The Large Hadronic Collider (LHC) is the
largest “atom smasher” and most complex scientific instrument ever
built. The LHC is the culmination of more than half a century of ever
more capable particle accelerators, mankind’s finest microscopes that
probe dimensions a billion times smaller than a single atom. With such
machines, particle physicists strive to discover what we and everything
else are ultimately made of.
The LHC was built by CERN, the European
nuclear research consortium near Geneva, Switzerland, at a cost of over
$10 billion, which doesn’t include the salaries of the army of
scientists who have worked on this program for decades. There are over
7000 physicists in the ATLAS and CMS collaborations alone. Last July,
with considerable fanfare, these groups announced strong evidence for
the discovery of the Higgs boson, which had eluded physicists for four
With the champagne bubbles long gone, the physics world is now asking: “What have you done for us lately?”
Physicists expected much more from the
LHC than just one new particle, and they promised much more to justify
their governments’ investments. If the price of discovery becomes $10
billion per particle, the LHC may become the last particle accelerator
The LHC’s deafening silence suggests a
terrifying answer: there is no new physics. After recording more
particle collisions, at higher energies, than all prior physics
experiments combined, LHC has found nothing noteworthy, at least so far.
Many physicists deem this the “nightmare
scenario” — just the same ole, same ole, without any of the hoped-for
discoveries, and providing nothing new to work on.
String Theory took a hit from this
absence of new physics. An LHC experiment searching for additional
spatial dimensions, the backbone of String Theory, turned up nothing.
The biggest disappointment is the
conspicuous absence of supersymmetric particles. For the last 30 years,
the theory of supersymmetry (SUSY) proclaimed that every known particle
type has a yet-undiscovered partner of the opposite spin. Each known
fermion should have a supersymmetric boson, and each known boson should
have a supersymmetric fermion. (Spin ½ fermions are the particles of
matter, while spin 1 bosons are the particles that carry forces.) Once
the LHC was built, so the mantra went, the 17 known fundamental
particles would be joined by 17 new particles that were presumably too
massive for prior accelerators to produce, as indicated in the following
SUSY was proposed to explain why gravity
is so much weaker than the other forces and to plug some embarrassing
holes in particle theories. Theorists’ calculations of the W and Z boson
masses from Higgs interactions were drastically higher than the
measured masses — supersymmetric partners were invoked to reduce the
interaction and fix this glaring error. Inventing something new that is
currently undetectable is a common approach theorists use to explain
what they don’t understand — sometimes, it actually works.
SUSY proponents claimed it would
eventually fix the most glaring theoretical physics error of all time:
quantum field theory calculations of dark energy (aka vacuum energy) are
up to 120 orders of magnitude higher than the value measured by cosmologists! It’s almost impossible to be more wrong than that.
Supersymmetric partners were also the
leading guess in resolving the mystery of dark matter, the invisible
stuff that accounts for over 80% of all matter in our universe.
Supersymmetric particles were thought to be massive enough, abundant
enough, and elusive enough to fit the bill.
Finally, SUSY is a prerequisite for
String Theory. SUSY could be true even if String Theory isn’t, but
without SUSY, String Theory is dead.
It’s fair to say that the vast majority
of all theoretical particle physicists’ efforts for the last 30 years
would be worthless if SUSY isn’t a true property of nature. Theorist
Mikhail Shifman has devoted almost his entire career to SUSY. Commenting
on how so many theorists could have been so wrong for so long he said:
“Of course, it is disappointing. We’re not gods.” I don’t think anyone
was under that delusion.
At this time, LHC has stopped generating
this type of data, and will be shut down for repairs and upgrading. It
will probably be running again and have more definitive data in 2016
(such schedules often slip). Unfortunately, particle physics can’t be
advanced in North America; the U.S. doesn’t have a competitive machine
and our two major accelerators have been closed.
If the renovated LHC continues to see no
evidence of supersymmetric particles, theorists might say: “SUSY
particles must be heavier than LHC can produce, so we need a bigger
accelerator.” But heavier SUSY particles wouldn’t cancel the
calculational discrepancies properly, making the theory more contrived
and much less appealing. And who is likely to fund a next generation
accelerator for $100 billion?
No one said this was going to be easy —
nature jealously guards her secrets. Physicists aren’t going to give up,
the LHC has many more years of life ahead, and despite current signs
they may eventually make remarkable discoveries.
A modest silver lining is that if SUSY
isn’t correct, we won’t have to deal with the awful names given to the
supersymmetric particles, such as squarks, sleptons, and winos.
February 11, 2013
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