Abstract:

One of the most pressing questions in both hot and cold QCD communities is what the physics mechanism responsible for modified parton densities in heavy nuclei is. One promising channel to address this question is the photoproduction of vector mesons, which is considered a clean probe to the nuclear parton structures.
We present a measurement of π+π−π+π− and π+π− photonuclear production in ultraperipheral Au+Au collisions at sNN=200 GeV. The data were collected in 2014 by the STAR experiment. The π+π−π+π− and π+π− final states, accompanied by mutual excitation of projectile and target, are observed to be greatly enhanced at low transverse momentum, which is consistent with coherent photoproduction. The π+π−π+π− invariant mass spectrum in coherent events exhibits a two-resonance structure around ∼1.5 and ∼1.7 GeV/c2 with widths of around 0.50 and 0.45 GeV/c2, likely corresponding to ρ(1450) and ρ(1700). Furthermore, a peak corresponding to ρ(2150) is observed. We also observe peaks corresponding to ρ(1450) and ρ(1700) in the π+π− final state and report the ratio of the branching fractions of the ρ(1450), ρ(1700) to π+π− and π+π−π+π−. We also present the ratios of ρ(1450), ρ(1700), and ρ(2150) to ρ0(770) coherent production cross sections. Further, we present the status on the analysis of π+π−π+π− polarization.



Figure 1: STAR Subsystems Relevant To UPC Collisions

 
Figure 2: Left Panel: p_T of 4pi final state with |y|<0.8, the black circles represent signal (Q=0), red circles represent scaled combinatorial background (Q=+- 2). Right panel is -|t| distribution of the signal with the combinatorial background subtracted. The fit function is sum of an exponential function (representing the coherently produced yield) and a dipole formfactor (representing the incoherently produced yield). The dipole single fit is the formfactor fit in the region where we don't expect coherently produced yield. 

Figure 3: dsigma/dM of 4-pion final state with statistical and bin-by-bin systematic uncertainties. The Fit function is superposition of 3 relativistic Breit-Wigner functions for spin-1 particles representing rho 1450, rho 1700 and likely rho2150 resonances including the mutual interference.  

Figure 4: dsigma/dy of 4-pion final state with statistical and bin-by-bin systematic uncertainties compared with scaled StarLight shape.    

Figure 5: dsigma/dM of 2-pion final state with statistical and bin-by-bin systematic uncertainties. The Fit function is superposition of 3 relativistic Breit-Wigner functions for spin-1 particles representing rho0 , omega and rho1700 resonances and one for spin-2 particles representing f2 1270. It further includes the mutual interferences and non-interfering background estimated from STAR e+e- data.  

Figure 6: dsigma/dy of 2-pion final state with statistical and bin-by-bin systematic uncertainties compared with scaled StarLight shape. 

Figure 7: Mass distribution of pion pairs from 4-pi final state. The pairs are separated by their mass where the heavier pair's mass correspond to the mass of rho0 meson. 

Figure 8: cos(theta) distribution of a pion in the centre-of-mass frame of the heavier pion pair from the 4-pion final state 

Figure 9: cos(theta) distribution of a pion in the centre-of-mass frame of the lighter pion pair from the 4-pion final state