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Brazilian Journal of Physics

Print version ISSN 0103-9733On-line version ISSN 1678-4448

Braz. J. Phys. vol.37 no.2c São Paulo June 2007

http://dx.doi.org/10.1590/S0103-97332007000500033 

SMALL-x PHYSICS AND DIFFRACTION

 

New CDF results on diffraction

 

 

Christina Mesropian, on behalf of the CDF Collaboration

The Rockefeller University 1230, York Avenue, New York, NY 10021, USA

 

 


ABSTRACT

We report new diffraction results obtained by the CDF collaboration in proton-antiproton collisions at the Fermilab Tevatron collider at Ös = 1.96 TeV. The first experimental evidence of exclusive dijet and diphoton production is presented. The exclusive results are discussed in context of the exclusive Higgs production at LHC. We also present the measurement of the Q2 and t dependence of the diffractive structure function.

Keywords: QCD; Diffraction; Exclusive production


 

 

I. INTRODUCTION

Diffractive reactions, which constitute a substantial fraction of the total cross section in hadron-hadron scattering, can be described in terms of the pomeron exchange, hypothetical object with the quantum numbers of the vacuum. The experimental signatures of the diffraction consist in particular kinematic configurations of the final states: presence of non-exponentially suppressed large rapidity gaps and/or presence of the intact leading particles. The diffractive processes became an important tool in understanding many interesting aspects of QCD such as low-x structure of the proton, behavior of QCD in the high density regime. Recently, a lot of attention was drawn to the possibility of discovering diffractively produced Higgs boson at the Large Hadron Collider (LHC).

Significant progress in understanding diffraction has been made at the Tevatron collider. CDF collaboration contributed extensively by studying wide variety of diffractive processes at three different center-of-mass energies: = 630 GeV, 1800 GeV - Run I of Tevatron, and 1960 GeV - Run II. Many important observations were made regarding the diffractive structure function of the pomeron, the breakdown of QCD factorization in hard diffraction between Tevatron and HERA, and the discovery of large rapidity gaps between two jets [1-3].

 

II. DIFFRACTIVE DIJET PRODUCTION

One of the diffractive processes studied during Run I and Run II is the hard single diffraction, which from the phenomenological point of view is described by assuming that a pomeron emitted by the incident antiproton undergoes a hard scattering with the proton. Comparing two samples of dijet events, diffractive, triggered by the presence of intact antiproton, detected in the Roman Pot Spectrometer (RPS), and non-diffractive, the diffractive structure function can be extracted. We extended our results from Run I by examining the Q2 dependence of the structure function, where Q2 is defined as an average value of the mean dijet ET. Fig. 1 shows the ratio of single diffractive dijet event rate to those of non-diffractive, , as a function of . In the range of 100 < Q2 < 10000 GeV2 no significant Q2 dependence is observed.

 

 

CDF also studied the Q2 dependence of t, four-momentum transfer squared, distributions in both soft and hard single diffractive processes. Fig. 2 shows t distributions for different Q2 values in the range 0 < | t| < 1 (GeV/c)2. The slope of the distributions at | t| = 0 (GeV/c)2 does not show any dependence on Q2.

 

 

III. EXCLUSIVE DIJET PRODUCTION

Observation of the Higgs boson is one of the main goals of LHC in CERN. Recently, considerable interest has been drawn to the subject of ''exclusive'' Higgs boson production pp ® p + H + p in double pomeron exchange (DPE), where central heavy object is produced alone, separated from outgoing hadrons by rapidity gaps. The exclusive double diffractive Higgs production has unique experimental advantages such as clean final state which contains only the Higgs boson and very forward leading nucleons, improved mass resolution, and small background from the direct production, due to the several suppression mechanisms. Although the cross section for the exclusive Higgs production is too small to be observed at the Tevatron, several processes mediated by the same mechanism but with higher production rates can be studied to calibrate theoretical predictions. In this proceedings we present results of exclusive dijet production in hadron colliders.

