More Lepton Flavor Violating Observables for LHCb's Run 2

The R_K measurement by LHCb suggests non-standard lepton non-universality (LNU) to occur in b -> s l+ l- transitions, with effects in muons rather than electrons. A number of other measurements of b -> s l+ l- transitions by LHCb and B-factories display disagreement with the SM predictions and, remarkably, these discrepancies are consistent in magnitude and sign with the R_K effect. Non-standard LNU suggests non-standard lepton flavor violation (LFV) as well, for example in B -> K l l' and B_s -> l l'. There are good reasons to expect that the new effects may be larger for generations closer to the third one. In this case, the B_s -> mu e decay may be the most difficult to reach experimentally. We propose and study in detail the radiative counterpart of this decay, namely B_s -> mu e gamma, whereby the chiral-suppression factor is replaced by a factor of order alpha/pi. A measurement of this mode would be sensitive to the same physics as the purely leptonic LFV decay and, depending on experimental efficiencies, it may be more accessible. A realistic expectation is a factor of two improvement in statistics for either of the B_{d,s} modes.Comment: 14 pages, 2 figures. v4: some typos corrected. Such corrections are not implemented in the journal versio

The Energy-Momentum Tensor(s) in Classical Gauge Theories

We give an introduction to, and review of, the energy-momentum tensors in classical gauge field theories in Minkowski space, and to some extent also in curved space-time. For the canonical energy-momentum tensor of non-Abelian gauge fields and of matter fields coupled to such fields, we present a new and simple improvement procedure based on gauge invariance for constructing a gauge invariant, symmetric energy-momentum tensor. The relationship with the Einstein-Hilbert tensor following from the coupling to a gravitational field is also discussed.Comment: 34 pages; v2: Slightly expanded version with some improvements of presentation; Contribution to Mathematical Foundations of Quantum Field Theory, special issue in memory of Raymond Stora, Nucl. Phys.

Les anomalies des désintégrations semileptoniques de b en s: Un étude complète de construction de modèles, définition de nouvelles observables et mesures via le détecteur LHCb

