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At least 1,693 records · Page 94
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DOE OSTI2017

Off-forward gluonic structure of vector mesons

Here, the spin-independent and transversity generalized form factors (GFFs) of the Φmeson are studied using lattice QCD calculations with light quark masses corresponding to a pion mass m π ~ 450(5) MeV . One transversity and three spin-independent GFFs related to the lowest moments of leading-twist spin-independent and transversity gluon distributions are obtained at six nonzero values of the momentum transfer up to 1.2 Ge V 2 . These quantities are compared with the analogous spin-independent quark GFFs and the electromagnetic form factors determined on the same lattice ensemble. In conclusion, the results show quantitative distinction between the spatial distribution of transversely polarized gluons, unpolarized gluons, and quarks and point the way towards further investigations of the gluon structure of nucleons and nuclei.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2017

On the Feynman-Hellmann theorem in quantum field theory and the calculation of matrix elements

In this paper, the Feynman-Hellmann theorem can be derived from the long Euclidean-time limit of correlation functions determined with functional derivatives of the partition function. Using this insight, we fully develop an improved method for computing matrix elements of external currents utilizing only two-point correlation functions. Our method applies to matrix elements of any external bilinear current, including nonzero momentum transfer, flavor-changing, and two or more current insertion matrix elements. The ability to identify and control all the systematic uncertainties in the analysis of the correlation functions stems from the unique time dependence of the ground-state matrix elements and the fact that all excited states and contact terms are Euclidean-time dependent. We demonstrate the utility of our method with a calculation of the nucleon axial charge using gradient-flowed domain-wall valence quarks on the $N_f=2+1+1$ MILC highly improved staggered quark ensemble with lattice spacing and pion mass of approximately 0.15 fm and 310 MeV respectively. We show full control over excited-state systematics with the new method and obtain a value of $g_A = 1.213(26)$ with a quark-mass-dependent renormalization coefficient.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2017

Using infinite-volume, continuum QED and lattice QCD for the hadronic light-by-light contribution to the muon anomalous magnetic moment

In our previous work, the connected and leading disconnected hadronic light-by-light contributions to the muon anomalous magnetic moment (g — 2) have been computed using lattice QCD ensembles corresponding to physical pion mass generated by the RBC/UKQCD Collaboration. However, the calculation is expected to suffer from a significant finite-volume error that scales like 1/L 2 where L is the spatial size of the lattice. In this paper, we demonstrate that this problem is cured by treating the muon and photons in infinite-volume, continuum QED, resulting in a weighting function that is precomputed and saved with affordable cost and sufficient accuracy. We present numerical results for the case when the quark loop is replaced by a muon loop, finding the expected exponential approach to the infinite volume limit and consistency with the known analytic result. Here, we have implemented an improved weighting function which reduces both discretization and finite-volume effects arising from the hadronic part of the amplitude.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2017

Updated constraints on self-interacting dark matter from Supernova 1987A

Here we revisit SN1987A constraints on light, hidden sector gauge bosons (“dark photons”) that are coupled to the standard model through kinetic mixing with the photon. These constraints are realized because excessive bremsstrahlung radiation of the dark photon can lead to rapid cooling of the SN1987A progenitor core, in contradiction to the observed neutrinos from that event. The models we consider are of interest as phenomenological models of strongly self-interacting dark matter. We clarify several possible ambiguities in the literature and identify errors in prior analyses. We find constraints on the dark photon mixing parameter that are in rough agreement with the early estimates of Dent et al. [arXiv:1201.2683.], but only because significant errors in their analyses fortuitously canceled. Our constraints are in good agreement with subsequent analyses by Rrapaj & Reddy [Phys. Rev. C 94, 045805 (2016).] and Hardy & Lasenby [J. High Energy Phys. 02 (2017) 33.]. We estimate the dark photon bremsstrahlung rate using one-pion exchange (OPE), while Rrapaj & Reddy use a soft radiation approximation (SRA) to exploit measured nuclear scattering cross sections. We find that the differences between mixing parameter constraints obtained through the OPE approximation or the SRA approximation are roughly a factor of ~2–3. Hardy & Laseby [J. High Energy Phys. 02 (2017) 33.] include plasma effects in their calculations finding significantly weaker constraints on dark photon mixing for dark photon masses below ~10 MeV. We do not consider plasma effects. Lastly, we point out that the properties of the SN1987A progenitor core remain somewhat uncertain and that this uncertainty alone causes uncertainty of at least a factor of ~2–3 in the excluded values of the dark photon mixing parameter. Further refinement of these estimates is unwarranted until either the interior of the SN1987A progenitor is more well understood or additional, large, and heretofore neglected effects, such as the plasma interactions studied by Hardy & Lasenby [J. High Energy Phys. 02 (2017) 33.], are identified.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2017

