Diffractive Studies at the Tevatron Gregory R. Snow University of Nebraska (For the D0 and CDF Collaborations) Introduction Hard Color Singlet Exchange (CSE) (forward jet - gap - forward jet) Hard Single Diffraction (forward gap - opposite dijets or W) Double Pomeron Exchange Signatures (forward gap - jets - forward gap) Improvements for Tevatron Run II Conclusions CMS Collaboration Meeting March 8, 2001 Workshop on Forward Physics and Luminosity Determination at the LHC Helsinki, Finland November 2, 2000 ICHEP98 Vancouver, BC, Canada July 23 - 29, 1998 International Europhysics Conference Tampere, Finland July 15 - 21, 1999

University of Nebraska (For the D0 Collaboration) D0 Run I Diffraction Results Gregory R. Snow University of Nebraska (For the D0 Collaboration) Introduction Hard Color Singlet Exchange (CSE) (forward jet - gap - forward jet) Hard Single Diffraction (forward gap - opposite dijets or W) Double Pomeron Exchange Signatures (forward gap - jets - forward gap) Diffractive W and Z Production (W/Z plus gap) Conclusions New-ish Nice result, need to publish New Small-x Workshop Fermilab September 17 - 20, 2003

So why am I here? Dino was my Ph.D. thesis advisor at RockU, somehow diffraction gets in your blood Oversaw some of this work as D0 Run I QCD co-convener 1998-2001 (with Schellman, Brandt, Elvira) and Editorial Board member/chair I have great appreciation for Andrew Brandt’s invention and leadership of the D0 diffraction effort Expanded D0’s Run I physics menu Several unique Ph.D. theses and publications resulted Run I pioneering work has led to Run II Forward Proton Detector – more comprehensive studies to come (see upcoming talks) I said “yes” to this talk

Conclusion before the talk So you’re at a cocktail party, and someone asks you: “What fraction of (insert your favorite QCD dijet, W/Z, direct photon, heavy flavor process here) events are accompanied by a striking rapidity gap in the calorimeter?” Your answer: “About ONE PERCENT. Come here often?”

QCD Physics from DØ QCD publications represent 25% of the 125 papers published by D from its Run 1 data sample (100 pb-1) Jet physics “Transverse Energy Distributions within Jets at 1.8 TeV” “Studies of Topological Distributions of Inclusive Three- and Four-Jet Events at 1800 GeV” “Measurement of Dijet Angular Distributions and Search for Quark Compositeness” “Determination of the Absolute Jet Energy Scale in the D Calorimeters” “The Dijet Mass Spectrum and a Search for Quark Compositeness at 1.8 TeV” “The Inclusive Jet Cross Section in Collisions at 1.8 TeV” “Limits on Quark Compositeness from High Energy Jets in Collisions at 1800 GeV” “The ratio of Jet Cross Sections at 630 GeV and 1800 GeV” “Ratios of Multijet Cross Sections at 1800 GeV” “High-pT Jets at 630 and 1800 GeV” Direct photon physics “The Isolated Photon Cross Section in the Central and Forward Rapidity Region at 1.8 TeV” “The Isolated Photon Cross Section at 1.8 TeV” QCD with W and Z “A Study of the Strong Coupling Constant Using W + Jets Processes” “Measurement of the inclusive differential cross section for Z bosons as a function of transverse momentum produced at 1.8 TeV” “Evidence of Color Coherence Effects in W+Jets Events at 1.8 TeV” “Differential production cross section of Z bosons as a function of transverse momentum at 1.8 TeV” “Differential Cross Section for W Boson Production as a Function of Transverse Momentum in Collisions at 1.8 TeV Rapidity gaps, hard diffraction, BFKL dynamics “Rapidity Gaps between Jets in Collisions at 1.8 TeV” “The Azimuthal Decorrelation of Jets Widely Separated in Rapidity” “Jet Production via Strongly-Interacting Color-Singlet Exchange” “Color Coherent Radiation in Multijet Events from Collisions at 1.8 TeV” “Probing Hard Color-Singlet Exchange at 630 Gev and 1800 GeV” “Hard Single Diffraction in Collisions at 630 and 1800 GeV” “Probing BFKL Dynamics in Dijet Cross Section at Large Rapidity Intervals at 1800 and 630 GeV” “Observation of Diffractively Produced W and Z Bosons in Collisions at 1800 GeV”

