balloon ligher than air with libgdx, impementation of FixtureDef.density, gravityScale, linearDamping?

I use libGdx/box2d’s physic. My intention is to create a balloon filled with helium (a body, what hovers above a surface, if we apply a little force to it – it moves fast but not far, if we push against a obstacle – it collide and jump back (restitution)).
I was suggested to use linearDamping or gravityScale to achieve similar behavoir.
So far I achieved a ball behavior: it hovers, it moves (not as intended, just simple move), but no restitution. Here my code:

        mBallDef = new BodyDef();
        mBallDef.type = BodyType.DynamicBody;
        mBallDef.position.set(x, y);

        mShape = new CircleShape();
        mShape.setRadius(radius);

        mFixDef = new FixtureDef();
        mFixDef.shape = mShape;
        mFixDef.density = 0.7f;
        mFixDef.friction = .5f;
        mFixDef.restitution = .8f;

        mBall = world.createBody(mBallDef);
        mBall.setActive(true);
        mBall.setAwake(true);

        mBallFix = mBall.createFixture(mFixDef);
        mBall.setGravityScale(.2f);
        mBall.setLinearDamping(5.5f);

Touch implementation (just moves a balloon up, it’s for test, nothing special)

Gdx.input.setInputProcessor(new InputHandler()
    {
        @Override
        public boolean touchDown(int screenX, int screenY, int pointer, int button) 
        {
            mBall.applyLinearImpulse(0, 15.5f, mBall.getWorldCenter().x + mShape.getRadius(),mBall.getWorldCenter().y - mShape.getRadius(), true);
            mBall.setAngularVelocity(0);

            return true;
        }

    });

When I use for gravity – .2f and for linearDamping – 5.5f, a ball hovers, but I loose a restitution for my balloon. The restitution is available only for linearDamping below 1.5f. And I was told that it’s not good practice to keep gravity far away from 1.
My experiment with FixtureDef.density failed, changing it to various values (grom -2 to 2) has no visible effect.
The questions are:
1) What’s use of density? It doesn’t provide visible affect.
2) What values I should declare to create balloon ligher than air?