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F. Myhrer

Publications and source records attributed to F. Myhrer.

At least 19 recordsLinked to original sources
arXiv2017

Low-Energy Effective Field Theory of Lepton-Proton Bremsstrahlung

We calculate the cross section for the lepton-proton bremsstrahlung process $l+p\to l^\prime +p+\gamma$ in effective field theory. This process corresponds to an undetected background signal for the proposed MUSE experiment at PSI. MUSE is designed to measure elastic scattering of low-energy electrons and muons off a proton target in order to extract a precise value for the proton's r.m.s. radius. We show that the commonly used {\it peaking approximation}, which is used to evaluate the {\it radiative tail} for the elastic cross section, is not applicable for muon proton scattering at the low-energy MUSE kinematics. We also correct a misprint in a commonly cited review article.

nucl-th↗
arXiv2017

Workshop on Excited Hyperons in QCD Thermodynamics at Freeze-Out (YSTAR2016) Mini-Proceedings

This Workshop brought top experts, researchers, postdocs, and students from high-energy heavy ion interactions, lattice QCD and hadronic physics communities together. YSTAR2016 discussed the impact of "missing" hyperon resonances on QCD thermodynamics, on freeze-out in heavy ion collisions, on the evolution of early universe, and on the spectroscopy of strange particles. Recent studies that compared lattice QCD predictions of thermodynamic properties of quark-gluon plasma at freeze-out with calculations based on statistical hadron resonance gas models as well as experimentally measured ratios between yields of different hadron species in heavy ion collisions provide indirect evidence for the presence of "missing" resonances in all of these contexts. The aim of the YSTAR2016 Workshop was to sharpen these comparisons and advance our understanding of the formation of strange hadrons from quarks and gluons microseconds after the Big Bang and in todays experiments at LHC and RHIC as well as at future facilities like FAIR, J-PARC and KL at JLab. It was concluded that the new initiative to create a secondary beam of neutral kaons at JLab will make a bridge between the hardron spectroscopy, heavy-ion experiments and lattice QCD studies addressing some major issues related to thermodynamics of the early universe and cosmology in general.

hep-ph↗
arXiv2016

Workshop on Physics with Neutral Kaon Beam at JLab (KL2016) Mini-Proceedings

The KL2016 Workshop is following the Letter of Intent LoI12-15-001 "Physics Opportunities with Secondary KL beam at JLab" submitted to PAC43 with the main focus on the physics of excited hyperons produced by the Kaon beam on unpolarized and polarized targets with GlueX setup in Hall D. Such studies will broaden a physics program of hadron spectroscopy extending it to the strange sector. The Workshop was organized to get a feedback from the community to strengthen physics motivation of the LoI and prepare a full proposal. Further details about the Workshop can be found on the web page of the conference: http://www.jlab.org/conferences/kl2016/index.html .

hep-ph↗
arXiv2016

Threshold pion production in proton-proton collisions at NNLO in chiral EFT

The reaction $NN \to NN π$ offers a good testing ground for chiral effective field theory at intermediate energies. It challenges our understanding of the first inelastic channel in nucleon-nucleon scattering and of the charge-symmetry breaking pattern in hadronic reactions. In our previous studies, we presented a complete calculation of the pion-production operator for s-wave pions up-to-and-including next-to-next-to-leading order (NNLO) in the formulation of chiral effective field theory, which includes pions, nucleons and $Δ(1232)$ degrees of freedom. In this paper we calculate the near threshold cross section for the $pp \to d π^{+}$ reaction by performing the convolution of the obtained operators with nuclear wave functions based on modern phenomenological and chiral potentials. The available chiral $NN$ wave functions are constructed with a cutoff comparable with the momentum transfer scale inherent in pion production reactions. Hence, a significant portion of the dynamical intermediate-range physics is thereby cut off by them. On the other hand, the NNLO amplitudes evaluated with phenomenological wave functions appear to be largely independent of the $NN$ model used and give corrections to the dominant leading order contributions as expected from dimensional analysis. The result gives support to the counting scheme used to classify the pion production operators, which is a precondition for a reliable investigation of the chirally suppressed neutral pion production. The explicit inclusion of the $Δ(1232)$ is found to be important but smaller than expected due to cancellations.

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arXiv2014

Comparing radiative and recoil corrections in neutron beta-decay and inverse beta-decay

The inverse $β$-decay reaction, anti-nu_e + p --> e^+ + n, for low-energy anti-neutrinos coming from nuclear reactors is of great current interest in connection with high-precision measurements of the neutrino mixing angle $θ_{13}$. We have previously derived analytic expressions, up to next-to-leading order in heavy-baryon chiral perturbation theory, for the radiative corrections (RCs) and the nucleon-recoil corrections both for this reaction and for the related neutron $β$-decay process. We investigate here the numerical consequences of these analytic expressions. We show that the recoil corrections are small for neutron $β$-decay, but for inverse $β$-decay, the recoil corrections are comparable in size to the RCs for typical energies of reactor anti-neutrinos, and they have opposite signs. It turns out that the RCs and the recoil corrections exhibit very different dependences on the neutrino energy.

