Search for a very light NMSSM Higgs boson produced in decays of the 125 GeV scalar boson and decaying into $\tau$ leptons in pp collisions at $\sqrt{s}=8$ TeV

A search for a very light Higgs boson decaying into a pair of τ leptons is presented within the framework of the next-to-minimal supersymmetric standard model. This search is based on a data set corresponding to an integrated luminosity of 19.7 fb$^{−1}$ of proton-proton collisions collected by the CMS experiment at a centre-of-mass energy of 8 TeV. The signal is defined by the production of either of the two lightest scalars, h$_{1}$ or h$_{2}$, via gluon-gluon fusion and subsequent decay into a pair of the lightest Higgs bosons, a$_{1}$ or h$_{1}$. The h$_{1}$ or h$_{2}$ boson is identified with the observed state at a mass of 125 GeV. The analysis searches for decays of the a$_{1}$ (h$_{1}$) states into pairs of τ leptons and covers a mass range for the a$_{1}$ (h$_{1}$) boson of 4 to 8 GeV. The search reveals no significant excess in data above standard model background expectations, and an upper limit is set on the signal production cross section times branching fraction as a function of the a$_{1}$ (h$_{1}$) boson mass. The 95% confidence level limit ranges from 4.5 pb at ${m}_{{\mathrm{a}}_1}$ $\left({m}_{{\mathrm{h}}_1}\right)=8$ GeV to 10.3 pb at ${m}_{{\mathrm{a}}_1}$ $\left({m}_{{\mathrm{h}}_1}\right)=5$ GeV.