The Standard Model of particle physics represents one of the triumphs of modern physics. With the discovery of the Higgs boson at the LHC, all of the particles in the Standard Model have now been observed. The main aim of this book is to provide a broad overview of our current understanding of particle physics. It is intended to be suitable for final-year undergraduate physics students and also can serve as an introductory graduate-level text. The emphasis is very much on the modern view of particle physics with the aim of providing a solid grounding in a wide range of topics.

Our current understanding of the sub-atomic Universe is based on a number of profound theoretical ideas that are embodied in the Standard Model of particle physics. However, the development of the Standard Model would not have been possible without a close interplay between theory and experiment, and the structure of this book tries to reflects this. In most chapters, theoretical concepts are developed and then are related to the current experimental results. Because particle physics is mostly concerned with fundamental objects, it is (in some sense) a relatively straightforward subject. Consequently, even at the undergraduate level, it is quite possible to perform calculations that can be related directly to the recent experiments at the forefront of the subject.

Pedagogical approach

In writing this textbook I have tried to develop the subject matter in a clear and accessible manner and thought long and hard about what material to include.

Relation of observables to experimental techniques

Every probe in the search for the QGP in relativistic heavy ion collisions tends to have a different experimental technique associated with it. In all cases the multiplicities in nuclear collisions are so large that all the detectors used are very highly segmented. For measuring the charged multiplicity or dn/dy,a segmented multiplicity detector, usually an array of proportional tubes with pad readout, or a silicon pad array were used in early experiments, while Time Projection drift Chambers (TPC) have become popular more recently. For measuring transverse energy flow, dET/dy, a hadron calorimeter is used. Some groups use an electromagnetic shower counter for this purpose. This has the advantage of being smaller, cheaper and higher in resolution than a full hadron calorimeter; but it has the disadvantage of being biased, since only π0 and η0 mesons are detected (via their two photon decay). Nuclear fragmentation products are detected by calorimeters in the projectile direction and by E, dE/dx scintillator arrays in the target fragmentation region. The particle composition and transverse momentum distributions are measured using magnetic spectrometers with particle identification. Typically, time-of-flight, gas and aerogel Cerenkov counters, and dE/dx are used to separate pions from kaons, protons, deuterons, etc. Drift chambers are generally utilized for charged particle tracking, although streamer chambers and TPCs are also in use. Lepton pair detectors are very specialized, and usually combine magnetic spectrometers with lepton identification (muons by penetration, and electrons by “gas” and “glass”).

The CCRS experiment at the CERN-ISR

A very interesting thing happened in the second round of ISR experiments, two first round experiments decided to combine their detectors and join forces. The Saclay–Strasbourg experiment (R102) had proposed a second spectrometer arm (Arm 2) to study what was produced opposite in azimuth to balance the transverse momentum of the high pT hadrons [281]. Then, together with the CCR experiment (R103), it was proposed [282] to add the CCR lead glass counters (PbGl) behind the Arm 2 to enable improved detection of single electrons and gamma rays at high pT as well as to continue the search for e+e- pairs. This became R105, the CERN–Columbia–Rockefeller–Saclay experiment, CCRS (Figure 7.1). This detector turned out to be very powerful and was rewarded with one major discovery, one near-miss and several excellent measurements.

The key features of this detector were the following:

1. (i)≥105 charged hadron rejection from electron identification in the Cerenkov counter combined with matching the momentum and energy of an electron candidate in the magnetic spectrometer and the PbGl;

2. (ii) minimum of material in the aperture to avoid external conversions;

3. (iii) zero magnetic field on the axis to avoid de-correlating conversion pairs;

4. (iv) rejection of conversions in the vacuum pipe (and small opening angle internal conversions) by requiring single ionization in a hodoscope of scintillation counters H′ close to the vacuum pipe, preceded by a thin track chamber to avoid conversions in the H′ counters;

5. (v) precision measurement of π0 and η, the predominant background source;

6. (vi) precision background determination in the direct single e± signal channel by adding an external converter, to distinguish direct single e± from e± from photon conversion.