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Methods of Experimental Particle Physics

Alexei Safonov

Lecture #9

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Today• Basics of particle detection and passage

of particles through matter

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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 HZZm+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)

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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)

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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

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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

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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

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

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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)

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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)

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Hadrons• Most interactions happening at hadron colliders

are strong interactions between quarks and gluons (e.g. qgqg 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

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Charged Hadrons• Charged pions (p±):

• M=139.57018±0.00035 MeV• Lifetime: (2.6033 ±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 (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

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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)

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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!

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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

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End of Lecture• Actually we got through a couple more

slides, but next time we will re-start from here to preserve the continuity

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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

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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)