All posts by natashahehir

I am an aspiring particle physicist and undergraduate at Queen Mary University of London.

The Higgs Boson

As mentioned in my post about the standard model of physics (which can be found here), one of the most important recent discoveries in particle physics is the Higgs boson particle discovered on 4th July 2012 – and as promised, here is its very own post!If you have even the smallest of interests in physics, then you have probably heard of the Higgs boson. So, what is it? Why is it so important?

Back in the 1960’s, theoretical physicists noticed that two of the four fundamental forces, the weak and electromagnetic, were very similar. Similar in such a way that they can be described by the same theory. This implies that light, consisting of electricity and magnetism, and some types of radioactivity all stem from one force called the electroweak force. By uniting these forces into one, the new unified theory’s basic equations correctly describe the combined electroweak force.

Remembering that each force has an accompanying force carrier particle, a boson, then if the unified theory correctly describes the electroweak force to a basic level, that means it should also be able to correctly describe the force carriers. And it does. Except for one big problem – all of the force carrier particles come out of the theory with no mass! None!

The boson for the electromagnetic force is the photon which is indeed massless. The problem arises from the weak force carriers, the W and Z bosons, as these are known and found particles which have a finite mass.

Now you see the problem.

Theorists Robert Brout, Franҫois Englert and Peter Higgs attempted to solve this problem with a proposal that would account for the mass. This was that the entire universe is filled with an invisible field, now known as the ‘Higgs field’, which gives the mass to particles when they interact with it. Particles that interact more with the field acquire more mass and particles that don’t interact with it at all are left massless, just like the photon. This is called the Brout – Englert – Higgs mechanism.

With every fundamental field there is an associated particle, just like the photon is to the electromagnetic field, so to prove this proposal they had to detect the associated particle to the Higgs field – the Higgs boson.

A great analogy for this concept is water. Water in a swimming pool fills all the space, but water is made of millions and millions of H2O molecules, just like the Higgs field is made up of loads of Higgs boson particles. Some fish swim through the water quickly and easily, barely interacting with the water at all, like the photons. On the other hand, if I was in the water doing my attempt at swimming (I can’t swim), I would be interacting with the water a lot more than the fish and moving a lot slower. In this analogy I would be a massive particle made massive by interacting a lot with the water similar to the W and Z bosons.

If this proposal was correct, agreeing to experimental data, it would vastly contribute to the understanding of particle physics on a subatomic level and explain how the particles, hence us and everything in our universe, have mass.

On 4th July 2012, ATLAS and CMS experiments at CERN’s Large Hadron Collider announced that they had detected a particle that was consistent with the predictions of the standard model for the Higgs boson. After more data was taken and analysed, it was indeed found to be the Higgs boson creating strong evidence for the Brout – Englert – Higgs mechanism.

On 8th October 2013, more than 50 years after the theory was proposed, Peter Higgs and Franҫois Englert (Robert Bruot had sadly passed away) were jointly awarded the Nobel prize in Physics “for the theoretical discovery of a mechanism that contributes to our understanding of the origin of mass of subatomic particles, and which recently was confirmed through the discovery of the predicted particle by the ATLAS and CMS experiments at CERNs LHC.”

A massive accomplishment for particle physics.

References:

https://home.cern/about

A great video from Fermilab:https://youtu.be/RIg1Vh7uPyw

Photo:http://www.openculture.com/2012/07/the_higgs_boson_and_its_discovery_explained_with_animation.html

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A Brief Introduction to the Standard Model of Particle Physics

Have you ever wondered what everything around us is made up of? 

In the past ten years one of the most significant achievements in physics has been the discovery that everything in the entire universe is made up from a small number of elementary particles and a few fundamental forces. Pretty amazing, huh? This is called the Standard Model of particle physics.

Our everyday objects, from water to the kitchen sink, are made of molecules, a bunch of atoms bonded together. Each atom consists of a nucleus at the centre with negatively charged electrons orbiting around it. The nucleus itself contains positively charged protons and neutrally charged neutrons. All elements can be described by how many protons, neutrons and electrons each atom has and hence all materials and elements can be described by this structure.

 
Figure 1: http://www.livescience.com 

Atoms compose every solid, liquid, gas and plasma you can think of, either being neutral atoms or ionised (having a net charge). Atoms are so small they are of the order 10^-10m, which is 10 Billionth of a metre!
 
