Anomalous Moment Research Articles

Revisiting the top quark chromomagnetic dipole moment in the SM

We revisit the anomalous chromomagnetic dipole moment in the Standard Model and show that its triple gluon vertex contribution, with the on-shell gluon ($q^2=0$), generates an infrared divergent pole. Consequently, the chromomagnetic dipole should not be perturbatively evaluated at $q^2=0$. Focusing on this top quark anomaly, denoted as $\hat{\mu}_t(q^2)$, we compute it with the off-shell gluon with a large momentum transfer, just as the $\alpha_s(m_Z^2)$ convention scale, for both spacelike $q^2=-m_Z^2$ and timelike $q^2=m_Z^2$ cases. We found that $\hat{\mu}_t(-m_Z^2)$ $=$ $-0.0224$ $-0.000925i$ and $\hat{\mu}_t(m_Z^2)$ $=$ $-0.0133$ $-0.0267i$. Our $\mathrm{Re}\thinspace\hat{\mu}_t(-m_Z^2)$ matches well with the current experimental value $\hat{\mu}_t^\mathrm{Exp}=-0.024_{-0.009}^{+0.013}(\mathrm{stat})_{-0.011}^{+0.016}(\mathrm{syst})$, and the $\mathrm{Im}\thinspace\hat{\mu}_t(-m_Z^2)$ part is induced by flavour changing charged currents.

Anomalous electromagnetic moments of τ lepton in γγ → τ+τ− reaction in Pb+Pb collisions at the LHC

We discuss the sensitivity of the γγ→τ+τ− process in ultraperipheral Pb+Pb collisions on the anomalous magnetic (aτ) and electric (dτ) moments of τ lepton at LHC energies. We derive the corresponding cross sections by folding the elementary cross section with the heavy-ion photon fluxes and considering semi-leptonic decays of both τ leptons in the fiducial volume of ATLAS and CMS detectors. We present predictions for total and differential cross sections, and for the ratios to γγ→e+e−(μ+μ−) process. These ratios allow to cancel theoretical and experimental uncertainties when performing precision measurements at the LHC. The expected limits on aτ with existing Pb+Pb dataset are found to be better by a factor of two comparing to current best experimental limits and can be further improved by another factor of two at High Luminosity LHC.

On the Second Dipole Moment of Dirac’s Particle

An analysis is presented of the possible existence of the second anomalous dipole moment of Dirac’s particle next to the one associated with the angular momentum. It includes a discussion why, in spite of his own derivation, Dirac has doubted about its relevancy. It is shown why since then it has been overlooked and why it has vanished from leading textbooks. A critical survey is given on the reasons of its reject, including the failure of attempts to measure and the perceived violations of time reversal symmetry and charge–parity symmetry. It is emphasized that the anomalous electric dipole moment of the pointlike electron (AEDM) is fundamentally different from the quantum field type electric dipole moment of an electron (eEDM) as defined in the standard model of particle physics. The analysis has resulted into the identification of a third type Dirac particle, next to the electron type and the Majorana particle. It is shown that, unlike as in the case of the electron type, its second anomalous dipole moment is real valued and is therefore subject to polarization in a scalar potential field. Examples are given that it may have a possible impact in the nuclear domain and in the gravitational domain.

Periodic fast radio bursts from forcedly precessing neutron stars, anomalous torque, and internal magnetic field for FRB 180916.J0158+65 and FRB 121102

ABSTRACT A recent discovery of the periodic activity of the repeating fast radio burst source FRB 180916.J0158+65 in the Canadian Hydrogen Intensity Mapping Experiment (CHIME) hints at possible origin of the FRB from a freely precessing neutron star with a magnetar magnetic field of about 1016 G. However, the absence of simultaneously detected high-energy emission in the Swift and AGILE observations imposes stringent constraints on the field magnitude and questions the possibility of such a progenitor. We show that consideration of forced precession of a neutron star does not encounter the difficulty. This kind of precession takes place even if the neutron star is not deformed and is brought about by the anomalous moment of electromagnetic forces induced by stellar rotation and determined by non-corotational currents. Contrary to what is expected for the currents of corotation, the anomalous torque calculated by the direct method appears to be non-zero. If the observed 16.35-d period corresponds to the period of stellar precession, the inferred internal magnetic field appears to be about 6 × 1014 G for rotational period 1 s. For another possibly periodic FRB 121102 with 157-d period, the magnetic field is even lower, 2 × 1014 G, thereby justifying earlier considerations and not ruling out the hypothesis of FRB origin from precessing neutron stars.

