Latest results on rare decays from LHCb

Rare flavour changing neutral current decays are sensitive indirect probes for new effects beyond the Standard Model (SM). In the SM, these decays are forbidden at tree level and are therefore loop-suppressed. In SM extensions, new, heavy particles can significantly contribute and affect both their branching fractions as well as their angular distributions. The rare decay $B^0\to K^{*0}(\to K^+\pi^-)\mu^+\mu^-$ is of particular interest, since it gives access to many angular observables, allowing to model-independently test the operator structure of the decay. A previous analysis of the angular distributions of the final state particles showed interesting tensions with SM predictions using the data sample taken by the LHCb detector during 2011. These proceedings will summarize latest results on rare decays from the LHCb experiment with emphasis on the angular analysis of the decay $B^0\to K^{*0}\mu^+\mu^-$, using the full Run I data sample of the LHCb experiment.Comment: 8 pages, 7 figures. Contribution to the proceedings of the 50th Rencontres de Moriond, Electroweak Session, 14-21 March 201

Lepton Flavour Universality tests in BB decays as a probe for New Physics

In the Standard Model (SM), the coupling of the electroweak gauge bosons to the leptons is lepton flavour universal. Tests of this property constitute sensitive probes for new physics models that violate lepton flavour universality. Recent tests of lepton universality in rare $b\to s\ell\ell$ decays and semileptonic $b\to c\tau\bar{\nu}_\tau$ transitions have shown some tensions with the precise SM predictions. These proceedings summarise the latest results on lepton flavour universality from the LHCb experiment.Comment: 6 pages, 5 figures. Contribution to the proceedings of the 53th Rencontres de Moriond, Electroweak Session, 10-17 March 201

Measurement of the Bs mixing phase in the decay Bs -> J/psi phi with the LHCb experiment

This work presents a method to extract the CP-violating phase phi_s using the Bs meson decay Bs -> J/psi phi. The determination of phi_s is one of the key measurements of the physics program of the LHCb experiment. The phase phi_s arises from the interference between Bs mixing and the decay and is predicted by the Standard Model to be phi_s = (-0.0363 +- 0.0017) rad. Physics processes beyond the Standard Model can affect the Bs mixing amplitude and lead to deviations from this prediction. The precision measurement of phi_s therefore constitutes an indirect search for New Physics. In the scope of this thesis an unbinned maximum likelihood method to extract phi_s has been implemented. Since the decay Bs -> J/psi phi is a pseudoscalar to vector-vector transition the extraction method depends on both decay time and decay angles. Acceptance and resolution effects are also taken into account. The procedure is successfully tested on fully simulated events and fast simulated data. It is used to analyze data taken by the LHCb experiment in 2010 which correspond to an integrated luminosity of 36 pb^-1. With a flavor blind analysis the decay width difference DG_s is determined to be DG_s = (0.084 +- 0.112 +- 0.014) ps^-1 under the assumption of no CP violation. Using a Bs production flavor dependent analysis two-dimensional confidence regions for phi_s and DG_s are extracted. The Standard Model prediction is found to agree within 1.0 standard deviations with the data. In one dimension the confidence interval for phi_s at 68.3% confidence level is determined to be [-2.78, -0.39] rad

Measurement of the properties of the Ξb∗ 0 baryon

We perform a search for near-threshold Ξb0 resonances decaying to Ξb−π+ in a sample of proton-proton collision data corresponding to an integrated luminosity of 3 fb−1 collected by the LHCb experiment. We observe one resonant state, with the following properties: mΞb∗0−mΞb−−mπ+=15.727±0.068stat±0.023systMeV/c2ΓΞb∗0=0.90±0.16stat±0.08systMeV.This confirms the previous observation by the CMS collaboration. The state is consistent with the JP = 3/2+ Ξb∗ 0 resonance expected in the quark model. This is the most precise determination of the mass and the first measurement of the natural width of this state. We have also measured the ratio σpp→Ξb∗0XℬΞb∗0→Ξb−π+σpp→Ξb−X=0.28±0.03stat.±0.01syst.

Observation of Λb0 → ψ(2S)pK− and Λb0 → J/ψπ+π−pK− decays and a measurement of the Λb0 baryon mass

The decays Λb0 → ψ(2S)pK− and Λb0 → J/ψπ+π−pK− are observed in a data sample corresponding to an integrated luminosity of 3 fb−1, collected in proton-proton collisions at 7 and 8 TeV centre-of-mass energies by the LHCb detector. The ψ(2S) mesons are reconstructed through the decay modes ψ(2S) → μ+μ− and ψ(2S) → J/ψπ+π−.The branching fractions relative to that of Λb0 → J/ψpK− are measured to be ℬΛb0→ψ2SpK−ℬΛb0→J/ψpK−=20.70±0.76±0.46±0.37×10−2, where the first uncertainties are statistical, the second are systematic and the third is related to the knowledge of J/ψ and ψ(2S) branching fractions. The mass of the Λb0 baryon is measured to be MΛb0=5619.65±0.17±0.17MeV/c2, where the uncertainties are statistical and systematic

Search for CP violation in D ± → K S 0 K ± and D s ± → K S 0 π ± decays

A search for CP violation in Cabibbo-suppressed D ± → K S 0 K ± and D s ± → K S 0 π ± decays is performed using pp collision data, corresponding to an integrated luminosity of 3 fb, recorded by the LHCb experiment. The individual CP-violating asymmetries are measured to be AD±CP→K0SK±=(+0.03±0.17±0.14)%AD±sCP→K0Sπ±=(+0.38±0.46±0.17)%, assuming that CP violation in the Cabibbo-favoured decays is negligible. A combination of the measured asymmetries for the four decay modes D (s) ± → K S 0 K ± and D (s) ± → K S 0 π ± gives the sum AD±→K0SK±CP+AD±s→K0Sπ±CP=(+0.41±0.49±0.26)%. In all cases, the first uncertainties are statistical and the second systematic. The results represent the most precise measurements of these asymmetries to date and show no evidence for CP violation

Measurement of CP violation parameters in B0→DK*0 decays

An analysis of B0→DK*0 decays is presented, where D represents an admixture of D0 and D¯0 mesons reconstructed in four separate final states: K−π+, π−K+, K+K− and π+π−. The data sample corresponds to 3.0  fb−1 of proton-proton collision, collected by the LHCb experiment. Measurements of several observables are performed, including CP asymmetries. The most precise determination is presented of rB(DK*0), the magnitude of the ratio of the amplitudes of the decay B0→DK+π− with a b→u or a b→c transition, in a Kπ mass region of ±50  MeV/c2 around the K∗(892) mass and for an absolute value of the cosine of the K*0 helicity angle larger than 0.4

First observation of a baryonic B+c decay

A baryonic decay of the B+c meson, B+c→J/ψpp¯π+, is observed for the first time, with a significance of 7.3 standard deviations, in pp collision data collected with the LHCb detector and corresponding to an integrated luminosity of 3.0  fb−1 taken at center-of-mass energies of 7 and 8 TeV. With the B+c→J/ψπ+ decay as the normalization channel, the ratio of branching fractions is measured to be B(B+c→J/ψpp¯π+)/B(B+c→J/ψπ+)=0.143+0.039−0.034(stat)±0.013(syst). The mass of the B+c meson is determined as M(B+c)=6274.0±1.8(stat)±0.4(syst)  MeV/c2, using the B+c→J/ψpp¯π+ channel