Chunk-by-chunk luminous region information

The quantities described below are used internally by the QFNDIP package (sec. 4.17 on page  ). They give the best possible precision and should therefore be preferred, when available, to QVXNOM,QVYNOM, QVZNOM described above.

XGETBP

= .TRUE. if the event-chunk beam information from QFGET_BP is available. The quantities listed below are defined only if:
XGETBP = .TRUE.

QVTXBP(I)

I=1,2,3: x,y,z position of luminous region centroid (cm). Notice: QVTXBP(3) is always equal to zero.

QVTEBP(I)

I=1,2,3: Estimated uncertainty on QVTXBP(I) (cm). Notice: QVTEBP(3) is always equal to 1.

QVTSBP(I)

I=1,2,3: x,y,z rms size of luminous region (cm), averaged over the year (or run period). The routine QBEAMX (see section 12.10) provides access to the metachunk-by-metachunk values of sigma_x.

Real data:

The beam position x and y and their uncertainties returned in QVTXBP and QVTEBP are taken from the ALPB bank in the run record, or from the beam.position file if the dataset was produced by JULIA version < 275.03 . The sizes of the luminous region returned in QVTSBP are taken from the ALRP database bank and are average values for the year (or run period).

Monte Carlo:

The beam position x and y returned in QVTXBP are generated so as to simulate the desired beam size distribution. The primary vertex position of the event is extracted from truth information and an offset is added according to the size of the luminous region. The smearing corresponding to the average chunk-by-chunk beam position uncertainty (QVTEBP) is included.

The ``desired'' beam size distribution is by default taken from measurements of the actual LEP conditions for the geometry year of the Monte Carlo file. This default may be changed by means of data cards ( BPER,BPWT,BSIZ,SIBE) as explained in section 4.15.

The horizontal size of the luminous region $\sigma_{x}^{}$ is thrown at random from the measured distribution, which is parametrized as the sum of two Gaussians (not a delta function). The uncertainties in x and y returned in QVTEBP, as well as the three sizes returned in QVTSBP, are taken from the ALRP database bank and are average values for the year (or run period).

Important remarks:

To know how the beam position information was obtained for a given event, use the subroutine QWHICH_BP (see Sec 12.17 on p. ).

Another way of obtaining the beam position is to use the LEP BOMs (for LEP2 data only). See the description of the data card BOBS in Sec 12.17 on p. ) and the subroutine QFILBP_STATUS to know how the beam spot was found for a given event in Sec 12.18 on p. .