Top Quark Physics at the Tevatron

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Top Quark Physics at the Tevatron

• Prof. Meenakshi Narain
• PY482, Spring 2002

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The Standard Model
leptons
quarks
electromagnetism
spin 1
weak force
strong force
H
Higgs
spin 0
3
History of Top Quark Searches

• x

1995 observation Tevatron
History of - direct and indirect mass
measurements - lower limits from searches C.
Quigg, Top-ology, Physics Today, May 1997
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Top Quark Physics

• Top-antitop quark pair production

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Top Quark Physics

• Single top quark production

(Drell-Yan)
(W-gluon fusion)
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Top Quarks Rate of production

• Cross Section prediction
• Pair production
• s(pp tt X) ? 4.7 - 6.2 pb
• Variations in resummation procedure
• QCD renormalization scale
• BC Berger and Contoponagos ,
• BCMN Bonciani et al,
• LS?N Laenen et. al. ,
• NLO Nason et.al
• Single top production
• s(pp Wg ? t X) 1.7? 0.2 pb (Stelzer et
  al.)
• s(pp W ? t X) 0.72?0.04 pb (Smith et
  al.)

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Top Quark Decay

• Top quark decays predominantly to W boson and a
  b quark (t?Wb, t?W-b)
• Width of the top quark
• ?(t ?bW) ? 1.55 GeV
• Corresponds to a lifetime
• ?(t) ? 0.4 x 10-24 s
• Time scale for confinement
• 1/?QCD? few x 10-24 s
• top quark decays before it can be hadronized

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Top Quark Event Types

• Depends on the W boson decay signature
• W ln or qq
• Relative ratios
• Branching fraction

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Producing particles

• Need Ecom to be large enough to produce a top
  quark
• Shoot particle beam on a target
• Ecom 2ÖEmc2 20 GeV for E 100 GeV m 1
  GeV/c2
• Collide two particle beams
• Ecom 2E 200 GeV for E 100 GeV

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Where do we find the quark and anti-quarks to
collide?

• Quarks are not found free in nature! But
  (anti)quarks are elements of (anti)protons.
• So, if we collide protons and anti-protons we
  should get some qq collisions.

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Energy available in a pp collision

• Proton structure functions give the probability
  that a single quark (or gluon) carries a
  fraction x of the proton momentum (which is 900
  GeV/c at the Tevatron)

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Energy available in a pp collision

• Ecom of parton collision varies
• Small Ecom ?
• very energetic secondaries,
• but boosted along beam direction
• Large Ecom ?
• can create massive object
• that decays to secondaries with
• large momentum component
• transverse to the beam. (pT)
• So, how do we collide protons and antiprotons ?
  Well, we have a special machine for that ...

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Our instrument
Fermilab
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The Tevatron
Tevatron

• Run started April 2002
• luminosity 2?1032cm-2s-1
• deliver 2-4 pb-1
• upgraded detectors

main injector
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Accelerating Particles

• Simplest accelerator
• Two parallel metal plates separated by a gap
• and connected to the terminals of a battery.
• Accelerators use more sophisticated voltage
  sources to give protons a few million
  electron-volts

The proton accelerates in the electric field
within the gap For the 10 Volt battery, the
energy gained by the proton
10 electron-Volts (eV)
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The Fermilab Accelerator

• Two basic types of components
• the magnets
• The magnets keep charged particles moving in a
  circular path.
• the RF cavities.
• The RF cavities pump energy into the particles
  each time they pass through the cavities.
  Particles complete many laps around the
  accelerator ring and receive a small boost in
  energy with every lap.

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Raising the Energy RF Cavities

• Radio Frequency Cavities (RF)
• The Tevatron accelerator uses electric fields
  that alternate at 53 MHz (million cycles per
  second), half the frequency of FM radio signals.
• The electric field pushes the particles at just
  the right time, similar to the way a parent
  pushes a child on a swing

Ions ride the EM-waves in RF cavities
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Bending Particles

• The path of charged particles bends in a magnetic
  field.
• A ring of electromagnets sends the particle beams
  in a circle.
• As the particles get more energetic, we need
  stronger electromagnets to keep them in their
  circular path.
• To build a more powerful accelerator would have
  required putting so much current into the
  existing electromagnets they might melt.
• A magnet technology had to be developed based on
  superconductivity