Measurement of the top quark forward-backward production asymmetry and the anomalous chromoelectric and chromomagnetic moments in pp collisions at sqrt{s} = 13 TeV

The parton-level top quark (t) forward-backward asymmetry and the anomalous chromoelectric left({hat{d}}_{mathrm{t}}right) and chromomagnetic left({hat{mu}}_{mathrm{t}}right) moments have been measured using LHC pp collisions at a center-of-mass energy of 13 TeV, collected in the CMS detector in a data sample corresponding to an integrated luminosity of 35.9 fb−1. The linearized variable {A}_{mathrm{FB}}^{(1)} is used to approximate the asymmetry. Candidate mathrm{t}overline{mathrm{t}} events decaying to a muon or electron and jets in final states with low and high Lorentz boosts are selected and reconstructed using a fit of the kinematic distributions of the decay products to those expected for mathrm{t}overline{mathrm{t}} final states. The values found for the parameters are {A}_{mathrm{FB}}^{(1)}={0.048}_{-0.087}^{+0.095}{left(mathrm{stat}right)}_{-0.029}^{+0.020}left(mathrm{syst}right),{hat{mu}}_{mathrm{t}}=-{0.024}_{-0.009}^{+0.013}{left(mathrm{stat}right)}_{-0.011}^{+0.016}left(mathrm{syst}right), and a limit is placed on the magnitude of left|{hat{d}}_{mathrm{t}}right| < 0.03 at 95% confidence level.

{hbox {BHLS}}_2, a new breaking of the HLS model and its phenomenology

Previous studies have shown that the Hidden Local Symmetry (HLS) Model, supplied with appropriate symmetry breaking mechanisms, provides an Effective Lagrangian (BHLS) able to encompass a large number of processes within a unified framework. This allowed one to design a global fit procedure which provides a fair simultaneous description of the e^+ e^- annihilation into six final states (pi ^+pi ^-, pi ^0gamma , eta gamma , pi ^+pi ^-pi ^0, K^+K^-, K_L K_S), the dipion spectrum in the tau decay and some more light meson decay partial widths. In this paper, additional breaking schemes are defined which improve the BHLS working and extend its scope so as to absorb spacelike processes within a new framework ({hbox {BHLS}}_2). The phenomenology previously explored with BHLS is fully revisited in the {hbox {BHLS}}_2 context with special emphasis on the phi mass region using all available data samples. It is shown that {hbox {BHLS}}_2 addresses perfectly the close spacelike region covered by NA7 and Fermilab data; it is also shown that the recent lattice QCD (LQCD) information on the pion form factor are accurately predicted by the {hbox {BHLS}}_2 fit functions derived from fits to only annihilation data. The contribution to the muon anomalous magnetic moment a_mu ^{mathrm{th}} of these annihilation channels over the range of validity of {hbox {BHLS}}_2 (up to simeq 1.05 GeV) is updated within the new {hbox {BHLS}}_2 framework and shown to strongly reduce the former BHLS systematics. The uncertainty on a_mu ^{mathrm{th}}(sqrt{s}< 1.05 , hbox {GeV}) is much improved compared to standard approaches relying on direct integration methods of measured spectra. Using the {hbox {BHLS}}_2 results, the leading-order HVP contribution to the muon anomalous moment is a_mu ^{mathrm{HVP-LO}}= 686.65 pm 3.01 +(+1.16,-0.75)_{mathrm{syst}} in units of 10^{-10}. Using a conservative estimate for the light-by-light contribution, our evaluation for the muon anomalous magnetic moment is a_mu ^{mathrm{th}}=left[ 11,659,175.96 pm 4.17 +(+1.16,-0.75)_{mathrm{syst}}right] times 10^{-10}. The relationship between the dispersive and LQCD approaches to the rho ^0–gamma mixing is also discussed which may amount to a shift of delta a_mu [pi pi ]_{rho gamma }=+(3.10pm 0.31) times 10^{-10} at LO+NLO, presently treated as additional systematics. Taking also this shift into account, the difference a_mu ^{mathrm{th}}-a_mu ^{mathrm{BNL}} exhibits a significance not smaller than 3.8 sigma .

