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2 Tetraquarks and Pentaquarks

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4 Tetraquarks and Pentaquarks Greig Cowan University of Edinburgh Tim Gershon University of Warwick IOP Publishing, Bristol, UK

5 ª IOP Publishing Ltd 2018 All rights reserved. No part of this publication may be reproduced, stored in a retrieval system or transmitted in any form or by any means, electronic, mechanical, photocopying, recording or otherwise, without the prior permission of the publisher, or as expressly permitted by law or under terms agreed with the appropriate rights organization. Multiple copying is permitted in accordance with the terms of licences issued by the Copyright Licensing Agency, the Copyright Clearance Centre and other reproduction rights organisations. Permission to make use of IOP Publishing content other than as set out above may be sought at permissions@iop.org. Greig Cowan and Tim Gershon have asserted their right to be identified as the authors of this work in accordance with sections 77 and 78 of the Copyright, Designs and Patents Act Multimedia content is available from About Book online ISBN (ebook) DOI / Version: Physics World Discovery ISSN (online) British Library Cataloguing-in-Publication Data: A catalogue record for this book is available from the British Library. Published by IOP Publishing, wholly owned by The Institute of Physics, London IOP Publishing, Temple Circus, Temple Way, Bristol, BS1 6HG, UK US Office: IOP Publishing, Inc., 190 North Independence Mall West, Suite 601, Philadelphia, PA 19106, USA

6 Contents Abstract Author biographies vi vii Tetraquarks and Pentaquarks 1 1 Introduction 1 The Standard Model and quantum chromodynamics 1 Hadrons and the quark model 3 Different types of hadrons 4 2 Background 5 Understanding hadrons through their quantum numbers 5 Discovery of the J/ψ meson 7 Resonances, Breit Wigner lineshape and phase changes 8 Controversy over claimed light pentaquarks 9 Heavy flavour hadrons, and the experimental facilities used to study them 10 Discovery of the X(3872) 13 3 Current directions 15 Tetraquarks 15 Pentaquarks 18 Doubly heavy baryons 20 4 Outlook 22 Models of exotic hadrons and how they can be distinguished 22 Experimental prospects 23 Additional resources 24 v

7 Abstract Since 2003 there has been an explosion in the observation of new hadronic states that cannot be classified by the well-tested quark model of mesons (quark-antiquark) and baryons (three quarks or antiquarks). The properties of these states indicate that they are combinations of four, five, or more, quarks and antiquarks, making them manifestly exotic. A recent high-profile example was the discovery of two pentaquark candidates by the LHCb collaboration at the CERN Large Hadron Collider (LHC). No scientific consensus has yet emerged to explain the exotic hadron spectrum, demanding a new set of experimental observations that will feed the development of theoretical models to describe the dynamics of multiquark states. In this Discovery book we will take you on a tour of quantum chromodynamics, explain about the latest research into the physics of exotic hadrons and describe the exciting opportunities that are offered by the next generation of particle physics experiments. vi

8 Author biographies Greig Cowan Greig Cowan is a particle physicist at the University of Edinburgh. Prior to the start of the CERN Large Hadron Collider (LHC) he deployed and commissioned the grid computing infrastructure that now underpins the success of the LHC physics programme. Since 2010 he has been a member of the LHCb collaboration at CERN, focussing on measuring the subtle differences between matter and antimatter in the physics of b quarks (so-called CP violation), searching for new states in the exotic hadron spectrum and measuring the properties of those that have been found. From he was an Ernest Rutherford research fellow, funded by the UK s Science and Technology Facilities Council (STFC). Tim Gershon Tim Gershon is a Professor of Physics at the University of Warwick. After completing his PhD at the University of Cambridge in 2000, his research career has taken him to experiments in Japan and California as well as CERN. He served as Physics Coordinator for the LHCb experiment for the calender years 2012 and In addition to work on hadron spectroscopy, he is highly active in studies of CP violation in the B system. vii