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STAR
Abstract:
We report on the measurement of the Central Exclusive Production of charged particle pairs h^{+}h^{-} (h = \pi, K, p) with the STAR detector at RHIC in proton-proton collisions at \sqrt{s} = 200 GeV. The charged particle pairs produced in the reaction pp\to p^\prime+h^{+}h^{-}+p^\prim are reconstructed from the tracks in the central detector, while the forward-scattered protons are measured in the Roman Pot system. Differential cross sections are measured in the fiducial region, which roughly corresponds to the square of the four-momentum transfers at the proton vertices in the range 0.04~\mbox{GeV}^2 < -t_1 , -t_2 < 0.2~\mbox{GeV}^2, invariant masses of the charged particle pairs up to a few GeV and pseudorapidities of the centrally-produced hadrons in the range |\eta|<0.7. The measured cross sections are compared to phenomenological predictions based on the Double Pomeron Exchange (DPE) model. Structures observed in the mass spectra of \pi^{+}\pi^{-} and K^{+}K^{-} pairs are consistent with the DPE model, while angular distributions of pions suggest a dominant spin-0 contribution to \pi^{+}\pi^{-} production. The fiducial \pi^+\pi^- cross section is extrapolated to the Lorentz-invariant region, which allows decomposition of the invariant mass spectrum into continuum and resonant contributions. The extrapolated cross section is well described by the continuum production and at least three resonances, the f_0(980), f_2(1270) and f_0(1500), with a possible small contribution from the f_0(1370). Fits to the extrapolated differential cross section as a function of t_1 and t_2 enable extraction of the exponential slope parameters in several bins of the invariant mass of \pi^+\pi^- pairs. These parameters are sensitive to the size of the interaction region.
