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Register a new usere+e- results from BABAR and implications for g-2
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Authors
Michel Davier
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The BABAR collaboration has nearly completed a program of precise measurements of the cross sections for the dominant channels of e+e- --> hadrons from threshold to an energy of 3-5 GeV using the initial-state radiation (ISR) method, i.e. the measurement of the cross sections e+e- --> gamma hadrons with the energetic photon detected at large angle to the beams. These data are used as input to vacuum polarization dispersion integrals, in particular the hadronic contribution to the muon g-2 anomaly. In addition to the recently measured pi+pi- cross section, giving the dominant contibution, many multihadronic channels have been investigated, with some recent examples presented here. We give preliminary results for the process e+e- --> K+K-(gamma) using 232 fb-1 of data collected with the BABAR detector at e+e- center-of-mass energies near 10.6 GeV. The lowest-order contribution to the hadronic vacuum polarization term in the muon magnetic anomaly is obtained for this channel: amu-KK-LO=(22.95 +-0.14(stat) +-0.22(syst)) 10^-10, which is about a factor of three more precise than the previous world average value.
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The persistent 3-4$sigma$ discrepancy between the experimental result from BNL for the anomalous magnetic moment of the muon and its Standard Model (SM) prediction, was confirmed recently by the MUON G-2 result from Fermilab. The combination of the two measurements yields a deviation of 4.2$sigma$ from the SM value. Here, we review an analysis of the parameter space of the electroweak (EW) sector of the Minimal Supersymmetric Standard Model (MSSM), which can provide a suitable explanation of the anomaly while being in full agreement with other latest experimental data like the direct searches for EW particles at the LHC and dark matter (DM) relic density and direct detection constraints. Taking the lightest supersymmetric particle (LSP) (the lightest neutralino in our case) to be the DM candidate, we discuss the case of a mixed bino/wino LSP, which can account for the full DM relic density of the universe and that of wino and higgsino DM, where we take the relic density only as an upper bound. We observe that an upper limit of ~ 600 GeV can be obtained for the LSP and next-to (N)LSP masses establishing clear search targets for the future HL-LHC EW searches, but in particular for future high-energy $e^+e^-$ colliders, such as the ILC or CLIC.
Numerous channels of the cross section e+e- --> hadrons have been measured by the BABAR experiment using the ISR method. For the pi+pi-(gamma) and K+K-(gamma) channels, BABAR has pioneered the method based on the ratio between the hadronic mass spectra and mu+mu-(gamma). Many systematic uncertainties cancel in the ratio, hence the precise measured cross sections. These measurements have been exploited for phenomenological studies, like the determination of the hadronic contribution to the anomalous magnetic moment of the muon (g-2)_mu.
The BaBar collaboration has an extensive program of studying hadronic cross sections in low-energy e+e- collisions, accessible via initial-state radiation. Our measurements allow significant improvements in the precision of the predicted value of the muon anomalous magnetic moment. These improvements are necessary for illuminating the current ~3.6 sigma difference between the predicted and the experimental values. We have published results on a number of processes with two to six hadrons in the final state. We report here the results of recent studies with final states that constitute the main contribution to the hadronic cross section in the energy region between 1 and 3 GeV, as e+e- to K+K-, pi+pi-, and e+e- to 4 hadrons.
With the long-standing tension between experiment and Standard-Model (SM) prediction in the anomalous magnetic moment of the muon, $a_mu=(g-2)_mu/2$, at the level of 3-4$sigma$, it is natural to ask if there could be a sizable effect in the electric dipole moment (EDM) $d_mu$ as well. In this context it has often been argued that in UV complete models the electron EDM, which is very precisely measured, excludes a large effect in $d_mu$. However, the recently observed 2.5$sigma$ tension in $a_e=(g-2)_e/2$, if confirmed, requires that the muon and electron sectors effectively decouple to avoid constraints from $muto egamma$. We briefly discuss UV complete models that possess such a decoupling, which can be enforced by an Abelian flavor symmetry $L_mu-L_tau$. We show that, in such scenarios, there is no reason to expect a correlation between the electron and muon EDM, so that the latter can be sizable. New limits on $d_mu$ improved by up to two orders of magnitude are expected from the upcoming $(g-2)_mu$ experiments at Fermilab and J-PARC. Beyond, a proposed dedicated muon EDM experiment at PSI could further advance the limit. In this way, future improved measurements of $a_e$, $a_mu$, as well as the fine-structure constant $alpha$ are not only set to provide exciting precision tests of the SM, but, in combination with EDMs, to reveal crucial insights into the flavor structure of physics beyond the SM.
A combined fit is performed to the BaBar and Belle measurements of the e+e- to pi+pi-psi(2S) cross sections for center-of-mass energy between threshold and 5.5 GeV. The resonant parameters of the Y(4360) and Y(4660) are determined. The mass is 4355^{+9}_{-10}pm 9 MeV/c^2 and the width is 103^{+17}_{-15}pm 11 MeV/c^2 for the Y(4360), and the mass is 4661^{+9}_{-8}pm 6 MeV/c^2 and the width is 42^{+17}_{-12}pm 6 MeV/c^2 for the Y(4660). The production of the Y(4260) in pi+pi-psi(2S) mode is found to be at 2sigma level, and B(Y(4260) to pi+pi-psi(2S))Gamma_{e+e-} is found to be less than 4.3 eV/c^2 at the 90% confidence level, or equal to 7.4^{+2.1}_{-1.7} eV/c^2 depending on it interferes with the Y(4360) constructively or destructively. These information will shed light on the understanding of the nature of the Y states observed in initial state radiation processes.
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