Use case for sparse analysis

Outline of use case

The main point for the $B_s \rightarrow \mu^+\mu^-$ is to measure the branching ratio. Due to the clean signature of a two-body decay with muons a very tight selection can be written corresponding to a low rate. For estimating the background, sidebands are required for the dimuon invariant mass distribution as well as separate samples of other 2-body B decays. The signal efficiency will somehow be measured from $B \rightarrrow J/\Psi X$ events.


The selection places requirements on the tracking for accuracy in the vertexing and particle ID for the muon identification. The category for the stream where this selection belongs is physics.


Number of streams from the detector

The signal and the $B \rightarrrow J/\Psi X$ control events will all come from the dimuon trigger. However the $B \rightarrow h^{+} h^{-}$ events that will be used as a control sample for the particle ID are part of the exclusive stream. As there might be a common stripping between the signal and the $B \rightarrow h^{+} h^{-}$ events individual stream would cause trouble with double counting. This arguments seems to favour a single stream that all subsequent stripping is done from.

Information required on trigger, stripping and luminosity

As this is a branching ratio measurement, strict control of the dataset used is required.

The branching ratio will be measured relative to a known braching ratio to avoid the uncertainty from the B cross section. The $B^{+} \rightarrow J/\Psi K^{+}$ channel is the most likely channel. Eventually one might want to normalise to $BR(B_{s} \rightarrow J/\Psi  \varphi)$ to avoid the uncertainty from the Bs production fraction. The relative trigger efficiency for the control and the signal channel will depend on trigger conditions so these need to be retrieved for every single run in the dataset - even for for runs where there might be no events selected. If this information is stored in a kind of run header we need to carry this one forward even for empty runs. If the information is external the information about the dataset selection should be kept separately.

Information on luminosity is only required to be able to state the size of the dataset used for the measurement. It will not be used in the actual measurement.

Number of streams in the stripping

If there is a low number of streams then the signal selection will form only a very small number of events inside a much larger stream. Hence the analysis will require more resources in terms of access to more data (what is the penalty from this?). If the selection can be combined with the other 2-body selections that anyway will be used as a control sample this might be less of an issue.

Information stored for each event

Given the very simple final state there seems to be a limited benefit of storing any extra information in the DST from the stripping.

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