Friday, 27 May 2016

CMS: Higgs to mu tau is going away

One interesting anomaly in the LHC run-1 was a hint of Higgs boson decays to a muon and a tau lepton. Such process is forbidden in the Standard Model by the conservation of  muon and tau lepton numbers. Neutrino masses violate individual lepton numbers, but their effect is far too small to affect the Higgs decays in practice. On the other hand, new particles do not have to respect global symmetries of the Standard Model, and they could induce lepton flavor violating Higgs decays at an observable level. Surprisingly, CMS found a small excess in the Higgs to tau mu search in their 8 TeV data, with the measured branching fraction Br(h→τμ)=(0.84±0.37)%.  The analogous measurement in ATLAS is 1 sigma above the background-only hypothesis, Br(h→τμ)=(0.53±0.51)%. Together this merely corresponds to a 2.5 sigma excess, so it's not too exciting in itself. However, taken together with the B-meson anomalies in LHCb, it has raised hopes for lepton flavor violating new physics just around the corner.  For this reason, the CMS excess inspired a few dozen of theory papers, with Z' bosons, leptoquarks, and additional Higgs doublets pointed out as possible culprits.

Alas, the wind is changing. CMS made a search for h→τμ in their small stash of 13 TeV data collected in 2015. This time they were hit by a negative background fluctuation, and they found Br(h→τμ)=(-0.76±0.81)%. The accuracy of the new measurement is worse than that in run-1, but nevertheless it lowers the combined significance of the excess below 2 sigma. Statistically speaking, the situation hasn't changed much,  but psychologically this is very discouraging. A true signal is expected to grow when more data is added, and when it's the other way around it's usually a sign that we are dealing with a statistical fluctuation...

So, if you have a cool model explaining the h→τμ  excess be sure to post it on arXiv before more run-2 data is analyzed ;)

Saturday, 14 May 2016

Plot for Weekend: new limits on neutrino masses

This weekend's plot shows the new limits on neutrino masses from the KamLAND-Zen experiment:

KamLAND-Zen is a group of buddhist monks studying a balloon filled with the xenon isotope Xe136. That isotope has a very long lifetime, of order 10^21 years, and undergoes the lepton-number-conserving double beta decay Xe136 → Ba136 2e- 2νbar. What the monks hope to observe is the lepton violating neutrinoless double beta decay Xe136 → Ba136+2e, which would show as a peak in the invariant mass distribution of the electron pairs near 2.5 MeV. No such signal has been observed, which sets the limit on the half-life for this decay at T>1.1*10^26 years.

The neutrinoless decay is predicted to occur if neutrino masses are of Majorana type, and the rate can be characterized by the effective mass Majorana mββ (y-axis in the plot). That parameter is a function of the masses and mixing angles of the neutrinos. In particular it depends on the mass of the lightest neutrino (x-axis in the plot) which is currently unknown. Neutrino oscillations experiments have precisely measured the mass^2 differences between neutrinos, which  are roughly (0.05 eV)^2 and (0.01 eV)^2. But oscillations are not sensitive to the absolute mass scale; in particular, the lightest neutrino may well be massless for all we know.  If the heaviest neutrino has a small electron flavor component, then we expect that the mββ parameter is below 0.01 eV.  This so-called normal hierarchy case is shown as the red region in the plot, and is clearly out of experimental reach at the moment. On the other hand, in the inverted hierarchy scenario (green region in the plot), it is the two heaviest neutrinos that have a significant electron component. In this case,  the effective Majorana mass mββ is around 0.05 eV.  Finally, there is also the degenerate scenario (funnel region in the plot) where all 3 neutrinos have very similar masses with small splittings, however this scenario is now strongly disfavored by cosmological limits on the sum of the neutrino masses (e.g. the Planck limit Σmν < 0.16 eV).

As can be seen in the plot, the results from KamLAND-Zen, when translated into limits on the effective Majorana mass, almost touch the inverted hierarchy region. The strength of this limit depends on some poorly known nuclear matrix elements (hence the width of the blue band). But even in the least favorable scenario future, more sensitive experiments should be able to probe that region. Thus, there is a hope  that within the next few years we may prove the Majorana nature of neutrinos, or at least disfavor the inverted hierarchy scenario.

