Elastic electron-proton scattering (e-p) and the spectroscopy of hydrogen atoms are the two methods traditionally used to determine the proton charge radius, r
. In 2010, a new method using muonic ...hydrogen atoms
found a substantial discrepancy compared with previous results
, which became known as the 'proton radius puzzle'. Despite experimental and theoretical efforts, the puzzle remains unresolved. In fact, there is a discrepancy between the two most recent spectroscopic measurements conducted on ordinary hydrogen
. Here we report on the proton charge radius experiment at Jefferson Laboratory (PRad), a high-precision e-p experiment that was established after the discrepancy was identified. We used a magnetic-spectrometer-free method along with a windowless hydrogen gas target, which overcame several limitations of previous e-p experiments and enabled measurements at very small forward-scattering angles. Our result, r
= 0.831 ± 0.007
± 0.012
femtometres, is smaller than the most recent high-precision e-p measurement
and 2.7 standard deviations smaller than the average of all e-p experimental results
. The smaller r
we have now measured supports the value found by two previous muonic hydrogen experiments
. In addition, our finding agrees with the revised value (announced in 2019) for the Rydberg constant
-one of the most accurately evaluated fundamental constants in physics.
The analysis of the nine 1-fold differential cross sections for the γr,vp→π+π−p photo- and electroproduction reactions obtained with the CLAS detector at Jefferson Laboratory was carried out with the ...goal to establish the contributing resonances in the mass range from 1.6 GeV to 1.8 GeV. In order to describe the photo- and electroproduction data with Q2-independent resonance masses and hadronic decay widths in the Q2 range below 1.5 GeV2, it was found that an N′(1720)3/2+ state is required in addition to the already well-established nucleon resonances. This work demonstrates that the combined studies of π+π−p photo- and electroproduction data are vital for the observation of this resonance. The contributions from the N′(1720)3/2+ state and the already established N(1720)3/2+ state with a mass of 1.745 GeV are well separated by their different hadronic decays to the πΔ and ρp final states and the different Q2-evolution of their photo-/electroexcitation amplitudes. The N′(1720)3/2+ state is the first recently established baryon resonance for which the results on the Q2-evolution of the photo-/electrocouplings have become available. These results are important for the exploration of the nature of the “missing” baryon resonances.
Precision measurement of the neutral pion lifetime Larin, I; Zhang, Y; Gasparian, A ...
Science (American Association for the Advancement of Science),
05/2020, Letnik:
368, Številka:
6490
Journal Article
Recenzirano
Odprti dostop
The explicit breaking of the axial symmetry by quantum fluctuations gives rise to the so-called axial anomaly. This phenomenon is solely responsible for the decay of the neutral pion π
into two ...photons (γγ), leading to its unusually short lifetime. We precisely measured the decay width Γ of the Formula: see text process. The differential cross sections for π
photoproduction at forward angles were measured on two targets, carbon-12 and silicon-28, yielding Formula: see text, where stat. denotes the statistical uncertainty and syst. the systematic uncertainty. We combined the results of this and an earlier experiment to generate a weighted average of Formula: see text Our final result has a total uncertainty of 1.50% and confirms the prediction based on the chiral anomaly in quantum chromodynamics.
We report a partial-wave analysis of new data on the double-polarization variable E for the reactions γp→π+n and γp→π0p and of further data published earlier. The analysis within the Bonn–Gatchina ...(BnGa) formalism reveals evidence for a poorly known baryon resonance, the one-star Δ(2200)7/2−. This is the lowest-mass Δ⁎ resonance with spin-parity JP=7/2−. Its mass is significantly higher than the mass of its parity partner Δ(1950)7/2+ which is the lowest-mass Δ⁎ resonance with spin-parity JP=7/2+. It has been suggested that chiral symmetry might be restored in the high-mass region of hadron excitations, and that these two resonances should be degenerate in mass. Our findings are in conflict with this prediction.
The Jefferson Lab frozen spin target Keith, C.D.; Brock, J.; Carlin, C. ...
