Direct photon cross section with conversions at CDF

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Direct photon cross section with

conversions at CDFD. Acosta, T. Affolder, M. G. Albrow, D. Ambrose, D. Amidei, K. Anikeev,J. Antos, G. Apollinari, T. Arisawa, A. Artikov, W. Ashmanskas, F. Azfar, P.Azzi-Bacchetta, N. Bacchetta, H. Bachacou, W. Badgett, A. Barbaro-Galtieri,V. E. Barnes, B. A. Barnett, S. Baroiant, M. Barone, G. Bauer, F. Bedeschi,S. Behari, S. Belforte, W. H. Bell, G. Bellettini, J. Bellinger, D. Benjamin, A.Beretvas, A. Bhatti, M. Binkley, D. Bisello, M. Bishai, R. E. Blair, C. Blocker,K. Bloom, B. Blumenfeld, A. Bocci, A. Bodek, G. Bolla, A. Bolshov, D. Bor-toletto, J. Boudreau, C. Bromberg, E. Brubaker, J. Budagov, H. S. Budd, K.Burkett, G. Busetto, K. L. Byrum, S. Cabrera, M. Campbell, W. Carithers, D.Carlsmith, A. Castro, D. Cauz, A. Cerri, L. Cerrito, J. Chapman, C. Chen, Y.C. Chen, M. Chertok, G. Chiarelli, G. Chlachidze, F. Chlebana, M. L. Chu, J.Y. Chung, W. H. Chung, Y. S. Chung, C. I. Ciobanu, A. G. Clark, M. Coca,A. Connolly, M. Convery, J. Conway, M. Cordelli, J. Cranshaw, R. Culbertson,D. Dagenhart, S. D’Auria, P. de Barbaro, S. De Cecco, S. Dell’Agnello, M.Dell’Orso, S. Demers, L. Demortier, M. Deninno, D. De Pedis, P. F. Derwent,C. Dionisi, J. R. Dittmann, A. Dominguez, S. Donati, M. D’Onofrio, T. Dorigo,N. Eddy, R. Erbacher, D. Errede, S. Errede, R. Eusebi, S. Farrington, R. G.Feild, J. P. Fernandez, C. Ferretti, R. D. Field, I. Fiori, B. Flaugher, L. R.Flores-Castillo, G. W. Foster, M. Franklin, J. Friedman, I. Furic, M. Gallinaro,M. Garcia-Sciveres, A. F. Garfinkel, C. Gay, D. W. Gerdes, E. Gerstein, S.Giagu, P. Giannetti, K. Giolo, M. Giordani, P. Giromini, V. Glagolev, D. Glen-zinski, M. Gold, N. Goldschmidt, J. Goldstein, G. Gomez, M. Goncharov, I.Gorelov, A. T. Goshaw, Y. Gotra, K. Goulianos, A. Gresele, C. Grosso-Pilcher,M. Guenther, J. G. da Costa, C. Haber, S. R. Hahn, E. Halkiadakis, C. Hall, R.Handler, F. Happacher, K. Hara, R. M. Harris, F. Hartmann, K. Hatakeyama,J. Hauser, J. Heinrich, M. Hennecke, M. Herndon, C. Hill, A. Hocker, K. D.Hoffman, S. Hou, B. T. Huffman, R. Hughes, J. Huston, C. Issever, J. Incan-dela, G. Introzzi, M. Iori, A. Ivanov, Y. Iwata, B. Iyutin, E. James, M. Jones,T. Kamon, J. Kang, M. K. Unel, S. Kartal, H. Kasha, Y. Kato, R. D. Kennedy,R. Kephart, B. Kilminster, D. H. Kim, H. S. Kim, M. J. Kim, S. B. Kim, S. H.Kim, T. H. Kim, Y. K. Kim, M. Kirby, L. Kirsch, S. Klimenko, P. Koehn, K.Kondo, J. Konigsberg, A. Korn, A. Korytov, J. Kroll, M. Kruse, V. Krutelyov,

S. E. Kuhlmann, N. Kuznetsova, A. T. Laasanen, S. Lami, S. Lammel, J. Lan-caster, K. Lannon, M. Lancaster, R. Lander, A. Lath, G. Latino, T. LeCompte,Y. Le, J. Lee, S. W. Lee, N. Leonardo, S. Leone, J. D. Lewis, K. Li, C. S. Lin,M. Lindgren, T. M. Liss, T. Liu, D. O. Litvintsev, N. S. Lockyer, A. Loginov, M.Loreti, D. Lucchesi, P. Lukens, L. Lyons, J. Lys, R. Madrak, K. Maeshima, P.Maksimovic, L. Malferrari, M. Mangano, G. Manca, M. Mariotti, M. Martin, A.Martin, V. Martin, M. Martinez, P. Mazzanti, K. S. McFarland, P. McIntyre,M. Menguzzato, A. Menzione, P. Merkel, C. Mesropian, A. Meyer, T. Miao,R. Miller, J. S. Miller, S. Miscetti, G. Mitselmakher, N. Moggi, R. Moore, T.Moulik, M. Mulhearn, A. Mukherjee, T. Muller, A. Munar, P. Murat, J. Nacht-man, S. Nahn, I. Nakano, R. Napora, F. Niell, C. Nelson, T. Nelson, C. Neu,M. S. Neubauer, C. Newman-Holmes, T. Nigmanov, L. Nodulman, S. H. Oh,Y. D. Oh, T. Ohsugi, T. Okusawa, W. Orejudos, C. Pagliarone, F. Palmonari,R. Paoletti, V. Papadimitriou, J. Patrick, G. Pauletta, M. Paulini, T. Pauly,C. Paus, D. Pellett, A. Penzo, T. J. Phillips, G. Piacentino, J. Piedra, K. T.Pitts, A. Pompos, L. Pondrom, G. Pope, T. Pratt, F. Prokoshin, J. Proudfoot,F. Ptohos, O. Poukhov, G. Punzi, J. Rademacker, A. Rakitine, F. Ratnikov, H.Ray, A. Reichold, P. Renton, M. Rescigno, F. Rimondi, L. Ristori, W. J. Robert-son, T. Rodrigo, S. Rolli, L. Rosenson, R. Roser, R. Rossin, C. Rott, A. Roy,A. Ruiz, D. Ryan, A. Safonov, R. St Denis, W. K. Sakumoto, D. Saltzberg,C. Sanchez, A. Sansoni, L. Santi, S. Sarkar, P. Savard, A. Savoy-Navarro, P.Schlabach, E. E. Schmidt, M. P. Schmidt, M. Schmitt, L. Scodellaro, A. Scrib-ano, A. Sedov, S. Seidel, Y. Seiya, A. Semenov, F. Semeria, M. D. Shapiro,P. F. Shepard, T. Shibayama, M. Shimojima, M. Shochet, A. Sidoti, A. Sill,P. Sinervo, A. J. Slaughter, K. Sliwa, F. D. Snider, R. Snihur, M. Spezziga,F. Spinella, M. Spiropulu, L. Spiegel, A. Stefanini, J. Strologas, D. Stuart, A.Sukhanov, K. Sumorok, T. Suzuki, R. Takashima, K. Takikawa, M. Tanaka,M. Tecchio, R. J. Tesarek, P. K. Teng, K. Terashi, S. Tether, J. Thom, A. S.Thompson, E. Thomson, P. Tipton, S. Tkaczyk, D. Toback, K. Tollefson, D.Tonelli, M. Tonnesmann, H. Toyoda, W. Trischuk, J. Tseng, D. Tsybychev, N.Turini, F. Ukegawa, T. Unverhau, T. Vaiciulis, A. Varganov, E. Vataga, S. Ve-jcik, G. Velev, G. Veramendi, R. Vidal, I. Vila, R. Vilar, I. Volobouev, M. vonder Mey, R. G. Wagner, R. L. Wagner, W. Wagner, Z. Wan, C. Wang, M. J.Wang, S. M. Wang, B. Ward, S. Waschke, D. Waters, T. Watts, M. Weber, W.C. Wester, B. Whitehouse, A. B. Wicklund, E. Wicklund, H. H. Williams, P.Wilson, B. L. Winer, S. Wolbers, M. Wolter, S. Worm, X. Wu, F. Wurthwein,U. K. Yang, W. Yao, G. P. Yeh, K. Yi, J. Yoh, T. Yoshida, I. Yu, S. Yu, J. C.Yun, L. Zanello, A. Zanetti, F. Zetti, and S. Zucchelli