Monday, 9 May 2016

Off we go

The LHC is back in action since last weekend, again colliding protons with 13 TeV energy. The weasels' conspiracy was foiled, and the perpetrators were exemplarily electrocuted. PhD students have been deployed around the LHC perimeter to counter any further sabotage attempts (stoats are known to have been in league with weasels in the past). The period that begins now may prove to be the most exciting time for particle physics in this century.  Or the most disappointing.

The beam intensity is still a factor of 10 below the nominal one, so  the harvest of last weekend is meager 40 inverse picobarns. But the number of proton bunches in the beam is quickly increasing, and once it reaches O(2000), the data will stream at a rate of a femtobarn per week or more. For the nearest future, the plan is to have a few inverse femtobarns on tape by mid-July, which would roughly double the current 13 TeV dataset. The first analyses of this chunk of data  should be presented around the time of the  ICHEP conference in early August. At that point we will know whether the 750 GeV particle is real. Celebrations will begin if the significance of the diphoton peak increases after adding the new data, even if the statistics is not enough to officially announce  a discovery. In the best of all worlds, we may also get a hint of a matching 750 GeV peak in another decay channel (ZZ, Z-photon, dilepton, t-tbar,...) which would help focus our model building. On the other hand, if the significance of the diphoton peak drops in August, there will be a massive hangover...

By the end of October, when the 2016 proton collisions are scheduled to end, the LHC hopes to collect some 20 inverse femtobarns of data. This should already give us a rough feeling of new physics within the reach of the LHC. If a hint of another resonance is seen at that point, one will surely be able to confirm or refute it with the data collected in the following years.  If nothing is seen... then you should start telling yourself that condensed matter physics is also sort of fundamental,  or that systematic uncertainties in astrophysics are not so bad after all...  In any scenario, by December, when first analyses of the full  2016 dataset will be released,  we will know infinitely more than we do today.

So fasten your seat belts and get ready for a (hopefully) bumpy ride. Serious rumors should start showing up on blogs and twitter starting from July.

Friday, 1 April 2016

April Fools' 16: Was LIGO a hack?


This post is an April Fools' joke. LIGO's gravitational waves are for real. At least I hope so ;) 

We have had recently a few scientific embarrassments, where a big discovery announced with great fanfares was subsequently overturned by new evidence.  We still remember OPERA's faster than light neutrinos which turned out to be a loose cable, or BICEP's gravitational waves from inflation, which turned out to be galactic dust emission... It seems that another such embarrassment is coming our way: the recent LIGO's discovery of gravitational waves emitted in a black hole merger may share a similar fate. There are reasons to believe that the experiment was hacked, and the signal was injected by a prankster.

From the beginning, one reason to be skeptical about LIGO's discovery was that the signal  seemed too beautiful to be true. Indeed, the experimental curve looked as if taken out of a textbook on general relativity, with a clearly visible chirp signal from the inspiral phase, followed by a ringdown signal when the merged black hole relaxes to the Kerr state. The reason may be that it *is* taken out of a  textbook. This is at least what is strongly suggested by recent developments.

On EvilZone, a well-known hacker's forum, a hacker using a nickname Madhatter was boasting that it was possible to tamper with scientific instruments, including the LHC, the Fermi satellite, and the LIGO interferometer.  When challenged, he or she uploaded a piece of code that allows one to access LIGO computers. Apparently, the hacker took advantage the same backdoor that allows the selected members of the LIGO team to inject a fake signal in order to test the analysis chain.  This was brought to attention of the collaboration members, who  decided to test the code. To everyone's bewilderment, the effect was to reproduce exactly the same signal in the LIGO apparatus as the one observed in September last year!

Even though the traces of a hack cannot be discovered, there is little doubt now that there was a foul play involved. It is not clear what was the motif of the hacker: was it just a prank, or maybe an elaborate plan to discredit the scientists. What is even more worrying is that the same thing could happen in other experiments. The rumor is that the ATLAS and CMS collaborations are already checking whether the 750 GeV diphoton resonance signal could also be injected by a hacker.

