Unravelling the Mysteries of Matter with the CERN Large Hadron...

Unravelling the Mysteries of Matter with theCERN Large Hadron Collider

An Introduction/Overview of Particle PhysicsIntroductory Lecture August 3rd 2014

Bobby Samir AcharyaInternational Centre for Theoretical Physics

and King’s College London

African School of Fundamental Physics and Applications 2014, Dakar, Senegal

Bobby Samir Acharya ICTP/KCL

ASP2014, UCAD, Dakar, Senegal

Welcome Everyone to the 3rd African School in FundamentalPhysics and its ApplicationsEspecially welcome to the students who have travelled from all overAfrica and beyond and for whom this school has been developed.Also welcome to the lecturers and the LOC and UCAD for makingthis possibleFinally: thank you to the numerous international institutes anduniversities for their support.



Particles, the Universe and the LHC

All known elementaryparticles

Energy of theUniverse Large Hadron Collider

An introduction and overview of particle physics. Also: whyand what the LHC is for. Two Mysteries: Dark Matter andthe Higgs Boson



Atoms and the Periodic Table

All the matter that we know about on and in the Earth has thissimple classification. Simple enough to fit on a t-shirt!This strongly suggests an underlying, simpler structureBobby Samir Acharya ICTP/KCL


Particles, Matter and the Universe

We, and the matter which makesup all of the Earth is made ofatoms

The planets and stars also ALLseem to be made of atoms

The galaxy contains hundreds ofbillions (i.e. ∼ 1011) of starswhich are basically alike: somuch of the galaxy is also bemade of atoms.

Remarkably, all of the galaxies(and there are ∼ 1011 of them)also look quite similar!

Hence, we learn that much of thematter of the entire Universe ismade of atoms

We will ADD some veryimportant details to this picturelater.



Abundance of Elements in the Universe

The heavier the atom, the less of it there isSimpler atoms are easier to make!



The Structure of Atoms

The structure and regularity of theperiodic table suggests that atomsare made of even simpler objects

Electrons were discovered byThompson in so-called cathode raytubes around 1897.A wire filamentwith a current passing through wasseen to emit particles with negativecharge. This showed that atomscontained ”electrons”.α-particles (now known to beHelium nuclei) were discovered asradiation emitted by variousradioactive elements and compounds

Rutherford used beams ofα-particles scattered off Gold toprove that atoms ”must” contain avery dense nucleus

The α-particles were deflected atlarge angles, proving that atoms had”structure” - dense nucleus.

Shortly after this, the Bohr model of

the atom (protons and neutrons in a

dense nucleus, surrounded by a cloud

of electrons) was ”established”.



The Atom

The atom consists of a verydense nucleus surrounded by a”cloud of electrons”

Atoms have sizes of order10−11 to 10−10 m.

Nucleus is 10−4 times smaller!

Protons are positively charged.Neutrons are neutral.

Electrons have minus thecharge of protons but are muchlighter (me ∼ mp/2000)

But are protons, neutronsand electrons fundamental?Are these the only particles?Bobby Samir Acharya ICTP/KCL


Charged particles in Electric and Magnetic Fields

Electric current is just movingcharge

You can create a magnetic fieldby coiling a current carrying wire

By the same token, a magneticfield exerts a force on chargedparticles

Strong electric and magneticfields can be used to acceleratecharged particles to high energies!

This is how particle acceleratorswork: you will learn this in week3!



The galaxy is a particle collider!

The galaxy has a strongmagnetic field and chargedparticles!

A high energy particle iscreated in a star and broughtto Earth by the galacticmagnetic field

It strikes the upper atmosphereof Earth

Multiple interactions occurbetween the particle and theatoms in the atmosphere

Creates a cosmic ray ”shower”



The galaxy as a particle collider!

This is a particular examplewhen the original particle isa proton

This example producespions, photons, muons,electrons and neutrinos!

A muon from a cosmic raylike this just passed throughyour body!

In fact, muons and pionswere discovered in cosmicrays



The Particle Zoo

As a result of cosmic ray and ground-based particle colliderexperiments we now know that protons and neutrons areactually made of more fundamental particles called quarks andgluons

We also know that protons and neutrons have many ‘cousins’which are also made of quarks and gluons. Hundreds of thesehave been discovered over the past six decades.

Neutrons, when not bound inside atoms, actually decay into aproton, an electron and a neutral (almost massless) particlecalled a neutrino.

The most remarkable fact, though, is that all of thesehundreds of particles and their properties are preciselydescribed by a very simple model.



The Standard Model of Particle Physics

The entire periodictable can be explainedby just the first andlast columns!Last column are theforce carrying particlesThere are three”families” of quarksand leptons.Why notjust one?Why these particularmasses?

Are there more than

just these particles?