Resurrecting Leather-Stocking: Pathfinding in Jacksonian America

Resurrecting Leather-Stocking: Pathfinding in Jacksonian America

Disk and Washer Coupled Cavity Linac Design and Cold-Model for Muon Linac

A disk and washer (DAW) coupled cavity linac (CCL) was developed for a mediumvelocity part in a muon linac to measure muon anomalous magentic moment and search for electric dipole moment. Muons were accelerated from v/c = β = 0.3 to 0.7 at an operational frequency of 1.3 GHz. In this paper, the cavity design, beam dynamics design, and cold-model measurements are presented.

Probing the anomalous electromagnetic dipole moments of the top quark in γp collision at the LHC, the HL-LHC and the HE-LHC

The determination of the electromagnetic dipole moments of the top quark is one of the most important goals of the top quark physics program in the collider experiments. For this reason, the top quark pair production to investigate the sensitivity on the electric and magnetic dipole moments of the top quark via the process pp→pγp→ptt¯X at the Large Hadron Collider (LHC), the High-Luminosity Large Hadron Collider (HL-LHC) and High-Energy Large Hadron Collider (HE-LHC) is discussed. We apply pure leptonic and semileptonic decays for top quark pair production in the final state. Moreover, we consider systematic uncertainties of 0,3% and 5%. The best limits obtained from the process pp→pγp→ptt¯X on the anomalous aA and aV couplings are |aA|=0.0200 and aV=[−0.9959;0.0003]. Thus, our results indicate that the process pp→pγp→ptt¯X is a very good perspective to probe the electric and magnetic dipole moments of the top quark at the HL-LHC and the HE-LHC.

Probing the nonstandard top-gluon couplings through tt¯γγ production at the LHC

In this paper, we investigate the anomalous chromoelectric and chromomagnetic dipole moments of the top quark through top pair production in association with two photons at the LHC. We first present the strategy to reconstruct this process assuming a different source for background processes. Then, we focus on the existing constraints from inclusive top-pair production from the Tevatron and LHC, adding the new LHC measurement. Afterwards, we introduce the new cross section ratio $R_{2\gamma/\gamma}=\sigma_{t\bar{t}\gamma\gamma}/\sigma_{t\bar{t}\gamma}$ and show the usefulness of this ratio in canceling most of the systematic uncertainties and its special functionality to constrain dipole moments. Finally, we use the scalar sum of the transverse momentum of jets, $H_{T}$, in order to define a signal-dominated region and obtain limits on these anomalous top couplings using different amounts of expected data from the LHC.

Lepton-flavor violation and two loop electroweak corrections to (g−2)μ in the B−L symmetric SSM

Charged lepton flavor violating processes are forbidden in the standard model (SM), hence the observation of charged lepton flavor transitions would represent a clear signal of new physics beyond the standard Model. In this work, we investigate some lepton flavor violating processes in the minimal supersymmetric extension of the SM with local $B-L$ gauge symmetry (B-LSSM). And including the corrections from some two loop diagrams to the anomalous dipole moments (MDM) of muon, we discuss the corresponding constraint on the relevant parameter space of the model. Considering the constraints from updated experimental data, the numerical results show that, new contributions in the B-LSSM enhance the MSSM predictions on the rates of $l_j-l_i$ transitions about one order of magnitude, and also enhance the MSSM prediction on the muon MDM. In addition, two loop electroweak corrections can make important contributions to the muon MDM in the B-LSSM.

