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A. Sirlin

Publications and source records attributed to A. Sirlin.

33 records · Page 2Linked to original sources
arXiv1999

Considerations Concerning the Radiative Corrections to Muon Decay in the Fermi and Standard Theories

The FAC, PMS, and BLM optimization methods are applied to the QED corrections to the muon lifetime in the Fermi V-A theory. The FAC and PMS scales are close to m_e, while the BLM scale nearly concides with the geometric average \sqrt{m_e m_μ}. The optimized expressions are employed to estimate the third order coefficient in the α(m_μ) expansion and the theoretical error of the perturbative series. Using arguments based on effective field theory and a simple examination of Feynman diagrams, it is shown that, if contributions of O(αm_μ^2/M_W^2) are neglected, the corrections to muon decay in the SM factorize into the QED correction of the Fermi V-A theory and the electroweak amplitude g^2/(1-Δr), both of which are strictly scale-independent. We use the results to clarify how the QED corrections to muon decay and the Fermi constant G_F should be used in the SM, and what is the natural choice of scales if running couplings are employed.

hep-ph↗
arXiv1999

Test of the Heavy Top Expansion in the Evaluation of MW and sin^2theta_eff

In order to test the accuracy of the Heavy Top-mass Expansion (HTE) employed in recent two-loop calculations of MW and sin^2theta_eff, we consider their contributions to subtracted quantities of the form MW_sub=MW(mh)-MW(mh0), where mh0 is a reference point. The results are compared with those obtained by a precise numerical evaluation of all the two-loop contributions involving both the Higgs boson and a fermion loop. For the choice mh0=65 GeV, and over the large range 65 GeV <mh< 1TeV, we find very small differences between the precise and HTE calculations, amounting to |deltaMW|<0.8 MeV and |delta s^2_eff|<1.2 x 10^-5. Although corrections involving light fermions are necessary for the consistency and test of existent calculations, we also discuss the separate contributions from the top-bottom isodoublet. In this case, the differences are larger, although still small, namely |deltaMW|<1.9 MeV and |delta s^2_eff|< 4.5 x 10^-5.

hep-ph↗
arXiv1998

Recent developments in the study of unstable particle processes

Developments in the analysis of W and quark propagators in the resonance region, and recent considerations concerning the mass and width of the Higgs boson are reviewed. Particular emphasis is placed on the instability of these fundamental particles, and on related issues of gauge dependence.

hep-ph↗
arXiv1998

Radiative Corrections to W and Quark Propagators in the Resonance Region

We discuss radiative corrections to W and quark propagators in the resonance region, |s-M^2| \lsim M*Gamma. We show that conventional mass renormalization, when applied to photonic or gluonic corrections, leads in next to leading order (NLO) to contributions proportional to [M*Gamma/(s-M^2)]^n, (n=1,2...), i.e. to a non-convergent series in the resonance region, a difficulty that affects all unstable particles coupled to massless quanta. A solution of this problem, based on the concepts of pole mass and width, is presented. It elucidates the issue of renormalization of amplitudes involving unstable particles and automatically circumvents the problem of apparent on-shell singularities. The roles of the Fried-Yennie gauge and the Pinch Technique prescription are discussed. Because of special properties of the photonic and gluonic contributions, and in contrast with the Z case, the gauge dependence of the conventional on-shell definition of mass is unbounded in NLO. The evaluations of the width in the conventional and pole formulations are compared and shown to agree in NLO but not beyond.

hep-ph↗
arXiv1998

Mass and Width of a Heavy Higgs Boson

The gauge dependence of the Higgs-boson mass and width in the on-shell scheme of renormalization is studied in the heavy-Higgs-boson approximation. The corresponding expansions in the pole scheme are analyzed adopting three frequently employed parametrizations. The convergence properties and other theoretical features of the on-shell and pole expansions, as well as their relative merits, are discussed.

hep-ph↗
arXiv1998

Radiative Corrections to W and Quark Propagators in the Resonance Region

It is shown that conventional mass renormalization, when applied to photonic or gluonic corrections to unstable particle propagators, leads to non-convergent series in the resonance region. A solution of this problem, based on the concepts of pole mass and width, is presented. In contrast with the Z case, the conventional on-shell definition of mass for W bosons and unstable quarks contains an unbounded gauge dependence in next-to-leading order. The on-shell and pole definitions of width are shown to coincide if terms of O(Γ^2) and higher are neglected, but not otherwise.