Thursday, 17 March 2016

Diphoton update

Today at the Moriond conference ATLAS and CMS updated their diphoton resonance searches. There's been a rumor of an ATLAS analysis with looser cuts on the photons where the significance of the 750 GeV excess grows to a whopping 4.7 sigma. The rumor had it that the this analysis would be made public today, so the expectations were high. However, the loose-cuts analysis was not approved in time by the collaboration, and the fireworks display was cancelled.  In any case,  there was some good news today, and some useful info for model builders was provided.











Let's start with ATLAS. For the 13 TeV results, they now have two analyses: one called spin-0 and one called spin-2. Naively, the cuts in the latter are not optimized not for a spin-2 resonance but rather for a high-mass resonance  (where there's currently no significant excess), so the spin-2 label should not be treated too seriously in this case. Both analyses show a similar excess at 750 GeV: 3.9 and 3.6 sigma respectively for a wide resonance. Moreover, ATLAS provides additional information about the diphoton events, such as the angular distribution of the photons, the number of accompanying jets, the amount of missing energy, etc. This may be very useful for theorists entertaining less trivial models, for example when the 750 GeV resonance is produced  from a decay of a heavier parent particle. Finally, ATLAS shows a re-analysis of the diphoton events collected at 8 TeV center-of-energy of the LHC. The former run-1 analysis was a bit sloppy in the interesting mass range; for example, no limits at all were given for a 750 GeV scalar hypothesis.  Now the run-1 data have been cleaned up and analyzed using the same methods as in run-2. Excitingly, there's a 2 sigma excess in the spin-0 analysis in run-1, roughly compatible with what one would expect given the observed run-2 excess!   No significant excess is seen for the spin-2 analysis, and the tension between the run-1 and run-2 data is quite severe in this case. Unfortunately, ATLAS does not quote the combined significance and the best fit cross section for the 750 GeV resonance.

For CMS, the big news is that the amount of 13 TeV data at their disposal has increased by 20%. Using MacGyver skills, they managed to make sense of the chunk of data collected when the CMS magnet was off due to a technical problem. Apparently it was worth it, as new diphoton events have been found in the 750 GeV ballpark. Thanks to that, and a better calibration,  the significance of the diphoton excess in run-2  actually increases up to 2.9 sigma!  Furthermore, much like ATLAS, CMS updated their run-1 diphoton analyses and combined them with the run-2 ones.  Again, the combination increases the significance of the 750 GeV excess. The combined significance quoted by CMS is 3.4 sigma,  similar for spin-0 and spin-2 analyses. Unlike in ATLAS, the best fit is for a narrow resonance, which is the more preferred option from the theoretical point of view.

In summary, the diphoton excess survived the first test.  After adding more data and improving the analysis techniques the significance slightly increases rather than decreases, as expected for a real particle.  The signal is now a bit more solid: both experiments have a similar amount of diphoton data and they both claim a similar significance of the  750 GeV bump.  It may be a good moment to rename the ATLAS diphoton excess as the LHC diphoton excess :)  So far, the story of 2012 is repeating itself: the initial hints of a new resonance solidify into a consistent picture. Are we going to have another huge discovery this summer?

Friday, 11 March 2016

750 GeV: the bigger picture

This Thursday the ATLAS and CMS experiments will present updated analyses of the 750 GeV diphoton excess. CMS will extend their data set by the diphoton events collected in the periods when the detector was running without the magnetic field (which is not essential for this particular study), so the amount of available data will slightly increase. We will then enter the Phase-II of the excitement protocol,  hopefully followed this summer by another 4-th-of-July-style discovery. To close the Phase-I, here's a long-promised post about the bigger picture. There's at least 750 distinct models that can accommodate the diphoton signal observed by ATLAS and CMS. However, a larger framework for physics beyond the Standard Model it which these phenomenological models can be embedded is a more tricky question. Here is a bunch of speculations.

Whenever a new fluctuation is spotted at the LHC one cannot avoid mentioning supersymmetry. However,  the 750 GeV resonance cannot be naturally interpreted in this framework, not the least because it cannot be identified as a superpartner of any known particles. The problem is that explaining the observed signal strength requires introducing new particles with large couplings, and the complete theory typically enters into a strong coupling regime at the energy scale of a few TeV. This is not the usual SUSY paradigm, with weakly coupled physics at the TeV scale followed by a desert up to the grand unification scale. Thus, even if the final answer may still turn out to be supersymmetric, it will not be the kind of SUSY we've been expecting all along. Weakly coupled supersymmetric explanations are still possible in somewhat more complicated scenarios with new very light sub-GeV particles and cascade decays, see e.g. this NMSSM model.

