Charged Lepton Pair Production At High Energy Reactions

The investigation of either the opposite-sign or the same-sign charged lepton pair production plays an important role in testing the standard model (SM) and probing new physics beyond the SM. In this thesis, B-meson rare decays and charged dilepton production in pp collisions are used to search for signals of new physics. B-meson rare decays induced by flavor-changing neutral current are very sensitive to standard model and new physics beyond the SM. B-meson semileptonic rare decays B+→π+l+l-(l= e,μ,τ) include flavor-changing neutral current b →d process. Precise study in SM is needed to test the SM and search for new physics beyond the SM with this transition. We perform an restudy of the B→π form factors, and then give the SM predictions of the branching fractions for B+→π+l+l- processes. In comparison with the LHCb data, it is found that the possible parameter space for new physics cannot be eliminated. The neutrino oscillations discovered in neutrino experiments indicate that neutrinos have non-zero mass. This fact provides the only one direct evidence of possible new physics beyond the SM. As a consequence, one of the important issues in neutrino physics today is to investigate the neutrino mass generation mechanism and clarify the character of neutrino masses. Seesaw model is one of the natural schemes to describe tiny neutrino mass. We study the contribution from heavy Majorana neutrino to the B-meson rare decay B+→π+μ+μ- and lepton number violating rare decays B±→π(?)μ±μ± in type (I) seesaw model. The Majorana nature of neutrinos lighter than 5 GeV can only be determined by these B meson rare decays. In order to search for more heavier Majorana neutrino, the charged dilepton production processes pp→μ±μ(?)(μ±)jj at the LHC are studied systematically. The main research contents of the paper are as follows,·A systematic study is given on the B-meson semi-leptonic rare decay B+→π+μ+μ-which has been discovered by the LHCb experiment. Firstly, a QCD light-cone sum rule calculation with a chiral current correlator is performed at twist-2 next-to-leading order to determine the B→π form factors f+B→(q2), f0B→π(q2) and fTB→π(q2) in the region 0≤q2≤12 GeV2. The pollution from the poorly known twist-3 pion light-cone distribution amplitude is eliminated due to the application of the chiral current correlator, and more stable QCD light-cone sum rule results are obtained. Secondly, to reduce the uncertainty in determining the high-q2 behaviors of the form factors by extrapolating the QCD light-cone sum rule results, the complementarity of the lattice QCD and QCD light-cone sum rule approaches as well as the general analyticity of the form factors are adopted, and lattice QCD data in the high-q2 region together with QCD light-cone sum rule results are simultaneously used to fit the Bourrely-Caprini-Lellouch parametrization. The behaviors of the form factors in the whole kinematical region are consistent with the investigation given by Ali, Parkhomenko and Rusov. Thirdly, we study the B+→π+l+l-)l=3,μ,τ) processes in SM with the obtained form factors and give some observables such as branching fractions, differential branching fractions and so on. Our SM prediction of the branching fraction for B+→π+μ+μ- decay channel could account for the LHCb data and is consistent with the other theoretical calculation results within uncertainties. However, the possible parameter space for new physics cannot be ruled out within the existing experimental and theoretical precision.· We study the contributions from heavy Majorana neutrino (mπ< mN<mB) to the B-meson semi-leptonic rare decay B+→π+μ+μ- and lepton number violating B-meson rare decays B±→π(?)μ+μ+ in type (Ⅰ) seesaw model. The constrains on the parameters of type (Ⅰ) seesaw model, heavy Majorana neutrino mass mN and neutrino mixing matrix element RμN, are acquired by using the existing SM predictions and LHCb measurement of the branching fraction for B+→π+μ+μ- process. Then, these obtained parameters are adopted to investigate the lepton number violating B-meson rare decays B±→π(?)μ+μ+ .The resulting branching fraction can account for the LHCb data. Although the branching fractions for the lepton number violating B-meson rare decays are very small, these lepton number violating processes are hopely to be discovered with the continuous improvement of experimental accuracy as signals of heavy Majorana neutrino. The obtained parameters for Majorana neutrino can also be used to search for other lepton number violating processes.· In type (I+II) seesaw model, we perform a systematic analysis on the existing bounds given by the theoretical predictions and experimental measurements for heavy Majorana neutrinos, and investigate the opposite-sign (same-sign) charged dilepton production pp→μ±μ(?)(μ±)jj with two nearly degenerate Majorana neutrinos at center-of-mass energy 14 and 100 TeV LHC, assuming that model parameters can concurrently satisfy data. The cross sections a, charge asymmetries Ac and CP asymmetries in polar dis-tribution ACPθ and in azimuthal distribution ACPΦ for pp→μ±μ(?)(μ±)jj are studied. It is found that AC、ACPθ and ACPΦ are not sensitive to the mass of the ligher Majo-rana neutrino mN1, and AθCP(Δφ)=ACPΦP(Δφ) where Δφis the net CP phase arising from the neutrino mixing parameters RμN1 and RμN2. At center-of-mass energy 14 TeV, preliminary background analysis is performed for pp→μ±μ(?)(μ±)jj processes. The results show that the corresponding C and CP asymmetries are still relatively obvious with the stringent selection cuts. Moreover, further study on the observability of heavy Majorana neutrinos and the related CP violation could be performed at the LHC with updated energy and luminosity.