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Posts Tagged ‘black holes’

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?

Figure 1: An example of a black hole (center) demonstrating Hawking radiation, which is when a black hole radiates, or emits,  particles (e & γ) through interaction with virtual particles.

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

Particles to the People!

Monday, February 22nd, 2010


This weekend our department had a Physics Fair, free to the public, where hundreds of parents and kids came and learned about the research we’re involved in. There were grad students and professors available from many research groups including plasma, condensed matter, astrophysics, particle physics, and more.


Hey that's my experiment!

I enjoyed interacting with the public and letting them know people from their community are involved in a project they’ve actually heard about in the news. Of course, many people who had heard of a “hadron collider”, heard about it because of “black hole” fear stories.  Not that anyone was really afraid, it’s just that newspapers liked to make eye-catching, sensational headlines (like shown here).

If that’s what it takes to get on the cover of some newspapers, I’ll take it.  It’s a starting point, and at least gets people talking.

We had a few things for kids to look at, including a cloud chamber to see particles from cosmic rays.

Another thing we had for kids was a “quark puzzle”, which was an improved design from previous fairs.  See it here:


Quark Puzzle! (click to see larger image)

With this, kids could put together up and down quarks in whatever combinations of 3 they wished to create ether a delta-minus, neutron, proton, or delta-plus-plus.  Then they pasted them together using a “gluon” glue stick.  The quarks fit together in such a way that they can only make a circle with quarks of all three colors: red, green, and blue.

I know, I know, it’s way low budget, but a surprising number of kids enjoyed pasting quarks together.  Some kids made pasted together a bunch of quarks and were really excited to be bringing home so many particles.


For Those Who Are Concerned

Wednesday, September 10th, 2008

If you need current, up-to-the-minute information on whether the LHC is indeed as safe as the physics community says it is, consider consulting this website:


(If you are seriously worried, you might prefer this one: http://public.web.cern.ch/Public/en/LHC/Safety-en.html It explains in detail all the reasons why we’re sure there’s no danger at all.)


More exciting than politics!

Saturday, August 23rd, 2008

Some weeks ago, New York Times columnist Gail Collins wrote a piece about the possibility of black-hole production at the LHC.  I think she took this far too seriously; the chance of the LHC making black holes is super-tiny, and these aren’t the sort of black holes that are going to eat anything anyway.  Today Collins (who really is one of my favorite columnists, very trenchant political commentary) returned to the matter.  This time she did talk to an actual physicist, Brown’s Greg Landsberg, who is the US CMS physics coordinator.  She’s still too hung up on it, but at least she gave us some publicity for the big September 10 date, and even said that LHC startup will be as exciting as the upcoming Democratic and Republican conventions.  For sure, it will be more suspenseful!


Doomsday, in the Court

Thursday, March 27th, 2008

And speaking of black holes, right on cue, it’s 1999 all over again, from MSNBC’s Cosmic Log (via Slashdot…):

Some folks outside the scientific mainstream have asked darker questions as well: Could the collider create mini-black holes that last long enough and get big enough to turn into a matter-sucking maelstrom? Could exotic particles known as magnetic monopoles throw atomic nuclei out of whack? Could quarks recombine into “strangelets” that would turn the whole Earth into one big lump of exotic matter?

Former nuclear safety officer Walter Wagner has been raising such questions for years – first about an earlier-generation “big bang machine” known as the Relativistic Heavy-Ion Collider, and more recently about the LHC.

Last Friday, Wagner and another critic of the LHC’s safety measures, Luis Sancho, filed a lawsuit in Hawaii’s U.S. District Court. The suit calls on the U.S. Department of Energy, Fermilab, the National Science Foundation and CERN to ease up on their LHC preparations for several months while the collider’s safety was reassessed.

“We’re going to need a minimum of four months to review whatever they’re putting out,” Wagner told me on Monday. The suit seeks a temporary restraining order that would put the LHC on hold, pending the release and review of an updated CERN safety assessment. It also calls on the U.S. government to do a full environmental review addressing the LHC project, including the debate over the doomsday scenario.

It’s an obvious question to ask who is going to get to review the situation in the next four months. All, and literally all, of the people nominally qualified to evaluate this kind of thing still aren’t even slightly more afraid of this than they were in 1999, after studying billions and billions of collision events at RHIC. And Walter Wagner (“the founder of a botanical garden in Hawaii”, according to Robert Crease, commenting on a letter Wagner wrote to Scientific American in 1999 — the one which started the avalanche) has been through this before, to no effect. But it’s nice to learn a few new things in this piece:

