How do I Avoid Using the Copy Constructor in A Setter

Apologies if this is a basic question, I am new to C++. Let us say I have 2 classes, A and B, defined as below:

class A {
    public:
        A(int i) { a = i; };
        A(A&) = default;
        int a;
};
class B {
public:
    B(A& a) : this_a(a) {};
    B(B&) = default;
    A& this_a;
    void set_a(A& a) {
        this_a = a;
    };
};

and let us say my main method is:

int main()
{
    A a1(5);
    A temp(5);
    B e(temp);
    cout << a1.a << endl;
    e.set_a(a1);
    e.this_a.a = 0;
    cout << a1.a << endl;
    cout << e.this_a.a << endl;

    return 0;
}

The output for this code is:

5
5
0

As far as I can tell, this is the output because in my setter, the copy constructor is being used for the A reference being stored in the B object. This is why:

int main()
{
    A a1(5);
    B e(a1);
    cout << a1.a << endl;
    e.this_a.a = 0;
    cout << a1.a << endl;

    return 0;
}

Outputs:

5
0

Is there any way for me to be able to write the setter in such a way that I can still access the member by reference?