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Methods of Experimental Particle Physics. Alexei Safonov Lecture #9. Today. Basics of particle detection and passage of particles through matter. Analyzing Data in HEP. Any discovery in HEP is effectively an observation of some new type of “events”

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Methods of Experimental Particle Physics

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  • Basics of particle detection and passage of particles through matter
analyzing data in hep
Analyzing Data in HEP
  • Any discovery in HEP is effectively an observation of some new type of “events”
    • Observe a flux of charged particles coming from the sky – cosmic rays
      • Need to “tag” charged particles with a detector
    • Observe “events” in which a charged pion decays to a muon
      • Need to “measure” both particles, but also identify them (you need to know that a muon is a muon and a pion is a pion)
        • May need to have a detector that can measure momenta and masses of the “before” and “after” particles
    • Observe predicted Higgs production in the channel HZZm+m-m+m- at the LHC
      • Find “events” with 4 muons, which you can pair in such a way that each pair has a mass of the Z boson
        • Need to “recognize” muons, measure their momentum to reconstruct the invariant masses of the pairs
        • But also need to “suppress” possible “background” events that can look similar to these events (and know how much is left)
particle detection
Particle Detection
  • As you saw, in pretty much all cases, you need to “reconstruct” an “event” by:
    • Detecting (“reconstructing”) particles
    • Measuring particle properties (momenta, mass, charge)
    • Identify their type (muon, electron, photon etc.)
    • Putting all this information together to recognize “signal” events, suppress background events as much as you can, know how to estimate what’s left
  • First three steps done using particle detectors that recognize and measure properties of the particles for you
    • The forth is done using computers (or rulers and calculators in the old times)
particle detectors
Particle Detectors
  • Particle detectors are designed and built thinking of what kind of particles you need to recognize and measure
  • “General purpose” detectors (like CMS, ATLAS, CDF and D0) consist of a combination of many individual detectors each registering or recognizing something and doing its own measurements
    • Muon chambers help “identify” muons and measure their momenta
  • Then you utilize all information to reconstruct “everything” that happened in this “event”
    • Having redundancy helps as you can compare the data from different detectors for consistency, which may for example help you catch “impostors”, like a pion which of your detectors took for a muon
basic principles
Basic Principles
  • All detectors utilize the knowledge about how different particles interact with matter:
    • Charged particles bend in the magnetic field
    • Charged particles ionize matter they pass through
    • Charged particles in certain media can emit light (scintillators or Cherenkov radiation)
    • Most charged and neutral particles will be destroyed by releasing their energy if you put a 100-ton steel cube in front of it
      • Kind of useless, but if you could find a way to measure how much energy passing particles release in your cube, you just built yourself a “calorimeter”
    • Some will escape (which is also a way to “tag” them):
      • For example, neutrinos won’t even notice your cube as they almost do not interact with matter
particles we care about at colliders
Particles We Care About at Colliders
  • “Interesting” particles like higgs and Z’s decay almost immediately
    • You can’t see them directly, but you can find their decay products and tell that there was a Higgs produced in this collision
  • A typical (incomplete) set of (meta-)stable particles, which you use as your “building blocks” to get back to Higgs:
    • Electrons, muons, photons, charged pions
      • In some sense neutrinos
    • More rare ones – charged and neutral kaons, protons, neutrons
particle data group
Particle Data Group
  • Annual review of particle physics
charged leptons
Charged Leptons
  • Electron:
    • The lightest charged lepton, Stable(!)
      • Bends in magnetic field!
    • M=0.510998928±0.000000011 MeV
    • Interactions:
      • Electromagentic, weak, can’t interact strongly (e.g. can’t emit a gluon)
  • Muon:
    • Second lightest charged lepton
    • M=105.6583715±0.0000035 MeV
    • Lifetime: (2.1969811±0.0000022)x10-6 s
    • Interactions:
      • Electromagentic, weak, can’t interact strongly (e.g. can’t emit a gluon)
neutral leptons
Neutral Leptons
  • Neutrinos are almost massless
    • Don’t have charge so they don’t interact electromagnetically
    • For the same reason they don’t bend in magnetic field
    • Very weakly interacting with matter
      • Most of the time will fly through the entire Earth without interacting at all
  • Consider them “invisible” particles in your experiments:
    • If something is missing (energy not balanced), assume it’s due to neutrino(s)
  • Most interactions happening at hadron colliders are strong interactions between quarks and gluons (e.g. qgqg scattering)
    • Quarks or gluons can never live by themselves due to color charge, so they pull partners out of the vacuum so that together the system is color-less
    • So outgoing quark or gluon becomes a spray of particles consisting of quarks and kept together by gluons
    • Can make many combinations:
      • Baryons: protons, neutrons
        • Three quarks each like uud
      • Mesons:
        • p-mesons consist of u,d quarks
        • r-mesons consist of u and d quarks too
        • K-mesons consist of s and u quarks
charged hadrons
Charged Hadrons
  • Charged pions (p±):
    • M=139.57018±0.00035 MeV
    • Lifetime: (2.6033 ±0.0005)x10-8s
      • At colliders, enough to be considered “stable”
    • Interacts: electromagentically, strongly, and weakly (e.g. decays into a muon via electroweak coupling)
  • Charged rho (r±):
    • M=775.49±0. 34 MeV
    • Lifetime: ~4.5×10-24 s (decays to p0p±)
    • Interacts:
      • Doesn’t matter as it decays so fast, in this case you will care about detecting pions
  • Proton (p):
    • Stable, M=938.272046±0.000021
    • Interacts:
      • Strongly, electromagnetically
neutral hadrons
Neutral Hadrons
  • Neutral pions(p0):
    • M=134.9766±0.0006 MeV
    • Lifetime: 8.52±0.18×10-17 s (decays mainly to two photons)
    • Interacts:
      • Again, doesn’t matter as you will care about photons
  • Neutrons:
    • M=939.565379±0.000021 MeV
      • Slightly heavier than a proton
    • Lifetime: 878.5 ± 0.8 s (practically stable)
    • Interacts:
      • Strongly, weakly (decay is so slow is because it’s the weak interaction)
charged particles
Charged Particles
  • All stable charged particles interact with charged particles in matter
    • Matter mainly consists of protons and electrons
    • Electrons are light, easy to kick them hard enough to separate from the atom:
      • Ionization!
pdg passage of particles through matter
PDG: Passage of Particles Through Matter
  • Section 30 of the “PDG Book” (using 2012 edition) provides a very detailed review
  • We will only walk over some of it, please see PDG and references therein for further details
end of lecture
End of Lecture
  • Actually we got through a couple more slides, but next time we will re-start from here to preserve the continuity
charged particles1
Charged Particles
  • Heavy (much heavier than electron) charged particles
    • Scattering on free electrons: Rutherford scattering
    • Account that electrons are not free (Bethe’s formula):
    • Energy losses: from moments of
                  • Ne is in “electrons per gram”
      • J=0: mean number of collisions
      • J=1: average energy loss – interesting one
energy loss
Energy Loss
  • Energy loss (MeV per cm of path length) depends both on the material and density
    • Convenient to divide by density [g/cm3] for “standard plots”
      • If you need to know actual energy loss, you should multiply what you see in the plot by density (rho)