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This is an opportunity to delve inside the nucleus. Protons and neutrons are both examples of hadrons - particles composed of quarks that therefore feel the strong force. In this module, discover some of the more exotic quark combinations and newly discovered particles that may give us the answer to questions such as 'What is dark matter?'.
Course requirements for Module 6: Particle Physics meets Nuclear Physics
Course requirements for Module 6: Particle Physics meets Nuclear Physics
6.1 The Standard Model (Questions)
6.2 Evidence of Dark Matter (Questions)
6.3 Searching for Dark Matter (Questions)
The Standard Model
The Standard Model
What particles make up everything around us? At the most fundamental level, particles are described by the Standard Model:
We divide particles into two groups based on a quantum property called spin. This describes the amount of rotation carried inside the particle, even if the particle is point-like and therefore can’t rotate in the classical way. These two groups are fermions and bosons.
Fermions, which make up all of the matter we see around us, have a half-integer number of spin quanta: 1/2, 3/2, 5/2,…
Bosons, which are generally responsible for interactions between different types of matter, have an integer number of spin quanta: 0, 1, 2,…
Fermions
Fermions
12 of our fundamental particles are fermions, the quarks and leptons, and are separated into three "families" or "generations". All fermions have corresponding antiparticles, which have the same masses but opposite charges and, for the quarks, anti-colours. One of the most common antiparticles, which we have already encountered, is the positron - the positive antiparticle of the negative electron.
Quarks
There are six flavours of quarks:
The three "up-type" quarks, which have a charge of +2/3e: the up, the charm, and the top quark, (-e is the charge of an electron).
The three "down-type" quarks, which have a charge of -1/3e: the down, the strange, and the bottom quark.
Protons and neutrons are composed of the lightest quarks in the first "family", the up and down quarks. Quarks in the second and third families are much heavier than the up and down quarks, so particles containing these quarks will be short-lived and decay to release the excess energy.
In addition to electric charge, quarks carry another type of charge called colour, which comes in three types: red, green, and blue. Quarks cannot exist individually and only exist naturally as colour neutral combinations, called hadrons.
Three quarks (red, blue, and green) can combine to form baryons, (such as protons and neutrons) which are fermions.
Alternatively, a colour combined with its anti-colour can form a quark anti-quark pair called mesons, (such as pions) which are in fact bosons.
Leptons
There are also six flavours of leptons:
The three charged leptons, all with a charge of -e: the electron, the muon, and the tau.
The three neutral leptons: the neutrinos.
The most familiar of the leptons is the electron. Muons and taus are heavier versions of electrons: whereas the electron mass is only 1/2000 of the mass of the proton, the tau is almost twice as heavy as the proton. The electron neutrino is extremely light (but not massless) and has no charge. There is still a lot we don’t know about the neutrinos. We don’t yet know the neutrino masses, or even if the tau neutrino is the heaviest.
Bosons
Bosons
The so-called interaction bosons are responsible for the fundamental forces of nature.
Photons (particles of light or other electromagnetic radiation) are responsible for the electromagnetic interaction that dominates much of our daily life, including keeping the electrons within the atom. The electromagnetic interaction between fundamental particles and even large scale objects happens by the exchange of photons back and forth.
The gluon is responsible for holding the atomic nucleus together through the strong interaction. It even holds the protons and neutrons themselves together, because the quarks constantly exchange gluons.
The weak interaction (governed by the Z and W bosons) determines, for example, how a neutron (made of two down quarks and an up quark) can change into a proton (composed of two up quarks and a down quark). In the atomic nucleus, the opposite transformation is also possible. Both of these decay types are known as beta decays: beta- or beta+ depending on whether the negative electron or its antiparticle, the positron, is emitted. In addition to these interactions, the Higgs boson is responsible for giving particles their mass.
Gravity is the one elusive fundamental force left. It is extremely weak, even in comparison to the weak interaction, and we think this interaction could be governed by a fundamental particle, the graviton. However, we have never observed the graviton and, even though there are plenty of ideas about the nature of this particle, in reality we don’t know. Perhaps we will find out soon by studying some of the heaviest and densest objects in the universe, such as black holes and neutron stars.
In the video below, Abby-Rhian Powell and Kayleigh Gates from the University of Glasgow explore more about quarks, the role of the strong force, and how current experiments are discovering new particles with different quark combinations.
As Abby and Kayleigh explained, although naturally existing fermions come in a quark triplet (baryons) or a quark-antiquark doublet (mesons), there are a number of human-made subatomic particles. These are not yet known to exist outside of high energy particle experiments, however, they could hypothetically be formed in the interior of neutron stars. Dr. Mikhail Bashkanov from the University of York explains more in the video below.
