The CMS collaboration at the CERN Large Hadron Collider has pulled off an extremely neat new measurement of the Higgs boson production rate - one which, for some reasons, is extraordinary in its own right.

Despite being the decay mode with the highest probability (two thirds of Higgs bosons die that way), the H->bb process is among the most elusive to put in evidence in LHC data, because b-quarks are quite commonplace there. 
A proton-proton collider is effectively a quark-gluon collider, and quarks and gluons feel the strong force, which physicists call "quantum chromodynamics", QCD. The "chromo" there alludes at the imaginative description of quarks and gluons as "coloured" objects, and their colour being the attribute that the force holding them together is sensitive to. Colour is the "charge" that QCD feels.

In subnuclear physics, the strength of an interaction is connected to how likely it is that it takes place. An electrically charged particle feels an electromagnetic fields, or generates its own, by absorbing or emitting photons. The emission/absorption process occurs with a certain probability, which physicists encode in a quantity called "coupling strength". For electromagnetic forces, the coupling strength equals 0.007 or so, but for low-energy chromodynamical interactions it can be as large as 0.4: coloured particles thus interact much more strongly. 

The interaction strength reflects in the probability that some process takes place when you collide particles. If you have quarks and gluons as projectiles, the interaction that will take place is almost always a QCD one, and the produced particles in the final state of the reaction are quarks and gluons, as these are the particles that QCD produces. b-quarks, which are heavier than the up and down quarks making up most of the nuclear matter, are no exception - they get produced with very large rates in proton-proton collisions.

Now, if you are to find a Higgs boson decay into b-quark pairs in LHC collisions, you want to select collisions that produce b-quark pairs. Unfortunately, because of its strength, QCD produces them at a rate which is ten millions times higher. Do we stand a chance to say "hey, this b-quark pair looks more like a Higgs decay to me" ? No, we don't. Unless...

We do not actually need to accept all b-quark pairs in our selection. We can in fact restrict ourselves to a narrow region of phase space, where the two b-quarks are so energetic that they travel close in angle to one another. Few Higgs decays are produced in this "boosted" regime, but they are comparatively much more frequently in that topology than b-quark pairs produced by regular QCD reactions. That's what CMS went for in the 2016 data.

Such a search would be impossible if it weren't for the recent advancements in the reconstruction of heavy particle decays producing two quarks inside single, energetic hadronic jets. What you have is a W boson, or a Z boson, or a H boson decaying into two quarks, the two quarks traveling very close in angle, then dressing up as messy hadronic jets, and landing close together in some region of the calorimeter system of your detector. Until 10 years ago, that mess of hadrons would only be catalogued as a single jet - no chance to discern in it the signal of the decay of a heavy boson. But recently it has been shown how to "prune" the jet signal components and evidence the two "sub-jets" that come from the heavy decay.

By using these techniques, and many other tricks, CMS can show how it is possible to see W and Z bosons in non-b-tagged jets, as you can see in the reconstructed mass spectrum below.

(The upper panel shows the data (black points) compared with backgrounds plus vector boson decay signals in blue; the lower panel shows what happens when you remove the background component - only the vector boson signals remain).

If you were unfamiliar with hadronic resonance searches, all the reaction the graph above might have extracted from you might be "Oh, ok, there's a bumplet there. Cool." But if, like me, you've spent twenty years of your life going after these elusive signals, you cannot but be awed. Extracting these signals from the QCD background is HARD! My PhD thesis, published in 1999, showed how it was possible to see a small signal of Z->bb decays in CDF data, and it was the first time one could see that process in a hadron collider. But that signal was way less prominent than the beautiful combined signal of W and Z decays to quarks above!

Now, what if we apply b-tagging to the sub-jets ? The result is shown below. Not surprisingly, the Z->bb signal is now prominent, and the W decay signal is almost all gone (W bosons do not yield b quarks in their decay, only lighter ones). But the histogram also shows that there is some sensitivity to boosted H->bb decays now!

Using the above data, CMS was able to measure that the Higgs boson production rate in this decay channel is compatible with expectations from the Standard Model, the ratio of the two quantities being assessed at 2.3+1.8-1.6 (it should be 1.0 if the SM is correct). Is that cool or not ?

For more information, please see the CMS publication with details of this new analysis here.


Tommaso Dorigo is an experimental particle physicist, who works for the INFN at the University of Padova, and collaborates with the CMS experiment at the CERN LHC. He coordinates the European network AMVA4NewPhysics as well as research in accelerator-based physics for INFN-Padova, and is an editor of the journal Reviews in Physics. In 2016 Dorigo published the book “Anomaly! Collider physics and the quest for new phenomena at Fermilab”. You can purchase a copy of the book by clicking on the book cover in the column on the right.