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Measurement of the central exclusive production of charged particle pairs in proton-proton collisions at $\sqrt{s} = 200$ GeV with the STAR detector at RHIC

The collaboration
2020.

Abstract (data 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 $pp \to p^{\prime}+h^{+}h^{-}+p^{\prime}$ are reconstructed from the tracks in the central detector and identified using the specific energy loss and the time of flight method, while the forward-scattered protons are measured in the Roman Pot system. Exclusivity of the event is guaranteed by requiring the transverse momentum balance of all four final-state particles. Differential cross sections are measured as functions of observables related to the central hadronic final state and to the forward-scattered protons. They are measured in a fiducial region corresponding to the acceptance of the STAR detector and determined by the central particles' transverse momenta and pseudorapidities as well as by the forward-scattered protons' momenta. This fiducial region roughly corresponds to the square of the four-momentum transfers at the proton vertices in the range $0.04~\mathrm{GeV}^2 < -t_1 , -t_2 < 0.2~\mathrm{GeV}^2$, 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.For $\pi^+\pi^-$ production, the fiducial 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.

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Data from Figure 8

10.17182/hepdata.94264.v1/t1

Differential fiducial cross section for CEP of $\pi^+\pi^-$ pairs as a function of the invariant mass of the pair. Systematic...

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Data from Figure 9 (left)

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Differential fiducial cross section for CEP of $K^+K^-$ pairs as a function of the invariant mass of the pair. Systematic...

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Data from Figure 9 (right)

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Differential fiducial cross section for CEP of $p\bar{p}$ pairs as a function of the invariant mass of the pair. Systematic...

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Data from Figure 10 (left)

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Differential fiducial cross section for CEP of $\pi^+\pi^-$ pairs as a function of the rapidity of the pair. Systematic uncertainties...

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Data from Figure 10 (middle)

10.17182/hepdata.94264.v1/t5

Differential fiducial cross section for CEP of $K^+K^-$ pairs as a function of the rapidity of the pair. Systematic uncertainties...

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Data from Figure 10 (right)

10.17182/hepdata.94264.v1/t6

Differential fiducial cross section for CEP of $p\bar{p}$ pairs as a function of the rapidity of the pair. Systematic uncertainties...

• #### Figure 11 (top, left)

Data from Figure 11 (top, left)

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Differential fiducial cross section for CEP of $\pi^+\pi^-$ pairs as a function of the difference of azimuthal angles of the...

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Data from Figure 11 (top, middle)

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Differential fiducial cross section for CEP of $K^+K^-$ pairs as a function of the difference of azimuthal angles of the...

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Data from Figure 11 (top, right)

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Differential fiducial cross section for CEP of $p\bar{p}$ pairs as a function of the difference of azimuthal angles of the...

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Data from Figure 11 (bottom, left)

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Differential fiducial cross section for CEP of $\pi^+\pi^-$ pairs as a function of the sum of the squares of the...

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Data from Figure 11 (bottom, middle)

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Differential fiducial cross section for CEP of $K^+K^-$ pairs as a function of the sum of the squares of the...

• #### Figure 11 (bottom, right)

Data from Figure 11 (bottom, right)

10.17182/hepdata.94264.v1/t12

Differential fiducial cross section for CEP of $p\bar{p}$ pairs as a function of the sum of the squares of the...

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Data from Figure 12 (top, left)

10.17182/hepdata.94264.v1/t13

Differential fiducial cross section for CEP of $\pi^+\pi^-$ pairs as a function of the invariant mass of the pair, for...

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Data from Figure 12 (top, right)

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Differential fiducial cross section for CEP of $\pi^+\pi^-$ pairs as a function of the invariant mass of the pair, for...

• #### Figure 12 (middle, left)

Data from Figure 12 (middle, left)

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Differential fiducial cross section for CEP of $K^+K^-$ pairs as a function of the invariant mass of the pair, for...

• #### Figure 12 (middle, right)

Data from Figure 12 (middle, right)

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Differential fiducial cross section for CEP of $K^+K^-$ pairs as a function of the invariant mass of the pair, for...

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Data from Figure 12 (bottom, left)

10.17182/hepdata.94264.v1/t17

Differential fiducial cross section for CEP of $p\bar{p}$ pairs as a function of the invariant mass of the pair, for...

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Data from Figure 12 (bottom, right)

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Differential fiducial cross section for CEP of $p\bar{p}$ pairs as a function of the invariant mass of the pair, for...

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Data from Figure 13 (left)

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Differential fiducial cross section for CEP of $\pi^+\pi^-$ pairs as a function of the invariant mass of the pair, for...

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Data from Figure 13 (right)

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Differential fiducial cross section for CEP of $\pi^+\pi^-$ pairs as a function of the invariant mass of the pair, for...

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Data from Figure 14 (top, left)

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Differential fiducial cross section for CEP of $\pi^+\pi^-$ pairs as a function of the $\cos{\theta^{CS}}$ of the positive charge pion...

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Data from Figure 14 (top, middle)

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Differential fiducial cross section for CEP of $K^+K^-$ pairs as a function of the $\cos{\theta^{CS}}$ of the positive charge kaon...

• #### Figure 14 (top, right)

Data from Figure 14 (top, right)

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Differential fiducial cross section for CEP of $p\bar{p}$ pairs as a function of the $\cos{\theta^{CS}}$ of the central state proton...

