Discovery of the Higgs Boson at the CERN Large Hadron Collider
Peter Watkins, University of Birmingham, UK
The size of things
The chart above shows the scale of the known universe and some of the equipment used to probe it.
Our planet is a tiny part of the universe. It orbits one of the 300 billion stars in our Milky Way galaxy, which in turn is one of about 100 billion galaxies in the known universe
Between 1 x E-36 to 1 x E-6s (anti-) quarks and gluons were found in the plasma state
Between 1 x E-6 to 1s the plasma cooled to allow protons and neutrons to form
Acknowledgements – I would like to thank Professor Watkins and his colleagues from the LHC for the use of their images throughout the article
What are the components of matter?
The fundamental mass particles
We don’t know why there is such a vast range of quark masses
A proton has a mass of 1.67262178 × E-27 kilograms (about 938 MeV) and is made up of two up quarks and a down quark
A neutron has a mass of 1.674927351(74) x E−27 kilograms (about 940 MeV) and is made up of two down quarks and one up quark
Forces – carried by particles
The forces between particles are all carried by particles too.
The electric force is due to the exchange of virtual photons.
Virtual particles briefly violate energy conservation due to the uncertainty principle from quantum mechanics.
The more precisely the position is determined, the less precisely the momentum is known in this instant, and vice versa.
–Heisenberg, uncertainty paper, 1927
5 December 1901 – 1 February 1976
The history of force unification
Terrestrial and celestial mechanics; Inertial vs. gravitational mass led to the law of Universal Gravitation – Isaac Newton 1687
Electricity and Magnetism led to electromagnetism (electromagnetic waves and photons). James Clerk Maxwell used work started by Faraday to produce the electromagnetic theory in 1860. The theory predicted the speed of light.
Electromagnetism and the weak force led to the electroweak force with intermediate W and Z bosons (1970-1983). The Boson masses are about 100 times larger than the proton.
Probing shorter distances reveals deeper irregularities
What do we know?
We know that all matter on the Earth consists of up and down quarks, electrons and electron neutrinos.
We know that photons are connected with the electromagnetic force
We know the gluon is connected with the strong force (gluons can change into quarks and anti-quarks and back again)
We know that the W Boson is connected to the weak force
We have a greater knowledge of the Higgs Boson (produced when the Higgs field is excited)
Unification of Forces
The aim is to describe everything by one single force which is not the case at the moment.
The fundamental forces act by exchanging particles:
Gravitation particle is believed to be the graviton (not seen yet) for gravity – solar system, galaxies …- it is an extremely weak force
Electromagnetism particle is the photon – atoms, electricity
The weak force particles are the W and Z Boson – beta decay and how stars generate energy
The strong force particle is the gluon – binds quarks inside the proton and the neutron
Discovery of the W+/– and Z0 Bosons
The “Standard Model”: the most precise theory there is!
“Feynman diagrams” allow us to calculate any processes with high precision
Physicists use Feynman diagrams to visualize particle production and decay. Follow this link to learn about Feynman diagrams.
May 11, 1918 – February 15, 1988
The graph that is bottom right was evidence for the Z Boson
Tested in many experiments since 1960s
The standard model
The Standard Model of particle physics is a theory concerning the electromagnetic, weak, and strong nuclear interactions, which mediate the dynamics of the known subatomic particles. It was developed throughout the latter half of the 20th century, as a collaborative effort of scientists around the world.
Higgs (or BEH) Field – Universal Higgs Field?
Major objective of the LHC –
What is the origin of Mass?
An idea from 50 years ago – Higgs field
Higgs boson is excitation of this field
Is it a universal quantum field?
Massive force carriers (like W and Z bosons) forbidden by the theory unless symmetry broken in very special way
Shape of the Higgs field
In early universe Higgs field was zero – all particles massless
… a lot happened and by the mid 90’s the Higgs boson was considered the most critical particle to be found experimentally
Search for the Higgs boson
Higgs boson properties was predicted by theory (except its mass) about 50 years ago
Once mass is known theory predicts how often it decays to different types of particles and how often it should be produced in proton-proton collisions at the LHC.
