Search for the rare decays B0 (s) - μ+μ− and K0 S - μ+μ− with 1 fb−1 at LHCb

This thesis is devoted to the explanation of the searches for the very rare decays B0 (s) ! + and K0 S ! + , performed using the 1 fb1 of data taken by LHCb during 2011 from the LHC proton{ proton collisions at an energy of p s=7 TeV. The most relevant theory aspects concerning these searches will be presented. Furthermore, the LHCb experiment will be described, and some of its most interesting results in the rare decays area reported. The B0 (s) ! + search has not resulted in any signal excess above background, but has produced world best upper limits in the branching ratios of both channels: B(B0 ! + ) < 1:0 109 and B(B0 s ! + ) < 4:5 109, both at 95% con dence level. Finally, no signi cant signal excess has been found in the search for K0 S ! + neither, and again an upper limit on its branching ratio has been set: B(K0 S ! + ) < 11:2 109 at 95% con dence level. This upper limit is a factor of 35 better than the previous world best

Sitio web sobre el acelerador lhc: recurso para el aula de “ciencias del mundo contemporáneo” lhc-closer.es

SITIO WEB SOBRE EL ACELERADOR LHC: RECURSOPARA EL AULA DE “CIENCIAS DEL MUNDOCONTEMPORÁNEO”lhc-closer.e

Discovering True Muonium at LHCb

We study the potential of the LHCb experiment to discover, for the first time, the $\mu^+\mu^-$ true muonium bound state. We propose a search for the vector $1^3S_1$ state, $\mathcal{T\!M}$, which kinetically mixes with the photon and dominantly decays to $e^+e^-$. We demonstrate that a search for $\eta \to \gamma \mathcal{T\!M}$, $\mathcal{T\!M}\to e^+e^-$ in a displaced vertex can exceed a significance of 5 standard deviations assuming statistical uncertainties. We present two possible searches: an inclusive search for the $e^+e^-$ vertex, and an exclusive search which requires an additional photon and a reconstruction of the $\eta$ mass.Comment: 12 pages, 5 figures; v2: edits from comments and review, matched to PRD versio

Sitio web sobre el acelerador LHC: recurso para el aula de “Ciencias del Mundo Contemporáneo”

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Exclusive displaced hadronic signatures in the LHC forward region

The LHCb detector provides accurate vertex reconstruction and hadronic particle identification, which make the experiment an ideal place to look for light long-lived particles (LLP) decaying into Standard Model (SM) hadrons. In contrast with the typical search strategy relying on energetic jets and a high multiplicity of tracks from the LLP decay, LHCb can identify LLPs in exclusive, specific hadronic final states. To illustrate the idea, we study the sensitivity of LHCb to an exotic Higgs decay h → SS, followed by the displaced decay of GeV-scale scalars into charged kaons S → K+K−. We show that the reconstruction of kaon vertices in narrow invariant mass windows can efficiently eliminate the combinatorial backgrounds from B-meson decays. While the same signal is extremely difficult to probe in the existing displaced jet searches at ATLAS/CMS, the LHCb search we propose can probe the branching ratio BR(h → SS) down to 0.1% (0.02%) level with 15 (300) fb−1 of data. We also apply this projected bound to two scenarios with Higgs portal couplings, where the scalar mediator S either couples to a) the SM quarks only, or b) to both quarks and leptons in the minimal flavor violation paradigm. In both scenarios we compare the reach of our proposed search with the expected constraints from ATLAS and CMS on the invisible Higgs width and with the constraints from rare B-decays studies at LHCb. We find that for 1 GeV < mS < 2 GeV and 0.5 mm . cτ . 10 mm our proposed search will be competitive with the ATLAS and CMS projections, while at the same time providing crucial information of the hadronic interactions of S, which can not be obtained from the indirect measurement of the Higgs invisible width.The work of XCV is supported by MINECO (Spain) through the Ram´on y Cajal program RYC-2016-20073 and by XuntaGal under the ED431F 2018/01 project. YT is supported in part by the National Science Foundation under Grant Number PHY-1620074. YT would like to thank Aspen Center for Physics for hospitality during the completion of this work, which is supported by National Science Foundation grant PHY-1607611.S

Real-time discrimination of photon pairs using machine learning at the LHC

ALP-mediated decays and other as-yet unobserved B decays to di-photon final states are a challenge to select in hadron collider environments due to the large backgrounds that come directly from the p p collision. We present the strategy implemented by the LHCb experiment in 2018 to efficiently select such photon pairs. A fast neural network topology, implemented in the LHCb real-time selection framework achieves high efficiency across a mass range of 4−20 GeV/c2. We discuss implications and future prospects for the LHCb experimentS

Observation of the Ξ−b→J/ψΛK−decay

The observation of the decay Ξ−b→J/ψΛK−is reported, using a data sample corresponding to an integrated luminosity of 3 fb−1, collected by the LHCb detector in ppcollisions at centre-of-mass energies of 7and 8 TeV. The production rate of Ξ−bbaryons detected in the decay Ξ−b→J/ψΛK−is measured relative to that of Λ0bbaryons using the decay Λ0b→J/ψΛ. Integrated over the b-baryon transverse momentum pT<25 GeV/cand rapidity 2.0 <y <4.5, the measured ratio is fΞ − b fΛ0 b B(Ξ − b → J/ψΛK− ) B(Λ0 b → J/ψΛ) = (4.19±0.29 (stat)± 0.15 (syst))×10−2, where fΞ−band fΛ0bare the fragmentation fractions of b →Ξ−band b →Λ0btransitions, and Brepresents the branching fraction of the corresponding b-baryon decay. The mass difference between Ξ−band Λ0bbaryons is measured to be M(Ξ − b )− M(Λ0 b ) = 177.08±0.47 (stat)± 0.16 (syst)MeV/c2.S

Measurement of the J/ψ pair production cross-section in pp collisions at √s=13 TeV

The production cross-section of J/ψ pairs is measured using a data sample of pp collisions collected by the LHCb experiment at a centre-of-mass energy of √s=13 TeV, corresponding to an integrated luminosity of 279 ±11 pb−1. The measurement is performed for J/ψ mesons with a transverse momentum of less than 10 GeV/c in the rapidity range 2.0 < y < 4.5. The production cross-section is measured to be 15.2 ± 1.0 ± 0.9 nb. The first uncertainty is statistical, and the second is systematic. The differential cross-sections as functions of several kinematic variables of the J/ψ pair are measured and compared to theoretical predictions.S

Measurements of prompt charm production cross-sections in pp collisions at √s=5 TeV

Production cross-sections of prompt charm mesons are measured using data from pp collisions at the LHC at a centre-of-mass energy of 5 TeV. The data sample corresponds to an integrated luminosity of 8.60 ± 0.33 pb−1 collected by the LHCb experiment. The production cross-sections of D 0, D +, D +s, and D ∗+ mesons are measured in bins of charm meson transverse momentum, p T, and rapidity, y. They cover the rapidity range 2.0 < y < 4.5 and transverse momentum ranges 0 < p T < 10 GeV/c for D 0 and D + and 1 < p T < 10 GeV/c for D +s and D ∗+ mesons. The inclusive cross-sections for the four mesons, including charge-conjugate states, within the range of 1 < p T < 8 GeV/c are determined to be σ(pp→D0X)=1004±3±54μb,σ(pp→D+X)=402±2±30μb,σ(pp→D+sX)=170±4±16μb,σ(pp→D∗+X)=421±5±36μb, where the uncertainties are statistical and systematic, respectively.S