Monday, May 16, 2022

An Eighth Predicted Decay Channel Of The Higgs Boson?

The CMS experiment at the Large Hadron Collider has searching for a signal of the predicted decay of Higgs bosons to charm anti-charm quark pairs finds results that are not inconsistent with the Standard Model prediction. 

Large backgrounds from multiple sources with significant uncertainties make securing a significant signal difficult, however. 

The money chart from the paper is this one, in which the predicted Higgs to charm decay signal residual after subtracting known backgrounds is shown in red bars in the lower part of the chart and the measurements are shown as crosses:

So, CMS did observe a signal (three bins are exactly what was expected and two show a slightly stronger signal than expected), but not a very strong one, because they don't have enough statistical power from their measurements to expect to see a very strong signal.

Previous results (with the predicted values show as percentage branching fractions) were as follows:

b-quark pairs, 57.7% (observed)
W boson pairs, 21.5% (observed)
gluon pairs, 8.57%
tau-lepton pairs, 6.27% (observed)
c-quark pairs, 2.89% 
Z boson pairs, 2.62% (observed)
photon pairs, 0.227% (observed)
Z boson and a photon, 0.153% (observed)
muon pairs, 0.021 8% (observed)
electron-positron pairs, 0.000 000 5%

The paper and its abstract are as follows:
A search for the standard model Higgs boson decaying to a charm quark-antiquark pair, H →cc¯, produced in association with a leptonically decaying V (W or Z) boson is presented. The search is performed with proton-proton collisions at s√ = 13 TeV collected by the CMS experiment, corresponding to an integrated luminosity of 138 fb−1. Novel charm jet identification and analysis methods using machine learning techniques are employed. 
The analysis is validated by searching for Z →cc¯ in VZ events, leading to its first observation at a hadron collider with a significance of 5.7 standard deviations. 
The observed (expected) upper limit on σ(VH)(H→cc¯) is 0.94 (0.50+0.22−0.15) pb at 95% confidence level (CL), corresponding to 14 (7.6+3.4−2.3) times the standard model prediction. For the Higgs-charm Yukawa coupling modifier, κc, the observed (expected) 95% CL interval is 1.1 <|κC|< 5.5 (|κc|< 3.4), the most stringent constraint to date.
CMS Collaboration, "Search for Higgs boson decay to a charm quark-antiquark pair in proton-proton collisions at s√ = 13 TeV" arXiv:2205.05550 (May 11, 2022) (submitted to Physical Review Letters; Report number: CMS-HIG-21-008, CERN-EP-2022-0811).

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