# Presentation: 200113

## How Charming is 

***Charles Plager*****, UCLA**
**On behalf of the CDF Collaboration**** **
**CERN Seminar**
**July 2****nd****, 2008**

**The Search for Top**
** ****	****Flavor Changing **
**	****Neutral Currents**
**t ****!**** Z c**
**at CDF Run II**

***How Charming is ***
***the Truth?***

## Outline

- The Tevatron and the CDF Experiment

- Top Quark Physics

- The Search for Top FCNC Decay

- Summary

## Tevatron Run II: 2001–2009 (2010?)

- 2 km

- Tevatron

- Fermi National Accelerator Laboratory – Aerial View

**315 ****¢**** 10****30**** cm**–2** s**–1**!**

- Proton-antiproton collider: √s = 1.96 TeV.
- 36×36 bunches, collisions every 396 ns.
- Record instantaneous peak luminosity:
- Luminosity goal: 5.5 – 6.5 fb–1 of integrated luminosity by 2009, running in 2010 currently under discussion.
- Two multi-purpose detectors: CDF and DØ.

## Tevatron Performance

- Tevatron continues to perform **very** well:
- More than 4.3 fb–1 delivered.
- More than 3.5 fb–1 recorded by CDF.

- Tevatron Store Number

**4.3 fb****–1**

**3.5 fb****–1**

**Run I Total **
**(~100 pb****-1****)**

- The Tevatron just delivered
- 56 pb-1 in ***single week****!*

## The CDF II Detector

*z*

*y*

*x*

- [CDF]

- Protons

- Antiprotons

- Central Muon Detector

- Forward Muon Detector

- Hadronic Wall Calorimeter

- Central Calorimeter (Em/Had)

- Plug Calorimeter (Em/Had)

- Solenoid Magnet

- Silicon Vertex Detectors

- Central Outer Tracker

- Luminosity Monitor

## Outline

- The Tevatron and the CDF Experiment

- Top Quark Physics

- The Search for Top FCNC Decay

- Summary

## Top Quark History

- CDF and DØ Run I announced the top quark discovery March, 1995.
- This discovery did not “*just happen*”:
- Other experiments had been looking for the previous 20 years with no (real) top quark discovery.
- PETRA (DESY): e+e-
- SppS (CERN):     p p
- LEP I (CERN):    e+e-
- Run I was in its fourth year (after three years of Run 0 and many years of designing, building, and commissioning the detectors).

-  TOP TURNS TEN

- TOP TURNS THIRTEEN

## A Quick Note About Scale

- Cross Sections at s = 1.96 TeV

- For those not
- intimately familiar
- with  Tevatron
- high pT Physics:
**Top:**
**1 **in** 10 Billion**
- Reducing and understanding
-  backgrounds is the key.

## Top Quark Review

- Top: the **Golden** quark ( ~ 175 GeV/c2)
- Only fermion with mass near EW scale.
- 40 times heavier than the bottom quark.
- Very wide (1.5 GeV/c2)
- The top quarks decay before they can hadronize.
- We can study the decay of the bare quark.
- Usually observed in pairs.
- Fundamental question:
- 	Is it the truth, the Standard Model (SM) truth, and nothing but the truth?
- Did we really find the top quark?
- Is it the SM top quark?
- Is it only the SM top quark?
- The top quark is an ideal place to look for Beyond the Standard Model Physics!

- tt Pair Lepton + Jets Decay

- -

- _

## New Era of Top Precision Physics!

- CDF and DØ now have more than thirty (**30** !!!) times as much integrated luminosity as we did when they discovered the top quark in    Run I!
- With the data we have recorded, we are now able to have large, ***very pure*** top samples.
- Of the almost 50 results that CDF sent to the winter conferences, ***more than half*** were in top physics!

- Double B-Tag W + Jets Candidates

## What Can We Study About Top Quarks?

- Production cross section
- Resonance production
- Production kinematics
- Spin polarization

- Top charge
- Top spin
- Top lifetime
- Top mass

- Branching ratios
- Rare decays
- Non-SM decays
- Decay kinematics
- W helicity
- |Vtb|

- Top physics
- is very rich.

