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Abstract
Some models of leptogenesis involve a quasi-degenerate pair of heavy neutrinos N1,2 whose masses can be small, O(GeV). Such neutrinos can contribute to the rare lepton-number-violating (LNV) decay {W}^{pm}to {mathrm{ell}}_1^{pm }{mathrm{ell}}_2^{pm }{left({q}^{prime}overline{q}right)}^{mp } . If both N1 and N2 contribute, there can be a CP-violating rate difference between the LNV decay of a W− and its CP-conjugate decay. In this paper, we examine the prospects for measuring such a CP asymmetry ACP at the LHC. We assume a value for the heavy-light neutrino mixing parameter |BℓN|2 = 10−5, which is allowed by the present experimental constraints, and consider 5 GeV ≤ MN≤ 80 GeV. We consider three versions of the LHC — HL-LHC, HE-LHC, FCC-hh — and show that small values of the CP asymmetry can be measured at 3σ, in the range 1% ≲ ACP ≲ 15%.
Highlights
PreambleIt is useful to make some preliminary remarks. For the decay W − → F , where F is the final state, the signal of CP violation will be a nonzero value of BR(W − → F ) − BR(W + → F) ACP = BR(W − → F ) + BR(W + → F)
N1,2 whose masses can be small, O(GeV)
JHEP04(2021)021 studies all use as motivation the neutrino minimal standard model [22,23,24,25], which combines the seesaw mechanism and leptogenesis, and even provides a candidate for dark matter
Summary
It is useful to make some preliminary remarks. For the decay W − → F , where F is the final state, the signal of CP violation will be a nonzero value of BR(W − → F ) − BR(W + → F) ACP = BR(W − → F ) + BR(W + → F). A second way of generating a CP-even phase difference is through oscillations of the heavy neutrinos. The neutrino is produced as Ni, it decays as Ni, leading to the fermion-number-violating and LNV process W − → This implies that (i) conjugate fields will be involved in the amplitudes, and (ii) the amplitudes will be proportional to the neutrino mass. In the third line, we have replaced Ni Nic by the neutrino propagator Another contribution to this process comes from a diagram like that of figure 1, but with 1 ↔ 2. In order to compute the rate for the decay, it will be necessary to integrate over the phase space of the final-state particles.
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