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:
to lowest order , hadronic @xmath5 decays can be described by external ( ) and internal ( ) w - emission , gluonic ( ) and electroweak ( ) penguins , as well as annihilation ( ) , w - exchange ( ) , and penguin annihilation ( ) diagrams .
neglecting ckm - matrix elements one might naively expect /@xmath9
, /@xmath10 , and /@xmath11  @xcite . , ,  ought to be very small compared to   as they are suppressed by @xmath12 . taking ckm matrix elements into account gronau
@xcite   have suggested an approximate hierarchy in orders of @xmath13 as follows : @xmath14    @xmath15 denotes the internal electroweak penguin diagram which is color suppressed .
gronau  assume that  is dominated by @xmath16  loop .
is then suppressed by @xmath18  in @xmath19  as compared to @xmath20  penguin amplitudes .
fleischer and mannel  @xcite  suggested @xmath21  for @xmath20  penguin amplitudes .
a similar @xmath22  ratio may be expected for @xmath19  penguins .
figures  [ fig : diagrams]a ) to d ) depict the four dominant diagrams .
the cleo ii experiment  @xcite has accumulated @xmath23  pairs . with typical efficiencies for two - body final states of @xmath24 , and backgrounds of only a fraction of an event per million @xmath1  pairs we are sensitive to branching fractions as low as a few times @xmath25 , in some cases .
predictions for the dominant  and  amplitudes translate into branching fractions at a level of one to few times @xmath26 .
we are therefore in a position to provide first experimental tests of theoretical predictions for absolute  @xcite , as well as ratios of branching fractions  @xcite .
many authors have proposed to use charmless hadronic @xmath5  decays to probe the ckm sector of the standard model .
for a recent review of this topic see for example ref .
. an experimental test of the hierarchy of decay amplitudes presented above is crucial to assess the experimental and theoretical feasibility of probing the standard model in this manner .
the cornell electron storage ring ( cesr ) is a symmetric @xmath27  collider operating at a center of mass energy near the @xmath28  resonances .
the hadronic cross section for continuum production of u , d , s , or c quark anti - quark pairs is about a factor of 3 higher than that for @xmath29 .
this continuum production is the dominant background to the @xmath5  decays of interest here . the cleo ii detector  @xcite boasts excellent charged and neutral particle detection .
the momenta of charged particles is measured in a tracking system consisting of a 6-layer straw tube chamber , a 10-layer precision drift chamber , and a 51-layer main drift chamber , all operating inside a 1.5 t superconducting solenoid .
the main drift chamber also provides a measurement of de / dx used for particle identification .
photons are detected using 7800 csi crystals , which are also inside the solenoid .
the return yoke is instrumented at various depths with proportional counters to identify muons .
cleo ii has accumulated a grand total of @xmath0  million @xmath1  pairs .
table  [ tab : results ] lists the measured branching fractions and upper limits for charmless hadronic @xmath5  decays to @xmath30  as well as final states containing an @xmath31  or @xmath32 .
this is an update to a previously published analysis  @xcite .
the main difference being a @xmath33  increase in data , and loosening of continuum background suppression cuts to increase the efficiency by @xmath34 .
furthermore , we have extended the analysis to look for modes containing @xmath31  or @xmath32 .    0.4 cm
. summary of preliminary cleo results for @xmath35 .
the results for @xmath36  and @xmath37  are cleo(1995 ) results ( prd 53 , 1039 ( 1996 ) ) .
an update for these final states is still in progress .
