.

The theory of the Higgs field is able to explain how elementary particles can have non-zero mas. Why different particles have different 1 masses remains a mystery. Theory of electroweak ? force, unifying electromagnetic and weak forces, developed by Sheldon Glashow. The Higgs boson and This theory introduced the W± and Z0 as carriers the origin of mass of the weak force. The 3 formulation required that Mass machanism shown 2012 these have zero mass, by Abdus Salam and inconsistent with the Steven Weinberg to force's short range. be able to account also for the masses of quarks and leptons.

1983 5 W± and Z0 first detected, 1972 1964 1967 by UA1 and UA2 1961 4 experiments. Mathematical consistency of electroweak theory, including the mass mechanism, formally 2 proven by Gerard 't Hooft 6 Higgs boson first detected, Mass mechanism, explaining how the carriers of the weak and Martinus Veltman by ATLAS and CMS force can have non-zero mass, proposed by six physicists, experiments. 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. Particle accelerators Two designs of particle accelerator commonly used are the linear accelerator Colliding 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 particles 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. The European Laboratory for

Particle Physics (CERN) hosts 0.5% 9% Physics the world's highest-energy Industrial research particle accelerator: the Large processing 44% Radiotherapy Hadron Collider. More than 30,000 Particle sources particle Electrons can be obtained by heating a metal wire 40% accelerators (thermionic emission), for example by passing an electric Ion worldwide current through it. implantation 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 6% protons and electrons. Biomedical 0.5% research X-ray synchrotrons Creating particle beams Charged particles can be accelerated to higher energies Collider parameters using electric fields, and can be steered using electric and magnetic fields. The particles can then be focused When two particles collide, their energies can be used in into beams. the creation of new particles. At a collision energy, E, the probability of creating a particle of type X, is represented Highest energies are achieved using radiofrequency by a quantity known as the cross section, ı(X,E). This is cavities. These are metal chambers, containing an electric conventionally measured in multiples of a unit called the field with peaks and troughs that switch many times a barn (b), where 1 b = 10-28 m2. The collision rate is expressed second. As a particle moves through a chamber, it's pushed in terms of a luminosity, L, measured in units of inverse along in much the way that a surfer is pushed by waves on barn per second (b-1 s-1). the sea. The peaks and troughs cause particles in a beam to group together in bunches. Beam area, A

electric field

Time Bunch of n separation, t particles

Particle energies are measured in multiples of the Crossing point electronvolt (eV), the energy gained by an electron accelerated through 1 volt. n2 kiloelectronvolt (keV) = 1,000 eV Luminosity ~ A × t megaelectronvolt (MeV) = 1,000,000 eV The rate, N(X), of collisions where a particle of type X is gigaelectronvolt (GeV) = 1,000,000,000 eV created is the product of cross section and luminosity: teraelectronvolt (TeV) = 1,000,000,000,000 eV N(X) = ı(X,E) × L. CMS

4 The Large Hadron Collider 2 TeV 6 .6 The Large Hadron Collider (LHC) is the highest-energy machine 5 9 in an arrangement of interlinked particle accelerators. Four main k m 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.

5

LARGE HADRON COLLIDER (2008)

6.911 km

4 ALICE LHCb SUPER PROTON 450 SYNCHROTRON GeV (1976)

ATLAS 2

PROTON SYNCHROTRON BOOSTER (1972)

m 57 1 1 1.4 GeV LINEAR ACCELERATOR erence mf : 6 (First beams: 1978) u 2 50 rc 8 i m MeV C

33m 90 25 keV GeV 3

Proton 1 PROTON SYNCHROTRON Proton 2 (1959)

CERN's current linear accelerator replaced an earlier machine, operated 1959 to 1992 Proton beam 1 Collisions travels in here Proton beams cross in the middle of the detector. Particles produced in resulting collisions travel through the different detector layers

Proton beam 2 travels in here

Detecting particles

25 m 46 m

mass: The ATLAS and CMS 7,000,000 kg experiments use giant detectors for precise measurements of magnetic field: 2 tesla particle collisions.

