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Fred Jegerlehner

Publications and source records attributed to Fred Jegerlehner.

At least 19 recordsLinked to original sources
arXiv2026

Lepton Magnetic Moments: What They Tell Us

Recently, the exciting new Fermilab (FNAL) Muon g-2 measurement impressively confirmed the final Brookhaven (BNL) result from 2004, and with a result four times more precise, has launched a new serious attack on the Standard Model (SM). On the theoretical side, ab initio lattice QCD (LQCD) calculations of hadronic vacuum polarization have made remarkable progress. They are now the new standard for studying the leading non-perturbative contributions, which have previously hindered matching with the precision required for full exploitation of the experimental results. The lattice results affected both leading hadronic contributions the hadronic vacuum polarization (HVP) and the hadronic light-by-light (HLbL) contributions by increasing the previously generally accepted $e^+e^-$ to hadrons based dispersion relation results. The shifts reduced the discrepancy between theory and experiment, leaving nothing missing. One of the most prominent signs of Beyond the Standard Model (BSM) physics has disappeared: the SM appears validated more than ever, in agreement with what other searches at the Large Hadron Collider (LHC) at CERN tell us! A triumph of the SM, even though the SM cannot explain known cosmological puzzles like dark matter or baryogenesis, and why neutrino masses are so tiny, the absence of strong CP violation, for example. I also argue that the discrepancy between the data-driven dispersive result and the lattice QCD results for the hadronic vacuum polarization can be largely explained by correcting the $e^+e^-$ data for 'rho-gamma' mixing effects.

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arXiv2024

The $η/η' \rightarrow π^+ π^- γ$ Decays within BHLS$_2$ and the Muon HVP

The departure of the latest FNAL experimental average for the muon anomalous magnetic moment $a_μ=(g_μ-2)/2$ measurements having increased from $4.2 σ$ to $5.0 σ$, with respect to the White Paper consensus, it may indicate a hint for new physics. As the most delicate piece of $a_μ$ is its leading order HVP part $a_μ^{HVP-LO}$, methods to ascertain its theoretical value are crucial to interpret this discrepancy. We propose to examine the dipion spectra from the $η/η' \rightarrow π^+ π^- γ$ decays in the Hidden Local Symmetry (HLS) context using its BHLS$_2$ broken variant. We thus have at our disposal a framework where the close relationship of the dipion spectra from the $η/η'$ and $τ$ decays and of the $e^+e^- \to π^+π^-$ annihilation can be simultaneously considered. A special focus is put to the high statistic dipion spectra from the $η$ decay collected by the KLOE/KLOE2 Collaboration and $η'$ decay collected by the BESIII Collaboration, and it is shown that the BHLS$_2$ framework provides a fair account of their dipion spectra. More precisely, it is first proven that a single Omnès representation real polynomial is requested, common to both the $η$ and $η'$ dipion spectra. Moreover, it is shown that fits involving the $η/η'/τ$ dipion spectra, and excluding the $e^+e^- \to π^+π^-$ annihilation data, allow for a prediction of the pion vector form factor data $F_π(s)$ which fairly agree with the usual dipion spectra collected in the $e^+e^- \to π^+π^-$ annihilation channel. Even if more precise $η/η'/τ$ dipion spectra would help to be fully conclusive, this confirms the Dispersive Approach results for $a_μ^{HVP-LO}$ and points towards a common non experiment-dependent origin to this tension with the now well accepted LQCD result.

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arXiv2023

Is the Higgs Boson the Master of the Universe?

The discovery of the Higgs particle has yielded a specific value for the mass of the Higgs boson, which, depending on some technical details in the calculation of the $\overline{\mathrm{MS}}$ parameters (relevant for the high energy range) from the physical parameters (measured in low energy range), allows the Standard Model (SM) to hold up to the Planck scale about $Λ_{\rm Pl} \sim 10^{19}~{\rm GeV}$. One then has the possibility that the Higgs boson not only provides mass for all SM-particles but very likely also has supplied dark energy that inflated the young universe shortly after the Big Bang. The SM Higgs boson is a natural candidate for the Inflaton, and the Higgs boson decays are able to reheat the universe after inflation. I argue that the structures of the SM evolve naturally from a Planck cutoff medium (ether) and thus find their explanation. That the SM is an emergent structure is also strongly supported by Veltman's derivation of the SM from some general principles, which we can understand as the result of a low-energy expansion. I emphasize the role of the hierarchy problem and the problem of the cosmological constant as causal for the Higgs inflation scenario. After the discovery of the Higgs boson at 125 GeV, and considering the absence of beyond the SM particles at the LHC, a new view on the SM of particle physics and its role in early cosmology has become indispensable. Very likely, the spectacular Higgs discovery turned out to have completed the SM in an unexpected way, revealing it as an inescapable emergence which shapes the early universe.

