In the preceding chapter we discussed the electromagnetic interaction, which was responsible for the energy loss and small angle scattering of charged particles, and for the production and interactions of photons. However, there are other types of processes where the nuclear interaction may represent the dominant mechanism. These include particle creation reactions, interactions at high energies or large momentum transfers, and interactions of neutral particles other than the photon. In this chapter we will examine some of the basic properties of nuclear interactions. We will not be directly concerned with the physics underlying subnuclear phenomena. Instead, our main concern will be to survey its overall features, principally total cross sections, particle production multiplicities, and angular distributions.

We will first discuss the strong interaction. The group of particles known as hadrons is influenced by this interaction. Next we will briefly discuss the weak interaction, which is responsible for the interactions of neutrinos in matter and for the decay of most quasistable particles.

Strong interactions

The group of particles known as hadrons are subject to the strong interaction in matter. The neutron is an ideal probe of this interaction since it has no appreciable electromagnetic interactions. We have seen that the cross sections for most electromagnetic interactions are strongly peaked in the forward direction and fall off with increasing energy. Thus, away from the forward direction, the high energy behavior of all hadrons is determined by the strong interaction.

One of the most commonly used particle detectors is the scintillation counter. A fraction of the energy lost by a charged particle can excite atoms in the scintillating medium. A small percentage of the energy released in the subsequent deexcitation can produce visible light. The technique has been used since the earliest investigations of radioactivity, when, for instance, Rutherford used scintillating ZnS crystals in his alpha particle scattering experiments.

In modern detectors light produced in the scintillator is propagated through light guides and directed onto the face of a photomultiplier tube. Photoelectrons emitted from the cathode of the tube are amplified to give a fast electronic pulse, which can be used for triggering or timing applications.

The scintillation process

We define a scintillator to be any material that produces a pulse of light shortly after the passage of a particle. The phenomenon is closely related to fluorescence, which is usually defined to be the production of a light pulse shortly following the absorption of a light quantum. “Shortly” here refers to time intervals on the order of 10 ns or less. Phosphorescence is a third phenomenon involving light emission, but in this case the molecules are left in a meta-stable state, and the emission may occur much later than the initiating event.

Both inorganic and organic scintillators have been discovered. The scintillation process is different for the two groups.

A device that measures the total energy deposited by a particle or group of particles is known as a calorimeter, in analogy with the laboratory instrument that measures the amount of deposited heat. We have already encountered several devices, such as sodium iodide scintillation counters and total absorption Cerenkov counters, that can be used as calorimeters for photon detection. We will consider properties of these “continuous” calorimeters again in Chapter 14. In this chapter we consider a class of calorimeters that periodically sample the development of a shower initiated by an incident particle. There are two major types of sampling calorimeters, depending on whether the incident particle initiates an electromagnetic or hadronic shower. Each type of calorimeter is optimized to maximize the rejection of the other type of shower.

Calorimeters have found wide use in particle physics experiments. Neutral particles can only be detected by using this method. Sampling calorimeters of very large size have been used as neutrino detectors. We have seen that at high energy, particle multiplicities grow with increasing energy, and the angular distribution of groups of the produced secondaries are highly collimated (jet effect). Under these conditions calorimeters can provide a useful trigger for interesting events based on the total energy deposited in a localized area. Calorimeters can easily be modularized and made to cover large solid angles. In addition, we shall see that the size of a calorimeter needed to measure the energy of a particle scales like ln(E), whereas the size of a magnetic deflection device would scale like E½.

This final chapter has three goals. First, we want to show how experimentalists have measured properties of subatomic particles. This section takes the form of a survey of some of the applicable techniques. Second, we want to discuss some of the considerations involved in measurements of particle interactions, such as total cross sections, elastic differential cross sections, polarization experiments, and new particle searches. Finally, we want to illustrate these measurements with examples of actual particle physics experiments.

Particle properties

In this section we will describe some of the methods used to measure the basic properties of the elementary particles. As mentioned in Chapter 1, these properties include charge, mass, spin, magnetic moment, lifetime, and branching ratios. Many specialized techniques have been developed for measuring some of these properties, particularly for the electron and nucleons. We will not attempt to survey all the applicable procedures for each particle, since many of the methods use techniques from atomic and molecular physics that fall outside the scope of this book. Instead we will follow the philosophy of the preceding chapters and discuss selected examples in more detail.

Charge

The sign of a particle's charge may be inferred from the direction of its deflection (if any) in a magnetic field of known orientation. The magnitude of the charge can be determined if the momentum of the particle and the strength of the magnetic field are known.

I have felt for some time that there should be a book that briefly ties together the most important topics in experimental particle physics. The biggest difficulty I have encountered in trying to do this is not that information concerning this subject is lacking, but rather that so much of it exists. Reports on experimental techniques and devices can be found scattered through specialized monographs, conference proceedings, data compilations, review papers, and journal articles. I have had to make enumerable, arbitrary selections in order to produce what I hope is a balanced overview of the subject in a book of reasonable length. I hope that the final product will be useful to graduate students and to others interested in an introduction to the subject and as a reference for practitioners in the field.

