Sunday, 21 December 2014

Weekend plot: rare decays of B mesons, once again

This weekend's plot shows the measurement of the branching fractions for neutral B and Bs mesons decays into  muon pairs:
This is not exactly a new thing. LHCb and CMS separately announced evidence for the B0s→μ+μ- decay in summer 2013, and a preliminary combination of their results appeared a few days after. The plot above comes from the recent paper where a more careful combination is performed, though the results change only slightly.

A neutral B meson is a  bound state of an anti-b-quark and a d-quark (B0) or an s-quark (B0s), while for an anti-B meson the quark and the antiquark are interchanged. Their decays to μ+μ- are interesting because they are very suppressed in the standard model. At the parton level, the quark-antiquark pair annihilates into a μ+μ- pair. As for all flavor changing neutral current processes, the lowest order diagrams mediating these decays occur at the 1-loop level. On top of that, there is the helicity suppression by the small muon mass, and the CKM suppression by the small Vts (B0s) or Vtd (B0) matrix elements. Beyond the standard model one or more of these suppression factors may be absent and the contribution could in principle exceed that of the standard model even if the new particles are as heavy as ~100 TeV. We already know this is not the case for the B0s→μ+μ- decay. The measured branching fraction (2.8 ± 0.7)x10^-9  agrees within 1 sigma with the standard model prediction (3.66±0.23)x10^-9. Further reducing the experimental error will be very interesting in view of observed anomalies in some other b-to-s-quark transitions. On the other hand, the room for new physics to show up  is limited,  as the theoretical error may soon become a showstopper.

Situation is a bit different for the B0→μ+μ- decay, where there is still relatively more room for new physics. This process has been less in the spotlight. This is partly due to a theoretical prejudice: in most popular new physics models it is impossible to generate a large effect in this decay without generating a corresponding excess in B0s→μ+μ-. Moreover,  B0→μ+μ- is experimentally more difficult:  the branching fraction predicted by the standard model is (1.06±0.09)x10^-10, which is 30 times smaller than that for  B0s→μ+μ-. In fact, a 3σ evidence for the B0→μ+μ- decay appears only after combining LHCb and CMS data. More interestingly, the measured branching fraction, (3.9±1.4)x10^-10, is some 2 sigma above the standard model value. Of course, this is  most likely a statistical fluke, but nevertheless it will be interesting to see an update once the 13-TeV LHC run collects enough data.

Saturday, 13 December 2014

Planck: what's new

Slides from the recent Planck collaboration meeting are now available online. One can find there preliminary results that include an input from Planck's measurements of the polarization of the  Cosmic Microwave Background (some which were previously available via the legendary press release in French). I already wrote about the new  important limits on dark matter annihilation cross section. Here I picked up a few more things that may be of interest for a garden variety particle physicist.

  • ΛCDM. 
    Here is a summary of Planck's best fit parameters of the standard cosmological model with and without the polarization info:

    Note that the temperature-only numbers are slightly different than in the 2013 release, because of improved calibration and foreground cleaning.  Frustratingly, ΛCDM remains  solid. The polarization data do not change the overall picture, but they shrink some errors considerably. The Hubble parameter remains at a low value; the previous tension with Ia supernovae observations seems to be partly resolved and blamed on systematics on the supernovae side.  For the large scale structure fans, the parameter σ8 characterizing matter fluctuations today remains at a high value, in some tension with weak lensing and cluster counts. 
  • Neff.
    There are also better limits on deviations from ΛCDM. One interesting result is the new improved constraint on the effective number of neutrinos, Neff in short. The way this result is presented may be confusing.  We know perfectly well there are exactly 3 light active (interacting via weak force) neutrinos; this has been established in the 90s at the LEP collider, and Planck has little to add in this respect. Heavy neutrinos, whether active or sterile, would not show in this measurement at all.  For light sterile neutrinos, Neff implies an upper bound on the mixing angle with the active ones. The real importance of  Neff lies in that it counts any light particles (other than photons) contributing to the energy density of the universe at the time of CMB decoupling. Outside the standard model neutrinos, other theorized particles could contribute any real positive number to Neff, depending on their temperature and spin. A few years ago there have been consistent hints of Neff  much larger 3, which would imply physics beyond the standard model. Alas, Planck has shot down these claims. The latest number combining Planck and Baryon Acoustic Oscillations is Neff =3.04±0.18, spot on 3.046 expected from the standard model neutrinos.  This represents an important constraint on any new physics model with very light (less than eV) particles. 
  • Σmν.
    The limit on the sum of the neutrino masses keeps improving and gets into a really interesting regime. Recall that, from oscillation experiments, we can extract the neutrino mass differences: Δm32 ≈ 0.05 eV and Δm12≈0.009 eV up to a sign, but we don't know their absolute masses.  Planck and others have already excluded the possibility that all 3 neutrinos have approximately the same mass. Now they are not far from probing the so-called inverted hierarchy, where two neutrinos have approximately the same mass and the 3rd is much lighter, in which case Σmν ≈ 0.1 eV. Planck and Baryon Acoustic Oscillations set the limit Σmν < 0.16 eV at 95% CL, however this result is not strongly advertised because it is sensitive to the value of the Hubble parameter. Including non-Planck measurements leads to a weaker, more conservative limit Σmν < 0.23 eV, the same as quoted in the 2013 release. 
  • CνB.
    For dessert, something cool. So far we could observe the cosmic neutrino background only through its contribution to the  energy density of radiation in the early universe. This affects observables that can be inferred from the CMB acoustic peaks, such as the Hubble expansion rate or the time of matter-radiation equality. Planck, for the first time, probes the properties of the CνB. Namely, it measures the  effective sound speed ceff and viscosity cvis parameters, which affect the growth of perturbations in the CνB. Free-streaming particles like the neutrinos should have ceff^2 =  cvis^2 = 1/3, while Planck measures ceff^2 = 0.3256±0.0063 and  cvis^2 = 0.336±0.039. The result is unsurprising, but it may help constraining some more exotic models of neutrino interactions. 

