The differential cross section for 2H(γ,d)π0 has been measured at deuteron center-of-mass angles of 90° and 136°. This work reports the first data for this reaction above a photon energy of 1 GeV, and permits a test of the apparent constituent counting rule and reduced nuclear amplitude behavior as observed in elastic ed scattering. Measurements were performed up to a photon energy of 4.0 GeV, and are in good agreement with previous lower energy measurements. Overall, the data are inconsistent with both constituent-counting rule and reduced nuclear amplitude predictions

Abbott, D.J.

Ahmidouch, A.

Armstrong, C.S.

Arrington, J.

Assamagan, K.A.

Baker, O.K.

Barrow, S.P.

Beatty, D.P.

Beck, D.H.

Beedoe, S.Y.

Beise, E.J.

Belz, J.E.

Bochna, C.

Bosted, P.E.

Brash, E.J.

Breuer, H.

Cadman, R.V.

Cardman, L.

Carlini, R.D.

Cha, J.

Chant, N.S.

Collins, G.

Cothran, C.

Cummings, W.J.

Danagoulian, S.

Duncan, F.A.

Dunne, J.A.

Dutta, D.

Eden, T.

Ent, R.

Filippone, B.W.

Forest, T.A.

Fortune, H.T.

Frolov, V.V.

Gao, H.

Geesaman, D.F.

Gilman, R.

Gueye, P.L.J.

Gustafsson, K.K.

Hansen, J.-O.

Harvey, M.

Hinton, W.

Holt, R.J.

Jackson, H.E.

Keppel, C.E.

Khandaker, M.A.

Kinney, E.R.

Klein, A.

Koltenuk, D.M.

Kumbartzki, G.

Lung, A.F.

Mack, D.J.

Madey, R.

Markowitz, P.

McFarlane, K.W.

McKeown, R.D.

Meekins, D.G.

Meziani, Z.-E.

Miller, M.A.

Mitchell, J.H.

Mkrtchyan, H.G.

Mohring, R.M.

Napolitano, J.

Nathan, A.M.

Niculescu, G.

Niculescu, I.

O'Neill, T.G.

Owen, B.R.

Pate, S.F.

Potterveld, D.H.

Price, J.W.

Rakness, G.L.

Ransome, D.

Reinhold, J.

Rutt, P.M

Salgado, C.W.

Savage, G.

Segel, R.E.

Simicevic, N.

Stoler, P.

Suleiman, R.

Tang, L.

Terburg, B.P.

van Westrum, D.

Vulcan, W.F.

Williamson, S.E.

Witkowski, M.T.

Wood, S.A.

Yan, C.

Zeidman, B.

