Science Blogs
_Have we reached the end of Particle Physics?_
(http://scienceblogs.com/startswithabang/2012/10/17/have-we-reached-the-end-of-particle-physics/)
Posted by _Ethan_ (http://scienceblogs.com/startswithabang/author/esiegel/)
on October 17, 2012
“The particle and the planet are subject to the same laws and what is
learned of one will be known of the other.” -James Smithson
The entirety of the known Universe — from the smallest constituents of the
atoms to the largest superclusters of galaxies — have more in common than
you might think.
(http://scienceblogs.com/startswithabang/files/2012/10/201103_VirgoGCM_andreo.jpeg)
Image credit: Rogelio Bernal Andreo of
http://blog.deepskycolors.com/about.html.
Although the scales differ by some 50 orders of magnitude, the laws that
govern the grandest scales of the cosmos are the very same laws that govern
the tiniest particles and their interactions with one another on the
smallest known scales.
(http://scienceblogs.com/startswithabang/files/2012/10/feynm5.gif)
Image credit: R. Nave of
http://hyperphysics.phy-astr.gsu.edu/hbase/particles/expar.html.
We study these two scales in entirely different ways; the largest scales
can only be studied with great telescopes, using the natural cosmic labora
tory of outer space, while the smallest scales require the largest, most
powerful machines ever constructed here on Earth: particle accelerators! And
of
all the particle accelerators ever built by humanity, the Large Hadron
Collider (LHC) is by far the most powerful.
(http://scienceblogs.com/startswithabang/files/2012/10/lhc10.jpeg)
Image credit: Maximilien Brice, © CERN.
Although many of us are still hoping that the LHC finds something new,
exciting and unexpected, it was constructed — first and foremost — to find
the last missing piece of _the Standard Model_
(http://scienceblogs.com/startswithabang/2012/07/03/the-biggest-firework-of-them-all-the-higgs/)
: _the_
(http://scienceblogs.com/startswithabang/2012/07/04/how-the-higgs-gives-mass-t
o-the-universe/) _Higgs_
(http://scienceblogs.com/startswithabang/2012/07/05/explaining-the-higgs-on-tv-last-night/)
_Boson_
(http://scienceblogs.com/startswithabang/2012/07/06/now-that-weve-got-the-higgs-whats-next/)
.
There are many types of fundamental particles in the Universe, but we can
divide them into three general categories: fermions (like quarks and
electrons), gauge bosons (like the photon), and the Higgs, a unique,
fundamental
scalar particle.
(http://scienceblogs.com/startswithabang/files/2012/07/FNAL_ESiegel.jpeg)
Image retrieved from Fermilab, modified by me.
I don’t know whether you followed physics news prior to the LHC, but if you
did, you’ll remember that there was wild speculation about what mass the
Higgs Boson was going to have. There’s a very good reason for this: all
these particles — through the physics of quantum field theory — have dramatic
effects on what we observe in this world.
(http://scienceblogs.com/startswithabang/files/2012/10/proton-naif.gif)
Image credit: DESY in Hamburg, from
http://www.desy.de/f/hera/engl/chap1.html.
For example, we normally think of protons and neutrons as being made up of
3 quarks apiece, but those three quarks only account for some 2% of the
total mass of those particles; the rest of that mass comes from all the other
particles, interacting via the laws of quantum field theory (QFT). All
these particles are so interdependent on one another that if the _top quark_
(http://en.wikipedia.org/wiki/Top_quark) — the heaviest of all standard
model particles (and some 180 times the mass of the proton) — were twice the
mass it actually is, every proton in the Universe would be 20% heavier than
the protons that actually exist!
So, too, the mass of the Higgs would be highly dependent on what else is in
the Universe, and what interactions actually happen according to the laws
of QFT.
(http://scienceblogs.com/startswithabang/files/2012/10/Higgs_feynman.jpg)
Image credit: David Kaplan.
The standard model, of course, does not include gravity. But the real
Universe has gravity, and we assume that whatever the full, fundamental theory
of the Universe is, it incorporates all of the known forces, gravity
included. When it comes to gravity, we typically consider General Relativity
as a
low-energy, large-scale (compared to the _Planck length_
(http://en.wikipedia.org/wiki/Planck_length) , at least) approximation of a
more fundamental,
fully quantum treatment of gravity, which is simply beyond the scope of
our theoretical tools.
(http://scienceblogs.com/startswithabang/files/2012/10/spacetimelu31.jpeg)
Image credit: Jim Mims of Science And Computer Science, from
http://www.alpcentauri.info/.
