|The energy imbalance (represented by ET with a slash through it) occurs whenever a Z boson decays into two neutrinos at the same time that a Higgs boson decays into two quarks (represented by the two jets).|
We can search for the Higgs boson in various ways. To search for it directly, particle physicists look for its decays into pairs of particles such as W+W–, ZZ or two photons. The combined mass of the pair is that of its parent particle, the Higgs boson. The discovery of the Higgs-like particle last summer at CERN was achieved in this way.
But the Higgs boson also decays into a pair of quarks, which look like messy sprays of energy in the detector called jets. Unfortunately, this type of search is much more difficult because this process is overwhelmed by noise from other, less interesting collisions produced by the strong force. Gaining access to Higgs-to-quarks decays, therefore, requires some tricks.
The first trick is to look for the Higgs boson indirectly. That is, we can look directly for the W or Z boson, which sometimes is produced alongside the Higgs particle. This doesn’t happen a lot—only about 10 to 20 percent of all Higgs bosons are produced in association with another particle. However, when it does happen, the signatures made in the detector are distinct enough that analysts can leverage the uniqueness of the collision to discriminate against the overwhelming noise.
So what’s the unique characteristic we’re looking for? That’s where the second trick comes in. A team at CDF has performed a search where the Z boson decays invisibly at the same time the Higgs boson decays into two quarks. To spot an invisible decay, one looks for what appears to be a “hole” in the detector. This is an area where you expect to see activity but see nothing. This so-called energy imbalance arises whenever a Z boson decays to a pair of neutrinos, which do not interact with the detector. By searching for such holes and by using an optimized procedure (an improvement of 14 percent over our previous analysis for identifying jets that originated from the Higgs boson decay), the CDF team was able to produce some of the most sensitive limits on the production of the Higgs boson when it decays into the bb final state.
—edited by Andy Beretvas