# Presentation: 363172

## High pT b-tagging at CDF:

**Challenge of *****b*****-tagging at a hadron machine**
**Lifetime-based *****b*****-tagging at CDF**
**Measuring efficiency in the data**
**Understanding contribution from non-*****b***** sources**
**Other CDF *****b*****-tagging techniques**
**Considerations for LHC experiments**
**Summary **

** **
**Christopher Neu**
**University of Pennsylvania**
**on behalf of the CDF Collaboration**** **
- Top2006 Workshop
- 13 January 2006
- University of Coimbra, Portugal

**High *****p******T***** *****b*****-tagging at CDF:**
**Measuring Efficiency and Understanding Mistags**

**Notes:**
PDF – fix problems 
define ctau

## b-Tagging at the Tevatron

- The **ability to identify jets originating from *****b***** quark production** is critical for several facets of the Tevatron RunII physics program – top, Higgs, exotic searches, QCD...
**Distinguishing jets from *****b***** quarks** from light flavor and charm:
- The **long lifetime** of the *b*
- The **large mass** of *B* hadrons
- The **energetic semileptonic decay** of *B* hadrons
- Given that we have some nice handles – ***b-*****tagging sounds easy, right?**
**Challenges** at a **hadron machine**:
**Busy environment** in tracking detectors
**Multiple interactions** within each crossing
**No *****Z*****→*****bb *****peak** with which to calibrate tagging algorithms
- Calibration samples are available – but **incomplete overlap** with interesting signal spectra
**Challenges** distinguishing bottom jets **from charm jets**:
- Charm has **nonzero lifetime**
**Intermediate mass** of charmed hadrons
**Similar semileptonic decay** spectrum to *B* sector

**Notes:**
Me: HF ratios – or how do we really measure charm efficiency?  Charm jet tag eff from MC – and then apply SF with different larger errors.  

## The CDF Detector: Crucial Components for Tagging

**The CDF Detector: Crucial Components for Tagging**

**Charged particle tracking: **
**Solenoid provides a 1.4T magnetic field**
- Good momentum resolution
**Silicon:** several subsystems
**SVXII:**
- 5 layers out to radius of 10.6cm
- |z| < 45cm
**L00:**
- Directly on beampipe
- Valuable for improved tracking – 4% increase in tag efficiency
**ISL:**
- Two layers at r = 20,28cm
- Provides forward silicon tracking
**COT:**
- Open drift chamber
- Good *p**T*, spatial resolution
**Calorimetry – **jets, electrons
**Muon system - **muons
**Trigger**
- Highly efficient for high pT leptons
- Also collects valuable inclusive lepton, jet samples

**Notes:**
What is ISL btwn 0 and 25 cm?

## CDF b-tagging Tools

**CDF *****b*****-tagging Tools**

**Ingredients for a useful tagger:**
**Tag efficiency for *****b*****-jets** in data, MC
**Mistag rate** in order to understand **contribution to tagged sample from non-*****b***** sources** – per-jet mistag probability
- Efficiency and mistag probability are not single-valued
- need to be examined as a function of jet- and event-level quantities
- CDF  has **several tagging tools**** **in use/development for RunII analyses:
**Identification of jets with a secondary vertex – SECVTX:**
- Exploits the long lifetime of the *b* quark
- Additional handle one can use is the *mass of the reconstructed secondary vertex*
**Jet Probability:** incorporates lifetime, mass information
- Assigns a per-jet probability that the jet was consistent with coming from a prompt source
**Soft lepton tagging:**** **looks for energetic electron or muon within a jet
**NN tagging algorithms:**
- Simultaneous incorporation of lifetime, mass, semileptonic decay information along with event level quantities
- Two versions under development
- One that attempts to increase purity within SECVTX selected sample
- Another that looks for tags in generic jet sample

**Main focus of this talk**

**Notes:**
Save time here – 

## Secondary vertex b-tagging at CDF

**SECVTX algorithm: **attempt to **construct a secondary vertex** among  **large impact parameter (*****d******0*****)** **tracks **using a two-pass scheme
**Pass1:**
- Starts** **with construction of** 2-track “seed” vertex**
**Attach** all remaining tracks that are consistent with seed.
- Construct the multitrack vertex, iteratively **pruning **away the attached tracks if they spoil vertex fit.
**Resulting candidate vertex required to have 3 or more tracks **
**Pass2: **tighter track *d**0* significance requirement
- Attempt to **vertex all** these tracks to a common point.
**Remove** any track that spoils the vertex fit, re-vertexing after each removal.
**Resulting candidate vertex required to have 2 or more tracks **
- Apply **vertex quality cuts**
- removal of ***K******s******,******Λ****** *****vertices**
- Removal of vertices in the **material portion of CDF** (beampipe, silicon ladders)
**If the vertex survives, the jet is “tagged” –**
**sign of transverse displacement **of secondary vertex wrt interaction point, ***L******xy****,**** ***determines _**positive tag**_ or _**negative tag**_.