- _

## Top Pair Decay Modes

- According to the SM, top quarks almost (?) always decay to Wb.
- When classifying the decay modes, we use the W decay modes:
- Leptonic
- Light leptons (e or )
- Tauonic ()
- Hadrons

## Important Tool: Lepton ID

- For many analyses, we need a very
- 	pure set of high pT electrons and muons.
- Electrons (as we reconstruct them):
- Have charged particle track.
- Leave almost all of their energy
- 	in the electromagnetic calorimeter.
- Ask for no other nearby tracks.
- We do not want leptons from
- 	(heavy flavor) jets.
- Muons:
- Have charged particle track.
- ~ Minimum ionizing (leave little energy in either the electromagnetic or hadronic calorimeter)
- Find a “stub” of a track in dedicated muon detector systems on outside of CDF.
- Ask for no other nearby tracks.

- K**§**,

## Important Tool: Jet Reconstruction

- We think of *partons*, but we reconstruct *jets.*
- We need to convert “*raw*” jets to *“corrected”* jets -                          Jet Energy Scale (JES) correction.
- Takes into account detector effects, neutral particles in jets, particles outside of the jet cone, underlying events, multiple interactions, ...

## Important Tool: B Jet Tagging

- Since we (often) expect t  W b,
- 	b jet tagging is a very important tool.
- Most backgrounds do not have  bottom quark jets.
- We rely on the long b quark lifetime.
- B hadrons can travel several millimeters before decaying.
- Use displaced vertices or many displaced tracks (impact parameter).

**b-tag**

**b-tag**

- jet

- MET

- jet

- CDF Event:
- Close-up View of Layer 00 Silicon Detector

**2.4 cm**

## Top Physics Finally Makes Prime Time

***The Big Bang Theory!***
**Mondays on CBS.**

**Top FCNC**

**Top Branching**
**Fractions**

## Outline

- The Tevatron and the CDF Experiment

- Top Quark Physics

- The Search for Top FCNC Decay

- Summary

## Top FCNC Outline

- The Search for Top FCNC Decay

- Introduction

- Acceptances

- Backgrounds

- Unblinding

- Fitting For Everything

- Search For Invisible Top Decays

- Direct FCNC Search

## Flavor Changing Neutral Currents

- Flavor changing neutral current (FCNC) interactions:
- Transition from a quark of flavor A and charge Q to quark of flavor B with the same charge Q.
- Examples: b → sγ, t → Hc, ...
- 1960s: only three light quarks (u,d,s) known,mystery in kaon system:
- Solution: “GIM Mechanism” (Glashow, Iliopoulos, Maiani, 1970)
- Fourth quark needed for cancellation in box diagram: prediction of charm quark.
- Cancellation would be exact if all quarks had the same mass: estimate of charm quark mass.

**10****8**** times smaller than...?**

**Flavor**
**Changing**
**Neutral**
**Current**

## Top Flavor Changing Neutral Currents

- FCNC possible e.g. via penguin diagrams.
- Suppression of this mode:
- GIM mechanism
- Cabibbo suppression
- Expected SM branching fraction (Br) for t → Zc as small as 10–14 .
- Any signal at the Tevatron or LHC: New Physics.

- Generic FCNC

- Penguin Diagram

## Top FCNC & New Physics

**W****+  **

**  ****H****+****?**

- FCNC are enhanced in many models of physics beyond the SM.
- Enhancement mechanisms:
- FCNC interactions at tree level.
- Weaker GIM cancellation by new particles in loop corrections.
- Examples:
- New quark singlets: Z couplings not flavor-diagonal → tree level FCNC.
- Two Higgs doublet models: modified Higgs mechanism.
- Flavor changing Higgs couplings allowed at tree level.
- Virtual Higgs in loop corrections.
- Supersymmetry: gluino/neutralino and squark in loop corrections.

## Previous Limits

- Run I Search:
- 110 pb-1 of data
- tt ! Zc Wb ! Z+≥4j
- Limit: Br (t ! Zc) < 33% at 95% C.L.

- Limit from LEP II
- search for single top production:
- 	                  e+ e- ! t c
- 634 pb-1
- Limit: Br (t ! Zc) < 13.7% at 95% C.L.
- 	) Best limit so far with **Z bosons**.

## Top FCNC Outline

- The Search for Top FCNC Decay

- Introduction

- Acceptances

- Backgrounds

- Unblinding

- Fitting For Everything

- Direct FCNC Search

- Search For Invisible Top Decays

## What do we mean by “invisible?”
Not (well) reconstructed as double b-tag lepton + jets.
What would happen if there were a large branching fraction to an invisible decay?  For example,
	 Br (t ! Invisible) = 10%?
Br (t ! Wb) = 90%
P (tt ! Wb Wb) = 81%
 ) For a purely invisible decay, we should have an 19% deficit when we look at the L + J event yield for a given theoretical cross section.

