Liquid Hydrogen Target Research Articles

Parity violation in neutron–proton capture—The NPDGamma experiment

The NPDGamma collaboration has recently completed the first phase of a measurement to determine the size of the weak nucleon–nucleon interaction from cold neutron capture on a liquid hydrogen target. In the framework of the nearly 30 year old DDH model [B. Desplanques, J.F. Donoghue, B.R. Holstein, Annals of Physics 124 (1980) 449], the measured process is explained in terms of the weak pion–nucleon coupling, while the framework of modern effective field theory parameterizes the measured process in terms of the S 1 3 - P 1 3 , long range transition (essentially the Danilov parameter ρ t ) [S.L. Zhu et al., Nuclear Physics A 748 (2005) 435; C.-P. Liu, Phys. Rev. C 75 (2007) 065501]. The couplings in terms of either model are directly proportional to the parity violating up-down asymmetry in the angular distribution of gamma rays with respect to the neutron spin direction in the reaction n ⇒ + p → d + γ . The asymmetry has a predicted size of 5 × 10 - 8 and the aim of the NPDGamma collaboration is to measure it to 20%. The first phase of the measurement was completed at the Los Alamos National Laboratory Neutron Science Center Spallation Source with a preliminary result of ( - 1.1 ± 2.1 stat . ± 0.2 sys . ) × 10 - 7 . Here, we report on the measurements and the results obtained so far. The experiment is currently being installed at the Spallation Neutron Source (SNS) at Oak Ridge National Laboratory, for the remainder of its run time.

Properties of the 7He ground state from 8He neutron knockout

The unbound nucleus 7He, produced in neutron-knockout reactions with a 240 MeV/u 8He beam in a liquid-hydrogen target, has been studied in an experiment at the ALADIN-LAND setup at GSI. From an R-matrix analysis the resonance parameters for 7He as well as the spectroscopic factor for the 6He(0+)+n configuration in its ground-state have been obtained. The spectroscopic factor is 0.61 confirming that 7He is not a pure single-particle state. An analysis of 5He data from neutron-knockout reactions of 6He in a carbon target reveals the presence of an s-wave component at low energies in the α+n relative energy spectrum.A possible low-lying exited state in 7He observed in neutron knockout data from 8He in a carbon target and tentatively interpreted as a Iπ=1/2− state, could not be observed in the present experiment. Possible explanations of the shape difference between the 7He resonance obtained in the two knockout reactions are discussed in terms of target-dependence or different reaction mechanisms at relativistic energies.

Near-Threshold Photoproduction ofΛ(1520)from Protons and Deuterons

Photoproduction of Lambda(1520) with liquid hydrogen and deuterium targets was examined at photon energies below 2.4 GeV in the SPring-8 LEPS experiment. For the first time, the differential cross sections were measured at low energies and with a deuterium target. A large asymmetry of the production cross sections from protons and neutrons was observed at backward K+/0 angles. This suggests the importance of the contact term, which coexists with t-channel K exchange under gauge invariance. This interpretation was compatible with the differential cross sections, decay asymmetry, and photon beam asymmetry measured in the production from protons at forward K+ angles.

Photoproduction of π0π+ pairs on proton and deuteron

The reaction of π0π+ photoproduction on free and quasi-free protons with γ-ray energies from 0.5 to 1.5 GeV has been studied. Its cross section was measured in the range θ π 0 ≥ 20°. The distributions of π0π+, π0n, and π+n invariant masses were analyzed. The experiment was performed using the Compton back-scattered γ-ray beam at the GRAAL facility (Grenoble, France), applying liquid deuterium and hydrogen targets and a large-aperture (almost 4π) detector. Particular attention was paid to analysis of systematic errors related to the neutron detection efficiency.

