# Presentation: 024283

## Jets at CDF

**Jets at CDF**

- Michele Gallinaro
- for the CDF collaboration

- Workshop on Low-x Physics – Lisbon, June 2006

## Outline

- _Jet algorithms_
- _Low P__T__ QCD_
- • underlying event
- _High P__T__ QCD_
- • Inclusive jet cross section:
-  Midpoint (cone) and KT
-  central/forward jets
- • Heavy flavour jets:
- b-jet cross section
- W/Z+heavy flavor
- • Photons+jets

- Only a selection of CDF results!

- Michele Gallinaro - "Jets at CDF" - June 2006

## CDF at the Tevatron

**Tevatron**
**√****s=1.96 TeV**

- 25pb-1/week

- 60,000 Ws/week

**Peak lum: **
**1.7 10****32**** cm****-2**** sec****-1**

**Run II data:**
**1.6 fb****-1**

**~4 MB events per bunch crossing**

- p

- p

- _

- Steel HAD CAL
- |η| < 3.6
- σ(E)/E ≈ 50%/√E

- Silicon
- Vertex Tracker

- Central Tracker
- |η| < 1.0
- σ(PT)/PT ≈ 0.15%

- Lead EM CAL
- |η| < 3.6
- σ(E)/E ≈ 14%/√E

- CDF

- Michele Gallinaro - "Jets at CDF" - June 2006

## Jet reconstruction algorithms

- Jets are collimated sprays of hadrons
- originating from the hard scattering
- what is a jet (theory)
- what can be measured (experiment)
- Different algorithms correspond to
- different observables  give different results!

**K****T**
- Cluster particle/towers
- Based on their relative pT
- Infrared and coll. safe
- No merging/splitting

**MidPoint (cone)**
- Cluster particle/towers
- Based on their proximity
- in the y-φ plane

- Michele Gallinaro - "Jets at CDF" - June 2006

## Jet energy scale

- Fractional systematic uncertainty vs PT

- Run II 2005

- Run II 2004

- Must correct calorimeter energy deposition for detector, algorithm and physics effects to obtain “true” jet energy

- central  region

- factor x2 decrease

- Michele Gallinaro - "Jets at CDF" - June 2006

## Underlying event

- Michele Gallinaro - "Jets at CDF" - June 2006

## Underlying event

**Jet #1 Direction**

** **

****

**“****Toward”**

** **

**“****Transverse”**

** **

**“****Transverse”**

** **

**“****Away”**

** **

**Leading Jet**

**Back-to-Back**

**Jet #1 Direction**

** **

****

**“****Toward”**

** **

**“****Transverse”**

** **

**“****Transverse”**

** **

**“****Away”**

** **

**Jet #2 Direction**

** **

- Physics results at the Tevatron/LHC rely on accurate MC modeling of all characteristics of the event
- Studies of the underlying event in jet events in Run I led to Pythia Tune A
- Run II studies being expanded and new MC tunes expected soon (summer)

- Michele Gallinaro - "Jets at CDF" - June 2006

## Jet shapes

-   Jet shapes ruled by multi-gluon emission from primary parton
- test of parton shower models
- sensitive to underlying event structure
- sensitive to quark and gluon mixture in the final state
- find excellent agreement with PYTHIA Tune A (enhanced gluon-ISR)

- _[Phys. Rev. D 71, 112002 (2005)](http://scitation.aip.org/getabs/servlet/GetabsServlet?prog=normal&id=PRVDAQ000071000011112002000001&idtype=cvips&gifs=yes)__ _

- Michele Gallinaro - "Jets at CDF" - June 2006

## Inclusive jet cross section

- Test of pQCD predictions
- Jet cross section measurements test pQCD over many orders of magnitude
- Constrain QCD parameters (PDFs, s)
- Search for new physics
- Search for breakdown of the standard model at very short distances
- compositeness (high-ET excess? high dijet mass events? new heavy particles? resonant structure in dijet mass spectrum?)

