Search

Abstract A sensitivity study for the search for the charged lepton flavor violating process $$\tau \rightarrow \gamma \mu $$ τ → γ μ at the Super $$\tau $$ τ -Charm Facility is performed with a fast simulation. With the expected performance of the current detector design and an integrated luminosity of $$1\,\textrm{ab}^{-1}$$ 1 ab - 1 corresponding to one-year of data taking, the sensitivity on the branching fraction (BF) of $$\tau \rightarrow \gamma \mu $$ τ → γ μ is estimated to be at the level of $$10^{-8}$$ 10 - 8 . The sensitivity under different detector performances are also studied. With ideal performance, the BF could be probed to be $$2.8\,\times \,10^{-8}$$ 2.8 × 10 - 8 at 90% confidence level. The sensitivity is expected to scale with the square root of the luminosity, therefore with a total luminosity of $$10\,\hbox {ab}^{-1}$$ 10 ab - 1 corresponding to ten-year of data taking, the sensitivity could reach $$8.8\,\times 10^{-9}$$ 8.8 × 10 - 9 , which is about one order of magnitude improvement upon the current best upper limit.

Abstract We report a feasibility study of pure leptonic decay $$D_s^+\rightarrow \mu ^+\nu _\mu $$ D s + → μ + ν μ by using a fast simulation software package at STCF. With an expected luminosity of $$1~\mathrm {ab}^{-1}$$ 1 ab - 1 collected at STCF at a center-of-mass energy of 4.009 GeV, the statistical sensitivity of the branching fraction is determined to be 0.3%. Combining this result with the $$c \rightarrow s$$ c → s quark mixing matrix element $$|V_{cs}|$$ | V cs | determined from the current global Standard Model fit, the statistical sensitivity of $$D_s^+$$ D s + decay constant, $$f_{D_s^+}$$ f D s + , is estimated to be 0.2%. Alternatively, combining the current results of $$f_{D_s^+}$$ f D s + calculated by lattice QCD, the statistical sensitivity of $$|V_{cs}|$$ | V cs | is determined to be 0.2%, which helps probe possible new physics beyond Standard Model. The unprecedented precision to be achieved at STCF will provide a precise calibration of QCD and rigorous test of Standard Model.

Abstract We report a sensitive study of measuring $$b\rightarrow s\gamma $$ b → s γ photon polarisation in $$D^{0}\rightarrow K_1(1270)^-e^+\nu _e$$ D 0 → K 1 ( 1270 ) - e + ν e with an integrated luminosity of $$\mathscr {L}$$ L = 1 ab $$^{-1}$$ - 1 at a center-of-mass energy of 3.773 GeV at a future Super Tau Charm Facility. More than 61,000 signals of $$D^{0}\rightarrow K_1(1270)^-e^+\nu _e$$ D 0 → K 1 ( 1270 ) - e + ν e are expected. Based on a fast simulation software package, the statistical sensitivity for the ratio of up-down asymmetry is estimated to be $$1.5\times 10^{-2}$$ 1.5 × 10 - 2 by performing a two-dimensional angular analysis in $$D^{0}\rightarrow K_1(1270)^-e^+\nu _e$$ D 0 → K 1 ( 1270 ) - e + ν e . Combining with measurements of up-down asymmetry in $$B\rightarrow K_1\gamma $$ B → K 1 γ , the photon polarisation in $$b\rightarrow s\gamma $$ b → s γ can be determined model-independently.