Correlated leading baryon-antibaryon production in
e
þ
e
!
c
c
!
þ
c
c
X
B. Aubert,
1
Y. Karyotakis,
1
J. P. Lees,
1
V. Poireau,
1
E. Prencipe,
1
X. Prudent,
1
V. Tisserand,
1
J. Garra Tico,
2
E. Grauges,
2
M. Martinelli,
3a,3b
A. Palano,
3a,3b
M. Pappagallo,
3a,3b
G. Eigen,
4
B. Stugu,
4
L. Sun,
4
M. Battaglia,
5
D. N. Brown,
5
B. Hooberman,
5
L. T. Kerth,
5
Yu. G. Kolomensky,
5
G. Lynch,
5
I. L. Osipenkov,
5
K. Tackmann,
5
T. Tanabe,
5
C. M. Hawkes,
6
N. Soni,
6
A. T. Watson,
6
H. Koch,
7
T. Schroeder,
7
D. J. Asgeirsson,
8
C. Hearty,
8
T. S. Mattison,
8
J. A. McKenna,
8
M. Barrett,
9
A. Khan,
9
A. Randle-Conde,
9
V. E. Blinov,
10
A. D. Bukin,
10,
*
A. R. Buzykaev,
10
V. P. Druzhinin,
10
V. B. Golubev,
10
A. P. Onuchin,
10
S. I. Serednyakov,
10
Yu. I. Skovpen,
10
E. P. Solodov,
10
K. Yu. Todyshev,
10
M. Bondioli,
11
S. Curry,
11
I. Eschrich,
11
D. Kirkby,
11
A. J. Lankford,
11
P. Lund,
11
M. Mandelkern,
11
E. C. Martin,
11
D. P. Stoker,
11
C. Buchanan,
12
B. L. Hartfiel,
12
H. Atmacan,
13
J. W. Gary,
13
F. Liu,
13
O. Long,
13
G. M. Vitug,
13
Z. Yasin,
13
V. Sharma,
14
C. Campagnari,
15
T. M. Hong,
15
D. Kovalskyi,
15
M. A. Mazur,
15
J. D. Richman,
15
T. W. Beck,
16
A. M. Eisner,
16
C. A. Heusch,
16
J. Kroseberg,
16
W. S. Lockman,
16
A. J. Martinez,
16
T. Schalk,
16
B. A. Schumm,
16
A. Seiden,
16
L. O. Winstrom,
16
C. H. Cheng,
17
D. A. Doll,
17
B. Echenard,
17
F. Fang,
17
D. G. Hitlin,
17
I. Narsky,
17
P. Ongmongkolkul,
17
T. Piatenko,
17
F. C. Porter,
17
R. Andreassen,
18
M. S. Dubrovin,
18
G. Mancinelli,
18
B. T. Meadows,
18
K. Mishra,
18
M. D. Sokoloff,
18
P. C. Bloom,
19
W. T. Ford,
19
A. Gaz,
19
J. F. Hirschauer,
19
M. Nagel,
19
U. Nauenberg,
19
J. G. Smith,
19
S. R. Wagner,
19
R. Ayad,
20,
†
W. H. Toki,
20
E. Feltresi,
21
A. Hauke,
21
H. Jasper,
21
T. M. Karbach,
21
J. Merkel,
21
A. Petzold,
21
B. Spaan,
21
K. Wacker,
21
M. J. Kobel,
22
K. R. Schubert,
22
R. Schwierz,
22
D. Bernard,
23
E. Latour,
23
M. Verderi,
23
P. J. Clark,
24
S. Playfer,
24
J. E. Watson,
24
M. Andreotti,
25a,25b
D. Bettoni,
25a
C. Bozzi,
25a
R. Calabrese,
25a,25b
A. Cecchi,
25a,25b
G. Cibinetto,
25a,25b
E. Fioravanti,
25a,25b
P. Franchini,
25a,25b
E. Luppi,
25a,25b
M. Munerato,
25a,25b
M. Negrini,
25a,25b
A. Petrella,
25a,25b
L. Piemontese,
25a
V. Santoro,
25a,25b
R. Baldini-Ferroli,
26
A. Calcaterra,
26
R. de Sangro,
26
G. Finocchiaro,
26
S. Pacetti,
26
P. Patteri,
26
I. M. Peruzzi,
26,
‡
M. Piccolo,
26
M. Rama,
26
A. Zallo,
26
R. Contri,
27a,27b
E. Guido,
27a,27b
M. Lo Vetere,
27a,27b
M. R. Monge,
27a,27b
S. Passaggio,
27a
C. Patrignani,
27a,27b
E. Robutti,
27a
S. Tosi,
27a,27b
M. Morii,
28
A. Adametz,
29
J. Marks,
29
S. Schenk,
29
U. Uwer,
29
F. U. Bernlochner,
30
H. M. Lacker,
30
T. Lueck,
30
A. Volk,
30
P. D. Dauncey,
31
M. Tibbetts,
31
P. K. Behera,
32
M. J. Charles,
32
U. Mallik,
32
C. Chen,
33
J. Cochran,
33
H. B. Crawley,
33
L. Dong,
33
V. Eyges,
33
W. T. Meyer,
33
S. Prell,
33
E. I. Rosenberg,
33
A. E. Rubin,
33
Y. Y. Gao,
34
A. V. Gritsan,
34
Z. J. Guo,
34
N. Arnaud,
35
M. Davier,
35
D. Derkach,
35
J. Firmino da Costa,
35
G. Grosdidier,
35
F. Le Diberder,
35
V. Lepeltier,
35
A. M. Lutz,
35
B. Malaescu,
35
P. Roudeau,
35
M. H. Schune,
35
J. Serrano,
35
V. Sordini,
35,
x
A. Stocchi,
35
G. Wormser,
35
D. J. Lange,
36
D. M. Wright,
36
I. Bingham,
37
J. P. Burke,
37
C. A. Chavez,
37
J. R. Fry,
37
E. Gabathuler,
37
R. Gamet,
37
D. E. Hutchcroft,
37
D. J. Payne,
37
C. Touramanis,
37
A. J. Bevan,
38
C. K. Clarke,
38
F. Di Lodovico,
38
R. Sacco,
38
M. Sigamani,
38
G. Cowan,
39
S. Paramesvaran,
39
A. C. Wren,
39
D. N. Brown,
40
C. L. Davis,
40
A. G. Denig,
41
M. Fritsch,
41
W. Gradl,
41
A. Hafner,
41
K. E. Alwyn,
42
D. Bailey,
42
R. J. Barlow,
42
