void check(){
       
      TFile *f1 = new TFile("KspipiCovariancematrix_cisi.root","read");
      TMatrixDSym CovMat = *(TMatrixDSym*)f1->Get("CovMat_stat");            
      
      delete f1;
      
      //perform Cholesky decomposition
      TDecompChol decompMat(16);
      decompMat.SetMatrix(CovMat);
      decompMat.Decompose(); 
        
      const TMatrixD a = decompMat.GetU();
      a.T();


}