Methods of Experimental Particle Physics

1. Methods of Experimental Particle Physics Alexei Safonov Lecture #9

2. Today • Basics of particle detection and passage of particles through matter

3. 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)

4. 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)

5. 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

6. 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

7. 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

8. Particle Data Group • http://pdg.lbl.gov/ • Annual review of particle physics

9. 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)

10. 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)

11. Hadrons • 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

12. 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

13. 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)

14. 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!

15. 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

16. End of Lecture • Actually we got through a couple more slides, but next time we will re-start from here to preserve the continuity

17. 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

18. 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)