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the hadroproduction of the particles with open charm is a rich area to explore the hadronization process of heavy quarks . in hadronic interactions ,
it is especially interesting to compare the forward production characteristics of a given charm particle species for different beam hadrons .
the hadronization process may introduce a distinction between those charm hadrons which have a valence quark or anti - quark in common with the beam compared to those which do not .
we assume that charm production can be factorized into perturbative and non - perturbative elements .
the first part describes the production of the pair @xmath5 which the @xmath6 fusion mechanism is the main diagram .
this is calculable in perturbative qcd ( pqcd ) , which does not predict any asymmetry between the @xmath7 and the @xmath8 produced .
next  to  leading order pqcd calculations introduce a small asymmetry in the quark momenta  @xcite .
the soft part , or hadronization , describes the process by which charmed quarks appear as hadrons in the final state .
only phenomenological models exist at this stage . for
@xmath1 beams a large asymmetry has been observed experimentally  @xcite .
we present here preliminary results for charm - anticharm asymmetries for @xmath2 production from three different beams .
charm decays are also important laboratories for understanding pqcd at the charm mass scale . state  of  the  art methods for calculating non - leptonic decay rates of the charmed hadrons employ heavy quark effective theory and the factorization approximation  @xcite .
nonetheless , the three  body cabibbo  suppressed decays of charmed baryons are prohibitively difficult to calculate .
measurements of relative branching fraction between simple charmed baryon states , both cabibbo  favored and cabibbo  suppressed , give additional insight into the structure of the decay amplitude and the validity of the factorization approximation . until now the only cabibbo  suppressed charmed baryon decay reported with significant statistics is @xmath9 and its resonant state , @xmath10  @xcite .
in this paper , we report the first observation of the cabibbo  suppressed @xmath11 and determine the branching fraction of this decay relative to the cabibbo  favored @xmath12 mode .
selex is a high energy hadroproduction experiment at fermilab using a multi  stage spectrometer designed for high acceptance for forward interactions ( @xmath13 ) .
the experiment aimed especially to understand charm production in the forward hemisphere and to study charmed baryon decays . using both a negative hyperon beam ( 50% @xmath0 , 50% @xmath1 ) and a proton beam ( 92% p , 8% @xmath14 ) , selex took 15.2 billion interaction events during the 19961997 fix target run , tagging 600 gev @xmath0 , @xmath1 and 540 gev @xmath15 beams with a beam trd .
the data were accumulated using a segmented charm target ( 5 foils : 2 cu , 3 c , each separated by 1.5 cm ) whose total thickness was 5% of interaction length for protons .
the spectrometer had silicon strip detectors to measure beam and outgoing tracks and provided precision primary and secondary vertex reconstruction .
transverse position resolution was 4 @xmath16 for the 600 gev beam and the average longitudinal vertex position resolution was 270 @xmath16 for primary and 560 @xmath16 for secondaries , respectively .
track momenta were measured after magnetic deflection by a system of proportional wire chambers ( pwc ) , drift chambers and silicon strip detectors .
track momentum resolution for a typical 100 gev track was @xmath17% .
the absolute momentum scale was calibrated to the @xmath18 mass . for @xmath19
decays the average mass resolution was 9 mev for @xmath20 momenta from 100 - 450 gev .
charged particle identification was done with the ring image @xmath21erenkov radiation ( rich ) detector  @xcite which distinguished @xmath22 from @xmath23 up to 165 gev .
the proton identification efficiency was greater than 95% above proton threshold ( @xmath24 gev ) and the pion mis  identification probability was about 4% .
interactions were selected by a scintillator trigger and an online software filter .
the charm trigger required at least 4 charged tracks and 2 hits in a scintillator hodoscope after the second analyzing magnet .
we triggered on about 1/3 of all inelastic interactions .
based on downstream tracking with particle identification information , the online filter selected events that had evidence for a secondary vertex among tracks completely reconstructed using the forward pwc spectrometer and the vertex silicon .
this filter reduced the data size by a factor of nearly 8 at a cost of about a factor of 2 in charm written to tape as normalized from a study of unfiltered @xmath18 and @xmath25 decays .
most of the charm loss came from selection cuts that were independent of charm species or kinematic variables and which improved the signal / noise in the final sample .
