# $Ds^{\pm}$ to $\tau$ question on Branching Ratio

Gold Member

## Main Question or Discussion Point

Hi I have some question...I want to find what fraction of some produced $D_s^\pm$ to decay into $\tau$'s.
I was looking around the pdg and I came across something that confused me.
Here:
http://pdg.lbl.gov/2014/listings/rpp2014-list-Ds-plus-minus.pdf
on page 4 it lists the semileptonic decays, and an example for the ratio to muons it gives: $Br(D_s^\pm \rightarrow \mu^\pm \nu_\mu)=(5.56 \pm 0.25)~ \times 10^{-3}$

However in this:
http://pdg.lbl.gov/2006/reviews/dsdecaycons_s034310.pdf
Equation (2) gives a totally different Branching ratio...
$Br(D_s^\pm \rightarrow \mu^\pm \nu_\mu)=0.0074\pm 0.0013= (7.4 \pm 1.3) \times 10^{-3}$

Why is this happening? (or what am I reading wrong?)

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Well, the muonic branching fraction won't tell you the tau branching fraction, which is about 6%. But besides that, why shouldn't the number change and become more precise over the last 8 years?

mfb
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5.6 is compatible with 7.4 +- 1.3 at a level of ~1.4 standard deviations. Nothing unusual.
If you check the http://pdg8.lbl.gov/rpp2014v1/pdgLive/BranchingRatio.action?parCode=S034&desig=7 [Broken], you'll see multiple measurements after 2006 going into this new, more precise average.

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Gold Member
Well, the muonic branching fraction won't tell you the tau branching fraction, which is about 6%.
I would take the value from pdg for the branching ratio, however I came across this incompatible values and I became more aware of using those numbers...

But besides that, why shouldn't the number change and become more precise over the last 8 years?
I think because the values are not compatible (with errors)

Gold Member
mfb
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They are perfectly compatible, see the post above (we posted nearly at the same time).

Table 6.6 gives 6.4% for tau neutrinos, you are looking at the wrong decay ($D^\pm$).

Gold Member
Oh I am sorry, for my last post, it's 100% wrong.

However @mfb I would like to ask how you can see the 1.4 std in the above... thanks

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mfb
Mentor
7.4-5.6 = 1.8 deviation