Particle Physics and the LHC

Pete Edwards

Department of Physics, Durham University

O g d en C en trefo r

F u n d a m en ta l P h y sic s

What is matter?

In Aristotle’s philosophy there were four elements

The concept of elements

In 1808 Dalton listed many elements we recognise today

The periodic table

In 1860’s Mendeleev arranged the elements by property into the periodic table

The periodic table

Not only was this a beautiful pattern it was also predictive

Some elements were missing and their properties could be predicted

All were later discovered

Turn of the 20th century

Thus by the end of the 19th century the idea of elements was well developed

The smallest piece of an element was known as an atom with atoms imagined as small spheres

All matter in the Universe could be described by around 100 different atoms – not too bad!

Enter the electron

In 1897 J J Thompson discovered the electron

It soon became clear that it not only played an important role in electricity but was also contained inside atoms

Atoms have sub-structure!

The plum pudding model

One of the first models of the atom to include electrons

Thompson imagined the electrons, with their negative charge, were stuck in a blob of positively charged material

The structure of atoms

In 1912 the first particle physics experiment was carried out

Fired radioactive particles at gold foil

Found most of the particles went straight through

But occasionally some did scatter back…..

This was totally unexpected!

Rutherford scattering

This showed that the atom has a dense positively charged nucleus surrounded by a cloud of electrons

Rutherford said “It was if someone had told me that having fired a pistol at a sheet of paper, the bullet had bounced back!”

The plum pudding model predicts that the average electric field is zero – no scattering

The dense positively charged nucleus leads to scattering from a ‘point like’ object whose size could be worked out

A new picture of matter

So in the 1930’s the Universe was a simple place

All matter was made of atoms

Atoms had a nucleus made from protons and neutrons surrounded by a cloud of electrons

All the known matter in the Universe could be described by three particles

Antimatter too!

• In 1932 Anderson discovered the positron• For every particle there is also an antiparticle• Antimatter just like ordinary matter but with opposite

charge• electron negative, positron positive!

Finally - Cosmic Rays

By 1926 it was clear that the Earth was bombarded by a high energy rain of protons from outer space – Cosmic Rays

Cosmic Ray research

Scientists quickly started to study cosmic rays using the new cloud chamber detectors and photographic emulsions located on mountain tops or flown in balloons

The particle explosion

New particles just kept coming…………

Muon() 1936

Kaon (K-plus)() 1947

Kaon (K-zero)() 1947

Lambda 1951

Sigma (sigma-plus) 1953

Pion (pi-minus)() 1947

XI (xi-minus) 1952

Anti-Lambda 1958

Sigma (sigma-minus) 1953

Kaon (K-minus)() 1947

Pion (pi-plus)() 1947

Pion (pi-zero)() 1949

XI (xi-zero) 1959

Neutrino() 1955

Finding patterns

Like Mendeleev, group particles with similar properties together

Patterns Sub-structure

In 1964 Murray Gell-Mann suggested that the many particles found could be made from just three quarks

He called them up, down and strange

But no free quarks seen……………….

Man-made cosmic rays

By the 1960’s particle accelerators were operating in America (Berkeley - West coast, Brookhaven - Long Island NY and SLAC – Stanford California) and Europe (CERN – Geneva)

Length 0.5mEnergy of electron beam

20kVLength 3200m

Energy of electron beam million times greater

Enter the quark

In 1967 used SLAC to scatter electrons off protons

Still the Rutherford scattering experiment, on a bigger scale!

Results showed that proton had sub-structure

Made up of three point like objects - quarks

The particles of matter

Model of atom today

Quarks and electrons are fundamental

As far as we can tell no further sub-structure

Proton – up up down (uud)Neutron – down down up (ddu)

All ordinary matter in the Universe is made up from these three particles

So that’s that?

Not quite!

We can describe ordinary matter with three particles – two quarks (u and d) and the electron

Remember to describe all the particles that were found using cosmic rays we needed a third quark – strange (s)

There was also another particle the muon – just like the electron but heavier

Where do these particles fit?