To summarize, Planck continues to deliver disappointing results, and there's still more to follow ;)

Monday, 1 December 2014

After-weekend plot: new Planck limits on dark matter

The Planck collaboration just released updated results that include an input from their  CMB polarization measurements. The most interesting are the new constraints on the annihilation cross section of dark matter:

Dark matter annihilation in the early universe injects energy into the primordial plasma and increases the ionization fraction. Planck is looking for imprints of that in the CMB temperature and polarization spectrum. The relevant parameters are the dark matter mass and  <σv>*feff, where <σv> is the thermally averaged annihilation cross section during the recombination epoch, and feff ~0.2 accounts for the absorption efficiency. The new limits are a factor of 5 better than the latest ones from the WMAP satellite, and a factor of 2.5 better than the previous combined constraints.

What does it mean for us?  In vanilla models of thermal WIMP dark matter <σv> = 3*10^-26 cm^3/sec, in which case dark matter particles with masses below ~10 GeV are excluded by Planck. Actually, in this mass range the Planck limits are far less stringent the ones obtained by the Fermi collaboration from gamma-ray observations of dwarf galaxies. However, the two are complementary to some extent. For example, Planck probes the annihilation cross section in the early universe, which can be different than today. Furthermore, the CMB constraints obviously do not depend on the distribution of dark matter in galaxies, which is a serious source of uncertainty for cosmic rays experiments.  Finally, the CMB limits extend to higher dark matter masses where gamma-ray satellites lose sensitivity. The last point implies that Planck can weigh in on the PAMELA/AMS cosmic-ray positron excess. In models where the dark matter annihilation cross section during the recombination epoch is the same as today, the mass and cross section range that can explain the excess is excluded by Planck. Thus, the new results make it even more difficult to interpret the positron anomaly as a signal of dark matter.

Wednesday, 19 November 2014

Update on the bananas

One of the most interesting physics stories of this year was the discovery of an unidentified 3.5 keV x-ray  emission line from galactic clusters. This so-called bulbulon can be interpreted as a signal of a sterile neutrino dark matter particle decaying into an active neutrino and  a photon. Some time ago I wrote about the banana paper that questioned the dark matter origin of the signal. Much has happened since, and I owe you an update. The current experimental situation is summarized in this plot:

To be more specific, here's what's happening.

  •  Several groups searching for the 3.5 keV emission have reported negative results. One of those searched for the signal in dwarf galaxies, which offer a  much cleaner environment allowing for a more reliable detection. No signal was found, although the limits do not exclude conclusively the original bulbulon claim. Another study looked for the signal in multiple galaxies. Again, no signal was found, but this time the reported limits are in severe tension with the sterile neutrino interpretation of the bulbulon. Yet another study failed to find the 3.5 keV line in  Coma, Virgo and Ophiuchus clusters, although they detect it in the Perseus cluster. Finally, the banana group analyzed the morphology of the 3.5 keV emission from the Galactic center and Perseus and found it incompatible with dark matter decay.
  • The discussion about the existence of the 3.5 keV emission from the Andromeda galaxy is  ongoing. The conclusions seem to depend on the strategy to determine the continuum x-ray emission. Using data from the XMM satellite, the banana group fits the background in the 3-4 keV range  and does not find the line, whereas this paper argues it is more kosher to fit in the 2-8 keV range, in which case the line can be detected in exactly the same dataset. It is not obvious who is right, although the fact that the significance of the signal depends so strongly on the background fitting procedure is not encouraging. 
  • The main battle rages on around K-XVIII (X-n stands for the X atom stripped of n-1 electrons; thus, K-XVIII is the potassium ion with 2 electrons). This little bastard has emission lines at 3.47 keV and 3.51 keV which could account for the bulbulon signal. In the original paper, the bulbuline group invokes a model of plasma emission that allows them to constrain  the flux due to the K-XVIII emission from  the  measured ratios of the strong S-XVI/S-XV and Ca-XX/Ca-XIX lines. The banana paper argued that the bulbuline model is unrealistic as it  gives inconsistent predictions for some plasma line ratios. The bulbuline group pointed out that the banana group used wrong numbers to estimate the line emission strenghts. The banana group maintains that their conclusions still hold when the error is corrected. It all boils down to the question whether the allowed range for the K-XVIII emission strength assumed by the bulbine group is conservative enough. Explaining the 3.5 keV feature solely by K-XVIII requires assuming element abundance ratios that are very different than the solar one, which may or may not be realistic.   
  •  On the other hand, both groups have converged on the subject of chlorine. In the banana  paper it  was pointed out that the 3.5 keV line may be due to the Cl-XVII (hydrogen-like chlorine ion) Lyman-β transition which happens to be at 3.51 keV. However the bulbuline group subsequently derived limits on the corresponding Lyman-α line at 2.96 keV. From these limits, one can deduce in a fairly model-independent way that the contribution of Cl-XVII Lyman-β transition is negligible.   

