In Horizon 2020, the general principle is that each partner has the right to transfer its own project results to a third party, such as its mother company.
However, if the transferee is located in a third country, i.e. a non-EU country non-associated with Horizon 2020 (such as the US), transfers will be possible in theory, but may be made subject to certain conditions. This will depend on whether your own Grant Agreement contains an optional clause to this effect (article 30.3 of the Model Grant Agreement).
If this optional clause is included in your Grant Agreement, the transfer of project results to a third party located in the US – or in any other non-associated country – will be subject to a prior, formal notification to the European Commission. Following this notification, the Commission will have the right to object to the transfer, if it considers that it is not in line with EU competitiveness interests, not consistent with ethical principles, or not consistent with security considerations. If the Commission objects, no transfer can take place; if it allows the transfer provided that certain conditions are met, no transfer can take place until these conditions are fulfilled.
If the optional article 30.3 is not included in your GA, transfers to third parties will be allowed, regardless of their location.
In any case, if a transfer is possible or allowed, please note that the transferee (US company) will still have to comply with certain obligations, such as the obligations to exploit, disseminate, and grant access rights to the project results (please see article 30.1 of the Model Grant Agreement).