New Combined ATLAS/CMS Higgs Mass Result

The ATLAS and CMS collaborations have finally released a joint result on their Higgs analyses. The paper uses the two channels where the final state invariant mass is most easily calculated and where backgrounds are expected to be small (Higgs to two gammas or Higgs to four charged leptons). These two channels don’t have have very many signal events, which limits the ability of the two experiments to make a precise measurement.

Combining results from different experiments is always tricky, since the analyzers have to be sure that systematic uncertainties are treated properly. Some systematics will be completely separate for each experiment, while others may be correlated. Regardless, the new result is mH = 125.09±0.24 GeV. The measurement is still statistics-limited, but is getting closer and closer to being limited by systematics. This is quite good, but still nowhere near the kind of precision to which the Z and W masses are known. To do that at the LHC, the experiments would need to improve systematics by more than an order of magnitude as well as collect many times more data. If we realistically hope to know the Higgs mass as precisely as for the W and Z bosons, we’ll need an e+e Higgs factor collider (i.e. the ILC or something similar).

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