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arXiv2021

The Standard Model of particle physics as a conspiracy theory and the possible role of the Higgs boson in the evolution of the early universe

I am considering Veltman's "The Infrared - Ultraviolet Connection" addressing the issue of quadratic divergences and the related huge radiative correction predicted by the electroweak Standard Model (SM) in the relationship between the bare and the renormalized theory, commonly called "the hierarchy problem" which usually is claimed that this has to be cured. After the discovery of the Higgs particle at CERN, which essentially completed the SM, an amazing interrelation of the leading interaction strengths of the gauge bosons, the top-quark and the Higgs boson showed up amounting that the SM allows for a perturbative extrapolation of the running couplings up to the Planck scale. The central question concerns the stability of the electroweak vacuum, which requires that the running Higgs self-coupling stays positive. Although several evaluations seem to favor the meta-stability within the experimental and theoretical parameter-uncertainties, one should not exclude the possibility that other experiments and improved matching conditions will be able to establish the absolute stability of the SM vacuum in the future. I will discuss the stable vacuum scenario and its impact on early cosmology, revealing the Higgs boson as the inflaton. It turns out that the Standard Model's presumed "hierarchy problem" and similarly the "cosmological constant problem" resolve themselves when we understand the SM as a low energy effective tail that is emergent from a cutoff-medium at the Planck scale. "The Infrared - Ultraviolet Connection" conveyed by the Higgs boson mass renormalization appears in a new light when the energy dependence of the SM couplings is taken into account. The bare Higgs boson mass square then changes sign below the Planck scale where it is activating the Higgs mechanism.

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arXiv2019

The Hierarchy Problem and the Cosmological Constant Problem Revisited -- A new view on the SM of particle physics

We argue that the Standard Model (SM) in the Higgs phase does not suffer from a "hierarchy problem" and that similarly the "cosmological constant problem" resolves itself if we understand the SM as a low energy effective theory emerging from a cutoff-medium at the Planck scale. We actually take serious Veltman's "The Infrared - Ultraviolet Connection" addressing the issue of quadratic divergences and the related huge radiative correction predicted by the SM in the relationship between the bare and the renormalized theory, usually called "the hierarchy problem" and claimed that this has to be cured. We discuss these issues under the condition of a stable Higgs vacuum, which allows to extend the SM up to the Planck cutoff. The bare Higgs boson mass then changes sign below the Planck scale, such that the SM in the early universe is in the symmetric phase. The cutoff enhanced Higgs mass term as well as the quartically enhanced cosmological constant term provide a large positive dark energy which triggers the inflation of the early universe. Reheating follows via the decays of the four unstable heavy Higgs particles, predominantly into top-antitop pairs, which at this stage are still massless. Preheating is suppressed in SM inflation since in the symmetric phase bosonic decay channels are absent at tree level. The coefficients of the shift between bare and renormalized Higgs mass as well as of the shift between bare and renormalized vacuum energy density exhibit close-by zeros at about $7.7 \times 10^{14}$ GeV and $3.1 \times 10^{15}$ GeV, respectively. The zero of of the Higgs mass counter term triggers the electroweak phase transition from the low energy Higgs phase and to the symmetric phase above the transition point. The scenario highly favors to understand the SM and its main properties as a natural structure emerging at long distance.