  1. Someone thinks the “inner workings” of ATLAS is what I always thought was the outside. I should be nicer about this, but it’s a little funny. While a cheap shot, I admit, I consider this lapse fair game, since the phrase “inner workings” certainly was meant to have a sinister ring in this context.
  2. But speaking of (not being) funny, physicists’ attempts at being wry often misfire. Michio Kaku, whom Boyle seems to have used as a source, provides a reasonable, if blustery, dismissal of strangelets — “We see no evidence of this bizarre theory” — but then trips up: “Once in a while, we trot it out to scare the pants off people. But it’s not serious.” Unfortunately, this comes across as insulting to people who are seriously concerned about the effect of science on the environment, and does nothing to inspire their trust in us. If we keep making “jokes” like this to reporters, then we deserve to waste all of the energy that we do fending off folks like Wagner. So let’s stop intentionally scaring people, even in jest.
  3. I’ve always complained that these same folks haven’t updated the conceptual basis for their paranoia (b.t.w. there is literally no factual basis, not even a hint — we’d be shouting it to the rooftops if there was merely a hint of a hint, believe me). But CERN did make a good faith attempt to update things in 2002-3, two years into the RHIC era — and I’m embarassed to admit that I’ve never seen it (but that said, no-one has ever brought it to my attention, and it certainly doesn’t percolate up to Google’s notice — but “CERN doomsday” does yields up this gem.) Someone asked me recently to check out the “Safety Concerns” section of the Wikipedia article on the LHC, and…well, I was busy. Live and learn

Anyway, I’ll be following this closely on the physiblogosphere. Stay tuned.


Talkin’ Black Hole Blues

Monday, March 24th, 2008

Interesting week last, especially last Monday when I and a theory colleague were asked to chat with a documentary producer for the National Geographic Channel. The producer (who has previously worked on a series about exploring time) is working on a show about black holes, with more focus on what space-time looks like around (and even inside) a black hole. He had heard something about the connections between RHIC physics and black holes and wanted to see whether we had anything to offer. Given our spotty past in explaining these kind of things without freaking out people who would otherwise be interested, my colleague and I actually managed to put together an interesting story (none of which implying any danger to anyone not living in a higher dimensional space). Turns out that there are three almost-distinct pictures of “black holes” being used in connection with high energy nuclear collisions (and at the LHC, it’ll only get higher!):

  1. “Real” gravitational black holes – it’s been argued many times over that, for normal gravitational physics, energy densities at RHIC aren’t capable of producing enough matter in a small enough space to induce gravitational collapse, and subsequent decay via Hawking radiation. However, the presence of large extra dimensions has the effect of dramatically lowering the Planck scale and allowing this sort of phenomenon to occur, leading to spectacular isotropic decays of high mass black holes into the kitchen sink of particles from the standard model and beyond. I have relatively little experience bantering about this (i.e. one should check out Backreaction for more details on blackholology), but this is certainly the most “popular” conception of black holes at colliders these days.
  2. Hawking-Unruh radiation – my colleague in this interview, Dima Kharzeev, and collaborators have put forth an interesting analogy between “minimum bias” particle production, i.e. the many low energy particles produced in essentially every proton proton collision and, scaled up, every nucleus-nucleus collision. In this scenario, the process of the incoming projectiles “stopping” each other, and thus slowing down, by construction leads to acceleration (well, deceleration in this case). Einstein’s equivalence principle (remember that you can’t tell the difference between an elevator accelerating upwards at 9.8m/s^2 and the Earth pulling you down by gravity at 9.8m/s^2…) allows them to connect this slowing down to the Unruh effect in a gravitational field, which predicts the quantum tunneling of particles with an effective temperature of T=a/2Pi. When numbers are put in, out pops the famous freezeout (or Hagedorn) temperature we measure at RHIC (and in proton-proton collisions for years). So in effect, all strong interactions measured in the laboratory make a “black hole”, but not one resulting from gravitational collapse. As an onlooker, I find this connection curious, but not isolated — over the years I’ve noticed many authors make a variety of connections between gravitational physics and strong interactions, but they always feel mysterious, and thus it’s unclear where to go next.
  3. “Dual Black Holes” – this is something I and many others (both amateurs and pros) have found intrinsically exciting for a few years. The famous AdS/CFT conjecture suggests that strong interactions involving strongly-coupled quarks and gluons are really better (and more easily) described as a theory living on the boundary of a 10 dimensional gravity theory, with 5 extended and 5 compact dimensions. In this picture, again, every collision involves a black hole, which controls it’s microscopic properties (e.g. the viscosity), but one that lives in a larger dimensional space, and is thus again not the result of gravitational collapse in 4 dimensions. As people who have followed this thread (e.g. via my various blogs) over the years may be aware, this connection is allowing the development a striking number of techniques relevant to actual heavy ion phenomenology — and carries no risk to the 4-dimensional world (which someone should have told the BBC in early 2005…). Of course, we’re all hoping that the extra dimensions actually have some ontological status beyond being a mere mathematical trick, but time will tell.

Anyway, there we had it: three kinds of black hole physics, all of which are probably connected in some way, and all of which are potentially connected to RHIC or the LHC. (Recheck this post soon for more links…it’s bedtime – ok it’s wednesday, but done!)