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arXiv2014

An update of muon capture on hydrogen

The successful precision measurement of the rate of muon capture on a proton by the MuCap Collaboration allows for a stringent test of the current theoretical understanding of this process. Chiral perturbation theory, which is a low-energy effective field theory that preserves the symmetries and the pattern of symmetry breaking in the underlying theory of QCD, offers a systematic framework for describing $μp$ capture and provides a basic test of QCD at the hadronic level. We describe how this effective theory with no free parameters reproduces the measured capture rate. A recent study has addressed new sources of uncertainties that were not considered in the previous works, and we review to what extent these uncertainties are now under control. Finally, the rationale for studying muon capture on the deuteron and some recent theoretical developments regarding this process are discussed.

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arXiv2014

Neutrino Emissivities from Deuteron-Breakup and Formation in Supernovae

Neutrino emissions from electron/positron capture on the deuteron and the nucleon-nucleon fusion processes in the surface region of a supernova core are studied. These weak processes are evaluated in the standard nuclear physics approach, which consists of one-nucleon and two-nucleon-exchange currents and nuclear wave functions generated by a high precision nucleon-nucleon potential. In addition to the cross sections for these processes involving the deuteron, we present neutrino emissivities due to these processes calculated for typical profiles of core-collapsed supernovae. These novel neutrino emissivities are compared with the standard neutrino emission mechanisms. We find that the neutrino emissivity due to the electron capture on the deuteron is comparable to that on the proton in the deuteron abundant region. The implications of the new channels involving deuterons for the supernova mechanism are discussed.

astro-ph.HE↗
arXiv2013

The muon capture rate on hydrogen and the values of $g_A$ and $g_{πNN}$

Motivated by the recent developments in the determination of the experimental values of the nucleon axial-vector coupling constant $g_A$ and the pion-nucleon coupling constant $g_{πNN}$, we carry out a heavy-baryon chiral perturbation calculation of the hyperfine-singlet $μp$ capture rate $Γ_0$ to next-to-next-to-leading order (N$^2$LO), with the use of the latest values of $g_A$ and $g_{πNN}$. The calculated N$^2$LO value is $Γ_0^{\rm theor}(μ^- p \to ν_μn) =718 \pm 7 \,s^{-1}$, where the estimated N$^3$LO contribution dominates the error. This value is in excellent agreement with the experimental value reported by the MuCap Collaboration.

nucl-th↗
arXiv2013

Pion production in nucleon-nucleon collisions in chiral effective field theory with Delta(1232)-degrees of freedom

A calculation of the pion-production operator up to next-to-next-to-leading order for s-wave pions is performed within chiral effective field theory. In the previous study [Phys. Rev. C 85, 054001 (2012)] we discussed the contribution of the pion-nucleon loops at the same order. Here we extend that study to include explicit Delta degrees of freedom and the 1/m_N^2 corrections to the pion-production amplitude. Using the power counting scheme where the Delta-nucleon mass difference is of the order of the characteristic momentum scale in the production process, we calculate all tree-level and loop diagrams involving Delta up to next-to-next-to-leading order. The long-range part of the Delta loop contributions is found to be of similar size to that from the pion-nucleon loops which supports the counting scheme. The net effect of pion-nucleon and Delta loops is expected to play a crucial role in understanding of the neutral pion production data.

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arXiv2013

Ordinary Muon Capture in Hydrogen Reexamined

The rate of muon capture in a muonic hydrogen atom is calculated in heavy-nucleon chiral perturbation theory up to next-to-next-to leading order. To this order, we present the systematic evaluation of all the corrections due to the QED and electroweak radiative corrections and the proton-size effect. Since the low-energy constants involved can be determined from other independent sources of information, the theory has predictive power. For the hyperfine-singlet $μp$ capture rate $Γ_0$, our calculation gives $Γ_0=710 \,\pm 5\,s^{-1}$, which is in excellent agreement with the experimental value obtained in a recent high-precision measurement by the MuCap Collaboration.

