History: BigBite Tracking and Drift Chamber Global Variables

This document describes the variables that are stored as Hall A Global Variables and are available for output to the Root Tree. They can also be examined in the analyzer using the gHaVars->Print() and gHaVars->PrintFull() commands. The subset of Global Variables that are actually written to the Root Tree is determined by a file described in the Hall A C++ Analyzer Documentation topic "Defining the Output".


There are three coordinate systems used:

Lab Coordinates - Hall coordinate system where y goes against gravity, z is in the direction of the beam, and x is to the left when looking down the beam

Target Coordinates - Origin at the center of the target. x goes with gravity, z is parallel to the ground in the direction towards the center of the BigBite drift chambers. y is to the left looking down z

Detector Coordinates - Origin specified by center of the first
plane of the drift chambers. x is "down" in the dispersion direction, z is in the nominal direction of particles such that z = 0 is defined by the first plane of the chamber. y is defined so we have a right handed coordinate system.

Global variables defined that are not specifically associated with the BigBite or Neutron Spectrometer arms.

Run and Event Information

tag	description
g.runnum	Run Number
g.runtype	CODA run type
g.runtime	CODA run time
g.evnum	Event Number
g.evtyp	Event Type
g.evlen	Event Length
g.helicity	Beam Helicity
g.timestamp	Time Stamp

Global variables associated with the individual chamber planes have labels of the form B.dc..tag. For example, the number of hits in plane v2 can be accessed using the label B.dc.v2.nhit.

(THaMWDCHit) TClonesArray* fHits

An fHits array is created for each chamber plane and contains the wire hit information for that plane.

tag	size	default	note	defining function	description
nhit	[1]	NULL	1	THaMWDCPlane::Decode	Number of hits
wire	[nhit]	NA		THaMWDCPlane::Decode	wire
rawtime	[nhit]	NA		THaMWDCPlane::Decode	Raw TDC time of hit
time	[nhit]	NA	2	THaMWDCPlane::Decode	Drift time (nsec)
offset	[nhit]	0.0		THaMWDCPlane::Decode	TDC offset parameter (nsec)
myref	[nhit]	0.0		THaMWDCPlane::Decode	Time of reference chan (nsec)
dist	[nhit]	0.0			?
track	[nhit]	-1			?
showercut	[nhit]	FALSE		THaMWDC::ConstructTracks	Boolean value if hit passed shower cuts

Note 1: To be added into the fHits array, the wire number must be valid. If the no neg ativ e flag is TRUE, the time must be between fMinTime and fMaxTime

Note 2: The driftime is computed as time = rawtime(T DC calibration)-T DC Of f set-Ref erenceC hannelT ime If the reference channel time is not valid, it is not subtracted off, but the hit is stored and fRefOkay is set to FALSE.

Double t fRawT[nWires], fCorT[nWires], fRefT[nRefWires], and Int t fTCounter[nWires]

Array sizes set to be the number of wires in the plane and map to the wire number. The index of the array is the wire number.

tag	size	default	note	defining function	description
rawtimeof	[nWires]	-1e35		THaMWDCPlane::Decode	Raw TDC Time
cortimeof	[nWires]	-1e35		THaMWDCPlane::Decode	Drift time (nsec)
NumHitsof	[nWires]	0		THaMWDCPlane::Decode	Number of hits on this wire
reftimeof	[nRefWires]	-2e35		THaMWDCPlane::Decode	Time of reference channel (nsec)

Additional Plane Data

tag	size	default	note	defining function	description
refchokay	[1]	true		THaMWDCPlane::Decode	All reference channels in plane are OK

Global variables associated with the BigBIte tracks are of the form B.tag.

(THaTrack) TClonesArray* fTracks

An fTracks array is created in the THaSpectrometer class. The tracks are defined in the BigBite detectorframe coordinate system with x towards higher momentum (downwards) z perpendicular to the wire chamber planes in the direction of the particle's momentum and y determined by a right-handed coordinate system. The x- and y- intercepts are defined at z=0. (The first chamber plane.)

Basic Tracking Variables

tag	size	default	note	defining function	description
tr.n	[1]	NULL		THaMWDC::ConstructTracks	Number of tracks found
tr.x	[tr.n]	NA		THaMWDC::ConstructTracks	Track X in detector coords at z=0 in meters
tr.y	[tr.n]	NA		THaMWDC::ConstructTracks	Track Y in detector coords at z=0 in meters
tr.xp	[tr.n]	NA		THaMWDC::ConstructTracks	Track slope dx/dz (tangent theta) in detector coords
tr.yp	[tr.n]	NA		THaMWDC::ConstructTracks	Track slope dy/dz (tanget phi) in detector coords
tr.chi2	[tr.n]	NA		THaMWDC::ConstructTracks	Chi squared (not reduced)
tr.ndof	[tr.n]	NA		THaMWDC::ConstructTracks	Number of degrees of freedom

Extended Tracking Variables

tag	size	default	note	defining function	description
B.tr.p	tr.n	NA		THaMWDC::FindVertices	Track momentum magnitude (GeV/c)
B.tr.px	tr.n	NA		THaMWDC::FindVertices	Track momentum magnitude (GeV/c) times directional cosine in x direction in Lab Coordinates
B.tr.py	tr.n	NA		THaMWDC::FindVertices	Track momentum magnitude (GeV/c) times directional cosine in y direction in Lab Coordinates
B.tr.pz	tr.n	NA		THaMWDC::FindVertices	Track momentum magnitude (GeV/c) times directional cosine in y direction in Lab Coordinates
B.tr.vx	tr.n	NA		THaMWDC::FindVertices	Track vertex x coord in lab coordinates (meters)
B.tr.vy	tr.n	NA		THaMWDC::FindVertices	Track vertex y coord in lab coordinates (meters)
B.tr.vz	tr.n	NA		THaMWDC::FindVertices	Track vertex z coord in lab coordinates (meters)
B.tr.fT.x	tr.n	NA		THaMWDC::FindVertices	Front track (before magnet interaction) x coordinate at z=0 in target coordinates (meters)
B.tr.fT.y	tr.n	NA		THaMWDC::FindVertices	Front track y coordinate at z=0 in target coordinates (meters)
B.tr.fT.xp	tr.n	NA		THaMWDC::FindVertices	Front track slope dx/dz (tangent theta) in target coords
B.tr.fT.yp	tr.n	NA		THaMWDC::FindVertices	Front track slope dy/dz (tangent phi) in target coords
B.tr.defang	tr.n	NA		THaMWDC::FindVertices	Deflection angle (radians)
B.tr.beam.x	tr.n	NA		THaMWDC::FindVertices	Track x coordinate at beam z in detector coords (meters)
B.tr.beam.y	tr.n	NA		THaMWDC::FindVertices	Track y coordinate at beam z in detector coords (meters)
B.tr.beam.z	tr.n	NA		THaMWDC::FindVertices	Track z coordinate at beam z in detector coords (meters)

Coarse Tracking Variables

B.ctr.*

These variables are identical to those above for B.tr.*, however, they are calculated using the coarse tracks. They are available when the SaveCoarseTracks() function is called in the THaBBe class.