Schoeffel, L

CERN Document Server

QCD ANALYSIS OF THE DIFFRACTIVE STRUCTUREFUNCTION FD(3)2L. SCHOEFFELCE-Saclay, F-91191 Gif-sur-Yvette Cedex, FranceE-mail: schoel@hep.saclay.cea.frThe proton diractive structure function FD(3)2measured in the H1 and ZEUSexperiments at HERA is analyzed in terms of perturbative QCD evolution. Thedata are well described by a QCD analysis in which point-like parton distributions,evolving according to the DGLAP equations, are assigned to the leading and sub-leading Regge exchanges. The gluon distributions are found to be quite dierentfor H1 and ZEUS. A global t analysis, where a higher twist component is takenfrom models, allows us to use data in the whole available range in diractive massand gives a stable answer for the leading twist contribution. An extrapolation tothe Tevatron range is compared with CDF data on single diraction.1 IntroductionIt is now experimentally well established at HERA1;2that a substantial frac-tion of ep events is contributable to diraction, i.e. color singlet exchange,initiated by a highly virtual photon. The idea of a point-like structure ofthe Pomeron exchange3opens the way to the determination of its parton(quark and gluon) distributions, where the Pomeron point-like structure canbe treated in a similar way as the proton one4.2 Extraction of parton distributions in the PomeronAs indicated in the following equation :FD(3)2(Q2; ; xlP) = flP=p(xlP)FlP2(Q2; ) + flR=p(xlP)FlR2(Q2; ) (1)the diractive structure function FD(3)2can be investigated in the frameworkof Regge phenomenology. In this parameterisation, FlP2can be interpretedas the Pomeron structure function and FlR2as an eective Reggeon structurefunction, with the restriction that it takes into account various secondaryRegge contributions which can hardly be separated. Moreover, it has beensuggested that the Q2evolution of these structure functions may be under-stood in terms of parton dynamics3, i.e. as coming from leading twist, per-turbative QCD contributions where parton densities are evolved according toDGLAP equations.ws-p8-50x6-00: submitted to World Scientic on June 7, 2001 1A simple prescription is adopted in which the parton distributions of boththe Pomeron and the Reggeon are parameterised in terms of non-perturbativeinput distributions at some low scale Q20= 3 GeV2.The pion structure function5is assumed for the sub-leading Reggeontrajectory with a free global normalization to be determined by the data.For the Pomeron, a quark avour singlet distribution (zSq(z;Q2) = u+u+ d+d+ s + s) and a gluon distribution (zG(z;Q2)) are parameterized interms of coecients C(S)jand C(G)jat Q20= 3 GeV2.zS (z;Q2= Q20) =243Xj=1C(S)jPj(2z   1)352 e0:01z 1(2)zG(z;Q2= Q20) =243Xj=1C(G)jPj(2z   1)352 e0:01z 1(3)where z = xi=IPis the fractional momentum of the Pomeron carried by thestruck parton, Pj() is the jthmember in a set of Chebyshev polynomials,which are chosen such that P1= 1, P2=  and Pj+1() = 2Pj() Pj 1().The trajectory intercepts are xed to lP= 1:20, lR= 0:62 for H1 andlP= 1:13 for ZEUS. No sub-leading trajectory is needed to t ZEUS data4.Only data points with Q2 3 GeV2,   0:65,MX> 2 GeV and y  0:45 areincluded in the t in order to avoid large higher twist eects and the regionthat may be most strongly aected by a non-zero value of R. The resultingparton densities of the Pomeron are presented in gure 1 with the values of2=dof .3 Global tsThen, we have tried to rene these QCD ts by extending the number of datapoints included into the procedure. The problem is that the cuts describedabove are essential to avoid the experimental domain where non-perturbativepower contributions could be large and we cannot suppress these cuts with-out subtracting in another way these power contributions (or