The hadronic interactions of the photon are studied in terms of new
measurements of the total hadronic cross section and di-jet production in
photon-photon collisions at LEP2.Comment: 3 pages, 3 figures, 1 table, to be published in proceedings
  ICHEP2000, Osaka, Japa

Wengler, Thorsten

English

arXiv

The hadronic interactions of the photon are studied in terms of new measurements of the total hadronic cross section and di-jet production in photon-photon collisions at LEP2

Wengler, T

The University of Manchester - Institutional Repository

The Hadronic picture of the photon

The hadronic interactions of the photon are studied in terms of new measurements of the total hadronic cross section and di-jet production in photon-photon collisions at LEP2.The hadronic interactions of the photon are studied in terms of new measurements of the total hadronic cross section and di-jet production in photon-photon collisions at LEP2

CERN Document Server

For Publisher’s usev1.0 - 7th December 1998.Ω00 ¡00tw-ichep: submitted to World Scientific on October 31, 2000 1For Publisher’s useTHE HADRONIC PICTURE OF THE PHOTONTHORSTEN WENGLERCERN, EP division, CH-1211 Geneve 23, Switzerlandemail: Thorsten.Wengler@cern.chThe hadronic interactions of the photon are studied in terms of new measurements of the total hadroniccross section and di-jet production in photon-photon collisions at LEP2.1 Total hadronic γγ cross sectionThe reaction of e+e−→ e+e− γ∗γ∗→ e+e−+hadrons is analysed for quasi-real photonsusing data collected by L3 and OPAL atLEP2. From this measurement the totalhadronic cross section σγγ(W ) is extractedusing a luminosity function for the photonflux and extrapolating to Q2=0 GeV2. TheL3 data 1 are shown in Figure 1, represent-ing an integrated luminosity of 392.6 pb−1taken at√s from 189 to 202 GeV. The to-tal hadronic cross section, σγγ , is shown asa function of the invariant mass W of thephoton-photon system. Also shown as thesolid line is the result of a fit using the pa-rameterisation proposed by Donnachie andLandshoff 2: σtot = As + Bs−η. The sec-ond term represents the Reggeon exchangedominant at low W , while at high W thePomeron exchange driven by the exponent prevails. If photons behave predominantlylike hadrons the values of  = 0.095± 0.002and η = 0.34± 0.02 obtained from a universalfit to hadron-hadron cross sections 3 shouldbe valid for σγγ(W ) as well. Indeed this isnot the case for the L3 data, as demonstratedby the dashed line in Figure 1. Instead a fitwith A, B, and  as free parameters yields = 0.250±0.016, which is more than a fac-tor of two higher than the universal value, arise significantly steeper than expected fromthe universal fit of hadron-hadron cross sec-tions. The L3 data is compared to an OPALmeasurement 4 using an integrated luminos-ity of 74.3 pb−1 taken at√s from 161 to183 GeV. Both measurements are consistentinside their respective uncertainties. A simi-lar fit as described above to the OPAL datahowever yields  = 0.101± 0.025, in agree-ment with the universal fit. The steeper riseobserved by L3 is mainly determined by theirdata points at highest and lowest W .020040060080050 100 150L3 preliminaryW γγ [GeV]σ γγ [nb] OPAL  1998 data  1999 dataFigure 1. The photon-photon total hadronic crosssection, σγγ (W ). The solid line is a fit to the L3 data,the dashed line uses the parameters from a universalfit to hadron-hadron cross sections (see text). Herethe L3 points do not include the dominant system-atic error due to MC model dependencies. With thiserror included the total uncertainties on the OPALand L3 measurements are of comparable size.2 Di-jet ProductionThe production of di-jets in the collisions oftwo quasi-real photons has been measured byALEPH and OPAL. Jet production is cal-culable in perturbative QCD. The measure-ments can therefore be used to study the per-tw-ichep: submitted to World Scientific on October 31, 2000 2For Publisher’s usePTdσ/dP TData (hadrons)NLO QCD (partons)10-11104 6 8 10 12 14 16 18 20Figure 2. Differential di-jet cross section as measuredby ALEPH using 59.2 pb−1 taken at√s =183 GeV.formance of NLO predictions for these pro-cesses. ALEPH 5 has analysed 59.2 pb−1taken at√s =183 GeV using the Durhamk⊥ clustering algorithm6 with Ycut =0.018,chosen to maximise the rate of di-jet events.Di-jet events are required to have at least twojets with |ηjet| < 1.4, and a minimum jettransverse momentum of 4 (3) GeV for thefirst (second) jet. The differential di-jet crosssection as a function of the highest pT jet inthe event is shown in Figure 2. The data