G. Jackson,
42
G. D. Lafferty,
42
T. J. West,
42
J. I. Yi,
42
J. Anderson,
43
A. Jawahery,
43
D. A. Roberts,
43
G. Simi,
43
J. M. Tuggle,
43
C. Dallapiccola,
44
E. Salvati,
44
R. Cowan,
45
D. Dujmic,
45
P. H. Fisher,
45
S. W. Henderson,
45
G. Sciolla,
45
M. Spitznagel,
45
R. K. Yamamoto,
45
M. Zhao,
45
P. M. Patel,
46
S. H. Robertson,
46
M. Schram,
46
P. Biassoni,
47a,47b
A. Lazzaro,
47a,47b
V. Lombardo,
47a
F. Palombo,
47a,47b
S. Stracka,
47a,47b
L. Cremaldi,
48
R. Godang,
48,
k
R. Kroeger,
48
P. Sonnek,
48
D. J. Summers,
48
H. W. Zhao,
48
X. Nguyen,
49
M. Simard,
49
P. Taras,
49
H. Nicholson,
50
G. De Nardo,
51a,51b
L. Lista,
51a
D. Monorchio,
51a,51b
G. Onorato,
51a,51b
C. Sciacca,
51a,51b
G. Raven,
52
H. L. Snoek,
52
C. P. Jessop,
53
K. J. Knoepfel,
53
J. M. LoSecco,
53
W. F. Wang,
53
L. A. Corwin,
54
K. Honscheid,
54
H. Kagan,
54
R. Kass,
54
J. P. Morris,
54
A. M. Rahimi,
54
S. J. Sekula,
54
N. L. Blount,
55
J. Brau,
55
R. Frey,
55
O. Igonkina,
55
J. A. Kolb,
55
M. Lu,
55
R. Rahmat,
55
N. B. Sinev,
55
D. Strom,
55
J. Strube,
55
E. Torrence,
55
G. Castelli,
56a,56b
N. Gagliardi,
56a,56b
M. Margoni,
56a,56b
M. Morandin,
56a
M. Posocco,
56a
M. Rotondo,
56a
F. Simonetto,
56a,56b
R. Stroili,
56a,56b
C. Voci,
56a,56b
P. del Amo Sanchez,
57
E. Ben-Haim,
57
G. R. Bonneaud,
57
H. Briand,
57
J. Chauveau,
57
O. Hamon,
57
Ph. Leruste,
57
G. Marchiori,
57
J. Ocariz,
57
A. Perez,
57
J. Prendki,
57
S. Sitt,
57
L. Gladney,
58
M. Biasini,
59a,59b
E. Manoni,
59a,59b
C. Angelini,
60a,60b
G. Batignani,
60a,60b
S. Bettarini,
60a,60b
G. Calderini,
60a,60b,
{
M. Carpinelli,
60a,60b,
**
A. Cervelli,
60a,60b
F. Forti,
60a,60b
M. A. Giorgi,
60a,60b
A. Lusiani,
60a,60c
M. Morganti,
60a,60b
N. Neri,
60a,60b
E. Paoloni,
60a,60b
G. Rizzo,
60a,60b
J. J. Walsh,
60a
D. Lopes Pegna,
61
C. Lu,
61
J. Olsen,
61
A. J. S. Smith,
61
A. V. Telnov,
61
F. Anulli,
62a
E. Baracchini,
62a,62b
G. Cavoto,
62a
R. Faccini,
62a,62b
F. Ferrarotto,
62a
F. Ferroni,
62a,62b
M. Gaspero,
62a,62b
P. D. Jackson,
62a
L. Li Gioi,
62a
M. A. Mazzoni,
62a
S. Morganti,
62a
G. Piredda,
62a
F. Renga,
62a,62b
C. Voena,
62a
M. Ebert,
63
T. Hartmann,
63
H. Schro
̈
der,
63
R. Waldi,
63
T. Adye,
64
B. Franek,
64
E. O. Olaiya,
64
F. F. Wilson,
64
S. Emery,
65
L. Esteve,
65
G. Hamel de Monchenault,
65
W. Kozanecki,
65
G. Vasseur,
65
Ch. Ye
`
che,
65
M. Zito,
65
M. T. Allen,
66
D. Aston,
66
D. J. Bard,
66
R. Bartoldus,
66
J. F. Benitez,
66
R. Cenci,
66
J. P. Coleman,
66
M. R. Convery,
66
J. C. Dingfelder,
66
J. Dorfan,
66
G. P. Dubois-Felsmann,
66
PHYSICAL REVIEW D
82,
091102(R) (2010)
RAPID COMMUNICATIONS
1550-7998
=
2010
=
82(9)
=
091102(8)
091102-1
Ó
2010 The American Physical Society
W. Dunwoodie,
66
R. C. Field,
66
M. Franco Sevilla,
66
B. G. Fulsom,
66
A. M. Gabareen,
66
M. T. Graham,
66
P. Grenier,
66
C. Hast,
66
W. R. Innes,
66
J. Kaminski,
66
M. H. Kelsey,
66
H. Kim,
66
P. Kim,
66
M. L. Kocian,
66
D. W. G. S. Leith,
66
S. Li,
66
B. Lindquist,
66
S. Luitz,
66
V. Luth,
66
H. L. Lynch,
66
D. B. MacFarlane,
66
H. Marsiske,
66
R. Messner,
66,
*
D. R. Muller,
66
H. Neal,
66
S. Nelson,
66
C. P. O’Grady,
66
I. Ofte,
66
M. Perl,
66
B. N. Ratcliff,
66
A. Roodman,
66
A. A. Salnikov,
66
R. H. Schindler,
66
J. Schwiening,
66
A. Snyder,
66
D. Su,
66
M. K. Sullivan,
66
K. Suzuki,
66
S. K. Swain,
66
J. M. Thompson,
66
J. Va’vra,
66
A. P. Wagner,
66
M. Weaver,
66
C. A. West,
66
W. J. Wisniewski,
66
M. Wittgen,
66
D. H. Wright,
66
H. W. Wulsin,
66
A. K. Yarritu,
66
C. C. Young,
66
V. Ziegler,
66
X. R. Chen,
67
H. Liu,
67
W. Park,
67
M. V. Purohit,
67
R. M. White,
67
J. R. Wilson,
67
M. Bellis,
68
P. R. Burchat,
68
A. J. Edwards,
68
T. S. Miyashita,
68
S. Ahmed,
69
M. S. Alam,
69
J. A. Ernst,
69
B. Pan,
69
M. A. Saeed,