It is produced very rarely (1 in E+13 collisions) and decays in about E-22 seconds. It travels a distance of about the size of a proton!
European Laboratory for Particle Physics
World’s largest particle physics laboratory – Multi-national laboratory near Geneva
The Large Hadron Collider (LHC)
The LHC is a 27km accelerator which collides counter-rotating beams of protons. Collision energy is 8TeV. In 2015 this will go up to 13 TeV
Tev = million million eV (1 ev is 1.6 x E-19J)
CERN laboratory on Swiss – French border
Every day, around 10 000 scientists from all over the world perform research at CERN
21 European Member States and around 60 additional countries collaborate in our scientific projects.
Building the LHC
In the main ring:
1746 superconducting magnets
… including 1232 15m SC dipoles
… weighing 27 tonnes each
… producing 8.36 Tesla
… and running at –270C
… needs 700,000 litres liquid He
… and 12 million litres liquid Nitrogen
The fastest racetrack on the planet – The protons will reach 99.9999991% speed of light, and go round the 27km ring 11,000 times per second
A very good vacuum with ten times less atmosphere than the moon inside the LHC beam pipes
The coldest place – The LHC operates at -271 C (1.9K), this is colder than outer space. A total of 36,800 tonnes are cooled to this temperature.
Could this be the largest refrigerator ever?
At four places the beams intersect
There are hot spots too! When the two beams of protons collide, they will generate temperatures 1000 million times hotter than the heart of the sun, but in a minuscule space
General Concept: E=mc^2
c = speed of light, m = particle mass, E = particle energy
Collide 2 protons with energy E = 4TeV each giving a total energy of 8 TeV. This can create particle X with mass equivalent of less than 8 TeV.
Actual interactions occur between quarks and gluons that carry part of proton energy. Most particles we create live only for a very short fraction of a second and then decay
Inside the Proton
A new era for particle physics
7,000 tonnes, 42m long, 22m wide, 22m high (About the height of a 5 storey building)
3000 Physicists from 176 Institutes in 38 Countries
The image above gives you some idea of the size of ATLAS (it isn’t really stuck between two buildings)
Charged Particle Tracking Detectors
Layers of silicon detectors
In particle physics, a calorimeter is an experimental apparatus that measures the energy of particles. Most particles enter the calorimeter and initiate a particle shower and the particles’ energy is deposited in the calorimeter, collected, and measured. The energy may be measured in its entirety, requiring total containment of the particle shower, or it may be sampled. Typically, calorimeters are segmented transversely to provide information about the direction of the particle or particles, as well as the energy deposited, and longitudinal segmentation can provide information about the identity of the particle based on the shape of the shower as it develops. Calorimetry design is an active area of research in particle physics.
First proton-proton collision took place on November 23rd, 2009
First 7 TeV on the 30th March 2010
Which collisions to record?
Proton bunches collide 40 million times per second producing 1000 million proton collisions per second. Only 400 collisions per second are recorded so the selection of which collisions to record is done in a millionth of a second e.g. is it to be an energetic electron or muon.
A Triggering device is an electronic circuit which is used to control another electronic circuit.
CP Module selects electron candidates
A great deal of computing power is needed for the processing of collision results
The Worldwide LHC Computing Grid (WLCG) is an international collaborative project that consists of a grid-based computer network infrastructure incorporating over 170 computing centres in 36 countries, as of 2012, world’s largest computing grid. It was designed by CERN to handle the prodigious volume of data produced by Large Hadron Collider (LHC) experiments.
By 2012, data from over 300 trillion (3 x E14) LHC proton-proton collisions had been analysed, and LHC collision data was being produced at approximately 25 petabytes per year. Over 100000 processors are used using ultra-high speed data transfers (millions of gigabytes of data per year).
Huge data volumes: 600 MB/s; 5,000 TB/year
Huge CPU requirements: 15 s/event
Data stored and analysed on world-wide LHC computing grid: 11 clouds across the globe
LHC Data Taking: 2010-2012
1 x E15 interactions
24/7 operation typically from March – October each year
Rate of interactions:
About 1 billion interactions per second
Fast “trigger” decision => record about 400 events/second
The image below shows jets with 1.9 and 1.7 TeV transverse momenta (pT)
Steps to a new discovery
1) Measure many collisions
2) Understand your detector very well
3) Select and analyse the ‘events’
Many events include known particles but do some collisions show new features?
