The ALPHA array QVTSBP (section 6.4) provides an average horizontal beam size sigma_x for each year. QBEAMX is designed to provide run/event-dependent sigma_x values for the real data and to provide corresponding information for Monte Carlo. The data is divided into ``metachunks'' of typically 275 qqbar events (about four beam position ``chunks'') for the measurements. The numerical values are read automatically by ALPHA at initialisation time and put in internal banks VBWP,WIDE and WIDN .
To use QBEAMX, one must make the above subroutine call during the event analysis stage .
Real data: The measured value of SIGX is taken from the WIDE bank. A bias correction (from the database bank VBWP) is applied.
Monte Carlo: The value of sigma_x thrown in the subroutine QFGET_BP is recovered by means of the entry QFMCSX. NEVE, the number of events in the metachunk, is thrown according to the distribution in the real data, as found in the WIDN bank. The value of sigma_x is smeared according to the resolution (as a function of NEVE, with parameters in VBWP) and the result is returned in SIGX. Thus the data and Monte Carlo should be directly comparable as far as SIGX is concerned.