boost threads return type operator()

I've been working with boost::threads to do some multithreading in my
code and have run into some questions that I haven't been able to find
answers to.

I'll include some sample code to illustrate my questions.

#include <boost/thread/thread.hpp>
#include <iostream>

using namespace std;

class mt
{
private:
    double _alpha;
    double _beta;
    double _x;
    double& _y1;

public:
    double _y2;

public:
    mt( double alpha, double beta, double x, double& y1 ):
        _alpha(alpha), _beta(beta),
        _x(x), _y1(y1) {};

    void operator()()
    {
        // Some function more complicated than this...
        _y1 = _alpha/_beta*_x;
        _y2 = _y1;
    };

    double GetY2() { return _y2; };
};

void main()
{
    double y1;
    double y2;
    mt a( 4,1,3,y1 );
    boost::thread thrd( a );
    thrd.join();
    y2 = a.GetY2();
    cout << y1 << endl;
    cout << y2 << endl;
}

If I understand everything correctly, when I call
boost::thread thrd( a ),
it creates a new (thread local) instance of mt with which to work with
which goes out of scope at
thrd.join(). That is why y1 gives a good answer, but y2 contains garbage.

Now for my question:

Is it possible to call something like:
    y = boost::thread thrd( a(x) );
    where I've redefines operator() to take a double "x"
    and return a double "y".
    Then I don't have to give x and y when I construct "a".

Basically, my question is how to return something from the () operator
inside the thread.

Please ask for clarification if I haven't been clear.