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The Tevatron

• Accelerates protons to 1 TeV
• uses 1000 superconducting magnets (_at_4.3 K -liquid
  He)
• Superconducting magnets produce a larger magnetic
  field at a lower operating cost than conventional
  magnets.
• Fields of 4-5 Tesla
• Tevatron
• largest liquid He facility 4500 liters/hr

The manufacture of the Tevatron magnets required
more than 135,000 pounds of niobium-titanium
alloy at the project's start in 1974, the
worldwide annual production was a few hundred
pounds. The manufacturing technology and capacity
developed in response to this demand was later
used to help make possible and commercially
viable a new medical diagnostic method magnetic
resonance imaging, MRI.
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Producing Anti-protons

• Antiprotons are produced in collisions of
  high-energy protons on a target.
• a rare process for every 106 p we get 10 anti-p
• A lithium lens focuses the antiprotons into a
  beam.

The beam goes into the Accumulator, which stores
large numbers of antiprotons until they are
needed. Magnets bend the antiprotons in a
circular path.
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Accelerating Antiprotons

• Because antiprotons have the opposite electric
  charge of protons, they bend in the opposite
  direction as they move through a magnetic field.
• antiprotons can circulate in the same accelerator
  as the protons, but in the opposite direction.
• This is exactly what is needed to make protons
  and antiprotons collide. You get two accelerators
  for the price of one!

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The Fermilab Tevatron

• To produce and detect top via proton-antiproton
  collisions at Fermilab, 7 accelerators and 2
  detectors were used

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Proton Anti-proton 121x121 bunches Protons/bun
ch 1011 Anti-protons/bunch
1010 Beam Energy
1 TeV/beam Luminosity 1032 cm-2 s-1 bunch
crossing time 132 ns Collision Rate
10 MHz
Anti-Proton
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Event Rates

• Protons anti-proton bunches collide at the rate
  of 10 MHz starting April 1, 2001.
• We expect 2-3 pp collisions per crossing
• Will record 50 events/sec (write ? 1 TByte/yr)
• During 1991-1996
• rate was 3.5 micro-seconds
• 286,000 bunch crossings/sec, 1-2 pp collisions
  per crossing
• max rate data written to tape - 5 Hz
• recorded about 65 million events (preferentially
  chosen for interesting physics analyses) there
  were 6x6 bunches
• collision
• out of these events - identified 50 top pair
  production events!

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Top Production a rare process

• Rate of dominant processes at the Tevatron

N ? ?L dt

• ? is ? to the probability that a given process
  will occur.
• L is a measure of the beam intensity (
  L1031/cm2s )
• ?L dt is a measure of the amount of data
  collected (100 pb-1)

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And one in every 3 billion collisions will
produce a tt
-
Tops Family Tree
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The D0 experiment
Protons and antiprotons collide in a flash of
energy. Other particles condense out that
energy. Physicists use detectors to "see" these
particles, including the top quark.
muon detector
calorimeter
tracker
5000 tons 400 physicists Run I 1992-1996 100
pb-1 ?80 publications ?100 Ph.D.s
beam pipe
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The CDF experiment
muon detector
calorimeter
tracker
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A Generic Detector System
Tracking chambers ? trajectory of charged
particles Calorimeters ? measure
energy Electronmagnetic e, photon
Hadronic pion, K, proton,neutrons Muon Chambers
? measure muon trajectory Magnets
? charged particles bend in
magnetic fields. Bend depends
on charge and momentum
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Recording the signals from the detector

• Particle physicists use sophisticated electronics
  to convert detector information into digital
  signals and use powerful computers to process
  this information. With over several million
  interactions per second at the Tevatron,
  physicists need all the help they can get!
• Physicists can store only one in 100,000
  collisions at the Tevatron for later analysis so
  they rely on specialized hardware and software
  called "triggers" to select the most interesting
  events. Even with this enormous reduction, the
  CDF and DØ experiments plan to record about 20
  megabytes per second which must be shipped over
  computer networks for storage.

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Data Recorded in1992-1996 Run

• In the experimental run that discovered the top
  quark, each experiment recorded 40 terabytes of
  data on 8000 tapes, a stack of tapes 500 feet
  tall, over twice the height of Wilson Hall !

