Event Shape Routines

A single routine is provided to calculate a variety of event shape variables.

      SUBROUTINE getevtshape (NPAR,PS,SCALE,SHAPE,SUME)

      DOUBLE PRECISION PS(4,10),SCALE,SHAPE(NOVAR),SUME
      INTEGER          NPAR
      INTEGER          NOVAR
      PARAMETER       (NOVAR=20)   ! the current number of variables

where the input parameters are

• NPAR the number of input particles,
• PS(4,11) the four vectors PS() from disphase,
• SCALE the scale to normalise to for some variables, usually Q.

the subroutine outputs

• SUME the total energy in the current region of the Breit frame,
• SHAPE(NOVAR) and array with all the event shapes. They are indexed by a letter code eg. SHAPE(itz) for current jet thrust.

The table below gives the indexes for all 14 of the available event shapes. The remaining 6 variables in SHAPE(20) are internal or obsolete and should not be used.

SHAPE(itz)	Current jet thrust
SHAPE(itz2)	2nd moment of current jet thrust
SHAPE(itm)	Thrust axis thrust
SHAPE(itm2)	2nd moment of thrust axis thrust
SHAPE(ijb)	Current jet broadening
SHAPE(ijb2)	2nd moment of current jet broadening
SHAPE(ijm)	Current jet mass
SHAPE(ic4)	C-parameter (normalised to energy)
SHAPE(ic5)	C-parameter (normalised to momentum)
SHAPE(ic6)	C-parameter (normalised to scale/2)
SHAPE(ied)	Energy deficit
SHAPE(iob)	Oblateness
SHAPE(itr)	Thrust to resultant axis
SHAPE(ijbr)	Jet broadening to resultant axis

The index variables are available by including the file index.inc. The routine is designed to be called from within disphase and passed the common library four-vectors ps() directly. No cut on the total energy in the current region (to ensure infrared safety) is performed within the routine; this quantity is returned and any cut is the responsibility of the user routine.