Finding the mass of a short lived particle
E^2 = p^2c^2+ m^2c^4 where m is rest mass, E is energy, p is momentum and c is the speed of light
Very Short-lived particles
W, Z or Higgs bosons decay very close to collision point so you can only detect them from their decay products
Z boson candidate
Bremsstrahlung i.e. “braking radiation” or “deceleration radiation” is electromagnetic radiation produced by the deceleration of a charged particle when deflected by another charged particle, typically an electron by an atomic nucleus. The moving particle loses kinetic energy, which is converted into a photon, thus satisfying the law of conservation of energy. The term is also used to refer to the process of producing the radiation. Bremsstrahlung has a continuous spectrum, which becomes more intense and whose peak intensity shifts toward higher frequencies as the change of the energy of the accelerated particles increases.
Higgs Boson Decay
How would we see the Higgs Boson?
July 4, 2012 – Higgs-like Boson Discovered
A joint seminar was held between CERN and the International Conference for High-Energy Physics in Melbourne, Australia.
500,000 watched a webcast of the event
TV channels broadcast to about 1,000,000,000 more
Someone was even watching in Antarctica!
Two experiments on the Large Hadron Collider, ATLAS and CMS, were presenting preliminary results from 2011 and 2012 data in their search for the Higgs Boson.
Higgs boson search challenges
Mass wasn’t known; It was rarely produced
Decay rates were predicted: H to Z Z (Z decays to e+e- or µ+µ-) or H to gg
Search for mass peaks in these decays
Two photon decay candidate
Two photon mass distributions
Both ATLAS and CMS experiments reported significant excesses of events in their mass plots around 125 GeV in several decay channels
Higgs to Z + Z then decay to four charged leptons (e.g. e-e+e-e+)
Discovery of a New Particle
Properties similar to those of Higgs boson
Signal strength consistent with expectation
Conclusion on Higgs
The Higgs field is required to explain why the W and Z bosons are massive
It predicts the existence of a spin zero boson
The ATLAS and CMS experiments at the LHC have discovered a new particle mass~125 GeV
Now comparing its detailed properties with those predicted for Standard Model Higgs boson
Currently the experimental results are consistent with these predictions
July 4th 2012 made many physicists including Peter Higgs very happy
Some of the questions and next steps
Dark matter is a type of matter in astronomy and cosmology hypothesized to account for effects that appear to be the result of mass where such mass cannot be seen.
Galaxies and clusters of galaxies rotate too fast –> dark matter
Where has all the anti-matter gone?
What else is out there?
Various ideas considered: New forces of nature; Extra dimensions of space Suggested by e.g. string theory; Microscopic black holes
The LHC experiments can look for all of these. Also sensitive to something “completely different”
For each ½-integer spin particle (Fermion) there is an integer spin partner (Boson) and vice versa:
Complete spectrum of partners to standard model particles;
Their spins are different by ½ unit;
They are heavier (or else we’d have seen them already).
Large Hadron Collider and its experiments ran very successfully in 2010 / 2011 at 7 TeV
Energy for 2012 collisions was at 8 TeV
Recorded p p collisions in 2011 ~ 5 fb^-1
Recorded p p collisions in 2012 ~ 22 fb^-1
Jan/Feb 2013 collisions of p Pb ions
The physicists are busy analysing all this recent data to see what else they can discover ….
Shutdown in 2013/2014 to prepare LHC for 13-14 TeV and upgrade the detectors
Excellent 7 minute video on CERN, Particle Physics and Higgs
Several 10 minute recordings of tours of LHC experiments and related interviews
Many puzzles remain
Many other analyses ongoing in parallel at LHC e.g. searches for Dark Matter particles
From 2015: collision energy increases from 8 to 13 TeV so there is a great chance to discover other new particles. More precise Higgs boson measurements can be made. Higher luminosity beams will allow many new searches.
Fundamental research has always been a driving force for innovation