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arXiv2013

Octet Spin Fractions and the Proton Spin Problem

The relatively small fraction of the spin of the proton carried by its quarks presents a major challenge to our understanding of the strong interaction. Traditional efforts to explore this problem have involved new and imaginative experiments and QCD based studies of the nucleon. We propose a new approach to the problem which exploits recent advances in lattice QCD. In particular, we extract values for the spin carried by the quarks in other members of the baryon octet in order to see whether the suppression observed for the proton is a general property or depends significantly on the baryon structure. We compare these results with the values for the spin fractions calculated within a model that includes the effects of confinement, relativity, gluon exchange currents and the meson cloud required by chiral symmetry, finding a very satisfactory level of agreement given the precision currently attainable.

nucl-th↗
arXiv2012

Radiative corrections to anti-neutrino proton scattering at low energies

For the low-energy anti-neutrino reaction, $\barν_e + p \to e^+ + n$, which is of great current interest in connection with on-going high-precision neutrino-oscillation experiments, we calculate the differential cross section in a model-independent effective field theory (EFT), taking into account radiative corrections of order $α$. In EFT, the short-distance radiative corrections are subsumed into well-defined low-energy constants the values of which can in principle be determined from the available neutron beta-decay data. In our low-energy EFT, the order-$α$ radiative corrections are considered to be of the same order as the nucleon recoil corrections, which include the "weak magnetism" contribution. These recoil corrections have been evaluated as well. We emphasize that EFT allows for a systematic evaluation of higher order corrections, providing estimates of theoretical uncertainties in our results.

hep-ph↗
arXiv2012

Pion production in nucleon-nucleon collisions in chiral effective field theory: next-to-next-to-leading order contributions

A complete calculation of the pion-nucleon loops that contribute to the transition operator for $NN\to NNπ$ up-to-and-including next-to-next-to-leading order (N$^2$LO) in chiral effective field theory near threshold is presented. The evaluation is based on the so-called momentum counting scheme, which takes into account the relatively large momentum of the initial nucleons inherent in pion-production reactions. We show that the significant cancellations between the loops found at next-to-leading order (NLO) in the earlier studies are also operative at N$^2$LO. In particular, the $1/m_N$ corrections (with $m_N$ being the nucleon mass) to loop diagrams cancel at N$^2$LO, as do the contributions of the pion loops involving the low-energy constants $c_i$, i=1...4. In contrast to the NLO calculation however, the cancellation of loops at N$^2$LO is incomplete, yielding a non-vanishing contribution to the transition amplitude. Together with the one-pion exchange tree-level operators, the loop contributions provide the long-range part of the production operator. Finally, we discuss the phenomenological implications of these findings. In particular, we find that the amplitudes generated by the N$^2$LO pion loops yield contributions comparable in size with the most important phenomenological heavy-meson exchange amplitudes.

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arXiv2010

Muon Capture on the Deuteron -- The MuSun Experiment

We propose to measure the rate \Rd\ for muon capture on the deuteron to better than 1.5% precision. This process is the simplest weak interaction process on a nucleus that can both be calculated and measured to a high degree of precision. The measurement will provide a benchmark result, far more precise than any current experimental information on weak interaction processes in the two-nucleon system. Moreover, it can impact our understanding of fundamental reactions of astrophysical interest, like solar pp fusion and the $ν+d$ reactions observed by the Sudbury Neutrino Observatory. Recent effective field theory calculations have demonstrated, that all these reactions are related by one axial two-body current term, parameterized by a single low-energy constant. Muon capture on the deuteron is a clean and accurate way to determine this constant. Once it is known, the above mentioned astrophysical, as well as other important two-nucleon reactions, will be determined in a model independent way at the same precision as the measured muon capture reaction.

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arXiv2009

Understanding the proton's spin structure

We discuss the tremendous progress that has been towards an understanding of how the spin of the proton is distributed on its quark and gluon constituents. This is a problem that began in earnest twenty years ago with the discovery of the proton ``spin crisis'' by the European Muon Collaboration. The discoveries prompted by that original work have given us unprecedented insight into the amount of spin carried by polarized gluons and the orbital angular momentum of the quarks.

hep-ph↗
arXiv2009

Two-pion-exchange and other higher-order contributions to the $pp\to ppπ^0$ reaction

Much effort has been invested on effective-field-theoretical studies of the near-threshold NN \to NNπreactions and, in order to deal with the rather large three-momentum transfers involved, the momentum counting scheme was proposed as an alternative to the usual Weinberg counting scheme. To examine the practical utility of the momentum counting scheme, we study here the contributions of higher-order terms to the pp\to ppπ^0 reaction. We discuss the consequences of the different treatments of the nucleon propagators in these two counting schemes. We also investigate certain higher chiral-order pion-rescattering terms and the five-point counter-term, which turn out to give important contributions.

nucl-th↗
arXiv2008

The spin content of the proton

Considerable progress has been made in our knowledge of the spin distribution within the proton. The recently measured limits on polarized gluons in the proton suggest polarized gluons contribute very modestly to the proton spin. We will show that a modern, relativistic and chirally symmetric description of the nucleon structure naturally explain the current proton spin data. Most of the "missing" spin is carried by confined quark and antiquarks' angular momentum.

hep-ph↗