**Displaced**
**tracks**

- d0

**Primary vertex**

**Prompt**
**tracks**

**Secondary **
**vertex**

- Drawing of the transverse plane of a single-top event – forward jet escapes down beampipe:

***L******xy***

- Here – **positive *****L******xy****** *****tag**.

**Notes:**
Pass2 track sel

## Contribution to b-Tag Sample from Light Flavor Jets 

- The **flight direction a *****B***** hadron** travels in during its lifetime is **correlated to the jet direction**
**Light flavor jets** should be consistent with **zero lifetime**
- However **fake tracks within a jet** with large impact parameter can help satisfy vertex requirements
**Sources **of fake tracks:
**Limited detector resolution**
**Long-lived light particle decays** (*Λ**, K**s*)
**Material interactions**
- Fake tracks within a jet from limited detector resolution** **should be **symmetric about the primary interaction point**
- Therefore light flavor vertices symmetric in *L**xy*
- This allows one to ***use the ensemble of negatively tagged jets*** as a ***prediction to the light flavor contribution to the positive tag rate***** (aka mistags)**

**Primary vertex**

**Secondary **
**vertex**

**Primary vertex**

**Tagging of *****b***** jet**

**Spurious tagging of light flavor jet**

**Displaced**
**tracks**

**Prompt**
**tracks**

**Displaced**
**tracks**

**Prompt**
**tracks**

**Secondary **
**vertex**

***L******xy****** > 0***

***L******xy******<0***

## Contribution to b-Tag Sample from Light Flavor Jets

- However what is needed is an **a priori prediction of the light flavor content** of the positively tagged jets in the signal data sample
**Procedure:**
- For *b*-tagging based top physics analyses, the focus is the ***W*****+jets data sample**
- Use **inclusive jet sample for calibration** of mistags
- Determine **per-jet mistag probability** in a number of different variables –
- Jet* E**T*, |*η*|, *φ*
- Jet track multiplicity
- Σ*E**T**jets*
- Use calibration jet samples to determine parameterization – then **apply to signal data sample**
**Sources of systematic error:**
- Extrapolation from calibration sample to signal sample
- Uncertainty on Σ*E**T**jet*s
- Trigger bias
- Result: can predict **mistag contribution to 8%**

**Notes:**
Distributions – blessed? Some definitely blessed – which talking about earlier – 

## Light Flavor Jet Tag Asymmetry 

**Light Flavor Jet Tag Asymmetry **

- The **mistag parameterization** only accounts for limited detector resolution source of the mistag sample
- Material interactions within the jet decay bias the distribution to positive *L**xy* values – introducing a l**ight flavor jet tag asymmetry **
**A**symmetry can be measured
- MC templates of pseudo-*c**τ* for *b*, *c*, and light flavor jets
- Fit to pseudo-*c**τ* distribution from generic jet sample

** ****N****light****+**** ****/ N****- ****= 1.27 +- 0.13**

**- Center of COT**

**Dijet MC**

**R****xy ****(cm)**

- 0.5 cm

**Notes:**
Need alpha times beta, alpha is for numerator, beta is for denom – just one number

## Summary: Mistags

**Mistag studies:**
- Data from inclusive jet samples
- Two SECVTX operating points – **Tight and Loose**
- Different points in **efficiency-versus-purity space**
**Loose operating point** is similar to proposed LHC taggers
- Relaxed track requirements wrt Tight SECVTX – larger mistags
- For a **central *****E******T****** = *****40 GeV jet**, the SECVTX mistag rate is ~1**%**

## Efficiency Measurement in the Data

**Understanding the tag efficiency in the Monte Carlo is simple**
- But what one really seeks is **the efficiency for tagging b-jets in the data**
**Strategy:**
- Measure the tag efficiency in data in a sample that is enriched in real b-jets
- Measure the tag efficiency in MC in a sample that models this HF-enriched data sample
- Calculate a ***b*****-tagging scale factor** = Ratio of data tag efficiency / MC tag efficiency
- Scale factor is a measure of how the MC differs from reality
**Two techniques currently employed at CDF:**
- Both use samples of dijets
- Enrich the HF content:
- One jet demanded to have a lepton – so-called “lepton-jet” – indicative of semileptonic B decay
- Other jet – recoil or “away-jet” – demanded to be tagged
- One method relies on **“muon-jets”** and fits the b- and non-b content using** templates of the relative *****p******T****** *****of the muon wrt jet axis = *****p******T******rel***
- One method considers **double tags** in events where the away jet is paired with an **“electron-jet” that is also tagged**

