1 Methods of Experimental Particle Physics Alexei Safonov Lecture #9

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

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  +  -  +  - 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) 3

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) 4

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 5

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 6

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 7

Particle Data Group Annual review of particle physics 8

Charged Leptons Electron: The lightest charged lepton, Stable(!) Bends in magnetic field! M= ± MeV Interactions: Electromagentic, weak, can’t interact strongly (e.g. can’t emit a gluon) Muon: Second lightest charged lepton M= ± MeV Lifetime: ( ± )x10 -6 s Interactions: Electromagentic, weak, can’t interact strongly (e.g. can’t emit a gluon) 9

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) 10

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:  -mesons consist of u,d quarks  -mesons consist of u and d quarks too K-mesons consist of s and u quarks 11

Charged Hadrons Charged pions (  ± ): M= ± MeV Lifetime: ( ±0.0005)x10 -8 s At colliders, enough to be considered “stable” Interacts: electromagentically, strongly, and weakly (e.g. decays into a muon via electroweak coupling) Charged rho (  ± ): M=775.49±0. 34 MeV Lifetime: ~4.5× s (decays to  0  ± ) Interacts: Doesn’t matter as it decays so fast, in this case you will care about detecting pions Proton (p): Stable, M= ± Interacts: Strongly, electromagnetically 12

Neutral Hadrons Neutral pions (  0 ): M= ± MeV Lifetime: 8.52±0.18× s (decays mainly to two photons) Interacts: Again, doesn’t matter as you will care about photons Neutrons: M= ± MeV Slightly heavier than a proton Lifetime: ± 0.8 s (practically stable) Interacts: Strongly, weakly (decay is so slow is because it’s the weak interaction) 13

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! 14

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 15

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

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 17

Energy Loss Energy loss (MeV per cm of path length) depends both on the material and density Convenient to divide by density [g/cm 3 ] for “standard plots” If you need to know actual energy loss, you should multiply what you see in the plot by density (rho) 18