We present new results from studies on diffractive dijet production and
exclusive production of dijet and diphoton obtained by the CDF Collaboration in
proton-antiproton collisions at the Fermilab Tevatron.Comment: 4 pages, To be submitted to the proceedings of the 41st Rencontres de
  Moriond - QCD and High Energy Hadronic Interactions, La Thuile, Italy, 18-25
  March 200

Terashi, Koji

U., /Rockefeller

English

arXiv

We present new results from studies on diffractive dijet production and exclusive production of dijet and diphoton obtained by the CDF Collaboration in proton-antiproton collisions at the Fermilab Tevatron

UNT Digital Library

arXiv:hep-ex/0605084 v1   17 May 2006 NEW DIFFRACTION RESULTS FROM THE TEVATRONK. TERASHI(for the CDF and DØ Collaborations)The Rockefeller University1230 York Avenue, New York, NY 10021, USAWe present new results from studies on diffractive dijet production and exclusive productionof dijet and diphoton obtained by the CDF Collaboration in proton-antiproton collisions atthe Fermilab Tevatron.1 IntroductionDiffractive events in p̄p collisions are characterized by the presence of a leading proton or an-tiproton which remains intact, and/or a rapidity gap, defined as a pseudorapiditya region devoidof particles. Diffractive events involving hard processes (“hard diffraction”), such as productionof high ET jets, have been studied to understand the QCD aspects of the exchanged object, thePomeron (a color singlet entity with vacuum quantum numbers). One of the most importantquestions in hard diffractive processes is whether or not they obey QCD factorization, in otherwords, whether the Pomeron has a universal, process independent, parton distribution function(PDF). Results on diffractive deep inelastic scattering (DDIS) from the ep collider HERA showthat QCD factorization holds in DDIS. However, single diffractive (SD) rates of W -boson, dijet,b-quark and J/ψ productions relative to non-diffractive (ND) ones measured in Run I at CDF1 are about an order of magnitude lower than expectations from PDFs determined at HERA,indicating a severe breakdown of QCD factorization in hard diffraction between Tevatron andHERA. The suppression relative to predictions based on DDIS PDFs has been further studiedby measuring “diffractive structure function” FDjj using diffractive dijet data in CDF2. TheFDjj was measured by looking at the ratio RSDND of SD to ND dijet event rates as a function ofBjorken-x variable (xBj) and multiplying the ratio by the known ND proton structure function.aThe pseudorapidity η of a particle is defined as η ≡ − ln(tan θ/2), where θ is the polar angle of the particlewith respect to the proton beam direction.FERMILAB-CONF-06-121-E    Bjx ) / NDξ∆Ratio  ( SD /  10-310-210-1110 / ndf 2χ  23.26 / 12Prob   0.0256Const     0.0009656± 0.01031 slope     0.04422± 1.027 10-310-210-1 6% (slope)± 20% (norm), ±overall syst. uncertainty: )/2jet2T+Ejet1T>=(E*T,  <E2>*T <E≡ 2Q<0.09pCALξ0.03< CDF Run II Preliminary2   100 GeV≈2Q2 400 GeV≈2Q2  1,600 GeV≈2Q2  3,000 GeV≈2Q2  6,000 GeV≈2Q2 10,000 GeV≈2Q 2 |t| (GeV/c)0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1/dt [arbitrary units]σd10102103104RPS inclusive)2~225GeV2RPS+Jet5 (Q)2~900GeV2RPS+Jet20 (Q)2~4,500GeV2RPS+Jet50 (Q)/2jet2T+Ejet1T>=(E*T,  <E2>*T <E≡ 2Q<0.08pRPSξ0.05< CDF Run II Preliminarystatistical uncertainties onlyFigure 1: Left: Ratio of SD to ND dijet rates as function of xBj of the parton in the antiproton for different Q2ranges. Right: t distributions (with arbitrary normalization) in diffractive dijet events with different Q2 values.Measuring the ratio RSDND thus provides information regarding the behavior of FDjj relative tothe ND proton PDF.Another area of great interest is exclusive production of dijets, diphoton and χc(b) meson atthe Tevatron. In leading order (LO) QCD, such exclusive processes can occur through exchangeof a color-singlet two gluon system between nucleons, leaving large rapidity gaps in the forwardregions. One of the two gluons takes part in a hard interaction while the other serves toneutralize the color. This type of production is generally suppressed by the Sudakov formfactor. However, it’s potentially a useful channel to search for the light Standard Model Higgsboson (predominantly decaying to bb̄) at the LHC, since exclusive bb̄ production is expectedto be significantly suppressed by a helicity selection (JZ = 0) rule. Our goal of studies is toestablish exclusive production experimentally and measure the cross sections of the exclusiveprocesses to calibrate