Search for the Photoexcitation of Exotic Mesons in theπ+π+π−System

A search for exotic mesons in the $\pi^{+}\pi^{+}\pi^{-}$ system photoproduced by the charge exchange reaction $\gamma p\to \pi^{+}\pi^{+}\pi^{-}(n)$ was carried out by the CLAS collaboration at Jefferson Lab. A tagged-photon beam with energies in the 4.8 to 5.4 GeV range, produced through bremsstrahlung from a 5.744 GeV electron beam, was incident on a liquid-hydrogen target. A Partial Wave Analysis (PWA) was performed on a sample of 83,000 events, the highest such statistics to date in this reaction at these energies. The main objective of this study was to look for the photoproduction of an exotic $J^{PC} = 1^{-+}$ resonant state in the 1 to 2 GeV mass range. Our PWA analysis, based on the isobar model, shows production of the $a_{2}(1320)$ and the $\pi_{2}(1670)$ mesons, but no evidence for the $a_{1}(1260)$, nor the $\pi_{1}(1600)$ exotic state at the expected levels. An upper limit of 13.5 nb is determined for the exotic $\pi_1(1600)$ cross section, less than 2% of the $a_2(1320)$ production.

The Main Injector Particle Production Experiment

It is currently impossible to apply the theory of strong interactions, QCD, to compute non-perturbative hadron interaction cross sections from first principles. Thus high quality hadronic particle production measurements are needed to fully understand the data from experiments that have hadronic interactions as a signal or background. This includes atmospheric and accelerator neutrino experiments, calorimetry and hadronic shower simulation. Such data can also be used to test observed regularities in particle production such as scaling relations in inclusive reactions with some precision. The spectrum of baryon resonances predicted by QCD includes many states that have not been observed. Measurements of pion, kaon, and proton beams on a liquid hydrogen target at beam momenta around 1 GeV/c will provide important input to searches for missing baryon resonances. We review status, results, and prospects of particle production measurements at Fermilab.

Persistent decoupling of valence neutrons toward the dripline: Study ofC20byγspectroscopy

The very neutron-rich nucleus $^{20}\mathrm{C}$ has been investigated by inelastic scattering on $^{208}\mathrm{Pb}$ and liquid hydrogen targets. Through distorted wave analysis, the reduced electric quadrupole transition probability $B(E2;{0}_{\mathrm{g}.\mathrm{s}.}^{+}\ensuremath{\rightarrow}{2}_{1}^{+})l18.4$ (stat) ${e}^{2} {\mathrm{fm}}^{4}$ and the neutron transition probability ${M}_{n}^{2}=292\ifmmode\pm\else\textpm\fi{}52$ (stat) ${\mathrm{fm}}^{4}$ have been derived. A simple shell model calculation has shown a need for a factor of about 0.4 decrease of the normal polarization charges to elucidate the results. This is interpreted as a decoupling of the valence neutrons from the nuclear core in carbon isotopes heavier than $^{14}\mathrm{C}$.

A thin gold coated hydrogen heat pipe-cryogenic target for external experiments at COSY

A gravity assisted Gold coated heat pipe (GCHP) with 5-mm diameter has been developed and tested to cool a liquid hydrogen target for external beam experiments at COSY. The need for a narrow target diameter leads us to study the effect of reducing the heat pipe diameter to 5 mm instead of 7 mm, to study the effect of coating the external surface of the heat pipe by a shiny gold layer (to decrease the radiation heat load), and to study the effect of using the heat pipe without using 20 layers of‘ super-insulation around it (aluminized Mylar foil) to keep the target diameter as small as possible. The developed gold coated heat pipe was tested with 20 layers of super-insulation (WI) and without super-insulation (WOI). The operating characteristics for both conditions were compared to show the advantages and disadvantages.

