After a long and occasionally difficult road to turning on, most people are just thrilled that the Large Hadron Collider is up and running smoothly. But already in its young life the LHC has collected enough data to yield impressive physics results. Unfortunately, those have mostly been of the form “we haven’t seen anything new yet.”
One new thing we would like to see is supersymmetry. The two big multi-purpose detectors at the LHC, named ATLAS and CMS, have both done searches for SUSY in the new data and come up empty-handed. That doesn’t mean it’s not there, actually; a “search for SUSY” is typically a search for a particular kind of signal, often in a particular kind of model. There is so much data already that it’s takes time and more than a bit of ingenuity to search through it effectively. But we could have seen evidence by now, and we haven’t.
Here is a paper from CMS, and a paper from ATLAS. There’s also a great blog post describing the results by Flip Tanedo at US/LHC Blogs. If you like your exclusion plots a bit sassier, check out Résonaances, where Jester reproduces a plot from Alessandro Strumia.
Here’s one way of thinking about the results, from the ATLAS paper (via Flip’s blog post). They look at collisions that produce a particular kind of signal — one lepton, jets (collimated collections of strongly-interacting particles produced by quark or gluon decay), and missing energy (indicating particles like neutrinos that aren’t measured by the experiment directly).
Supersymmetry predicts lots of particles and lots of parameters, so you can’t realistically explore the whole parameter space and put it on a single plot. Instead, you fix some parameters and set others to zero, leaving a two-dimensional space of possibilities. In this case, the horizontal axis is a scalar mass parameter, and the vertical axis is a fermion mass parameter, in terms of which everything else is determined (within this highly constrained and frankly unrealistic parameterization).
The solid-color regions are the places where previous experiments (the Tevatron and LEP) had already ruled them out. The dark black line is the new limit from CMS, and the dark red line is the new limit from ATLAS. Everything below those lines is ruled out.
So: the LHC has ruled out a lot of parameter space that was previously allowed for supersymmetry. Which is too bad. Except that it’s very difficult to quantify what one means by “a lot” in this case. There are many different ways to parameterize the theories, and many different searches one can do. Nature could be surprising us; it wouldn’t be the first time. Bottom line: it’s too bad we haven’t found SUSY yet, but there’s certainly no reason to declare it dead.
But supersymmetry might just be out of reach, or completely irrelevant. Theorists have to keep an open mind, and watch what happens as the experimenters push forward. My hope has always been that we’ll discover something that nobody thought of ahead of time — that possibility is very much open!
If you are thinking about going back to the drawing board, how far back would that be? The 1920’s perhaps, when an electron stopped being a rotating orbiting planet and became a point particle probability? Then you might have to give up all the jargon accumulated over 90 years!
I take it you’ve not yet seen the (more imressive?) ATLAS 0-lepton results, part of which have been released in preliminary form at ASPEN last Thursday.
See slides 12, 21 and 23 of
http://indico.cern.ch/contributionDisplay.py?sessionId=31&contribId=44&confId=103979
Is there any consensus on what (s)particles will show up first, if there is SUSY?
Is like the splitting of identical energylevels in molecular quantum chemistry, where one goes up as much (or a bit more) than the other goes down? That is to say, will the lightest SUSY ‘thingie’ be the twin of the heaviest normal stuff, which I guess in the top, or is it not as simple as that?
Regrets, I’ve had a few,
but then again,
too few dimensions….
Strings have been up and down and over and out
But I know one thing:
Each time the Superstrings are
Flat on their face
They just pick themselves up and get
back in the race.
That’s Strings! That’s what all the sci mags say,
Shot down in April, back up in May.
And if the sparticles are never found,
They’re gonna roll up in an 11 or 13 dimensional ball
And die. My, my.
It’s a Frank Sinatra universe, folks. Do not deny. 😉
Sili:
Good questions. I think the gluinos and squarks have the best chance of being detected early, simply because they are coloured, and therefore should be produced relatively copiously in the LHC collisions. Other superpartners have to be produced via weak or electromagnetic interactions, and therefore require more data to be taken.
In simple models of SUSY breaking, there are some rules about what happens to the masses, a bit like you suggest. However, ‘realistic’ models of SUSY breaking have a hidden sector which actually does the breaking, and this is then communicated to the standard model fields via ‘messengers’. Quite generically, the result is that the coloured superpartners tend to be heavier, with the sleptons, charginos and neutralinos lighter. This is roughly because gauge interactions drive the masses up, so stronger gauge interactions lead to larger masses.
Thanks. I have to admit that I certainly don’t understand the details of that answer, but there’s plenty of fodder for googling now.
I’m rather grateful that as a chemist I only have to deal with electrons.
Here is a link to a related article from Physics World:
http://physicsworld.com/cws/article/news/45182
Subtext: string theorists get nervous? Probably way too soon to say that but maybe a twitch or two ?
I must be in a dreadful mood as I’m picking on all of Sean’s posts tonight (sorry!). You say “One new thing we would like to see is supersymmetry.” Like? I think it is dangerous when we attach good/bad terms to finding or excluding a result or theory. The conclusion of the post is far better.
