public function findByReferenceUser($Reference, $User)
{
    $qb = $this->createQueryBuilder('a')
        ->select('a')
        ->join('a.Reference', 'r')
        ->where('r.id = :refId')
        ->setParameter("refId", $Reference->getId())
        ->join('a.User', 'u')
        ->andWhere('u.id = :userId')
        ->setParameter("userId", $User->getId());

    return $qb->getQuery()->getOneOrNullResult();
}

$list_article = $RArticle->findByReferenceUser($reference, $panier->getUser());
    foreach ($list_article as $key => $article)
        echo gettype($article);

$list_article = $RArticle->findAll();
    foreach ($list_article as $key => $article)
        echo gettype($article);