diff --git a/src/particles/diagnostics/CovarianceMatrixMath.H b/src/particles/diagnostics/CovarianceMatrixMath.H
index 63aefd772..8771082ee 100644
--- a/src/particles/diagnostics/CovarianceMatrixMath.H
+++ b/src/particles/diagnostics/CovarianceMatrixMath.H
@@ -161,7 +161,7 @@ namespace impactx::diagnostics
      */
     amrex::ParticleReal
     TraceMat (
-        amrex::Array2D<amrex::ParticleReal, 1, 6, 1, 6> A
+        amrex::Array2D<amrex::ParticleReal, 1, 6, 1, 6> const & A
     )
     {
         int const dim = 6;
@@ -182,8 +182,8 @@ namespace impactx::diagnostics
      */
     amrex::Array2D<amrex::ParticleReal, 1, 6, 1, 6>
     MultiplyMat (
-        amrex::Array2D<amrex::ParticleReal, 1, 6, 1, 6> A,
-        amrex::Array2D<amrex::ParticleReal, 1, 6, 1, 6> B
+        amrex::Array2D<amrex::ParticleReal, 1, 6, 1, 6> const & A,
+        amrex::Array2D<amrex::ParticleReal, 1, 6, 1, 6> const & B
     )
     {
         amrex::Array2D<amrex::ParticleReal, 1, 6, 1, 6> C;
diff --git a/src/particles/diagnostics/EmittanceInvariants.H b/src/particles/diagnostics/EmittanceInvariants.H
index 2d35db001..3766bd9a4 100644
--- a/src/particles/diagnostics/EmittanceInvariants.H
+++ b/src/particles/diagnostics/EmittanceInvariants.H
@@ -43,7 +43,7 @@ namespace impactx::diagnostics
             amrex::ParticleReal,
             amrex::ParticleReal>
     KineticInvariants (
-        amrex::Array2D<amrex::ParticleReal, 1, 6, 1, 6> Sigma
+        amrex::Array2D<amrex::ParticleReal, 1, 6, 1, 6> const & Sigma
     );
 
     /** Returns the three eigenemittances
@@ -68,7 +68,7 @@ namespace impactx::diagnostics
             amrex::ParticleReal,
             amrex::ParticleReal>
     Eigenemittances (
-        amrex::Array2D<amrex::ParticleReal, 1, 6, 1, 6> Sigma
+        amrex::Array2D<amrex::ParticleReal, 1, 6, 1, 6> const & Sigma
     );
 
 } // namespace impactx::diagnostics
diff --git a/src/particles/diagnostics/EmittanceInvariants.cpp b/src/particles/diagnostics/EmittanceInvariants.cpp
index 397323c54..d8ab1daf5 100644
--- a/src/particles/diagnostics/EmittanceInvariants.cpp
+++ b/src/particles/diagnostics/EmittanceInvariants.cpp
@@ -37,7 +37,7 @@ namespace impactx::diagnostics
             amrex::ParticleReal,
             amrex::ParticleReal>
     KineticInvariants (
-        amrex::Array2D<amrex::ParticleReal, 1, 6, 1, 6> Sigma
+        amrex::Array2D<amrex::ParticleReal, 1, 6, 1, 6> const & Sigma
     )
     {
         using namespace amrex::literals;
@@ -98,7 +98,7 @@ namespace impactx::diagnostics
             amrex::ParticleReal,
             amrex::ParticleReal>
     Eigenemittances (
-        amrex::Array2D<amrex::ParticleReal, 1, 6, 1, 6> Sigma
+        amrex::Array2D<amrex::ParticleReal, 1, 6, 1, 6> const & Sigma
     )
     {
         BL_PROFILE("impactx::diagnostics::Eigenemittances");