By CMS Collaboration

The standard model (SM) of particle physics is by far the most successful theory describing the experimental observations of particle interactions. Most of the SM interactions preserve the “flavor” of the elementary particles, with the exception of the exchange of the heavy, charged W boson. However, a variety of theoretical models that aim to extend the SM predict the existence of a new heavy, neutral boson (Z’) decaying to lepton (𝓁) pairs, with a mass near the TeV scale, i.e., about one thousand times heavier than protons. Such a new particle would have similar properties to the well-known SM Z boson, which is also a heavy neutral boson about 90 times heavier than the proton, but would play a role in new interactions, not foreseen by the SM itself. Among these models, scenarios where the new Z’ boson interacts with beauty (b) and strange (s) quarks (like those depicted in Fig. 1) are of particular interest because the existence of such a Z’ boson may have implications in low-energy b → s𝓁𝓁 interactions. Indeed, recent measurements of these interactions at the LHC might be interpreted as hints for the presence of physics beyond the SM, as also described in a recent newsletter of the CERN Experimental Physics Department.

Feynman diagram Feynman diagram

Figure 1:  Example Feynman diagrams of a new Z’ neutral vector boson produced in association with (left) two b quarks, via bb → Z’, or (right) one b quark, via sb → Z’, and decaying to a pair of oppositely-charged muons (µ+µ). The Z’ boson couples to the bb (sb) quark pair with a strength proportional to gb (gb δbs), and to the µ+µ pair with a strength proportional to g𝓁.

A Z’ boson produced in association with at least one b quark and decaying into a pair of oppositely-charged muons would leave a unique signature in the CMS detector. In events with two highly energetic muons and at least one jet arising from the b quarks involved in the Z’ production mechanism, like the event shown in Fig. 2, the new boson would appear as a narrow “bump” in the reconstructed dimuon invariant mass distribution, on top of the SM processes, that we hereby refer to as our “background”.

Figure 2: Event produced in proton-proton collisions at a center-of-mass energy of 13 TeV, collected with the CMS detector. The event contains two oppositely-charged muons with large transverse momenta (red lines), and two b quark jets (cyan cones). The missing transverse energy is represented by the pink arrow. The green lines represent tracks from charged particles. The energy deposits in the electromagnetic (ECAL) and hadronic (HCAL) calorimeters are represented by orange and blue towers, respectively: the length of a tower is proportional to the amount of deposited energy. The signature of this event is consistent with the production of a new neutral vector boson (Z') with a mass of about 2.4 TeV in association with two b quarks, with the Z' boson then decaying to two muons. Try to rotate and zoom!

The mass of this potential new Z’ boson is unknown, which makes the hunt for such a bump a very difficult task. Furthermore, previous results from the CMS experiment suffered from the large background arising from SM processes involving the top quark, which can lead to a similar signature in the detector and have a much higher probability than the potential new physics signal.

This search adopted a strategy aimed at suppressing this source of background as much as possible while probing a wide range of signal Z’ mass hypotheses. For this purpose, we exploited the fact that the invariant mass distribution obtained from the reconstructed top quark decay products has an endpoint at the top quark mass value. As a result, we were able to reduce this background by more than a factor of 300, while retaining most of the predicted signal for Z’ mass hypotheses larger than a few hundred GeV.

The residual SM background contribution was estimated directly in data, by fitting the reconstructed dimuon invariant mass distribution in regions centered around the signal Z’ mass hypothesis being probed. The signal, if present, would have appeared as a narrow bump at the center of the window, as depicted in Fig. 3.

dimuon mass

Figure 3: Dimuon invariant mass (mμμ) distributions in events with one b quark jet. The dimuon invariant mass distribution as obtained in data (black circles) is shown together with the functional forms used to fit the SM background, and the distribution expected for an hypothetical signal Z’ with a mass of 500 GeV. The signal distribution is scaled by a factor of 0.5 to improve visibility.

The search did not show any bump, nor any significant excess, that would hint at the presence of new physics.  Therefore, we used the results to set constraints on specific models that feature the presence of a narrow Z’ boson dimuon resonance, like the model considered in Fig. 4, as well as model-independent constraints aimed at facilitating the work of our theory colleagues, since they can be more easily reinterpreted in terms of any model that predicts a new heavy neutral particle decaying into a pair of muons and produced in association with b quarks.


Figure 4: Observed (solid) and expected (dashed) exclusion limits at 95% confidence level in the gbg𝓁 plane for a Z’ model like the one depicted in the diagrams of Fig. 1, for δbs= 0.25 and several Z’ mass hypotheses. The exclusion curves extend only up to coupling values at which the Z’ width is equal to half of the dimuon invariant mass resolution, marked by the dotted curves. Beyond these coupling values, the assumption that the Z’ width is narrow with respect to the dimuon invariant mass resolution is not considered valid. The enclosed regions are excluded.

Knowing that there is no Z’ boson in the explored phase space will help steer the searches for new physics that CMS will perform in the future!

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