[ cols="<,<,<,<,<,<",options="header " , ]     in this paper we provide only a brief description of the analysis .
further details can be found in refs .
two kinematic variables , @xmath38   are used to form a two dimensional signal plus sideband region . using the beam energy in @xmath39   improves the mass resolution by an order of magnitude , resulting in @xmath40mev .    the main continuum background suppression is obtained by requiring @xmath41 .
the angle @xmath42  here is the angle between the candidate axis and the sphericity axis of the rest of the event .
candidate @xmath5  daughters from continuum background tend to be the leading particles in two back to back 5  gev `` jets '' .
background therefore peaks towards @xmath43 .
signal is flat in @xmath44  as the two @xmath5 s are approximately at rest in the labframe , leading to uncorrelated directions for the decay products .
this difference in `` event shape '' between @xmath1  signal and continuum background is exploited further using a fisher discriminant technique ( @xmath45 ) described in detail in ref .
the yield is determined using a maximum likelihood fit for the fraction of signal and background events out of the total number of events . as input to the fit @xmath46 , and
@xmath47   information are used .
the angle @xmath48  is the @xmath5 decay angle with respect to the z - axis in the labframe .
decays of @xmath32  are reconstructed in @xmath49 .
the search for @xmath50 includes @xmath51 as well as @xmath52 .
the @xmath53  mass is used as further input to the maximum likelihood fit where applicable . and @xmath54   is used as part of @xmath45 rather than in the fit in those cases .
mass distributions for @xmath5  decays to @xmath55  and @xmath56  are shown in figure  [ fig : mass ] .
additional cuts are applied to suppress backgrounds in these plots .
the curves are the pdf used in the fit normalized to the fit result times the efficiency of the additional cuts applied .
we have measured the branching fractions for exclusive @xmath5  decays to the final states @xmath58  and @xmath59 .
all three of these are @xmath60  transitions .
it is very instructive to compare the square root of the three measured branching fractions with each other , as well as the diagrams that are expected to contribute . for simplicity
we ignore diagrams that are expected to be suppressed by @xmath61 . for completeness
, we have also listed the upper limits in @xmath62  and the central value from the fit in @xmath63 .
@xmath64    the amplitude @xmath65  enters due to @xmath66  mixing .
it refers to the flavor singlet penguin diagram .
we have followed ref .
@xcite in our choice of mixing angle of @xmath67 . for this choice of @xmath68
there is no flavor octet contribution  in @xmath69 .
varying this angle within its known range  @xcite makes no difference to the general arguments presented here .
the branching fraction in @xmath70  sets the scale by providing a direct measurement of the  amplitude .
measured branching fractions and upper limits for all other @xmath57   transitions are consistent with being dominated by the measured  amplitude .
in particular , we see no need to invoke a @xmath71 or glueball component , nor anomalous coupling of two gluons to @xmath32 in order to explain the relative size of these branching fractions  @xcite .
theoretical predictions of absolute branching fractions have large uncertainties due to factorization hypothesis and poorly known form factors . keeping that in mind , theoretical predictions   @xcite  of @xmath72   agree surprisingly well with our experimental result .
let us look at eq .
( 2 ) in some more detail . to @xmath73 ,
the only non - trivial weak phases in the ckm - matrix are those of @xmath74  and @xmath75 .
the relative weak phase between  and  for @xmath57  transitions is thus the phase of @xmath74 .
the ratio of @xmath76   may therefore provide constraints on the poorly known phase of @xmath74  as was pointed out in ref .
@xcite .
the ratio of flavor singlet to flavor octet gluonic penguin diagrams @xmath77  is rather difficult to estimate theoretically . neglecting  and
, we find that our current upper limit on @xmath78  is consistent with the naive expectation of @xmath79 .
this may provide a more stringent limit on @xmath80  as we increase our data set . similarly , a significant discrepancy between the ratio of branching fractions for @xmath81  and @xmath82  may in the future provide a lower limit on @xmath83@xmath83 .
construction of an amplitude quadrangle for these modes may in certain cases even provide information on the relative phases of these amplitudes .
while we do see some excess of events above background in @xmath85 and @xmath86 , the respective statistical significance of @xmath87  and @xmath88  is quite marginal .
both of these decay modes are expected to be dominated by simple external w - emission ( ) diagrams .
factorization may therefore be less questionable here than in the @xmath60  transitions discussed above .    using the cleo measurement @xmath89  @xcite   we can use the factorization hypothesis , isgw ii , and the qcd factor @xmath90  @xcite  to predict the branching fractions @xmath91  and @xmath92   respectively  @xcite .
