Search for lepton flavour violation at LHCb

AbstractWe report the first searches for lepton flavour violating τ decays at a hadron collider. In particular, a search for the decay τ→μμμ is presented, using data corresponding to an integrated luminosity of 1.0 fb-1 collected at 7 TeV by LHCb in 2011. The upper limit on the branching ratio could be set, B(τ−→μ+μ−μ−)<8.0×10−8 at 90 % confidence level (the inclusion of the charge conjugate process is included).In addition, we report on the search for heavy Majorana neutrinos in lepton number violating B−→π+μ−μ− decay (as well as in its charge conjugate). In this case, the whole LHCb dataset is used (1.0 fb-1 at s=7 TeV and 2 fb-1 at s=8 TeV). No signal is found, and a limit on the branching ratio is set, B(B−→π+μ−μ−)<4×10−9 at 95 % confidence level. This limit is then used to set limits on the coupling between the hypothetical Majorana neutrino and muons

Measurement of CP violation in B-0 -> D+D- decays

The CP violation observables S and C in the decay channel B-0 -> D+D- are determined from a sample of proton-proton collisions at center-of-mass energies of 7 and 8 TeV, collected by the LHCb experiment and corresponding to an integrated luminosity of 3 fb(-1). The observable S describes CP violation in the interference between mixing and the decay amplitude, and C parametrizes direct CP violation in the decay. The following values are obtained from a flavor-tagged, decay-time-dependent analysis: S = -0.54(-0.16)(+0.17) (stat) +/- 0.05(syst) ,C=0.26(-0.17)(+0.18) (stat) +/- 0.02(syst). These values provide evidence for CP violation at a significance level of 4.0 standard deviations. The phase shift due to higher-order standard model corrections is constrained to a small value of Delta phi = -0.16(-0.21)(+0.19) rad

Constraints on the unitarity triangle angle γ from Dalitz plot analysis of B0 →dK+π- decays

The first study is presented of CP violation with an amplitude analysis of the Dalitz plot of $B^0 \to D K^+ \pi^-$ decays, with $D \to K^+ \pi^-$, $K^+ K^-$ and $\pi^+ \pi^-$. The analysis is based on a data sample corresponding to $3.0\,{\rm fb}^{-1}$ of $pp$ collisions collected with the LHCb detector. No significant CP violation effect is seen, and constraints are placed on the angle $\gamma$ of the unitarity triangle formed from elements of the Cabibbo-Kobayashi-Maskawa quark mixing matrix. Hadronic parameters associated with the $B^0 \to D K^*(892)^0$ decay are determined for the first time. These measurements can be used to improve the sensitivity to $\gamma$ of existing and future studies of the $B^0 \to D K^*(892)^0$ decay

Observation of charmonium pairs produced exclusively in pp collisions

The CP-violating asymmetry a s sl is studied using semileptonic decays of B0 s and B0 s mesons produced in pp collisions at a centre-of-mass energy of 7 TeV at the LHC, exploiting a data sample corresponding to an integrated luminosity of 1.0 fb−1 . The reconstructed final states are D± s µ ∓, with the D± s particle decaying in the φπ± mode. The D± s µ ∓ yields are summed over B0 s and B0 s initial states, and integrated with respect to decay time. Data-driven methods are used to measure efficiency ratios. We obtain a s sl = (−0.06 ± 0.50 ± 0.36)%, where the first uncertainty is statistical and the second systematic

Model-independent evidence for J /ψp contributions to Λb0 →j /ψpK- decays

The data sample of Λ0b→J/ψpK− decays acquired with the LHCb detector from 7 and 8 TeV pp collisions, corresponding to an integrated luminosity of 3  fb−1, is inspected for the presence of J/ψp or J/ψK− contributions with minimal assumptions about K−p contributions. It is demonstrated at more than nine standard deviations that Λ0b→J/ψpK− decays cannot be described with K−p contributions alone, and that J/ψp contributions play a dominant role in this incompatibility. These model-independent results support the previously obtained model-dependent evidence for P+c→J/ψp charmonium-pentaquark states in the same data sample

