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1Particles are detected through their interaction with [blank_start]matter[blank_end]. Detectors can stop particles [blank_start]completely[blank_end] or let them pass through. Particles can interact through short range interaction with the [blank_start]atomic nucleus[blank_end] via the [blank_start]strong nuclear[blank_end] interaction or via the [blank_start]weak[blank_end] interaction. If the energy of a hadron is [blank_start]high enough[blank_end], new particles
can be produced which can induce the creation of [blank_start]hadronic showers[blank_end]. Interactions of high-energetic photons with matter depend on the [blank_start]energy[blank_end]. A charged particle can excite and ionize atoms along its path giving rise to energy [blank_start]loss[blank_end] via ionization. A charged particle can also emit electromagnetic radiation as it passes through material. For high energetic particles, the [blank_start]alteration[blank_end] of electromagnetic radiation and pair production can lead to electromagnetic showers.
Hadrons interact with [blank_start]nuclei[blank_end] due to the nuclear force. At higher energies, [blank_start]Inelastic[blank_end] scattering dominates: final state particles differ from the initial particles. The interaction probability is parametrized by the [blank_start]nuclear absorption[blank_end] (interaction) length λc : mean distance travelled before an inelastic collision. The nuclear absorption length [blank_start]decreases[blank_end] with the mass of the atom. Thus, to build compact detectors, we need [blank_start]heavy[blank_end] materials like iron and lead. High-energetic hadrons induce cascades of [blank_start]nuclear interactions[blank_end] with the nuclei of [blank_start]detector atoms[blank_end]: [blank_start]hadronic[blank_end] showers. We need approximately [blank_start]10[blank_end] nuclear absorption lengths to contain 99% of the shower energy. We need sufficient energy to produce secondary hadrons in fixed-target like interactions, typically Ehad > 0.5 GeV.
Depending on the energy, a photon will be absorbed (ionising the atom - [blank_start]photoelectric[blank_end] effect), scattered with electrons from the shell ([blank_start]compton[blank_end] effect) or produce electron-positron pairs (pair production). As a charged particle passes through matter it leaves a trail of ionization electrons resulting from [blank_start]electromagnetic interactions[blank_end]. The average energy loss due to ionization is described by the Bethe-Bloch equation and
depends on material (Z,A), velocity, and charge of the particle. The critical energy EC decreases with Z of the material and increases with the mass of the particle. The energy loss by radiation is parametrised by the
radiation length X0, the average [blank_start]thickness[blank_end] of material that reduces the [blank_start]mean[blank_end] energy of an electron to 1/e. Radiaton lengths decrease with Z. Ergo, compact detectors are build with high-Z materials.
The alternation of bremstrahlung and [blank_start]pair production[blank_end] leads to particle cascades: electromagnetic showers. The shower maximum is at about 5-10 radiation lengths. In order to contain the shower, we need 20-25
radiation lengths. Electrons/positrons and photons with energies above EC can be identified as EM showers.
Charged-particle detectors typically detect the ionisation [blank_start]track[blank_end] of charged particles: tracking detector. Charged-particle detectors are usually immersed in a homogeneous [blank_start]magnetic[blank_end] field. The Lorentz force leads to
a circular orbit of curvature radius ρ. We reconstruct particle [blank_start]momentum[blank_end] from track curvature but the momentum resolution degrades with increasing momentum.
Charged particles can travel [blank_start]faster[blank_end] than the speed of light in a material: c' = c/n where n is the index of
refraction. Scintillators convert excitation by charged particles into [blank_start]light[blank_end], which can then be detected by photodetectors. Organic: Fast and cheap but low-Z. Example: Polystyrene. Inorganic: Slower, high-Z and very efficient but expensive. Example: Sodium Iodide.
