Saturday, 4 October 2014

Weekend Plot: Stealth stops exposed

This weekend we admire the new ATLAS limits on stops - hypothetical supersymmetric partners of the top quark:

For a stop promptly decaying to a top quark and an invisible neutralino, the new search excludes the mass range between m_top and 191 GeV. These numbers do not seem impressive at first sight, but let me explain why it's interesting.

No sign of SUSY at the LHC could mean that she is dead, or that she is resting hiding. Indeed, the current experimental coverage has several blind spots where supersymmetric particles, in spite of being produced in large numbers, induce too subtle signals in a detector to be easily spotted. For example, based on the observed distribution of events with a top-antitop quark pair accompanied by large missing momentum, ATLAS and CMS put the lower limit on the stop mass at around 750 GeV. However, these searches are inefficient if the stop mass is close to that of the top quark, 175-200 GeV (more generally, for m_top+m_neutralino ≈ m_stop). In this so-called stealth stop region,  the momentum carried away by the neutralino is too small to distinguish stop production from the standard model process of top quark production. We need another trick to smoke out light stops. The ATLAS collaboration followed theorist's suggestion to use spin correlations. In the standard model, gluons couple  either to 2 left-handed or to 2 right-handed quarks. This leads to a certain amount of correlation between  the spins of the top and the antitop quark, which can be seen by looking at angular distributions of the decay products of  the top quarks. If, on the other hand, a pair of top quarks originates from a decay of spin-0 stops, the spins of the pair are not correlated. ATLAS measured spin correlation in top pair production; in practice, they measured the distribution of the azimuthal angle between the two charged leptons in the events where both top quarks decay leptonically. As usual, they found it in a good agreement with the standard model prediction. This allows them to deduce that there cannot be too many stops polluting the top quark sample, and place the limit of 20 picobarns on the stop production cross section at the LHC, see the black line on the plot. Given the theoretical uncertainties, that cross section corresponds to the stop mass somewhere between 191 GeV and 202 GeV.

So, the stealth stop window is not completely closed yet, but we're getting there.

29 comments:

Anonymous said...

There is a related theoretical study here: http://arXiv.org/abs/arXiv:1407.1043

Robert L. Oldershaw said...

How about a new "confinement" gambit. There are zillions of sparticles but they are all confined to unobservable extra dimensions so you cannot see them?

Hey, it works for theologians!

Alex Small said...

Jester,

Since you keep reporting on negative results for different BSM theories, I am curious what sorts of BSM ideas you think are worth exploring?

Jester said...

Currently, there's no strong motivations for any particular BSM framework. Various theories of dark matter are worth exploring, because dark matter is one thing from beyond the SM that we're sure it exists (though of course we don't know if dark matter is accessible at the LHC). Pragmatically, we should explore as wide range of theories and signatures as possible... we may always find something unexpected.

Ervin Goldfain said...

Jester,

Your answer to Alex' question is well formulated and makes a lot of sense.

I would only add that, in my opinion, one must continue to explore the inner structure of the Standard Model (SM)- as we know it today - to better understand its dynamics and properties. Both SM and the Renormalization Group are non-trivial frameworks leading to complex systems of non-linear equations. Little is known about the behavior of strongly coupled gauge theory (aside from lattice simulations) and non-perturbative phenomena. Many questions remain open on the Higgs sector...and so on.

Sven said...

Hi,
indeed the question *which* BSM theory should be pursued is an important and complicated one (DM and possibly neutino masses alone are sufficient justification that we must look for BSM physics). In the absense of any clear BSM hint from the LHC, one should look at the various pros and cons of the BSM alternatives, and also which measurements they predicted (or not). In this sense the prediction of the Higgs boson mass by SUSY theories remains very interesting. :-)

Anonymous said...

Jester,

are we all 100% completely fully sure that dark matter is BSM? Would anybody bet his life on it?

Jester said...

Yes, i would. In the worst case dark matter is right-handed neutrinos, but formally that's still BSM.

Alex Small said...

