CodeWars Sum By Factors C++ No idea how to make it work

I try to solve Sum by Factors on CodeWars and I have no idea how to get it working with C++. My code prints just the highest prime factor for every number. Good solution is: I = {12, 15}; // result = "(2 12)(3 27)(5 15)" and mine is Expected: equal to (2 12)(3 27)(5 15) Actual: (3 12)(3 27)(5 15). Can anybody take a look at this code and help me find correct solution?

I don't want to rewrite for instance Python code to Cpp, just trying to improve myself.

#include <string>
#include <vector>
#include <utility>
#include <cmath>
#include <numeric>

class SumOfDivided
{
public:
    static std::string sumOfDivided(std::vector<int> &lst)
    {
      std::vector<std::pair<int, int>> v;
      std::string rtn = "";

      int sum = std::accumulate(lst.begin(), lst.end(), 0);
      v.emplace_back(std::make_pair(primeFactors(std::ref(sum)), std::ref(sum)));

      for(auto& elem : lst)
        v.emplace_back(std::make_pair(primeFactors(std::ref(elem)), std::ref(elem)));

      sort(v.begin(), v.end());

      for(auto& elem : v)
        rtn += "(" + std::to_string(elem.first) + " " + std::to_string(elem.second) + ")";

      return rtn;
    }

    static int primeFactors(int& n)
    { 
      int helper = 2;
      int number = abs(n);

      while(number > helper)
        if(number % helper == 0)
        {
          number /= helper;
          helper = 2;
        }
        else
          helper++;

      return helper;
    }
};