Abstract
We have observed exclusive photon pair production in proton-antiproton collisions at
\sqrt{s} = 1.96 TeV, using 1.11 +- 0.07 fb^{-1} of integrated luminosity
taken by the Run II Collider Detector at Fermilab.
We select events with two electromagnetic showers, each with transverse energy
E_T > 2.5 GeV and
pseudorapidity |\eta| < 1.0, with no other particles detected in -7.4 < \eta < +7.4.
The two showers have similar E_T and have \Delta\phi ~ \pi; 34 have two
tracks, consistent with the QED
process p \bar{p} -> p + e^+e^- + \bar{p} by two-photon exchange, while
43 events have no charged tracks. From the distribution in the number of
electromagnetic clusters in wire chambers
at shower maximum (6 X_0) we conclude that < 16% (at 95% C.L.) of these events are exclusive \pi^0\pi^0. The contribution of events with p(\bar{p}) dissociation is negligible.
The cross section of p\bar{p} -> p+\gamma\gamma+\bar{p} with |\eta(\gamma)| < 1.0 and
E_T(\gamma) > 2.5 GeV is \sigma^{|\eta|<1, E_T>2.5
GeV}_{\gamma\gamma excl.}=2.48 +- 0.42(stat) +- 0.41(syst) pb.
This agrees with predictions for
the process \pom+\pom \rightarrow \gamma\gamma, where \pom is a pomeron, through an
intermediate quark loop.
Public note
Selection of blessed plots
- Trigger rate versus instantaneous luminosity.
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- Log10 (ADC counts of PMT with largest signal) in BSC-1 West.
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- Log10 (ADC counts of PMT with largest signal) in Miniplug West.
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- Log10 (ADC counts of PMT with largest signal) in Central EM cal.
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- Exclusive efficiency (= probability of no detected interaction) as a function of bunch luminosity (data taking period 9).
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- Invariant mass of exclusive e⁺e⁻ events compared to MC.
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- Azimuthal angle difference (from back to back) between the photons compared to MC.
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- Transverse momentum of photon pairs compared to MC.
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- Invariant mass of exclusive photon pair events compared to MC.
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- Number of CES strip chamber shower distribution (sum of both sides) for photon pair candidates compared to photon pair and neutral pion pair MC.
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- Chi2 value fit of data to background signal composition versus fraction of exclusive neutral pion pair background.
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Eventdisplays
Acknowledgements
The authors are very grateful to the Durham theory group for useful discussions, especially to V.A. Khoze, M.G. Ryskin, W.J. Stirling and L.A. Harland-Lang.
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 Instituto
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 Foundation; the A.P. Sloan Foundation; the
Bundesministerium f\"ur Bildung und Forschung, Germany; the Korean World Class
University Program, the National Research Foundation of Korea; the Science and
Technology Facilities Council and the Royal Society, UK; the Institut National
de Physique Nucleaire et Physique des Particules/CNRS; the Russian Foundation
for Basic Research; the Ministerio de Ciencia e Innovaci\'{o}n, and Programa
Consolider-Ingenio 2010, Spain; the Slovak R\&D Agency; the Academy of Finland;
and the Australian Research Council (ARC).
General Remarks
The preliminary results were blessed in the QCD group on Friday the 19th of May 2011.
Please contact us
if you have any questions about our results.
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