Article
Reference
Central Pseudorapidity Gaps in Events with a Leading Antiproton at the Fermilab Tevatron pp Collider
CDF Collaboration
CLARK, Allan Geoffrey (Collab.), et al.
Abstract
We report a measurement of the fraction of events with a large pseudorapidity gap Δη within the pseudorapidity region available to the proton dissociation products X in p¯+p→p¯+X. For a final state p¯ of fractional momentum loss ξp¯ and 4-momentum transfer squared tp¯ within 0.06
CDF Collaboration, CLARK, Allan Geoffrey (Collab.), et al . Central Pseudorapidity Gaps in Events with a Leading Antiproton at the Fermilab Tevatron pp Collider. Physical Review Letters , 2003, vol. 91, no. 01, p. 011802
DOI : 10.1103/PhysRevLett.91.011802
Available at:
http://archive-ouverte.unige.ch/unige:38053
Disclaimer: layout of this document may differ from the published version.
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Central Pseudorapidity Gaps in Events with a Leading Antiproton at the Fermilab Tevatron pp Collider
D. Acosta,14T. Affolder,25H. Akimoto,50M. G. Albrow,13D. Ambrose,37D. Amidei,28K. Anikeev,27J. Antos,1 G. Apollinari,13T. Arisawa,50A. Artikov,11T. Asakawa,48W. Ashmanskas,10F. Azfar,35P. Azzi-Bacchetta,36 N. Bacchetta,36H. Bachacou,25W. Badgett,13S. Bailey,18P. de Barbaro,41A. Barbaro-Galtieri,25V. E. Barnes,40
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011802-1 0031-9007=03=91(1)=011802(6)$20.00 2003 The American Physical Society 011802-1
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A. Zanetti,47F. Zetti,25and S. Zucchelli31
(CDF Collaboration)
1Institute of Physics, Academia Sinica, Taipei, Taiwan 11529, Republic of China
2Argonne National Laboratory, Argonne, Illinois 60439, USA
3Istituto Nazionale di Fisica Nucleare, University of Bologna, I-40127 Bologna, Italy
4Brandeis University, Waltham, Massachusetts 02254, USA
5University of California at Davis, Davis, California 95616, USA
6University of California at Los Angeles, Los Angeles, California 90024, USA
7University of California at Santa Barbara, Santa Barbara, California 93106, USA
8Instituto de Fisica de Cantabria, CSIC-University of Cantabria, 39005 Santander, Spain
9Carnegie Mellon University, Pittsburgh, Pennsylvania 15213, USA
10Enrico Fermi Institute, University of Chicago, Chicago, Illinois 60637, USA
11Joint Institute for Nuclear Research, RU-141980 Dubna, Russia
12Duke University, Durham, North Carolina 27708, USA
13Fermi National Accelerator Laboratory, Batavia, Illinois 60510, USA
14University of Florida, Gainesville, Florida 32611, USA
15Laboratori Nazionali di Frascati, Istituto Nazionale di Fisica Nucleare, I-00044 Frascati, Italy
16University of Geneva, CH-1211 Geneva 4, Switzerland
17Glasgow University, Glasgow G12 8QQ, United Kingdom
18Harvard University, Cambridge, Massachusetts 02138, USA
19Hiroshima University, Higashi-Hiroshima 724, Japan
20University of Illinois, Urbana, Illinois 61801, USA
21The Johns Hopkins University, Baltimore, Maryland 21218, USA
22Institut fu¨r Experimentelle Kernphysik, Universita¨t Karlsruhe, 76128 Karlsruhe, Germany
23High Energy Accelerator Research Organization (KEK), Tsukuba, Ibaraki 305, Japan
24Center for High Energy Physics, Kyungpook National University, Taegu 702-701, Korea; Seoul National University, Seoul 151-742, Korea; and Sung Kyun Kwan University, Suwon 440-746, Korea
25Ernest Orlando Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA
26University College London, London WC1E 6BT, United Kingdom
27Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA
28University of Michigan, Ann Arbor, Michigan 48109, USA
29Michigan State University, East Lansing, Michigan 48824, USA
30Institution for Theoretical and Experimental Physics, ITEP, Moscow 117259, Russia
31University of New Mexico, Albuquerque, New Mexico 87131, USA
32Northwestern University, Evanston, Illinois 60208, USA
33The Ohio State University, Columbus, Ohio 43210, USA
011802-2 011802-2
34Osaka City University, Osaka 588, Japan
35University of Oxford, Oxford OX1 3RH, United Kingdom
36Universita di Padova, Istituto Nazionale di Fisica Nucleare, Sezione di Padova, I-35131 Padova, Italy
37University of Pennsylvania, Philadelphia, Pennsylvania 19104, USA
38Istituto Nazionale di Fisica Nucleare, University and Scuola Normale Superiore of Pisa, I-56100 Pisa, Italy
39University of Pittsburgh, Pittsburgh, Pennsylvania 15260, USA
