Evidence for a Soft Nuclear Equation-of-State from Kaon Production in Heavy-Ion Collisions

Abstract

The production of pions and kaons has been measured in 197 Au+ 197 Au collisions at beam energies from 0.6 to 1.5 A·GeV with the Kaon Spectrometer at SIS/GSI. The K + meson multiplicity per nucleon is enhanced in Au+Au collisions by factors up to 6 relative to C+C reactions whereas the corresponding π ratio is reduced. The ratio of the K + meson excitation functions for Au+Au and C+C collisions increases with decreasing beam energy. This behavior is expected for a soft nuclear equation-of-state.

Phase transitions in dense matter and the maximum mass of neutron stars

“…In particular, the softest of our three EoSs, BSk19, will then be able to support a neutron star as massive as PSR J1614−2230, provided the phase transition begins at a density n N as low as 0.2 fm −3 . This shows, incidentally, that the existence of a two-solar mass neutron star is not necessarily incompatible with the soft nuclear-matter EoS that is suggested by the measurements of the kaon and pion productions in heavy-ion collisions (Fuchs et al 2001;Sturm et al 2001;Hartnack et al 2006;Xiao et al 2009). …”

“…Since the softest of our Skyrme functionals that is compatible with the measured mass of PSR J1614−2230 is BSk20 , the question arises as to whether it is not too stiff to be compatible with the analysis of K + production (Fuchs et al 2001;Sturm et al 2001;Hartnack et al 2006) and π − /π + production ratio (Xiao et al 2009) that have been measured in heavy-ion collisions. In particular, the former analysis suggests that the EoS of symmetric nuclear matter is much softer than what is obtained with BSk20 over the range 2n 0 n 3n 0 , whereas the latter analysis concludes that over the range 2n 0 n 3.5n 0 the symmetry energy rises significantly less steeply than predicted for the APR EoS, on which BSk20 is based.…”

On the Maximum Mass of Neutron Stars

“…Alternatively, measurements of the kaon and pion productions in heavy-ion collisions [42][43][44][45] seem to suggest a very soft EoS. Various exotic mechanisms such as a "fifth force" 46 or variations of the gravitational constant 47 have been proposed to account simultaneously for both this result and the existence of massive NSs such as PSR J1614−2230…”

Neutron star properties with unified equations of state of dense matter

“…The BSk21 EDF is also favoured by recent measurements of atomic masses . Although the functional underlying the softest EoS based on the EDF BSk19 seems to be favoured by the analysis of heavy-ion collision experiments (Fuchs et al 2001;Sturm et al 2001;Hartnack et al 2006;Xiao et al 2009), the corresponding EoS appears to be ruled out by astrophysical observations. However, as discussed by Chamel et al (2013), this apparent discrepancy could be easily resolved by considering the occurrence of a transition to an exotic phase in NS cores.…”