Most Dangerous Substance:Strange Matter

                             

                       Strange Matter:The Mystery

 INTRODUCTION

 
It is generally believed that if a nucleus or collection of nuclei is subjected to a high enough pressure, a transition to quark matter occurs. Above a critical pressure, Pn individual nucleon bound-aries dissolve and a system with baryon number A is best described as 3A quarks and not as A nucleons. The quarks are still confined to the system as a whole but they are no longer parts of three quark units. In this quark matter phase, quarks have very different wave functions than they do in nuclei, and hadrons, as we know them, do not exist.

  

 
Quark matter will always contain a significant fraction of strange quarks. (The higher mass quarks, charm, etc., will not enter the discussion because their masses are much larger than any chemical potentials we ever consider.) Suppose we keep a giant nucleus or collection of nuclei, with total baryon number A, at a pressure above Pc long enough for the weak interactions to establish the favored numbers of up, down and strange quarks. The conversion to strange quarks lowers the Gibbs potential by lowering the energies of individual quarks. The excess energy can be carried away by photon or neutrino emission without changing the baryon number. Now imagine slowly reduc-ing the pressure to zero. If, as the pressure is lowered, the Gibbs potential of the quark matter with up, down and strange quarks rises above that of compressed nuclei, then weak interactions will eliminate the strange quarks and at zero pressure the system will be ordinary nuclei with a total baryon number A. However, another outcome is possible. It is conceivable that the Gibbs potential of quark matter containing a large fraction of strange quarks is below that of compressed nuclei for all pressures. In this case, the system will not return to nuclei but will stay quark matter even after the external pressure has gone to zero. This possibility was first entertained by E. Witten who pointed out that we may have misidentified the true, zero pressure, ground state of the strong interactions which may be "strange matter" and not iron.2 Immediately an objection can be raised. If strange matter has a lower energy per baryon than ordinary nuclei, then why do ordi-nary nuclei exist? Why do they not convert to strange matter? The answer is that they are metastable and the rate to convert is es-sentially zero. For a nucleus to convert, it must change roughly one-third of its quarks to strange. If it tries to do this one quark at a time, it will convert protons and neutrons to lambdas which is energetically unfavorable. Strange matter is postulated to be stable in bulk but, as we will see, it is not stable for very low baryon number. (There are no known stable strange baryons.) For a large nucleus to become strange matter it needs to convert many quarks simultaneously to strange. This requires a high-order weak inter-action and gives a negligible rate. Only under high pressure, where nucleon boundaries are dissolved, can a system convert one quark
at a time. So, for example. the interiors of neutron stars may be stable strange matter. At present, detailed calculations are incapable of deciding if strange matter is stable or not. No QCD based calculation schemes predict the energy per baryon number of strange matter to even 100 MeV accuracy. I will assume that strange matter is stable and explore the consequences of that assumption, relying as little as possible on detailed calculations. In the next section I will discuss the general properties of strange matter. We will see that since strange matter has a low electric charge to baryon ratio, it is stable against fissioning and can, in principle, be found in lumps ranging from nuclear to stellar dimensions. I will explore the general properties of very large lumps which can be treated as bulk systems and very small lumps which must be viewed as consisting of distinct quarks. We will see that strange matter can be stable above a certain critical baryon number, of order 100, but unbound for lower baryon number. Strange matter, produced in the early universe, was originally proposed as a dark matter candidate. Its virtue as a dark matter candidate is that, because of its extreme density, relatively few lumps are required to close the universe and these stable lumps would be very difficult to detect astronomically. However, in Sec-tion 3 I will show that any lump produced early in the history of the universe would have evaporated by the time the universe was one second old. It is still possible that strange matter exists in what are usually called neutron stars. In Section 4 I will discuss the properties of strange stars and contrast them with conventional neutron stars. The main differences lie in the mass radius relation for small mass and in the properties of the solid crust of the star which a strange star may not even possess. Finally, in Section 5 I will survey some techniques for searching for strange matter here on Earth today.

What happens when strange matter comes in contact with or-dinary matter?

A neutron which enters strange matter will lower its energy by falling apart and releasing its quarks to the quark phase. If many neutrons enter, eventually the relative numbers of up and down quarks will become higher than the most favored equilibrium configuration. However, weak interactions can change up and down quarks to strange and reestablish the favored distri-butions. A lump of strange matter has an insatiable appetite for neutrons and grows fat by eating them. Strange matter carries a small positive hadronic charge and is neutralized by electrons. Even for very large lumps, the electrons extend beyond the edge of the strange matter because the electrons are only electromagnetically bound while the hadronic surface of strange matter falls off in a few Fermis. A proton or positive ion approaching a lump first passes through a film of electrons. In this region there is an outward electric force repelling the projectile. For a positively charged particle to enter strange matter it must overcome a Coulomb barrier which may be tens of MeV. A lump of strange matter moving at galactic velocities would not have strong interactions with material which it bumps into. A small lump of strange matter could be found resting in some material on the earth without interacting with the material.

SEARCHING FOR STRANGE MATTER

I have argued that any strange matter produced very early in the history of the universe would have evaporated before the universe was one second old but that strange matter may exist in stars today. Collision of such stars or other astrophysical events may have produced small lumps of strange matter which permeate the galaxy. Regardless of any astrophysical or cosmological considerations, if strange matter is more stable than iron, it is worthwhile considering ways to search for strange matter. Perhaps some lumps are to be found on Earth today. Any lump found on Earth could not have a baryon number much above 1016 corresponding to a radius of a few hundred Fermis. Larger lumps are too heavy; they could not be supported by ordinary materials and would sink to the center of the earth. One proposaF is to use heavy ion activation to search for small impurities of strange matter in laboratory samples of ordinary mat-ter. Lumps of strange matter carry a positive hadronic charge and are neutralized by electrons which extend beyond the hadronic material. Like an ordinary nucleus, a lump of strange matter pre-sents a Coulomb barrier to an incoming positive ion. However, strange matter has a much lower charge to baryon ratio than nuclei and, for typical values of the parameters in -model calculations, it presents a lower Coulomb barrier than nuclei. An ion whose energy is just below the Coulomb barrier for ordinary nuclei may have enough energy to enter a lump of strange matter. An ion which does enter a lump will fall apart and release its excess binding energy to the lump. If the lump is large it will not fission and it will radiate away the excess binding energy as well as the kinetic energy of the projectile. To find an impurity of strange matter in a sample, you could use the sample as a target of a heavy ion beam whose energy is just below the Coulomb barrier for ordinary nuclear reactions. If a beam nucleus enters a lump, a striking signal would appear in an otherwise quiet system. For example, if the binding energy of strange matter relative to gold is 20 Me V, then a single gold nu-cleus, just below the Coulomb barrier, would release 5 GeV after entering a lump of strange matter. This energy would show up as an isotropic photon burst which would be hard to miss. A discus-sion of sensitivities and of a preliminary experiment carried out at the Super HILAC can be found in the survey of Inspire. Small lumps of strange matter whizzing around in our galaxy may occasionally pass through the Earth. DeRujula and Glashow8 have analyzed a variety of detection methods sensitive to different mass ranges and fluxes. For example, underground proton decay detectors are' sensitive to the light produced by strange lumps pass-ing directly through them. They estimate that a lump whose radius is larger than 10-10 cm could produce a detectable signal in the IMB detector. Lumps larger than 10-2 cm passing through Earth would deposit so much energy that they may be detected as "epi-linear earthquakes." They analyze this and other possible means of detection of strange lumps passing through Earth.

Source: Vigyan TV India,Inspire

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