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:
the cross section for the interaction of two quasi - real photons is described by three components .
the first `` soft '' component is described by the vector dominance model ( vdm ) , parametrized in hadronic phenomenology by regge poles .
a second `` direct '' component , the point like reaction @xmath4 , can be calculated in qed .
finally , there is a  hard component " ( resolved or anomalous qcd component ) which requires knowledge of the quark and gluon parton density functions of the photon .
the direct and hard contributions to two - photon interactions can be measured in open heavy flavour production . in this case
a hard physical scale is given by the c- or b - quark mass . at lep energies , the direct and single resolved processes , shown in figure  [ fig : feynman ] , are predicted to give comparable contributions to the charm production cross section @xcite , whereas at low energies the direct process dominates .
the main contribution to the resolved photon cross section is the photon - gluon fusion process @xmath5 .
contributions to charm production arising from vdm processes and from doubly resolved processes are expected to be small@xcite .
this letter presents the measurement of the @xmath6 cross section as a function of the two - photon centre - of - mass energy @xmath7 in the interval 5  gev  @xmath8  70  gev .
the data correspond to a total integrated luminosity @xmath9 410 pb@xmath2 , collected with the l3 detector  @xcite at centre - of - mass energies @xmath10  gev .
the inclusive charm production cross section @xmath11 ) was measured by l3 at @xmath12  gev  @xcite .
charm quarks are identified by their decay into electrons  @xcite .
the pythia @xcite monte carlo is used to model the processes .
quarks other than b are taken as massless in the corresponding matrix elements  @xcite .
the resolved process uses the sas1d photon structure function @xcite .
the two - photon luminosity function is implemented in the equivalent photon approximation ( epa )  @xcite with a cutoff @xmath13 , where @xmath14 is the mass of the @xmath15 meson .
the background sources are @xmath16 , @xmath17 , @xmath18 and @xmath19 .
these processes are generated by jamvg  @xcite , pythia , koralz  @xcite and koralw  @xcite respectively .
the detector simulation is performed using the geant @xcite and gheisha @xcite packages .
the monte carlo events are reconstructed in the same way as the data .
time dependent detector inefficiencies , as monitored during the data taking period , are also simulated .
the measurement of the @xmath20 cross section is performed on the high statistics electron sample of 2434 events discussed in reference @xcite .
the backgrounds from annihilation processes and two - photon production of tau pairs are estimated to be 0.75% and are subtracted from the data .
the background from @xmath21 events is modelled with the pythia monte carlo assuming equal contributions from direct and resolved processes .
it is also subtracted from the data assuming our measured cross section  @xcite . the charm purity after background subtraction is 75% .
the analysis is restricted to events with visible mass @xmath22 gev .
@xmath23 is calculated from the four - momentum vectors of the measured particles , tracks and calorimetric clusters including those from the small angle luminosity monitor .
the @xmath23 distribution is corrected for trigger efficiency using data with a set of independent triggers .
it varies from 94@xmath24 at @xmath25 gev to 98@xmath24 at @xmath26 gev .
table  [ tab : numbevents ] shows the selected data events in bins of and the different contributions of the signal and the background predicted by the pythia monte carlo .
a comparison of the visible mass distribution of the data after final selection with the expectations of the pythia monte carlo at @xmath27 gev is shown in figure  [ fig : wvis9899 ] .      the hadronic final state is not always fully contained in the detector acceptance .
an unfolding procedure is hence applied to obtain the @xmath0 centre - of - mass energy @xmath28 from @xmath23 .
figure  [ fig : wvisvswgg ] shows the correlation of the @xmath23 average value , @xmath29 , with @xmath30 as predicted by the monte carlo .
the unfolding corrects for both missing particles and detector resolution by the relation : @xmath31 where @xmath32 is the content of the @xmath33-th interval of @xmath28 listed in table  [ tab : cross ] , and @xmath34 is the content of the @xmath35-th interval of @xmath23 listed in table  [ tab : numbevents ] .
