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.

The material in this book is an enlarged version of a highly successful, short course of lectures given to graduate students in the Nuclear Physics Laboratory, Oxford. The course was designed to interest both nuclear structure and elementary particle physicists.

I am an experimental high energy physicist, and although as an undergraduate I suffered a course in statistics, it was only later during my research work, when I had to deduce something from my own data, that I learned how to use statistics. I also discovered that there were many things that one learned by experience and which were not explicitly mentioned in text-books; I tried to incorporate these aspects of the subject in my lectures. The emphasis of my lectures was thus not on formal proofs or on a rigorous treatment of the subject, but rather on practical applications and on how to use statistics to obtain the best results from one's data and to know the limitations of the results. In short, this was a course given by a non-statistician to non-statisticians.

My course did not attempt to cover every single example of statistics problems that can arise in nuclear and particle physics. The aim was rather to explain as fully as possible the different techniques that are available for attacking data analysis problems, to explain their relative merits and drawbacks, and to try to give the students sufficient confidence in their own ability to tackle any new problems that they might encounter. The book has maintained this approach.

Why do we do experiments?

There are basically two different types of results of experiments that scientists perform in order to learn about the physical world. In one type, we set out to determine the numerical value of some physical quantity, while in the second we are testing whether a particular theory is consistent with our data. These two types are referred to as ‘parameter determination’ and ‘hypothesis testing’ respectively. (Of course, in real life situations there is a degree of overlap between the two: a parameter determination may well involve the assumption that a specific theory is correct, while a particular theory may predict the value of a parameter.) For example, a parameter determination experiment could consist of measuring the velocity of light, while a hypothesis testing experiment could check whether the velocity of light has suddenly increased by several percent since the beginning of this year.

In this chapter, we are mainly concerned with various aspects of calculating the accuracy of parameter determination type experiments. We will have more to say about hypothesis testing experiments in Chapter 2.

Why estimate errors?

When we performed parameter determination experiments at school, we considered that the job was over once we obtained a numerical value for the quantity we were trying to measure. At university, and even more so in every-day situations in the laboratory, we are concerned not only with the answer but also with its accuracy.