and also for many processes like jet- or vector meson-production. Here we will concentrate to diffractive inclusive and dijet measurements.

Transcription

1 INCLUSIVE AND DIJET DIFFACTIVE PODUCTION AT HEA Alice Valkárová on behalf of H an ZEUS Collaborations Institute of Particle an Nuclear Physics Faculty of Mathematics an Physics of Charles University VHolešovičkách, 8 Praha 8 Czech epublic The results of two methos of ientifying of iffractive events, Large apiity Gap metho an measuring of proton in forwar spectrometers, were compare by H an ZEUS experiments. The HEA combine iffractive ouble-iffrential reuce cross sections in events with a leaing proton were obtaine. The iffractive ijet prouction in events with a leaing proton was measure an compare to NLO QCD calculations. Introuction The bulk of the cross section of harons is ominate by long range forces, where a satisfactory unerstaning of quantum chromoynamics ( QCD still remains a challenge. The first observe events in eep inelastic scattering (DIS at HEA containing a large gap in the pseuorapiity istributions of final state harons, were ientifie as iffractive ep interactions. In iffractive interactions the proton stays intact or issociates into a low mass state (Y, while the photon may issociate into a haronic state X, γ p Xp (Y. The systems are separate by a large rapiity gap (LG. The iffractive exchange object (pomeron has vacuum quantum numbers an carries a fraction of the initial proton longituinal momentum an a momentum transfer t from the incoming to the outgoing proton. Another possibility which was use by both H an ZEUS experiments to ientify iffractive interactions was etection of the forwar proton by proton spectrometers - roman pots, place close to the beam pipe. HEA has mae it possible to stuy iffraction in a wie range of exchange photon virtualities Q from the region of photoprouction, Q GeV,totheDISregionwithQ GeV, an also for many processes like jet- or vector meson-prouction. Here we will concentrate to iffractive inclusive an ijet measurements. Comparison of the selection methos Methos use for the ientification of iffractive events - LG an measuring of the forwar proton in the forwar spectrometers (FS have ifferent systematics. The FS metho selects events in which the proton scatters elastically, whereas the LG metho oes not istinguish between the case where the scattere proton remains intact or where it issociates into a system of low mass M Y. In the case of H measurements are correcte to M Y <.6 GeV while in ZEUS case to M Y = m p. It is therefore natural to ask if both selection methos an both

2 r D(3 (LPS/ r D(3 (LG ZEUS ZEUS LPS/LG Average fit =. =.65 =.3 =.98 =.9 =.55 =.65 =.7 =.3 = GeV 3.9 GeV.5 GeV =.37 =.4 =.8 =.69 4 GeV =. =.48 = = Q =4 GeV Figure : The ratio of the reuce iffractive cross sections as obtaine with the LPS an LG methos, before the subtraction of the proton issociative backgroun. <.6 GeV / H H LG/FPS HEA II H LG/FPS HEA I <.6 GeV / H H LG/FPS HEA II H LG/FPS HEA I <.6 GeV / H H LG/FPS HEA II H LG/FPS HEA I LG/FPS HEA II Norm. uncert..7 Q (GeV Figure : The ratio of the reuce iffractive cross section for M Y <.6 GeV an t < GeV obtaine using the H LG ata to that for M Y = M p an t < GeV.esultsareshownasafunctionofQ, β an. experiments give a compatible results. The selection methos were compare by both ZEUS 3 an H 4. In Fig. the ratio of the reuce cross sections values obtaine with ZEUS forwar proton spectrometer LPS to those obtaine with the LG metho is shown. The ratio is inepenent of, Q an β, whereβ = x/. The normalisation ifference is ascribe to about 5% proton-issociative contribution in the LG sample. The opposite ratios (reuce cross sections measure with LG ivie by those measure with FPS forwar spectrometer of experiment H are shown in Fig.. The contamination of issociation events in the sample of LG interactions is about % not epening on the variable plotte. It is however evient that the precise knowlege an corrections for proton issociation processes are a key factor in unerstaning an comparing of results obtaine by both experiments. The ouble ifferential reuce cross sections measure using proton spectrometers of H (FPS an ZEUS (LPS were compare an combine to a HEA reuce cross section shown in Fig.3. The goo agreement of both ata samples in shape in normalisation is observe. This way two experiments calibrate each other resulting in a significant reuction of the systematic

