SCT Experiment

# The SCT Physics

The goal of the SCT experiment is to comprehensively study the processes of production and decay of the charmed hadrons and $$\tau$$ lepton using a data sample about two orders of magnitude larger than those collected in the CLEOc and BESIII experiments. Pairs of $$\tau$$ leptons and charm quarks are produced in electron-positron annihilation near the threshold, as shown on diagrams:

Charm quark cannot exist in a free state and has to produce a charmed hadron, like $$J/\psi$$ meson ($$c\bar{c}$$ quark composition), $$D^0$$ meson ($$c\bar{u}$$ quark composition), $$\Lambda_c^+$$ baryon ($$udc$$ quark composition) and many others, during the hadronization process. Threshold production provides the best experimental environment for a comprehensive study of these particles. The SCT research program includes hundreds of measurements and covers:

• Precise tests of the electroweak (EW) model in the charm and lepton sectors;
• Precise measurements of nonperturbative quantum chromodynamics (QCD) phenomena;
• Direct and indirect searches for phenomena beyond the Standard Model.

The SCT experiment involves the use of a polarized electron source providing 80% longitudinal polarization of the electrons beam. This feature expands capability of studying baryons and tau lepton, in particular, tests of CP sysmmetry in their production and decays. The highlights of the SCT physics program are listed on the cards below:

## Charm

• Measurement of the strong phases of $$D^0$$ decay amplitudes
• Measurement of the absolute branching fractions of charmed hadrons
• Searches for rare and forbidden decays of charmed hadrons
• Measurement of the CP symmetry breaking in charm decay and production

## Tau

• Precision measurement of the $$\tau$$ lepton properties
• Measurement of the Michel parameters, tests of the lepton universality
• Precision measurement of hadronic $$\tau$$ decays
• Search for CP symmetry breaking in $$\tau$$ production and decays

## QCD

• Physics of highly-exited quarkonium
• Baryon interaction at threshold
• Search for glueballs in decays of $$J/\psi$$ and $$\psi(2S)$$
• Measurement of the total cross section of $$e^+e^-\to \mathrm{hadrons}$$

## The SCT energy range

The energy range of the SCT factory ($$\sqrt{s}$$ between 3 GeV and 7 GeV) allows production of all leptons and light hadrons, and most of charmed hadrons.

One particularly interesting thing to measure is the total probability (cross sections) of $$e^+e^-$$ annihilation to hadrons as a function of the center-of-mass energy, and the $$R$$ ratio - ratio of the annihilation cross sections to hadrons and a muon pair:

$$R \equiv \frac{\sigma(e^+e^-\to\mathrm{hadrons})}{\sigma(e^+e^-\to\mu^+\mu^-)}$$.

The plot below summarizes current status of the $$R$$ measurement between $$2$$ and $$7$$ GeV. Tooltips show particularly interesting energy points and ranges, and highlight main research topics.

## The SCT annual yield

Target luminosity of the SCT collider is $$10^{35}~\mathrm{cm}{}^{-2}\mathrm{s}{}^{-1}$$. This machine will provide a large amount of data. The table below shows a possible scenario of data collection during one experimental season:

Annual yields of the SCT experiment. The first column shows process of interest, $$E$$ is the center-of-mass energy, $$\sigma$$ is the cross section of the process, $$L$$ denotes integrated luminosity collected at energy point, and $$N$$ stands for the number of recorded events.
Process $$\mathbf{E\ (GeV)}$$ $$\mathbf{\sigma\ (nb)}$$ $$\mathbf{L\ (fb^{-1})}$$ $$\mathbf{N}$$ Topics
$$e^+e^-\to J/\psi$$ $$3.097$$ $$\approx3400$$ $$300$$ $$10^{12}$$ Light meson spectroscopy, rare decays,
CP symmetry, Weinberg angle
$$e^+e^-\to \tau^+\tau^-$$ $$3.555$$ $$\approx0.2$$ $$50$$ $$10^{8}$$ Hadronic and leptonic decays of tau, CP symmetry,
flavor universality, flavor number violation
$$e^+e^-\to \psi(2S)$$ $$3.686$$ $$\approx640$$ $$150$$ $$10^{11}$$ Light meson spectroscopy
$$\psi(3770) \to D\bar{D}$$ $$3.770$$ $$\approx6$$ $$300$$ $$10^{9}$$ D mesons decays, charm mixing, CP symmetry
$$\psi(4160) \to D_s^+D_s^-$$ $$4.170$$ $$\approx6$$ $$100$$ $$10^{8}$$ $$D_s$$ meson decays
$$e^+e^- \to \Lambda_c^+ \Lambda_c^-$$ $$4.650$$ $$\approx0.2$$ $$100$$ $$10^{8}$$ Charmed baryons production and decays

For a more decailed discussion of the SCT physics program see the conceptual design and materials of dedicated workshops (Orsay 2018, Moscow 2019, Guangzhou 2020, Novosibirsk 2021).