121 research outputs found
Search for Light Gauge Bosons of the Dark Sector at the Mainz Microtron
A new exclusion limit for the electromagnetic production of a light U(1)
gauge boson {\gamma}' decaying to e^+e^- was determined by the A1 Collaboration
at the Mainz Microtron. Such light gauge bosons appear in several extensions of
the standard model and are also discussed as candidates for the interaction of
dark matter with standard model matter. In electron scattering from a heavy
nucleus, the existing limits for a narrow state coupling to e^+e^- were reduced
by nearly an order of magnitude in the range of the lepton pair mass of 210
MeV/c^2 < m_e^+e^- < 300 MeV/c^2. This experiment demonstrates the potential of
high current and high resolution fixed target experiments for the search for
physics beyond the standard model.Comment: 4 pages, 7 figure
Measurement of polarization-transfer to bound protons in carbon and its virtuality dependence
We measured the ratio of the transverse to longitudinal
components of polarization transferred from electrons to bound protons in
by the process at the
Mainz Microtron (MAMI). We observed consistent deviations from unity of this
ratio normalized to the free-proton ratio,
, for both -
and -shell knocked out protons, even though they are embedded in averaged
local densities that differ by about a factor of two. The dependence of the
double ratio on proton virtuality is similar to the one for knocked out protons
from and , suggesting a universal behavior.
It further implies no dependence on average local nuclear density
Search for light massive gauge bosons as an explanation of the anomaly at MAMI
A massive, but light abelian U(1) gauge boson is a well motivated possible
signature of physics beyond the Standard Model of particle physics. In this
paper, the search for the signal of such a U(1) gauge boson in
electron-positron pair-production at the spectrometer setup of the A1
Collaboration at the Mainz Microtron (MAMI) is described. Exclusion limits in
the mass range of 40 MeV up to 300 MeV with a sensitivity in the mixing
parameter of down to are presented. A large
fraction of the parameter space has been excluded where the discrepancy of the
measured anomalous magnetic moment of the muon with theory might be explained
by an additional U(1) gauge boson.Comment: 4 pages, 3 figure
Observation of Lambda H-4 hyperhydrogen by decay-pion spectroscopy in electron scattering
At the Mainz Microtron MAMI, the first high-resolution pion spectroscopy from
decays of strange systems was performed by electron scattering off a Be-9
target in order to study the ground-state masses of Lambda-hypernuclei.
Positively charged kaons were detected by a short-orbit spectrometer with a
broad momentum acceptance at zero degree forward angles with respect to the
beam, efficiently tagging the production of strangeness in the target nucleus.
In coincidence, negatively charged decay-pions were detected by two independent
high-resolution spectrometers. About 10^3 pionic weak decays of hyperfragments
and hyperons were observed. The pion momentum distribution shows a
monochromatic peak at p_pi ~ 133 MeV/c, corresponding to the unique signature
for the two-body decay of hyperhydrogen Lambda H-4 -> He-4 + pi-, stopped
inside the target. Its binding energy was determined to be B_Lambda = 2.12 +-
0.01 (stat.) +- 0.09 (syst.) MeV with respect to the H-3 + Lambda mass
Beam-Recoil Polarization Measurement of π0 Electroproduction on the Proton in the Region of the Roper Resonance
The helicity-dependent recoil proton polarizations P[superscript '][subscript x] and P[superscript ′][subscript z] as well as the helicity-independent component P[subscript y] have been measured in the p([→ over e],e[superscript ′][→ over p])π[superscript 0] reaction at four-momentum transfer Q[superscript 2]≃0.1 GeV[superscript 2], center-of-mass proton emission angle θ[superscript *][subscript p]≃90°, and invariant mass W≃1440 MeV. This first precise measurement of double-polarization observables in the energy domain of the Roper resonance P[subscript 11](1440) by exploiting recoil polarimetry has allowed for the extraction of its scalar electroexcitation amplitude at an unprecedentedly low value of Q[superscript 2], establishing a powerful instrument for probing the interplay of quark and meson degrees of freedom in the nucleon
Components of polarization-transfer to a bound proton in a deuteron measured by quasi-elastic electron scattering
We report the first measurements of the transverse ( and ) and
longitudinal () components of the polarization transfer to a bound
proton in the deuteron via the reaction,
over a wide range of missing momentum. A precise determination of the electron
beam polarization reduces the systematic uncertainties on the individual
components, to a level that enables a detailed comparison to a state-of-the-art
calculation of the deuteron that uses free-proton electromagnetic form factors.
We observe very good agreement between the measured and the calculated
ratios, but deviations of the individual components. Our results
cannot be explained by medium modified electromagnetic form factors. They point
to an incomplete description of the nuclear reaction mechanism in the
calculation
The influence of Fermi motion on the comparison of the polarization transfer to a proton in elastic and quasi-elastic scattering
A comparison between polarization-transfer to a bound proton in quasi-free
kinematics by the A knockout reaction and that in elastic
scattering off a free proton can provide information on the characteristics of
the bound proton. In the past the reported measurements have been compared to
those of a free proton with zero initial momentum. We introduce, for the first
time, expressions for the polarization-transfer components when the proton is
initially in motion and compare them to the H data measured at the Mainz
Microtron (MAMI). We show the ratios of the transverse () and longitudinal
() components of the polarization transfer in , to those of elastic scattering off a "moving proton", assuming
the proton's initial (Fermi) momentum equals the negative missing momentum in
the measured reaction. We found that the correction due to the proton motion is
up to 20\% at high missing momentum.
However the effect on the double ratio
is largely canceled out, as
shown for both H and C data. This implies that the kinematics is not
the primary cause for the deviations between quasi-elastic and elastic
scattering reported previously
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