21 m Basics of particle detection 15 m mass: 12,500,000 When a particle passes through a piece of material, it kg may interact with the electrons and quarks from which magnetic field: 4 tesla 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 ATLAS and CMS both examples of particle detectors. They measure energy The detectors of ATLAS and CMS, the two general-purpose deposited by photons, at different points, over a short time experiments at the Large Hadron Collider, have a similar interval. They then generate electrical signals to identify basic design. Each is roughly cylindrical in shape, and built points where this energy is above a threshold. up in layers. The two detectors differ in the technology Bunches of protons cross up to 40,000,000 times a used. Detector sizes reflect the distances over which second in experiments at the Large Hadron Collider. The particles must be measured for accurate determination of experiment detectors must be reset after each crossing. paths, momentum and energy. Muon detectors A 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.

muon neutrino

proton neutron

photon electron

Tracking detectors Calorimeters Tracking detectors are optimised for measurement of position, and allow Calorimeters are detectors reconstruction of the flight paths of charged particles. These detectors are optimised for measuring particle energies, intended to have little effect on a particle's speed and direction, and so and include large blocks of dense material. must contain minimal material. After a particle enters a calorimeter, its interactions can produce new particles, High-precision tracking detectors are often built from thin layers of silicon, which can themselves interact. This each divided into pixels or strips. The technology is like that used in results in a shower of particles, which camera sensors, but satisfying requirements for large area, fast response, extinguishes as all of the original particle's and radiation resistance. A detector layer records a hit in each pixel or strip energy is lost to the calorimeter material. that is crossed by a charged particle, and where the particle interacts. The detector records a signal for each Larger tracking detectors are typically built from chambers, or other block, dependent on the amount of energy containers, filled with gas. A charged particle crossing the gas results in a absorbed. hit being recorded in a nearby sensor wire. Thinner devices, called Particle paths are reconstructed by fitting curves to hits in different electromagnetic calorimeters, are used to detector layers. Paths can then be combined to identify particles that have measure the energies of photons, electrons a common origin, for example coming from a decay. and positrons. Thicker devices, called hadronic calorimeters, are used to measure Tracking detectors are usually placed in a magnetic field. A charged particle the energies of charged and neutral follows a curved path in the field, and the curvature gives the particle's hadrons. momentum (product of mass and velocity). On 4th July 2012, a discovery was Discovery cautiously announced: "CERN experiments of the Higgs observe particle consistent with long-sought Higgs boson". boson

80.5

measurements and their uncertainty Before the discovery 80.45 The mass of the Higgs boson ranges of values can't be directly calculated in the for the mass, mH, of the 115 GeV < m < 127 GeV Standard Model, but can be linked to 80.4 Higgs boson H other quantities. By early 2012, it had been significantly constrained 600 GeV < m < 1000 GeV by measurements of the masses H of the and W boson. 80.35

These were consistent with the W boson (GeV) Mass of Higgs boson having a mass in the range 115 GeV to 127 GeV, and were 80.3 incompatible with higher values.

155 160 165 170 175 180 185 190 195

Mass of top quark (GeV)

Results from CDF collaboration

_ 1 bb WW

The Standard Model also allowed calculations of how a Higgs boson would decay, depending on its mass. As a result 10-1 ττ gg of quantum effects, a decay is possible ZZ _ into short-lived particles that nominally cc 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. 10-2 To balance energy in these cases, one of the particles from the decay must have

Fraction of decays Fraction a mass lower than its nominal mass. Z Such a particle is said to be virtual, and γγ γ its symbol is sometimes written with a star superscript. The example decay 10-3 mentioned might then be written as: 100 120 140 160 180 200 H0 ¤ Z0 Z0*