3 r D( H FPS HEA II ZEUS LPS =.8 = HEA (prel < t <.55 GeV.8.56 Q (GeV HEA Diffraction Working Group July Figure 3: The reuce cross section measure by ZEUS with LPS spectrometer, H with FPS spectrometer an HEA combine reuce cross section. uncertainties. 3 Dijets in events with a leaing proton. In previous ijet analyses at HEA, iffractive events have been selecte on the basis of the presence of LG. H collaboration measure the cross section for the iffractive DIS processes ep >jjx p, with two jets an a leaing proton measure in the FPS 8.Theijetswithtransversemomenta in the CMS system Pt > 5 GeV an P t > 4 GeV, in the central pseuorapiity region. <η<.5, were compare with next-to-leaing (NLO QCD preictions base on DPDFs H fit 6 B an H fit 7 Jets previously measure by H 5,6.TheNLOQCDcalculations (program NLOjet++ 7 werecorrectefortheharonization. TheresultsforthePt cross section of the leaing jet an (where is the longituinal momentum entering the har subprocess shown in Fig. 4 a an b agree within errors with NLO calculations in shape an normalization very well. Using the same sample of ifractive central ijets 8, the istribution of the momentum transfer from the incoming to outcoming proton, t, wasmeasureanfittebytheformula exp(bt. The value of B =5.89±.5 GeV agrees with the previous inclusive t measurements 4 satisfactorily, as it is shown in Fig. 4 c. Two central jets in events with a leaing proton measure with the Very Forwar Proton Spectrometer (VFPS were analyse by experiment H 9. The mean value of the Pt of two central jets (Pt > 5.5 GeV an P t > 4 GeV an is compare with NLO QCD calculations in Fig. 5. It is seen that within errors agree the ata with moel quite well.

4 [pb/gev] * /P T, two central jets H FPS Data NLO Fit 7 Jets NLO Fit 6 B LO Fit 7 Jets H 6 8 * P [GeV] T, [pb] / 5 two central jets H FPS Data NLO Fit 7 Jets NLO Fit 6 B H.5 ] - B [GeV H FPS two central jets H FPS inclusive DIS - H - Figure 4: The ifferential cross section for two central jets shown as a function of P t an. The t slope compare to the H inclusive iffractive DIS ata as a function of. VFPS DIS Dijets VFPS DIS Dijets - /<Pt* jet > (pb.gev H Preliminary H Data (Prel. NLO DPDF Jets 7 (x <Pt* jet > (GeV (pb / 8 6 H Preliminary H Data (Prel. NLO DPDF Jets 7 (x Figure 5: The ifferential cross section for ijets shown as a function of the mean Pt jet an as measure using VFPS spectrometer. / [pb] one central + one forwar jet 6 4 H FPS Data NLO Fit 7 Jets NLO Fit 6 B H * [pb/ ] / one central + one forwar jet H FPS Data NLO Fit 7 Jets NLO Fit 6 B H * [ ] Figure 6: The ifferential cross section for prouction ofone cemtral an one forwar jet shown as a function of an Φ.

5 The search for physics beyon DGLAP evolution was stuie by measurement of DIS iffractive forwar jets 8.TheselectionofeventswiththeprotonintheHFPSgivesauniqueopportunity to measure jets in the forwar region close to the proton irection. The one central jet with Pt > 3.5 GeVanoneforwarjetwithPt > 3.5 GeVintheforwarpseuorapiityregion. <η<.8 wereselecte. SincetheNLOQCDbaseontheDGLAPapproachescribesthe ifferential cross section for an Φ (where Φ is the ifference if azimuthal angles of both jets well, (see Fig. 6, there is no hint for any observation of beyon DGLAP approach effects. Summary The ratio of FS to the LG cross section (ZEUS or LG to the FS cross section (H is inepenent of, Q an β within uncertainties, confirming that contributions from proton issociation in the LG measurement o not significantly alter the measure, Q an β epenencies. The amount of iffractive issociation backgroun in LG sample is about %. The HEA reuce cross section for iffractive prouction with leaing proton was obtaine reucing the uncertainties of measurements of both experiments. The cross sections of ijets measure in events with a leaing proton agree well with NLO calculations. There is no hint for the observation of beyon DGLAP approach effects in events with leaing proton an forwar jets. eferences. ZEUS Coll., M.Derrick et al., Phys.Lett. B 35 ( H Coll., T.Ahme et al., Nucl. Phys. B49 (994, ZEUS Coll.,S.Chekanov et al.,np B86 (9. 4. H Coll.,F.D.Aaron et al.,epj C7 ( H Coll., A.Aktas et al., Eur.Phys. J. C48,( H Coll., A.Aktas et al., JHEP 7 ( Z.Nagy, Z.Trocsanyi, Phys. ev. Lett. 85 ( H Coll. F.D.Aaron et al., DESY--66 (, submitte to EPJC. 9. J.Delvax for H coll., Dijet Prouction in DIS using VFPS at HEA, proceeings of DIS workshop, Newport News, Virginia, USA (.