Particle Physics and the LHC Pete Edwards Department of Physics, Durham University
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 20 th 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
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
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 0.5m Energy of electron beam 20kV Length 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=mc 2 !
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
Inside the LEP tunnel LEP was 27000m in circumference Four bunches of electrons and positrons circulated inside the vacuum pipe One ten thousandth of a second for a complete circuit About one electron-positron collision 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 Number of different neutrinos = 2.994 ± 0.011 20 000 000 Rate
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 = mc 2
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
M Z = 91.1875 +/- 0.0021 GeV
Results from LEP It should be around here! 95% Ruled Out
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 PHYSICS
AT THE TeV SCALE
WE NEED TO PROBE THESE ENERGIES: ENTER THE LHC
Supersymmetry: symmetry of fermions and bosons fermions bosons
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
The CERN LHC
A proton-proton collider in the LEP tunnel
… with protons of energy 7000 GeV
THE BIGGEST EXPERIMENT IN HUMAN HISTORY
ATLAS and CMS
Origin of mass
2,000 scientists from 34 countries
General purpose detector
1,800 scientists from over 150 institutions
LHCb and ALICE
Studying the differences between matter and anti-matter
LHCb will detect over 100 million b and b-bar mesons each year
Heavy ion collisions, to create quark-gluon plasmas
50,000 particles in each collision
90 institutes in 30 countries
These experiments will produce Petabytes of data 1 PByte = 1,000,000 GByte
10 PBytes of data a year
(10 Million GBytes = 14 Million CDs)
Concorde (15 Km) Mt. Blanc (4.8 Km) One year’s data from LHC would fill a stack of CDs 20km high
Large amounts of data ……………………
Example from LHC: starting from this event… … we are looking for this “signature” Selectivity: 1 in 10 13 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
Alice Status First cosmics in TPC Sector
ATLAS Construction status: on track for collisions towards the end of 2007
Inner Detector Complete integrated Pixel end-cap with 6.6 M channels at CERN
Transverse slice through CMS detector Click on a particle type to visualise that particle in CMS Press “escape” to exit
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.