One-neutron removal reactions ofC18andC19on a proton target

We have studied the one-neutron removal reactions of $^{18}\mathrm{C}$ at 81 MeV/nucleon and $^{19}\mathrm{C}$ at 68 MeV/nucleon on a liquid hydrogen target. Transverse-momentum distributions of the core fragments $^{17}\mathrm{C}$ and $^{18}\mathrm{C}$, and their partial cross sections, are measured using coincidences with the $\ensuremath{\gamma}$ rays. These are compared to theoretical calculations based on the continuum-discretized coupled-channels (CDCC) method. We have found that the transverse-momentum distributions of $^{17}\mathrm{C}$ are well described by the CDCC calculations, and the agreement provides evidence for the spin-parity assignments of ${J}^{\ensuremath{\pi}}=1/{2}^{+}$ for the 0.21-MeV state and ${J}^{\ensuremath{\pi}}=5/{2}^{+}$ for the 0.33-MeV state in $^{17}\mathrm{C}$. The ${J}^{\ensuremath{\pi}}=(2,3){}^{+}$ assignment for the 4.0-MeV state in $^{18}\mathrm{C}$ is obtained for the first time based on the momentum distribution of $^{18}\mathrm{C}$. The measured partial cross sections are consistent with the CDCC calculations combined with shell-model spectroscopic factors. The present study demonstrates that the transverse-momentum distribution obtained in a one-neutron removal reaction on a proton target is as useful as the conventional longitudinal-momentum distribution measured with the heavier Be or C targets for determining the orbital angular momentum of a removed neutron.

Compass and the Nucleon Spin Puzzle

The COMPASS (COmmon Muon and Proton Apparatus for Structure and Spectroscopy) experiment has been in operation at CERN since 2002, carrying on an ambitious experimental program on the spin structure of the nucleon and on hadron spectroscopy. The spin structure of the nucleon has been investigated by impinging a 160 GeV/c momentum μ+ beam on solid polarized targets. In 2002, 2003, 2004, and 2006 a polarized deuteron target (6LiD) was used, while in 2007 data were collected on a NH3 polarized proton target. In this article I will focus on the contribution of COMPASS to the problem of the nucleon spin. I will not mention the hadron program, searching for glueballs and exotics in central production and diffractive processes, which just started in 2008, with a first run with a 190 GeV/c momentum pion beam scattering off a liquid Hydrogen target.

SEARCH FOR K−pn AND K−pp BOUND STATES AT SPRING-8/LEPS

K−pn and K−pp bound states are searched for at SPring-8/LEPS in the d(γ, K+) and d(γ, K+π−) reactions with photon energies from 1.5 to 2.4 GeV. The data was collected during the 2002-2003 run period utilizing liquid hydrogen and liquid deuterium targets. Using the K+ and K+π− missing mass spectra from the deuteron, we hope to determine upper limits for K−pn and K−pp bound state production.

Inelastic scattering studies ofC16reexamined

Previous inelastic scattering experiments populating the ${2}_{1}^{+}$ state in $^{16}\mathrm{C}$ using $^{208}\mathrm{Pb}$ and liquid hydrogen targets have been reanalyzed. Exploiting the different sensitivities of the two probes for the neutron and proton distributions, the neutron and proton deformation lengths and transition strengths have been derived by means of distorted wave calculations. The determined $B(E2)$ value is consistent with the results of lifetime measurements while the large difference in the neutron and proton transition strengths shows relatively enhanced neutron excitations.

Lithium isotopes beyond the drip line

The unbound isotopes 10Li, 12Li and 13Li have been observed after nucleon-knockout reactions at relativistic energies with 11Li and 14Be beams impinging on a liquid hydrogen target. The channels Li9+n, Li11+n and Li11+2n were analysed in the ALADIN-LAND setup at GSI. The 10Li data confirm earlier findings, while the 12Li and 13Li nuclei were observed for the first time. The Li11+n relative-energy spectrum shows that the ground state of 12Li can be described as a virtual s-state with a scattering length of −13.7(1.6) fm. A broad energy spectrum was found for the Li11+2n channel. Based on the assumption that the relative-energy spectrum is dominated by a correlated background presumably stemming from initial correlations in the 14Be ground-state, evidence for a 13Li resonance at 1.47(31) MeV above the Li11+2n threshold with a width around 2 MeV has been found.

