Event-shape and multiplicity dependence of freeze-out radii in pp collisions at √{s } = 7 TeV

Two-particle correlations in high-energy collision experiments enable the extraction of particle source radii by using the Bose-Einstein enhancement of pion production at low relative momentum q ∝ 1/ R. It was previously observed that in pp collisions at √{s} = 7TeV the average pair transverse momentum k _T range of such analyses is limited due to large background correlations which were attributed to mini-jet phenomena. To investigate this further, an event-shape dependent analysis of Bose-Einstein correlations for pion pairs is performed in this work. By categorizing the events by their transverse sphericity S _T into spherical ( S _T > 0:7) and jet-like ( S _T < 0:3) events a method was developed that allows for the determination of source radii for much larger values of k _T for the first time. Spherical events demonstrate little or no background correlations while jet-like events are dominated by them. This observation agrees with the hypothesis of a mini-jet origin of the non-femtoscopic background correlations and gives new insight into the physics interpretation of the k _T dependence of the radii. The emission source size in spherical events shows a substantially diminished k _T dependence, while jet-like events show indications of a negative trend with respect to k _T in the highest multiplicity events. Regarding the emission source shape, the correlation functions for both event sphericity classes show good agreement with an exponential shape, rather than a Gaussian one.