the charm events were selected by the following requirements ; ( 1 ) fits for both primary and secondary vertex have @xmath26 5 , ( 2 )
longitudinal separation l between primary and secondary vertices is greater than 8 times the combined longitudinal error @xmath27 , and ( 3 ) the reconstructed momentum vector from the secondary vertex points back to the primary vertex with good quality @xmath28 , and ( 4 ) @xmath29 for k identification and @xmath30 for @xmath15 identification , where @xmath31 is the likelihood function based on rich information .
additional cuts will be explicitly described if applied .
the production properties of charm quarks require measurements of charm hadrons .
pqcd calculations can be probed experimentally using measurements of single - charm - particle inclusive distributions as a function of @xmath3 and asymmetries , either integrated or as a function of @xmath3 .
previous charm hadroproduction experiments showed evidence of a large enhancement in the forward production of charmed particles that contain a quark or an antiquark in common with the beam ( _ leading particles _ ) over those that do not ( _ non - leading particles _ ) , in the meson sector .
recently , this study has been extended to the production of baryons by a @xmath0 beam  @xcite .
according to our simulations and our measurements of @xmath18 decays , the selex spectrometer acceptance is charge independent and we have a smooth , generally constant acceptance as a function of @xmath3 for @xmath32 .
we show in fig .
[ integrated_a ] the integrated charm production asymmetry for @xmath33 and @xmath34 for @xmath0 , @xmath1 and proton beams in the kinematic region @xmath32 .
the raw asymmetry for charm baryons is much stronger than for charm mesons in this forward @xmath3 region for all 3 beam types .
the effect is even more pronounced when the charm baryons are produced by a baryon beam .
one typical explanation of this observed asymmetry is that longitudinal momentum is added to the produced charm quark if it recombines with a valence quark from the incoming particle , forming the leading particle shown in fig .
[ stfigure ] .
this is incorporated , for example , in the pythia simulation program  @xcite , where the effect is overestimated for typical model parameters  @xcite . with this scenario in mind ,
we expect to find a harder @xmath3 spectrum for leading particles than non - leading particles
.    4.2 truein    -.2 cm    4.2 truein    -.2
cm    the @xmath3 dependence of the corrected number of events from each beam is shown in figure  [ xf ] .
the curves are fits of the standard parametrization @xmath35 to the data .
the @xmath3 distributions for @xmath2 show an @xmath3 dependence for all three beam types that is harder than that reported for d mesons from a @xmath36 beam .
the @xmath2 is a leading particle for each of the three beam particles reported here , and the values of @xmath37 are all consistent . further insight into
the mechanism will come from comparisons of charm meson production characteristics from these three beam hadrons . that work is in process .
4.2 truein    -.2 cm
the study of cabibbo  suppressed charm decays can provide useful insights into the weak interaction mechanism for non - leptonic decays  @xcite .
the observed final state may arise either from direct quark mixing at the decay stage or , in some cases , from quark rearrangement in final - state scattering . by comparing the strengths of cabibbo  suppressed decays to their cabibbo  favored analogs , one can , in a systematic way , assess the contributions of the various mechanisms .
[ decayfigure ] shows a simple spectator diagram with external @xmath39emission for @xmath38 decaying into a cabibbo  allowed and a cabibbo  suppressed mode .
the other cabibbo  allowed @xmath40 mode interchanges @xmath41 and @xmath42 quark lines and produces a @xmath43 pair from the vacuum instead of a @xmath44 pair .
4.2 truein    -.2 cm    since the decay process is similar in the two modes except flavor changing through the cabibbo  kobayashi
 maskawa matrix element ( @xmath45 , we expect @xmath46 , where @xmath47 is the cabibbo angle and @xmath48 is a coefficient of order one containing information about differences in the two decay mechanisms over the allowed phase space . to the extent that the relative branching ratio is different from this , we may argue for enhancement or suppression of one of the two modes .
4.2 truein    -.2 cm    fig .
[ cspkifigure ] shows the first observation of the cabibbo 
suppressed @xmath49 decay mode .
the inset of the figure shows the invariant mass distribution from reconstructed @xmath50 candidates with @xmath51 gev and the sum of transverse momentum square ( @xmath52 ) of decaying particles greater than 0.3 ( gev@xmath53 .
the large peak is the charm baryon decay , @xmath54 and the bump in the right corner shows the cabibbo  suppressed @xmath55 decay .
the analysis cuts off because of a maximum mass cut in the analysis of this mode during this first pass through the data .
we observed @xmath56 events for @xmath55 with the @xmath57 mass at ( @xmath58 ) mev .
