1 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 ICHEP98 Vancouver, BC, Canada July , 1998 Workshop on Forward Physics and Luminosity Determination at the LHC Helsinki, Finland November 2, 2000 International Europhysics Conference Tampere, Finland July , 1999 CMS Collaboration Meeting March 8, 2001

2 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 Small-x Workshop Fermilab September , 2003 New New-ish Nice result, need to publish

3 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 (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

4 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?”

5 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”

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

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

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

9 N events n cal DØ Central Gaps between Dijets 2-D multiplicity (n cal vs. n trk ) between two leading 2-D multiplicity (n cal vs. n trk ) between two leading jets (E T > 12) for (a) 1800 GeV and (b) 630 GeV jets (E T > 12) for (a) 1800 GeV and (b) 630 GeV Redundant detectors important to extract Redundant detectors important to extract rapidity gap signal rapidity gap signal Dijet events with one interaction per bunch crossing Dijet events with one interaction per bunch crossing selected using multiple interaction flag selected using multiple interaction flag Negative binomial fit to “QCD” multiplicity f S = (N data - N fit )/N total Rapidity gap fraction

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

11 Hard Single Diffraction  Measure minimum multiplicity here  Two topologies Measure multiplicity here Central jets Forward jets Gap fractions (central and forward) at  s = 630 and 1800 GeV Single diffractive  distributions Comparisons with Monte Carlo to investigate Pomeron structure Physics Letters B531, 52 (2002) Jet E T ’s > 12, 15 GeV

12  s = 1800 GeV  s = 630 GeV Forward jets Central jets L0 scintillator tiles used now  or Measure Multiplicity here 2-D Multiplicity distributions

Forward Jets Solid lines show show HSD candidate events Dashed lines show non-diffractive events 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 E T Hard Single Diffraction

14 Hard Single Diffraction Signal Data 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 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  /1800 Central Jets 4.1  Fwd/Cent Jets 3.0  Fwd/Cent Jets 1.3  0.1 Gap Fraction # diffractive Dijet Events / # All Dijets 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.

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

16 Double Gaps at 1800 GeV |Jet  | 15 GeV Demand gap on one side, and measure multiplicity on opposite side Gap Region 2.5<|  |<5.2

17 Double Gaps at 630 GeV |Jet  | 12 GeV Demand gap on one side, and measure multiplicity on opposite side Gap Region 2.5<|  |<5.2

18 Publication (hep-ex/ ) accepted by Phys. Lett. B Diffractively Produced W and Z Electron from W decay, with missing E T May expect jets accompanying W or Z 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)

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

20 Peak in (0,0) bin indicates diffractive Z   Minimum side n cal L0 n L0 Diffractively Produced Z’s n cal Plot multiplicity in 3<|  |<5.2 (minimum side) 9 of 811 in (0,0)

21 M T =70.4 E T =36.9 E T =35.2 Standard W Events Diffractive W Candidates E T =35.1 E T =37.1 M T =72.5 Compare diffractive W characteristics to all W’s Electron E T Missing E T Transverse mass Good agreement given lower diffractive statistics

22 Fraction of diffractively produced W/Z Sample Diffractive Probability Background All Fluctuates to Data Central W( )% 1 x  Forward W( )% 6 x  All W( – 0.17)% 3 x  All Z ( )% 5 x  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 { Opposite trend compared to hard diffractive dijet case

23 W+Jet Rates Jet E T Data Quark Hard Gluon >8GeV (10 ± 3)% 14-20% 89 % >15GeV (9 ± 3)% 4-9 % 53 % >25GeV (8 ± 3)% 1-3 % 25 % 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. The W+Jet rates are consistent with a quark dominated pomeron and inconsistent with a hard gluon dominated one.

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

25 Calculate  =  p/p for W boson events using calorimeter energy deposition prescription : Diffractive W Boson  Sum over all particles in event: those with largest E T and closest to gap given highest weight in sum (particles lost down beam pipe at –  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  =  p p Most events  < 10%, mean is 5%

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