  We consider contributions of the open-string singlet~(OSS) interaction to the
proton-proton scattering. Modelling baryon and meson as instanton and open
string in the effective QCD string model, the typical pomeron is identified
with a massive spin-2 closed string. According to the open-closed string
duality, the massive spin-2 closed string is accompanied by the open-string
singlet interaction with universal coupling. The flat space 4-point open-string
singlet amplitudes and cross sections are computed in the Regge limit.
Including both the open and closed string pomerons, we fit the differential
cross-section with experimental data of the proton-proton and proton-antiproton
scattering from the CDF, E-710, D0, LHC-ATLAS and LHC-TOTEM collaborations. The
fitting results show the universality of the string mass scale~(or string
tension) and consistency of the string model between the open and closed string
parameters in describing the $pp$ and $p\bar{p}$ scattering in the Regge limit.
The fitting of the string model to experimental data gives the string scale
$M_{S}\simeq 1.7-1.8$ GeV and the corresponding glueball pole mass $m_{g}\simeq
1.6-1.7$ GeV. This lies within experimental values of the observed $2^{++},
I=0$ "meson" masses.
