Particle Accelerators

What is an accelerator?

An accelerator is a device to produce beams of charged particles and accelerate them to high velocity or energy (much greater than that for electrons or nuclei in atoms) with:

• Nearly constant velocity or energy
• Parallel trajectories
• Small beam cross section

and to deliver these particles to a target where they scatter from the target constituents and:

• Reveal the structure of the constituents (a microscope)
• Create new particles for study
• Modify the character of the target (e.g. a tumor)

source

vacuum tube with force stations

target

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Why build accelerators?

In the 1930’s, physicists wanted to understand the structure of the atomic nucleus, and the strong force that binds it together. Naturally occurring radioactive decays give particles of energies of a few MeV; need to produce particles of higher energy.

In 1931, E.O. Lawrence made the first modern particle accelerator – the cyclotron – with successive versions reaching over 100 MeV.

The need for higher energy is two-fold:

1. Creation of new nuclei/particles of higher mass than the natural elements requires energy : E = mc². Nowadays we seek new states of matter at the millions of MeV level. Today’s accelerators deliver beams of ~10 TeV.

2. In analogy with the microscope, seeing finer detail within subatomic particles requires a wavelength λ smaller than the size of the object. deBroglie told us that λ = h/p, and the momentum p ~ E. Modern accelerators probe structure at the 10^-19 m level.

1 eV = energy gained by electron by 1 V battery.

1 keV = 1000 eV

1 MeV = 10⁶ eV

1 GeV = 10⁹ eV

1 TeV = 10¹² eV

electron binding in atoms ≈ 1 eV

proton binding in nuclei ≈ 1 MeV

proton mass energy ≈1 GeV

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What can we accelerate?

All accelerators are based on acceleration of charged particles by electric forces.

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We accelerate elementary particles and nuclei which have small mass (F = ma, so a = F/m) to get beams of high velocity (and energy).

Bringing a charged particle from nearly at rest to high energy requires that it be sufficiently stable to not decay in flight.

The particles must exist in sufficient abundance to give high intensity beams (the collision processes are rare).

⇒ This limits the available particles to electrons and positrons, protons, nuclei (atoms stripped of their electrons) and antiprotons (though making them in abundance is hard). Maybe in future can use muons (but they decay in a microsecond!).

proton of charge q

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M87 – giant elliptical long exposure

short exposure shows a jet emerging from galaxy

Magnified jet viewed in radio

Magnified jet viewed in IR

Nature’s accelerators

Several views of the M87 galaxy in the Virgo cluster

As usual, Nature is more clever than we are: many galaxies show ‘jets’ of light, X-rays resulting from electrons accelerated to high velocity by the spinning black hole at their center. The light results from radiation by the electrons curling around very strong magnetic fields.

But assembling a supermassive black hole on earth is not recommended.

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How to accelerate?

We surf the wave!

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v

E_z

z

How to accelerate?

Make the rf wave travel at the speed of the particles, so get a continuous push. Synchronize the particle bunches just ahead of the peak of the wave. Thus a particle that is a little too low in velocity falls behind where E is larger, and so gets a larger push to catch up with the bunch.

The rf wave is confined in a set of ‘cavities’ whose shape controls the speed of the wave.

Energy gain = force x distance = q E l Energy gain/meter of 30 – 100 MeV/m is possible using rf cavities.

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How to contain the beams?

The particles tend to stray from the straight and narrow. Any deviation of particle direction from the desired axis would lead to beam blow-up.

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Such divergence occurs due to the particle source, mutual repulsion of beam particles, imperfections in the accelerator, etc.

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We need a way to restore directions to the desired axis.

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How to contain the beams?

Strong focusing principle:

Quadrupole magnets focus the beams, similar to lenses in ordinary optics (4 poles rather than 2 for dipole) and magnetic field pattern as shown. Field grows linearly with distance from center. Particle is deflected toward center. Particle is deflected away from center. So focus horizontally and defocus vertically (and vice versa).

Ray hits the horizontal focusing element further out and bends more than at the defocussing element. So get net focussing effect. Same in vertical plane.

Alternate F and D elements to oscillate the particles around the beam axis (betatron oscillations).

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Betatron oscillations

Individual particles oscillate around their central orbit, but stay confined.

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Clever idea #1

Conserve real estate and save on components by bending the beams into a circular path so that they repeatedly traverse the same rf accelerating cavities and quadrupoles. Do this by passing the beams through bending magnets.