You can find out more about the discovery of the d* hexaquark in the 'Find out more' section at the bottom of the page.
Activity 6.1
Activity 6.1
Log in to Isaac Physics and answer the questions:
Fermions
Bosons
Quark Matter
Particles and Interactions 1, 2, 5, 7, 8
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Dark Matter
Dark Matter
Astronomers believe that most of the matter in the universe is invisible to us. It is non-luminous, so it doesn’t emit or absorb light, and therefore cannot be seen directly. This dark matter cannot yet be described in terms of any of the particles that we know, so what makes us think that it exists at all?
The presence of this dark matter has been inferred indirectly from the motions of astronomical objects. In 1933, Fritz Zwicky discovered that the galaxies were orbiting each other so fast that they should have been thrown outwards into space. Instead, they displayed stable orbits and remained within the cluster, meaning that some invisible mass must be providing the gravitational attraction needed to keep the cluster bound together.
In the 1960s, Vera Rubin (figure 1) and her team studied galaxy rotation curves and showed that the velocity of material at the edge of galaxies wasn’t decreasing as expected. They showed that most galaxies were missing a significant amount of matter that could not be explained by stars, gas or other known objects. This is shown in figure 2, below. Find out about the Rubin Observatory being built in her name in the 'Find Out More' section at the bottom of the page.
Figure 1: Vera Rubin in 2009
Figure 2: Observed and expected rotation curves of spiral galaxy Messier 33
Mario De Leo, CC BY-SA 4.0, via Wikimedia Commons
Dark Matter in the Milky Way
Dark Matter in the Milky Way
By looking at its rotation curve, we can estimate the amount of dark matter in our galaxy, the Milky Way.
Figure 3: A rotation curve for our galaxy, produced using the RadioSALSA 3m telescope located in Sweden, by a University of York physics undergraduate student.
The plot above shows a rotation curve for our galaxy. Note that pc is a unit of astronomical distance called a parsec: 1 pc = 3.086 x 1016m. We can see that the velocity increases until about 3.5 kpc, beyond which the velocity starts to plateau. This flat relationship extends to the edge of the galaxy (about 50kpc).
The rotation curve of a galaxy describes how the circular velocity (the speed in a circle) of stars and gas in the galaxy varies with distance from the centre. We can use Newton’s law of gravitation to describe the gravitational force, F, experienced by an object with mass m at a distance r from the centre of the galaxy:
where G is the gravitational constant (G=6.67x10-11Nm2kg-2) and M is the mass interior to the radius r. For objects moving in a circle around the centre of the galaxy with velocity v, the centripetal force is given by:
Combining these two equations and rearranging, we get:
Astronomers can estimate the value of the circular velocity using radio observations of neutral hydrogen. Neutral hydrogen is found throughout the galaxy as gas clouds. These clouds give off radio-frequency photons, which can be easily detected on Earth. By measuring the Doppler shift of these clouds (which tells us whether the clouds are moving towards or away from us and how fast), it is possible to calculate their circular velocities and therefore create a rotation curve.
Work your way through the Isaac Physics questions below (6.2) to see if you can find evidence of dark matter for yourself. You should find that a certain percentage of the mass of the galaxy cannot be accounted for. This is the contribution attributed to dark matter!
Activity 6.2
Activity 6.2
Log in to Isaac Physics and answer the questions:
Evidence of Dark Matter
Don't forget to log in to Isaac Physics!
What is Dark Matter?
What is Dark Matter?
The origin of dark matter in the Universe is a key unsolved question for science. Understanding dark matter may be the first step to a deeper understanding of the origins of the universe, with consequences far beyond physics and astronomy.
Dark matter isn't actually "dark" - it is named because it does not interact with the electromagnetic field, such as visible light. So dark matter does not emit light, and it doesn’t even absorb light, and it is therefore very difficult to detect.
Major efforts are underway to explain dark matter, with recent approaches including searches for weakly interacting exotic particles beyond the standard model of particle physics. However, to date, suitable exotic candidates have not revealed themselves in particle accelerators or in sensitive detectors placed deep underground. We have compiled a few of the ideas that researchers have - or have had - of what dark matter might be. Yet, until it is detected, it is difficult to narrow the field down.
Candidates for Dark Matter
Candidates for Dark Matter
Weakly Interacting Massive Particles (WIMPs)
Weakly Interacting Massive Particles (WIMPs)
A popular idea for what dark matter might be is a hypothetical kind of new non-baryonic particle potentially existing beyond the Standard Model, which would interact primarily via gravity and the weak nuclear force. These would be relatively heavy compared to particles in the standard model. They must interact so weakly that existing detectors don’t have the sensitivity to see them.
