Run 2 Turbo PV refitting bug

The issue

In order to an unbiased (or less biased) measure of the decay time of a long lived particle, PVs must be refitted excluding the signal tracks. This is enabled by setting `ReFitPVs = True` on the appropriate algorithm (CombineParticles, FilterDesktop, etc). However, in order to trigger the generation of refitted PVs, the PVs must be accessed by the algorithm, eg, by a `BPV` LoKi functor. When refitted PVs are generated, they are saved to the output mDST along with the particle -> PV relations tables and so are accessible for user analysis. If the refitted PVs aren't generated, they can't be recovered offline on the mDST Turbo output.

The majority of Turbo lines requesting PV refitting in some algorithm in their sequence didn't do so in conjunction with a `BPV` functor, and so the refitted PVs aren't persisted, and are lost.

The effect

If one or more tracks from your signal candidate are included in the PV fit, this will pull the position of the PV towards those tracks and so will bias any displacement variables calculated wrt the PV. This is particularly important for time dependent analyses, as the decay time will be biased. This could potentially bias measurements of, eg, lifetimes, mixing, time dependent CPV, etc.

Mitigating effects

The PV fit only includes tracks for which the $\mathrm{IP} \chi^2 < 9$ (see here and here). Thus, if you have strong enough displacement cuts (eg, $\mathrm{IP} \chi^2$) in your selection, the signal tracks will never have been included in the PV fit anyway. Note however that there is a difference between the $\mathrm{IP} \chi^2$ used by the PV fitter to include/exclude tracks and the $\mathrm{IP} \chi^2$ that you'll get offline.

Unaffected lines

These are lines that requested PV refitting, and did so correctly. Lines that didn't request PV refitting aren't shown.

Line Comment

Affected lines

These are lines that requested PV refitting in some way, but did it incorrectly.

-- MichaelAlexander - 2019-06-19

This topic: LHCb > WebHome > LHCbComputing > Run12Performance > TurboPVRefittingBug
Topic revision: r1 - 2019-06-19 - MichaelAlexander
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