F = ma ⇒ qvB = mv²/R or E(GeV) = 0.3 B(T) x R(m)

The world’s most powerful accelerator – LHC at CERN, starting this year – has average magnetic field of 5.5T (8.3T in the magnets), and will accelerate protons to 7 TeV (7000 GeV). Thus R = 4.3 km.

If we wanted 100 TeV, would need a circular ring 125 km in diameter!

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(at large velocities ≈ c)

Limits on circular accelerators

When charged particles are accelerated, they radiate EM waves (for example acceleration in radio antennas creates the radio signal). Charges going in a circle are accelerated (centripetal), so they emit synchrotron radiation.

Energy radiated per turn ~ E⁴/(R m⁴) (E = beam energy, m=particle mass)

Lessons: power radiated grows very rapidly with beam energy. Making the circle larger helps but not very fast. And the radiation for electron accelerators is very much more than for protons (m_p/m_e = 2000).

For the LEP electron accelerator at CERN (100 GeV electron beams with R = 4.3 km) an electron loses 4% of its energy every time around the circle, and requires 20 MW of rf power just to break even. This is about the end of the road for circular electron accelerators, so must consider linear accelerators.

Radiation is along particle direction, like a searchlight – X-rays, UV and visible light. Used to study biological processes and materials in ‘light sources’.

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Old way of using beams

Accelerate protons in Tevatron to 100’s of GeV

Extract protons from Tevatron and send to a target.

A spray of many particles of all types are produced in the collisions in the target – π mesons, K mesons, photons, protons, neutrons etc.

Select and focus one stream of secondary particles and send to the experiment.

Tevatron

Target station

In the collision of beam and target particles, we must conserve momentum so produced particles must move to right, limiting the energy available to produce new particles.

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Clever idea #2

Use 2 opposing beams colliding head-on. Now the net momentum is zero, so no energy is wasted on unneeded motion of final particles.

The full energy of the beams is available for creating new particles.

Colliding beam accelerator

New complications: must accelerate 2 beams, and control both carefully to bring them into collision at the same very small spot.

In circular collider, if oppositely charged particles like p and p (Tevatron), e⁺ e⁻ (LEP), need just one set of magnets to guide both beams. If same sign particles (RHIC, LHC), need two sets of magnets.

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Fermilab proton-antiproton collider

Main Injector (antiprotons)

Antiproton accumulator

Main Injector (protons)

1. Three circular accelerators (8, 120, 1000 GeV), plus two injector pre-accelerators

3. MI shoots protons into Tevatron.

Collisions at the experiment! 2 TeV available energy for particle creation.

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Linac

Booster

Antiproton Accumulator

Main Injector

Antiproton production target

Tevatron

DØ

CDF

Chicago

1.3 miles

Fermilab complex

Five accelerators in the complex for successive energy gains.

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BNL RHIC complex

rf accelerating station

Accelerates nuclei (e.g. Au). Both beams are positively charged, so need two magnet rings since directions of beams are opposite.

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Stanford Linear Accelerator Center

3 mile long linac, accelerating electrons or positrons to 50 GeV, used for fundamental studies of quarks until 2008. Now being converted to a free electron laser light source.

The Linac crosses above the foothills near San Francisco Bay, across the San Andreas fault and I-280.

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CERN and the Large Hadron Collider

The 5.5 mile diameter underground tunnel originally housed the e⁺e⁻ collider LEP. It is now being readied for 14 TeV proton-proton collisions at the LHC.

Mt. Blanc

Lake Geneva

City of Geneva

Airport

LHC

Tunnel housing LHC is ~100m underground. The energy stored in the beams is equivalent to an aircraft carrier moving at 12 knots, enough to melt 1 ton of Cu (be careful where the beam goes!!)

(LHC will not make black holes that swallow the earth!)

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International Linear Collider

We strongly believe that the LHC will discover new phenomen. To explain those new discoveries, we will need a complementary electron-positron collider operating at 500 – 1000 TeV. The ILC is being designed to do that job.

31 km

Two opposing linear (to eliminate synchrotron radiation) accelerators (e⁻ & e⁺) of about 10 km length each, bringing beams to a collision spot about 6 nm high by 100 nm wide. Initially each linac has E=250 GeV, upgradable to 500 GeV (1 TeV collisions).