69
S. B. Zain,
69
A. Soffer,
70
S. M. Spanier,
71
B. J. Wogsland,
71
R. Eckmann,
72
J. L. Ritchie,
72
A. M. Ruland,
72
C. J. Schilling,
72
R. F. Schwitters,
72
B. C. Wray,
72
B. W. Drummond,
73
J. M. Izen,
73
X. C. Lou,
73
F. Bianchi,
74a,74b
D. Gamba,
74a,74b
M. Pelliccioni,
74a,74b
M. Bomben,
75a,75b
L. Bosisio,
75a,75b
C. Cartaro,
75a,75b
G. Della Ricca,
75a,75b
L. Lanceri,
75a,75b
L. Vitale,
75a,75b
V. Azzolini,
76
N. Lopez-March,
76
F. Martinez-Vidal,
76
D. A. Milanes,
76
A. Oyanguren,
76
J. Albert,
77
Sw. Banerjee,
77
B. Bhuyan,
77
H. H. F. Choi,
77
K. Hamano,
77
G. J. King,
77
R. Kowalewski,
77
M. J. Lewczuk,
77
I. M. Nugent,
77
J. M. Roney,
77
R. J. Sobie,
77
T. J. Gershon,
78
P. F. Harrison,
78
J. Ilic,
78
T. E. Latham,
78
G. B. Mohanty,
78
E. M. T. Puccio,
78
H. R. Band,
79
X. Chen,
79
S. Dasu,
79
K. T. Flood,
79
Y. Pan,
79
R. Prepost,
79
C. O. Vuosalo,
79
and S. L. Wu
79
(
B
A
B
AR
Collaboration)
1
Laboratoire d’Annecy-le-Vieux de Physique des Particules (LAPP), Universite
́
de Savoie,
CNRS/IN2P3, F-74941 Annecy-Le-Vieux, France
2
Universitat de Barcelona, Facultat de Fisica, Departament ECM, E-08028 Barcelona, Spain
3a
INFN Sezione di Bari, I-70126 Bari, Italy
3b
Dipartimento di Fisica, Universita
`
di Bari, I-70126 Bari, Italy
4
University of Bergen, Institute of Physics, N-5007 Bergen, Norway
5
Lawrence Berkeley National Laboratory and University of California, Berkeley, California 94720, USA
6
University of Birmingham, Birmingham, B15 2TT, United Kingdom
7
Ruhr Universita
̈
t Bochum, Institut fu
̈
r Experimentalphysik 1, D-44780 Bochum, Germany
8
University of British Columbia, Vancouver, British Columbia, Canada V6T 1Z1
9
Brunel University, Uxbridge, Middlesex UB8 3PH, United Kingdom
10
Budker Institute of Nuclear Physics, Novosibirsk 630090, Russia
11
University of California at Irvine, Irvine, California 92697, USA
12
University of California at Los Angeles, Los Angeles, California 90024, USA
13
University of California at Riverside, Riverside, California 92521, USA
14
University of California at San Diego, La Jolla, California 92093, USA
15
University of California at Santa Barbara, Santa Barbara, California 93106, USA
16
University of California at Santa Cruz, Institute for Particle Physics, Santa Cruz, California 95064, USA
17
California Institute of Technology, Pasadena, California 91125, USA
18
University of Cincinnati, Cincinnati, Ohio 45221, USA
19
University of Colorado, Boulder, Colorado 80309, USA
20
Colorado State University, Fort Collins, Colorado 80523, USA
21
Technische Universita
̈
t Dortmund, Fakulta
̈
t Physik, D-44221 Dortmund, Germany
22
Technische Universita
̈
t Dresden, Institut fu
̈
r Kern- und Teilchenphysik, D-01062 Dresden, Germany
23
Laboratoire Leprince-Ringuet, CNRS/IN2P3, Ecole Polytechnique, F-91128 Palaiseau, France
24
University of Edinburgh, Edinburgh EH9 3JZ, United Kingdom
25a
INFN Sezione di Ferrara, I-44100 Ferrara, Italy
25b
Dipartimento di Fisica, Universita
`
di Ferrara, I-44100 Ferrara, Italy
26
INFN Laboratori Nazionali di Frascati, I-00044 Frascati, Italy
27a
INFN Sezione di Genova, I-16146 Genova, Italy
27b
Dipartimento di Fisica, Universita
`
di Genova, I-16146 Genova, Italy
28
Harvard University, Cambridge, Massachusetts 02138, USA
29
Universita
̈
t Heidelberg, Physikalisches Institut, Philosophenweg 12, D-69120 Heidelberg, Germany
30
Humboldt-Universita
̈
t zu Berlin, Institut fu
̈
r Physik, Newtonstr. 15, D-12489 Berlin, Germany
31
Imperial College London, London, SW7 2AZ, United Kingdom
32
University of Iowa, Iowa City, Iowa 52242, USA
33
Iowa State University, Ames, Iowa 50011-3160, USA
34
Johns Hopkins University, Baltimore, Maryland 21218, USA
B. AUBERT
et al.
