Recent Results on Jet Physics and as XXI Physics in Collision Conference Seoul, Korea June 28, 2001 Presented by Michael Strauss The University of Oklahoma PIC 2001 Michael Strauss The University of Oklahoma

PIC 2001 Michael Strauss The University of Oklahoma Outline Introduction and Experimental Considerations Jet and Event Characteristics Low ET Multijet Studies Subjet Multiplicities Cross Sections Three-to-Two Jet Ratio Ratio at Different Center-of-Mass Energies Inclusive Production DiJet Production PIC 2001 Michael Strauss The University of Oklahoma

Motivation for Studying Jets Investigates pQCD Compare with current predictions pQCD is a background to new processes Investigates parton distribution functions (PDFs) Initial state for all proton collisions Investigates physics beyond the Standard Model d2s dET dh ET Central h region pdf ? Compositeness ? PIC 2001 Michael Strauss The University of Oklahoma

Developments in Jet Physics (with proton initial states) Inclusion of error estimates in the PDFs covariance matrices More rigorous treatment of experimental errors calculation of virtual corrections Progress toward NNLO predictions jet algorithms workshop More consistent ET calculations between experiments at the Tevatron PIC 2001 Michael Strauss The University of Oklahoma

Cone Definition of Jets Centroid found with 4-vector addition Cone Definition R=0.7 in h-f Merging and splitting of jets required if they share energy Rsep required to compare theoretical predictions to data (Rsepis the minimum separation of 2 partons to be considered distinct jets) R=2 + 2 h = -ln[tan(q/2)] 2R 1.3R D0 uses Ellis and Soper’s definition PIC 2001 Michael Strauss The University of Oklahoma

PIC 2001 Michael Strauss The University of Oklahoma KT Definition of Jets KT Definition cells/clusters are combined if their relative kT2 is “small” (D=1.0 or 0.5 is a scaling parameter) Infrared Safe Same definition for partons, Monte Carlo and data Allows subjet definitions min(dii, dij) = dij  Merge min(dii, dij) = dii  Jet PIC 2001 Michael Strauss The University of Oklahoma

PIC 2001 Michael Strauss The University of Oklahoma KT and Cone Algorithm Use CTEQ4M and Herwig Match KT jets with cone jets DO Preliminary DO Preliminary 99.9% of Jets have DR<0.5 pT of KT algorithm is slightly higher PIC 2001 Michael Strauss The University of Oklahoma

KT Algorithm and Subjets For subjets, define “large” KT (ycut = 10-3) Theoretically it is infrared safe. Cone algorithm is not because of soft radiation in NNLO calculations. The same algorithm can be applied to partons from fixed-order or resummed calculations, and to MC particles and to data. In the cone algorithm, R_sep is an ad-hoc parameter used to accommodate the difference between jet definitions at the parton and detector levels. An arbitrary procedure must be used to split and merge jets when using the cone algorithm Increasing ycut PIC 2001 Michael Strauss The University of Oklahoma

Jet Selection Criteria Central Tracking EM Calorimeter Hadronic Calorimeter Inner Muon Magnet Outer Muon e g jet m noise Typical selections on EM fraction, hot cells, missing ET, vertex position, etc. > 97% efficient > 99% pure PIC 2001 Michael Strauss The University of Oklahoma

Jet Energy Corrections no distinction between jets of different kinds Response functions Noise and underlying event “Showering” Resolutions: Uncertainty on ET Estimated with dijet balancing or simulation d2s dET dh ET d2s dET dh Response function restores energy loss in calorimeter Noise and underlying event added Showering correction removes effect where energy from particle inside the cone is lost outside of the cone Resolution causes energy. I.e. in cross section plot. ET Observed Cross Section Important for cross section measurement PIC 2001 Michael Strauss The University of Oklahoma

Jet and Event Quantities Low ET Multijet Studies Subjet Multiplicity PIC 2001 Michael Strauss The University of Oklahoma

