Strangeness production

Strangeness production is a signature and a diagnostic tool of quark–gluon plasma (or QGP) formation and properties. Unlike up and down quarks, from which everyday matter is made, strange quarks are formed in pair-production processes in collisions between constituents of the plasma. The dominant mechanism of production involves gluons only present when matter has become a quark–gluon plasma. When quark–gluon plasma disassembles into hadrons in a breakup process, the high availability of strange antiquarks helps to produce antimatter containing multiple strange quarks, which is otherwise rarely made. Similar considerations are at present made for the heavier charm flavor, which is made at the beginning of the collision process in the first interactions and is only abundant in the high-energy environments of CERN's Large Hadron Collider. Strangeness production is a signature and a diagnostic tool of quark–gluon plasma (or QGP) formation and properties. Unlike up and down quarks, from which everyday matter is made, strange quarks are formed in pair-production processes in collisions between constituents of the plasma. The dominant mechanism of production involves gluons only present when matter has become a quark–gluon plasma. When quark–gluon plasma disassembles into hadrons in a breakup process, the high availability of strange antiquarks helps to produce antimatter containing multiple strange quarks, which is otherwise rarely made. Similar considerations are at present made for the heavier charm flavor, which is made at the beginning of the collision process in the first interactions and is only abundant in the high-energy environments of CERN's Large Hadron Collider. The majority of ordinary matter in the universe is found in atomic nuclei, which are made of neutrons and protons. These neutrons and protons are made up of smaller particles called quarks. For every type of matter particle there is a corresponding antiparticle with the same mass and the opposite charge. It is hypothesized that during the first few instants of the universe, it was composed of almost equal amounts of matter and antimatter, and thus contained nearly equal number of quarks and antiquarks. Once the universe expanded and cooled to a critical temperature of approximately 2×1012 K, quarks combined into normal matter and antimatter. Antimatter annihilated with matter up to the small initial asymmetry of about one part in five billion, leaving the matter around us. Free and separate individual quarks and antiquarks have never been observed in experiments—quarks and antiquarks are always found in groups of three (baryons), or bound in quark–antiquark pairs (mesons). Free quarks probably existed in the extreme conditions of the very early universe until about 30 microseconds after the Big Bang, in a very hot gas of free quarks, antiquarks and gluons. This gas is called a quark–gluon plasma (QGP), since the quark-interaction charge (color charge) is mobile and quarks and gluons move around. This is possible because at a high temperature the early universe is in a different vacuum state, in which normal matter cannot exist but quarks and gluons can, they are deconfined (able to exist independently as separate unbound particles). In order to recreate this deconfined phase of matter in the laboratory it is necessary to exceed a minimum temperature, or its equivalent, a minimum energy density. Scientists achieve this using particle collisions at extremely high speeds, where the energy released in the collision can raise the subatomic particles' energies to an exceedingly high level, sufficient for them to briefly form a tiny amount of quark-gluon plasma which can be studied for a brief fraction of a second before it cools again. In this way, it is possible to study conditions akin to those in the early Universe at the age of 10–40 microseconds. Discovery of this new QGP state of matter has been announced both at CERN and at Brookhaven National Laboratory (BNL). At this time comprehensive experimental evidence about its properties is being assembled. The process of the formation of quark–gluon plasma lasts little longer than the time the light takes to pass through the volume occupied by the atomic nucleus used to produce the ultra-high pressure and temperature in the highly-energetic collision. After this brief time the hot drop of quark plasma evaporates in a process called hadronization. The extremely limited duration of the plasma makes it challenging (difficult) to study free quarks in quark–gluon plasma. The diagnosis and the study of the properties of quark–gluon plasma can be undertaken using quarks not present in matter seen around us. The experimental and theoretical work relies on the idea of strangeness enhancement. This was the first observable of quark–gluon plasma proposed in 1980 by Johann Rafelski and Rolf Hagedorn. Unlike the up and down quarks, strange quarks are not brought into the reaction by the colliding nuclei. Therefore, any strange quarks or antiquarks observed in experiments have been 'freshly' made from the kinetic energy of colliding nuclei. Conveniently, the mass of strange quarks and antiquarks is equivalent to the temperature or energy at which protons, neutrons and other hadrons dissolve into quarks. This means that the abundance of strange quarks is sensitive to the conditions, structure and dynamics of the deconfined matter phase, and if their number is large it can be assumed that deconfinement conditions were reached. One cannot assume that under all conditions the yield of strange quarks is in thermal equilibrium. In general, the quark-flavor composition of the plasma varies during its ultra short lifetime as new flavors of quarks such as strangeness are cooked up inside. The up and down quarks from which normal matter is made are easily produced as quark-antiquark pairs in the hot fireball because they have small masses. On the other hand the next lightest quark flavor, strange quarks, will reach its high quark–gluon plasma thermal abundance only on the most violent collisions generating high temperatures and that at the end of the cooking process. This is only possible due to a new process, the gluon fusion, as shown by Rafelski and Müller in 1981. The top section of the figure shows gluon fusion in form of Feynman diagrams: gluons are the wavy lines; strange quarks are the solid lines; time runs from left to right. The bottom section is the process where the heavier quark pair arises from the lighter pair of quarks shown as dashed lines. The gluon fusion process occurs almost ten times faster than the quark based strangeness process, and allows achievement of the high thermal yield where the quark based process would fail to do so during the duration of the 'micro-bang'. The gluon collisions here are occurring within the thermal matter phase and are thus different from the high energy processes that can ensue in the early stages of the collisions when the nuclei crash into each other. The heavier, charm and bottom quarks are produced there dominantly. The study in relativistic nuclear (heavy ion) collisions of charmed and soon also bottom hadronic particle production beside strangeness will provide complementary and important confirmation of the mechanisms of formation, evolution and hadronization of quark gluon plasma. These newly cooked strange quarks find their way into a multitude of different final particles that emerge as the hot quark–gluon plasma fireball breaks up, see the scheme of different processes in figure. Given the ready supply of antiquarks in the 'fireball', one also finds a multitude of antimatter particles containing more than one strange quark. On the other hand, in a system involving a cascade of nucleon-nucleon collisions, multi-strange antimatter are produced less frequently considering that several relatively improbable events must occur in the same collision process. For this reason one expects that the yield of multi-strange antimatter particles produced in the presence of quark matter is enhanced compared to conventional series of reactions. Strange quarks also bind with the heavier charm and bottom quarks which also like to bind with each other. Thus in presence of a large number of these quarks quite unusually abundant exotic particles can be produced, some of these have never been observed before. This should be the case in the forthcoming exploration at the new Large Hadron Collider at CERN of the particles that have both charm and strange quarks, and even bottom quarks as components.