theoretical calculations of exclusive Higgs production at the LHC.2 Run II Diffraction MeasurementsIn Run II, CDF has studied various topics on diffraction, including Q2 dependence of FDjj inSD and productions of exclusive dijet and exclusive diphoton, for which the results will bediscussed below. Two “Miniplug” (MP) calorimeters cover the forward pseudorapidity region3.6 < |η| < 5.2, and 7 stations of scintillation counters, called Beam Shower Counters (BSC),mounted around the beam pipe, extend the coverage to the very forward region of 5.4 < |η| < 7.4.The Roman Pots (RP) used in Run I were re-installed and are being operated to trigger onleading antiprotons in the kinematic range 0.03 < ξ < 0.1 and 0 < |t| < 3 GeV2, where ξ is thefractional momentum loss of the antiproton and t is the four momentum transfer squared.3 Diffractive Dijet ProductionTriggering on a leading antiproton in the RP in conjunction with at least one jets in calorimeters,diffractive dijet events have been studied. Using a ND dijet sample triggered only on the jetrequirement, the ratio RSDND is measured as a function of xBj , as shown in Fig. 1 (left). Thisfigure shows the ratios for different Q2 values obtained from different jet ET triggers. Here Q2is defined as the square of average value of the mean dijet ET . In the range 100 < Q2 < 10000GeV2 no significant Q2 dependence is observed, which indicates QCD evolution of the Pomeroncould be similar to that of the proton.A Q2 dependence of the t in diffractive dijet events is also examined. Fig. 1 (right) showsthe t distributions (with arbitrary normalization) for different Q2 values spanned over the widerange. The slope at t = 0 GeV/c2 appears to be quite independent of Q2 and is close to the onein standard diffractive t distribution. Measurement of the slope values is currently under way.X / Mjj = MjjR0 0.2 0.4 0.6 0.8 1 1.2Arbitrary10-410-310-210-1DPE data (stat. only)H1-fit2)⊕POMWIG (CDFBackgroundPOMWIG + BackgroundH1-fit2⊕CDFCDF Run II PreliminaryX / Mjj = MjjR0 0.2 0.4 0.6 0.8 1 1.2Arbitrary020004000CDF Run II PreliminaryDPE data (stat. only)H1-fit2⊕POMWIG: CDFExHuMEBest Fit to Data| < 5.9gapη3.6 < | > 10 GeVjet2TE < 5 GeVjet3TE 0.4 %± = 14.1 exclF(stat. only)X/Mjj = MjjR0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 11 - F-0.200.20.40.60.8CDF Run II Preliminary Systematic Uncertainty 11-F = POMWIG + BackgroundinclMCH1-fit2)⊕(CDF<0.4jj at Rinclnormalized to Data : stat. error only2F]〉<0.4)jj(Rbc/inclF〈 / bc/incl = 1 - [F11 - F)incl / Dataincl = 1 - (MC21 - FFigure 2: Left: Rjj of the data (points) and POMWIG MC prediction (thick solid histogram) composed of DPEdijet events (thin solid histogram) and non-DPE background events (dashed histogram). The data and the MCprediction are normalized to a same area. Center: best fit (solid histogram) to the data (points) obtained usinginclusive (dashed line) and exclusive ExHuME (shaded histogram) MC predictions for the Rjj shape. Right: Oneminus the ratio F , obtained from heavy flavor jet ratio (F1) and inclusive data-POMWIG comparison study (F2),as a function of Rjj . Details of the plot are described in the text.4 Search for Exclusive Dijet ProductionWe have implemented in Run II a dedicated trigger that requires a BSC gap on the proton-side in addition to the requirements for the leading antiproton in the RP and at least onecalorimeter tower with ET > 5 GeV (RP+GAP+ST5). Requiring offline an additional gap inthe MP on the p-side, we have obtained a large amount of double Pomeron exchange (DPE) dijetevents. For those events we examine “dijet mass fraction”, Rjj, defined as the invariant massMjj of the two highest ET jets divided by the mass MX of the whole system (except leadingnucleons); Rjj = Mjj/MXb. This observable should be sensitive to how much the event energyis concentrated in the dijet. The Rjj of exclusive dijet events is expected to be peaked aroundRjj ∼ 0.8 and have a long tail towards low Rjj due to hadronization of partons causing energyleak from jet cones and also the presence of gluon radiations in the initial and final states.The signal search is performed by comparing the data with predictions for inclusive DPEdijet events in the Rjj distribution shape and looking for an excess of events in high Rjj region.We use POMWIG Monte Carlo event generator 4 (with detector simulation) to simulate theDPE dijets. Fig. 2 (left) shows a comparison of the Rjj shapes between the data and the MCprediction composed of DPE dijet events and non-DPE background events. The data and theMC prediction are normalized to same