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arXiv2018

The Role of Mesons in Muon g-2

The muon anomaly $a_μ=(g_μ-2)/2$ showing a persisting 3 to 4 $σ$ deviation between the SM prediction and the experiment is one of the most promising signals for physics beyond the SM. As is well known, the hadronic uncertainties are limiting the accuracy of the Standard Model prediction. Therefore a big effort is going on to improve the evaluations of hadronic effects in order to keep up with the 4-fold improved precision expected from the new Fermilab measurement in the near future. A novel complementary type experiment planned at J-PARC in Japan, operating with ultra cold muons, is expected to be able to achieve the same accuracy but with completely different systematics. So exciting times in searching for New Physics are under way. I discuss the role of meson physics in calculations of the hadronic part of the muon g-2. The improvement is expected to substantiate the present deviation $Δa_μ^{\rm New \ Physics}=Δa_μ^{\rm Experiment}- Δa_μ^{\rm Standard \ Model}$ to a 6 to 10 standard deviation effect, provided hadronic uncertainties can be reduce by a factor two. This concerns the hadronic vacuum polarization as well as the hadronic light-by-light scattering contributions, both to a large extent determined by the low lying meson spectrum. Better meson production data and progress in modeling meson form factors could greatly help to improve the precision and reliability of the SM prediction of $a_μ$ and thereby provide more information on what is missing in the SM.

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arXiv2018

The Muon g-2 in Progress

Two next generation muon $g-2$ experiments at Fermilab in the US and at J-PARC in Japan have been designed to reach a four times better precision from 0.54 ppm to 0.14 ppm and the challenge for the theory side is to keep up in precision as far as possible. This has triggered a lot of new research activities. The main motivation is the persisting 3 to 4 $σ$ deviation between standard theory and experiment. As Standard Model predictions almost without exception match perfectly all other experimental information, the deviation in one of the most precisely measured quantities in particle physics remains a mystery and inspires the imagination of model builders. Plenty of speculations are aiming to explain what beyond the Standard Model effects could fill what seems to be missing. Here very high precision experiments are competing with searches for new physics at the high energy frontier lead by the Large Hadron Collider at CERN. Actually, the tension is increasing steadily as no new states are found which could accommodate the $g_μ-2$ discrepancy. With the new muon $g-2$ experiments this discrepancy would go up at least to 6 $σ$, in case the central values do not move, up to 10 $σ$ could be reached if the present theory error could be reduced by a factor of two. Interestingly, the new $α$ from Berkeley by R. H. Parker et al. Science 360, 191 (2018): $α^{-1}({\rm Cs18})=137.035999046(27)$ gives an $a_e$ prediction $a_e=0.00115965218157(23)$ such that $a_e^{\rm exp}-a_e^{\rm the}=(-84\pm36)\times 10^{-14}$ shows a $- 2.3~ σ$ deviation now.

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arXiv2017

Variations on Photon Vacuum Polarization

I provide updates for the theoretical predictions of the muon and electron anomalous magnetic moments, for the shift in the fine structure constant $α(M_Z)$ and for the weak mixing parameter $\sin^2 Θ_W(M_Z)$. Phenomenological results for Euclidean time correlators, the key objects in the lattice QCD approach to hadronic vacuum polarization, are briefly considered. Furthermore, I present a list of isospin breaking and electromagnetic corrections for the lepton moments, which may be used to supplement lattice QCD results obtained in the isospin limit and without the e.m. corrections.

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arXiv2017

Muon g-2 Theory: the Hadronic Part

I present a status report of the hadronic vacuum polarization effects for the muon $g-2$, to be considered as an update of [1]. The update concerns recent new inclusive $R$ measurements from KEDR in the energy range 1.84 to 3.72 GeV. For the leading order contributions I find $a_μ^{\mathrm{had}(1)}=(688.07\pm 4.14)[688.77\pm3.38]\times 10^{-10}$ based on $e^+e^-$data [incl. $τ$ data], $a_μ^{\mathrm{had}(2)}= (-9.93\pm 0.07) \times 10^{-10}$ (NLO) and $a_μ^{\mathrm{had}(3)}= (1.22\pm 0.01) \times 10^{-10}$ (NNLO). Collecting recent progress in the hadronic light-by-light scattering I adopt $π^0,η,η'$ [$95 \pm 12$] + axial--vector [$8 \pm ~3$] + scalar [$-6\pm ~1$] + $π,K$ loops [$-20\pm 5$] + quark loops [$22\pm ~4$] + tensor [$1\pm ~0$] + NLO [$3\pm ~2$] which yields $ a^{(6)}_μ(\mathrm{lbl},\mathrm{had})=(103 \pm 29) \times 10^{-11}.$ With these updates I find $a_μ^{\rm exp}-a_μ^{\rm the}=(31.3\pm 7.7)\times 10^{-10}$ a 4.1 $σ$ deviation. Recent lattice QCD results and future prospects to improve hadronic contributions are discussed.

