Exclusive pi+ pi- production at 7TeV

Abstract: We report a measurement of the exclusive production of pairs of charged pions in proton-proton collisions, dominated by the process $pp \rightarrow p^{(*)} \pi^{+}\pi^{-} p^{(*)}$, where $\rm p^{(*)}$ stands for a diffractively dissociated proton, the $\pi^{+} \pi^{-}$ pair is emitted at central rapidities $y$, and the incident protons stay intact or dissociate without detection ${\rm p^{(*)}}$. The measurement is performed with the CMS detector at the LHC, using a data sample corresponding to an integrated luminosity of 450~$\mu$b$^{-1}$ collected at a center-of-mass energy of $\sqrt{s}$ = 7 TeV in 2010. The cross section measured in the phase space defined by pion transverse momentum $p_{\rm T}>0.2$~GeV/c and rapidity $|y|< 2 $ is found to be $20.5~\pm~0.3~(\rm {stat})~(\pm~3.1~\rm {syst})~\pm$~0.8~(lumi)~$\mu$b. The differential cross sections for $\pi^{+}\pi^{-}$ pairs as a function of the pion pair invariant mass, $p_{\rm T}$, and $y$, as well as a single-pion differential cross section as a function of pion $p_{\rm T}$ are also measured and compared to several phenomenological predictions.