Query regarding b-tagging ...

[Shankha Banerjee]

Dear Experts,

I have a naive query.

I am dealing with events where a Higgs decays to b-quarks. So all my events have two b-quarks at the parton level. I use Herwig++ for generating my MC events.

Now, I am using Delphes 3.3.2 which has a b-tagging efficiency given by this formula :

0.80*tanh(0.003*pt)*(30/(1+0.086*pt)),

which is roughly of the order of 70% for 50 GeV < pT_b < 250 GeV.

Now, I have roughly 7200 events in my sample, so should I expect to see :

~(1-0.7)^{2 ~ 650 0 b-tagged jets events
~ 0.7x(1-0.7)x2 ~ 3000 1 b-tagged jets events
~ 0.7*0.7 ~ 3500 2 b-tagged jets events ?}

However, I see ~ 1500 0 b-tagged jets events

~ 3500 1 b-tagged jet events
~ 2000 2 b-tagged jet events

Am I missing something here ? Is this expected ?

Are there some other factors playing a role here.

I can clearly see that the njet distribution (where its all jets),
peaks at 2.

It will be really helpful if you can help me with understanding this.

Thanks a lot.

Best regards,
Shankha.

[Michele Selvaggi]

Hi Shankha,

I did not check precisely the numbers, but you might get some additional inefficiency due to:

• jet reconstruction (you might loose some jets in the process although this should be very rare)
• sometimes Delphes does not find the b inside the jet.

What you observe as final efficiency is the (probability that the jet matches a b-parton)*(nominal b-tagging efficiency) .

Michele

[Shankha Banerjee]

Hi Michele,

Thanks a lot for your response.

For me, I get less number of 2 b-jets than what I numerically expect.

For 0 b-jets, I get more than twice. So, I can try to match it with
the prescription that you suggested.

You say that "sometimes Delphes does not find the b inside the jet."
Can I know this probability ? Can you please point me out to the part
of the .tcl card where this is written ?

Can you please look at the numbers I wrote in my first query ? Are these
reasonable ?

Sorry for this naive query again.

Best regards,

Shankha.

[Alexandre Mertens]

Hello,

It indeed happens that the jet direction is different from the b-quark direction, due to ISR emission for instance, the effect depends on the pt...

There is two extra reasons for the numbers that you observe: the Pt and eta cuts.
In particular, the pt of the b-jets is in general lower than 50 GeV, which mean lower acceptance and lower efficiency. So, the quoted numbers make sense to me.

I hope that this helps,

Cheers,
Alexandre