- Not (well) reconstructed as double b-tag lepton + jets.
- What would happen if there were a large branching fraction to an *invisible* decay?  For example,
- 	 Br (t ! *Invisible*) = 10%?
- Br (t ! Wb) = 90%
- P (tt ! Wb Wb) = 81%
-  ) For a purely invisible decay, we should have an 19% deficit when we look at the L + J event yield for a given theoretical cross section.

- Search for Invisible Top Decays

**XY**

## Feldman-Cousins Acceptance Bands

## Search for Invisible Top Decays, cont.

- From Cacciari et al. (hep-ph: 0804.2800)  assuming CTEQ PDFs.
- Expected Limits:
- Observed Limits:

**World’s First**
**Measurement!**

**Better**
**Than L3’s**
**Published Limit!**

## Top FCNC Outline

- Search For Invisible Top Decays

- The Search for Top FCNC Decay

- Introduction

- Acceptances

- Backgrounds

- Unblinding

- Fitting For Everything

- Direct FCNC Search

## Top FCNC Direct Search: Roadmap

- Basic question: how often do top quarks decay into Zc?
- Measure (or set limit) on branching fraction, Br (t → Zc).
- Normalize to lepton + jets          top pair decays.
- Selection of decay channels fortt → Zc Wb:
- 	Z → charged leptons: very clean signature, lepton trigger.
- W → hadrons: large branching fractions, no neutrinos .) Event can be fully reconstructed
- Final signature: Z + ≥4 jets.

**Z Decay Modes:**

**W Decay Modes:**

## Search for FCNC: Ingredients

- ’

## Top Mass Reconstruction

- For our signal, we have three hadronic masses to reconstruct:
- W mass
- t ! Wb mass
- t ! Z c mass
- To improve resolution, we correct the W and Z daughters so that the masses are correct.
- Rescale the daughters within their resolutions.
- Smaller mass resolution            ) Better signal separation.

- t ! Wb mass
- resolution:
- 20 GeV ) 16 GeV!

- Signal MC with partons correctly matched
- to reconstructed objects.

## Mass 2

- We do not know which partons are reconstructed as which jets.
- 	) Loop over all 12 permutations and take lowest 2 value.

- Signal-like

- Background-like

- High side tail of 2

## Round 1: Blind Analysis

- Event signature: Z → l+ l– + 4 jets.
- Motivation for blind analysis: Avoid biases by looking into the data too early.
- Blinding & unblinding strategy:
- Initial blinded region: Z + ≥ 4 jets.
- Later: add control region in Z + ≥ 4 jets from high side tail of mass 2.
- Optimization of analysis on data control regions and Monte Carlo (MC) simulation only.
- Very last step: “opening the box”,
*	**i.e.,* look into signal region in data.
- Counting experiment:
- 	) Compared expected background to observed events.

## Top FCNC Outline

- Search For Invisible Top Decays

- The Search for Top FCNC Decay

- Introduction

- Backgrounds

- Unblinding

- Fitting For Everything

- Direct FCNC Search

- Acceptances

## Lepton + Track Z Candidates

- Use isolated track (instead of tight lepton) for second lepton.
- Doubles acceptance.
- Almost all backgrounds have real leptons.
- Base Event Selection:
- Tight lepton + track lepton Z candidate.
- At least four jets (|| < 2.4, corrected ET > 15 GeV).

- Tight Leptons

## To B-Tag or not to B-Tag?

- Advantage of requiring b-tag:
-  ) Better discrimination against main background (Z + jets).
- Disadvantage:
-  ) Reduction of data sample size.

- Solution: Use both!
- Split sample in ***tagged*** (at least one tagged jet) and ***anti-tagged*** (no tagged jets).
- Optimize cuts individually for tagged and anti-tagged samples.
- Combine samples in limit calculation.

## Acceptance Calculation: Catch 22?

## Solution: Running Acceptance

- “Running” Acceptance Correction

- Acceptance and tt depend on       .
- Our limit code recalculates acceptance as a function of branching fraction.
- Normalization to double-tagged top pair cross section measurement:
- Smallest overlap (              )  between acceptances.