Although the Standard Model (SM) of particle physics gives an overall excellent description of the observations, a few results, mainly obtained by the LHCb detector at CERN, point towards deviations in the transitions from quark b to quarks s and c. If confirmed, these anomalies would give a clear signal of physics beyond the SM, as they violate lepton flavour universality (LFU). In this context, new measurements and their theoretical predictions are crucial to define possible new physics scenarios. This thesis concerns both aspects and is therefore divided into two parts. The first one is devoted to theoretical considerations on the b to s transition as well as models explaining the anomalies. The second part presents a new measurement, namely the search for the decays of Bs mesons to two muons and a photon at LHCb.Radiative leptonic decays are promising to test the SM because the additional photon not only enlarges the branching ratio by lifting the chiral suppression factor, but also offers a sensitivity to other operators. Using the language of effective field theory (EFT), the decay of the Bs meson into two leptons and a photon is studied and new methods to reduce the theoretical uncertainty on its branching ratio are proposed. Besides, the behaviour of this decay at large dilepton mass gives the possibility of an indirect measurement where the total branching ratio is measured as a background of the corresponding non-radiative channel. Furthermore, if the violation LFU is experimentally confirmed, one also expects violation of lepton flavour. Measuring such violation, which would be an undeniable sign of new physics, is yet very challenging. Radiative decays can however support this search by offering additional channels with potentially larger branching ratios.The interpretation of the B anomalies in term of shifts in the EFT coefficients put a few scenarios forward. These scenarios can then be interpreted in term of new physics models. One such model is based on the consideration of an additional symmetry group acting horizontally between the SM generations. This model is severely constrained by low energy observables, such as meson mixings and leptonic decays, but allowing for a mass degeneracy between the new group’s bosons explains all b → s anomalies while passing other experimental constraints. The absence of new physics in low energy observables can also be interpreted as the presence of leptoquarks, for which the interaction between two quark- or two lepton-currents only arises at the one loop level. A model based on a vector leptoquark can for example give an explanation to both b → s and b → c anomalies. Interestingly, ultraviolet completions of these models contain natural Dark Matter candidates, hence relating two outstanding problems of particle physics.The analysis presented in the second part is particularly challenging because the probed decay is both very rare and radiative. The main difficulty lies in the presence of a large combinatorial background due to light meson decays. Tackling it while keeping a high efficiency on the signal selection requires the use of two successive multivariate analyses. The signal is then normalised to a similar decay of the Bs where no new physics is expected. Measuring a ratio of yields instead of an unique branching ratio allows for a partial cancellation of experimental uncertainties. On the other hand, this procedure requires a good knowledge of each selection step efficiencies for the two channels. These efficiencies are extracted from Monte-Carlo simulations or, when possible, directly from the data. Even if no significant excess is found, this analysis will allow to set the first limit on the total branching ratio.Bien que le Modèle Standard (SM) de la physique des particules permette une excellente description des observations, quelques résultats, principalement obtenus par le détecteur LHCb du CERN, montrent des signes de déviations dans les transitions du quark b aux quarks s et c. Si ces anomalies sont confirmées, elles donneront un signal clair de physique au-delà du SM car elles violent l’universalité de la saveur leptonique. Dans ce contexte, de nouvelles mesures et leurs prédictions théoriques sont cruciales pour définir de potentiels scénarios de nouvelle physique (NP). Cette thèse s’intéresse à tous ces aspects et est divisée en deux parties. La première propose des considérations théoriques sur la transition b → s ainsi que des modèles expliquant les anomalies. La seconde partie présente une nouvelle mesure, la recherche de désintégrations de mésons Bs en deux muons et un photon à LHCb.Les désintégrations leptoniques radiatives sont particulièrement prometteuses pour tester le SM, car l’ajout du photon n’entraîne pas seulement un agrandissement du rapport de branchement (BR), mais offre aussi une sensibilité à de nouveaux opérateurs. La désintégration du Bs en deux leptons et un photon est alors étudiée en utilisant une théorie effective des champs, et de nouvelles méthodes permettant de réduire l’incertitude théorique sur son BR sont proposées. Le comportement de cette désintégration lorsque la masse du dilepton est grande donne par ailleurs la possibilité de réaliser une mesure indirecte, où le BR est mesuré comme bruit de fond de son équivalent non-radiatif. En outre, si la violation de l’universalité leptonique est confirmée, on s’attend aussi à une violation de la saveur leptonique. La mesure de cette dernière, signe indéniable de NP, est néanmoins très difficile. Les désintégrations radiatives permettent cependant de simplifier cette recherche en offrant des canaux supplémentaires.L’interprétation des anomalies en termes de modification des coefficients de la théorie effective met en avant quelques scénarios. Ceux-ci peuvent alors être interprétés en terme de modèles de NP. L’un d’eux se base sur un nouveau groupe de symétrie qui agit horizontalement entre les générations du SM. Ce modèle est fortement contraint par des observables de basse énergie, mais attribuer différentes masses aux bosons du nouveau groupe permet d’expliquer les anomalies b → s tout en satisfaisant les contraintes expérimentales. L’absence de NP dans les observables de basse énergie peut en outre suggérer la présence de leptoquarks, pour lesquels l’interaction entre deux courants de quarks ou de leptons n’arrive qu’à une boucle. Un modèle basé sur un leptoquark vecteur explique par exemple à la fois les anomalies b → s et b → c. Il est intéressant de noter que les complétions ultraviolettes de ces différents modèles contiennent des candidats pour la Matière Noire, reliant ainsi deux problèmes prépondérants de la physique des particules.L’analyse présentée dans la seconde partie est rendue particulièrement difficile par la rareté et le caractère radiatif de la désintégration étudiée. La difficulté principale réside dans la présence d’un important bruit de fond combinatoire dû à la désintégration de mésons légers. Le prendre en compte tout en gardant une grande efficacité sur la sélection du signal nécessite d’utiliser deux analyses multivariées successives. Le signal est alors normalisé à une désintégration similaire du Bs où aucune NP est attendue. Mesurer le rapport des efficacités au lieu d’un unique BR permet une suppression partielle des incertitudes expérimentales. En revanche, cette procédure nécessite une bonne connaissance des efficacités de chaque étape de la sélection. Celles-ci sont extraites de simulation Monte-Carlo ou, quand c’est possible, directement des données. Même si aucun excès significatif n’est observé, cette analyse permettra d’établir la première limite sur le BR total

The discrepancies in b→sb\to s semi-leptonic decays: A complete study from model-building aspects, to the definition of novel observables, to their measurement with the LHCb detector.

