1 Low-x Meeting July 2008 Held here Don Lincoln Fermi National Accelerator Laboratory for the DØ collaboration Jet Physics at DØ

2 ● Inclusive Jet Cross- Section ● Photons & Jets ● W + charm

3 The Fermilab Tevatron Interactions/ crossing Bunch crossing (ns)  Ldt (pb -1 /week) 3    Peak L (cm -2 s -1 )  s (TeV) 36  36 6  6 Bunches in Turn Run IIbRun IIaRun I Results based on ~ 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 fb -1 by the end of the run.

4 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 Jets of particles originate from hard collisions between quark and gluons fragmentation parton distribution parton distribution Jet Underlying event Photon, W, Z etc. Hard scattering ISR FSR p p

5 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 E T 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”

6 Jet Events at the Tevatron Three jet event at D0 1 st leading Jet (p T ~624 GeV) 2 nd leading Jet (p T ~594 GeV) 3 rd leading jet M jj = 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!

7 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)

8 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

9 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

10 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

11 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

12 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

13 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

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

15 Direct photons come unaltered from the hard sub-process Allows to understand hard scattering dynamics ElectroMagnetic Shower Detection Higher granularity EM 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

16 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

17 D0 Collab., Phys. Lett. B 639, 151 (2006) Results consistent with NLO theory p T 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

18 Also fragmentation: Dominant production at low p T  (< 120 GeV) is through Compton scattering: qg → q+   + jet + X Event selection |   | < 1.0 (isolated) p T > 30 GeV |  jet | < 0.8 (central), 1.5 < |  jet | < 2.5 (forward) p T jet > 15 GeV 4 regions:  g.  jet >0,<0, central and forward jets MET< 12.5 GeV p T (cosmics, W → e ) Probe PDF's in the range < x < 0.8 and p T  = 900 < Q 2 < 1.6 x 10 5 GeV [hep-ex], [Re]submitted to PLB, very near acceptance Inclusive  + jets Production

19 Neural net used to distinguish photons and determine photon purity Inclusive  + jets Production

20 Cross section determined in four rapidity bins and over large Pt range Inclusive  + jets Production

21 Inclusive  + jets Production Similar p T 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 p T  range) Data & Theory agree qualitatively A quantitative difference is observed in the central/forward ratios Need improved and consistent theoretical description for  + jet

22 W + c-jet Production s (90%) or d (10%) c c W-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 p T >20 GeV, MET > 20 GeV |  jet | 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) [hep-ex] Accepted to PLB – D0 Phys. Rev. Lett. 100, (2008) - CDF % difference in CTEQ vs MRST % uncertainty on the CTEQ6.5M set

23 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

24 ● integrated over all p T and all bins with |y| < 2.5 ● no significant deviation from the theoretical prediction ● recently accepted by – Physics Letters B – arXiv: v1 [hep-ex] – Fermilab-Pub-08/062-E ● CDF’s recently published result: – Phys. Rev. Lett. 100, ● jet p T 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% ± (stat.) (syst.) Result

25 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 announced at ICHEP. Stay tuned!