MOLLER - Measurment of a Lepton-Lepton Electroweak Reaction

The purpose of this page is to introduce the reader to the basic concepts of the MOLLER experiment. No prior knowledge is asssumed. We will start with a brief summary, then describe the lepton-lepton electroweak reaction, and move on to the concepts of parity and parity violation. We will then discuss Cherenkov radiation and Cherenkov detectors. Finally, we will cover the MOLLER experimental design.

A list of the topics covered on this page is:

• Lepton-lepton electroweak reaction
• Parity
• Parity Violating Asymmetry (Apy)
• Cherenkov Radiation
• Cherenkov Detectors
• MOLLER design

MOLLER Summary

MOLLER is a cutting-edge experiment that studies the electroweak reaction (mix of electromagnetic and weak-nuclear) between leptons, specifically electrons. To measure these interactions, MOLLER uses detectors that capture Cherenkov radiation which is emitted by the electrons. By carefully analyzing the small changes in the intensity of this radiation we will be able to calculate a quantity called the weak mixing angle.

When we say a small change we mean small. We're talking about a difference that is so small, it's like finding a few extra grains of sand on a beach, the change in intensity will be just a few parts in a billion.

This page will introduce you to the basic concepts of the MOLLER experiment. The next section will describe the lepton-lepton electroweak reaction.

What is the Lepton-Lepton Electroweak Reaction?

The video below describes the Lepton-Lepton Electroweak reaction.

So you can see that the lepton-lepton electroweak reaction is a reaction between two leptons. In MOLLER it will be two electrons. When these electrons interact, there could be an electromagnetic interaction (where they exchange a photon) or a weak interaction (where they exchange a Z^0 boson). The mixing of these interactions is why it is called the electroweak reaction. And MOLLER's measurment of the parity-violating asymmetry (Apy) will allow us to derive a quantity called the weak mixing angle (opens in a new tab), also called the Weinberg angle.

Its actually not quite that simple. If your interested in learning more about electroweak theory, you can start by reading here Electroweak (opens in a new tab). And an excelent introductory book is Introduction to elementary particle physics (opens in a new tab) By David Griffiths.

Ok, so know we know what is meant by 'Lepton-lepton' and 'Electroweak.' But MOLLER will measure the Parity Violating Asymmetry in the lepton-lepton electroweak reaction. So, what is parity? and what is parity violation?

Parity is a concept in physics that refers to the spatial inversion of a physical system. This means system remains the same when its spatial coordinates are inverted. Then we say that the system has parity symmetry. However, the weak force violates this symmetry, which means that some processes involving the weak force do not look the same when their spatial coordinates are inverted. What does that really mean?

The video below describes the concept of parity and parity violation.

So, in the context of the MOLLER experiment, the Apy refers to the difference in scattering probabilities of the electrons when their spins are flipped. By measuring this asymmetry, the experiment will gain insight into the weak mixing angle, a crucial parameter in the electroweak theory.

This brings us to the question of how do we actually measure the Apy in the lepton-lepton electroweak reaction? We can do this by using Cherenkov radiation.

Cherenkov Radiation and Cherenkov Detectors

Cherenkov Radiation

Cherenkov radiation happens when a charged particle, like an electron, zips through a medium faster than light can travel in that same medium. As a result, the particle gives off a cone-shaped burst of electromagnetic radiation, which we call Cherenkov radiation. It's like a sonic boom, but for light.

This video below describes Cherenkov radiation.

If you're curious to learn more about Cherenkov radiation, you can start by checking out this wikipedia article (opens in a new tab).

Cherenkov detectors

Alright, so that's what Cherenkov radiation is. But what does this have to do with the MOLLER?

MOLLER is equipped with detectors that are sensitive to Cherenkov radiation. As the beam's electrons become scattered, they pass through these Cherenkov detectors, all the while emitting Cherenkov radiation. By analyzing the patterns of Cherenkov radiation intensity we can derive the value of the weak mixing angle.

MOLLER's main detectors are called thin quartz detectors. They are designed to pick up all or most of the Cherenkov radiation emitted by charged particles passing through them. They do this because of a physical feature called total internal reflection. The radition is then captured by photomultiplier tubes (PMTs), and recorded by a data acquisition system.

The video below discusses how these types of Cherenkov detectors work.

So, now we understand that MOLLER will measure the weak mixing angle using Cherenkov detectors. The detectors will detect Cherenkov radiation emitted by electrons. This radiation will bounce around inside of the detector until it exits a beveled end and enters a photomultiplier tube. By switching the direction of the electron's spin, we can measure a small change in the intensity of the Cherenkov radiation. This is how the measurment of a lepton-lepton electroweak reaction will work.

MOLLER Design

Now that we understand the basic physics concepts of how MOLLER will measure the weak mixing angle we can look at a high level overview of the entire MOLLER experiment.

Key components of the experiment include a Beam, a hydrogen target, a magnetic spectrometer for sorting the scattered electrons, Cherenkov detectors to capture the light emitted, and additional detectors like the Showermax for gathering more data. This section will give more detail about each of these components.

The Beam

The beam in the MOLLER experiment is a stream of 11 GeV electrons generated by a particle accelerator. These electrons are "spun" so that they have a specific direction, or polarization. During the experiment, the spin direction is rapidly switched back and forth (a process called helicity reversal). This beam is aimed at a hydrogen target.

The Target

Next is the 1.25 m liquid hydrogen target, where the beam electrons interact with target electrons, leading to scattering events.

The Spectrometer

Following the target is the spectrometer system, consisting of resistive toroidal magnets with a 7-fold azimuthal symmetry, which separates and focuses the scattered electrons into 7 different channels. This helps to optimize the measurment by blocking one the scattered electrons and excepting the other. As the electroweak reaction always scatters two electrons (one from the beam and one from the target).

Spectrometer Image (opens in a new tab) from Mike Bevins.

The Detectors

Thin Quartz Detectors

After the electrons have passed through the spectrometer, they are detected by the Cherenkov detectors. The main detectors in the MOLLER experiment are made of thin quartz and arranged in a specific geometry to effectively capture Cherenkov radiation emitted by the scattered electrons.

These detectors are organized into six concentric rings, each divided into at least 21 individual segments around the azimuth. The rings have different radial distances to capture the various scattered electrons. The second-largest ring primarily captures the Moller electron signal, while the second-smallest ring mainly captures the unradiated elastic scattered signal from electron-proton interactions. This strategic arrangement allows us to study the Cherenkov radiation patterns and gather crucial information about the underlying physics in the experiment.

Main detectors

Images (opens in a new tab) from Larry Bartoszek.

Additional Detectors

Additional detectors including ShowerMax, pion detectors (acrylic calorimeters), and large and small angle monitors, which help monitor backgrounds and calibrations. Together, these components form a comprehensive experimental setup that allows scientists to measure the Apy in the lepton-lepton electroweak reaction.

ShowerMax detector.	Pion detector.

Dr. McNulty's group is responsible for the design and construction of the ShowerMax detector. This is the detector that you would most likely be working on if you were to join the group. The next page has much more detailed information about ShowerMax.