Soft and hard interactions in pp̅ collisions at √s =1800 and 630 GeV

TLDR: In this article, the authors present a study of p (p) over bar collisions at roots=1800 and 630 GeV collected using a minimum bias trigger by the CDF experiment in which the data set is divided into two classes corresponding to "soft" and "hard" interactions.

Abstract: We present a study of p (p) over bar collisions at roots=1800 and 630 GeV collected using a minimum bias trigger by the CDF experiment in which the data set is divided into two classes corresponding to "soft" and "hard" interactions. For each subsample, the analysis includes measurements of the multiplicity, transverse momentum (p(T)) spectrum, and the average p(T) and event-by-event p(T) dispersion as a function of multiplicity. A comparison of results shows distinct differences in the behavior of the two samples as a function of the center of mass (c.m.) energy. We find evidence that the properties of the soft sample are invariant as a function of c.m. energy.

Figures

FIG. 16. Same as Fig. 14 for thehard samples.

FIG. 13. Same as Fig. 11 for thehard samples.

FIG. 5. Same as Fig. 4 for thesoftsamples. The continuous line in the ratio plot shows the upper limit of the systematic uncertainties. The lower limit overlaps the data points.

FIG. 6. Same as Fig. 4 for thehardsamples. The continuous line in the ratio plot shows the upper limit of the systematic uncertainties. The lower limit overlaps the data points.

FIG. 11. Mean transverse momentum vs multiplicity for the full MB samples at 1800 and 630 GeV. On the bottom the ratio of the two curves is shown.

FIG. 12. Same as Fig. 11 for thesoft samples.

Full text

Article
Reference
Soft and hard interactions in pp collisions at s√=1800 and 630 GeV
CDF Collaboration
CLARK, Allan Geoffrey (Collab.), et al.
Abstract
We present a study of pp collisions at s√=1800 and 630 GeV collected using a minimum bias
trigger by the CDF experiment in which the data set is divided into two classes corresponding
to “soft” and “hard” interactions. For each subsample, the analysis includes measurements of
the multiplicity, transverse momentum (pT) spectrum, and the average pT and event-by-event
pT dispersion as a function of multiplicity. A comparison of results shows distinct differences
in the behavior of the two samples as a function of the center of mass (c.m.) energy. We find
evidence that the properties of the soft sample are invariant as a function of c.m. energy.
CDF Collaboration, CLARK, Allan Geoffrey (Collab.), et al. Soft and hard interactions in pp
collisions at s√=1800 and 630 GeV. Physical review. D. Particles, fields, gravitation, and
cosmology, 2002, vol. 65, no. 07
DOI : 10.1103/PhysRevD.65.072005
Available at:
http://archive-ouverte.unige.ch/unige:38033
Disclaimer: layout of this document may differ from the published version.
1 / 1

