Top partners

Many extensions of the Standard Model predict that the top quark has a heavier partner. Together, these particles control unreasonable predictions of the mass of the Higgs boson made by the familiar Standard Model.

One consequence of quantum mechanics is that particles can change their identity, as long as they don’t do it for very long. As an example, the recently discovered Higgs boson can temporarily morph into a top quark-antiquark pair before recombining into a Higgs boson. The particle is then free to turn into pairs of Z bosons, W bosons and (probably weirdest of all) Higgs bosons. This behavior has been established for all known subatomic particles. So far, so good.

While this frenzied morphing is common, the impact it has on something like the mass of the Higgs boson is less clear. It turns out that this behavior is predicted to drive the mass of the Higgs boson to values far, far, higher than have been measured. Since the measurement is quite precise, the mismatch between prediction and what is observed is perhaps the single most pressing question facing LHC physicists and almost guarantees that the LHC will discover something completely new. If it doesn’t, it may be that the Standard Model of particle physics will need rethinking.

Given the incredible success of the Standard Model, it is theoretically popular to devise additions to the known theory that will tame the crazy predictions of quantum mechanics. Two of the most popular ones are to invoke supersymmetry and models in which the Higgs boson consists of smaller particles still. We need to remain open to all credible solutions.

In supersymmetry, known particles in the fermion family have undiscovered cousins in the boson family and vice versa. For instance, the fermionic top quark is predicted to have a bosonic stop squark. These two particles conspire together to cancel the huge mass of the Higgs boson predicted by the traditional Standard Model.

The idea that a Higgs boson is composed of other particles also predicts a partner of the fermionic top quark, but unlike a supersymmetric partner, it is also a fermion, not a boson. This model also says that the top quark, which has an electric charge of +2/3, has a partner with electric charge of +5/3. A particle with charge of +5/3 has never been observed, but such a particle is expected to decay in a striking way: The top partner would decay into a top quark and a W boson. Since these top partners would be produced in pairs, such an interaction would result in events with a top quark-antiquark pair and a W+W- pair.

CMS physicists have looked for events with these characteristics. No evidence for them was found. Researchers were able to establish that if these exotic top partners exist, they have a mass more than 800 times that of a proton. This limit is more than twice as large as the best previous measurement.

Don Lincoln

These US CMS scientists contributed to this analysis.
The CMS detector has been running with a minimal muon detection system, which was nevertheless instrumental in the discovery of the Higgs boson. In order to increase redundancy in the future, additional detectors have been added to the existing cathode strip chamber system. The US CMS people pictured here have made important contributions in manufacturing these extra detectors.