Recent BABAR Results on Two-photon Physics Page: 2 of 5
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1. Introduction
Two-photon production of a resonance R is studied at e+e- colliders in the process e+e- -
e+e-R. The electrons in this process are scattered predominantly at small angles. For the pseu-
doscalar meson production, the effect of strong interactions is described by only one form factor
F(qi, q) depending on the squared momentum transfers to the electrons.
Two-photon processes are usually studied in so called no-tag mode with both final electrons
undetected. In this case the virtual photons emitted by electrons are practically real, the momentum
transfers squared are close to zero. In no-tag mode the meson-photon transition form factor at zero
q2's and the meson two-photon width are measured.
In the single tag-mode one of the final electrons is detected. The corresponding virtual photon
is highly off-shell. From the measurement of the cross section richer information is extracted: the
dependence of the meson form factor on Q= _-q.
In this report we present results of no-tag and single-tag measurements performed with the
BABAR detector at the PEP-II e+e- collider. The results are based on data with integrated lumi-
nosity of about 500 fb collected at the center-of-mass energy of 10.6 GeV.
No-tag two-photon events are selected by the requirement that the transverse momentum of
detected hadron system is low. The single-tag events are selected with the detected and identified
electron and with the fully reconstructed pseudoscalar meson, w0, ij, i', or 1Jc. It is required that
the transverse momentum of the electron-plus-meson system be low and the missing mass in an
event be close to zero.
2. Measurement of Tle and ne(2S) parameters in the no-tag mode
The KSK+2T mass spectrum for no-tag events is shown in Fig. 1(a). The r7c, J/y, and ne(2S)
peaks are clearly seen over a non-resonant smooth background. The J/y's are produced in the
initial state radiation process e+e- - J/yy. An evidence for the Xc - KSK+WT decay is also
seen in Fig. lb. From the fit to the mass spectrum the following 1Jc parameters are determined [1]:
m = 2982.2 0.4 1.5 MeV/c2, F = 31.7 1.2 0.8 MeV, (2.1)
F(ne - yy)B(ne - KKK) = 0.379 0.009 0.031 keV. (2.2)
These are the most precise measurements of the 1Jc mass and width to date. The obtained value of
F(ne - yy)B(ne - KKK) agrees with the CLEO measurement 0.407 0.022 0.028 keV [2].
From the fit to the KSK+WT mass spectrum in the vicinity of the ne(2S) resonance the follow-
ing values of the ne(2S) mass and width are obtained:
m = 3638.3 1.5 0.6 MeV/c2, F'= 14.2 4.4 2.5 MeV. (2.3)
These results are preliminary. They are in reasonable agreement with the previous BABAR mea-
surements [3]: m = 3630.8 3.4 1.0 MeV/c2 and F = 17.0 8.3 2.5 MeV, obtained using 88
fb-1 data. The current PDG values for these parameters are m = 3637 4 MeV/c2 and F = 14 7
MeV [4]. The measured value of the ne(2S) width is also in good agreement with an estimation
based on a quark model: F(ne(2S) - gg) F f(ne(1S) - gg)F(y(2S) - ee)/F(y(1S) - ee) _
12.1 1.0 MeV.2
Recent Results on Two-photon Physics at BABAR
V. P. Druzhinin
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Druzhinin, Vladimir P. Recent BABAR Results on Two-photon Physics, article, June 12, 2013; United States. (https://digital.library.unt.edu/ark:/67531/metadc828382/m1/2/: accessed March 29, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.