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Co-ordinates System
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Reconstructing an Electron
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Identifying an Electron
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Identifying an Electron
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Neutrinos

• Neutrinos do not interact with the detector.
• Momentum conservation tells us
• But dont measure pz. Therefore

invisible
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Identifying Muons
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Identifying Muons
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Quarks are detected as Jets of hadrons

• Quarks do not exist as free particles. qq pairs
  are pulled from the vaccum to produce stable
  particles mesons, baryons

Quarks hadronize single quark appears as a
jet (spray) of hadrons in the detector
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Jet Production and Reconstruction
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Identifying the b quark
top Higgs SUSY Technicolor
b
Semileptonic decays of the b-quark B(b ?? X) ?
20 ? detect muons in jets
1
OR
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Top Event with b-jet (D0)
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Top Event with b-jet (D0)
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Identifying the b quark
top Higgs SUSY Technicolor
b
B(b ?? X) ? 20 ? detect muons
1
life time ? 1.5 ps ? c? ? 0.5 mm
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precise tracking ?silicon microstrip detectors
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B-tagging using displaced vertex

• Compute distance in the transverse plane between
  the secondary and primary vertex
• (idealized sketch of vertex tagging in transverse
  plane)
• positive tags decay of a long lived particle at
  the primary vertex.
• Negative tags indication of algorithm
  performance

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D? Tracker
Solenoid (2 T)
Fiber Tracker
et1.7
Silicon Tracker

800,000 channels 1400 silicon elements area
4.7 m2 10 ?m single hit resolution
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Displaced Vertices
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How do we look for the top quarks?

• Collect events of types
• Dilepton
  Lepton jets
• (ee, em, mm 2 jets missing ET).
  (e, m 3 or 4 jets missing ET ).
• All jets (5 or 6 jets, b-tags, event shape, NN).
• (2...n jet processes x106 larger than tt
• Wn-jet x103 larger !

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Searching for the needle in haystack

• For example, the ljets final state is swamped by
  backgrounds from Wn-jets ?select events with
  an electron pT
• To select a tt enriched samples exploit
  differences between tt and background events

A) Kinematic Distributions B) Final State
Particle Content
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Kinematic Distributions

• Some distributions for tt?e?jets events and
  major backgrounds
• ...various other variable e.g. HT, b-jets etc

High pT leptons
Many jets
Large pT
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Identifying top quark events

• Look for events with kinematics different from
  those of the backgrounds
• Large Lepton pT
• Large Missing pT
• multiple jets with high pT
• sum of pT of all jets in the event
• etc
• Use the fact that there are two b-jets in the
  event
• Jets in Wjets background events arise mostly
  from fragmentation of gluons and light quarks
  (W?cs)

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Look for our friend the b-quark

• Exploit the existence of b-quarks in top events
• Wjets events have only little heavy flavor,
  mainly from g ? bb/cc
• top decays are rich in heavy flavor
• tt? Wb ? c ? s
• l?
• ? Wb ? c ? s
• ud/cs
• ? 2b quarks and 2.5 c quarks per top decay
• soft lepton tag
• B(b?l?c) ? B(c?l?s) ? 10?
• ? tag b-quark jet with secondary ? (D?)

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And.
top!

• Clear excess in the jet multiplicity spectrum
• Signal region ?3 jets
• Dependence of background on numbers of jets is
  logarithmic

Background region
Signal region
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Significance of Signal

• How confident are we that we see something new?
• We observe n17 events
• Without top we expect l3.8?0.6 events

Number of observed events is Poisson distributed

• If there were no top, the probability to observe
  at least 17 events is

including uncertainties in efficiency etc...
For a Gaussian curve that is the area under the
tail outside 4.6 s
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Discovery!
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A Top Quark event picture
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What we learn
W helicity
Production Cross Section Production
Kinematics Resonance Production Spin
Correlations Mass
Decay modes Branching ratios CKM matrix element
Vtb Rare decays Non-SM decays
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Measure Mass

• Why is top so massive?
• Why does any fundamental particle have mass?
• Is the answer to these questions connected with
  the interactions that govern the behavior of
  particles ?
• Fundamental parameter of SM
• Affects predictions of SM
• via radiative corrections
• BB mixing
• W and Z mass
• measurements of MW,
• mt constrain MH

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Measure Cross Section

• Rate of production
• Compare to theory
• Deviations indicate non-SM modes of production.
• e.g Maybe production via high mass intermediate
  state?

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Spin Correlation

• Significant asymmetry exists in same-spin vs.
  opposite-spin top quark pairs
• expect 70 tt opposite helicity
• Non-zero measurement
• Confirms top quark spin 1/2

No correlation
Correlation!
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New Resonance?

