Divide event into two hemispheres

Input arguments:

ICLASS

described in 10.3.

IVEC

Track number of vector which defines the ``hemi''spheres.

COSCUT

The cosine of the opening angle of a cone around IVEC. Tracks inside this cone belong to the same side, and all other ones belong to the opposite side. The word ``hemisphere'' is correct if COSCUT = 0.

Results:

'same-s'

The 4-momentum sum of tracks on the same side as IVEC.

'opp-s'

The 4-momentum sum of tracks on the side opposite to IVEC.

The two output vectors can be used to assign tracks to one of the the two hemispheres with the lock algorithm (10.2.3).

In the following example, the event is divided into two hemispheres according to the thrust axis. Then, each hemisphere is boosted separately into the rest frame of all contributing tracks.

      DIMENSION IVECT(2)
C---Thrust axis
      CALL QJTHRU (THRU, 'THRUST', KRECO)
      ITHRU = KPDIR ('THRUST', KRECO)
C---Two hemispheres:
      CALL QJHEMI ('SAME', 'OPPO', KRECO, ITHRU, 0.)
      IVECT(1) = KPDIR ('SAME', KRECO)
      IVECT(2) = KPDIR ('OPPO', KRECO)
C---Lock all tracks in the 'oppo' hemisphere:
      CALL QLOCK (IVECT(2))
C---Loop over both hemispheres:
      DO 10 IHEMI = 1, 2
C---Transform all selected tracks into the rest frame of IVECT(IHEMI):
      CALL QTCLAS (KRECO, IVECT(IHEMI))
C---Now, do the analysis. For example:
C---Plot the thrust in the boosted frame.
      CALL QJTHRU (THRUB, ' ', IVECT(IHEMI))
      CALL QHF1 (4711, THRUB, 1.)
C---QLREV: locked tracks -> unlocked tracks and vice versa.
C---This selects tracks in the hemisphere 'OPPO' for next loop.
      CALL QLREV (KRECO)
   10 CONTINUE

Note that in the above example, two of the maximum six Lorentz frames are in use. They can be dropped by the statement CALL QVDROP (' `, IVECT(IHEMI)) inside the loop (see 9.2.6).