Sneha Malde

Abstract:

Measurements are presented of electroweak boson production using data from $pp$ collisions at a centre-of-mass energy of $\sqrt{s} = 8\mathrm{\,Te\kern -0.1em V}$. The analysis is based on an integrated luminosity of $2.0\mathrm{\,fb}^{-1}$ recorded with the LHCb detector. The bosons are identified in the $W\rightarrow\mu\nu$ and $Z\rightarrow\mu^{+}\mu^{-}$ decay channels. The cross-sections are measured for muons in the pseudorapidity range $2.0 < \eta < 4.5$, with transverse momenta $p_{\rm T} > 20{\mathrm{\,Ge\kern -0.1em V\!/}c}$ and, in the case of the $Z$ boson, a dimuon mass within $60 < M_{\mu^{+}\mu^{-}} < 120{\mathrm{\,Ge\kern -0.1em V\!/}c^{2}}$. The results are \begin{align*} \sigma_{W^{+}\rightarrow\mu^{+}\nu} &= 1093.6 \pm 2.1 \pm 7.2 \pm 10.9 \pm 12.7{\rm \,pb} \, , \sigma_{W^{-}\rightarrow\mu^{-}\bar{\nu}} &= \phantom{0}818.4 \pm 1.9 \pm 5.0 \pm \phantom{0}7.0 \pm \phantom{0}9.5{\rm \,pb} \, , \sigma_{Z\rightarrow\mu^{+}\mu^{-}} &= \phantom{00}95.0 \pm 0.3 \pm 0.7 \pm \phantom{0}1.1 \pm \phantom{0}1.1{\rm \,pb} \, , \end{align*} where the first uncertainties are statistical, the second are systematic, the third are due to the knowledge of the LHC beam energy and the fourth are due to the luminosity determination. The evolution of the $W$ and $Z$ boson cross-sections with centre-of-mass energy is studied using previously reported measurements with $1.0\mathrm{\,fb}^{-1}$ of data at $7\mathrm{\,Te\kern -0.1em V}$. Differential distributions are also presented. Results are in good agreement with theoretical predictions at next-to-next-to-leading order in perturbative quantum chromodynamics.

Abstract:

Search for the lepton-flavour violating decay D⁰ → e^±μ^∓

A search for the lepton-flavour violating decay D⁰ → e^±μ^∓ is made with a dataset corresponding to an integrated luminosity of 3.0 fb⁻¹ of proton–proton collisions at centre-of-mass energies of 7 TeV and 8 TeV, collected by the LHCb experiment. Candidate D⁰ mesons are selected using the decay D^∗+ → D⁰π⁺ and the D⁰ → e^±μ^∓ branching fraction is measured using the decay mode D⁰ → K⁻π⁺ as a normalization channel. No significant excess of D⁰ → e^±μ^∓ candidates over the expected background is seen, and a limit is set on the branching fraction, 𝓑(D⁰ → e^±μ^∓) < 1.3 × 10⁻⁸, at 90% confidence level. This is an order of magnitude lower than the previous limit and it further constrains the parameter space in some leptoquark models and in supersymmetric models with R-parity violation.

Abstract:

First observation of the decay B_s⁰ → K_S⁰K^∗(892)⁰ at LHCb

A search for B_(s)⁰ → K_S⁰K^∗(892)⁰ decays is performed using pp collision data, corresponding to an integrated luminosity of 1.0 fb⁻¹, collected with the LHCb detector at a centre-of-mass energy of 7 TeV. The B_s⁰ → K_S⁰K^∗(892)⁰ decay is observed for the first time, with a significance of 7.1 standard deviations. The branching fraction is measured to be
𝓑(B_s⁰ → K̅⁰K^∗(892)⁰) + 𝓑(B_s⁰ → K⁰K̅^∗(892)⁰)=(16.4 ± 3.4 ± 2.3) × 10⁻⁶,
where the first uncertainty is statistical and the second is systematic. No evidence is found for the decay B⁰ → K_S⁰K^∗(892)⁰ and an upper limit is set on the branching fraction, 𝓑(B⁰ → K̅⁰K^∗(892)⁰) + 𝓑(B⁰ → K⁰K̅^∗(892)⁰) < 0.96 × 10⁻⁶, at 90 % confidence level. All results are consistent with Standard Model predictions.

Abstract:

The Dalitz plot analysis technique is used to study the resonant substructures of B−→D+π−π− decays in a data sample corresponding to 3.0  fb−1 of pp collision data recorded by the LHCb experiment during 2011 and 2012. A model-independent analysis of the angular moments demonstrates the presence of resonances with spins 1, 2 and 3 at high D+π− mass. The data are fitted with an amplitude model composed of a quasi-model-independent function to describe the D+π− S wave together with virtual contributions from the D∗(2007)0 and B*0 states, and components corresponding to the D∗2(2460)0, D∗1(2680)0, D∗3(2760)0 and D∗2(3000)0 resonances. The masses and widths of these resonances are determined together with the branching fractions for their production in B−→D+π−π− decays. The D+π− S wave has phase motion consistent with that expected due to the presence of the D∗0(2400)0 state. These results constitute the first observations of the D∗3(2760)0 and D∗2(3000)0 resonances, with significances of 10σ and 6.6σ, respectively.

Abstract:

A model-dependent amplitude analysis of the decay B0 → D(KS0π+π−)K∗0 is performed using proton-proton collision data corresponding to an integrated luminosity of 3.0 fb−1, recorded at √s=7 and 8 TeV by the LHCb experiment. The CP violation observables x± and y±, sensitive to the CKM angle γ, are measured to bex−=−0.15±0.14±0.03±0.01,y−=0.25±0.15±0.06±0.01,x+=0.05±0.24±0.04±0.01,y+=−0.65−0.23+0.24±0.08±0.01, where the first uncertainties are statistical, the second systematic and the third arise from the uncertainty on the D → KS0π+π− amplitude model. These are the most precise measurements of these observables. They correspond to γ = (80−22+21)° and rB0=0.39±0.13, where rB0 is the magnitude of the ratio of the suppressed and favoured B0 → DK+π− decay amplitudes, in a Kπ mass region of ±50 MeV around the K*(892)0 mass and for an absolute value of the cosine of the K*0 decay angle larger than 0.4.[Figure not available: see fulltext.]