Search for Heavy, Long-Lived Particles that Decay to Photons at CDF II
Search for Heavy, Long-Lived Particles that Decay to Photons at CDF II
A. Abulencia,24 J. Adelman,13 T. Affolder,10 T. Akimoto,55 M.G. Albrow,17 S. Amerio,43 D. Amidei,35
A. Anastassov,52 K. Anikeev,17 A. Annovi,19 J. Antos,14 M. Aoki,55 G. Apollinari,17 T. Arisawa,57 A. Artikov,15
W. Ashmanskas,17 A. Attal,3 A. Aurisano,42 F. Azfar,42 P. Azzi-Bacchetta,43 P. Azzurri,46 N. Bacchetta,43
W. Badgett,17 A. Barbaro-Galtieri,29 V.E. Barnes,48 B.A. Barnett,25 S. Baroiant,7 V. Bartsch,31 G. Bauer,33
P.-H. Beauchemin,34 F. Bedeschi,46 S. Behari,25 G. Bellettini,46 J. Bellinger,59 A. Belloni,33 D. Benjamin,16
A. Beretvas,17 J. Beringer,29 T. Berry,30 A. Bhatti,50 M. Binkley,17 D. Bisello,43 I. Bizjak,31 R.E. Blair,2
C. Blocker,6 B. Blumenfeld,25 A. Bocci,16 A. Bodek,49 V. Boisvert,49 G. Bolla,48 A. Bolshov,33 D. Bortoletto,48
J. Boudreau,47 A. Boveia,10 B. Brau,10 L. Brigliadori,5 C. Bromberg,36 E. Brubaker,13 J. Budagov,15 H.S. Budd,49
S. Budd,24 K. Burkett,17 G. Busetto,43 P. Bussey,21 A. Buzatu,34 K. L. Byrum,2 S. Cabreraq,16 M. Campanelli,20
M. Campbell,35 F. Canelli,17 A. Canepa,45 S. Carilloi,18 D. Carlsmith,59 R. Carosi,46 S. Carron,34 B. Casal,11
M. Casarsa,54 A. Castro,5 P. Catastini,46 D. Cauz,54 M. Cavalli-Sforza,3 A. Cerri,29 L. Cerritom,31 S.H. Chang,28
Y.C. Chen,1 M. Chertok,7 G. Chiarelli,46 G. Chlachidze,17 F. Chlebana,17 I. Cho,28 K. Cho,28 D. Chokheli,15
J.P. Chou,22 G. Choudalakis,33 S.H. Chuang,52 K. Chung,12 W.H. Chung,59 Y.S. Chung,49 M. Cilijak,46
C.I. Ciobanu,24 M.A. Ciocci,46 A. Clark,20 D. Clark,6 M. Coca,16 G. Compostella,43 M.E. Convery,50 J. Conway,7
B. Cooper,31 K. Copic,35 M. Cordelli,19 G. Cortiana,43 F. Crescioli,46 C. Cuenca Almenarq,7 J. Cuevasl,11
R. Culbertson,17 J.C. Cully,35 S. DaRonco,43 M. Datta,17 S. D’Auria,21 T. Davies,21 D. Dagenhart,17
P. de Barbaro,49 S. De Cecco,51 A. Deisher,29 G. De Lentdeckerc,49 G. De Lorenzo,3 M. Dell’Orso,46 F. Delli Paoli,43
L. Demortier,50 J. Deng,16 M. Deninno,5 D. De Pedis,51 P.F. Derwent,17 G.P. Di Giovanni,44 C. Dionisi,51
B. Di Ruzza,54 J.R. Dittmann,4 M. D’Onofrio,3 C. Dörr,26 S. Donati,46 P. Dong,8 J. Donini,43 T. Dorigo,43
S. Dube,52 J. Efron,39 R. Erbacher,7 D. Errede,24 S. Errede,24 R. Eusebi,17 H.C. Fang,29 S. Farrington,30
I. Fedorko,46 W.T. Fedorko,13 R.G. Feild,60 M. Feindt,26 J.P. Fernandez,32 R. Field,18 G. Flanagan,48 R. Forrest,7
S. Forrester,7 M. Franklin,22 J.C. Freeman,29 I. Furic,13 M. Gallinaro,50 J. Galyardt,12 J.E. Garcia,46
F. Garberson,10 A.F. Garfinkel,48 C. Gay,60 H. Gerberich,24 D. Gerdes,35 S. Giagu,51 P. Giannetti,46 K. Gibson,47
J.L. Gimmell,49 C. Ginsburg,17 N. Giokarisa,15 M. Giordani,54 P. Giromini,19 M. Giunta,46 G. Giurgiu,25
V. Glagolev,15 D. Glenzinski,17 M. Gold,37 N. Goldschmidt,18 J. Goldsteinb,42 A. Golossanov,17 G. Gomez,11
G. Gomez-Ceballos,33 M. Goncharov,53 O. González,32 I. Gorelov,37 A.T. Goshaw,16 K. Goulianos,50 A. Gresele,43
S. Grinstein,22 C. Grosso-Pilcher,13 R.C. Group,17 U. Grundler,24 J. Guimaraes da Costa,22 Z. Gunay-Unalan,36
C. Haber,29 K. Hahn,33 S.R. Hahn,17 E. Halkiadakis,52 A. Hamilton,20 B.-Y. Han,49 J.Y. Han,49 R. Handler,59
F. Happacher,19 K. Hara,55 D. Hare,52 M. Hare,56 S. Harper,42 R.F. Harr,58 R.M. Harris,17 M. Hartz,47
K. Hatakeyama,50 J. Hauser,8 C. Hays,42 M. Heck,26 A. Heijboer,45 B. Heinemann,29 J. Heinrich,45 C. Henderson,33
