EPJ A Hadrons and Nuclei EPJ.org your physics journal Eur. Phys. J. A (2015) 51: 114 DOI 10.1140/epja/i2015-15114-0 Melting hadrons, boiling quarks Johann Rafelski 51 Eur. Phys. J. A (2015) : 114 THE EUROPEAN DOI 10.1140/epja/i2015-15114-0 PHYSICAL JOURNAL A Review Melting hadrons, boiling quarks Johann Rafelski1,2,a 1 CERN-PH/TH, 1211 Geneva 23, Switzerland 2 Department of Physics, The University of Arizona Tucson, Arizona, 85721, USA Received: 12 August 2015 / Revised: 23 August 2015 Published online: 22 September 2015 c The Author(s) 2015. This article is published with open access at Springerlink.com Communicated by T.S. B´ır´o Abstract. In the context of the Hagedorn temperature half-centenary I describe our understanding of the hot phases of hadronic matter both below and above the Hagedorn temperature. The first part of the review addresses many frequently posed questions about properties of hadronic matter in different phases, phase transition and the exploration of quark-gluon plasma (QGP). The historical context of the discovery of QGP is shown and the role of strangeness and strange antibaryon signature of QGP illustrated. In the second part I discuss the corresponding theoretical ideas and show how experimental results can be used to describe the properties of QGP at hadronization. The material of this review is complemented by two early and unpublished reports containing the prediction of the different forms of hadron matter, and of the formation of QGP in relativistic heavy ion collisions, including the discussion of strangeness, and in particular strange antibaryon signature of QGP. 1 Introduction This presentation connects and extends a recent retro- spective work, ref. [1]: Melting Hadrons, Boiling Quarks; The years 1964/65 saw the rise of several new ideas which From Hagedorn temperature to ultra-relativistic heavy-ion in the following 50 years shaped the discoveries in funda- collisions at CERN; with a tribute to Rolf Hagedorn. This mental subatomic physics: report complements prior summaries of our work: 1986 [2], 1991 [3],1996 [4], 2000 [5], 2002 [6], 2008 [7]. 1) The Hagedorn temperature TH; later recognized as the A report on “Melting Hadrons, Boiling Quarks and melting point of hadrons into TH” relates strongly to quantum chromodynamics (QCD), 2) Quarks as building blocks of hadrons; and the theory of quarks and gluons, the building blocks of 3) The Higgs particle and field escape from the Goldstone hadrons, and its lattice numerical solutions; QCD is the theorem, allowing the understanding of weak interac- quantum (Q) theory of color-charged (C) quark and gluon tions, the source of inertial mass of the elementary par- dynamics (D); for numerical study the space-time contin- ticles. uum is discretized on a “lattice”. Telling the story of how we learned that strong inter- The topic in this paper is Hagedorn temperature T H actions are a gauge theory involving two types of parti- and the strong interaction phenomena near to T .I H cles, quarks and gluons, and the working of the lattice present an overview of 50 years of effort with emphasis numerical method would entirely change the contents of on: this article, and be beyond the expertise of the author. I a) Hot nuclear and hadronic matter. recommend instead the book by Weinberg [8], which also shows the historical path to QCD. The best sources of b) Critical behavior near T . H the QCD relation to the topic of this article are: a) the c) Quark-gluon plasma (QGP). book by Kohsuke Yagi and Tetsuo Hatsuda [9] as well as, d) Relativistic heavy ion (RHI) collisions1. b) the now 15 year old monograph by Letessier and the au- e) The hadronization process of QGP. thor [6]. We often refer to lattice-QCD method to present f) Abundant production of strangeness flavor. QCD properties of interest in this article. There are books and many reviews on lattice implementation of gauge the- a e-mail: [email protected] ories of interacting fields, also specific to hot-lattice-QCD 1 We refer to atomic nuclei which are heavier than the α- method. At the time of writing I do not have a favorite to particle as “heavy ions”. recommend. Page 2 of 58 Eur. Phys. J. A (2015) 51: 114 Immediately in the following subsect. 1.1 the famous 5) In relativistic heavy ion collisions the kinetic en- Why? is addressed. After that I turn to answering the ergy of ions feeds the growth of quark population. These How? question in subsect. 1.2, and include a few reminis- quarks ultimately turn into final state material particles. cences about the accelerator race in subsect. 1.3. I close This means that we study experimentally the mechanisms this introduction with subsect. 