Particle Colliders

8/14/2019 Particle Colliders

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IISc

Rohini M Godbole

Centre for High Energy Physics

Indian Institute of Science, Bangalore.

Miranda House Symposium

Feb 2, 2009

Accelerators and High Energy

Physics


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OutlineThe essential link betweenAccelerators and Particle Physics

alternatively called ' High Energy Physics'

A small tour through History and different environments for expts.

What are the current and future accelerators that Particle Physicists

are looking at

Really futuristic Accelerators and Colliders: e + e - ,

Role being played by Indian Physicists? (most of the

talks in this symposium)


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Particle Physics

i) What are the elementary constituents of matter?

ii) What holds them together?iii)What is the correct mathematical framework to describe how these

constituents are put together to form matter that we observe around us

and describe its behavior under different conditions?

Particle physicists have arrived at an accepted set of (almost)complete answers to these questions.

Accelerators have played an indispensible role in this.

This talk will try to give you a flavor of this role that they have played.

Subject of particle physics has developed in the last 110 years

(for example: e-

, neutron was discovered in 1897 and 1936 resp.)What does it deal with?


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Fundamental Constituents

Currently Accepted PictureCurren

tly Accepted Picture


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Particle Physics (con.)Four Basic Forces of Nature:

Gravitational Force: Force holding us on the earth

Electromagnetic Force: Force holding the electrons in the atoms

Weak Force: Force responsible for the decay of radioactive nuclei.

Strong Force1 : Force responsible for holding the nucelons

(proton/neutron) together in a nucleus.

Strong Force : Force which holds the quarks and gluons in a

Nucleon.


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Particle Physics (con.)


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Same Picture : better drawn!

Distance scales at which Accelerators

have helped us reveal new structures

AND

along with the development of the

theoretical models of fundamental

constituents and interactions among

them, have helped us conclude that

quarks and leptons have a size smaller

than 10 -18 meters,if at all they are

NOT point-like !!


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Tools of seeing structure


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The beginning

The first elementary particle to be discovered was the electron

about 110 years ago (1897: J.J. Thompson)

What was the apparatus: A cathode ray tube:

What does a cathode ray tube do?Accelerate the electrons

emitted by the hot filament, using potential difference.

So this is the classic accelerator used for the discovery of the

FirstFundamental particle the electron.

The love affair has never stopped.


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The Discovery of the Nucleus

How was the Nucleus discovered?Beam of MeV energy

particles ..

Already a factor of a

Million!!

Target

Detector The beam was scattered from the

target and scintillations on the zinc

sulphide screen were counted.

E. Rutherford

Beam,Target and

Detector:

HEP experiment of today!!


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Why did the Nucleus appear to be a point to

Rutherford?

Nucleus had arrived:

Picture of an atom made up of a 'point' nucleus and electrons

orbiting around it (1911)

Chadwick found neutrons in 1936.

Remember that the resolving power of a microscope isproportional to the wavelength of light

if something is smaller than it will appear as a point


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Why did the Nucleus appear to be a point to

Rutherford? (con.) In Rutherford's experiment the 'light' was the particle

Photon of energy E = h has wavelength

= c/ = h c / h = h c/E

where c is the velocity of light

Wave Particle Duality means that a particle with momentum P

and energy E will have wavelength

~ / PRadioactive Nuclei emit ray with energy ~ MeV

10 -10 - 10 -11 cm.

So for this probe anythingSo for this probe anything


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Resolution of a structure depends on the

wavelength of the probe.

To Probe structure of

matter through scattering

experiments one needs

particles with smaller and

smaller wavelengths and

hence higher and higher

energies

Hence Fundamental Particle Physics is synonymous with

High Energy Physics and thus intimately connected with the

world of Accelerators.


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Two ways in which the high energy is useful

I I I

Revealing structures at

smaller and smaller distances

1) Since E = m c 2 , to produce

new particles whose masses

and other properties were predicted

in the theories that were

formulated

2) Check the predictions of thetheory for interactions of particles

in the laboratory under controlled

conditions.


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Where were the early high energy beams available?

Nature's own Accelerator : Cosmic Ray Experiments:

Early discoveries of 'NEW' particles first

happened in Cosmic Ray Experiments:

1) Discovery of the Positron, antiparticle

of the electron (1931)2) Discovery of the or the 'strange '

particles 'kaons' for example.

Indian Physicists participated in

these early experiments withCosmic Rays.

Even today the area continues to bring

surprises and also interesting

information.


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Synergy between Particle Physics and Accelerators

Beginning from Rutherford, particle physicists wanted always BEAMSwith higher and higher energy.

Quotation from Rutherford:

It has long been my ambition to have available for studies a copious

supply of atoms and electrons which have energies transcending those

of the , particles from the radioactive bodies

Gamow's work on Barrier penetration proved 0.5 MeV may even be

enough for nuclear disintegration.

