# Presentation: 626775

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

**Christopher Neu**
- on behalf of the
- CDF and D Collaborations
**XLIIIrd 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 *****W/Z *****+ jets**
**Recent Tevatron progress**
**Summary and future**

**Outline:**

## Importance of W/Z + Jet Physics

- Page

**Importance of *****W/Z***** + Jet Physics**

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

***Why study W/Z +jet production?***

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

***V***

***V***

***q***

***q***

***q’***

***q***

***q***

***q’***

***q***

***q******(’)***

***q******(‘)***

***q***

**Notes:**
heard much earlier today about the Tevatron jet and photon+jet physics programs

LO and NLO simulations are what we as experimentalists want to use – specifically in creating background models for 
searches..

## The CDF and D Experiments

- Page

**The CDF and D******** Experiments**

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

**Common features:**

## W + Inclusive Jets

- Christopher Neu

- Page

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

***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 GeV, || < 2.0
- Cone algorithm, R= 0.4

**Fake W bkgd**

**Real W bkgd**

**W+jets signal**

**W+jets signal**

**Fake W bkgd**

**Real W bkgd**

**W+jets signal**

**Fake W bkgd**

**Real W bkgd**

**W+jets signal**

**Fake W bkgd**

**Real W bkgd**

**S/B ~ 10/1**

**S/B ~ 1.2/1**

**Notes:**
Study of W+jets production
Wev , then consider samples with >=1, >=2, >=3, >=4 jets
MET is characteristic shape for real W’s and fake W’s.  Most such events come from W+jets production (signal here).
Maximum likelihood fits of MET are used to get contributions from signal, background for MET>30

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)
Not corrected for underlying event, out-of-cone, splash out

IR safety: add infinitely soft gluon emissions and see if the jets found are still the same.  If they are then the jet alg is “IR safe”.
JetClu, Midpoint are not IR safe.  Something like SISCone is.

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

One can construct total cross sections from these different inclusive jet multiplicities...

## W + Inclusive Jets

- Christopher Neu

- Page

**PRD 77, 011108(R)**

**NLO prediction more **
**accurate than LO!**
- Relative rates within each prediction consistent with data.

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

**MLM** :
- ALPGEN (LO) +
- Herwig (shower) + MLM matching

**SMPR** :
- MadGraph (LO) +
- Pythia (shower) + CKKW matching

**MCFM** :
- MCFM (NLO)

**Notes:**
Total cross sections as a function of minimum jet multiplicity compared to some recent theory predictions.
This is important since we want to see if the LO, NLO models are reliable for giving us the rates of these
events...we want to use the rate predictions in other analyses where W+jets is a background.

Three theory predictions:
MLM = ALPGEN v2.12 (LO) + Herwigv6.5 + MLM matching + CTEQ5L
SMPR = MadGraph v4 (LO) + Pythiav6.3 + CKKW + CTEQ6L1
MCFM = NLO, no showering, no UE + CTEQ6.1M  ----- just W+1-jet and W+2-jet

We talk about matching more in the next slide – stay tuned.

Diff PDFs used for each but all CTEQ5/6, so no major difference.

Data errors: inner is statistical, outer is total (stat+syst)
 - mostly driven by JES at low jet ET and understanding bkgds at high jet ET

Theory errors: LO: varying the renormalization scale x0.5-x2.0
		  NLO: pdfs and scale...smaller impact!
Conclusions:
 - Both LO total cross section preds as a function of nth jet ET undershoot data
 - NLO is improved wrt the data measurement
 - Ratio of n/n-1 is pretty consistent for LO and NLO preds and consistent with data
 - Consistency of n/n-1 points indicate that the LO and NLO are both getting the hard outgoing parton physics right

NLO does a good job but we are limited in the multiplicity of outgoing partons we can consider.
So LO cannot be abandoned - How do these LO calculations proceed?

## W + Inclusive Jets

- Page

**PRD 77, 011108(R)**

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

**MLM** :
- ALPGEN (LO) +
- Herwig (shower) + MLM matching

**SMPR** :
- MadGraph (LO) +
- Pythia (shower) + CKKW matching

**MCFM** :
- MCFM (NLO)

**Can examine differential cross sections **
**within each multiplicity bin....**

**NLO prediction more **
**accurate than LO!**
- Relative rates within each prediction consistent with data.

**Notes:**
Total cross sections as a function of minimum jet multiplicity compared to some recent theory predictions.
This is important since we want to see if the LO, NLO models are reliable for giving us the rates of these
events...we want to use the rate predictions in other analyses where W+jets is a background.

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

We talk about matching more in the next slide – stay tuned.

Diff PDFs used for each but all CTEQ5/6, so no major difference.

Data errors: inner is statistical, outer is total (stat+syst)
 - mostly driven by JES at low jet ET and understanding bkgds at high jet ET

Theory errors: LO: varying the renormalization scale x0.5-x2.0
		  NLO: pdfs and scale...smaller impact!
Conclusions:
 - Both LO total cross section preds as a function of nth jet ET undershoot data
 - NLO is improved wrt the data measurement
 - Ratio of n/n-1 is pretty consistent for LO and NLO preds and consistent with data
 - Consistency of n/n-1 points indicate that the LO and NLO are both getting the hard outgoing parton physics right

NLO does a good job but we are limited in the multiplicity of outgoing partons we can consider.
So LO cannot be abandoned - How do these LO calculations proceed?

