You will be presented with the full text of a science research paper. Please write an abstract for this paper. Your response should include the abstract and no additional text.

Paper text:
measuring purely leptonic decays of heavy mesons allows the determination of meson decay constants , which connect measured quantities , such as the @xmath5 mixing ratio , to ckm matrix elements .
currently , it is not possible to determine @xmath6 experimentally from leptonic @xmath7 decays , so theoretical calculations of @xmath6 must be used .
the most promising of these calculations involves lattice qcd @xcite @xcite @xcite , though there are other methods @xcite @xcite @xcite @xcite @xcite .
measurements of pseudoscalar decay constants such as @xmath8 provide checks on these calculations and help discriminate among different models .
= 3.0 in    the decay diagram for @xmath9 is shown in fig .
[ dptomunu ] .
the decay rate is given by @xcite @xmath10 where @xmath11 is the @xmath12 mass , @xmath13 is the mass of the final state lepton , @xmath14 is a ckm matrix element equal to 0.2205 @xcite , and @xmath15 is the fermi coupling constant .
various theoretical predictions of @xmath8 range from 190 mev to 350 mev . because of helicity suppression
, the electron mode @xmath16 has a very small rate . the relative widths are @xmath17 for the @xmath18 , @xmath19 and @xmath20 final states , respectively .
unfortunately the mode with the largest branching fraction , @xmath21 , has at least two neutrinos in the final state and is difficult to detect .
the cleo - c detector is equipped to measure the momenta and direction of charged particles , identify charged hadrons , detect photons and measure with good precision their directions and energies .
muons above 1.1 gev can also be identified .
the detector is almost cylindrically symmetric with everything but the muon detector inside a superconducting magnet coil run at a current that produces an almost uniform 1.0 t field .
the detector consists of a six - layer wire drift chamber at small radius that is low mass , suitable for these relatively low energies .
it is followed by a 47-layer drift chamber ; both chambers use a gas mixture of 60% helium and 40% propane . these two devices measure charged track three - momenta with excellent accuracy .
the drift chamber also measures energy loss , de / dx , that is used to identify charged tracks below about 0.7 gev / c .
after the drift chamber there is a ring imaging cherenkov detector ( rich ) @xcite , that identifies charged particles over most of their momentum range .
the rich is surrounded by a thallium doped csi crystal array consisting of about 8000 tapered crystals 30 cm long and about 5x5 @xmath22 at the rear .
in this study we use 60 pb@xmath0 of cleo - c data recorded at the @xmath23 resonance ( 3.770 gev ) .
these events consist of a mixture of @xmath24 , @xmath25 and continuum events .
there also may be small amounts of @xmath26 pairs and two - photon events .
we examine all the recorded events and retain those containing at least one charged @xmath27 candidate in the modes listed in table  [ tab : drecon ] .
the selection criteria are described in detail in what follows .
all acceptable track candidates must have a helical trajectory that approaches the event origin within a distance of 0.005 m in the azimuthal projection and 0.05 m in the polar projection , where the azimuthal projection is in the bend view of the solenoidal magnet .
each track must posses at least 50% of the hits expected to be on a track and its polar direction must have cosine with respect to the beam direction of magnitude @xmath28 .
we use both charged particle ionization loss in the drift chamber ( de / dx ) and rich information to identify kaons and pions used to fully reconstruct @xmath27 mesons .
the rich is used for momenta larger than 0.55 gev .
information on the angle of detected cherenkov photons is translated into a liklihood of a given photon being due to a particular particle .
contributions from all photons associated with a particular track are then summed to form an overall liklihood denoted as @xmath29 for each particle hypothesis . to differentiate between pion and kaon candidates , we use the difference : @xmath30 ) .
usually this cut is set at zero except for muon candidates where the difference between @xmath31 ) is set at 10 , to ensure a high , well understood efficiency . to utilize the de / dx information
we calculate @xmath32 as the difference between the expected ionization loss for a pion and the measured loss divided by the measurement error .
similarly , @xmath33 is defined in the same manner using the expected ionization for a kaon .
we use both the rich and de / dx information for @xmath34 meson tag candidate tracks in the following manner : ( a ) if neither the rich nor de / dx information is available , then the track is accepted as both a pion and a kaon candidate .