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arXiv2017

Photon radiation in $e^+e^-\to$ hadrons at low energies with CARLOMAT 3.1

We present a sample of results for the cross sections of several processes of low energetic $e^+e^-$ annihilation into final states containing pions accompanied by one or two photons, or a light lepton pair. The results, which have been obtained with a new version of a multipurpose Monte Carlo program CARLOMAT, labelled 3.1, demonstrate new capabilities of the program which, among others, include a possibility of taking into account either the initial or final state radiation separately, or both at a time, and a possibility of inclusion of the electromagnetic charged pion form factor for processes with charged pion pairs. We also discuss some problems related to the $U(1)$ electromagnetic gauge invariance.

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arXiv2015

Leading-order hadronic contribution to the electron and muon g-2

I present a new data driven update of the hadronic vacuum polarization effects for the muon and the electron $g-2$. For the leading order contributions I find $a_μ^{\mathrm{had}(1)}=(686.99\pm 4.21)[687.19\pm 3.48]\times 10^{-10}$ based on $e^+e^-$data [incl. $τ$ data], $a_μ^{\mathrm{had}(2)}= (-9.934\pm 0.091) \times 10^{-10}$ (NLO) and $a_μ^{\mathrm{had}(3)}= (1.226\pm 0.012) \times 10^{-10}$ (NNLO) for the muon, and $a_e^{\mathrm{had}(1)}=(184.64\pm 1.21)\times 10^{-14}$ (LO), $a_e^{\mathrm{had}(2)}=(-22.10\pm 0.14)\times 10^{-14}$ (NLO) and $a_e^{\mathrm{had}(3)}=(2.79\pm 0.02)\times 10^{-14}$ (NNLO) for the electron. A problem with vacuum polarization undressing of cross-sections (time-like region) is addressed. I also add a comment on properly including axial mesons in the hadronic light-by-light scattering contribution. My estimate here reads $a_μ[a_1,f_1',f_1] \sim ({ 7.51 \pm 2.71}) \times 10^{-11}\,.$ With these updates $a_μ^{\rm exp}-a_μ^{\rm the}=(32.73\pm 8.15)\times 10^{-10}$ a 4.0 $σ$ deviation, while $a_e^{\rm exp}-a_e^{\rm the}=(-1.10\pm 0.82)\times 10^{-12}$ shows no significant deviation.

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arXiv2015

The hierarchy problem and the cosmological constant problem in the Standard Model

We argue that the SM in the Higgs phase does not suffer form a "hierarchy problem" and that similarly the "cosmological constant problem" resolves itself if we understand the SM as a low energy effective theory emerging from a cut-off medium at the Planck scale. We discuss these issues under the condition of a stable Higgs vacuum, which allows to extend the SM up to the Planck length. The bare Higgs boson mass then changes sign below the Planck scale, such the the SM in the early universe is in the symmetric phase. The cut-off enhanced Higgs mass term as well as the quartically enhanced cosmological constant term trigger the inflation of the early universe. The coefficients of the shift between bare and renormalized Higgs mass as well as of the shift between bare and renormalized vacuum energy density exhibit close-by zeros at some point below the Planck scale. The zeros are matching points between short distance and the renormalized low energy quantities. Since inflation tunes the total energy density to take the critical value of a flat universe Omega_tot=rho_tot/rho_crit=Omega_Lambda+Omega_matter+Omega_radiation}=1 it is obvious that Omega_Lambda today is of order Omega_tot given that 1>Omega_matter, Omega_radiation>0, which saturate the total density to about 26 % only, the dominant part being dark matter(21 %).

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arXiv2014

Comment on H --> gamma gamma and the role of the decoupling theorem and the equivalence theorem

I am commenting on the paper Gastmans, Wu and Wu, which recently obtained new support by a paper by Christova and Todorov, about a new calculation of the H --> gamma gamma rate mediated by the W boson loop, and the lack of decoupling of the heavy states which mediate the decay. We remind the reader that the heavy Higgs limit is dominated by the contribution from the longitudinal W bosons, which in the limit m_H >> M_W are represented by the charged Higgs ghosts according to the equivalence theorem. The corresponding contribution is missing in Gastmans, Wu and Wu as well as in the new paper by Christova and Todorov.

