Measurements of |Vus| and Searches for Violation of Lepton Universality and CPT in Tau Decays at BaBar Page: 4 of 6
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Measurements of IVusl and Searches for Violation of Lepton Universality and CPT in Tau Decays at BABAR
method illustrative formula Ath Ref.
K 3 'F(K 3) = Vus f+(0)]2 x (well known constants) 0.58% 
K 2 (K/ir) K2= x (well known constants) 0.50% 
F(ir2) Vu f e
tau K/r '(> Kv) _ f2 x (well known constants) 0.50% 
F(r -> irv) V Kwell
G fK Vs2mr m3 2
S-> Kv exclusive (t -> K-vz) = Gf16 mzkt 1 z ( 4 Sj 5EW 1.27% 
2 -> XS v inclusive - RV+A gRzSU3 breaking 0.23%-0.47% [9, 10]
unitarity Vus2 = -Vubl2
Table 1: Combinations of experimental measurements to compute Vns : Ath identifies the theo-
retical uncertainties due to QCD as determined by the reference(s) on the right. Rtis Fr ->
strange hadronic final states)/F(r -> evv), RZV+A F(r - non-strange hadronic final states)/F(r ->
fit. Since there is a large discrepancy between the measurements of 2 -> K-K+K-v by BABAR 
and Belle , a PDG-style error scale factor (5.44) has been used.
We define the branching fraction of the tau to a strange hadronic final state as described in
Table 2 (which is slightly different from the not-yet-finalized HFAG-tau definition ).
In the literature, e.g. Refs. [9, 10], Res is obtained by summing all strange decay modes,
while R ,v+A is obtained by difference using unitarity for sum of the tau branching fractions, i.e.
R ,v+A = R ,h - Res with R ,h = (1 - Be - B ) /Be, where Be, B are the leptonic tau branching frac-
tions computed assuming SM lepton universality and using also the tau lifetime world average .
This method implicitly assigns all possibly not-yet-measured or neglected hadronic tau branching
fractions to R ,v+A. To prevent that, we rather determine R ,v+A by summing up all non-strange
modes. We obtain Vus = 0.2166 0.0023, which is 3.576 away from Vus = 0.2255 0.0010
obtained from CKM unitarity using Ref. . We still use the improved Be assuming SM lep-
ton universality to compute R ,v+A and Res. Figure 1 reports comparison with other Vus de-
terminations. When using the literature definition for R ,v+A, there are no significant changes:
Vus = 0.2163 0.0023, 3.666 away from unitarity. By inflating measurement errors in the
global fit with a procedure similar to the one used by PDG for error scale factors, we obtain
Vus = 0.2166 0.0025, corresponding to a slightly reduced discrepancy of 3.296.
5. Test of CPT conservation from tau mass measurements
The BABAR collaboration has recently published a tau mass measurement  obtained by fit-
ting the end-point of the reconstructed minimum tau mass for 2 -> 3wv candidates. The measure-
ment is robust against background contamination, is statistically precise but is ultimately systemat-
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Lusiani, Alberto & /Pisa, Scuola Normale Superiore /INFN, Pisa. Measurements of |Vus| and Searches for Violation of Lepton Universality and CPT in Tau Decays at BaBar, article, June 12, 2013; United States. (digital.library.unt.edu/ark:/67531/metadc845185/m1/4/: accessed April 24, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.