area. We have examined various Pomeron PDFs andunderlying events (Pomeron remnants) in the MC, and find that it is hard to reproduce the dataexcess at high Rjj by any of those changes. Two exclusive dijet production models implementedin ExHuME 5 and DPEMC 6 (Exclusive DPE mode) have also been studied. Fig. 2 (center)shows the best fit to the data Rjj shape obtained from the inclusive POMWIG and the exclusiveExHuME predictions for the Rjj in events with dijets of ET > 10 GeV and a third jet veto ofEjet3T < 5 GeV. The third jet veto is introduced because the exclusive MC generates only aLO gg → gg process. The fit shows the data excess can be well described by the presence ofexclusive dijets. The exclusive DPE model in DPEMC also provides a good agreement with thedata in the excess shape.According to Ref.3, LO process of exclusive dijet is dominated by gg → gg and contributionsfrom gg → qq̄ are strongly suppressed by the helicity selection rule. To check this prediction, wesearch for the suppression of heavy flavor (b and c) jets in exclusive-dominant (high Rjj) region.We use a 200 pb−1 data sample triggered on the requirements of the RP+GAP+ST5 and atleast one displaced vertex track with pT > 2 GeV/c. The displaced track requirement effectivelybThe system mass MX is obtained from 4-momenta of all calorimeter (massless) towers and the dijet massMjj is calculated from the calorimeter tower energies inside the R=0.7 cones of jets.Figure 3: Left: Exclusive production of e+e− pair through two photon exchange γγ → e+e− in p̄p collision. Right:Exclusive production of γγ pair via gluon-gluon fusion gg → γγ with a quark loop in p̄p collisionenhance heavy flavor contents in the sample. We measure the ratio F of heavy flavor jets to alljets as a function of Rjj of the events. The result is presented in Fig. 2 (right) as “1−F” versusRjj. The heavy flavor to all jet ratio (F1 in the figure) is normalized by the weighted average ofthe F1 in the range Rjj < 0.4 so that correlated systematic uncertainties are canceled out. Weobserve the increasing trend in 1 − F1 with increasing Rjj, which could be a manifestation ofthe JZ = 0 selection rule. The result is compared with the inclusive dijet result by showing the“1 − F2”, where F2 is the ratio of the inclusive MC predicted events (normalized to the data atRjj < 0.4) to the data. The 1−F2 is thus equivalent to the fraction of the observed excess in thedata. The 1 − F ratios are consistent with each other in both magnitude and Rjj dependence.5 Search for Exclusive Diphoton ProductionCDF has also performed search for exclusive diphoton production 7 (Fig. 3) using Run II data.The data used in the search is obtained by a trigger which requires two electromagnetic (EM)towers and BSC gaps in both forward directions. Requiring all the calorimeters to be emptyabove noise (except for the triggered two EM towers), we have observed 10 events containingtwo electron candidates with ET > 5 GeV (the two EM towers containing a single track withpT > 1 GeV/c each) and nothing else in the CDF detectors. The observed events appear to beconsistent with QED-mediated dielectron production p̄+ p→ p̄+ e+e− + p through two photonexchange γγ → e+e−. The LPAIR Monte Carlo generator 8 predicts 9 ± 3 events which areconsistent with the observed events, though backgrounds in the data are not estimated yet.In the same dataset the search finds 3 events with two ET > 5 GeV photon candidates (thetriggered EM towers associated with no tracks) and nothing else in the detectors. The ExHuMEMonte Carlo generator for exclusive diphoton p̄ + p → p̄ + γγ + p via gg → γγ predicts 1+3−1events. Background estimation is currently under way.References1. F. Abe et al., Phys. Rev. Lett. 78 2698 (1997), 79 2636 (1997);T. Affolder et al., Phys. Rev. Lett. 84 232 (2000), 87, 241802 (2001).2. T. Affolder et al., Phys. Rev. Lett. 84, 5043 (2000).3. V.A. Khoze, A.D. Martin, and M.G. Ryskin, Eur. Phys. J. C 23 311 (2002), C 26 229(2002), C 25 391 (2002), C 34 327 (2004).4. B. E. Cox and J. R. Forshaw, Comput. Phys. Commun. 144, 104 (2002).5. J. Monk and A. Pilkington, hep-ph/0502077.6. M. Boonekamp and T. Kucs, Comput. Phys. Commun. 167, 217 (2005).7. V.A. Khoze, A.D. Martin, M.G. Ryskin and W. J. Stirling, Eur. Phys. J. C 38 475,(2005).8. J. Vermaseren, Nucl. Phys. B 229, 347 (1983).

New diffraction results from the Tevatron

http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.342.1064

New Diffraction Results from the Tevatron

Abstract

Similar works

Full text

Available Versions

UNT Digital Library