Soft and hard interactions in pp
¯
collisions at
冑
sÄ 1800 and 630 GeV
D. Acosta,
12
T. Affolder,
23
H. Akimoto,
45
M. G. Albrow,
11
P. Amaral,
8
D. Ambrose,
32
D. Amidei,
25
K. Anikeev,
24
J. Antos,
1
G. Apollinari,
11
T. Arisawa,
45
A. Artikov,
9
T. Asakawa,
43
W. Ashmanskas,
8
F. Azfar,
30
P. Azzi-Bacchetta,
31
N. Bacchetta,
31
H. Bachacou,
23
S. Bailey,
16
P. de Barbaro,
36
A. Barbaro-Galtieri,
23
V. E. Barnes,
35
B. A. Barnett,
19
S. Baroiant,
5
M. Barone,
13
G. Bauer,
24
F. Bedeschi,
33
S. Belforte,
42
W. H. Bell,
15
G. Bellettini,
33
J. Bellinger,
46
D. Benjamin,
10
J. Bensinger,
4
A. Beretvas,
11
J. P. Berge,
11
J. Berryhill,
8
A. Bhatti,
37
M. Binkley,
11
D. Bisello,
31
M. Bishai,
11
R. E. Blair,
2
C. Blocker,
4
K. Bloom,
25
B. Blumenfeld,
19
S. R. Blusk,
36
A. Bocci,
37
A. Bodek,
36
G. Bolla,
35
Y. Bonushkin,
6
D. Bortoletto,
35
J. Boudreau,
34
A. Brandl,
27
S. van den Brink,
19
C. Bromberg,
26
M. Brozovic,
10
E. Brubaker,
23
N. Bruner,
27
E. Buckley-Geer,
11
J. Budagov,
9
H. S. Budd,
36
K. Burkett,
16
G. Busetto,
31
A. Byon-Wagner,
11
K. L. Byrum,
2
S. Cabrera,
10
P. Calafiura,
23
M. Campbell,
25
W. Carithers,
23
J. Carlson,
25
D. Carlsmith,
46
W. Caskey,
5
A. Castro,
3
D. Cauz,
42
A. Cerri,
33
A. W. Chan,
1
P. S. Chang,
1
P. T. Chang,
1
J. Chapman,
25
C. Chen,
32
Y. C. Chen,
1
M.-T. Cheng,
1
M. Chertok,
5
G. Chiarelli,
33
I. Chirikov-Zorin,
9
G. Chlachidze,
9
F. Chlebana,
11
L. Christofek,
18
M. L. Chu,
1
J. Y. Chung,
28
Y. S. Chung,
36
C. I. Ciobanu,
28
A. G. Clark,
14
A. P. Colijn,
11
A. Connolly,
23
M. Convery,
37
J. Conway,
38
M. Cordelli,
13
J. Cranshaw,
40
R. Culbertson,
11
D. Dagenhart,
44
S. D’Auria,
15
F. DeJongh,
11
S. Dell’Agnello,
13
M. Dell’Orso,
33
S. Demers,
36
L. Demortier,
37
M. Deninno,
3
P. F. Derwent,
11
T. Devlin,
38
J. R. Dittmann,
11
A. Dominguez,
23
S. Donati,
33
J. Done,
39
M. D’Onofrio,
33
T. Dorigo,
16
N. Eddy,
18
K. Einsweiler,
23
J. E. Elias,
11
E. Engels, Jr.,
34
R. Erbacher,
11
D. Errede,
18
S. Errede,
18
Q. Fan,
36
H.-C. Fang,
23
R. G. Feild,
47
J. P. Fernandez,
11
C. Ferretti,
33
R. D. Field,
12
I. Fiori,
3
B. Flaugher,
11
G. W. Foster,
11
M. Franklin,
16
J. Freeman,
11
J. Friedman,
24
Y. Fukui,
22
I. Furic,
24
S. Galeotti,
33
A. Gallas,
16,
*
M. Gallinaro,
37
T. Gao,
32
M. Garcia-Sciveres,
23
A. F. Garfinkel,
35
P. Gatti,
31
C. Gay,
47
D. W. Gerdes,
25
P. Giannetti,
33
P. Giromini,
13
V. Glagolev,
9
D. Glenzinski,
11
M. Gold,
27
J. Goldstein,
11
I. Gorelov,
