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

[ChrisVer]

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?)

 

[Vanadium 50]

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]

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 , you'll see multiple measurements after 2006 going into this new, more precise average.

 

[ChrisVer]

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)

 

[ChrisVer]

Also checking here:
http://www-donut.fnal.gov/internal/theses/Emily_thesis.pdf
Table 6.6 , page 158 of the text, which gives the tau to 0.0007 (not 5.5% or 0.055 as in the pdg)

 

[mfb]

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 ($D^\pm$).

 

[ChrisVer]

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

 

[mfb]

7.4-5.6 = 1.8 deviation
1.8/1.3=1.38 or about 1.4.

A better estimate would take the uncertainty on the more precise value into account. The uncertainties could have some positive correlation, however (from systematic effects), so not taking it into account it is a conservative estimate.