40Purdue University, West Lafayette, Indiana 47907, USA
41University of Rochester, Rochester, New York 14627, USA
42Rockefeller University, New York, New York 10021, USA
43Instituto Nazionale de Fisica Nucleare, Sezione di Roma, University di Roma I, ‘‘La Sapienza,’’ I-00185 Roma, Italy
44Rutgers University, Piscataway, New Jersey 08855, USA
45Texas A&M University, College Station, Texas 77843, USA
46Texas Tech University, Lubbock, Texas 79409, USA
47Istituto Nazionale di Fisica Nucleare, University of Trieste, Udine, Italy
48University of Tsukuba, Tsukuba, Ibaraki 305, Japan
49Tufts University, Medford, Massachusetts 02155, USA
50Waseda University, Tokyo 169, Japan
51University of Wisconsin, Madison, Wisconsin 53706, USA
52Yale University, New Haven, Connecticut 06520, USA (Received 15 February 2003; published 3 July 2003)
We report a measurement of the fraction of events with a large pseudorapidity gapwithin the pseudorapidity region available to the proton dissociation productsX inppp!ppX. For a final stateppof fractional momentum lossppand 4-momentum transfer squaredtpp within0:06< pp <0:09 andjtppj<1:0[0.2]GeV2 atps
1800[630] GeV, the fraction of events with >3 is found to be 0:2460:001stat 0:042syst[0:1840:001stat 0:043syst]. Our results are compared with gap fractions measured in minimum biasppp collisions and with theoretical expectations.
DOI: 10.1103/PhysRevLett.91.011802 PACS numbers: 13.85.Ni
In a previous Letter [1], we reported a measurement of the fraction of events with a central pseudorapidity gap [2] produced in ppp collisions at ps
1800 and 630 GeV. Here, we present results from a similar mea- surement performed in a subsample of ppp events con- taining a leading (high longitudinal momentum) antiproton (Fig. 1). Large pseudorapidity gaps are pre- sumed to be due to Pomeron (P) exchange and are the signature for diffraction [3]. The process with a leading beam particle in the final state, which is kinematically associated with an adjacent pseudorapidity gap, is known as single diffraction dissociation (SD), while that with a central gap as double diffraction dissociation (DD). The process in Fig. 1 is a combination ofpp-p SD andP-pDD and will be referred to in this Letter as SDD.
Low transverse momentum (pT) processes have tradi- tionally been treated theoretically in the framework of Regge theory [3]. The introduction of a linear Pomeron (P) trajectory,t 0 0t, with intercept 0 1 >1, enables the theory to correctly predict certain salient features of the high energy behavior of hadronic interactions, such as the rise of the total cross section and the shrinking of the forward elastic scattering peak with increasing energy. However, the success of the theory in describing diffraction has been limited. While the shape of the SD and DD distributions as a function ofare correctly described, the normalization was found to be suppressed relative to the theoretical predictions by about an order of magnitude as the energy increases from
ps 20to 1800 GeV [1,4,5]. Proposals made to address this
issue are divided into two general groups: (a) those based on ‘‘damping’’ of the SD cross section at small (anti)pro- ton fractional momentum loss or on a changing Pomeron intercept as a function ofps
[6] or as a function of[7], and (b) those in which only the overall normal- ization is suppressed asps
increases [5,8–10]. The models of group (a) cannot be applied to SDD. In a parton model approach developed by Bjorken [11] to describe events with a large pseudorapidity gap between two jets pro- duced by a colorless two-gluon exchange, a suppression of the overall normalization relative to the QCD calcu- lation was predicted due to additional partonic color exchanges in the same event which spoil the diffractive pseudorapidity gap signature. In this model, which be- longs to group (b), the color exchanges would simulta- neously spoil all diffractive rapidity gaps in an event and therefore the ratio of two-gap to one-gap events in a given
η ηmin max
p
0 η
ln M12 ln M22 ln1/ξp ln s’
ln s
p p
IP tp M1 p IP t
M2
FIG. 1 (color online). Schematic diagram and event topology in pseudorapidity space of a SDD (single diffraction plus gap) interaction, ppp!ppGAPppM1GAPM2, with a leading outgoing antiproton of fractional momentum losspp, associated with a pseudorapidity gap pp ln1
p
p, and a gap within the region ofspanned bylns0lnsln1
p p.