Supersymmetry has been suggested independently in 1971 by Juri Gol’fand and Evgeni Likhtman, in 1973 by Dmitri Volkov and V. Akulov, and in 1974 by Julius Wess and Bruno Zumino. In 1976 Peter van Nieuwenhuizen, Sergio Ferrara, Daniel Z. Freedman, Stanley Deser, and Bruno Zumino suggested a local supersymmetry called supergravity. In 1981 Edward Witten has shown that supersymmetry can solve several shortcomings of Grand Unified theories. In 1984 Michael Green and John Schwarz have shown that string theory and supersymmetry can be combined. This is the superstring theory. In 1995 Edward Witten has shown that the membrane concept can agree the 11-dimensional supergravity with the 10-dimensional superstring theory. Both theories are limit cases of an 11-dimensional M-theory.
Supersymmetric theories predicted that the elementary particles of the standard theory of particle physics (leptons, quarks, photon, gluons, W- and Z-boson, Higgs boson) have supersymmetric partners. This supersymmetric particles (called neutralinos, photino, gluinos, Winos, Zinos, squarks, and sleptons) were all predicted to have rest masses between 50 and 300 GeV (billion electron volts).
Now the ATLAS Collaboration of the LHC (Large Hadron Collider) presented data (arXiv: 1102.2357) which do not confirm the gluino. It would have been detected if its rest mass were less than 700 GeV.
I am not so surprised that signs of light supersymmetric particles have not been detected. I predict that supersymmetry will not be confirmed. My arguments are the following.
(1) The main reason for supersymmetry is that it can explain some shortcomings of minimal Grand Unified Theories, i. e. the mass-hierarchy problem (i. e. the fact that W- and Z-boson do not have rest masses of 10^15 GeV, although they should have “eaten” (coupled to) the Higgs bosons of Grand Unification) and the non-observation of the proton decay (lower limit: mean proton lifetime of 10^33 years).
But this argument requires that there is Grand Unification.
In 1997 I suggested (Modern Physics Letters A 12, 3153 – 3159 = hep-ph/9708394) a generalization of quantum electrodynamics, called quantum electromagnetodynamics. This theory is based on the gauge group U(1) x U'(1). In contrast to QED it describes electricity and magnetism as symmetrical as possible. Moreover it explains the quantization of electric charge. It includes electric and magnetic charges (Dirac magnetic monopoles) and two kinds of photon, the conventional Einstein electric photon and the hypothetical Salam magnetic photon. The electric-magnetic duality of this theory reads:
electric charge — magnetic charge
electric current — magnetic current
electric conductivity — magnetic conductivity
electric field strength — magnetic field strength
electric four-potential — magnetic four-potential
electric photon — magnetic photon
electric field constant — magnetic field constant
dielectricity number — magnetic permeability
Because of the U(1) x U'(1) group structure and the Dirac quantization condition e * g = h (unit electric charge times unit magnetic charge is equal to the Planck constant), this theory is hard to agree with Grand Unification. Although a group such as SU(5) x SU'(5) is in principle not impossible.
(2) Another reason for supersymmetry is that it can explain the existence of (anti-symmetrical) fermions in an otherwise symmetrical theory (such as Special Relativity and General Relativity).
However, it has long been known that a generalization of General Relativity which includes anti-symmetry is Einstein-Cartan theory. The affine connection of this theory includes not only the non-Lorentz invariant symmetrical Christoffel symbol but also the Lorentz invariant anti-symmetrical Torsion tensor.
Within the framework of a quantum field theory, the Torsion tensor corresponds to a spin-three boson called tordion, which was introduced in 1976 by F. W. Hehl et al.: Reviews of Modern Physics 48 (1976) 393 – 416.
In 1999 I discussed (International Journal of Modern Physics A 14, 2531-2535 = arXiv: gr-qc/9806026) the properties of the tordion. Moreover I sugested that the electric-magnetic duality is analogous to the mass-spin duality. This analogy reads:
electric charge — magnetic charge – mass — spin
electric field constant — magnetic field constant — gravitational constant — reduced Planck constant
electric four-potential — magnetic four-potential — metric tensor — torsion tensor
electric photon — magnetic photon — graviton — tordion
(3) Supersymmetric theories including superstring and M theory have not much predictive power. For example, so far no one has shown that these theories predict the empirically obvious Naturkonstanten-Gleichung (fundamental equation of unified field theory, Modern Physics Letters A 14, 1917-1922 = arXiv: astro-ph/9908356):
ln (kappa * c * H * M) = −1 / alpha
where kappa is the Einstein field constant, c is the speed of light, H is the Hubble constant, M is the Planck mass, and alpha is the fine-structure constant. By using the WMAP−5 value
H = (70.5 +/- 1.3) km / (s * Mpc)
(E. Komatsu et al.: Astrophys. J. Suppl. Series 180 (2009) 330 – 376) the left-hand side yields
ln (kappa * c * H * M) = – 137.025(19)
which is within the error bars equal to
– 1 / alpha = – 137.035 999 679(94)
The Naturkonstanten-Gleichung predicts the Hubble constant to be
H = 69.734(4) km / (s * Mpc)
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