B-tag

[teddy]

I've some question about the b-tagging. I find that in Jet finder there is a parameter ParameterR (call it Rjet ), also in b-tagging there is another parameter delta R call it dRbjet. As far as I know, Rjet control the Cone size of the reconstructed jet, and the dRbjet define the size of cone in which around the jet we search the b-related generated particle to determine whether the jet is a b-jet candidate.

The question is that what will happen if I set dRbjet > Rjet ? Will one jet which is not originate from b-quark but is very close to a b-jet be tagged as a b-jet? (So is it required that we should set dRbjet <= Rjet ?)

Thanks very much

[Michele Selvaggi]

Hi,

I have never checked this quantitatively, but your statements seems correct. It is safe to take dRbjet <= Rjet, otherwise you could increase the probability to artificially tag jets as b in situations when they are not coming from b. Now how big is the effect I don't know…

Cheers,
Michele

[teddy]

Replying to mselvaggi:

Hi,

I have never checked this quantitatively, but your statements seems correct. It is safe to take dRbjet <= Rjet, otherwise you could increase the probability to artificially tag jets as b in situations when they are not coming from b. Now how big is the effect I don't know…

Cheers,
Michele

Hi:

Thanks for your reply!

I still have some problem in understanding the b-tagging procedure.

Assume that there is one gluon emitting from a b-quark, and in some case we could assume that in the final results, there are two jets one is originated from the b-quark and one is from the gluon emitting from b-quark, and they are a little close to each other , assume dRjj = 0.6, (using ParameterR = 0.5 in Jet Reconstruction) and the b-quark is in the middle of these two jet such that dRj1b<0.5 and dRj2b<0.5. (also using dR=0.5 in b-tagging) So when these two jets are passed to b-bag module, are they both considered as b-jet candidates? And Further according some random number to determine whether they are tagged as b-jet?

Thanks
best!

teddy

[Michele Selvaggi]

Hi Teddy,

sorry for the late reply. Although in principle your concern is valid, in practice if you use anti-kT it should never happen that dRjj=0.6.
This is a feature of anti-kT jets that they never overlap, thus dRjj > 1.0 by construction. Therefore if you choose dRbjet < parameterR the case you describe should never happen.

Cheers,
Michele

[teddy]

Replying to mselvaggi:

Hi Teddy,

sorry for the late reply. Although in principle your concern is valid, in practice if you use anti-kT it should never happen that dRjj=0.6.
This is a feature of anti-kT jets that they never overlap, thus dRjj > 1.0 by construction. Therefore if you choose dRbjet < parameterR the case you describe should never happen.

Cheers,
Michele

Hi Michele

Thank you so much! It seems that I should look deeply into the jet algorithm and understand it better! Thanks!

Best!

Teddy