# Presentation: 698497

## Low-x Meeting 2008
6-10 July 2008

Held here

Don Lincoln
Fermi National Accelerator Laboratory

for the DØ collaboration

**Low-x Meeting 2008**
**6-10 July 2008**
**Held here**
**Don Lincoln**
*Fermi National Accelerator Laboratory*
***for the DØ collaboration***

**Jet Physics at DØ**

## Inclusive Jet Cross-Section

Photons & Jets

W + charm

***Inclusive Jet Cross-Section***
***Photons & Jets***
***W + charm***

## The Fermilab Tevatron

- Results based on ~0.7 – 1.0 fb-1

- Highest-energy accelerator currently in operation
- only place where Top quarks have been produced
- Data delivered > 4.4 fb-1
- expect to reach 6- 8 fb-1 by the end of the run.

## Jet Production in pQCD

- Proton remnants form the _Underlying Event (U.E.)_
- _We compare to pQCD calculations_ to NLO (     )

- _Jets_  of particles originate from hard collisions between quark and gluons

## Jet Measurements at the Tevatron

- D0 Run II jet results presented here use the
-  _Additional midpoint seeds_ between pairs of close jets _improve IR safety_
-  _4-vector sum scheme_ instead of sum ET
-  Split/merge after stable proto-jets found

-  _Jet Energy Scale:_ 2-3% at CDF
-                                    1-2% at D0
-   (after 7 years of hard work using MC tuned
-    to data, +jet & dijet event balance)
-  _Energy Resolution:_ unsmearing procedure using /ET measured from dijet data.

-  Midpoint cone algorithm (R=0.7)

-  Main Systematics to Jet Measurements

-  Compare data and theory at the “particle level”

## Jet Events at the Tevatron

- _Three jet event at D0_

- 1st leading Jet (pT ~624 GeV)

- 2nd leading Jet (pT ~594 GeV)

- 3rd  leading jet

- Mjj=1.22 TeV

- DØ
- CDF

**(at HERA)**

**LHC**

- Complementary to HERA and fixed target experiments

- DØ jet coverage |****| < 2.4
- → very forward jets are available!

## DØ calorimeter

- Calorimeter is the most important detector for jet measurements
- Liquid-Argon/Uranium calorimeter:
- Stable response, good resolution
- Partially compensating (*e/**** *~ 1)
- Gaps covered with scintillator tiles

- Calorimeter structure divides the measurement in three regions:
**Central calorimeter** (easiest)
**Intercryostat region **(challenging)
**End caps **(fine segmentation)

## Inclusive cross section results

- Largest data set from Run II with the widest rapidity coverage (|y| <2 .4) and smallest uncertainties to date
- Uncertainties competitive with (better than) Run I and CDF
- Jet spectrum presented at **particle level **with** midpoint cone** (*R*cone = 0.7)
- Compared to next-to-leading order (NLO) theory with CTEQ6.5M PDFs and non-perturbative corrections from Pythia

## Comparison to theory

- Comparing data and theory, the general tendency is to favor MRST2004 PDFs or the lower edge of CTEQ6.5 uncertainty ⇒ less high-*x* gluon
- CTEQ6.5 reduced PDF uncertainties by ~×2 compared to CTEQ6.1

## Improvement since 2006

- The uncertainties have improved by up to factor two and more in the central region since preliminary JES (2006)
- Forward regions not published before, but improvement over factor ten

## Final results

- Good agreement between data and theory at all rapidities; MRST2004 PDFs and the lower end of CTEQ6.5 PDF uncertainty favored
- Scale uncertainty in next-to-leading order (NLO) theory                comparable to experimental uncertainty at low *p**T*

## DØ and CDF comparison

- The DØ and CDF data are compatible within uncertainties
- Note that the CTEQ6.1 PDF band in the CDF plot is twice as wide as the CTEQ6.5 PDF band in the DØ plot
- Central values of the theory slightly different

## Uncertainty correlations

- Leading sources are from JES:
**EM energy scale** (*Z ** **e**+**e*** calibration)
**Photon energy scale** (MC description of *e */  response, material budget)
**High *****p******T***** extrapolation** (fragmentation in Pythia/Herwig, PDFs)
**Rapidity decorrelation** (uncertainty in -dependence)
**Detector showering** (goodness of template fits)
- Only five highest out of **23 correlated systematics** shown

## Inclusive Jets:  Summary

- Detailed inclusive jet cross section measurement over eight orders of magnitude in range *p**T  *= 50  600 GeV with wide rapidity coverage (six bins in *|y*| < 2.4)
- Good agreement with NLO pQCD calculations observed, with reduced high *x* gluon favored compared to CTEQ6.5M
- Uncertainty correlations studied in detail and correlations found to be high; 23+1 sources provided for global PDF fits
- Request from CTEQ and MRSW groups for data to be incorporated to global PDF fits
**Submitted to PRL.  Final acceptance imminent.**

## Photon Production

- Direct photons come unaltered from the hard sub-process Allows to understand hard scattering dynamics

** ****Electromagnetic Shower Detection **

** ****Higher Granularity**
** ****Detector**

** ****Preshower **

** ****EM Calorimeter **

-  EM shower with very little energy in
-   hadronic calorimeter
-  Geometric isolation
-  No associated track
-  R(, Jet) > 0.7 (cone jets, R = 0.7)

- _Photon Identification_

- _Background Estimation_

-  Origins: Neutral mesons: o, 
-               + Instrumental: EM jets
-  Shower shape quantities in NN
-    to estimate purity.

