Z->bb Studies / Overview

Objective:
Find a good way of determining the background to Z->bb.

Must be determined from data!

Two weeks ago I showed a new method,
and tested it on:

A small Pass2 di-jet data set

The Pass1 multi-jet data set

This week:

Full Pass2 di-jet data set...

Comparison of two d(phi) cuts...

Double-tagged
data

We're trying to pull a signal
out of a HUGE background
with low systematics... !!!

background?

Z->bb MC
Normalized to
cross-section * lum

A Heavy 2-jet Dataset

I'm playing around with the following dataset:

“BID” skim – PASS2

Loose mu, pT > 4 GeV

Matched to JCCB jet, dR < .7

JES 5.3 (results shown include mu corrections)

90M events -> higgs_multijet rootuples

~350/pb ?

Invariant mass spectrum looks pretty smooth

Many triggers are overlapped

MC: Pass2 -> ~200k Z->bb, ~200k bb

I wasn't able to update all the plots in this talk with the full dataset in time...

Most plots here are for ~100/pb of data

A TRF-based Method

Select a di-jet data sample with a leading b-tag and d(phi)>2.9

Calculate the TRF :
prob. to tag the second jet,
as a func. of pT, 3 eta regions

Apply the TRF back to the same single-tag sample

Correct TRF for Z in single-tag sample

Compare to d(phi)<2.9 sample

|eta|<1.1

|eta|<1.5,>1.1

|eta|<2.5,>1.5

1

2

and then make these corrections
(discussed later...)

TRF Method (cont.)

What were the assumptions?

There's no Z->bb in the single-tag sample

Wrong, but we can correct for this !

Inv. mass spectrum of the cc/bb background is the same as that of the light QCD background

This depletion indicates a shift?

We can correct for this by comparing to the d(phi)<2.9 data sample

Data
Background

Difference

Gaussian Fit

3

Z TRF Correction

Initial Z peak measured in data

Get ratio of Z peak size in single/double-tag Z->bb MC (~6.5)

Scale initial Z peak back to the amount of Z->bb expected in the single-tagged data

Re-calculate the TRF after subtracting expected single-tagged Z peak from the single-tagged data

Re-apply the new TRF to the single-tagged data

Fraction of Z->bb
in single-tagged

data sample

Z->bb MC *
50% trig. eff.

4

Comparison to d(phi)<2.9 data

There is less Z->bb (lower S/B) in
d(phi)<2.9 data

Z->bb peak should be smaller

Background effects, like from differences in the bb/cc vs. QCD inv. mass spectrum, should not be reduced (maybe even increased)

We can use the difference in shape between the +- d(phi)=2.9 cut to correct for background biases...

This is the difference between the two d(phi) cuts

5

Full Pass2...

I have rootuples for all of Pass2 but the analysis is not completely done for them

Have to re-derive a TRF's, corrections, etc.

Needs to be more automated!

Here's the result for d(phi)>2.9

I've also lowered the pT cut from 25 to 20 GeV

I've not yet compared to d(phi)<2.9 and made the necessary corrections!

Should be compared to step 4...

Summary

Full Pass2 data set has been analyzed

Z peak in d(phi)>2.9 is larger than in d(phi)<2.9

It needs some more work...

What are the systematic errors involved?

How dependent are the results on the exact TRF method used?

Bin sizes, number of eta regions, etc. ???

What's the trig. eff. for Z->bb, averaged over all Pass2 data?

Can we use the Andre trick ? :

Take the amount of bb in inv. mass region of Z collected on a known trigger with known lum. and known Z->bb eff.

Find the amount of expected Z->bb collected on that trigger

Ratio of Z->bb to bb should stay the same for all triggers

This method could be an important cross-check of results obtained with the CDF-method

Andy Haas

2004-07-07T15:59:13