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Mikolaj Misiak

Publications and source records attributed to Mikolaj Misiak.

At least 19 recordsLinked to original sources
arXiv2024

Current Status of the Standard Model Prediction for the B_s -> mu^+ mu^- Branching Ratio

The rare decay B_s -> mu^+ mu^- provides an important constraint on possible deviations from the Standard Model in b-s-l-l interactions. The present weighted average of its branching ratio measurements amounts to (3.34 \pm 0.27) x 10^-9, which remains in good agreement with the theoretical prediction of (3.64 \pm 0.12) x 10^-9 within the Standard Model. In the present paper, we review calculations that have contributed to this prediction, and discuss the associated uncertainties.

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arXiv2024

Two-Loop QCD Anomalous Dimensions of Flavour-Changing Four-Quark Operators Within and Beyond the Standard Model

We calculate the two-loop QCD anomalous dimension matrix (ADM) gamma^(1)_NDR in the NDR-MSbar scheme for all the flavour-changing four-quark dimension-six operators that are relevant in both the Standard Model and its extensions. Both current-current and penguin diagrams are included. Some of our NDR-MSbar results for Delta F=1 operators overlap with the previous calculations, but several others have never been published before. In the case of Delta F=2 operators, our results are compatible with the ones obtained by Ciuchini et al. in the Regularization-Independent renormalization scheme, but differ from their NDR-MSbar results. In order to explain the difference, we calculate the ADM of Delta F=2 operators again, extracting it from the ADM of Delta F=1 operators.

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arXiv2017

NNLO QCD counterterm contributions to $B$ \to $X_s γ$ for the physical value of $m_c$

One of the most important O($α_s^2$) corrections to the $B \to X_s γ$ branching ratio originates from interference of contributions from the current-current and photonic dipole operators. Its value has been estimated using an interpolation in the charm quark mass between the known results at $m_c=0$ and for $m_c \ll m_b/2$. An explicit calculation for the physical value of $m_c$ is necessary to remove the associated uncertainty. In the present work, we evaluate all the ultraviolet counterterm contributions that are relevant for this purpose.

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arXiv2017

Weak Radiative Decays of the B Meson and Bounds on $M_{H^\pm}$ in the Two-Higgs-Doublet Model

In a recent publication [6], the Belle collaboration updated their analysis of the inclusive weak radiative B-meson decay, including the full dataset of (772 +_ 11) x 10^6 BBbar pairs. Their result for the branching ratio is now below the Standard Model prediction [7,8], though it remains consistent with it. However, bounds on the charged Higgs boson mass in the Two-Higgs-Doublet Model get affected in a significant manner. In the so-called Model-II, the 95% C.L. lower bound on $M_{H^\pm}$ is now in the 570-800 GeV range, depending quite sensitively on the method applied for its determination. Our present note is devoted to presenting and discussing the updated bounds, as well as to clarifying several ambiguities that one might encounter in evaluating them. One of such ambiguities stems from the photon energy cutoff choice, which deserves re-consideration in view of the improved experimental accuracy.

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arXiv2015

The (Q_7,Q_{1,2}) contribution to B -> X_s gamma at O(alpha_s^2)

Interference between the photonic dipole operator Q_7 and the current-current operators Q_{1,2} gives one of the most important QCD corrections to the B -> X_s gamma decay rate. So far, the O(alpha_s^2) part of this correction has been known in the heavy charm quark limit only (m_c >> m_b/2). Here, we evaluate this part at m_c=0, and use both limits in an updated phenomenological study. Our prediction for the CP- and isospin-averaged branching ratio in the Standard Model reads B^SM_{s gamma} = (3.36 +_ 0.23) * 10^-4 for E_gamma > 1.6 GeV.

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arXiv2014

Tree-level contributions to B -> Xs gamma

Weak radiative decay B -> X_s gamma is known to be a loop-generated process. However, it does receive tree-level contributions from CKM-suppressed b -> u ubar s gamma transitions. In the present paper, we evaluate such contributions together with similar ones from the QCD penguin operators. For a low value of the photon energy cutoff E_0 ~ m_b/20 that has often been used in the literature, they can enhance the inclusive branching ratio by more than 10%. For E_0 = 1.6 GeV or higher, the effect does not exceed 0.4%, which is due to phase-space suppression. Our perturbative results contain collinear logarithms that depend on the light quark masses m_q (q=u,d,s). We have allowed m_b/m_q to vary from 10 to 50, which corresponds to values of m_q that are typical for the constituent quark masses. Such a rough method of estimation may be improved in the future with the help of fragmentation functions once the considered effects begin to matter in the overall error budget for BR(B -> X_s gamma).

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arXiv2014

B_{s,d} -> l+ l- in the Standard Model with Reduced Theoretical Uncertainty

We combine our new results for the O(alpha_em) and O(alpha_s^2) corrections to B_{s,d} -> l^+ l^-, and present updated branching ratio predictions for these decays in the standard model. Inclusion of the new corrections removes major theoretical uncertainties of perturbative origin that have just begun to dominate over the parametric ones. For the recently observed muonic decay of the B_s meson, our calculation gives BR(B_s -> mu^+ mu^-) = (3.65 +_ 0.23) * 10^(-9).

