A hallmark of graphene is its unusual conical band structure that leads to a zero-energy band gap at a singleElectrons in metals and semiconductors undergo many complex interactions, and most theoretical treatments make use of the quasiparticle approximation, in which independent electrons are replaced by electron-and holelike quasiparticles interacting through a dynamically screened Coulomb force. The details of the screening are determined by the valence band structure, but the band energies are modified by the screened interactions. A complex self-energy function describes the energy and lifetime renormalization of the band structure resulting from this interplay.Bohm and Pines (1) accounted for the short-range interactions between quasiparticles through the creation of a polarization cloud formed of virtual electron-hole pairs around each charge carrier, screening each from its neighbors. The long-range interactions manifest themselves through plasmons, collective charge density oscillations of the electron gas that can propagate through the medium with their own band-dispersion relation.These plasmons can in turn interact with the charges, leading to strong self-energy effects. Lundqvist predicted the presence of new composite particles called plasmarons formed by the coupling of the elementary charges with plasmons (2). Their distinct energy bands should be observable using angle-resolved photoemission spectroscopy (ARPES), but so far have only been observed by optical (3, 4) and tunneling spectroscopies (5), which probe the altered density of states.Understanding the coupling between electrons and plasmons is important because of new "plasmonic" devices proposed to merge photonics and electronics. Graphene in particular has been proposed as a promising candidate for such devices (6-8). Plasmarons have been predicted to occur in graphene and to be observable in ARPES (9, 10), yet their detailed dispersion and interaction with defects remain unknown. Here we present a

Graphene is described at low energy by a massless Dirac equation whose eigenstates have definite chirality. We show that the tendency of Coulomb interactions in lightly doped graphene to favor states with larger net chirality leads to suppressed spin and charge susceptibilities. Our conclusions are based on an evaluation of graphene's exchange and random-phase-approximation correlation energies. The suppression is a consequence of the quasiparticle chirality switch which enhances quasiparticle velocities near the Dirac point.

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