Central Exclusive Production in the STAR Experiment at RHIC

TL;DR

This paper reports on the measurement of Central Exclusive Production of charged pion and kaon pairs in polarized proton-proton collisions at RHIC, utilizing Roman Pot detectors and TPC for full event kinematics control.

Contribution

First measurement of CEP of charged meson pairs at RHIC using Roman Pot detectors and dE/dx discrimination in the TPC.

Findings

Observed exclusive $ ho^+ ho^-$ and $K^+K^-$ production channels.

Achieved full kinematic control of the reaction using Roman Pot and TPC data.

Provided new data on diffractive processes at $ oot s=200$ GeV.

Abstract

The STAR experiment at the Relativistic Heavy Ion Collider (RHIC) performs studies of diffractive processes with the focus on the exclusive production of particles in central range of rapidity. In 2015 STAR collected 18 pb$^{-1}$ of data in polarized proton+proton collisions at $\sqrt{s}$=200 GeV to measure Central Exclusive Production (CEP) process $pp\to pXp$ through Double Pomeron Exchange (DPE) mechanism. The intact protons moving inside the RHIC beampipe after the collision were measured in silicon strip detectors (SSD), which were placed in the Roman Pot vessels. This enables full control over interaction kinematics and verification of the exclusivity of the reaction by measuring the total (missing) transverse momenta of all final state particles: the central diffractive system in the Time Projection Chamber (TPC) and the forward protons in the Roman Pots. With the use of ionization energy loss in the TPC, d$E$/d$x$, it was possible to discriminate various production channels in $pp\to pXp$ reaction. This paper presents results on exclusive production of two charged particles ($\pi^{+}\pi^{-}$ and $K^{+}K^{-}$) in mid-rapidity region, $-1<\eta<1$, with small squared four-momentum transfer of forward protons, $0.03<-t<0.3~(\textrm{GeV}/c)^{2}$, obtained using 2.5% of full statistics.