D Low ET Multijet events ET of Leading Jet At high-ET, NLO QCD does quite well, but the number of jets at low- ET does not match as well. (Comparison with Pythia) Each jet’s ET>20 GeV. Theory normalized to 2-jet data >40 GeV. Looks at low x gluon radiation The normalization corresponds to a K-factor in Pythia of 1.31 Looking also at Jetrad and Herwig PIC 2001 Michael Strauss The University of Oklahoma

D Low ET Multijet events Strong pT ordering in DGLAP shower evolution may suppress “spectator jets” in Pythia BFKL has diffusion in log(pT) (DATA-THEORY)/THEORY PIC 2001 Michael Strauss The University of Oklahoma

D Subjet Multiplicity Using KT Algorithm Monte Carlo Perturbative and resummed calculations predict that gluon jets have higher subjet multiplicity than quark jets, on average. Linear Combination: <M> = fg Mg + (1-fg) MQ DO Preliminary Mean Jet Multiplicity Gluon Jet Fraction Quark Jet Fraction PIC 2001 Michael Strauss The University of Oklahoma

D Subjet Multiplicity Using KT Algorithm Assume Mg, MQ independent of √s Measure M at two √s energies and extract the g and Q components DO Preliminary PIC 2001 Michael Strauss The University of Oklahoma

D Subjet Multiplicity Using KT Algorithm Raw Subjet Multiplicities Extracted Quark and Gluon Mutiplicities DO Preliminary DO Preliminary Higher M  more gluon jets at 1800 GeV PIC 2001 Michael Strauss The University of Oklahoma

D Subjet Multiplicity Using KT Algorithm HERWIG prediction =1.91±0.16(stat) Largest uncertainty comes from the gluon fractions in the PDFs Coming soon as a PRD PIC 2001 Michael Strauss The University of Oklahoma

ZEUS Subjet Multiplicity Comparison at hadron level Unfolded using Ariadne MC NLO QCD describes data Sensitive to as PIC 2001 Michael Strauss The University of Oklahoma

PIC 2001 Michael Strauss The University of Oklahoma as from ZEUS Subjets nsub-1   Proportional to as Major Systematic Errors Model dependence (2-3%) Jet energy scale (1-2%) Major Theoretical Errors Variation of renormalization scale PIC 2001 Michael Strauss The University of Oklahoma

PIC 2001 Michael Strauss The University of Oklahoma Cross Sections Inclusive cross sections Rapidity dependence KT central inclusive R32 630/1800 ratio of jet cross sections Di-Jets as Conclusions PIC 2001 Michael Strauss The University of Oklahoma

PIC 2001 Michael Strauss The University of Oklahoma Jet Cross Sections How well are pdf’s known? Are quarks composite particles? What are appropriate scales? What is the value of as? Is NLO (as3) sufficient? PIC 2001 Michael Strauss The University of Oklahoma

CTEQ Gluon Distribution Studies Momentum fraction carried by quarks is very well known from DIS data Fairly tight constraints on the gluon distribution except at high x Important for high ET jet production at the Tevatron and direct photon production PIC 2001 Michael Strauss The University of Oklahoma

Experimental Differential Cross Section Theoretical cross section Physics variables are q and x Detector measures ET and h Counting experiment with detector inefficiencies PIC 2001 Michael Strauss The University of Oklahoma

CDF Inclusive Jet Cross Section 0.1 < |h| < 0.7 Complete c2 calculation PRD 64, 032001 (2001) PIC 2001 Michael Strauss The University of Oklahoma

x-Q2 Measured Parameter Space Q2 (GeV2 ) From D Inclusive Cross Section Measurement PIC 2001 Michael Strauss The University of Oklahoma

D Inclusive Jet Cross Section Five rapidity regions Largest systematic uncertainty due to jet energy scale Curves are CTEQ4M d2 dET d (fb/GeV) PRL 86, 1707 (2001) ET (GeV) PIC 2001 Michael Strauss The University of Oklahoma

