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Cosmic rays from galaxies far far away

22 September 2017

Cosmic rays with energies a million times greater than the protons accelerated in the Large Hadron Collider are coming from far outside our own galaxy, say scientists from the Pierre Auger Collaboration.

Ever since the existence of cosmic rays with individual energies of several Joules[1] was established in the 1960s, speculation has raged as to whether such particles are created in our galaxy or in distant extragalactic objects. The 50-year-old mystery has been solved using cosmic particles with a mean energy of 2 Joules recorded by the Pierre Auger Observatory in Argentina — the largest cosmic-ray observatory ever built. 

The observations, published today in Science, have found that at a mean energy of 2 Joules, the cosmic rays’ rate of arrival is ~6% greater from one half of the sky, with the excess lying 120˚ away from the Galactic centre.

Spokesperson for the Pierre Auger Collaboration, Professor Karl-Heinz Kampert from the University of Wuppertal in Germany, said: "We are now considerably closer to solving the mystery of where and how these extraordinary particles are created, a question of great interest to astrophysicists. Our observation provides compelling evidence that the sites of acceleration are outside the Milky Way.”

Emeritus spokesperson for the Pierre Auger Collaboration, Emeritus Professor Alan Watson from the School of Physics and Astronomy at the University of Leeds said: “I consider this to be one of the most exciting results that we have obtained and one which solves a problem targeted when the Observatory was conceived by Jim Cronin and myself over 25 years ago”.

Cosmic rays are the nuclei of elements from hydrogen (the proton) to iron. Above 2 Joules, the rate of their arrival at the top of the atmosphere is only about one per square kilometre per year — equivalent to one hitting the area of a football pitch about once per century. Such rare particles are detectable because they create showers of electrons, photons and muons through successive interactions with the nuclei in the atmosphere.  These showers spread out, sweeping through the atmosphere at the speed of light in a disc-like structure, similar to a dinner-plate, several kilometres in diameter. 

The showers contain more than ten billion particles and are detected at the Pierre Auger Observatory through the Cherenkov light they produce in a few of the 1,600 detectors spread over 3,000 km2 of Western Argentina — an area comparable to that inside the M25 around London.  The times of arrival of the particles at the detectors, measured with GPS receivers, are used to find the arrival directions of events to within ~1˚. 

By studying the distribution of the arrival directions of more than 30,000 cosmic particles, the Pierre Auger Collaboration has discovered an anisotropy, significant at 5.2 standard deviations (a chance of about two in ten million), in a direction where the distribution of galaxies is relatively high.

Although this discovery clearly indicates an extragalactic origin for the particles, the actual sources have yet to be pinned down.  The direction of the excess points to a broad area of sky rather than to specific sources as the magnetic field of our galaxy causes even particles as energetic as these to be deflected by a few 10s of degrees. The direction, however, cannot be associated with putative sources in the plane or centre of our galaxy for any realistic configuration of the galactic magnetic field.

Cosmic rays of even higher energy than the bulk of those used in this study exist, some even with the kinetic energy of well-struck tennis ball.  As the deflections of such particles are expected to be smaller, the arrival directions should point closer to their birthplaces. 

These highly energised cosmic rays are even rarer and further studies are underway to use them to try to pin down which extragalactic objects are the sources. Knowledge of the nature of the particles will aid this identification and work on this problem is targeted in the upgrade of the Pierre Auger Observatory to be completed in 2018.  


[1] 1 Joule = ~ 6 x 1018 eV


Further information:


Professor Alan Watson is available for interviews and additional information at or +44 07870 109602

The research paper “Observation of a large-scale anisotropy in the arrival directions of cosmic rays above 8 × 1018 eV” was published today in Science.

Pierre Auger Collaboration involves over 400 scientists from 18 countries.

Related Articles:

Scientific American - High-Energy Cosmic Rays Come from Outside Our Galaxy
ScienceNews - Ultrahigh energy cosmic rays come from outside the Milky Way

National Teaching Fellowship for Physics Lecturer

14 September 2017

Dr Samantha Pugh has been awarded a National Teaching Fellowship, one of the most prestigious awards for higher education teaching.

The Higher Education Academy (HEA) announced Dr Samantha Pugh, Lecturer in STEM Education in the School of Physics and Astronomy and Faculty Lead for Teaching Enhancement, has been made a National Teaching Fellow in recognition of excellence in teaching. This brings Leeds’ tally to 26 – the highest of any university.