Figure 2: http://www.futurism.com 

What happens when we try and look on an even smaller scale? What are protons and neutrons?

The smallest particles we currently know of have two different types, Quarks and Leptons. Protons and neutrons are both made of quarks, and electrons are a type of lepton. Since we can’t actually say what quarks and leptons are made up of – and are the smallest particles we know of – these are our elementary particles.
 
Figure 3: https://www.spec2000.net/06-atomicphysics.htm 

These elementary particles have no discernible size or structure but we do know some properties of the two types.

Quarks have an electrical charge of -1/3 or +2/3 of the charge of an electron denoted ‘e’. They also come in 6 flavours; Up, Down, Strange, Charm, Bottom and Top. Leptons, however, have an electrical charge of either zero or ‘e’. They also come in six flavours; Electron, Muon, Tau, Electron Neutrino, Muon Neutrino and Tau Neutrino.

They can be organised into 3 generation with the first generation quarks creating the most stable particles, protons and neutrons.
Each generation of particles gets heavier in mass. The heavier they are the more energy is needed to bind them and can make the resultant hadrons more unstable with short lifetimes.

 
Figure 4: http://qmplus.qmul.ac.uk/pluginfile.php/663925/mod_resource/content/2/03-Basics.pdf 

Another difference between the two types is that Leptons exist as free particles. However, quarks existing as free particles has never been observed. They always form bound states of quarks called Hadrons of which there are two types; Baryons and Mesons.

Baryons consist of 3 quarks. For example, a proton is a baryon and is made up of three quarks; two up and one down quark giving a net charge of +e and similar for a neutron which consists of one up and two down quarks giving a net charge of zero, both as expected. Mesons only consist of a quark and an antiquark such as a pion which has an up quark and a down antiquark. There are many different types of hadrons as can be seen in figure 5.

 
Figure 5: http://qmplus.qmul.ac.uk/pluginfile.php/663925/mod_resource/content/2/03-Basics.pdf:

All of these quarks and leptons have an antiparticle partner. These antiparticles have the same mass as their corresponding partners but have opposite charge and angular momentum. This is antimatter. An example you may have heard of is the anti-electron known as the positron. When these partners meet, they annihilate each other conserving charge and momentum.

These elementary particles make up everything in the universe together with four forces. The four forces are Eletromagnetism, Strong Interaction, Weak Interaction and Gravity. They cause particles to interact with each other through force carriers called gauge bosons.
They interact by exchanging the corresponding gauge boson to the force.

The electromagnetic force is the exchange of a photon; a particle representing a packet of light, this force holds atoms and molecules together and is the force between charges. The strong interactive force is the exchange of a gluon which transmits the force that binds together quarks in a hadron and is responsible for holding together all nuclei. The weak interaction is the exchange of intermediate vector bosons W and Z. This interaction changes one quark flavour into another and is the cause of the transmutation of a proton to a neutron which creates deuterium fusion that takes place in the sun. Its role in changing quarks is also involed in many nuclear decays. Gravity is the weakest of the four forces, it is an attractive force between two masses where a massless graviton particle is exchanged. This particle has not yet been directly observed. Despite being the weakest of the four, the force of gravity is the most dominant in the universe for shaping the structures of stars and galaxies.

The most recently discovered scalar particle, the Higgs boson, is needed in the standard model to explain why the W and Z bosons have mass and hence why all particles have mass, however this particle deserves its own post entirely.

All of this information, and hence what the entire universe around us is made up of, can be summed up very neatly in the picture below – the standard model of particle physics.

 


Figure 6: http://qmplus.qmul.ac.uk/pluginfile.php/663925/mod_resource/content/2/03-Basics.pdf

Sources:

http://hyperphysics.phy-astr.gsu.edu/hbase/particles/expar.html

http://qmplus.qmul.ac.uk/pluginfile.php/663925/mod_resource/content/2/03-Basics.pdf

http://hepwww.rl.ac.uk/pust/masterclass/mclass05/pphysicstoday1.pdf

The physics of the atomic bombs

On August 6th and August 9th 1945, the nature of warfare was irrevocably changed and the world had become a far more dangerous place. Two atomic bombs, named ‘Little Boy’ and ‘Fat Man’, were dropped on the Japanese cities of Hiroshima and Nagasaki killing at least 129,000 people. An estimated half of all the deaths occurred on the first day with large numbers dying from radiation sickness, burns, illness and malnutrition in the following months. This remains the only use of nuclear weaponry to date, but the threat of another nuclear war has remained ever since. Many people are aware of the impact of these weapons, but may not know how they work. This is the physics behind the atomic bombs.

