Using collider physics to investigate high energy neutrinos

Abstract

For a long time it was assumed that the charmed baryon-to-meson ratio was universal reflecting the universality of the charm fragmentation fractions. Due to this assumption all the event generators and MC simulations used the fragmentation fraction measured at Hera and LEP in $e^+e^-$ collisions for all collision types and energy scales. Recently a measurement by ALICE proved that this ratio depends on the transverse momentum of the particles as well as on the collision energy \cite{Acharya:2017kfy}. The new measurements show that the ratio is a factor 4-6 higher than what was measured in e^+e^- collisions. The detailed understanding of this effect is of fundamental importance in physics. Particularly in recent years the IceCube experiment discovered the existence of astronomical neutrinos. One of the main contributors to the background signal are atmospheric neutrinos coming from charmed hadron decays in the Earths atmosphere. This background is modelled by using MC generators where the fragmentation fractions are assumed universal and set to the value measured in e^+e^- collisions. The breaking of the universality will change the neutrino flavour content and it might affect the correct representation of such a background. In this study the Pythia8 event generator is tuned such that it reproduces the Lambda_{c}^{+}/D^0 ratio as measured in 7 TeV pp collisions by ALICE. Then the changes in the neutrino energy distribution are studied. The tuning is based on new colour reconnection modes made for these type of collisions. By taking the new mode that takes the full QCD colour rules into account the baryon-to-meson ratio is reproduced well. The differential production cross-section of the Lambda_{c}^{+} baryon improved between 20 and 30% while the D^0 cross-section agrees 10% less with data. There is an indication that the neutrino energy distribution changes but for the high energetic neutrinos the uncertainties are too large to be able to draw conclusions. We tried obtaining more statistics at these energies by putting a minimum momentum transfer for the parton interaction but this did not give the desired results. Hence more events must be generated and further tuning is necessary.