Z->bb Studies / Overview

Objective:
Find a good way of determining the background to Z->bb.
Must be determined from data!

Last week:

reviewed the TRF method

compared peak in d(phi)>2.9 to <2.9

tested it on some Pass2 data (100/pb)

showed a sneak peak of the full Pass2 data set (no pun intended!)

This week:

Full Pass2 data set

Some cross checks with MC samples

Cross-check with 0-1 tag TRF

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

~300/pb ?

Many triggers are overlapped

MC: Pass2 ->

~200k Z->bb

~200k bb (direct- Pythia, pT>40GeV)

Cuts:

No Bad Lum Block

Jet Quality Cuts

Taggable jets

Exactly 2 good, taggable, jets with (corrected) pT>20 GeV

Good primary vertex (>=4 tracks)

Often separate by d(phi) of the jets, >< 2.9

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

Compare to shift from 0->1 tag 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!

Use measured Z-peak to remove its contribution from the original TRFs

Turns out to be a small correction

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

looking at the shift from the 0->1 tag spectra in data

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

The peak is not shifted very much

This is confirmed by following MC simulations of this correction

Fraction of Z->bb
in single-tagged

data sample

Z->bb MC

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

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

The result is still consistent with
Z->bb MC

But statistics aren't great this way...
since we subtracted off Z->bb signal!

5

0->1 Tag Invariant Mass Spectra

Investigate the single b-tagged data sample

Not much contamination from Z->bb there

Much less bb there as well

Compare to slide 9: 10% bb, instead of 50%

Starts out around 1% in untagged data

Each b-tag application increases bb/QCD by about a factor of ~5-10

Background from “random” tagging can be estimated in the same way as for 1->2 tags

Calculate a TRF, derived on the 0-tag sample

Apply to the 0-tag sample

The difference shows the same behavior as the bias we saw before in the 1->2 tag TRF comparison

This is not due to Z->bb, which would be small

6

Subtracting the 0->1 Tag Spectrum

We actually used this fact to correct the background to Z->bb in the past!
(see my ADM talk of May. '03)

Compare:

0->1 tag difference (solid circles)
(scaled to the number of 2-tag / 1-tag events)

0->1 tag difference correcting for Z->bb in single-tag sample using MC
(scaled to the number of 2-tag / 1-tag events)

The 1->2 tag difference (open circles)

The real Z->bb peak is the difference between the 1->2 tag difference and the 0->1 tag difference...

Resulting fit is consistent with MC,
within statistical errors!

MC Cross-checks

Suyong made a very good suggestion last week that we try to test the method on MC samples

This is slightly difficult, because large statistics of QCD MC are not available
(we would need ~1 million events)

First investigate what the bb MC looks like:

The effect of the pT>40 GeV parton cut is clearly seen

Basic shape and normalization look good

Calculate the bb MC TRF
(for the 3 eta regions)

MC Cross-checks

Apply the TRF back to the bb MC

We get back the same thing
(within statistical errors)

OK, so at least there's no big bug in the TRF code! :)

Data

Background

Difference

MC Cross-checks

We want to test the Z->bb TRF correction...

Look at the pT spectra of the b-jets in bb vs. Z->bb events

Note how Z->bb influences mostly lower pT jets, as expected

Calculate the TRF for the Z->bb

Compared to the bb, it's very similar
(the plot is for the|eta|<1.1 TRF)

There are differences for jets far from the Z peak... weird events?

bb

Z->bb

MC Cross-checks

Here's the simulated data (points), made from Z->bb plus the bb.

Background is calculated from
bb scaled six times + Z->bb
without the Z TRF correction

We scale the bb by six to simulate the ratio of “QCD” to Z->bb in the single-tag data sample where TRFs are calculated

The difference is also shown, it works!

Now go back and do the Z TRF correction:

The peak is not shifted very much

Summary

The full Pass2 data set has been analyzed, roughly 300/pb, 90M events!

Some checks have been performed on the TRF methods using MC samples

The method is internally consistent in simulations, so far...

Could do more:

Add/subtract MC Z->bb peak from data... see what happens?

Try QCD (light-jet) MC?

The QCD to bb inv. mass spectrum bias is similar in the 0->1 tag samples

Can be used to understand the shape of the bb background to Z->bb!

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 ? : (see last week's talk...)

Andy Haas

2004-07-07T15:59:13