38 research outputs found
Search for a Dark Photon in Electroproduced e + e − pairs with the Heavy Photon Search experiment at JLab
The Heavy Photon Search experiment took its first data in a 2015 engineering run using a 1.056 GeV, 50 nA electron beam provided by CEBAF at the Thomas Jefferson National Accelerator Facility, searching for a prompt, electroproduced dark photon with a mass between 19 and 81 MeV/c2. A search for a resonance in the e+e− invariant mass distribution, using 1.7 days (1170 nb−1) of data, showed no evidence of dark photon decays above the large QED background, confirming earlier searches and demonstrating the full functionality of the experiment. Upper limits on the square of the coupling of the dark photon to the standard model photon are set at the level of 6×10−6. Future runs with higher luminosity will explore new territory
The HPS electromagnetic calorimeter
The Heavy Photon Search experiment (HPS) is searching for a new gauge boson, the so-called “heavy photon.” Through its kinetic mixing with the Standard Model photon, this particle could decay into an electron-positron pair. It would then be detectable as a narrow peak in the invariant mass spectrum of such pairs, or, depending on its lifetime, by a decay downstream of the production target. The HPS experiment is installed in Hall-B of Jefferson Lab. This article presents the design and performance of one of the two detectors of the experiment, the electromagnetic calorimeter, during the runs performed in 2015–2016. The calorimeter's main purpose is to provide a fast trigger and reduce the copious background from electromagnetic processes through matching with a tracking detector. The detector is a homogeneous calorimeter, made of 442 lead-tungstate (PbWO4) scintillating crystals, each read out by an avalanche photodiode coupled to a custom trans-impedance amplifier
Constraints on the Onset of Color Transparency from Quasielastic ¹²C(e, e′p) up to Q² = (14.2 GeV /c)²
Quasielastic scattering on 12C(e,e′p) was measured in Hall C at Jefferson Lab for spacelike four-momentum transfer squared Q2 in the range of 8–14.2(GeV/c)2 with proton momenta up to 8.3GeV/c. The experiment was carried out in the upgraded Hall C at Jefferson Lab. It used the existing high-momentum spectrometer and the new super-high-momentum spectrometer to detect the scattered electrons and protons in coincidence. The nuclear transparency was extracted as the ratio of the measured yield to the yield calculated in the plane wave impulse approximation. Additionally, the transparency of the 1s1/2 and 1p3/2 shell protons in 12C was extracted, and the asymmetry of the missing momentum distribution was examined for hints of the quantum chromodynamics prediction of color transparency. All of these results were found to be consistent with traditional nuclear physics and inconsistent with the onset of color transparency
A Direct Measurement of Hard Two-Photon Exchange with Electrons and Positrons at CLAS12
One of the most surprising discoveries made at Jefferson Lab has been the
discrepancy in the determinations of the proton's form factor ratio between unpolarized cross section measurements and the
polarization transfer technique. Over two decades later, the discrepancy not
only persists but has been confirmed at higher momentum transfers now
accessible in the 12-GeV era. The leading hypothesis for the cause of this
discrepancy, a non-negligible contribution from hard two-photon exchange, has
neither been conclusively proven or disproven. This state of uncertainty not
only clouds our knowledge of one-dimensional nucleon structure but also poses a
major concern for our field's efforts to map out the three-dimensional nuclear
structure. A better understanding of multi-photon exchange over a wide phase
space is needed. We propose making comprehensive measurements of two-photon
exchange over a wide range in momentum transfer and scattering angle using the
CLAS12 detector. Specifically, we will measure the ratio of positron-proton to
electron-proton elastic scattering cross sections, using the proposed positron
beam upgrade for CEBAF. The experiment will use 2.2, 4.4, and 6.6 GeV lepton
beams incident on the standard CLAS12 unpolarized hydrogen target. Data will be
collected by the CLAS12 detector in its standard configuration, except for a
modified trigger to allow the recording of events with beam leptons scattered
into the CLAS12 central detector. The sign of the beam charge, as well as the
polarity of the CLAS12 solenoid and toroid, will be reversed several times in
order to suppress systematics associated with local detector efficiency and
time-dependent detector performance. The proposed high-precision determination
of two-photon effects will be...Comment: Experimental Proposal E12+23-008 submitted to Jefferson Lab PAC 51,
34 pages, 18 figure
Search for axion-like particles through nuclear Primakoff production using the GlueX detector
We report on the results of the first search for the production of axion-like
particles (ALP) via Primakoff production on nuclear targets using the GlueX
detector. This search uses an integrated luminosity of 100
pbnucleon on a C target, and explores the mass region of 200
< < 450 MeV via the decay . This mass range is
between the and masses, which enables the use of the measured
production rate to obtain absolute bounds on the ALP production with
reduced sensitivity to experimental luminosity and detection efficiency. We
find no evidence for an ALP, consistent with previous searches in the quoted
mass range, and present limits on the coupling on the scale of (1 TeV). We
further find that the ALP production limit we obtain is hindered by the peaking
structure of the non-target-related dominant background in GlueX, which we
treat by using data on He to estimate and subtract these backgrounds. We
comment on how this search can be improved in a future higher-statistics
dedicated measurement
When Color meets Gravity; Near-Threshold Exclusive Photoproduction on the Proton
The proton is one of the main building blocks of all visible matter in the
universe. Among its intrinsic properties are its electric charge, mass, and
spin. These emerge from the complex dynamics of its fundamental constituents,
quarks and gluons, described by the theory of quantum chromodynamics (QCD).
Using electron scattering its electric charge and spin, shared among the quark
constituents, have been the topic of active investigation until today. An
example is the novel precision measurement of the proton's electric charge
radius. In contrast, little is known about the proton's inner mass density,
dominated by the energy carried by the gluons, which are hard to access through
electron scattering since gluons carry no electromagnetic charge. In the
present work we chose to probe this gluonic gravitational density using a small
color dipole, the particle, through its threshold photoproduction.
From our data we determined, for the first time, the proton's gluonic
gravitational form factors, which encode its mass density. We used a variety of
methods and determined in all cases a mass radius that is notably smaller than
the electric charge radius. In some cases, the determined radius is in
excellent agreement with first-principle predictions from lattice QCD. This
work paves the way for a deeper understanding of the salient role of gluons in
providing gravitational mass to visible matter.Comment: Under peer revie
First Measurement of the EMC Effect in B and B
The nuclear dependence of the inclusive inelastic electron scattering cross
section (the EMC effect) has been measured for the first time in B and
B. Previous measurements of the EMC effect in nuclei showed
an unexpected nuclear dependence; B and B were measured to
explore the EMC effect in this region in more detail. Results are presented for
Be, B, B, and C at an incident beam energy of
10.6~GeV. The EMC effect in the boron isotopes was found to be similar to that
for Be and C, yielding almost no nuclear dependence in the EMC
effect in the range . This represents important, new data supporting
the hypothesis that the EMC effect depends primarily on the local nuclear
environment due to the cluster structure of these nuclei.Comment: Submitted to PR