Eur. Phys. J. Special Topics 229, 1{140 (2020) c The Author(s) 2020 THE EUROPEAN https://doi.org/10.1140/epjst/e2019-900263-x PHYSICAL JOURNAL SPECIAL TOPICS Review Discovery of Quark-Gluon Plasma: Strangeness Diaries Johann Rafelski1,2 ;a 1 CERN-TH, 1211 Geneva 23, Switzerland 2 Department of Physics, The University of Arizona, Tucson, AZ, 85721, USA Received 4 November 2019 Published online 24 January 2020 Abstract. We look from a theoretical perspective at the new phase of matter, quark-gluon plasma (QGP), the new form of nuclear matter created at high temperature and pressure. Here I retrace the path to QGP discovery and its exploration in terms of strangeness production and strange particle signatures. We will see the theoretical arguments that have been advanced to create interest in this determining signa- ture of QGP. We explore the procedure used by several experimental groups making strangeness production an important tool in the search and discovery of this primordial state of matter present in the Universe before matter in its present form was formed. We close by looking at both the ongoing research that increases the reach of this observable to LHC energy scale pp collisions, and propose an interpretation of these unexpected results. It is very appropriate that you did reconstruct your version of the QGP discovery. Your quotations concerning me are correct and reproduce well my opinion, which I have not changed. CERN found good evidence for deconfinement, and it was at all appropriate to say that in public, independently from the status of RHIC at the time. Luciano Maiani CERN Director General 1 January 1999{31 December 2003. a e-mail: [email protected] 2 The European Physical Journal Special Topics Dedications (alphabetic): 1. Rolf Hagedorn, who would have been 100 years old on 20th July 2019, had influence at CERN essential in the quest to unlock the scientific opportunities relativistic heavy ion collisions offer. I had the privilege to have been marked by Hagedorn's magic wand in a decisive way. 2. Jean Letessier, Jean's 80th birthday in December of 2018 provided the initial motivation to prepare a manuscript that includes a review of the highlights of our research achievements. I worked with Jean on the topic of this review for 20 years, from a first meeting in Summer 1992, arranged by Rolf Hagedorn, to the end of 2013. After many years of fruitful collaboration, Jean is the friend and colleague with whom I have published the largest number of research papers. 3. Berndt M¨uller, with whom I studied physics in Frankfurt, and with whom I published many influential works on strangeness and strong field physics. 4. Helga Rafelski (3 September 1949 { 5 November 2000); she would have turned 70 as I write this today. Preamble This review introduces the strangeness signature of QGP in the format of a personal diary: diary means that I use some (unpublished) material buried in my history chest box. This includes some unpublished manuscripts from arXiv with some added insight into the reasons why these works are unpublished. All told, about half of this review presentation relies on earlier written personal records; the other half is freshly written with added verifiable references or records. Here is an example how this works { in November 1993 I prepared a short write-up describing the ongoing and future work I hoped to carry out collaborating with Jean Letessier: The LPTHE laboratory at the University Paris submitted this to CNRS in order to secure funding to allow me to work in Paris in Spring 1994. This roadmap focused on the ongoing experimental program at the CERN-SPS, defining our ensuing and enduring collaboration: STRANGE ANTIBARYONS The high temperature phase of quantum-chromodynamics (QCD), the quark-gluon plasma (QGP) is characterized by { color deconfinement, and { partial restoration of chiral symmetry. This picture is supported by numerical simulations of lattice SU(3)-gauge theory and by high temperature QCD perturbation theory. In QGP the production of strange particles is expected to be efficient due to: (a) Lower energy threshold: The most efficient conventional strangeness producing reactions p + p ! p + Λ + K+; π + π ! K + K¯ require c.m. energy of 700 MeV, in the QGP the energy for strange quark pair production G + G ! s +s ¯; q +q ¯ ! ss¯ is 2ms ' 300 MeV. The reduction in threshold is very important in the presence of a thermalized phase space distribution, where at best temperatures T = 160−250 MeV can be reached today. Discovery of Quark-Gluon Plasma: Strangeness Diaries 3 (b) Increased strangeness density: In a HG the density of strange hadrons is '0:1 fm−3. By contrast, in the QGP the density of strange quarks is 3=2 −3 ns = ns¯ ≈ 6(T ms=2π) exp(−ms=T ) ≈ 0:3 fm : (c) Anti-strangeness can