uncertainties in the formfactor and factorization hypothesis are not reflected in the errors quoted here .
furthermore , contributions from anything other than the diagram are neglected in this kind of comparison . keeping this in mind ,
we conclude that the central value from the fit to the experimental data as shown in table  [ tab : results ] compares well with these predictions .
we do not see any evidence for @xmath93  or @xmath94 .
the dominant contributions to these decays are due to ( @xmath95 ) , and  respectively .
the penguin diagrams in both cases are @xmath19  penguins .
theoretical predictions for these modes range from less than @xmath96  to few times @xmath25@xcite .
finally , we see no evidence for @xmath97 .
this is not surprising as this decay can only proceed via  or  diagrams .
theoretical predictions for this process are at the level of at most a few times @xmath98@xcite .
we can therefore conclude that an overall consistent picture of charmless hadronic @xmath5  decays to two pseudoscalar mesons is starting to emerge .
cleo has measured the dominant @xmath99  transitions at levels consistent with theoretical predictions .
no signals are found in any of the decay modes that are expected to be suppressed .
many thanks to all the other people involved in the search for @xmath5  decays to two charmless pseudoscalar mesons at cleo : peter gaidarev , jim alexander , peter kim , andrei gritsan , jean roy , and jim smith .
further thanks go to lawrence gibbons for information concerning the factorization test , and to robert fleischer , karl berkelman and jim smith for many useful discussions .
99 m. gronau , o.f .
hernndez , d. london , and j.l .
rosner , ; m. gronau , o.f .
hernndez , d. london , and j.l .
y.kubota a.deandrea _ et al .
_ , , ; l .- l.chau , , n.g.deshpande and j.trampetic ; m.bauer , b.stech , m.wirbel , .
d. zeppenfeld , ; m.j.savage , m.b.wise , , erratum : ; m.gronau and d.london , ; m. gronau , o.f .
hernndez , d. london , and j.l .
rosner , ; r.fleischer , _
int.j.mod.phys._*a12 * , 2459 ( 1997 ) .
f.wrthwein , ph.d . , cornell jan .
1995 ; p.gaidarev , ph.d . , cornell 1997 .
dighe , m. gronau , and j.l .
rosner , ; erratum : * b377 * 325 ( 1996 ) .
f.j . gilman , r. kaufman , ; * 37 * 3348(e ) ( 1988 ) .
i.halperin , a.zhitnitsky , hep - ph/9704412 ; d.atwood , a.soni , hep - ph/9704357 .
r.fleischer , t.mannel , hep - ph/9704423 .
a.j.buras , . using the measured value
@xmath100  ( t.e.browder hep - ph/9611373 ) makes no difference .
l.gibbons , private communication .
m.ciuchini , e.franco , g.martinelli , l.silvestrini , hep - ph/9703353 .
d.du , l.guo , d.zhang predict ( hep - ph/9706214 ) @xmath101 .
multiplying this by @xmath102  leads to @xmath103 .
using @xmath0  million @xmath1  pairs accumulated with the cleo detector we have measured @xmath2 , @xmath3 , and @xmath4 . 
 these constitute the first observations of exclusive @xmath5  decays to charmless hadronic final states . 
 furthermore , a measurement of @xmath6 , as well as upper limits on various other @xmath5  decays to two charmless pseudoscalar mesons are presented . in particular , an upper limit of @xmath7 @ @xmath8c.l . 
 is placed . 
 all of these results are still preliminary , and averaging over charge conjugate modes is always implied .    17.0 cm 25.0 cm -0.25 cm -2.5 cm    # 1#2#3#4#1 * # 2 * 
 , # 3 ( # 4 )    calt-68 - 2121 +    * b decays to two charmless pseudoscalar mesons at cleo *    frank wrthwein    _ caltech _    
 _ m / c 256 - 48 _    _ pasadena , ca 91125 , usa _    _ 
 email : fkw@cithe502.cithep.caltech.edu_