Nuclear instruments & methods in physics research. Section A, Accelerators, spectrometers, detectors and associated equipment,
08/2012, Letnik:
684
Journal Article
Recenzirano
Odprti dostop
A frozen spin polarized target, constructed at Jefferson Lab for use inside a large acceptance spectrometer, is described. The target has been utilized for photoproduction measurements with polarized ...tagged photons of both longitudinal and circular polarization. Protons in TEMPO-doped butanol were dynamically polarized to approximately 90% outside the spectrometer at 5T and 200–300mK. Photoproduction data were acquired with the target inside the spectrometer at a frozen-spin temperature of approximately 30mK with the polarization maintained by a thin, superconducting coil installed inside the target cryostat. A 0.56T solenoid was used for longitudinal target polarization and a 0.50T dipole for transverse polarization. Spin-lattice relaxation times as high as 4000h were observed. We also report polarization results for deuterated propanediol doped with the trityl radical OX063.
The Central Time-of-Flight detector for the Jefferson Laboratory 12-GeV upgrade is being designed with linear-focused photomultiplier tubes that require a robust magnetic shield against the CLAS12 ...main 5-T solenoid fringe fields of 100
mT (1
kG). Theoretical consideration of a ferromagnetic cylinder in an axial field has demonstrated that its shielding capability decreases with increasing length. This observation has been confirmed with finite element analysis using POISSON model software. Several shields composed of coaxial ferromagnetic cylinders have been studied. All difficulties caused by saturation effects were overcome with a novel dynamical shield, which utilizes a demagnetizing solenoid between the shielding cylinders. Basic dynamical shields for ordinary linear-focused 2-in. photomultiplier tubes were designed and tested both with models and experimental prototypes at different external field and demagnetizing current values. Our shield design reduces the 1
kG external axial field by a factor of 5000.
A new highly segmented start counter for the CLAS detector Sharabian, Y.G.; Battaglieri, M.; Burkert, V.D. ...
Nuclear instruments & methods in physics research. Section A, Accelerators, spectrometers, detectors and associated equipment,
2006, 2006-1-00, Letnik:
556, Številka:
1
Journal Article
Recenzirano
Odprti dostop
The design, construction and performance of a highly segmented Start Counter are described. The Start Counter is an integral part of the trigger used in photon beam running with CLAS in Hall B at the ...Thomas Jefferson National Accelerator Facility (TJNAF). The Start Counter is constructed of 24 2.2-mm-thick single-ended scintillation paddles, forming a hermetic hexagon around the target region. This device measures the interaction time of the incoming photon in the target by detecting the outgoing particles. The counter provides complex trigger topologies, shows good efficiency and achieved a time resolution of 350
ps.
We report a high-precision determination of the
1
S
0 neutron–neutron scattering length (
a
nn
) using the
2H(
π
−,
nγ)
n reaction. The value obtained in the present work is −18.50± 0.05 ...(statistical) ± 0.44 (systematic) ± 0.30 (theoretical) fm, which is consistent with the values from previous measurements. Combining our result with previous measurements reduces the total uncertainty in the world average of
a
nn
to ±0.4 fm, matching the accuracy to which the charge-symmetric parameter
a
pp
is determined.
The PRad windowless gas flow target Pierce, J.; Brock, J.; Carlin, C. ...
Nuclear instruments & methods in physics research. Section A, Accelerators, spectrometers, detectors and associated equipment,
07/2021, Letnik:
1003
Journal Article
Recenzirano
Odprti dostop
We report on a windowless, high-density, gas-flow target at Jefferson Lab that was used to measure rp, the root-mean-square charge radius of the proton. The target achieved its design goal of an ...areal density of 2 × 1018 atoms/cm2, with the gas uniformly distributed over the 4 cm length of the cell and less than 1% residual gas outside the cell. This design eliminated scattering from the end caps of the target cell, a problem endemic to previous measurements of the proton charge radius in electron scattering experiments, and permitted a precise, model-independent extraction of rp by reaching unprecedentedly low values of Q2, the square of the electron’s transfer of four-momentum to the proton.
PDF