Mass and The Higgs Boson. Original Slide from 2011.See next slide for update

There is one particle predicted by the Standard Model whichhas not yet been discoveredThis is the so-called Higgs boson, after Peter Higgs who madesignificant contributions to this part of the Standard ModelAccording to the model, it is the Higgs boson which isresponsible for giving the quarks and leptons their massThe heavier the particle, the stronger its interactions with theHiggs particle, so the top quark has the strongest interactionwith the Higgs, the electron the weakest one.One of the main reasons for the LHC physics programme is tofind the Higgs boson, or prove that it doesn’t exist!It’s discovery would shed enormous light on the nature andorigin of mass and ”complete” the story of the StandardModelBobby Samir Acharya ICTP/KCL


Mass and The Higgs Boson. Slide updated 2013

There is one additional particle predicted by the StandardModel which was not yet mentioned.This is the so-called Higgs boson, after Peter Higgs who madesignificant contributions to this part of the Standard Model in1964According to the model, it is the Higgs boson which isresponsible for giving the quarks and leptons their massThe heavier the particle, the stronger its interactions with theHiggs particle, so the top quark has the strongest interactionwith the Higgs, the electron the weakest one.One of the main reasons for the LHC physics programme wasto find the Higgs boson, or prove that it doesn’t exist!It’s discovery would shed enormous light on the nature andorigin of mass and ”complete” the story of the StandardModelThe Higgs boson was discovered at the LHC in 2012 andPeter Higgs and Francois Englert were awarded the 2013Nobel Prize for Physics.



The LHC and The Higgs Boson

Much data from the LHC collected in 2011 and 2012

The first evidence for the existence of the Higgs boson hasemerged

A new particle has been discovered

So far its properties are remarkably consistent with the Higgsboson

More data is being collected and analysed to determine if theStandard Model is correct

These are very exciting times. More about the LHC and theHiggs boson later in the course.



Dark Matter

Einstein’s theory of gravity has been tested with remarkableaccuracy in various systems within the galaxyBut, if you estimate the mass of galaxies by the number ofatoms that they contain, there’s a huge problemGalaxies of that mass would not move in the way that thegalaxies have been observed to be movingThere is now very strong evidence that there is additional,”dark” matter in the Universe, beyond protons, neutrons,electrons, photons and neutrinosIn fact there seems to be about five times more dark matterthan ”atomic matter”!There are some plausible arguments that particles made ofdark matter could be produced directly at the LHC (more onthis later)



Cosmic History

We understand most of thehistory of the Universe interms of the Standard Modelof particle physics



The Large Hadron Collider

The basic idea is to :

accelerate beams of protons to as high energy as possiblesmash them into each other as often as possibleCreate new particles like the Higgs boson or dark matterDetect these particles in ”particle detectors” which surroundthe ”collisions”

The production of ”new particles” is rare, so the morecollisions we have the better

The LHC ran successfully in 2011 and 2012. Will restart in2015 at even higher energy!



The LHC Accelerator complex

A circular, 27 km tunnel,roughly 100 m undergroundTwo proton beams, oneclockwise, one anti-clockwiseFour collision pointsUp to a billion collisions persecond!!



The LHC Accelerator complex

About 1600superconducting magnets(most weigh more than20 tonnes!) used toaccelerate the protonbeamsAbout 100 tonnes ofliquid helium required tokeep the magnets atoperating temperature ofminus 271 CentigradeThe LHC is the coolestplace in the Universe!The magnetic field isabout 8 Tesla (about 105

times that of the Earth!)



The LHC beam and Energy

Each proton beam is made of bunches of protons. Up to 2808bunches with 1011 protons each. Energy of each proton 7000GeV.Note: One GeV = about 1.6× 10−10 Joules and 1 J = 1Watt second.Energy per beam = 2808× 1011 × 7000 GeV= 2808× 1011 × 7000× 1.6× 10−10J = 362 MJEquivalent to 87 Kilograms of TNT, which has about 1016

times as many protons as the LHC beam!Equivalent to the Kinetic Energy of a small aircraft carriermoving at 40 km per hour!The energy in the magnets is about 1010 Joules. (A typicalhouse in Europe consumes about 2000 Joules per second onaverage).



A Particle Detector



Particle Detection in ATLAS



Processes and Production Rates at the LHC

M.Nessi - CERN 7

TeV production rates

Whatever accelerator might cover theTeV scale, it must produce a highenough event rate to be statisticallysignificant

Nrate = ! * Luminosity * BR

Beam Energy needed to produce newmassive particles such as the Higgsboson --> TeV

Beam Intensity needed becausesome of the processes that onewould like to study are very rare(e.g. small !.B for decay modes

visible above background) -->L=1034 cm-2 sec-1

Higgs showcase: m=500 GeV, ! ~ pb,BR(to 4 µ)~10-3

N~few/day -> L=1034 cm-2 sec-1



Producing Higgs Bosons with the LHC

Once produced,the Higgs decaysto lighterStandard ModelParticlesTwo photons, twotau leptons, two Zbosons,...We then measurethese decayproducts and”reconstruct” theHiggs!



Many ideas to be explored.....

Theoretical physicists have come up with many ideas forphysics beyond the Standard Model

These predict new particles and phenomena for the LHC:

Supersymmetry predicts a new particle for every StandardModel particle eg a super-electron (or selectron).

The LHC may discover extra dimensions. These are predictedby string theory and could lead to the discovery of tiny blackholes!

So far, these ideas have not been discovered at the LHC, solimits on such models have been obtained!



Unravelling the Mysteries of Matter with the CERN Large Hadron...

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