Rugby 2014

Discovery of the Higgs Boson at the CERN Large Hadron Collider

Peter Watkins, University of Birmingham, UK

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The size of things

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http://scaleofuniverse.com/

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

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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?

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The fundamental mass particles

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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

http://en.wikipedia.org/wiki/Proton

http://en.wikipedia.org/wiki/Neutron

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.

http://hyperphysics.phy-astr.gsu.edu/hbase/uncer.html#c1

The more precisely the position is determined, the less precisely the momentum is known in this instant, and vice versa.

–Heisenberg, uncertainty paper, 1927

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5 December 1901 – 1 February 1976

http://en.wikipedia.org/wiki/Werner_Heisenberg

Fundamental forces

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The history of force unification

Terrestrial and celestial mechanics; Inertial vs. gravitational mass led to the law of Universal Gravitation – Isaac Newton 1687

http://en.wikipedia.org/wiki/Isaac_Newton

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http://en.wikipedia.org/wiki/Newton’s_law_of_universal_gravitation

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.

http://en.wikipedia.org/wiki/Michael_Faraday

http://en.wikipedia.org/wiki/James_Clerk_Maxwell

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http://en.wikipedia.org/wiki/Electromagnetism

http://abyss.uoregon.edu/~js/21st_century_science/lectures/lec04.html

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.

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http://www.particlezoo.net/

http://en.wikipedia.org/wiki/W_and_Z_bosons

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

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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

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The strong force particle is the gluon – binds quarks inside the proton and the neutron

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Discovery of the W+/– and Z0 Bosons

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http://www.hep.manchester.ac.uk/u/hanl/lecture/Lecture3_WZ.PDF

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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.

http://atlas.physicsmasterclasses.org/en/zpath_lhcphysics2.htm

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May 11, 1918 – February 15, 1988

http://en.wikipedia.org/wiki/Richard_Feynman

The graph that is bottom right was evidence for the Z Boson

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Tested in many experiments since 1960s

The standard model

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http://en.wikipedia.org/wiki/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.

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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?

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Massive force carriers (like W and Z bosons) forbidden by the theory unless symmetry broken in very special way

Shape of the Higgs field

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In early universe Higgs field was zero – all particles massless

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… 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!

CERN
European Laboratory for Particle Physics

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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

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Every day, around 10 000 scientists from all over the world perform research at CERN

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21 European Member States and around 60 additional countries collaborate in our scientific projects.

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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

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The accelerator

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

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A very good vacuum with ten times less atmosphere than the moon inside the LHC beam pipes

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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.

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Could this be the largest refrigerator ever?

Collision points

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At four places the beams intersect

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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

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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

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Inside the Proton

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A new era for particle physics

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ATLAS Detector

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

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The image above gives you some idea of the size of ATLAS (it isn’t really stuck between two buildings)

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Charged Particle Tracking Detectors

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Layers of silicon detectors

Electromagnetic Calorimeter

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More Calorimeters

http://en.wikipedia.org/wiki/Calorimeter_(particle_physics)

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.

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Muon Detectors

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First proton-proton collision took place on November 23rd, 2009

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First 7 TeV on the 30th March 2010

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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.

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http://en.wikipedia.org/wiki/Triggering_device

A Triggering device is an electronic circuit which is used to control another electronic circuit.

CP Module selects electron candidates

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A great deal of computing power is needed for the processing of collision results

http://en.wikipedia.org/wiki/Worldwide_LHC_Computing_Grid

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).

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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

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LHC Data Taking: 2010-2012

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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)

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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

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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

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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.

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Higgs Boson Decay

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How would we see the Higgs Boson?

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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.

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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.

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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

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Two photon mass distributions

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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+)

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Discovery of a New Particle

Properties similar to those of Higgs boson

Signal strength consistent with expectation

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http://en.wikipedia.org/wiki/Fran%C3%A7ois_Englert

http://en.wikipedia.org/wiki/Peter_Higgs

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

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July 4th 2012 made many physicists including Peter Higgs very happy

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Some of the questions and next steps

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DARK MATTER?

http://en.wikipedia.org/wiki/Dark_matter

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.

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Galaxies and clusters of galaxies rotate too fast –> dark matter

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Where has all the anti-matter gone?

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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”

Supersymmetry (SUSY)

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).

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Summary

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

http://atlas.ch

www.phdcomics.com/comics.php

Excellent 7 minute video on CERN, Particle Physics and Higgs

www.sixtysymbols.com

Several 10 minute recordings of tours of LHC experiments and related interviews

Outlook

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

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Supersymmetry

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