Abstract
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 sqrt{s} = 7TeV the average pair transverse momentum kT 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 ST into spherical (ST > 0:7) and jet-like (ST < 0:3) events a method was developed that allows for the determination of source radii for much larger values of kT 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 kT dependence of the radii. The emission source size in spherical events shows a substantially diminished kT dependence, while jet-like events show indications of a negative trend with respect to kT 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.
Highlights
S = 7 TeV the average pair transverse momentum kT range of such analyses is limited due to large background correlations which were attributed to mini-jet phenomena
This paper presents two-particle correlation functions (CFs) as a function of the pair relative three-momentum q = (p1 − p2)i (p1 − p2)i and the source radius parameter for different intervals of dNch/dη and kT for jet-like and spherical event topologies
The main detectors used for this analysis are: the Inner Tracking System (ITS) [17], the Time Projection Chamber (TPC) [18], the Time-Of-Flight detector (TOF) [19], and the V0 [20]
Summary
TeV were analyzed, which were recorded by the ALICE experiment at the LHC [15, 16] during the 2010 running period. The ITS is a six-layer cylindrical silicon detector used for precise vertex and track reconstruction close to the interaction point It provides full azimuthal coverage and spans the pseudorapidity range |η| < 0.9. The TPC is the main tracking detector in ALICE and measures the specific ionization energy loss of particles in the TPC gas for particle identification (PID). It covers the whole azimuth and provides a radial coverage of 159 possible space points for tracks. An offline event selection is applied to reject beam-halo induced events and beam-gas collisions Accepted events have their primary vertex reconstructed within ±8 cm from the center of the detector along the beam line in order to ensure uniform tracking performance. Two-track effects such as merging and splitting are minimized using pion pair selection criteria as described in [7] and are known to be negligible in this pT range for q greater than about 50 MeV/c, which is much less than the expected width of the Bose-Einstein correlation peak
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