We review hard diffractive results and prospects at the Tevatron with an
emphasis on factorization breaking in diffractive processes. Upper limits on
the exclusive di-jet and chi_c^0 production cross sections at CDF and the
status of the D0 Forward Proton Detectors are discussed.Comment: 6 pages, 6 figures, presented at the XXXV International Symposium on
  Multiparticle Dynamics 2005, August 9-15, 2005, Kromeriz, Czech Republi

Peters, Krisztian

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arXiv

Krisztian Peters

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Hard Diffractive Results and Prospects at the Tevatron

We review hard diffractive results and prospects at the Tevatron with an emphasis on factorization breaking in diffractive processes. Upper limits on the exclusive di-jet and {chi}{sub c}{sup 0} production cross sections at CDF and the status of the D0 Forward Proton Detectors are discussed

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arXiv:hep-ex/0601031 v1   17 Jan 2006Hard Diffractive Results and Prospects at theTevatron1Krisztian PetersDepartment of Physics & Astronomy, University of Manchester,Manchester M13 9PL, UKon behalf of the DØ and CDF CollaborationsAbstract. We review hard diffractive results and prospects at the Tevatron with an emphasison factorization breaking in diffractive processes. Upperlimits on the exclusive di-jet andχ0cproduction cross sections at CDF and the status of the DØ Forward Proton Detectors are discussed.DIFFRACTION AT THE TEVATRONDiffractive events are mediated by the exchange of color singlets with vacuum quantumnumbers and have clear experimental signatures. These are on one hand rapidity gaps:the absence of particles in some regions of rapidity in contrast to non-diffractive eventswhere gaps are filled by additional soft parton interactionswhich yields an exponentialsuppression of rapidity gaps. On the other hand tagged protons or anti-protons:p orp̄ scattered at small angle and measured in Roman Pots far away from the interactionpoint. Depending on these rapidity gaps and tags, the main diffract ve event topologiesat the Tevatron are: single diffraction (SD), double diffraction (DD) or double Pomeronexchange (DPE). Single diffraction is characterized by a leading proton or anti-protonwhich escapes the collision intact and the presence of a further particle or a di-jet result-ing from a hard scattering separated by a rapidity gap. Double diffraction is characterizedby a gap in the central region and dissociated protons and anti-protons. In DPE there isa gap on both, the proton and anti-proton side, with a centralproduced di-jet or otherparticles. Proton and anti-proton remains intact.In RUN II the CDF Forward Proton Detectors (FPDs) include Roman Pot detectorsapproximately 57m from the interaction point. These consist of three stations and eachstation comprises one scintillation fiber detector and one trigger counter. Beam ShowerCounters, a set of scintillation counters around the beam pipe, are used to reject non-diffractive (ND) background at the trigger level. This makes it possible to collect diffrac-tive data at high luminosities. The energy flow in the event inthe very forward directionis measured by Miniplug Calorimeters. These consist of alternating layers of lead platesand liquid scintillator readout. It has a towerless geometry without dead regions due to1 Presented at the XXXV International Symposium on Multiparticle Dynamics 2005, August 9-15, 2005,Kromeriz, Czech RepublicFERMILAB-CONF-06-011-E(a)     Bjx)/ND]ξ∆Ratio [(SD/10-410-310-210-1110CDF Run II Preliminary / ndf 2χ  4.616 / 12Prob   0.9696Constant  0.09523± -4.55 Slope     0.04685± -0.9466 10-310-210-1 25% norm. uncertainty± > = 14 GeVjetT<E (0.02 to 0.10) (x 1)ξ  (0.05 to 0.10) (x 10)ξ  (0.02 to 0.05) (x 0.1)ξ  CDF Run I (b)     Bjx)/ND]ξ∆Ratio [(SD/10-310-210-1110-310-210-1 25% overall norm. uncertainty±2>*T <E≡ 2QCDF Run II Preliminary [8, 12] GeV)∈ *T (E2 100 GeV≈2Q [18,25] GeV)∈ *T (E2 400 GeV≈2Q [35,50] GeV)∈ *T (E2 1600 GeV≈2QFIGURE 1. Ratio of diffractive to non-diffractive di-jet event ratesa a function ofxB j at CDF fordifferentξ ranges and compared to Run I (a) and for differentQ2 values.the lack of internal mechanical boundaries. The DØ FPDs are described in the following.FACTORIZATION IN DIFFRACTIONOne of the central issues of diffraction is whether hard diffractive processes obey QCDfactorization. As Collins proved [1] for the general class of diffractive DIS processes,the cross section can be described as the convolution of a hard sc ttering matrix element(process dependent) and parton density functions (processindependent). The questionarises if this factorization theorem is more general, are paton densities really universalin diffractive exchange? Can we use them for different collider processes and energies?To answer this question is fundamental for the understanding of diffraction and it isalso important to extrapolate Tevatron results to the LHC. The general strategy to provefactorization is to extract parton density functions (PDFs) and compare predictions tomeasurements of other processes and experiments.Before the extraction of