This paper is posted at Purdue e-Pubs.

http://docs.lib.purdue.edu/physics articles/504

Direct photon cross section with conversions at CDF

D. Acosta,14 T. Affolder,7 M. G. Albrow,13 D. Ambrose,36 D. Amidei,27 K. Anikeev,26 J. Antos,1 G. Apollinari,13

T. Arisawa,50 A. Artikov,11 W. Ashmanskas,2 F. Azfar,34 P. Azzi-Bacchetta,35 N. Bacchetta,35 H. Bachacou,24

W. Badgett,13 A. Barbaro-Galtieri,24 V. E. Barnes,39 B. A. Barnett,21 S. Baroiant,5 M. Barone,15 G. Bauer,26

F. Bedeschi,37 S. Behari,21 S. Belforte,47 W. H. Bell,17 G. Bellettini,37 J. Bellinger,51 D. Benjamin,12 A. Beretvas,13

A. Bhatti,41 M. Binkley,13 D. Bisello,35 M. Bishai,13 R. E. Blair,2 C. Blocker,4 K. Bloom,27 B. Blumenfeld,21 A. Bocci,41

A. Bodek,40 G. Bolla,39 A. Bolshov,26 D. Bortoletto,39 J. Boudreau,38 C. Bromberg,28 E. Brubaker,24 J. Budagov,11

H. S. Budd,40 K. Burkett,13 G. Busetto,35 K. L. Byrum,2 S. Cabrera,12 M. Campbell,27 W. Carithers,24 D. Carlsmith,51

A. Castro,3 D. Cauz,47 A. Cerri,24 L. Cerrito,20 J. Chapman,27 C. Chen,36 Y. C. Chen,1 M. Chertok,5 G. Chiarelli,37

G. Chlachidze,13 F. Chlebana,13 M. L. Chu,1 J.Y. Chung,32 W. -H. Chung,51 Y. S. Chung,40 C. I. Ciobanu,20 A. G. Clark,16

M. Coca,40 A. Connolly,24 M. Convery,41 J. Conway,43 M. Cordelli,15 J. Cranshaw,45 R. Culbertson,13 D. Dagenhart,4

S. D’Auria,17 P. de Barbaro,40 S. De Cecco,42 S. Dell’Agnello,15 M. Dell’Orso,37 S. Demers,40 L. Demortier,41

M. Deninno,3 D. De Pedis,42 P. F. Derwent,13 C. Dionisi,42 J. R. Dittmann,13 A. Dominguez,24 S. Donati,37

M. D’Onofrio,16 T. Dorigo,35 N. Eddy,20 R. Erbacher,13 D. Errede,20 S. Errede,20 R. Eusebi,40 S. Farrington,17

R. G. Feild,52 J. P. Fernandez,39 C. Ferretti,27 R. D. Field,14 I. Fiori,37 B. Flaugher,13 L. R. Flores-Castillo,38

G.W. Foster,13 M. Franklin,18 J. Friedman,26 I. Furic,26 M. Gallinaro,41 M. Garcia-Sciveres,24 A. F. Garfinkel,39

C. Gay,52 D.W. Gerdes,27 E. Gerstein,9 S. Giagu,42 P. Giannetti,37 K. Giolo,39 M. Giordani,47 P. Giromini,15

V. Glagolev,11 D. Glenzinski,13 M. Gold,30 N. Goldschmidt,27 J. Goldstein,34 G. Gomez,8 M. Goncharov,44 I. Gorelov,30

A.T. Goshaw,12 Y. Gotra,38 K. Goulianos,41 A. Gresele,3 C. Grosso-Pilcher,10 M. Guenther,39 J. Guimaraes da Costa,18

C. Haber,24 S. R. Hahn,13 E. Halkiadakis,40 C. Hall,18 R. Handler,51 F. Happacher,15 K. Hara,48 R. M. Harris,13

F. Hartmann,22 K. Hatakeyama,41 J. Hauser,6 J. Heinrich,36 M. Hennecke,22 M. Herndon,21 C. Hill,7 A. Hocker,40

K. D. Hoffman,10 S. Hou,1 B.T. Huffman,34 R. Hughes,32 J. Huston,28 C. Issever,7 J. Incandela,7 G. Introzzi,37 M. Iori,42

A. Ivanov,40 Y. Iwata,19 B. Iyutin,26 E. James,13 M. Jones,39 T. Kamon,44 J. Kang,27 M. Karagoz Unel,31 S. Kartal,13

H. Kasha,52 Y. Kato,33 R. D. Kennedy,13 R. Kephart,13 B. Kilminster,40 D. H. Kim,23 H. S. Kim,20 M. J. Kim,9

S. B. Kim,23 S. H. Kim,48 T. H. Kim,26 Y. K. Kim,10 M. Kirby,12 L. Kirsch,4 S. Klimenko,14 P. Koehn,32 K. Kondo,50

J. Konigsberg,14 A. Korn,26 A. Korytov,14 J. Kroll,36 M. Kruse,12 V. Krutelyov,44 S. E. Kuhlmann,2 N. Kuznetsova,13

A.T. Laasanen,39 S. Lami,41 S. Lammel,13 J. Lancaster,12 K. Lannon,32 M. Lancaster,25 R. Lander,5 A. Lath,43

G. Latino,30 T. LeCompte,2 Y. Le,21 J. Lee,40 S.W. Lee,44 N. Leonardo,26 S. Leone,37 J. D. Lewis,13 K. Li,52 C. S. Lin,13

M. Lindgren,6 T. M. Liss,20 T. Liu,13 D. O. Litvintsev,13 N. S. Lockyer,36 A. Loginov,29 M. Loreti,35 D. Lucchesi,35

P. Lukens,13 L. Lyons,34 J. Lys,24 R. Madrak,18 K. Maeshima,13 P. Maksimovic,21 L. Malferrari,3 M. Mangano,37

G. Manca,34 M. Mariotti,35 M. Martin,21 A. Martin,52 V. Martin,31 M. Martınez,13 P. Mazzanti,3 K. S. McFarland,40

P. McIntyre,44 M. Menguzzato,35 A. Menzione,37 P. Merkel,13 C. Mesropian,41 A. Meyer,13 T. Miao,13 R. Miller,28

J. S. Miller,27 S. Miscetti,15 G. Mitselmakher,14 N. Moggi,3 R. Moore,13 T. Moulik,39 M. Mulhearn,26 A. Mukherjee,13

T. Muller,22 A. Munar,36 P. Murat,13 J. Nachtman,13 S. Nahn,52 I. Nakano,19 R. Napora,21 F. Niell,27 C. Nelson,13

T. Nelson,13 C. Neu,32 M. S. Neubauer,26 C. Newman-Holmes,13 T. Nigmanov,38 L. Nodulman,2 S. H. Oh,12 Y. D. Oh,23

T. Ohsugi,19 T. Okusawa,33 W. Orejudos,24 C. Pagliarone,37 F. Palmonari,37 R. Paoletti,37 V. Papadimitriou,45

J. Patrick,13 G. Pauletta,47 M. Paulini,9 T. Pauly,34 C. Paus,26 D. Pellett,5 A. Penzo,47 T. J. Phillips,12 G. Piacentino,37