First observation of D0-D-0 oscillations in D0 →k+π-π+π- decays and measurement of the associated coherence parameters

Charm meson oscillations are observed in a time-dependent analysis of the ratio of D0→K+π−π+π− to D0→K−π+π−π+ decay rates, using data corresponding to an integrated luminosity of 3.0  fb−1 recorded by the LHCb experiment. The measurements presented are sensitive to the phase-space averaged ratio of doubly Cabibbo-suppressed to Cabibbo-favored amplitudes rK3πD and the product of the coherence factor RK3πD and a charm mixing parameter y′K3π. The constraints measured are rK3πD=(5.67±0.12)×10−2, which is the most precise determination to date, and RK3πDy′K3π=(0.3±1.8)×10−3, which provides useful input for determinations of the CP-violating phase γ in B±→DK±, D→K∓π±π∓π± decays. The analysis also gives the most precise measurement of the D0→K+π−π+π− branching fraction, and the first observation of D0-¯D0 oscillations in this decay mode, with a significance of 8.2 standard deviations

Evidence for Exotic Hadron Contributions to Λb0 →j /ψpπ- Decays

A full amplitude analysis of Λ0b→J/ψpπ− decays is performed with a data sample acquired with the LHCb detector from 7 and 8 TeV pp collisions, corresponding to an integrated luminosity of 3  fb−1. A significantly better description of the data is achieved when, in addition to the previously observed nucleon excitations N→pπ−, either the Pc(4380)+ and Pc(4450)+→J/ψp states, previously observed in Λ0b→J/ψpK− decays, or the Zc(4200)−→J/ψπ− state, previously reported in B0→J/ψK+π− decays, or all three, are included in the amplitude models. The data support a model containing all three exotic states, with a significance of more than three standard deviations. Within uncertainties, the data are consistent with the Pc(4380)+ and Pc(4450)+ production rates expected from their previous observation taking account of Cabibbo suppression

Measurement of the track reconstruction efficiency at LHCb

The determination of track reconstruction efficiencies at LHCb using J/ψ → µ +µ − decays is presented. Efficiencies above 95% are found for the data taking periods in 2010, 2011, and 2012. The ratio of the track reconstruction efficiency of muons in data and simulation is compatible with unity and measured with an uncertainty of 0.8% for data taking in 2010, and at a precision of 0.4% for data taking in 2011 and 2012. For hadrons an additional 1.4% uncertainty due to material interactions is assumed. This result is crucial for accurate cross section and branching fraction measurements in LHCb

Measurement of the Z+b-jet cross-section in pp collisions at= 7 TeV in the forward region

: The associated production of a Z boson or an off-shell photon γ ∗ with a bottom quark in the forward region is studied using proton-proton collisions at a centre-of-mass energy of 7 TeV. The Z bosons are reconstructed in the Z/γ∗ → µ +µ − final state from muons with a transverse momentum larger than 20 GeV, while two transverse momentum thresholds are considered for jets (10 GeV and 20 GeV). Both muons and jets are reconstructed in the pseudorapidity range 2.0 10 GeV, and σ(Z/γ∗ (µ +µ −) + b-jet) = 128 ± 36 (stat) ± 22 (syst) ± 5 (lumi) fb (0.2) for pT(jet) > 20 GeV. Keywords: Hadron-Hadron Scatterin

Observation of Λb0→ ψ(2S)pK−and Λb0→ J/ψπ+π−pK−decays and a measurement of the Λb0baryon mass

The decays Λ b 0  → ψ(2S)pK− and Λ b 0  → 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 Λ b 0  → J/ψpK− are measured to be B(Λ0b→ψ(2S)pK−)B(Λ0b→J/ψpK−)=(20.70±0.76±0.46±0.37)×10−2, B(Λb0→ψ(2S)pK−)B(Λb0→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 Λ b 0 baryon is measured to be M(Λ0b)=5619.65±0.17±0.17 MeV/c2, M(Λb0)=5619.65±0.17±0.17 MeV/c2, where the uncertainties are statistical and systematic