In this work a new improved coupling function by Xaplanteris et al. (2021) based on Quantum Field Theory is applied for spectral analysis of cosmic rays. This new coupling function had already been confirmed in the
application of cosmic rays with the calculation of primary cosmic ray intensity during Forbush decreases and Ground Level Enhancements but this is the first time that it is used for the calculation of spectral index.
To describe the rigidity spectrum of the GCR during a Forbush decrease, a power law in rigidity is often used. The exponent of this power law, the spectral index, describes the hardness of the GCR spectrum. The spectrum
of the Galactic Cosmic Ray (GCR) becomes harder during the Forbush decrease main phase. For this analysis the Forbush decrease of the cosmic ray intensity observed on June 2015 was chosen. Daily cosmic ray data of the
neutron monitor stations obtained from the high-resolution neutron monitor database-(NMDB) were used for calculating the cosmic ray density and anisotropy variations. For the spectral analysis of the galactic cosmic
rays the technique of Wawrzynczak and Alania (2010) is applied, based on the coupling coefficient method. The obtained results of the Fd analysis by using the new coupling function by Xaplanteris et al. (2021) are
compared with the results obtained based on the coupling function by Clem and Dorman (2000).