J. Piedra,8 K.T. Pitts,20 A. Pompos,39 L. Pondrom,51 G. Pope,38 T. Pratt,34 F. Prokoshin,11 J. Proudfoot,2 F. Ptohos,15

O. Poukhov,11 G. Punzi,37 J. Rademacker,34 A. Rakitine,26 F. Ratnikov,43 H. Ray,27 A. Reichold,34 P. Renton,34

M. Rescigno,42 F. Rimondi,3 L. Ristori,37 W. J. Robertson,12 T. Rodrigo,8 S. Rolli,49 L. Rosenson,26 R. Roser,13

R. Rossin,35 C. Rott,39 A. Roy,39 A. Ruiz,8 D. Ryan,49 A. Safonov,5 R. St. Denis,17 W. K. Sakumoto,40 D. Saltzberg,6

C. Sanchez,32 A. Sansoni,15 L. Santi,47 S. Sarkar,42 P. Savard,46 A. Savoy-Navarro,13 P. Schlabach,13 E. E. Schmidt,13

M. P. Schmidt,52 M. Schmitt,31 L. Scodellaro,35 A. Scribano,37 A. Sedov,39 S. Seidel,30 Y. Seiya,48 A. Semenov,11

F. Semeria,3 M. D. Shapiro,24 P. F. Shepard,38 T. Shibayama,48 M. Shimojima,48 M. Shochet,10 A. Sidoti,35 A. Sill,45

P. Sinervo,46 A. J. Slaughter,52 K. Sliwa,49 F. D. Snider,13 R. Snihur,25 M. Spezziga,45 F. Spinella,37 M. Spiropulu,7

L. Spiegel,13 A. Stefanini,37 J. Strologas,30 D. Stuart,7 A. Sukhanov,14 K. Sumorok,26 T. Suzuki,48 R. Takashima,19

K. Takikawa,48 M. Tanaka,2 M. Tecchio,27 R. J. Tesarek,13 P. K. Teng,1 K. Terashi,41 S. Tether,26 J. Thom,13

A. S. Thompson,17 E. Thomson,32 P. Tipton,40 S. Tkaczyk,13 D. Toback,44 K. Tollefson,28 D. Tonelli,37 M. Tonnesmann,28

H. Toyoda,33 W. Trischuk,46 J. Tseng,26 D. Tsybychev,14 N. Turini,37 F. Ukegawa,48 T. Unverhau,17 T. Vaiciulis,40

A. Varganov,27 E. Vataga,37 S. Vejcik III,13 G. Velev,13 G. Veramendi,24 R. Vidal,13 I. Vila,8 R. Vilar,8 I. Volobouev,24

PHYSICAL REVIEW D, VOLUME 70, 074008

1550-7998=2004=70(7)=074008(12)$22.50 70 074008-1 2004 The American Physical Society

M. von der Mey,6 R. G. Wagner,2 R. L.Wagner,13 W.Wagner,22 Z. Wan,43 C.Wang,12 M. J.Wang,1 S. M.Wang,14 B.Ward,17

S. Waschke,17 D. Waters,25 T. Watts,43 M. Weber,24 W. C. Wester III,13 B. Whitehouse,49 A. B. Wicklund,2 E. Wicklund,13

H. H.Williams,36 P.Wilson,13 B. L.Winer,32 S.Wolbers,13 M.Wolter,49 S.Worm,43 X.Wu,16 F.Wurthwein,26 U. K. Yang,10

W. Yao,24 G. P. Yeh,13 K. Yi,21 J. Yoh,13 T. Yoshida,33 I. Yu,23 S. Yu,36 J. C. Yun,13 L. Zanello,42 A. Zanetti,47

F. Zetti,24 and S. Zucchelli3

(CDF Collaboration)

1Institute of Physics, Academia Sinica, Taipei, Taiwan 11529, Republic of China2Argonne National Laboratory, Argonne, Illinois 60439, USA

3Istituto Nazionale di Fisica Nucleare, University of Bologna, I-40127 Bologna, Italy4Brandeis University, Waltham, Massachusetts 02254, USA

5University of California at Davis, Davis, California 95616, USA6University of California at Los Angeles, Los Angeles, California 90024, USA

7University of California at Santa Barbara, Santa Barbara, California 93106, USA8Instituto de Fisica de Cantabria, CSIC-University of Cantabria, 39005 Santander, Spain

9Carnegie Mellon University, Pittsburgh, Pennsylvania 15213, USA10Enrico Fermi Institute, University of Chicago, Chicago, Illinois 60637, USA

11Joint Institute for Nuclear Research, RU-141980 Dubna, Russia12Duke University, Durham, North Carolina 27708, USA

13Fermi National Accelerator Laboratory, Batavia, Illinois 60510, USA14University of Florida, Gainesville, Florida 32611, USA

15Laboratori Nazionali di Frascati, Istituto Nazionale di Fisica Nucleare, I-00044 Frascati, Italy16University of Geneva, CH-1211 Geneva 4, Switzerland

17Glasgow University, Glasgow G12 8QQ, United Kingdom18Harvard University, Cambridge, Massachusetts 02138, USA

19Hiroshima University, Higashi-Hiroshima 724, Japan20University of Illinois, Urbana, Illinois 61801, USA

21The Johns Hopkins University, Baltimore, Maryland 21218, USA22Institut fur Experimentelle Kernphysik, Universitat Karlsruhe, 76128 Karlsruhe, Germany

23Center for High Energy Physics: Kyungpook National University, Taegu 702-701, Korea; Seoul National University,Seoul 151-742, Korea;

and SungKyunKwan University, Suwon 440-746, Korea24Ernest Orlando Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA

25University College London, London WC1E 6BT, United Kingdom26Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA

27University of Michigan, Ann Arbor, Michigan 48109, USA28Michigan State University, East Lansing, Michigan 48824, USA

29Institution for Theoretical and Experimental Physics, ITEP, Moscow 117259, Russia30University of New Mexico, Albuquerque, New Mexico 87131, USA

31Northwestern University, Evanston, Illinois 60208, USA32The Ohio State University, Columbus, Ohio 43210, USA

33Osaka City University, Osaka 588, Japan34University of Oxford, Oxford OX1 3RH, United Kingdom

35Universita di Padova, Istituto Nazionale di Fisica Nucleare, Sezione di Padova, I-35131 Padova, Italy36University of Pennsylvania, Philadelphia, Pennsylvania 19104, USA

37Istituto Nazionale di Fisica Nucleare, University and Scuola Normale Superiore of Pisa, I-56100 Pisa, Italy38University of Pittsburgh, Pittsburgh, Pennsylvania 15260, USA

39Purdue University, West Lafayette, Indiana 47907, USA40University of Rochester, Rochester, New York 14627, USA41Rockefeller University, New York, New York 10021, USA

42Instituto Nazionale de Fisica Nucleare, Sezione di Roma, University di Roma I, ‘‘La Sapienza,’’ I-00185 Roma, Italy43Rutgers University, Piscataway, New Jersey 08855, USA