Physical Review C

English

Digital Repository at the University of Maryland

RAPID COMMUNICATIONSPHYSICAL REVIEW C, VOLUME 60, 052201Coherent p0 photoproduction on the deuteron up to 4 GeVD. G. Meekins,1,2 D. J. Abbott,2 A. Ahmidouch,3 C. S. Armstrong,1 J. Arrington,4 K. A. Assamagan,5 O. K. Baker,2,5S. P. Barrow,6 D. P. Beatty,6 D. H. Beck,7 S. Y. Beedoe,8 E. J. Beise,9 J. E. Belz,10 C. Bochna,7 P. E. Bosted,11E. J. Brash,12,17 H. Breuer,9 R. V. Cadman,7 L. Cardman,2 R. D. Carlini,2 J. Cha,5 N. S. Chant,9 G. Collins,9C. Cothran,13 W. J. Cummings,14 S. Danagoulian,8 F. A. Duncan,9 J. A. Dunne,2 D. Dutta,15 T. Eden,5 R. Ent,2B. W. Filippone,4 T. A. Forest,7 H. T. Fortune,6 V. V. Frolov,16 H. Gao,7 D. F. Geesaman,14 R. Gilman,17 P. L. J. Gueye,5K. K. Gustafsson,9 J-O. Hansen,14 M. Harvey,5 W. Hinton,5 R. J. Holt,7 H. E. Jackson,14 C. E. Keppel,2,5M. A. Khandaker,18 E. R. Kinney,19 A. Klein,20 D. M. Koltenuk,6 G. Kumbartzki,17 A. F. Lung,9 D. J. Mack,2 R. Madey,3,5P. Markowitz,21 K. W. McFarlane,18 R. D. McKeown,4 Z-E. Meziani,22 M. A. Miller, 7 J. H. Mitchell,2H. G. Mkrtchyan,23 R. M. Mohring,9 J. Napolitano,16 A. M. Nathan,7 G. Niculescu,5 I. Niculescu,5 T. G. O’Neill,14B. R. Owen,7 S. F. Pate,24 D. H. Potterveld,14 J. W. Price,16 G. L. Rakness,19 R. D. Ransome,17 J. Reinhold,14 P. M. Rutt,17C. W. Salgado,18 G. Savage,5 R. E. Segel,15 N. Simicevic,7 P. Stoler,16 R. Suleiman,3 L. Tang,2,5 B. P. Terburg,7D. van Westrum,19 W. F. Vulcan,2 S. E. Williamson,7 M. T. Witkowski,16 S. A. Wood,2 C. Yan,2 and B. Zeidman141Department of Physics, College of William and Mary, Williamsburg, Virginia 231872Physics Division, Thomas Jefferson National Accelerator Facility, Newport News, Virginia 236063Department of Physics, Kent State University, Kent, Ohio 442424Department of Physics, California Institute of Technology, Pasadena, California 911255Department of Physics, Hampton University, Hampton, Virginia 236686Department of Physics, University of Pennsylvania, Philadelphia, Pennsylvania 191047Department of Physics, University of Illinois at Urbana-Champaign, Urbana, Illinois 618018Department of Physics, North Carolina A&T State University, Greensboro, North Carolina 274119Department of Physics, University of Maryland, College Park, Maryland 2074210TRIUMF, Vancouver, British Columbia, Canada V6T 2A311Department of Physics, American University, Washington, D.C. 2001612Department of Physics, University of Regina, Regina, Saskatchewan, Canada S2S 0A213Department of Physics, University of Virginia, Charlottesville, Virginia 2290114Physics Division, Argonne National Laboratory, Argonne, Illinois 6043915Department of Physics, Northwestern University, Evanston, Illinois 6020116Department of Physics, Rensselaer Polytechnic Institute, Troy, New York 1218017Department of Physics, Rutgers University, New Brunswick, New Jersey 0890318Department of Physics, Norfolk State University, Norfolk, Virginia 2350419Department of Physics, University of Colorado at Boulder, Boulder, Colorado 8030920Department of Physics, Old Dominion University, Norfolk, Virginia 2350421Department of Physics, Florida International University, University Park, Florida 3319922Department of Physics, Temple University, Philadelphia, Pennsylvania 1912223Yerevan Physics Institute, Yerevan, Armenia24Department of Physics, New Mexico State University, Las Cruces, New Mexico 88003~Received 18 March 1999; published 27 September 1999!The differential cross section for2H(g,d)p0 has been measured at deuteron center-of-mass angles of 90°and 136°. This work reports the first data for this reaction above a photon energy of 1 GeV, and permits a testof the apparent constituent counting rule and reduced nuclear amplitude behavior as observed in elasticedscattering. Measurements were performed up to a photon energy of 4.0 GeV, and are in good agreement withprevious lower energy measurements. Overall, the data are inconsistent with both constituent-counting rule andreduced nuclear amplitude predictions.@S0556-2813~99!50910-2#PACS number~s!: 25.20.Lj, 24.85.1p, 25.10.1sioasfro-apy isfere asnre--ibleeateMany previously measured exclusive hadronic reactcross sections were found to obey a power-law scalingpredicted by the constituent-counting rules~CCR! @1#. Theserules should apply when the energy and momentum tranin the reaction are sufficiently large such that any macscopic hadronic effect~such as constituent binding or motion! can be neglected and the reaction proceeds by hscatterings only. Processes such as H(g,g)p @2#, H(g,p)p0@3#, and electron-proton scattering@4,5# seem to follow, atleast for certain center-of-mass angles, these power-law0556-2813/99/60~5!