Factorization and non-local 1/m b corrections in the decay B X s gamma

In this thesis, a systematic analysis of the bar B to X_sgamma photon spectrum in the endpoint region is presented. The endpoint region refers to a kinematic configuration of the final state, in which the photon has a large energy m_b-2E_gamma = O(Lambda_QCD), while the jet has a large energy but small invariant mass. Using methods of soft-collinear effective theory and heavy-quark effective theory, it is shown that the spectrum can be factorized into hard, jet, and soft functions, each encoding the dynamics at a certain scale. The relevant scales in the endpoint region are the heavy-quark mass m_b, the hadronic energy scale Lambda_QCD and an intermediate scale sqrt{Lambda_QCD m_b} associated with the invariant mass of the jet. It is found that the factorization formula contains two different types of contributions, distinguishable by the space-time structure of the underlying diagrams. On the one hand, there are the direct photon contributions which correspond to diagrams with the photon emitted directly from the weak vertex. The resolved photon contributions on the other hand arise at O(1/m_b) whenever the photon couples to light partons. In this work, these contributions will be explicitly defined in terms of convolutions of jet functions with subleading shape functions. While the direct photon contributions can be expressed in terms of a local operator product expansion, when the photon spectrum is integrated over a range larger than the endpoint region, the resolved photon contributions always remain non-local. Thus, they are responsible for a non-perturbative uncertainty on the partonic predictions. In this thesis, the effect of these uncertainties is estimated in two different phenomenological contexts. First, the hadronic uncertainties in the bar B to X_sgamma branching fraction, defined with a cut E_gamma > 1.6 GeV are discussed. It is found, that the resolved photon contributions give rise to an irreducible theory uncertainty of approximately 5 %. As a second application of the formalism, the influence of the long-distance effects on the direct CP asymmetry will be considered. It will be shown that these effects are dominant in the Standard Model and that a range of -0.6 < A_CP^SM < 2.8 % is possible for the asymmetry, if resolved photon contributions are taken into account.

Bayesian Approach to Spectral Function Reconstruction for Euclidean Quantum Field Theories

We present a novel approach to the inference of spectral functions from Euclidean time correlator data that makes close contact with modern Bayesian concepts. Our method differs significantly from the maximum entropy method (MEM). A new set of axioms is postulated for the prior probability, leading to an improved expression, which is devoid of the asymptotically flat directions present in the Shanon-Jaynes entropy. Hyperparameters are integrated out explicitly, liberating us from the Gaussian approximations underlying the evidence approach of the maximum entropy method. We present a realistic test of our method in the context of the nonperturbative extraction of the heavy quark potential. Based on hard-thermal-loop correlator mock data, we establish firm requirements in the number of data points and their accuracy for a successful extraction of the potential from lattice QCD. Finally we reinvestigate quenched lattice QCD correlators from a previous study and provide an improved potential estimation at T2.33TC.

Hard thermal loop benchmark for the extraction of the nonperturbative QQ[over ¯] potential

The extraction of the finite temperature heavy quark potential from lattice QCD relies on a spectral analysis of the Wilson loop. General arguments tell us that the lowest lying spectral peak encodes, through its position and shape, the real and imaginary parts of this complex potential. Here we benchmark this extraction strategy using leading order hard-thermal loop (HTL) calculations. In other words, we analytically calculate the Wilson loop and determine the corresponding spectrum. By fitting its lowest lying peak we obtain the real and imaginary parts and confirm that the knowledge of the lowest peak alone is sufficient for obtaining the potential. Access to the full spectrum allows an investigation of spectral features that do not contribute to the potential but can pose a challenge to numerical attempts of an analytic continuation from imaginary time data. Differences in these contributions between the Wilson loop and gauge fixed Wilson line correlators are discussed. To better understand the difficulties in a numerical extraction we deploy the maximum entropy method with extended search space to HTL correlators in Euclidean time and observe how well the known spectral function and values for the real and imaginary parts are reproduced. Possible venues for improvement of the extraction strategy are discussed.

Review of lattice results concerning low-energy particle physics

We review lattice results related to pion, kaon, D- and B-meson physics with the aim of making them easily accessible to the particle-physics community. More specifically, we report on the determination of the lightquark masses, the form factor f+(0), arising in semileptonic K → π transition at zero momentum transfer, as well as the decay-constant ratio fK / fπ of decay constants and its consequences for the CKM matrix elements Vus and Vud. Furthermore, we describe the results obtained on the lattice for some of the low-energy constants of SU(2)L × SU(2)R and SU(3)L×SU(3)R Chiral Perturbation Theory and review the determination of the BK parameter of neutral kaon mixing. The inclusion of heavy-quark quantities significantly expands the FLAG scope with respect to the previous review. Therefore, we focus here on D- and B-meson decay constants, form factors, and mixing parameters, since these are most relevant for the determination of CKM matrix elements and the global CKM unitarity-triangle fit. In addition we review the status of lattice determinations of the strong coupling constant αs.

