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

showed results from the full Pass2 data set

showed some MC checks of the TRF method

This week:

Try the CDF method

Try a new variant of the CDF method

Double-tagged data

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

Background?

Z->bb MC

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:

Good 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

I use SVT Loose b-id

This skim is really big!
(Loose mu -> medium mu) is bad for us!

I propose we make our own skim:
- no mu requirement
- 2 jets>15GeV,
- not more than 2 jets

Look at the ratio of single-tag to double-tag events as a function of inv. mass

Do this in the “control data set”, which contains events failing final analysis cuts

I use dijet events, but take d(phi)<2.9

Then apply this “TRF” back to the single-tag data sample in d(phi)>2.9

You get too few events expected as background!

The normalization is off by ~15%:
There is more bb in d(phi)>2.9 than in
d(phi)<2.9, so the average tag rate is higher?

After a re-scaling, the background looks a lot like the data

But background statistics are very poor using this method

The CDF “TRF”

From g->bb, but not an issue
because no events are below
50 GeV with d(phi)>2.9

Re-scaled background

Background?

Z->bb MC

The CDF Method

The CDF Method (cont.)

The resulting difference between data and background (even after the re-scaling) shows little resemblance to the expected MC Z->bb peak

There is a clear shift of data towards higher masses

A possible explanation is the re-scaling, which is done to account for less bb in the
d(phi)<2.9 sample. If the shapes of bb and “QCD” are not identical, then a shift could occur. We saw this already when comparing the 0-1 tag spectrum with the 1-2 tag ...

The Z->bb in the d(phi)<2.9 sample was not accounted for

There should be less than in the d(phi)>2.9 sample, but we scaled it up when we had to re-scale the background!

Difference between data and background
using the CDF method... hmmm. :)

The real trouble with the CDF Method is that the d(phi)<2.9 sample is very different from the
d(phi)>2.9 sample

Try getting the “TRF” from the d(phi)>2.9 sample, but use 0->1 tag data

The background still has to be re-scaled, to the integral of the 2-tag data

The resulting diff between data and background looks a lot like the MC Z->bb

Statistics of background are very good, since there's lots of 0-tag events

There's less shift in inv. mass since the shift from 0->1 is already applied when using the “TRF” to go from 1->2 tags

There is not as much interference from Z->bb in the “TRF” since the 1-tag sample has very little (compared to the 2-tag d(phi)<2.9 sample)

A Modified CDF Method

Several background estimation methods have been compared

I think the basic CDF method has serious flaws

A variant of it using the 0-1 tag data in the same d(phi) and Njets ranges has some hope

The TRF method makes the fewest assumptions and has the most statistical power, but is certainly the most complicated

We need to come up with a smarter skim
for Z->bb...

We should also take a look at our new triggers and make sure that they allow us to compute the
Z->bb background without added bias

We may need a large and unbiased, untagged data sample! :)

Resulting Z->bb peak using
the TRF method... derived last week

Summary

Andy Haas

2004-07-07T15:59:13