# Presentation: 319174

## W/Z + Jets and W/Z + Heavy Flavor Jets

**Christopher Neu**
**Rencontres de Moriond **QCD and High Energy Interactions
- 12 March 2008
- La Thuile - Aosta Valley, Italy

***W/Z***** + Jets and *****W/Z***** + Heavy Flavor Jets**
**at the Tevatron**

**Importance of *****V *****+ jets**
**Recent Tevatron progress**
**Summary and future**

**Outline:**

## Importance of Boson + Jet Physics

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**Importance of Boson + Jet Physics**

- Important tests of Quantum Chromodynamics (QCD)
- Recent LO and NLO simulations need experimental verification
- Signature shared with top, Higgs and other searches

***Why study boson +jet production?***

***V*****=*****W***** or *****Z***

***V***

***V***

***q***

***q***

***q’***

***q***

***q***

***q’***

***q***

***q******(’)***

***q******(‘)***

***q***

## The CDF and D0 Experiments

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**The CDF and D0 Experiments**

- Charged particle tracking  in axial magnetic field
- Electromagnetic and hadronic calorimetry
-  Muon detection
- Luminosity
- Three level event trigger

**Common features:**

## W + Inclusive Jets

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***W***** + Inclusive Jets**

**CDF *****W*****+jets analysis: **320/pb
***W***** selection: **seek *W* *e ***
*e*: ET > 20 GeV, || < 1.1
**: missing energy MET > 30 GeV
- MT (*W*) > 20  GeV/c2
**Jet definition:**
- Corrected ET > 20, || < 2.0
- JetClu w/ cone R= 0.4
- R(e, jet) > 0.52
**Major backgrounds:**
- leptonic (W, Zee, WW, W, tt
- multijet (Fake W signature + jets)

- PRD 77, 011108(R)

**NLO prediction more accurate.**
**Differential cross sections **
**also available...**

- MLM :   ALPGEN v2.12 (LO) + Herwig v6.5 + MLM + CTEQ5L
- SMPR:   MadGraph v4 (LO) + Pythia v6.3 + CKKW + CTEQ6L1
- MCFM: NLO, no showering + CTEQ6.1M

**Notes:**
Merging algorithms designed to remove ambiguities introduced when 
V+n-parton process actually makes an event with n_j != n jets. Need
a convention for how to handle this so as not to double count.

Jet corrections: measured jet ET is on average equal to the summed ET’s 
of the particles in the jet cone.  Corrections take into account:
  - imprecise response of CDF calorimeter (L0)
  - non-uniformity in CDF calorimeter (L1)
  - multiple interactions  (L4)
  - non-linearity or un-instrumented regions (L5)

MLM = ALPGEN v2.12 (LO) + Herwigv6.5 + MLM matching + CTEQ5L
SMPR = MadGraph v4 (LO) + Pythiav6.3 + CKKW + CTEQ6L1
MCFM = NLO, no showering + CTEQ6.1M

Backgrounds are different as a function of ET, multiplicity.
High ET is bkgd dominated.  Low ET less so.  Bkgd increases
as njets increases.

Conclusions:
 - LO total cross section preds as a function of nth jet ET undershoot data
 - Ratio of n/n-1 is pretty consistent for LO and NLO preds
 - NLO is improved wrt the data measurement

## W + Inclusive Jets

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***W***** + Inclusive Jets**

- Difficult to calculate. Simplification strategies:
- Generate 22 and use parton shower
- Generate 2N processes but ignore
- 	virtual corrections (loops)
- Neither alone is ideal, so hybrid of the two is employed
- Ambiguities arise
- Possibility for double counting
- “SMPR” and “MLM” refer to
- 	algorithms for removing overlaps
*W+n*-jet cross section better reproduced
- 	by SMPR treatment:
- overlap removal or tuned UE in Pythia?
- Recent PRD-RC:

**Notes:**
Multi-particle final states are hard to calculate in a complete sense.

Conclusions (NLO good, CKKW good) hold for Z+jets??

