The universe after Planck

Here's the 2013 rendition of the cosmological Mona Lisa, 

or, in our jargon, the multipole expansion of the Cosmic Microwave Background  (CMB) power spectrum. Compared to previous experiments, what distinguishes Planck is that it measures the power spectrum all the way from the largest angular scales down to less than 0.1 degrees.  Before, to cover the entire range, one had to combine several different CMB experiments: WMAP, SPT, ACT, which was more vulnerable to  unknown systematic errors (indeed, the results from SPT and ACT were not completely consistent). Thanks to Planck, the errors are reduced, especially at large multipoles,  and the confidence in the results is strengthened.

For the general public, the most palatable piece of information is about the  composition of the Universe. The dough is made of 69.2±1.0% dark energy, 25.8±0.4% dark matter, and 4.82±0.05% baryons, after combining Planck with other cosmological measurements. As you can  see, the errors are of order 1 in 100, which represents a factor of 2 increase in precision compared to WMAP-9. The central values have shifted a bit: there's additional 2% of dark matter at the expense of dark energy, but the change is consistent within 2 sigma with previously quoted errors.

From a particle physicist's point of view the single most interesting observable  from Planck is the notorious Neff. This observable measures the effective number of degrees of freedom with sub-eV mass that coexisted with the photons in the plasma at the time when the CMB was formed (see e.g. my older post for more explanations). The standard model predicts  Neff≈3, corresponding to the 3 active neutrinos. Some models beyond the standard model featuring sterile neutrinos, dark photons, or axions could lead to Neff > 3, not necessarily an integer.  For a long time various experimental groups have claimed Neff much larger than 3, but with an error too large to blow the trumpets. Planck was supposed to sweep the floor and it did.  They find Neff=3.3±0.5, that is no hint of anything interesting going on. The gurgling sound you hear behind the wall is probably your colleague working on sterile neutrinos committing a ritual suicide.  
  
Another number of interest for particle theorists is the sum of neutrino masses. Recall that oscillation experiments tell us only about the mass differences, whereas the absolute neutrino mass scale is still unknown.  Neutrino masses larger than 0.1 eV would produce an observable imprint into the CMB. In fact, the SPT experiment recently made a  claim that  the sum of neutrino masses is 0.32±0.11 eV, a 3 sigma evidence for a non-zero value.  That would be huge, if confirmed. Well, no such luck:
Planck sees no hint of neutrino masses and puts the 95% CL limit at 0.23 eV. (Check out the comment section for a more informed statement).

Literally,  the most valuable Planck result is the measurement of the spectral index ns, as it may tip the scale for the  Nobel committee to finally hand out the prize for inflation.  Simplest models of inflation (e.g., a scalar field φ with a φ^n potential slowly changing it vacuum expectation value) predicts the spectrum of primordial density fluctuations that is adiabatic (the same in all components) and Gaussian (full information is contained in the 2-point correlation function). Much as previous CMB experiments, Planck does not see any departures from that hypothesis. A more quantitative  prediction of simple inflationary models is that the primordial spectrum of fluctuations is almost but not exactly scale-invariant. More precisely, the spectrum is of the form P∼(k/k0)^(ns-1), with ns close to but typically slightly smaller than 1, the size of ns-1 being dependent on how quickly (i.e. how slowly) the inflaton field rolls down its potential. The previous result from WMAP-9  ns=0.972±0.013 (ns=0.9608±0.0080 after combining with other cosmological observables) was already a strong hint of a red-tilted spectrum. The Planck result ns=0.9603±0.0073 (ns=0.9608±0.0054 after combination)  pushes the departure of ns-1 from zero past the magic 5 sigma significance. This number can of course also be fitted in more complicated models or in alternatives to inflation, but it is nevertheless a strong support for the most trivial  version of inflation.  

I was a bit surprised by how much emphasis in today's press conferences was put on the small glitches at low multipoles. It seems that Planck people are also a bit frustrated by the fact that their results are nothing but a triumphant confirmation  of old paradigms. Even at the LHC nobody would make a big deal of a 2.5 sigma anomaly, and in the present case we're in the area of astrophysics where error bars are treated more loosely ;-)  Moreover, according to Planck, the quadrupole mode in  the fluctuation spectrum is aligned with the ecliptic plane, which suggests some unknown background or pesky systematics at large angular scales. Of course, many a theorist will come up with a beautiful explanation  of  the low multipole anomaly. But not because  it's convincing, but because there's nothing else to ponder on...


In summary, the cosmological results from  Planck are really impressive. We're looking into a pretty wide range of  complex physical phenomena occurring billions of years ago. And, at the end of the day, we're getting a perfect description with a fairly simple model. If this is not a moment to exclaim "science works bitches", nothing is.  Particle physicists, however, can find little inspiration in the Planck results. For us, what Planck has observed is by no means an almost perfect universe... it's the perfectly boring universe.

 
Mind that this post is an outsider's perspective from the angle of particle physics. For a better insight into cosmological aspects of the Planck results see for example here. Note that Planck dumped 28 full-grown papers today, so browsing through it will take some time, and there may be some hidden treasures at the bottom of the chest...   