PDFs, diffractive fractions already yield some insight. Both,CDF and DØ, measured for different processes the fractions of events with one gapto all events. It was found that all ratios are at the order of 1% at the Tevatron whichwould support a more general factorization theorem. However the ratio of 1% yields anuniform gap suppression w.r.t. HERA where the diffractive rates are approximately 10times higher. This discrepancy indicates already the breakdown of QCD factorization.CDF measured in Run I the diffractive structure function of the anti-proton from SDdi-jets and the result was compared with expectations from diffractive DIS measure-ments of H1 [2]. These events can be described in terms of a Pomeron emitted fromthe anti-proton and scattering with a parton from the proton. The diffractive structurefunction was obtained by measuring the ratio of the diffractive and non-diffractive crosssection. The product of this ratio with the known non-diffractive structure function givesthe result on the diffractive structure function. Althoughthe shapes of the structure func-tions from HERA and Tevatron have similar shapes, there is a normalization discrepancyof a factor of 10. This result again confirms the breakdown of QCD factorization betweenthe Tevatron and HERA as already expected from the result on the diffractive fractions.(a) X/Mjj = MjjR0 0.2 0.4 0.6 0.8 1 Prescale/0.05×#Events 110102103104105106 (Prescale 280)pSD<7.5) (Prescale 5)gapη(5.5<p + GAPpDPE = SD<7.5) (Prescale 5)gapη(3.6<p + GAPpDPE = SDEnergy Scale UncertaintyCDF Run II Preliminary-1L = 26 pbDijet Mass Fraction < 0.1pξ : 0.03 < pSD > 10 GeVjet2TE(b) Di-muon + photon invariant mass (GeV)2.8 3 3.2 3.4 3.6 3.8 4Events/bin00.511.522.533.54Events/bindataFakeEvtCDF Run 2 PreliminaryFIGURE 2. Di-jet mass fraction for different rapidity gap selections(a) and di-muon plus photoninvariant mass in the exclusive sample compared to Monte Carlo predictions (b) at CDF.One possibility to explain these observations is that, since there are more spectator par-tons inpp̄ collisions w.r.t.γ∗p collisions, the rate of gap destructions due to soft partonicinteractions is larger at the Tevatron. One attempt to describe this rate is made with theintroduction of the concept of the “gap survival probability” |S|2 [3, 4]. The observationthat the shapes of the two structure functions from HERA and the Tevatron are simi-lar supports this concept. The gap survival probability factor orrects the normalizationdiscrepancy.In Run II the diffractive di-jet sample was collected with a dedicated trigger whichselects events with at least one calorimeter tower above the5 G V ET threshold anda threefold Roman Pot Spectrometer coincidence. Calorimeter information is used todetermine the momentum loss of the anti-proton,ξ p̄ =∑ET e−η√s. (1)SD and background regions are selected according to the measuredξ p̄ values. A largenumber of events are atξ p̄ ∼ 1 which are due to the overlap of at least one ND con-tribution. A plateau is observed in theξ p̄ distribution which results from a distributionproportional to 1/ξ p̄ as expected for diffractive production.In Fig. 1 the ratio of SD to ND event rates is plotted versus BjorkenxB j . In Fig. 1(a)this ratio is integrated over three differentξ regions and compared to the Run I result.There is noξ dependence observable between 0.03 and 0.1. Furthermore the slope andnormalization agrees and thus confirms the Run I result. In Fig. 1(b) the same ratiois plotted for three differentQ2 values, whereQ2 is the mean jet transverse energy〈(E1T + E2T)/2〉2. There is no appreciableQ2 dependence in the observable region of100 < Q2 < 1600 GeV2. Thus both structure functions seem to have a similarQ2evolution which does not disfavor any of the two mechanisms of hard diffraction,namely the existence of a hard Pomeron (exchange of a colorless object) or a soft colorrearrangement in the final state.Using the di-jet sample CDF extracted also DPE di-jet production events. Theseevents are characterized by a leading anti-proton, two jetsin the central pseudorapidityregion and a large rapidity gap on the outgoing proton side. If factorization holds the ratioof DPE to SD,RDPESD (xp,ξp) has to be equal to the ratio of SD to ND,RSDND(x p̄,ξ p̄) (in LOQCD) for a fixedxB j andξ value. Although the collected events have differentξ rangesfor the proton and anti-proton, the weighted average ofRSDND(x p̄,ξ p̄) is flat in ξ and theratio was extrapolated toξ = 0.02. At thisξ values the above mentioned ratios differ by afactor of 5 [5]. The deviation from unity yields again a breakdown of factorization. Sincethe formation of a second gap is less suppressed this result is coherent with the conceptof the gap survival probability. The number of spectator partons does not changed withthe formation of a second gap, i.e. one does not have to pay theprice for the gap twotimes.In the same manner as was previously done by the SD/ND ratio, the diffractivestructure function can be extracted from the DPE/SD ratio. The obtained result nowapproximately agrees with expectations from H1 leading again to the above mentionedconclusions.SEARCH FOR EXCLUSIVE EVENTSSince the CDF