M. Herndon,59 J. Heuser,26 D. Hidas,16 C.S. Hillb,10 D. Hirschbuehl,26 A. Hocker,17 A. Holloway,22 S. Hou,1
M. Houlden,30 S.-C. Hsu,9 B.T. Huffman,42 R.E. Hughes,39 U. Husemann,60 J. Huston,36 J. Incandela,10
G. Introzzi,46 M. Iori,51 A. Ivanov,7 B. Iyutin,33 E. James,17 D. Jang,52 B. Jayatilaka,16 D. Jeans,51 E.J. Jeon,28
S. Jindariani,18 W. Johnson,7 M. Jones,48 K.K. Joo,28 S.Y. Jun,12 J.E. Jung,28 T.R. Junk,24 T. Kamon,53
P.E. Karchin,58 Y. Kato,41 Y. Kemp,26 R. Kephart,17 U. Kerzel,26 V. Khotilovich,53 B. Kilminster,39 D.H. Kim,28
H.S. Kim,28 J.E. Kim,28 M.J. Kim,17 S.B. Kim,28 S.H. Kim,55 Y.K. Kim,13 N. Kimura,55 L. Kirsch,6 S. Klimenko,18
M. Klute,33 B. Knuteson,33 B.R. Ko,16 K. Kondo,57 D.J. Kong,28 J. Konigsberg,18 A. Korytov,18 A.V. Kotwal,16
A.C. Kraan,45 J. Kraus,24 M. Kreps,26 J. Kroll,45 N. Krumnack,4 M. Kruse,16 V. Krutelyov,10 T. Kubo,55
S. E. Kuhlmann,2 T. Kuhr,26 N.P. Kulkarni,58 Y. Kusakabe,57 S. Kwang,13 A.T. Laasanen,48 S. Lai,34 S. Lami,46
S. Lammel,17 M. Lancaster,31 R.L. Lander,7 K. Lannon,39 A. Lath,52 G. Latino,46 I. Lazzizzera,43 T. LeCompte,2
E. Lee,53 J. Lee,49 J. Lee,28 Y.J. Lee,28 S.W. Leeo,53 R. Lefèvre,20 N. Leonardo,33 S. Leone,46 S. Levy,13
J.D. Lewis,17 C. Lin,60 C.S. Lin,17 M. Lindgren,17 E. Lipeles,9 A. Lister,7 D.O. Litvintsev,17 T. Liu,17
N.S. Lockyer,45 A. Loginov,60 M. Loreti,43 R.-S. Lu,1 D. Lucchesi,43 P. Lujan,29 P. Lukens,17 G. Lungu,18
L. Lyons,42 J. Lys,29 R. Lysak,14 E. Lytken,48 P. Mack,26 D. MacQueen,34 R. Madrak,17 K. Maeshima,17
K. Makhoul,33 T. Maki,23 P. Maksimovic,25 S. Malde,42 S. Malik,31 G. Manca,30 F. Margaroli,5 R. Marginean,17
C. Marino,26 C.P. Marino,24 A. Martin,60 M. Martin,25 V. Marting,21 M. Mart́ınez,3 R. Mart́ınez-Ballaŕın,32
T. Maruyama,55 P. Mastrandrea,51 T. Masubuchi,55 H. Matsunaga,55 M.E. Mattson,58 R. Mazini,34 P. Mazzanti,5
K.S. McFarland,49 P. McIntyre,53 R. McNultyf ,30 A. Mehta,30 P. Mehtala,23 S. Menzemerh,11 A. Menzione,46
P. Merkel,48 C. Mesropian,50 A. Messina,36 T. Miao,17 N. Miladinovic,6 J. Miles,33 R. Miller,36 C. Mills,10
M. Milnik,26 A. Mitra,1 G. Mitselmakher,18 A. Miyamoto,27 S. Moed,20 N. Moggi,5 B. Mohr,8 C.S. Moon,28
http://arxiv.org/abs/0704.0760v1
R. Moore,17 M. Morello,46 P. Movilla Fernandez,29 J. Mülmenstädt,29 A. Mukherjee,17 Th. Muller,26 R. Mumford,25
P. Murat,17 M. Mussini,5 J. Nachtman,17 A. Nagano,55 J. Naganoma,57 K. Nakamura,55 I. Nakano,40 A. Napier,56
V. Necula,16 C. Neu,45 M.S. Neubauer,9 J. Nielsenn,29 L. Nodulman,2 O. Norniella,3 E. Nurse,31 S.H. Oh,16
Y.D. Oh,28 I. Oksuzian,18 T. Okusawa,41 R. Oldeman,30 R. Orava,23 K. Osterberg,23 C. Pagliarone,46
E. Palencia,11 V. Papadimitriou,17 A. Papaikonomou,26 A.A. Paramonov,13 B. Parks,39 S. Pashapour,34
J. Patrick,17 G. Pauletta,54 M. Paulini,12 C. Paus,33 D.E. Pellett,7 A. Penzo,54 T.J. Phillips,16 G. Piacentino,46
J. Piedra,44 L. Pinera,18 K. Pitts,24 C. Plager,8 L. Pondrom,59 X. Portell,3 O. Poukhov,15 N. Pounder,42
F. Prakoshyn,15 A. Pronko,17 J. Proudfoot,2 F. Ptohose,19 G. Punzi,46 J. Pursley,25 J. Rademackerb,42
A. Rahaman,47 V. Ramakrishnan,59 N. Ranjan,48 I. Redondo,32 B. Reisert,17 V. Rekovic,37 P. Renton,42
M. Rescigno,51 S. Richter,26 F. Rimondi,5 L. Ristori,46 A. Robson,21 T. Rodrigo,11 E. Rogers,24 S. Rolli,56
R. Roser,17 M. Rossi,54 R. Rossin,10 P. Roy,34 A. Ruiz,11 J. Russ,12 V. Rusu,13 H. Saarikko,23 A. Safonov,53
W.K. Sakumoto,49 G. Salamanna,51 O. Saltó,3 L. Santi,54 S. Sarkar,51 L. Sartori,46 K. Sato,17 P. Savard,34
