How Not To Write An Abstract

One of the minor annoyances of life and one of my pet peeves is when the abstract for an academic journal article announces that it has reached an important conclusions on a hot issue identified in the abstract, but doesn't tell you the conclusion that it reached even though it could be stated in a few words. 

The following article fits the bill (although I do applaud its implementation of the emerging practice of listing only the corresponding lead author and the collaboration for whom the author speaks, rather than every participant in the author line).

It announces that it tested lepton universality violation in the decay of charm quarks, which is a potential violation of the Standard Model suggested by other experiments, but doesn't say in the abstract what it concluded.

Leptonic and semileptonic decays in the charm sector have been well studied in recent years. With the largest data sample near DD¯ threshold, precision measurements of leptonic and semileptonic decays of charm meson and baryon are perfromed at BESIII. Test for letpon flavor universality is also performed. Sensitivity for rare leptonic and semileptonic charm decays is significantly improved taking advantage of the huge statistics in LHCb and the B factories.

S. F. Zhang (On behalf of the BESIII Collaboration) "Experimental study for leptonic and semileptonic decays in the charm sector" (June 21, 2019).

The conclusion, however, is a notable negative result, disfavoring results that have appeared to differ from the Standard Model assumption that charged leptons are identical in all respects except mass, including weak force transition probabilities. As the concluding summary in the paper explains:

In summary, BESIII has improved the precision of decay constants, form factors and CKM matrix elements in the charm sector with recent measurements. Meanwhile, LFU test at a very high precision (1.5% for Cabbibo favoured decays and 4% for Cabbibo suppressed decays) has been performed while no evidence of violation is found. Search for charm semileptonic decays to scalar mesons were performed at BESIII and the current results are in favor of the SU(3) nonet tetraquark description of a0(980), f0(500) and f0(980). Moreover, our sensitivity to rare charm leptonic and semileptonic decays has been improved by several magnitudes with the huge statistics at LHCb, and strong constraints have been set for various new physics models with recent measurements. With more data coming from BESIII, LHCb and BelleII, experiment study of charm leptonic and semileptonic decays will be further improved in the future.

Another of my minor peeves with regard to arXiv, is that it doesn't have categories that distinguish between proposed experiments and searches, such as this one related to Belle II, this one proposing an ATLAS search, and this one at a proposed LHeC experiment, and actual experimental results.

Analysis

For what it is worth, I would really like to see a good review article attempting to reconcile results like this one that do not find LFU (lepton flavor universality) violation to a high precision with the results that do not, rather than cherry picking one or the other. I've blogged a lot of papers going each way, but haven't had the time or mental space to really try to determine if the results are simply contradictory or if there is something special about the apparently LFU violating cases from the LFU observing cases.

I am particularly critical of papers that have tried to combine multiple LFU violating results (at low individual significance) to get a higher significance, without considering either look elsewhere effects or actually non-LFU violating experimental results. My intuition is that the statistical significance of the LFU violating experiments considered in that manner is much lower than actually claimed.

But, part of that analysis requires some discrimination regarding what experiments should and should not be included as similar enough to be considered as part of the same global average.

For example, is it correct to lump decays of beauty mesons with decays of charmed mesons in this analysis? You can inappropriately exaggerate the significance of a result by making a distinction without a difference. But, it is also possible that there is some good theoretical reason for there to be LFU violation in some experiments but not in others.

Likewise, it is also possible that apparently LFU violation in multiple seemingly independent measurements of similar decays is actually subject to correlated systemic errors because everyone in the field doing similar experiments is inclined as a consequence of a common educational background and sub-disciplinary culture to make discretionary choices in setting up an experiment that can lead to systemic error in the same way.

So, an author of such a review article needs to have considerable wisdom and understanding regarding both the larger theoretical issues, and practical methodological details of these experiments, to reach insightful and correct conclusions about the likelihood of the existence or non-existence of LFU violation from a comprehensive review of experiments that test LFU.