Each time you see a diphoton peak you want to cry Higgs, since this is how the 125 GeV Higgs boson was first spotted. Many theories predict an extended Higgs sector with multiple heavy scalar particles, but again such a framework is not the most natural one for interpreting the 750 GeV resonance. There are two main reasons. One is that different Higgs scalars typically mix, but the mixing angle in this case is severely constrained by Higgs precision studies and non-observation of 750 GeV diboson resonances in other channel. The other is that, for a 750 GeV Higgs scalar, the branching fraction into the diphoton final state is typically tiny (e.g., ~10^-7 for a Standard-Model-Higgs-like scalar) and a complicated model gymnastics is needed to enhance it. The possibility that the 750 GeV resonance is a heavy Higgs boson is by no means excluded, but I would be surprised if this were the case.  

It is more tempting to interpret the diphoton resonance as a bound state of new strong interactions with a confinement scale in the TeV range. We know that the Quantum Chromodynamics (QCD) theory, which describes the strong interactions of the Standard Model quarks, gives rise to many scalar mesons and higher-spin resonances at low energies. Such a behavior is characteristic for a large class of similar theories.  Furthermore,  if the new strong sector contains mediator particles  that carry color and electromagnetic charges, the production in gluon fusion and decay into photons is possible for the composite states, see e.g. here.  The problem is that, much as for QCD, one would expect not one but an entire battalion of resonances. One needs to understand how the remaining resonances predicted by typical strongly interacting models could have avoided detection so far.

One way this could happen is if the 750 GeV resonance is a scalar that, for symmetry reasons, is much lighter than most of the particles in the strong sector. Here again our QCD may offer us a clue, as it contains pseudo-scalar particles, the so-called pions,  which are a factor of 10 lighter than the typical mass scale of other resonances. In QCD, pions are Goldstone bosons of the chiral symmetry spontaneously broken by the vacuum quark condensate. In other words, the smallness of the pion mass is  protected by a symmetry, and general theorems worked out in the 60s  ensure the quantum stability of such an arrangement. The similar mechanism can be easily implemented in other strongly interacting theories,  and it is possible to realize the 750 GeV resonance as a new kind of pion, see e.g. here.   Even the mechanism for decaying into photons -- via chiral anomalies -- can be borrowed directly from QCD. However, the symmetry protecting the 750 GeV scalar could also be completely different that the ones we have seen so far. One example is the dilaton, that is   a Goldstone boson of a spontaneously broken conformal symmetry, see e.g. here. This is a theoretically interesting possibility, since approximate conformal symmetry often arises as a feature of strongly interacting theories. All in all, the 750 GeV particle may well be  a pion or dilaton harbinger of new strong interactions at a TeV scale. One can then further speculate that the Higgs boson also originates from that sector, but that is a separate story that may or may not be true.

Another larger framework worth mentioning here is that of extra dimensions. In the modern view, theories with the new 4th dimension of space are merely an effective description of strongly interacting sectors discussed above. For example, the famous Randall-Sundrum model, with the Standard Model living in a section of a 5D AdS5 space, is a weakly coupled dual description of strongly coupled theories with a conformal symmetry and a large N gauge symmetry. These models thus offer a calculable way to embed the 750 GeV resonance in a strongly interacting theory. For example, the dilaton can be effectively described in the Randall-Sundrum model as the radion - a scalar particle corresponding to fluctuations of the size of the 5th dimension, see e.g. here. Moreover, the Randall-Sundrum framework  provides a simple way to realize the 750 GeV particle as a spin-2 resonance. Indeed, the model always contains massive Kaluza-Klein  excitations of the graviton, whose couplings to matter can be much stronger than that of the massless graviton. This possibility have been relatively less explored so far, see e.g.  here,  but that may change next week...

Clearly, it is impossible to say anything conclusive at this point. More data in multiple decay channels is absolutely necessary  for a more concrete picture to emerge. For me personally, a confirmation of the 750 GeV excess would be a strong hint for new strong interactions at a few TeV scale. And if this is indeed the case,  one may seriously think that our  40-years-long brooding about the hierarchy problem has not been completely misguided...

