Session FF: Proton Puzzles II

Design and Installation Status of the Liquid Hydrogen Cryotarget Assembly for the MUSE Experiment

A fundamental challenge faced by the scientific community is to resolve the proton radius puzzle: the unexplained discrepancy observed between muon spectroscopic measurements and atomic measurements of the proton charge radius. The MUon Scattering Experiment (MUSE) at the Paul Scherrer Institute is likely to play an instrumental role towards resolving this puzzle. It will be the first elastic scattering experiment that will use a simultaneous beam of muons and electrons at very low Q^{2}, thus allowing a direct comparison of the proton radius from the two leptonic probes at a sub-percent level. An essential and arguably the most complex part of MUSE is the liquid hydrogen cryotarget assembly. Its design is guided by several experimental and safety constraints. The experiment primarily requires a liquid hydrogen target of very stable density, a vertically movable ladder consisting of three targets in addition to the liquid hydrogen target for alignment and background studies, and large vacuum windows to provide high acceptance. In this talk, I will present the design of the cryotarget assembly, the fabrication technique for the target cells and safety tests. I will also discuss the installation status of the cryotarget assembly.   

The MUSE FPGA Trigger* Shraddha Dogra Rutgers

The MUSE experiment measures the elastic scattering of electrons and muons from a liquid hydrogen target at the PiM1 beam line of the Paul Scherrer Institute in Villigen, Switzerland. PiM1 provides a mixed beam of electrons, muons, and pions, so the particle rate in the scattered particle detectors is dominated by pion decays and pion scattering from the target. To efficiently obtain data on the much lower rate elastic electromagnetic scattering, we employ a variety of strategies based on segmented fast scintillators that send signals to FPGAs so that the species of all beam particles can be determined and scattering events from the target can be identified. I will discuss the MUSE triggering strategy and performance.

  

Beam Characterization for MUSE at PSI

The Muon Scattering Experiment (MUSE) at Paul Scherrer Institute (PSI) is being prepared to resolve the proton radius puzzle – the six-standard deviation discrepancy between proton charge radius measurements with electronic and muonic probes, respectively. MUSE is designed to measure the proton charge radius with elastic electrons and muons scattering simultaneously and with both charge polarities. For an accurate determination of the lepton scattering angle, event-by-event beam particle tracking is required to reconstruct the incoming particle track. A telescope of Gas Electron Multipliers (GEM), exposed to a high flux of beam particles is used to reconstruct the incoming tracks with high spatial resolution while representing minimal material for the beam to pass through. Additional GEM elements are placed at the intermediate focus between the two dipoles of the PiM1 beamline to study the beam properties. The status of the GEM performance will be reported.   

Beam Particle Identification and Momentum Determination in MUSE

The Proton Radius Puzzle has been an important physics question and generated rich discussion since the muonic hydrogen experiment found a smaller than expected proton radius. The MUSE collaboration will measure the elastic scattering of electrons and muons from a liquid hydrogen target at the PiM1 beam line of the Paul Scherrer Institute in Villigen, Switzerland. We will determine whether the radius of the proton is the same when determined from the two different particle types in a scattering experiment. I will discuss how we identify beam particles and determine the beam momentum in the experiment, along with relevant beam line simulations.   

Target Simulation for the Muon Scattering Experiment (M$\mu$SE) at the Paul Scherrer Institut

In 2010, a novel method of spectroscopic measurements on muonic hydrogen resulted in a 4\% smaller proton radius than previously observed, and at an order of magnitude improvement in precision. This measurement, and a second in 2013, established the so-called "Proton Radius Puzzle". Now, the MUSE collaboration will simultaneously measure, for the first time, electron and muon scattering of both polarities from a liquid hydrogen target to measure the proton radius. To produce a competitive measurement, much work has been done to understand how systematic uncertainties and backgrounds affect the measured lepton-proton elastic scattering cross sections, nuclear form factors, and ultimately the extracted proton radius. To this end, simulation is an invaluable tool for understanding sources of error and guiding experimental design. In this talk, I will review the status of the target simulation for the MUSE experiment and discuss how simulation results have informed the design of the target chamber, liquid hydrogen cells, and background subtraction techniques.    

Searching for Diffractive Contributions to the Forward $\pi^0$ Transverse Single-Spin Asymmetry in $\sqrt{s}=200$ GeV Polarized $pp$ Collisions

The observation of large transverse single-spin asymmetries, $A_N$, has been around since 1976, but their
origin is not yet fully understood. Recent measurements of $\pi^0$ $A_N$ at STAR carried out with the
Forward Meson Spectrometer (FMS), an electromagnetic calorimeter covering a forward pseudorapidity range
of $2.6<\eta<4$, have shown that more-isolated $\pi^0$s have larger asymmetries than those which are
less-isolated. In 2015, Roman Pot (RP) detectors were installed which allow for the identification of
outgoing protons at near-beam rapidities. These RP protons can be used to select for $\pi^0$s in the FMS
which could have originated from diffractive-type processes. In particular, a sample of highly-isolated
$\pi^0$s is observed in the FMS, in coincidence with the forward-going proton deflecting into a RP,
and nothing more in the forward direction. This presentation will focus on this
class of events and its properties, along with other FMS and RP correlations, venturing into these
unexplored possible contributions to the large $\pi^0$ $A_N$.   

Very Forward Neutral Particle Measurement in the RHICf experiment

In June of 2017, we installed an electro-magnetic calorimeter in the most forward area of the STAR experiment and took 510 GeV polarized proton collision data for neutral particle production (neutron, photon, neutral pion) at pseudorapidity > 6. The cross section measurement will give us new inputs to develop high-energy collision models which is essential to understand air-shower from ultra-high energy cosmic rays. The asymmetry measurement will enable us to understand the hadron collision mechanism based on QCD. The data were taken with three detector positions in order to cover wide kinematic regions. STAR detector data were also recorded for combined data analysis. We will present evaluation of the experimental data and status of the data analysis.