The atomic bombs derive their immense destructive power from the sudden release of energy that occurs from splitting the nuclei of fissile elements in their core. The two bombs that were dropped on Japan used different cores and a different assembly method. Little Boy was dropped on Hiroshima and used a uranium core in a gun-type assembly method, while Fat Man used a plutonium core in an implosion assembly method. I will explain the difference in these methods a little later, but I think a good place to start the deconstruction of these bombs is the process of fission.

The nuclei of atoms consist of protons and neutrons. The number of protons determine the element, (Uranium has 92 and Plutonium has 94) while the number of neutrons determine the isotope. The isotopes chosen for the atomic bombs were uranium-235 and plutonium-239. These were chosen because they readily undergo fission. U-235 is a valuable isotope for nuclear weaponry because it can function as the primary fuel for a weapon. When U-235 absorbs a neutron it breaks into two new atoms plus three new neutrons and some binding energy. Two of those neutrons become absorbed by a uranium-238 atom. However, the remaining neutron does collide with another U-235; it then fissions and releases two neutrons. Both of those collide with U-235 atoms which then releases between one and three neutrons. This begins the chain reaction that grows exponentially. To ensure that enough neutrons are produced to cause the chain reaction requires a critical mass of fissionable material. The more fissionable material you have, the better the chance of a chain reaction event occurring.


(www.edmodo.com)

The difference between the two bombs is in the process of detonation. Little boy which used the uranium isotope 235 was constructed using a gun type design. This design involves firing one amount of U-235 at another to combine the two masses. This creates a critical mass that then sets off the chain reaction that will eventually detonate the bomb. The collision of the two masses has to happen quick enough to avoid spontaneous fission; which would cause the bomb to fizzle and fail to detonate. Fat Man, powered by plutonium, could not use the same gun-type design. This was due to the plutonium extracted from the nuclear reactors not being pure plutonium-239, but contained traces of the plutonium-240 isotope. The higher fission rate of 240 meant that in a gun type design it would cause spontaneous fission before the two masses collide which would lower the energy. Instead they used a central mass of plutonium, known as a plutonium pit, placed in a shell of conventional explosives, know as an explosive lens. These explosions detonate at precise times to cause a spherical shock wave, squeezing the plutonium and increasing the pressure and density of the substance. This increase in density allows the plutonium to reach its critical mass, firing neutrons and initiating the fission chain reaction.

(https://en.m.wikipedia.org/wiki/Nuclear_weapon#Fission_weapons)

Following the success of the atomic bombs, physicists went on to evolve this weaponry creating fusion or hydrogen bombs. In these bombs, the energy is not produced by fission but fusion. Fusion bombs contain a fission bomb inside them that creates the high temperature and pressure needed for fusion to take place. In these conditions the hydrogen isotopes deuterium and tritium can readily fuse and release enormous amounts of energy.

In many ways the construction of the nuclear bomb was an accomplishment of the practical uses of physics, and an example of the power of nuclear physics, but the fact that this accomplishment of human endeavour was used to create weaponry of such great devastation will forever remain a scar on humanity.

Neutrino Mystery

The 2015 Nobel prize for physics went to Takaaki Kajita and Arthur B. McDonald for the discovery of neutrino oscillations, showing that neutrinos have mass. This turned out to be the solution to one of the biggest mysteries in particle physics which begins with radioactive decay.

In radioactive decay, atomic nuclei would spit out a particle leaving behind a new nucleus with less energy. It seemed that the nucleus was loosing energy which is not possible from the law of conservation. This law states that energy cannot be gained or lost in any event, the energy you had before is always equal to the energy after. On the 4th December 1940 physicist Wolfgang Pauli postulated that the missing energy went to a third particle in the decay – the neutrino, which would be emitted with the electron. Neutrinos have no charge and were thought to have no mass so solid objects seem as empty space to neutrinos, meaning they can pass through matter without interacting. This made them incredibly difficult to detect.