- Acc. Ratio

- L+J yield

## Top FCNC Outline

- Acceptances

- Search For Invisible Top Decays

- The Search for Top FCNC Decay

- Introduction

- Unblinding

- Fitting For Everything

- Direct FCNC Search

- Backgrounds

## Expected Backgrounds

**Standard Model Top Pair Production**
-  Small background: no real Z, need extra jets from gluon radiation and/or “fake lepton.”
-  Dilepton channel (tt → Wb Wb → lb lb): dilepton invariant mass can fall into Z mass window.
-  Lepton + Jets channel (tt → Wb Wb → lb qq’b): misreconstruct one jet as a lepton (“fake”), invariant mass of lepton and fake lepton can fall into Z mass window.
-  Large fraction of heavy flavor jets: more important in b-tagged samples.
-  Estimated from MC simulation.

- How do you search for a signal that is likely not there? Understand the background!
- Standard model processes that can mimic Z + ≥4 jets signature:
- Z+Jets: Z boson production in association with jets → dominant background for top FCNC search, most difficult to estimate
- Standard model top pair production→ small background
- Dibosons: WZ and ZZ diboson production → small background
- W+Jets, WW: negligible
- Top FCNC background estimate: mixture of data driven techniques and MC predictions

**Diboson Production: WZ, ZZ**
-  Small background (similar in size to standard model tt production).
-  Small cross section but real Z.
-  Need extra jets from gluon radiation.
-  ZZ: Heavy flavor contribution from
-    Z → bb decay.
-  Estimated from MC simulation.

## Z+Jets Production

- MC tool for Z+Jets: ALPGEN
- Modern MC generator for multiparticle final states
- “MLM matching” prescription to remove overlap between jets from matrix element and partons showers
- Comparing ALPGEN with data:
- Leading order generator: no absolute prediction for cross section.
- After normalization to total Z yield, still underestimates of number of events with large jet multiplicities.
- Our strategy: only shapes of kinematic distributions from MC, normalization from control samples in data.
- Normalize to the high side tail of mass 2 in data.

- Blinded

- Blinded

## Base Selection Background Estimate

- Fit from high side of **2** tail :
- 	**130 ****§**** 28 total background events.**
- Background tagging rate:
- 5 of 31 events are tagged.
- Combine with data-based method in lower jet bins.
-  	**)**** 15% ****§**** 4% background event **
**           ****tag rate.**

## Optimized Signal Region Selection

- Optimized for best average expected limit.
- Systematic uncertainties are taken into account, but do not affect limit very strongly.

**Expected  Limit: **
**6.8% ****§**** 2.9%**

## Top FCNC Outline

- Backgrounds

- Acceptances

- Search For Invisible Top Decays

- The Search for Top FCNC Decay

- Introduction

- Fitting For Everything

- Direct FCNC Search

- Unblinding

## First Look

- Before we unblind the signal regions, we want to check our base predictions:
- So far, so good...  Let’s open the box!

## Open the Signal Box

- Opening the box with 1.1 fb–1
- Event yield consistent with background only.
- Fluctuated about 1σ high:  slightly *“unlucky.”*
- Result:
- Expected limit: 6.8% ± 2.9%.

- –***  ******Or is it the first hint of a signal?!***

## Slide 47

## Top FCNC Outline

- Unblinding

- Backgrounds

- Acceptances

- Search For Invisible Top Decays

- The Search for Top FCNC Decay

- Introduction

- Direct FCNC Search

- Fitting For Everything

## Round 2: Is That The Best We Can Do?

- More             :
- Add 70% more data (1.9 fb–1).
- Fit 2 Shape:
- Previous version: counting experiment.
- Template fit to √χ2 shape: exploit full shape information, less sensitive to background normalization.
- Build on previous experience:
- Same event selection
- Same acceptance algebra
- Same method of calculating (most) systematic uncertainties

## Differences From Counting Experiment

- Advantages:
- Absolute estimation of Z + jets background is difficult.  This drove the counting experiment.
- Since we are fitting:
- No absolute Z + jets background estimation needed.
- No estimate of Z + jets tagging fraction needed.
-  ) Let these both float in the fit.
- Smaller backgrounds are fixed to SM expectations.
- Disadvantages:
- Counting experiment does not have shape systematic uncertainties.
- Counting experiment: Only worry about ratios of acceptances.
- Fit 2: We need to understand and account for this.

