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edit: @annav has noted an informative blog post summarizing the theoretical (but not experimental error) issues related to this measurement.

There is some excitement about a bump in an electron-positron opening angle spectrum from 8Be internal pair conversion that it might be a new particle - mediator of a possible new force. eg. Feng et al. 2016.

In Krasznahorkay et al. 2016 (ref. 6 of Feng 2016) the bump can be seen in Fig. 4:

enter image description here

However, Gulyás et al (2015) (ref. 30 of Feng et al. 2016) shows a measure of the sensitivity as a function of the opening angle, and the improved design with 5 lepton telescopes is still fairly bumpy:

The resulted angular correlation for the uncorrelated events gave us the experimental response curve. Reasonably good agreement was obtained with the results of the MC simulations as presented in Fig. 5.

enter image description here enter image description here

My Question: Recalling another electron positron "bump" experiment (see below), and it's final conclusion after three different experiments by three different groups at two laboratories finally got together with sufficient wine, what is the evidence so far that this bump is not just a calibration error, an incorrect efficiency correction, or something similar?

From this review or "retrospective" presentation - refer to the original document for more background.

enter image description here

above: slide #4 from here.

enter image description here

above: slide #26 from here.

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    $\begingroup$ Most particle physicists would give a high probability for one of the explanations you offer. The crux is that if such a particle exists, it should show up in e+e- laboratory experiments $\endgroup$ – anna v Aug 24 '16 at 13:49
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    $\begingroup$ All these big pictures make the question a little hard to handle. Not that it's bad, really, but it would certainly be an improvement if you can trim it down a bit. (I think the screenshots of the slides could be safely omitted, for example.) $\endgroup$ – David Z Aug 24 '16 at 13:50
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    $\begingroup$ i changed the imgur to medium $\endgroup$ – anna v Aug 24 '16 at 13:57
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    $\begingroup$ Related, related. $\endgroup$ – rob Aug 24 '16 at 14:00
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    $\begingroup$ @rob Remember that you are not looking at the boson but at the scattering mediated by it. Momentum transfer is only limited by the boson mass if you don't have enough energy. Both SLC and LEP spent a lot of time and effort on weak interaction (80-90 GeV carriers) and weak coupling. A circa 20 MeV boson gives much larger 'geometric' component to the cross-section and there was plenty of energy. So the couple constant would have to be tiny for them to miss it. But how does it effect low energy decays strongly enough to show? Not impossible with a running coupling constant, but ... $\endgroup$ – dmckee --- ex-moderator kitten Aug 24 '16 at 14:31

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