If a neutrino was emitted when a nucleus decayed it implied that a neutrino colliding with a nucleus could stablalise it and the neutrino would decay, meaning that neutrinos must be emitted in nuclear fusion. It was theorised that the main source of energy from the sun was nuclear fusion so the sun would be emitting a huge number of neutrinos every second and detecting these neutrinos would be strong evidence to that theory.

However Ray Davis’s experiment was only detecting roughly a third of the neutrinos that were expected by the theory of John Bahcall. People were very sceptical of the experiment as trillions of neutrinos went through his tank every second and only ten evidences of the neutrinos were expected to be detected each week. When Davis’s results came through as roughly 1/3 of the expected neutrinos, people were convinced his experiment was wrong and not the theory.

In Japan, physicists were conducting an unrelated experiment to look for a rare kind of nuclear decay and unintentionally found that the number of atmospheric neutrinos their equipment was detecting was also smaller than expected. These atmospheric neutrinos are produced when cosmic rays from space hit the earths atmosphere, spraying out particles including the atmospheric neutrinos. The standard model says there are three flavours of neutrino: electron neutrinos (which are produced by the sun and the only neutrinos that Ray Davis’s experiment was capable of detecting), muon neutrinos and tau neutrinos.

People then began to think that the theory was wrong, but John Bahcall went over his theory and insisted it was mathematically correct, making the solar neutrino problem the biggest mystery in particle physics.

A new theoretic proposal was that neutrinos were able to change back and forth from different flavours, called neutrino oscillations. Suggesting that in the time it takes for the neutrinos to travel from the sun to the earth they can change from electron neutrinos to muon and tau neutrinos (neutrinos the current experiments could not detect). However in order for them to change, time must pass and in order for the neutrinos to have a sense of time they must have a mass – changing the standard model which stated they were massless.

Later in Japan, scientists completed Super Kamiokande, a huge detector that is capable of detecting the electron and tau neutrinos. It could also detect the direction in which the neutrinos were coming from. As the neutrinos were supposedly massless, meaning they have no sense of time or distance,  they expected to have an equal number of neutrinos detected from above as below eventhough neutrinos travelling from below were travelling further. Once plotting the results they found that only half of the neutrinos detected above were detected from below.

This was evidence that the neutrinos have a sense of distance which implies they cannot be travelling at the speed of light and do indeed have a mass. The solution to the solar neutrino problem.

A  link to a great short film about the the ghost particle recommended by my nuclear physics lecturer can be found below.

http://www.dailymotion.com/video/x20rn7s_nova-the-ghost-particle-discovery-science-universe-documentary_tv

Women in Physics

I believe getting women into physics is really important and so I am starting my blog with a short overview of three inspiring women who made great contributions to the study of physics.


Marie Curie
the first woman to win a Nobel prize! She discovered two new elements, Polonium and Radium. Curie had lumps of a fairly common mineral called pitchblend which she knew was radioactive (Radioactivity refers to particles which are emitted from nuclei due to nuclear instability), however there were no traces of the only known radioactive element uranium in it and so realised a new element must be hidden in the mineral. In order to find the new elements she had to grind the pitchblend and extract them. Radium and Polonium are extremely radioactive and it was only because she was dealing with such tiny amount that Curie lived to as long as 67, when it was eventually the cause of her death.

Maria Goeppert Mayer was a theoretical physicist who was a Nobel laureate for proposing the nuclear shell model of the atomic nucleus. The nuclear shell model describes the structure of the nucleus in terms of energy levels. In the nuclear shell model, each nucleon moves in a central potential well created by other nucleons, just as the electrons orbit a potential well created by the nucleus in the atomic shell model. The orbits form a series of shells of increasing energy. Nuclei with completely filled outer shells are most stable. 

Lise Meitner
most notably contributed to the understanding of nuclear fission. She was one of the first to articulate a theory of how the nucleus of an atom could be split into smaller parts. Meitner, along with scientists Otto Hahn and Fritz Strassman, were firing neutrons into heavy Uranium nuclei and always seemed to end up with something lighter. Meitner discovered that the extra neutron was not sticking to the uranium but causing it to split into two lighter nuclei (Barium and Krypton). It was soon realised that this process had the potential to produce large amounts of energy. Nuclear fission was then used as a source for nuclear power and, regrettably for her, led to the first atomic bomb.

This is just a very brief description of some of the amazing things these female physicists have achieved in a time when it was much harder for women to become established.