Symfony how to mongo-odm-aggregation-bundle - mongodb

Question

I am confused to ask this question but I can not find a solution to my problem.
I use the mongo-odm-agregation-bundle to perform an aggregate on my data.
I don't know how to use correctly this bundle, the documentation is not sufficiently explicit and the result is not that i would expect.
So, in mongoDB my code is for the aggregate :
id: { Epreuve:"$EPREUVE", month: { $month: "$DATE" },
day: { $dayOfMonth: "$DATE" }, year: { $year: "$DATE" }},
total: { $sum: "$SCORE" },
nbmots: {$sum: "$NBMOTS"},
moymots: {$avg : "$NBMOTS"},
moytemps:{$avg: "$CHRONOS"},
position: { $sum: 1 },
And the result is :
{
"_id" : {
"Epreuve" : "Verbe",
"month" : NumberInt(2),
"day" : NumberInt(21),
"year" : NumberInt(2017)
},
"total" : NumberLong(430),
"nbmots" : NumberLong(16),
"moymots" : 16.0,
"moytemps" : 147.24,
"position" : 1.0
}
In Symfony, i use this sample to test :
$expr = new \Solution\MongoAggregation\Pipeline\Operators\Expr;
$aq = $this->get('doctrine_mongodb.odm.default_aggregation_query')->getCollection('PortailBundle:DataScoreMotsMeles')->createAggregateQuery();
$result = $aq->match(['SESSION'=>$currentSession])
->group(['_id' => [ 'Epreuve' => "EPREUVE", ],
'Score' => $expr->sum("SCORE"),
'nbMots'=> $expr->sum("NBMOTS"),
'moyMots'=> $expr->avg("NBMOTS"),
'moyTemps'=> $expr->avg("CHRONOS"),
'count' => $expr->sum(1)])
->sort(['count' => -1])
->limit(10)
->getQuery()
->aggregate()
->toArray();
The result is :
Array ( [0] => Array ( [_id] => Array ( [Epreuve] => **EPREUVE** ) [Score] => **0** [nbMots] => **0** [moyMots] => [moyTemps] => [count] => 3 ) )
The problem is the result is 0 each time.
It is normal because i use :
$expr->sum("NBMOTS")
instead of :
$expr->sum('$NBMOTS')
But if i use '$NBMOTS' it doesn't works. So how i do ? I need your help.