the matrix @xmath36 is constructed by considering for each monte carlo event its reconstructed and generated  @xcite : @xmath37 where @xmath38 is the likelihood of observing the measured value given a generated value of and @xmath39 is the generated distribution after the selection cuts .
the unfolding matrix is determined using charm events generated with pythia .
the unfolding uncertainty is estimated with pythia using five flavour events instead of charm events and using the phojet@xcite monte carlo generator with all quark flavours .
this comparison leads to an estimated uncertainty of 5% .
charm production in the phojet generator is implemented with a high @xmath40 threshold which prevents the use of phojet for charm efficiency studies .
after unfolding , the events are corrected for efficiency using the ratio between selected and generated charm events in each interval .
the events with 3  gev  @xmath41  5  gev and @xmath42  70  gev are used for unfolding , but are excluded from the measurement due to their large correction factors and unfolding uncertainty .
the cross section @xmath43 is measured from the number of events corrected for the efficiency with the pythia monte carlo in each @xmath44 interval and the integrated luminosity .    to extract the cross section @xmath45 of two real photons ,
the photon flux  @xcite is calculated and the hadronic two - photon process is extrapolated to @xmath46 , where @xmath47 is the virtuality of a photon .
this is done through the following relation : @xmath48 for each interval a numerical integration is performed over its width and the unmeasured @xmath49 of the scattered electrons .
table  [ tab : cross ] gives the efficiencies and the cross sections @xmath50 and @xmath45 as a function of @xmath28 .
the systematic uncertainties arise mainly from the charm purity estimate , followed by the unfolding procedure , the charm efficiency , the beauty cross section  @xcite , the photon flux and the trigger efficiency , all summarized in table  [ tab : systematics ] .
the photon flux uncertainty is estimated by the comparison of the two models  @xcite .
the uncertainty due to the other background processes is negligible . due to the unfolding
, the data points are correlated .
the correlation matrix is given in table  [ tab : correlation ] .
figure  [ fig : dsigdwgg ] shows the differential cross section @xmath51 as a function of @xmath7 measured in the interval 5  gev  @xmath8  70  gev .
the expected slope from the pythia monte carlo is steeper than that of the data .
figure  [ fig : sig_ggcc_qcd ] compares the measured cross section @xmath52 as a function of @xmath44 with nlo qcd calculations  @xcite .
the calculations use massive quarks in the matrix elements .
the charm mass , @xmath53 , is fixed to 1.2 gev , the renormalization and factorization scales are set to @xmath53 and @xmath54 , respectively , the qcd parameter @xmath55 is set at 227.5 mev , and the grs - ho  @xcite photon parton density function is used . using this set of input parameters ,
the nlo qcd predictions reproduce well the energy dependence and the normalization .
the calculation with @xmath56 gev results in about 50% lower cross section values , except the first point , where it is lower by 25% .
a change in the renormalization scale from @xmath53 to @xmath54 decreases the qcd prediction by 10% and 30% at low and high @xmath44 respectively .
we compare also the measured charm cross section with the total cross section of hadron production in two - photon collisions  @xcite , scaled by an arbitrary factor @xmath57 .
the slope of @xmath58 ) is clearly larger than that of @xmath59 _ hadrons _ ) .    a more quantitative comparison results from fits to the data .
a parametrisation  @xcite of the form @xmath60 , `` pomeron @xmath61 reggeon '' , with @xmath62 describes well the energy behaviour of all the total hadron - hadron cross sections with universal values @xmath63 and @xmath64@xcite .
a fit to our data with @xmath65 , @xmath66 and @xmath67 being free parameters and with fixed @xmath68 yields in the interval 5  gev
@xmath8  70  gev :
open charm production in @xmath0 collisions is studied with data collected at @xmath1 centre - of - mass energies from 189 gev to 202 gev corresponding to a total integrated luminosity of 410 pb@xmath2 . 
 the charm cross section @xmath3 ) is measured for the first time as a function of the two - photon centre - of - mass energy in the interval from 5  gev to 70  gev and is compared to nlo qcd calculations .