If you look outside your window at the clouds, the stars, the planets, all that you will see is made of matter. However, when the universe was born, there were equal amounts of matter and antimatter. So where has all the antimatter gone?
The answer, if one is found, will be at the Large Hadron Collider (LHC), the world’s most powerful particle physics experiment, now taking a breather while engineers refit it to make it even more powerful by 2015. Then, it will be able to spot tinier, much more shortlived particles than the Higgs boson, which itself is notoriously shortlived.
While it ran from 2008 to early 2013, the LHC was incredibly prolific. It smashed together billions of protons in each experiment at speeds close to light’s, breaking them open. Physicists hoped the things that’d tumble out might show why the universe has come to prefer matter over antimatter.
In fact, from 2013 to 2015, physicists will be occupied gleaning meaningful results from each of these experiments because they simply didn’t have enough time to sift through all of them while the machine was running.
They will present their results as papers in scientific journals. Each paper will will be the product of analysis conducted on experimental data corresponding to some experiment, each with some energy, some luminosity, and other such experimental parameters central to experimental physics.
One such paper was submitted to a journal on April 23, 2013, titled ‘First observation of CP violation in the decays of B_s mesons‘. According to this paper, its corresponding experiment was conducted in 2011, when the LHC was smashing away at 7 TeV centre-of-mass (c.o.m) collision energy. This is the energy at the point inside the LHC circuit where two bunches of protons collide.
The paper also notes that the LHCb detector was used to track the results of the collision. The LHCb is one of seven detectors situated on the LHC’s ring. It has been engineered to study a particle known as the beauty quark, which is more than 4.2 times heavier than a proton, and lasts for about one-hundred-trillionth of a second before breaking down into lighter particles, a process mediated by some of nature’s four fundamental forces.
The beauty is one of six kinds of quarks, and together with other equally minuscule particles called bosons and leptons, they all make up everything in the universe: from entire galaxies to individual atoms.
For example, for as long as it lives, the beauty quark can team up with another quark or antiquark, the antimatter counterpart, to form particles called mesons. Generally, mesons are particles composed of one quark and one antiquark.
Why don’t the quark and antiquark meet and annihilate each other in a flash of energy? Because they’re not of the same type. If a quark of one type and an antiquark of another type meet, they don’t annihilate.
The B_s meson that the April 23 paper talks about is a meson composed of one beauty antiquark and one strange quark. Thus the notation ‘B_s’: A B-meson with an s component. This meson violates a law of the universe physicists long though unbreakable, called the charge-conjugation/parity (CP) invariance. It states that if you took a particle, inverted its charge (‘+’ to ‘-‘ or ‘-‘ to ‘+’), and then interchanged its left and right, its behaviour shouldn’t change in a universe that conserved charge and parity.
Physicists, however, found in the 2011 LHCb data that the B_s meson was flouting the CP invariance rule. Because of the beauty antiquark’s and strange quark’s short lifetimes, the B_s meson only lasted for so long before breaking down into lighter particles, in this case called kaons and pions.
When physicists calculated the kaons‘s and pions‘s charges and compared it to the B_s meson’s, they added up. However, when they calculated the kaons‘s and pions‘s left- and right-handednesses, i.e. parities, in terms of which direction they were spinning in, they found an imbalance.
A force, called the weak force, was pushing a particle to spin one way instead of the other about 27 per cent of the time. According to the physicists’ paper, this result has been reached with a confidence-level of more than 5-sigma. This means that some reading in the data would disagree with their conclusion not more than 0.00001 per cent of the time, sufficient to claim direct evidence.
Of course, this wouldn’t be the first time evidence of CP violation in B-mesons had been spotted. On 17 May, 2010, B-mesons composed of a beauty antiquark and a down quark were shown shown to decay at a much slower rate than B-antimesons of the same composition, in the process outlasting them. However, this is the first time evidence of this violation has been found in B_s mesons, a particle that has been called “bizarre”.
While this flies in the face of a natural, intuitive understanding of our universe, it is a happy conclusion because it could explain the aberration that is antimatter’s absence, one that isn’t explained by a theory in physics called the Standard Model.
Here was something in the universe that was showing some sort of a preference, ready to break the symmetry and uniformity of laws that pervade the space-time continuum.
Physicists know that the weak force, one of the fundamental forces of nature like gravity is, is the culprit. It has a preference for acting on left-handed particles and right-handed antiparticles. When such a particle shows itself, the weak force offers to mediate its breakdown into lighter particles, in the process resulting in a preference for one set of products over another.
But in order to fully establish the link between matter’s domination and the weak force’s role in it, physicists have to first figure out why the weak force has such biased preferences.
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This post originally appeared in The Copernican science blog at The Hindu on April 25, 2013.