Measurement of transition form factors in two-photon collisions at BESIII

The two-photon physics program of the BESIII Collaboration is mainly motivated by the need of new measurements of transition form factors as input for the Standard Model calculations of $ \mathop a\nolimits_\mu ^{HLbL} $, the hadronic light-by-light scattering contribution to the anomalous magentic moment of the muon. The large data sets acquired at BESIII allow to study the transition form factors of pseudoscalar mesons in the relevant region of momentum transfer for $ \mathop a\nolimits_\mu ^{HLbL} $. In this presentation the status of the respective measurements for π0,η and η’ mesons are discussed, and prospects for studies of multi-meson systems as well as of doubly-virtual transition form factors are given.

Probing anomalous off-shell electroweak dipole moment of the top quark in single top quark production associated with Z or W bosons

An extension of the previous studies of anomalous Wtb interaction with offshell W boson was considered in associated single top quark production with W or Z bosons. More general variant with covariant derivative DµW−µ b−(fLWPL + fRWPR)t for the same processes is considered in this paper.

Model-independent sensibility studies for the anomalous dipole moments of theντat the CLIC basede+e−andγe−colliders

To improve the theoretical prediction of the anomalous dipole moments of the $\tau$-neutrino, we have carried out a study through the process $\gamma e^- \to \tau \bar\nu_\tau \nu_e$, which represents an excellent and useful option in determination of these anomalous parameters. To study the potential of the process $\gamma e^- \to \tau \bar\nu_\tau \nu_e$, we apply a future high-energy and high-luminosity linear electron-positron collider, such as the CLIC, with $\sqrt{s}=380, 1500, 3000\hspace{0.8mm}GeV$ and ${\cal L}=10, 50, 100, 200, 500, 1000, 1500, 2000, 3000\hspace{0.8mm}fb^{-1}$, and we consider systematic uncertainties of $\delta_{sys}=0, 5, 10\%$. With these elements, we present a comprehensive and detailed sensitivity study on the total cross-section of the process $\gamma e^- \to \tau \bar\nu_\tau \nu_e$, as well as on the dipole moments $\mu_{\nu_\tau}$ and $d_{\nu_\tau}$ at the $95\%$ C.L., showing the feasibility of such process at the CLIC at the $\gamma e^-$ mode with unpolarized and polarized electron beams.

Dilepton azimuthal correlations in toverline{t} production

The dilepton azimuthal correlation, namely the difference ϕ between the azimuthal angles of the positive and negative charged lepton in the laboratory frame, provides a stringent test of the spin correlation in toverline{t} production at the Large Hadron Collider. We introduce a parameterisation of the differential cross section dσ/dϕ in terms of a Fourier series and show that the third-order expansion provides a sufficiently accurate approximation. This expansion can be considered as a ‘bridge’ between theory and data, making it very simple to cast predictions in the Standard Model (SM) and beyond, and to report measurements, without the need to provide the numbers for the whole binned distribution. We show its application by giving predictions for the coefficients in the presence of (i) an anomalous top chromomagnetic dipole moment; (ii) an anomalous tbW interaction. The methods presented greatly facilitate the study of this angular distribution, which is of special interest given the 3.2(3.7)σ deviation from the SM next-to-leading order prediction found by the ATLAS collaboration in Run 2 data.