Back to the particle accelerators

By the 1970’s large circular accelerators being built

Creating New Particles

positron (e+)electron (e-)

muon (-)

antimuon (+) E=mc2 !

Back to the particle accelerators

One such accelerator was SPEAR a ring which collided electrons and positrons together

In 1974 evidence for a fourth quark – charm (c) was seen at the SPEAR

In 1975 evidence for a particle like the electron and the muon but much heavier – the tau ()

Even more quarks!

As accelerator energies increased still further yet another quark was discovered in 1977 – bottom (b)

By now theorists were convinced that a pattern was beginning to emerge with families consisting of pairs of quarks and their matching electron like particles

WHERE IS TOP (t)?

Will it ever end?

Whilst the Americans built an accelerator to find the top quark

At CERN LEP was built – a huge accelerator to collide beams of electrons and positrons

LEP

Inside the LEP tunnel

LEP was 27000m in circumference

Four bunches of electrons andpositrons circulated inside thevacuum pipe

One ten thousandth of a second for a complete circuit

About one electron-positroncollision per second

Energy of electron beam ten million times greater than TV

ALEPH – a LEP particle detector

Will it ever end?

In 1991 experiments at LEP proved that there are only three families of quarks and their associated electron like particles or leptons

Found no evidence for quark sub-structure

What about the sixth quark?

Top quark discovered in 1995 at Fermilab in USA

Rat

e

Number of different neutrinos= 2.994 ± 0.011

20 000 000

The matter particles

First Generation

Ordinary Matter

Second Generation

Cosmic Rays

Third Generation

Accelerators

The Standard Model

Some open questions

• Why are the four forces in nature so different?

• How can we model them?

• Do they arise from only one fundamental force?

• How do the fundamental particles get mass?

• How can we test the mechanism for the origin of mass?

• ……………………………

How do the fundamental particles get their mass?

New concept needed: Higgs mechanism

The Higgs mechanism

New field postulated that fills all space: the Higgs field

All fundamental particles obtain their masses from interacting with the Higgs field

The Higgs boson is the field quantum of the Higgs field (like the photon is the quantum of the e.m. field)

To understand the Higgs mechanism, imagine that a room full of physicists chattering quietly is like space filled with the Higgs field ...

... a well-known scientist walks in, creating a disturbance as he moves across the room and attracting a cluster of admirers with each step ...

... this increases his resistance to movement, in other words, he acquires mass, just like a particle moving through the Higgs field...

So to summarize

• The Higgs field fills all space and interacts with fundamental particles

• the interaction generates a mass for the particle

What about the Higgs Boson?

….it is the field quantum of the Higgs field, in the same way as the photon is the field quantum of the electromagnetic field

…let’s go back to our example

... if a rumor crosses the room ...

... it creates the same kind of clustering, but this time among the scientists themselves. In this analogy, these clusters are the Higgs particles.

Is there any evidence that this idea is correct?

• The Higgs boson is the only fundamental particle postulated in the Standard Model that has not been seen yet at accelerators

• We need to find the Higgs boson in order to prove that the Higgs field exist, and hence to show that the above explanation for the origin of mass is indeed correct

Why haven’t we seen the Higgs boson yet?

• The Standard Model does not predict the mass of the Higgs boson: the Higgs mass is an unknown parameter

• High-energy accelerators are needed to produce heavy particles: E = mc2

• Searches so far gave rise to a lower bound on the mass of the Higgs boson

Clues on the Higgs mass from precision physics

• We have measured many observables very precisely

• Measuring the Z mass to this accuracy is like measuring your body weight with an error of 1 gram

• the weight of a lungful of air

MMZZ = 91.1875 +/- 0.0021 GeV = 91.1875 +/- 0.0021 GeV

6

4

2

02 0 10 0 4 00

Mos

t lik

ely

m [GeV ] H

Results from LEP

95%95%Ruled Ruled OutOut It should be

It should be

around here!

around here!

