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A. Garcia

Publications and source records attributed to A. Garcia.

At least 73 records · Page 4Linked to original sources
arXiv2012

Weak Interaction Studies with 6He

The 6He nucleus is an ideal candidate to study the weak interaction. To this end we have built a high-intensity source of 6He delivering ~10^10 atoms/s to experiments. Taking full advantage of that available intensity we have performed a high-precision measurement of the 6He half-life that directly probes the axial part of the nuclear Hamiltonian. Currently, we are preparing a measurement of the beta-neutrino angular correlation in 6He beta decay that will allow to search for new physics beyond the Standard Model in the form of tensor currents.

nucl-ex↗
arXiv2012

Measurement of the neutron $β$-asymmetry parameter $A_0$ with ultracold neutrons

We present a detailed report of a measurement of the neutron $β$-asymmetry parameter $A_0$, the parity-violating angular correlation between the neutron spin and the decay electron momentum, performed with polarized ultracold neutrons (UCN). UCN were extracted from a pulsed spallation solid deuterium source and polarized via transport through a 7-T magnetic field. The polarized UCN were then transported through an adiabatic-fast-passage spin-flipper field region, prior to storage in a cylindrical decay volume situated within a 1-T $2 \times 2π$ solenoidal spectrometer. The asymmetry was extracted from measurements of the decay electrons in multiwire proportional chamber and plastic scintillator detector packages located on both ends of the spectrometer. From an analysis of data acquired during runs in 2008 and 2009, we report $A_0 = -0.11966 \pm 0.00089_{-0.00140} ^{+0.00123}$, from which we extract a value for the ratio of the weak axial-vector and vector coupling constants of the nucleon, $λ= g_A/g_V = -1.27590 \pm 0.00239_{-0.00377}^{+0.00331}$. Complete details of the analysis are presented.

nucl-ex↗
arXiv2012

Discovery of the Optical/Ultraviolet/Gamma-ray Counterpart to the Eclipsing Millisecond Pulsar J1816+4510

The energetic, eclipsing millisecond pulsar J1816+4510 was recently discovered in a low-frequency radio survey with the Green Bank Telescope. With an orbital period of 8.7 hr and minimum companion mass of 0.16 Msun it appears to belong to an increasingly important class of pulsars that are ablating their low-mass companions. We report the discovery of the gamma-ray counterpart to this pulsar, and present a likely optical/ultraviolet counterpart as well. Using the radio ephemeris we detect pulsations in the unclassified gamma-ray source 2FGL J1816.5+4511, implying an efficiency of ~25% in converting the pulsar's spin-down luminosity into gamma-rays and adding PSR J1816+4510 to the large number of millisecond pulsars detected by Fermi. The likely optical/UV counterpart was identified through position coincidence (<0.1") and unusual colors. Assuming that it is the companion, with R=18.27+/-0.03 mag and effective temperature >15,000 K it would be among the brightest and hottest of low-mass pulsar companions, and appears qualitatively different from other eclipsing pulsar systems. In particular, current data suggest that it is a factor of two larger than most white dwarfs of its mass, but a factor of four smaller than its Roche lobe. We discuss possible reasons for its high temperature and odd size, and suggest that it recently underwent a violent episode of mass loss. Regardless of origin, its brightness and the relative unimportance of irradiation make it an ideal target for a mass, and hence a neutron star mass, determination.

astro-ph.HE↗
arXiv2012

Precision Measurement of the 6He Half-Life and the Weak Axial Current in Nuclei

Studies of 6He beta decay along with tritium can play an important role in testing ab-initio nuclear wave-function calculations and may allow for fixing low-energy constants in effective field theories. Here, we present an improved determination of the 6He half-life to a relative precision of 3x10^(-4). Our value of 806.89 \pm 0.11(stat)^{+0.23}_{-0.19}(syst) ms resolves a major discrepancy between previous measurements. Calculating the statistical rate function we determined the ft-value to be 803.04 ^{+0.26}_{-0.23} s. The extracted Gamow-Teller matrix element agrees within a few percent with ab-initio calculations.

nucl-ex↗
arXiv2011

Near-infrared thermal emissivity from ground based atmospheric dust measurements at ORM

We present an analysis of the atmospheric content of aerosols measured at Observatorio del Roque de los Muchachos (ORM; Canary Islands). Using a laser diode particle counter located at the Telescopio Nazionale Galileo (TNG) we have detected particles of 0.3, 0.5, 1.0, 3.0, 5.0 and 10.0 um size. The seasonal behavior of the dust content in the atmosphere is calculated. The Spring has been found to be dustier than the Summer, but dusty conditions may also occur in Winter. A method to estimate the contribution of the aerosols emissivity to the sky brightness in the near-infrared (NIR) is presented. The contribution of dust emission to the sky background in the NIR has been found to be negligible comparable to the airglow, with a maximum contribution of about 8-10% in the Ks band in the dusty days.

