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Particle theorists and are investigating the bizarre properties of the densest known state of – matter – which may exist only in objects about to collapse into holes Students Jack Cheyne and Greg Gowan

SuperconductingJack Cheyne, Greig Cowan and Mark Alford

hat happens as you squeeze matter? We all know that, under pressure, substances Wrearrange themselves into denser forms, or phases. Gases turn into liquids, and liquids typically turn into solids. But that is nothing compared with the changes that happen when the crushing force of , in the aftermath of a , squeezes a massive into a ball about 20 , cores are almost as discrete set of quantum with half- kilometres across. mysterious to us as the ocean floor was to integer spin, and since the 1950s we have The of such a compact star is primitive Man. But armed with the known that such particles in a metal can so high that themselves cannot Standard Model, we can make some become superconducting by forming pairs exist. Atomic nuclei swallow their - reasonable guesses about what might be in which two spins cancel. In the past few wasting clouds and are pressed found there. In the Standard Model, the years a growing number of nuclear and together into a liquid of and ultimate constituents of matter are quarks theorists have become convinced . But protons and neutrons still and , so at high enough pressure that something similar will happen in have a modicum of internal structure: in we expect a liquid of quarks and electrons quark matter. Since the each of them, three quarks are bound to form, which is the quark matter. between quarks is associated with their so- together by the strong force as described No one knows whether the core of a called colour charge (, green or ), by the Standard Model of Particle . compact star achieves such pressures, but and described by a theory called quantum In the core of the compact star, at the it certainly comes close. And in spite of the chromodynamics (QCD), this special highest that are known to exist in difficulties, astronomers are gradually quark state is known as a colour , many physicists speculate that the obtaining more and more information superconductor. relentless piston of gravity will finally crush about compact : their , Unlike an electrical superconductor, these last vestiges of structure, leaving the , size, cooling rate, and so on. So however, colour-superconducting quark densest form of matter that our theories theorists are beginning to ask themselves: matter comes in many varieties, each of can describe – quark matter. how would quark matter be expected to which is a separate phase of matter. This is behave? And building on that, could we because quarks, unlike electrons, come in Inside a star tell from Earth-based observations of many species. They can have one of three The core of a compact, or is distant compact stars whether they have different colours, and in the core of a one of the hardest places in the quark matter in their cores? compact star we expect three different 16 to study experimentally. Sheathed in For theorists, one of the surprising flavours of quark (up, down and strange) nuclear matter that is still glowing from results of their calculations is that quark to exist. This means that in pairing up the heat of the supernova, and scattered matter has its own rich set of distinct there are many choices: will the red down out in space thousands of light years from forms. Quarks, like electrons, belong to a quarks pair with the green up quarks, or >> website: www.physics.wustl.edu/~alford/high_density.html

250 EARLY UNIVERSE r-modes. But we see plenty of fast-spinning neutron stars, such as millisecond , indicating that the matter inside these 170 Quark-gluon neutron stars is too viscous to allow r-mode flows. So we may soon be able to 100 rule out some of these superfluid phases. To do this definitively will require better r-mode calculations, such as those being

Hadron gas performed by Peter Jones at Oxford and Colour superconductor Nils Andersson at Southampton University. TEMPERATURE (MeV) TEMPERATURE Neutron stars All this work on high-density quark matter fits into a more general quest to Atomic nuclei 10 normal nuclear density build the ‘phase diagram’ for QCD – a DENSITY road map of the behaviour of matter under extreme conditions of ultra-high A QCD phase diagram density and/or ultra-high temperature. At and hard to observe, theorists and sufficiently high temperatures, nuclei astronomers are now optimistic that such ‘ionise’ into a plasma of quarks – the signatures exist. One example is simply the quark-gluon plasma (QGP). This happens rate at which a compact star cools. The at about a million million degrees (170 compact star starts out, freshly created by MeV in particle-physics units). a supernova, 10,000 times hotter than the Surprisingly, it is easier to produce core of our . Its surface glows with such temperatures, albeit briefly, in X-rays, which can be observed by the laboratory, than it is to achieve The Crab hosts a telescopes such as NASA’s Chandra. the densities required to make neutron star – its core of the hardest objects to study colour superconducting quark matter. Signatures of quark matter Accelerators such as RHIC at Brookhaven Over thousands of years a compact star National Laboratory and the Large the blue strange quarks? gradually cools, and, by measuring the Hadron Collider being constructed at It is very hard to answer this question spectra and ages of such stars in our CERN can collide heavy nuclei to make definitively, because lattice QCD – the , observers can construct a tiny flashes of QGP whose products can usual brute-force computational approach temperature-versus- cooling curve. be studied to learn more about that state. – is unusable at high quark density. The crucial point is that the star cools not However, it is hard to make a cool lump of Physicists are currently trying to get a via the from its surface, but high-density matter this way, so rough idea of the special properties of mainly by emitting from its observations of compact stars still have a each phase, and the signatures by which interior. The exact rate of the cooling vital role to play. Together, particle experimental observations can tell us therefore depends on the heat capacity physicists and astronomers hope to collect which is favoured. This is a worldwide and emissivity of the star’s enough data to allow them to draw the effort involving researchers in Germany, interior, and it turns out that the existence phase boundaries in the QCD phase Italy, Japan, the US, and many other of a colour superconducting phase in the diagram, and see whether they match countries. The UK is involved too. At the core can greatly influence these theorists’ . F University of Wales Swansea, Simon properties. Colour superconducting quark Hands is finding ways to apply lattice matter cools very slowly, in a way that is Jack Cheyne and Greig Cowan are QCD methods to quark matter. And at consistent with observed cooling curves. postgraduate students at Glasgow University Glasgow University and Washington A more exotic signature is connected and Dr M. Alford is assistant professor at University, St Louis in the US, with Jürgen with the dynamics of the rotating compact Washington University, Saint Louis, US. Berges of the ITP at Heidelberg University, star. If its interior has low enough viscosity, Emails: [email protected], s t we are exploring new methods for dense- then unstable flows (‘r-modes’) can occur, [email protected] and d u o n matter calculations. We investigate the and these would rapidly carry off the [email protected] i r i d lowest-density forms of colour angular , causing its spin to e f l u i n r l e superconductivity that have the best slow down very quickly. Most forms of l p a u t s chance of existing in Nature, and we try to colour superconducting quark matter are Inside a s n y o i n g envisage how these phases might manifest actually superfluids (superfluidity is when neutron star r r c t t C u themselves in observable signatures. a fluid loses all viscosity). They would, u d n n s e o Although compact stars are remote therefore, permit rapid spindown via o t N c o r r e p

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