Note the role played by theoretical developments to set the challenges

to accelerator development right from the inception !!


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Synergy between Particle Physics and Accelerators

Then the goal was modest: 0.5 MeV

1) Cockcroft Walton : Electrostatic Acceleration

(ancestor of the Van de Graaf generator , Pelletron etc.

used in Nuclear Physics, Solid State Physics... For HEP

today it is used in early stages to get the initial beam)Limitation to the energy.

2) Cyclotrons : Using Magnetic Resonance Acceleration.

3) Synchrocyclotrons: Frequency Modulated Resonance

Acceleration.

Higher energies required by HEP possible using (2) and (3) ..

basically


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Beginning of artificial acceleration.

It began with Cockcroft and Walton: Ordinary electrostatic acceleration:

The first accelerator acceleratedprotons to 0.5 MeV and caused

artificial disintegration of nuclei

in the Cavendish Laboratory.

The first W-C accelerator

One in use today at Fermilab.


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Cyclotrons , Synchrotrons

Super Proton Synchrotron...

Circular accelerators:

Vacuum chamber of the firstcyclotron by Lawrence and

Livingston

The Tevatron Ring.

Proton, Anti-proton Collider.


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Deep Inelastic Scattering:

Rutherford's experiment with accelerated electrons and protons/nucleiStanford Linear Accelerator : 2 mile long. (SLAC)

e - ( E')

e - (E) Target T Hofstadter: same for Nuclei.

Revealed that protons are

made of quarks.

DIS


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Different Ways of Using Beams

Making anti-particles


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Accelerators and Colliders.

Modes of operation: Fixed Target Machines and Colliders.

1) Fixed Target Machines:

a) Electron, muon and neutrino beams incident on nucleon and

nuclear targets.

b) Proton, Pion Beams incident on a target consisting of nucleons or

heavy nuclei. Early experiments : Bubble Chamber Experiment

used Hydrogen/Deuterium Targets.

c) Beam dump experiments : dump high energy proton beam in a

heavy target, producing lot of mesons which in their decays

produce and , i. e. , muons and neutrinos. Thus one gets high

energy beams of neutrinos and muons to be used in (a).

Of course beam dump is used to also get pion and kaon beams as well.


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Accelerators and Colliders (con.)

2) Colliders:

Beams of accelerated particles collide against each other.

a) p p and p colliders : Began with the CERN PS (Proton

Synchrotron) which was a pp machine, then came along S Smachine where the protons and anti-protons collided against each

other. (Circular Colliders)

b) e + e - colliders have electron and positrons colliding againsteach other

c) ep colliders where an electron or positron beam collides with a

proton beam.

p

p

p


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Fixed Target Machine : Energy available for particle production much lessthan the beam energy. Why? The beam has energy E

b(say), target is at

rest , so ET

= MT .

Total Center of mass energy of this system is

s=2MTEb

Colliders: Mostly consist of two beams with equal energy (momentum)

colliding with each other. Thus the center of mass frame is the same as

the laboratory frame The total cms energy is given by

s=2Eb

For a given beam energy the fixed target mode loses a lot of energy in

the motion of the center of mass.

Fixed Target Machines vs Colliders.


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Colliders vs Fixed Target Machines

Collider Mode more efficient (for a given beam energy) for new particle

production.

Fixed target mode used in the early days for studies of structure as well

particle production.

Colliders became more popular when good collimating techniques

allowed making intense beams .. (even then anti-proton beam a special

case).

But energy is only one consideration. Other important characteristic of

a machine is the Luminosity of collisions, i.e., the number of

collisions per unit area, per unit time. Obviously easier to achievehigher luminosity with fixed targets which can be big in size.

For example, for neutrinos, the interaction strength is so small that

targets need to be huge!! So withbeams, fixed target expts. only option.


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Colliders using anti-protons.

Creating intense, focused beams of anti-protons was a difficult jobThe developments leading up to construction of the S S had

indicated that it will be possible to achieve energies required to make

the W and Z , only if we could achieve luminosities that are

appreciable, for the beam energies then available.

pp

pp

Van der Meer got the Nobel Prize along with Rubia for

making the focusing of the beams possible, using the method of

phase space cooling. Rubia led the experiment that found the

evidence for the W and Z, but would not have been possible

without the luminosity.

Thus developments in HEP experiments and Accelerator

designs continue to go hand in hand.