higher-twistterms). However, some parametrizations for higher-twist contributions havebeen proposed6;7after a dedicated analysis of H1 data.We use these parametrizations to subtract the higher twist componentfrom the FD(3)2measurements at high  and we redo the QCD analysis withall data points included. The resulting parton densities of the Pomeron are inws-p8-50x6-00: submitted to World Scientic on June 7, 2001 200.10.200.511.500.10.200.511.500.10.20 0.2 0.4 0.6 0.8 1 00.511.50 0.2 0.4 0.6 0.8 1Figure 1. Quark avour singlet (zS, left) and gluon (zG, right) distributionsof the Pomeronderived from H1 diractive data (2=dof = 177:1=154 = 1:15, full lines) and ZEUS dirac-tive data (2=dof = 29:3=(30  6) = 1:22, dashed lines).perfect agreement with those presented in gure 1. Moreover, the statisticalsignicance of the global QCD ts is increased.4 Extrapolation to Tevatron and comparison with CDF dataThe QCD ts we have obtained from HERA data allow us to make directcomparisons for measurements at the Tevatron. It is quite interesting to beable to directly test factorization breaking between HERA and the Tevatronusing the measurements performed at both accelerators. We thus compare theextrapolations of the H1 and ZEUS QCD ts to the recent CDF diractivejet cross-section measurement8. We note a large discrepancy both in shapeand normalization between H1 predictions and CDF data, clearly showingfactorization breaking. However, the ZEUS ts are more compatible in nor-malization with the CDF measurement even if the shape is not describedws-p8-50x6-00: submitted to World Scientic on June 7, 2001 3properly. More precise data from HERA and more detailed comparisons withTevatron are thus needed to study precisely factorization breaking betweenboth experiments.5 ConclusionsWe have proposed of a new NLO DGLAP t to the diractive structure func-tions. We have obtained parton distributions for the Pomeron using H1 andZEUS data. They both require a large gluon component inside the Pomeronbut the gluon densities are quite dierent. Using a parametrisation for thehigher twist contribution to FD2allows us to get a description of the full setof data using the NLO DGLAP evolution equations, and leads to the sameparton distributions as before. The same dierence leads to diverging extrap-olations for single diraction at the Tevatron. The ZEUS-based extrapolationleads to a possible compatibility with factorization at least in the small (large diractive mass) range.AcknowledgmentsThis work has been done in collaboration with C.Royon, J.Bartels, H.Jung andR.Peschanski. We thank A.Brandt, D.Goulianos, P.Marage, P.Newman, andP.Van Mechelen for useful discussions and a careful reading of the manuscript.References1. H1 Collaboration, Z. Phys. C 76,613 (1997).2. ZEUS Collaboration, Eur.Phys.J. C 6 , 43 (1999).3. G. Ingelman, P. Schlein, Phys. Lett. B 152, 256 (1985).4. C.Royon, L.Schoeel, J.Bartels, H.Jung, R.Peschanski, Phys. Rev. D63, 74004 (2001).5. M. Gluck, E. Reya, A. Vogt, Z. Phys. C 53, 651 (1992).6. J.Bartels, J.Ellis, H.Kowalski, M.Wuestho, Eur.Phys.J. C 7, 443 (1999).J.Bartels, C.Royon, Mod. Phys. Lett. A 14, 1583 (1999).7. A.Bialas, R.Peschanski, C.Royon, Phys. Rev. D 57 , 6899 (1998).S.Munier, R.Peschanski, C.Royon, Nucl. Phys. B 534, 297 (1998).8. CDF Coll., preprint FERMILAB-Pub-/055-E, subm. to Phys.Rev.Lett.ws-p8-50x6-00: submitted to World Scientic on June 7, 2001 4

QCD analysis of the diffractive structure function $F_{2} ^{D(3)}$

Abstract

Similar works

Full text

Available Versions

CERN Document Server

QCD analysis of the diffractive structure function F2D(3)F_{2} ^{D(3)}F2D(3)​

Abstract

Similar works

Full text

Available Versions

CERN Document Server

QCD analysis of the diffractive structure function $F_{2} ^{D(3)}$