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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e+ e- collider vs hadron collider (con)
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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e+ e- collider vs hadron collider (con)
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.
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