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Ab initio QCD Calculation of the Proton Mass *November 23, 2008*

*Posted by keithkchan in Journal club.*

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In Science this week, physicists report that they successfully calculated the proton mass using lattice Quantum Chromodynamics (QCD). This marks the efforts of physicists in the last 30 years to calculate the mass of nucleons from first principles. We believe that about 90% of the mass in the nucleons is due to the strong interaction between quarks and gluons, and the mass of the quarks and/or EM interaction between the quarks contribute the remaining. Because of the strong coupling the QCD, this has not been confirmed satisfactorily. The latest calculation by Durr et al is a vivid demonstration of the validity of the standard model.

In lattice QCD, the spacetime is discretized, so that the calculations are reduced to finite integrals that can be done on the computer. The difficulties in lattice QCD are that the vacuum is strongly fluctuating, so that to describe it accurately, many snapshots are required. Another obstacle is that extremely high computational power is required to include the influence of the the quark-antiquark pair on the gluon vacuum. In last couple of years, researchers develop better mathematical description and more accurate formulas to describe the quark-antiquark pair. The above simulation from Derek Leinweber is the visualization of the fluctuating vacuum due to quarks and gluons. So finally, researchers are able to calculate the mass of the proton on a computer with the needed physics included and put the error under control. The is another triumph of the standard model. Cheers =:).

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