Basic Measurements: What do we want to measure?

1. 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

2. 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)

3. 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 v

4. CDF Run 1 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. -

5. 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.

6. 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

7. Lifetime:Time dilation, lab distance: Distribution of decays at distance x is exponential: Slope depends on D, hence on c , measure slope/Dto 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. Fundamental Particle Properties

8. 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.

9. 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|

10. 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… don’t 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).

11. What Makes Particle Detection Possible? Next time-- Passage of particles through matter: How we “see” particles