A. Savoy-Navarro,44 T. Scheidle,26 P. Schlabach,17 E.E. Schmidt,17 M.P. Schmidt,60 M. Schmitt,38 T. Schwarz,7
L. Scodellaro,11 A.L. Scott,10 A. Scribano,46 F. Scuri,46 A. Sedov,48 S. Seidel,37 Y. Seiya,41 A. Semenov,15
L. Sexton-Kennedy,17 A. Sfyrla,20 S.Z. Shalhout,58 M.D. Shapiro,29 T. Shears,30 P.F. Shepard,47 D. Sherman,22
M. Shimojimak,55 M. Shochet,13 Y. Shon,59 I. Shreyber,20 A. Sidoti,46 P. Sinervo,34 A. Sisakyan,15 A.J. Slaughter,17
J. Slaunwhite,39 K. Sliwa,56 J.R. Smith,7 F.D. Snider,17 R. Snihur,34 M. Soderberg,35 A. Soha,7 S. Somalwar,52
V. Sorin,36 J. Spalding,17 F. Spinella,46 T. Spreitzer,34 P. Squillacioti,46 M. Stanitzki,60 A. Staveris-Polykalas,46
R. St. Denis,21 B. Stelzer,8 O. Stelzer-Chilton,42 D. Stentz,38 J. Strologas,37 D. Stuart,10 J.S. Suh,28 A. Sukhanov,18
H. Sun,56 I. Suslov,15 T. Suzuki,55 A. Taffardp,24 R. Takashima,40 Y. Takeuchi,55 R. Tanaka,40 M. Tecchio,35
P.K. Teng,1 K. Terashi,50 J. Thomd,17 A.S. Thompson,21 E. Thomson,45 P. Tipton,60 V. Tiwari,12 S. Tkaczyk,17
D. Toback,53 S. Tokar,14 K. Tollefson,36 T. Tomura,55 D. Tonelli,46 S. Torre,19 D. Torretta,17 S. Tourneur,44
W. Trischuk,34 S. Tsuno,40 Y. Tu,45 N. Turini,46 F. Ukegawa,55 S. Uozumi,55 S. Vallecorsa,20 N. van Remortel,23
A. Varganov,35 E. Vataga,37 F. Vazquezi,18 G. Velev,17 G. Veramendi,24 V. Veszpremi,48 M. Vidal,32 R. Vidal,17
I. Vila,11 R. Vilar,11 T. Vine,31 I. Vollrath,34 I. Volobouevo,29 G. Volpi,46 F. Würthwein,9 P. Wagner,53
R.G. Wagner,2 R.L. Wagner,17 J. Wagner,26 W. Wagner,26 R. Wallny,8 S.M. Wang,1 A. Warburton,34 D. Waters,31
M. Weinberger,53 W.C. Wester III,17 B. Whitehouse,56 D. Whiteson,45 A.B. Wicklund,2 E. Wicklund,17
G. Williams,34 H.H. Williams,45 P. Wilson,17 B.L. Winer,39 P. Wittichd,17 S. Wolbers,17 C. Wolfe,13
T. Wright,35 X. Wu,20 S.M. Wynne,30 A. Yagil,9 K. Yamamoto,41 J. Yamaoka,52 T. Yamashita,40 C. Yang,60
U.K. Yangj,13 Y.C. Yang,28 W.M. Yao,29 G.P. Yeh,17 J. Yoh,17 K. Yorita,13 T. Yoshida,41 G.B. Yu,49 I. Yu,28
S.S. Yu,17 J.C. Yun,17 L. Zanello,51 A. Zanetti,54 I. Zaw,22 X. Zhang,24 J. Zhou,52 and S. Zucchelli5
(CDF Collaboration∗)
1Institute of Physics, Academia Sinica, Taipei, Taiwan 11529, Republic of China
2Argonne National Laboratory, Argonne, Illinois 60439
3Institut de Fisica d’Altes Energies, Universitat Autonoma de Barcelona, E-08193, Bellaterra (Barcelona), Spain
4Baylor University, Waco, Texas 76798
5Istituto Nazionale di Fisica Nucleare, University of Bologna, I-40127 Bologna, Italy
6Brandeis University, Waltham, Massachusetts 02254
7University of California, Davis, Davis, California 95616
8University of California, Los Angeles, Los Angeles, California 90024
9University of California, San Diego, La Jolla, California 92093
10University of California, Santa Barbara, Santa Barbara, California 93106
11Instituto de Fisica de Cantabria, CSIC-University of Cantabria, 39005 Santander, Spain
12Carnegie Mellon University, Pittsburgh, PA 15213
13Enrico Fermi Institute, University of Chicago, Chicago, Illinois 60637
14Comenius University, 842 48 Bratislava, Slovakia; Institute of Experimental Physics, 040 01 Kosice, Slovakia
15Joint Institute for Nuclear Research, RU-141980 Dubna, Russia
16Duke University, Durham, North Carolina 27708
17Fermi National Accelerator Laboratory, Batavia, Illinois 60510
18University of Florida, Gainesville, Florida 32611
19Laboratori Nazionali di Frascati, Istituto Nazionale di Fisica Nucleare, I-00044 Frascati, Italy