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Data from Figure 14 (bottom, left)

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Differential fiducial cross section for CEP of $\pi^+\pi^-$ pairs as a function of the $\phi^{CS}$ of the positive charge pion...

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Data from Figure 14 (bottom, middle)

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Differential fiducial cross section for CEP of $K^+K^-$ pairs as a function of the $\phi^{CS}$ of the positive charge kaon...

• #### Figure 14 (bottom, right)

Data from Figure 14 (bottom, right)

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Differential fiducial cross section for CEP of $p\bar{p}$ pairs as a function of the $\phi^{CS}$ of the central state proton...

• #### Figure 15 (top, left)

Data from Figure 15 (top, left)

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Differential fiducial cross section for CEP of $\pi^+\pi^-$ pairs as a function of the rapidity of the pair, for invariant...

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Data from Figure 15 (top, middle)

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Differential fiducial cross section for CEP of $\pi^+\pi^-$ pairs as a function of the difference of azimuthal angles of the...

• #### Figure 15 (top, right)

Data from Figure 15 (top, right)

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Differential fiducial cross section for CEP of $\pi^+\pi^-$ pairs as a function of the sum of the squares of the...

• #### Figure 15 (middle, left)

Data from Figure 15 (middle, left)

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Differential fiducial cross section for CEP of $\pi^+\pi^-$ pairs as a function of the rapidity of the pair, for invariant...

• #### Figure 15 (middle, middle)

Data from Figure 15 (middle, middle)

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Differential fiducial cross section for CEP of $\pi^+\pi^-$ pairs as a function of the difference of azimuthal angles of the...

• #### Figure 15 (middle, right)

Data from Figure 15 (middle, right)

10.17182/hepdata.94264.v1/t32

Differential fiducial cross section for CEP of $\pi^+\pi^-$ pairs as a function of the sum of the squares of the...

• #### Figure 15 (bottom, left)

Data from Figure 15 (bottom, left)

10.17182/hepdata.94264.v1/t33

Differential fiducial cross section for CEP of $\pi^+\pi^-$ pairs as a function of the rapidity of the pair, for invariant...

• #### Figure 15 (bottom, middle)

Data from Figure 15 (bottom, middle)

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Differential fiducial cross section for CEP of $\pi^+\pi^-$ pairs as a function of the difference of azimuthal angles of the...

• #### Figure 15 (bottom, right)

Data from Figure 15 (bottom, right)

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Differential fiducial cross section for CEP of $\pi^+\pi^-$ pairs as a function of the sum of the squares of the...

• #### Figure 16 (top, left)

Data from Figure 16 (top, left)

10.17182/hepdata.94264.v1/t36

Differential fiducial cross section for CEP of $\pi^+\pi^-$ pairs as a function of the $\cos{\theta^{CS}}$ of the positive charge pion...

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Data from Figure 16 (top, middle)

10.17182/hepdata.94264.v1/t37

Differential fiducial cross section for CEP of $\pi^+\pi^-$ pairs as a function of the $\cos{\theta^{CS}}$ of the positive charge pion...

• #### Figure 16 (top, right)

Data from Figure 16 (top, right)

10.17182/hepdata.94264.v1/t38

Differential fiducial cross section for CEP of $\pi^+\pi^-$ pairs as a function of the $\cos{\theta^{CS}}$ of the positive charge pion...

• #### Figure 16 (bottom, left)

Data from Figure 16 (bottom, left)

10.17182/hepdata.94264.v1/t39

Differential fiducial cross section for CEP of $\pi^+\pi^-$ pairs as a function of the $\phi^{CS}$ of the positive charge pion...

• #### Figure 16 (bottom, middle)

Data from Figure 16 (bottom, middle)

10.17182/hepdata.94264.v1/t40

Differential fiducial cross section for CEP of $\pi^+\pi^-$ pairs as a function of the $\phi^{CS}$ of the positive charge pion...

• #### Figure 16 (bottom, right)

Data from Figure 16 (bottom, right)

10.17182/hepdata.94264.v1/t41

Differential fiducial cross section for CEP of $\pi^+\pi^-$ pairs as a function of the $\phi^{CS}$ of the positive charge pion...

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Data from Table 2

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Integrated fiducial cross sections with statistical and systematic uncertainties for CEP of $\pi^+\pi^-$, $K^+K^-$ and $p\bar{p}$ pairs in two ranges...

• #### Table 3 (A)

Data from Table 3 (A)

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Results of the fit to extrapolated $d\sigma/dm(\pi^+\pi^-)$ in two ranges of azimuthal angle difference $\Delta\varphi$ between forward-scattered protons. The fit...

• #### Table 3 (B)

Data from Table 3 (B)

10.17182/hepdata.94264.v1/t44

Results of the fit to extrapolated $d\sigma/dm(\pi^+\pi^-)$ in two ranges of azimuthal angle difference $\Delta\varphi$ between forward-scattered protons. The fit...

• #### Table 4 (A)

Data from Table 4 (A)

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Ratios of integrated cross sections of resonance production to cross sections for f2(1270) production, in the pi+pi- channel. The results...

• #### Table 4 (B)

Data from Table 4 (B)

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Ratios of integrated cross sections of resonance production in two ranges of $\Delta\varphi$, in the pi+pi- channel. The results are...

• #### Table 5

Data from Table 5

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Exponential slope of the $t$-distribution in three ranges of $m(\pi^+\pi^-)$ and two ranges of $\Delta\varphi$. The results are obtained for...