Dr Pugh said:

“It’s really wonderful to be recognised with a National Teaching Fellowship. I’m very grateful to the University of Leeds for giving me the opportunities and encouragement. I would like to also acknowledge all of the amazing colleagues and students that I have the pleasure of working with, who have made this award possible.”

Professor Tom Ward, Deputy Vice-Chancellor: Student Education said:

“The University’s innovative programme of research-based education is committed to developing independent, critical thinkers who make a difference to the world around them

“Leeds is a Russell Group university that has always placed an emphasis on recognising and rewarding teaching excellence alongside its exceptional research expertise. The National Teaching Fellowship Scheme is an important recognition of that excellence and it is with great pleasure that I congratulate Samantha.”

Dr Pugh’s work focuses on helping students to improve their prospects in the long term. This includes working with them to embed specific skills into their courses that employers will recognise, and involving both employers and students in how courses are shaped, particularly through the Group Industrial Project in Physics. She also established the popular Peer Assisted Learning Scheme across the Faculty of Mathematics and Physical Sciences.

She established Industrial Advisory Boards in Chemistry, Maths and Physics: a major step in enhancing employer engagement with the University. She is also renowned for inspiring and mentoring colleagues to shape teaching through pedagogic research and scholarship at Leeds and beyond.

Dr Pugh has also secured a prestigious Leeds Excellence and Innovation Fellowship sabbatical at the Leeds Institute for Teaching Excellence, leading the project Reimagining University Assessment by Learning from Secondary Education.

Further information

The HEA manages the National Teaching Fellowship Scheme on behalf of the Higher Education Funding Council for England, the Higher Education Funding Council for Wales, and the Department for the Economy (DfE) in Northern Ireland.

High Education academy has praised the impact of Dr Pugh’s work.

Dr Robert Purdy authors new book on particle physics

11 September 2017

Dr Robert Purdy, School of Physics and Astronomy, has authored a new book on particle physics.

The publication is a comprehensive introduction to particle physics, aimed at third-year undergraduates. 

There are two versions of the publication, The Fundamentals of Particle Physics, published in the UK, and Particle Physics: An Introduction, published in the US.

It is written to complement the third-year module Dr Purdy, who is also part of the Theoretical Physics Research Group, leads: Theoretical Elementary Particle Physics. This particular module provides an in-depth introduction to theoretical particle physics. 

It is a basis for further study in particle physics, astrophysics, detector physics and other areas of science and technology, which require elementary knowledge of particle physics concepts.

About the publication

Particle Physics remains one of the most popular aspects of modern physics to study. It seeks to understand the Universe at its most fundamental level by examining the building blocks from which the Universe is ultimately constructed. Specifically, The Fundamentals of Particle Physics considers what symmetries exist in nature and how they are represented. As a result, many of its underlying principles can be applied to other areas of physics. Understanding Particle Physics is therefore essential for physics undergraduate students. 

This book provides a comprehensive overview of particle physics, with an emphasis on the underlying principles and mathematical structures. In particular, the role of symmetries in nature is explored, as well as how this leads to an understanding of the particle Universe. The book has three main aims. Firstly, to introduce the key concepts of particle physics. Secondly, to provide a solid understanding of the Standard Model of particle physics, as well as looking beyond to potential future developments in the field. Thirdly, to present the necessary tool-kit required for further exploration in particle physics, with each chapter accompanied by exercises to support understanding.

Further information

Dr Purdy’s book is available to purchase on Amazon 

School of Physics and Astronomy scores highly for student satisfaction

9 August 2017

The School of Physics and Astronomy scored 90% for overall student satisfaction in the latest National Student Survey (NSS).

The University overall has also performed well, achieving a score of 89%.

The NSS gathers feedback from mostly final-year undergraduate students about their experience studying their course at their institution. The University of Leeds had a 70% response rate.  

Further information

More detailed results are available on the HEFCE website.

Professor Cliff Jones wins prestigious Katharine Burr Blodgett Medal and Prize

30 June 2017

Cliff Jones, of the School of Physics and Astronomy at the University of Leeds, has been awarded one of the Institute of Physics' (IoP) gold medals for 2017.

IoP gold medals reward outstanding and sustained contributions to physics by people of international reputation.

Specifically, Professor Jones has been recognised for his contributions to the organisation or application of physics in an industrial or commercial context.

Cliff's industrial achievements include being co-inventor of the Zenithal Bistable Display and founding Displaydata Ltd.

Read more about Professor Jones' award and achievements on the Institute of Physics website.


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