Hi All.

In case you have been away from the Wonderful World of Physics for the past few weeks there is now evidence for the Standard Model Brout­-Englert­-Higgs Boson, with a mass of approximately 125 GeV/c², from the ATLAS, CMS, CDF, DZero, and the combined CDF+Zero experiments [Moriond 2012 Conference, FNAL press release]. This is really exciting, and measurements of Higgs-related processes will definitely have a profound impact on the viability of Beyond the Standard Model theories like supersymmetry and technicolor.

Enough about Higgs, though. Of the many, MANY reasons for constructing the Large Hadron Collider and the Detector Experiments, one of my personal favorites is

to search for evidence of quantum gravity in TeV-scale proton collisions.

We know pretty well that gravity exists. (If you have issue with this, buy two apples and while eating one let go of the other.) We also know things like electrons, muons, & photons exist. (Flip on a light switch or buy a Geiger counter.) What we are less sure about is how, on an elementary level, are electrons, muons, & photons affected by gravity?

Over the past few decades, there has been a ton of research investigating this very question, resulting in very fruitful and fascinating discoveries. For example: black holes can radiate photons and other gauge bosons by interacting with particles that have spontaneously been produced through quantum mechanical fluctuations. This is the famous Hawking radiation (See Fig. 1) [3]. Two other examples that come to mind both attempt to explain why gravity appears to be so much weaker than either the strong nuclear force (QCD) or the electroweak force (EWK). Their argument is that all Standard Model particles are restricted to three spatial dimensions, whereas new physics, include quantum gravity, exists in more than three spatial dimensions. The difference between the two theories is that the Large Extra Dimensions (or ADD) model supposes that all additional spatial dimensions are very small (<10^-20 cm) but that each dimension is not too difference from what we experience everyday (See Fig. 2) [4,5]. The Randall-Sundrum model, on the other hand, proposes that there exists only a single extra dimension but that this spatial dimension is “warped” and unlike anything we have ever experienced [6,7]. I have not even mentioned string theory, but I am sure you can imagine that the list goes on for a while.

Figure 2: In the ADD (Large Extra Dimension) model, an electron (e-) and positron (e+) may annihilate and produce a graviton (G) and photon (γ). A defining feature is that the Standard Model particles (e±,γ) are restricted to the move in 3 spatial dimensions, whereas the graviton may propagate in additional dimensions.

Microscopic Black Holes

Despite the number of models trying to describe gravity at the most elementary level, there is actually a phenomenon that is surprisingly common to most all of them: they all predict the existence of microscopic black holes, or at least something very close to it. Now here is where I can easily dig myself a hole, so I want to be clear. The black hole-like objects these models predict are vastly different from the star-devouring black holes we have grown to know and love. Those exist at the center of galaxies and other places like that. The most obvious difference is that astronomical black holes are, well, astronomically huge. The black holes that I am talking about, if they exist, are significantly smaller than a proton.  The term “microscopic” makes these things sound much bigger than they are. Secondly, the masses of micro-black holes are comparable to the energy of the LHC; consequently, they will evaporate (via Hawking radiation) and disintegrate (decay) within moments of being produced. In the off chance that a stable micro-black hole is generated, then after about 10^-25 seconds the thing will decay and burst into a blaze of  glory quarks & gluons (See Figs. 1 (above) & 3 (below)). Research has also concluded that these things are harmless and CERN has gone out of its way to inform the public of this.

Figure 3: "-->--" is the path the microscopic black hole travels (exaggerated) while evaporating, before decaying. Click to enlarge.

Admittedly, the fun part of writing this post was trying figure out a way to describe just how a microscopic black hole event, if it existed, would look in an LHC collider detector. Hawking radiation is straight forward enough to draw (Fig. 1), but things are a bit more involved when you want to show that some of those photons and Z bosons decaying into, say, electrons and positrons. So I got a little carried away and drew things by hand. Figure 3 shows a “typical” a micro-black hole, if they exist, briefly zipping around the detector radiating photons (γ), Z’s, W±’s, and gluons (g), before bursting into a bunch more bosons all at once. These bosons will then do whatever particles normally do in a particle detector and make a mess (shower and hadronize). A very distinguishing feature that I want to highlight is the number of particles that are produced in a single micro-black hole event, this is called particle multiplicity. If they exist, then the average micro-black hole event will result in a very high multiplicity (number) of final-state particles.