At least, it has been for generations. But there is a new idea gaining
traction in recent years when it comes to making a quantum theory of gravity:
_asymptotic safety_ (http://arxiv.org/abs/0709.3851) . Without going into
any mathematical detail (and with full disclosure that I myself don’t
understand it as well as I’d like), you can think of it as a mathematical
trick
that allows you to incorporate gravitation into your QFT. (For a little more
detail, see _here_ (http://arxiv.org/pdf/0709.3851v2.pdf) , and for a lot
more, see _the Weinberg original_
(http://books.google.com/books?id=pxA4AAAAIAAJ&pg=PA790&lpg=PA790&dq=weinberg+"Ultraviolet+divergences+in+quantum+theorie
s+of+gravitation"&source=bl&ots=dFF-LiJhO0&sig=Cu3bYeeDSATy34YC2cfknPcyuT4&h
l=en&sa=X&ei=xW5_UNDMN8W9iwLOo4CABA&ved=0CCgQ6AEwAQ#v=onepage&q=weinberg%20"
Ultraviolet%20divergences%20in%20quantum%20theories%20of%20gravitation"&f=fa
lse) .)
There’s a very important reason we care about this: if we understand how to
incorporate gravity into our quantum field theories, and we’ve measured
the masses of all the standard model particles except one, we can
theoretically predict what the mass of that one remaining particle needs to be
in
order for physics to work properly at all energies!
(http://scienceblogs.com/startswithabang/files/2012/10/particles.gif)
Image credit: Harrison Prosper at Florida State University.
We can do this because demanding that the Universe be stable constrains
that last free parameter — the mass of the Higgs boson — to be one particular
value. If the mass turns out to be that value, then that’s indicative
that, if asymptotic safety is a valid idea, there are no new particles in the
Universe that couple to the Standard Model. In other words, there are no
new particles to be found by building colliders in the Universe, all the way
up to Planck energies, some 15 orders of magnitude more energetic than
those probed by the LHC.
But if we can predict that mass, and the actual mass of the Higgs boson
turns out to be anything else, either higher or lower, then that means there
must be something new in the Universe in order for physics to be
self-consistent. Now, here’s the truly amazing thing: _that mass was
calculated back
in 2009_
(http://www.sciencedirect.com/science/article/pii/S0370269309014579) , before
the LHC was turned on.
(http://scienceblogs.com/startswithabang/files/2012/10/Shaposh.jpg)
Image credit: From Phys. Lett. B's paper by Mikhail Shaposhnikov &
Christof Wetterich.
You can read _the abstract here_ (http://arxiv.org/abs/0912.0208) and the
_full article here_ (http://arxiv.org/pdf/0912.0208v2.pdf) , but what’s
truly amazing is that we’ve now found the Higgs, and we know its mass. Want
to see what this paper, nearly 3 years old now, predicted for the mass of
the Higgs? (Highlights, below, are mine.)
(http://scienceblogs.com/startswithabang/files/2012/10/Shaposh_abstract.jpg)
Image credit: Mikhail Shaposhnikov & Christof Wetterich.
Holy. Crap.
So I want you to understand this correctly, because this could be huge. If
asymptotic safety is right, and the work done in this paper is right, then
an observation of a Higgs Boson with a mass of 126 GeV, with a very small
uncertainty (±1 or 2 GeV), would be damning evidence against supersymmetry,
extra dimensions, technicolor, or any other theory that incorporates any
new particles that could be found by any accelerator that could be built
within our Solar System.
Fast-forward to this past July, when _the discovery of the Higgs Boson_
(http://scienceblogs.com/startswithabang/2012/07/05/explaining-the-higgs-on-tv-
last-night/) — confirmed to be a single, fundamental scalar particle of
spin-0 — was announced. What was its mass, again?
(http://scienceblogs.com/startswithabang/files/2012/10/vixrasignaldp.png)
Image credit: Vixra blog, of combined CMS/ATLAS Higgs signal.
According to the combined ATLAS+CMS data (both major detectors), a Higgs of
mass somewhere between 125 and 126 GeV was detected with a (robust)
significance of 6-σ, with an uncertainty of around ±1 GeV. In other words,
those
of you who followed the excitement in July may have witnessed the last
fundamental particle physics discovery we will ever make. There still may be
more out there, but the Higgs Boson could have very well been the last
unfound fundamental particle accessible to colliders.
Yes, there are still more _questions to answer_
(http://scienceblogs.com/startswithabang/2012/07/06/now-that-weve-got-the-higgs-whats-next/)
, more
physics to learn and more to explore even with the LHC, including questions
about dark matter, the origin of neutrino mass, and the lack of strong
CP-violation. But there might not be _anything more to learn_
(http://indico.cern.ch/conferenceDisplay.py?confId=198224) — at least, in
terms of
fundamental, new particles — from doing particle physics at higher and higher
energies.
--
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