Small Anomaly, Might bring Monumental Changes

Updated: Jul 21, 2021


Science is the almighty study of the universe, but yet the only study where we don't really know what can change everything. It's basically the physical manifestation of the Trial and Error method with a twist of genius and crazy, and it has very weird ways to introduce itself. One of them is a very tiny dancer, forgetting its routine from a very tiny angle. Let's talk about how this tiny friend of ours probably introduced us to a new side of this science.


Universe is governed by four fundamental forces i.e. gravity, electromagnetic force, weak nuclear force and strong nuclear force. From the explosion in Hiroshima-Nagasaki to the movement of our Star System in respect to the Centre of Milky Way, everything can be explained by these four fundamental forces, making them the corner-stone of the modern scientifical era.


What’s wrong? :

An experiment conducted by fermi labs presented an anomaly that couldn’t be explained by our four fundamental forces. So, let’s see what’s up.




A part of the standard model, specifically quantum electro dynamics, describes how a particle will tend to rotate to align with a magnetic field. The strength of that interaction is defined by something called ‘The G-factor’ which is coherently different from the TV show, ’The X factor’ (the basic difference is that is that in Gs case, Simon isn’t making sarcastic comments every five seconds).



QED predicts the G-factor for the electron that matches experimental data to one part to a billion, and that’s some precise stuff. So, if this is so accurate for electrons, surely it will be solid for other particles as well, right? Well, no.


Muons:

The Muon (protagonist of this experiment) is a heavier version of an electron. It is classified as a lepton and has a charge of -1e. It has a very short lifespan and in about 2 microseconds it decays into electrons.


What’s wrong with muons:

Historically our experimental data of the muon G-factor did not match the QED calculations and that meant that the calculations were missing something. And that something was maybe something beyond the standard model of physics. When theory and experiments don’t agree, generally there is an undiscovered particle or force at work.


The wobble:

Every particle with electric charge has a quantum spin and they generate a magnetic field. Now what happens when you place an object with magnetic in a secondary magnetic field? The object will tend to rotate and align with the other field. Now for a particle, this torque will depend on angular momentum, its charge and mass. An electron also has a magnetic dipole but its dipole movement is different from the classical one because of G, for an electron G = 2. So, an electron will respond to a magnetic field twice as strongly compared to an equivalent classical charge.



QED tells us what this difference is. In theory electromagnetic interactions result due to the interaction of charged particles communicating by exchanging virtual photons. This phenomenon can occur in many different ways. In the simplest case is of an electron being deflected by a photon, you get a value of G=2 which was also calculated by Dirac in his equation. But in another set of cases where a virtual photon is produced which is a slightly more complicated event, allowed Julian Schwinger to calculate a slightly higher value of g=2.0011614. Over time more and more complicated interactions were added and slightly altered this value. The latest calculations rely on supercomputers to add thousands of complicated cases like these and get us our g-factor which is 2.001159652181643. So, the anomaly here is the leftover bit which is g-2. For an electron that calculation perfectly agrees with the theoretical number.

So, what’s the next best thing to do? Do the same experiment with other particles, like muons. But the muon’s g-factor is altered slightly due to the presence of virtual particle in the vacuum of space. So, when we add up all possible cases for the muon interaction, we end up with something which is slightly off the experimental data.


Why?:

So, the question here is that why would we get the wrong value for the muon but the right value for the electron? Probably because muons are thicc. Simply put, the bigger the particle is, the more likely it is to interact with virtual particle. In general, you might think that vacuum is completely empty, but in actuality, due to the uncertainty principle, there are particles which pop in and out of existence for just a blip, they are called virtual particles.

So, the muon is way more likely to be disturbed due to these virtual particles. Therefore, it is also way more likely to encounter any completely unknown virtual particle that might be playing peek-a-boo there. Even accounting for all the known virtual particles, the muon g-factor is slightly off. So what physicists hope is that an unknown particle is at work here.

So, the question here is that why would we get the wrong value for the muon but the right value for the electron? Probably because muons are thicc. Simply put, the bigger the particle is, the more likely it is to interact with virtual particle. In general, you might think that vacuum is completely empty, but in actuality, due to the uncertainty principle, there are particles which pop in and out of existence for just a blip, they are called virtual particles.

So, the muon is way more likely to be disturbed due to these virtual particles. Therefore, it is also way more likely to encounter any completely unknown virtual particle that might be playing peek-a-boo there. Even accounting for all the known virtual particles, the muon g-factor is slightly off. So what physicists hope is that an unknown particle is at work here.



Until now, experiments conducted on muon g-factor in Brookhaven labs resulted in 3.7 sigma difference from the theory. Physicists however have higher expectations than 3.7(sounds like Asian parents), they need a sigma 5 difference for anything to be declared a discovery.

The fermi lab experiment is designed to be more sensitive than the former experiments and reach 5 sigma difference. In the experiment, muons are sent flying at nearly the speed of light around magnetic tube with a 50 feet diameter, the muons interact with the magnetic field and rotate due to them being dipoles, like a top. We call this Larmor precession and its frequency depends on the g-factor, this also governs the frequency of what the muon’s decay into. So, by measuring the energies of those particles which are produced after the decay, particularly positrons, researchers can determine the precession rate and measure the g-factor. Due to the fermi lab experiment, the anomalous difference is considered to be 4.2 sigma so not yet a complete discovery but very close. The g-2 experiment team worked very hard to clear all systematic errors and if everything goes right, this might a faint glimmer in the current theoretical wasteland of physics.



Another anomalous experiment:

Another experiment conducted in the LHCb also hinted towards a new fundamental force. LHCb is a high precision detector and is trying to pick up very rare events that don’t occur commonly. What the team at LHCb is looking for is decay of a particular type of meson which is a combination of a quark and an anti-quark. They are specifically looking for a B-meson which has B quark in it. (We all are made of up and down quarks.) The b-quarks are very rare and they decay rapidly and when they decay, they form another pair of quarks, s-quark and as they do so they produce 2 leptons, the muon pair or the electron pair. In particular the number of times the b-quark decays into either an electron and positron pair or a muon and its anti-particle pair is important. The ratio of muon pair to electron pair produced while decay should be one i.e. roughly equal, but this is not the case, the ratio in the experiment is found to be 0.846. Now because of lepton universality principle this should not be the case. So, the simplest explanation is that there is some virtual particle which is stopping the muon route to go through and there are many possible suggestions of this particle such as lepto-quarks or z-prime particles or vector quarks.



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