The Qweak Experiment: a Search for New Physics at the TeV Scale

A new precision measurement of the parity violating analyzing power in longitudinally polarized electron scattering from the proton at very low Q 2 at an incident energy of 1.16 GeV is in the final stages of preparation for execution at Jefferson Laboratory (JLab). There exists an unique opportunity to carry out the first ever precision measurement of the weak charge of the proton, Q W p = 1 − 4 sin 2 θ W , by making use of the technical advances that have been made at JLab's world-leading parity violating electron scattering program and by using the results of earlier experiments to remove hadronic contributions. A 2200 hour measurement of the parity violating asymmetry in elastic electron-proton scattering at Q 2 = 0.03 (GeV/c) 2 employing 180 μ A of 85% polarized beam on a 0.35 m long liquid hydrogen target will determine the weak charge of the proton with 4% combined statistical and systematic errors. The Standard Model makes a firm prediction of Q W p , based on the ‘running’ of the weak mixing angle sin 2 θ W from the Z o pole down to lower energies. Any significant deviation of sin 2 θ W from its Standard Model prediction at low Q 2 would constitute a signal of new physics. In the absence of new physics, the envisaged experiment will provide a 0.3% determination of sin 2 θ W , making this a very competitive measurement of the weak mixing angle. Complementary to the present experiment is a measurement of the weak charge of the electron in parity violating Møller scattering at 11 GeV, currently under consideration, with the upgraded CEBAF at JLab. The objective of that experiment would be a measurement of sin 2 θ W with a precision comparable to or better than any individual measurement at the Z o pole.

Single $\mathrm{\ensuremath{\pi^{-}}}$ production in np collisions for excess energies up to 90 MeV

The quasifree reaction $np\to pp\pim$ was studied in a kinematically complete experiment by bombarding a liquid hydrogen target with a deuteron beam of momentum 1.85 GeV/c and analyzing the data along the lines of the spectator model. In addition to the three charged ejectiles the spectator proton was also detected in the large-acceptance time-of-flight spectrometer COSY-TOF. It was identified by its momentum and flight direction thus yielding access to the Fermi motion of the bound neutron and to the effective neutron 4-momentum vector $\mathbb{P}_n$ which differed from event to event. A range of almost 90 MeV excess energy above threshold was covered. Energy dependent angular distributions, invariant mass spectra as well as fully covered Dalitz plots were deduced. Sizeable $pp$ FSI effects were found as were contributions of $p$ and $d$ partial waves. The behavior of the elementary cross section $\sigma_{01}$ close to threshold is discussed in view of new cross section data. In comparison with existing literature data the results provide a sensitive test of the spectator model.

Unbound excited states in 19,17C

The neutron-rich carbon isotopes 19,17C have been investigated via proton inelastic scattering on a liquid hydrogen target at 70 MeV/nucleon. The invariant mass method in inverse kinematics was employed to reconstruct the energy spectrum, in which fast neutrons and charged fragments were detected in coincidence using a neutron hodoscope and a dipole magnet system. A peak has been observed with an excitation energy of 1.46(10) MeV in 19C, while three peaks with energies of 2.20(3), 3.05(3), and 6.13(9) MeV have been observed in 17C. Deduced cross sections are compared with microscopic DWBA calculations based on p-sd shell model wave functions and modern nucleon–nucleus optical potentials. Jπ assignments are made for the four observed states as well as the ground states of both nuclei.