Roman Pots have been installed at the end of Run I, CDF collected inRun II two orders of magnitude more di-jet data which made a study of exclusive di-jetproduction in DPE possible. The strategy was to obtain an inclusive DPE di-jet eventsample and look for exclusive signature using the di-jet mass fractionR j j = M j j/MXwhere the di-jet mass is divided by the mass of the rest of the system excluding theproton and anti-proton. In Fig. 2(a) the obtained number of events is plotted versus thedi-jet mass fraction where no gap, a narrow gap and a wide gap was required on theproton side. The result is a smoothly falling spectrum all the way down toR j j = 1 and asimilar event yield at high mass fraction regardless of the gap requirements. From this itfollows that no significant excess due to exclusive di-jets is een at highR j j. An upperlimit on the exclusive di-jet production cross section is calculated based on events withR j j > 0.8. For example requiring a minimum jet transverse energy of 10 GeV, this upperlimit isEminT = 10 GeV : σ(R j j > 0.8) < 1.1±0.1(stat)±0.5(sys) nb. (2)There are also other production channels available in DPE, we mention here theexclusiveχ0c production. This process is of particular interest, since the χ0c quantumnumbers are very similar to the ones of the Higgs boson, thus it can be used to testand normalize the predictions for exclusive Higgs production at the LHC. CDF didan analysis in Run II where theχ0c further decays in aJ/ψ + γ. 93 pb−1 of di-muontriggered data was used and events have been selected in theJ/ψ mass window. Alarge rapidity gap on both the proton and anti-proton side was required. 10 events havebeen found which are exclusiveχ0c (→ J/ψ + γ) candidates. In Fig. 2(b) the di-muonplus photon invariant mass of the 10 events is compared with asample of generatedχ0cFIGURE 3. The Forward Proton Detector at DØ. Quadrupole Pots are namedP or A when placed onthe proton or the anti-proton side, respectively. Dipole Pots are named D.events passed through a detector simulation. The invariantm ss is consistent with thatof theχ0c , although the mean mass is higher and the distribution broader in the data thanin the simulation. This may be due to the simulation or there may be contributions fromcosmic events, higher massχc mesons orJ/ψ + π0 events. Since it is very difficult tofully understand the background one may calculate an “upperlimit” on exclusiveχ0cproduction assuming that the observed 10 events are allJ/ψ +γ events. This upper limitis:σ = 49±18(stat)±39(sys) pb. (3)DØ FORWARD PROTON DETECTORSThe DØ Forward Proton Spectrometers have been installed andrecently commissioned.These are 9 momentum spectrometers with 2 scintillating fiber detectors each, asschematically shown in Fig. 3. There is a dipole spectrometer located on the scatteredanti-proton side behind the dipole magnets approximately 58 m away from the inter-action point. They have in the range of|t| ≈ 0−1 GeV2 andξ ≈ 0.03− 0.07 a verygood acceptance. Eight quadrupole spectrometers are on both side of the main detectorapproximately 23 and 31 m away from the interaction point, behind the quadrupole mag-nets and the separators. They have a very good acceptance in the region of:|t| ≈ 0.8−3.0GeV2 andξ ≈ 10−3−0.05. The position detectors housed inside Roman Pots operatemillimeters away from the beam, however outside of the ultrahigh vacuum. They enablethe reconstruction of the high energy protons and anti-protons directly, thus providing afirst time possibility of tagging both the protons and anti-protons and measuring theirξandt dependence at the Tevatron.The DØ scintillating fiber detectors have 6 layers of scintillating fiber channels andone trigger scintillator layer. The fibers are oriented within ±45o to reconstruct hits andobtain redundancy. Furthermore every second channel is offset by 2/3 fiber for a finerhit resolution. All 18 detectors regularly brought close tothe beamline and diffractivesamples being collected.In a small dedicated test run of the FPDs in a stand alone mode the slope of theelastic cross section of proton anti-proton scattering wasmeasured. In Fig. 4 the result(not normalized) is plotted and compared with theory predictions [6]. DØ access a newFIGURE 4. The slope of the elasticpp̄ cross section measured at DØ and compared to predictions of[6] (solid line).kinematic domain with these measurements and the result of the slope agrees wellwith the model of Bocket al. [6]. This measurement is being redone using the fullyintegrated FPDs. The alignment and detector understandingof the Roman Pot detectorsis in progress and more physics results are expected soon.REFERENCES1. J. C. Collins,Phys. Rev. D57, 3051–3056 (1998),hep-ph/9709499.2. T. Affolder, et al.,Phys. Rev. Lett. 84, 5043–5048 (2000).3. J. D. Bjorken,Phys. Rev. D47, 101–113 (1993).4. E. Gotsman, E. M. Levin, and U. Maor,Phys. Lett. B309, 199–204 (1993),hep-ph/9302248.5. T. Affolder, et al.,Phys. Rev. Lett. 85, 4215–4220 (2000).6. M. Block, R. Fletcher, F. Halzen, B. Margolis, and P. Valin, Phys. Rev. D41, 978 (1990).

Hard Diffractive Results and Prospects at the Tevatron

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