| The Standard Model(SM)of particle physics is the most successful theory which describes the elementary particles in nature and their interactions so far.A large amount of experimental tests have supported its reliability since its establishment.However,the SM also has some deficiencies,such as the problem of dark matter candidates,the anomalies in flavor physics and so on,some of which may indicate the possible existence of New Physics(NP)beyond the SM.Given that the current direct detections at colliders have not provided any unambiguous NP signals,one would expect the probability of detecting indirectly NP through the various low-energy flavor physics processes.Within the SM,there is no flavor-changing neutral current(FCNC)at tree level,which means that the processes involving the FCNC transitions are the main ground to examine the SM and search for possible NP.Most studies of the FCNCs focus on the processes involving K and B mesons,such as the K0-(?)0 and Bd(s)0-(?)d(s)0 mixings,as well as the rare decays like KL0→/π0v(?),K+→π+v(?),K+→π+μ(?),B→Xsv(?),B→Xsγ,B→Xsμ+μ-and so on.However,much fewer studies have been done on the FCNCs in the charm physics,principally because the theoretical values of the corresponding SM predictions are very tiny for both the D0-(?)0 mixing and the rare charmed-hadron weak decays.This is in sharp contrast to the FCNC processes of K and B mesons.Within the SM,the FCNCs related to the D mesons involve the light down-type quarks in the loops,being quite different from the ones related to the K and B mesons(here the loop diagrams are dominated by the top-quark contribution).This makes the former suppressed strongly by the Glashow-Iliopoulos-Maiani(GIM)mechanism.Nevertheless,extensions of the SM may invalidate such a suppression,and can even make their contributions being of several orders of magnitude larger than that of the SM.As a consequence,the charm FCNCs are also the important probe of NP.This dissertation concerns mainly the studies of NP searches in the charm physics through the low-energy ep scattering processes.In chapter 1,we briefly introduce the significance and the current situation of the charm physics.Chapter 2 emphatically introduces the theoretical basis of the charm physics,including the SM of particle physics,the renormalization group and the FCNCs,and also recapitulates the difficulties of the theoretical models used to describe the FCNCs in the charm physics.Chapters 3 and 4 are the main body of this dissertation,with the specific contents given as follows:Firstly,we investigate the NP effects in the charm physics through the low-energy scattering processes e-p→e-(μ-)Λc.These scattering processes can be realized in the ep scattering experiments by adopting our properly selected experimental setups.In the framework of the general low-energy effective Lagrangian,we show that the low-energy scattering processes can provide more competitive potentials for NP searches than the traditional charmed-hadron weak decays and the high-pT searches of dilepton invariant mass tails at high-energy colliders.In the three specific Leptoquark(LQ)models R2,U3 and(?)1,we also show that the low-energy scattering processes are promising to be observed,and provide also a possible way to distinguish the scalar and vector LQ models in future experiments.Secondly,based on the last observation that the scalar and vector LQ models can be discriminated from each other through the two scattering processes,we proceed to investigate if all the LQ models can be disentangled from each other through the low-energy ep scattering processes.We find that four spin asymmetries ALe,ALp,AL3ep and AL6ep can be defined in terms of the cross sections of the low-energy scattering processes(?)-p→e-Λc,e-(?)→e-Λc and(?)→e-Λc,with the initial electron and/or proton longitudinally polarized,and the LQ models can be then effectively distinguished from each other by analyzing the measured values of these four spin asymmetries.However,this cannot be achieved by the transversely polarized scattering processes unless high experimental accuracy can be ensured in the future experiments.Nevertheless,these transversely polarized scattering processes are far from being in vain,because they provide us with a unique opportunity to directly measure the imaginary part of the Wilson coefficients(WCs)in the general low-energy effective Lagrangian.Through our properly selected experiment setups,we also find that all of these polarized scattering processes are promisingly observed;even in the worst-case scenario—no any experimental signals are observed at all,we can also impose severe constraints on the WCs through these polarized scattering processes.Compared with the traditional charmed-hadron weak decays and the high-pT searches of dilepton invariant mass tails at high-energy colliders,the low-energy polarized scattering processes provide more competitiveness for searching potential NP.With the rapid development of experiments,we expect that the proposed mechanisms in this dissertation will be very helpful for detecting and verifying the various NP scenarios.Finally,our summary and prospect are made in chapter 5.Meanwhile,for the convenience of discussions,we provide in the appendices the Fierz identities as well as the analytic expressions of the amplitudes squared of the unpolarized and polarized scattering processes. |
