Higgs Data Offers Joy and Pain For Particle Physicists 186
scibri writes "So now that we've pretty much found the Higgs Boson, what's next? Well: 'There's going to be a huge massacre of theoretical ideas in the next couple of years,' predicts Joe Lykken, a theoretical physicist at Fermilab. The data has shored up the standard model, but technicolor is dead and supersymmetry is starting to look pretty ropey now. Theorists are now poking at the mathematical chinks in the standard theory in the hopes of being the first to find a deeper truth about how the Universe works."
Re:Did we really find it? (Score:5, Informative)
It is not confirmed, but it is not expected to not be confirmed, so nothing lost by starting on the theoretical work ahead of the confirmations. In the unlikely case it turns out to be something else, we can just start over.
Re:Did we really find it? (Score:5, Informative)
It's unknown but really likely. There is definitely a particle at around 125 GeV but there certainly is a (very small) chance it could be something else.
The standard model predicts a number of different ways the Higgs Boson can decay and what probability it has for each type of decay.
The most common easy to measure decay modes are:
Higgs -> Two Photons (high energy gamma rays)
Higgs -> Two W Bosons -> 4 leptons (electrons or muons)
So what they are actually seeing is the decay products and they measure the energy of each component of the decay and add that up to find the original energy of the Higgs.
The measurement of the two photons is called the "gamma-gamma" channel or "diphoton" channel. They call the 4 lepton channel the "golden channel" because it's a pretty clean signal with a low "background" (noise). That is, they get a good signal to noise ratio from the 4 lepton channel.
The theory says that the two photons should happen a certain % of the time and the 4 leptons should happen a different % and the other decay modes should happen with other probabilities.
One of the reasons to believe they have found the Higgs boson and not some other particle is that the decay relative rates for each type of decay are pretty close to what the theory suggests.
The best way to study the Higgs would be to produce lots of them accurately without producing other particles. The best-known way to do that is with a linear collider that smashes leptons (usually electrons) together. They can tune the energy of the collisions to the exact value to produce Higgs. This is how the W boson was studied so accurately at SLAC. A new international linear collider (ILC) would need to be built to reach the energy levels needed to make the Higgs. Luckily, it's a pretty low and easy to reach energy compared to what it could have been which makes an ILC somewhat reasonable to build.
Re:Did we really find it? (Score:5, Informative)
From what I understand it was only one single experiment that showed us something that we think is where/what the Higgs Boson would look like.
Has it been reproduced or confirmed?
That's not very definitive. Can anybody else around well versed in particle physics tell us if the Higgs has really been found or not?
I think that the announcement is based on a couple of years of data collected by two different teams using different methods, so calling it a single experiment seems a bit of an over simplification. See Higgs Discovery: The Data [profmattstrassler.com] blog entry by Matt Strassler.
Re:Did we really find it? (Score:5, Informative)
The LHC was built to find any new physics, not just the Higgs. The fact that we've been able to rule out SUSY for large mass ranges is part of that. To measure the specific properties of one particle though does need something a bit more purpose-built. They'll be able to measure a lot about the Higgs boson but not anywhere near as much as a linear collider could measure.
Also, for part of the year they stop injecting protons and instead inject lead nucului. This is meant to measure extremely messy but very high energy collisions that should generate quark-gluon plasmas.
Re:Did we really find it? (Score:5, Informative)
Discrediting a theory isn't a permanent thing. Any theory can be brought back if evidence warrants it. Even Einstein's "biggest blunder," the cosmological constant, is now the most popular theory to explain the universe's accelerating expansion.
Re:The real takeaway (Score:5, Informative)
More like, we will find that quantum physics and standard model don't actually differ, but only in observation.
They differ by virtue of belonging to different categories of things.
Quantum physics is a general framework that encapsulates a number of particular physical theories, including quantum electrodynamics (interaction between charged particles and photons), quantum electroweakdynamics or whatever it's called (throw in the W and Z bosons and neutrinos on top of quantum electrodynamics), quantum chromodynamics (interaction between quarks, bearing a charge called "color", and gluons, the force quanta for the field generated by that charge), and the standard model (quantum electroweakandchromodynamics). So the standard model is a quantum theory, and thus falls under the general heading of "quantum physics" (as do atomic physics, nuclear physics, most if not all of what's called "condensed matter physics", and so on).
Decay channels not rates (Score:5, Informative)
One of the reasons to believe they have found the Higgs boson and not some other particle is that the decay relative rates for each type of decay are pretty close to what the theory suggests.
Actually that is not really true because we do not have enough statistics to measure these rates with any accuracy. In fact the "most likely" value for diphoton rates for both ATLAS and CMS are quite a bit higher than the Standard Model predicts but the accuracy is sufficiently low that they are not yet inconsistent with the SM values. So really the rate measurements are currently far too inaccurate to have any idea whether this is a Higgs boson or not but things are improving rapidly as we gain statistics.
What is far more important at the moment are the decay channel observations. Since it decays into photons, W and Z bosons we know it must be either a spin-0 or spin-2 particle and it cannot be a fermion (spin-0.5). The Higgs should be spin-0 so this is consistent but not conclusive. Essentially it decays into the particles it should do and it _potentially_ has the correct spin. We can get a more accurate determination of the spin i.e. whether it is spin-0 or spin-2 by looking at the angle between the two leptons (electron or muon) produced in the WW decay channel - expect results from ATLAS and CMS on this soon.
However by the end of the year the rate measurements should be a lot more accurate and things will possibly start to get interesting if the current diphoton rates stay where they are but we end up with less uncertainty on the measurement.
Pink elephants (Score:5, Informative)
http://vimeo.com/41038445 [vimeo.com]
Enjoy!
Re:Did we really find it? (Score:4, Informative)
The "v" in the context of W decay to "lv" is a neutrino ("v" is a close approximation to the lowercase N or Nu in greek ν)
Re:Did we really find it? (Score:4, Informative)
only one single experiment
Actually, it was two different experiments: CMS [wikipedia.org] and ATLAS [wikipedia.org]. The LHC is the big ring-like structure that accelerates particles around it; CMS and ATLAS are two detectors at different points on that ring that watch as the particles collide with each other. Both CMS and ATLAS have detected a new particle, with the same mass (~125 GeV), with about the same significance (~5-sigma, or about a 1-in-50-million chance of getting that result by chance).
The mass is about what the standard model of particle physics predicts for the Higgs boson, so it looks very much like this new particle is it. But the physicists are being careful not to state outright that they've found it, because there are certain properties that the Higgs is expected to have - charge, spin and parity, I think - which they haven't been able to measure yet. When they've got enough data to measure those, and if they match what the Higgs is supposed to have, then they'll state that they've definitely detected it.
Re:Stopped reading at "Mathematical chinks" (Score:4, Informative)