Physicists finally calculated where the proton’s mass comes from

诚信胡子 2018-12-02

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PARTICLE PHYSICS

Physicists finally calculated where the proton’s mass comes from

Only 9 percent of the subatomic particle’s bulk comes from the mass of its quarks亚原子粒子只有9%的体积来自夸克的质量

BY EMILY CONOVER 6:00AM, NOVEMBER 26, 2018

MASSIVE UNDERTAKING Using a technique called lattice QCD, scientists figured out how protons (illustrated here in the nucleus of an atom) get their mass.利用一种叫做晶格QCD的技术，科学家们发现了质子(如图所示，在原子核中)是如何获得质量的。

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A proton’s mass is more than just the sum of its parts. And now scientists know just what accounts for the subatomic particle’s heft.现在科学家们知道亚原子粒子的重量是由什么引起的。

Protons are made up of even smaller particles called quarks, so you might expect that simply adding up the quarks’ masses should give you the proton’s mass. However, that sum is much too small to explain the proton’s bulk. And new, detailed calculations show that only 9 percent of the proton’s heft comes from the mass of constituent quarks.新的，详细的计算表明，只有9%的质子来自组成夸克的质量 The rest of the proton’s mass comes from complicated effects occurring inside the particle, researchers report in the Nov. 23 Physical Review Letters.

Quarks get their masses from a process connected to the Higgs boson, an elementary particle first detected in 2012 夸克的质量来自于与希格斯玻色子(Higgs boson)相连接的过程。希格斯玻色子是一种基本粒子，于2012年首次被发现(SN: 7/28/12, p. 5). But “the quark masses are tiny,” says study coauthor and theoretical physicist Keh-Fei Liu of the University of Kentucky in Lexington. So, for protons, the Higgs explanation falls short.

Instead, most of the proton’s 938 million electron volts of mass is due to complexities of quantum chromodynamics, or QCD, the theory which accounts for the churning of particles within the proton. Making calculations with QCD is extremely difficult, so to study the proton’s properties theoretically, scientists rely on a technique called lattice QCD, in which space and time are broken up into a grid, upon which the quarks reside.

Using this technique, physicists had previously calculated the proton’s mass (SN: 12/20/08, p. 13). But scientists hadn’t divvied up where that mass comes from until now, says theoretical physicist André Walker-Loud of Lawrence Berkeley National Laboratory in California. “It’s exciting because it’s a sign that … we’ve really hit this new era” in which lattice QCD can be used to better understand nuclear physics.

‌In addition to the 9 percent of the proton’s mass that comes from quarks’ heft, 32 percent comes from the energy of the quarks zipping around inside the proton, Liu and colleagues found. (That’s because energy and mass are two sides of the same coin, thanks to Einstein’s famous equation, E=mc2.) Other occupants of the proton, massless particles called gluons that help hold quarks together, contribute another 36 percent via their energy.质子的其他组成部分，即帮助夸克凝聚在一起的无质量粒子胶子，通过它们的能量贡献了另外36%。

The remaining 23 percent arises due to quantum effects that occur when quarks and gluons interact in complicated ways within the proton.剩下的23%是由于夸克和胶子在质子内以复杂的方式相互作用时产生的量子效应引起的。 Those interactions cause QCD to flout a principle called scale invariance.这些相互作用导致QCD忽略了一个称为比例不变性的原则。 In scale invariant theories, stretching or shrinking space and time makes no difference to the theories’ results.在尺度不变理论中，空间和时间的拉伸或收缩对理论结果没有影响。 Massive particles provide the theory with a scale, so when QCD defies scale invariance, protons also gain mass.

The results of the study aren’t surprising, says theoretical physicist Andreas Kronfeld of Fermilab in Batavia, Ill. Scientists have long suspected that the proton’s mass was made up in this way. But, he says, “this kind of calculation replaces a belief with scientific knowledge.”

Citations

Y.B. Yang et al. Proton mass decomposition from the QCD energy momentum tensor. Physical Review Letters. Vol. 121, November 23, 2018, p. 212001. doi: 10.1103/PhysRevLett.121.212001.