A mystery for the WIMP hypothesis is why the dark matter should be out in the halo beyond the visible matter rather than concentrated in the center of galaxies. Efforts to detect these WIMPS are similar to those used to find neutrinos. Many billions of neutrinos pass the Earth every second, mostly originating in the Sun - however because these neutrinos only act very weakly they are very difficult (but not impossible) to detect. If these hypothetical particles were created in the Big Bang and are gravitationally bound, cosmological models predict that these particles should concentrate towards the center of galaxies.
Massive Astronomical Compact Halo Object (MACHO)
Massive Astronomical Compact Halo Object (MACHO)
These were one of the first proposed candidates for dark matter. Effectively a combination of hidden, but heavy baryonic matter and black holes. These could be objects that give off very little EM radiation such as brown dwarves, neutron stars and black holes. These objects are known to exist and we can discover them as they pass in front of background stars and gravitationally bend the light from those stars, acting as a lens. However, from these observations, we now know that MACHOs can account for only a tiny proportion of the dark matter galactic halo. Therefore, scientists have concluded that we need another type of dark matter.
Axions
Axions
Physicists originally came up with axions to help fix a problem with the strong nuclear force. The problem is that particles that feel the strong force don’t behave differently when either their electric charges are switched, or if they get flipped upside down - hence researchers came up with a way to explain this. As a side effect, their explanation also suggested that the universe may be full of new hypothetical particles called axions. Although the individual particles have a very low mass, the universe-forming Big Bang could have churned out axions in abundance — maybe enough to constitute dark matter!
Dark photons
Dark photons
A dark photon is a hypothetical force-carrying particle, similar to a regular photon. Unlike a regular photon, these dark photons would have a mass. If they exist in the quantities needed to account for all the dark-matter mass in the universe, dark photons would exist everywhere and would move together like a wave. Dark photons would hardly interact with normal matter, but as waves, they would exert a very weak force.
Kaluza-Klein particles
Kaluza-Klein particles
The Kaluza-Klein theory could provide us with fascinating consequences. It suggests the existence of an invisible 5th dimension, instead of the 4-dimensions that we know (height, width, depth and time). What has this got to do with dark matter? If we extend our standard model into 5-dimensions, the hypothetical particles we would find have been proposed as dark matter candidates. These particles could interact both via electromagnetism and gravity. However, as it is in a dimension we can't see, we wouldn't observe it just by looking at the sky. The particle should be able to be detected in experiments as it should decay into particles we can measure – neutrinos and photons - yet powerful particle accelerators like the Large Hadron Collider are yet to detect it.
The Search for Dark Matter
The Search for Dark Matter
Much of the search for dark matter takes place in underground laboratories. Working underground presents a number of challenges, not the least the difficulty of moving detectors, equipment and scientists a kilometre underground safely. So why would we do this? In the video below, Prof. Sean Paling introduces us to one such facility, Boulby Underground Laboratory.
The challenge for dark matter detectors is that dark matter usually passes straight through detectors, with any hint of its passage being incredibly rare. With a device that’s big enough and sensitive enough, a few interactions registered could confirm the existence of dark matter and allow us to explore its properties. In the video below, Dr XinRan Liu from the University of Edinburgh speaks about some of the experiments taking place at Boulby Underground Laboratory.
A cutting-edge detector looking for dark matter is the LUX-ZEPLIN (LZ) detector. This is a combination of two previous world leading dark matter programs. First, the ZEPLIN detectors (I, II, III) were built and operated at Boulby Underground Laboratory from the late 1990s until 2011. These pioneered the technology that has since come to dominate dark matter detection. LUX was the main US-based xenon experiment, and was world leading from 2013. It was located at the Sanford Underground Research Facility, which is where the new LZ detector is based.
The LZ detector is made of a very large volume of liquid xenon. Xenon has the useful property that should a particle of dark matter collide with one of the xenon atoms, small bursts of light will be emitted. Very sensitive light detectors are constantly watching the xenon, recording any signals that might occur.
Xenon has numerous properties that make it ideal as a dark matter detector. It scintillates, which is the technical name for its property to give out light when struck by other particles. Moreover, it is then transparent to its own scintillation light, which means that we can get the signal out of all parts of a large detector. It is a noble element, which means we can easily purify large quantities to a very high degree. And xenon has a very high atomic mass, which makes it an ideal target for dark matter interactions as each atom has many neutrons and protons to act as targets for the dark matter to scatter from. Finally, xenon naturally has several stable isotopes, that is, atoms of xenon which all have 54 protons naturally occur with various different numbers of neutrons, ranging from 70 to 82. There are different ways dark matter could interact with these different isotopes, and having a detector made from all of them makes the experiment sensitive to a wider range of types of dark matter. Some of the isotopes are candidates for an exciting process known as neutrino-less double beta decay, which is a whole different area of physics, but which LZ is also sensitive to - a bonus for free!
