p14 Z->bb for ICHEP

Method reminder

Signal expectation

Cross-checks

Double-tagged data

We're trying to pull a signal
out of a HUGE background !!!

Background?

Z->bb MC

Andy Haas (Columbia U. / Nevis Labs)

Per Jonsson (Imperial, UK)

The 0->1 shift

Derive a TRF from the untagged data

Apply back to the untagged data

You don't get back the single-tagged inv. mass spectrum !!!

There is a shift in inv. mass, coming from correlations between the jet kinematics and b-tagging rates within events

difference between b- / light- jet energy scale?

difference between bbbar / light-qqbar cross-section vs. inv. mass?

???

The shift is not the result of Z->bb, to first order

Measure this shift, and call it the “0->1 shift”

The effect of Z->bb is accounted for, and we call it the “corrected 0->1 shift”

(details on later slide...)

The 1->2 inv. mass difference

Derive a TRF from the single-tagged data

Apply back to the single-tagged data

You don't get back the double-tagged inv. mass spectrum !!!

There is a shift in inv. mass, coming from correlations between the jet kinematics and b-tagging rates within events

This shift is assumed to be the result of the same effects which caused the 0->1 shift

After removing the 0->1 shift, there is also evidence of a remaining peak, assumed to be from Z->bb

The effect of Z->bb is accounted for, and we call it the “corrected diff”

(details on next slide...)

Correcting for Z->bb in 0, 1 – tag samples

An initial measurement of the Z->bb peak is made

The fraction of Z->bb in the untagged and single-tagged samples is estimated from the fraction measured in the double-tagged sample, using Z->bb MC

The MC is only used for estimating the b-tagging rates! It is not used for measuring the shape of the signal at all!!!

This Z->bb fraction is then removed from each event used to derive the TRFs in the untagged and single-tagged samples

Each event is weighted when deriving a TRF by 1-f, where “f” is the fraction of Z->bb events in that sample for that inv. mass bin

Since the corrections are made to the TRFs, they correct the 0->1 shift and also the expected background in the double-tagged sample

The correction affects slightly the observed size and shape of the excess in the double-tagged sample

Therefore, this new excess is then used to re-derive the correction, which changes slightly the size and shape of the expected excess again, so this new excess is used to derive the correction...

After 3 iterations, the size and shape of the excess is stable

Signal Observed vs. Expectation

810+-230 events observed

What do we expect from Z->bb in the SM ?

Selection efficiency is: 732/244250 = 0.3%

Expected events is: 0.3% * 1180 *300 = 1061

However, this does not account for trigger efficiency

We are not selecting by trigger, so are using a giant OR of terms

Main term is muon+jet triggers, like MU_JT20_L2MO: MUW scint + CJT(1,3)

MUW scint term is 82% efficient per loose offline muon, averaged over phi (large degredation at bottom hole), as determined from J/Psi in p14 muon certification note

L2/L3 terms are >95% efficient after offline selections

Final trigger efficiency is estimated to be:
80+-20%

So, we finally get: 1061 *0.80 = 850 events

L2MU:
97% efficient

Cross-checks

A variety of checks on the signal peak have been performed

Removing the muon-JES corrections

peak gets wider

shifts down in mass

MC does not shift much because it is not selected for offline muons (to increase statistics)

We've tested for the influence of W->cs in the untagged and single-tagged samples

We've reversed the dphi cut (to <2.9)

peak mostly goes away, consistent with MC

Cross-checks

We've just selected either central (|eta|<1) or forward (|eta|>1) jet events

results are consistent with MC

We've widened the dphi cut to >2.5

gives an even nicer result

consistent with MC

Andy Haas

2004-07-07T15:59:13