20University of Geneva, CH-1211 Geneva 4, Switzerland
21Glasgow University, Glasgow G12 8QQ, United Kingdom
22Harvard University, Cambridge, Massachusetts 02138
23Division of High Energy Physics, Department of Physics,
University of Helsinki and Helsinki Institute of Physics, FIN-00014, Helsinki, Finland
24University of Illinois, Urbana, Illinois 61801
25The Johns Hopkins University, Baltimore, Maryland 21218
26Institut für Experimentelle Kernphysik, Universität Karlsruhe, 76128 Karlsruhe, Germany
27High Energy Accelerator Research Organization (KEK), Tsukuba, Ibaraki 305, Japan
28Center for High Energy Physics: Kyungpook National University,
Taegu 702-701, Korea; Seoul National University, Seoul 151-742,
Korea; SungKyunKwan University, Suwon 440-746, Korea
29Ernest Orlando Lawrence Berkeley National Laboratory, Berkeley, California 94720
30University of Liverpool, Liverpool L69 7ZE, United Kingdom
31University College London, London WC1E 6BT, United Kingdom
32Centro de Investigaciones Energeticas Medioambientales y Tecnologicas, E-28040 Madrid, Spain
33Massachusetts Institute of Technology, Cambridge, Massachusetts 02139
34Institute of Particle Physics: McGill University, Montréal,
Canada H3A 2T8; and University of Toronto, Toronto, Canada M5S 1A7
35University of Michigan, Ann Arbor, Michigan 48109
36Michigan State University, East Lansing, Michigan 48824
37University of New Mexico, Albuquerque, New Mexico 87131
38Northwestern University, Evanston, Illinois 60208
39The Ohio State University, Columbus, Ohio 43210
40Okayama University, Okayama 700-8530, Japan
41Osaka City University, Osaka 588, Japan
42University of Oxford, Oxford OX1 3RH, United Kingdom
43University of Padova, Istituto Nazionale di Fisica Nucleare,
Sezione di Padova-Trento, I-35131 Padova, Italy
44LPNHE, Universite Pierre et Marie Curie/IN2P3-CNRS, UMR7585, Paris, F-75252 France
45University of Pennsylvania, Philadelphia, Pennsylvania 19104
46Istituto Nazionale di Fisica Nucleare Pisa, Universities of Pisa,
Siena and Scuola Normale Superiore, I-56127 Pisa, Italy
47University of Pittsburgh, Pittsburgh, Pennsylvania 15260
48Purdue University, West Lafayette, Indiana 47907
49University of Rochester, Rochester, New York 14627
50The Rockefeller University, New York, New York 10021
51Istituto Nazionale di Fisica Nucleare, Sezione di Roma 1,
University of Rome “La Sapienza,” I-00185 Roma, Italy
52Rutgers University, Piscataway, New Jersey 08855
53Texas A&M University, College Station, Texas 77843
54Istituto Nazionale di Fisica Nucleare, University of Trieste/ Udine, Italy
55University of Tsukuba, Tsukuba, Ibaraki 305, Japan
56Tufts University, Medford, Massachusetts 02155
57Waseda University, Tokyo 169, Japan
58Wayne State University, Detroit, Michigan 48201
59University of Wisconsin, Madison, Wisconsin 53706
60Yale University, New Haven, Connecticut 06520
(Dated: November 3, 2018; Version 5.1)
We present the first search for heavy, long-lived particles that decay to photons at a hadron
collider. We use a sample of γ+jet+missing transverse energy events in pp̄ collisions at
1.96 TeV taken with the CDF II detector. Candidate events are selected based on the arrival time
of the photon at the detector. Using an integrated luminosity of 570 pb−1 of collision data, we
observe 2 events, consistent with the background estimate of 1.3±0.7 events. While our search
strategy does not rely on model-specific dynamics, we set cross section limits in a supersymmetric