Observation of charmless baryonic decays B ( s ) 0 → p p ¯ h + h ' –

Decays of B 0 and B 0 s mesons to the charmless baryonic final states p¯ph + h' – , where h and h' each denote a kaon or a pion, are searched for using the LHCb detector. The analysis is based on a sample of proton-proton collision data collected at center-of-mass energies of 7 and 8 TeV, corresponding to an integrated luminosity of 3 fb –1 . Four-body charmless baryonic B 0 s decays are observed for the first time. The decays B 0 s →p¯pK + K – , B 0 s →p¯pK ± π ∓ , B 0 →p¯pK ± π ∓ and B 0 →p¯pπ + π – are observed with a significance greater than 5 standard deviations; evidence at 4.1 standard deviations is found for the B 0 →p¯pK + K – decay and an upper limit is set on the branching fraction for B 0 s →p¯pπ + π – . Branching fractions in the kinematic region m(p¯p) < 2850 MeV/c 2 are measured relative to the B 0 →J/ψ(→p¯p)K*(892) 0 channel.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2017

First lattice QCD study of the gluonic structure of light nuclei

The role of gluons in the structure of the nucleon and light nuclei is investigated using lattice quantum chromodynamics (QCD) calculations. The first moment of the unpolarised gluon distribution is studied in nuclei up to atomic number $A=3$ at quark masses corresponding to pion masses of $m_\pi\sim 450$ and $806$ MeV. Nuclear modification of this quantity defines a gluonic analogue of the EMC effect and is constrained to be less than $\sim 10$% in these nuclei. This is consistent with expectations from phenomenological quark distributions and the momentum sum rule. In the deuteron, the combination of gluon distributions corresponding to the $b_1$ structure function is found to have a small first moment compared with the corresponding momentum fraction. The first moment of the gluon transversity structure function is also investigated in the spin-1 deuteron, where a non-zero signal is observed at $m_\pi \sim 806$ MeV. In conclusion, this is the first indication of gluon contributions to nuclear structure that can not be associated with an individual nucleon.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2017

Sea quarks contribution to the nucleon magnetic moment and charge radius at the physical point

We report a comprehensive analysis of the light and strange disconnected-sea quarks contribution to the nucleon magnetic moment, charge radius, and the electric and magnetic form factors. The lattice QCD calculation includes ensembles across several lattice volumes and lattice spacings with one of the ensembles at the physical pion mass. We adopt a model-independent extrapolation of the nucleon magnetic moment and the charge radius. We have performed a simultaneous chiral, infinite volume, and continuum extrapolation in a global fit to calculate results in the continuum limit. We find that the combined light and strange disconnected-sea quarks contribution to the nucleon magnetic moment is μ M (DI) = –0.022(11)(09)μ N and to the nucleon mean square charge radius is &lang;r 2 &rang; E (DI) = –0.019 (05)(05) fm 2 which is about 1/3 of the difference between the &lang;r$^{2}_{p}$&rang; E of electron-proton scattering and that of a muonic atom and so cannot be ignored in obtaining the proton charge radius in the lattice QCD calculation. Here, the most important outcome of this lattice QCD calculation is that while the combined light-sea and strange quarks contribution to the nucleon magnetic moment is small at about 1%, a negative 2.5(9)% contribution to the proton mean square charge radius and a relatively larger positive 16.3(6.1)% contribution to the neutron mean square charge radius come from the sea quarks in the nucleon. For the first time, by performing global fits, we also give predictions of the light and strange disconnected-sea quarks contributions to the nucleon electric and magnetic form factors at the physical point and in the continuum and infinite volume limits in the momentum transfer range of 0 ≤ Q 2 ≤ 0.5 GeV 2 .

