Particle Identification

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PARTICLE IDENTIFICATION

Elementary particles are studied by looking at the production and decay of particles in high-energy collisions, where the initial state energy is converted (via E = mc²) into the mass of new particles. These collisions and decays obey the laws of probability so that to build up a picture of reality, a library of the events must be accumulated that are the observed outcomes of collisions. A collision will typically produce a number of particles whose identities must be unraveled to label a particular event correctly.

Elementary particles have a wide range of masses, interactions, and average lifetimes against decay. Table 1 gives a representative selection of particles—for each charged particle there is an antiparticle with the opposite electric charge. The average distance to decay is calculated allowing for relativistic time dilation at a momentum equal to 10 times the particle mass.

Ionization of the Detector Medium

Electrically charged particles ionize (remove electrons from) matter as they pass through it. This disturbance can be used to detect the path they follow. If the particle detector, typically a large volume of a suitable gas such as argon, is placed in a magnetic field, the trajectory is deflected into a circular path (by the same law that makes electric motors turn when an electric current (moving charge) passes

TABLE 1

Masses, Interactions, and Average Lifetimes against Decay for Selected Particles
	Charge	Mass	Absorption	Mean	Average
	(proton	(proton	length in	lifetime	distance to
Particle	charge = 1)	mass = 1)	iron (cm)	(sec)	decay (cm)
credit: Courtesy of David H. Saxon.
Protron(ρ)	+1	1	20	stable	infinite
Electron(e^-)	-1	0.0005	2	stable	infinite
Muon(μ^-)	-1	0.113	Very long	2 × 10^-6	600,000
Pion(π⁺)	+1	0.149	30	3 × 10^-8	8,000
Kaon(κ⁺)	+1	0.527	30	1 × 10^-8	4,000
Photon(γ)	0	0	2	stable	infinite
κ⁰	0	0.531	—	9 × 10^-11	30
φ⁰	0	1.086	—	1 × 10^-22	4 × 10^-11
B_s	0	5.724	—	1 × 10^-12	0.4

through a magnetic field). The radius of curvature depends on the particle momentum (mass times velocity with relativistic corrections). Thus the momenta and directions of charged particles can be measured.

The density of ionization along the track depends on the particle velocity. (Slow particles have a longer time available to disturb each atom as they pass and so are more efficient at ionizing gases.) So, for a known measured momentum the particle velocity will depend on the mass. Thus, if the momentum is measured by the curvature of a track, and the ionization density provides the information about the velocity, the ionization can be compared to that expected for e⁺, μ⁺, π⁺, κ⁺, and ρ, and the particle's identity can be inferred. This measurement is rather delicate as the differences in ionization are not large compared to the sample-by-sample fluctuations obtained from measurements taken while moving down the track. So, other methods are preferred.

Cherenkov Radiation

A direct method of inferring the particle velocity, and hence the mass, once the momentum is known, is to look for Cherenkov radiation. It is not possible for a particle to travel faster than light in a vacuum, but in a material medium, light has a reduced velocity, and a particle may exceed that (reduced) light velocity in the medium. In doing so, it gives out a flash of blue/ultraviolet light, analogous to the sonic boom of a plane traveling faster than sound. This has been used to infer velocities, and hence particle identities, most notably in the case of electrons, which have a dramatically lower mass than any other charged particles and so travel much faster for the same momentum.

Electrons and Muons

Electrons and muons are produced only rarely in particle collisions and often arise from the decay of heavy quarks. It is therefore important that they are identified efficiently and unambiguously. We take advantage of their very different interaction lengths in solid material (Table 1). Compared to more commonly produced particles such as pions and kaons, electrons are absorbed after only a short distance in material, and muons pass through great thicknesses without being affected.

Figure 1 shows an event produced in an e⁺e^- collision. (The incident particles enter at right angles to the plane displayed and annihilate at the center of the detector to produce new matter.) A back-to-back quark-antiquark pair is produced. Each one materializes as a jet of charged and neutral particles. The circular tracks of charged particles passing through a gas are seen (some of them identified by ionization measurements as kaons), followed by signals from a detector made of successive layers of passive metal absorber interleaved with active detector layers that show whether interactions have occurred. (Such a detector is known as a calorimeter because one can measure the total energy of the incident particle by adding up the signals from all the layers.) The electron has interactions in the inner detector layer only, the pions and kaons give signals also in the outer detector layer.

Vertex Detectors

Neutral particles leave no tracks. One can detect them only by absorption in a calorimeter (which treats photons just like electrons) or by their decay in flight to charged particles. The blow-up in Figure 1 shows a complex chain of events. By placing several layers of extremely precise position measurements close to the production vertex, one can reconstruct the outgoing particle trajectories and show whether they originated from the primary interaction point or from a separate vertex arising from the decay in flight of a heavier particle. Such vertex detectors are readily made using silicon-microchip technology and can measure to a precision of a few millionths of a meter.

In Figure 1a, B̄_s is produced at the interaction point (IP) and decays after a few millimeters to a D_s⁺ plus a π^-. The D_s⁺ travels a further 0.4 mm and decays to κ⁺ κ^- π⁺. (Note that the π⁺ track, for example, does not point at the B_s decay and so could not have been produced there.)

Mass of Decaying Particle

From the measured momenta and directions of the outgoing kaons one can reconstruct the mass a

FIGURE 1

possible parent particle that produced them. Finding a mass consistent (within the accuracy of the measurement) with the expected Φ⁰ mass, one identifies the κ⁺ κ^- pair as originating from a Φ⁰ itself produced in the D_s⁺ decay. The Φ⁰ lives only fleetingly and so travels a negligible distance before itself decaying to the κ⁺ κ^- pair.