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arXiv2013

Three-loop QCD corrections to B_s -> mu^+ mu^-

The decay B_s -> mu^+ mu^- in the Standard Model is generated by the well-known W-box and Z-penguin diagrams that give rise to an effective quark-lepton operator Q_A at low energies. We compute QCD corrections of order alpha_s^2 to its Wilson coefficient C_A. It requires performing three-loop matching between the full and effective theories. Including the new corrections makes C_A more stable with respect to the matching scale mu_0 at which the top-quark mass and alpha_s are renormalized. The corresponding uncertainty in |C_A|^2 gets reduced from around 1.8% to less than 0.2%. Our results are directly applicable to all the B_{s(d)} -> l^+ l^- decay modes.

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arXiv2012

$\bar{B}\to X_s γ$ in the Two Higgs Doublet Model up to Next-to-Next-to-Leading Order in QCD

We compute three-loop matching corrections to the Wilson coefficients $C_7$ and $C_8$ in the Two Higgs Doublet Model by applying expansions for small, intermediate and large charged Higgs boson masses. The results are used to evaluate the branching ratio of $\bar{B}\to X_s γ$ to next-to-next-to leading order accuracy, and to determine an updated lower limit on the charged Higgs boson mass. We find $\mhplus \ge 380 $GeV at 95% confidence level when the recently completed BABAR data analysis is taken into account. Our results for the charged Higgs contribution to the branching ratio exhibit considerably weaker sensitivity to the matching scale $μ_0$, as compared to previous calculations.

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arXiv2011

Rare B-Meson Decays

Rare decays of the B-meson that arise due to loop-mediated FCNC transitions are known to provide important constraints on beyond-SM theories. Basic properties of several such decays are reviewed here.

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arXiv2011

Anomalous Wtb coupling effects in the weak radiative B-meson decay

We study the effect of anomalous Wtb couplings on the B -> Xs gamma branching ratio. The considered couplings are introduced as parts of gauge-invariant dimension-six operators that are built out of the Standard Model fields only. One-loop contributions from the charged-current vertices are assumed to be of the same order as the tree-level flavour-changing neutral current ones. Bounds on the corresponding Wilson coefficients are derived.

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arXiv2010

Completing the Calculation of BLM corrections to B -> Xs gamma

Perturbative O(alpha_s^2) corrections to BR(B -> Xs gamma) in the BLM approximation receive contributions from two-, three- and four-body final states. While all the two-body results are well established by now, the other ones have remained incomplete for several years. Here, we calculate the last contribution that has been missing to date, namely the one originating from interference of the current-current and gluonic dipole operators (K_18^{(2)beta_0} and K_{28}^{(2)beta_0}). Moreover, we confirm all the previously known results for BLM corrections to the photon energy spectrum that involve the current-current operators (e.g., K_{22}^{(2)beta_0} and K_{27}^{(2)beta_0}). Finally, we also confirm the recent findings of Ferroglia and Haisch on self-interference of the gluonic dipole operator (K_{88}^{(2)beta_0}).

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arXiv2009

B -> Xs gamma - Current Status

Our current knowledge of BR(B -> Xs gamma) is briefly summarized, with particular attention to uncertain non-perturbative effects.

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arXiv2009

QCD Calculations of Radiative B Decays

The current status of B -> Xs gamma decay rate calculations is summarized. Missing ingredients at the NNLO level are listed. The global normalization factor and non-perturbative effects are discussed. Arguments are presented that results for the cutoff-enhanced perturbative corrections have been misused in Ref. [15] by applying them in the region E_gamma \in [1.0, 1.6] GeV, which means that the corresponding numerical effect on BR(B -> Xs gamma, E_gamma > 1.6 GeV) is unreliable.

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arXiv2007

NNLO QCD corrections to the B -> X_s gamma matrix elements using interpolation in m_c

One of the most troublesome contributions to the NNLO QCD corrections to B -> X_s gamma originates from three-loop matrix elements of four-quark operators. A part of this contribution that is proportional to the QCD beta-function coefficient beta_0 was found in 2003 as an expansion in m_c/m_b. In the present paper, we evaluate the asymptotic behaviour of the complete contribution for m_c >> m_b/2. The asymptotic form of the beta_0-part matches the small-m_c expansion very well at the threshold m_c = m_b/2. For the remaining part, we perform an interpolation down to the measured value of m_c, assuming that the beta_0-part is a good approximation at m_c=0. Combining our results with other contributions to the NNLO QCD corrections, we find BR(B -> X_s gamma) = (3.15 +_ 0.23) x 10^-4 for E_gamma > 1.6 GeV in the B-meson rest frame. The indicated error has been obtained by adding in quadrature the following uncertainties: non-perturbative (5%), parametric (3%), higher-order perturbative (3%), and the interpolation ambiguity (3%).

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arXiv2006

Four-Loop Anomalous Dimensions for Radiative Flavour-Changing Decays

We evaluate the complete four-loop anomalous dimension matrix that is necessary for determining the effective flavour-changing neutral current couplings qbar-q'-gamma and qbar-q'-g at the next-to-next-to-leading order in QCD. The resulting O(alpha_s^2(mu_b)) correction to the B -> X_s gamma branching ratio amounts to around -2.9% for mu_b = 5 GeV, and -4.4% for mu_b = 2.5 GeV

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arXiv2005

Three-Loop Mixing of Dipole Operators

We calculate the complete three-loop O(alpha_s^3) anomalous dimension matrix for the dimension-five dipole operators that arise in the Standard Model after integrating out the top quark and the heavy electroweak bosons. Our computation completes the three-loop anomalous dimension matrix of operators that govern low-energy |Delta F| = 1 flavor-changing processes, and represents an important ingredient of the next-to-next-to-leading order QCD analysis of the Bbar -> X_s gamma decay.

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