- raw ET=567 GeV
- =-0.19

- raw ET=588 GeV
- =+0.43

- Michele Gallinaro - "Jets at CDF" - June 2006

## Inclusive Jets: Run I legacy

- Run I
- cone jet finding algorithm
- apparent excess at high pT, but within the overall systematic errors
- is it “new physics” or parton distribution function effect ?
- Between Run I and Run II
- improved jet finding algorithms:
- 	- MidPoint cone algorithm
- 	- kT algorithm

- Michele Gallinaro - "Jets at CDF" - June 2006

## Jet cross section

- higher cross section than Run I
- increased pT range
- probes physics at small distances ≈10-19m
- tail sensitive to new physics
- tests of pQCD over several orders of magnitude
- sensitive to PDFs (gluon @ high-x)

- Extend measurement to “forward” region
- constrain gluon PDF further (sensitive to different Q2, depending on rapidity)
- central jets are more likely to signal new physics
- forward/central ratio free of syst. uncertainties

- Michele Gallinaro - "Jets at CDF" - June 2006

## Inclusive jet cross section

- use “central jets”(0.1<|y|<0.7)
- correct jets from pile-up events
- correct for calorimeter response
- inclusive jet cross section at particle level (free from detector effects)

- pT range increased by ~200 GeV/c compared to Run I
- test pQCD predictions over 8 orders of magnitude

- RunII reach

- Michele Gallinaro - "Jets at CDF" - June 2006

## Inclusive jet cross section: kT algorithm

- measurements in the forward region have the power to reduce PDF uncertainties

- Inclusive jet cross section: kT algorithm

- Michele Gallinaro - "Jets at CDF" - June 2006

## b-jet production

*b*-jets are signatures of many important new physics processes
- different processes (*b, bb, ****+b, Z+b*) probe different production mechanisms

*_*

- Leading Order

- Next to Leading Order

- gluon splitting

- flavor excitation

- Flavor creation

- other radiative corrections...

- Michele Gallinaro - "Jets at CDF" - June 2006

## b-tagging

- reconstruct secondary vertex from B hadron decays (b-tagging)
- exploit good tracking capabilities of the CDF detector to tag b-jets by reconstructing secondary vertices
- shape of secondary vertex mass used to extract *b*-purity from data

- Michele Gallinaro - "Jets at CDF" - June 2006

## Inclusive b-jet cross section

- test pQCD over more than 6 orders of magnitude
- systematic uncertainties dominated by jet energy scale and fraction of tagged jets which are true *b*-jets
- agreement with NLO pQCD within systematic uncertainties
- large uncertainties related to renormalization and factorization scales suggest that not-included higher order contributions might play a major role in *b*-jet production calculations

- Michele Gallinaro - "Jets at CDF" - June 2006

## Z + b-jet production

- probe *b* content of proton
- background for searches for new physics                     e.g. Higgs ZH→Z*bb*

- _

- Michele Gallinaro - "Jets at CDF" - June 2006

## Z + b-jet production

- No assumptions made on charm content – template fit based on mass of charged tracks at secondary vertex
- Cross section (pTjet>20GeV/c, |jet|<1.5):                     (Z0+*b*-jet)=0.96±0.32(stat)±0.14(syst) pb                                     and ratio
- (Z0+*b*-jet)/(Z0+jet) = 0.0237±0.0078(stat)±0.0033(syst) consistent with NLO predictions 0.48 pb and  0.018±0.004
- consistent with NLO predictions, but still statistically limited
- (more soon)

- Michele Gallinaro - "Jets at CDF" - June 2006

## W/Z + jets production

- W/Z + jets is a possible signature of:
- Top pairs and single top
- Higgs boson
- Supersymmetric particles
- QCD production of W/Z + jets                                          is a large background for                                            these processes
- presence of W/Z ensures high Q2
-  test pQCD in a multijet environment
- large BR to leptons: easy to detect experimentally

- Michele Gallinaro - "Jets at CDF" - June 2006

## W/Z + jets production

- Aim for definition independent of theoretical and detector effects

- restricted W kinematics phase space reduces theoretical dependence, without compromising results