PHYSICAL REVIEW D
82,
091102(R) (2010)
RAPID COMMUNICATIONS
091102-2
35
Laboratoire de l’Acce
́
le
́
rateur Line
́
aire, IN2P3/CNRS et Universite
́
Paris-Sud 11,
Centre Scientifique d’Orsay, B.P. 34, F-91898 Orsay Cedex, France
36
Lawrence Livermore National Laboratory, Livermore, California 94550, USA
37
University of Liverpool, Liverpool L69 7ZE, United Kingdom
38
Queen Mary, University of London, London, E1 4NS, United Kingdom
39
University of London, Royal Holloway and Bedford New College, Egham, Surrey TW20 0EX, United Kingdom
40
University of Louisville, Louisville, Kentucky 40292, USA
41
Johannes Gutenberg-Universita
̈
t Mainz, Institut fu
̈
r Kernphysik, D-55099 Mainz, Germany
42
University of Manchester, Manchester M13 9PL, United Kingdom
43
University of Maryland, College Park, Maryland 20742, USA
44
University of Massachusetts, Amherst, Massachusetts 01003, USA
45
Massachusetts Institute of Technology, Laboratory for Nuclear Science, Cambridge, Massachusetts 02139, USA
46
McGill University, Montre
́
al, Que
́
bec, Canada H3A 2T8
47a
INFN Sezione di Milano, I-20133 Milano, Italy
47b
Dipartimento di Fisica, Universita
`
di Milano, I-20133 Milano, Italy
48
University of Mississippi, University, Mississippi 38677, USA
49
Universite
́
de Montre
́
al, Physique des Particules, Montre
́
al, Que
́
bec, Canada H3C 3J7
50
Mount Holyoke College, South Hadley, Massachusetts 01075, USA
51a
INFN Sezione di Napoli, I-80126 Napoli, Italy
51b
Dipartimento di Scienze Fisiche, Universita
`
di Napoli Federico II, I-80126 Napoli, Italy
52
NIKHEF, National Institute for Nuclear Physics and High Energy Physics, NL-1009 DB Amsterdam, The Netherlands
53
University of Notre Dame, Notre Dame, Indiana 46556, USA
54
Ohio State University, Columbus, Ohio 43210, USA
55
University of Oregon, Eugene, Oregon 97403, USA
56a
INFN Sezione di Padova, I-35131 Padova, Italy
56b
Dipartimento di Fisica, Universita
`
di Padova, I-35131 Padova, Italy
57
Laboratoire de Physique Nucle
́
aire et de Hautes Energies, IN2P3/CNRS, Universite
́
Pierre et Marie Curie-Paris6,
Universite
́
Denis Diderot-Paris7, F-75252 Paris, France
58
University of Pennsylvania, Philadelphia, Pennsylvania 19104, USA
59a
INFN Sezione di Perugia, I-06100 Perugia, Italy
59b
Dipartimento di Fisica, Universita
`
di Perugia, I-06100 Perugia, Italy
60a
INFN Sezione di Pisa, I-56127 Pisa, Italy
60b
Dipartimento di Fisica, Universita
`
di Pisa, I-56127 Pisa, Italy
60c
Scuola Normale Superiore di Pisa, I-56127 Pisa, Italy
61
Princeton University, Princeton, New Jersey 08544, USA
62a
INFN Sezione di Roma, I-00185 Roma, Italy
62b
Dipartimento di Fisica, Universita
`
di Roma La Sapienza, I-00185 Roma, Italy
63
Universita
̈
t Rostock, D-18051 Rostock, Germany
64
Rutherford Appleton Laboratory, Chilton, Didcot, Oxon, OX11 0QX, United Kingdom
65
CEA, Irfu, SPP, Centre de Saclay, F-91191 Gif-sur-Yvette, France
66
SLAC National Accelerator Laboratory, Stanford, California 94309 USA
67
University of South Carolina, Columbia, South Carolina 29208, USA
68
Stanford University, Stanford, California 94305-4060, USA
69
State University of New York, Albany, New York 12222, USA
70
Tel Aviv University, School of Physics and Astronomy, Tel Aviv, 69978, Israel
71
University of Tennessee, Knoxville, Tennessee 37996, USA
72
University of Texas at Austin, Austin, Texas 78712, USA
73
University of Texas at Dallas, Richardson, Texas 75083, USA
74a
INFN Sezione di Torino, I-10125 Torino, Italy
74b
Dipartimento di Fisica Sperimentale, Universita
`
di Torino, I-10125 Torino, Italy
75a
INFN Sezione di Trieste, I-34127 Trieste, Italy
75b
Dipartimento di Fisica, Universita
`
di Trieste, I-34127 Trieste, Italy
76
IFIC, Universitat de Valencia-CSIC, E-46071 Valencia, Spain
77
University of Victoria, Victoria, British Columbia, Canada V8W 3P6
78
Department of Physics, University of Warwick, Coventry CV4 7AL, United Kingdom
79
University of Wisconsin, Madison, Wisconsin 53706, USA
(Received 15 June 2010; published 9 November 2010)
We present a study of
649
35
e
þ
e
!
c
c
events produced at
ffiffiffi
s
p
10
:
6 GeV
containing both a
þ
c
baryon and a
c
antibaryon. The number observed is roughly 4 times that expected if the leading charmed
hadron types are uncorrelated, confirming an observation by the CLEO Collaboration. We find a 2-jet
CORRELATED LEADING BARYON-ANTIBARYON
...
PHYSICAL REVIEW D
82,
091102(R) (2010)
RAPID COMMUNICATIONS
091102-3
topology in these events but very few additional baryons, demonstrating that the primary
c
and
c
are
predominantly contained in a correlated baryon-antibaryon system. In addition to the charmed baryons we
observe on average
2
:
6
0
:
2
charged intermediate mesons, predominantly pions, carrying 65% of the
remaining energy.
DOI:
10.1103/PhysRevD.82.091102
PACS numbers: 13.66.Bc, 13.60.Rj, 13.87.Fh
Baryon production in high-energy jets from
e
þ
e
annihilations has presented a series of challenges to our
understanding of strong interactions. Its observation led to
the competing notions of ‘‘primary’’ and ‘‘local’’ baryon
correlations [
1
]. In the former, the
e
þ
and
e
annihilate
into a primary diquark-antidiquark, rather than a quark-
antiquark, pair. The diquark and antidiquark then hadron-
ize into jets containing a leading baryon
N
1
and a leading
antibaryon
N
2
, respectively, but no other (anti)baryons.
N
1
and
N
2
would then share two quark flavors and typi-
cally have high, antiparallel momenta and large values of
variables characterizing their separation, such as invariant
mass or rapidity difference
j
y
j
, where
y
0
:
5ln
½ð
E
þ
p
k
Þ
=
ð
E
p
k
Þ
,
E
is the baryon energy, and
p
k
is the
projection of its momentum on the thrust axis.
Alternatively, an
N
1
N
2
pair might be produced locally, in
an individual step of a hadronization cascade, with a
smaller value of
j
y
j
. Most experimental studies of
baryon-antibaryon pairs have shown
j
y
j
distributions
that peak at small values [
2
].
