[Brahms-l] Fwd: STAR new paper on photoproduction in ultra peripheral collisions at RHIC

FYI

Flemming Videbaek

> Subject: STAR new paper on photoproduction in ultra peripheral  
> collisions at RHIC
>
> Dear Barbara, Wit, Flemming and Steve,
>
> I am pleased to announce that the STAR collaboration will submit, in  
> a week's time, a new paper on photoproduction in ultra peripheral Au 
> +Au collisions at RHIC to Physical Review C. The title and abstract  
> are enclosed below.
>
> Best regards,
>
> Nu Xu
>
> -----------------------------------------------------------------------
> Nu Xu
>
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> =============================================================
>
> Title: "Observation of $\pi^{+}\pi^{-}\pi^{+}\pi^{-}$  
> Photoproduction in Ultra-Peripheral Heavy Ion Collisions at STAR"
>
> Abstract: "We present a measurement of $\pi^{+}\pi^{-}\pi^{+}\pi^{-} 
> $ photonuclear production in ultra-peripheral Au-Au collisions at $ 
> \sqrt{s_{\rm {NN}}}$ = 200 GeV from the STAR experiment. The $\pi^{+} 
> \pi^{-}\pi^{+}\pi^{-}$ final states are observed at low transverse  
> momentum and are accompanied by mutual nuclear excitation of the  
> beam particles. The strong enhancement of the production cross  
> section at low transverse momentum is consistent with coherent  
> photoproduction. The $\pi^{+}\pi^{-}\pi^{+}\pi^{-}$  invariant mass  
> spectrum of the coherent events exhibits a broad peak  around 1540 $ 
> \pm$ 40 MeV/$c^2$ with a width of 570 $\pm$ 60 MeV/$c^2$, in  
> agreement with the photoproduction data for the $\rho^{0}$(1700).   
> We do not observe a corresponding peak in the $\pi^{+}\pi^{-}$ final  
> state and measure an upper limit for the ratio of the branching  
> fractions of the $\rho^{0}$(1700) to $\pi^{+}\pi^{-}$ and $\pi^{+} 
> \pi^{-}\pi^{+}\pi^{-}$ of 2.5% at 90% conconfidence level.  The ratio 
>  of $\rho^{0}$(1700) and $\rho^{0}$(770) coherent production cross  
> sections is measured to be 13.4 $\pm$ 0.8stat. $\pm$  4.4syst.%."



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