the statistical significance for the signal , @xmath59 , is @xmath60 in which @xmath61 is the number of signal events and @xmath62 is the number of background events under the signal region .    in this first reconstruction pass , hyperons ( @xmath63 )
were identified only inclusively in the limited decay interval ( @xmath64 . the hyperon candidate track was identified with a track having @xmath65 gev for which no reconstructed track segments were observed in the 14 chambers along the trajectory after the second analyzing magnet .
this category of tracks ( kinks by disappearance ) had a unique @xmath66 identification for positive kinks but had an ambiguity for negative kinks between @xmath67 and @xmath68 .
reflections in 3-body modes are therefore expected and are taken into account , based on data in the true mode .
[ brfigure ] shows two cabibbo
 favored @xmath57 modes decaying to @xmath69 and @xmath70 , respectively .
for these modes , we require additional kinematical cuts : ( a ) the transverse component of the reconstructed parent particle momentum with respect to the line of flight less than 0.3 gev , and ( b ) the momentum of the @xmath14 is greater than 10 gev .
the shaded area in @xmath69 and @xmath71 is the estimated reflection from @xmath72 and @xmath73 , respectively .
the shape is determined by a monte carlo simulation and the area is normalized to the observed number of signal events in the @xmath2 data .    to estimate the total acceptance for decay modes of interest
, we embedded monte carlo charm decays events in data events .
we generated charm events with an average transverse momentum @xmath74 gev and longitudinal momentum spectrum as observed for the @xmath57 data .
detector hits , including resolution and multiple coulomb scattering smearing effects , produced by these embedded tracks were ord into the hit banks of the underlying data event .
the new ensemble of hits was passed through the selex off - line reconstruction .
the total acceptance of each mode was determined from the number of fitted signal events from the charmed particle mass spectrum .
4.2 truein    to check the acceptance and particle identification , we measured two well  measured relative decay fractions , @xmath75 and @xmath76 where the first error is statistical and the second is the systematic difference between charge  conjugate states .
the results agree well with the world averages  @xcite .
for the @xmath77 decays , we applied a tighter cut @xmath78 ( @xmath79 ) to suppress background from @xmath80 since the lifetime of @xmath77 is 2.4 times longer than that of @xmath81 .    to check the hyperon decay acceptance ,
we take into account two cabibbo - favored modes @xmath12 and @xmath82 .
we measure the relative branching fraction to be @xmath83 .
the error is statistical only .
this result is comparable to the cleo measurement , @xmath84  @xcite .
the number of events and estimated acceptance for these three @xmath57 modes with the same set of cuts are summarized in table  [ tab xc+ ] .
.summary of observed events and estimated acceptance for @xmath57 modes . [ cols= " < , > , > " , ]     [ tab xc+ ]    the preliminary branching fraction of the cabibbo  suppressed decay @xmath11 relative to the cabibbo 
favored @xmath12 is measured to be @xmath85 which corresponds to @xmath86 .
we observed a large production asymmetry in favour of @xmath2 over @xmath87 for all three beams in the region @xmath88 the asymmetry is stronger for baryon beams than for the @xmath1 beam .
we report the observation of the cabibbo  suppressed decay @xmath11 at mass = ( @xmath89 ) gev with @xmath56 signal events .
the relative branching fraction of the decay @xmath11 to the cabibbo - favored @xmath12 is measured to be @xmath90 .
carnegie mellon university , fermilab , university of iowa , university of rochester , university of hawaii , university of michigan - flint , petersburg nuclear physics institute ( russia ) , institute for theoretical and experimental physics ( moscow ) , institute for high energy physics ( protvino ) , moscow state university , university of sao paulo , centro brasileiro de pesquisas fisicas , universidade federal de paraiba , insitute of high energy physics ( beijing ) , university of bristol , tel aviv university , max planck institut fuer kernphysik ( heidelberg ) , university of trieste and infn , university of rome and infn , universidad autonoma de san luis potosi , bogazici university
we present preliminary results on various aspects of charm baryon studies at the 1996 - 1997 fixed target experiment of fermilab studying charm produced from incident @xmath0 , proton , and @xmath1 beams at 600 gev . 
 first results include the comparison of hadroproduction asymmetries for @xmath2 production from the 3 beams as well @xmath3 distributions and the first observation of the cabbibo - suppressed decay @xmath4 . 
 the relative branching fraction of the cabbibo - suppressed mode to the 3-body cabbibo - favored modes is also presented .