67 research outputs found
The improved 10th order QED expression for a_{\mu} : new results and related estimates
New estimates of the 10th order QED corrections to the muon anomalous
magnetic moment are presented. The estimates include the information on
definite improved 10th order QED contributions to , calculated by
Kinoshita and Nio. The final estimates are in good agreement with the ones,
given recently by Kinoshita.Comment: Presented at Working Group 4 of 7th Int. Workshop on Neutrino
Factories and Superbeams NuFact05, Frascati 21-26 June 2005; 3 double column
pages, LaTe
Hadronic Contributions to the Muon Anomaly in the Constituent Chiral Quark Model
The hadronic contributions to the anomalous magnetic moment of the muon which
are relevant for the confrontation between theory and experiment at the present
level of accuracy, are evaluated within the same framework: the constituent
chiral quark model. This includes the contributions from the dominant hadronic
vacuum polarization as well as from the next--to--leading order hadronic vacuum
polarization, the contributions from the hadronic light-by-light scattering,
and the contributions from the electroweak hadronic vertex.
They are all evaluated as a function of only one free parameter: the
constituent quark mass. We also comment on the comparison between our results
and other phenomenological evaluations.Comment: Several misprints corrected and a clarifying sentence added. Three
figures superposed and two references added. Version to appear in JHE
Hadronic contribution to the muon g-2: a Dyson-Schwinger perspective
We summarize our results for hadronic contributions to the anomalous magnetic
moment of the muon (), the one from hadronic vacuum-polarisation (HVP)
and the light-by-light scattering contribution (LBL), obtained from the
Dyson-Schwinger equations (DSE's) of QCD. In the case of HVP we find good
agreement with model independent determinations from dispersion relations for
as well as for the Adler function with deviations well
below the ten percent level. From this we conclude that the DSE approach should
be capable of describing with similar accuracy. We also
present results for LBL using a resonance expansion of the quark anti-quark
T-matrix. Our preliminary value is .Comment: Contribution to the proceedings of 'International school of nuclear
physics, 33rd course', Erice-Sicily: 16 - 24 September 201
Heavy mass expansion, light-by-light scattering and the anomalous magnetic moment of the muon
Contributions from light-by-light scattering to (g_\mu-2)/2, the anomalous
magnetic moment of the muon, are mediated by the exchange of charged fermions
or scalar bosons. Assuming large masses M for the virtual particles and
employing the technique of large mass expansion, analytical results are
obtained for virtual fermions and scalars in the form of a series in (m_\mu
/M)^2. This series is well convergent even for the case M=m_\mu. For virtual
fermions, the expansion confirms published analytical formulae. For virtual
scalars, the result can be used to evaluate the contribution from charged
pions. In this case our result confirms already available numerical
evaluations, however, it is significantly more precise.Comment: revtex4, eps figure
Dispersion relations for hadronic light-by-light scattering and the muon g
The largest uncertainties in the Standard Model calculation of the anomalous magnetic moment of the muon (g – 2)μ come from hadronic effects, and in a few years the subleading hadronic light-by-light (HLbL) contribution might dominate the theory error. We present a dispersive description of the HLbL tensor, which is based on unitarity, analyticity, crossing symmetry, and gauge invariance. This opens up the possibility of a data-driven determination of the HLbL contribution to (g – 2)μ with the aim of reducing model dependence and achieving a reliable error estimate.
