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Flip Tanedo | USLHC | USA

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The Post-Higgs Hangover: where’s the new physics?

Now that the good people at CERN have finished their Higgs-discovery champagne, many of us have found ourselves drawn to harder drinks. While the Higgs is the finishing touch on the elegant edifice of the Standard Model, it’s the culmination of theoretical physics from the 1960s. Where’s all the exciting new physics that we’d been expecting “just around the corner” at the terascale?

My generation of particle physicists entered graduate school expecting a cornucopia of supersymmetry and extra dimensions at the TeV scale just waiting for us to join the party—unfortunately those hopes and dreams have yet come up short. While the book has yet to be written on whether or not the Higgs branching ratios are Standard Model-like, two recent experimental updates in collider and dark matter physics have also turned up empty.

No Z’ at 1 TeV

The first is the search for Z’ (“Z prime”) resonances, these are “smoking gun” signatures of a new particle which behaves like a heavy copy of the Z boson. Such particles are predicted by several models of new physics. There was some very cautious excitement after the 2011 data showed a 2σ bump in the dilepton channel around 1 TeV (both at CMS and ATLAS):

The horizontal axis is the mass of the hypothetical particle (measured by the momenta of the two leptons it supposedly decays to) in GeV, while the vertical axis is the rate at which these two lepton events are seen. (The other lines are examples for what one would expect for a Z’ from different models, for our purposes we can ignore them.) A bump would be indicative of a new particle causing a resonance: an increased rate in the observation of two leptons with a given energy. You can see something that is beginning to “kinda-sorta” look like a bump around 1 TeV. Of course, 2σ signals come and go with statistics—and this is indeed what happened with this year’s data [CMS EXO-12-015]:

Bummer. (Again, one really doesn’t have much right to be disappointed—that’s just the way the statistics works.)

Still no WIMP dark matter

Another area where we have good reason to expect new physics is dark matter. Astrophysical observations have given very strong evidence that the dark matter that gravitationally seeds our galaxies is composed of some new particle that is not described by the Standard Model. One nice feature is that astrophysical and cosmological data tell us the dark matter density in our galaxy, from which we can deduce a relation between the dark matter mass and its interaction strength.

Physicists observed that one particularly interesting scenario is when the dark matter particle interacts via the weak force—the sector of our the Standard Model that gets tied up with electroweak symmetry breaking and the Higgs. In this case, the dark matter mass should be right around a few hundred GeV, right in the ballpark of the LHC. To some, this is very suggestive evidence that dark matter may be related to electroweak physics. This class of models got a cute name: WIMPs, for weakly interacting massive particles. There are other types of dark matter, but until fairly recently WIMPs were king because they fit so nicely with models of new physics that were already modifying the electroweak scale.

Unfortunately, the flagship dark matter detector, XENON, recently released a sobering summary of its latest data at the Dark Attack conference in Switzerland. Yes, that’s really the conference name. XENON is a wonderful piece of detector technology that any particle physicist would be proud of. Their latest data-taking run found only two events (what’s expected from background). The result is the following plot:

How to read this plot: the horizontal axis is the mass of the WIMP particle. You get to pick this (or your model of new physics predicts this).  The vertical axis is the cross section, which measures the number of dark matter–detector interactions that such a WIMP is expected to undergo. The large boomerang-shaped lines are the limits set by the experiment—as the red text says, for a mass of around 55 GeV, it rules out cross sections that are above a certain number. For “garden variety” interaction channels, this number is already much smaller than the ball park estimate for the weak force.

The blob at the bottom right is some fairly arbitrary slice of the supersymmetry parameter space, but this is really just there for illustrative purposes and shouldn’t be interpreted as any kind of exclusion of supersymmetry. The other lines are other past experiments. The circles at the top left are slightly controversial ‘signals’ that have been ruled out within the WIMP paradigm by the last few direct detection experiments (XENON and CDMS).

The story is not necessarily as dour as the plot seems to indicate. There are many clever ways to get dark matter, not all of them WIMP-like. In fact, even the above plot is limited to the “spin-independent” coupling—an assumption about the particular way that dark matter interacts with nuclei. But these WIMP searches will eventually hit a brick wall around 2017: that’s when the XENON 1T (“one ton”) experiment will be sensitive to cross sections that are three orders of magnitude smaller than the current bounds. At that level of sensitivity, you end up with a lot of background noise from cosmic neutrinos which, as far as the detector is concerned, behave very much like dark matter. (They’re not.) Looking for a dark matter signal against this background is like looking for a needle in a stack of needles.

Where do we stand?

Between the infamous magnet quench of 2008 to the sobering exclusion plots of the last couple of years, an entire generation of graduate students and young postdocs is internalizing the idea that finding new physics will not be as simple as turning on the LHC as some of us had believed as undergrads. Despite our youthful naivete, the LHC is also still in its infancy with a 14 TeV run coming after its year-long shutdown. The above results are sobering, but they just mean that there wasn’t any low-hanging fruit for us to gobble up right away.

  • All the new research coming out is so interesting. I’m not surprised though that the results aren’t as quick in coming as all have hoped. I’m looking forward to reading the newest research about Dark Matter! I think it will go a long way into helping us understand how our universe works! Great article!

  • The findings in this article are spot on. I’m interested to see what the graduates students and young postdocs have to say in response. Follow up article?

  • We’re getting closer and closer at understanding how the universe works and what it is made of. There are scientists who argue that the current set of physics is a dead end road and that we should accept the limitations of Einsteins way of thinking and move on. Where do you stand in that?
    Very interesting article! i’ll be keeping an eye on your website.

  • Wouter

    hi Flip,
    you are still the only one to put an effort in to communicate the real physics to interested scientists outside the community. And very Feunmanish too. Big thanks.
    Question: in all drawings I’ve seen so far, the interaction between a massive particle and the Higgs field is made to look like a Brownian process: a particle weaving it’s way between random collisions. Since its path is non-linear (spending part of the time moving in the y direction), it’s speed in the x-direction is reduced (more interaction: slower at given energy). But if that were the case, then the net distance travelled would be proportional to the sqrt of time (like it is for diffusion processes). I’d like a better insight at what slows down a particle with given energy when it Higgs-interacts. Where does it spend the ‘lost time’ when not moving in the x-direction it initially travelled (while not changing direction in 3D)? Put differently, I never heard of an interaction without change in momentum.
    I hope the mathematics might allow an other & nicer analogy.


  • Deep thought! Thanks for ctonirubting.

  • Cesar

    Hi!, I’m a big fan of the research going on at the LHC although I must mention that I’m not a physics person. Nevertheless, I try to read and ask as much as possible to those who are physicists and actually know what the LHC can an cannot do for physics and humanity. One of the things I was once told by a very good physics’ student was about the role that the LHC could play in finding out more of what we need to know in order to make interstellar travel possible. This person told me that the LHC and the discovery of the Higgs boson could actually lead to theoretical advances required to find our the principles that had to be met to reach interstellar travel.

    I cam across this article (http://edition.cnn.com/2013/05/13/opinion/opinion-time-travel-paul-davies/index.html?hpt=hp_c4) from Paul Davies where he suggested that the LHC could put us in track to learn what we must in order to manipulate blackholes and wormholes for travelling purposes. I would like to have an opinion of this suggestion and the role that the LHC could actually play in that.

    Thanks a lot!!!!

  • John Avers

    Just wondering if there has been any findings to support the existence of an ether where all matter is a form of pure energy interacting with the ether so as to cause various wave phenomena? Just wondering.

  • Bomi

    Please write a new post for us about the gauge invariance

  • Bomi