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Measurement of Delta G / G in COMPASS

Figure 2.1a shows the gluon momentum fraction tex2html_wrap_inline2240 from our Monte Carlo studies of the open charm production by photon-gluon fusion (see par. 3.1.2 of the COMPASS proposal for a description of Monte Carlo event generators) integrated over the quasi-real photon energy spectrum tex2html_wrap_inline2284 from 35 to 85 GeV. The input unpolarized gluon distribution is the Duke-Owens set 1.1 and tex2html_wrap_inline2286  GeV/c tex2html_wrap_inline2256 . Other distributions, like the MRSA, give similar spectra. The tex2html_wrap_inline2240 distribution is basically a convolution of the unpolarized gluon distribution with the elementary photon-gluon fusion cross section and the quasi-real photon spectrum. In the tex2html_wrap_inline2284 range of 35-85 GeV this distribution is peaked at tex2html_wrap_inline2296 , it starts at tex2html_wrap_inline2298 and extends over tex2html_wrap_inline2300 , with a good coverage in the tex2html_wrap_inline2302 region. According to recent parametrizations, tex2html_wrap_inline2304 has a maximum in the range of our proposed measurement.

   figure701
Figure: tex2html_wrap_inline2240 distribution of gluons in open charm production: a) tex2html_wrap_inline2242  GeV; b) dashed line tex2html_wrap_inline2244  GeV, solid line tex2html_wrap_inline2246  GeV.

With increasing tex2html_wrap_inline2284 , tex2html_wrap_inline2240 decreases; by splitting the covered tex2html_wrap_inline2284 interval in smaller tex2html_wrap_inline2284 bins (or using a higher beam energy as mentioned in the proposal) we cover different tex2html_wrap_inline2240 regions. Figure 2.1b shows the tex2html_wrap_inline2240 distributions for two different tex2html_wrap_inline2284 bins of 30-45 GeV and 75-90 GeV, respectively. It can be seen how the low tex2html_wrap_inline2284 bin covers the high tex2html_wrap_inline2240 region and is peaked at tex2html_wrap_inline2332 while the high tex2html_wrap_inline2284 bin covers the low tex2html_wrap_inline2240 region and is peaked at tex2html_wrap_inline2338 . By splitting our tex2html_wrap_inline2284 interval, for instance into 4 tex2html_wrap_inline2284 bins, the measurement error on the tex2html_wrap_inline2344 asymmetry for each single tex2html_wrap_inline2284 bin will increase roughly by a factor of 2, each tex2html_wrap_inline2284 bin having now a tex2html_wrap_inline2350 . Such an analysis will also allow us to determine approximately the tex2html_wrap_inline2304 shape in this tex2html_wrap_inline2240 range.

It has to be added that the error estimates reported in the proposal were based on the tex2html_wrap_inline2356 decays only (with and without tex2html_wrap_inline2358 tagging). We can expect a considerable improvement on the tex2html_wrap_inline2344 measurement including other D decay channels, giving tex2html_wrap_inline2364 smaller than 4 %, and hence a better sensitivity on tex2html_wrap_inline2260 .


next up previous contents
Next: Measurement of Delta G Up: Delta G / G Previous: Delta G / G

Lars Schmitt
Wed May 22 16:44:09 METDST 1996