Physical Review D Aritcle |
The Sources of b-Quarks at the Tevatron and their CorrelationsRick FieldJanuary 1, 2002 |
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| Note: Click on the figures to enlarge the figure and to access an encapsulated postscript (EPS) version of the figure. | |||||
It is important to have good leading order (or leading-log order) estimates of hadron-hadron collider observables. Of course, precise comparisons with data require beyond leading order calculations. If the leading order estimates are within a factor of two of the data, higher order calculations might be expected to improve the agreement. On the other hand, if the leading order estimates are off by more than about a factor of two of the data, one cannot expect higher order calculations to improve the situation. In this case, even if the higher order corrections were large enough to bring agreement, one could not trust a perturbative series in which the second term is greater than the first. If a leading order estimate is off by more than a factor of two, it usually means that one has overlooked some important physics. For this reason good leading-log order estimates are important. |
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In this analysis the leading-log order QCD hard scattering Monte-Carlo models of
HERWIG,
ISAJET,
and PYTHIA
are used to study the sources of b-quarks at the Tevatron.
The reactions responsible of producing b-quarks are separated into three categories; flavor
creation, flavor excitation, and shower/fragmentation. Flavor creation corresponds to the
production of a  pair by gluon fusion or by the annihilation of
light quarks via the
two 2-to-2 parton subprocesses,  , and  ,
and is illustrated in the above figure.
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| The data from CDF and D0 for the integrated inclusive b-quark cross section for y < 1 at 1.8 TeV are compared with the QCD Monte-Carlo model predictions for flavor creation in the above figure, where y is the rapidity of the b-quark. Here the parton distribution functions CTEQ3L have been used for all three Monte-Carlo models and, as is well know, the leading order predictions are roughly a factor of four below the data. The leading order estimates of the flavor creation contribution to b-quark production at the Tevatron are so far below the data that higher order corrections (even though they may be important) cannot be the whole story. | |||||
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An additional source of b-quarks at the Tevatron comes from the scattering of a b or  quark out of
the initial-state into the final-state by a gluon or by a light quark or antiquark via the subprocesses;
 ,  ,
and  , plus the corresponding three
subprocesses.
This is referred to as "flavor excitation" and is illustrated in the above figure. Flavor excitation
is, of course, very sensitive to the number of b and quarks within the
proton (i.e. the structure functions).
The b and quarks are generated through the Q2 evolution of the
structure functions. Even with
no "intrinsic" pairs within the proton, at high
Q2 pairs are produced by gluons
and populate the proton "sea". The number of pairs within
the proton is related, through the Q2 evolution,
to the gluon distribution within the proton. None of the structure functions considered in this analysis
include "intrinsic" pairs within the proton. The
pair content within the proton is generated entirely through the Q2
evolution of the structure functions.
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Another source of b-quarks at the Tevatron comes from reactions which have a
in the
final-state but with only gluons and light quarks and light antiquarks participating in the 2-to-2 hard
parton scattering subprocess (i.e. no heavy quarks in the 2-to-2 hard scattering subprocess). This is
referred to as "shower/fragmentation" and is illustrated in the above figure. Here the subprocesses
are all QCD 2-to-2 gluon, light quark, and light antiquark subprocesses. The "shower/fragmentation"
contribution comes from pairs produced within parton showers or
during the fragmentation process. This category includes the "gluon splitting" subprocess,
 , as modeled by the QCD leading-log Monte-Carlo models.
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The data from CDF and D0 on the integrated inclusive b-quark total cross-section are compared with
the QCD Monte-Carlo model predictions in the above figure with the predictions of PYTHIA.
The four curves correspond to the
contribution to b-quark production from flavor creation, flavor excitation, shower/fragmentation,
and the resulting overall total. After adding the contributions from all three sources
PHYTHIA (CTEQ3L) agrees fairly well with the data. The QCD Monte-Carlo model leading-log estimates
of the flavor excitation and the shower/fragmentation contributions to production are uncertain
and should not be taken too seriously. However, it seems likely that at the Tevatron all three sources of
b-quarks, flavor creation, flavor excitation, and shower/fragmentation are important. One does not expect
precise agreement from leading-log estimates. On the other hand, the qualitative agreement shown indicates that
probably nothing unusual is happening in b-quark production at the Tevatron. Furthermore, in Run II at the Tevatron we
should be able to isolate experimentally the individual contributions to b-quark production by studying
correlations in detail.
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