astro-ph.IM↗
arXiv2011

Branches of 33S(p,γ)34Cl at Oxygen-Neon Nova Temperatures

Recent simulations of classical novae on oxygen-neon white-dwarf stars indicate that the isotopic ratio 32S/33S has the potential to be a remarkable indicator of presolar grains of nova origin. The 33S(p,γ)34Cl reaction influences this ratio directly by destroying 33S in novae. Additionally, β-delayed γ-rays from the metastable state of 34Cl (t_{1/2} = 32 min) have been suggested to be potential nova observables. We have measured the branches for known 33S(p,γ)34Cl resonances that are activated at temperatures relevant to oxygen-neon novae. We provide the first reliable uncertainties on these branches and the first upper limits for several previously unmeasured branches.

astro-ph.SR↗
arXiv2011

A New Limit on Time-Reversal Violation in Beta Decay

We report the results of an improved determination of the triple correlation $D P \cdot(p_{e}\times p_ν)$ that can be used to limit possible time-reversal invariance in the beta decay of polarized neutrons and constrain extensions to the Standard Model. Our result is $D=(-0.96\pm 1.89 (stat)\pm 1.01 (sys))\times 10^{-4}$. The corresponding phase between g_A and g_V is $ϕ_{AV} = 180.013^\circ\pm0.028^\circ$ (68 % confidence level). This result represents the most sensitive measurement of D in beta decay.

nucl-ex↗
arXiv2011

Absolute Determination of the 22Na(p,g) Reaction Rate in Novae

Gamma-ray telescopes in orbit around the Earth are searching for evidence of the elusive radionuclide 22Na produced in novae. Previously published uncertainties in the dominant destructive reaction, 22Na(p,g)23Mg, indicated new measurements in the proton energy range of 150 to 300 keV were needed to constrain predictions. We have measured the resonance strengths, energies, and branches directly and absolutely by using protons from the University of Washington accelerator with a specially designed beamline, which included beam rastering and cold vacuum protection of the 22Na implanted targets. The targets, fabricated at TRIUMF-ISAC, displayed minimal degradation over a ~ 20 C bombardment as a result of protective layers. We avoided the need to know the stopping power, and hence the target composition, by extracting resonance strengths from excitation functions integrated over proton energy. Our measurements revealed that resonance strengths for E_p = 213, 288, 454, and 610 keV are stronger by factors of 2.4 to 3.2 than previously reported. Upper limits have been placed on proposed resonances at 198-, 209-, and 232-keV. We have re-evaluated the 22Na(p,g) reaction rate, and our measurements indicate the resonance at 213 keV makes the most significant contribution to 22Na destruction in novae. Hydrodynamic simulations including our rate indicate that the expected abundance of 22Na ejecta from a classical nova is reduced by factors between 1.5 and 2, depending on the mass of the white-dwarf star hosting the nova explosion.

nucl-ex↗
arXiv2010

Solar fusion cross sections II: the pp chain and CNO cycles

We summarize and critically evaluate the available data on nuclear fusion cross sections important to energy generation in the Sun and other hydrogen-burning stars and to solar neutrino production. Recommended values and uncertainties are provided for key cross sections, and a recommended spectrum is given for 8B solar neutrinos. We also discuss opportunities for further increasing the precision of key rates, including new facilities, new experimental techniques, and improvements in theory. This review, which summarizes the conclusions of a workshop held at the Institute for Nuclear Theory, Seattle, in January 2009, is intended as a 10-year update and supplement to Reviews of Modern Physics 70 (1998) 1265.

nucl-ex↗
arXiv2010

Determination of the Axial-Vector Weak Coupling Constant with Ultracold Neutrons

A precise measurement of the neutron decay $β$-asymmetry $A_0$ has been carried out using polarized ultracold neutrons (UCN) from the pulsed spallation UCN source at the Los Alamos Neutron Science Center (LANSCE). Combining data obtained in 2008 and 2009, we report $A_0 = -0.11966 \pm 0.00089_{-0.00140}^{+0.00123}$, from which we determine the ratio of the axial-vector to vector weak coupling of the nucleon $g_A/g_V = -1.27590_{-0.00445}^{+0.00409}$.

nucl-ex↗
arXiv2010

Direct measurements of 22Na(p,g)23Mg resonances and consequences for 22Na production in classical novae

The radionuclide 22Na is a potential astronomical observable that is expected to be produced in classical novae in quantities that depend on the thermonuclear rate of the 22Na(p,g)23Mg reaction. We have measured the strengths of low-energy 22Na(p,g)23Mg resonances directly and absolutely using a radioactive 22Na target. We find the strengths of resonances at E_p = 213, 288, 454, and 610 keV to be higher than previous measurements by factors of 2.4 to 3.2, and we exclude important contributions to the rate from proposed resonances at E_p = 198, 209, and 232 keV. The 22Na abundances expected in the ejecta of classical novae are reduced by a factor of ~ 2.