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ILC systems

Show one-half of ILC – the electron linac. Positron side is nearly identical.

1. Source provides the polarized electrons
2. Pre-acceleration linac to 5 GeV
3. Damping ring to make the beam cross section very small

4. Bunch compressor squeeze bunch along beam direction
5. Main linac to accelerate to full energy
6. Bring beams to collision and remove them safely to dump

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ILC Sources

Electron source: Shine a laser (polarized light) on GaAs crystal and eject polarized electrons. Pre-accelerate to 5 GeV.

Positron source: Pass the accelerated electron beam through a magnet that wiggles the electrons in a helix, emitting polarized photons (synchrotron radiation). Let the photons strike a target creating positrons (and electrons). Send the polarized positrons to their damping ring and linac. Original electrons continue down their original path in the linac.

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ILC Damping Rings

Damping rings function is to reduce the size of the beam (in transverse positions and velocities). Bend the beams, emitting synchrotron radiation and reducing all components of momentum. Then boost just the momentum along the beam direction. Fractional transverse momenta are lowered.

3 initial particle rays with finite divergence

magnet bend

radiated photons carry off energy

reduced energy particles with same divergence

rf accel; increase longitudinal momentum

final rays back to original energy but smaller divergence

Test damping ring in Japan has achieved the small beam size required for ILC.

R&D needed to control buildup of electrons in rings that destroy the small size.

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Main Linacs (heart of ILC)

Ultra-pure Nb superconducting cavities made from 9 clam shells welded together. Inject rf wave to provide 35 MeV gain in 1 meter.

Require cavities with 35 MV/m and low loss (high Q). Some have reached this specification. But reliably preparing the very smooth surfaces needed is still a problem.

chemical etch

electro- polish

ILC specs

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Generating the rf electric field

Need 800 of these (they last for about 50,000 hours.

Klystron tube : convert pulse to 10 MW rf wave at 1.3 MHz.

Modulator : convert AC power from grid to 120 kV, 140 A pulses, 1 msec long.

26 cavities (1 quadrupole focus magnet) in three cryomodules, all fed by 1 klystron.

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Beam particles induce electromagnetic fields and currents in the walls of the enclosure: “wakefields”. If the beam is exactly on central axis, these fields cancel at the beam location.

If not, they create an net force on the beam that tries to blow it up.

Wakefield from head of bunch gives extra kick to tail of bunch, skewing the beam.

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Herding the cats

Herding the cats – need some negative feedback

Controlling the wakefields:

1. Align the beam cavities, magnets very accurately (micron level) – won’t work completely due to ground motion and environmental noise.
2. So need very sensitive detectors to measure the beam positions and give feedback to reposition the beams on the proper axis (on 1 sec level).
3. Wakefield mitigation by making the tail of the beam a little lower in energy so the quadrupole focussing restores the tail more than the head.

Wakefields can also affect subsequent bunches of particles; control by making the walls a bit resistive to damp the currents before the next beam bunch arrives.

Beam quality (size) control is a tricky business and gives much complexity.

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Hopes for the future

Machines like LHC and ILC are pushing the limits of technology and cost.

• Making magnets with > 10 Tesla fields is not presently possible. So circular machines must grow as energy grows.
• Synchrotron energy grows rapidly as energy increases – ultimately a limit for proton accelerators as well as electrons.
• Electron linacs like ILC are limited by the available accelerating gradients in the cavities: We run into intrinsic materials breakdowns for > 50 MV/m; so can increase energy only by making longer accelerator.

Increased size ⇒ cost (most of cost is related to civil construction, or components that fill the accelerator length).

How might accelerators of the future evade today’s limits?

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Plasma accelerators (good wakefields?)

R&D effort to develop plasma wakefield accelerators with accelerating gradients ~1000 times that available with conventional RF cavities.

Plasma = A hot soup of electrons and ions, ordinary atoms dissociated by heat or electricity. For accelerator use, create the plasma by intense lasers, or electrical discharge, and contain the plasma within the beam tube.

Variant A: Send a conventional beam of electrons (or positive particles) into a plasma. The beam expels the plasma electrons and causes periodic regions of high and low electron density = plasma wakefield. Electrons ejected at point A are attracted to the ion excess at B. The electron pattern is wavelike and gives electric fields that accelerates the beam.