Bright muon source driven by GeV electron beams from a compact laser wakefield accelerator

We report here a systematic quantitative study on the generation and characteristics of an active muon source driven by the interaction of an electron beam within the energy range of 1–10 GeV from laser wakefield acceleration (LWFA) with a tungsten target, using Monte Carlo simulations. The 10 GeV electron beam, achievable in the near future, from LWFA using femtosecond multi-PW lasers is employed to drive the bright source of muon pairs in a compact setup. We show that a highly directional and intense source of short-pulsed GeV muon pairs (μ−μ+) have a peak brightness of 5 × 1017 pairs s–1cm–2sr–1 and sub-100 ps duration could be produced using a quasi-monoenergetic 10 fs, 10 GeV electron bunch with a 1-mrad divergence and 100 pC charge. The muon pairs are emitted from a point-like source with a well-defined position and timing; the source has a size and geometric emittance of about 1 mm and 40 μm, respectively. Such muon sources can greatly benefit applications in muon radiography as well as studies on anomalous dipole moments, rare decays of muons, neutrino oscillations, and an injector for a future compact muon collider.

Model-independent sensitivity estimates for the electromagnetic dipole moments of the τ -lepton at the CLIC

We establish model independent bounds on the anomalous magnetic and electric dipole moments of the tau-lepton using the two-photon processes $\gamma\gamma \to \tau^+\tau^-$ and $\gamma\gamma \to \tau^+\tau^-\gamma$. We use ${\cal L}=10, 50, 100, 300, 500, 1000, 1500, 2000, 3000 \hspace{0.8mm}fb^{-1}$ of data collected with the future $e^+e^-$ linear collider such as the CLIC at $\sqrt{s}=380, 1500, 3000 \hspace{0.8mm}GeV$ and we consider systematic uncertainties of $\delta_{sys}=0, 3, 5\hspace{1mm}\%$. Precise bounds at $95\%$ C. L. on the anomalous dipole moments to the tau-lepton $-0.00015\leq \tilde{a}_\tau \leq 0.00017$ and $|\tilde{d}_\tau(ecm)|=9.040\times 10^{-19}$ are set from our study. Our results show that the processes under consideration are a very good prospect for probing the dipole moments of the tau-lepton at the future $e^+e^-$ linear collider at the $\gamma\gamma$ mode.

Analysis of the anomalous electromagnetic moments of the tau lepton in γp collisions at the LHC

In this study, we investigate the potential of the process pp→pγ⁎p→pτν¯τq′X at the LHC to examine the anomalous electromagnetic moments of the tau lepton. We obtain 95% confidence level bounds on the anomalous coupling parameters with various values of the integrated luminosity and center-of-mass energy. The improved bounds have been obtained on the anomalous coupling parameters of electric and magnetic moments of the tau lepton aτ and |dτ| compared to the current experimental sensitivity bounds. The γp mode of photon reactions at the LHC have shown that it has great potential for the electromagnetic dipole moments studies of the tau lepton.

CP-violating top quark couplings at future linear e^+e^- colliders

We study the potential of future lepton colliders to probe violation of the CP symmetry in the top quark sector. In certain extensions of the Standard Model, such as the two-Higgs-doublet model (2HDM), sizeable anomalous top quark dipole moments can arise, which may be revealed by a precise measurement of top quark pair production. We present results from detailed Monte Carlo studies for the ILC at 500 hbox { GeV} and CLIC at 380 hbox { GeV} and use parton-level simulations to explore the potential of high-energy operation. We find that precise measurements in e^+e^- rightarrow tbar{t} production with subsequent decay to lepton plus jets final states can provide sufficient sensitivity to detect Higgs-boson-induced CP violation in a viable two-Higgs-doublet model. The potential of a linear e^+e^- collider to detect CP-violating electric and weak dipole form factors of the top quark exceeds the prospects of the HL-LHC by over an order of magnitude.

Fermilab muon g-2 experiment

The Fermilab muon g-2 experiment will measure the muon anomalous magnetic moment aμ to 140 ppb – a four-fold improvement over the earlier Brookhaven experiment. The measurement of aμ is well known as a unique test of the standard model with broad sensitivity to new interactions, particles and phenomena. The goal of 140 ppb is commensurate with ongoing improvements in the SM prediction of the anomalous moment and addresses the longstanding 3.5σ discrepancy between the BNL result and the SM prediction. In this article I discuss the physics motivation and experimental technique for measuring aμ, and the current status and the future work for the project.