44Texas A&M University, College Station, Texas 77843, USA45Texas Tech University, Lubbock, Texas 79409, USA

46Institute of Particle Physics, University of Toronto, Toronto M5S 1A7, Canada47Istituto Nazionale di Fisica Nucleare, University of Trieste / Udine, Italy

48University of Tsukuba, Tsukuba, Ibaraki 305, Japan49Tufts University, Medford, Massachusetts 02155, USA

D. ACOSTA et al. PHYSICAL REVIEW D 70 074008

074008-2

50Waseda University, Tokyo 169, Japan51University of Wisconsin, Madison, Wisconsin 53706, USA

52Yale University, New Haven, Connecticut 06520, USA(Received 19 April 2004; published 21 October 2004)

We present a measurement of the isolated direct photon cross section in p �p collisions at��s

p�

1:8 TeV and j�j< 0:9 using data collected between 1994 and 1995 by the Collider Detector at Fermilab(CDF). The measurement is based on events where the photon converts into an electron-positron pair inthe material of the inner detector, resulting in a two track event signature. To remove �0 ! �� and�! �� events from the data we use a new background subtraction technique which takes advantage ofthe tracking information available in a photon conversion event. We find that the shape of the crosssection as a function of photon pT is poorly described by next-to-leading-order QCD predictions, butagrees with previous CDF measurements.

DOI: 10.1103/PhysRevD.70.074008 PACS numbers: 13.85.Qk, 12.38.Qk

I. INTRODUCTION

The CDF Collaboration recently published a measure-ment of the direct photon cross section [1]. This analysisfound that the shape of the cross section as a function ofpT is poorly described by next-to-leading-order (NLO)QCD calculations [2], and that the discrepancy persists at��s

p� 1800 GeV and 630 GeV. A similar conclusion has

been reached by the D0 collaboration [3,4], and by otherhadron-hadron experiments [5]. CDF and D0 both ob-serve a cross section excess at low pT , while at high pTCDF finds a deficit where D0 agrees with the theoreticalprediction. The difference between the CDF and D0 mea-surements at high pT is within the combined systematicuncertainties of the two experiments.

Photon measurements in hadron collisions are compli-cated by the large number of �0 ! �� and �! ��events produced in these experiments. These backgroundsare traditionally suppressed by requiring that the photonbe isolated from other energy in the calorimeter, but thisrequirement also eliminates some of the direct photonsignal. Special calculations which take the isolation re-quirement into account have been developed in order tocompare these measurements to NLO QCD [2].

To remove the remaining meson events from the datasample, experimentalists have relied upon understandingthe shape and development of electromagnetic (EM)showers in the calorimeter . At CDF two techniques areused: a shower transverse profile method, and a preshowermethod [1]. The data sets are based on photon triggers,where a high ET EM shower is found in the centralcalorimeter with no associated charged tracks.

In this article we report on a new measurement of thedirect photon cross section at CDF based on events wherethe photon converts to an e�e� pair in the detectormaterial prior to passing through the central trackingchamber. The EM showers in these events have tracksassociated with them, and so are explicitly rejected byconventional photon measurements. Furthermore, the ad-dition of tracking information to the event makes possible

a new background subtraction technique which is system-atically independent from the standard calorimetermethods.

The primary motivation for studying the direct photoncross section is the potential to extract information aboutthe parton distribution function (PDF) of the gluon insidethe proton, due to the large contribution of gq! �qdiagrams to the process [6]. This program has been frus-trated by differences between the measurements and cal-culations which are difficult to explain by altering thegluon PDF alone [7–9]. The direct photon cross sectionmeasurement with conversions therefore serves as a crosscheck of conventional photon techniques, as well as ademonstration of a new method for future high pT photonstudies.

II. DETECTOR AND DATA SETS

The data was collected at the Fermilab TeVatron col-lider between 1994 and 1995 (Run 1b) with a center-of-mass energy of 1.8 TeV. A detailed description of CDF inRun 1 may be found elsewhere [10]. Here we brieflydescribe those detector components critical for the con-version measurement. The central tracking system con-sists of a silicon vertex detector (SVX), a vertex TPC(VTX), and a large central tracking chamber (CTC).These detectors are located inside a 1.4 T solenoidalmagnet. The transverse momenta of charged particles inthe tracking system are measured primarily by the CTC,which has a momentum resolution of ��pT�=p2

T �0:002 GeV�1. Outside the tracking system are the CDFcalorimeters, which are subdivided in � [11,15] and �into projective towers which point to the nominal p �pinteraction point at the center of the detector. The centralregion (j�j< 1:1) is instrumented with the central elec-tromagnetic (CEM), central hadronic (CHA), and wallhadronic (WHA) calorimeters. EM showers in the CEMgenerally deposit their energy in two or three towers in �,and these towers are referred to as a CEM cluster. Theenergy resolution of the CEM is

DIRECT PHOTON CROSS SECTION WITH. . . PHYSICAL REVIEW D 70 074008

074008-3

��E�E

�

��13:5%��E sin�

p

�2� �1:6%�2

s

where � is the polar angle of the shower measured withrespect to the proton beam direction. The CEM isequipped with a layer of crossed wire and strip gaschambers (CES) located at a depth of six radiation lengths(the typical shower maximum) to measure the transverseshape of the shower. A second layer of wire chambers,known as the CPR, is located between the solenoid andthe CEM. The CPR is used as a preshower detector inconventional photon measurements, with the 1.1 X0 ra-diation lengths of the solenoid acting as the convertingmaterial.

We use a three level trigger system to collect the twodata sets used in the photon cross section measurementwith conversions. The first data sample, known as the8 GeV electron data, requires a cluster in the CEM of atleast 8 GeVat Level 1. Level 2 requires an associated trackfound by the fast hardware track finder (CFT) with pT >7:5 GeV, and an associated CES cluster found by a hard-ware cluster finder (XCES). This trigger applies severalelectron identification requirements at Level 3, includingrequirements on the transverse shape of the shower seenin the CES, the geometric matching between the showerand the track, the lateral sharing of the shower energyover the several CEM towers, and the electromagneticfraction of the shower. The integrated luminosity of thisdata set is 73:6 pb�1.

The second data sample, known as the 23 GeV photondata, requires an 8 GeV CEM cluster at Level 1, but atLevel 2 this requirement is increased to 23 GeV. The Level2 trigger also applies an isolation requirement to the CEMcluster by requiring that the neighboring calorimetertowers have ET < 4 GeV. The 23 GeV photon triggerdoes not require that a track be found by the CFT, and itdoes not apply any electron identification requirements,although at least one CES cluster must be found withmore than 0.5 GeV of energy at Level 3. Note that thistrigger was designed to collect nonconversion photons(hence its name), but since it does not veto photon can-didates which have associated tracks, we can use it tosearch for conversion events as well. The integrated lu-minosity is 83:7 pb�1.

Inner detector photon conversions are characterized bytwo opposite sign CTC tracks which pass near each otherin the material of the beampipe, SVX, VTX, or innercylinder of the CTC . Two conversion identification re-quirements are applied to the raw CTC tracks. The firstrequires that the absolute value of the difference betweenthe track cot�s be less than 0.05. The second requires thatthe absolute value of the distance between the tracksin the transverse plane at the radial location where theyare parallel be less than 0.3 cm. The distributions for thesetwo quantities are shown in Fig. 1. At least one of these

tracks is required to point at a CEM cluster, and the softertrack is required to have pT > 0:4 GeV.

