Sample Composition of the l+SVT Triggers

Introduction

Outline

• The invariant mass, M(l,SVT),
• The signed impact parameter of the lepton or SVT track.

Fits to signed impact parameter distributions

The shape of the signed impact parameter distribution of the lepton is described quite well by a model that includes these four components. The fits are shown here and here.

Comparison with background subtraction

Trigger-side Dilution

How to explain this in a talk

1. The l+SVT triggers provide us with a very large sample of semileptonic b and c decays. This can be illustrated with this figure or this figure.
2. We can isolate the bottom component by cutting on M(l,SVT) and subtracting the non-b background. This can be illustrated using this figure or this figure.
3. Opposite side flavor tags can be studied in this way: we now have enough statistics to subdivide samples of tagged events, measuring the relative dilution of each sub-sample. An example is the soft muon tag, in which we can bin the dilution by both muon subsystem and pT-rel.
4. The b-decay on the trigger side can mix and can undergo wrong-sign sequential decays. These effects will reduce the 'raw' dilution measured for an opposite side tag. We can estimate the dilution on the trigger side using a Monte Carlo and apply a correction to obtain the 'true' dilution of the opposite side tag.
5. We have enough statistics to optimize b-flavor tags with this sample. After the optimization has been performed, we can then perform cross-checks using more fully reconstructed channels. For example, we can cross check with the dilution observed in B0 --> l,D+ events. We can also measure the dilution in B --> J/psi K+ events, which would be relevant for a sin(2 beta) analysis. These are statistically independent samples and dilutions measured using them have not been biased by the optimization procedure.
6. Ultimately, we can predict the dilutions that would result when the flavor tags are applied to a sample of Bs decays. The systematic uncertainty of this prediction is not expected to dominate a Bs mixing analysis.
7. A long term goal has been to use Monte Carlo to describe opposite side tags in detail. Because we understand the sample composition, we may be able to learn about the physical processes that produce taggable b hadrons on the opposite side of the event. If this is the case, then we can eventually use Monte Carlo to describe many aspects of the tagging analyses.

Try saying something like this:

In Run-II we are able to carry out detailed studies of b-flavor tagging using high statistics control samples. The lepton+SVT trigger provides us with a high statistics sample of semileptonic b-decays that are well suited for optimizing the performance of opposite-side flavor tags. In 60 /pb we have about 150,000 semileptonic b-decays which is large enough to study the performance of flavor tags that have been sub-divided into several categories. An example is the soft muon tag in which we study the performance in each muon subsystem separately and also measure the dilution as a function of the independent variable pT-rel (the pT of the soft muon with respect to an opposite-side jet). Measuring the performance like this will allow us to parameterize the relative dilution of a tag that can be used on an event-by-event basis in a likelihood fit.

The statistics available with this inclusive sample far exceeds those of the more fully reconstructed b-decay channels like B0 --> D+,l-,nu. However, we can optimize and measure dilutions in the inclusive l+SVT samples and compare them with those measured using the statistically inferior l+D samples as a cross check. Assuming we obtain consistent results, this gives weight to the argument that we can determine the dilution in a sample like l+Ds, which is essential if we are to publish believable limits on Bs oscillations. Furthermore, since the flavor tags are studied with an independent, high statistics control sample, we can be sure that the optimization procedure has not biased the result.

This is one of the messages that we want to convey when we discuss flavor tagging at CDF in Run-II. Bs mixing remains a high-profile analysis and we have to be sure that we don't blow our credibility with 'results' from half-baked tagging analyses.

Supporting documentation

Blessed results

Signed impact parameter distribution of lepton with 4<M(l,track)<5 GeV. There are about 146,000 semileptonic b-decays in the shaded region.

sd0_emu_m4.eps
sd0_emu_m4.gif

Signed impact parameter distribution of lepton with 2<M(l,track)<4 GeV:

fit_sd0_m.eps
fit_sd0_m.gif

The same, but with a logarithmic y axis:

fit_sd0_m_logy.eps
fit_sd0_m_logy.gif

Signed impact parameter distribution of lepton with M(l,track)<4 GeV:

fit_sd0.eps
fit_sd0.gif

The same, but with a logarithmic y axis:

fit_sd0_logy.eps
fit_sd0_logy.gif

Numbers of events in the various components (54.3 /pb for e+SVT, 63.2 /pb for mu+SVT):

Table_2.gif
Table_2.tex

Fitted yields (not cross sections) of the various components:

Table_1.gif
Table_1.tex

Relative uncertainties in percent due to input parameters in the model:

Table_3.gif
Table_3.tex

Table_4.gif
Table_4.tex

Comparison of fitted and subtracted yields. The corrected yield is the fitted yield corrected for the expected fraction of signal with d0(SVT)<0.

Table_5.gif
Table_5.tex

Mass of muon+SVT compared with Monte Carlo normalized by fitted yields. Detector resolution has not been simulated in the Monte Carlo, as is evident from the J/psi peak:

mass_plot.eps
mass_plot.gif

This is another rendition of the same thing but with b and c components identified explicitly. This could be useful to motivate the requirement of 2<M(l,SVT)<4 GeV to reject the direct charm component.

another_mass_plot.eps
another_mass_plot.gif

Systematic uncertainties on the trigger-side dilution correction factor:

Table_6.gif
Table_6.tex

Last updated: Mon Sep 15 12:49:08 CDT 2003 mjones@fnal.gov