Despite the size of the LZ detector, we still only expect to see very few events per year as a result of dark matter interactions. If our detector was above ground, it would be swamped with millions of background noise events from the cosmic radiation that is constantly bombarding the Earth’s surface. We go deep underground as a natural shield against these sources of background. This is just the start of trying to escape sources of background events! Everything is radioactive - an average human naturally emits around 1000 gamma rays every second. We need to control the radiation of the laboratory within which the detector is sited, and of the detector itself. One particular problem is radon, which can escape from detector materials and into the xenon, through the process of emanation. When it decays, it can produce events mimicking a dark matter signal. The approach is to build increasingly radiation free shells of shielding, to reduce the radioactive background until we have the least radioactive cubic metre in the known universe at the heart of our detector. Dark matter, by its very nature, will be unaffected by our efforts to block it, so this central volume is where we search.
Find out more about how the detector works in the video below and in the 'Find out more' section at the bottom of the page.
Dark matter and the d* hexaquark
Dark matter and the d* hexaquark
The discovery of the d* hexaquark has opened up a new possibility for dark matter. This new six-quark particle is made from the very same light quarks which combine in trios to make up protons and neutrons. However, an important difference is that the six quarks in a d* result in a boson particle, which means they can group together in very different ways. The research suggests that, for the conditions available shortly after the Big Bang, many d* hexaquarks could have grouped together as the universe cooled and expanded to form the fifth state of matter - Bose-Einstein condensate. This gives a new possibility for what dark matter is and a potential explanation of how the matter in the Universe became dominated by dark matter. This is explained by Prof. Dan Watts from the Univeristy of York in the video below.
The next step in investigating this new dark matter candidate will be to obtain a better understanding of how the d* hexaquarks interact - when do they attract and when do they repel each other. Unlike the underground detectors we have looked at so far, to test this theory of dark matter and search for d* hexaquarks we need to look into the cosmos. Data from telescopes such as the Fermi Gamma-ray Space Telescope is being used to look for evidence of d* hexaquark Bose-Einstein condensates out in space, and new experiments are being developed to create d* hexaquarks inside an atomic nucleus to see if their properties are different to when they are in free space.
You can find out more about the d* hexaquark, including its possible role in neutron stars, in the 'Find out more' section at the bottom of the page.
Activity 6.3
Activity 6.3
Log in to Isaac Physics and answer the questions:
Candidates for Dark Matter
Boulby Underground Science Facility
The Lux Zeplin Detector
The d*hexaquark
Don't forget to log in to Isaac Physics!
Find out more (optional)
Find out more (optional)
More about Boulby Underground Laboratory
More about Boulby Underground Laboratory
Find out more about Boulby Underground Laboratory in this video: Prof. Sean Paling, Director and Senior Scientist at the lab, continues his tour and explains about some of the other research taking place deep underground.
Case Study: Studying high energy physics
Case Study: Studying high energy physics
My name is Emily, and I am a second year PhD student in the High Energy Physics department at University College London. My time is spread over two research projects.
Firstly, I am a member of the Lux Zeplin (LZ) collaboration. The LZ experiment is designed to hunt for dark matter particles, which we believe make up the majority of our universe. The LZ detector basically consists of a big bucket of liquid xenon, and if a dark matter particle interacts with the xenon, we see light. The challenge is that dark matter seems to interact very rarely and radioactivity in materials around us interact much more frequently. One particular problem is radon, which can escape from detector materials and into the xenon, through the process of emanation. When it decays, it can produce events mimicking a dark matter signal. I spend the majority of my time analysing this radon background, as well conducting on-site visits to Sanford Underground Research Facility, USA, to assist in detector operation and maintenance.
My second project is the Cold Radon Emanation Facility (CREF), which is based at Rutherford Appleton Laboratory, Harwell. The facility is part of a global effort to mitigate radon emanation from detector construction materials in next generation rare-event searches. The facility aims to perform radon emanation studies of materials with sufficient sensitivity and at (cold) temperatures of relevance for next generation rare-event detectors. The latter condition is unique to CREF, and will enable a reduction in the uncertainties on screenings conducted at room temperature due to the cold suppression of radon diffusion within and subsequent emanation out of materials. My work for CREF includes the commissioning of the facility, and detector data analysis.
