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arXiv 1107.3192nucl-thnucl-ex
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中能重离子碰撞中的磁效应

Magnetic effects in heavy-ion collisions at intermediate energies

Li Ou, Bao-An Li

摘要:

在同位旋相关的Boltzmann-Uhling-Uhlenbeck输运模型IBUU 11下,研究了束流能量为200 - 2000 MeV/核子的重离子反应中内电磁场的时间演化和空间分布.虽然磁场可以达到大约$7\times 10^{16}$ G,这明显高于磁星的估计表面磁场($\sim 10^{15}$ G),但它对核子观测值几乎没有影响,因为洛伦兹力通常比核力弱得多。然而,非常有趣的是,由类似弹丸(类似靶)的旁观者产生的磁场对正/负π介子在向前(向后)的快速性上有很强的聚焦/发散效应。因此,微分$π^-/π^+$比作为快度的函数被磁场显著改变,而正负π介子的总多重数保持大致相同。当束流能量大于1 GeV/核子时,当总$π^-$与$π^+$多重数的积分比不相等时,微分$π^-/π^+$比对核对称能$E_{\rm{sym}}(ρ)$的密度依赖性很敏感.我们的研究结果表明,磁效应应仔细考虑在未来的研究中使用的差分$π^-/π^+$比作为探针的$E_{\rm{sym}}(ρ)$在超饱和密度。

英文摘要:

The time-evolution and space-distribution of internal electromagnetic fields in heavy-ion reactions at beam energies between 200 and 2000 MeV/nucleon are studied within an Isospin-dependent Boltzmann-Uhling-Uhlenbeck transport model IBUU11. While the magnetic field can reach about $7\times 10^{16}$ G which is significantly higher than the estimated surface magnetic field ($\sim 10^{15}$ G) of magnetars, it has almost no effect on nucleon observables as the Lorentz force is normally much weaker than the nuclear force. Very interestingly, however, the magnetic field generated by the projectile-like (target-like) spectator has a strong focusing/diverging effect on positive/negative pions at forward (backward) rapidities. Consequently, the differential $π^-/π^+$ ratio as a function of rapidity is significantly altered by the magnetic field while the total multiplicities of both positive and negative pions remain about the same. At beam energies above about 1 GeV/nucleon, while the integrated ratio of total $π^-$ to $π^+$ multiplicities is not, the differential $π^-/π^+$ ratio is sensitive to the density dependence of nuclear symmetry energy $E_{\rm{sym}}(ρ)$. Our findings suggest that magnetic effects should be carefully considered in future studies of using the differential $π^-/π^+$ ratio as a probe of the $E_{\rm{sym}}(ρ)$ at supra-saturation densities.

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