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2017

Baryon-baryon interactions and spin-flavor symmetry from lattice quantum chromodynamics

Lattice quantum chromodynamics is used to constrain the interactions of two octet baryons at the SU(3) flavor-symmetric point, with quark masses that are heavier than those in nature (equal to that of the physical strange quark mass and corresponding to a pion mass of $\approx 806~\tt{MeV}$). Specifically, the S-wave scattering phase shifts of two-baryon systems at low energies are obtained with the application of L\"uscher's formalism, mapping the energy eigenvalues of two interacting baryons in a finite volume to the two-particle scattering amplitudes below the relevant inelastic thresholds. The values of the leading-order low-energy scattering parameters in the irreducible representations of SU(3) are consistent with an approximate SU(6) spin-flavor symmetry in the nuclear and hypernuclear forces that is predicted in the large-$N_c$ limit of QCD. The two distinct SU(6)-invariant interactions between two baryons are constrained at this value of the quark masses, and their values indicate an approximate accidental SU(16) symmetry. The SU(3) irreducible representations containing the $NN~({^1}S_0)$, $NN~({^3}S_1)$ and $\frac{1}{\sqrt{2}}(\Xi^0n+\Xi^-p)~({^3}S_1)$ channels unambiguously exhibit a single bound state, while the irreducible representation containing the $\Sigma^+ p~({^3}S_1)$ channel exhibits a state that is consistent with either a bound state or a scattering state close to threshold. These results are in agreement with the previous conclusions of the NPLQCD collaboration regarding the existence of two-nucleon bound states at this value of the quark masses.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2017

Unified scenario for composite right-handed neutrinos and dark matter

In this study, we entertain the possibility that neutrino masses and dark matter (DM) originate from a common composite dark sector. A minimal effective theory can be constructed based on a dark SU(3) D interaction with three flavors of massless dark quarks; electroweak symmetry breaking gives masses to the dark quarks. By assigning a Z 2 charge to one flavor, a stable “dark kaon” can provide a good thermal relic DM candidate. We find that “dark neutrons” may be identified as right handed Dirac neutrinos. Some level of “neutron-anti-neutron” oscillation in the dark sector can then result in non-zero Majorana masses for light standard model neutrinos. A simple ultraviolet completion is presented, involving additional heavy SU(3) D -charged particles with electroweak and lepton Yukawa couplings. At our benchmark point, there are “dark pions” that are much lighter than the Higgs and we expect spectacular collider signals arising from the UV framework. This includes the decay of the Higgs boson to ττℓℓ', where ℓ(ℓ ') can be any lepton, with displaced vertices. Finally, we discuss the observational signatures of this UV framework in dark matter searches and primordial gravitational wave experiments; the latter signature is potentially correlated with the H → ττℓℓ' decay.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2018

Improved search for heavy neutrinos in the decay π → e ν

In this study, a search for massive neutrinos has been made in the decay π + → e + ν. No evidence was found for extra peaks in the positron energy spectrum indicative of pion decays involving massive neutrinos (π → e + ν h ). Upper limits (90 % C.L.) on the neutrino mixing matrix element |U ei | 2 in the neutrino mass region 60–135 MeV/c 2 were set, which are an order of magnitude improvement over previous results.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2018

Test of lepton flavor universality by the measurement of the B 0 → D * – τ + ν τ branching fraction using three-prong τ decays

The ratio of branching fractions R(D* – )≡B(B 0 →D* – τ + ν τ )/B(B 0 →D* – μ + ν μ ) is measured using a data sample of proton-proton collisions collected with the LHCb detector at center-of-mass energies of 7 and 8 TeV, corresponding to an integrated luminosity of 3 fb –1 . The τ lepton is reconstructed with three charged pions in the final state. A novel method is used that exploits the different vertex topologies of signal and backgrounds to isolate samples of semitauonic decays of b hadrons with high purity. Using the B 0 →D*π + π – π + decay as the normalization channel, the ratio B(B 0 →D* – τ + ν τ )/B(B 0 →D* – π + π – π + ) is measured to be 1.97±0.13±0.18, where the first uncertainty is statistical and the second systematic. An average of branching fraction measurements for the normalization channel is used to derive B(B 0 →D* – τ + ν τ )=(1.42±0.094±0.129±0.054)%, where the third uncertainty is due to the limited knowledge of B(B 0 →D* – π + π – π + ). A test of lepton flavor universality is performed using the well-measured branching fraction B(B0→D*–μ+νμ) to compute R(D* – )=0.291±0.019±0.026±0.013, where the third uncertainty originates from the uncertainties on B(B0→D* – π + π – π + ) and B(B 0 →D* – μ + ν μ ). This measurement is in agreement with the Standard Model prediction and with previous measurements.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2013