## Z/* + Inclusive Jets

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***Z/****************** + Inclusive Jets**

**CDF *****Z /******************+jets analysis: **1.7/fb
***Z /************* *****selection**: seek *Z /*******e*+*e*-
- Two ET > 25 GeV electrons
- 66 < Mee < 116 GeV/c2
- At least one *e* w/ || < 1.1, else 1.2 < || < 2.8
**Jet definition:**
- Corrected pT > 30, |y| < 2.1
- MidPoint  w/ R=0.7
- R(e, jet) > 0.7
**Major backgrounds:  S/B ~ 7/1**
- Multijet
*W*+jets

## Z /* + Inclusive Jets

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***Z /****************** + Inclusive Jets**

- NLO predictions for jet pT differential cross section are accurate
- NLO predictions for jet multiplicity also accurate
- NLO-to-LO scaling appears to be a safe thing to do here

**Notes:**
MCFM + CTEQ6.1M

## Z /* + Inclusive Jets

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***Z /****************** + Inclusive Jets**

**D0 *****Z /******************+jets analysis: 950**/pb
- Analysis also uses *Z /*******e*+*e*-
- similar *e* kinematic criteria
- Jets down to 15 GeV/c
**Purpose here: **compare Pythia and Sherpa+CKKW event generators
- CKKW proved useful in *W*+jets
- True here as well?
- Data reproduced better with Sherpa+CKKW than with Pythia

- Sherpa v 1.0.6 + CTEQ6L
- Pythia v 6.319 and CTEQ6L1

**Notes:**
D0 cone algorithm 

## Z /* + Inclusive Jets

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***Z /****************** + Inclusive Jets**

**Sherpa + CKKW represents data better in **
**	****- **Z pT, Jet multiplicity
- 	- pT of jet 1,2,3
- 	- (jet, jet), (jet, jet)

## Identifying b Jets

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**Identifying *****b***** Jets**

***B***** hadron lifetime**: ~1.5 ps
- Large boost (v ~ 0.95*c*) means the *B* lifetime is really long in the lab frame
*B* travels macroscopic distance before decaying
**Exploit the long lifetime - **
- Reconstruct charged particle tracks
- See if they intersect at a common point
- Require the common point be  significantly displaced from the primary p-p collision point

**Long-lifetime yields secondary decay vertex**
**“*****b*****-tagging”**

***L******2d***

**Displaced**
**tracks**

*d**0*

**Primary vertex,**
**aka p/pbar collision point**

**Prompt**
**tracks**

**Secondary **
**vertex**

##  Tagging: b’s and Non-b’s

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** ****Tagging*****:***** *****b*****’s and Non-*****b*****’s**

**Primary vertex**

**Secondary **
**vertex**

**Primary vertex**

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

**Spurious tagging of light flavor jet:**
**“****mistag”**

**Displaced**
**tracks**

**Prompt**
**tracks**

**Displaced**
**tracks**

**Prompt**
**tracks**

**Secondary **
**vertex**

***L******2d****** > 0***

***L******2d******<0***

**Tag efficiency for *****b***** jets              **

**Tag efficiency for LF jets              **

**Jet E****T**** (GeV)**

**Jet E****T**** (GeV) **

**Fractional ******

**Fractional ******

**Notes:**
Three different operating points for the secondary vertex tagger
	- Loose: high efficiency
	-Tight: middle ground
	- Ultratight: high purity
What about charm jets?  Not shown here but for UT the per-c-jet efficiency is ~2% 
CDF 	L: 0.48, 0.01, 0.12
	T: 0.43, 0.005, 0.10
	UT: 0.20, 0.0005, 0.02
CMS:  0.5, 0.007, 0.09