Higgs: more of the same

Everything's clear now.  The most important Higgs search channels have been updated by both ATLAS and CMS (the only interesting channel that has not is the decay to b-quarks).  We're not going to learn much new for the next 2 years.  The CERN director, heeding to Résonaances' call,  announced Habemus Higgsam.

   
So, what's new since last week?

• Finally, CMS came out with their long awaited update of the Higgs→γγ search. It is clear why they have been so shy: new calibration of the electromagnetic calorimeter completed after last summer forced them to revise the  significance of the Higgs signal in the diphoton channel. The current significance is merely 3.2 sigma, and from that it's easy to deduce that they wouldn't have been able to claim a formal 5 sigma discovery on the 4th  July had they done their sums back then. But that's of course irrelevant now, as there's no shadow of a doubt the Higgs is there at 125 GeV. The real game changer is the (related) fact the  signal strength relative to the SM one measured by CMS is μ=0.8±0.3 and, unlike the one in the ATLAS and previous CMS measurements, does not show any excess over the standard model prediction. Naively combining the signal strength measured by ATLAS and CMS one gets the disappointing μ=1.2±0.2. Move on folks, nothing to see here.  See Matt's blog for a more in-depth discussion of the CMS diphoton update. 
• One visible consequence of the CMS updated is that  the preference for negative Yukawa couplings, displayed by the Higgs data before Moriond, completely vanished.  In the plot, borrowed from here,  is a section of the Higgs coupling space where one freely varies the coupling to W and Z bosons (denoted as "a"), and, independently, the coupling to the standard model fermions (denoted as "c"). Previously, the Higgs data somewhat  preferred  the bizarre region near a=1, c=-1  over the standard-model-like region near a=c=1, as the former was leading to an enhancement of the  Higgs→γγ rate. This is no more, only the standard-model-like island remains.   
• Another important update last week was from ATLAS in the WW→2l2ν channel.  Again no surprises here: the signal significance almost 4 sigma, the rate μ=1.0±0.3.
• To say that the Higgs is standard-model-like is an understatement.  This bastard screams and spits standard model.  After the Moriond updates the standard model gives an absolutely perfect fit to the combined data (previously it was disfavored at 80% confidence level, mostly due to the late diphoton excess).  Not even a single cliffhanger to makes us wait for  the next episode.....  If there's anything non-standard about the Higgs couplings to matter it is hiding very well and will be tricky to  uncover at the LHC, even after the energy upgrade. 
• It's worth stressing again that is has been firmly established that the Higgs couples to mass of  W and Z bosons, the statement which can be reinterpreted that the Higgs gives mass to W and Z bosons.  After Moriond the evidence for that is strengthened even further.  The LHC Higgs data alone (blue in the plot) imply this coupling should be within ~30% of the standard-model value at 95% confidence level. If one also takes into account precision observables from LEP  (red in the plot) the allowed range shrinks to ~10% (although the latter number is a bit less robust, as fine-tuned new physics effects could alter the conclusions). Although announcing the Higgs is a Higgs at this particular moment of history was to a large extent arbitrary,  it was by no means premature.   
• We see the Higgs, of which the corollary is that we don't see the invisible Higgs. The invisible decays could arise for example if the Higgs couples to dark matter, which is the case in a large class of well-motivated models. In the simplest situation, when the Higgs couplings to known particles take the standard model values, an invisible width would lead to a universal reduction of the rates in all decay channels. That is severely constrained by global fits, as seen in the plot (solid blue): the invisible branching fraction more than about 15-20% is strongly disfavored. Note that for the sake of this analysis "invisible" is "anything not measured", thus any non-standard decay channel is constrained this way. But that doesn't mean there's no point looking for exotic Higgs decays. In the end, we've already observed much more rare decays: the branching fraction to 2 photons is around 0.2%, and to 4 leptons  - around 0.01%. So 10% of invisible or some other freak decays seems like a goal well within reach of experimentalists.       
• A fun fact for dessert. In CMS the measured central values of the Higgs rates in the 3 most sensitive channel all fall slightly below 1, while in ATLAS they are all  slightly above one.  The new motto for ATLAS: we try harder ;-)
  
  

Higgs: what have we learned

Last Wednesday at the Moriond Conference the LHC dumped several new Higgs results based on the entire collected dataset of about 25 fb-1.  So, what do we know that we didn't know before?    