Search for Bottomonium States in Exclusive Radiative Υ ( 2 S ) Decays

We search for bottomonium states in $\Upsilon(2S)$ → ($\bar{bb}$) gamma decays with an integrated luminosity of 24.7fb -1 recorded at the $\Upsilon(2S)$ resonance with the Belle detector at KEK, containing (157.8 ± 3.6) X 10 6 $\Upsilon(2S)$ events. The ($\bar{bb}$) system is reconstructed in 26 exclusive hadronic final states composed of charged pions, kaons, protons, and $K^0_S$ mesons. We find no evidence for the state recently observed around 9975 MeV ($X_{\bar{bb}}$) in an analysis based on a data sample of 9.3 X 10 6 $\Upsilon(2S)$ events collected with the CLEO III detector. We set a 90 % confidence-level upper limit on the branching fraction $Β$[$\Upsilon(2S)$ → $X_{\bar{bb}\gamma}$] X $\sum_i$ $B$[$X_{\bar{bb}}$ → $h_i$] $<$ 4.9 X 10 -6 , summed over the exclusive hadronic final states employed in our analysis. This result is an order of magnitude smaller than the measurement reported with CLEO data. We also set an upper limit for the $\eta_b(1S)$ state of $B$[$\Upsilon(2S)$ → $\eta_b(1S)\gamma$] X $\sum_i$B[$\eta_b(1S)$ → $h_i$] $<$ 3.7 X 10 -6 .

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2015

Indications for a Critical End Point in the Phase Diagram for Hot and Dense Nuclear Matter

Here, excitation functions for the Gaussian emission source radii difference ( R out 2 - R side 2 ) obtained from two-pion interferometry measurements in Au + Au ( s N N = 7.7 – 200 GeV ) and Pb + Pb ( s N N = 2.76 TeV ) collisions are reported on for a broad range of collision centralities. The observed nonmonotonic excitation functions validate the finite-size scaling patterns expected for the deconfinement phase transition and the critical end point (CEP), in the temperature versus baryon chemical potential ( T , μ B ) plane of the nuclear matter phase diagram. A finite-size scaling (FSS) analysis of these data suggests a second order phase transition with the estimates T cep ~ 165 MeV and μ B cep ~ 95 MeV for the location of the critical end point. The critical exponents ( ν ≈ 0.66 and γ ≈ 1.2 ) extracted via the same FSS analysis place this CEP in the 3D Ising model universality class.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2016

Unitary limit of two-nucleon interactions in strong magnetic fields

In this study, two-nucleon systems are shown to exhibit large scattering lengths in strong magnetic fields at unphysical quark masses, and the trends toward the physical values indicate that such features may exist in nature. Lattice QCD calculations of the energies of one and two nucleons systems are performed at pion masses of m π ~ 450 and 806 MeV in uniform, time-independent magnetic fields of strength |B| ~ 10 19 – 10 20 Gauss to determine the response of these hadronic systems to large magnetic fields. Fields of this strength may exist inside magnetars and in peripheral relativistic heavy ion collisions, and the unitary behavior at large scattering lengths may have important consequences for these systems.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2016

Probing the Dark Sector with Dark Matter Bound States

A model of the dark sector where O ( few GeV ) mass dark matter particles χ couple to a lighter dark force mediator V , m V ≪ m χ , is motivated by the recently discovered mismatch between simulated and observed shapes of galactic halos. Such models, in general, provide a challenge for direct detection efforts and collider searches. We demonstrate that for a large range of coupling constants and masses, the production and decay of the bound states of χ , such as 0 - + and 1 - - states, η D and Υ D , is an important search channel. We show that e + e - → η D + V or Υ D + γ production at B factories for α D > 0.1 is sufficiently strong to result in multiple pairs of charged leptons and pions via η D → 2 V → 2 ( l + l - ) and Υ D → 3 V → 3 ( l + l - ) ( l = e , μ , π ) . The absence of such final states in the existing searches performed at BABAR and Belle sets new constraints on the parameter space of the model. We also show that a search for multiple bremsstrahlung of dark force mediators, e + e - → χ χ ¯ + n V , ending in missing energy and multiple leptons, will further improve the sensitivity to self-interacting dark matter.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2016

Measurement of the Difference of Time-Integrated C P Asymmetries in D 0 → K – K + and D 0 → π – π + Decays

A search for CP violation in D 0 → K – K + and D 0 → π – π + decays is performed using pp collision data, corresponding to an integrated luminosity of 3 fb –1 , collected using the LHCb detector at center-of-mass energies of 7 and 8 TeV. The flavor of the charm meson is inferred from the charge of the pion in D * + → D 0 π + and D * – → D ¯ 0 π – decays. The difference between the CP asymmetries in D 0 → K – K + and D 0 → π – π + decays, Δ A CP ≡ A CP ( K – K + ) – A CP (π – π + ), is measured to be [–0.10 ± 0.08 (stat) ± 0.03 (syst)]%. This is the most precise measurement of a time-integrated CP asymmetry in the charm sector from a single experiment.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