D Inclusive Jet Cross Section MRSTU includes no intrinsic parton transverse momentum and therefore has effectively increased gluon distributions at all x relative to MRST. CTEQ4HJ CTEQ4M MRSTg MRST PIC 2001 Michael Strauss The University of Oklahoma

PIC 2001 Michael Strauss The University of Oklahoma Gluon PDF Conclusions c2 determined from complete covariance matrix Best constraint on gluon PDF at high x Currently being incorporated in new global PDF fits PIC 2001 Michael Strauss The University of Oklahoma

Inclusive Cross Section Using KT Algorithm D = 1.0 D Preliminary Predictions IR and UV safe Merging behavior well-defined for both experiment and theory PIC 2001 Michael Strauss The University of Oklahoma

Comparison with Theory Normalization differs by 20% or more No significant deviations of predictions from data When first 4 data points ignored, probabilities are 60-80% PDF c/dof Prob MRST 1.12 31 MRSTg 1.38 10 MRSTg 1.17 25 CTEQ3M 1.56 4 CTEQ4M 1.30 15 CTEQ4HJ 1.13 29 D Preliminary PIC 2001 Michael Strauss The University of Oklahoma Strauss The University of Oklahoma

CDF as from Inclusive Cross Section as2X(0) is LO prediction as3X(0)k1 is NLO prediction X(0) and k1 determined from JETRAD MS scheme used Jet cone algorithm used with Rsep = 1.3 as determined in 33 ET bins ET (GeV) PIC 2001 Michael Strauss The University of Oklahoma Michael Strauss The University of Oklahoma

CDF as from Inclusive Cross Section Experimental systematic uncertainty Largest at low ET is underlying event Largest at high ET is fragmentation and pion response PIC 2001 Michael Strauss The University of Oklahoma Michael Strauss The University of Oklahoma

CDF as from Inclusive Cross Section m scale is the major source of theoretical uncertainty ET/2 < m < 2ET PDF affects as CTEQ4M minimizes c2 Theoretical uncertainties each ~ 5% PIC 2001 Michael Strauss The University of Oklahoma

ZEUS Inclusive Jet Production PIC 2001 Michael Strauss The University of Oklahoma

ZEUS Inclusive Jet Production Measured cross section slightly above NLP pQCD in forward section PIC 2001 Michael Strauss The University of Oklahoma

ZEUS Inclusive Jet Production as Results: Uses various fits of ds/dQ2 and ds/dET Full phase-space High-Q2 region (Q2>500 GeV) High-ET region (>14GeVT) Uses mu_R=ET, and mu_R=Q, uses MRST99 and CTEQ4 PIC 2001 Michael Strauss The University of Oklahoma

R32: Motivation and Method Study the rate of soft jet emission (20-40 GeV) QCD multijet production - background to interesting processes Predict rates at future colliders Improve understanding of the limitations of pQCD Identify renormalization sensitivity Does the introduction of additional scales improve agreement with data ? Measure the Ratio with HT for all jets with ET > 20, 30, 40 GeV for <3 and ET > 20 GeV for <2 PIC 2001 Michael Strauss The University of Oklahoma

PIC 2001 Michael Strauss The University of Oklahoma Inclusive R32 Features: Rapid rise HT<200GeV Levels off at high HT Interesting: 70% of high ET jet events have a third jet above 20 GeV 50% have a third jet above 40 GeV PIC 2001 Michael Strauss The University of Oklahoma

R32 Sensitivity to Renormalization Scale ET>20 GeV, h<2 show greatest sensitivity to scale PIC 2001 Michael Strauss The University of Oklahoma

PIC 2001 Michael Strauss The University of Oklahoma R32 Results Jet emission best modeled using the same scale i.e. the hard scale for all jets Best scale is that which minimizes 2 for all criteria R=0.6ETmax, for 20 GeV thresholds R= HT, .3 for all criteria Introduction of additional scales unnecessary. ET>20 GeV, h<2 PRL 86, 1955 (2001) PIC 2001 Michael Strauss The University of Oklahoma

