I overridden module-catalog/product/price/amount/default.phtml file in my theme

    <?php 
        echo $this->helper('Test\ProductDetail\Helper\Data')->DisplayDiscountLabel($_product);
    ?>

In helper file I have following code ;

     namespace Test\ProductDetail\Helper;


     class Data extends \Magento\Framework\App\Helper\AbstractHelper
    {
    public function DisplayDiscountLabel($_product)
    {
       $originalPrice = $_product->getPrice();
        $finalPrice = $_product->getFinalPrice();

        $percentage = 0;
        if ($originalPrice > $finalPrice) {
            $percentage = number_format(($originalPrice - $finalPrice) * 100 / $originalPrice,0);
        }

        if ($percentage) {
            return "(".$percentage."%)";
        }

       }
    }

but problem is that in phtml file I am not getting value of **$_product**?