## W + Inclusive Jets

- Christopher Neu

- Page

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

- LO calculation procedure: Generate *pp**W*+N partons at tree level, ignore loop corrections, employ parton shower
- Ambiguities arise:
- Possibility for double counting if Nparton  Njet
- SMPR and MLM refer to algorithms for avoiding/removing overlaps

**PRD 77, 011108(R)**

**MLM** :
- ALPGEN (LO) +
- Herwig (shower) + MLM matching

**SMPR** :
- MadGraph (LO) +
- Pythia (shower) + CKKW matching

**MCFM** :
- MCFM (NLO)

**W+********2 jets**

**At LO, MadGraph+Pythia+CKKW **
**provides better performance.**

**Notes:**
Multi-particle final states are hard to calculate in a complete sense – necessitated marriage of ME and PS tools.  

NLO only available up to W+2 outgoing partons.
LO: 2N generated, loops ignored, parton shower adds in radiation effects.
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.

Conclusions:
 - Calculations for W+N are not easy – LO 2N +PS is one way to go
 - ME calculations are similar – it is the PS and matching that are being tested here
 - Indications are that the Pythia+CKKW is superior to Herwig+MLM...although it would be nice to disentangle the two effects
 - NLO does a good job seemingly although the outgoing partons are not showered here (approximation made to make a “jet”) and NLO is only available for a limited number of outgoing partons

CKKW: matching procedure that casts the outgoing jets into two distinct categories; those above some scale are assigned to ME, those below some scale are assigned to PS
MLM: make sample with N outgoing partons, shower, make jets; keep event if N_MC_jets = N_partons
You typically make 0p,1p,>=2p samples – events from each are necessary to describe exactly N-jet physics.  
For highest parton multiplicty sample, events are allowed to have N_MC_jets >= N_partons

Conclusions on good hold for Z+jets??

## Z /* + Inclusive Jets

- Christopher Neu

- Page

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

**Validity of NLO predictions borne out in *****Z /******************+jets? **
***Z /************* *****selection**: seek *Z /*******e*+*e*-
- Two ET > 25 GeV electrons
- 66 < Mee < 116 GeV/c2
**Jet definition:**
- Corrected pT > 30, |y| < 2.1
- Cone algorithm  w/ R=0.7
**Major backgrounds:  S/B ~ 7/1**
- QCD multijets
*W *+ jets
- ttbar, diboson
*Z+***, *Z***

**NLO prediction once again**
**more accurate than LO!**

**Can’t see **the NLO
- prediction points -
- close overlap with data!

**hep-ex/0711.3717 (Submitted to PRL) **

**Notes:**
CDF performed a similar analysis in Z+jets in 1.7/fb of data.

Look first at total inclusive cross sections as a function of jet multiplicity n. 

Systematic uncertainties: JES, electron acceptance in MC, bkgds

Conclusions:
 - NLO prediction from MCFM once again doing a good job for differential cross section as a function of jet pT.
This is slightly different in that every jet in the events is an entry in these curves....but you get the point.  NLO
prediction from MCFM is doing a good job.  

## Z /* + Inclusive Jets

- Christopher Neu

- Page

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

- Differential cross section:
- No LO comparison here
**    ****NLO prediction**
**reliable – as in*****W*****+jets**
- Analysis would benefit from increased statistics to further populate the *Z*+2-jets sample
- NLO for *Z*+3-jets would be valuable as well.

**hep-ex/0711.3717 (Submitted to PRL) **

## Z /* + Inclusive Jets

- Page

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

**D******** *****Z /************(*********ee*****)+jets analysis: 950**/pb
**Purpose here: **compare Pythia (*pp**W*+1+ internal PS) and Sherpa (*pp**W*+N+ internal PS + CKKW matching) event generators
- Test of different prediction techniques
- Some confidence in CKKW from CDF *W*+jets LO studies...true here as well?

**Pythia**

**Sherpa**

## Z /* + Inclusive Jets

- Christopher Neu

- Page

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

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

**Not unexpected given **
**the nature of Pythia’s calculation.**

**Pythia**

**Sherpa**

## Summary so far...

*W/Z*+1,2 jet **NLO predictions** from MCFM **look reliable**
- NLO predictions **not yet in hand **for *W/Z*+3 jet
- Technique of calculating/generating ***pp**********W*****+N+ parton shower + matching **scheme (ala ALPGEN, MadGraph, Sherpa) **superior to Pythia+PS alone**
**Differences among available tools still need to be understood**
***W/Z***** + heavy flavor (*****b,c*****) jets also**
**	****important**
- background to top, Higgs, others
*W*+*c* production has unique features