A

B

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Plasma accelerators

Plasma wakefield is standing wave that can accelerate subsequent bunches of particles.

Experiment at SLAC showed that some 42 GeV electrons (accelerated over 3 miles by conventional rf) were doubled in energy in 84 cm of plasma!

But only a small fraction of beam is accelerated. And beam size is rather large. More R&D!

Beam energy →

42 GeV

84 GeV

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Laser plasma accelerators

Variant B. Create a plasma in a thin tubular channel using lasers. A subsequent high power laser pulse is confined to the tube, and its electric fields accelerate plasma electrons from rest. (no prior accelerator!) Earlier problems of keeping the laser light focussed, and the effect of light outrunning the particles have been solved for 3 cm long acceleration cells.

Group at Berkeley has succeed in accelerating electrons from rest in the plasma to 1 GeV in 3.3 cm.

30 GeV/m (1000xILC!)

The beam has small spatial and energy spread, so R&D will now focus on capturing the beam and accelerating further in subsequent cells.

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Diffusion into broader society

New ideas and tools invented for a particular scientific purpose have a way of finding applications in broader contexts.

• Quantum mechanics, devised to explain the workings of the atoms, led to transistors, computers, lasers …
• Studies of nuclear spin transitions led to MRI tomography
• Superconductivity led to high field magnets, train levitation, cryo- treatments …

So it has been for particle accelerators. The ability of high energy particle beams to probe small structures and create new forms of matter now find use elsewhere:

ACCELERATORS IN USE WORLDWIDE

Particle/nuclear ~120

Synchroton light sources ~50

Medical radioisotopes ~200

Radiotherapy ~7500

Biomedical research ~1000

Industrial processing/R&D >1500

Ion implantation etc. >7000

TOTAL ~17,500

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Synchrotron light source

Experimental beam lines

Synchrotron radiation is used to study protein structure, materials properties, environmental effects, chemical reactions, nanofabrication and much more. Typically a few GeV electron accelerator is used.

Electron accelerator

many beamlines

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Much of the cutting edge research in biology, solid state physics, materials science, chemistry & environmental science is done at light sources around the world – operating on principles established in particle physics accelerators.

Accelerators for society

Most medical imaging for diagnostics came from accelerator technology:

• Computer assisted tomography
• MRI
• PET scans

Accelerators are also used for:

• making nuclear isotopes for diagnostics/treatment
• microlithography of electronic circuits
• ion implantation for electronics
• sterilizing foods

Possible future uses:

• Linac induced nuclear reactors – safe, clean, non-uranium fuels, efficient
• Security – naval vessel protection, container scanning etc.

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Metastasized

prostate cancer

seen in PET scan

Proton cancer treatment

Traditional X-ray radiation does not penetrate far, loses much of its energy near the surface (burns), has broad spectrum of energy.

Protons from 50 – 200 MeV penetrate to any place in the body. They lose little energy until the last few cm, so dose is concentrated on tumor.

Proton facilities are large and costly – only two in the US. This is a place where the plasma wakefield acceleration R&D could pay off handsomely by drastically reducing the size and cost of the accelerator.

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Commercial electron linac for cancer treatment

Loma Linda (California)

- synchrotron source

- built/commissioned at Fermilab

- world leading patient throughput

Medical accelerators

~ 50 m

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Proton beam induced nuclear reactors

For some years, people have advanced the notion of high energy proton initiation of nuclear reactors (Energy Amplifier). This idea is re-vitalized with the advent of high power proton linacs based on the ILC SC cavities.

Proton linac working at several GeV, ~10 MW power creates fast neutrons in the Th reactor core. Th²³² has 14x10⁹ yrs half life and 5x abundance of U²³⁸.

Th captures fast neutrons (~100 keV); is a fertile (breeder) nucleus; n + Th²³² → Th²³³ → (β) Pa²³³ → (β) U²³³.

U²³³ fissions

10 MW p linac

Linac beam power of ~ 10 MW, consuming ~40 MW from grid Reactor produces ~700 MW to the grid.

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Proton beam induced nuclear reactors

EA viability still debated by nuclear community; much R&D still needed on accelerator systems and system issues.

Breeder reactions are subcritical – k factor ~ 0.98 (safe).

Buildup of isotopes in operation.

Fission products are relatively short lived.

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Who develops accelerator science?

