C++ - std::unique_ptr in vector<> is nullptr

I want to store Particle objects in a vector object, so I can access it later. These particles (Electrons and Protons) are inherited from the Particle class which contains a toString() virtual method. This toString() method is then overrided in Electron and Proton classes.

When I read the vector container, I want to access to the toString() method specific to Electron or Proton, and not to Particle.

Apparently, one way is to use std::unique_ptr. Here is the part of the code I try to run:

int main(){
    /**/
    std::vector<std::unique_ptr<Particle>> particles(nbParticles);

    particles.push_back(std::unique_ptr<Electron>( new Electron(1.0, 2.0, 3.0)));
    particles.push_back(std::unique_ptr<Proton>(new Proton(1.0, 2.0, 3.0)));
    particles.push_back(std::unique_ptr<Particle>(new Particle(0.0, 0.0, 1.0, 2.0, 3.0)));

    if (particles[0]==nullptr){
        std::cout<< "index=0 : nullptr"<<std::endl; //There is a null_ptr at particles[0]
    }

    if (particles[2]==nullptr){
        std::cout<< "index=2 : nullptr"<<std::endl; //There is not a null_ptr at particles[2]
    }

    std::cout<<particles[0]->toString()<<std::endl; //This is what I'm trying to do
    /**/
}

A pointer to a Particle object seems to be fine, but not to an Electron or Proton. I guess there is something wrong with the constructors ?

class Particle
{
public:
    Particle();
    Particle(double mass, double charge, double posX, double posY, double posZ);
    virtual std::string toString() const;
}

class Electron : public Particle
{
public:
    Electron(double PosX, double PosY, double PosZ);
    virtual std::string toString() const;
}

class Proton : public Particle
{
public:
    Proton(double PosX, double PosY, double PosZ);
    virtual std::string toString() const;
}

and the definitions:

Particle::Particle(double mass, double charge, double posX, double posY, double posZ) :
    m_mass(mass), m_charge(charge),
    m_posX(posX), m_posY(posY), m_posZ(posZ) {}


Electron::Electron(double PosX, double PosY, double PosZ) :
    Particle(9.109E-31, -1.602E-19, PosX, PosY, PosZ){}

Proton::Proton(double PosX, double PosY, double PosZ) :
    Particle(9.109E-31, +1.602E-19, PosX, PosY, PosZ){}