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Higgs Properties in the Fourth Generation MSSM: Boosted Signals Over the 3G Plan

The generalization of the MSSM to the case of four chiral fermion generations (4GMSSM) can lead to significant changes in the phenomenology of the otherwise familiar Higgs sector. In most of the 3GMSSM parameter space, the lighter CP-even $h$ is $\sim 115-125$ GeV and mostly Standard Model-like while $H,A,H^\pm$ are all relatively heavy. Furthermore, the ratio of Higgs vevs, $\tan β$, is relatively unconstrained. In contrast to this, in the 4GMSSM, heavy fourth generation fermion loops drive the masses of $h,H,H^\pm$ to large values while the CP-odd boson, $A$, can remain relatively light and $\tan β$ is restricted to the range $1/2 \lsim \tan β\lsim 2$ due to perturbativity requirements on Yukawa couplings. We explore this scenario in some detail, concentrating on the collider signatures of the light CP-odd Higgs at both the Tevatron and LHC. We find that while $gg \to A$ may lead to a potential signal in the $τ^+τ^-$ channel at the LHC, $A$ may first be observed in the $γγ$ channel due to a highly loop-enhanced cross section that can be more than an order of magnitude greater than that of a SM Higgs for $A$ masses of $\sim 115-120$ and $\tanβ<1$. We find that the CP-even states $h,H$ are highly mixed and can have atypical branching fractions. Precision electroweak constraints, particularly for the light $A$ parameter space region, are examined in detail.