Accelerator science is a very rich mixture of disciplines

• Classical and quantum physics
• Applied mathematics
• Computer science
• Materials science
• Electrical engineering
• Mechanical and civil engineering

People have come from all of these fields.

There are no dedicated educational programs for accelerator science in US universities. Some universities situated near major accelerators train students from other disciplines (including Stony Brook) with supervisors from Labs.

Stony Brook and Brookhaven Laboratory offer a nearly unique pairing of a laboratory with world class facilities, and university with good students and strong departments.

Scientists in both institutions are working to develop an Accelerator Science graduate program.

But there aren’t enough of them.

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Conclusion

• Accelerators arising from needs in basic research have increased in power by million-fold in 75 years, and have revealed the microscopic world in great detail.
• Many wonderful new ideas enabled this growth, but we are still limited by fundamental physical parameters – materials breakdown, magnet strength etc. Current research on plasma wakefield acceleration is promising.
• Accelerators have entered the mainstream of society, particularly in medicine and electronics industry. Trend toward miniaturization and cost reduction will only enhance this.

Particle Physics�

Accelerators & Detectors

• Luminosity and cross-sections
• Fixed target vs collider
• linac vs circular
• Detectors: fixed target, collider
• Detector elements

High Energies in Accelerators

• Produce new particles
• e.g. W, Z,
• … Higgs ?
• Probe small scale structure
• p=h/λ, e.g. proton structure

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Accelerators

• Electric Fields to accelerate stable charged partilces to high energy
• Simplest Machine – d.c. high V source
• 20MeV beam
• High frequency a.c. voltage
• Time to give particles successive kicks

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> MeV Energy speed ~c, hence length of tubes same

Linear Accelerator - Linac

Fermilab linac

Synchrotron

• pp,ep collider – need different magnets
• p anti-p, or e^-e⁺
• One set of magnets,one vacuum tube
• LEP (e+e-), Tevatron(p anti-p)
• Need to produce anti-particles
• Positron – OK, anti-protons difficult
• from proton nucleus collisons

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radius

B field (bending) and

E-field (accelerating cavity)

Synchronised with particle

velocity

Magnets

• International Linear Collider plan for 35 MV/m
• Length for 500 GeV beams ?

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Accelerating Cavities

Niobium, superconducting

1200 dipole superconducting (1.9K) magnets, 14.3m long, 8.35 T

Proton energy 7 TeV,

minimum ring circumference ?

Energy considerations: �1)Fixed Target vs Collider

• Linac Energy
• length & voltage per cavity
• Synchrotron Energy
• Radius, max B-field
• Synchrotron radiation

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Higher E = bigger machine

• Energy
• Achieve higher sqrt(s) at collider
• Direct new particle searches
• Stable particles
• Colliding beam expts use p,e^- (muons?)
• Rate
• Higher luminosity at fixed target

2) Linac vs synchrotron

Energy: Fixed Target Experiment

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b at rest:E_b=m_b

for

Energy: Colliding Beam

Symmetric beams – lab frame =CM frame Particle & anti-particle collision

Synchrotron Radiation

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ρ is radius of curvature of orbit

So for relativistic particles β≈1

Limits energy for a electron/positron machine

< ~ 100GeV/beam

Also a useful source of high energy photons for material studies

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Diamond Synchrotron started operation recently in Oxfordshire

Hence, LHC proton collider

Energy lost as particles

bent to travel in circle

Synchrotron: Beam Stability

• Particles accelerated in bunches LHC N=10¹⁰
• Particle accelerated just enough to keep radius constant – in reality…
• Synchrotron Oscillations
• Movement of particles wrt bunch
• out of phase with ideal, stability ensured

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Particle B arriving early receives a larger RF pulse

moves to a larger orbit and arrives later next time

Particle C arriving late received smaller acceleration, smaller orbit, earlier next time

V

Early

C

Focussing

• Particles also move in transverse plane
• Betatron oscillations
• Origin - natural divergence of the originally injected beam and small asymmetries in magnetic fields.
• Beams focussed using quadropole magnets.

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N.B. Dipoles=bending, Quadropoles=focussing

Focussing in vertical/ horizontal planes

Force towards centre of magnet.

Alternate vertical / horizontal

net focussing effect in both planes.