1.4 where the organization leading to the conversion of the colliding ion kinetic en- and contents of this review will be explained. ergy into mass of matter. One can wonder aloud if this sheds some light on the reverse process: Is it possible to convert matter into energy in the laboratory? 1.1 What are the conceptual challenges of the The last two points show the potential of “applica- QGP/RHI collisions research program? tions” of QGP physics to change both our understanding of, and our place in the world. For the present we keep Our conviction that we achieved in laboratory experi- these questions in mind. This review will address all the ments the conditions required for melting (we can also other challenges listed under points 1), 2), and 3) above; say, dissolution) of hadrons into a soup of boiling quarks however, see also thoughts along comparable foundational and gluons became firmer in the past 15-20 years. Now we lines presented in subsects. 7.3 and 7.4. can ask, what are the “applications” of the quark-gluon plasma physics? Here is a short wish list: 1) Nucleons dominate the mass of matter by a factor 1.2 From melting hadrons to boiling quarks 1000. The mass of the three “elementary” quarks found in nucleons is about 50 times smaller than the nucleon With the hindsight of 50 years I believe that Hagedorn’s mass. Whatever compresses and keeps the quarks within effort to interpret particle multiplicity data has led to the nucleon volume is thus the source of nearly all of mass the recognition of the opportunity to study quark decon- of matter. This clarifies that the Higgs field provides the finement at high temperature. This is the topic of the mass scale to all particles that we view today as elemen- book [1] Melting Hadrons, Boiling Quarks; From Hage- tary. Therefore only a small %-sized fraction of the mass dorn temperature to ultra-relativistic heavy-ion collisions of matter originates directly in the Higgs field; see sect. 7.1 at CERN; with a tribute to Rolf Hagedorn published at for further discussion. The question: What is mass? can be Springer Open, i.e. available for free on-line. This article studied by melting hadrons into quarks in RHI collisions. should be seen as a companion addressing more recent de- 2) Quarks are kept inside hadrons by the “vacuum” velopments, and setting a contemporary context for this properties which abhor the color charge of quarks. This book. explanation of 1) means that there must be at least two How did we get here? There were two critical mile- different forms of the modern æther that we call “vac- stones: uum”: the world around us, and the holes in it that are I) The first milestone occurred in 1964–1965, when called hadrons. The question: Can we form arbitrarily big Hagedorn, working to resolve discrepancies of the statis- holes filled with almost free quarks and gluons? was and tical particle production model with the pp reaction data, remains the existential issue for laboratory study of hot produced his “distinguishable particles” insight. Due to matter made of quarks and gluons, the QGP. Aficionados a twist of history, the initial research work was archived of the lattice-QCD should take note that the presentation without publication and has only become available to a of two phases of matter in numerical simulations does not wider public recently; that is, 50 years later, see chapt. 19 answer this question as the lattice method studies the en- in [1] and ref. [10]. Hagedorn went on to interpret the ob- tire Universe, showing hadron properties at low tempera- servation he made. Within a few months, in Fall 1964, he ture, and QGP properties at high temperature. created the Statistical Bootstrap Model (SBM) [11], show- 3) We all agree that QGP was the primordial Big- ing how the large diversity of strongly interacting particles Bang stuff that filled the Universe before “normal” mat- could arise; Steven Frautschi [12] coined in 1971 the name ter formed. Thus any laboratory exploration of the QGP “Statistical Bootstrap Model”. properties solidifies our models of the Big Bang and allows II) The second milestone occurred in the late 70s and us to ask these questions: What are the properties of the early 80s when we spearheaded the development of an ex- primordial matter content of the Universe? and How does perimental program to study “melted” hadrons and the “normal” matter formation in early Universe work? “boiling” quark-gluon plasma phase of matter. The in- 4) What is flavor? In elementary particle collisions, we tense theoretical and experimental work on the thermal deal with a few, and in most cases only one, pair of newly properties of strongly interacting matter, and the confir- created 2nd, or 3rd flavor family of particles at a time.