The heart of the quark matter

06 June 2008 (Volume 3 Issue 8)

The magnetic field may have a strong effect on the ground state of the matter in a neutron star

Figure 1: A neutron star is possibly the only place in nature where color superconductivity can form.

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In a theoretical study on quarks investigating the ground state of matter at the extremely high densities found in neutron stars, physicists at Brookhaven National Laboratory (BNL), USA, have determined the importance of the magnetic field¹.

Quarks are usually found in groups of three, forming protons or neutrons—which in particle physics are known as baryons. However, when the density becomes high, it is more appropriate to describe quarks as forming a liquid. At extremely high densities, as present almost exclusively in neutron stars, quarks can pair and form a state known as color superconductivity (Fig. 1).

Analogous to carriers in solid-state superconductors, quarks form so-called Cooper pairs, and an amount of energy Δ is necessary to split the pair. But unlike carriers, there are several types of quarks, which are distinguished by two properties known as ‘flavor’ and ‘color’. “Since there are nine quarks (three colors and flavors), there are many patterns of the Cooper pairing,” says Kenji Fukushima of the RIKEN BNL Research Center. “Although we knew that the ground state is a color superconductor, we did not know which pairing pattern is the right one.” The pairing strongly depends on factors including density, temperature, and external field.

Fukushima and his colleague, Harmen Warringa of BNL’s physics department, estimated that the maximum magnetic field in a neutron star can reach 10¹⁸ gauss, which is enough to provide an energy comparable with the average Δ. “The effects of the density and temperature have been well investigated, but only little is known about the magnetic field, though the magnetic field exists in compact stars in general,” says Fukushima.

The two scientists calculated the variation of three types of Δ—corresponding to different pairs of the up, down and strange quark flavors—with magnetic field. An important component of their calculations was that the system was electrically and color-neutral, which is realistic for a compact star. They found that all three types of Δ oscillate in a magnetic field, and for particularly high fields the color superconductor is formed only by down–strange pairs. “We found that the magnetic field has a significant effect on the ground state with neutrality imposed, which was surprising to us.”

The results could also be important for predicting the life of magnetars, neutron stars with extremely powerful magnetic fields, as the oscillations in the Δ—due to a decaying magnetic field—could influence the cooling rate and therefore the time of their evolution.

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1. Fukushima, K. & Warringa, H. Color superconducting matter in a magnetic field. Physical Review Letters 100, 032008 (2008). | article |