+ve particle

into paper

Cooling

• Initially particles have a wide spread of momentum and angle of emission at production
• Need to “cool” to bunch
• One methods – stochastic cooling used at CERN for anti-protons
• Sense average deviation of particles from ideal orbit
• Provide corrective kick
• Note particles travelling at c and so does does electrical signal !

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Particle

accelerator

Cross-Sections

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We perform an experiment:

How many pions do we expect to see ?

• Duration of expt(t)
• Volume of target seen by beam (V)
• Density of p in target (ρ)
• Beam incident /sec/Unit area (I)
• Solid angle of detector (ΔΩ)
• Efficiency of experiment (trigger/analysis) (ε)
• (I t) (Vρ) ΔΩ ε
• (1/Area)(N_o) ΔΩ ε

Smashing beam into a target

The constant of proportionality – the bit with the real physics in !

– is the differential cross-section

ΔN

Integration over 4π gives total cross-section

Can divide total xsec into different reactions e.g.

xsec measured in barn, pb etc…

Luminosity

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For colliding beams no V (target volume) term.

Require two narrow beams with complete overlap at collision point

Typical beam sizes 10-100μm in xy and cm in z

Interaction rate is

n₁,n₂ are number of particles in a bunch

f is the frequency of collisions

e.g. rotation in circular collider, this can be high, LHC 40 MHz!

a is the bunch area of overlap at collision point (100% overlap)

jn s^-1

is known as the luminosity

LHC plans up to 10³⁴ cm^-2 s^-1

Number of events = lumi x xsec x time

Typically good machine running time is ~1/3 yr (1x10⁷s)

Linac – one shot machine

Synchrotron – particles circulate for many hours

Fixed target luminosity can be higher

e.g. 10¹² p on 1m long liquid-H target gives~10³⁷cm^-2 s^-1

Electrons vs Protons ?

• Useful centre-of-mass energy electron vs proton
• Proton is composite, ~10% root(s) useful energy
• 100 GeV LEP, 1TeV Tevatron had similar reach

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• Electron-positron much cleaner environment
• No extra particles
• Can detect missing energy e.g. neutrinos, new neutral particles
• Proton
• Higher energies, less synchrotron radiation
• Electron-positron – “high precision machine”
• Proton-proton – “discovery machine”

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Tevatron

Event

LEP

Event

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A typical modern particle physics experiment

DELPHI experiment @ LEP collider

Example Particle Detector- ATLAS

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Detector Components:

Tracking systems, ECAL/HCAL, muon system

+ magnet – several Tesla - momentum measurement

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Tracking: Spatial Resolution 5-200μm

ECAL:

HCAL:

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Energy Resolution

Time Resolution:

LHC 40Mz=25ns

Elements of Detector System

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• Sensitive Detector Elements: e.g.
• Tracking - silicon sensors, gaseous ionisation detectors
• Calorimeters – lead, scintillators
• Electronic readout:e.g.
• Custom designed integrated circuits, custom pcbs,
• Cables, power supplies.
• Support Services: e.g.
• Mechanical supports
• Cooling
• Trigger System
• LHC 40 MHz, write to disk 2kHZ
• Which events to take ?
• Parallel processing, pipelines
• Trigger levels
• Add more detector components at higher levels

Computing in HEP

Each event 100kB-1MB

1000MB/s, 1PB/year

Cannot analyse on

single cluster

Worldwide computing

Grid

Example Neutrino Detector

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But not all detectors look like previous examples

Example – neutrino detector

• Very large volume
• Low data rate

Super-Kamiokande

half-fill with water

50,00 tonnes of water

11000 photomultiplier tubes

Neutrinos interact

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Chereknov light cone given off

and detected by photomultipliers

Accelerator Summary

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Considerations for an accelerator.

• Reaction to be produced
• Energy required
• Luminosity required
• Events expected

Particles are accelerated by electric field cavities.

Achievable Electric fields few MV/m

Higher energy = longer machine

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Fixed target expt. – not energy efficient but sometimes unavoidable

(e.g. neutrino expts)

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Particles are bent into circles by magnetic fields.

Synchrotron radiation – photons radiated as particle travels in circle

E lost increases with γ⁴, so heavy particles or bigger ring

Or straight line…

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Synchrotron oscillations controlled by rf acceleration

Quadropole magnets used to focus beams in transverse plane

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Linac – repetition rate slower as beams are not circulating

Synchrotron – beams can circulate for several hours

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