The characteristic signature of this process is presence of intact leading nucleons with exclusive dijet final state. At CDF the leading antiproton is detected by RPS, and the presence of leading proton is inferred from the large forward rapidity gap with D h < 3. The observable, sensitive to the amount of event energy concentrated in dijet is Rjj, the dijet mass fraction, defined as the invariant mass of the two highest ET jets, Mjj, divided by the mass MX of the whole system with the exception of the leading particles. Rjj of exclusive dijet is expected to be peaked around Rjj ~ 0.8 and have a long tail towards low value due to the hadronization of partons causing energy spills from the jet cones and gluon radiation in initial and final states.

The search for the exclusive dijets is performed by comparing 310 pb-1 of data with Monte Carlo, MC, expectations. We use POMWIG [4] MC event generator with detector simulation to obtain DPE dijets, see Fig. 3 which shows a comparison of the shape of Rjj for data and MC results, composed of DPE dijet events and non-DPE background. The evaluation of different Pomeron PDFs and underlying events (Pomeron remnants) in MC showed that these variations cannot account for the excess observed in data at high Rjj . Two different exclusive dijet production models [5, 6] implemented in ExHuME [7] and DPEMC [8] MC simulations have been also studied. Fig. 4 shows the Rjj distribution for the data and the best fit to the data shape obtained from the inclusive POMWIG and exclusive ExHuME predictions in events with dijet ET > 10 GeV and a veto on third jet. As can be seen from this plot, the data excess at high Rjj can be well described by the exclusive dijet production. The exclusive MC DPEMC, not shown here, also provides a good agreement with data.

 

 

 

 

One of the crucial advantages of exclusive central production is the suppression at the leading order of the background sub-process gg ® , as m2/M ® 0 (Jz=0 selection rule). This condition is satisfied when the quarks are light or when the dijet mass is much larger than the quark mass. The suppression mechanism can be used to extract exclusive dijets by identifying jets originating from quarks and looking for the suppression of quark jets relative to all jets at high Rjj. CDF has performed this study by using 200 pb-1 of data triggered by the presence of anti-proton in RPS, forward gap on proton side, dijets in the central region and at least one displaced vertex track with pT > 2 GeV/c.The last requirement effectively enhances the heavy flavor content of the sample. Fig. 5 shows the normalized ratio of heavy flavor jets to all jets as a function of Rjj of DPE b-jet events. The decreasing trend of the normalized ratio towards high Rjj could indicate the suppression of heavy flavor jets in the exclusive signal region. The observed behavior of heavy flavor jet production relative to the inclusive jets is consistent with the results of exclusive dijet signal search in inclusive DPE data sample.

 

 

The cross section of exclusive dijet production can be measured by multiplying the cross section of the DPE sample by the fraction of exclusive events and by accounting for acceptances. Fig. 6 shows the exclusive cross section obtained from the data sample compared with predictions from two Monte Carlo simulations, ExHuME and DPEMC. The data clearly favors the ExHuME.

 

 

IV. EXCLUSIVE DIPHOTON PRODUCTION

Another process which is closely related to exclusive Higgs production is exclusive diphoton production ® . CDF has performed search for exclusive g g using data obtained with the trigger requiring presence of two electromagnetic (EM) towers and forward gaps in both forward directions. 16 events containing two electron candidates are found after requiring all calorimeters to be empty, except for two trigger EM towers with ET > 5 GeV.The observed events are consistent with QED-mediated dielectron production ® p + e+e-+ through two photon exchange. In the same dataset 3 candidate events are found, by requiring all calorimeters to be empty, and no tracks to be associated with two EM trigger towers. The exclusive production estimates [5], implemented in ExHuME Monte Carlo generator predict exclusive g g events. The background estimates are still in process.

 

V. CONCLUSIONS

CDF experiment continues carrying strong program on diffractive physics during Run II. The studies of diffractive structure functions were extended by measuring Q2 dependence in the wide range of the values from 100 to 10000 GeV2. The t distribution dependence on Q2 was also presented. New results on exclusive dijet production demonstrate an excess of data events over backgrounds obtained from Monte Carlo predictions and are consistent with predictions from exclusive dijet production Monte Carlo simulations. The results from the exclusive dijet and diphoton production are an important calibration tools for theoretical models describing diffractive Higgs production at LHC.

 

Acknowledgments

I would like to thank the organizers of XXXVI International Symposium on Multiparticle Dynamics for a kind invitation, warm hospitality, and for an exciting conference.

 

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Received on 2 November, 2006; revised version received on 23 March, 2007

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