( b ) if de / dx is available and rich is not then we insist that pion candidates have @xmath35 and kaon candidates have @xmath36 ( c ) if rich information is available and de / dx is not available , then we require that @xmath37 for pions and @xmath38 for kaons .
( d ) if both de / dx and rich information are available , we require that @xmath39 for pions and @xmath40 for kaons .
we reconstruct @xmath41 s by first selecting photon candidates from energy deposits in the crystals not matched to charged tracks that have deposition patterns consistent with that expected for photons .
pairs of photon candidates are kinematically fit to the nominal @xmath41 mass .
we require the pull , the difference between the raw and fit mass normalized by its uncertainty , to be less than three for accepted @xmath41 candidates .
@xmath42 are formed from a pair of charged pions which are kinematically fitted to come from a single vertex .
we also require that the invariant mass of the two pions be within 4.5 times the width of the @xmath43 mass peak which is 0.004 gev r.m.s .
tagging modes are fully reconstructed by first evaluating the difference in the energy of the decay products with the beam energy .
we then normally require the absolute value of this difference to be within 0.02 gev of zero , approximately twice the r.m.s .
width , and then look at the reconstructed @xmath34 beam constrained mass defined as @xmath45 where @xmath46 runs over all the final state particles .
we also use the charge - conjugate @xmath12 tags and search for @xmath47 ; in the rest of this paper we will not mention the charge - conjugate modes explicitly , but they are always used .    the @xmath48 distribution for all @xmath34 tagging modes considered in this data sample are shown in fig .  [ drecon ] and listed in table  [ tab : drecon ] along with the number of signal and background events .
the event numbers are determined from fits of the @xmath48 distributions to gaussian signal functions plus a background shape parameterized as @xmath49 order polynomial for the @xmath50 , @xmath51 and @xmath52 and from fits to double gaussian signal functions plus @xmath49 order polynomial for @xmath53 and @xmath54 tags where we see a small tail on the higher mass side .
the models generally predict @xmath55 to be 10 - 15% larger than @xmath8 .
cleo previously measured @xmath55 as ( @xmath56 mev @xcite , and we are consistent with these predictions as well .
we look forward to more data to improve the precision .
99 c. davies _ et al .
lett . * 92 * , 022001 ( 2004 ) [ hep - lat/0304004 ] ; c. davies ,  lattice qcd , " in heavy flavour physics , scottish graduate textbook series , institute of physics 2002 , eds c. t. h. davies and s. m. playfer [ hep - ph/025181 ] ; a. kron feld , 
heavy quarks and lattice qcd , " [ hep - lat/0310063 ] .
z. g. wang _
_ ,  decay constants of the pseudoscalar mesons in the framework of the coupled schwinger - dyson equation and bethe - salpeter equation , " [ hep - ph/0403259 ] ( 2004 ) ; l. salcedo _
et al . _ , braz .
j .phys . * 34 * , 297 ( 2004 ) [ hep - ph/0311008 ] .
g. rong ,  recent bes results on @xmath1 and d meson productions and decays , " presented at xxxixth rencontres de moriond , electroweak interactions and unified theories , march 2004 , [ hep - ex/0406027 ] .
it appears that they only consider background modes with muons as they have a muon detector .
however , modes with pions could also contribute due to fakes , as the rate for hadrons around 900 mev faking muons can be quite sizable .
in 60 pb@xmath0 of data taken on the @xmath1 resonance with the cleo - c detector , we find 8 @xmath2 event candidates that are mostly signal , containing only 1 estimated background . 
 using this statistically compelling sample , we measure preliminary values of @xmath3 , and determine @xmath4 mev .     international conference on high energy physics , aug 2004 , beijing ]