Track pairs satisfying these requirements are fitted to aconversion vertex. The fit requires that the tracks meet at apoint in space where they are parallel, which improvesthe spatial and momentum resolutions of the recon-structed photon candidate. In addition, the vertex fit par-tially corrects for a pT bias present in the raw conversiontracks. This bias occurs when the spatial separation of theconversion tracks in the inner CTC superlayers is less thanthe two track resolution of the device. The final require-ment of the conversion selection is that the fitted conver-sion radius is required to be between 2 and 30 cm. Theradius of conversion distribution of the 8 GeV electrondata is shown in Fig. 2.

Conversion candidate events at CDF divide naturallyinto two sets based on their detector topology. In 1-towerconversions both tracks point to the same CEM tower, andin 2-tower conversions the tracks point to separate towers.1-tower conversions have the potential to confuse theelectron identification requirements applied by the8 GeV electron trigger at Level 3, due to the presence oftwo EM showers in the same CES chamber. Therefore inthe 8 GeVelectron data we require that the conversion be2-tower. Conversely, in the 23 GeV photon data we requirethat the conversion be 1-tower, in order to insure that thetwo data sets have no events in common.

FIG. 1. The two conversion identification requirements.Events between the vertical lines are accepted as conversioncandidates, while events in the tails are dominated by fakes. Noother selection requirements have been applied to the data inthese plots. Top: the spatial separation in the transverse planebetween the tracks at the radial location where they are parallel.If the tracks cross each other they are considered to havenegative separation. Bottom: the difference in the track cot�s.


074008-4

In a 1-tower event, the CEM cluster measures thesummed ET of both tracks. In a 2-tower event, however,the softer track is outside the high ET cluster formed bythe first track, and its own ET usually falls below theclustering threshold of the CEM reconstruction algo-rithm. In this case, only the higher ET cluster is found,but the pT of both tracks is measured by the CTC. Whendividing the conversion events into pT bins, we use thesummed ET measurement for 1-tower events, while for2-tower events we use the ET measurement of the higherenergy track plus the pT measurement of the lower energytrack.

The offline data reduction proceeds as follows. For the8 GeV electron data we require a 2-tower conversion atj�j< 0:9 and jz0j< 60 cm, where z0 is the position of theprimary event vertex along the beamline measured fromthe center of the detector. The conversion must be asso-ciated with a fiducial CEM cluster, and we reapply theelectron identification requirements imposed by the Level3 trigger. The reconstructed CEM cluster must have ET >8:0 GeV, and at least one of the conversion tracks musthave pT > 6:0 GeV. To suppress the contribution of �0

and � events we make two isolation requirements. Thefirst requires that the amount of energy found in a cone of

radius R ��2 � ��2

pless than 0.4 centered on the

highest ET shower be less than 1 GeV, excluding theenergy in the CEM cluster itself. The second requiresthat no extraneous tracks with pT > 0:4 GeV point tothe CEM cluster. Finally we require that the missing

energy (E6 T) in this data set be less than 25 GeV in orderto suppress a background due to W ! e� events.

For the 23 GeV photon data we require a 1-towerconversion at j�j< 0:9 and jz0j< 60 cm with a fiducialCEM cluster and the same isolation requirements. TheCEM cluster must have ET > 28 GeV and the conversionmust have at least one track with pT > 8:0 GeV. This dataset has no electron identification requirements, and no E6 Trequirement.

There is one complication to the cone 0.4 isolationrequirement in the case of a 2-tower conversion. If thesoft conversion track lands outside the CEM cluster, butwithin the 0.4 cone, then the cone energy sum is artifi-cially enhanced by the energy of this track. To remove thisenergy the tower hit by the soft track and its closestneighbor in � are excluded from the cone energy sum.In this case the area of the cone is slightly reduced, and toaccount for this the energy requirement is reduced from1.0 GeV to 0.87 GeV. This occurs in about 2/3 of all2-tower events.

III. BACKGROUND SUBTRACTION

A. �0 and � backgrounds

Most �0 ! �� and �! �� events are rejected by theisolation requirements. Those that remain are statisticallysubtracted from the data by a new technique based onE=p. E=p is the ratio of the ET measured in the CEM andthe pT measured by the CTC. For a 1-tower conversion theET is the two track summed energy measured by theCEM, and the pT is the sum of the two vertexed trackmomenta. For a 2-tower conversion, the CEM clustermeasures only the ET of the higher energy track, and inthis case the E=p ratio includes only the ET and vertexedpT of that track.

Under this definition, the E=p distribution for a directphoton conversion should be a narrow peak centered on1.0 whose width is determined by the CTC and CEMresolutions. In a �0 or � event, however, the secondunconverted photon usually showers in the same CEMcluster as the high ET conversion electron. Therefore in ameson event the ET measures the �0 energy, and the pTmeasures the energy of one of the decay photons. Sincetwo-body decay kinematics are understood, the shape ofthe meson E=p distribution is relatively easy to calculatewith a Monte Carlo simulation.

To predict the signal E=p distribution we generatedirect photon events using PYTHIA version 6.115 [11].The prompt photon is tracked through a material map ofthe CDF inner detector where it is allowed to convert intoan electron-positron pair. The two tracks pass through theremaining material, where they are allowed to undergobremsstrahlung, and through the tracking chamber andcalorimeter. To simulate the pT and ET measurements thetrue track parameters are smeared by the known resolu-tions of the CTC and CEM.

Radius of conversion (8 GeV electron data)

Central Tracking Chamberinner cylinder

Vertex TPC

SiliconVertexDetector

Dalitz

after vertexing

before vertexing

FIG. 2. Vertexed and unvertexed radius of conversion distri-bution in 8 GeV electron data. The peak at r � 0 labeled’Dalitz’ is due to �0 ! e�e�� decays and � ! e�e� events.The 2 cm< r < 30 cm data selection requirement has beenreleased in this plot.


074008-5

We find that for the purpose of predicting the mesonE=p distribution, it is adequate to simulate single mesons,rather that complete events, because the mesons in thedata are highly isolated. The generated mesons decay totwo photons which are tracked through the detector in thesame manner as the PYTHIA direct photons. The twomeson samples are combined using a �=�0 productionratio of 0:69� 0:08, which we measured in the data usinga sample of double conversion (�0=�! �� !e�e�e�e�) events, as shown in Fig. 3. Fortunately, theE=p distributions of �0 and � are very similar (due tosimilar decay kinematics), so the production ratio used inthe Monte Carlo has little effect on their combined E=pshape. We also use the double conversion events to mea-sure the meson pT spectrum to be used in the MonteCarlo. We find that a power law with an exponent ofnegative six gives a good description of the data.

We extract the number of signal candidates in each pTbin by performing a �2 fit of the E=p distributionsobserved in the data to the Monte Carlo signal andbackground templates. In the fit only the normalizationsof the signal and background are allowed to float.Examples of two fits are shown in Fig. 4.

As seen in Fig. 4, the narrow signal peak is quitedistinct from the broad background distribution, and thereis good agreement between data and Monte Carlo in thehigh E=p tail region. This indicates that the Monte Carlo

prediction of the E=p shape of the background is robust.Most of the fits return a confidence level greater than 5%,with a flat distribution up to 100%. In some pT bins,however, the narrow signal peak is shifted slightly withrespect to the Monte Carlo prediction, and these fits havea large �2. This effect is due to a pT bias associated with

π0,η

γ + π0,η

• data

16.5 GeV < pT < 18 GeV

π0,η

γ + π0,η

• data

30 GeV < pT < 34 GeV

FIG. 4. An example of the E=p background subtraction fit intwo pT bins. The 16.5 to 18 GeV bin is from the 8 GeV electron(2-tower) data, and the 30 to 34 GeV bin is from the 23 GeVphoton (1-tower) data.