## pTrel templates drawn from MC
Charm template very similar to that of light-flavor jets
b template similar for tagged and untagged b-jets 
Used to fit for b and non-b content in untagged and tagged data sample

Systematic errors: main source is extrapolation to higher jet ET
Result: SF = 0.915 +- 0.017(stat) +- 0.060(sys)

- Charm template very similar to that of light-flavor jets
*b* template similar for tagged and untagged *b*-jets
**Used to fit for *****b***** and non-*****b *****content** in untagged and tagged data sample
- Systematic errors: main source is **extrapolation to higher jet *****E******T***
- Result: **SF = 0.915 +- 0.017(stat) +- 0.060(sys)**

***b*****-Tag Efficiency: Muon *****p******T******rel***** ****Method**

| Pretag b-fraction | 0.779 +- 0.009 +- 0.015 |
| --- | --- |
| Tagged b-fraction | 0.990 +- 0.016 +- 0.002 |
| Data tag efficiency | 0.392 +- 0.007 +- 0.008 |
| MC tag efficiency | 0.4278 +- 0.0019 |

- Statistical errors only

## b-Tag Efficiency: Electron Method and Comparison

**SF****combined**** = 0.909 +- 0.060(stat+sys)**

- HF-enriched electron-jet sample contains both semileptonic B decays and conversions
- Use single tag rate in electron jet to **algebraically solve** for HF content of untagged sample
**Conversions** provide a complementary sample with similar topology with which one can understand the real HF content of the away-jet tagged sample
- Main sources of systematic error: **extrapolation** to higher jet *E**T** *, ***b,c***** fraction** in electron jets
- Result: **SF = 0.890 +- 0.028(stat) +- 0.072(sys)**
- Combination of electron and muon methods:

**Notes:**
Show the HF fractions and efficiency – table like p9

## Summary: Efficiency

**Efficiency studies:**
- ttbar Pythia MC studies
*b*-tagging SF has been applied
**Loose SECVTX operating point** – used in several top complete/ongoing top analyses
- For a **central *****E******T****** = *****60 GeV b-jet in top decay**, the Loose SECVTX tag efficiency is **~52%**
**Efficiency decrease at large |*****η*****|** is due mostly to tracking efficiency in the forward region – which are currently seeking to improve
**Charm efficiency:**
- Measured in MC, similar SF
- Efficiency ranges from 5-10% as a function of jet *E**T*

**Notes:**
Drops faster for tight rather than loose – 

## b-Tagging at D0

**D0 in RunII also has secondary vertex b-tagging in RunII **
**Benchmarks:**
- Efficiency for a 60 GeV b-jet is ~45%
- Mistag rate for 40 GeV jet is ~0.3%
- This is best compared to the CDF **SECVTX Tight operating point**:
- CDF Tight SECVTX efficiency for a 60 GeV b-jet is ~45%
- CDF Tight SECVTX mistag rate for 40 GeV jet is ~0.4% for central jets –

**CDF and D0 tagging algorithms have similar efficiency and mistag rates.**

## Looking Ahead to b-Tagging at LHC Experiments

**Good amount of experience** has been gained at the Tevatron experiments
- Fairly successful *b*-tagging tools have been developed
**This is not to mean however that all the problems are easy to solve**
- There are **many issues that deserve attention** for the future experiments:
**Alignment **of the silicon tracking detector
- Understanding of the **charge deposition models** for particles as they traverse the silicon detector
- Understanding the **material content around the interaction point**
**Tracking simulation** and its relation to reality
**Trigger effects** – ensure that enough calibration data is collected at appropriate *ET, **η* range for the physics one wants to do

## Summary

- Several critical portions of the Tevatron RunII physics program rely on the ability to identify jets originating from b quark production
- CDF has several b-tagging tools in use, including the secondary vertex tagger discussed here in particular
- With any *b*-tagging tool it is important to understand and quantify
- Efficiency for tagging b-jets in the data
- The rate at which non-b jets are tagged
- CDF has made progress in understanding these issues
- Tagger development for the LHC experiments can build upon the knowledge we have developed at the Tevatron

## Backup

## Backup – Muon Method Jet ET Dependence