Although the Standard Model (SM) of particle physics gives an overall excellent description of the observations, a few results, mainly obtained by the LHCb detector at CERN, point towards deviations in the transitions from quark $b$ to quarks $s$ and $c$. If confirmed, these anomalies would give a clear signal of physics beyond the SM, as they violate lepton flavour universality (LFU). In this context, new measurements and their theoretical predictions are crucial to define possible new physics scenarios. This thesis concerns both aspects and is therefore divided into two parts. The first one is devoted to theoretical considerations on the $b$ to $s$ transition as well as models explaining the anomalies. The second part presents a new measurement, namely the search for the decays of $B_s^0$ mesons to two muons and a photon at LHCb. Radiative leptonic decays are promising to test the SM because the additional photon not only enlarges the branching ratio by lifting the chiral suppression factor, but also offers a sensitivity to other operators. Using the language of effective field theory (EFT), the decay of the $B_s^0$ meson into two leptons and a photon is studied and new methods to reduce the theoretical uncertainty on its branching ratio are proposed. Besides, the behaviour of this decay at large dilepton mass gives the possibility of an indirect measurement where the total branching ratio is measured as a background of the corresponding non-radiative channel. Furthermore, if the violation LFU is experimentally confirmed, one also expects violation of lepton flavour. Measuring such violation, which would be an undeniable sign of new physics, is yet very challenging. Radiative decays can however support this search by offering additional channels with potentially larger branching ratios. The interpretation of the $B$ anomalies in term of shifts in the EFT coefficients put a few scenarios forward. These scenarios can then be interpreted in term of new physics models. One such model is based on the consideration of an additional symmetry group acting horizontally between the SM generations. This model is severely constrained by low energy observables, such as meson mixings and leptonic decays, but allowing for a mass degeneracy between the new group’s bosons explains all $b\to s$ anomalies while passing other experimental constraints. The absence of new physics in low energy observables can also be interpreted as the presence of leptoquarks, for which the interaction between two quark- or two lepton-currents only arises at the one loop level. A model based on a vector leptoquark can for example give an explanation to both $b\to s$ and $b\to c$ anomalies. Interestingly, ultraviolet completions of these models contain natural Dark Matter candidates, hence relating two outstanding problems of particle physics. The analysis presented in the second part is particularly challenging because the probed decay is both very rare and radiative. The main difficulty lies in the presence of a large combinatorial background due to light meson decays. Tackling it while keeping a high efficiency on the signal selection requires the use of two successive multivariate analyses. The signal is then normalised to a similar decay of the $B_s^0$ where no new physics is expected. Measuring a ratio of yields instead of an unique branching ratio allows for a partial cancellation of experimental uncertainties. On the other hand, this procedure requires a good knowledge of each selection step efficiencies for the two channels. These efficiencies are extracted from Monte-Carlo simulations or, when possible, directly from the data. Even if no significant excess is found, this analysis will allow to set the first limit on the total branching ratio

The Dark Side of 4321

International audienceThe evidence of Dark Matter (DM) is one of the strongest observational arguments in favor of physics beyond the Standard Model. Despite expectations, a similar evidence has been lacking so far in collider searches, with the possible exception of B-physics discrepancies, a coherent set of persistent deviations in a homogeneous dataset consisting of b → c and b → s semi-leptonic transitions. We explore the question whether DM and the B discrepancies may have a common origin. We do so in the context of the so-called 4321 gauge model, a UV-complete and calculable setup that yields a U$_{1}$ leptoquark, the by far most successful single mediator able to explain the B anomalies, along with other new gauge bosons, including a Z′. Adding to this setup a ‘minimal’ DM fermionic multiplet, consisting of a 4 under the 4321’s SU(4), we find the resulting model in natural agreement with the relic-density observation and with the most severe direct-detection bounds, in the sense that the parameter space selected by B physics is also the one favored by DM phenomenology. The DM candidate is a particle with a mass in the WIMP range, freeze-out dynamics includes a co-annihilator (the ‘rest’ of the 4 multiplet), and the most important gauge mediator in the DM sector is the Z′

