Perturbative <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:mi>χ</mml:mi> <mml:mi>EFT</mml:mi> </mml:mrow> </mml:math> calculations of the deuteron and triton up to <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML">

We extend previous studies of the deuteron and triton ground-state energies to next-to-next-to-leading order <a:math xmlns:a="http://www.w3.org/1998/Math/MathML"> <a:mrow> <a:mo>(</a:mo> <a:msup> <a:mrow> <a:mi mathvariant="normal">N</a:mi> </a:mrow> <a:mn>2</a:mn> </a:msup> <a:mi>LO</a:mi> <a:mo>)</a:mo> </a:mrow> </a:math> in chiral effective field theory, employing a power counting in which subleading interactions are treated perturbatively. Triton calculations are performed using the no-core shell model, and we demonstrate converged perturbative results for regulator cutoffs up to <c:math xmlns:c="http://www.w3.org/1998/Math/MathML"> <c:mrow> <c:mi mathvariant="normal">Λ</c:mi> <c:mo>≈</c:mo> <c:mn>1200</c:mn> </c:mrow> </c:math> MeV. We analyze exceptional cutoffs in the <e:math xmlns:e="http://www.w3.org/1998/Math/MathML"> <e:mrow> <e:msup> <e:mrow/> <e:mn>3</e:mn> </e:msup> <e:msub> <e:mi>P</e:mi> <e:mn>0</e:mn> </e:msub> </e:mrow> </e:math> and <f:math xmlns:f="http://www.w3.org/1998/Math/MathML"> <f:mrow> <f:msup> <f:mrow/> <f:mn>3</f:mn> </f:msup> <f:msub> <f:mi>S</f:mi> <f:mn>1</f:mn> </f:msub> <f:msup> <f:mo>–</f:mo> <f:mn>3</f:mn> </f:msup> <f:msub> <f:mi>D</f:mi> <f:mn>1</f:mn> </f:msub> </f:mrow> </f:math> nucleon-nucleon channels and find a resulting cutoff dependence in the triton ground-state energy at <g:math xmlns:g="http://www.w3.org/1998/Math/MathML"> <g:mrow> <g:msup> <g:mrow> <g:mi mathvariant="normal">N</g:mi> </g:mrow> <g:mn>2</g:mn> </g:msup> <g:mi>LO</g:mi> </g:mrow> </g:math> . The effect associated with the exceptional cutoff in the <i:math xmlns:i="http://www.w3.org/1998/Math/MathML"> <i:mrow> <i:msup> <i:mrow/> <i:mn>3</i:mn> </i:msup> <i:msub> <i:mi>P</i:mi> <i:mn>0</i:mn> </i:msub> </i:mrow> </i:math> channel can be mitigated by redefining the leading-order wave function within the freedom allowed by the effective field theory. The same approach applied in the <j:math xmlns:j="http://www.w3.org/1998/Math/MathML"> <j:mrow> <j:msup> <j:mrow/> <j:mn>3</j:mn> </j:msup> <j:msub> <j:mi>S</j:mi> <j:mn>1</j:mn> </j:msub> <j:msup> <j:mo>–</j:mo> <j:mn>3</j:mn> </j:msup> <j:msub> <j:mi>D</j:mi> <j:mn>1</j:mn> </j:msub> </j:mrow> </j:math> channel remedies the effect of this exceptional cutoff on the ground state energy of the deuteron, but not for the triton.