High (energy physics exploration) by East-west (collaboration on heavy ion collision experiments)

There is more than potential new elementary particles to understand fundamental interactions

This short note describes the long collaborative effort between Arizona and Krak´ow, showing some of the key strangeness signatures of quarkgluon plasma. It further presents an annotated catalog of foundational questions defining the research frontiers which I believe can be addressed in the foreseeable future in the context of relativistic heavy ion collision experiments. The list includes topics that are specific to the field, and ventures towards the known-to-be-unknown that may have a better chance with ions as compared to elementary interactions. 

Some 70 years ago the development of relativistic particle accelerators heralded a new era of laboratory-based systematic exploration and study of elementary particle interactions...  

The outcomes of this long quest are on one hand the standard model (SM) of particle physics, and on another, the discovery of the primordial deconfined quark-gluon plasma (QGP). These two foundational insights arose in the context of our understanding of the models of particle production and more specifically, the in-depth understanding of strong interaction processes. To this point we recall that in the context of SM discovery we track decay products of e.g. the Higgs particle in the dense cloud of newly formed strongly interacting particles. In the context of QGP we need to understand the gas cloud of hadrons into which QGP decays and hadronizes. Hadrons are always all we see at the end. They are the messengers and we must learn to decipher the message.

Charting the future frontier(s) of particle production

Jan Rafelski   (Submitted on 25 Aug 2016)

Exotic states of nuclear matter matter too

The year 1964/65 saw the rise of several new ideas which in the following 50 years shaped the discoveries in fundamental subatomic physics: 1. The Hagedorn temperature TH ; later recognized as the melting point of hadrons into 2. Quarks as building blocks of hadrons; and, 3. The Higgs particle and field escape from the Goldstone theorem, allowing the understanding of weak interactions, the source of inertial mass of the elementary particles. The topic in this paper is Hagedorn temperature 

T_H

 and the strong interaction phenomena near to TH . I present an overview of 50 years of effort with emphasis on: a) Hot nuclear and hadronic matter; b) Critical behavior near 

T_H

 ; c) Quark-gluon plasma (QGP); d) Relativistic heavy ion (RHI) collisions1 ; e) The hadronization process of QGP; f) Abundant production of strangeness flavor... 

A report on ‘Melting Hadrons, Boiling Quarks and T_H’ relates strongly to quantum chromodynamics (QCD), the theory of quarks and gluons, the building blocks of hadrons, and its lattice numerical solutions; QCD is the quantum (Q) theory of color-charged (C) quark and gluon dynamics (D); for numerical study the space-time continuum is discretized on a ‘lattice’. Telling the story of how we learned that strong interactions are a gauge theory involving two types of particles, quarks and gluons, and the working of the lattice numerical method would entirely change the contents of this article, and be beyond the expertise of the author. I recommend instead the book by Weinberg [8], which also shows the historical path to QCD... 

Our conviction that we achieved in laboratory experiments the conditions required for melting (we can also say, dissolution) of hadrons into a soup of boiling quarks and gluons became firmer in the past 15-20 years. Now we 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 1000. The mass of the three ‘elementary’ quarks found in nucleons is about 50 times smaller than the nucleon mass. Whatever compresses and keeps the quarks within the nucleon volume is thus the source of nearly all of mass of matter. This clarifies that the Higgs field provides the mass scale to all particles that we view today as elementary. Therefore only a small %-sized fraction of the mass of matter originates directly in the Higgs field; see Section 7.1 for further discussion. The question: What is mass? can be studied by melting hadrons into quarks in RHI collisions.  

2) Quarks are kept inside hadrons by the ‘vacuum’ properties which abhor the color charge of quarks. This explanation of 1) means that there must be at least two different forms of the modern æther that we call ‘vacuum’: the world around us, and the holes in it that are called hadrons. The question: Can we form arbitrarily big holes filled with almost free quarks and gluons? was and remains the existential issue for laboratory study of hot matter made of quarks and gluons, the QGP. Aficionados of the lattice-QCD should take note that the presentation of two phases of matter in numerical simulations does not answer this question as the lattice method studies the entire Universe, showing hadron properties at low temperature, and QGP properties at high temperature.  

3) We all agree that QGP was the primordial Big-Bang stuff that filled the Universe before ‘normal’ matter formed. Thus any laboratory exploration of the QGP properties solidifies our models of the Big Bang and allows us to ask these questions: What are the properties of the primordial matter content of the Universe? and How does ‘normal’ matter formation in early Universe work?  

4) What is flavor? In elementary particle collisions, we deal with a few, and in most cases only one, pair of newly created 2nd, or 3rd flavor family of particles at a time. A new situation arises in the QGP formed in relativistic heavy ion collisions. QGP includes a large number of particles from the second family: the strange quarks and also, the yet heavier charmed quarks; and from the third family at the LHC we expect an appreciable abundance of bottom quarks. The novel ability to study a large number of these 2nd and 3rd generation particles offers a new opportunity to approach in an experiment the riddle of flavor.  

5) In relativistic heavy ion collisions the kinetic energy of ions feeds the growth of quark population. These quarks ultimately turn into final state material particles. This means that we study experimentally the mechanisms leading to the conversion of the colliding ion kinetic energy 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? The last two points show the potential of ‘applications’ of QGP physics to change both our understanding of, and our place in the world. For the present we keep these questions in mind. This review will address all the other challenges listed under points 1), 2), and 3) above; however, see also thoughts along comparable foundational lines presented in Subsections 7.3 and 7.4... 

Melting Hadrons, Boiling Quarks

Johann Rafelski

(Submitted on 13 Aug 2015 (v1), last revised 16 Sep 2015 (this version, v2))

 Snapshot of two colliding lead ions just after impact (simulation).

At a special seminar on 10 February 2000, spokespersons from the experiments on CERN's Heavy Ion programme presented compelling evidence for the existence of a new state of matter in which quarks, instead of being bound up into more complex particles such as protons and neutrons, are liberated to roam freely.

Theory predicts that this state must have existed at about 10 microseconds after the Big Bang, before the formation of matter, as we know it today, but until now it had not been confirmed experimentally. Our understanding of how the universe was created, which was previously unverified theory for any point in time before the formation of ordinary atomic nuclei, about three minutes after the Big Bang, has with these results now been experimentally tested back to a point only a few microseconds after the Big Bang. (CERN Bulletin 07/00; 14 February 2000)