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Fix in Inc2mu2muLongLivedDown and B2DetachedDimuonAndJpsi Lines

We found a very strange decay time distribution for LLP's in our stripping line that selects B->2 (LLP -> mumu) decays. The reason for the strange decay time distribution of the decay products of the B->2 (LLP -> mumu) stripping line lies fundamentally in the vertexing. Building a vertex from two neutral LLP’s (long-lived particles) results in a very poor vertex resolution, visible in the vertex chi2. This has an immediate effect on the decay time distribution of the LLP’s. That means we cannot really use this line for selecting our signal that requires up to 2 LLP’s coming from a B meson. Once it gets to 3 LLP’s coming from the B, there is enough information to achieve a better vertex resolution and thus more precise decay time distributions. We came up with a mitigation strategy: Slightly reducing our physics case we can live with having only one of the two decay products of the B being an LLP, while the other is prompt.We found the B2MuMuMuMuB2JpsiKS stripping line, which already existed in Stripping 34 (and before). However, it is applying a very wide Jpsi veto which removes also a lot of our signal. So we would like to use that line, but removing the Jpsi veto, trying to get away with a less broad/no Jpsi veto offline. This issue also affects our newly written Incl2mu2muLongLivedDown line. There the effect is even enhanced as one of the LLP’s decays into downstream tracks. So also here the fix requiring the two long tracks to directly come from the B will have to be applied.

The two plots below show the problematic behaviour and how it is fixed. The bad B vertex fit is affecting the LLP decay time (a1_2_TAU) in the "B24mu" sample. This is fixed with requiring one dimuon to be actually not displaced. B_VertexChi2Decaytime

In total this fix is therefore changing two lines: Inc2mu2muLongLivedDown and B2DetachedDimuonAndJpsi. No other line is affected. We tested rates and multiplicity and found these numbers:

 |                                              *Decision name*|*Rate,%*|*Accepted*| *Mult*|*ms/evt*|

 |_StrippingSequenceStreamDimuon_                              |  0.6100|       122|       |  50.502|
 |!StrippingMultiLepton_Incl2mu2muLongLivedDownLine            |  0.0100|         1|  1.000|   4.348|
 |!StrippingMultiLepton_Incl2mu2muLongLivedDownLine_TIMING     |  0.0100|         1|  1.000|   0.145|
 |!StrippingB2MuMuMuMuB2DetachedDimuonAndJpsiLine              |  0.0017|         1|  1.000|   0.023|
 |!StrippingB2MuMuMuMuB2DetachedDimuonAndJpsiLine_TIMING       |  0.0017|         1|  1.000|   0.022|

Incl2mu2muLongLivedDownLine and B2MuMuMuMuB2DetachedDimuonAndJpsiLine tested with DaVinci/v44r11p5
Edited by Emilio Xose Rodriguez Fernandez

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