Study of Central Exclusive Production of $π^+π^-$, $K^+K^-$ and $p \bar{p}$ Pairs in Proton-Proton Collisions at $\sqrt{s} = 510$ GeV with the STAR Detector at RHIC

TL;DR

The study presents the first measurement of Central Exclusive Production of $\pi^+\pi^-$, $K^+K^-$, and $p\bar{p}$ pairs in proton-proton collisions at $\sqrt{s}=510$ GeV with STAR at RHIC, focusing on double IPomeron exchange in a clean exclusive topology. By tagging forward protons with Roman Pots and reconstructing the central hadron pairs in the STAR detector, the analysis obtains differential fiducial cross sections as functions of $\Delta\varphi$, $|t_1+t_2|$, $m(h^+h^-)$, and $y(h^+h^-)$, and compares them to GRANIITTI predictions that include both continuum and resonant contributions. The results show resonance structures consistent with DIPE, including the first observation of $f_0(980)$ in the $K^+K^-$ channel, and indicate strong dependence on the azimuthal separation of the forward protons, guiding future tuning of CEP models. Overall, the findings validate the DIPE-dominated CEP mechanism at RHIC energies and provide precise benchmarks for nonperturbative QCD descriptions and resonance production in exclusive processes.

Abstract

We report on the first measurement of the Central Exclusive Production (CEP) process in proton-proton (pp) collisions:, $pp \ \rightarrow \ p h^+ h^- p$ (where $h = π, K, p$) at the center-of-mass energy $\sqrt{s} = 510$ GeV with the STAR experiment at RHIC. At this energy, the process is dominated by a double I$\!$Pomeron exchange mechanism. Hence, it provides a clean environment for investigating I$\!$Pomeron interactions by measuring fully reconstructed final states involving only two hadrons and two forward scattered protons. The oppositely charged hadron pairs are measured within the central detector of STAR. The forward scattered protons are measured in the Roman Pot system allowing the verification of the event's exclusivity. Differential fiducial cross sections within the STAR acceptance are presented as a function of the difference in the azimuthal angle between the outgoing protons. The invariant masses of the charged hadron pairs are measured up to approximately 3 GeV and the square of the four-momentum transfer ($t_1$ and $t_2$) of the two forward-scattered protons in the range $0.3 \text{ GeV}^2 < -t_1 , -t_2 < 1.6 \text{ GeV}^2$. The differential fiducial cross sections of the forward protons as a function of the $|t_1 + t_2|$ are also presented. All results for the $π^+π^-$ pair are presented in three mass ranges. A comparison with GRANIITTI Monte Carlo predictions are also presented, where the spectra include continuum and resonant contributions. The observed spectra are consistent with double I$\!$Pomeron exchange, including resonances seen in previous studies, while the $f_0(980)$ resonance is reported here for the first time in the $K^+K^-$ decay channel.

Figures (13)

• Figure 1: Diagrams of CEP of $h^+h^-$ in the DI$\!$PE process. The resonant (left) and continuum production (middle) are shown. The two-gluon approximation of continuum production in DI$\!$PE in perturbative QCD (right) is also shown. I$\!$P omerons are represented by double zigzag lines and gluons by spiral lines.
• Figure 2: The schematic view of the STAR experiment. The main sub-detectors, including the TPC, the TOF, the BBC, the VPD and the BEMC are highlighted.
• Figure 3: The layout of the RP setup at STAR (not to scale) for measuring forward protons. Top $(x,z)$ and side $(y,z)$ views are shown. Two sets of RPs, labeled (W1, W2) and (E1, E2) were installed between the DX and D0 magnets, at $\pm$15.8 m and $\pm$17.6 m, on either side of the IP. The detector package has transverse size $5\times 8\; \mathrm{cm}^2$ and a depth 3.5 cm. The silicon sensor is $400\;\mu\mathrm{m}$ thick, while the trigger scintillator is 5 mm thick. The strips in the silicon detectors are $\approx 100\; \mu$m wide. Two dipole magnets, DX and D0, which bend the beams into and out of the IP, are also shown. The top view shows highlighted Roman Pot stations E1, E2, W1, W2 and dipole magnets DX, D0. The side view depicts Roman Pots. Taken from Ref. Wlodek.
• Figure 4: The RP fiducial volume shown with black solid lines on top of the combined distributions of forward protons’ momenta $p_y$ vs. $p_x$ reconstructed with the East and West RP stations.
• Figure 5: The distribution $p_{\textsf{T}}^{\text{miss}}$ for $\pi^+\pi^-$ CEP event candidates with the $p_{\textsf{T}}^{\text{miss}}$ cut illustrated by the black dashed line. The non-exclusive background is fitted using a second-degree polynomial with the constant term set to zero. The fit is depicted by the red line and projected into the signal region.