~N~AT_U~R~E~v_o_L_._33_7_2_FE_B_R_u_A_R_Y_I9_H_9 ______NEVVSAN0VI8NS------~~~5 Nuclear physics quark nuggets has been used by Brugger et al. \ who have adapted Rutherford scat­ tering, bombarding material samples with Engineering with quark matter beams of '"U and zospb nuclei. Any back­ scattering of the beams by angles greater Charles Alcock than 90° indicates the presence of scatter­ ing centres (the nuggets) more massive THE possibility that small drops of 'quark these collapsed stars, but some argue that than the projectile nuclei. matter' might be stable was first put strange matter is. For his proposal, Brugger et al. applied this elegant tech­ forward by Witten'. More detailed cal­ Madsen adduces the conclusion of Alpar' nique to various natural samples (includ­ culations by Farhi and Jaffe' showed that that the radio pulsars which exhibit glitches ing a meteorite) and derive upper limits to the hypothesis was certainly plausible; (abrupt, very small period reductions the abundance of quark nuggets: 1 per 1010 also these authors named stable quark followed by a few months of relaxation) for nuggets with A=l03 down to 1 per 1014 matter 'strange matter' because of the must be made of neutron matter, not at A=10'. The technique is less readily important role played by strange quarks, a stange matter. This conclusion rests on the applied for higher atomic number because name which has stuck. The strange-matter success of neutron-star models for the the electrostatic potential of the nuggets is hypothesis has several interesting con­ glitch phenomenon and the absence of a not a simple Coulomb form. sequences, principally in astrophysics (for model for glitches in the context of strange Shaw et al. 6 propose a much more direct a review see ref. 3). Two new papers, that matter. Because strange matter absorbs approach to the study of strange matter. of Brugger et al. on page 434 of this issue4 neutrons, one seed of strange matter will They propose that high-energy heavy-ion and one by Madsen', address the question convert an active neutron star into a collisions of the kind studied in present of how much strange matter there is in our 'strange' star. Madsen's analysis rests on CERN and Brookhaven National Labora­ part of the Universe. And Shaw et al. on the rate at which these seeds would be tory experiments might make small quark page 436 of this issue' suggest a novel accreted from the interstellar medium. nuggets. They further describe a scheme scheme for producing small lumps of Should they exist in the interstellar for isolating, slowing down and storing strange matter, collecting and then growing medium, quark nuggets will rain steadily nuggets which are produced in the experi­ the lumps; this is a proposal for engineering onto stars, where friction slows them ment. Any nugget which is captured may with quark matter. down. Thus a nugget can become bound then be "grown" by the addition of low Quark matter is a hypothetical phase in to a star. If the star is a neutron star, the energy neutrons. This growth phase which quarks are not confined in individ­ nugget will act as a seed which will grow releases of order 20 megaelectron volts of ual hadrons such as the proton (which until the entire star is converted to strange energy per captured neutron - potentially consists of two 'up' quarks and one matter. Furthermore, if the star is the a useful source of power, as this is nearly 'down') and the neutron (one up and two progenitor of a neutron star (a high-mass 10 times the energy yield per neutron in a down quarks), but are free to move about normal star which eventually explodes as a conventional fission reactor. In this regard within the phase. Strange matter is a supernova, compressing its core to a Shaw et al. are proposing a first attempt at quark-matter phase which contains three neutron star), then any nuggets it captures engineering with quark matter. flavours of quarks - up, down and will be present later when the neutron star There are uncertainities in this propo­ strange-and a few electrons. By hypoth­ is made. Madsen shows that this latter sal. First, it is not clear that the CERN and esis, strange matter is absolutely stable, process is the most interesting and places Brookhaven experiments do indeed make which means that its mass (energy) per limits on the number of quark nuggets in a quark phase. Second, this phase will quark is lower than that of iron (which is the interstellar medium. This is a model­ contain plenty of up and down quarks but the most stable ordinary nuclear matter) dependent limit, of course, but it does strange quarks are produced only as ss and cannot decay without violating energy show how to approach issues of funda­ pairs, and the strange quarks must sepa­ conservation (see figure). This hypothesis mental physics using astrophysics. rate from their anti-strange counterparts. is consistent with phenomenological A more direct method for seeking Third, strange matter is a low-temperature models of the strong interaction, but good phase and these experiments produce a first-principle computations of the mass .t:"'"' Ouark-gluon I very hot plasma (see figure). And fourth, ;! plasma Critical point . per baryon are not available. the isolation scheme proposed looks for Because strange matter is a bulk phase, > negatively charged nuggets, whereas .,,<:-"'.;:; Dilute hadron gas .1, it might be found in lumps of atomic ~ t strange matter is almost certainly positively 57 mass A in the range 10'-10 , but with :0>'" charged in its most stable form. atomic number (charge) Z much less than 'c ~ Despite these reservations, the propo­ -1 :.=::::'"'" A/2. These positively charged lumps are Dilute gas of quark nuggets '"'"' sal does not seem to be an expensive -2 :~ E often known as quark nuggets. They :m'=> addition to experiments that are already in behave chemically as ordinary atoms with -8 -6 -4 -2 0 2 4 progress. As the payoff in physics would the same Z and are, roughly speaking, log ~IMeVI be so large if strange matter were detec­ monstrous isotopes. A quark nugget may As shown in this hypothetical phase diagram, ted, the ideas should be further explored grow by absorbing neutrons; charged par­ strange matter is a high-density, low-temperature And the new engineering ideas raise ticles such as protons are not readily phase (density increases with chemical potential J.1). interesting long-term prospects for energy absorbed because of Coulomb repulsion. At high temperature, strange matter evaporates into generation. D ordinary hadrons (dotted line), much as a solid For theorists, not being able to recognize I. Witten. E. Phys. Rev. D30. 272-285 (1984). sublimates into vapour on heating; this transition is 2. Farhi. E. & Jaffe. R.L Phys. Rev. D30, 2379-2390 (1984). the true ground state of the strong inter­ not a phase transition. At higher temperature, the 3. Alcock, C. & Olinto, A. Rev. nuc/. part. Sci. 38. 161-184 actions (is it iron or strange matter?) is hadron gas undergoes a phase transition (solid line) (1988). to a hot gas of quarks and 'gluons'. Cosmic or 4. Brugger. M. eta/. Nature 337.434-436 (1989). very disturbing. It makes sense, therefore, 5. Madsen. L Phys. Rev. Lett. 61.2909-2912 (1988). to search for quark nuggets or use experi­ experimental production of strange matter occurs 6. Shaw. G.L. Shin, M .. Dalitz. R.M. & Desai, M. Nature ments to settle the question. Madsen' by producing the hot quark-gluon plasma with the 337. 436-439 ( 1989). hope that rapid cooling will trap portions of the 7. Alpar. M.A. Phys. Rev. Lett. 58. 2152·-2155 (1987). proposes that observations of pulsars matter in the cold, condensed phase. A detailed should help. Conventionally it is suggested Charles Alcock is at the Institute of Geophysics mechanism for accomplishing this has not been and Planetary Physics, Lawrence Livermore that neutrons are the stable form of matter proposed. [Temperature Tin megaelectron volts; 1 11 National Laboratory, PO Box 808, Livermore, in the enormous gravitational field of MeV= 10 K.) (From ref. 3.) California 94550, USA.