rpp2010-rev-tau-branching-fractions

rpp2010-rev-tau-branching-fractions - 1 BRANCHING FRACTIONS...

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–1– τ BRANCHING FRACTIONS Revised April 2010 by K.G. Hayes (Hillsdale College). Since the previous edition of this Review , there have been 11 published papers that have contributed to the τ Listings, including 5 each from the BaBar and Belle collaborations. Six of these papers have provided new upper limits on the branching fractions for neutrinoless τ -decay modes. Of the 59 neutrinoless τ -decay modes in the τ Listings, 2 are new and 24 have had improved limits set. The upper limits have been reduced by factors that range between 1.2 and 8.8 with the average reduction being a factor of 2.7. There are now 22 measurements and 12 upper limits from Belle and BaBar on branching fractions of conventional τ -decay modes, up from 1 measurement and 3 upper limits in the 2006 edition of this Review. Nine of these measurements are used in the constrained Ft to τ branching fractions, and 16 are for τ -decay modes for which older non- B -factory measure- ments exist. ±or those 16 measurements, the new B -factory measurements have on average about forty times the number of events as the most precise earlier measurements, and the statistical uncertainties on the B -factory measurements are on average about eight times smaller. However, the systematic un- certainties now greatly exceed the statistical uncertainties of all B -factory branching fraction measurements of major τ -decay modes. ±or example, the average ratio of systematic to statisti- cal uncertainty of the B -factory measurements of τ branching fractions larger than 10 3 is 10.4, while the average ratio for branching fractions smaller than 10 4 is 0.8. Thus, the total uncertainty on the branching fraction measurements from B - factories is on average only about 3.6 times smaller than the previous most precise non- B -factory measurements. Comparing the 16 B -factory τ branching fraction measure- ments to the earlier non- B -factory measurements reveals inter- esting systematic discrepancies between the two sets of measure- ments. ±igure 1 shows a histogram of the normalized di²erence (( B -factory value minus non- B -factory value)/estimated uncer- tainty in the di²erence) for the 16 measurements. The value used for the non- B -factory measurement is the value listed in CITATION: K. Nakamura et al. (Particle Data Group), JPG 37 , 075021 (2010) (URL: http://pdg.lbl.gov) July 30, 2010 14:34
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–2– the latest edition of this Review prior to the Frst B -factory measurement for that decay mode. ±ifteen of the 16 B -factory branching fraction measurements are smaller than the non- B -factory values. The average normalized di²erence between the two sets of measurements is -1.36 (-1.68 for the 8 Belle measurements and -1.05 for the 8 BaBar measurements). –3 –1 –2 0 +1 0 2 4 6 8 BaBar Belle Number per 0.5 Sigma difference +2 +3 Figure 1: Distribution of the normalized dif- ference between the 16 B -factory measurements of conventional τ -decay branching fractions and non- B -factory measurements. The Belle and BaBar collaborations have each published 8 measure- ments.
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This note was uploaded on 06/07/2011 for the course PHYS 4132 taught by Professor Kutter during the Spring '11 term at University of Florida.

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rpp2010-rev-tau-branching-fractions - 1 BRANCHING FRACTIONS...

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