27
A. T. Goshaw,
10
Y. Gotra,
34
K. Goulianos,
37
C. Green,
35
G. Grim,
5
P. Gris,
11
C. Grosso-Pilcher,
8
M. Guenther,
35
G. Guillian,
25
J. Guimaraes da Costa,
16
R. M. Haas,
12
C. Haber,
23
S. R. Hahn,
11
C. Hall,
16
T. Handa,
17
R. Handler,
46
W. Hao,
40
F. Happacher,
13
K. Hara,
43
A. D. Hardman,
35
R. M. Harris,
11
F. Hartmann,
20
K. Hatakeyama,
37
J. Hauser,
6
J. Heinrich,
32
A. Heiss,
20
M. Herndon,
19
C. Hill,
5
A. Hocker,
36
K. D. Hoffman,
35
R. Hollebeek,
32
L. Holloway,
18
B. T. Huffman,
30
R. Hughes,
28
J. Huston,
26
J. Huth,
16
H. Ikeda,
43
J. Incandela,
11,†
G. Introzzi,
33
A. Ivanov,
36
J. Iwai,
45
Y. Iwata,
17
E. James,
25
M. Jones,
32
U. Joshi,
11
H. Kambara,
14
T. Kamon,
39
T. Kaneko,
43
M. Karagoz Unel,
39,
*
K. Karr,
44
S. Kartal,
11
H. Kasha,
47
Y. Kato,
29
T. A. Keaffaber,
35
K. Kelley,
24
M. Kelly,
25
D. Khazins,
10
T. Kikuchi,
43
B. Kilminster,
36
B. J. Kim,
21
D. H. Kim,
21
H. S. Kim,
18
M. J. Kim,
21
S. B. Kim,
21
S. H. Kim,
43
Y. K. Kim,
23
M. Kirby,
10
M. Kirk,
4
L. Kirsch,
4
S. Klimenko,
12
P. Koehn,
28
K. Kondo,
45
J. Konigsberg,
12
A. Korn,
24
A. Korytov,
12
E. Kovacs,
2
J. Kroll,
32
M. Kruse,
10
S. E. Kuhlmann,
2
K. Kurino,
17
T. Kuwabara,
43
A. T. Laasanen,
35
N. Lai,
8
S. Lami,
37
S. Lammel,
11
J. Lancaster,
10
M. Lancaster,
23
R. Lander,
5
A. Lath,
38
G. Latino,
33
T. LeCompte,
2
K. Lee,
40
S. Leone,
33
J. D. Lewis,
11
M. Lindgren,
6
T. M. Liss,
18
J. B. Liu,
36
Y. C. Liu,
1
D. O. Litvintsev,
11
O. Lobban,
40
N. S. Lockyer,
32
J. Loken,
30
M. Loreti,
31
D. Lucchesi,
31
P. Lukens,
11
S. Lusin,
46
L. Lyons,
30
J. Lys,
23
R. Madrak,
16
K. Maeshima,
11
P. Maksimovic,
16
L. Malferrari,
3
M. Mangano,
33
M. Mariotti,
31
G. Martignon,
31
A. Martin,
47
J. A. J. Matthews,
27
P. Mazzanti,
3
K. S. McFarland,
36
P. McIntyre,
39
M. Menguzzato,
31
A. Menzione,
33
P. Merkel,
11
C. Mesropian,
37
A. Meyer,
11
T. Miao,
11
R. Miller,
26
J. S. Miller,
25
H. Minato,
43
S. Miscetti,
13
M. Mishina,
22
G. Mitselmakher,
12
Y. Miyazaki,
29
N. Moggi,
3
E. Moore,
27
R. Moore,
25
Y. Morita,
22
T. Moulik,
35
M. Mulhearn,
24
A. Mukherjee,
11
T. Muller,
20
A. Munar,
33
P. Murat,
11
S. Murgia,
26
J. Nachtman,
6
V. Nagaslaev,
40
S. Nahn,
47
H. Nakada,
43
I. Nakano,
17
C. Nelson,
11
T. Nelson,
11
C. Neu,
28
D. Neuberger,
20
C. Newman-Holmes,
11
C.-Y. P. Ngan,
24
H. Niu,
4
L. Nodulman,
2
A. Nomerotski,
12
S. H. Oh,
10
Y. D. Oh,
21
T. Ohmoto,
17
T. Ohsugi,
17
R. Oishi,
43
T. Okusawa,
29
J. Olsen,
46