interaction would be unaffected. This approach should also hold for lowpT diffractive processes. In this Letter, we examine this issue by studyingppp!ppXwith/
without a gapwithin therange of the systemX in addition to the gap of nominal [2] valuepp lnpp expected to be associated with the leading final state antiproton.
Our study is based on our ps
1800 (630) GeV in- clusive SD data described in Ref. [12] ([13]). The events were collected in the 1995–1996 Tevatron Run 1C by triggering the Collider Detector at Fermilab (CDF) on an antiproton detected in a Roman Pot Spectrometer (RPS) [12]. The average instantaneous luminosity during event collection was 0:210301:51030cm2sec1
at
ps
1800 (630) GeV. The components of CDF [14]
relevant to this study are the central tracking chamber (CTC), the calorimeters, and two scintillation beam- beam counter (BBC) arrays. The CTC tracking efficiency varies from 60% for pT 300 MeV to over 95% for pT >400 MeVwithinjj<1:2, and falls monotonically beyond jj 1:2 approaching zero at jj 1:8. The calorimeters have projective tower geometry and cover the regions jj<1:1 (central), 1:1<jj<2:4 (plug), and 2:2<jj<4:2 (forward). The tower di- mensions, where is the azimuthal angle, are approxi- mately0:115for the central and0:15for the plug and forward calorimeters. The BBC arrays cover the region3:2<jj<5:9.
The events were required to have a reconstructed RPS track of0:06< pp<0:09andjtppj<1:0[0.2]GeV2 at ps
1800 [630] GeV, a hit on BBCp (proton-side BBC) to exclude double Pomeron exchange events, and no more than one reconstructed vertex within 60 cm from the center of the detector along the beam direction.
The vertex requirement was imposed to reject overlap events due to multiple interactions in the same beam- beam crossing, since additional interactions would most likely spoil the pseudorapidity gap signature of a diffrac- tive event.
At
ps
1800(630) GeV, the averageppvalue of 0.075 of our data samples corresponds toP-pcollision energies
of
s0
p pps
p 493 (173) GeV, at which the proton dissociation products cover the nominal range from the maximum of ln
ps
7:5 (6.5) down to lnps
4:9 (3:9). Thus, the CDF calorimeter coverage,jj<4:2, is well suited for the present study.
Our analysis is similar to that used in evaluating the DD fraction in minimum bias events collected with a BBC coincidence trigger [1]. The method we use is based on the approximately flat dependence of the event rate on expected for SDD events compared to the exponential dependence expected for the normal SD events where rapidity gaps within the diffractive cluster X are due to random multiplicity fluctuations. Thus, in a plot of event rate versus, the SDD signal will appear as a flattening of an exponentially falling distribution at large[11].
In order to independently monitor detector effects in the
positive and negative directions, we look for max (min), theof the particle closest to0in the proton (antiproton) direction, and measure experimental gaps overlapping 0, 0expmaxmin (see Fig. 1).
For this purpose, a particle is defined as a reconstructed track in the CTC, a calorimeter tower with energy above a given threshold, or a BBC hit. The tower energy thresh- olds used, chosen to lie comfortably above noise level, are ET 0:2 GeV for the central and plug andE1 GeV for the forward calorimeters. The calorimeter noise was measured using beam-beam crossing events with no re- constructed vertex. At the calorimeter interfaces near jj 0, 1.1, and 2:4, where the noise level was found to be higher, we use higher thresholds of up to 0.3 GeV.
The average number of calorimeter towers per unit with EnoiseT above threshold in an event is0:07, small compared to the corresponding average particle density of3in the data. The fraction of SDD to total number of events based on0expis obtained directly from the data and corrected for (a) contamination from SD events, (b) the effect of the unobserved (below threshold) par- ticles, and (c) the triggering efficiency (acceptance) of BBCp for SD and SDD events. These corrections are made using a hadron-level Monte Carlo (MC) simulation.
