QCD at the Tevatron M. Martínez IFAE-Barcelona Results from CDF & D0 Collaborations

Main Injector (new) Tevatron DØCDF Chicago   p source Booster Tevatron

The Detectors CDF D0

Outline Jet Physics –Jet algorithms –Inclusive Jet production –Angular distributions –Soft gluon radiation Direct Photon Production Drell-Yan –Z & W production –W charge asymmetry –Boson+Jet(s) Production –Angular distributions QCD & New Physics I will talk about…..but not about… B physics (we got overdose yesterday) Fragmentation Functions Diffraction Hadron Spectroscopy QCD coherence effects …………....my apologies if I did not cover your favorite topic..

QCD : Asymptotic freedom & Confinement At high Q (short distances) perturbation theory can be used to compute partonic cross sections At low Q (large distances) pQCD breaks down (and we rely on phenomenological models) String model for hadronization Quarks confined inside hadrons

QCD Factorization Partonic cross section: calculated to a given order in pQCD PDFs of parton inside the proton: needs experimental input (universal  can be used to compute different processes) QCD: free quarks and gluons are not allowed…

Jet Physics Final state parton will produce a parton shower (multigluon emissions) Parton shower evolves until Q ~1 GeV where hadronization process takes place partons are thus revealed through collimated flows of hadrons called jets Parton-to-hadron transition depends on the model for gluon shower and hadronization To compute cross sections involving quarks and gluons in the final state first one needs to precisely define JETs phase spacematrix elementsJet algorithm

Dijet Event in CDF Detector E T = 666 GeV  = 0.43 E T = 633 GeV  = Dijet Mass = 1.36 TeV (probing distance ~ m) CDF (  -r view)  

Cone algorithm   NLO pQCD diagram Convenient to define jets in  space (shape invariant against longitudinal boost) CDF

Run I Cone algorithm 1.Seeds with E > 1 GeV 2.Draw a cone around each seed and reconstruct the “proto-jet” 3.Draw new cones around “proto-jets” and iterate until stability is achieved 4.Look for possible overlaps T merged if common transverse energy between jets is more than 75 % of smallest jet….. T pQCD NLO does not have overlaps (at most two partons in one jet) Therefore it uses larger cone R’ = Rsep x R to emulate experimental procedure -> arbitrary parameter

Run I Results Run I data compared to pQCD NLO Observed deviation in tail …….. was this a sign of new physics ?

gluon PDFs at high-x Important gluon-gluon and gluon-quark contributions at high- Gluon pdf at high-x not well known …room for SM explanation….

Notes on Run I Jet algorithm Cone algorithm not infrared safe: Cone algorithm not collinear safe: The jet multiplicity changed after emission of a soft parton below threshold (no jets) above threshold (1 jet) Fixed-order pQCD calculations will contain not fully cancelled infrared divergences: Replacing a massless parton by the sum of two collinear particles the jet multiplicity changes -> Inclusive jet cross section at NNLO -> Three jet production at NLO -> Jet Shapes at NLO three partons inside a cone

Run II -> MidPoint algorithm 1.Define a list of seeds using CAL towers with E > 1 GeV 2.Draw a cone of radius R around each seed and form “proto-jet” 3.Draw new cones around “proto- jets” and iterate until stable cones 4.Put seed in Midpoint  for each pair of proto-jets separated by less than 2R and iterate for stable jets 5.Merging/Splitting Cross section calculable in pQCD T

Inclusive jet p T cross section Agreement with theory within systematic uncertainties (dominated by jet-energy scale) NLO uncertainty due to high x

Cross section vs rapidity Measurements on large |Y| range help to constrain gluon at high x Good agreement with NLO pQCD New D0 measurements dominated by large uncertainty on jet energy scale (16 % -> will improve in the near future) x Run I

Cone-based algorithms can be modified to be infrared/collinear safe  Midpoint Cone-based jet algorithms include an “experimental” prescription to resolve situations with overlapping cones This is emulated in pQCD theoretical calculations by an arbitrary increase of the cone size : R  R’ = R * 1.3  Nature (QCD ?) prefers to separate partons into jets according to their relative transverse momentum Kt algorithm preferred by theory see Olga´s talk about CDF results with Kt in different  regions (…the ultimate measurement…) Motivation for the K T algorithm