W. Orejudos,
23
C. Pagliarone,
33
F. Palmonari,
33
R. Paoletti,
33
V. Papadimitriou,
40
D. Partos,
4
J. Patrick,
11
G. Pauletta,
42
M. Paulini,
23,‡
C. Paus,
24
D. Pellett,
5
L. Pescara,
31
T. J. Phillips,
10
G. Piacentino,
33
K. T. Pitts,
18
A. Pompos,
35
L. Pondrom,
46
G. Pope,
34
F. Prokoshin,
9
J. Proudfoot,
2
F. Ptohos,
13
O. Pukhov,
9
G. Punzi,
33
A. Rakitine,
24
F. Ratnikov,
38
D. Reher,
23
A. Reichold,
30
P. Renton,
30
A. Ribon,
31
W. Riegler,
16
F. Rimondi,
3
L. Ristori,
33
M. Riveline,
41
W. J. Robertson,
10
T. Rodrigo,
7
S. Rolli,
44
L. Rosenson,
24
R. Roser,
11
R. Rossin,
31
C. Rott,
35
A. Roy,
35
A. Ruiz,
7
A. Safonov,
5
R. St. Denis,
15
W. K. Sakumoto,
36
D. Saltzberg,
6
C. Sanchez,
28
A. Sansoni,
13
L. Santi,
42
H. Sato,
43
P. Savard,
41
A. Savoy-Navarro,
11
P. Schlabach,
11
E. E. Schmidt,
11
M. P. Schmidt,
47
M. Schmitt,
16,
*
L. Scodellaro,
31
A. Scott,
6
A. Scribano,
33
A. Sedov,
35
S. Segler,
11
S. Seidel,
27
Y. Seiya,
43
A. Semenov,
9
F. Semeria,
3
T. Shah,
24
M. D. Shapiro,
23
P. F. Shepard,
34
T. Shibayama,
43
M. Shimojima,
43
M. Shochet,
8
A. Sidoti,
31
J. Siegrist,
23
A. Sill,
40
P. Sinervo,
41
P. Singh,
18
A. J. Slaughter,
47
K. Sliwa,
44
C. Smith,
19
F. D. Snider,
11
A. Solodsky,
37
J. Spalding,
11
T. Speer,
14
P. Sphicas,
24
F. Spinella,
33
M. Spiropulu,
8
L. Spiegel,
11
J. Steele,
46
A. Stefanini,
33
J. Strologas,
18
F. Strumia,
14
D. Stuart,
11
K. Sumorok,
24
T. Suzuki,
43
T. Takano,
29
R. Takashima,
17
K. Takikawa,
43
P. Tamburello,
10
M. Tanaka,
43
B. Tannenbaum,
6
M. Tecchio,
25
R. J. Tesarek,
11
P. K. Teng,
1
K. Terashi,
37
S. Tether,
24
A. S. Thompson,
15
E. Thomson,
28
R. Thurman-Keup,
2
P. Tipton,
36
S. Tkaczyk,
11
D. Toback,
39
K. Tollefson,
36
A. Tollestrup,
11
D. Tonelli,
33
H. Toyoda,
29
W. Trischuk,
41
J. F. de Troconiz,
16
J. Tseng,
24
D. Tsybychev,
11
N. Turini,
33
F. Ukegawa,
43
T. Vaiciulis,
36
J. Valls,
38
S. Vejcik III,
11
G. Velev,
11
G. Veramendi,
23
R. Vidal,
11
I. Vila,
7
R. Vilar,
7
I. Volobouev,
23
M. von der Mey,
6
D. Vucinic,
24
R. G. Wagner,
2
R. L. Wagner,
11
N. B. Wallace,
38
Z. Wan,
38
C. Wang,
10
M. J. Wang,
1
S. M. Wang,
12
B. Ward,
15
S. Waschke,
15
T. Watanabe,
43
D. Waters,
30
T. Watts,
38
R. Webb,
39
H. Wenzel,
20
W. C. Wester III,
11
A. B. Wicklund,
2
E. Wicklund,
11
T. Wilkes,
5
H. H. Williams,
32
P. Wilson,
11
PHYSICAL REVIEW D, VOLUME 65, 072005
0556-2821/2002/65共7兲/072005共12兲/$20.00 ©2002 The American Physical Society65 072005-1

