[Rivet] Stable particle lifetimes

[Peter Skands]

Addendum:

In the ALEPH paper (which I believe *is* in Rivet), Barate et al., 
Physics Reports, Volume 294, Issues 1-3, February 1998, Pages 1-165,

it is mentioned on page 19, under Corrections for Detector Effects, 
"all particles having mean lifetimes less than 10^-9 s required to 
decay, and all other particles being treated as stable", which 
translates to c*tau >= 30 cm. This can therefore be added to any Rivet 
analyses that cites that ALEPH physics reports paper.

I don't think the difference between 10 and 30 cm is important. But the 
one between 10mm and 10cm *is*, since all the normally long-lived 
strange particles K0S, Lambda0, Sigma+, Sigma-, Xi0, Xi-, and Omega- lie 
in that window.

Cheers,
Peter

On 2/2/11 1:57 PM, Peter Skands wrote:
> Hi guys,
>
> While it seems relatively standard to use c*tau > 10mm at hadron
> colliders (as is also mentioned in the Rivet manual under the analyses),
> it was so far not completely clear to me what stable-particle definition
> to use for the LEP measurements, and I think you have been in doubt too
> - at least the Rivet manual does not give any explicit recipe.
>
> As you know, this can impact multiplicity distributions and also
> momentum fraction distributions (an undecayed particle with high x vs
> two or more lower-x daughters), which we rely on for tuning
> hadronization parameters.
>
> I just wanted to let you know that at least in one LEP paper I've been
> using (one not in Rivet so far), I now actually found the exact
> definition they use. Of course, that's only really for sure for *that*
> paper, but it's better than nothing, and the answer was a bit surprising
> to me.
>
> The paper is L3, http://arxiv.org/abs/hep-ex/0406049v1
>
> On p.26 where they discuss hadron multiplicities and x spectra, they say
>
> "In this correction procedure, we assume all particles with mean
> lifetime greater than 3.3 × 10−10 s to be stable."
>
> Now, if I am multiplying correctly by the speed of light, that
> translates to 10cm, not 10mm!
>
> I briefly spoke to Leif about this, and he figures that could be
> consistent with people in those days basically only setting the pi0
> stable (and K0Long, I guess). He also mentioned that, in principle,
> Rivet could be able to at least check the settings used for the run
> (e.g., by seeing if there are any stable K0S in the actual event
> records) and give an error message if it looks like the user put some
> particle stable that he wasn't supposed to?
>
> Anton and Stefan: this may mean that we want to be running the
> generators with a cut at 100mm instead of 10mm when we do LEP analyses.
> Rivet/Stefan, I'd also like us to look at possibly including this L3
> analysis in Rivet. It is extremely useful since it uses b-tagging to
> separate out light-flavor fragmentation from b-fragmentation, making it
> possible to tune those separately.
>
> As a side remark, the paper also mentions explicitly that all QED shower
> effects have been corrected for, both ISR and also FSR. That's something
> we should put in a comment in the manual if we include that analysis in
> Rivet.
>
> Anyway, just wanted to let you know, and ask if you have learned
> anything more about the definitions used for the analyses available in
> Rivet so far?
>
> For reference, I am attaching plots illustrating the x and Nch spectra
> for Pythia 8 with 10mm and 100mm, respectively. As you can see, there is
> a noticeable affect at high x in the fragmentation spectrum, and there
> is a HUGE effect on the Nch distribution.
>
> Cheers,
> Peter