Several mechanisms to describe baryon production and
correlations have been implemented in Monte Carlo ha-
dronization models [
3
]. In the
JETSET
[
4
] color-flux-tube
model, a tube break can result in a diquark-antidiquark
(rather than
q
q
) pair, producing an
N
1
N
2
pair locally. An
intermediate meson is introduced between
N
1
and
N
2
with some probability (50% by default [
5
]) to match the
measured
j
y
j
distributions. In the
HERWIG
[
6
] model, an
individual, color-singlet cluster may fragment into a
baryon-antibaryon pair but not a multibody state with
additional mesons. The model does not reproduce the
measured
j
y
j
distributions when tuned to other observ-
ables [
2
]. The
UCLA
[
7
] area-law model includes
N
1
N
2
pairs with any number of intermediate mesons, and sup-
presses higher-mass intermediate meson systems by means
of a tunable parameter.
Direct evidence of primary production and/or inter-
mediate mesons would be of great interest, but previous
searches for the latter using three-particle correlations [
8
]
or baryon flavor correlations [
9
] were generally
inconclusive.
At center-of-mass (c.m.) energies
ffiffiffi
s
p
much larger than
four baryon masses, the assumption of local baryon num-
ber conservation implies that an
e
þ
e
!
q
q
event con-
taining a leading baryon
N
1
in the
q
jet and a leading
antibaryon
N
2
in the
q
jet must also contain an antibaryon
N
3
in the
q
jet and a baryon
N
4
in the
q
jet. However, if the
N
1
N
3
N
4
N
2
mass is a large fraction of
ffiffiffi
s
p
, these four-
baryon events would be suppressed and other processes
might be visible—in particular, primary baryon production
events with exactly two baryons, one in each jet. At
ffiffiffi
s
p
10 GeV
, charmed (
c
) baryons are of particular interest,
since any high-momentum
c
or
c
baryon must be a leading
particle in an
e
þ
e
!
c
c
event, and any
N
c
1
N
3
N
4
N
c
2
mass exceeds
6
:
5 GeV
=c
2
. The CLEO Collaboration re-
ported an excess by a factor of
3
:
5
0
:
6
[
10
] in the number
of events at
ffiffiffi
s
p
¼
10
:
6 GeV
with both a
þ
c
and a
c
,
where their expectation is derived assuming local baryon
number conservation in the
JETSET
model and from
observed events with a
þ
c
and a
D
or
D
0
meson. This
excess is evidence that the baryon production is correlated
between the
c
and
c
jets and is consistent with primary
baryon production, but does not exclude the possibility of
local baryon production with correlation between the jets.
The two cases can be distinguished experimentally: local
production would require an additional baryon and anti-
baryon (
N
4
and
N
3
) in the event, so events with exactly one
þ
c
, exactly one
c
, and no additional baryons would
imply primary production. CLEO investigated this and
did not observe a strong signal for additional protons in
the
þ
c
c
candidate events, but due to a limited data
sample and the lack of a limit on additional neutrons
they were unable to exclude local baryon production.
In this paper we exploit the particle identification capa-
bilities of the
BABAR
detector [
11
] to select a sample of
þ
c
c
X
events in which the
þ
c
and
c
are produced at
high momentum in opposite hemispheres, and study their
characteristics in detail. We use
220 fb
1
of data collected
*
Deceased.
†
Now at Temple University, Philadelphia, Pennsylvania 19122, USA.
‡
Also with Universita
`
di Perugia, Dipartimento di Fisica, Perugia, Italy.
x
Also with Universita
`
di Roma La Sapienza, I-00185 Roma, Italy.
k
Now at University of South Alabama, Mobile, Alabama 36688, USA.
{
Also with Laboratoire de Physique Nucle
́
aire et de Hautes Energies, IN2P3/CNRS, Universite
́
Pierre et Marie Curie-Paris6,
Universite
́
Denis Diderot-Paris7, F-75252 Paris, France.
**
Also with Universita
`
di Sassari, Sassari, Italy.
B. AUBERT
et al.
PHYSICAL REVIEW D
82,
091102(R) (2010)
RAPID COMMUNICATIONS
091102-4
at
ffiffiffi
s
p
¼
10
:
54
–
10
:
58 GeV
. We identify the charged tracks
in the
X
system, looking for additional (anti)protons, and
search for higher-mass baryons that could be a source of
the
þ
c
c
X
events. We consider charged tracks measured
in the silicon vertex tracker (SVT) and drift chamber
(DCH), and identified as pions, kaons, or protons using
the DCH and the detector of internally reflected Cherenkov
light. The identification algorithm used here [
12
,
13
]is
over 99% efficient for pions and kaons (protons) within
the acceptance with momenta between 0.15 and 0.5
ð
1
:
2
Þ
GeV
=c
, with misidentification rates below 0.5%. At
higher momenta it remains over 90% efficient, with
misidentification rates generally below 1%.
We construct
þ
c
candidates in the
pK
þ
and
pK
0
S
decay modes and
c
in the corresponding charge-
conjugate modes. We consider a pair of oppositely charged
tracks as a
K
0
S
!
þ
candidate if a vertex fit returns a
2
with a confidence level (C.L.) exceeding 0.01, the
vertex is displaced by 2.5–60 cm from the interaction point
(IP) calculated for each event from the set of well-
measured tracks in the SVT, the angle
K
S
between the
K
0
S
candidate’s momentum and the IP-to-vertex direction
satisfies
cos
K
S
>
0
:
97
, and the
þ
invariant mass is in
the range
491
:
8
–
503
:
8 MeV
=c
2
. All combinations of a
K
0
S
and a well-measured (
15
hits in the DCH and
5
in the
SVT) proton are considered
þ
c
!
pK
0
S
candidates. A
combination of well-measured
p
,
K
, and
þ
tracks is
considered a
þ
c
!
pK
þ
candidate if its vertex fit
yields
C
:
L
:>
0
:
001
.
We require
p
, the momentum of the
þ
c
candidate in
the
e
þ
e
c.m. frame, to exceed
2
:
3 GeV
=c
, so that the rate
of
þ
c
from
ð
4
S
Þ
decays [
12
,
14
] is negligible. We select
events containing at least one
þ
c
candidate and at least
one
c
candidate, requiring each candidate to have mass
within
190 MeV
=c
2
of the fitted
þ
c
peak. We then form
þ
c
c
pairs provided that they have no common tracks in
their decay chains. For these 21 000 pairs we show the
candidate
pK
þ
and
pK
0
S
invariant mass distributions in
Fig.