model with eχ01 → γ eG and place the world-best 95% C.L. lower limit on the eχ01 mass of 101 GeV/c2
at τχ̃0
= 5 ns.
PACS numbers: 13.85.Rm, 12.60.Jv, 13.85.Qk, 14.80.Ly
∗With visitors from aUniversity of Athens, bUniversity of
Bristol, cUniversity Libre de Bruxelles, dCornell University,
eUniversity of Cyprus, fUniversity of Dublin, gUniversity of Ed-
inburgh, hUniversity of Heidelberg, iUniversidad Iberoamericana,
jUniversity of Manchester, kNagasaki Institute of Applied Science,
lUniversity de Oviedo, mUniversity of London, Queen Mary Col-
lege, nUniversity of California Santa Cruz, oTexas Tech University,
Searches for events with final state photons and miss-
ing transverse energy (E/T ) [1] at collider experiments
are sensitive to new physics from a wide variety of mod-
els [2] including gauge mediated supersymmetry breaking
(GMSB) [3]. In these models the lightest neutralino (χ̃01)
decays into a photon (γ) and a weakly interacting, stable
gravitino (G̃) that gives rise to E/T by leaving the detec-
tor without depositing any energy. The observation of an
eeγγE/T candidate event by the CDF experiment during
Run I at the Fermilab Tevatron [4] has increased the in-
terest in experimental tests of this class of theories. Most
subsequent searches have focused on promptly produced
photons [5, 6], however the χ̃01 can have a lifetime on the
order of nanoseconds or more. This is the first search
for heavy, long-lived particles that decay to photons at a
hadron collider.
We optimize our selection requirements using a GMSB
model with a standard choice of parameters [7] and vary
the values of the χ̃01 mass and lifetime. However, the final
search strategy is chosen to be sufficiently general and
independent of the specific GMSB model dynamics to
yield results that are approximately valid for any model
producing the same reconstructed final state topology
and kinematics [8]. In pp̄ collisions at the Tevatron the
inclusive GMSB production cross section is dominated
by pair production of gauginos. The gauginos decay
promptly, resulting in a pair of long-lived χ̃01’s in asso-
ciation with other final state particles that can be identi-
fied as jets. For a heavy χ̃01 decaying inside the detector,
the photon can arrive at the face of the detector with a
time delay relative to promptly produced photons. To
have good sensitivity for nanosecond-lifetime χ̃01’s [8], we
search for events that contain a time-delayed photon, E/T ,
and ≥ 1 jet. This is equivalent to requiring that at least
one of the long-lived χ̃01’s decays inside the detector.
This Letter summarizes [9] the first search for heavy,
long-lived particles that decay to photons at a hadron
collider. The data comprise 570±34 pb−1 of pp̄ collisions
collected with the CDF II detector [10] at
s = 1.96 TeV.
Previous searches for nanosecond-lifetime particles using
non-timing techniques yielded null results [11].
A full description of the CDF II detector can be found
elsewhere [10]. Here we briefly describe the aspects of the
detector relevant to this analysis. The magnetic spec-
trometer consists of tracking devices inside a 3-m diame-
ter, 5-m long superconducting solenoid magnet that op-
erates at 1.4 T. An eight-layer silicon microstrip detector
array and a 3.1-m long drift chamber with 96 layers of
sense wires measure the position (~xi) and time (ti) of the
pp̄ interaction [12] and the momenta of charged particles.
