Accelerators and High Energy Physics
Rohini M Godbole
Centre for High Energy Physics Indian Institute of Science, Bangalore. IISc
Miranda House Symposium Feb 2, 2009
Outline ➢The essential link between Accelerators 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)
History History of Particle Physics as a subject is essentially the path taken by the quest ' What is everything around us made up of ? ' The answer has changed through the centuries beginning from the basic idea of the 'Panchmahabhootas' (five elements) : Earth,
Water, Fire, Wind and Space) TO
Atoms .... Nuclei .... quarks,leptons,W/Z......
Particle Physics
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? 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.
Fundamental Constituents Currently Accepted Picture
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 Force : Force responsible for holding the nucelons 1
(proton/neutron) together in a nucleus.
➢Strong Force : Force which holds the quarks and gluons in a Nucleon.
Particle Physics (con.)
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 pointlike !!
Tools of seeing structure
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 First Fundamental particle the electron. The love affair has never stopped.
The Discovery of the Nucleus Target
How was the Nucleus discovered?
Beam of MeV energy particles .. Already a factor of a Million!! E. Rutherford
Beam,Target and Detector:
Detector
HEP experiment of today!! The beam was scattered from the
target and scintillations on the zinc sulphide screen were counted.
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 is proportional to the wavelength of light
if something is smaller than it will appear as a point
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 ~ ℏ / P Radioactive Nuclei emit ray with energy ~ MeV 10 10 10 11 cm. So for this probe anything < 10 11 cm. will be a 'point'. cm. will be a 'point'
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.
Two ways in which the high energy is useful I Revealing structures at smaller and smaller distances
I I 2 , to produce 1) Since E = m c new particles whose masses and other properties were predicted in the theories that were formulated 2) Check the predictions of the theory for interactions of particles in the laboratory under controlled conditions.
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 ¹ or the 'strange ' particles 'kaons' for example. Indian Physicists participated in these early experiments with Cosmic Rays. Even today the area continues to bring surprises and also interesting information.
Synergy between Particle Physics and Accelerators Beginning from Rutherford, particle physicists wanted always BEAMS with 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 !!
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
Beginning of artificial acceleration. It began with Cockcroft and Walton: Ordinary electrostatic acceleration: The first accelerator accelerated protons to 0.5 MeV and caused artificial disintegration of nuclei in the Cavendish Laboratory. The first WC accelerator
One in use today at Fermilab.
Circular accelerators: Vacuum chamber of the first cyclotron by Lawrence and Livingston
Cyclotrons , Synchrotrons Super Proton Synchrotron...
Proton, Antiproton Collider.
The Tevatron Ring.
Deep Inelastic Scattering: Rutherford's experiment with accelerated electrons and protons/nuclei Stanford Linear Accelerator : 2 mile long. (SLAC) Revealed that protons are made of quarks.
e ( E') DIS
µ e (E)
Target T
Hofstadter: same for Nuclei.
Different Ways of Using Beams Making antiparticles
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.
Accelerators and Colliders (con.) 2) Colliders:
Beams of accelerated particles collide against each other. a) p p and p p colliders : Began with the CERN PS (Proton
pp Synchrotron) which was a pp machine, then came along S S machine where the protons and antiprotons collided against each other. (Circular Colliders) b) e + e colliders have electron and positrons colliding against each other c) ep colliders where an electron or positron beam collides with a proton beam.
Fixed Target Machines vs Colliders. Fixed Target Machine : Energy available for particle production much less than the beam energy. Why? The beam has energy Eb (say), target is at rest , so E T = M T .Total Center of mass energy of this system is
s= 2 MT∗Eb 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=2 Eb For a given beam energy the fixed target mode loses a lot of energy in the motion of the center of mass.
Colliders vs Fixed Target Machines Collider Mode more efficient (for a given beam energy) for new particle production. But energy is only one consideration. Other important characteristic of a machine is the Luminosity ℒ of collisions, i.e., the number of collisions per unit area, per unit time. Obviously easier to achieve higher 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 with beams, fixed target expts. only option. 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 antiproton beam a special case).
Colliders using antiprotons. Creating intense, focused beams of antiprotons was a difficult job pp had The developments leading up to construction of the S S indicated that it will be possible to achieve energies required to make pp the W and Z , only if we could achieve luminosities that are appreciable, for the beam energies then available. 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.
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 particles pp are the (anti)quarks, gluons. Hence on the average only 1/6 th energy of the proton is available to the colliding partons. energy at constituent level effective E (e+ e ) ~ 6 E (pp) cm
cm
✗ e e environment is much cleaner to study. Theoretically +
better understood experimentally easier to handle ( no through going energy into beam pipes)
e e collider vs hadron collider (con) +
Hadronic Collider
Leptonic Collisions
e+
e
Particles with strong and electro magnetic interactions can be directly produced.
p
3
p
4
Only particles with electroweak charges can be produced directly.
e e collider vs hadron collider (con) +
In case of protons the (anti)quarks and gluons carry about 0.2 to 0.3 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
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 energy exploratory physics and e+ e colliders are better for detailed, precision studies or discoveries where it depends on very clean, background free environment.
Tevatron: Go to www.fnal.gov or www.cern.ch
Indian HEP groups participate in these experiments .. the collider is running at present , studying top quark physics.
Time Type GeV Comments
1996 2003 BES Tau factory
e+ x e
1.5 1.5
Old PEPII Ring High Luminosity
ep collider HERA in Hamburg, DESY. www.desy.de will give you more information.
Now also a collider colliding heavy ions against each other called RHIC is running in Brookhaven. Indian participation there too.
pep – II Ring for BABAR www.slac.stanford.edu will give more information. Has done high luminosity and hence high precision B physics giving information on matterantimatter asymmetry in the nature and hence clues to that in the Universe. Similar machine in Japan as well, Indian HEP groups participate in these experiments
Energies of accelerators through the years
More modern colliders and the physics they did
What the Colliders have taught us? Gf , Mw, Mz
CERN SppS discovered W and Z Rubia, van der Meer Nobel Prize
Precision measurements of W/Z LEP,Tevatron
Accurate prediction of top mass
Tested at LEPII, Tevatron test checked a deep theoretical issue in the formulation of the Standard Model of particle physics.
t'Hooft and Veltman Nobel Prize
Further precision measurements of Mw,Mz LEPII, Tevatron LHC to look for Higgs
Predict Mh
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
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 Next e+e collider can not be circular. ● Has to be linear ● Luminosities required are large, because expected crosssections are small. ● Being Linear these are single pass, not storage colliders. ●
●
High density bunches.
●
BeamBeam Interactions gives rise to emission of radiation called
'Beamstrahlung' ●
This gives rise to photonphoton 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: 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 BELLE , BABAR explored high luminosity, low energy frontier.
The Large Hadron Collider is exploring the high energy, high luminosity frontiers. International Linear Collider part of the High energy and high luminosity frontier which we hope will 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 need young people to participate in this adventure.