1(a)
. Clear
þ
c
signals are visible over modest back-
grounds. The peak mass values, rates, and momentum
distributions are consistent with previous measurements
[
12
,
14
,
15
]. We plot the invariant mass of the
c
candidate
versus that of the
þ
c
candidate in Fig.
1(b)
. Horizontal and
vertical bands are visible, corresponding to events with a
real
c
or
þ
c
, respectively, and there is a substantial
enhancement where they overlap.
The opening angle
between the
þ
c
and
c
momenta
in the c.m. frame is sensitive to their production mecha-
nism. We expect
þ
c
c
pairs from gluon splitting
(
e
þ
e
!
q
qg
!
q
qc
c
)or
e
þ
e
!
c
cg
events with a
very hard gluon to have relatively small
, but also a
suppressed selection efficiency due to the
p
requirement.
In the 21 000 events selected,
values are concentrated
near 180
, consistent with dominance of 2-jet
e
þ
e
!
c
c
events. Only seven events have
<
90
, one of which is in
the signal region defined below. Since the small-
back-
ground may have different characteristics from that at large
, we require
>
90
. This criterion also removes events
with a hard initial state photon, the study of which would
be interesting with a larger data sample and a different
analysis approach, as has been done by Belle [
16
].
About 3% of the events have two
þ
c
(or two
c
)
candidates, due to the two
pK
þ
combinations in
the decay chains
þþ
c
!
þ
c
ð
pK
þ
Þ
þ
and
þ
c
!
þ
c
ð
pK
þ
Þ
þ
. We include all combinations in the
sample and account for the kinematic overlap through the
background subtraction. We define a circular
þ
c
c
X
signal region centered at our peak mass values with a
radius of
12 MeV
=c
2
, which contains 919 entries. Using
the single-
þ
c
=
c
bands [
13
], we estimate an expected
background in the signal region of
245
5
events with one
real
þ
c
or
c
and one fake. Using events with both
masses at least
40 MeV
=c
2
from the fitted
þ
c
mass, we
estimate
25
1
expected background events with fake
þ
c
and
c
, giving a
þ
c
c
X
signal of
N
þ
c
c
¼
649
35
events.
We can calculate an expected number of signal events,
n
exp
, under the assumption that the
c
and
c
hadron types are
uncorrelated so that all signal events are four-baryon
events. Then
n
exp
¼
Cn
2
1
=
4
N
c
c
, where
n
1
¼
420 000
is
the number of single
þ
c
=
c
observed in the data,
N
c
c
¼
3
10
8
is the number of
e
þ
e
!
c
c
events expected for
our integrated luminosity, and the factor
C
accounts for the
correlation between the
þ
c
and
c
reconstruction effi-
ciencies. This formulation is independent of the
þ
c
branching fractions and average efficiencies. In the simple
case where the efficiencies of the
þ
c
and
c
in
þ
c
c
X
events are uncorrelated, no correction is needed (
C
¼
1
)
and
n
exp
¼
n
2
1
=
4
N
c
c
. More generally,
0
<C<
1
="
for an
average acceptance times efficiency of
"
: in the extreme
case of maximal correlation
C
¼
1
="
, and in the extreme
2.25
2.30
2.35
Mass (GeV/c
2
)
0
2
4
6
8
Entries / (2 MeV/c
2
)
pK
π
pK
S
2.25
2.30
2.35
pK
−
π
+
or pK
S
Mass (GeV/c
2
)
2.25
2.30
2.35
pK
+
π
−
or
pK
S
Mass (GeV/c
2
)
x10
2
(a)
(b)
1.5
5.5
15.5
0.5
2.5
8.5
44.5
25.5
133
76.5
FIG. 1. (a) Invariant mass distributions for the
þ
c
=
c
candi-
dates in selected events, reconstructed in the
pK
(gray) and
pK
0
S
(black) decay modes. (b) Invariant mass of the
c
candi-
date vs that of the
þ
c
candidate in the same event, in
5 MeV
=c
2
square bins.
CORRELATED LEADING BARYON-ANTIBARYON
...
PHYSICAL REVIEW D
82,
091102(R) (2010)
RAPID COMMUNICATIONS
091102-5
case of maximal anticorrelation
n
exp
¼
C
¼
0
.At
B
A
B
AR
there might be correlations because of the asymmetric
beam energies and detector layout. We evaluate this cor-
rection using the
JETSET
,
HERWIG
, and
UCLA
models, adjust-
ing their charm fragmentation parameters and reweighting
the resulting
p
distributions to reproduce our measured
distribution for inclusive
þ
c
[
12
]. Combined with smooth
parametrizations of our efficiencies as functions of momen-
tum and polar angle, the models give values of
C
ranging
from 0.63 to 1.65, with a mean of 1.05. Even allowing for
the large model dependence, the full range of
n
exp
¼
100
–
250
events is well below the observed
649
35
, con-
firming the enhanced rate
N
þ
c
c
=n
exp
4
reported by the
CLEO Collaboration [
10
].
We investigate the structure of the
þ
c
c
X
events using
the
þ
c
and
c
candidates along with additional charged
tracks that have at least ten points measured in the DCH,
five in the SVT, and extrapolate within 5 mm of the beam
axis. We subtract appropriately scaled distributions in the
background regions from those in the signal region to
obtain distributions for
þ
c
c
X
events. Figure
2(a)
shows
the distribution of the number of additional tracks, as well
as the numbers of identified
K
and
p=
p
among them.
Were each
c
baryon compensated by a light antibaryon,
then—assuming that half the antibaryons have an antipro-
ton in the final state and accounting for
p=
p
detection
efficiency—we would expect 45% of these events to con-
tain one identified
p=
p
and another 20% to contain both an
identified
p
and a
p
; we observe only 3.4% and 0.6%,
respectively. Figure
2(b)
shows the distribution of missing
mass, calculated from the four-momenta of the initial
e
þ
and
e
, the reconstructed
þ
c
and
c
, and all additional
tracks interpreted as pions. A typical
N
c
1
nXn
N
c
2
event,
containing both a neutron and an antineutron, would have a
missing mass well in excess of
2 GeV
=c
2
.
The distributions in Fig.
2
indicate that the majority of
the
þ
c
c
X
events do not contain additional baryons, and
therefore that the conservation of baryon number is real-
ized with the primary
c
and
c
hadrons. In the background-
subtracted sample of
649
35
þ
c
c
X
signal events,
there are
28
6
additional identified
p=
p
candidates.
