Particle Polarization in High Energy Physics

9 November 2023

Particle Polarization in High Energy Physics: An Introduction and Case Studies on Vector Particle Production at the LHC, by Pietro Faccioli and Carlos Lourenço, Springer

Particle Polarization in High Energy Physics

At the end of two pedagogical seminars that Pietro Faccioli gave in April 2013 at CERN and HEPHY, Vienna, on the topic “Angular momentum and decay distributions in high energy physics: an introduction and use cases for the LHC”, several people, including myself, encouraged him to turn his slides into a textbook on particle polarisation. Ten years later I received a copy of Particle Polarization in High Energy Physics: An Introduction and Case Studies on Vector Particle Production at the LHC from Carlos Lourenço, co-author and Pietro’s long-term colleague. During this decade, much has been learned about particle polarisation and related topics, in particular thanks to measurements made at the LHC. As someone who had a front-row seat to observe this progress in the context of polarisation measurements in the CMS experiment, I can attest to the importance and timeliness of this book.

Throughout the first four chapters, the authors guide the reader through a mosaic of relatively easy paths that introduce important concepts, including among others: helicity conservation, parity properties, polarisation frames and their transformations, frame-independent polarisation, and the Lam–Tung relation. Throughout the narrative, they often present real or simulated examples of caveats that can induce irreversible distortions in the measured distributions, potentially biasing the experimental results or their interpretation. The second half of the book (running to another 150 pages) targets a more expert audience, interested, for example, in acquiring the background knowledge needed to study cascade decays to vector particles or smearing effects of higher-order QCD (“non-planar”) processes. Appendix B, in particular, with page-long equations and no figures, must have been prepared “on demand” for people studying rare Z and W radiative decays with LHC data.

The pedagogical style of the text and the quality of the figures have clearly benefitted from the multiple interactions that the authors had with many people through physics schools, university seminars and workshops. The reader can also easily appreciate that the authors contributed to the field of particle polarisation with several original ideas, both regarding the development of robust data-analysis methods and their phenomenological interpretations. It is particularly eye-opening to see how easy it is to obtain biased experimental results if the analysis methods follow simplified approaches, ignoring the intrinsic multidimensionality of polarisation measurements. While thetext is very well written, the aspect that most distinguishes this book from others on similar topics is the presence of several beautiful figures, providing a welcome visual presentation of non-trivial concepts.

The authors contributed to the field of particle polarisation with several original ideas

Thanks to the CERN-supported open-access publication, the book can be directly downloaded by anyone who is interested. Although many readers will prefer a paper copy, the PDF file has the advantage that the reader can very easily navigate within the book by clicking on the many links connecting the text to figures, equations, cited references, and even to words in the very useful index. It is particularly practical to be one click away from an equation shown, sometimes, a hundred pages earlier.

Given the steady increase in the size of data samples being collected by the LHC experiments and the role that the polarisation aspects play in precision measurements of Standard Model processes, as well as in improving the efficiency of searches for new particles, the authors may soon be tempted to write a sequel. In such a future edition, it would be good to include a list of exercises for the interested reader, based on the authors’ behind-the-scenes knowledge and including realistic “traps” that readers should avoid. This would strengthen even further the role of the book as a guide for students and researchers involved in analysis of experimental data or in the interpretation of results.

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