• A general purpose tool for search for heavy
  objects decaying to top pairs
• Dynamical models of EWSB
• color octet resonances ? tt (mass ? several
  hundred GeV.)
• Technicolor gg ??T? (tt, gg)
• Topcolor qq ? V8? (tt, bb)
• ? peak in tt invariant mass

Run1 Data
Expected in Run2
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Tevatron Physics

• Exciting possibilities
• Measure production rates, spin correlations
• Isolate Single To quark events
• Measure Top Quark Mass with greater precision
• Clues to origin of mass?
• lots more studies.
• The next 5-10 years will hopefully lead to
  findings which may change the course of particle
  physics. ?

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References

• For this talk I have drawn heavily from the
  Fermilab web site and CDF/D0 experiments
• FNAL http//www.fnal.gov
• D0 http//www-d0.fnal.gov
• CDF http//www-cdf.fnal.gov
• A nice decription of HEP maybe found at
• http//www-ed.fnal.gov/projects/exhibits/searching
  /

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HEP Spin offs and Challenges

• Accelerator and Detector technology
• Leading edge technology
• Magnets super conducting technology, largest
  liquid He plant, few hundred thousand
  amperes of currents are transferred from power
  supplies to magnet cryostats
• vacuum systems - a vacuum of 10-9 10-10 torr
• (At this level, electrons can travel about a few
  thousand million kilometers before meeting a
  stray molecule of gas).
• Few thousand components need to be controlled,
  synchronized and monitored
• forefront of real-time distributed systems
  development.
• Particle detectors are the physicists' electronic
  eyes. These detectors are complex, precise, and
  sensitive devices.
• Our detectors will handle as much (or more?)
  information as state of Illinois
  telecommunications network, recording the results
  of about 10 million proton anti-proton collisions
  a second . Picking out interesting events demands
  new solutions in ultra-fast electronics.

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HEP Spin offs

• Cancer therapy
• Proton and ion synchrotrons are being developed
  for use in cancer therapy at several places.
• Simualtion tools which predict interaction of
  radiation with matter are being used to
  understand radiation doses for various tissues
• Detectors
• Uses of multiwire proportional chambers,
  developed by G Charpak at CERN (for which he was
  awarded the Nobel Prize in 1992)
• in commercial lorry and container inspection
  systems (at Le Havre and the Eurotunnel
  terminal). Spectacular drugs seizures have
  recently been reported confirming the
  effectiveness of these devices.
• research use in medical imaging (positron
  emission tomography body scanners) in studies
  of animal metabolism in astrophysics to study
  cosmic showers.
• electronics, measuring instruments, new
  manufacturing processes and
• materials, food preservation techniques,
  destruction of toxic products
• and more

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HEP Spin offs WWW (_at_CERN)

• The most widely used device developed at CERN is
  undoubtedly the World Wide Web.
• Initially designed to help communication within
  the particle physics community.
• Physics research groups now consist of several
  hundreds of members scattered in numerous
  research institutes and universities.
• They needed to have access to common databases,
  exchange and edit common documents such as
  scientific articles, reports, engineering designs
  and so on.
• They all had access to the Internet but needed an
  appropriate software tool which was developed for
  them by CERN.
• As is well known, its convenience made it spread
  rapidly to the whole academic world and from
  there to universal use.

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Reference Sites

• I have borrowed heavily from the following sites
• http//www.fnal.gov/pub/tour.html
• http//www-d0.fnal.gov (D0 experiment)
• http//www-d0.fnal.gov/newd0/hep_public.html
• http//www-cdf.fnal.gov (CDF experiment)
• http//public.web.cern.ch/Public/
• http//cmsdoc.cern.ch/cms/TRIDAS/html/outreach.htm
  l
• http//pdg.lbl.gov
• http//ParticleAdventure.org/

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Raising the energy

• Linear accelerator
• use a succession of acceleration gaps arranged
  one after another
• Boost in energy from each gap is added on top of
  the previous one
• The more gaps a linac has, the higher the energy
  it can give to a proton, the longer the linac
  gets.
• When a linac runs out of real estate, it reaches
  its energy limit
• So, if we could take the proton emerging from a
  gap and sent it back to the beginning once again
  - this time with a higher starting energy - it
  will emerge at a higher energy that before.
• We could do this many times - each time raising
  the energy to a higher level.

