Browsing by Subject "Particle correlations and fluctuations"

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  • The ALICE collaboration; Acharya, S.; Brucken, E. J.; Chang, B.; Hilden, T. E.; Kim, D.; Litichevskyi, V.; Mieskolainen, M. M.; Parkkila, J. E.; Rak, J.; Räsänen, S. S.; Saarinen, S.; Slupecki, M.; Snellman, T. W.; Trzaska, W. H.; Vargyas, M.; Viinikainen, J.; Yoon, J. H.; Zhou, Z.; Zhu, Y. (2019)
    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 proportional to 1/R. It was previously observed that in pp collisions at root s = 7 TeV 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.
  • The CMS collaboration; Sirunyan, A. M.; Eerola, P.; Kirschenmann, H.; Pekkanen, J.; Voutilainen, M.; Järvinen, T.; Karimäki, V.; Kinnunen, R.; Lampén, T.; Lassila-Perini, K.; Lehti, S.; Lindén, T.; Luukka, P.; Tuominen, E.; Tuominiemi, J.; Talvitie, J.; Tuuva, T. (2018)
    Measurements of differential cross sections d sigma/d phi* and double-differential cross sections d(2)sigma/ld phi*d/y/ for inclusive Z boson production are presented using the dielectron and dimuon final states. The kinematic observable phi* correlates with the dilepton transverse momentum but has better resolution, and y is the dilepton rapidity. The analysis is based on data collected with the CMS experiment at a centre-of-mass energy of 8 TeV corresponding to an integrated luminosity of 19.7 fb(-1). The normalised cross section (1/sigma) d sigma/d phi*, within the fiducial kinematic region, is measured with a precision of better than 0.5% for phi* <1. The measurements are compared to theoretical predictions and they agree, typically, within few percent.
  • The CMS collaboration; Sirunyan, A. M.; Tumasyan, A.; Eerola, P.; Forthomme, Laurent; Kirschenmann, H.; Österberg, K.; Voutilainen, M.; Bharti, M.; Brücken, Erik; Garcia, F.; Havukainen, J.; Karimäki, V.; Kim, Minsuk; Kinnunen, R.; Lampén, T.; Lassila-Perini, K.; Laurila, S.; Lehti, S.; Lindén, T.; Luukka, P.; Siikonen, H.; Tuominen, E.; Tuominiemi, J.; Tuuva, T. (2021)
    Measurements of the differential cross sections of Z + jets and gamma + jets production, and their ratio, are presented as a function of the boson transverse momentum. Measurements are also presented of the angular distribution between the Z boson and the closest jet. The analysis is based on pp collisions at a center-of-mass energy of 13 TeV corresponding to an integrated luminosity of 35.9 fb(-1) recorded by the CMS experiment at the LHC. The results, corrected for detector effects, are compared with various theoretical predictions. In general, the predictions at higher orders in perturbation theory show better agreement with the measurements. This work provides the first measurement of the ratio of the differential cross sections of Z + jets and gamma + jets production at 13 TeV, as well as the first direct measurement of Z bosons emitted collinearly with a jet.