Towards the continuum limit in transport coefficient computations

The analytic continuation needed for the extraction of transport coefficients necessitates in principle a continuous function of the Euclidean time variable. We report on progress towards achieving the continuum limit for 2-point correlator measurements in thermal SU(3) gauge theory, with specific attention paid to scale setting. In particular, we improve upon the determination of the critical lattice coupling and the critical temperature of pure SU(3) gauge theory, estimating r0Tc ≃ 0.7470(7) after a continuum extrapolation. As an application the determination of the heavy quark momentum diffusion coefficient from a correlator of colour-electric fields attached to a Polyakov loop is discussed.

Measurement of the parity-violating asymmetry parameter αb and the helicity amplitudes for the decay Λ 0 b →J/ψ⁰ with the ATLAS detector

A measurement of the parity-violating decay asymmetry parameter, αb , and the helicity amplitudes for the decay Λ 0 b →J/ψ(μ + μ − )Λ 0 (pπ − ) is reported. The analysis is based on 1400 Λ 0 b and Λ ¯ 0 b baryons selected in 4.6  fb −1 of proton–proton collision data with a center-of-mass energy of 7 TeV recorded by the ATLAS experiment at the LHC. By combining the Λ 0 b and Λ ¯ 0 b samples under the assumption of CP conservation, the value of α b is measured to be 0.30±0.16(stat)±0.06(syst) . This measurement provides a test of theoretical models based on perturbative QCD or heavy-quark effective theory.

Search for heavy, top-like quark pair production in the dilepton final state in pp collisions at. root s=7 TeV

Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)

Search for pair production of heavy top-like quarks decaying to a high-pT W boson and a b quark in the lepton plus jets final state at sqrts=7 TeV with the ATLAS detector

A search is presented for production of a heavy up-type quark (t') together with its antiparticle, assuming a significant branching ratio for subsequent decay into a W boson and a b quark. The search is based on 4.7 fb(-1) of pp collisions root s = 7 TeV recorded in 2011 with the ATLAS detector at the CERN Large Hadron Collider. Data are analyzed in the lepton + jets final state, characterized by a high-transverse-momentum isolated electron or muon, large missing transverse momentum and at least three jets. The analysis strategy relies on the substantial boost of the W bosons in the t'(t') over bar signal when m(t') greater than or similar to 400 GeV. No significant excess of events above the Standard Model expectation is observed and the result of the search is interpreted in the context of fourth-generation and vector-like quark models. Under the assumption of a branching ratio BR(t' -> W b) = I, a fourth-generation t' quark with mass lower than 656 GeV is excluded at 95% confidence level. In addition, in light of the recent discovery of a new boson of mass similar to 126 GeV at the LHC, upper limits are derived in the two-dimensional plane of BR(t' -> Wb) versus BR(t' -> Ht), where H is the Standard Model Higgs boson, for vector-like quarks of various masses.

Measurement of the Bottom Quark Contribution to Nonphotonic Electron Production in p plus p Collisions at root s=200 GeV

The contribution of B meson decays to nonphotonic electrons, which are mainly produced by the semileptonic decays of heavy-flavor mesons, in p + p collisions at root s = 200 GeV has been measured using azimuthal correlations between nonphotonic electrons and hadrons. The extracted B decay contribution is approximately 50% at a transverse momentum of p(T) >= 5 GeV/c. These measurements constrain the nuclear modification factor for electrons from B and D meson decays. The result indicates that B meson production in heavy ion collisions is also suppressed at high p(T).

Measurement of the top quark pair production cross section in the lepton plus jets channel in proton-antiproton collisions at root s=1.96 TeV

Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)

Search for heavy quarks decaying into a top quark and a W or Z boson using lepton + jets events in pp collisions at √s = 7 TeV

Results are presented from a search for the pair-production of heavy quarks, QQ̄, that decay exclusively into a top quark and a W or Z boson. The search is performed using a sample of proton-proton collisions at √s = 7 TeV corresponding to an integrated luminosity of 5.0 fb-1, collected by the Compact Muon Solenoid experiment. The signal region is defined using a sample of events containing one electron or muon, missing transverse momentum, and at least four jets with large transverse momenta, where one jet is likely to originate from the decay of a bottom quark. No significant excess of events is observed with respect to the standard model expectations. Assuming a strong pair-production mechanism, quark masses below 675 (625) GeV decaying into tW (tZ) are excluded at the 95 % confidence level.[Figure not available: see fulltext.] © 2013 CERN for the benefit of the CMS collaboration.

