It was reported last week that the LHC will be shut down for a few weeks due to a weasel gnawing through a power cable. The weasel, as you might expect, did not survive.
The two big LHC experiments ATLAS and CMS just released some of their first results from LHC Run 2, which is the newest run and the first at close to the design energy of 14 TeV.
I missed streaming the talks live and haven’t had time to track down the videos or slides yet, but everyone is buzzing about one thing in particular: a small excess seen in both experiments in the diphoton channel at a center of mass energy near 1.5 TeV (note: going over the slides now and it looks like Nature has embarrassingly completely misunderstood the plots. The peak is at 750 GeV). The result is quoted at a few sigma significance for each experiment, so the combined result won’t be high enough to show anything definitive at will be lowered somewhat by the “look elsewhere” effect, where the true significance of an excess in such a global search is actually lower than the apparent local significance (i.e. if you make enough measurements you’ll always find some large deviation from your expectation).
A peak in the diphoton channel potentially points to a new heavy neutral boson. This could be one of the many predicted bosons in various models of beyond the Standard Model physics or it could be something completely new. If the result holds up and it really is some new elementary particle (it’s far heavier than even the top, so if the excess is real, it’s probably not a bound state of known particles), it would be very exciting news for the field. That would be the first truly new particle found since the development of the Standard Model. Figuring out what it is and where it comes from would then clearly be one of the highest priority tasks for experimentalists over the next few decades.
There still aren’t any signs of supersymmetric particles like squarks and gluinos, making it even harder to fit minimal supersymmetry with current measurements.
In what may be the biggest non-Higgs news yet to come out of the LHC, the LHCb experiment has a brand new preprint reporting on a measurement of what looks like a couple new resonances consistent with pentaquarks – a theoretical particle that appears as a bound state of four quarks and an antiquark (or four antiquarks and a quark). The paper has been submitted to PRL, which in high energy physics is the most prestigious journal. Hopefully everything checks out and this really is a new discovery.
The analysis looks at decays of the neutral Λb baryon, which has a quark content of udb, a mass of 5.6 GeV and a lifetime of 1.4 ps. One of its more prominent decay modes is to a J/ψ (charmonium meson) and a regular Λ baryon, which generally decays to a pion and a nucleon. This paper looks at a similar final state of Λb→J/ψ K p. The presence of the kaon means that the Kp system could not be from a Λ, since a Λ isn’t heavy enough. They also only consider the case where the J/ψ decays to muons only, allowing its invariant mass to be reconstructed very accurately.
LHCb shows that they can reconstruct the invariant mass peak of the Λb using the three hadron final state. Where things get interesting is the two-particle invariant masses. Instead of looking at all three particles, they look at the invariant mass of the Kp system and the J/ψ p system. The Kp system shows many different peaks and significant deviation from the expectation just from random throws in phase space. This is expected since many excited Λ resonances exist that can decay into this mode. In the J/ψ p no such peaks are expected. However, there is a prominent bump in the spectrum around 4.4-4.5 GeV that looks very statistically significant. LHCb finds that the two spectra are modeled pretty well if they add two new resonances, one at 4450 MeV and a wider one at 4380. Looking in more detail at these peaks, they also find that things are best modeled by particles with total spins of J= 3/2 and 5/2 (apparently they can’t really tell which particle has which spin) and with opposite parities. Since everything here appears to point to new particle, they have been labeled Pc(4450)+ and Pc(4380)+. The widths indicate that these particles have lifetimes of order 10-23 s or so, which is indicative of strong decays. Strong decays preserve the quark content, so it seems likely that these particles would need to be pentaquarks with a quark content of uudc̅c.
Physicists thought that they found pentaquarks in the 2000s, but further studies suggested that that discovery was really just a statistical fluctuation. I think that this past experience means that we should be careful before trumpeting a definitive discovery, but this paper looks pretty convincing to me (I’m not an expert on precision b physics though). If this really is a new discovery, then we should continue to see better and better precision as more data is taken. If Λb particles can be created in large numbers in a lower energy collider such as one of the b factories, that might be the best way to really pin down the properties of this possible new class of particles.
A new preprint has appeared on the arXiv detailing the current status of searches for supersymmetry in light of the results from LHC Run 1. The preprint appears to be a chapter from a recent review of Run 1. The authors go over the various results that potentially relate to SUSY searches and what they mean for theory. In short, while SUSY hasn’t been found,, there are still plenty of ways to fit SUSY into current data. The constrained MSSM, which is the most common benchmark model for SUSY, is basically totally ruled out at this point.
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.
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.
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).