The physics blogosphere has been on fire1,2,3,4,5,6.7 regarding new results announced yesterday by the LHC‘s LHCb team.8 The collaboration have announced a 3.5 sigma signal indicating possible CP violations in the decay of charmed mesons, a signal not predicted by the Standard Model.
Okay, I perceive the collective shrugs among you. Perhaps some definitions are in order. I’ll start from the end of the previous paragraph and work my way to the front.
The Standard Model of particle physics is a model describing the electromagnetic, weak, and strong nuclear interactions. Pieced together from the 60’s through the mid-70’s, the Standard Model is well-tested to a high degree of precision, having predicted the existence of numerous particles subsequently detected in particle accelerator experiments. It successfully describes those three fundamental forces as different manifestations of an underlying Grand Unified interaction, indistinguishable from one another in the early universe immediately following the Big Bang, then branching off into separate interactions as the universe cooled.
But, despite its success, the Standard Model is incomplete. It does not incorporate General Relativity, and thus provides no explanation for gravity. Furthermore, it fails to account for more recently discovered phenomena such as dark matter or dark energy, and provides no explanation for the hierarchy problem. Theorists have been striving to address these issues by attempting to construct a “theory of everything” which would combine the Standard Model with General Relativity in some manner. These efforts have largely focused on highly speculative models such as string theory, M-theory, and loop quantum gravity, but such models have not yet lent themselves well to experimental validation or falsification. In fact, one of the most promising extensions to the Standard Model over the last few decades, supersymmetry, has had substantial setbacks of late due to the latest datasets to come out of LHC and Tevatron experiments (but that is a story for another posting).
And any experimental observations which defy predictions of the Standard Model provide fodder for the construction of new theories.
So, what is this about charmed mesons? Let’s talk quarks for a moment. (Sorry about the names. Physicists got really cutesy with naming conventions in the 60’s.) Everyone (hopefully) learns in school that atoms are made of electrons whizzing around a nucleus, and that the nucleus is composed of protons and neutrons. Well, it was figured out by the 60’s that protons and neutrons themselves are composite particles, each of which is made of up of three smaller particles called quarks, which in turn are bound to each other by particles called gluons. There are different kinds of quarks. Protons and neutrons are made up of combinations of the two most common kinds of quarks, the “up” and “down” quarks. (See what I mean about the names?) Up and down quarks have heavier, highly unstable cousins called “charm” and “strange” quarks. (Yeah, it goes on.) Then there are still heavier quarks called “top” and “bottom” quarks. (It used to be more bizarre. Originally, these two were called “truth” and “beauty.”) The zoo of particles made up of various combinations of these quarks is a big part of what is observed in particle accelerator experiments carried out at the Large Hadron Collider. Or, more specifically, their decay products, since these particles tend to be highly unstable.
I mentioned earlier that protons and neutrons are each made up of three quarks. Any particle made up of quarks is called a hadron (hence the name “Large Hadron Collider”). Hadrons made of three quarks (such as protons and neutrons) are called baryons. Particles made up of two quarks (always in quark-antiquark pairs) are called mesons. And this experiment specifically involved the decay of a particular type of meson, called the D0 meson, which consists of a charm quark and an anti-up quark.
Now, there are two main mechanisms by which this particle would be likely to decay:
1. The charm quark decays into a W+ boson and a strange quark, and the W+ then decays into an up quark and an anti-strange quark. These last two quarks combine to form a K+ meson, and the other strange quark combines with the original anti-up quark to form a K- meson.
Now, it turns out that an anti-D0 decays to the exact same combinations of particles. According to the Standard Model, the ratio of decays to kaon/anti-kaon pairs to that of pion/anti-pion pairs should be roughly equal for D0 and anti-D0 mesons, but the data yielded by the LHCb experiment indicate an asymmetry between the D0 and anti-D0 decay modes. This is a rather startling result.
This is where CP violations9 come into play. In particle physics, there are three fundamental symmetries of interest: charge symmetry (replacing each particle in an interaction with an oppositely-charge equivalent – i.e., its anti-particle), parity symmetry (looking at the mirror image of a system), and time reversal symmetry (how the interaction behaves going forward or back in time). Each of these symmetries correspond to a mathematical operator which acts upon the Lagrangian of a given system (the mathematical expression describing the dynamics of a system, defined as the kinetic energy minus the potential energy), and these operators are represented by the symbols C, P, and T. If applying a given operator (or combination of the operators) to an interaction results in no change, then the corresponding symmetry is said to be conserved. Otherwise, there is a violation of that symmetry.