/052201~4!/$15.00 60 0522nser-rdre-dictions in a region where the total center-of-mass-energa few GeV. However, in this region, the momentum transper gluon exchange may not be sufficient to consider theshard exchanges@6#. This suggests that soft wave functioeffects should not be neglected at these energies. Theduced nuclear amplitude~RNA! approach attempts to remove part of these effects—the soft components responsfor quark binding within the nucleons—by dividing out thempirical nucleon form factors@7,8#.Presently, a number of theoretical efforts seem to indic©1999 The American Physical Society01-1estti-Nthalinio-roththphaayndino-epn-at12nntloneCressroi-a15eofon-ngletorurendcmc--ctedon-verintheerteringasss to.retionea-2ions,eap-lossRAPID COMMUNICATIONSD. G. MEEKINS et al. PHYSICAL REVIEW C 60 052201that the observed power-law dependence may not be a rof quark and gluon degrees of freedom as described byCCR @9#. Nonetheless, it is very much of interest to invesgate the reason why the predictions of the CCR and Rmodels seem to be accurate in some cases, and not in oOf special interest is the question of whether nuclear retions follow these predictions. One such reaction,2H(g,p)n,has been reported to follow the predicted power-law scaat a center-of-mass angle of 90°, in a photon energy regbetween 1.0 and 4.0 GeV@10–12#. Perhaps even more impressive is that recent measurements of elastic electdeuteron scattering seem to follow the predictions of bothRNA and CCR models@13#.To investigate whether this agreement extends to onuclear reactions, we have measured one of the simplesttonuclear reactions involving a nucleus in the initial and finstate, the2H(g,d)p0 reaction. Photonuclear reactions mbe the optimal choice for this investigation, because Lashoff terms~which must be considered in hadron-hadronteractions@14,15#! cannot contribute, and the effective mmentum transfer@16# and differential cross sections can blarge compared to similar electronuclear cross sections.For the exclusive process A1B→C1D at high energyand large transverse momentum, dimensional analysisdicts the following constituent counting rule for the differetial cross section@1#:dsdt}s2(n22)f ~uc.m.!, ~1!where s and t are the Mandelstam variables,uc.m. is thecenter-of-mass scattering angle,f (uc.m.) depends on detailsof the dynamics of the process, andis the total number ofelementary fields~photon, quark, etc.! in the initial and finalstates. For the2H(g,p)n reaction,n22 is 11,n22 is 12 forthe 2H(e,e8d) reaction, and for the2H(g,d)p0 reactionn22 is 13. As mentioned above, data at a center-of-mangle of 90° for the2H(g,p)n reaction are in agreemenwith the CCR prediction above a photon energy of onlyGeV @10–12#. However, data at center-of-mass angles of 5and 36° do not agree with these predictions@12#. Further-more, while the RNA analysis describes the electrodeuteron elastic scattering cross section above a mometransfer squared of 2 GeV2 @7,13#, the data are also weldescribed by conventional calculations including mesexchange currents@17#. The RNA analysis also does not giva good description of the2H(g,p)n data, even though it isexpected to approach scaling at lower energies than the Cmodel. Previous data for the D (g,d)p0 reaction were lim-ited to photon energies belowEg.1 GeV @19#, and werenever tested against these predictions.Here we report on a substantial extension of existingsults for the2H(g,d)p0 reaction at deuteron center-of-maangles~angle between the incoming and outgoing deutein the center of mass! of 90° and 136°. In the present experment, an electron beam passed through a 4 or 6 %radiationlength copper radiator to create an untagged photon beincident on a cryogenic liquid deuterium target of 12 orcm length. Electron beam energies between 0.8 and 4.0 G05220ultheAers.c-gnn-eero-l--re-ss°-um-R-nm,V,and beam currents between 10 and 30mA were used. TheHigh Momentum Spectrometer~HMS! in Hall C at ThomasJefferson National Accelerator Facility, with a solid angle6.7 msr and a momentum acceptance of610% was used todetect the deuterons. The photon energy could be recstructed from the measured momentum and scattering aof the final state deuteron. Plastic scintillators were usedform a trigger and to provide time-of-flight information foparticle identification. Drift chambers were used to measthe trajectory of the particle from which the momentum ascattering angle of the deuteron were determined. A 6.35thick tungsten collimator was installed in front of the spetrometer. Although