Hadronic branching fraction measurements are approaching O(1%) precision levels, at which proper treatment of Final State (electromagnetic) Radiation (FSR) plays a significant role. We present a simultaneous determination of averages for the D0 branching fractions to the final states K−π+, π+π− and K+ K− with a uniform treatment FSR in these modes. In particular, the measurements have all been corrected, if necessary, to correspond to h+ h− (nγ), with a uniform modeling of FSR.
Many (but not all) of the measurements in use apply the PHOTOS package to estimate the FSR contributions. However, PHOTOS itself has evolved over the past 15 years, and an experiment's configuration of PHOTOS within their Monte Carlo suite affects the reliability of the PHOTOS output, particularly whether interference effects are included when the final state includes multiple charged particles. In some cases, interference in FSR has been turned off because of problems in J/psi decays with a low value of the PHOTOS parameter XPHCUT.
A crude history of the PHOTOS 2.XX releases used in the current measurements, along with the status of interference and correction of the measurements is
The following photon energy and hadronic invariant mass spectra are shown for the different PHOTOS versions and configurations in use in many of the current measurements. For 2.00, you can see the interference term present in π+π−, but not in K−π+For the two body mass plots shown, a typical experimental smearing of 10 MeV has been applied to illustrate the size of the FSR tail relative to experimental resolution. The arrow at 30 MeV in the photon energy spectrum indicates where the spectrum becomes important relative to experimental resolution (i.e., very quickly!).
The FSR photons energy spectrum extends to very sizeable energies, causing a very long low side mass tail on the hadronic invariant mass. Without accounting for this effect in experimental efficiency determinations, the K−π+ branching fraction would be biased low in the range of 2-3% for typical selection criteria. Absolute π+π− and K+K− rates would be affected at levels of approximately 4-6% and 1-2%, respectively. The most precise determinations of the branching fractions for these modes are via ratios to K−π+, for which there is some cancellation of the effect.
To obtain new average branching fractions for these three modes, we have performed a simultaneous average of the branching fraction measurements for B(D0 → K−π+) and the branching ratio measurements B(D0 → π+π−) / B(D0 → K−π+) and B(D0 → K+K−) / B(D0 → K−π+). All measurements have been corrected to correspond to the use of PHOTOS 2.15 with interfence effects included. Typically, experiments have either reported the change in branching fraction with and without incorporation of FSR, or have made explicit requirements on kinematic variables like the K−π invariant mass. In these cases, any needed correction is straightforward to evaluate. The correction is very difficult to evaluate for two measurements (from E791 and CLEO II) in which signal yields were obtained from fits to kinematic variables, but no information regarding is provided regarding the effects of FSR. These measurements are older measurements with uncertainties significantly larger than recent measurements, so have been excluded from the averages presented here. FOCUS measurements of the B(D0 → π+π−) / B(D0 → K−π+) and B(D0 → K+K−) / B(D0 → K−π+) branching ratios do contribute significantly to current world averages, so using information provided by the authors, we have implemented a toy Monte Carlo procedure to evaluate the necessary corrections.
In the averaging procedure, we have assumed that the dominant uncertainty in the FSR corrections come from the fact that the mesons are treated like structureless particles, so no contribution from structure-dependent terms in the decay process (eg. radiation off individual quarks). Internal studies done by various experiments have indicated that in Kπ decay, the PHOTOS corrections agree with data at the 20-30\% level. We therefore attribute a 25% uncertainty to the FSR prediction from potential structure-dependent contributions. For the other two modes, the only difference in structure is the final state valence quark content. While radiative corrections typically come in with a 1/M dependence, one would expect the additional contribution from the structure terms to come in on time scales shorter than the hadronization time scale. In this case, you might expect LambdaQCD to be the relevant scale, rather than the quark masses, and therefore that the amplitude is the same for the three modes. In treating the correlations among the measurements this is what we assume. We also assume that PHOTOS and structure amplitudes are relatively real with constructive interference. The uncertainties largely cancel in the branching fraction ratios, and on the final average branching fractions, the FSR uncertainty on Kπ dominates. Note that because of the relative sizes of FSR in the different modes, the ππ/Kπ branching ratio uncertainty is positively correlated with the Kπ branching fraction uncertanty, while the KK/Kπ branching ratio uncertainty is negatively correlated.