Muons from collisions or cosmic rays are identified by a
pUniversity of California Irvine, qIFIC(CSIC-Universitat de Valen-
cia),
system of drift chambers situated outside the calorime-
ters in the region with pseudorapidity |η| < 1.1 [1].
The calorimeter consists of projective towers with elec-
tromagnetic and hadronic compartments. It is divided
into a central barrel that surrounds the solenoid coil
(|η| < 1.1) and a pair of end-plugs that cover the region
1.1 < |η| < 3.6. Both calorimeters are used to identify
and measure the energy and position of photons, elec-
trons, jets, and E/T . The electromagnetic calorimeters
were recently instrumented with a new system, the EM-
Timing system (completed in Fall 2004) [13], that mea-
sures the arrival time of electrons and photons in each
tower with |η| < 2.1 for all energies above ∼5 GeV.
The time and position of arrival of the photon at the
calorimeter, tf and ~xf , are used to separate the photons
from the decays of heavy, long-lived χ̃01’s from promptly
produced photons or photons from non-collision sources.
We define the corrected arrival time of the photon as
tγc ≡ tf − ti −
|~xf − ~xi|
The tγc distribution for promptly produced, high energy
photons is Gaussian with a mean of zero by construction
and with a standard deviation that depends only on the
measurement resolution assuming that the pp̄ production
vertex has been correctly identified. Photons from heavy,
long-lived particles can have arrival times that are many
standard deviations larger than zero.
The analysis preselection is summarized in Table I. It
begins with events passing an online, three-level trigger
by having a photon candidate in the region |η| < 1.1
with ET> 25 GeV and E/T> 25 GeV. Offline, the high-
est ET photon candidate in the fiducial region of the
calorimeter is required to have ET > 30 GeV and to
pass the standard photon identification requirements [5]
with a minor modification [14]. We require the event
to have E/T > 30 GeV where the trigger is 100% effi-
cient. We require at least one jet with |ηjet| < 2.0 and
> 30 GeV [15]. Since a second photon can be identi-
fied as a jet, the analysis is sensitive to signatures where
one or both χ̃01’s decay inside the detector. To ensure
a high quality ti and ~xi measurement, we require a ver-
tex with at least 4 tracks,
tracks
pT > 15 GeV/c, and
|zi| < 60 cm; this also helps to reduce non-collision back-
grounds. For events with multiple reconstructed vertices,
we pick the vertex with the highest
tracks
pT . To re-
duce cosmic ray background, events are rejected if there
are hits in a muon chamber that are not matched to any
track and are within 30◦ of the photon. After the above
requirements there are 11,932 events in the data sample.
There are two major classes of background events: col-
lision and non-collision photon candidates. Collision pho-
tons are presumed to come from standard model interac-
tions, e.g., γ+jet+mismeasured E/T , dijet+mismeasured
E/T where the jet is mis-identified as a γ, and W → eν
where the electron is mis-identified as a γ. Non-collision
Preselection Requirements Cumulative (individual)
Efficiency (%)
> 30 GeV, E/T > 30 GeV 54 (54)
Photon ID and fiducial, |η| < 1.0 39 (74)*
Good vertex,
tracks
pT > 15 GeV/c 31 (79)
|ηjet| < 2.0, Ejet
> 30 GeV 24 (77)
Cosmic ray rejection 23 (98)*
Requirements after Optimization
E/T > 40 GeV, E
> 35 GeV 21 (92)
∆φ(E/T , jet) > 1 rad 18 (86)
2 ns < tγc < 10 ns 6 (33)
TABLE I: The data selection criteria and the cumulative
and individual requirement efficiencies for an example GMSB
model point at mχ̃0
= 100 GeV/c2 and τχ̃0
= 5 ns. The ef-
ficiencies listed are, in general, model-dependent and have a
fractional uncertainty of 10%. Model-independent efficiencies
are indicated with an asterisk. The collision fiducial require-
ment of |zi| < 60 cm is part of the good vertex requirement
(95%) and is estimated from data.
backgrounds come from cosmic rays and beam effects
that can produce photon candidates, E/T , and sometimes
the reconstructed jet. We separate data events as a func-
tion of tγc into several control regions that allow us to
estimate the number of background events in the final
signal region by fitting to the data using collision and
non-collision shape templates as shown in Fig. 1.
Collision photons are subdivided in two subclasses:
correct and incorrect vertex selection [13]. An incorrect
vertex can be selected when two or more collisions occur
in one beam bunch crossing, making it possible that the
highest reconstructed
tracks
pT vertex does not produce
the photon. While the fraction of events with incorrect
vertices depends on the final event selection criteria, the
tγc distribution for each subclass is estimated separately
using W → eν data where the electron track is dropped
from the vertexing. For events with a correctly associ-
ated vertex, the tγc distribution is Gaussian and centered
at zero with a standard deviation of 0.64 ns [13]. For
those with an incorrectly selected vertex the tγc distribu-
tion is also Gaussian with a standard deviation of 2.05 ns.
