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eep underground proton-decay charge must be zero; otherwise the direc- a sufficiently long lifetime, of order a year detectors in the Soudan iron mine tionality and timing of the signal would be in its rest frame, that it not decay en route D in Minnesota and under the Mont destroyed by galactic magnetic fields. from the source. (Lorentz dilation in- Blanc have recorded very energetic muons Secondly, the mass of the primary must be creases the observed lifetime of a rapidly coming from the direction of Cygnus X-3 less than its energy by a factor of about 104; moving particle by the ratio of its energy with its 4.79-hour periodicity. These ob- otherwise differences in travel times of to its mass.) The known neutral particles servations, if confirmed, present a very primaries with different energies would with such properties are photons, neu- challenging puzzle. What is the primary wash out the periodicity of the primaries trinos, and neutrons, but arguments cosmic-ray particle that produces the and hence that of the muons. (A 1OO-GeV- presented in the main text appear to rule muons at the earth, and how is such a mass particle, for example, would arrive these out. Briefly, the reported flux of particle produced in Cygnus X-3? One of about 1 hour sooner if it had an energy of muons is too high to be attributed to the more coherent explanations is that the 12 TeV than if it had an energy of 10 TeV gamma rays (high-energy photons), the primaries originate as exotic hadrons (1 GeV = 109 eV and 1 TeV = 1012 eV).) observed dependence of the muon flux on (strongly interacting particles, not yet To produce muons with sufficient energy zenith angle rules out neutrinos, and neu- made in laboratories) chipped off the neu- to penetrate the overlying rock and reach trons would decay in flight unless their tron star in Cygnus X-3, a star itself made the great depths of the detectors (equiva- energy was unacceptably large. entirely of matter containing a substantial lent to 2 to 5 kilometers of water), the The only remaining possibility is a fraction of strange quarks (Fig. 1). energies of the primaries are likely to be in previously unobserved particle, a ‘cygnet.’ The detection of a periodic muon signal the range 10 to 100 TeV; the mass of the The large flux of muons (comparable to deep underground constrains the prop- primaries is therefore likely to be at most 1 the observed flux of gamma rays), and erties of the primary. Firstly, its electric to 10 GeV. Lastly, the primary must have hence of cygnets, suggests that cygnets are 50 Spring 1986 LOS ALAMOS SCIENCE Star? Does Cygnus X-3 Contain a Strange Consultant to the Theoretical Division from the University of Illinois made at a rapid rate through strong inter- ness of 1, as when a lambda decays into a A more likely possibility is that the cygnet actions rather than through the slower nucleon. is accelerated from a neutron star bound to electromagnetic or weak interactions. One How might cygnets be made in Cygnus charged particles in the form of an exotic promising candidate for this strongly in- X-3? To generate the high-energy gamma nucleus. Free cygnets could then be re- teracting particle is the H particle, earlier rays believed responsible for the extensive leased by fragmentation of such a nucleus proposed by Robert L. Jaffe of MIT, com- air showers observed, Cygnus X-3 must when it strikes a particle in the atmosphere posed of two up, two down, and two have an accelerator capable of producing of the companion star, in a process similar strange quarks in a closely bound state; the charged particles with energies up to 1016 to proton-nucleus fragmentation observed His thus a particle with a strangeness of 2 eV. Cygnets might be produced as the in the laboratory. and a baryon number of 2, the same quan- energetic charged particles accelerated The next question is how exotic nuclei tum numbers as two lambda particles. If from a neutron star interact with the at- might be produced and emitted from a the mass of the H is less than that of the mosphere of the companion star. How- neutron star. A first possibility is that they lambda (1.1 16 GeV) plus that of the neu- ever, since the cross section for this proc- are made by bombardment of the surface tron (0.938 GeV), then the lifetime of the ess would have to be large to produce them of the neutron star by particles accelerated H could be sufficiently long for it to be a in quantities comparable to those of the onto it. (In the electromagnetic accelera- candidate for the primary, since in this gamma rays, we would expect to have seen tion process electron-positron pairs will be case it