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Low-x Meeting 20086-10 July 2008

Held here

Don LincolnFermi National Accelerator

Laboratory

for the DØ collaboration

Jet Physics at DØ

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● Inclusive Jet Cross-Section

● Photons & Jets

● W + charm

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The Fermilab Tevatron

82.32.5Interactions/

crossing

3963963500Bunch crossing (ns)

50173 Ldt (pb-1/week)

3 10329 10311.6 1030Peak L (cm-2s-1)

1.961.961.8s (TeV)

36 3636 366 6Bunches in Turn

Run IIbRun IIaRun I

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.

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Quark and gluon density is described by PDFs.

Proton remnants form the Underlying Event (U.E.)

We compare to pQCD calculations to NLO ( )

Jet Production in pQCD

3s

Jets of particles originate from hard collisions between quark and gluons

fragmentation

partondistributio

n

partondistributio

n

Jet

Underlyingevent

Photon, W, Z etc.

Hard scattering

ISR FSR

p

p

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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”

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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

DØCDF

(at HERA)

LHC

GeV)980(EGeV)980(E

pTeV1.96sp

Complementary to HERA and fixed target experiments

DØ jet coverage || < 2.4

→ very forward jets are available!

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D0 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)

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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 (Rcone = 0.7)

● Compared to next-to-leading order (NLO) theory with CTEQ6.5M PDFs and non-perturbative corrections from Pythia

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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

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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

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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 pT

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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

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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 pT 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

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Inclusive Jets: Summary● Detailed inclusive jet cross section

measurement over eight orders of magnitude in range pT = 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

● [Re]submitted to PRL. Final acceptance imminent.

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

ElectroMagnetic Shower Detection

Higher granularityEM 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.

Photon Production

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•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

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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

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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], [Re]submitted to PLB, very near acceptance

Inclusive + jets Production

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Neural net used to distinguish photons and determine photon purity

Inclusive + jets Production

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

Inclusive + jets Production

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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

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W + c-jet Production

s (90%) or

d (10%)

cc

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•W + (light) jet •WZ, ZZ rarely produce charge correlated jets• tt, tb, W+bc and W+b suppresed (small x-sec)0802.2400 [hep-ex] Accepted to PLB – D0 Phys. Rev. Lett. 100, 091803 (2008) - CDF

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

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Results from We 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

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● integrated over all pT and all bins with |y| < 2.5

● 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

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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

Several new results intended to be announcedat ICHEP. Stay tuned!