• data

signal fit

background fit

γγ → e+e-e+e-

η/π0 = 0.687 ± 0.076

FIG. 3. The diphoton mass spectrum of double conversionevents in the data. The data is fit to two Lorentzians plus a thirdorder polynomial. The polynomial is shown as the dotted line.The �0 and � peaks are visible at 0.135 GeV and 0.547 GeV,respectively. The ratio of the areas of the two peaks, along withthe Monte Carlo prediction for the ratio of acceptances, givesan �=�0 production ratio of 0:687� 0:076.


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conversion tracking which occurs when the spatial sepa-ration of the two tracks in the inner layers of the CTC isbelow the hit resolution. A hit level simulation of thetracking system reproduces this effect, but it is not simu-lated by our fast Monte Carlo, so the templates do notreproduce this. Studies show that the E=p shift is nolarger than 1% in the 8 GeV electron data, and 2% inthe 23 GeV photon data [12].

To determine a systematic uncertainty on the back-ground subtraction due to this effect, we multiply theE=p of each event in the 8 GeV electron data by scalefactors of 1.01 and 0.99. We then perform the �2 fit again,and we take the change in the number of signal candidatesas a systematic uncertainty. Similarly we use scale factorsof 1.02 and 0.98 in the 23 GeV data to determine theuncertainty. For the 8 GeV electron data this error is�12=� 10% at 10 GeV and decreases to less than �5%above 20 GeV, while the error is less than �5% for the23 GeV photon data.

B. Other backgrounds

We consider two other potential sources of background.The first is fake conversions, where two random trackssatisfy the conversion identification requirements. In thiscase the soft conversion track is likely to be a hadron. Astudy of the E=p of the soft conversion tracks finds noevidence for hadronic contamination, so we neglect thisbackground.

A second source of background is due to high pTprompt electrons, such as those produced in W ! e�events. These electrons often have a co-linear brems-strahlung photon, and this photon may convert in thedetector material and produce one or two soft tracks.The soft tracks can form a high pT conversion candidatewhen combined with the prompt electron. This back-ground is the motivation for the E6 T requirement appliedto the 8 GeV electron data, which would otherwise havesignificant W ! e� contamination above 25 GeV. In the23 GeV photon data this background is less significantbecause these events are unlikely to satisfy the 1-towertopology.

To account for remaining prompt electron backgroundsin both data sets, including any remaining W electrons,we have searched for hits in the SVX and VTX detectorsin events where the conversion occurs outside these de-tectors. These studies have indicated that in the 8 GeVelectron data there is no significant prompt electron con-tamination below 25 GeV, and above 25 GeV we adopt aone sided 10% systematic uncertainty. In the 23 GeVphoton data we adopt a one sided 3% systematic uncer-tainty in all pT bins.

IV. ACCEPTANCE AND EFFICIENCY

The acceptance is evaluated with the PYTHIA directphoton Monte Carlo, and includes the fiducial require-

ments, the 1-tower and 2-tower topological requirements,and the ET and pT requirements on the CEM clusters andtracks. For the 2-tower data (8 GeV electron trigger), theacceptance decreases from 33% at 10 GeV to 6.5% at65 GeV, due to the fact that the 2-tower geometry iskinematically disfavored at high pT . The 1-tower data(23 GeV photon trigger), on the other hand, is favored athigh pT , and its acceptance increases from 35% at 30 GeVto 43% at 65 GeV.

The efficiency of the remaining selection requirementsare measured in the data with a variety of complementarydata sets [12]. To measure the conversion identificationefficiency, we relax the requirement on the difference intrack cot� from 0.05 to 0.15 and the requirement on theseparation in the transverse plane from 0.3 cm to 1.0 cm.In this loose sample the conversions in the material of theCTC inner cylinder still dominate the fake conversions atthat large radius. Since fake conversions have a smoothradius-of-conversion distribution, we remove them with asideband subtraction, and the efficiency is the ratio of thenumber of real conversions found with the standard re-quirements to the number found with the relaxed require-ments. We find an efficiency of 97:4� 2:0%, where theuncertainty is determined by variations seen when divid-ing the data into pT bins .

The efficiency of the event z0 requirement is measuredto be 93:7� 1:1% in minimum bias data. The 8 GeVelectron trigger efficiency is measured with a prescaled5 GeVelectron trigger and an inclusive muon data set, andhas an asymptotic efficiency of 91:4� 0:9%. The 23 GeVphoton trigger efficiency is measured with prescaled10 GeVand 23 GeV photon triggers, and has an efficiencyof 91:4� 4:3% [13]. The electron identification efficiencyis measured with the nontrigger electron in Z! e�e�

data, and is found to be 84:3� 3:0%.The CTC tracking efficiency is measured by embed-

ding Monte Carlo tracks into the hit data of real events,and then measuring the efficiency with which the track-ing reconstruction code finds the new track in the event.We find that the tracking efficiency has a plateau value of96� 2% per track above 400 MeV.

The isolation requirement efficiency is measured bychoosing random locations in the calorimeter in mini-mum bias data and adding up the energy found within acone radius of 0.4. This procedure assumes that the energyin the calorimeter due to the underlying event and mul-tiple p �p interactions is similar to the energy found inminimum bias data. This assumption has been checked bycomparing with the energy found in the calorimeter in70 GeV photon data. In that study we measured theaverage energy found in the calorimeter 90� away fromthe photon in azimuth. We found that the photon data hadslightly more energy than the minimum bias data, and wetake the difference as a systematic error on the isolationcut efficiency. A second study with lower energy photon


074008-7

data gave similar results, and we concluded that theunderlying event energy does not depend on the pT ofthe photon. However, the underlying event energy doescorrelate well with the number of primary event verticesreconstructed by the VTX. Therefore we measure theisolation cut efficiency as a function of the number ofinteractions, and then convolute the measured values withthe number-of-vertices distribution observed in the con-version data. The efficiency is found to be 85:9� 0:4%.

The no-extra-track requirement efficiency is evaluatedwith electrons in Z! e�e� data, and is found to be

89:6� 0:5%. The missing energy efficiency is evaluatedwith unisolated conversion candidates. These events arepredominantly di-jet events where the true E6 T is zero, sothe measured E6 T is due to the calorimeter resolution. Theefficiency decreases from 1.0 at 20 GeV to 89% at 65 GeV.

The efficiencies are summarized in Table I. The totalacceptance times efficiency for the two data sets is plottedin Fig. 5.

V. TOTAL CONVERSION PROBABILITY

The final element of the photon cross section measure-ment with conversions is the total probability that thephoton converts in the CDF inner detector. The conver-sion probability has been evaluated in several ways. Thestandard technique relies on a material map measured inthe data with an inclusive conversion data set, calibratedwith an a priori determination of the amount of materialin the CTC inner cylinder. This method gives a conversionprobability of 5:17� 0:28%, and we refer to this result asthe standard material scale. Other material measurementsbased on the hard bremsstrahlung rate in W ! e� andJ= ! e�e� events give results in agreement with thisnumber [14].

