Many years ago Dr. Richard P. Feynman did in his writings this question.
By means of a simple experiment of magnetic resonance (MRI), any student can see that there is a negative particle in the atomic nucleus. In 2005 I published the answer "The Negatron" in:
https://www.researchgate.net/publication/268503521_Teora_QEDa_-_El_tomo_y_su_ncleo?ev=prf_pub
see also:
https://www.researchgate.net/publication/282359307_TCS_Theory_-_Space_properties_quarks_and_particles-2?ev=prf_pub
The current physics theory called "standard model", says absolutely nothing about it.
Book Teoría QEDa - El átomo y su núcleo.
Data TCS Theory - Space properties quarks and particles-2