Searching for new physics with Z bosons

Nearly twice as many events as expected occured when the Z boson decayed into four leptons.

Nearly twice as many events as expected occured when the Z boson decayed into four leptons.

The search for physics beyond the Standard Model has been frustrating. Even the opening of the new energy frontier at the LHC has not yet revealed obvious new phenomena, demonstrating the robustness of the current Standard Model. Yet, we know this model is incomplete and so the searches go on. Now, we’re taking a closer look at the data we already have in hand to try to find a chink in the armor.

Not knowing where that chink might lie, we make assumptions and probe. For instance, we might expect new heavy particles to decay preferentially to the heaviest known particles around. For example, in some models, gravitons can decay preferentially to Z boson pairs and so may have been missed by previous searches at the Tevatron and LHC. Events with Z pairs have distinctive signatures, so CDF physicists performed a search for events compatible with the decay of some new heavy particle.

A Z boson can decay into a pair of charged leptons, or into a pair of neutrinos, or into a pair of jets. We concentrated on three distinct final states of the Z boson pair. In each case, one of the Z bosons decays into a pair of electrons or muons. In the first case, the other Z also decays into a pair of charged leptons, also known as the four-lepton mode. In the second case, the other Z decays into a pair of neutrinos, and in the third case, it decays into a pair of jets.

Limits on the production rate of a new heavy particle decaying to a pair of Z bosons, as a function of mass.

Limits on the production rate of a new heavy particle decaying to a pair of Z bosons, as a function of mass.

In the four-lepton channel, we observed eight events, when 4.9 were expected. Unexpectedly, the Z pairs in four of the events had invariant mass around 325 GeV (see the top graph). It was surprising to see such large masses, and that the masses of each Z pair were so similar. Could this be a long awaited signature of a new particle?

What makes physics powerful is that the complete picture must be consistent. If the events observed in the four-lepton final state were from a new physics source, we would expect to see the same phenomenon in the other, higher-statistics, final states. When we looked at the second and third Z pair final states, we found 27 and 645 events when the expected yields are 26 ± 2.4 and 690 ± 47 respectively. Furthermore, the mass distributions showed no peaks corresponding to a mass of 325 GeV/c2.

Aidan Robson, University of Glasgow; Victoria Giakoumopoulou, University of Athens; and Pavel Murat, Fermilab, carried out the analysis.

Aidan Robson, University of Glasgow; Victoria Giakoumopoulou, University of Athens; and Pavel Murat, Fermilab, carried out the analysis.

Sadly, we concluded that the four-lepton events did in fact originate from Standard Model sources and we set limits on the production of new heavy particles using all three channels together (see the bottom graph). The pursuit of a sign of physics beyond the Standard Model goes on.