Basic Measurements: What do we want to measure?

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Basic Measurements What do we want to measure?
Fundamental Measurements From Quarks to Lifetimes

• Prof. Robin D. Erbacher
• University of California, Davis

References R. Fernow, Introduction to
Experimental Particle Physics, Ch. 15
D. Green, The Physics of Particle
Detectors, Ch. 13
http//pdg.lbl.gov/2004/reviews/pardetrpp.pdf
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Fundamental Particle Properties

• Charge Charge of a particle can be determined
  two ways
• Sign of charge Direction of deflection in a
  magnetic field
• Magnitude of charge
• Infer from knowledge of momentum and B-field
  strength
• Charge-dependent quantity, such as ionization
  energy loss, or Rutherford scattering cross
  section
• Direction tracking detectors, B-field
• Momentum tracking detectors, B-field
• Ionization energy loss sampling w/
  scintillation, TOF (for ?)
• (Example combine ? from time of flight (TOF)
  with dE/dx and use Bethe Bloch equation to get
  charge)

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Fundamental Particle Properties

• Mass Complicated mainly specialized techniques
• One Example
• Measure two independent mass-dependent
  quantities Momentum often one ionization,
  range, or velocity
• Momentum/range tracking detectors, B-field
• Ionization/velocity scintillation, TOF/ dE/dx,
  C, TOF
• Example (Fernow) Use conservation of energy and
  momentum to measure mass of muon neutrino ??
• Use knowledge of mass of pion and muon, and
  measure momentum and B-field strength accurately
• Scintillator stops ?s, magnets guide ?s, silicon
  gives momentum

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Fundamental Particle Properties

• Mass Complicated mainly specialized techniques
• Second Example
• Measure most quantities in an event, reconstruct
  mass
• Jet energies, lepton momenta, missing ET for
  examples
• Jet energies em and hadron calorimeters
  (fragmentation, etc)
• Momenta tracking detectors, B-field
• Missing ET all of the above, plus missing info
  corrections
• Example Measure top quark mass from tt pair
  production events
• Use best combination (?2) of partons
• to reconstruct top mass to best
• resolution possible.

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Fundamental Particle Properties

• Spin Spins complicated for decaying particles
• Ground state particles, electrons and nucleons
• Hyperfine structure in optical spectroscopy,
  atomic/molecular beam
• experiments, bulk matter measurements using NMR.
• Other low energy particles
• Various techniques eg charged pions determined
  by relating the
• cross section for reaction to the cross section
  for the inverse reaction.
• High energy interactions
• Spins can be found from the decay angular
  distributions, and from the
• production angular distributions for particle
  interactions.
• Example Measure top quark pair spin correlations
  using angles of decay products.

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Fundamental Particle Properties

• Magnetic Moment Closely related to spin
• Ground state particles, electrons and nucleons
• Again use optical spectroscopy, atomic/molecular
  beam
• experiments, bulk matter measurements using NMR.
• Muons
• Original measurement of g-factor done at CERN
  storage rings including
• a precise demonstration of relativistic time
  dilation. Details of these,
• and current g-2 experiments (BNL) leave for
  homework.
• Measuring the ??hyperon
• Fermilab protons on beryllium target, ?s 8
  polarized, sent through
• magnet and spin precession measured, giving
  , and hence ?.
• Keys to measurement ?s produced inclusively w/
  large cross section,
• large detector acceptance, high energy ? long
  decay length

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Fundamental Particle Properties

• Lifetime Time dilation, lab distance
• Distribution of decays at distance x is
  exponential
• Slope depends on ?D, hence on c? , measure
  slope/?D to get lifetime ?.
• Example Lifetime fraction of the new particle
  X(3872)
• Not quite a lifetime measurement, since
• need to know branching ratios and
• production. Measure fraction of X that
• are long-lived (from B meson decays)
• versus prompt.
• Measuring muon lifetime
• Senior lab course measure the muon
• lifetime in the lab. Leave setup
• and procedures for homework exercise.

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Fundamental Particle Properties

• Total Cross Section (prod rate) Two main methods
• 1) Measure every event (4? colliders bubble
  chambers)
• Often called a counting experiment
• Example Top Pair Production
• Rate of production of tt pairs one of
• first things to measure upon discovery
• 2) Transmission Experiment
• Measure particle intensity before and
• After target and extract cross section.
• Used at fixed target experiments, most often.

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Fundamental Measurements

• New Particle Searches Many categories/methods
• -Counting excess events over Standard Model
  background
• -Fits kinematic distributions to expected shapes
• 1) Expected Particles
• Searching for particles that are predicted
• by theory, or expected by data. May or
• may not know mass or other properties.
• (W, Z, J/psi, top, Higgs)
• Example Single Top Production
• Never yet observed, but expected by
• electroweak production, Vtb

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Fundamental Measurements

• New Particle Searches Many categories/methods
• (Counting excess events, or fits to
  distributions)
• 2) Completely New Phenomena
• Beyond Standard Model, unexpected. Some-
• times theories exist, sometimes not. Difficult
• little information to optimize the search.
• Carefully control background dont want
• false positive!
• Example Search for Z bump hunts
• Look for excess, usually in tails of
• distributions. Statistics of small
• numbers.
• Problem optimize
• differently for discovery than for
• searches (setting limits).

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What Makes Particle Detection Possible?
Next time-- Passage of particles through matter
How we see particles