this collimator will not stop highmomentum deuterons, it was used as a cut on reconstruquantities. Deuteron identification was obtained by recstructing the mass from the time-of-flight measurement oa 2-m flight path between two pairs of scintillator planesthe detector hut and from the reconstructed momentum ofparticle. This method identifies deuterons well at the lowphoton energies, as shown in Fig. 1~a!. At higher photonenergies, the ratio of protons@produced largely by the2H(g,p)n reaction# to deuterons entering the spectromewas large, and a larger tail from protons strongly interactin the first scintillator planes exists under the deuteron mpeak, as shown in Fig. 1~b!. This tail was subtracted in thedata analysis, and the uncertainty in the procedure addthe systematic uncertainty for the higher photon energiesBackground contributions from the target windows weremoved by placing cuts on the reconstructed target posiFIG. 1. Mass spectrum as determined from time-of-flight msurement and momentum reconstruction in the HMS:~a! at a pho-ton beam energy of 1.4 GeV,~b! at a photon beam energy of 3.GeV. The deuteron peaks can be clearly identified. In~a! the shadedarea indicates protons, probably undergoing secondary interactwhich were identified with energy loss in the scintillators. At thhigher energy~b!, the background under the deuteron peak isproximated by the sum of the shaded areas because energycannot be used to separate protons from deuterons.1-2ateanentinargifibtigran.osthhlprctrgth3%es--liicAtin-allo-get,tionron-ur-ea--itht toing.or-andisandtheheovetedrac-oornhesatis-t isn astheRAPID COMMUNICATIONSCOHERENTp0 PHOTOPRODUCTION ON THE DEUTERON . . . PHYSICAL REVIEW C 60 052201and subtracting the yield obtained with a cell of identicdimensions that was filled with liquid hydrogen to simulabremsstrahlung produced in the deuterium. Deuteriumhydrogen data were taken alternately during the experimThe yield from electroproduction was measured by repeathe procedure without the radiator. This background wsubtracted from the photoproduction yield with an enedependent correction factor to take into account the modcation of the electron beam flux and energy distributionthe radiator@20#.The photon energy bin limits were chosen to kinemacally eliminate deuterons associated with more than sinpion production processes, and to eliminate the bremsstlung endpoint, for which the photon flux is less well knowThe former is not alwaysa priori possible for this experi-ment, since the two pion and single pion kinematic threshare only separated by;25 MeV in photon energy in theworst case. However, we verified that the differential crosections did not depend on the photon energy cut athigher energies, and compared the measured bremsstraspectra with the theoretical spectra assuming single-pionduction. These theoretical bremsstrahlung spectra wereculated using a code@18# based on the the thick-targebremsstrahlung calculations of Matthews and Owens@21# incombination with the Landau spectra mimicking the eneloss tails in the radiator. The absolute uncertainty inbremsstrahlung photon flux is estimated to be less thanA typical example of an endpoint spectrum for th2H(g,d)p0 reaction, with a normalized theoretical bremstrahlung spectrum weighted bys28.6 ~the empirical energydependence of the cross section! a d smeared for spectrometer resolution for this process, is shown in Fig. 2. The socurve in the figure is in good agreement with the data whFIG. 2. Bremsstrahlung endpoint spectrum for the2H(g,d)p0reaction at a photon energy of 1.4 GeV and a center-of-mass aof 90°. The solid curve shows the~normalized! theoretical brems-strahlung spectrum for single-pion production weighted withs28.6.05220ldt.gsy-y-leh-ldseungo-al-ye.dhshow no indications of two pion production processes.possible competing process, in our measurement indisguishable fromp0 photoproduction, would be coherent reCompton scattering from the deuteron, but the ratio of anagous processes in hydrogen H(g,g)p to H(g,p)p0 in a simi-lar energy range is only a few percent@2# in the worst case.We applied a deuteron absorption correction~15–20 %! tocompensate for the inelastic deuteron reactions in the tarspectrometer windows, and detector stack. This correcwas constructed from measured proton-proton and neutproton cross sections, parametrized asA0.75(spp1snp). Weassign a 5% systematic uncertainty for this procedure. Fthermore, the validity of this procedure was checked by msuring 2H(e,e8d) in coincidence at two values of momentum transfer and comparing the calculated attenuation wthe reduction of this coincidence cross section with respecthe world data set on elastic electron-deuteron scatterThe results agree much better than the 5% uncertainty. Crections were also applied for the computer dead-timethe tracking efficiency. The overall systematic uncertaintyfound to range between 6% at the lower photon energies20% at the highest photon energy, and is dominated byattenuation correction and the background correction. Tbackground subtraction is related to events observed abthe photon endpoint and in a continuum in the reconstrucM2 spectra. These events were mostly removed by subttion of the tail under the deuteron missing mass@ ee Fig.1~b!