Beautiful Asymmetry
Posted by on Wednesday, October 7, 2020 Under: Particle Physics
This has been a most interesting day for the physics community.
As most of you are aware by now, the Nobel Prize in Physics was awarded this morning to three eminent researchers in the field of black hole physics, with two being astronomers and astrophysicists, and the third having worked on the theory of black holes.
However there was also a less publicized announcement today from the LHCb collaboration, with the discovery of time dependent matter-antimatter asymmetry being observed in certain rare beautiful decays. Perhaps I should take a step back here and explain what all of those terms mean...
A lot of progress has been made over the past twenty years in the fields of cosmology, astrophysics, and high energy particle physics. We know more about the natural world on both smallest and largest scales than at any time in human history. But there are still a number of mysteries in the Universe that continue to plague us.
One of these is matter-antimatter asymmetry. Everything that we see around us is made of matter. Everything from the computer or smartphone that you are reading this on, to the primordial stars whose light is just reaching us now is formed of atomic matter. But for the past century we have known about another group of particles known as antimatter. It has the same mass as ordinary matter, but with opposite electrical charge, and when matter and antimatter collide they annihilate each other. And the existence of antimatter can not be disputed, with particle accelerators producing billions of antiprotons for experiments and hospitals now producing a significant number of antimatter electrons (called positrons) every day in their PET scanners. It exists.
But it is also the subject of a great mystery. The laws of physics as we know them say that there is a symmetry between matter and antimatter. If you create matter out of the vacuum, then an equal amount of antimatter must be generated. And the only way to destroy matter is to destroy an equal quantity of antimatter. In every reaction, matter and antimatter remain balanced.
So where is all of the antimatter?
Astronomers and astrophysicists have probed the entire visible Universe, and no significant quantities have ever been found. The Big Bang model correctly predicts most of the properties of the Universe, but cannot explain why the Universe seems to have only matter in it, with no antimatter for balance. It is a mystery.
One possibility is that there is some reaction that we do not yet understand in which matter can be produced without antimatter, or at a greater rate than antimatter. It is this possibility that the LHCb has been probing with this latest research.
The team at LHCb generated billions of strange B mesons, usually denoted Bs0, which are composed of a strange quark and a beauty or bottom antiquark. They then carefully measured the particles that were generated when these B-mesons decayed, and specifically looked for decays that violated the expected symmetry. And what they discovered was that at certain times in the B-mesons lifetime, it would violate CP-symmetry, which is necessary for violating matter-antimatter symmetry as well.
This time dependence arises from another interesting property of B-mesons, their oscillations. For reasons that are beyond the scope of this article, B-mesons are not fully matter or anti-matter. The quark and antiquark can interact through the weak nuclear force, and change their flavour. The strange quark and beauty antiquark will transform into a strange antiquark and a beauty quark. And this combination should behave in a similar way to the original B-meson, but according to this experiment it does not. When the particle oscillates into its own anti-particle, its decay properties change. It has violated CP and matter-antimatter symmetry.
This effect has been observed before in lighter particles, but never at a sufficiently high rate to explain the missing antimatter in the Universe. This is the first time that it has been observed in these strange beauty meson decays, but at this point we do not have enough data to explain this effect. It could be an odd property of this particular particle, in which case the mystery would remain. Or it might be the first clues to a new physical phenomenon or a new high energy effect that has greater implications. We just don't know yet.
It is an interesting result, but still a long way from explaining the matter-antimatter symmetry in the Universe. And so the search continues.
As most of you are aware by now, the Nobel Prize in Physics was awarded this morning to three eminent researchers in the field of black hole physics, with two being astronomers and astrophysicists, and the third having worked on the theory of black holes.
However there was also a less publicized announcement today from the LHCb collaboration, with the discovery of time dependent matter-antimatter asymmetry being observed in certain rare beautiful decays. Perhaps I should take a step back here and explain what all of those terms mean...
A lot of progress has been made over the past twenty years in the fields of cosmology, astrophysics, and high energy particle physics. We know more about the natural world on both smallest and largest scales than at any time in human history. But there are still a number of mysteries in the Universe that continue to plague us.
One of these is matter-antimatter asymmetry. Everything that we see around us is made of matter. Everything from the computer or smartphone that you are reading this on, to the primordial stars whose light is just reaching us now is formed of atomic matter. But for the past century we have known about another group of particles known as antimatter. It has the same mass as ordinary matter, but with opposite electrical charge, and when matter and antimatter collide they annihilate each other. And the existence of antimatter can not be disputed, with particle accelerators producing billions of antiprotons for experiments and hospitals now producing a significant number of antimatter electrons (called positrons) every day in their PET scanners. It exists.
But it is also the subject of a great mystery. The laws of physics as we know them say that there is a symmetry between matter and antimatter. If you create matter out of the vacuum, then an equal amount of antimatter must be generated. And the only way to destroy matter is to destroy an equal quantity of antimatter. In every reaction, matter and antimatter remain balanced.
So where is all of the antimatter?
Astronomers and astrophysicists have probed the entire visible Universe, and no significant quantities have ever been found. The Big Bang model correctly predicts most of the properties of the Universe, but cannot explain why the Universe seems to have only matter in it, with no antimatter for balance. It is a mystery.
One possibility is that there is some reaction that we do not yet understand in which matter can be produced without antimatter, or at a greater rate than antimatter. It is this possibility that the LHCb has been probing with this latest research.
The team at LHCb generated billions of strange B mesons, usually denoted Bs0, which are composed of a strange quark and a beauty or bottom antiquark. They then carefully measured the particles that were generated when these B-mesons decayed, and specifically looked for decays that violated the expected symmetry. And what they discovered was that at certain times in the B-mesons lifetime, it would violate CP-symmetry, which is necessary for violating matter-antimatter symmetry as well.
This time dependence arises from another interesting property of B-mesons, their oscillations. For reasons that are beyond the scope of this article, B-mesons are not fully matter or anti-matter. The quark and antiquark can interact through the weak nuclear force, and change their flavour. The strange quark and beauty antiquark will transform into a strange antiquark and a beauty quark. And this combination should behave in a similar way to the original B-meson, but according to this experiment it does not. When the particle oscillates into its own anti-particle, its decay properties change. It has violated CP and matter-antimatter symmetry.
This effect has been observed before in lighter particles, but never at a sufficiently high rate to explain the missing antimatter in the Universe. This is the first time that it has been observed in these strange beauty meson decays, but at this point we do not have enough data to explain this effect. It could be an odd property of this particular particle, in which case the mystery would remain. Or it might be the first clues to a new physical phenomenon or a new high energy effect that has greater implications. We just don't know yet.
It is an interesting result, but still a long way from explaining the matter-antimatter symmetry in the Universe. And so the search continues.
In : Particle Physics
I am a theoretical physicist & mathematician, with training in electronics, programming, robotics, and a number of other related fields.