We include systematic correlations in the CLEO II measurements, and for the ALEPH measurements we take the systematic uncertainties in the D* direction relative to the jet axis as correlated. The CLEO-c K−π+ and K+K− measurements are correlated via the number of neutral DD pairs in the CLEO-c ψ(3770) sample, which is determined in analysis yielding the K−π+ branching fraction, and used to normalize the absolute yield in the K+K− analysis. The correlation coefficient is 0.32 for the combined statistical and systematic uncertainties, and 0.34 for the statistical-only uncertainties.
The fit for the three branching fractions has a final χ2 of 8.5 for 13 - 3 degrees of freedom. The statistical and systematic covariance matrices (including the correlations noted above) have been summed to obtain the full covariance matrix used in this fit. If we evaluate the χ2 using the covariance matrices corresponding to just the B(D0 → K−π+) measurements, to just the B(D0 → π+π−) / B(D0 → K−π+), and to just the B(D0 → K+K−) / B(D0 → K−π+), we obtain 4.23, 0.15 and 0.79, respectively.
The table below summarizes the branching fraction averages (in percent). The errors listed are statistical, systematic excluding FSR, and the FSR-related uncertainty. Note that the FSR uncertainty is comparable in magnitude to the statistical uncertainty on the average.
|B(D0 → K−π+)||(3.949 ± 0.023 ± 0.040 ± 0.025)%|
|B(D0 → π+π−)||(0.1425 ± 0.0019 ± 0.0018 ± 0.0014)%|
|B(D0 → K+K−)||(0.3941 ± 0.0038 ± 0.0050 ± 0.0024)%|
|B(D0 → K−π+)||1.00||0.72||0.74|
|B(D0 → π+π−)||0.72||1.00||0.53|
|B(D0 → K+K−)||0.74||0.53||1.00|
The following figure shows the comparison of the Kπ measurements to the average. For each measurement and for the average branching fraction, the statistical uncertainties, the combined statistical and systematic uncertainties excluding FSR uncertainties, and the total uncertainties are separately indicated.
|Experiment||BF (rescaled) [%]||correction [%]||PHOTOS version/Interf.||Reference|
|CLEO-c||3.891 ± 0.035 ± 0.065(0.027)||--||2.15/Yes||PRD 76, 112001,2007 [Spires]|
|BaBar||4.035 ± 0.037 ± 0.074(0.024)||0.69%||2.02/No||PRL 100, 051802, 2008 [Spires]|
|CLEO II||3.920 ± 0.154 ± 0.168(0.032)||2.80%||none||PRL 80, 3193, 1998 [Spires]|
|ALEPH||3.930 ± 0.091 ± 0.125(0.032)||0.79%||2.0/No||Phys.Lett.B403:367-376,1997 [Spires]|
|ARGUS||3.490 ± 0.123 ± 0.288(0.024)||2.33%||none||Phys Lett B340, 125, 1994 [Spires]|
|CLEO II||3.960 ± 0.080 ± 0.171(0.015)||0.38%||2.0/No||PRL 71, 3070, 1993 [Spires]|
|ALEPH||3.730 ± 0.351 ± 0.455(0.034)||3.12%||none||Phys Lett, B266, 218, 1991 [Spires]|
|Experiment||ππ/Kπ (rescaled)||correction [%]||PHOTOS version/Interf.||Reference|
|CLEO-c||0.0363 ± 0.0010 ± 0.0008(0.0001)||0.25%||2.02/No||PRL 96, 081802, 2006 [Spires]|
|CDF||0.03594 ± 0.00054 ± 0.00043(0.00015)||--||2.15/Yes||PRL 94, 122001,2005 [Spires]|
|FOCUS||0.0364 ± 0.0012 ± 0.0006(0.0002)||3.10%||none||PL B555, 167, 2003 [Spires]|
|Experiment||KK/Kπ (rescaled)||correction [%]||PHOTOS version/Interf.||Reference|
|CDF||0.0992 ± 0.0011 ± 0.0012(0.0001)||--||2.15/Yes||PRL 94, 122001,2005 [Spires]|
|FOCUS||0.0982 ± 0.0014 ± 0.0014(0.0001)||-1.12%||none||PL B555, 167, 2003 [Spires]|
|Experiment||BF (rescaled) [%]||correction [%]||PHOTOS version/Interf.||Reference|
|CLEO-c||0.411 ± 0.008 ± 0.009||0.64%||2.02/No||PRD 77, 091106, 2008 [Spires]|