#. Some of these events however, may be due to pgleFIG. 3. The data from the present work in combination with texisting data for the2H(g,d)p0 reaction. For a center-of-masangle of 90°~a! and for a center-of-mass angle of 136°~b!. Theerrors bars on the present work include both systematic and sttical uncertainties. Data from Imanishiet al. @19# are shown aspluses, a datum extracted from the SLAC NE-17 experimenshown as a star, and data from the present experiment are showsolid circles. Solid curves are RNA calculations normalized todata at 1.6 GeV.1-3tteinheyaolalo36onnodamofr-esa,tore-omes inionyCRtheusgetsuleap-ns.e aionheNa-rtandRAPID COMMUNICATIONSD. G. MEEKINS et al. PHYSICAL REVIEW C 60 052201reconstruction of momenta resulting from deuterons scaing inside the spectrometer. The systematic uncertaquoted includes a contribution from the sensitivity to tchoice of background subtraction procedure.The differential cross sections13ds/dt as determinedfrom our data for center-of-mass angles (uc.m.) of 90° and136° are shown in Fig. 3@18#. The data at the lowest energare in good agreement with earlier measurements by Imishi et al. @19# and extend to a photon energy of 4 GeV~2.4GeV! for uc.m.590° ~136°!. An unpublished2H(g,d)p0 da-tum @18# extracted from the SLAC NE17 experiment@11# isalso shown, and agrees well with the new data. The scurves in the figure are RNA calculations arbitrarily normized to the data at 1.6 GeV. It is clear that the data at bangles are inconsistent with the RNA approach. The 1data are consistent with the CCR predicteds213 scaling,while the 90° are in sharp disagreement with this predictiFurthermore, we note that while the data at 136° doextend to as high an incident photon energy as the 90°do, they do cover a similar range in effective momentutransfer (1<Q2<6) GeV2 . The recent measurementsthe deuteron electric form factorA(Q2) are consistent withboth the CCR and RNA predictions in a similar foumomentum transfer range 2<Q2<6 GeV2 @13#. Data forthese two reactions pose a sharp contrast as both procinvolve a deuteron in the initial and final states.The invariant cross section,ds/dt, for the 136° data wasfound to scale as213.160.2 and in the case of the 90° dat05220r-tyn-id-th°.ttasesds/dt;s29.660.4. This variation of the power ofs withcenter-of-mass scattering angle is also seen in other phoactions. The2H(g,p)n @12# and H(g,g)p @9# reactions werealso reported to scale with varying powers ofs for differentcenter-of-mass angles. When viewed collectively, data frthese photoprocesses may indicate that nuclear processthis energy range are still dominated by soft wave functeffects@6,9#. Similarly, the elasticedscattering results belowQ256 GeV2 may be more appropriately described bmeson-exchange calculations than by the RNA and Cmodels@17#.In summary, we have extended the sparse data set on2H(g,d)p0 reaction up to photon energies where previoreal photon experiments on hydrogen and deuterium tarstarted to show consistency with constituent counting rpredictions. The data at a center-of-mass angle of 136°pear consistent with constituent counting rule predictioThe data at 90° in the center-of-mass are the first abovphoton energy of 1 GeV to show such a dramatic deviatfrom the CCR and RNA predictions.We acknowledge the outstanding work of the staff of taccelerator division and Hall C at the Thomas Jeffersontional Accelerator Facility. This work was supported in paby research grants from the U.S. Department of Energythe U.S. National Science Foundation.y,@1# S.J. Brodsky and G.R. Farrar, Phys. Rev. Lett.31, 1153~1973!; V. Matveevet al., Lett. Nuovo Cimento7, 719~1973!;G.P. 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Coherent π0 photoproduction on the deuteron up to 4 GeV

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Coherent π0 photoproduction on the deuteron up to 4 GeV

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