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arXiv2014

The Standard model as a low-energy effective theory: what is triggering the Higgs mechanism?

The discovery of the Higgs by ATLAS and CMS at the LHC not only provided the last missing building block of the electroweak Standard Model, the mass of the Higgs has been found to have a very peculiar value about 126 GeV, which is such that vacuum stability is extending up to the Planck scale. This may have much deeper drawback than anticipated so far. The impact on the running of the SM gauge, Yukawa and Higgs couplings up to the Planck scale has been discussed in several articles recently. Here we consider the impact on the running masses and we discuss the role of quadratic divergences within the Standard Model. The change of sign of the coefficient of the quadratically divergent terms showing up at about mu_0 ~ 1.4 x 10^16 GeV may be understood as a first order phase transition restoring the symmetric phase, while its large negative values at lower scales triggers the Higgs mechanism, running parameters evolve in such a way that the symmetry is restored two orders of magnitude before the Planck scale. Thus, the electroweak phase transition takes place at the scale mu_0 and not at the electroweak scale v ~ 250 GeV. The SM Higgs system and its phase transition could play a key role for the inflation of the early universe. Also baryogenesis has to be reconsidered under the aspect that perturbative arguments surprisingly work up to the Planck scale.

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arXiv2014

Higgs inflation and the cosmological constant

The Higgs not only induces the masses of all SM particles, the Higgs, given its special mass value, is the natural candidate for the inflaton and in fact is ruling the evolution of the early universe, by providing the necessary dark energy which remains the dominant energy density. SM running couplings not only allow us to extrapolate SM physics up to the Planck scale, but equally important they are triggering the Higgs mechanism. This is possible by the fact that the bare mass term in the Higgs potential changes sign at about mu_0 = 1.4x10^16 GeV and in the symmetric phase is enhanced by quadratic terms in the Planck mass. Such a huge Higgs mass term is able to play a key role in triggering inflation in the early universe. In this article we extend our previous investigation by working out the details of a Higgs inflation scenario. We show how different terms contributing to the Higgs Lagrangian are affecting inflation. Given the SM and its extrapolation to scales mu>mu_0 we find a calculable cosmological constant V(0) which is weakly scale dependent and actually remains large during inflation. This is different to the Higgs fluctuation field dependent Delta V(phi), which decays exponentially during inflation, and actually would not provide a sufficient amount of inflation. The fluctuation field has a different effective mass which shifts the bare Higgs transition point to a lower value mu'_0 = 7.7x10^14 GeV. The vacuum energy V(0) being proportional to M_Pl^4 has a coefficient which vanishes near the Higgs transition point, such that the bare and the renormalized cosmological constant match at this point. The role of the Higgs in reheating and baryogenesis is emphasized.

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arXiv2014

About the role of the Higgs boson in the evolution of the early universe

After the discovery of the Higgs particle the most relevant structures of the SM have been verified and for the first time we know all parameters of the SM within remarkable accuracy. Together with recent calculations of the SM renormalization group coefficients up to three loops we can safely extrapolate running couplings high up in energy. Assuming that the SM is a low energy effective theory of a cutoff theory residing at the Planck scale, we are able to calculate the effective bare parameters of the underlying cutoff system. It turns out that the effective bare mass term changes sign not far below the Planck scale, which means that in the early universe the SM was in the symmetric phase. The sign-flip, which is a result of a conspiracy between the SM couplings and their screening/antiscreening behavior, triggers the Higgs mechanism. Above the Higgs phase transition the bare mass term in the Higgs potential must have had a large positive value, enhanced by the quadratic divergence of the bare Higgs mass. Likewise the quartically enhanced positive vacuum energy density is present in the symmetric phase. The Higgs system thus provides the large dark energy density in the early universe, which triggers slow-roll inflation, i.e. the SM Higgs is the inflaton scalar field. Reheating is dominated by the decay of the heavy Higgses into (in the symmetric phase) massless top/anti-top quark pairs. The new scenario possibly could explain the baryon-asymmetry essentially in terms of SM physics

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