For macroscopic objects it's clear that - once observed - the observed property does exist for a while, even if we are no longer observing it. That has to do with the complexity and stability of such objects. A stone is a stone, a tree is a tree. Macroscopic objects are to complex to get transformed somehow and to get reorganized under the influence of our observation. And the influence from the observation onto the object is negligible. So to have needles or leaves is an intrinsic property of trees (as long as the lumberjack does not arrive).
Each electron has a charge and due to this an electric field. The electron has also a magnetic dipole moment and a related intrinsic spin. The two fields are observable from a finite distance only and the observation instruments are heavy influencing the electron. So it could not be excluded completely that the observation creates the property. Than - in a strict sense - this are not intrinsic properties. But somehow I believe, they are. So which are the intrinsic properties of particles and why?
Answer
Symmetries as the definition of particle charges
Modern realistic particle physics theories are constructed from the requirement that there is such symmetry group which defines the quantities which are conserved in all processes which are described by theory (free propagation, interactions). This symmetry group is given as the direct product of subgroups of kinematic symmetry (the Poincare group) and intristic (interaction) symmetry (like Standard model symmetry).
From mathematical point of view, the particle is defined as irreducible representations of the Poincare group. By using Wigner classification of irreducible representation and (in some sense) experimental facts, we may classify all of free particles by the spin $s$ and mass $m$ (if particle is massive) or by helicity $h$ (if the particle is massless). These quantities (charges) are most important quantities of the particle, and they exist independently of the measurement.
Most of interactions are described by the Standard model, which local (i.e., possesses interactions) symmetry group is just $$ G_{\text{local}}\sim SU_{c}(3)\times SU_{L}(2)\times U_{Y}(1), $$ and its adjoint representations are realized by gauge fields. The property of the given particle to interact through these fields (experimental fact, which defines not the value of charges quantity, but the existence of interaction) defines charges under subgroups of SM symmetry group.
There is also experimental fact that in the case of low energies the number of particles of given type is conserved, which defines global symmetry group - baryon and leptons symmetry group: $$ G_{\text{global}}\sim U_{B}(1)\times U_{e}(1)\times U_{\mu}(1)\times U_{\tau}(1), $$ so that the full group of intristic symmetry of most interactions is $$ G_{\text{intr}} \sim G_{\text{local}}\times G_{\text{global}} $$
Particle charges as intristic properties
Intristic properties of the elementary particle is set of most fundamental characteristics which defines it.
Existense of independent on experimental measurements kinematic intristis quantities (mass and spin, or helicity) of the elementary particle comes from the only one experimental fact: our world is in good approximation Poincare-symmetrical. Thus each particle which lives in our world has mass and spin or helicity. This means that these quantities are intristic properties of the particle: by setting values of mass and spin (or helicity) we deal with the given free particle.
Values of local symmetry group charges (more correctly - their ratios) of elementary particles in Standard model are surprisingly fixed by SM itself through selfconsistency requirement (more presisely - by the requirement of the unitarity). For example, the fact of quantization of charge follows from such requirement.
So in the beginning the only thing which we need from the experiment is to fix: the particle interacts through such gauge field, or it doesn't. In some sense we just need to collect experimental data about the property of all particles to interact or not to interact, and then the fundamental requirements such as the unitarity will fix their ratios. Finally, since experimentally we see that interaction of each particle are defined only by these charges, and for each given particle (with given mass and spin or with given helicity) these property is unchanged, we than conclude from the definition that these charges are intristic properties of elementary particle.
Values of intristic global charges, such as lepton and baryon charges, comes from the fact of measurement of conserved number of particles of the given type. I.e., we just see that the summary number of electrons and electron neutrinos minus the summary number of positrons and electron antineutrino is conserved in all interactions, and this defines the electronic lepton number as intristic property.
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