D Cross Section Ratio: s(630)/s(1800) vs xT Ratio of the scale invariant cross sections : at different cm energies ( 630 and 1800 GeV) Ratio allows substantial reduction in uncertainties (in theory and experiment). May reveal: Scaling behavior Terms beyond LO ( as2 ) s ss = (ET3/2p) (d2s/dETdh) vs XT = ET / (s / 2 ) ET ss XT QCD 2 s(630)/s(1800) 1 0.0 xT 0.4 Naive Parton model PIC 2001 Michael Strauss The University of Oklahoma

D Inclusive Cross Section s = 1800 GeV s = 630 GeV PIC 2001 Michael Strauss The University of Oklahoma

PIC 2001 Michael Strauss The University of Oklahoma Cross Section Ratio (630)/(1800) is 10-15% below NLO QCD predictions Top plot: varying choice of pdf has little effect Bottom plot: varying R scale is more significant Better agreement where R different at 630 and 1800 (unattractive alternative !) Higher order terms will provide more predictive power! Published in PRL 86, 2523 (2001) PIC 2001 Michael Strauss The University of Oklahoma

PIC 2001 Michael Strauss The University of Oklahoma CDF DiJet Provides precise information about initial state partons Cone of R=0.7 Both Jets: ET>10 GeV Jet 1: 0.1<|h|<0.7 Jet 2: Four h regions 0.1<|h|<0.7 0.7<|h|<1.4 1.4<|h|<2.1 2.1<|h|<3.0 PIC 2001 Michael Strauss The University of Oklahoma

CDF DiJet Cross Section PDF c2/dof MRST 2.68 MRST 3.63 MRST 4.49 CTEQ4M 2.88 CTEQ4HJ 2.43 Mrstd has average kt of 0.64 GeV All < 1% Probability PIC 2001 Michael Strauss The University of Oklahoma

PIC 2001 Michael Strauss The University of Oklahoma ZEUS DiJet kT algorithm used ET > 8 GeV (leading) ET > 5 GeV (other) -1<h<2 (leading) 470<Q2<20000 GeV2 Phys Lett B507, 70 (2001) PIC 2001 Michael Strauss The University of Oklahoma

PIC 2001 Michael Strauss The University of Oklahoma ZEUS DiJet R2+1 parameterized as: R2+1 (MZ) = A1as(MZ) + A1as2(MZ) PIC 2001 Michael Strauss The University of Oklahoma

PIC 2001 Michael Strauss The University of Oklahoma ZEUS as Summary Dijets has lowest total error of all Zeus measurements. All measurements consistent with PDG value of 0.1185±20 Cdf was .1129 +/- .0001 +/- .008 plus theory PIC 2001 Michael Strauss The University of Oklahoma

Great reach at high x and Q2, A great place to look for new physics! Tevatron Run II Run II: Ecm = 1.96 TeV, L  2fb-1 expect: ~100 events ET > 490 GeV and ~1K events ET > 400 GeV Run I: Ecm = 1.8 TeV, L  0.1fb-1 yielded 16 Events ET> 410 GeV Great reach at high x and Q2, A great place to look for new physics! PIC 2001 Michael Strauss The University of Oklahoma

Conclusions from Jet Physics Growing sophistication in jet physics analysis Error matrices New jet algorithms Better corrections PDF refinements Results generally agree with NLO QCD and PDF’s Cross section measurements will continue to refine PDF’s as measurements agree with PDG Low ET physics still require theoretical refinements Jet physics should continue to provide fruitful developments High ET region can reveal compositeness and other new physics Low ET region reveals soft parton distributions in proton NNLO and other theoretical refinements needed Results needed for “discovery” measurements PIC 2001 Michael Strauss The University of Oklahoma