B. L. Winer,
28
D. Winn,
25
S. Wolbers,
11
D. Wolinski,
25
J. Wolinski,
26
S. Wolinski,
25
S. Worm,
38
X. Wu,
14
J. Wyss,
33
W. Yao,
23
G. P. Yeh,
11
P. Yeh,
1
J. Yoh,
11
C. Yosef,
26
T. Yoshida,
29
I. Yu,
21
S. Yu,
32
Z. Yu,
47
A. Zanetti,
42
F. Zetti,
23
and S. Zucchelli
3
共CDF Collaboration兲
1
Institute of Physics, Academia Sinica, Taipei, Taiwan 11529, Republic of China
2
Argonne National Laboratory, Argonne, Illinois 60439
3
Istituto Nazionale di Fisica Nucleare, University of Bologna, I-40127 Bologna, Italy
4
Brandeis University, Waltham, Massachusetts 02254
5
University of California at Davis, Davis, California 95616
6
University of California at Los Angeles, Los Angeles, California 90024
7
Instituto de Fisica de Cantabria, CSIC–University of Cantabria, 39005 Santander, Spain
8
Enrico Fermi Institute, University of Chicago, Chicago, Illinois 60637
9
Joint Institute for Nuclear Research, RU-141980 Dubna, Russia
10
Duke University, Durham, North Carolina 27708
11
Fermi National Accelerator Laboratory, Batavia, Illinois 60510
12
University of Florida, Gainesville, Florida 32611
13
Laboratori Nazionali di Frascati, Istituto Nazionale di Fisica Nucleare, I-00044 Frascati, Italy
14
University of Geneva, CH-1211 Geneva 4, Switzerland
15
Glasgow University, Glasgow G12 8QQ, United Kingdom
16
Harvard University, Cambridge, Massachusetts 02138
17
Hiroshima University, Higashi-Hiroshima 724, Japan
18
University of Illinois, Urbana, Illinois 61801
19
The Johns Hopkins University, Baltimore, Maryland 21218
20
Institut fu
¨
r Experimentelle Kernphysik, Universita
¨
t Karlsruhe, 76128 Karlsruhe, Germany
21
Center for High Energy Physics, Kyungpook National University, Taegu 702-701, Korea, Seoul National University,
Seoul 151-742, Korea,
and SungKyunKwan University, Suwon 440-746, Korea
22
High Energy Accelerator Research Organization (KEK), Tsukuba, Ibaraki 305, Japan
23
Ernest Orlando Lawrence Berkeley National Laboratory, Berkeley, California 94720
24
Massachusetts Institute of Technology, Cambridge, Massachusetts 02139
25
University of Michigan, Ann Arbor, Michigan 48109
26
Michigan State University, East Lansing, Michigan 48824
27
University of New Mexico, Albuquerque, New Mexico 87131
28
The Ohio State University, Columbus, Ohio 43210
29
Osaka City University, Osaka 588, Japan
30
University of Oxford, Oxford OX1 3RH, United Kingdom
31
Universita di Padova, Istituto Nazionale di Fisica Nucleare, Sezione di Padova, I-35131 Padova, Italy
32
University of Pennsylvania, Philadelphia, Pennsylvania 19104
33
Istituto Nazionale di Fisica Nucleare, University and Scuola Normale Superiore of Pisa, I-56100 Pisa, Italy
34
University of Pittsburgh, Pittsburgh, Pennsylvania 15260
35
Purdue University, West Lafayette, Indiana 47907
36
University of Rochester, Rochester, New York 14627
37
Rockefeller University, New York, New York 10021
38
Rutgers University, Piscataway, New Jersey 08855
39
Texas A&M University, College Station, Texas 77843
40
Texas Tech University, Lubbock, Texas 79409
41
Institute of Particle Physics, University of Toronto, Toronto, Canada M5S 1A7
42
Istituto Nazionale di Fisica Nucleare, University of Trieste/Udine, Italy
43
University of Tsukuba, Tsukuba, Ibaraki 305, Japan
44
Tufts University, Medford, Massachusetts 02155
45
Waseda University, Tokyo 169, Japan
46
University of Wisconsin, Madison, Wisconsin 53706
47
Yale University, New Haven, Connecticut 06520
共Received 26 October 2001; published 5 April 2002兲
D. ACOSTA et al. PHYSICAL REVIEW D 65 072005
072005-2