The MC generation of SD events is described in Ref. [4].
For SDD events, a diffractivepp and a cluster ofMX2 s are generated as for a SD interaction, and a DD interac- tion is assumed to take place in theP-pcollision, which is treated as in Ref. [1] and boosted to the lab frame. The same thresholds are used for particles in the MC as for towers in the data, after dividing the generated particle ET by an -dependent calibration coefficient of average value 1:6 representing the ratio of true to measured calorimeter energy. The MC generator includes the calo- rimeter noise, and for charged particles it is followed by a detector simulation.
Figure 2 shows Lego histograms of events versusmax and min for (a) data and (b) MC generated events, as well as MC events for (c) only SD and (d) only SDD
at
ps
1800 GeV. Similar results are obtained at s
p 630 GeV. The observed structure in the distri- butions along maxmin is caused by the variation of the tower energy thresholds with jj. The bins at jmaxminj 3:3contain all events within the BBC range of3:2<jmaxminj<5:9.
Figure 3 presents the number of events as a function of 0expfor the 1800 GeV data (points) and for a fit to the data using a mixture of MC generated SD and SDD contributions (solid histogram). The SD contribution (dashed histogram) exhibits an approximately exponen- tial fall with increasing0exp, as expected. The region of 0exp>3is dominated by the SDD signal and is used to extract the gap fraction (ratio of SDD to total number of events). The approximately flat behavior expected for the SDD distribution in this region is modulated by the -dependent tower thresholds used, causing the observed bumps and dips, and by the BBCp acceptance for SDD
011802-4 011802-4
events, which decreases with increasing0exp. The MC simulation reproduces these features of the data.
At
ps
1800 [630] GeV, the fraction of events with 0exp>3 is 15:90:1% [17:50:2%], of which the contribution of background SD events, estimated us- ing the MC simulation, is1:2%[2:4%]. The quoted errors are statistical. The amount of SD background in the region 0exp>3depends on the tower energy calibration coef- ficients and thereby on the calorimeter tower energy thresholds used in the MC. For example, increasing these thresholds has the effect of decreasing the multi- plicity in the MC generated events, resulting in larger rapidity gaps and hence larger SD background in the region of0exp>3. The systematic uncertainty in the background is estimated by raising (lowering) the tower thresholds in the MC by a factor of 1.25 (evaluated from a multiplicity uncertainty of10%), which increases (de- creases) the background by a factor of 1.6.
The correction factors needed to account for the ef- fect of unobserved particles and thus convert the mea- sured gap fractions to gap fractions corresponding to the nominal gap definition [2], 0 lns0s0=M12M22
[lnM2i= s0s0
p <0; i1;2], where s01 GeV2, were evaluated using the SDD Monte Carlo simulation and found to be 0:81 [0:73] at
ps
1800 [630] GeV.
Systematic errors are obtained by varying the tower en- ergy thresholds by 25%. These errors are correlated with the errors in the SD background contamination and therefore a combined systematic uncertainty of 15%
[23%] was evaluated for both these effects and applied to the extracted nominal gap fractions.
The BBCp acceptance, evaluated from the SDD MC simulation, is0:680:06syst[0:810:04syst], where the error is due to a 20% systematic uncertainty assigned to the fraction of the diffraction dissociation mass clus- ters which do not trigger the BBCp. For SD events, the BBC acceptance is 0:980:01syst [0:980:01syst].
Including all systematic errors, the acceptance-corrected SDD fractions for nominal gaps 0 >3 are 0:174 0:001stat 0:030syst [0:1380:001stat 0:032syst] at
ps
1800[630] GeV.
The 0 >3 fractions are extrapolated to all SDD gaps of >3 using the shape of the gap distribution of Eq. (1), which is based on Regge theory and factoriza- tion. This equation, which was used in the MC simulation, is obtained from the equation for SD by replacing theP-p total cross section factor with the P-pDD factorf g (see Ref. [1]).
