Instead of going to bed, I write this last sequel before CERN shares more information on search after Higgs boson so far. Actually, I'm on bed and midnight has passed so there is less than 9 hours until we hear more. But certain things have apparently leaked already and I would like to update here my previous article.
After more than 10 years of gathering and analyzing data produced by the U.S. Department of Energy's Tevatron collider, scientists from the CDF and DZero collaborations have found their strongest indication to date for the long-sought Higgs particle. Squeezing the last bit of information out of 500 trillion collisions produced by the Tevatron for each experiment since March 2001, the final analysis of the data does not settle the question of whether the Higgs particle exists, but gets closer to an answer. The Tevatron scientists unveiled their latest results on July 2, two days before the highly anticipated announcement of the latest Higgs-search results from the Large Hadron Collider in Europe. Their results look like this:
What does above graph say? Tevatron scientists found that the observed Higgs signal in the combined CDF and DZero data in the bottom-quark decay mode has a statistical significance of 2.9 sigma. This means there is only a 1-in-550 chance that the signal is due to a statistical fluctuation. Not enough. At the same time Philip Gibbs made his unofficial global Higgs combination with the new results from D0 included. The significance at 125.5 GeV has crept up to 4.4 sigma. Still not enough.
If the Higgs really is lurking at 125 GeV, Nature is giving us a very nice opportunity, because the Higgs should (if it’s the simple Standard-Model version) decay into a variety of different particles, and we can study each one separately. Every experimental possibility is a different “channel.” Here are the predictions for the simple Higgs at this mass. (Note that in some cases these particles quickly decay themselves) To make sure that what you’ve really found is the Higgs, you’d like to check that it decays into all the various channels with the right percentages. Not all final states are equally easy to find, however. When the Higgs decays into quarks or gluons, it releases a spray of jets that tend to get lost in all the background noise of other processes. The same is true if it decays into W’s or Z’s or taus and then those decay into quarks and gluons, which happens a lot. Our favorite processes, then, are when the Higgs decays all the way to leptons and photons. Indeed, a lot of the excitement from last December (including the ATLAS plot above) came from looking at two-photon events, even though those are expected to happen less than one percent of the time. Decays into four charged leptons (electrons or muons), which can happen when the Higgs goes to two Z’s and each Z decays into charged leptons, happen something like 0.01% of the time, but they’re so easy to spot that they’re also a favorite channel.
Now, something that has been hinted in previous article already. The Tevatron experiments are most sensitive to the Higgs boson decaying into a pair of b-quarks and produced in association with a W or Z boson. What they're testing is thus the Higgs couplings to electroweak gauge bosons and to b-quarks, both of which are central to establishing the higgsy nature of the newly discovered particle. In particular, the Tevatron data are suggesting that the particle indeed decays frequently into b-quarks (which, according to the Standard Model, should happen about 60% of the times). Thus, the Tevatron provides an important piece of the puzzle that, at the moment, is not available from the LHC. Actually, the rate observed in the VH→bb channel is 2±0.7 larger than predicted by the Standard Model, adding up to other intriguing hints of a non-standard Higgs behavior!
Second, the value of Higgs at 125 GeV is not "right". This is not SM Higgs as it is just too heavy. In reality, 125GeV sits exactly where the Minimal Supersymmetric Standard Model allows the Higgs boson to sit but it sits outside the interval that allows the Standard Model to be a complete and consistent theory of all non-gravitational interactions in nature.
And then few hours before the main event, CERN conference, video leaked out. Trailer if you want. Video leaked with CMS spokesman Joe Incandela talking about what they have seen. Cern say that this is one of several videos they have made, one for each of the possible outcomes, as though it's a presidential election and they've written one speech for victory and one for defeat. That sounds a bit odd. In video, we can hear Joe saying following:
We've observed a new particle. We have quite strong evidence that there's something there. Its properties are still going to take us a little bit of time. But we can see that it decays to two photons, for example, which tells us it's a boson, it's a particle with integer spin. And we know its mass is roughly 130 times the mass of the proton. And this is very significant. This is the most massive such particle that exists, if we confirm all of this, which I think we will.
And this is very, very significant. It's something that may, in the end, be one of the biggest observations of any new new phenomena in our field in the last 30 or 40 years, going way back to the discovery of quarks, for example. We see very, very strong evidence of the decay to two photons, and a very very narrow peak in the distribution. We see also the evidence of the decay to two Z-particles, which are like heavy photons, in this particular theory of elementary physics. And then we've studied the number of other channels that have reported, but these are less sensitive and are therefore less conclusive at the moment. But we are very excited. I'm extremely tired at the moment, so I may not appear to be as excited as I really am, but the significance of this observation could be very very great.
It could be ultimately seen that its properties are very consistent with the Standard Model Higgs, or it could be found out that its properties don't exactly match the predictions for the Standard Model. And if that's the case, then we have something really quite profound here. It could be a gateway, if you like, to the next phase of exploring the deepest fabric of the universe, which is pretty profound when you think about it.
And the other thing I would like to say is that obviously all of this is extremely preliminary. What we've looked for is a few grains of sand on a beach, in one sense. I did some calculations, and if you replaced every event, every collision of the beams that we've scanned or had take place in our experiment over the last two years, if you let each one of those be represented by a grain of sand, you'd have enough sand to fill an Olympic-sized swimming pool. And the number of events that we've collected now that we claim represent this observation are on the order of tens, or dozens. So it's an incredibly difficult task, and it takes a lot of care and cross-checking. We're re-calibrating, and we'll have better results, even on the current data, when we release at the end of the month. But it's very exciting.
First thing I found exciting here is that Joe talks about something 130 mass of proton. That's around 125 GeV (it is really 133 times the proton mass). He does not claim this really is Higgs - he is careful in choosing words and warns all this data is preliminary. In second part of video, not quoted above, Joe goes to say we need to study further properties of this particle and warn this may be Higgs-like particle. He leaves open this new particle may lead us to SUSY or extra dimensions. Yeehaw!
In 7 hours in 30 minutes show starts. I do not expect Higgs boson to be confirmed, but rather some particle which may be Higgs boson, but not SM Higgs. That would be consistant with everything said so far on this subject. And do not get surprised when they say it will take couple of years to figure it out when it comes to all properties (though Joe believes they may have something until the end of the this year) - this has been said before - even last year by pretty much everyone in this business. If you thought your life would suddenly change and all would be known then you need reality check and stop following mainstream media.
Credits: Fermilab, CERN, Philip Gibbs, Sean Carroll, Lubos Motl