Mass of Higgs boson (GeV) Results from LHC Higgs Cross Section Working Group γ _ Observation of a new particle t Protons were first circulated in the Large Hadron Higgs boson Collider on 10th September 2008. Nine days later, an t electrical connector between two of the accelerator's helium-cooled superconducting magnets failed under test. This resulted in a rapid release of helium gas, t γ 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. lepton Particle collisions are often referred to as events. Z0 A direct search for a Higgs boson aims to select events with a Higgs boson (signal) and to reject those Higgs boson anti-lepton 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 lepton characteristics. Z0*

anti-lepton

11

10

9

Signal from One strategy is to consider decays of the Higgs 8 boson where all of the particles from the decay Higgs boson decay to two Z bosons can be measured. This is the case for decays to two 7

photons and to two Z bosons. In the latter case, the 6 Z bosons can each decay to a charged lepton and

its anti-particle, and it is these that are detected. Events 5 Measurements of the energy and direction of the 4 particles from the decay allow reconstruction of the mass of the original particle. In a search, particles 3 consistent with the signal decay are selected, but may actually come from a background process. 2 The plot of reconstructed masses typically has a Estimated 1 background smooth shape for the background, with a bump corresponding to the signal, if present. 0 100 110 120 130 140 150

Reconstructed mass (GeV) 1500 Result from ATLAS collaboration, July 2012

Experimental data

1000 Signal from Higgs boson decay Results from ATLAS and CMS, on 4th July 2012, showed to two photons bumps in the distributions of masses reconstructed from two photons and two Z bosons. These indicated the existence of a previously undetected particle, 500 with a mass of about 125 GeV. The fact that the new Weighted events Weighted Estimated particle decayed into bosons meant that it must itself background be a boson. The measured mass was in the allowed range for the Higgs boson, from the constraints of the Standard Model. 0 110 120 130 140 150

Reconstructed mass (GeV) Result from CMS collaboration, July 2012 Measuring the Higgs boson

Decays

Measured properties of Since the discovery of the Higgs boson, many more events have been the Higgs boson have 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 been found to agree has also allowed observation of other decays: to two W bosons, to two with expectations from bottom quarks, and to two tau leptons. The rates at which the different decays occur have been measured. Within large uncertainties, they agree the Standard Model. with the rates predicted by the Standard Model.

Higgs decay to two bottom quarks CMS _ b H0 b ATLAS

Higgs decay to two tau leptons

τ+ H0 τ-

Higgs decay to two photons

γ H0 γ

Higgs decay to two W bosons

W H0 W*

Higgs decay to two Z bosons

Z0 H0 Z*0

-0.5 0.0 0.5 1.0 1.5 2.0

measured rate / rate expected from Standard Model Preliminary results from the ATLAS and CMS experiments, 2014 Spin 250

If a particle decays, the way in 200 Measurement which it spins affects the angular distribution of the emitted Expected from particles. Measurements have 150 Standard Model been made of emission angles Events for photons from decays of Higgs 100 bosons. The resulting distribution has large uncertainties, but is consistent with a decaying 50 particle that has zero spin. This is as expected for the Higgs boson in Uncertainty in the Standard Model. 0 background subtraction

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

Modulus of cosine of photon emission angle

Improving measurements

Measurements 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. International Linear Collider The International Linear Collider has been proposed for colliding electrons and positrons at energies from Upgrades to 0.25 TeV to 1 TeV. Collisions of this type, between the Large Hadron Collider elementary particles, are simpler to analyse than collisions between composite particles, such as protons. In this sense, A series of upgrades is planned for the Large Hadron they produce better data for precision measurements. Collider, defining a rich physics programme up to the Decay rates of the Higgs boson could potentially be early 2030s. The upgrades will raise the energy of proton measured with an accuracy at the per-cent level. collisions to 14 TeV, and will allow higher luminosity. The result will be to increase the rate at which Higgs bosons If approved, construction of the International Linear are produced, giving more data. Collider could be completed for the late 2020s. The machine would then have an operational life of twenty to A drawback of higher luminosity is that it means more thirty years. 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 Main linear accelerator for different decays of the Higgs boson to be measured to for positrons better than 10%.

Damping Rings

Main linear accelerator for electrons

31 km length = 310 fields

not to scale © ILC / form one The Standard Model Beyond the successfully describes a wealth of experimental data, Standard but seems to explain only Model 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.