d5
dtppdtdppddc t
4petpp1ln1=
2
0
4pet1 2
20 s00
s
: (1)
Here, c is the center of the gap , 0 the P-p coupling, the ratio of the triple-Pomeron to the P-p couplings, andlnss00
0lnss
0ln1
p
pthe rapidity space occupied by particles;
s00
p will be referred to below as
‘‘diffractive subenergy.’’ For numerical evaluations we use [1] 0:1040:002, 0 0:25 GeV2, 0:17,
tpp 6:57 GeV1 4:1 mb1=2 F1tpp, where F1tpp is the nucleon form factor. The variabletis not measured and therefore is integrated over in the calculations. Owing to the increased phase space resulting from releasing the requirement that the rapidity gap overlap 0, the
102 103 104 105
0 1 2 3 4 5 6 7
∆η0exp=ηmax-ηmin
events
√s=1800 GeV DATA
SDD + SD MC SD MC
FIG. 3 (color online). The number of events as a function of 0expmaxmin for data at
ps
1800GeV (points), for SD plus SDD (SDgap) MC generated events (solid line), and for only SD MC events (dashed line).
√s=1800 GeV
01 23
0 1 2 3 10
102 103 104 105
DATA
events
(a)
ηm
ax -ηmin
01 23
0 1 2 3 10
102 103 104 105
sum MC
norm events
(b)
ηm
ax -ηmin
01 23
0 1 2 3 10
102 103 104 105
SD MC
norm events
(c)
ηm
ax -ηmin
01 23
0 1 2 3 10
102 103 104 105
SDD MC
norm events
(d)
ηm
ax -ηmin
FIG. 2. The number of events as a function of max and min, theof the track or hit tower closest to0in the proton and antiproton direction, respectively, at ps
1800 GeV: (a) data; (b) Monte Carlo simulation; (c),(d) the individual contributions of Monte Carlo generated SD and SDD (SD gap) events. The MC distributions are normalized by a two- component fit to the data using distributions (c) and (d).
>3fractions are found to be larger than the0>3 ones by a factor of 1.44 [1.40] atps
1800 [630] GeV.
The evaluation of this factor is performed analytically and therefore no error is assigned to it; the effect of the uncertainty in the parameter, which controls the shape of thedistribution in Eq. (1), is<1%.
Our results for the ratioRSDDSD of the number of events with a gap of >3to the total number of SD events are 0:2460:001stat 0:042syst[0:1840:001stat 0:043syst] at
ps
1800 [630] GeV. These ratios are plotted in Fig. 4 at
s0
p 493[173] GeV, the average value of the diffractive massMX, and compared with double- diffractive to total cross section ratiosRDDT DD=T, where DD is obtained from [1] and T is set to the Pomeron exchange contribution, 20ss
0, to conform with the definition ofRSDDSD . The vertical error bars are mainly due to systematic effects, which are correlated among all points. The dashed lines represent predictions based on Regge theory and factorization normalized to the SD cross section at
ps
22 GeV(see [10]). The solid lines are predictions from the ‘‘renormalized gap proba- bility’’ model [10], in which the Regge cross section is factorized into two parts, one representing the ppp total cross section at the diffractive subenergy multiplied by n, wherenis the number of gaps, and a factor interpreted as the gap probability distribution normalized to unity over all available phase space. The bands around the solid lines represent a 10% uncertainty due to the factor[15].
The data are in good agreement with the renormalized gap model predictions.
In summary, we have measured the fraction of events with a pseudorapidity gap >3within the diffractive cluster X of the process ppp !ppX and found it to be 0:2460:001stat 0:042syst[0:1840:001stat 0:043syst] for 0:06< pp<0:09 and jtppj<1:0 [0.2]
GeV2 at ps
1800 [630] GeV. These values are higher than expectations from double-diffractive fractions in minimum bias events, lower than expectations from Regge theory and factorization, and in good agreement with predictions based on the renormalized gap probabil- ity model [10].
We thank the Fermilab staff and the technical staffs of the participating institutions for their vital contributions.
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 fuer Bildung und Forschung, Germany; and the Korea Science and Engineering Foundation.
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10-1 1
103 sub-energy √s--, (GeV)
gap fraction ∆η> 3.0
CDF: two-gap/one-gap Regge prediction Renorm-gap prediction
2-gap
1-gap
102
FIG. 4 (color online). Ratios of SDD (single-diffraction plus gap) to single-diffractive rates (solid circles) and double- diffractive to total cross sections (open circles) as a function of the collision energy of the subprocess, Pomeron-proton, and
p
pp, respectively. The uncertainties are mainly due to sys- tematic effects, which are highly correlated among all four data points. The dashed lines are predictions from Regge theory and the solid lines from the renormalized gap probabil- ity model [10].
011802-6 011802-6