## Shape Uncertainties

- What do we mean by *“shape uncertainties”*?
- We considered many choices for shape uncertainties.
- The two dominant effects were much larger than all others.
- Factorization/Renormalization (Q2) scale for Z + jets MC.
- Jet energy scale uncertainties.

## Shape Uncertainties: Q2

- ALPGEN: two Q2 “knobs” to turn.
- Factorization/renormalization scale:
- Vertex Q2 (for evaluation of α*S*):
- We turn both at the same time.
- Not enough to explain data.

## Shape Uncertainties: JES

- We need to convert “*raw*” jets to *“corrected”* jets
- 	) Jet Energy Scale correction (JES)
- Takes into account detector effects, neutral particles in jets, particles outside of the jet cone, underlying events, multiple interactions, ...

## Template Morphing

- Now that we have JES shifts, how do we incorporate this in our machinery?
- 	) Implemented *compound horizontal template morphing*.
- Horizontal morphing is simply interpolating between two normalized cumulative distribution functions (*i.e*., the normalized integral of the histogram).

- The **green **C.D.F. curve is the 75% interpolation between the **blue** and **red** C.D.F. curves.

*“**Everything You Always Wanted To Know About *
*Template Morphing But Were Afraid To Ask.”*

## Does Morphing Work?

- Test with Gaussians
- Easy to verify it is working as expected.
- Works on much more complicated shapes.
- Squares
- Half-circles
- mass 2 shapes

## Signal and Control Regions

- FCNC Signal

- Z+Jets Background

*“**How do we control shape uncertainties without hiding a small signal?”*
- Solution: add control region with little signal acceptance:
- Constrain shape uncertainties without “morphing away” signal.
- Definition: At least one optimized *E**T* or *m*T cut failed (do not look at any b-tagging information).

## Constraining Z + Jets Background

- We have validated that the MC works fairly well in a jet bin, but we do not trust it across jet bins.
- 	) No absolute Z + jet constraints.
-  Use MC to predict the ratio of Z + jets acceptance in the two signal regions to the control region.

**Passed all four E****T**
** ****and m****T**** Cuts**

**Failed at least one cut**

**Expected Background Distributions**

- ) 20% constraint

- ) No constraint!

## Fitting 2 Roundup

- No absolute Z + jet background estimate needed.
- For the template fit, we need to deal with shape uncertainties.
- Find dominant sources ) JES
- Morphing of JES templates in fitter.
- Do not want to *“morph away”* a real signal ) Control region.
- Use control region also for Z + jet constraints.
- Investigated effect of shape **not** being from JES ) Small effect.

## Feldman-Cousins in Five Minutes

- How are we going to interpret our results?
- Feldman-Cousins answers the question:
*	**“**What range of true values are likely to lead to this measured value?”*
- Why use Feldman-Cousins?
- Guarantees coverage.
- Data tell us whether we should report a measurement or a limit.
- Our method incorporates systematic uncertainties easily.

## Top FCNC Feldman-Cousins Bands

## Pseudo-Experiments (PEs)

***Pseudo-experiment:*** Generate all necessary numbers/templates to emulate data from an experiment.
- 1. Generate random numbers to simulate all systematic uncertainties.
- Pay attention to correlations.
- Vary **all** systematic uncertainties.
- Verify all numbers are physical.
- Morph all templates appropriately.
- 2. Generate numbers of background
-     and signal events.
- For each type of event, use
- 	templates to generate mass 2.
- Fit as if data.
- Repeat!

## FC Band Construction In A Nutshell

- Likelihood Ratio for B(t ! Zq) = 0.0150

- 95% of PEs

- Use *Likelihood Ratio Ordering Principle*:

- PEs generated with all statistical
-  and systematic uncertainties.

## Expected Limit

- =

- x

- x

## The Fit to the Data

## F.C. 95% C.L. Limit

## Outline

- The Tevatron and the CDF Experiment

- Top Quark Physics

- The Search for Top FCNC Decay

- Summary

## Summary

- CDF and the Tevatron are running very well.
- Thanks Tevatron!
- We just finished Run II’s first search for Top FCNC t ! Z c.
- Using **1.9 fb****-1**,
- 	we have the world’s best limit:
- 	**Br (t ****!**** Z c) < 3.7%** at 95% C.L.
- Using data-based background techniques will be very important for the LHC.

## Money Plot

## New Era of Precision Top Physics!

- 2006 PDG Top Entry

- 2008 PDG Top Entry

- 2010 PDG Top Entry

## Thank You!

## Slide 71