## Inclusive + X Production

- Neural net based on shower shape variables used in distinguishing photon

- Purity estimated by fitting NN templates to data
- Systematic uncertainty includes varying fit range

- Inclusive **+ X Production

## Inclusive + X Production

- D0 Collab., Phys. Lett. B 639, 151 (2006)

-  Results consistent with NLO theory
-  pT dependence similar to former
-   observations (UA2, CDF)

- Measurements based on higher stats, ~3 fb-1 with ~300 GeV reach, coming soon

-  Signal fraction is extracted from
-   data fit to signal and background
-   MC isolation-shape templates
-  Data-Theory agree to within ~20%
-   within errors

- Inclusive **+ X Production

## Inclusive + jets Production

- Also fragmentation:

- Dominant production at low pT(< 120 GeV) is through Compton scattering: qg  q+

- ___+jet + X Event selection_
-  || < 1.0 (isolated)
-  pT > 30 GeV
-  |jet| < 0.8 (central), 1.5 < |jet| < 2.5 (forward)
-  pTjet> 15 GeV
-   4 regions: g.jet>0,<0, central and forward jets
-  MET < 12.5 GeV + 0.36pT (cosmics, W   e)

- Probe PDF's in the range 0.007 < x < 0.8 and
-  pT= 900 < Q2 < 1.6 x 105 GeV2

- 0804.1107 [hep-ex], Submitted to PLB, very near acceptance

- Inclusive **+ jets Production

## Inclusive + jets Production

- Neural net used to distinguish photons and determine photon purity

- Inclusive **+ jets Production

## Inclusive + jets Production

- Cross section determined in four rapidity bins and over large Pt range

- Inclusive **+ jets Production

## Inclusive + jets Production

- Inclusive **+ jets Production

-  Similar pT dependence than inclusive
-   photons in UA2, CDF, and D0
-  Shapes very similar for all PDFs
-  Measurements cannot be simultaneously
-   accommodated by the theory

-  Most errors cancel in ratios between
-   regions (3-9% across most pT range)
-  Data & Theory agree qualitatively
-  A quantitative difference is observed
-   in the central/forward ratios

- Need improved and consistent theoretical description for + jet

## W + c-jet Production

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

**c**

**c**

**W****-**

-  _W+c-jet _is background to top pair, single top, Higgs.
-  It can signal the presence of new physics
-  Direct sensitivity to s-quark PDF

- _Data Selection_
-  L = 1 fb-1
-  W(l)isolated lepton pT>20 GeV, MET>20 GeV
- |jet| < 2.5, pTjet>20 GeV
-  Muon-in-jet with opposite charge to W is a c-jet candidate

- Systematic errors largely cancel in the ratio

- _Background_
-  WZ, ZZ rarely produce charge correlated jets
-  tt, tb, W+bc and W+b suppresed (small x-sec)

- 0802.2400 [hep-ex] Accepted by PLB – _D0_
- Phys. Rev. Lett. 100, 091803 (2008) - _CDF_

-        % difference in CTEQ vs MRST
-         % uncertainty on the CTEQ6.5M set

## Results from W  e  and W    channels

- jet pT is corrected to the particle level
- measurement compared with the theory
- ALPGEN: for tree level matrix element calculation
- PYTHIA: for parton shower
- uncertainty due to CTEQ 6.5M PDFs is 6.6%
- both channels show consistent results

## integrated over all pT and all bins with |η| < 2.5

no significant deviation from the theoretical prediction 

- no significant deviation from the theoretical prediction

- recently accepted by
- Physics Letters B
- arXiv:0803.2259v1 [hep-ex]
- Fermilab-Pub-08/062-E
- CDF’s recently published result:
- Phys. Rev. Lett. 100, 091803

- jet pT is corrected to the particle level
- measurement compared with the theory
- ALPGEN: for tree level matrix element calculation
- PYTHIA: for parton shower
- uncertainty due to CTEQ 6.5M PDFs is 6.6%

-    **0.074 ± 0.019 (stat.) + 0.012 -0.014 (syst.)**

- Result

## Summary

- The Tevatron experiments are entering the era of    precision QCD measurements based on samples in excess of 1 fb-1
- Good agreement with pQCD within errors is observed for jet production measurements
- An improved and consistent theoretical description is   needed for +jets
- W + charm production measurements are consistent with theoretical prediction