w C O N F C j d o g / a l b Fermi National Accelerator Laboratory / Y 4 Q O S ' 3 3 Diffractive Dijet Search with Roman Pots at CDF P.L. Melbsit For the CDF Collaboration The Rockefeller University New York, New York 121 Fermi National Accelerator Laboratory P.O. Box 5, Batavia, Illinois 651 August 1996 Presented at 1996Annual Divisional Meeting (DPF 96) of the Division of Particles and Fields of the American Physical Society, Minneapolis, Minnesota, August 115, 1996. 3Operatedby Universities ResearchAssociation Inc. under Contract No. DEACO276CH3with the United States Department of Energy & @ S T R f m O N OF W t S QOCUME~JT ISVNLIMfhEE)
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Submitted to DPF96 (Augnst Minneapolis, MN) August 9,1996 DIFFRACTWE DUET SEARCH WITH ROMAN POTS AT CDF P.L. MClbe The Rockefeller University, New York, N Y loo2f (Representing the CDF Collaboration) We present the results of a search for diffractive dijets produced in p p collisions at 6 d. 8 TeV from data collected by the Collider Detector at Fermilab using a Roman pot trigger. The dijet events exhibit additional diffractive characteristics such as rapidity gaps and boosted centerofmasssystems. 1 Introduction Within Regge theory the pomeron trajectory is used to describe high energy Mractive processed. Assuming that the pomeron is made up of quarks and gluons and that the Hractive cross section can be factorised into a pomeron flux times a pomeronproton total cross section, then the structure of the pomeron can be determined. Just as jets are used to probe the quark/gluon structure of the proton, diffractive dijets can be used to probe the structure of the pomeron. There is evidence from other experiments? that the pomeron has a hard parton distribution where the momentum is predominately shared by two gluons ( hardgluon ) or two quarks ( hardquark ), which may be modelled as p(1 p ), where p is the fraction of the pomeron s momentum carried by the part ons. We model Mractive interactions using POMPY I!, assuming a quark/gluon structure of the pomeron and a pomeron flux to generate ((, t ), where is the fraction of the proton s momentum carried by the pomeron and t is the 4 momentum transfer squared. PYTIIIA then simulates the pomeronproton interaction that produces dijets. < 2 Analysis Three Roman pot detectors were installed and used together with the CDF detector during the run 1C Tevatron run. Each Roman pot contained trigger scintillators and z, y position detectors using scintillating fibers read out by multichannel photomultiplier tubes. For diffractive events in which the 1
* b antiproton loses 5% to 1% of its momentum by the emission of a pomeron (.5 < ( <.1), the antiproton gets bent out of the beam into the Roman pot detectors. The 2,y track measured in the three Roman pots is projected back to the event vertex, using the accelerator magnetic transport matrix, to determine the (, t of the antiproton. The data sample used in the search for dxractive dijets was taken during a special low luminosity (a few x loz9 cm2s1) running period in which a triple Roman pot coincidence triggered the CDF readout. There are 1.53 million events with a good z,y reconstructed track and.5 < 6 <.1, consistent with coming from the CDF event vertex. These events show the expected correlations between the CDF event and the f measured in the Roman pots 55 meters away. Figure 1 (left plot) shows the expected correlation between the event mass,measured using calorimeter towers, and &, where the centerofmass energy available in a pomeronproton collision is 18& GeV. Figure 1 (right plot) shows the correlation between the detected (occupied tower with the largest q) and ln(, where the she of the rapidity gap is expected to scale with lnt. In addition to the tagged antiproton, rapidity gaps are also observed in the forward detectors on the same side. InelusiveOiff Colorimeter Moss vs Romanpct C Figure 1: Calorimeter Mass vs InclusiveOiff Colorimeter Tower T va ln(e) and qmar vs in((). (The Lines are linear fits to the data) The Roman pot cross section at 18 GeV was measured by C D F in 1989. The cross section for.5 < 6 <.1 is expected to be.64 mb, of which the pomeroninduced diffractive component is bp.15 mb and the remaind is a nondiffractive background possibly due to Reggeon exchange. Within our Roman pot triggered sample, there are 2546 Gets defined as two jets with E+ > 1 GeV. There is significantly lower activity observed (m pidie gaps) in the forward detectors on the Roman pot side (west) compared to the east side for these Roman pot dijet events, while nondiffrsctive events are west/east symmetric. Figure 2 shows dijet variables for the dijets with Roman pot tracks com 2
+ pared to a nondiffractive sample (superimposed). The mean G e t 9, (VI 7h)/2, shows the expected boost away from the Roman pots compared to the psymmetric nonmractive sample. The leading jet ET is similar for both Roman pot and nondiffractive dijets. The 41 4 2 distribution is more peaked than the nondxractive dijets, which could be explained by the lower 4 in the dif ractive interaction producing a smaller fraction of 3jet events. Diffractive Dijet C a n d i d a t e s 2 2 175 v) (E.> 1 CmV. O.O5CC<O. 1. ltlc.5 t I :.rso n 1 12s E 1 2 1 7s 5 25 v) OI c Mean q 25 z 2 e B n E z Lead Jet i (GeV) 16 1 4 1 12 1 8 5 4 16 E, eo 6 2 5 e,*, Romanpot Figure 2: Dijet Sample Kinematic Distributions (nondiffractiveis dashed histogram). The &jet production rate is u =.64mbx (2546 d i j e t s ) / ( l. 5 3 ~lo6 i d ) lpb. The rates predicted by POMPYT for a hardgluon pomeron structure is 6pb assuming the standard flux and.7pb assuming a flux that scales with the soft singlemractive cross section'. The comparison with the POMPYT diffractive simulation must be interpreted with the provision that we have not proven that all these G e t s with Roman pot tracks are due to pomeron exchange (diffractive), as opposed to other colorless Regge exchanges. References 1. K. Goalianos, Phys. Lett. B3b8 (1995) 379,and references therein. 2. A. Bnrndt ef ul. (UA8), Phys. Lett. B297 (1992) 417; H1 Collaboration, DESY 9536 (1995); L.E. Sinclai~(ZEUS),HEPEX/9661 (1996). 3. F. Abe et ul. (CDFCollaboration), Phys. Rev. D5,5535 (1994). 3