Wednesday, 25 February 2015

Persistent trouble with bees

No, I still have nothing to say about colony collapse disorder... this blog will stick to physics for at least 2 more years. This is an update on the anomalies in B decays reported by the LHCbee experiment. The two most important ones are:

  1. The  3.7 sigma deviation from standard model predictions in the differential distribution of the B➝K*μ+μ- decay products.
  2.  The 2.6 sigma violation of lepton flavor universality in B+→K+l+l- decays. 

 The first anomaly is statistically more significant. However, the theoretical error of the standard model prediction is not trivial to estimate and the significance of the anomaly is subject to fierce discussions. Estimates in the literature range from 4.5 sigma to 1 sigma, depending on what is assumed about QCD uncertainties. For this reason, the second anomaly made this story much more intriguing.  In that case, LHCb measures the ratio of the decay with muons and with electrons:  B+→K+μ+μ- vs B+→K+e+e-. This observable is theoretically clean, as large QCD uncertainties cancel in the ratio. Of course, 2.7 sigma significance is not too impressive; LHCb once had a bigger anomaly (remember CP violation in D meson decays?)  that is now long gone. But it's fair to say that the two anomalies together are marginally interesting.      

One nice thing is that both anomalies can be explained at the same time by a simple modification of the standard model. Namely, one needs to add the 4-fermion coupling between a b-quark, an s-quark, and two muons:

with Λ of order 30 TeV. Just this one extra coupling greatly improves a fit to the data, though other similar couplings could be simultaneously present. The 4-fermion operators can be an effective description of new heavy particles coupled to quarks and leptons.  For example, a leptoquark (scalar particle with a non-zero color charge and lepton number) or a Z'  (neutral U(1) vector boson) with mass in a few TeV range have been proposed. These are of course simple models created ad-hoc. Attempts to put these particles in a bigger picture of physics beyond  the standard model have not been very convincing so far, which may be one reason why the anomalies are viewed a bit skeptically. The flip side is that, if the anomalies turn out to be real, this will point to unexpected symmetry structures around the corner.

Another nice element of this story is that it will be possible to acquire additional relevant information in the near future. The first anomaly is based on just 1 fb-1 of LHCb data, and it will be updated to full 3 fb-1 some time this year. Furthermore, there are literally dozens of other B decays where the 4-fermion operators responsible for the anomalies could  also show up. In fact, there may already be some hints that this is happening. In the table borrowed from this paper we can see that there are several other  2-sigmish anomalies in B-decays that may possibly have the same origin. More data and measurements in  more decay channels should clarify the picture. In particular, violation of lepton flavor universality may come together with lepton flavor violation.  Observation of decays forbidden in the standard model, such as B→Keμ or  B→Kμτ, would be a spectacular and unequivocal signal of new physics.

Saturday, 7 February 2015

Weekend Plot: Inflation'15

The Planck collaboration is releasing new publications based on their full dataset, including CMB temperature and large-scale polarization data.  The updated values of the crucial  cosmological parameters were already made public in December last year, however one important new element is the combination of these result with the joint Planck/Bicep constraints on the CMB B-mode polarization.  The consequences for models of inflation are summarized in this plot:

It shows the constraints on the spectral index ns and the tensor-to-scalar ratio r of the CMB fluctuations, compared to predictions of various single-field models of inflation.  The limits on ns changed slightly compared to the previous release, but the more important progress is along the y-axis. After including the joint Planck/Bicep analysis (in the plot referred to as BKP), the combined limit on the tensor-to-scalar ratio becomes r < 0.08.  What is also important, the new limit is much more robust; for example, allowing for a scale dependence of the spectral index  relaxes the bound  only slightly,  to r< 0.10.

The new results have a large impact on certain classes models. The model with the quadratic inflaton potential, arguably the simplest model of inflation, is now strongly disfavored. Natural inflation, where the inflaton is a pseudo-Golsdtone boson with a cosine potential, is in trouble. More generally, the data now favors a concave shape of the inflaton potential during the observable period of inflation; that is to say, it looks more like a hilltop than a half-pipe. A strong player emerging from this competition is R^2 inflation which, ironically, is the first model of inflation ever written.  That model is equivalent to an exponential shape of the inflaton potential, V=c[1-exp(-a φ/MPL)]^2, with a=sqrt(2/3) in the exponent. A wider range of the exponent a can also fit the data, as long as a is not too small. If your favorite theory predicts an exponential potential of this form, it may be a good time to work on it. However, one should not forget that other shapes of the potential are still allowed, for example a similar exponential potential without the square V~ 1-exp(-a φ/MPL), a linear potential V~φ, or more generally any power law potential V~φ^n, with the power n≲1. At this point, the data do not favor significantly one or the other. The next waves of CMB polarization experiments should clarify the picture. In particular, R^2 inflation predicts 0.003 < r < 0.005, which is should be testable in a not-so-distant future.

Planck's inflation paper is here.

Wednesday, 4 February 2015

B-modes: what's next

The signal of gravitational waves from inflation is the holy grail of cosmology. As is well known, at the end of a quest for the holy grail there is always the Taunting Frenchman....  This is also the fate of the BICEP quest for primordial B-mode polarization imprinted in the Cosmic Microwave Background by the gravitational waves.  We've already known, since many months, that the high intensity of the galactic dust foreground does not allow BICEP2 to unequivocally detect the primordial B-mode signal. The only open question was how strong limits on the parameter r - the tensor-to-scalar ratio of primordial fluctuations - can be set. This is the main result of the recent paper that combines data from the BICEP2, Keck Array, and Planck instruments. BICEP2 and Keck are orders of magnitude more sensitive than Planck to CMB polarization fluctuations. However, they made measurements only at one frequency of 150 GhZ where the CMB signal is large. Planck, on the other hand, can contribute  measurements at higher frequencies where the galactic dust dominates, which allows them to map out the foregrounds in the window observed by BICEP. Cross-correlating the Planck and BICEP maps allows one to subtract the dust component, and extract the constraints on the parameter r. The limit quoted by BICEP and Planck,  r < 0.12, is however worse than  r < 0.11 from Planck's analysis of temperature fluctuations. This still leaves a lot of room for the primordial B-mode signal hiding in the CMB.  

So the BICEP2 saga is definitely over, but the search for the primordial B-modes is not.  The lesson we learned is that single frequency instruments like BICEP2 are not good in view of large galactic foregrounds. The road ahead is then clear: build more precise multi-frequency instruments, such that foregrounds can be subtracted. While we will not send a new CMB satellite observatory anytime soon, there are literally dozens of ground based and balloon CMB experiments already running or coming online in the near future. In particular, the BICEP program continues, with Keck Array running at other frequencies, and the more precise BICEP3 telescope to be completed this year. Furthermore, the SPIDER balloon experiment just completed the first Antarctica flight early this year, with a two-frequency instrument on board. Hence, better limits on r are expected already this year. See the snapshots below, borrowed from these slides, for a compilation of upcoming experiments.

Impressive, isn't it? These experiments should be soon sensitive to r~0.01, and in the long run to r~0.001. Of course, there is no guarantee of a detection. If the energy scale of inflation is just a little below 10^16 GeV, then we will never observe the signal of gravitational waves. Thus, the success of this enterprise crucially depends on Nature being kind. However the high stakes make  these searches worthwhile. A discovery, would surely count among the greatest scientific breakthrough of 21st century. Better limits, on the other hand, will exclude some simple models of inflation.  For example, single-field inflation with a quadratic potential is already under pressure. Other interesting models, such as natural inflation, may go under the knife soon. 

For quantitative estimates of future experiments' sensitivity to r, see this paper.

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!