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The Deficiency of Black Holes at the LHC

by John G. Cramer

Alternate View Column AV-158
Keywords: CERN, large, hadron, collider, LHC, black, hole, production, proton, collisions, deficiency, CMS, collaboration
Published in the July-August-2011 issue of Analog Science Fiction & Fact Magazine;
This column was written and submitted 2/25/2011 and is copyrighted ©2011 by John G. Cramer.
All rights reserved. No part may be reproduced in any form without
the explicit permission of the author.

The Large Hadronic Collider (LHC) at the CERN Laboratory located  at the eastern edge of Geneva , Switzerland , went into operation in late 2008, with dual proton beams circulated in the main ring for the first time on September 10, 2008.  Nine days later, a disaster occurred.  A slip-joint providing current to the superconducting magnets ceased to be superconducting, dumping large quantities of energy into the liquid helium cooling system, and six tons of liquid helium boiled away to become a high pressure gas.  The LHC monitoring mechanisms designed to limit damage in case of accidents initially failed to detect the event, with the result that a wave of high-pressure helium gas destroyed a string of about 53 of the LHC’s superconducting magnets and damaged many others.

After more than a year of shutdown, the LHC limped back into operation on November 20, 2009.  On March 30, 2010, the first 7.0 trillion electron-volts (TeV) collisions, half of the original design collision energy, took place.  The facility has continued normal scheduled operation with 7.0 TeV proton-proton collisions since that time, and has also spent a month running lead-lead collisions at 574 TeV.  It is currently scheduled to run at the 7.0 TeV proton-proton collision energy through the end of 2012.

In 2008, when the machine was preparing for operation, the big story in the media was not that the LHC was about to operate, but that it was possible that the Earth was about to be devoured by black holes produced at the LHC.  I perhaps contributed to that hysteria with my May-2003 AV column “The CERN LHC: A Black Hole Factory?”  In that column, I discussed the possibility that the LHC might produce small black holes in some of its high energy proton-proton collisions.  This was based on theoretical papers in the physics literature (see the references below) involving the hypothesis that gravity is such a weak force because it extends into several extra dimensions, while the other three forces do not.  These theories predicted that the proton-proton collisions at the LHC might produce tiny black holes with masses of 1 TeV or so.  The same theories also predicted that, if created, the microscopic black holes would be super-hot objects that would almost instantaneously dissipate themselves into a thermal cloud of lighter particles, quarks, gluons, electrons, positrons, neutrinos, and photons.

Reporters, bloggers, and others seized on the idea that the LHC might produce black holes and ignored the predictions of their super-brief lifetime.  Instead, they imagined a scenario in which LHC-produced black holes would began to suck in nearby matter, growing larger, and ultimately devouring the Earth.  Lawsuits, ultimately unsuccessful, were filed to prevent the LHC from beginning its operation.

The LHC has now been operating at 7.0 TeV for about a year.  Therefore, it’s time for an update on black hole production, or the lack thereof.  First, however, let me review the physics ideas that lead some theorists to predict that the LHC might produce microscopic black holes.

A black hole is an object that has acquired enough mass for its size to be completely confined by the force of gravity.  The velocity of escape from its surface exceeds the speed of light.  To put it another way, the gravitational force at its surface is so strong that energy cost of moving a lump of mass from its surface to some distance away exceeds the total mass-energy (E = m c2) of the mass lump.  There is good astrophysical evidence for the existence of super-dense objects assumed to be black holes, because mass falling into such objects produces lots of energy.  The energy-squandering behavior of quasars and active galactic nuclei, the x-rays emitted from certain binary star systems, and the high velocities of stars near the center of our own galaxy all suggest the presence of a black hole energy source.  It is also well established from theoretical modeling that a star with sufficient mass, after it burns through its supply of hydrogen, should collapse to a black hole following a supernova explosion.

There's no upper limit to the mass and size of a black hole.  In a certain sense, our entire universe can be considered to be a black hole, with us inside.  On the other end of the size scale, the mass of a black hole can be no smaller than a Planck mass, which is (hc/2pG)½.  Here, h is Planck's constant, c is the speed of light, and G is Newton 's gravitational constant.  The small value of G makes the Planck mass fairly large (22 micrograms or 1.22 × 1016 TeV) compared to fundamental particles.  As mentioned above, these small black holes should have a very short lifetime and should “evaporate” in a cloud of light particles due to the Hawking radiation process.