A second technique compares the number of �0 Dalitzdecays (�0 ! e�e��) to the number-of �0 ! �� decays.In some Dalitz events the on shell photon subsequentlyconverts in the inner detector material. Then the event hasa four track topology, with the invariant mass being the�0 mass. Similarly, in some �� events both photonsconvert in the detector material, giving the same fourtrack signature. In the four track event sample, the Dalitzevents can be separated from the �� events because twoof the Dalitz electrons are prompt. Since the four trackDalitz events undergo one conversion in the detectormaterial, while the four track �� events undergo twoconversions, the Dalitz-to-�� ratio gives the conversionprobability, after accounting for the branching ratios ofthe two decays. This method gives a conversion probabil-ity of 8:02� 0:73�stat� � 0:73�sys�%, which is signifi-cantly higher than the standard result quoted above.

TABLE I. Summary of signal efficiencies. The asterisks indicate to which data set each efficiency applies.

source 8 GeV electron 23 GeV photon efficiency

z0 * * 0:937� 0:011Conversion ID * * 0:974� 0:020Level 1 trigger * * 1.0Level 2 trigger (8 GeV) * 91:4� 0:9% above 16 GeVLevel 2 trigger (23 GeV) * 0:914� 0:043Level 3 electron ID * 0:849� 0:030Tracking (CTC) * * 0:96� 0:02 per trackIsolation * * 0:859� 0:004No extra tracks * * 0:896� 0:005E6 T * 1.0 below 20 GeV, 0.89 at 65 GeV

Acceptance × efficiency

8 GeV electron data (2-tower)

23 GeV photon data (1-tower)

FIG. 5. The total acceptance times efficiency for both con-version data sets. The decrease in the acceptance at high pT forthe 8 GeV electron data set is due to the 2-tower requirement,which becomes geometrically disfavored. The total conversionprobability is not included here.


074008-8

Several other data sets also give evidence for a largerconversion probability. At CDF the reconstructed mass ofdi-muon resonances such as the J= , �2s�, ��1s�,�2s�,and �3s� depend on the amount of material in the innerdetector due to muon dE=dx energy losses. We correct themuon momenta for the expected energy loss by assumingthe standard material scale. However, after the correctionthe reconstructed masses are less than the PDG massesfor all five resonances. For the J= the mass shift is morethan 20 times larger than the statistical error, while forthe ��3s� the shift is only 1.2 times the statistical error.The dominant systematic uncertainty is due to the factthat the reconstructed J= mass depends on the amountof material the muons pass through [14]. After adopting asystematic uncertainty to account for this, the measuredvalue of the J= mass agrees with the PDG value withinerrors. These effects do not prove that the standard ma-terial scale is too small, but they are consistent with thathypothesis.

There is also some evidence from W electrons for alarger material scale. In Run 1b the peak of the E=pdistribution in W electrons in the data is shifted to theright with respect to the Monte Carlo simulation whenassuming the standard material scale. Although this ef-fect is not adequately understood, the Monte Carlo E=ppeak can be made to agree with the data by increasing thematerial [14].

In summary, the evidence concerning the total conver-sion probability is ambiguous. Rather than choose be-tween two conflicting results, we adopt the approach ofchoosing a central value and systematic uncertaintywhich encompasses all possibilities. This value is 6:60�1:43% . We make one adjustment to the conversion proba-bility to account for the effective loss of material due tothe requirement on the radial location of the conversion(rcnv > 2:0 cm). The final value of the effective conver-sion probability is 6:40� 1:43%. The uncertainty on theconversion probability dominates all other errors on thephoton cross section. However, while the conversionprobability affects the overall normalization of the crosssection, it does not affect the shape of the cross section asa function of pT . This is assured by the fact that the pair-production cross section does not vary significantly overthe pT range considered here.

VI. SYSTEMATIC UNCERTAINTIES

In this section we briefly summarize the systematicuncertainties on the photon cross section measurementwith conversions. (A detailed discussion is given inRef. [12].) In Section III we discussed the systematicuncertainties we adopt to account for shortcomings inthe Monte Carlo E=p model, and backgrounds due toprompt electrons. Both of these uncertainties dependon pT .

For the 8 GeVelectron data the remaining pT dependentsystematic uncertainties are as follows. We take a system-atic uncertainty to account for a possible time dependenceon the trigger efficiency. This uncertainty is determinedby counting the fraction of events in our final samplewhich occur before the midpoint of Run 1b. This fractionis 53.6%, and depends on pT . We take 3.6% as the uncer-tainty. Secondly, the conversion identification efficiencyvaries by 2.0% when dividing the data into pT bins, andwe take this as a systematic uncertainty.

We also adopt the following pT independent systematicuncertainties. The cross section uncertainty due the totalconversion probability is �27=�17%. The CEM energyscale uncertainty results in a cross section error of 3.0%.The integrated luminosity is measured to 4.1%, and theasymptotic trigger efficiency is known to 1.4%. There arealso uncertainties due to the tracking efficiency (2.0%),the electron identification efficiency (3.5%), and the z0requirement efficiency (1.2%).

The systematic uncertainties on the 23 GeV photon datasample are similar, except there is no trigger time depen-dence, no electron identification uncertainty, and theasymptotic trigger efficiency is known to 4.7%.

The total pT independent systematic uncertainty is�28=�18% for both data sets. The total pT dependent

TABLE II. The pT dependent systematic uncertainty for allpT bins. The correlated systematic uncertainty is +28/-18% forboth data sets.

pT (GeV) pT dep. sys. err. (%)8 GeV electron (2-tower) data:

10–11 �10:6=�12:811–12 �9:3=�11:612–13 �9:4=�9:313–14 �8:5=�8:614–15 �6:7=�7:3

15–16.5 �6:7=�6:916.5–18 �5:7=�6:018–20 �7:6=�7:820–22 �7:0=�6:1

22–24.5 �4:3=�5:824.5–27 �5:1=�11:927–30 �5:7=�11:330–34 �4:1=�11:134–39 �4:1=�11:039–45 �5:6=�11:545-52 �4:1=�10:852-65 �8:8=�13:3

23 GeV photon (1-tower) data:30–34 �2:3=�4:934–39 �2:8=�4:939–45 �3:9=�5:645–52 �5:0=�4:752–65 �4:7=�8:2


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systematic uncertainty in each pT bin is listed in Table II.The total systematic uncertainty is listed in Table III.

VII. CROSS SECTION MEASUREMENT

The cross section is calculated according to

d�2

dpTd��

Nsignal

A & �pT �� RL

(1)

A & is the acceptance times efficiency shown in Fig. 5multiplied by the effective conversion probability of6.40%. We measure the average cross section between�0:9<�< 0:9, so �� is 1.8. �pT is the bin width, andRL is the integrated luminosity of 73:6 pb�1 for the

8 GeVelectron data and 83:7 pb�1 for the 23 GeV photondata.

The final result for both data sets is listed in Table III.In the pT region where the data sets overlap (30 GeV<pT < 65 GeV) the two measurements are in good agree-ment with each other. This comparison is an importantcross check on the acceptance and efficiency calculationsof the two data sets, since they differ by up to a factorof 9.

Since the two data sets are in agreement we can com-bine the measurements in the region of overlap. However,the 23 GeV photon data would dominate the combinedcross section (due to much smaller errors), so instead wesimply adopt the 23 GeV photon data above 30 GeV. Thishybrid cross section is compared to NLO QCD and thestandard CDF measurement (referred to as CES-CPR) inFig. 6. The theory curve is taken from the authors ofReference [2]. The calculation uses the CTEQ5M partondistribution functions, and the renormalization, factori-zation, and fragmentation scales have been set to the pTof the photon. This calculation takes into account thesuppression of the bremsstrahlung diagrams due to theisolation requirement on the photon. In the lower half ofFig. 6 the measurements are shown as (data-theory)/theory.