hep-ph↗
arXiv1998

Differences Between the Pole and On-Shell Masses and Widths of the Higgs Boson

The differences between the on-shell mass and width of the Higgs boson and their pole counterparts are evaluated in leading order. For a heavy Higgs boson, they are found to be sensitive functions of the gauge parameter and become numerically large over a class of gauges that includes the unitary gauge. For a light Higgs boson, the differences remain small in all gauges. The pinch-technique mass and width are found to be close to their pole counterparts over a large range of Higgs boson masses.

hep-ph↗
arXiv1997

The Role of M(W) in Precision Studies of the Standard Model

Recent calculations have significantly decreased the scheme and residual scale dependence of basic radiative corrections of the Standard Electroweak Model. This leads to a theoretically accurate prediction of the W-boson mass M(W), as well as a reduced upper bound for the Higgs boson mass M(H). The implications of a precise M(W) measurement on the M(H) estimate are emphasized.

hep-ph↗
arXiv1996

Precise calculation of MW, sin^2 theta_MSbar, and sin^2 theta_eff

The two-loop O(g^4 mt^2/mw^2) corrections are incorporated in the theoretical calculation of MW, sin^2 theta_MSbar(MZ), and sin^2 theta_eff, as functions of MH. The analysis is carried out in a previously proposed MSbar formulation and two novel on-shell resummation schemes. It is found that the inclusion of the new effects sharply decreases the scheme and residual scale dependence of the calculations. QCD corrections are incorporated in two different approaches. Comparison with the world average of sin^2 theta_eff leads to MH= 127 +143 -71 GeV and MW= 80.367 +/- 0.048 GeV, with small variations among the six calculations.

hep-ph↗
arXiv1996

Analysis of the Z^0 Resonant Amplitude in General R_ξGauges

The Z^0 resonant amplitude is discussed in general R_ξgauges. When the original on-shell definition of the Z^0 mass M is employed, a gauge dependence of M emerges in the next-to-leading approximation which, although small, is of the same magnitude as the current experimental error. In the following order of expansion, these unphysical effects are unbounded. The gauge dependence of M disappears when modified, previously proposed definitions of mass or self-energies, are used. The relevance of these considerations to the concept of the mass of unstable particles is pointed out.

hep-ph↗
arXiv1995

LEADING VACUUM-POLARIZATION CONTRIBUTIONS TO THE RELATION BETWEEN POLE AND RUNNING MASSES

The vacuum--polarization contributions of ${\cal O}(b^{n-1} \al_s^n)$ to the relation between the pole--mass $M$ of a quark and the \MS parameter $\mh (M)$ are evaluated by a straightforward method. They are found to approximate very well the exact answer, known through ${\cal O}(\al_s^2)$, thus providing a simple physical interpretation. Results are also given for the cases when the vacuum--polarization contributions are defined by the pinch technique prescription and specific background field gauges. Assuming that the terms $n \geq 3$ are also dominant, we evaluate $M_t/\hat{m}_t(M_t)$, compare the results with those of optimization methods, briefly discuss $M_b/\mh_b (M_b)$, and estimate the irreducible errors in the perturbative series. Implications for the electroweak amplitude $Δρ$ are emphasized. An update of the QCD corrections to this amplitude, including an estimate of the theoretical error, is given.

hep-ph↗
arXiv1994

Evidence for Bosonic Electroweak Corrections in the Standard Model

We present strong indirect evidence for the contribution of bosonic electroweak corrections in the Standard Model. Although important conceptually, these corrections give subleading contributions in current high energy experiments, and it was previously thought that they are difficult to detect. We also discuss the separate contribution of the Higgs boson.

hep-ph↗
arXiv1994

QCD Corrections to $Δρ$

We discuss a simple method to evaluate the QCD corrections to $Δρ$. It assumes that the perturbative expansion in terms of $\ms$ parameters is meaningful and, unlike other studies, exploits significant available information concerning $\chass$ corrections. This approach leads to an enhancement of $\sim26\%$ relative to the conventional evaluation. QCD corrections to the $Z^0\to b\bar b$ amplitude are also considered. Implications for electroweak physics are briefly discussed.

hep-ph↗