LHC point 1 at CERN, where the ATLAS detector is.
Part I, Part II
So there’s this new particle around that’s thought to be the Standard Model Higgs boson. The next big news on this particle is coming very soon. Two weeks from now, on November 12th, a conference in Kyoto called Hadron Collider Physics (HCP) will begin. The LHC experiments have been in a frenzy trying to get new exciting results to show at this conference. The LHC has been doing extremely well: it has produced for its experiments in 2012 more than three times the amount of data it produced in 2011, and it will keep going until the end of the year.
The dataset to analyse is bigger than it was at the last big announcement on July 4th. This means more sensitivity to things yet unseen, and more precision to things already seen. The discovery announced on July 4th has left a number of open questions. The answer to these questions might very well be that there is new unsuspected physics just around the corner…
The Higgs boson as a gateway to new physics
The Higgs boson would be the final piece completing the Standard Model of particle physics. We have however, numerous reasons to think the Standard Model isn’t all there is to Nature. Most obvious is its complete lack of power to explain gravity, dark matter or dark energy. But also, it leaves a lot of unanswered questions such as why do the fundamental particles have the specific masses that they do, and why they are organized in three generations. At the LHC, we are on the lookout for hints of something beyond the Standard Model. This new particle we just found might be our quickest road to it.
In part I of this series of posts, I was a bit wrong in claiming that the new particle observed has no spin. I found out upon discussion with my colleagues that it is also possible that this new particle has spin 2. A spin 2 particle is a pretty weird beast, maybe even weirder than a spin 0 particle. There aren’t any spin 2 particles in the current roster of fundamental particles, except for maybe a very famous but hypothetical one: the graviton.
Remember that in part I, I said that the spin of a fundamental particle has rather counter-intuitive implications on how that particle behaves. Spin 1/2 particles can’t pileup on top of each other in the same state while spin 1 particles can. Spin 2 particles can also carry interactions between spin 1/2 particles. Spin 0 particles permeate all of space and can generate rapid expansion of the Universe under special circumstances… Spin 2 particles can also carry interactions, and they are the only particles that can carry interactions that have no balancing act.
What do I mean by this? Think about the electromagnetic force. There are positive charges and negative charges, but mix the two charges together (in an atom for example) and you end up with something neutral. This is what I mean by balancing act. The strong force has color charges and the weak force has hypercharge and isospin and these two forces can also “cancel out” in their own weird ways. Gravity is much simpler and much more annoying. You can never get rid of it by cancelling it away. Everything attracts everything else. It’s very counter-intuitive, but only a particle with a more complicated spin configuration like spin 2 can be responsible for such an apparently simpler force, that is if gravity even is carried by a fundamental particle.
We know that this new particle we saw in the LHC collisions can decay to a pair of photons. Photons are spin 1 particles, but they can’t have 0 spin. Since spin has to be conserved during decays, the fact that we end up with two photons implies that the original particle has either spin 0 (two photons of opposite spins come out) or spin 2 (two photons of the same spin come out).
Speaking of the decay to two photons, one thing that has been quite striking to me is how frequent it has been observed to be. If this particle really is the Higgs, it should not happen that often. We have seen more pairs of photons that we expected to see if the particle really is the Higgs boson. Does that throw the Higgs boson out the window? Not yet. It is a bit early to say that this anomaly is significant. It may just vanish when we become more sensitive by acquiring more data. Nevertheless, if the anomaly persists, it may be our very first clue to what physics are like beyond the Standard Model.
It turns out that another mode of decay of the new particle has been showing up too often. The decay to a pair of Z bosons has also been seen with an unexpectedly high frequency, but that anomaly is even less significant. More light will soon be shed on this at the HCP conference, even if no definitive statement is made yet.
What is also interesting about the decay to a pair of Z bosons is that it can yield some more information on the spin of the particle. I don’t think this is the kind of thing we can expect in the HCP results, but by closely inspecting the two Z bosons we can tell whether it is a spin 0 or a spin 2 particle. It’s just a very difficult measurement, and we might not even have enough data to do it yet.
Now, the Higgs boson of the Standard Model is also predicted to decay to taus and b quarks. These decay modes have yet to be seen, but they may be announced at HCP. There is also the possibility that the new particle does not decay by these modes. That would certainly be an indication that we are dealing with something else than the Higgs boson. Interestingly, a spin 2 particle would be forbidden to decay to taus, since taus are spin 1/2 particles (it just wouldn’t add up). The decays to taus and b quarks might also be weaker than expected (to compensate for the excesses in photons and Z’s?) but that would also be a deviation from the Standard Model expectations. For more on the spin of the new particle, I strongly recommend this article on Quantum Diaries. Actually, I recommend the whole blog
Getting into wild guesses, it is possible that what decays to photons is a different particle than what decays to Z bosons. An excess in the production of pairs of W bosons has also been seen, so maybe that’s another, different particle. Unfortunately, not all decays modes provides us with the same information. We can get more detailed spin information from W’s, Z’s and taus than we can from photons. We can get better mass measurements from Z’s and photons than for W’s and taus. It’s a tough game to make sure all these phenomena are tied to the same particle, but we can get reasonably certain once we have good accuracy on mass and spin measurements.
I haven’t talked at all about what the new physics might be if not for the Standard Model. There are lots of ideas out there, from extra dimensions to supersymmetry, from composite quark models to technicolor. Some of these ideas have already been driven to near-extinction by experimental results from the LHC and other experiments. I don’t feel the need to talk about them because at this point, we really don’t know. Lots of people out there have a favorite theory, and they will be eager to tell you what physics beyond the Standard Model may look like, but not me. I’m hoping for the unexpected. In some sense, it’s even more emotionally driven than the attachment to one particular theory, because then everything is possible. But in the end, experiments will have the final word, and the scientific community will listen.
So the Higgs boson of the Standard Model have a lot of very precise predictions about what it decays into, how frequently and what spin it should have. That makes room for unexpected findings, since all these predictions have to pan out. This is when science gets exciting, because verifying a prediction that’s never been checked before is when huge discoveries are made. I strongly sense I will have more entries to this series to write after HCP. Stay tuned!