We present a study of pp
¯
collisions at
冑
s⫽1800 and 630 GeV collected using a minimum bias trigger by the
CDF experiment in which the data set is divided into two classes corresponding to ‘‘soft’’ and ‘‘hard’’ inter-
actions. For each subsample, the analysis includes measurements of the multiplicity, transverse momentum
(p
T
) spectrum, and the average p
T
and event-by-event p
T
dispersion as a function of multiplicity. A compari-
son of results shows distinct differences in the behavior of the two samples as a function of the center of mass
共c.m.兲 energy. We find evidence that the properties of the soft sample are invariant as a function of c.m. energy.
DOI: 10.1103/PhysRevD.65.072005 PACS number共s兲: 13.85.Hd, 12.38.Mh, 13.87.Fh
I. INTRODUCTION
Hadron interactions are often classified as either ‘‘hard’’
or ‘‘soft’’ 关1,2兴. Although there is no formal definition for
either, the term ‘‘hard interactions’’ is typically understood to
mean high transverse energy (E
T
) parton-parton interactions
associated with such phenomena as high E
T
jets, while the
soft component consists of everything else. Whereas pertur-
bative QCD provides a reasonable description of high E
T
jet
production, there is no equivalent theory for the low E
T
mul-
tiparticle production processes that dominate the inelastic
cross section. Some QCD inspired models 关2兴 attempt to de-
scribe these processes by the superposition of many parton
interactions extrapolated to very low momentum transfers. It
is not known, however, if such a superposition or some col-
lective multiparton process is at work.
The study of low-E
T
interactions usually involves collect-
ing data using minimum bias 共MB兲 triggers, which, ideally,
sample events in fixed proportion to the production rate—in
other words, in their ‘‘natural’’ distribution. Lacking a com-
prehensive description of the microscopic processes 关3兴 in-
volved in low-E
T
interactions, our knowledge of the details
of low transverse momentum (p
T
) particle production rests
largely upon empirical connections between phenomenologi-
cal models and data collected with MB triggers at many
center-of-mass 共c.m.兲 energies. Such comparisons are further
complicated by the difficulty in isolating events of a purely
‘‘soft’’ or purely ‘‘hard’’ nature.
This paper adopts a novel approach in addressing this
issue using samples of pp
¯
collisions at
冑
s⫽1800 and 630
GeV collected with a MB trigger. The analysis first divides
the full minumum bias samples into two subsamples, one
highly enriched in soft interactions, the other relatively de-
pleted of soft interactions. We then compare inclusive distri-
butions and final state correlations between the subsamples
and as a function of c.m. energy in order to gain insight into
the mechanisms of particle production in soft interactions.
The results in the isolated soft sample exhibit some interest-
ing properties, in particular an unpredicted invariance with
c.m. energy.
II. DATA SET AND EVENT SELECTION
Data samples have been collected with the Collider De-
tector at Fermilab 共CDF兲 experiment at the Fermilab Teva-
tron Collider. The CDF apparatus has been described else-
where 关4兴; here only the parts of the detector utilized for the
present analysis are discussed.
Data at 1800 GeV were collected with a minimum bias
trigger during runs 1A and 1B, and at 1800 and 630 GeV
during run 1C. This trigger requires coincident hits in scin-
tillator counters located at 5.8 m on either side of the nomi-
nal interaction point and covering the pseudo-rapidity
关
␩
⫽
⫺ log„tan(
␪
/2)… where
␪
⫽ angle with respect to the proton
direction兴 interval 3.2⬍
兩
␩
兩
⬍ 5.9, in coincidence with a
beam-crossing signal.
The analysis uses charged tracks reconstructed within the
central tracking chamber 共CTC兲. The CTC is a cylindrical
drift chamber covering a
␩
interval of about three units with
full efficiency for
兩
␩
兩
⭐1 and p
T
⭓0.4 GeV/c.
The inner radius of the CTC is 31.0 cm and the outer
radius is 132.5 cm. The full CTC volume is contained in the
superconducting solenoidal magnet which operates at 1.4 T
关5兴. The CTC has 84 sampling wire layers, organized in 5
axial and 4 stereo ‘‘superlayers’’ 关6兴. Axial superlayers have
12 radially separated layers of sense wires, parallel to the z
axis 共the beam axis兲, that measure the r-
␾
position of a
track. Stereo superlayers have 6 sense wire layers, with an
⬃3° stereo angle, that measure a combination of r-
␾
and z
information. The stereo angle direction alternates at each ste-
reo superlayer. Axial and stereo data are combined to form a
three-dimensional track.
The spatial resolution of each point measurement in the
CTC is less than 200
␮
m; the transverse momentum reso-
lution, including multiple scattering effects, is
␴
p
T
/p
T
2
⭐0.003 (GeV/c).
Inside the CTC inner radius, a set of time projection
chambers 共VTX兲关7兴 provides r-z tracking information out to
a radius of 22 cm for
兩
␩
兩
⬍ 3.25. The VTX is used in this
analysis to find the z position of event vertices, defined as a
set of tracks with p
T
greater than about 50 MeV/c that con-
verge to the same point along the z axis. Reconstructed ver-
tices are classified as either ‘‘primary’’ or ‘‘secondary’’ based
upon several parameters: the number of converging track
segments 共with a minimum of four within
兩
␩
兩
⬍ 3), the total
number of hits used to form a segment, forward-backward
symmetry and vertex isolation. Isolated, higher multiplicity
vertices with highly symmetric topologies are typically clas-
sified as primary; lower mulitiplicity, highly asymmetric ver-
tices or those with few hits in the reconstructed tracks are
typically classified as secondary. Systematic uncertainties in-
troduced by the vertex classification scheme are discussed in
Sec. VI.
The transverse energy flux was measured by a calorimeter
*
Now at Northwestern University, Evanston, IL 60208.
†
Now at University of California, Santa Barbara, CA 93106.
‡
Now at Carnegie Mellon University, Pittsburgh, PA 15213.
SOFT AND HARD INTERACTIONS IN pp
¯
COLLISIONS...
PHYSICAL REVIEW D 65 072005
072005-3