nucl-ex↗
arXiv2010

Vus and neutron beta decay

We discuss the effect of the recent change of $V_{\rm us}$ by three standard deviations on the standard model predictions for neutron beta decay observables. We also discuss the effect the experimental error bars of $V_{\rm us}$ have on such predictions. Refined precision tests of the standard model will be made by a combined effort to improve measurements in neutron beta decay and in strangeness-changing decays. By itself the former will yield very precise measurements of $V_{\rm ud}$ and make also very precise predictions for $V_{\rm us}$.

hep-ph↗
arXiv2010

Evidence against manifest right-handed currents in neutron beta decay

Bounds and presence of manifest right handed currents in neutron beta decay are reviewed. Assuming the unitarity of the Cabibbo-Kobayashi-Maskawa matrix, current experimental situation imposes very stringent limits on the mixing angle, $-0.00077<ζ<0.00089$, and on the mass eigenstate, $M_2({\rm GeV})\in(291.4,439.9)$, in contradiction with the established lower bound on $M_2$.

hep-ph↗
arXiv2010

Apparent Faster-Than-Light Pulse Propagation in Interstellar Space: A new probe of the Interstellar Medium

Radio pulsars emit regular bursts of radio radiation that propagate through the interstellar medium (ISM), the tenuous gas and plasma between the stars. Previously known dispersive properties of the ISM cause low frequency pulses to be delayed in time with respect to high frequency ones. This effect can be explained by the presence of free electrons in the medium. The ISM also contains neutral hydrogen which has a well known resonance at 1420.4 MHz. Electro-magnetic theory predicts that at such a resonance, the induced dispersive effects will be drastically different from those of the free electrons. Pulses traveling through a cloud of neutral hydrogen should undergo "anomalous dispersion", which causes the group velocity of the medium to be larger than the speed of light in vacuum. This superluminal group velocity causes pulses containing frequencies near the resonance to arrive earlier in time with respect to other pulses. Hence, these pulses appear to travel faster than light. This phenomenon is caused by an interplay between the time scales present in the pulse and the time scales present in the medium. Although counter-intuitive, it does not violate the laws of special relativity. Here, we present Arecibo observations of the radio pulsar PSR B1937+21 that show clear evidence of anomalous dispersion. Though this effect is known in laboratory physics, this is the first time it has been directly observed in an astrophysical context, and it has the potential to be a useful tool for studying the properties of neutral hydrogen in the Galaxy.

astro-ph.GA↗
arXiv2008

Radiative corrections to the three-body region of the Dalitz plot of baryon semileptonic decays with angular correlation between polarized emitted baryons and charged leptons

We have calculated the radiative corrections to the Dalitz plot of baryon semileptonic decays with angular correlation between polarized emitted baryons and charged leptons. This work covers both charged and neutral decaying baryons and is restricted to the so-called three-body region of the Dalitz plot. Also it is specialized at the center-of-mass frame of the emitted baryon. We have considered terms up to order (alpha/pi)(q/M_1)^0, where q is the momentum-transfer and M_1 is the mass of the decaying baryon, and neglected terms of order (alpha/pi)(q/M_1)^n$ for n >= 1. The expressions displayed are ready to obtain numerical results, suitable for model-independent experimental analyses.

hep-ph↗
arXiv2008

Electron-capture branch of 100Tc and tests of nuclear wave functions for double-beta decays

We present a measurement of the electron-capture branch of $^{100}$Tc. Our value, $B(\text{EC}) = (2.6 \pm 0.4) \times 10^{-5}$, implies that the $^{100}$Mo neutrino absorption cross section to the ground state of $^{100}$Tc is roughly one third larger than previously thought. Compared to previous measurements, our value of $B(\text{EC})$ prevents a smaller disagreement with QRPA calculations relevant to double-$β$ decay matrix elements.

nucl-ex↗
arXiv2007

The Araucaria Project. The Distance to the Local Group Galaxy WLM from Cepheid Variables discovered in a Wide-Field Imaging Survey

We have conducted an extensive wide-field imaging survey for Cepheid variables in the Local Group irregular galaxy WLM. From data obtained on 101 nights, we have discovered 60 Cepheids which include 14 of the 15 Cepheid variables previously detected by Sandage and Carlson. Down to a period of 3 days, our Cepheid survey in WLM should be practically complete. Importantly, we have found for the first time a long-period Cepheid (P=54.2 days) in this galaxy, alleviating the puzzle that WLM with its many blue, massive stars does not contain Cepheids with periods longer than about 10 days. Our data define tight period-luminosity relations in V, I and the reddening-free Wesenheit magnitude ${\rm W}_{\rm I}$ which are all extremely well fit by the corresponding slopes of the LMC Cepheid PL relation, suggesting no change of the PL relation slope down to a Cepheid metal abundance of about -1.0 dex, in agreement with other recent studies. We derive a true distance modulus to WLM of 25.144 $\pm$0.03 (r) $\pm$0.07 (s) mag from our data, in good agreement with the earlier 24.92 $\pm$ 0.21 mag determination of Lee, Freedman and Madore (1993a) from Cepheid variables. The quoted value of the systematic uncertainty does not include the contribution from the LMC distance which we have assumed to be 18.50 mag, as in the previous papers in our project.

astro-ph↗