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Consider the following function dsPrimakovdtnucleus:

(*Helm*)
mMo=96;
Zmo=42;
Amo = 96;
sval = 0.9;
fmtoGeVm1 = 5;
R1 = Sqrt[(1.23*Amo^(1/3) - 0.6)^2 + 7/3 Pi^2*0.52^2 - 5*sval^2];
FHelm[Z_, q2_] = 3*Z*BesselJ[1, Sqrt[q2]*R1*fmtoGeVm1]/(Sqrt[q2]*R1*fmtoGeVm1)*Exp[-((q2*sval^2*fmtoGeVm1^2)/2)];
aEMval = 1/137;
(*Differential cross section*)
sinv[Eg_, mt_] = 2*mt*Eg + mt^2;
dsPrimakovdtnucleus[ma_, t_, Eg_] = (2*aEMval*FHelm[Zmo, -t]^2*mZ^4/(t^2*(mZ^2 - s)^2 (t - 4*mZ^2)^2) (ma^2*t (mZ^2 + s) -ma^4 mZ^2 - t*((mZ^2 - s)^2 + s*t)) /. {s -> sinv[Eg, mZ]} // Simplify) /. {mZ -> mMo};

Here, t is a variable, while ma, Eg are positive parameters, with Eg > Egmin, where

Egmin[ma_] = (ma^2 + 2*mMo*ma)/(2*mMo);

I need to integrate dsPrimakovdtnucleus over t which ranges within tmin and tmax:

tmin[ma_, Eg_] = 
  ma^4/(4*sinv[Eg, 
       mt]) - ((sinv[Eg, mt] - mt^2)/(2*Sqrt[sinv[Eg, mt]]) + 
      Sqrt[(sinv[Eg, mt] + ma^2 - mt^2)^2/(4*sinv[Eg, mt]) - ma^2])^2/.{mt->mMo};
tmaxTemp[ma_, Eg_, mt_] = 
  ma^4/(4*sinv[Eg, 
       mt]) - ((sinv[Eg, mt] - mt^2)/(2*Sqrt[sinv[Eg, mt]]) - 
      Sqrt[(sinv[Eg, mt] + ma^2 - mt^2)^2/(4*sinv[Eg, mt]) - ma^2])^2;
tmaxLimit[ma_, Eg_] = 
  Assuming[Eg > 0 && mt > 0, 
   Simplify[Normal@Series[tmaxTemp[ma, Eg, mt], {ma, 0, 4}]]];
tmax[ma_, Eg_] = 
  If[ma/Eg > 0.01, Evaluate[tmaxTemp[ma, Eg, mt]/.{mt->mMo}], 
   Evaluate[tmaxLimit[ma, Eg]]];

Note that I have expanded tmaxTemp in the limit ma/Eg << 1.

The integral is

sPrimakovNucleus[ma_, Eg_] := 
 If[Eg > Egmin[ma], 
  NIntegrate[
   dsPrimakovdtnucleus[ma, t, Eg], {t, 
    tmin[ma, Eg], tmax[ma, Eg]}], 0]

It behaves well until the ratio ma/Eg becomes small. Say, sPrimakovNucleus[1, 10] returns 0.73 without any issues, while sPrimakovNucleus[1, 50] returns

NIntegrate::ncvb: NIntegrate failed to converge to prescribed accuracy after 9 recursive bisections in t near {t} = {-0.00010003}. NIntegrate obtained 0.129474 and 0.122071 for the integral and error estimates.

The reason is that the integrand includes a pole, and the upper integration limit becomes very close to it, being regularized by the limit tmaxLimit. If it is very small (i.e., ma/Eg << 1), then, probably, Mathematica fails.

Could you please tell me how to avoid this problem?

Edit: added missing definitions.

Update: the problem was that once ma/Eg becomes much less than one, only a very narrow integration domain contributes to the total value. So I may manually reduce tmax[ma, Eg] to say 0.1 tmax[ma,Eg]. It would be better however to automatize this procedure...

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  • $\begingroup$ Do we need definitions for mMo and Zmo to be able to test this to see if a fix works? $\endgroup$
    – Bill
    Commented Feb 2, 2023 at 2:04
  • $\begingroup$ @Bill : oh, I am totally sorry. Just added them. $\endgroup$ Commented Feb 2, 2023 at 7:06
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    $\begingroup$ Change all decimal numbers to exact rationals to allow precision greater than MachinePrecision. Then add two options to NIntegrate, WorkingPrecision->64 and MaxRecursion->32 That allowed me to evaluate sPrimakovNucleus[1, 50] without warning or error messages. Experiment with those two option values to see what you really need. $\endgroup$
    – Bill
    Commented Feb 3, 2023 at 7:05

1 Answer 1

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Your integrand has an exponential-decay factor, which causes "a very narrow integration domain [to contribute] to the total value" when ma/Eg is small. A simple change of variables $t=-e^u$ fixes it.

sPrimakovNucleus[ma_, Eg_] := 
 If[Eg > Egmin[ma], 
  NIntegrate[
   -Exp[u] dsPrimakovdtnucleus[ma, -Exp[u], Eg],
   {u, Log[-tmin[ma, Eg]], Log[-tmax[ma, Eg]]}
   ], 
  0];
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