system 关8兴 covering from ⫺ 4.2 to 4.2 in
␩
. The system
consists of three subsystems, each with separate electromag-
netic and hadronic compartments: the central calorimeter,
covering the range
兩
␩
兩
⬍ 1; the end-plug, covering 1⬍
兩
␩
兩
⬍ 2.4; and the forward calorimeter, covering 2.2⬍
兩
␩
兩
⬍ 4.2.
Energy measurements are made within projective ‘‘towers’’
that span 0.1 units of
␩
and 15° in aximuth (
␾
) within the
central calorimeter, and 5° in the end-plug and forward calo-
rimeters.
The 1800 GeV data sample consists of subsamples col-
lected during three different time periods. Approximately
1 700000 events were collected in run 1A at an average lu-
minosity of 3.3⫻ 10
30
s
⫺ 1
cm
⫺ 2
, 1 500 000 in run 1B at an
average luminosity of 9.1⫻ 10
30
s
⫺ 1
cm
⫺ 2
and 106000 in
run 1C at an average luminosity of 9.0⫻ 10
30
s
⫺ 1
cm
⫺ 2
.
The 630 GeV data set consists of about 2 600 000 events re-
corded during run 1C at an average luminosity of
1.3⫻ 10
30
s
⫺ 1
cm
⫺ 2
.
Additional event selection conducted offline removed the
following events: 共i兲 events identified as containing cosmic-
ray particles as determined by time-of-flight measurements
using scintillator counters in the central calorimeter; 共ii兲
events with no reconstructed tracks; 共iii兲 events exhibiting
symptoms of known calorimeter problems; 共iv兲 events with
at least one charged particle reconstructed in the CTC to
FIG. 1. Multiplicity distributions for the full MB samples at
1800 and 630 GeV; data are plotted in KNO variables. In the bot-
tom panel the ratio of the two above distributions is shown. The two
continuous lines delimit the band of all systematic uncertainties 共see
Sec. VI of text兲.
FIG. 2. Same as Fig. 1 for the soft samples.
FIG. 3. Same as Fig. 1 for the hard samples.
FIG. 4. Transverse momentum distributions for the full MB
samples at 1800 and 630 GeV. In the bottom panel the ratio of the
two distributions is shown. The two continuous lines delimit the
band of all systematic uncertainties 共see Sec. VI of text兲.N
track
refers to the number of charged tracks in a unit
␩
interval.
D. ACOSTA et al. PHYSICAL REVIEW D 65 072005
072005-4