D0 Hard Diffraction Studies Understand the Pomeron via …… Or W/Z

Calorimeter tower thresholds used in rapidity gap analyses DØ Detector (Run I) (nL0 = # tiles in L0 detector with signal 2.3 < |h| < 4.3) Calorimeter tower thresholds used in rapidity gap analyses beam Central Gaps EM Calorimeter ET > 200 MeV |h| < 1.0 Forward Gaps E > 150 MeV 2.0 < || < 4.1 Had. Calorimeter E > 500 MeV 3.2 < || < 5.2 L0 Detector End Calorimeter Central Calorimeter EM Calorimeter Central Drift Chamber (ntrk = # charged tracks with |h| < 1.0) Hadronic Calorimeter (ncal = # cal towers with energy above threshold)    = 0.1  0.1 Three different rapgap tags

Central Gaps between Dijets Two forward jets with |h | > 1.9, Dh > 4.0 and rapidity gap in |h | < 1.0 Signature for color singlet exchange Study gap fraction fS as function of Center of mass energy (1800, 630) Jet ET Dh Compare with Monte Carlo color singlet models Low energy run (630 GeV) very illuminating No tracks or cal energy jet h f |h | < 1.0

DØ Central Gaps between Dijets Nevents ncal 2-D multiplicity (ncal vs. ntrk) between two leading jets (ET > 12) for (a) 1800 GeV and (b) 630 GeV Redundant detectors important to extract rapidity gap signal Dijet events with one interaction per bunch crossing selected using multiple interaction flag fS = (Ndata- Nfit)/Ntotal Rapidity gap fraction Negative binomial fit to “QCD” multiplicity

Example of comparison to different DØ Central Gaps between Dijets Fraction of events with central gap fS630 = 1.85  0.09  0.37 % fS1800 = 0.54  0.06  0.16 % (stat) (sys) Sys. error dominated by background fit uncertainties Ratio (630/1800) fS630/fS1800 = 3.4  1.2 Example of comparison to different color singlet models s = 1800 GeV In the soft-color scenario, one can extract a ratio of gap survival probabilities { Gap fraction vs. second-highest jet ET 1.5  0.1 Data consistent with a soft color rearrangement model preferring initial quark states, inconsistent with two-gluon, photon, or U(1) models

Hard Single Diffraction Measure multiplicity here Measure minimum multiplicity here -4.0 -1.6 h 3.0 5.2 Jet ET’s > 12, 15 GeV Forward jets Central jets Two topologies -5.2 -3.0 -1. h 1. 3.0 5.2 Gap fractions (central and forward) at s = 630 and 1800 GeV Single diffractive x distributions Comparisons with Monte Carlo to investigate Pomeron structure Physics Letters B531, 52 (2002)

or Measure Multiplicity here -4.0 -1.6 -1.0 h 1.0 3.0 5.2 s = 1800 GeV s = 630 GeV 2-D Multiplicity distributions Central jets Forward jets L0 scintillator tiles used now Forward jets Central jets

Hard Single Diffraction 1800 Forward Jets Event Characteristics Fewer jets in diffractive events Jets are narrower and more back-to-back (Diffractive events have less overall radiation) Gap fraction has little dependence on average jet ET Solid lines show show HSD candidate events Dashed lines show non-diffractive events

Hard Single Diffraction Data Signal Measured Gap Data Set Fraction 1800 Forward Jets (0.65  0.04)% 1800 Central Jets (0.22  0.05)% 630 Forward Jets (1.19  0.08)% 630 Central Jets (0.90  0.06)% Data Sample Ratio 630/1800 Forward Jets 1.8  0.2 630/1800 Central Jets 4.1  1.0 1800 Fwd/Cent Jets 3.0  0.7 630 Fwd/Cent Jets 1.3  0.1 Gap Fraction # diffractive Dijet Events / # All Dijets Background Signal extraction Forward 1800 GeV example Signal: 2D falling exponential Background: 4 parameter polynomial surface Lessons Forward Jets Gap Fraction > Central Jets Gap Fraction 630 GeV Gap Fraction > 1800 GeV Gap Fraction Publication compares data with POMPYT M.C. using different quark and gluon structures. This data described well by a Pomeron composed dominantly of quarks, or a reduced flux factor convoluted with a gluonic Pomeron with both soft and hard components.

