底夸克是带有电荷−1⁄3 e个第三代夸克，又称为美夸克。虽然量子色动力学描述每一种夸克个方法侪类似，由于底夸克带有交关大个裸质量（约为4.2 GeV/c2，:135稍微多过质子质量的四倍)，而且CKM矩阵个元素Vub搭Vcb个数值交关小，因此底夸克拥有独特个标签。
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