iscompared to an NLO calculation of Klasenet al.7, performed at the level of QCD par-tons. The agreement of the calculation withthe data is good, except at the lowest pjetT .OPAL 8 has analysed 384 pb−1 taken at√s from 189 GeV to 202 GeV. Jets are foundusing the inclusive k⊥6 clustering algorithm.Jets entering the analysis have a pseudora-pidity |ηjet| < 2.0. The two jets with thehighest EjetT in each event are taken. An av-erage transverse energy of the two leading jetsin the event of E¯jetT > 5 GeV is required. Theadditional condition |EjetT,1 − EjetT,2|/(EjetT,1 +EjetT,2) < 1/4 keeps low EjetT jets from enter-ing the distributions and ensures asymmetricEjetT thresholds for the two jets. The differ-ential cross section as a function of E¯jetT (notshown) is reasonably well described by boththe LO MC models PHOJET and PYTHIAand the NLO calculation 7. Di-jet eventshave the particular advantage that the frac-tion of the photon momentum, xγ , enteringthe hard scattering can be estimated fromthe di-jet system. The observable quantityx±γ ≡ (∑jets=1,2(E±pz))/(∑hadrons(E±pz))is defined for this purpose. OPAL has for thefirst time measured differential cross sectionsas a function of xγ , where the data has beenfully unfolded for detector effects. Here xγrefers to both x+γ and x−γ entering the distri-bution. The region of small xγ is expectedto be dominated by resolved photon interac-tions, and is hence particularly sensitive tothe gluon density in the photon. The OPALdata is shown in Figure 3 in three regionsof E¯jetT . The data is compared to three pre-dictions of the PHOJET generator using theGRV LO, SaS1D, and LAC1 parton distri-bution functions respectively. The sensitiv-ity of this observable to the different partondistributions is clearly visible. The alreadydisfavoured LAC1 set is shown to demon-strate the effect of a high gluon density andclearly overshoots the data especially at lowE¯jetT . GRV and SaS1D on the other handseem to underestimated the gluon density. Apreliminary study of the underlying event us-data [pb] NLO [pb]5 GeV < E¯jetT < 7 GeVxγ > 0.75 111.0±3.8 206.5xγ < 0.75 205.5±4.8 84.47 GeV < E¯jetT < 11 GeVxγ > 0.75 77.4±2.6 87.0xγ < 0.75 71.5±2.2 32.611 GeV < E¯jetT < 25 GeVxγ > 0.75 32.0±2.5 27.4xγ < 0.75 15.8±1.7 7.5Table 1. OPAL di-jet cross sections compared toNLO predictions in regions of xγ and E¯jetT.tw-ichep: submitted to World Scientific on October 31, 2000 3For Publisher’s useOPAL Preliminary0204060801001200 0.2 0.4 0.6 0.8 1xγdσdijet /dx γ   [pb]OPAL dataPHOJET 1.10GRV LOSaS 1DLAC 15 <  E jetT  < 7 GeV− 5 <  E jetT  < 7 GeV−7 <  E jetT  < 11 GeV− 11 <  E jetT  < 25 GeV−10-111010 2-2 -1.5 -1 -0.5 0log10( xγ)dσdijet /dlog10(xγ)   [pb]010203040500 0.2 0.4 0.6 0.8 1xγdσdijet /dx γ   [pb]05101520250 0.2 0.4 0.6 0.8 1xγdσdijet /dx γ   [pb](a)(b)(c) (d)Figure 3. OPAL differential di-jet cross section as a function of xγ in several regions of E¯jetT.ing PYTHIA with and without multiple in-teractions show a visible effect only at thelowest values of E¯jetT and xγ . Table 1 com-pares the OPAL measurement with an NLOcalculation 7 for small and large xγ in thethree regions of E¯jetT . It should be stressedthat hadronisation corrections are not takeninto account in this calculation. The NLOcalculation predicts much too large a crosssection for xγ > 0.75 and small E¯jetT , whilebeing too low by about a factor of two forxγ < 0.75. With increasing E¯jetT the discrep-ancy for xγ > 0.75 largely disappears, whilefor xγ < 0.75 the NLO prediction remainstoo low by a factor of two for the highestxγ > 0.75 considered. This also suggests thatthe parton density functions used in the NLOcalculation (GRV) underestimate the gluondensity in the photon.References1. L3 Coll., L3 note 2570, Abstract 622 sub-mitted to this conference.2. A. Donnachie and P.V. Landshoff,Phys. Lett. B296 (1992) 227.3. PDG, Eur. Phys. J. C3 (1998) 1.4. Opal Coll., G. Abbiendi et al.,Eur. Phys. J. C14 (2000) 199.5. ALEPH Coll., ALEPH 2000-052, Ab-stract 265 submitted to this conference.6. S. Catani, Yu.L. Dokshitzer, M.H. Sey-mour and B.R. Webber, Nucl. Phys.B406 (1993) 187; S.D. Ellis, D.E. Soper,Phys. Rev. D48 (1993) 31607. M. Klasen, T. Kleinwort and G. Kramer,Eur. Phys. J. Direct C1 (1998) 1;M. Klasen, private communication.8. OPAL Coll., OPAL PN443, Abstract 583submitted to this conference.tw-ichep: submitted to World Scientific on October 31, 2000 4

The Hadronic Picture of the Photon

Abstract

Similar works

Full text

Available Versions

The University of Manchester - Institutional Repository

CERN Document Server