Diffraction at DØ Andrew Brandt University of Texas, Arlington Intro and Run I Hard Diffraction Results Run II and Forward Proton Detector Physics Colloquium November 5, 2003 UTA E  

Main Injector & Recycler Tevatron Booster p p p source Chicago DØ CDF pp Run I ( ) :  s =1.8 TeV Run Ic (1995):  s =630 GeV Run II ( ? ) :  s =1.96 TeV Fermilab Tevatron

Jet Proton remnant   p spectator partons

Elastic Scattering The particles after scattering are the same as the incident particles The cross section can be written as: This has the same form as light diffracting from a small absorbing disk, hence the name diffractive phenomena Elastic “dip” Structure from Phys. Rev. Lett. 54, 2180 (1985).

Diffraction at Tevatron p p  p p p p  p (p) + X p p  p (p) + j j p p  p p + j j

Learning about the Pomeron QCD is theory of strong interactions, but 40% of total cross section is attributable to Pomeron exchange -- not calculable and poorly understood Does it have partonic structure? Soft? Hard? Super-hard? Quark? Gluon? Is it universal -- same in ep and ? Is it the same with and without jet production? Answer questions in HEP tradition -- collide it with something that you understand to learn its structure Note: variables of diffraction are t and  ~ M 2 with FPD measure without FPD just measure 

Diffractive Variables p Beam pFpF P For TeV Pomeron Exchange Non-diffractive For ( Note : ) GeV

Ingelman-Schlein Factorization allows us to look at the diffractive reaction as a two step process. Hadron A emits a Pomeron (pomeron flux) then partons in the Pomeron interact with hadron B. The Pomeron to leading order is proposed to have a minimal structure of two gluons in order to have quantum numbers of the vacuum A A* B J1J1 J2J2 P X G. Ingelman and P. Schlein, Phys. Lett. B 152, 256 (1985)

Ingelman-Schlein II The flux factor term has been derived by Donnachie and Landshoff after comparison to global data: The remaining cross-section can be found from standard factorization processes to be The only unknown is the structure function of parton a (with momentum fraction  ) in the Pomeron so measurements of the cross section allow us to probe this structure function

Balitsky, Fadin, Kuraev and Lipatov Starting with two reggeized gluons we can add perturbative corrections of real ladder gluons and virtual radiative gluons to get a gluon ladder Mathematically, each successive order of correction adds a power of log s to the perturbative expansion and at sufficient energies will “break” the perturbation BFKL Proposes to fix this by isolating in each order the contribution with the highest power of log s and resumming these leading terms (leading logarithmic approximation) BFKL

Color Evaporation (SCI) This theory attempts to account for rapidity gaps in diffractive events without resorting to the use of a Pomeron Model has been successfully applied to onium production (charmonium, J/psi) Proposes that allowing soft color (non- perturbative) interactions can change the hadronization process such that color is bleached out and rapidity gaps appear Soft Color shows same exponential t- dependence as Ingelman-Schlein due to primordial k T of the partons Suggests a formation rate of gaps in gluon- gluon sub processes which is less than or equal to the formation rate in quark-quark sub processes

      Color Evaporation II R. Enberg, G. Ingelman, and N. Timneanu, Phys. Rev. D 64, (2001).

Pomeron Structure 1) UA8 shows partonic structure of pomeron (diffractive dijet production) consistent with hard structure (like gg or qq) and perhaps a super-hard component 2) HERA DIS with large gap shows a quark component in pomeron, F 2 D shows pomeron dominantly gluonic 3) HERA diffractive jet and structure function analysis indicate dominantly hard gluonic structure 4) Comparison of HERA and Tevatron data crucial for understanding pomeron

60’s: First evidence for hadronic diffraction, S matrix Regge theory, Pomeron 70’s: DIS, High p T processes. Parton model, QCD, c, τ, b, gluon 80’s: Ingelman-Schlein, BFKL 90’s: Hard Diffraction (UA8), Rapidity Gaps (Bjorken), HERA (diffraction in ep), Tevatron 40 years of Diffraction

DØ Detector (Run I) EM Calorimeter L0 Detector beam (n l0 = # tiles in L0 detector with signal 2.3 < |  | < 4.3) Central Gaps EM CalorimeterE T > 200 MeV|  | < 1.0 Forward Gaps EM Calorimeter E > 150 MeV2.0 < |  | < 4.1 Had. CalorimeterE > 500 MeV3.2 < |  | < 5.2) Central Drift Chamber (n trk = # charged tracks with |  | < 1.0) End Calorimeter Central Calorimeter (n cal = # cal towers with energy above threshold) Hadronic Calorimeter

Hard Color-Singlet Studies jet    QCD color-singlet signal observed in ~ 1 % opposite-side events (p ) Publications DØ: PRL 72, 2332(1994) CDF: PRL 74, 885 (1995) DØ: PRL 76, 734 (1996) Zeus: Phys Lett B369, 55 (1996) (7%) CDF: PRL 80, 1156 (1998) DØ: PLB 440, 189 (1998) CDF: PRL 81, 5278 (1998) H1: Eur.Phys.J. C (2002) Color-Singlet fractions at  s = 630 & 1800 GeV Color-Singlet Dependence on: , E T,  s (parton-x) Jill Perkins (UTA), Ph. D. on color singlet at  s = 630

Hard Color-Singlet Exchange Count tracks and EM Calorimeter Towers in |  | < 1.0 jet    (E T > 30 GeV,  s = 1800 GeV) Measured fraction (~1%) rises with initial quark content : Consistent with a soft color rearrangement model preferring initial quark states Inconsistent with two-gluon, photon, or U(1) models Measure fraction of events due to color-singlet exchange Phys. Lett. B (1998)

1800 and 630 GeV Single Diffractive Multiplicities  s = 1800 GeV  s = 630 GeV Peak at zero multiplicity striking indication of rapidity gap (diffractive) signature in events with hard scattering (two jets with E T >12 GeV) B. Abbott et al. (DØ Collaboration), Phys. Lett. B 531, 52 (2002)

POMPYT Monte Carlo p p  p (or p) + j j * Model pomeron exchange POMPYT26 (Bruni & Ingelman) * based on PYTHIA *define pomeron as beam particle P p * Structure Functions 1) Hard Gluon xG(x) ~ x(1-x) 2) Flat Gluon (flat in x) 3) Quark xG(x) ~ x(1-x) 4) Soft Gluon xG(x) ~ (1-x) 5 p p P  = 1 - x p (momentum loss of proton)

Data gap fractions ~1%, higher for  s=630 than 1800 GeV Data fraction < MC hard gluon or quark fractions (normalization difference), ratios imply this is not a simple normalization factor—would require significant soft gluon structure to save Ingelman-Schlein type model; rates favor SCI model

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

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