Observation of Exclusive Dijet Production at the Fermilab Tevatron p-pbar Collider Page: 4 of 27
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Exclusive dijet production in pp collisions is a process
in which both the antiproton and proton escape the in-
teraction point intact and a two-jet system is centrally
-* fl' + (jeti +.jet2) +p.
This process is a particular case of dijet production in
double Pomeron exchange (DPE), a diffractive process in
which the antiproton and proton suffer a small fractional
momentum loss, and a system X containing the jets is
p +p-* [p' +1Py] +[p' +1P]-*s' +X +p', (2)
where P designates a Pomeron, defined as an exchange
consisting of a colorless combination of gluons and/or
quarks carrying the quantum numbers of the vacuum.
In a particle-like Pomeron picture (e.g. see ), the
system X may be thought of as being produced by the
collision of two Pomerons, IP and P,,
JPP +PP X - Yr/P + (jets + jet2) +Yr/,, (3)
where in addition to the jets the final state generally con-
tains Pomeron remnants designated by Yr/P and Yr/p.
Dijet production in DPE is a sub-process to dijet produc-
tion in single diffraction (SD) dissociation, where only the
antiproton (proton) survives while the proton (antipro-
ton) dissociates. Schematic diagrams for SD and DPE
dijet production are shown in Fig. 1 along with event
topologies in pseudorapidity space (from Ref. ). In
SD, the escaping p is adjacent to a rapidity gap, defined
as a region of pseudorapidity devoid of particles . A
rapidity gap arises because the Pomeron exchanged in a
diffractive process is a colorless object of effective spin
J > 1 and carries the quantum numbers of the vacuum.
In DPE, two such rapidity gaps are present.
Dijet production in DPE may occur as an exclusive
process  with only the jets in the final state and no
Pomeron remnants, either due to a fluctuation of the
Pomeron remnants down to zero or with a much higher
cross section in models in which the Pomeron is treated
as a parton and the dijet system is produced in a 2 - 2
process analogous to ryy - jet + jet .
In a special case exclusive dijets may be produced
through an intermediate state of a Higgs boson decay-
ing into bb:
IPP + IPP H (I> jet1) + (b jet2). (4)
burgh EH9 3JZ, United Kingdom, kUniversity of Heidelberg, D-
69120 Heidelberg, Germany, 1Universidad Iberoamericana, Mexico
D.F., Mexico, "'University of Manchester, Manchester M13 9PL,
England, nNagasaki Institute of Applied Science, Nagasaki, Japan,
University de Oviedo, E-33007 Oviedo, Spain, pQueen Mary, Uni-
versity of London, London, El 4NS, England, 4Texas Tech Univer-
sity, Lubbock, TX 79409, rIFIC(CSIC-Universitat de Valencia),
46071 Valencia, Spain,
tP 0 - lp
FIG. 1: Illustration of event topologies in pseudorapidity,
tj, and associated Pomeron exchange diagrams for dijet pro-
duction in (a) single diffraction and (b) double Pomeron ex-
change. The shaded areas on the left side represent "underly-
ing event" particles not associated with the jets [from Ref. ].
a) g9 Jet
(b) g H
FIG. 2: Leading order diagrams for (a) exclusive dijet and
(b) exclusive Higgs boson production in pp collisions.
Exclusive production may also occur through a t-
channel color-singlet two gluon exchange at leading order
(LO) in perturbative quantum chromo-dynamics (QCD),
as shown schematically in Fig. 2 (a), where one of the two
gluons takes part in the hard scattering that produces the
jets, while the other neutralizes the color flow . A simi-
lar diagram, Fig. 2 (b), is used in  to calculate exclusive
Higgs boson production.
Exclusive dijet production has never previously been
observed in hadronic collisions. In addition to providing
information on QCD aspects of vacuum quantum num-
ber exchange, there is currently intense interest in using
measured exclusive dijet production cross sections to cal-
ibrate theoretical predictions for exclusive Higgs boson
production at the Large Hadron Collider (LHC). Such
predictions are generally hampered by large uncertain-
ties due to non-perturbative suppression effects associ-
ated with the rapidity gap survival probability. As these
effects are common to exclusive dijet and Higgs boson
production mechanisms, dijet production potentially pro-
vides a "standard candle" process against which to cali-
brate the theoretical models [6, 7].
In Run I (1992-96) of the Fermilab Tevatron pp col-
lider operating at 1.8 TeV, the Collider Detector at Fer-
milab (CDF) collaboration made the first observation of
dijet production by DPE using an inclusive sample of
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Aaltonen, T.; Phys., /Helsinki Inst. of; Adelman, J.; /Chicago U., EFI; Akimoto, T.; U., /Tsukuba et al. Observation of Exclusive Dijet Production at the Fermilab Tevatron p-pbar Collider, article, December 1, 2007; Batavia, Illinois. (https://digital.library.unt.edu/ark:/67531/metadc902230/m1/4/: accessed April 19, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.