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From the signed impact parameter a signed significance
can be derived by dividing the measured signed impact parameter
by the estimate of its resolution.
Well measured tracks with large impact parameters lead to large
values for the significance while badly measured tracks
with large resolution remain in the core of the distribution.
In fig.17 the significance
distributions of tracks is shown for a sample of events
at large photon virtualities [43].
A detailed description of the analysis based on such signatures
is given in section 6.2.2.
The selected tracks are
required to have transverse momenta of larger than 500 MeV.
No lepton or jet criteria are explicity imposed.
Consequently, the relative contribution from charm and beauty events is
significantly smaller than in fig.16.
Only those tracks with an impact parameter of less than
0.1 cm enter the significance distributions and
(fig.17a and c).
This selection suppresses contributions from decays of long lived
particles containing strangeness and allows to achieve a reasonable
discrimination of the charm and beauty components.
Studies show that the significance distribution from strange
particle decays is almost symmetric at small impact parameters.
The distribution
(fig.17a)
denotes the significance of the track with the highest significance
in the event. Only those events are used in which there is only
one selected track.
The significance
(fig.17c)
is defined for events with two or more selected tracks and where
the track has the second highest significance of all selected tracks.
In addition, it is required that the significance
has the same sign as
in the same event.
The distributions are dominated by light quark events and
exhibit large tails to both negative and positive
values of significance with only a small asymmetry due to the long lived
charm and beauty decays.
In the 'negative subtraction method' the negative bins in the
significance distributions are mirrored at and subtracted from
the positive. This way, effects that lead to significance distributions which are
symmetric around zero are removed.
In particular, the uncertainties due to the impact parameter resolution
and the light quark normalization are substantially reduced.
The subtracted distributions are shown in figs.17b
and d. The contributions from charm and beauty are
determined by a fit which is performed
simultaneously to both the subtracted and
distributions
and the total number of inclusive events before track selection.
The shapes for the
,
and
distributions are taken from Monte Carlo
simulations and their normalization is fitted to the data.
Only the statistical errors of the data and Monte Carlo simulation are
considered in the fit.
The Monte Carlo
,
and
contributions in each
-
interval
are allowed to be scaled by factors
,
and
,
respectively. The fit to the
and
distributions mainly constrains
and
, whereas the overall normalization constrains
.
The
and
quark fractions are distinguished in the fit by
their different shapes in the
and
distributions.
The precise simulation of the simulated shapes is crucial for inclusive
lifetime tag analyses. In particular, the size of the -fraction
obtained from the fits is directly dependent on the decomposition
of the simulated charm sample into events with short-lived and
with long-lived charmed hadrons (e.g.
),
since the long-lived charmed hadrons have lifetime distributions and track
multiplicities which are similar to those of beauty hadrons.
In existing H1 analyses charm fragmentation universality is assumed
and the fragmentation fractions as described
in [159] are used which are based on measurements
at LEP, CLEO and ARGUS. Measurements of charm fragmentation at HERA
are discussed in detail in section 6.1.6.
Results consistent with the negative subtraction method have been found using alternative methods, such as the multi-impact parameter method (MIP) and the method of deterministic annealing.