- Christopher Neu

- Page

**W/Z**

**W/Z**

**l ****/ l,******

****** ****/ l, ******

## Tagging: b’s and Non-b’s

- Page

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

**Primary vertex**

**Secondary **
**vertex**

**Primary vertex**

**Tagging of real *****b***** jet:**
- long lifetime+
- large boost =
**secondary vertex**

**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 u/d/s/g 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

**Goals: **
- Measure *W+b*-jet production
- 	cross section
- Use measurement to improve
- 	background estimate for Higgs
- 	search
*W* and jets selection here similar to
*	**W* + inclusive jets analysis
- key difference: 1 or 2 jets only
- Here we need to identify jets that
- 	are likely *b*’s (via **high purity **
**	****tagging**) and determine how many
- 	are really *b*’s via **vertex mass:**
**	****	**-  invariant mass of charged particle
- 		    tracks in secondary vertex

- Christopher Neu

- Page

***M******B-hadrons****** > M******C-hadrons****** > M******LF-hadrons***
**	****	****     ****so**
**	*****M******b****** vert****** > M******c****** vert****** > M******LF******vert***

- From simulation

**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%)
*Total contribution: ~*180 tagged *b* jets
**Result: **
- 	xBR = 2.74  0.27 (stat)  0.42 (syst)  pb
**Prediction: **
- 	 xBR = 0.78 pb
- 		              (ALPGEN )

- Christopher Neu

- Page

- 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.

- ~1000 tagged jets
- among which ~700 are
- consistent with coming from a *b* quark

**New result!**
**x3.5 mismatch**

**High purity **
***b*****-tagging at work!**

- NB:

**Publication in preparation.**

**Notes:**
x3.5 mismatch btwn ALPGEN and data for b-jet cross section in W+b-jet production
x1.4 mismatch btwn ALPGEN and data for differential cross section as a function of jet ET...different measures but 
conclusion is clear: something is not being done correctly in the ALPGEN W+b-jets prediction.

On the way:  inclusive measurements (ala W+jets)
		differential measurements (ala W+jets)
		comparison to NLO

## Z + b-Jets

- Page

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

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

- Differential cross sections with comparisons to  LO, NLO predictions
- Dividing by (Z) puts LO, NLO on equal footing
-  Pythia does a good job at low jet ET

## Z + b-Jets

- Page

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

- ALPGEN (LO) and MCFM (NLO) undershoot data in several bins
- Pythia surprisingly on target in some regimes – LO predictions are  in other analyses low (eg, Z+jets).

**Publication in preparation.**

**Notes:**
ALPGEN + Pythia?  If so then Tune A / UE goodness is in both the Pythia and ALPGEN predictions.

## W/Z + b-Jets: Summary

- Page

***W/Z***** + *****b-*****Jets: Summary**

- More studies for *W+b*-jets are forthcoming
- Need to understand NLO predictions
- In *Z+b*-jets it is strange that the NLO prediction undershoots data
- Borne out in *W+b*-jets?

- Raw NLO predictions corrected for
- underlying event and hadronization effects.

## W + Single c Production

- Christopher Neu

- Page

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

- W+single *c *production unique
- no *W*+single *b* analogue at Tevatron
**Exploit feature:  **charm semileptonic daughter and *W* have opposite charge

**c**

**c**

**W****-**

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

**μ**

**Different tagging algorithm:**
- Soft Muon Identification
- used to identify **charm hadron decays!**

**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

**Good agreement!**

** ****hep-ex/0711.2901  - submitted to PRL**

## W + Single c Production

- Page

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

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

- LO prediction
- reasonably good.

- Impact of systematic errors drawn on theory curve.
- Statistical uncertainty drawn for data points .

**Publication in preparation.**

## Conclusions

- Page

**Conclusions**

- W/Z + jets physics plays an **important role** in current collider physics programs
- Current NLO predictions for W/Z + look to be **accurate**, higher multiplicities desirable
***W/Z*****+b-jets**  studies have indicated deficiencies in both LO and NLO predictions; **more study and more data  is needed **
*W*+single *c *studies indicate **good agreement **with LO predictions

## Backup Slides

- Page

**Backup Slides**

## W + Inclusive Jets

- Page

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

**MLM** :
- ALPGEN (LO) +
- Herwig (shower) + MLM matching

**SMPR** :
- MadGraph (LO) +
- Pythia (shower) + CKKW matching

**MCFM** :
- MCFM (NLO)

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

NLO only available up to W+2 outgoing partons.
LO: 2N generated, loops ignored, parton shower adds in radiation effects.
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.

Conclusions:
 - Calculations for W+N are not easy – LO 2N +PS is one way to go
 - ME calculations are similar – it is the PS and matching that are being tested here
 - Indications are that the Pythia+CKKW is superior to Herwig+MLM...although it would be nice to disentangle the two effects
 - NLO does a good job seemingly although the outgoing partons are not showered here (approximation made to make a “jet”) and NLO is only available for a limited number of outgoing partons

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

## Identifying b Jets

- Page

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

***B***** hadron lifetime**: ~1.5 ps
- Large boost (v ~ 0.95*c*) means the *B* lifetime is long in the lab frame
*B* travels macroscopic distance before decaying which we can detect
**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**

## W + b-Jets

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