The first three chapters give an overview of the subject and discuss the electromagnetic and nuclear interactions of particles. A knowledge of particle interactions is necessary for an understanding of how detectors work, besides being interesting in their own right. The next three chapters are concerned with three nearly universal aspects of particle physics experiments: beams, targets, and fast electronics. Chapters 7 through 12 contain more detailed discussions of various types of detectors. Whenever possible I have attempted to enumerate the advantages and disadvantages of each detector and to specify the factors that limit its performance. The last three chapters are concerned with integrating detectors into a coherent system.

Particle physics is the study of the properties of subatomic particles and of the interactions that occur among them. This book is concerned with the experimental aspects of the subject, including the characteristics of various detectors and considerations in the design of experiments. This introductory chapter begins with a description of the particles and interactions studied in particle physics. Next we briefly review some important material from relativistic kinematics and scattering theory that will be used later in the book. Then we give a brief preview of the various aspects of particle physics experiments, before discussing each topic in greater detail in subsequent chapters. Finally, we give a short discussion of some of the tasks involved in analyzing the data from an experiment.

Particle physics

Particle physics is the branch of science concerned with the ultimate constituents of matter and the fundamental interactions that occur among them. The subject is also known as high energy physics or elementary particle physics. Experiments over the last 40 years have revealed whole families of short-lived particles that can be created from the energy released in the high energy collisions of ordinary particles, such as electrons or protons. The classification of these particles and the detailed understanding of the manner in which their interactions leads to the observable world has been one of the major scientific achievements of the twentieth century.

The notion that matter is built up from a set of elementary constituents dates back at least 2000 years to the time of the Greek philosophers.

Most particle physics experiments require a beam of particles of a certain type. Usually these particles are provided by a high energy accelerator. Thus we will begin this chapter with a brief description of the characteristics of particle accelerators. These divide into two major classes, depending on whether the particle beam collides with a fixed target or with another beam of particles. We then discuss some properties of secondary beams from fixed target accelerators and the rudiments of beam transport theory. Since an important property of the beam for the experimentalist is the intensity, we will discuss flux monitoring. This is followed by a description of alternate sources of particles. The chapter concludes with a discussion of radiation protection.

Particle accelerators

A particle experimentalist is primarily concerned with four properties of the particle beam: the energy, the flux of particles, the duty cycle of the accelerator, and the fine structure in the intensity as a function of time. The duty cycle is defined to be the fraction of the time that the accelerator is delivering particles to the experiment. A detailed description of the components and acceleration process in various types of accelerators is beyond the scope of this book. However, we will give a brief overview in order to introduce some of the terminology.

The beam in an accelerator starts in either an electron gun or an ion source.

Certain types of fast pulse electronics, such as discriminators and coincidence units, are used almost universally in particle physics experiments. In this chapter we review some important features of these and other electronic equipment, strictly from the point of view of a user.

Fast pulse instrumentation

An important function of fast electronics in particle physics experiments is to decide if the spatial and temporal patterns of detector signals satisfy the requirements of the event trigger. Fast in this context generally means circuits capable of processing pulses at a 100-MHz repetition rate. Most detectors produce analog signals. Discriminators are used to convert these analog signals into standardized logic levels. Logic units are available that can perform the logical operations: AND, NAND, OR, NOR, and NOT. The input and output signal amplitudes of these devices correspond to two possible states: 0 to 1 (or T or F). The logic unit signals can be joined together so that the final output is only true when a predetermined pattern of input signals is present. This output pulse can be used to signal the occurrence of a physical event of interest.

The need for certain electronic devices such as discriminators and logic units in practically every experiment lead to the establishment of the NIM standard. Devices that satisfy the NIM requirements must be housed in standard sized modules with standard rear connectors. Up to 12 units can be plugged into a NIM bin.

A well-designed trigger is an essential ingredient for a successful particle physics experiment. The trigger must efficiently pass the events under study without permitting the data collection systems to become swamped with similar but uninteresting background events. Since the design of a trigger depends critically on the intent of the experiment and is strongly influenced by the choice of beam parameters, target, geometry, and so forth, it is impossible to give a prescription here on how to set up a trigger for any situation. Instead, we must content ourselves in this chapter with considering some general classes of trigger elements and with examining some specific examples in more detail. It should be mentioned that some experiments do not use a trigger. For example, neutrino experiments sometimes accept any event that occurs within a gate following the acceleration cycle.

General considerations

A trigger is an electronic signal indicating the occurrence of a desired temporal and spatial correlation in the detector signals. The desired correlation is determined by examining the physical process of interest in order to find some characteristic signature that distinguishes it from other processes that will occur simultaneously. Most triggers involve a time correlation of the form B · F, where B is a suitably delayed signal indicating the presence of a beam particle and F is a signal indicating the proper signature in the final state. The time coincidence increases the probability that the particles all come from the same event.