Sunday, 18 January 2015

Weekend plot: spin-dependent dark matter

This weekend plot is borrowed from a nice recent review on dark matter detection:
It shows experimental limits on the spin-dependent scattering cross section of dark matter on protons. This observable is not where the most spectacular race is happening, but it is important for constraining more exotic models of dark matter. Typically, a scattering cross section in the non-relativistic limit is independent of spin or velocity of the colliding particles. However, there exist reasonable models of dark matter where the low-energy cross section is more complicated. One possibility is that the interaction strength is proportional to the scalar product of spin vectors of a dark matter particle and a nucleon (proton or neutron). This is usually referred to as the spin-dependent scattering, although other kinds of spin-dependent forces that also depend on the relative velocity are possible.

In all existing direct detection experiments, the target contains nuclei rather than single nucleons. Unlike in the spin-independent case, for spin-dependent scattering the cross section is not enhanced by coherent scattering over many nucleons. Instead, the interaction strength is proportional to the expectation values of the proton and neutron spin operators in the nucleus.  One can, very roughly, think of this process as a scattering on an odd unpaired nucleon. For this reason, xenon target experiments such as Xenon100 or LUX are less sensitive to the spin-dependent scattering on protons because xenon nuclei have an even number of protons.  In this case,  experiments that contain fluorine in their target molecules have the best sensitivity. This is the case of the COUPP, Picasso, and SIMPLE experiments, who currently set the strongest limit on the spin-dependent scattering cross section of dark matter on protons. Still, in absolute numbers, the limits are many orders of magnitude weaker than in the spin-independent case, where LUX has crossed the 10^-45 cm^2 line. The IceCube experiment can set stronger limits in some cases by measuring the high-energy neutrino flux from the Sun. But these limits depend on what dark matter annihilates into, therefore they are much more model-dependent than the direct detection limits.  

Saturday, 10 January 2015

Weekend Plot: axion hunting

We just had a good laugh about the discovery of sharp-turning axions.  However, I should not leave you with the impression that there's something fishy in general with axion searches. On the contrary: while axions are hypothetical particles, they are arguably more motivated than all sorts of sthings and inos out there. Axions may explain the absence of a large  electric dipole moment of the neutron (the so-called strong-CP problem of QCD), and at the same time they may account for a part or all of dark matter in the Universe. The current state of experimental searches for axions is summarized in this plot:
The axes correspond to the two most relevant axion parameters: the mass, and the strength of its coupling to two photons (more precisely, the dimension-5 pion-like couplings to a photon field strength tensor and the dual tensor). In general, these can be independent parameters; however in particular models they are related. For example, in the KSVZ model - one of the QCD axion models solving the strong CP problem - one has m∼1/f and  g∼1/f, where f is the axion symmetry breaking scale. It follows that m and g are proportional to each other, as indicated by the green line in the plot. The yellow band is, roughly, the range predicted by other QCD axion models; therefore it is a natural beacon for axion searches. The CDM2 region in the band is where the QCD axion may account for all of dark matter in the universe.  

Currently, two experiments are at the forefront of axion searches. The CERN Axion Solar Telescope (CAST) attempts to detect axions produced in the Sun's core. The telescope is really a scavenged LHC magnet that converts axions into observable x-ray photons. CAST constrains the axion-photon coupling g  at the level of 10^-10/GeV for axion masses up to 1 eV (for larger masses the conversion length is too large for this setup). The Axion Dark Matter eXperiment (ADMX) targets axions of a different provenience. They search for conversion of axions that make the dark matter halo using a resonant microwave cavity within a magnet. They can probe much smaller values of the axion-photon couplings, but only for a small range of masses limited by the geometry of the cavity. Interestingly, both experiments have managed to probe parts of the QCD axion preferred region. Next stages of ADMX and CAST as well as the future IAXO telescope will continue to nibble the yellow band, although the dark matter preferred region may remain beyond our reach for quite some time...

Friday, 9 January 2015

Do-or-die year

The year 2015 began as any other year... I mean the hangover situation in particle physics. We have a theory of fundamental interactions - the Standard Model - that we know is certainly not the final  theory because it cannot account for dark matter, matter-antimatter asymmetry, and cosmic inflation. At the same time, the Standard Model perfectly describes any experiment we have performed here on Earth (up to a few outliers that can be explained as statistical fluctuations).  This is puzzling, because some these experiments are in principle sensitive to very heavy particles, sometimes well beyond the reach of the LHC or any future colliders. Theorists cannot offer much help at this point. Until recently,  naturalness was the guiding principle in constructing new theories, but  few have retained confidence in it. No other serious paradigm has appeared to replace naturalness. In short, we know for sure there is new physics beyond the Standard Model, but have absolutely no clue what it is and how big energy is needed to access it.

Yet 2015 is different because it is the year when LHC restarts at 13 TeV energy.  We should expect high-energy collisions some time in summer, and around 10 inverse femtobarns of data by the end of the year. This is the last significant energy jump most of us may experience before retirement, therefore this year is going to be absolutely crucial for the future of particle physics. If, by next Christmas, we don't hear any whispers of anomalies in LHC data, we will have to brace for tough times ahead. With no energy increase in sight, slow experimental progress, and no theoretical hints for a better theory, particle physics as we know it will be in deep merde.

You may protest this is too pessimistic. In principle, new physics may show up at the LHC anytime between this fall and the year 2030 when 3 inverse attobarns of data will have been accumulated. So the hope will not die completely anytime soon. However, the subjective probability of making a discovery will decrease exponentially as time goes on, as you can see in the attached plot. Without a discovery, the mood will soon plummet, resembling something of the late Tevatron, rather than the thrill of pushing the energy frontier that we're experiencing now.

But for now, anything may yet happen. Cross your fingers.

Thursday, 1 January 2015

2014 Mad Hat awards

New Year is traditionally the time of recaps and best-ofs. This blog is focused on particle physics beyond the standard model where compiling such lists is challenging, given the dearth of discoveries or even   plausible signals pointing to new physics.  Therefore I thought I could somehow honor those who struggle to promote our discipline by finding new signals against all odds, and sometimes against all logic. Every year from now on, the Mad Hat will be awarded to the researchers who make the most outlandish claim of a particle-physics-related discovery, on the condition it gets enough public attention.

The 2014 Mad Hat award unanimously goes to Andy Read, Steve Sembay, Jenny Carter, Emile Schyns, and, posthumously, to George Fraser, for the paper Potential solar axion signatures in X-ray observations with the XMM–Newton observatory. Although the original arXiv paper sadly went unnoticed, this remarkable work was publicized several months later by the Royal Astronomical Society press release and by the article in Guardian.

The crucial point in this kind of endeavor is to choose an observable that is noisy enough to easily accommodate a new physics signal. In this particular case the observable is x-ray emission from Earth's magnetosphere, which could include a component from axion dark matter emitted from the Sun and converting to photons. A naive axion hunter might expect the conversion signal should be observed by looking at the sun (that is the photon inherits the momentum of the incoming axion), something that XMM cannot do due to technical constraints. The authors thoroughly address this point in a sentence in Introduction, concluding that it would be nice if the x-rays could scatter afterwards at the right angle. Then the signal that is searched for is an annual modulation of the x-ray emission, as the magnetic field strength in XMM's field of view is on average larger in summer than in winter. A seasonal dependence of the x-ray flux is indeed observed, for which axion dark matter is clearly the most plausible explanation.

Congratulations to all involved. Nominations for the 2015 Mad Hat award are open as of today ;) Happy New Year everyone!