Stopping a long-running asynchronous computation

I'm writing a program that will perform a long-running computation, gradually improving a solution. On top of that, there's a simple UI that basically allows the user to stop the computation (letting the worker finish a step it's working on) and shows the best solution. Here's a simplified version of what I have, I'm using async/await for the first time. Is there anything you would improve, or any other comments?

Worker code

public class SearchEngine
{
    //...
    public async Task<IProblemSolution> Search(CancellationTokenSource cts)
    {
        await Task.Factory.StartNew(() => {
            BestSolution = Problem.RandomSolution();
            while (!cts.IsCancellationRequested)
            {
                IProblemSolution s = Problem.RandomSolution(); // for simplification 
                if (s.ObjectiveValue() < BestSolution.ObjectiveValue())
                    BestSolution = s;
            }
        });
        return BestSolution;
    }
}

UI

    private async void StartButton_Click(object sender, RoutedEventArgs e)
    {
        if (CancelTokenSrc != null)
            return;

        CancelTokenSrc = new CancellationTokenSource();
        Engine = new SearchEngine(new SampleProblem());

        StopButton.IsEnabled = true;
        StartButton.IsEnabled = false;
        Task<IProblemSolution> t = Engine.Search(CancelTokenSrc);
        await t;

        ResultLabel.Content = t.Result.ObjectiveValue().ToString();
        StopButton.IsEnabled = false;
        StartButton.IsEnabled = true;
        CancelTokenSrc = null;
    }

    private void StopButton_Click(object sender, RoutedEventArgs e)
    {
        if (CancelTokenSrc != null)
            CancelTokenSrc.Cancel();
    }