Measurement of the inelastic pppp cross-section at a centre-of-mass energy of 13 TeV

The cross-section for inelastic proton-proton collisions at a centre-of-mass energy of 13 TeV is measured with the LHCb detector. The fiducial cross-section for inelastic interactions producing at least one prompt long-lived charged particle with momentum p > 2 GeV/c in the pseudorapidity range 2 < η < 5 is determined to be σ$_{acc}$ = 62.2 ± 0.2 ± 2.5 mb. The first uncertainty is the intrinsic systematic uncertainty of the measurement, the second is due to the uncertainty on the integrated luminosity. The statistical uncertainty is negligible. Extrapolation to full phase space yields the total inelastic proton-proton cross-section σ$_{inel}$ = 75.4 ± 3.0 ± 4.5 mb, where the first uncertainty is experimental and the second due to the extrapolation. An updated value of the inelastic cross-section at a centre-of-mass energy of 7 TeV is also reported

Measurement of C ⁣PC\!P asymmetries in two-body B(s)0B_{(s)}^{0}-meson decays to charged pions and kaons

International audienceThe time-dependent CP asymmetries in B0→π+π- and Bs0→K+K- decays are measured using a data sample of p p collisions corresponding to an integrated luminosity of 3.0  fb-1, collected with the LHCb detector at center-of-mass energies of 7 and 8 TeV. The same data sample is used to measure the time-integrated CP asymmetries in B0→K+π- and Bs0→π+K- decays. The results are Cπ+π-=-0.34±0.06±0.01, Sπ+π-=-0.63±0.05±0.01, CK+K-=0.20±0.06±0.02, SK+K-=0.18±0.06±0.02, AK+K-ΔΓ=-0.79±0.07±0.10, ACPB0=-0.084±0.004±0.003, and ACPBs0=0.213±0.015±0.007, where the first uncertainties are statistical and the second systematic. Evidence for CP violation is found in the Bs0→K+K- decay for the first time

Measurement of the ratio of the B0→D∗−τ+ντB^0 \to D^{*-} \tau^+ \nu_{\tau} and B0→D∗−μ+νμB^0 \to D^{*-} \mu^+ \nu_{\mu} branching fractions using three-prong τ\tau-lepton decays

International audienceThe ratio of branching fractions R(D*-)≡B(B0→D*-τ+ντ)/B(B0→D*-μ+νμ) is measured using a data sample of proton-proton collisions collected with the LHCb detector at center-of-mass energies of 7 and 8 TeV, corresponding to an integrated luminosity of 3  fb-1. For the first time, R(D*-) is determined using the τ-lepton decays with three charged pions in the final state. The B0→D*-τ+ντ yield is normalized to that of the B0→D*-π+π-π+ mode, providing a measurement of B(B0→D*-τ+ντ)/B(B0→D*-π+π-π+)=1.97±0.13±0.18, where the first uncertainty is statistical and the second systematic. The value of B(B0→D*-τ+ντ)=(1.42±0.094±0.129±0.054)% is obtained, where the third uncertainty is due to the limited knowledge of the branching fraction of the normalization mode. Using the well-measured branching fraction of the B0→D*-μ+νμ decay, a value of R(D*-)=0.291±0.019±0.026±0.013 is established, where the third uncertainty is due to the limited knowledge of the branching fractions of the normalization and B0→D*-μ+νμ modes. This measurement is in agreement with the standard model prediction and with previous results

First observation of B+→Ds+K+K−B^{+} \to D_s^{+}K^{+}K^{-} decays and a search for B+→Ds+ϕB^{+} \to D_s^{+}\phi decays

International audienceA search for B$^{+}$ → D$_{s}^{+}$ K$^{+}$K$^{−}$ decays is performed using pp collision data corresponding to an integrated luminosity of 4.8 fb$^{−1}$, collected at centre-of-mass energies of 7, 8 and 13 TeV with the LHCb experiment. A significant signal is observed for the first time and the branching fraction is determined to b