New theoretical ideas suggest that gravity, the weakest of the four forces, could become stronger and more comparable in strength to the other forces at small distances, because of the effects of hypothetical extra dimensions used only by gravity.  In this scenario, as the effective value of G grows larger, the Planck mass drops, minimum size black holes become less massive, and the energy required to produce black holes can decrease many orders of magnitude, to around 1 TeV.  This is well within range of LHC collision energies.  In this scenario the LHC would be a "black hole factory", an accelerator that makes large quantities of microscopic black holes.  The mass and production probability of such black holes depends in part on how many extra dimensions gravity is allowed to expand into, and calculations usually consider between two to six such extra dimensions.

As Steven Hawking showed in the 1970s, a black hole should have a surface temperature that depends on the curvature of its surface.  A star-mass black hole has a large radius, a very small curvature, and a temperature of only a few degrees Kelvin.  On the other hand, a mini black hole like those that the LHC might produce would be extremely hot, because its very small radius, (millimeters or less) would produce a correspondingly large temperature, up to 1.5 × 1014 K or 80 GeV.  At such a high surface temperature, the black hole would evaporate very rapidly into lighter particles: photons, electrons, and quarks, with energies ranging from 80 GeV down.

So the question is, after one year of LHC operation is there any evidence that the LHC is producing mini black holes?  One of the three large detectors located at the collision points of the LHC is the Compact Muon Solenoid (CMS) experiment.   On December 15, 2010, the CMS Collaboration released a paper in which they considered the evidence for black hole production from 10 trillion 7.0 TeV proton-proton collisions tracked by their detector at the LHC.  The format of their 26 page paper is itself interesting, because it includes 9 pages of physics discussion, 3 pages of references, and 14 pages listing the authors and institutions that are participants in the CMS Collaboration.   Such is the labor-intensive state of experimental high energy physics.

What would a black hole look like, as seen by the CMS detector?  It is estimated that if a black hole with a mass of around 1 TeV were produced, about 75% of its mass-energy would immediately evaporate into quarks and gluons, because these particles have many color degrees of freedom, making their Hawking evaporation more probable.  The resulting quarks and gluons would produce jets of strongly interacting particles tracked by the detector.  The remainder of the mass-energy would show up as photons and weakly-interacting particles  (positive and negative taus, muons, and electrons, as well as neutrinos, and Z and W bosons).  Some scenarios also predict energy loss as gravitational shock waves and stable non-interacting and non-accreting remnants.  The production of black holes at the LHC would be show up as “democratic” many-particle decays showing no preferred direction, with the final state particles carrying many GeV of kinetic energy.  The CMS Collaboration has searched for such events in their data.

Of course, random events that have nothing to do with black hole production can show characteristics similar to the “signal” that is the subject of the black hole search.  This event background is simulated with calculations using the Monte Carlo technique and is compared with the observed data.  Similar calculations based on theoretical models are also used to predict deviations from the background that would result from black hole production.  These show up as excess events at transverse energies beginning at 1.5 TeV and ending at 2.5 TeV, where the highest energy CMS data points are measured.

The net result of this investigation is that up to the end of 2010, no evidence for the production of black holes at the LHC has been observed.  The authors conclude that if there are 6 extra gravitational dimensions, the minimum black hole mass must be greater than 4.0 to 4.5 TeV.  If there are 4 extra gravitational dimensions, the minimum black hole mass must be greater than 3.8 to 4.4 TeV.   If there are only 2 extra gravitational dimensions, the minimum black hole mass must be greater than 3.4 to 4.0 TeV.

Of course, these results are somewhat limited by statistics and are greatly limited by the collision energies currently available at the LHC, which are only half of the design collision energy.  There could be black hole production that only shows up at higher collision energies.

The current plan at CERN is to operate the LHC at the present proton-proton collision energy of 7 TeV until the end of 2012.  Then there will be a one-year shutdown to replace suspicious current joints and make other improvements, and in about 2014 the LHC will resume operation with proton-proton collision energies of 14 TeV.  In that operating range, a new search for black hole production will be done, looking for black holes in the mass range up to around 9 or 10 TeV.

Watch this column for further results.

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 This page was created by John G. Cramer on 07/08/2011.