Algorithm to estimate momentum of Velo track from parent mass constraint

Goal

Use Velo track in reconstruction of decays with (at least) double cascade topology: (P -> (C1 -> GC1 GC2 ..) C2 ...) where GC1 is a Velo track

Motivation

Reconstruct charged particles with nanosecond lifetimes, such as

\Sigma^+

,

\Sigma^-

,

\Xi^-

and

\Omega^-

, with high efficiency in double cascade topologies.

Usage

1. Build C1 with a standard particle combiner.
2. Use this algorithm to estimate momentum of Velo track using C1 mass constraint, and refit the Velo track with the PID assigned in the chain. Note that the algorithm duplicates the decay chain due to the 2-fold ambiguity of the mass constraint.
3. Build P with output of 2. and other basic particles.
4. Use DTF with C1 mass constraint and PV constraint to select mass window for P.
5. Use DTF with P mass constraint and PV constraint to select mass window for C1.
6. Use DTF with PV constraint, C1 and P mass constraints to perform selection with kinematic and topological variables.

Why this algorithm?

The DTF will profit from the estimate of the algorithm.
With this workflow, the momentum resolution

\delta p / p

of the

\Xi^-

Velo track in the decay chain

\Omega_b^-\to(\Xi_c^+\to\Xi^-\pi^+\pi^+)K^-\pi^-

is in the order of 5 %. The efficiency of this approach is much improved compared to using the Velo track "as is" in the DTF.

Dependencies

LHCb!4995 (merged)