BuSca. Buffer Scanner for LLP searches
Require lhcb-datapkg/ParamFiles!90 to run CI tests
Developers: V. Kholoimov, A. Oyanguren, B. Jashal, J. Zhuo, V. Svintozelskyi
This is a prototype algorithm for searches at the HLT1 level. It is based on the reconstruction of downstream tracks and vertices, selecting candidates and projecting them in 2D histograms of flight distance (FD) and mass at 30 MHz. The main feature of the BuSca algorithm is that it does not persist the data and therefore saves the bandwidth. Hot spots are searched for indicating a probability that new particles are encountered in that region, providing a guide for new HLT1 and HLT2 lines development.
BuSca structure
Filling 2D histograms online: FD vs mass and Armenteros-Podolanski plot.
1. BuSca monitoring system
Selection procedure
BuSca uses one hidden layer neural network, based on tracks and vertex quality. NN is trained on MinBias and signal channels.
| NN structure | NN Output |
|---|---|
|
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BuSca histograms
- The mass vs FD plot: gives the opportunity to see not only the mass of the possible new particle, but also the mean place where it decayed. SM particles with masses greater than 1.5 GeV are not expected to decay in the downstream region. This is due to the fact that most of the resonances produced in proton-proton collisions or in B decays have very short lifetimes and decay inside the VELO detector.
- The Armenteros-Podolanski plot: it is a representation of the
p_{T}of the parent particle in a two-body decay, versus the asymmetry of the longitudinal momentum of the decay products. They appear as semi-ellipses, whose parameters provide information on the masses of the parent and the child particles.
Allen implementation is based on !1556 (merged)
Binning scheme (FD vs Mass)
Adaptative binning is used based on the mass and FD resolutions:
\sigma_m = 0.02\cdot m ; [MeV]
\sigma_{FD} = 80 - 0.02\cdot FD; [mm]
Bin width is configurable in terms of number of \sigma: BW = k\cdot\sigma
See details: https://indico.cern.ch/event/1361613/contributions/5852549/attachments/2822381/4929073/BuSca_19_03.pdf
Output example
| FD vs mass | Armenteros-Podolanski |
|---|---|
|
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The left peak corresponds to K^0_S and Λ^0 particles, while an example of a new particle is shown at 3 GeV. |
K_S^0 pairs create a bottom big ellipse, with center at (0; 0), while Λ^0 and \bar{\Lambda}^0 form two symmetric ellipses at left and right parts of the plot. The resulting ellipses are additionally highlighted with coloured lines. The top ellipse indicates a new particle with mass 3GeV. |
2. Trigger mechanism
HLT1DownstreamBuSca
Line is prepared and turned off until a signal is detected.
Configuration: switch mechanism, mass and FD windows.
Turning on in mass = [2800; 3200] MeV and FD = [500; 2300] mm.
Armenteros-Podolansi plot for triggered events:
HLT2
A passthrough line will be prepared.