## W + b-Jets

**CDF *****W+b*****-jets analysis**: 1.9/fb
**Goals:**
- Measure jet cross section
- Compare to theory
- Use measurement in background
- 	estimate for Higgs search
*W* selection:
*p**T* > 20 isolated central *e* or **
- Missing energy MET > 25 GeV
- Jet selection:
- 1 or 2 corrected ET>20 GeV,
- 	|| < 2.0 JetClu R=0.4 jets
- Ultratight secondary vertex tags

- Christopher Neu

- Page

- Extract species content via
**vertex mass, *****M******ver*****t**:
- - Mass of tracks in vertex
- - Correlated to mass of decaying hadron

**Notes:**
We choose to use Ultratight btagging in this analysis
	x10 redux in LF, x4 redux in c at expense of x2 redux in b eff

## W + b-Jets

***W***** + *****b-*****Jets**

- Largest backgrounds: S/B ~ 3/1
- ttbar  (40% of total bkgd)
- single top (30%)
- Fake *W *(15%)
*WZ *(5%)
**Result: **
- 	xBR = 2.74  0.27 (stat)  0.42 (syst)  pb
**Prediction: **
- 	 xBR = 0.78 pb
- 	ALPGEN v2.10’ + Pythia v6.325 + MLM + CTEQ6L

- Christopher Neu

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- This cross section is for *b* jets from *W+b*-jet production in events with a high pT central lepton, high pT neutrino and 1 or 2 total jets.

- Shape comparisons?
- NLO comparison?
- Differential cross sections?
- Inclusive measurement?

- x3.5 mismatch

**Request list:**

## W + b-Jets

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***W***** + *****b-*****Jets**

## Z + b-Jets

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***Z***** + *****b-*****Jets**

- Similar CDF analysis for *Z+b*-jets: 2/fb
- Utilize *Z** ee, ***
- Jet definition:
- Corrected ET > 20 GeV, || < 1.5
- JetClu R=0.7
- Secondary vertex tags

- Differential cross sections with comparisons to  LO, NLO predictions
-  Pythia does a good job at low jet ET
- ALPGEN and MCFM undershoot data in several bins

## Z + b-Jets

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***Z***** + *****b-*****Jets**

***V+b*****-Jets Summary:**

## W + Single c Production

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***W***** + Single *****c***** Production**

- CDF W+single c analysis: 1.8/fb
*W* selection:
*p**T* > 20 isolated *e* or **
- Missing energy MET > 25 GeV
- Jet selection:
- 1 or 2 corrected ET > 10 GeV jets
- identify semileptonic decay
**Exploit feature:  **charm semileptonic daughter and *W* have opposite charge

**c**

**c**

**W****-**

**s** (90%) or
**d** (10%)

- μ

**Soft Muon b-Tagging:**

**Notes:**
L4 corrections here not L5!

## W + Single c Production

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***W***** + Single *****c***** Production**

**Backgrounds**: *W+u*, Fake *W*, Drell-Yan  (NOS = NSS for bkgd)
**Result:**xBR = 28.5  8.2 (stat)  4.4 (syst)  1.7 (lum) pb
**Prediction: **ALPGEN
- 	 xBR = 22.2  1.2 (PDF)  3.8 (scale)  pb

- LO prediction
- reasonably good.

## W + Single c Production

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***W***** + Single *****c***** Production**

- Similar analysis completed at D0: 1/fb
- Measures
- 	which can be compared to the
- 	LO prediction: 0.040  0.003 (PDF)

- LO prediction
- reasonably good.

## Conclusions

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**Conclusions**

- Boson + jets physics plays an important role in current collider physics programs
- Understanding V+jets is crucial for the success of many current and future measurements and searches
- Current NLO predictions for V + inclusive jets have been experimentally verified and are shown to be more accurate than the LO counterparts
- V+b-jets  studies have indicated deficiencies in LO, NLO predictions; more study is needed
- W+single c studies indicate good agreement with LO predictions

## Backup Slides

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**Backup Slides**

## W + Inclusive Jets

- Christopher Neu

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***W***** + Inclusive Jets**

## Z + Inclusive Jets

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***Z***** + Inclusive Jets**