-  LO W+*n* parton prediction reproduces shape well

- first bin agrees by construction

- Michele Gallinaro - "Jets at CDF" - June 2006

## Dijet resonance (W/Z+)

- New physics searches using dijets depend critically on a good understanding of the jet-jet invariant mass resolution

- one source is W/Z→qq→jet jet
- trigger on photon (no jet threshold)
- 	 does not bias the W/Z mass peak
- W/Z(qq) channels are very similar to those used in the Higgs searches
- UA2 saw peak: at Tevatron larger bkg

- Tevatron is the only machine to test the Higgs sector until LHC

- Dijet resonance (W/Z+)

- Michele Gallinaro - "Jets at CDF" - June 2006

## Neural network used for S/B separation

-  found best 10 to maximize S/√B

- (j1-j2)

- (j1-)

- (j2-)

- sphericity

**W/Z+******** (signal)**
****** ****dijet**

- signal eff.: ~70%
- bkg. eff.: ~20%

- NN used to improve S/√B

- Michele Gallinaro - "Jets at CDF" - June 2006

## No W/Z+ signal in 184 pb-1

**Fit from Data**

**Extrapolation**

**Turn on**

- Signal

- Region

- Use data for background shape
-  no evidence for signal yet found

- (SM prediction 20.5 pb)

- CDF preliminary

- NN optimized

- kinematic cuts

- No W/Z+ signal in 184 pb-1

- Michele Gallinaro - "Jets at CDF" - June 2006

## Summary

- Tevatron program has rich program with significant impact on the understanding of high-Q2 QCD
- jets, photons, bosons+jets, heavy flavor+jets
- tune MC generators, parton showering, etc.
- constrain PDFs, especially gluon at high-x
- measure cross sections of QCD processes (new physics, bkg)

- Many analyses may benefit from large datasets
- dijet and bbar cross sections, photon+heavy flavor, etc.

- Latest results correspond to 1fb-1 of data
- progress in understanding of soft PT physics (UE, hadronization, etc.)
- theory agrees with jet cross section over >8 orders
- NLO predictions consistent with b-jet cross section

- Michele Gallinaro - "Jets at CDF" - June 2006

## W/Z + jets production

- W+*n* parton LO calculation + parton shower + hadronisation:
- • W + ≥ n jets cross-section
- • jet kinematics for ≥ n jets

- Issues:
- dependence on Q2 scale
- dependence on cuts
- overlap of samples

-  LO W + ≥*n* parton cross section vs data in good agreement (large uncertainties)

- Michele Gallinaro - "Jets at CDF" - June 2006

## Prompt diphoton cross section

- QCD background to searches for new physics with  signature
- Sensitive to initial state soft gluon radiation in the transverse momentum of  system (qT)
- Main processes qq→, gg→ (low  mass), also one or both ’s from fragmentation of hard parton
- Require isolation to reduce background from 0,
- Residual background removed statistically based on shower shape

- DIPHOX
- All processes at NLO ( added by CDF)
- Fixed order calculation
- ResBos
- Direct contributions at NLO; fragmentation LO
- Initial state soft gluon resummation
- PYTHIA (scaled to data by factor 2)
- LO + parton shower

- Michele Gallinaro - "Jets at CDF" - June 2006

## Prompt diphoton cross section

- DIPHOX breaks at small qT (no soft gluon resummation)
- ResBos is OK at small qT, but fails at small  (importance of NLO fragmentation contribution)
- Statistical uncertainties dominant – hope to have results with larger data sample soon

- Michele Gallinaro - "Jets at CDF" - June 2006

## Inclusive jet cross section

- both kT and MidPoint show good agreement with NLO predictions
- measurements in the forward region have the power to reduce PDF uncertainties

- Michele Gallinaro - "Jets at CDF" - June 2006

## W/Z + jets production

- Michele Gallinaro - "Jets at CDF" - June 2006