The tγc distributions for both non-collision backgrounds
are estimated separately from data using events with
no reconstructed tracks. Photon candidates from cos-
mic rays are not correlated in time with collisions, and
therefore their tγc distribution is roughly flat. Beam halo
photon candidates are produced by muons that origi-
nate upstream of the detector (from the p direction) and
travel through the calorimeter, typically depositing small
amounts of energy. When the muon deposits significant
energy in the EM calorimeter, it can be misidentified as a
photon and cause E/T . These photons populate predomi-
nantly the negative tγc region, but can contribute to the
signal region. Since beam halo muons travel parallel to
the beam line, these events can be separated from cosmic
ray events by identifying the small energy deposited in
the calorimeter towers along the beam halo muon trajec-
tory.
The background prediction uses control regions out-
side the signal time window but well within the 132 ns
time window that the calorimeter uses to measure the
energy. The non-collision background templates are nor-
malized to match the number of events in two time win-
dows: a beam halo-dominated window at {−20, −6} ns,
selected to be 3σ away from the wrong vertex collision
background, and a cosmic rays-dominated window at
{25, 90} ns, well away from the standard model and
beam halo contributions. The collision background is
estimated by fitting events in the {−10, 1.2} ns window
with the non-collision contribution subtracted and with
the fraction of correct to incorrect vertex events allowed
to vary. In this way the background for the signal region
is entirely estimated from data samples. The systematic
uncertainty on the background estimate is dominated by
our ability to calibrate the mean of the tγc distribution
for prompt photons. We find a variation of 200 ps on
the mean and 20 ps on the standard deviation of the dis-
tribution by considering various possible event selection
criteria. These contribute to the systematic uncertainty
of the collision background estimate in the signal region
and are added in quadrature with the statistical uncer-
tainties of the final fit procedure.
We estimate the sensitivity to heavy, long-lived parti-
cles that decay to photons using GMSB models for dif-
ferent χ̃01 masses and lifetimes. Events from all SUSY
processes are simulated with the pythia Monte Carlo
program [16] along with the detector simulation [17]. The
acceptance is the ratio of simulated events that pass all
the requirements to all events produced. It is used in
the optimization procedure and in the final limit setting
and depends on a number of effects. The fraction of χ̃01
decays in the detector volume is the dominant effect on
the acceptance. For a given lifetime this depends on the
boost of the χ̃01. A highly boosted χ̃
1 that decays in
the detector typically does not contribute to the accep-
tance because it tends to produce a photon traveling in
the same direction as the χ̃01. Thus, the photon’s arrival
time is indistinguishable from promptly produced pho-
tons. At small boosts the decay is more likely to happen
inside the detector, and the decay angle is more likely
to be large, which translates into a larger delay for the
photon. The fraction of events with a delayed photon ar-
rival time initially rises as a function of χ̃01 lifetime, but
falls as the fraction of χ̃01’s decaying outside the detector
begins to dominates. In the χ̃01 mass region considered
(65 ≤ mχ̃0
≤ 150 GeV/c2), the acceptance peaks at a
lifetime of around 5 ns. The acceptance also depends on
the mass as the boost effects are mitigated by the ability
to produce high energy photons or E/T in the collision, as
discussed in Ref. [8].
The total systematic uncertainty of 10% on the ac-
Photon Corrected Time of Arrival (ns)
-20 0 20 40 60 80
)-1 + Jet data (570 pb
E + γ
Standard Model
Beam Halo
Cosmics
GMSB Signal MC
Photon Corrected Time of Arrival (ns)
-20 0 20 40 60 80
FIG. 1: The time distribution for photons passing all but
the final timing requirement for the background predic-
tions, data, and a GMSB signal for an example point at
= 100 GeV/c2, τχ̃0
= 5 ns. A total of 1.3±0.7 back-
ground events are predicted and 2 (marked with a star) are
observed in the signal region of 2 < tγc < 10 ns.
ceptance is dominated by the uncertainty on the mean
of the tγc distribution (7%) and on the photon ID effi-
ciency (5%). Other significant contributions come from
uncertainties on initial and final state radiation (3%), jet
energy measurement (3%), and the parton distribution
functions (1%).