Hard Single Diffraction distributions using (0,0) bin Prescription based on calorimeter energy deposition s = 1800 GeV forward central s = 630 GeV forward = Dp p used to extract  distributions, fractional energy loss of diffracted beam particle   0.2 for s = 630 GeV Shaded bands indicate variance due to calorimeter energy scale uncertainties

Double Gaps at 1800 GeV |Jet h| < 1.0, ET > 15 GeV Gap Region 2.5<|h|<5.2 Demand gap on one side, and measure multiplicity on opposite side

Double Gaps at 630 GeV |Jet h| < 1.0, ET > 12 GeV Gap Region 2.5<|h|<5.2 Demand gap on one side, and measure multiplicity on opposite side

Diffractively Produced W and Z Electron from W decay, with missing ET Rapidity gap Process probes quark content of Pomeron   W  e Z  e+e- considered and require single interaction to preserve possible rapidity gaps (reduces available stats considerably) May expect jets accompanying W or Z Publication (hep-ex/0308032) accepted by Phys. Lett. B

Diffractively Produced W’s  -1.1 0 1.1 3.0 5.2  Minimum side 68 of 8724 in (0,0) nL0 L0 ncal Central and forward electrons considered  -2.5 -1.1 0 1.1 3.0 5.2  Minimum side Peaks in (0,0) bins indicate diffractive W 23 of 3898 in (0,0) Plot multiplicity in 3<||<5.2 (minimum side) nL0 ncal L0

Diffractively Produced Z’s  -2.5 -1.1 0 1.1 3.0 5.2  Minimum side 9 of 811 in (0,0) Plot multiplicity in 3<||<5.2 (minimum side) nL0 L0 ncal ncal Peak in (0,0) bin indicates diffractive Z ncal

Standard W Events Diffractive W Candidates Compare diffractive W characteristics to all W’s Standard W Events Diffractive W Candidates ET=35.2 ET=35.1 Electron ET ET=36.9 ET=37.1 Good agreement given lower diffractive statistics Missing ET MT=70.4 MT=72.5 Transverse mass

Fraction of diffractively produced W/Z Diffractive W/Z signals extracted from fits to the 2-D multiplicity distributions, similar to hard single diffraction dijet analysis Small correction to fitted signal from residual contamination from multiple interaction events NOT rejected by single interaction requirement (based on # vertices, L0 timing) Corrections due to jets misidentified as electrons and Z’s which fake W’s very small Sample Diffractive Probability Background All Fluctuates to Data Central W (1.08 + 0.19 - 0.17)% 1 x 10-14 7.7s Forward W (0.64 + 0.18 - 0.16)% 6 x 10-8 5.3s All W (0.89 + 0.19 – 0.17)% 3 x 10-14 7.5s All Z (1.44 + 0.61 - 0.52)% 5 x 10-6 4.4s Opposite trend compared to hard diffractive dijet case Partonic models don’t describe the entirety of the data (give more Forward than Central) Forward W isn’t described well by any normalization. Soft Color seems to do best, but not evolved enough CDF was 3.8sig signal, significant existence for Z even with few events {

W+Jet Rates Jet ET Data Quark Hard Gluon It is instructive to look at W+Jet rates for rapidity gap events compared to POMPYT Monte Carlo, since we expect a high fraction of jet events if the Pomeron is dominated by the hard gluon NLO process. Jet ET Data Quark Hard Gluon >8GeV (10 ± 3)% 14-20% 89 % >15GeV (9 ± 3)% 4-9 % 53 % >25GeV (8 ± 3)% 1-3 % 25 % The W+Jet rates are consistent with a quark dominated pomeron and inconsistent with a hard gluon dominated one.

W/Z Cross Section Ratio RD = (WD ) / ( ZD ) = R*(WD/W)/ (ZD/Z) where WD/W and ZD/Z are the measured gap fractions from this analysis and R=(W)/ (Z) = 10.43 ± 0.15(stat) ± 0.20(sys) ± 0.10(NLO) B. Abbott et al. (D0 Collaboration), Phys. Rev D 61, 072001 (2000) Substituting in these values gives RD = 6.45 + 3.06 - 2.64 This value of RD is somewhat lower than, but consistent with, the non-diffractive ratio.

Diffractive W Boson x Calculate x = Dp/p for W boson events using calorimeter energy deposition prescription: = Dp p Sum over all particles in event: those with largest ET and closest to gap given highest weight in sum (particles lost down beam pipe at –h do not contribute Use only events with rapidity gap {(0,0) bin} to minimize non-diffractive background Correction factor 1.5 ± 0.3 derived from Monte Carlo used to calculate  Most events  < 10%, mean is 5%

Hard Single Diffraction Central gap 630 GeV Forward 630 GeV All Z All W 1.0% Hard Single Diffraction Central 630 GeV Central W Central gap 1800 GeV Forward 1800 GeV Forward W Central 1800 GeV OK, so it’s right – about 1% Tomorrow, I’ll have a few words to say about augmenting this plot at LHC (CMS) energies