The astronomers seem pretty sure DM isn't made out of standard model particles. They seem pretty sure that GR is correct on astronomical and cosmological length scales. If the data and methods for those suppositions are sound, BSM particles are pretty much what it has to be.

Chris said...

Dark matter is not baryonic - the ratio of 2nd to 3rd acoustic peak of the CMB tells us that.

Carlos said...

Let me stress something that perhaps needs to be said regarding the LHC limits on direct stop production. You mention that the current limit is of the order of 700 GeV, unless the stop mass is close to the top mass. I would say that, assuming that the neutralino is lighter than the stop and provided the neutralino mass is larger than 250 GeV, there is essentially no limit on the stop mass coming from direct stop production. This means that the real bound on the stop mass is 250 GeV, unless the mass is close to the top mass in which case it can be even smaller. Let me also emphasize that 250 GeV is far away from 700 GeV (actually it is far closer to the top mass, 173 GeV), so it is wrong to say that it is 700 GeV unless the spectrum is compressed or the stop mass is close to the top mass.

Cédric Bardot said...

Talking about BSM and understanding better the Higgs sector and the issue of the stability of the electroweak vacuum, I hope Jester will find some time to comment about the experimental search for a more conservative U(1)B-L gauge symmetry that could extend the SM.
@Sven, there are also interesting Higgs mass post-dictions* from Solid Theoretical Research In Natural Geometric Structures** without SUSY that could be interesting...
(*arxiv.org/abs/1208.1030 and arxiv.org/abs/1408.5367;
**STRINGS definition according to Juan Maldacena in physics.princeton.edu/strings2014/slides/Maldacena.pdf ;-)

Cormac said...

Lovely post - I hope the title didn't raised a few flags at the NSA!

Anonymous said...

Jester, Chris,

yes, the CMP peaks height ratios point towards dark matter. But could there not be another explanation for the peak heights?

I am astonished and amazed that you would bet your life on such an indirect argument, with all the pitfalls due to measurement errors and possibly forgotten terms in the oscillation equations. I wish you that you are right!

Jester said...

There are numerous independent arguments for dark matter: galactic rotation curves, dynamics of galaxy clusters, large scale structure, CMB peaks, bullet cluster, baryon acoustic oscillations. Any one of these would not be completely convincing on its own, but taken together they are just enough to bet a life on :)

Alex Small said...

The most convincing evidence is that the independent signals don't just point to missing mass. They also point to roughly the same amount of missing mass.

Theo Nieuwenhuizen said...

@ Jester "dark matter is one thing from beyond the SM that we're sure it exists"
I strongly doubt the existence of WIMPs or axions. They would mess up the Galaxy, LCDM heavily fails at galactic scales. The problem is not "dirty gastrophysics" but a failed theory poorly rescued by more and more epicycles.
@ Jester "There are numerous independent arguments for dark matter: galactic rotation curves, dynamics of galaxy clusters, large scale structure, CMB peaks, bullet cluster, baryon acoustic oscillations."
True, there must be something out there. We still have a candidate in active neutrinos (a fit of lensing data for the galaxy cluster A1689 predicts eV ranged masses) and sterile ones of either eV range or perhaps 7 keV.
At the galactic scale, there may be an important role for dark baryons, related to nonlinear structure formation at those "small" scales.

Chris said...

In my book sterile neutrinos are BSM.

Also, don't forget that the acoustic oscillation peak ratio was a *prediction* of non-baryonic DM. It is most certainly not a measurement error (if i am not mistaken, at least 4 collaborations have seen it) and anything you might have to add to the oscilation equations will be BSM stuff, too.

I am singling out this one measurement because even MOND people acknowledge that nonbaryonic DM is the only way to explain it.

Filippo said...

Primordial black holes are not excluded as DM candidates, and I wouldn't say they are BSM.

andrew said...

Dark matter phenomena - absolutely.

DM v. inaccuracies in the conventional equations of gravity or how we approximate them? Not so sure. There are solid cases to be made for each.

I also think that there is a great deal to be said for probing the why's of the SM.