125 GeV

measured mass 1019 GeV Contributions to the self-energy from conversion and emission processes down to a chosen distance can be bare mass calculated in the Standard Model. The bare mass can be 0.511 MeV 0.469 MeV deduced as the difference between the measured mass and the self-energy. measured bare mass mass HIGGS BOSON 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 ELECTRON 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.

COMPOSITE HIGGS A Higgs boson that is a bound state of two fermions would have a certain Several possibilities for eliminating the fine-tuning problem are being size. This would define a lower limit for investigated by the ATLAS and CMS experiments. evaluating self-energy contributions, potentially reducing the bare mass. SUPERSYMMETRY Supersymmetry introduces a fermion partner for each boson of the EXTRA DIMENSIONS Standard Model, and a boson partner for each fermion. The Higgs self-energy The mass of the Higgs boson could contributions from a Standard Model particle and its supersymmetry partner be distributed over more spatial would tend to cancel out. The bare mass would then become similar to the dimensions than the three experienced measured mass. in everyday life. The measured mass _ ~_ would then be only a fraction of the t t true mass, which could be close to the bare mass. H0 H0 H0 H0 + ~ 0 The different possibilities aren't mutually exclusive, and each can lead to a variety of different models. ~ t t Stars in a galaxy move in orbits about the galactic centre. Orbital speeds measured for stars further from a galaxy's centre tend to Dark matter be higher than calculated from the distribution of visible matter. The most widely accepted explanation is that the galaxy contains and dark energy 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.

measured rotational velocity velocity rotational

ca lculated

distance from centre

26.8% Astronomical observations indicate that the Universe Dark matter is expanding, at a rate that's increasing with time. The 68.3% undetected energy that drives this accelerating expansion Space-based Dark is given the name of dark energy. It's thought to be thinly surveys are able energy spread throughout the Universe. to measure the 4.9% energy and matter Matter content of the made of Universe. atoms

V al on ue si s f is rom Planck m the future

The Standard Model provides no explanation of either dark matter or dark energy. the present 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 the past 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. Lifelong particle-physics Being a devotee particle physicist Other careers

UK researchers on the ATLAS and CMS experiments make key contributons to studies of the Higgs boson and beyond. People who train as particle physicists develop specialist and general skills that make them Groups from eighteen UK institutes are involved highly sought after. Some spend their working lives in particle- in experiments at the Large Hadron Collider. physics research, some combine Their work is mostly funded by the Science and research with consultancy work, Technology Facilities Council. Additional support some decide to change career. comes from organisations such as the Royal Particle physicists have gone on to Society and the European Research Council, and work in areas as varied as fashion, big-data analytics, asset finance, from industrial partnerships. publishing, and Formula-1 racing.

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 A standard route to research fellowships, which can include working in experimental funding for equipment and travel. Fixed-term contracts particle physics is or permanent position undergraduate study in physics, followed by a doctorate in particle physics. At undergraduate level, students take lecture courses in particle physics, First job – and may carry out related Subsequent employment may be literature reviews and University or through further fixed-term contracts, projects. research laboratory or through a permanent position. Permanent University positions usually combine teaching and research, with possible progression from Lecturer to DPhil or PhD Reader to Professor.

Undergratuate dregree Non-physics background At doctoral level, students receive a through grounding in all aspects of particle physics. They then specialise in one technical area – for example As well as the standard route, people also detectors, electronics, come to work in particle physics after or computing – and also University study in a variety of other undertake a physics study. subjects. These include engineering, CAREER PATH computer science, and mathematics. 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 bit Every day is different, and the variety of the work is Michaela one of the aspects that I enjoy most. Queitsch-Maitland How I became interested in particle physics My interest in particle physics began when I heard a PhD Student, Particle Physics Group, talk by Frank Close at the Hay Festival, many years ago. University of Manchester I found it amazing how much it was possible to learn about physics on such an unimaginably small scale. What I do I work on the ATLAS experiment, studying Alternative career Higgs decays to two photons. In a typical day, If I weren't able to be a physicist, I'd like a job I write computer code, chat with colleagues, that involved football — although I was never good drink coffee, attend meetings, write more enough to actually be a footballer! 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, Julie Kirk most of your friends tend to be physicists. This is far from a bad thing — but it can lead Research Physicist, to some very geeky conversations in the pub! Rutherford Appleton Laboratory How I became interested in What I do particle physics In the ATLAS experiment, we only have resources As an undergraduate, I took part in the CERN to record and analyse a few hundred of the tens Summer Student Programme. This was the of millions of particle collisions that occur each start of a serious interest in particle physics, second. I work on the electronics that decides which and convinced me to do a PhD in the subject. collisions to keep. Once a collision is discarded it's lost forever, so we need to be very sure that we're Advice to younger self keeping the right ones! Stop procrastinating, and leaving everything to the last minute! Fitting in a social life Working at CERN had a big effect on my family life, as Curious fact I met my husband there. I'm now based in the United I have a black belt in karate. 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. Rebecca Lane Monica Vazquez Acosta PhD Student, High Energy Physics Group, Research Associate, High Energy Imperial College London Physics Group, Imperial College London

