Gravity in the Next Dimension: Micro Black Holes at ATLAS

What would gravity look like if we lived in a 6-dimensional space-time? Models of TeV-scale gravity theorize that the fundamental scale of gravity, M_D, is much lower than what’s measured here in our normal, 4-dimensional space-time. If true, this could explain the large difference between the scale of electroweak interactions (order of 100 GeV) and gravity (order of 10¹⁶ GeV), an important open question in particle physics. There are several theoretical models to describe these extra dimensions, and they all predict interesting new signatures in the form of non-perturbative gravitational states. One of the coolest examples of such a state is microscopic black holes. Conveniently, this particular signature could be produced and measured at the LHC!

Sounds cool, but how do you actually look for microscopic black holes with a proton-proton collider? Because we don’t have a full theory of quantum gravity (yet), ATLAS researchers made predictions for the production cross-sections of these black holes using semi-classical approximations that are valid when the black hole mass is above MD. This production cross-section is also expected to dramatically larger when the energy scale of the interactions (pp collisions) surpasses MD. We can’t directly detect black holes with ATLAS, but many of the decay channels of these black holes include leptons in the final state, which IS something that can be measured at ATLAS! This particular ATLAS search looked for final states with at least 3 high transverse momentum (p_t) jets, at least one of which must be a leptonic (electron or muon) jet (the others can be hadronic or leptonic). The sum of the transverse momenta, is used as a discriminating variable since the signal is expected to appear only at high p_t.

This search used the full 3.2 fb^-1 of 13 TeV data collected by ATLAS in 2015 to search for this signal above relevant Standard Model backgrounds (Z+jets, W+jets, and ttbar, all of which produce similar jet final states). The results are shown in Figure 1 (electron and muon channels are presented separately).  The various backgrounds are shown in various colored histograms, the data in black points, and two microscopic black hole models in green and blue lines. There is a slight excess in the 3 TeV region in the electron channel, which corresponds to a p-value of only 1% when tested against the background only hypothesis. Unfortunately, this isn’t enough evidence to indicate new physics yet, but it’s an exciting result nonetheless! This analysis was also used to improve exclusion limits on individual extra-dimensional gravity models, as shown in Figure 2. All limits were much stronger than those set in Run 1.

So: no evidence of microscopic black holes or extra-dimensional gravity at the LHC yet, but there is a promising excess and Run 2 has only just begun. Since publication, ATLAS has collected another 10 fb^-1 of sqrt(13) TeV data that has yet to be analyzed. These results could also be used to constrain other Beyond the Standard Model searches at the TeV scale that have similar high pt leptonic jet final states, which would give us more information about what can and can’t exist outside of the Standard Model. There is certainly more to be learned from this search!

References and further reading:

• CERN Physics Briefing: Extra dimensions, gravitons, and tiny black holes
• Douglas M. Gingrich, “Collider searches for non-pertubative low-scale gravity states” (arXiv:1509.07180)
• Elizabeth Winstanely, “Black holes, TeV-scale gravity, and the LHC” (arXiv:1206.5409)
• M. Shifman, “Large Extra Dimensions: becoming acquainted with a an alternative paradigm” (arXiv:0907.3074)

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