  We introduce an effective theory for quantum critical points (QCPs) in heavy
fermion systems, involving a change in carrier density without symmetry
breaking. Our new theory captures a strongly coupled metallic QCP, leading to
robust marginal Fermi liquid transport phenomenology, and associated linear in
temperature ($T$) "strange metal" resistivity, all within a controlled large
$N$ limit. In the parameter regime of strong damping of emergent bosonic
excitations, the QCP also displays a near-universal "Planckian" transport
lifetime, $\tau_{\mathrm{tr}}\sim\hbar/(k_BT)$. This is contrasted with the
conventional so-called "slave boson" theory of the Kondo breakdown, where the
large $N$ limit describes a weak coupling fixed point and non-trivial transport
behavior may only be obtained through uncontrolled $1/N$ corrections. We also
compute the weak-field Hall coefficient within the effective model as the
system is tuned across the transition. We further find that between the two
plateaus, reflecting the different carrier densities in the two Fermi liquid
phases, the Hall coefficient can develop a peak in the critical crossover
regime, like in recent experimental findings, in the parameter regime of weak
boson damping.