And, outside DM if you are looking for the highest percentage of papers where experiment doesn't produce the expected result, the answer has to be in QCD. Maybe we're just operationalizing the equations wrong. But, there are more than enough anomalous QCD results out there for us to be missing something - probably something subtle - but I wouldn't be at all surprised to find out that there are some corners that we've missed.

There's lots of just plain brute force work to be done pinning down SM (or maybe even BSM) neutrino physics - CP violation parameter, Majorana v. Dirac, normal v. inverted hierarchy, absolute neutrino masses, the quadrants of one of the theta angles, and neutrinoless double beta decay exclusions, just to start with. One you have those data points, you have a complete set of reasonably accurately known SM parameters and you can really get serious about exploring deeper inner workings.

Another tempting experimental target in terms of SUSY and BSM physics is the experimentally measure the running of the three gauge coupling constants in order to see if they more closely match SM or SUSY. Some of the distinctions should be accessible at LHC energies. My intuition is also that we will probably find out that once the gauge coupling beta functions are modified to incorporate a running of the quantum gravity Newton's constant term that voila, we will find out that there is SM gauge unification at a renormalized Planck's constant value. A little 1% tweak here, and 1% tweak there over all of the orders of magnitude from TeV to the the GUT scale is really all it takes.

vmarko said...

Andrew,

Just a small note --- gravity is not renormalizable, so it would be really hard to make sense of "running of the quantum gravity Newton's constant".

Anonymous said...

vmarko,

Gravity is perturbatively non-renormalizable, but it may be renormalizable in a "would be" non-perturbative theory.One may speculate that Newton's constant runs with the energy scale in such a non-perturbative model.

vmarko said...

Anonymous,

AFAIK, the concept of renormalizability is always connected to the perturbation expansion. Given a full nonperturbative theory, you can perform a perturbation expansion at two different energy scales, and compare the corresponding coupling constants, thereby deriving their flow.

The theory is called renormalizable if the comparison between the couplings at two different scales can be made at all; otherwise the theory is nonrenormalizable.

The fact that general relativity is (perturbatively) nonrenormalizable means that one cannot compare couplings at two different energy scales (the number of the coupling constants is different at different energies, and the RGE-s cannot be formulated).

Given that, I don't understand what does it mean to say that Newton's constant runs with the energy scale in a nonperturbative theory. In order to even recognize what is Newton's constant, one needs to perform the perturbation expansion of this full theory, and one cannot construct a RGE for it, due to (perturbative) nonrenormalizability.

Anyway, we are getting off-topic here... :-)

Anonymous said...

vmarko,

Asymptotic safety scenarios in quantum gravity are related to my comment above, see for example:

http://www.scholarpedia.org/article/Asymptotic_Safety_in_quantum_gravity

and

http://www.physics.utoronto.ca/~manber/On%20the%20running%20of%20the%20coupling%20constants%209.pdf

vmarko said...

Anonymous,

Ok, I agree, in the asymptotic safety approach to QG one can indeed make sense of the running of G. But I'd say that AS is probably the only model with this property --- i.e. it's not a generic thing for QG, but specific to AS only.

Best, :-)
Marko

Ervin Goldfain said...

"But I'd say that AS is probably the only model with this property --- i.e. it's not a generic thing for QG, but specific to AS only".

There are few Quantum Gravity scenarios with fancy names that may directly (or indirectly) relate to Asymptotic Safety: Causal Dynamic Triangulation (CDT), Horava-Lifshitz gravity, Quantum Einstein Gravity (QEG) and models based on multifractional spacetimes.

But the "jury is still out" and is presently unclear if either one of these scenarios will stand the test of time.

vmarko said...

It wasn't my intention to get dragged into a discussion about this, but...

CDT can have AS only as an effective field theory approximation, which breaks down at small enough scales (since CDT is fundamentally piecewise-linear). So flow of G is not defined beyond the approximation scale.

Horava gravity has big problems with the scalar mode.

QEG and "multifractional spacetimes" are just frameworks and buzzwords, rather than well-defined precise models of QG.

Ervin Goldfain said...

vmarko,

Please read my last sentence, I am not disagreeing with you.

ruhul said...

The astronomers seem pretty sure DM isn't made out of standard model particles.