The CES-CPR measurement and the conversion mea-surement agree with each other both in shape and innormalization. The total systematic uncertainty on theconversion measurement is larger ( � 30=� 20%) thanthe CES-CPR measurement (18% at 10 GeV and 11% at115 GeV) due to the large uncertainty on the total con-version probability. Nevertheless, for both measurementsthe total systematic uncertainties are primarily pT inde-

TABLE III. Summary of the conversion cross section measurement in both data sets. The 8 GeV electron data has an integratedluminosity of 73:6 pb�1, and the 23 GeV photon data has an integrated luminosity of 83:7 pb�1. �� is 1.8, and the effectiveconversion probability, which is not included in the acceptance � efficiency shown here, is 6.40%. The NLO QCD theory wascalculated by the authors of Reference [2], and uses the CTEQ5M parton distribution functions with all scales set to the pT of thephoton.

pT hpTi d�=dpTd� stat sys NLO QCD(GeV) (GeV) A & Nsignal (pb/GeV) error (%) error (%) (pb/GeV)

8 GeV electron (2-tower) data:10–11 10.5 0.067 7152 12590 2.2 �30=�22 1096811–12 11.5 0.094 7761 9771 2.1 �29=�22 743412–13 12.5 0.106 6111 6773 2.2 �29=�20 520313–14 13.5 0.109 4320 4659 2.6 �29=�20 374314–15 14.5 0.108 3195 3483 2.9 �29=�20 2758

15–16.5 15.7 0.105 3059 2289 2.8 �29=�20 196316.5–18 17.2 0.096 1846 1509 3.5 �28=�19 132818–20 18.9 0.086 1391 950 4.1 �29=�20 88820–22 20.9 0.077 863 658 5.1 �29=�19 577

22–24.5 23.2 0.068 596 413 6.0 �28=�19 36924.5–27 25.7 0.063 344 258 7.7 �28=�22 23827–30 28.3 0.056 272 207 8.8 �28=�21 15830–34 31.9 0.047 136 85.9 13.5 �28=�21 94.634–39 36.3 0.041 101 58.5 14.4 �28=�21 49.139– 45 41.6 0.036 63.9 34.5 18.7 �28=�22 26.245–52 48.1 0.032 21.7 11.4 53.3 �28=�21 13.452–65 57.8 0.030 16.6 5.0 33.5 �29=�23 5.7

23 GeV photon (1-tower) data:30–34 31.9 0.212 723 88.4 4.8 �28=�19 94.634–39 36.3 0.220 564 53.1 5.3 �28=�19 49.139– 45 41.6 0.238 316 22.9 7.4 �28=�20 26.245–52 48.1 0.250 225 13.3 8.5 �29=�19 13.452–65 57.8 0.261 131 4.0 11.3 �29=�21 5.7


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pendent, so that both techniques give a much more precisemeasurement of the shape of the cross section as a func-tion of pT . The agreement of the conversion and CES-CPR measurements on the shape is remarkable, since thetwo techniques have little in common with each other.They use independent data samples, independent back-ground subtraction techniques, and have different accep-tances, efficiencies, and systematic uncertainties.

Figure 7 shows the conversion measurement alone as(data-theory)/theory. To compare the shape of the data tothe calculation, the uncertainty bars in this plot are thecombined statistical and pT dependent systematic uncer-tainties. The data show a steeper slope than the calcula-tion which is unexplained by the systematic uncertaintiesof the measurement. Other analyses have concluded thatthis type of shape difference is difficult to resolve simplyby changing the renormalization, fragmentation, andfactorization scales of the calculation, or the set ofparton distribution functions [1]. Since two independentexperimental techniques are in agreement on the shape,this is further evidence that refinements to the calculationare needed before these measurements can provide usefulconstraints on the gluon distribution of the proton.

ACKNOWLEDGMENTS

We would like to thank Werner Vogelsang for providingthe theoretical calculations used in this paper. Wealso thank the Fermilab staff and the technical staffsof the participating institutions for their vital con-tributions. This work was supported by the U.S.Department of Energy and National ScienceFoundation; the Italian Istituto Nazionale di FisicaNucleare; the Ministry of Education, Culture, Sports,Science and Technology of Japan; the Natural Sciencesand Engineering Research Council of Canada; theNational Science Council of the Republic of China; theSwiss National Science Foundation; the A. P. SloanFoundation; the Bundesministerium fuer Bildung undForschung, Germany; the Korean Science andEngineering Foundation and the Korean ResearchFoundation; the Particle Physics and AstronomyResearch Council and the Royal Society, UK; theRussian Foundation for Basic Research; the ComisionInterministerial de Ciencia y Tecnologia, Spain; worksupported in part by the European Community’sHuman Potential Programme under ContractNo. HPRN-CT-20002, Probe for New Physics; and thiswork was supported by Research Fund of IstanbulUniversity Project No. 1755/21122001.

Direct photon cross-section (pp– → γX)

|η| < 0.9, √s = 1.8 TeV

Conversion data onlyError bars are stat. ⊕ pT depen. sysData norm. error is +28/-18%Theory is NLO QCD, CTEQ5M, µ=pT

FIG. 7. The isolated photon cross section from conversionscompared to NLO QCD. The error bars shown here are thecombined statistical and pT dependent systematics, in order tocompare the shape of the measured cross section to theory.

Direct photon cross-section (pp– → γX)

|η| < 0.9, √s = 1.8 TeVconversionsCES-CPR

NLO QCD CTEQ5M, µ=pT

statistical errors only

FIG. 6. The isolated direct photon cross section. The result ofthe conversion technique is compared with CES-CPR andtheory. For the conversion measurement the 8 GeV electrondata is shown below 30 GeV, and the 23 GeV photon data above.The theory curve is from the authors of Reference [2], and usesthe CTEQ5M parton distribution functions with the all scalesset to the pT of the photon. Only the statistical error bars areshown here.


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[1] CDF Collaboration, D. Acosta et al., Phys. Rev. D 65,112003 (2002).

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[3] D0 Collaboration, B. Abbott et al., Phys. Rev. Lett. 84,2786 (2000).

[4] D0 Collaboration,V. M. Abazov et al., Phys. Rev. Lett. 87,251805 (2001).

[5] Fermilab E706 Collaboration, L. Apanasevich et al.,Phys. Rev. Lett. 81, 2642 (1998).

[6] W. Vogelsang and A. Vogt, Nucl. Phys. B 453, 334(1995).

[7] J. Huston et al., Phys. Rev. D 51, 6139 (1995).[8] J. Huston et al., Phys. Rev. D 58, 114034 (1998).

[9] L. Apanasevich et al., Phys. Rev. D 59, 074007(1999).

[10] CDF Collaboration, F. Abe et al., Nucl. Instrum.Methods Phys. Res., Sect. A 271, 387 (1988).

[11] T. Sjostrand et al., Comput. Phys. Commun. 135, 238(2001).

[12] C. Hall, Ph.D. thesis, Harvard University, 2002, availableat http://www-cdf.fnal.gov/grads/thesis.html.

[13] D. Partos, Ph.D. thesis, Brandeis University, 2001, avail-able at http://www-cdf.fnal.gov/grads/thesis.html.

[14] CDF Collaboration, T. Affolder et al., Phys. Rev. D 64,052001 (2001).

[15] Pseudorapidity (�) is defined by � � � lntan��=2�,where � is the polar angle measured from the beamline.


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Direct photon cross section with conversions at CDF

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