Revisit particle combination and vertex fitting
The current vertex fitter implements a lot of branching in order to determine how to process the information of the combined particles. Some of the particle types (like TrackWithoutVelo
and Resonance
are not covered, despite being defined). This makes introducing further developments quite hard. There is currently no accurate distinction among the different particle objects that might be encountered. The following summarizes the different cases that should be covered on the particle combination:
Definitions
-
\iota \equiv \text{is basic}
: the object is a basic particle, therefore it is not composed by any other particles. -
\varepsilon \equiv \text{can be extrapolated}
: the object has a state that can be extrapolated to any position inz
, including the information of the covariance matrix. -
\upsilon \equiv \text{has vertex}
: the object is a composite or resonance with valid vertex information (position and covariance matrix). -
\tau \equiv \text{has track}
: the object represents a charged particle, with track information.
The relations \upsilon\Rightarrow\varepsilon
and \tau\Rightarrow\varepsilon
hold at any point.
Types of objects
Nickname | Description | \iota |
\varepsilon |
\upsilon |
\tau |
---|---|---|---|---|---|
T |
track-like | \checkmark |
\checkmark |
\times |
\checkmark |
N |
neutral | \checkmark |
\times |
\times |
\times |
C |
composite | \times |
\checkmark |
\checkmark |
\times |
C' |
composite | \times |
\times |
\times |
\times |
R |
resonance | \times |
\checkmark |
\checkmark |
\times |
R' |
resonance | \times |
\times |
\times |
\times |
The difference between C
and R
(similarly for C'
and R'
) is established at run-time based on the lifetime of the processed particle. If it is considered to decay promptly, the composite is built according to R
and C
otherwise. This distinction is needed in order to provide and forward vertex information in cases where a short-lived particle is combined with another particle that doesn't have an accurate pointing information (like a photon in B_s^0\to K^{\ast0}\gamma
). A combination of particles yields C'
or R'
if the vertex can not be determined (like when combining two photons, for example).
Note that all the particles must have a well-defined 4-momenta. The 4-momenta is corrected when doing the vertex fit, since for particles that can not be extrapolated one might be able to correct the 4-momenta based on the vertex reconstructed using the remaining particles of the combination. Examples of the different combinations and particle types are:
B_s^0\to\mu^+\mu^-
Here the B_s^0
is of type C
and the muons are of type T
.
B_s^0\to K^{\ast0}(892)(\to K^{\pm}\pi^{\mp})\gamma
Here the B_s^0
is of type C
, the K^{\ast0}(892)
is of type R
, kaons and pions are of type T
and the photon is of type N
.
B_s^0\to K^{\ast0}(892)(\to K_S^0(\to \pi^+\pi^-)\pi^0)\gamma
Here the B_s^0
is of type C'
, the K^{\ast0}(892)
is of type R'
, the K_S^0
is of type C
, charged pions are of type T
and the neutral pion and photon are of type N
.
\Lambda_b^0\to\Lambda(\to p\pi^-)\gamma
Here the \Lambda_b^0
is of type C'
, the \Lambda
is of type C
, protons and pions are of type T
and the photon is of type N
.
The combination
The result of a combination yields an object with a vertex (C
or R
) only if one of the following two conditions are satisfied:
- We are combining at least two objects that can be extrapolated. In this case we must do the vertex fit using the information from all the objects that can be extrapolated.
- We are combining a single resonance with vertex information plus a set of objects that can not be extrapolated, a case where the vertex should be set to that of the resonance.
If the combination yields an object without a vertex the momenta is determined from the raw value of the momenta of the particles in the combination (which should have already assigned some pointing information, specially for neutral objects). Otherwise, the momenta is calculated from the vertex-corrected momenta of all the particles. The vertex-corrected momenta is the value of the momenta assuming that particles come from the point of closest approach to all the extrapolated objects. In the vertex fit, the information is treated differently depending whether particles have tracking or vertex information, and that's why we must also make the distinction \tau/\upsilon
.
(Created in the context of !2639 (closed))