Derivation of the Lamb Shift using an Effective Field Theory

We rederive the $O(\alpha^5)$ shift of the hydrogen levels in the non-recoil ($m_e/m_P \to 0$) limit using Nonrelativistic QED (NRQED), an effective field theory developed by Caswell and Lepage (Phys. Lett. 167B, 437 (1986)). Our result contains the Lamb shift as a special case. Our calculation is far simpler than traditional approaches and has the advantage of being systematic. It also clearly illustrates the need to renormalize (or ``match'') the coefficients of the effective theory beyond tree level.Comment: 15 pages, 11 Postscript figures, uses Latex2e and epsf.te

Off-diagonal Yukawa Couplings in the s-channel Charged Higgs Production at LHC

The search for the heavy charged Higgs (mH+ > mtop) has been mainly based on the off-shell top pair production process. However, resonance production in s-channel single top events is an important channel to search for this particle. In a previous work, it was shown that this process, i.e., qq' -> H+ -> tb + h.c., can lead to comparable results to what is already obtained from LHC searches through gb -> tH- process. What was obtained was, however, based on diagonal Yukawa couplings between incoming quarks assuming cs as the main incoming pair due to the CKM matrix element being close to unity. The aim of this paper is to show that off-diagonal couplings, like cb, may lead to substantial contributions to the cross section, even if the corresponding CKM matrix element is two orders of magnitude smaller. For this reason, the cross section is calculated for each initial state including all diagonal and off-diagonal terms, and all is finally added together to get the total cross section which is observed to be ~ 2.7 times larger than what is obtained from cs initial state. Results are eventually reflected into 95% C.L. exclusion and $5\sigma$ discovery contours at different integrated luminosities of LHC. A reasonable coverage of the parameter space is obtained by the 95% C.L. exclusion contour.Comment: 12 pages, 10 figure