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I get this error when trying to integrate a seemingly simple expression:

eradius = 2.8179403267*10^-15;
n = 10^-10/(1.6*10^-19);
gamma = 1600/0.511;
lb = 0.549267;
theta2 = 0.046892183133281855;
eta3 = -(theta1*lb) - (theta1*d);
rho2 = lb/theta2;

CSR3linear = 
  (0.22*eradius*n)/(gamma*rho2^(2/3)*(s/lb (sigma3end - sigma3beg) + sigma3beg)^(4/3));

kick3 = NIntegrate[(eta3 + s^2/(2 rho2))*CSR3linear, {s, 0, lb}] /. 
        {theta1 -> {0.06}, 
         d -> {8.60435},
         sigma3beg -> {1.300512*10^-004},
         sigma3end -> {1.472289*10^-005}};

NIntegrate::inumr: The integrand has evaluated to non-numerical values for all sampling points in the region with boundaries {{0,0.549267}}.

What am I doing wrong here? Also, the choice to have the parameters theta1, d sigma3beg and sigma3end as lists in the integration is because its actually a long list of numbers for each parameter (~20) that would take forever to input separately and record each output; would much prefer the solution to outputed in one list.

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You try to integrate before theta1 etc. are given numeric values. If I understand correctly how you wish to deal with the lists of parameters, then use Map to apply NIntegrate to each integrand:

kick3 = Map[
  NIntegrate[#, {s, 0, lb}] &,
  (eta3 + s^2/(2 rho2))*
    CSR3linear /. {theta1 -> {0.06}, d -> {8.60435}, 
    sigma3beg -> {1.300512*10^-004}, sigma3end -> {1.472289*10^-005}}
  ]
(*
  {-3.91073*10^-6}
*)
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