With 95% confidence

LEP tells us about the Higgs boson of the Standard Model:

• it has a mass of more than 114.3 GeV

• it has a mass of less than 186 GeV

“It should be just around the corner”

Can the Standard Model (SM) be the whole story?

• The SM provides a tremendously successful description of the physics that we have tested at experiments

• But it is incomplete and has further serious shortcomings:

it does not contain gravity,

does not allow the unification of the fundamental forces,

does not have a candidate for dark matter in the Universe, …

Can the Standard Model (SM) be the whole story?

• SM gives no answer:

EXPECT NEW PHYSICSAT THE TeV SCALE

WE NEED TO PROBE THESE ENERGIES: ENTER THE LHC

Supersymmetry: symmetry of fermions and bosons

fermionsfermions bosonsbosons

Properties of Supersymmetry (SUSY)

• Predicts partner particles to all SM particles (differ by ½ in their spins), Extended Higgs sector (four Higgs bosons)

• No superpartners have been found yet ( -> so far only lower limits on their masses)

• SUSY automatically incorporates gravity

• Unification of forces at high energy

• Lightest SUSY particle is an attractive candidate for dark matter in the Universe

What is the mass of the Higgs boson?

• Standard Model: Higgs mass is an unknown parameter, but high-precision physics allows us to obtain indirect bounds

• Supersymmetry: the mass of the lightest Higgs boson is a prediction of the model, has to be lighter than about 130 GeV in the minimal model

-> Higgs physics is a powerful test of SUSY

What if there is no Higgs?

• The Standard Model without a Higgs breaks down (i.e. shows unphysical behaviour) in the energy domain of about 1 TeV (= 1000 GeV)

• This energy region will be probed by the LHC

No matter what the mechanism is that gives particles mass, we will definitely see signatures of it at the LHC

The CERN LHC

4 Large Experiments

The world’s most powerful particle accelerator - 2007

•A proton-proton collider in the LEP tunnel

•… with protons of energy 7000 GeV

•THE BIGGEST EXPERIMENT IN HUMAN HISTORY

The CERN LHC

ATLAS and CMS

• General purpose

• Origin of mass

• Supersymmetry

• 2,000 scientists from 34 countries

ATLAS • General purpose

detector• 1,800 scientists from

over 150 institutions

CMS

LHCb and ALICE

• Studying the differences between matter and anti-matter

• LHCb will detect over 100 million b and b-bar mesons each year

LHCb

• Heavy ion collisions, to create quark-gluon plasmas

• 50,000 particles in each collision

• 1000 people• 90 institutes in 30

countries

ALICE

These experiments will produce

Petabytes of data

1 PByte = 1,000,000 GByte

Concorde(15 Km)

Mt. Blanc(4.8 Km)

One year’s data from LHC

would fill a stack of CDs 20km high

• 10 PBytes of data a year • (10 Million GBytes = 14

Million CDs)

• Large amounts of data ……………………

Example from LHC: starting from this event…

…we are looking for this “signature”Selectivity: 1 in 1013

Like looking for 1 person in a thousand world populations

Or for a needle in 20 million haystacks!

• ~100,000,000 electronic channels

• 800,000,000 proton-proton interactions per second

• 0.0002 Higgs per second

ALICE

Size: 16 x 26 meters

Weight: 10,000 tons

First cosmicsin TPC Sector

Alice Status

ATLASConstruction status:on track for collisionstowards the end of 2007

Inner Detector

Complete integrated Pixel end-capwith 6.6 M channels at CERN

CMS Assembly

CMS Solenoid

Transverse slice through CMS detectorClick on a particle type to visualise that particle in CMS

Press “escape” to exit

LHCb pit

Conclusions ??

• Particle physics is about to enter new territory where ground-breaking discoveries are expected

• We are likely to find out about the origin of mass and the unification of the fundamental interactions, we may be able to produce dark matter in the laboratory

• Expect the unexpected – what we will find could

dramatically change our current picture of the structure of matter, space and time

So what has particle physics ever done for us?

The future

The LHC at CERN is due to start operating in 2007.