We determine the kinematic and tγc selection require-
ments that define the final data sample by optimizing
the expected cross section limit without looking at the
data in the signal region. To compute the expected 95%
confidence level (C.L.) cross section upper limit [18], we
combine the predicted GMSB signal and background esti-
mates with the systematic uncertainties using a Bayesian
method with a flat prior [19]. The expected limits are op-
timized by simultaneously varying the selection require-
ments for E/T , photon ET , jet ET , azimuth angle be-
tween the leading jet and E/T (∆φ(E/T , jet)), and t
c . The
∆φ(E/T , jet) requirement rejects events where the E/T is
overestimated because of a poorly measured jet. While
each point in χ̃01 lifetime vs. mass space gives a slightly
different optimization, we choose a single set of require-
ments because it simplifies the final analysis, while only
causing a small loss of sensitivity. The optimized require-
ments are summarized in Table I. As an example, the ac-
ceptance for mχ̃0
= 100 GeV/c2 and lifetime τχ̃0
= 5 ns
is estimated to be (6.3±0.6)%.
After all kinematic requirements, 508 events are ob-
served in the data before the final signal region time re-
quirement. Their time distribution is shown in Fig. 1.
Our fit to the data outside the signal region predicts total
backgrounds of 6.2±3.5 from cosmic rays, 6.8±4.9 from
beam halo background sources, and the rest from the
)2 mass (GeV/c
65 70 75 80 85 90 95 100 105 110
0 1χ∼
1.0 pb
0.5 pb
0.3 pb
0.2 pb
0.13 pb
FIG. 2: The contours of constant 95% C.L. upper cross section
limits for a GMSB model [7].
standard model. Inside the signal time region, {2, 10} ns,
we predict 1.25±0.66 events: 0.71±0.60 from standard
model, 0.46±0.26 from cosmic rays, and 0.07±0.05 from
beam halo. Two events are observed in the data. Since
the result is consistent with the no-signal hypothesis, we
set limits on the χ̃01 lifetime and mass. Figure 2 shows the
contours of constant 95% C.L. cross section upper limit.
Figure 3 shows the exclusion region at 95% C.L., along
with the expected limit for comparison. This takes into
account the predicted production cross section at next-
to-leading order [20] as well as the uncertainties on the
parton distribution functions (6%) and the renormaliza-
tion scale (2%). Since the number of observed events
is above expectations, the observed limits are slightly
worse than the expected limits. These limits extend at
large masses beyond those of LEP searches using photon
“pointing” methods [11].
In conclusion, we have performed the first search for
heavy, long-lived particles that decay to photons at a
hadron collider using data collected with the EMTim-
ing system at the CDF II detector. There is no excess
of events beyond expectations. As our search strategy
does not rely on event properties specific solely to GMSB
models, we can exclude any γ+jet+E/T signal that would
produce more than 5.5 events. We set cross section limits
using a supersymmetric model with χ̃01 → γG̃, and find
a GMSB exclusion region in the χ̃01 lifetime vs. mass
plane with the world-best 95% C.L. lower limit on the
χ̃01 mass of 101 GeV/c
2 at τχ̃0
= 5 ns. Future improve-
ments with similar techniques should also provide sen-
sitivity to new particle decays with a delayed electron
signature [2]. By the end of Run II, an integrated lumi-
nosity of 10 fb−1 is possible for which we estimate a mass
reach of ≃ 140 GeV/c2 at a lifetime of 5 ns.
)2 mass (GeV/c
65 70 75 80 85 90 95 100 105 110
0 1χ∼
)-1+1jet analysis with EMTiming (570 pb
Predicted exclusion region
Observed exclusion region
ALEPH exclusion upper limit
χ∼GMSB 
)=15β, tan(Λ=2messM
>0µ=1, messN
65 70 75 80 85 90 95 100 105 110
FIG. 3: The exclusion region at 95% C.L. as a function of eχ01
lifetime and mass for a GMSB model [7]. The predicted and
the observed regions are shown separately and are compared
to the most stringent published limit from LEP searches [11].
We thank the Fermilab staff and the technical staffs
of the participating institutions for their vital contribu-
tions. This work was supported by the U.S. Department
of Energy and National Science Foundation; the Italian
Istituto Nazionale di Fisica Nucleare; the Ministry of
Education, Culture, Sports, Science and Technology of
Japan; the Natural Sciences and Engineering Research
Council of Canada; the National Science Council of the
Republic of China; the Swiss National Science Founda-
tion; the A.P. Sloan Foundation; the Bundesministerium
für Bildung und Forschung, Germany; the Korean Sci-
ence and Engineering Foundation and the Korean Re-
search Foundation; the Particle Physics and Astronomy
Research Council and the Royal Society, UK; the Russian
Foundation for Basic Research; the Comisión Interminis-
terial de Ciencia y Tecnoloǵıa, Spain; in part by the Eu-
ropean Community’s Human Potential Programme un-
der contract HPRN-CT-2002-00292; and the Academy
of Finland.
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and pT = p sin θ where E is the energy measured by
the calorimeter and p the momentum measured in the
tracking system. E/T = | −
EiT ~ni| where ~ni is a unit
vector that points from the interaction vertex to the ith
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http://arxiv.org/abs/hep-ph/0605193
http://arxiv.org/abs/hep-ex/0611010