Study of heavy-flavor quarks produced in association with top-quark pairs at s √ =7 TeV using the ATLAS detector

Using a sample of dilepton top-quark pair (tt ¯ ) candidate events, a study is performed of the production of top-quark pairs together with heavy-flavor (HF) quarks, the sum of tt ¯ +b+X and tt ¯ +c+X , collectively referred to as tt ¯  + HF . The data set used corresponds to an integrated luminosity of 4.7  fb −1 of proton-proton collisions at a center-of-mass energy of 7 TeV recorded by the ATLAS detector at the CERN Large Hadron Collider. The presence of additional HF (b or c ) quarks in the tt ¯ sample is inferred by looking for events with at least three b -tagged jets, where two are attributed to the b quarks from the tt ¯ decays and the third to additional HF production. The dominant background to tt ¯  + HF in this sample is tt ¯ +jet events in which a light-flavor jet is misidentified as a heavy-flavor jet. To determine the heavy- and light-flavor content of the additional b -tagged jets, a fit to the vertex mass distribution of b -tagged jets in the sample is performed. The result of the fit shows that 79 ± 14 (stat) ± 22 (syst) of the 105 selected extra b -tagged jets originate from HF quarks, 3 standard deviations away from the hypothesis of zero tt ¯  + HF production. The result for extra HF production is quoted as a ratio (R HF ) of the cross section for tt ¯  + HF production to the cross section for tt ¯ production with at least one additional jet. Both cross sections are measured in a fiducial kinematic region within the ATLAS acceptance. R HF is measured to be [6.2±1.1(stat)±1.8(syst)]% for jets with p T >25  GeV and |η|<2.5 , in agreement with the expectations from Monte Carlo generators.

Machine learning techniques for heavy-flavour baryon production measurements at the LHC

Il quark-gluon plasma (QGP) è uno stato della materia previsto dalla cromodinamica quantistica. L’esperimento ALICE a LHC ha tra i suoi obbiettivi principali lo studio della materia fortemente interagente e le proprietà del QGP attraverso collisioni di ioni pesanti ultra-relativistici. Per un’esaustiva comprensione di tali proprietà, le stesse misure effettuate su sistemi collidenti più piccoli (collisioni protone-protone e protone-ione) sono necessarie come riferimento. Le recenti analisi dei dati raccolti ad ALICE hanno mostrato che la nostra comprensione dei meccanismi di adronizzazione di quark pesanti non è completa, perchè i dati ottenuti in collisioni pp e p-Pb non sono riproducibili utilizzando modelli basati sui risultati ottenuti con collisioni e+e− ed ep. Per questo motivo, nuovi modelli teorici e fenomenologici, in grado di riprodurre le misure sperimentali, sono stati proposti. Gli errori associati a queste nuove misure sperimentali al momento non permettono di verificare in maniera chiara la veridicità dei diversi modelli proposti. Nei prossimi anni sarà quindi fondamentale aumentare la precisione di tali misure sperimentali; d’altra parte, stimare il numero delle diverse specie di particelle prodotte in una collisione può essere estremamente complicato. In questa tesi, il numero di barioni Lc prodotti in un campione di dati è stato ottenuto utilizzando delle tecniche di machine learning, in grado di apprendere pattern e imparare a distinguere candidate di segnale da quelle di fondo. Si sono inoltre confrontate tre diverse implementazioni di un algoritmo di Boosted Decision Trees (BDT) e si è utilizzata quella più performante per ricostruire il barione Lc in collisioni pp raccolte dall’esperimento ALICE.

Spin alignment measurements of the K*(0)(892) and phi(1020) vector mesons in heavy ion collisions at root S(NN)=200 GeV

We present the first spin alignment measurements for the K*(0)(892) and phi(1020) vector mesons produced at midrapidity with transverse momenta up to 5 GeV/c at root s(NN) = 200 GeV at RHIC. The diagonal spin-density matrix elements with respect to the reaction plane in Au+Au collisions are rho(00) = 0.32 +/- 0.04 (stat) +/- 0.09 (syst) for the K*(0) (0.8 < p(T) < 5.0 GeV/c) and rho(00) = 0.34 +/- 0.02 (stat) +/- 0.03 (syst) for the phi (0.4 < p(T) < 5.0 GeV/c) and are constant with transverse momentum and collision centrality. The data are consistent with the unpolarized expectation of 1/3 and thus no evidence is found for the transfer of the orbital angular momentum of the colliding system to the vector-meson spins. Spin alignments for K(*0) and phi in Au+Au collisions were also measured with respect to the particle's production plane. The phi result, rho(00) = 0.41 +/- 0.02 (stat) +/- 0.04 (syst), is consistent with that in p+p collisions, rho(00) = 0.39 +/- 0.03 (stat) +/- 0.06 (syst), also measured in this work. The measurements thus constrain the possible size of polarization phenomena in the production dynamics of vector mesons.