To clarify the situation we need more replies to comments on replies, and maybe also  better data from future x-ray satellite missions. The significance of the detection depends, more than we'd wish, on dirty astrophysics involved in modeling the standard x-ray emission from galactic plasma. It seems unlikely that the sterile neutrino model with the originally reported parameters will stand, as it is in tension with several other analyses. The probability of the 3.5 keV signal being of dark matter origin is certainly much lower than a few months ago. But the jury is still out, and it's not impossible to imagine that more data and more analyses will tip the scales the other way.

Further reading: how to protect yourself from someone attacking you with a banana.

Saturday, 8 November 2014

Weekend Plot: Fermi and 7 dwarfs

This weekend the featured plot is borrowed from the presentation of Brandon Anderson at the symposium of the Fermi collaboration last week:

It shows the limits on the cross section of dark matter annihilation into b-quark pairs derived from gamma-ray observations of satellite galaxies of the Milky Way. These so-called dwarf galaxies are the most dark matter dominated objects known, which makes them a convenient place to search for dark matter. For example, WIMP dark matter annihilating into charged standard model particles would lead to an extended gamma-ray emission that could be spotted by the Fermi space telescope. Such emission coming from dwarf galaxies would be a smoking-gun signature of dark matter annihilation, given the relatively low level of dirty astrophysical backgrounds there (unlike in the center of our galaxy). Fermi has been looking for such signals, and a year ago they already published limits on the cross-section of dark matter annihilation into different final states.  At the time, they also found a ~2 sigma excess that was intriguing, especially in conjunction with the observed gamma-ray excess from the center of our galaxy. Now Fermi is coming back with an updated analysis using more data and better calibration. The excess is largely gone and, for the bb final state, the new limits  (blue) are 5 times stronger than the previous ones (black). For the theoretically favored WIMP annihilation cross section (horizontal dashed line), dark matter particle  annihilating into b-quarks is excluded if its mass is below ~100 GeV. The new limits are in tension with the dark matter interpretation of the galactic center excess (various colorful rings, depending who you like). Of course, astrophysics is not an exact science, and by exploring numerous uncertainties one can soften the tension. What is more certain is that a smoking-gun signature of dark matter annihilation in dwarf galaxies is unlikely to be delivered in the foreseeable future.    

Sunday, 19 October 2014

Weekend Plot: Bs mixing phase update

Today's featured plot was released last week by the LHCb collaboration:

It shows the CP violating phase in Bs meson mixing, denoted as φs,  versus the difference of the decay widths between the two Bs meson eigenstates. The interest in φs comes from the fact that it's  one of the precious observables that 1) is allowed by the symmetries of the Standard Model, 2) is severely suppressed due to the CKM structure of flavor violation in the Standard Model. Such observables are a great place to look for new physics (other observables in this family include Bs/Bd→μμ, K→πνν, ...). New particles, even too heavy to be produced directly at the LHC, could produce measurable contributions to φs as long as they don't respect the Standard Model flavor structure. For example, a new force carrier with a mass as large as 100-1000 TeV and order 1 flavor- and CP-violating coupling to b and s quarks would be visible given the current experimental precision. Similarly, loops of supersymmetric particles with 10 TeV masses could show up, again if the flavor structure in the superpartner sector is not aligned with that in the  Standard Model.

The phase φs can be measured in certain decays of neutral Bs mesons where the process involves an interference of direct decays and decays through oscillation into the anti-Bs meson. Several years ago measurements at Tevatron's D0 and CDF experiments suggested a large new physics contribution. The mild excess has gone away since, like many other such hints.  The latest value quoted by LHCb is φs = - 0.010 ± 0.040, which combines earlier measurements of the Bs → J/ψ π+ π- and  Bs → Ds+ Ds- decays with  the brand new measurement of the Bs → J/ψ K+ K- decay. The experimental precision is already comparable to the Standard Model prediction of φs = - 0.036. Further progress is still possible, as the Standard Model prediction can be computed to a few percent accuracy.  But the room for new physics here is getting tighter and tighter.

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.