QVSRCH : Secondary vertices and b-tagging

This routine from T. Mattison performs a primary vertex finding , a 2-jet finding and a secondary vertices finding with an estimation of b quark tagging variables .

To use it properly , one must read the complete description of the method and of the package in the Aleph Note 92-173 (December 1992).

QVSRCH cannot be used when reading a Nano-Dst, since it needs the track error matrices.

Input arguments:

BPOS(3)

Beam position. BPOS(2) must be an accurate beam y coordinate, i.e., QVYNOM or QVTXBP(2) (output from GET_BP). BPOS(1),BPOS(3) are used for X and Z track cuts and as additional constraints on primary vertex

BSIZ(3)

Size of luminous region (1 sigma). Fixed values of .0200, .0020, 2.0 will work OK. Any negative element of BSIZ will suppress internal primary finding, in which case PVTX() and EPVTX() become input variables.

Output arguments:

PVTX(3)

Primary vertex in Aleph XYZ coordinates

EPVTX(3)

Error (not variance) in PVTX

DJET(3,2)

Two normalized XYZ direction vectors for two jets

BTAG(2)

B-tagging variable for the two hemispheres it is basically the half the chi-square difference between the hypothesis that all tracks in the hemisphere come from primary, or that some tracks come from the secondary and some from primary a cut at 10, or at 20 on the sum, is reasonable

SVTX(3,2)

Two secondary vertices in coordinate systems that are used internally, (one system per jet).

• Origins are at PVTX(), and are oriented by DJET(,)
• The third coordinate is the decay length in the jet direction
• The first coordinate is R-Phi-like, the second is Z-like
ESVTX(3,2)
Errors on SVTX() in the internal rotated system where geometrical correlations are small and ignored . May be negative, indicating a problem, not necessarily severe, in finding the vertex
AVTX(3,2)
Two secondary vertices in ALEPH XYZ coordinates
CAVTX(3,3,2)
Two covariance matrices for AVTX()
JERR
Error flag
• 0 All OK
• +1 for secondary vertex 1 errors questionable
• +2 for secondary vertex 2 errors questionable
• +3 for questionable errors on both secondary vertices
• +4 for no good tracks in jet 1
• +8 for no good tracks in jet 2
• +12 for no good tracks in either jet
• +16 for questionable primary vertex
• +32 for not enough tracks for primary vertex
• +64 for no jets found

QVSRCH performs an internal jet finding. If you don't want it, a data card:

NQVJ

will suppress the internal jet finding, in which case DJET becomes an input variable which must be filled in by the user.