This is really important because in a typical proton-proton collision, things are not as busy. To clarify: plenty of things happen in proton collisions; micro-black hole events are just a bit busier. When protons collide, only two or three primary particles are produced and these then decay in predictable ways. In addition, the incident protons fragment and hit the side walls (“end caps”) of the detectors.

Figure 4: Typical proton-proton collision at the Large Hadron Collider as seen from a Detector Experiment. Click to enlarge.

This is it though. This is how experimentalists test whether these gravity-motivated theories correctly describe nature. What differentiates microscopic black hole events from any other proton-proton event is the number of final-state particles seen by the detector. In other words: particle multiplicity! There are not too many Standard Model processes that will result in, say, 10~15 final-state particles. If suddenly a experiment group sees a bunch of 15-particle events, then more refined searches can be performed to determine the root cause of this potential signal of new physics.

Recent Results from ATLAS and CMS

The most recent results from the ATLAS and CMS Experiments on their searches for microscopic black holes are both from March 2012. In these papers, ATLAS reports using 1.3 fb^-1 of data, which is the equivalent of 91 trillion proton-proton collisions; CMS reports using a whopping 4.7 fb^-1, or the equivalent of 329 trillion collisions. Both groups have opted to look for events with a large number of final-state particles, specifically in the central/barrel region of the detector in order to sidestep the fact that fragmenting protons increase the multiplicity in the detectors’ side walls (end caps). ATLAS, in particular, requires that two of the final-state particles are muons with the same electric charge. This subtle requirement actually has a significant impact on the search by minimizing the number of Standard Model processes that may mimic the signal, but at the cost of reducing the number of expected micro-black hole events. In order to optimize their search, CMS sums the magnitudes of all final-state particles’ momenta. This is a bit clever because with so many additional particles this sum is expected to be significantly larger than for a typical Standard Model process.

Sadly, as you have probably guessed, neither group has seen anything like a micro-black hole. At any rate, here is a really cool micro-black hole candidate observed by with the CMS detector. It is most likely NOT an actual mico-black hole event, just a couple Standard Model processes that passed all the analysis requirements. Pretty, isn’t it.

Figure 5: A candidate microscopic black hole event observed with the Compact Muon Solenoid Experiment. Click to enlarge.

Happy Colliding

- richard (@bravelittlemuon)

Partial Bibliography

1. ATLAS Collaboration, Search for strong gravity signatures in same-sign dimuon final states using the ATLAS detector at the LHC, Phys. Lett. B 709 (2012) 322-340, arXiv:1111.0080v2
2. CMS Collaboration,Search for microscopic black holes in pp collisions at sqrt(s) = 7 TeV, Submitted to the Journal of High Energy Physics,  arXiv:1202.6396v1
3. S. Hawking, Particle Creation by Black Holes, Commun. Math. Phys. 43 (1975) 199–220, euclid.cmp/1103899181
4. N. Arkani-Hamed, S. Dimopoulos, and G. Dvali, The hierarchy problem and new dimensions at a millimeter, Phys. Lett. B 429 (1998) 263–267, arXiv:hep-ph/9803315v1
5. N. Arkani-Hamed, S. Dimopoulos, and G. Dvali, Phenomenology, astrophysics and cosmology of theories with submillimeter dimensions and TeV scale quantum gravity, Phys. Rev. D 59 (1999) 086004, arXiv:hep-ph/9807344v1
6. L. Randall and R. Sundrum, Large Mass Hierarchy from a Small Extra Dimension, Phys. Rev. Lett. 83 (1999) 3370–3373, arXiv:hep-ph/9905221v1
7. L. Randall and R. Sundrum, An Alternative to Compactification, Phys. Rev. Lett. 83(1999) 4690–4693, arXiv:hep-th/9906064v1
8. S. Dimopoulos and R. Emparan, String balls at the LHC and beyond, Phys. Lett. B 526(2002) 393–398, arXiv:hep-ph/0108060v1
9. R. Casadio, S. Fabi, B. Harms, & O. Micu, Theoretical survey of tidal-charged black holes at the LHC, arxiv.org/abs/0911.1884v1