These
p=
p
candidates include background from two
main sources: interactions in the detector material and
misidentified pions or kaons. We expect five protons
from material interactions. We also expect about 12 pions
or kaons misidentified as protons, based on the numbers
and momenta of the observed additional
and
K
tracks. In cross-checks these expectations are found to be
consistent with the data within uncertainties: there are
8
4
more identified
p
than
p
(with the excess attributed
to material interactions), and there are
7
3
events seen
with exactly one additional identified
p=
p
and an event
missing mass below
750 MeV
=c
2
(inconsistent with a
missing second baryon, and so attributed to a misidentified
kaon or pion). Subtracting the expected contributions from
these two background sources, correcting for efficiency,
and assuming equal
p
and
n
production rates, we estimate
that we observe
13
8
true four-baryon events. This is
well below the rate of 100 to 250 four-baryon events
expected for uncorrelated production, let alone the ob-
served rate of
649
35
events, indicating that the four-
baryon process is strongly suppressed and that the primary
production process dominates.
None of the reconstructed events is consistent with the
two-body process
e
þ
e
!
þ
c
c
. However, the signal
could arise from the pair production of
c
baryons if one
or both are excited states that decay to
þ
c
=
c
:
e
þ
e
!
N
c
1
N
c
2
!
þ
c
c
X
. Combining
þ
c
=
c
candidates with
one or two additional tracks assigned the pion mass hy-
pothesis gives the invariant mass distributions in Fig.
3
.
The points represent sideband-subtracted signal events and
the histograms the single-
þ
c
=
c
sidebands with entries
reweighted to reproduce the number of the
þ
c
=
c
in
signal events and their momentum and polar angle distri-
butions in the lab frame. Peaks are visible in the sideband
data for the
þþ
=
0
c
ð
2455
Þ
,
þþ
=
0
c
ð
2520
Þ
, and the excited
þ
c
states at 2593, 2625, 2765, and
2880 MeV
=c
2
. We find
no unexpected peaks in our
þ
c
ð
Þ
,
þ
c
K
,or
þ
c
p
mass
distributions. The points are consistent with the histo-
grams, indicating similar
c
baryon compositions in the
two event types. Only two events are kinematically con-
sistent with
e
þ
e
!
N
c
1
N
c
2
for these known
N
c
.
Distributions of
and the decay angles in the
þ
c
rest
frames are consistent with multihadron events, and not
with very heavy states decaying into a
þ
c
and more than
two pions. We conclude that
e
þ
e
!
N
c
1
N
c
2
processes
represent a small fraction of our sample. From the fits in
Fig.
3
, we estimate that
35
3%
of the
þ
c
and
29
2%
of the additional pions in our sample are decay products of
heavier
c
baryons.
02
4
68
Multiplicity
0
40
80
120
160
Events
Tracks
K
±
p/
p
-1
0
1
23
4
Missing Mass (GeV/c
2
)
0
20
40
60
80
(a)
(b)
FIG. 2. Background-subtracted distributions for the 649
þ
c
c
X
events in the data: (a) the numbers of additional tracks,
identified
K
and identified
p=
p
; and (b) missing mass, with
imaginary masses given negative real values. Most events have
no identified
K
or
p=
p
and the corresponding zero-multiplicity
points are off the vertical scale in (a).
B. AUBERT
et al.
PHYSICAL REVIEW D
82,
091102(R) (2010)
RAPID COMMUNICATIONS
091102-6
Having established the presence of a category of events
containing a
c
baryon, a
c
baryon, no other (anti)baryons,
and several intermediate mesons, we study the number
and structure of these mesons. We exclude events with
an identified
p=
p
or a missing mass squared below
0
:
25 GeV
2
=c
4
. We estimate that the sample contains a
further
5
5
four-baryon events in which no
p=
p
is
detected; we take these to have the same distributions as
the events with an identified
p=
p
and subtract an appro-
priately scaled contribution to correct for them. In
this sample of
619
35
events, we study a number of
quantities including the
þ
c
=
c
and additional track mo-
menta, polar angles, rapidities, and opening angles. Their
inclusive distributions are quite similar to those in the
single-
þ
c
=
c
sample and similar to those in all hadronic
events. In particular, signing the thrust axis such that the
þ
c
rapidity is positive, the
þ
c
and
c
rapidities cluster
near
þ
1
:
1
and
1
:
1
units, respectively, with the additional
tracks of each charge distributed broadly and symmetri-
cally in between.
These 619 events contain only
45
10
identified
K
of
which about 20 are expected to be misidentified pions. The
events show no mass peak for
K
0
S
candidates reconstructed
from pairs of tracks not included in the
þ
c
or
c
(includ-
ing tracks that do not extrapolate within 5 mm of the beam
axis). The
K
:
ratio is thus much lower than the value 0.3
typical of hadronic events, which might be due to the
limited energy available and the fact that our
c
baryons
are nonstrange (the lighter
c
-
s
baryons do not decay into
þ
c
). The
þ
,
K
, and
K
þ
K
invariant mass dis-
tributions show no significant resonant structure; in par-
ticular, there is no evidence for the
0
. This implies a
vector:pseudoscalar meson ratio much lower than the value
near one typical of hadronic events, and suggests that most
tracks not from
c
baryon decays represent distinct inter-
mediate mesons.
The intermediate meson multiplicity is distributed
broadly. We verify that the contribution from decays of
heavier
c
baryons is not concentrated in any particular
region in Fig.
2(a)
, but due to the limited sample size we
do not attempt to correct the distribution. We observe an
average of 2.7 additional charged tracks per event.
Correcting for
c
baryon decays and tracking efficiency
gives
2
:
6
0
:
2
charged intermediate mesons per event,
where the uncertainty includes both statistical and system-
atic effects. The uncertainty is dominated by the track
acceptance in these events, evaluated with a set of simula-
tions based on the observed
and
K
distributions. On
average, the
c
and
c
baryons carry 75% of the event energy,
and the intermediate charged mesons account for about
65% of the remainder. This and the broad distribution of
missing masses in Fig.
2(b)
suggest the presence of addi-
tional neutral mesons. If intermediate
0
are produced at
half the
rate, as in typical hadronic events, the average
intermediate meson multiplicity would be
3
:
9
0
:
3
.