Thursday, 11 February 2016

LIGO: what's in it for us?

I mean us theoretical particle physicists. With this constraint, the short answer is not much.  Of course, every human being must experience shock and awe when picturing the phenomenon observed by LIGO. Two black holes spiraling into each other and merging in a cataclysmic event which releases energy equivalent to 3 solar masses within a fraction of a second.... Besides, one can only  admire the ingenuity that allows us to detect here on Earth a disturbance of the gravitational field created in a galaxy 1.3 billion light years away. In more practical terms, the LIGO announcement starts the era of gravitational wave astronomy and thus opens a new window on the universe. In particular, LIGO's discovery is a first ever observation of a black hole binary, and we should soon learn more about the ubiquity of astrophysical systems containing one or more black holes. Furthermore, it is possible that we will discover completely new objects whose existence we don't even suspect. Still, all of the above is what I fondly call dirty astrophysics on this blog,  and it does not touch upon any fundamental issues. What are the prospects for learning something new about those?

In the long run, I think we can be cautiously optimistic. While we haven't learned anything unexpected from today's LIGO announcement, progress in gravitational wave astronomy should eventually teach us something about fundamental physics. First of all, advances in astronomy, inevitably brought by this new experimental technique,  will allow us to better measure the basic parameters of the universe. This in turn will provide us information about aspects of fundamental physics that can affect the entire universe, such as e.g. the dark energy. Moreover, by observing phenomena occurring in strong gravitational fields and of which signals propagate over large distances, we can place constraints on modifications of Einstein gravity such as the graviton mass (on the downside,  often there is no consistent alternative theory that can be constrained).

Closer to our hearts, one potential source of gravitational waves is a strongly first-order phase transition. Such an event may have occurred as the early universe was cooling down.  Below a certain critical temperature a symmetric phase of the high-energy theory may no longer be energetically preferred, and the universe enters a new phase where the symmetry is broken. If the transition is violent (strongly first-order in the physics jargon), bubbles of the new phase emerge, expand, and collide, until they fill the entire visible universe. Such a dramatic event produces gravitational waves with the amplitude that may be observable by future experiments.   Two examples of phase transitions we suspect to have occurred are the QCD phase transition  around T=100 MeV, and the electroweak phase transition around T=100 GeV. The Standard Model predicts that neither is first order, however new physics beyond the Standard Model may change that conclusion. Many examples of required  new physics have been proposed to modify the electroweak phase transition, for example models with additional Higgs scalars, or with warped extra dimensions.  Moreover, the phase transition could be related to symmetry breaking in a hidden sector that is very weakly or not at all coupled (except via gravity) to ordinary matter.  Therefore, by observing or putting limits on phase transitions in the early universe we will obtain complementary information about the fundamental theory at high energies.    

Gravitational waves from phase transitions are typically predicted to peak at frequencies much smaller than the ones probed by LIGO (35 to 250 Hz). The next generation of gravitational telescopes will be more equipped to detect such a signal thanks to a much larger arm-length (see figure borrowed from here). This concerns especially the eLISA space interferometer which will probe millihertz frequencies. Even smaller frequencies can be probed by pulsar timing arrays which search for signals of gravitational waves using stable pulsars for an antenna.  The worry is that the  interesting signal may be obscured by astrophysical backgrounds, such as (oh horror) gravitational wave emission from white dwarf binaries. Another  interesting beacon for future experiments is to detect gravitational waves from inflation (almost discovered 2 years ago via another method by the BICEP collaboration).  However, given the constraints from the CMB observations,  the inflation signal may well be too weak even for the future  giant space interferometers like DECIGO or BBO.

To summarize, the importance of the LIGO discovery for the field  of particle physics is mostly the boost it gives to  further experimental efforts in this direction.  Hopefully, the eLISA project will now take off, and other ideas will emerge. Once gravitational wave experiments become sensitive to sub-Hertz frequencies, they will start probing the parameter space of interesting theories beyond the Standard Model.  

Thanks YouTube! It's the first time I see a webcast of a highly anticipated event running smoothly in spite of 100000 viewers. This can be contrasted with Physical Review Letters who struggled to make one damn pdf file accessible ;)