could not undergo the rapid decay cygnets produced in laboratory accelerator produced, and if, for example, positrons into a lambda and a neutron. Decay of the experiments. (The cygnet mass should be are accelerated away, then the electrons H into two neutrons would be very slow relatively low, so the energy threshold for will be accelerated back to the surface, at since it involves a change in strangeness of producing them should be well below the energies of a TeV or greater, and cause 2, a rarer process than a change in strange- energies available at current accelerators.) substantial spallation of the surface.) This LOS ALAMOS SCIENCE Spring 1986 51 Cygnus X-3—A Strange Neutron Star? Exotic Charged Underground Particles Cygnets Detector Neutron Muons Comparion Star FROM STRANGE QUARK MATTER TO MUONS Fig. 1. A scenario for the observation, in of a strange neutron star cause ejection of star. The cygnets released travel un- underground detectors, of muons with the exotic charged nuclei, which are accel- deflected to the earth’s atmosphere, where directionality and periodicity of Cygnus erated outward and fragment as they pass they produce muons that penetrate to the X-3. Particles accelerated onto the surface through the atmosphere of the companion underground detectors. exceptionally long, at least the time re- a large density fluctuation. If the strange quired for diffusion, of order 105 years. state is lower in energy per baryon than the The final possibility is that the entire neu- normal state of nuclear matter, then once tron star is made of strange matter, and formed the seed will begin to convert the surface spallation throws exotic nuclei up matter around it into strange matter, as a into the beam of particles accelerated away fire spreads through flammable material. from the neutron star. The ‘burning’ front would first convert the Neutron stars may very well be made of liquid core of the neutron star to exotic matter containing a substantial fraction of matter; the heat ahead of the front would strange quarks if, as Edward Witten of melt the crust of the neutron star, as well Princeton conjectured, the absolute as melt the nuclei in the crust into normal Matter ground state of matter might not be the fluid nuclear matter, and within an hour Density familiar material nuclei are made of, but or so the entire star would be converted rather is ‘strange quark matter’ in which into a strange neutron star. Fig. 2. If, as illustrated here, the minimum the quarks, a substantial fraction of which One important consequence of this in the energy per baryon versus density are strange, are not confined within indi- scenario is that if the compact star in curve for normal nuclear matter is higher vidual nucleons but are free to roam Cygnus X-3 is a strange neutron star, then than that for strange quark matter, then throughout. By having less zero-point, or many, if not all, neutron stars should also, normal matter, which in its ground state Fermi, energy, such matter could be stable as a result of the same burning process, be sits at the minimum of the normal matter compared to ordinary nuclear matter (Fig. strange. Strange neutron stars are expected curve, would be unstable against transition 2). (We need not worry about ordinary to cool more rapidly than normal stars to strange quark matter. This transition nuclei turning into strange nuclei if strange since they can emit neutrinos more could result in a neutron star composed matter is the lowest energy state only when rapidly. This enhanced cooling should be entirely of strange quark matter. a finite percentage of the baryons are observable in measurements with future x- strange.) ray telescopes of the surface temperatures mechanism might produce exotic nuclei Imagine then a neutron star being of neutron stars. from normal nuclei, but one is faced with formed (of normal nucleons) in the core of The Cygnus X-3 muon data sugest the the question of why, if correct, it has never a supernova explosion. At the very high existence of a new and unusual particle been observed in the laboratory. A second densities in the center (an order of magni- produced in a new and unusual way. If possibility is that exotic nuclei are tude above the density of laboratory nu- future measurements confirm these data, produced in the core of the neutron star clei, some 3 X 104 grams per cubic cen- the underground experiments will have and then diffuse to the surface. But the timeter), a seed of strange quark matter led to a remarkable discovery of new lifetimes of the exotic nuclei must then be can form either spontaneously or through physical phenomena.

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