Measurement of the total photoabsorption cross section on a proton in the energy range 600–1500 MeV at the GRAAL

The total photoabsorption cross section on a free proton was measured at the GRAAL facility in the energy range Eγ = 600−1500 MeV. The large-aperture LAGRANγE detector and a liquid hydrogen target were used in the experiment performed with a back-scattered Compton gamma beam. To improve the accuracy, two alternative methods were employed. First, a subtraction method of using empty-target measurements allowed the cross section σtot to be evaluated directly because of a low level of the electromagnetic background. Second, an algorithm for evaluating σtot on the basis of summing the dominating partial cross sections was developed. Experimental results obtained for σtot by the two methods are compared with existing data.

Generalized parton distributions and Deeply Virtual Compton Scattering on proton at CLAS

Two measurements of target and beam spin asymmetries for the reaction e p → e p γ were performed with CLAS at Jefferson Laboratory. Polarized 5.7 GeV electrons were impinging on a longitudinally polarized ammonia and liquid hydrogen target respectively. These measurements are sensitive to Generalized Parton Distributions. Sizable sin ϕ azimuthal angular dependences were observed in both experiments, indicating the dominance of leading twist terms and the possibility of extracting combinations of Generalized Parton Distributions on the nucleon.

Study of N=20 shell gap with 1H(28Ne,27,28Ne) reactions

This paper reports on the 1H(28Ne,28Ne) and 1H(28Ne,27Ne) reactions studied at intermediate energy using a liquid hydrogen target. From the cross section populating the first 2+ excited state of 28Ne, and using the previously determined B(E2) value, the neutron quadrupole transition matrix element has been calculated to be Mn=13.8 ±3.7 fm2. In the neutron knock-out reaction, two low-lying excited states were populated in 27Ne. Only one of them can be interpreted by the sd shell model while the additional state may intrude from the fp shell. These experimental observations are consistent with the presence of fp shell configurations at low excitation energy in 27,28Ne nuclei caused by a vanishing N=20 shell gap at Z=10.

Double radiative pion capture on hydrogen and deuterium and the nucleon's pion cloud

We report measurements of double radiative capture in pionic hydrogen and pionic deuterium. The measurements were performed with the RMC spectrometer at the TRIUMF cyclotron by recording photon pairs from pion stops in liquid hydrogen and deuterium targets. We obtained absolute branching ratios of $[3.02\ifmmode\pm\else\textpm\fi{}0.27(\mathrm{stat}.)\ifmmode\pm\else\textpm\fi{}0.31(\mathrm{syst}.)]\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}5}$ for hydrogen and $[1.42\ifmmode\pm\else\textpm\fi{}{}_{0.12}^{0.09}(\mathrm{stat}.)\ifmmode\pm\else\textpm\fi{}0.11(\mathrm{syst}.)]\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}5}$ for deuterium and relative branching ratios of double radiative capture to single radiative capture of $[7.68\ifmmode\pm\else\textpm\fi{}0.69(\mathrm{stat}.)\ifmmode\pm\else\textpm\fi{}0.79(\mathrm{syst}.)]\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}5}$ for hydrogen and $[5.44\ifmmode\pm\else\textpm\fi{}{}_{0.46}^{0.34}(\mathrm{stat}.)\ifmmode\pm\else\textpm\fi{}0.42(\mathrm{syst}.)]\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}5}$ for deuterium. For hydrogen, the measured branching ratio and photon energy-angle distributions are in fair agreement with a reaction mechanism involving the annihilation of the incident ${\ensuremath{\pi}}^{\ensuremath{-}}$ on the ${\ensuremath{\pi}}^{+}$ cloud of the target proton. For deuterium, the measured branching ratio and energy-angle distributions are qualitatively consistent with simple arguments for the expected role of the spectator neutron. A comparison between our hydrogen and deuterium data and earlier beryllium and carbon data reveals substantial changes in the relative branching ratios and the energy-angle distributions and is in agreement with the expected evolution of the reaction dynamics from an annihilation process in $S$-state capture to a bremsstrahlung process in $P$-state capture. Lastly, we comment on the relevance of the double radiative process to the investigation of the charged pion polarizability and the in-medium pion field.