Tag Archives: CERN

The LHC Starts 13 TeV Collisions

Collisions at the LHC have started again and are now at a center of mass energy of 13 TeV for protons on protons. This was the target energy for the upcoming physics run this year, so this means that the accelerator group was successful in getting everything up to the desired energy. It’s still a little bit lower than the design energy of 14 TeV, but is much better than the 7-8 TeV that the initial runs used. The extra energy allows for a lot of analyses to be done better. Searches for exotic particles can reach a wider range of masses. While cross sections for most processes fall with energy, it is also the case that for some important analyses the backgrounds fall faster than the signal. Thus, a better measurement can be made with lower statistics because the sample has a better purity.

Collisions Restart at the LHC

The LHC has started proton-proton collisions again as the accelerator is prepared for its upcoming physics run at 13 TeV. The beams are just being kept at the injection energy of 450 GeV, but runs like this are probably important for both getting the detectors ready and also helping tune the beam so that things go smoothly when they start ramping up the energy.

New CMS Search for a Beyond the Standard Model Higgs

The CMS collaboration posted a preprint for a search for a beyond the Standard Model Higgs boson decaying into pairs of intermediate vector bosons (Ws and Zs). In many common extensions of the Standard Model, such as in the MSSM (Minimal Supersymmetric Standard Model), there is an expanded Higgs sector that includes a number of scalar and pseudoscalar Higgs bosons. The extra Higgs bosons can have electric charges (and can even be doubly-charged).

This search looks for neutral Higgs bosons with masses that are quite a bit higher than the Higgs found a couple years ago. The analysis looks at several final states from Higgs decays to WW and ZZ, such as ZZ to four charged leptons. As usual, the general feature expected from a new particle is a bump in the invariant mass spectrum compared to the Standard Model prediction. The results are consistent with the Standard Model, so they set a limit on the cross section for these events, and can exclude a heavy Higgs up to around a 1 TeV mass (obviously this requires some assumptions on the couplings to other particles, so this isn’t a general exclusion for all scalar particles).

LHC Back On

Earlier today, the beam people at CERN were able to successfully inject protons beams going both directions in the LHC at 450 GeV. This is the first time the beam has been circulated in the accelerator since the long shutdown started in 2013. Their job over the next couple months will be to increase the energy and to increase the number of bunches until everything is ready for physics running at the detectors (collisions at 6.5 TeV per beam).

LHC To Restart This Weekend?

CERN is reporting that the Large Hadron Collider is ready to resume running. There was a minor problem with an electrical connection that has been fixed, so hopefully everything is ready for collisions at a center of mass energy of 13 TeV, just below the design energy of 14 TeV.

The beam is expected to start this weekend, as long as no other problems arise. Physics beam won’t happen quite yet though. It sounds like there is going to be an extended period of beam tuning, with actual collisions at full beam energy starting in June.

Higher energy allows for a great deal of new work to be done. In some important analyses, such as Higgs searches, the background can fall faster with energy than the signal, boosting the overall statistical power of results. Furthermore, higher energy increases the possible reach of searches for new physics. Many hope for new physics at the TeV scale in order to avoid fine-tuning in the electroweak sector of the Standard Model (such as from heavy quark loop corrections to the Higgs mass). In collisions at these energies, the protons can’t be treated as individual particles, but rather than collections of quarks and gluons (collectively called partons in this context). While the center of mass energy of a proton-proton collision will be 13 TeV, the actual amount of energy accessible in a collision will be smaller. If, for example, we take two typical quarks with about 30% of the total momentum, the center of mass energy would instead be around 4 TeV. Gluons typically carry even less energy (the actual momentum distributions of quarks and gluons in a proton are called “parton distribution functions”). So, in order to access TeV-scale physics, it would be useful to have an overall energy around an order of magnitude higher.

The BBC on the LHC

The BBC recently published a post by one of its science editors entitled “What is the point of the Large Hadron Collider?” The author goes over several justifications for such large projects. There is the idealistic view: Big science projects are important for improving our understanding of the universe, which to many is important to our society in its own right. There is also the pragmatic view: We don’t know what will come out of the research but at the very least many important inventions and a lot of technological progress have been made as a result of such projects in the past.

I would say that both points of view are compelling. The latter is obviously important, but the former is also a good point to make. A great deal of what people devote resources to in modern society is largely extraneous. We already spend a great deal of money on things that we want but don’t actually need. I would say that large science projects are a way to devote a tiny fraction of our resources to projects that in some way advance human society. Science, like literature, music, visual arts, theater, architecture, cuisine, and many other things, is an important part of what makes our society and our culture what they are today.

New Combined ATLAS/CMS Higgs Mass Result

The ATLAS and CMS collaborations have finally released a joint result on their Higgs analyses. The paper uses the two channels where the final state invariant mass is most easily calculated and where backgrounds are expected to be small (Higgs to two gammas or Higgs to four charged leptons). These two channels don’t have have very many signal events, which limits the ability of the two experiments to make a precise measurement.

Combining results from different experiments is always tricky, since the analyzers have to be sure that systematic uncertainties are treated properly. Some systematics will be completely separate for each experiment, while others may be correlated. Regardless, the new result is mH = 125.09±0.24 GeV. The measurement is still statistics-limited, but is getting closer and closer to being limited by systematics. This is quite good, but still nowhere near the kind of precision to which the Z and W masses are known. To do that at the LHC, the experiments would need to improve systematics by more than an order of magnitude as well as collect many times more data. If we realistically hope to know the Higgs mass as precisely as for the W and Z bosons, we’ll need an e+e Higgs factor collider (i.e. the ILC or something similar).

Grad Student Designs Lego LHC Model

An intrepid graduate student on one of the experiments at the Large Hadron Collider at CERN has proposed an LHC Lego set. The proposed set includes tiny models of all four of the principal LHC experiments (ALICE, ATLAS, CMS, and LHCb) as well as some miniature beamline models.

You can find the set at Lego’s Ideas page. If it can attain 10,000 supporters by February 2016 then Lego will review the proposal for possible approval and sales.