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e+ e- collider vs hadron collider

For e+ e- colliders the initial beam energy is very

accurately known as the colliding particles are the e + / e-

For pp or machines the colliding fundamental particlesare the (anti-)quarks, gluons. Hence on the average only

1/6 th energy of the proton is available to the collidingpartons. energy at constituent level effective

Ecm

(e+ e - ) ~ 6 Ecm

(pp)

e+

e-

environment is much cleaner to study. Theoreticallybetter understood experimentally easier to handle ( no

through going energy into beam pipes)

pp


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e+ e- collider vs hadron collider (con)

e+

e -

p3

p4

Hadronic ColliderLeptonic Collisions

Particles with strong and

electro magnetic interactions

can be directly produced.

Only particles with electro-weak charges

can be produced directly.


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In case of protons the (anti-)quarks and gluons carry about 0.2 to0.3 of the total energy. Hence the Total energy available for particle

production less than the total (anti-)proton energies.

But protons can be accelerated to higher energies much more easily.

Hence

1) e+ e- colliders limited energy for producing new particles

but clean environment and hence great for precision studies

2) Hadronic Colliders messy environment, all the beam energy

not available for particle production, BUT great for

extending the horizons of energy and discovering new aspects


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Hadrons easier to store, can be accelerated to higher energies and

suffer no energy loss in circular orbit.

Thus in general hadron colliders are better for highest energyexploratory physics and e+ e- colliders are better for detailed,

precision studies or discoveries where it depends on very clean,

background free environment.


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Tevatron:

Go to www.fnal.gov or www.cern.ch

Indian HEP groups participatein these experiments .. the

collider is running at present ,

studying top quark physics.
http://www.fnal.gov/http://www.fnal.gov/


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1996

- 2003 BES e+ x e -

1.5 1.5

Old PEP-II Ring

High LuminosityTau factory

Time Type GeV Comments


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ep collider HERA in Hamburg, DESY.

www.desy.de will give you more information.

Now also a collider

colliding heavy ionsagainst each other

called RHIC is

running in

Brookhaven. Indianparticipation there too.
http://www.desy.de/http://www.desy.de/


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pep II Ring for BABAR

www.slac.stanford.edu will give more information.

Has done high luminosity and hence high precision Bphysics giving information on matter-antimatter asymmetry in

the nature and hence clues to that in the Universe.

Similar machine in Japan aswell, Indian HEP groups

participate in these

experiments
http://www.slac.stanford.edu/http://www.slac.stanford.edu/


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Energies of accelerators through the years


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More modern colliders and the physics they did


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What the Colliders have taught us?

Gf , Mw,

Mz

Precision measurements of W/ZRubia, van der

Meer Nobel Prize

Further precision measurements of Mw,Mz

CERN SppS discovered W and Z

LEP,TevatronAccurate prediction of top mass

t'Hooft and Veltman

Nobel Prize

Tested at LEP-II, Tevatron test checked a

deep theoretical issue in the formulation

of the Standard Model of particle physics.

LHC to look for Higgs

Predict Mh

LEP-II, Tevatron

h h llid h h


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What the Colliders have taught us?

Precision study of

the Higgs

Next LinearCollider

LC

Deeper understanding of the Symmetry

breaking in Standard Model, Supersymmetry,.....

LHC and LC should have overlap in operations


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Large Hadron Collider


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Large Hadron Collider

Indian participation

1) in LHC machine building

2) pp experiment CMS detector

3) Heavy Ion Detector ALICE

4) Phenomenological studies

for both the experiments

Next e+ e Collider


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Next e+ e- Collider

Next e+e- collider can not be circular.

Has to be linear Luminosities required are large, because expected cross-sections

are small. Being Linear these are single pass, not storage colliders.

High density bunches. Beam-Beam Interactions gives rise to emission of radiation called

'Beamstrahlung'

This gives rise to photon-photon interactions at each beam collisions

In turn can give rise to large backgrounds In 1992 with Manuel Drees we pointed this out and it has affected

the Designs of Linear Colliders.

Interaction between particle physicists and accelerator physicists of

utmost importance.

Far Future:


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Far Future:

World wide Study Groups

http://hp0.cts.iisc.ernet.in/

Meetings/LCWG/

Indian Linear Collider

Working Group

Discussions on Machine,

detector studies going on.

Conclusions
http://hp0.cts.iisc.ernet.in/http://hp0.cts.iisc.ernet.in/


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Conclusions

BELLE , BABAR explored high luminosity, low energy frontier.

The Large Hadron Collider is exploring the high energy, highThe Large Hadron Collider is exploring the high energy, high

luminosity frontiers.luminosity frontiers.International Linear Collider part of theInternational Linear Collider part of the

High energy and high luminosity frontier which we hope willHigh energy and high luminosity frontier which we hope willbe explored next.be explored next.

Intense proton beam facility will explore the high intensity

frontier to study issues in strong interactions.

Future holds interesting physics and we needFuture holds interesting physics and we need

young people to participate in this adventure.young people to participate in this adventure.

Particle Colliders

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