Frequently asked questions

Q1. What are the contributions in this paper?

The authors present a study of pp collisions at s√=1800 and 630 GeV collected using a minimum bias trigger by the CDF experiment in which the data set is divided into two classes corresponding to “ soft ” and “ hard ” interactions. 

Q2. What is the VTX used in this analysis?

The VTX is used in this analysis to find the z position of event vertices, defined as a set of tracks with pT greater than about 50 MeV/c that converge to the same point along the z axis. 

Q3. How many events are in the minimum bias sample?

After all event selection cuts, 2 079 558 events remain in the full minimum bias sample at As51800 GeV ~runs 1A 11B11C), and 1 963 157 in that at As5630 GeV ~run 1C!. 

Q4. What is the r-f position of a track?

Axial superlayers have 12 radially separated layers of sense wires, parallel to the z axis ~the beam axis!, that measure the r-f position of a track. 

Q5. What is the dispersion expected to decrease with increasing multiplicity?

The dispersion is expected to decrease with increasing multiplicity and to converge to zero when m→` if only statistical fluctuations are present. 

Q6. What can be the effect of misclassification or identification of vertices?

Misclassification or identification of vertices can strongly influence the pT and multiplicity distributions, particularly the latter. 

Q7. What is the effect of the correction for gamma conversions, secondary particle interactions and?

The systematic uncertainty due to the correction for gamma conversions, secondary particle interactions and particle decays is estimated to be about 1%, almost independent of multiplicity and pT . 

Q8. What is the effect of particle correlations in the multibody final state?

Large non-statistical fluctuations of the mean event pT are a consequence of particle correlations in the multibody final state @26#. 

Q9. What is the effect of the inverse multiplicity on the mean event pT?

the mean pT increases at low multiplicity even in the soft sample, which should be highly depleted in high ET events. 

Q10. What is the effect of using widely different efficiency corrections on the distributions?

Theimpact of using widely different efficiency corrections on the multiplicity and pT distributions is—at most—as large as the statistical uncertainty. 

Q11. What is the dispersion versus inverse multiplicity for the soft and hard samples?

The dispersion versus inverse multiplicity for the soft and hard samples, shown in Figs. 15 and 16, confirms that this effect is related to the contribution of jet production which, as discussed in @27#, increases eventby-event fluctuations. 

Q12. What is the ratio of the dispersion in the soft sample at the two energies?

The ratio of the dispersion in the soft sample at the two energies is flat as a function of multiplicity, a feature not exhibited by the hard sample. 

Q13. How much is the deviation in the ratio of pT distributions at the two energies?

The deviation in the ratio of pT distributions at the two energies is almost constant at about 10% up to a pT around 11 GeV/c , increasing to 15% as pT increases. 

Q14. How much slope of the inclusive pT distribution increases for minimum bias samples?

It is known that for minimum bias samples, the slope of the inclusive pT distribution increases steadily by some power of log s up to Tevatron energies @10,15#.