Calorimeters measure the energy [blank_start]deposited[blank_end] in a material through electromagnetic or hadronic showers. Sampling calorimeters consist of alternating layers of showering material [blank_start]absorbers[blank_end] and of sampling material
where the shower products are [blank_start]detected[blank_end]. The absorber is typically a large Z material, e.g. lead. In the sampling material, the low-energetic shower products create ionisation or scintillation [blank_start]signals[blank_end]. Example: Liquid nobel gases or scintillators. The energy resolution improves with the particle energy.- Amatter
- Bcompletely
- Catomic nucleus
- Dstrong nuclear
- Eweak
- Fhigh enough
- Ghadronic showers
- Henergy
- Iloss
- Jalteration
- Knuclei
- LInelastic
- Mnuclear absorption
- Ndecreases
- Oheavy
- Pnuclear interactions
- Qdetector atoms
- Rhadronic
- S10
- Tphotoelectric
- Ucompton
- Velectromagnetic interactions
- Wdeposited
- Xabsorbers
- Ydetected
- Zsignals
- AAfaster
- ABlight
- ACtrack
- ADmagnetic
- AEmomentum
- AFpair production
- AGthickness
- AHmean
2Charged hadron: Track in tracking detector, [blank_start]hadronic[blank_end] shower in calorimeter
Neutral hadron: [blank_start]hadronic[blank_end] shower in calorimeter
Electron/positron: track in tracking detector, [blank_start]EM[blank_end] shower in calorimeter
Photon: [blank_start]EM[blank_end] shower in calorimeter
Muon: Track in tracking detector, [blank_start]track[blank_end] in calorimeter- Atrack
- Bhadronic shower
- CEM shower
- DEM
- Ehadronic
- FEM
- Ghadronic
- Hhadronic
- IEM
- Jhadronic
- KEM
3The Electric charge, Q, the total charge of the particles before and after the interaction takes place is [blank_start]conserved[blank_end] in all interactions. The Lepton number is conserved in all interactions and each [blank_start]generation[blank_end] has a separately conserved lepton number. The Baryon number is conserved in all interactions as the number of (quarks - anti-quarks) is [blank_start]constant[blank_end]. The net quark flavour corresponds to the [blank_start]difference[blank_end] between the number of quarks and antiquarks. All Quark flavour numbers are conserved in electromagnetic and strong interactions but not necessarily in [blank_start]weak[blank_end] interactions.
Each particle has a fixed intrinsic angular momentum - its [blank_start]spin[blank_end] - expressed with two quantum numbers: s and ms. The total angular momentum is conserved in all interactions.
Parity [blank_start]reverses[blank_end] the spatial coordinates. The Parity operator has the eigen values +1 (even) and −1 (odd). Parity is conserved in Strong and Electromagnetic interactions, but not necessarily in weak interactions. Parity is a multiplicative quantum number. Quarks and leptons have intrinsic P = +1 assigned. Antifermions have [blank_start]opposite[blank_end] parity as fermions. For bosons, P(particle) = P(antiparticle).
Charge conjugation, C, changes a particle into its [blank_start]antiparticle[blank_end]. C is as well a multiplicative quantum number. C is conserved in Strong and Electromagnetic interactions but not necessarily in weak interactions. Most particles are not eigenstates of C: Only particles that are their own [blank_start]antiparticles[blank_end] (e.g. π0, γ) are eigenstates of C.
Time Reversal T reverses the time component of processes. Can be checked by measuring reaction rates of a process and its reverse process. Time reversal [blank_start]symmetry[blank_end] is present in Strong and Electromagnetic interactions. The product of Charge-Parity and Parity, CP, is conserved in strong and EM interactions and nearly conserved in weak interactions. A small degree of
CP-violation is important to create a [blank_start]dominance[blank_end] of matter over antimatter in the early universe. In a relativistic Quantum field theory, the laws of physics are expected to be [blank_start]invariant[blank_end] under the combined operations of Charge-Parity, Parity and Time-Reversal (CPT). ⇒ if a process does not [blank_start]conserve[blank_end] CP it violates as well time-reversal symmetry T.- Aconserved
- Bgeneration
- Cconstant
- Ddifference
- Eweak
- Fspin
- Greverses
- Hopposite
- Iantiparticle
- Jantiparticles
- Ksymmetry
- Ldominance
- Minvariant
- Nconserve
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- A level EPP Quiz on W3, created by A P on 09/05/2021.
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- A P
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- English
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