The new physics reach of null tests with $$D \rightarrow \pi \ell \ell $$ and $$D_s \rightarrow K \ell \ell $$ decays

Abstract

| c| = | u| = 1 processes are unique probes of flavor physics in the up-sector within and beyond the Standard Model (SM). SM tests with rare semileptonic charm meson decays are based on an approximate CP-symmetry, a superior GIM-mechanism, angular distributions, leptonuniversality and lepton flavor conservation. We analyze the complete set of null test observables in D → π ( ) and D s → K ( ) decays, ( ) = e, μ, and find large room for new physics safely above the SM contribution. We identify signatures of supersymmetry, leptoquarks and anomaly-free U (1) -models with generation-dependent charges, for which we provide explicit examples. Z -effects in c → u ( ) transitions can be sizable if both left-handed and right-handed couplings to quarks are present.

Charm physics confronts high-pT lepton tails

“…On the other hand, the interpretation of hadron weak decays requires calculations of hadronic matrix elements in lattice QCD which in the charm sector are becoming available with increasing precision [6,[10][11][12][13][14][15][16]. Neutral-current decays are a priori more sensitive to NP because of the strong GIM suppression of the short-distance contributions in the up-quark sector [17][18][19][20][21][22][23]. Nonetheless, these are typically dominated by long-distance hadronic effects, which are difficult to treat from first principles [23][24][25][26][27], hampering a direct theoretical interpretation of the data in terms of short-distance physics.…”

“…Short-distance SM contributions to c → u ( ) transitions are strongly suppressed by the GIM mechanism. As a result, the main SM contributions to rare D-meson decay amplitudes are due to long-distance effects [17,20,22,23,106,107]. While this will be a limiting factor once the experimental measurements become more precise, at present one can obtain bounds on short-distance NP entering in D 0 → ( ) and D (s) → P ( ) by assuming that the experimental limits are saturated by short-distance NP contributions [20,22,23].…”

“…The differential branching ratios for D → π ( ) and D s → K ( ) decays are studied in refs. [20,22]. Currently, the best limits are obtained using D + → π + ( ) decays, for which an expression analogous to that in eq.…”

“…(32) can be found in ref. [22]. 7 Barring cancellations among WCs, we derive the following 95% CL limits at the charm-mass scale 8…”

“…8 For the ee channel, we use the same hadronic coefficients as the ones provided in ref. [22] for the LFV case, given that both experimental limits are obtained from the same BaBar analysis [109] using the same kinematical regimes and that lepton mass effects are negligible. Table 6.…”