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Raising the energy

• Circular accelerator (synchrotron)
• Since charged particle bend in magnetic fields,
  we can use magnets to bend the protons
  trajectory and send it circling back to the
  beginning
• Each time the proton goes around the circle and
  through the accelerating gaps it gains more
  energy
• To keep it on the same circular path as it gains
  energy, we must make the magnetic field slightly
  stronger each time it goes around.
• We synchronize the increase in the magnetic field
  with the proton's increasing energy.
• To keep our proton and billions of others like it
  circling together, we focus them into a tight
  beam.
• Voila! We have made a circular accelerator called
  a synchrotron
• Fermilab's Tevatron give particles energies of
  nearly a trillion electron volts, or one TeV, and
  is the world's most powerful.

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Stage 1 Cockroft Walton

• Electrostatic accelerator
• uses large capacitors to produce the static E
  field
• electrons are added to H2 atom
• H2 ion, attracted to positive voltage
• accelerated to 750 keV
• ? 30x energy of electron beam in TV tube

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Stage 2 LINAC

• Accelerates H2 ion from 750 keV to 4 MeV
• 500 ft long
• Consists of tanks filled with small tubes, called
  drift tubes, spaced further and further apart.
• An electric field is applied to the tubes
  repeatedly reversing in direction.
• Particles travel through the drift tubes, hiding
  in them when the electric field is in a direction
  that would slow them down and emerging into the
  gaps between the drift tubes when the field is in
  the direction to speed them up.
• Before entering next stage the electrons are
  stripped off, leaving only the protons

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Stage 2 LINAC
Linac Tanks
Linac Drift Tubes
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Stage 2 LINAC RF Cavities
Radio Frequency Cavities
Ions ride the EM-waves in RF cavities, except
when in drift tubes
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Stage 3 Booster

• Accelerates protons from 4 MeV to 8 GeV
• a synchrotron (circular accelerator)
• 500 ft diameter, 200 ft underground
• start accln when 2.5x1012 protons
• protons travel around the accelerator 20,0000
  times
• radio frequency cavities where the protons are
  accelerated
• The cylindrical tubes on top of each cavity are
  power amplifiers.
• normally cycles twelve times in rapid succession,
  loading twelve pulses, or bunches of protons,
  into the Main Ring, the next stage of the
  acceleration process.

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Stage 4 (old) Main Ring

• Accelerates protons to 150 GeV
• 4 miles circumference
• tunnel 10 ft diameter, 20 ft below ground
• 1000 conventional copper coil magnets bend and
  focus protons
• Main Ring
• also sends protons to a target to create
  anti-protons

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Stage 4 (now) Main Injector

• Replacement of the main ring
• Accelerates protons and anti-protons from 8 to
  150 GeV
• Tangent to the tunnel which houses the main ring
  and tevatron
• superconducting magnet technology
• produces 120 GeV protons for creation of
  anti-protons
• decelerates anti-protons for resuse in the
  Tevatron
• The recycling of antiprotons will provide a
  tenfold increase in collisions.

Main Injector
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Stage 5 The Tevatron

• Accelerates protons to 1 TeV
• housed in the same tunnel as the main ring
• uses 1000 superconducting magnets (_at_4.3 K -liquid
  He)
• Superconducting magnets produce a larger magnetic
  field at a lower operating cost than conventional
  magnets.
• Fields of 4-5 Tesla
• Tevatron
• largest liquid He facility 4500 liters/hr

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Stage 6 Anti-protons

• Creating anti-protons
• extract protons with energy of 120 GeV from the
  Main Injector
• protons collide with a nickel target, produce a
  wide range of secondary particles including many
  antiprotons.
• For every 106 p we get 10 anti-p
• Lithium lens used to focus the antiprotons
• They are transported to the Debuncher ring where
  they are reduced in size, increasing the density
  of anti-p.
• They are then transferred to the Accumulator ring
  for storage.
• Finally, when 50-150x1010 anti-p are stacked,
  the antiprotons are reinjected into the Main
  Injector and passed down into the Tevatron.

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The Tevatron. protons anti-protons

• The accumulated stack of 8 GeV antiprotons, plus
  a new batch of 8 GeV protons from the Booster,
  are accelerated to 1000 GeV by the Main Injector
  and the superconducting Tevatron working in
  tandem.

Main Ring (ps and anti-ps) 150 GeV
Tevatron (ps and anti-ps) 900 GeV
The two counter-rotating beams are focused and
brought into collision at the CDF and DÆ
detectors.
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Run II
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Why high energy?
Smallest length scale probed
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The Fermilab Tevatron
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Higgs mass constraint

• Measure
• top mass
• W mass
• Constrain
• Higgs mass
• low mass ?
• Direct limit
• gt102.6 GeV
• (LEP)

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Top Pair Production