The new type of event observed in our data might be
explained by either primary diquark-antidiquark produc-
tion or the production of multiple intermediate mesons
between a baryon and antibaryon. Neither the
JETSET
nor
the
HERWIG
model produces events of the type observed,
although both might be adapted to include one or both
of the above processes.
JETSET
does produce
N
c
1
M
N
c
2
events, where
M
is a single meson, often a vector decaying
into two or three pions, but the event characteristics are far
from consistent with the data. Multiple intermediate meson
processes occur naturally in the
UCLA
model, which also
predicts an enhanced
þ
c
c
X
fraction due to events of this
type, with suppressions of kaons and vector mesons. The
version of the
UCLA
model used does not describe the
observed events in detail, having an average of only 1.8
intermediate mesons with a distribution peaked at low
values, but the results presented here should encourage
development of this and other relevant models.
In summary, we isolate a sample of
649
35
e
þ
e
!
c
c
events containing both a
þ
c
and a
c
with high
momentum in opposite hemispheres, and study these
events in detail. The number of events is estimated to be
about 4 times that expected if the leading
c
and
c
hadron
types are uncorrelated, confirming an observation by the
CLEO Collaboration. Taking advantage of the particle
identification capabilities of the
BABAR
detector and the
2.5
2.6
2.7
Λ
c
π
±
or
Λ
c
π
±
Mass (GeV/c
2
)
0
20
40
60
80
100
Entries / (4 MeV/c
2
)
Signal (background-subtracted)
Sideband (weighted)
2.6
2.7
2.8
2.9
Λ
c
π
+
π
−
or
Λ
c
π
+
π
−
Mass (GeV/c
2
)
0
10
20
30
Entries / (8 MeV/c
2
)
(a)
(b)
FIG. 3 (color online). Invariant mass distributions for
(a)
þ
c
and
c
and (b)
þ
c
=
c
þ
combinations.
The points with errors represent the background-subtracted
þ
c
c
X
events, and the weighted histograms are from the
single-
þ
c
=
c
sidebands.
CORRELATED LEADING BARYON-ANTIBARYON
...
PHYSICAL REVIEW D
82,
091102(R) (2010)
RAPID COMMUNICATIONS
091102-7
large data sample, we are further able to establish that
almost all of these events contain no additional baryons.
They do contain
2
:
6
0
:
2
additional charged intermediate
mesons on average, and events with zero additional mesons
do not contribute significantly. Our event sample exhibits
distributions of momentum, angle, rapidity, and
c
baryon
type similar to those in typical hadronic events, but con-
tains fewer kaons and vector mesons. This is direct evi-
dence for a new class of multihadron events, in which
baryon number is conserved by a leading baryon and
antibaryon, rather than locally along the hadronization
chain.
We are grateful for the excellent luminosity and machine
conditions provided by our PEP-II colleagues, and for the
substantial dedicated effort from the computing organiza-
tions that support
BABAR
. The collaborating institutions
wish to thank SLAC for its support and kind hospitality.
This work is supported by DOE and NSF (USA), NSERC
(Canada), CEA and CNRS-IN2P3 (France), BMBF and
DFG (Germany), INFN (Italy), FOM (The Netherlands),
NFR (Norway), MES (Russia), MEC (Spain), and STFC
(United Kingdom). Individuals have received support from
the Marie Curie EIF (European Union) and the A. P. Sloan
Foundation.
[1] See, e.g., P. Oddone, in Proceedings of the 12th SLAC
Summer Institute on Particle Physics: The Sixth Quark
(SSI 84), Stanford, California, 1984, edited by A. Mosher
(SLAC-R-281), p. 442.
[2] M. Althoff
et al.
(TASSO Collaboration),
Phys. Lett.
139B
, 126 (1984)
; H. Aihara
et al.
(TPC Collaboration),
Phys. Rev. Lett.
57
, 3140 (1986)
; D. Buskulic
et al.
(ALEPH Collaboration),
Z. Phys. C
64
, 361 (1994)
;P.
Abreu
et al.
(DELPHI Collaboration),
Phys. Lett. B
416
,
247 (1998)
; G. Abbiendi
et al.
(OPAL Collaboration),
Eur.
Phys. J. C
13
, 185 (2000)
.
[3] We use default parameter values for all models unless
otherwise noted.
[4] In
PYTHIA
v. 6.22, T. Sjostrand
et al.
,
Comput. Phys.
Commun.
135
, 238 (2001)
.
[5] B. Andersson, Cambridge Monogr. Part. Phys., Nucl.
Phys., Cosmol.
7
, 1 (1997).
[6] G. Marchesini
et al.
,
Comput. Phys. Commun.
67
, 465
(1992)
.
[7] S. Chun and C. Buchanan,
Phys. Rep.
292
, 239 (1998)
;S.
Abachi
et al.
,
Eur. Phys. J. C
49
, 569 (2007)
.
[8] P. Abreu
et al.
(DELPHI Collaboration),
Phys. Lett. B
318
,
249 (1993)
.
[9] P. D. Acton
et al.
(OPAL Collaboration),
Phys.
Lett. B
305
, 415 (1993)
; P. Abreu
et al.
(DELPHI
Collaboration),
Phys. Lett. B
490
, 61 (2000)
;G.
Abbiendi
et al.
(OPAL Collaboration),
Eur. Phys. J. C
64
, 609 (2009)
.
[10] A. Bornheim
et al.
(CLEO Collaboration),
Phys. Rev. D
63
, 112003 (2001)
.
[11] B. Aubert
et al.
(
BABAR
Collaboration),
Nucl. Instrum.
Methods Phys. Res., Sect. A
479
, 1 (2002)
.
[12] B. Aubert
et al.
(
B
A
B
AR
Collaboration),
Phys. Rev. D
75
,
012003 (2007)
.
[13] B. L. Hartfiel, Ph.D. thesis, University of California-Los
Angeles, 2005; SLAC-R-823 (unpublished).
[14] R. Seuster
et al.
(Belle Collaboration),
Phys. Rev. D
73
,
032002 (2006)
.
[15] B. Aubert
et al.
(
B
A
B
AR
Collaboration),
Phys. Rev. D
72
,
052006 (2005)
.
[16] G. Pakhlova
et al.
(Belle Collaboration),
Phys. Rev. Lett.
101
, 172001 (2008)
.
B. AUBERT
et al.
PHYSICAL REVIEW D
82,
091102(R) (2010)
RAPID COMMUNICATIONS
091102-8