Dijet Angular Distribution Current uncertainties on jet energy scale and gluon PDFs at high x makes difficult to claim new physics from the tail of the Pt distribution….. ……how about QCD dynamics ? similar to Rutherford scattering (dominant t-channel gluon exchange) The presence of quark compositeness at scale  would add terms like We define then..this also tells you gluon has spin 1.. Dijet mass > 635 GeV Rutherford

The Underlying Event A typical Tevatron event consists of : hard interaction initial soft gluon radiation interaction between remnants Underlying Event contribution must be removed from the jets before comparing to NLO QCD predictions Precise measurements at low Pt require good modeling of the underlying event Crucial for a proper estimation of QCD backgrounds in Searches for New Physics…

Mean track multiplicity vs leading jet Pt Underlying Event Studies transverse region sensitive to soft underlying event activity Good description of the underlying event by PYTHIA after tuning the amount of initial state radiation, MPI and selecting CTEQ5L PDFs (known as PYTHIA Tune A)

Studies on Jet Fragmentation Jet shape dictated by multi-gluon emission form primary parton Test of parton shower models and their implementations Sensitive to quark/gluon final state mixture and run of strong coupling Sensitive to underlying event structure in the final state Gluons radiate more than quarks (QCD color charges) Gluon jets Broader

Quark and Gluon Jets Jet narrows as Pt increases gluon/quark mixture running coupling

Jet shapes PYTHIA Tune A describes the data (enhanced ISR + MPI tuning) PYTHIA default too narrow MPI are important at low Pt HERWIG too narrow at low Pt We know how to model the UE at 2 TeV (at least for QCD jet processes)

Direct Photon Production jet  Using prompt photons one can precisely study QCD dynamics: Well known coupling to quarks Give access to lower Pt Clean: no need to define "jets" constrain of gluon PDF ? Experimentally difficult because of ~30 % background from decays    00 Preshower detector Shower maximum detector jet

Pt Distribution of Photons  NLO well below the data Initial state gluon radiation dominates the low pt region of the spectrum…. Add initial kt smearing ? Add Parton Shower ? Resummed calculation ! 4 GeV kT Correction NLO+Parton shower resummed calculation (good agreement) DØ data

Drell-Yan Processes Drell-Yan processes allow to: show the validity of QCD Factorization study the effect of initial state soft gluon radiation in case of W production…constrain u/d quark PDFs

Drell-Yan Processes

W & Z Production Cross Section NNLO pQCD calculation describes the data very well…

W Charge Asymmetry W+ boosted towards proton direction (+  ) constrains d(x)/u(x) positive rapidities

Pt Distribution of Z  Z

Z+jet(s) Production

W+jet(s) Production at CDF One of the most relevant processes at Tevatron: Stringent test of QCD NLO predictions Background for Top/Higgs Physics Reasonable agreement with LO (but large scale dependence) incl. W+n jets LO program interfaced to Herwig for parton shower and hadronization (…..enhanced LO QCD prediction….) describes the mean features of the data Jet spectra Z+jets Run 1

W+jet(s) Production Background to top and Higgs Physics Stringent test of pQCD predictions Test Ground for ME+PS techniques (Special matching  MLM, CKKW to avoid double counting on ME+PS interface) Alpgen + Herwig LO  large uncertainty W + 1 parton +PS W+ 2 partons

W+ jet(s) Production 1 st jet in W + 1p 2 nd jet in W + 2p 3 rd 4 th ME+PS implementation tested using the N th jet spectrum in W+N jet events (more sensitive one) Dijet Mass in W+2jets Energy-scale NLO now available for W+2jets

Angular Distributions Angular distribution dominated by spin-1/2 quark exchange "Nice" test of SM predictions…

QCD & New Physics (I) (Top Quark Production) Relies on good understanding of all background processes..

Multi-jets + Missing Et Final States QCD Z  + 3 jets W  e  + 2/3 jets W   + 2 jets Top ………. Big  s QCD & New Physics (search for SUSY) Backgrounds Channel with the best sensitivity for mSUGRA searches…...needs good understanding of QCD…

Final Remark & References The Tevatron offers unique opportunities to study in detail QCD processes that will be fundamental background for the search of new physics at both Tevatron and the LHC Books you should carry with you: