National Accelerator Laboratory

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1 Fermi National Accelerator Laboratory FERMILAB-Conf-98/117-E CDF Diffractive Dijet and W Production in CDF Konstantin Goulianos For the CDF Collaboration The Rockefeller University New York, New York 11 Fermi National Accelerator Laboratory P.O. Box 5, Batavia, Illinois 651 Aril 1998 Published Proceedings of the LAFEX International School on High Energy Physics Lishe 98, Worksho on Diffractive Physics, Rio de Janeiro, Brazil, February 16-, 1998 Oerated by Universities Research Association Inc. under Contract No. DE-AC-76CH3 with the United States Deartment of Energy

2 Disclaimer This reort was reared asanaccount of work sonsored by an agency of the United States Government. Neither the United States Government nor any agency thereof, nor any of their emloyees, makes any warranty, exressed or imlied, or assumes any legal liability or resonsibility for the accuracy, comleteness, or usefulness of any information, aaratus, roduct, or rocess disclosed, or reresents that its use would not infringe rivately owned rights. Reference herein to any secic commercial roduct, rocess, or service by trade name, trademark, manufacturer, or otherwise, does not necessarily constitute or imly its endorsement, recommendation, or favoring by the United States Government or any agency thereof. The views and oinions of authors exressed herein do not necessarily state or reect those of the United States Government or any agency thereof. Distribution Aroved for ublic release; further dissemination unlimited.

3 CDF/PUB/CDF/PUBLIC/4537 RU 98/E-4 March 5, 1998 Diractive Dijet and W Production in CDF 1 Konstantin Goulianos The Rockefeller University, New York, NY 11, U.S.A. Abstract. Results on diractive dijet and W-boson roduction from CDF are reviewed and comared with redictions based on factorization of the diractive structure function of the roton measured in dee inelastic scattering at HERA. INTRODUCTION In this aer, we summarize the ublished CDF results on diractive dijet [1] and W -boson [] roduction and comare them with redictions based on factorization of the diractive structure function of the roton measured in e +! e + +[! X] dee inelastic scattering (DIS) at HERA [3,4]. Our comarison between CDF and HERA results has been reorted in three revious aers [5{7], arts of which are reroduced here. Hadronic diraction is believed to be mediated by the exchange of the omeron, which carries the quantum numbers of the vacuum. In the framework of QCD, omeron exchange must involve the exchange of q q and/or gg airs in a color-singlet state. However, the question arises whether the omeron, although virtual, has a unique artonic structure, as do real hadrons. Such a structure, if it exists, could be robed in hard diractive rocesses. Figure 1 shows (a) a schematic diagram and (b) the event toology for diractive roduction. Since there is no color exchanged between the colorless omeron and the arent nucleon, a raidity ga (region devoid of articles) emerges as a characteristic signature of omeron exchange. Such gas can be used to tag diractive roduction. Another way of tagging diraction is rovided by the recoil or. The CDF Collaboration has obtained results for hard diraction in collisions at s = 18 GeV using both tagging techniques. The results we reort here are were obtained using the raidity ga method. At HERA, where 8 GeV electrons are brought into collision with 8 GeV rotons ( s 3 GeV), diraction has been studied both in hotoroduction and in high Q DIS. The H1 and ZEUS Collaborations have measured the diractive structure function of the roton and its internal factorization 1) Presented at \Diractive Physics, LAFEX International School on High Energy Physics (LISHEP-98), Rio de Janeiro, Brazil, 1- February 1998."

4 (a) t, ξ IP X X x DIFFRACTIVE CLUSTER IP RAPIDITY GAP -ln ξ roton (b) η max ln(/ ) T FIGURE 1. (a) Schematic diagram and (b) event toology for diraction dissociation. roerties. Figure shows the kinematics of a DIS diractive collision. In analogy with the F (Q ;x) structure function, the 4-variable diractive e' e q S W t, ξ ' M FIGURE. Schematic diagram of a diractive DIS collision involving a virtual hoton, emitted by an electron, and a virtual omeron, emitted by a roton. structure function of the roton, F D(4) (Q ;x;;t), is dened through the cross section equation d 4 dq dx d dt = 4 xq 4 D(4) f(y) F (Q ;x;;t) (1) where x and y are the Bjorken DIS variables, f(y), y + y =[(1 + R)], and is the fraction of the roton's momentum taken by the omeron. The t-integrated diractive structure function, F D(3),was determined from the data by assuming R =inf(y). The reorted measured values for F D(3) corresond to cross sections given by d 3 dq d d jtj<1 4 Q 4 D(3) f(y) F (Q ;;) () where x= reresents the fraction of the momentum of the omeron carried by the interacting quark. Below we will use the measured F D(3) structure

5 function to redict the rate for diractive W roduction at the Tevatron and comare the rediction with the value measured by CDF. THE CDF DETECTOR The comonents of the CDF detector relevant to studies of diractive dijet and W -boson roduction using raidity gas are [8] the Beam-Beam Counters (BBC), the Central Tracking Chamber (CTC), and the calorimeters. The BBC consist of a square array of 16 scintillation counters laced at z osition of 6 m from the center of the detector. The calorimeters have atower geometry with segmentation of.1 units in and 15 (5 for jj > 1:1) in. The -coverage is: BBC 3: < jj < 5:9 CTC jj < 1:8 CAL: central jj < 1:1 CAL: lug 1:1 < jj < :4 CAL: forward : < jj < 4: A \article" is dened as a hit in a BBC, a track with P T > 3 MeV in the CTC, or a calorimeter tower with measured E T > MeV (corrected E T > 3 MeV), excet for the region :4 < jj < 4: for which the requirementis atower energy of E>1:5 GeV. DIFFRACTIVE DIJET PRODUCTION CDF searched for diractive dijet roduction in a samle of 335 dijet events with a single-vertex (to exclude events due to multile interactions), in which the two leading jets have E T > GeV and are both at <1:8 or> 1:8. No requirement was imosed on the resence or kinematics of extra jets in an event. Figure 3 shows the correlation of the BBC and forward (jj > :4) calorimeter tower multilicities in the -region oosite the dijet system. The excess in the - bin is attributed to diractive roduction. After subtracting the non-diractive background and correcting for the single-vertex selection cut, for detector live-time accetance and for the raidity ga accetance (:7 :3), calculated using the POMPYT Monte Carlo rogram [1] with omeron < :1, the \Ga-Jet-Jet" fraction (ratio of diractive to nondiractive dijet events) was found to be R GJJ =[:75 :5(stat) :9(syst)]% = (:75 :1)% > GeV, jj jet > 1:8, 1 >, <:1) (E jet T Figure 4 shows omeron- distributions of dijet events generated by a POMPYT Monte Carlo simulation for < :1 and a hard gluon omeron structure. The jets were required to have E jet T > GeV and be in the region 1:8 < jj < 3:5 with 1 >.

6 1 NUMBER OF EVENTS TOWERS BBC HITS FIGURE 3. Tower versus BBC multilicity for dijet events with both jets at >1:8 or <1:8. EVENTS ξ OF POMERON FIGURE 4. Monte Carlo omeron distributions for diractive dijet events with jet E T > GeV and 1:8 < jj < 3:5 generated by POMPYT using a hard-gluon omeron structure. The shaded area reresents the subset of Monte Carlo events with zero BBC and forward calorimeter multilicities, corresonding to the data in the (,) bin of Fig. 3 DIFFRACTIVE W PRODUCTION CDF made the rst observation [] of diractive W s and measured the W roduction rate using a samle of 846 events with an isolated central e + or e, (jj < 1:1) of E T > GeV and missing transverse energy 6E T > GeV. In searching for diractive events, CDF studied the correlations of the BBC multilicity, N BBC, with the sign of the electron-, e, or the sign of its charge, C e. In a diractive W! e event roduced in a collision with a omeron emitted by the roton, a raidity ga is exected at ositive (-direction), while the leton is boosted towards negative (angle-ga correlation). Also, since the omeron is quark-avor symmetric, and since, from energy considerations, mainly valence quarks from the articiate in roducing the W,aroximately twice as many electrons as ositrons are exected (charge-ga correlation). Figure 5 shows the BBC versus tower multilicity for two event samles characterized by the correlation between the seudoraidity of the BBC whose multilicity is lotted, BBC, and the e or C e,asfollows: (a) doubly-correlated events, for which e C e > and e BBC <, and (b) doubly-anticorrelated events, for which e C e > and e BBC >. Monte Carlo simulations show that diractive W events are exected to have low BBC or tower multilicities (in the range -3), and that there should be 4 times as many doubly-correlated than doubly-anticorrelated events. This diractive signature is satised by the small number of events at low multilicities in Fig. 5.

7 The robability that the observed excess is caused by uctuations in the nondiractive background was estimated to be 1:1 1,4. Figure 6 shows omeron- distributions of W events generated by POMPYT. Correcting for accetance, the ratio of diractive to non-diractive W roduction is: R W =[1:15 :51(stat) :(syst)]% ( <:1) ENTRIES 4419 (a) EVENTS BBC MULTIPLICITY TOWER MULTIPLICITY ENTRIES BBC MULTIPLICITY 5 (b) TOWER MULTIPLICITY FIGURE 5. Tower versus BBC multilicity forw events: (a) (anglecharge)-correlated; (b) (anglecharge)-anticorrelated. ξ OF POMERON FIGURE 6. Monte Carlo omeron distributions for diractive W events generated by POMPYT using a hardquark omeron structure: (solid line) all events; (dotted line) events with a central electron; (shaded area) events withacentral electron and, 1 or hits in the (anglecharge)-correlated BBC (corresonding to the signal). THE GLUON FRACTION OF THE POMERON By combining the diractive W and dijet results, CDF extracted the gluon fraction of the omeron, f g. Assuming the standard omeron ux, the measured W and dijet fractions trace curves in the lane of D versus f g, where D is the total momentum fraction carried by the quarks and gluons in the omeron. Figure 7 shows the 1 curves corresonding to the results. From the diamond-shaed overla of the W and dijet curves, CDF obtained f g =:7 :. This result, which is indeendent of the omeron ux normalization, agrees with the result obtained by ZEUS [3] from DIS and dijet hotoroduction (dashed-dotted line in Fig. 7). For the D-fraction, CDF obtained the value D =:18 :4. In the next section we willshow that the decrease of the D-fraction from HERA to the Tevatron can be accounted for by the omeron ux renormalization factor [9]. The dashed lines are the 1 curves of the UA8 diractive dijet results [1]. To comare UA8 with CDF, the UA8 fractions must rst be multilied by the ratio of the renormalization factors at the two energies, which is [9] D CDF =D UA8 :7. Within the errors, the results of the two exeriments are in good agreement.

8 MOMENTUM FRACTION OF HARD PARTONS IN POMERON CDF- DIJETS UA8 ZEUS CDF-W GLUON FRACTION OF HARD PARTONS IN POMERON FIGURE 7. Momentum fraction versus gluon fraction of hard artons in the omeron evaluated by comaring measured diractive rates with Monte Carlo redictions based on the standard omeron ux and assuming that only hard omeron artons articiate in the diractive rocesses considered. Results are shown for ZEUS (dashed-dotted), UA8 (dashed) and the CDF-dijet and CDF-W measurements. The CDF W result is shown for two (dotted) or three (solid) light quark avors in the omeron. From the shaded diamond-shaed region of overla of the CDF W and dijet roduction bands a gluon fraction of f g =:7 : and a momentum sum rule discreancy factor of D =:18 :4 were extracted [1]. The latter can be considered to be a discreancy in the omeron ux, if the momentum sum rule for the omeron is assumed. The measured value of the omeron ux discreancy is in general agreement with the renormalized ux rediction [9].

9 FROM HERA TO THE TEVATRON The diractive structure function F D(3) measured in DIS at HERA (Eq. ) can be used directly to calculate the rate of diractive W -boson roduction in collisions at s = 18 GeV. Such a calculation yields [5] R W =6:7% for the ratio of diractive to non-diractive W roduction (a similar result has been obtained [1] by L. Alvero, J. Collins, J. Terron and J. Whitmore). The measured value [] R W =(1:15 :55)% is smaller than the redicted 6.7% by a factor of :17 :8. The deviation of this factor from unity reresents a breakdown of factorization. Assuming that the raidity ga robability (-distribution) in F D(3) (Eq. ) scales to the total ga robability (integrated over all available hase sace), it has been shown [5] that the ratio of the scaling factors from HERA to the Tevatron is :19. This ratio agrees with the discreancy factor of :17 :8 between the measured R W and the standard factorization rediction, as well as (or equivalently) with the momentum sum rule discreancy D = :18 :4 obtained from the comarison of the diractive W and dijet rates. Thus, the diractive structure function with a scaled ga robability rovides a \transatlantic diractive bridge" connecting HERA with the Tevatron! The scaling of the ga robability in the diractive structure function is equivalent to the omeron ux renormalization scheme roosed to unitarize the soft diraction trile-omeron amlitude [9]. Thus, the breakdown of factorization in hard diraction is traced back to the breakdown observed in soft diraction [5{7,9,13]. In both cases, soft and hard, factorization breaks down to reserve unitarity. The interesting fact is that the breakdown occurs in a way that resects a scaling law, namely the scaling of the (;t)-dierential ga robability tothe total ga robability, integrated over all available (;t)-sace for any given rocess [7]. CONCLUSION Exeriments at HERA and at Colliders show that the omeron has a hard artonic structure, which consists of 7% gluons and 3% quarks. Predictions using the diractive structure function of the roton measured in DIS at HERA are larger than the W and dijet rates measured at the Tevatron by a factor of 6. This breakdown of factorization can be accommodated by scaling the raidity ga robability distribution, which aears in the diractive structure function as a -deendent factor multilying a function of Q and, to the total ga robability. The scaling of the ga robability is equivalent to the omeron ux renormalization hyothesis introduced [9] to account for the s-deendence of the soft single diraction dissociation cross section.

10 REFERENCES 1. F. Abe et al., CDF Collaboration, Measurement of diractive dijet roduction at the Fermilab Tevatron, Phys. Rev. Lett. 79 (1997) F. Abe et al., CDF Collaboration, Observation of diractive W-boson roduction at the Fermilab Tevatron, Phys. Rev. Lett. 78 (1997) ZEUS Collaboration: M. Derrick et al., Measurement of the diractive structure function in dee inelastic scattering at HERA, Z. Phys. C68 (1995) 569; M. Derrick et al., Diractive hard hotoroduction at HERA and evidence for the gluon content of the omeron, Phys. Lett. B356(1995) H1 Collaboration: T. Ahmed et al., Phys. Lett. B348(1995) 681; ib. A measurement and QCD analysis of the diractive structure function F D(3),Contribution to ICHEP'96, Warsaw, Poland, July 1996; ib. Inclusive measurement of diractive dee-inelastic e scattering, he-ex/ Aug K. Goulianos, Factorization and scaling in hard diraction, Proceedings of the 5 th International Worksho on Dee Inelastic Scattering and QCD (DIS-97), Chicago, USA, Aril 1997, (AIP Conference Proceedings 47, J. Reond and D. Krakauer, Eds.); he-h/ Aug K. Goulianos, Results on Diraction, Proceedings of the XVII th International Conference on Physics in Collision, Bristol, UK, 5-7 June 1997; heex/ Aug K. Goulianos, From HERA to the Tevatron: a scaling law in hard diraction, to aear in Proceedings of the XXVII International Symosium on Multiarticle Dynamics, Laboratori Nazionali di Frascati-INFN, Frascati (Rome)-Italy, 8-1 Setember F. Abe et al., CDF Collaboration, The Collider Detector at Fermilab, Nucl. Instrum. Meth. A71(1988) 387; Amidei et al., ib. A35(1994) K. Goulianos, Renormalization of hadronic diraction and the structure ofthe omeron. Phys. Lett. B358(1995) 379; Erratum B363 (1995) P. Schlein, Evidence for Partonic Behavior of the Pomeron, Proceedings of the International Eurohysics Conference on High Energy Physics, Marseille, France, -8 July 1993 (Editions Frontieres, Eds. J. Carr and M. Perrottet). 11. F.Abe et al., CDF Collaboration, Measurement of single diraction dissociation at s = 546 and 18 GeV, Phys. Rev. D5 (1994) J. Whitmore, Diractive arton distribution functions and factorization tests, Proceedings of the 5 th International Worksho on Dee Inelastic Scattering and QCD (DIS-97), Chicago, USA, Aril 1997, (AIP Conference Proceedings 47, J. Reond and D. Krakauer, Eds.). 13. K. Goulianos and J. Montanha, Factorization and scaling in soft diraction, aer in these Proceedings.