Vector Boson Fusion

Question

I have been reading about the production mechanisms for the Higgs at the LHC. It is always mentioned that for Vector Boson Fusion, the initial quarks cause jets that are back to back and with a higher transverse momentum pT than jets in other processes. Is there a kinematic reason for why the jets have such a high pT? Is it because the initial quarks have high pT due to the large energies at the LHC?

Answer 1

I would have thought that it's because (1) the initial state quarks are in the final state and (2) they each must have a very large momentum transfer in order to produce an on-shell Higgs. Small momentum transfer collisions won't put enough energy into the (virtual) VB's that fuse to make the Higgs. That large momentum transfer is going to produce large pT's.

There's a nice set of diagrams here . The only one with both quarks in the final state is the VBF one, so the large pT would distinguish those events.

(Pure speculation on my part on much of this. I haven't done phenomenology for 25 years.)

Answer 2

Don't quote me on this just yet (I want to check some sources), but my thought would be that it's because the VBF cross section is most dominant around the W and Z resonances which are at $80\text{ GeV}$ and $91\text{ GeV}$ respectively. That's a lot of energy tied up in those bosons, and in turn in the Higgs that they collide to produce. So when that Higgs decays, it's going to produce particularly high-momentum jets.

In other Higgs production processes, you wouldn't get the resonance around the large masses of the vector bosons because the virtual particles involved aren't so massive. The exception would be gluon fusion with a top quark loop, but that's also suppressed by the low gluon density at large Bjorken $x$.