The lokuttara citta experiences the dhamma which does not arise and fall away, it experiences nibbana.
There are four paramattha dhammas: citta, cetasika, rūpa and nibbāna. Citta, cetasika and rupa are realities which arise and fall away, they are conditioned dhammas (sankhāra dhammas). Nibbāna does not arise and fall away. It has no conditions through which it arises, it is an unconditioned dhamma (visankhāra dhamma). We cannot experience the unconditioned reality unless paññā is developed to the degree that it can experience the conditioned dhammas as they are: impermanent, dukkha and anattā (not self).
Do both magga-citta and phala-citta directly experience nibbāna?
The magga-citta and the phala-citta are lokuttara cittas, thus they have nibbāna as the object. When the magga-citta has fallen away, it is succeeded immediately by the phala-cittas which experience the same object. When one performs kāmāvacara kusala kamma (kusala kamma of the sensuous plane of consciousness) the vipāka does not follow immediately. Even if the vipāka were to arise soon after the kamma, it could never arise in the same process of citta. It is different with the lokuttara citta. The magga-citta has to be followed immediately by the phala-cittas, which are two or three moments of citta, depending on the individual.