What I do What I do I work on the CMS experiment, searching for Higgs I work on the CMS experiment, searching for decays to two tau leptons. My work is entirely Higgs decays to tau leptons. computer based, and relies heavily on efficient programming. Memorable experience One of the most exciting moments in my The best bit career was when the first proton beams were Seeing results that I've produced pop up in journal injected into the Large Hadron Collider. After articles, talks, and posters is very nice! years of preparation, this was the start of a new era in particle physics. What friends and family think about my work Awe at the fact that I work at CERN, complete Where I started incomprehension about what I actually do. I studied theoretical physics at the Universidad Autónoma de Madrid. After Making school physics more appealing to girls graduating, I pursued a career in experimental There is a clear stereotype that needs to be broken. high-energy physics, as I was interested in I think that it can help if female scientists make testing theories that I'd learned as a student. themselves accessible to school students, by visiting schools and helping with outreach activities. Alternative career I'm interested in advanced radiotherapy Where I started techniques for cancer treatment. I studied for my undergraduate degree at the University of Oxford. I was drawn to the place itself Curious fact more than anything, with its interesting traditions I come from the Canary Islands, where we and history. have two world-leading observatories.

Alan Barr Associate 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!" Carl Gwilliam Research Associate, Particle Physics Group, University of Liverpool

What I do Christos Anastopoulos I work on the ATLAS experiment, searching for a Higgs Research Fellow, Particle Physics decay that, so far, hasn't been seen. and Particle Astrophysics Group, What family and friends think about my work University of Sheffield The strangest reaction that I've ever had is: "Physicist — do you put the bubbles in Fizzy drinks?" What I do I work on the ATLAS experiment, where How I became interested in particle physics I'm searching for Higgs decays to two Z0 My interest in physics was driven by a great teacher. I bosons. I spend a lot of time with research remember him explaining radioactive decay with M&M students, discussing their results, and sweets. We did an A-level module on particle physics, and planning analysis strategies. I was captivated by the thought of looking into the very building blocks of nature. The best bit Interacting with interesting, clever people.

The worst bit Long meetings.

What family and friends think about my work Elisabetta Pianori They're very interested in knowing more about both the human and scientific Research Fellow, Elementary Particle aspects. Physics Group, University of Warwick Fitting in a social life What I do Sometimes it's hard to balance my social I work on the real-time selection of interesting life against the excitement of searching collisions in the ATLAS experiment, and on studies for new physics. This gets easier with of the Higgs boson. Every day is different. Some days experience. I'm needed at the experiment control room; some days are filled with meetings; some days I work by Making school physics more myself in my office, trying to solve problems! appealing For both boys and girls, we need to make The best bit the subject exciting. Outreach activities are very important in this respect. What I love most is the people. Knowing and learning is great, but it's even better when done Advice to younger self together. Relax, work and things will be fine.

The worst bit Curious fact Also the people! Trying to get hundreds of physicists I drink a lot of coffee, at every possible time to work together is very hard. We each tend to have of the day. strong opinions.

Useful skills from particle physics I've developed strong analytical skills, and a creative approach to problem solving. the-higgs-boson-and-beyond.org