The individual symmetries are rather routinely violated by well-known interactions (at least for C and P). Things get more interesting when the symmetries are combined. For example, CPT symmetry is thought to never be violated. CP symmetry (which is of interest in this experiment), is sometimes violated, although this is still pretty rare. And, as it turns out, replacing D0 mesons with anti-D0 mesons in the experiment in question here is equivalent mathematically to performing a CP transformation on the system, since the charges are reversed, as is the “handedness” of the particles (but a discussion of isospin should wait for another day).
And it is a good thing that CP symmetry can be violated. Otherwise, we wouldn’t exist.
More explanation is in order here. Empty space is never truly empty. Even in the harshest vacuum of deep space, at the tiniest of scales, the universe is frothing with pairs of particles and their corresponding anti-particles spontaneously popping into existence, then promptly annihilating each other, returning their energy to the zero-point energy of the vacuum. Quantum theory doesn’t permit empty space to be energy-free (at least when a potential is present – ANY potential). The ground state for a vacuum has to be non-zero, and it is from this non-zero vacuum energy that the spontaneous creation of virtual particle pairs occurs. The very beginnings of the cosmos, the Big Bang, can be regarded as the same process writ large. When the Big Bang, er, banged, so to speak, an equal number of matter and antimatter particles should have been created, which would have then promptly annihilated each other, leaving us as no-shows. What seems to have actually happened is that, initially, slightly more matter than antimatter was created. The bulk of the matter and antimatter interacted, destroying each other, leaving the small excess of matter behind which constitutes the matter we see in our universe today.
All of which makes our existence possible.
Back in the 60’s, the legendary Soviet physicist (and famed political dissident) Andrei Sakharov figured out how this baryonic matter-antimatter asymmetry could happen.10 And his explanation depended upon CP symmetry being violated (which, mathematically, is identical to a T-symmetry violation).
Now, keep in mind that this is not the first time that CP violations have been observed. It is simply the first time they have been observed with charmed particles, and the first time that the violation has been this far out of line with Standard Model predictions. Nor is it necessarily the case that this particular CP violation is responsible for baryonic asymmetry. In fact, the last time a CP violation was reported, Sean Carroll published a screed11 on his blog lamenting how quick commentators were to associate such violations with the Sakharov conditions. But, the fact of the matter is that the more we learn about the conditions under which CP violations can occur, the better equipped we are to come up with models to explain the mechanism for such violations, and thus to grapple with the baryonic asymmetry problem.
And that 3.5 sigma signal being reported? Keep in mind that we are talking about a signal just barely strong enough to be considered as possibly being more than a statistical fluke. A 3 sigma signal is pretty much the baseline for something to be considered worth looking at. A 5 sigma signal screams “We have something here!” The LHCb team is a long way from that threshold. They’ll have to collect much more data to see if the signal recedes back into the noise, or if it steps more firmly into the foreground to shout “New physics here!”
Update: Dec. 4
Anna Phan has an update on this at Quantum Diaries.
1. “LHCb has evidence of new physics! Maybe.“, Resonaances.
2. Alexey Petrov, “LHCb reports observation of CP-violation in charm. Welcome New Physics? Or not?“, Quantum Diaries.
3. Anna Phan, “A New Surprising Result!“, Quantum Diaries.
4. Sean Carroll, “New Physics at LHC? An Anomaly in CP Violation“, Cosmic Variance.
5. Tommaso Dorigo, “CP Violation in Charm Decays: 3.5 Sigma From LHCb!“, A Quantum Diaries Survivor.
6. Matt Strassler, “A Notable Discrepancy at the LHCb Experiment“, Of Particular Significance
7. “Charm and beauty: LHCb has it all!“, Quantum Diaries
8. Mat Charles, “Search for CP violation in two-body charm decays at LHCb“, Presentation at the Hadron Collider Physics Symposium 2011, Paris, Nov. 14, 2011
9. Anna Phan, “What exactly is CP violation?“, Quantum Diaries.
10. Sakharov, A.D. (1967). “Violation of CP Invariance, C Asymmetry and Baryon Asymmetry of the Universe”. Zhurnal Eksperimentalnoi i Teoreticheskoi Fiziki, Pisma 5: 32. (Russian). (Translated in Journal of Experimental and Theoretical Physics Letters5, 24 (1967).)
11. Sean Carroll, “Marketing CP Violation“, Cosmic Variance