Lisa Randall CERN 2007

Black Holes and Quantum Gravity at the LHC
The talk focused on models with higher dimensions of quantum gravity in the context of a low quantum gravity scale. How low ? Well, as low as one can hope for - about 1 TeV or so. Naturally, at the LHC one would expect quite dramatic signatures.

Should LHC be looking at black hole production or elsewhere ?
The questions experimentalists have to ask themselves at the start of a project like the LHC, which will explore unknown new energy scale and domains: - “Are we optimizing existing searches for the signatures we might have access to ?” and “Are we sure we are not missing possible searches ?”
[+/-] Click here to expand

One interesting question, connected to the scope of Lisa’s talk, is: “If there is new physics, but it lies at a higher energy scale than the one directly accessible by the machine, how do we maximize our chances to see it ?”

Historically, the reason that black holes appear so promising as compared with other possible signatures is the predicted huge cross section for their production if there is a low quantum gravity scale. Lisa ventured to compute that if quantum gravity turns on at a scale of a TeV, one gets 100 pb which for 100 fb-1 luminosity would yield ten million events.

The basic reason why this cross section is so large compared to the production of a particle with TeV mass in a typical beyond the SM theory is the lack of any small couplings, such as gauge couplings in the cross section and absence of phase space suppression factors. However, this estimate ignores several major considerations and uncertainties in the black hole production and decay cross sections.

There is no suppression from gauge couplings, so it is indeed a large signal. Also, the signature is spectacular, since these objects are predicted to decay into large multiplicity final state, with highly spherical distributions. Very distinctive, unmistakable new physics.

But the problem is that the idyllic picture is not very realistic. The onset of a non-perturbative regime where black holes are produced and decay with those signatures is much above the QG scale, and this appears to be above the reach of even the LHC.

The Large Hadron Collider (LHC) at CERN has not emitted the first burp yet, and it is already criticized for being a midget. In any case, at threshold one would not see the striking signatures, but maybe something can be saved.

Randall was very clear in stating that the LHC is unlikely to make classical BH states decaying with Hawking radiation. She appeared to be interested in assessing the damage: and the answer is that, if you have a low quantum gravity scale and you cross it, you will have a change in the two-particle final states. Things are not calculable, but there appear to still be distincitve experimental signatures that are capable of distinguishing among different models.

You have to go well above M, the energy scale of quantum gravity, to be sure to hit the striking signatures publicized in the past. The parton distribution functions of the proton drop rapidly with the fraction of parton momentum, and since we are by necessity near threshold, the value of the latter is very important in determining what the rate of the new process is going to be. To make matters worse, M is convention-dependent. Factors of - fly around easily, and although one knows these are only conventions and what one cares for is just the actual threshold, there is a big difference between 1 TeV and 2 TeV for the LHC. So the picture is fuzzy.


Bubble Chamber: Leonard Susskind & Lisa Randall

Lisa discussed some of the models and the resulting conventions and equations for the schwarzschild radius, the energy scale, and the other main characteristics.

One point which looked important is that in the models considered, the black hole lifetime is bigger than the inverse of the energy scale of quantum gravity. This drives some of the phenomenology of the black hole decay. Another point is that every degree of freedom should carry an insignificant amount of energy with respect to the total; and since we are never going to get far above threshold at the LHC, we will have to be careful to call what we produce a real classical black hole. These things have low entropy close to threshold, and the multiplicity of the decay will be affected.

A critical factor in the computation of the number of particles emitted in the black hole decay is the assumption of the dimensionality of the space: particles emitted in the bulk have more directions in which to oscillate. Furthermore, since the threshold for producing black holes is not M, but a higher energy, even if we did see a black hole, we would not be able to extract M from the total cross section, because of inelasticity effects: not all the energy of the colliding partons goes in the creation of the black hole, due to initial state radiation.

The difficult question to answer is in fact, what fraction of the energy gets trapped inside the horizon? It is of course important since the PDF fall rapidly with energy. What is clear is that the inelasticity effectively increases the threshold. The reduction in cross section due to this effect is enormous, and it is the lack of considering it, which has brought some overoptimistic predictions in the past.

So, the upshot is that BH production threshold is higher than originally thought. It means a lower production cross section, a lower reach in black hole mass, and it translates into lower entropy reach as well. The conclusion of Lisa Randall is that we will not produce classical thermal black holes at the LHC. What will we still be able to produce, then ? And what kind of multiplicities should we expect ?

Lisa discussed the calculation of the multiplicity of final state particles. She said that the calculation is totally unreliable. But one thing stays clear: low multiplicity final states will dominate even if we call it black holes. So we have to face the facts, and study 2-body final states: jets and leptons. Can they be distinguished from backgrounds by rate, kinematics, bra size? Yes. For jets, transversality is the key. QCD is dominated by t-channel exchange, i.e., forward scattering. Black hole events are isotropic. So this is really becoming like any other compositeness search: massive states produced at low rapidity.

While describing a scenario where the LHC will have to walk the walk of unclear kinematical analyses rather than being hit in the face by those firework-like signatures that experimentalists have started to dream more and more frequently as of late, Randall was careful to insist that the LHC is indeed a powerful machine, although she fell short of declaring it will make everything clear about quantum gravity.

After discussing the signature of black holes, Randall delved into the possible signatures of the same kind from alternative models of quantum gravity, such as a weakly coupled string theory. There one apparently expects a resonance behaviour, followed by a dramatic drop in transverse cross section, which can be used to distinguish the stringy behavior from the simple production of a new Z’ boson, ...

In addition to the resonance, you would also see a drop in the quantity. This could also allow to distinguish models: you could decide you are finding a stringy state, and you could even distinguish different stringy models, because the correlation between and the cross section is different for different models. In summary, black holes are not as spectacular as advertised in the past. However, they may still provide lots of information about quantum gravity, through careful studies of processes.

Lisa also said she would love to see these studies done by Atlas and CMS: energy-dependent angle studies in dijet production.

Tommaso Dorigo asked the question: "I know from previous blogging on the issue that when one reaches a quantum gravity regime, the QCD cross section of dijet production has to go down, but Lisa had not discussed this feature." She explained that before one reaches the regime when QCD 2-particle cross section gets reduced, the cross section has to go up, in any case. So the dijet cross section reduction that Sabine has first studied happens at a regime that LHC will fail to cover.

Source: Lisa Randall: black holes out of reach of LHC
by Tommaso Dorigo @ A Quantum Diaries Survivor.
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Lisa Randall: Unification in warped extra dimensions and bulk holography
Lisa Randall: Smashing open the Universe @ Prospect Magazine.
Event probability for Production of Single Top Quarks using Matrix Elements

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