Our dispersive approach defines unambiguously the pion-pole and the pion-box contribution to the HLbL tensor. Using Mandelstam double-spectral representation, we have proven that the pion-box contribution coincides exactly with the one-loop scalar-QED amplitude, multiplied by the appropriate pion vector form factors. Using dispersive fits to high-statistics data for the pion vector form factor, we obtain αμπ-box=−15.92×10−11. A first model-independent calculation of effects of ππ intermediate states that go beyond the scalar-QED pion loop is also presented. We combine our dispersive description of the HLbL tensor with a partial-wave expansion and demonstrate that the known scalar-QED result is recovered after partial-wave resummation. After constructing suitable input for the γ*γ* → ππ helicity partial waves based on a pion-pole left-hand cut (LHC), we find that for the dominant charged-pion contribution this representation is consistent with the two-loop chiral prediction and the COMPASS measurement for the pion polarizability. This allows us to reliably estimate S-wave rescattering effects to the full pion box and leads to αμπ-box+αμ,J=0ππ,π-pole LHC=−241×10−11
Hadronic light-by-light corrections to the muon g-2: the pion-pole contribution
The correction to the muon anomalous magnetic moment from the pion-pole
contribution to the hadronic light-by-light scattering is considered using a
description of the pi0 - gamma* - gamma* transition form factor based on the
large-Nc and short-distance properties of QCD. The resulting two-loop integrals
are treated by first performing the angular integration analytically, using the
method of Gegenbauer polynomials, followed by a numerical evaluation of the
remaining two-dimensional integration over the moduli of the Euclidean loop
momenta. The value obtained, a_{mu}(LbyL;pi0) = +5.8 (1.0) x 10^{-10},
disagrees with other recent calculations. In the case of the vector meson
dominance form factor, the result obtained by following the same procedure
reads a_{mu}(LbyL;pi0)_{VMD} = +5.6 x 10^{-10}, and differs only by its overall
sign from the value obtained by previous authors. Inclusion of the eta and
eta-prime poles gives a total value a_{mu}(LbyL;PS) = +8.3 (1.2) x 10^{-10} for
the three pseudoscalar states. This result substantially reduces the difference
between the experimental value of a_{mu} and its theoretical counterpart in the
standard model.Comment: 27 pages, Latex, 3 figures. v2: version to be published in Phys. Rev.
D, Note added and references updated (don't worry, sign has not changed
The Muon Anomalous Magnetic Moment: A Harbinger For "New Physics"
QED, Hadronic, and Electroweak Standard Model contributions to the muon
anomalous magnetic moment, a_mu = (g_mu-2)/2, and their theoretical
uncertainties are scrutinized. The status and implications of the recently
reported 2.6 sigma experiment vs.theory deviation a_mu^{exp}-a_mu^{SM} =
426(165) times 10^{-11} are discussed. Possible explanations due to
supersymmetric loop effects with m_{SUSY} \simeq 55 sqrt{tan beta} GeV,
radiative mass mechanisms at the 1--2 TeV scale and other ``New Physics''
scenarios are examined.Comment: 24 page
The renormalization group inspired approaches and estimates of the tenth-order corrections to the muon anomaly in QED
We present the estimates of the five-loop QED corrections to the muon anomaly
using the scheme-invariant approaches and demonstrate that they are in good
agreement with the results of exact calculations of the corresponding
tenth-order diagrams supplemented by the additional guess about the values of
the non-calculated contributions.Comment: LATEX 15 pages, figures available upon request; preprint
CERN-TH.7518/9
The Muon g-2
The muon anomalous magnetic moment is one of the most precisely measured
quantities in particle physics. In a recent experiment at Brookhaven it has
been measured with a remarkable 14-fold improvement of the previous CERN
experiment reaching a precision of 0.54ppm. Since the first results were
published, a persisting "discrepancy" between theory and experiment of about 3
standard deviations is observed. It is the largest "established" deviation from
the Standard Model seen in a "clean" electroweak observable and thus could be a
hint for New Physics to be around the corner. This deviation triggered numerous
speculations about the possible origin of the "missing piece" and the increased
experimental precision animated a multitude of new theoretical efforts which
lead to a substantial improvement of the prediction of the muon anomaly
a_mu=(g_mu-2)/2. The dominating uncertainty of the prediction, caused by strong
interaction effects, could be reduced substantially, due to new hadronic cross
section measurements in electron-positron annihilation at low energies. Also
the recent electron g-2 measurement at Harvard contributes substantially to the
progress in this field, as it allows for a much more precise determination of
the fine structure constant alpha as well as a cross check of the status of our
theoretical understanding.Comment: 134 pages, 68 figure
- …