More about the LUX-ZEPLIN detector
More about the LUX-ZEPLIN detector
Weaving the Search for Dark Matter: A detailed video on the LZ Grid construction
Dark Matter Day Q&A with LZ Researchers: A general interest Q&A with LZ scientists on dark matter and a discussion of the challenges of building detectors to find it
LUX-ZEPLIN inner detector transported nearly a mile underground: A video showing the inner cryostat vessel (ICV) being constructed on the surface and then lowered underground
Dark Matter Day Q&A with Kevin Lesko: An introduction to dark matter and LZ by Kevin Lesko, a previous Spokesperson for the LZ collaboration
Finding the d* hexaquark
Finding the d* hexaquark
An experiment looking for the d* hexaquark was led by the University of York using the Crystal Ball detector at the Mainz Microtron (known as MAMI). MAMI is a particle accelerator located at the Johannes Gutenberg University in Mainz, Germany. Like many other particle accelerators, MAMI is used by physicists from around the world to investigate nuclear and particle physics. The experiment searched for the d* hexaquarks by focusing an intense gamma ray beam on a liquid deuterium target. You may remember that deuterium is an isotope of hydrogen with one proton and one neutron in the nucleus (called a deuteron). Deuterium is found naturally in seawater, where it constitutes approximately 0.0153% of hydrogen, or can be concentrated to form so-called heavy water. Hitting a deuteron with a photon causes the deuterium nucleus to split into a proton and a neutron in a process called photodisintegration. By using sufficiently high energy gamma ray photons, the quark substructure of the deuterons can be investigated. By measuring the final state of the protons and neutrons from the photodisintegration of the deuterium nuclei, it is possible to determine that d* hexaquarks were formed during this process.
The existence of the d* hexaquark was inferred by observing the degree of spin polarisation of the outgoing proton and neutron. For the first time, both were almost completely polarised at photon energies corresponding to the mass of the d* hexaquark. This cannot be explained by current conventional theories. The results provide important new information for the emerging field of multiquark states and potentially for astrophysics, where the d* hexaquark may play an important role in neutron stars and even provide a new candidate for the mysterious dark matter. You can read more about the experiment in Signatures of the d*(2380) Hexaquark in d (γ,p→n), published in the APS Physics, Physical Review Letters (2020).
The process is shown in the diagram below:
More on d* hexaquarks and dark matter
More on d* hexaquarks and dark matter
Dr Mikhail Bashkanov and Prof. Dan Watts explain their research in a Syzygy Live event for the York Festival of Ideas.
d* hexaquarks and neutron stars
d* hexaquarks and neutron stars
The d* hexaquark may have a role to play in the structure of neutron stars. Neutron stars are the remnants left after the gravitational collapse of massive stars during a supernova event.
The masses of neutron stars are typically of the order of one to two times the mass of the Sun, with radii of 10 − 14 km. Neutron stars are therefore among the densest objects in the universe. Their density is typically 3 - 5 times larger than the density of an atomic nucleus. Different models have been created to predict the size of neutron stars. However, recent measurements of neutron star radii done by NASA’s NICER mission, combined with information coming from gravitational waves, are challenging these models. They show larger neutron stars than originally predicted.
Research by the University of York shows that models including the presence of the d* hexaquarks in the centre of neutron stars lead to maximum masses compatible with these recent observations. So how might these d* hexaquarks form in a neutron star? Usually, protons make up about 15% of neutron star cores. Therefore, it seems plausible that, under extreme pressure in the centre of a neutron star, proton-neutron pairs would combine into a new particle - the d* hexaquark. This would have a smaller size than the protons and neutrons on their own, so could be favourable. Taking all of the protons and adding an equal amount of neutrons would allow up to 30% of neutron star matter to be converted into d* hexaquarks. Current predictions suggest that up to 20% of neutron star matter could actually be in the form of these particles. Whilst we are still a long way from proving that these d*hexaquarks are present in these large neutron stars, this initial data is an exciting starting point for further scientific investigation. You can read more about d* hexaquarks and neutron stars in The d*(2380) in Neutron Stars - A New Degree of Freedom? published in Physics Letter B (2018).
Rubin Observatory
Rubin Observatory
The Vera C. Rubin Observatory, previously referred to as the Large Synoptic Survey Telescope, is currently under construction in Chile. The first light of the telescope, (the first use of the telescope to take an astronomical image when it has been constructed) is expected to be January 2025.
This telescope is to conduct a 10-year Legacy Survey of Space and Time, addressing four areas of astronomy:
Probing dark energy and dark matter
Taking an inventory of the solar system
Exploring the transient optical sky
Mapping the Milky Way
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