be more abundant than u;¯ d¯: Since strange valence quarks are practically completely absent in the nuclear mat- ter entering a relativistic heavy ion collision, the available phase space for strange antiquarks is not suppressed. Non-strange antiquarks, on the other hand, are sup- pressed due to the presence of a non-vanishing baryo-chemical potential µB in a baryon-rich QGP, as it forms at AGS or SPS energies. The predicted phase space ratio in QGP: ns¯ 1 2 ≈ (ms=T ) K2(ms=T ) exp(µB =3T ): nu¯ + nd¯ 2 Both these consequences (a), (b) of deconfinement and chiral symmetry restoration favor the production of multiply strange hadrons. The condition (c) further allows to expect that this enhancement is increasing with increasings ¯ content of hadrons. This observation is certainly contrary to the normal hadronic reactions in which (e.g. in p{p collisions) the yield of strange antibaryons is falling rapidly withs ¯ content. The corresponding experimentally measured cross sections are inputs in hadronic cascade models. Consequently, the abundance of strange antibaryons from dynamical hadronic models is expected to be opposite to the expectations from QGP, and the yields com- puted are considerably smaller than obtainable in QGP models. Contents Dedications...................................... 2 Preamble ....................................... 2 Contents ....................................... 3 1 A new phase of matter .................................. 5 1.1 Why we are interested in quark-gluon plasma .................. 5 1.1.1 A new and interdisciplinary field of physics ............... 5 1.1.2 Quantum vacuum structure and quark confinement ........... 6 1.1.3 Hagedorn (temperature physics) Frontier ................ 9 1.1.4 Superdense nuclei or QCD matter? .................... 10 1.1.5 Strangeness: a natural tool to study QGP . 13 1.2 Establishing (ultra)relativistic heavy ion collisions beams . 17 1.2.1 Heavy ions at CERN ............................ 18 1.2.2 (Ultra)relativistic heavy ion collisions in USA . 23 1.3 Was quark-gluon plasma really discovered? ................... 25 1.3.1 Strangeness is getting ready ........................ 25 1.3.2 Strangeness in the race for QGP ..................... 26 1.3.3 Particles from a hot fireball ........................ 29 1.3.4 The CERN February 2000 announcement . 33 1.3.5 SQM2000 Meeting in Berkeley ...................... 37 1.3.6 BNL announces ideal flow ......................... 38 1.3.7 Can QGP be experimentally recognized? . 41 1.4 Non-strangeness signatures of QGP: J/Ψ psions/charmonium . 42 1.5 After the CERN quark-gluon plasma discovery . 45 1.5.1 Enhancement of multi-strange baryons at CERN-SPS . 45 1.5.2 NATO support for strangeness and SHARE . 45 1.5.3 How does SHM work? ........................... 48 1.5.4 Threshold and energy dependence of strangeness enhancement . 50 1.5.5 Addressing small volume effects ...................... 51 4 The European Physical Journal Special Topics 1.5.6 Strangeness enhancement at collider energies . 53 1.5.7 Systematics of ALICE-LHC strangeness results . 54 2 Strangeness in Quark-Gluon plasma .......................... 57 2.1 Strangeness production .............................. 58 Strangeness as an observable ........................... 58 Kinetics of strangeness production and evolution . 60 Gluons in plasma .................................. 62 Approach to absolute chemical equilibrium ................... 63 2.2 Strange antibaryon production .......................... 66 2.2.1 High p? recombinant enhancement .................... 66 Strangeness production in quark-gluon plasma . 67 Strange hadron formation from quark-gluon plasma . 67 2.2.2 Strange antibaryons; has anyone noticed? . 70 2.3 Soft and strange hadronic observable of QGP at RHIC . 72 Flavor flow from quark-gluon plasma ....................... 72 Why flavor{strangeness? .............................. 73 Strange signatures of quark-gluon plasma .................... 74 Paths to observe multistrange (anti) baryons . 77 Lessons from early experimental results ..................... 82 2.4 Strangeness production with running QCD parameters . 88 Production of strangeness ............................. 90 Fireball dynamics and initial conditions ..................... 94 2.5 A picture with STAR at RHIC .......................... 98 Hadronic probes of QGP .............................. 99 Strangeness signatures of deconfinement . 100 3 Soft hadron data analysis and interpretation . 102 3.1 Statistical hadronization model (SHM) . 102 Particle spectra ...................................103 Counting (strange) particles in hot matter . 105 Measuring chemical potentials . 107 Experimental particle ratios ............................108 Phase space saturation ...............................109