Recent results on Central Exclusive Production with the STAR detector at RHIC

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 process $pp\to pXp$ (CEP)
through Double \Pom omeron Exchange (D\Pom E) mechanism, which is
expected to be dominant at this center-of-mass energy. The intact
protons moving inside the RHIC beampipe after the collision were measured
in silicon strip detectors, which were placed in the Roman Pot vessels. This
enables full control over interaction kinematics and clear verification
of the exclusivity of the reaction by measuring the total (missing)
transverse momentum 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$x$, as well as velocity measured with TPC and
barrel Time-Of-Flight detector (TOF) it was possible to discriminate
various production channels in $pp\to pXp$ reaction. We shall present
preliminary results on exclusive production of two ($\pi^{+}\pi^{-}$,
$K^{+}K^{-}$, $p\bar{p}$) and four ($\pi^{+}\pi^{-}\pi^{+}\pi^{-}$)
charged particles in mid-rapidity region, $|\eta|<1$, with small squared
four-momentum transfer of forward protons, $0.03<|t_{1}|,|t_{2}|<0.3~(\textrm{GeV}/c)^{2}$.