• One thing I learned is that Higgs remains exciting enough to make me wake up at 8am. But barely so. Indeed, the time of the revolution is over, we are now entering a 20-year long period of incremental progress and painful squeezing of experimental errors. The best evidence of declining enthusiasm is that it took me 3 days to scribble a summary for Résonaances.  Also the experimenters seem to be less excited, as they didn't bother to update all the important channels in time for Moriond. Soon the Higgs will become just an annoying background to more interesting searches ;-)
•   The more we look the more standard-model-like it seems.  Early on in the game there were several glitches, but with time all the Higgs observables are converging toward the standard model prediction. The only worth-mentioning discrepancy that remains is the h→γγ rate measured by ATLAS. But even that is getting less compelling. The diphoton rate normalized to the standard model one drifted from 1.80-0.36+0.42 last December to 1.65-0.30+0.34 in Moriond. That means the significance of the excess didn't grow after adding more data, which is usually a bad sign. Besides, ATLAS still struggles with the twin peaks -- the 2.5 GeV discrepancy between Higgs masses measured in the γγ and ZZ→4l channels -- which suggests that an unknown systematic error or at least an unlucky fluctuation affects one or both of these measurements. Anyway, next week the air will be clearer when CMS says what they have to say. Except for ATLAS γγ, the rates in the remaining channels updated for Moriond (ATLAS ZZ, CMS WW ττ) are within 1 sigma uncertainty from the standard model prediction. 
•  The single most interesting piece of news was the CMS update in the h→ττ decay channel. Almost 3 sigma evidence for a signal brings up to 4 the number of channels where we clearly observe the Higgs signals (I'm not counting  the bb channel, as the Tevatron no longer claims an evidence there). It's also the first direct evidence that the Higgs couples to fermions, although indirectly we knew that before (the rate of Higgs production via gluon-gluon fusion is roughly consistent with the process being  mediated by a top quark loop). It's also an evidence that Higgs couples to down-type fermions, which excludes a chunk of the  parameters o 2-Higgs doublet model    
• I'm starting to get really annoyed by the endless talk of  "Higgs spin determination".  As if there was any serious issue at stake.  I've said it in the previous post, and I'll say it again: there's  absolutely no way the 126 GeV particle has any other spin than zero.  We don't even have a working model for a light spin-2 resonance that would be theoretically sound and not excluded experimentally. At some level, too much  caution may become unscientific.   Well, in the end we can't be 100% sure whether Newton's law is responsible for the motion of the planets in the solar system; it might be 7 dwarfs pulling the string in exactly the way as to mimic the inverse square law....  The question is a bit better posed for "Higgs parity determination", as in that case we at least have a sensible model for a pseudoscalar. But even then, a pseudoscalar  simultaneously reproducing the observed rates  in the γγ, ZZ, WW, and ττ channels is absolutely improbable. Of course, the above doesn't mean that the measurements involved in "spin and parity determination" are useless.  On the contrary,  it is perfectly conceivable that the Higgs has some (subleading) non-standard interactions with matter and those measurements are in a position to reveal them.  However,  they should not be viewed as measuring the spin of the Higgs  but as testing whether differential distributions for Higgs production and decay agree with the standard model.  A better way to present results is the one in the CMS plot here that shows the limits on the variable fa3 -- the relative contribution of the pseudoscalar interactions to the h→ZZ decay amplitude squared. From that we read off that fa3 larger than 58% is excluded at 95% confidence level, already a useful piece of information for constraining Higgs interactions.
•  It's also worth pausing for a moment on Higgs searches in channels where we should not see a signal, because the standard model either forbids that particular decay or predicts too small a rate to be detectable with the current amount of data. In this category are the h→Zγ, h→μμ, and h→invisible searches  presented in Moriond. Quite generally, this is a very  promising direction for new physics searches, probably more promising than measuring the Higgs couplings. Indeed, the precision with  which we can measure the Higgs couplings is limited not only by statistics and experimental uncertainty but also by theory. Predicting the Higgs rates in a hadronic collider with a precision better than 10% may be tough, and we're already not too far from that level of experimental precision.  On the other hand, it's entirely possible that Higgs has some non-standard interactions that open a new decay channel or lead to an order-of-magnitude enhancement of the existing one. Observing such a rare decay process would be a clear smoking gun of new physics. Obviously, no luck so far, but I personally consider searching for rare non-standard Higgs decays our best chance for finding new physics at the LHC. 
• The idea  of the sausage game is take a movie title and replace one word with sausage, for example  Harry Potter and the Deathly Sausage, Star Wars: The Sausage Strikes Back, Saving Private Sausage, etc.  The organizers of Moriond propose a similar parlour game with the Higgs.  Indeed, the Higgs jumps on us everyday from  the arXiv, from newspapers and TV, sometimes I open the fridge and see the Higgs. That is very tiring in the long run, especially if you have to listen to a string of Higgs talks after a day of skiing. So, to make the talks more attractive to listeners,  last year the speakers were asked to replace Higgs with the BEH sound; this year the word was SMS which, I must say, is less captivating. The rumor is that next year Higgs will be replaced by "Little Bunny".  The talk about  "Combined searches for Little Bunny in ATLAS and CMS" will definitely attract some a lot of attention :-)    
  

When shall we call it Higgs?

In one week from now we'll have a new release of Higgs boson results from the LHC.  Although in private it's always the Higgs, in official communications the particle discovered at the LHC last year is being called different names: sometimes the 125 GeV particle, sometimes a scalar boson (as opposed to scalar fermions), and most often a Higgs-like boson.  This caution was understandable at the early stage, given the fresh memory of faster-than-light neutrinos.  However, since it's been walking and quacking like a duck for more than half a year now, there's a discussion among experimenters when they will be allowed to drop the derogatory "-like" suffix. 

So, at which point a particle becomes to a Higgs?  

One issue discussed at length in this context is the spin.  This is however moot as we already know with great certainty that the answer is 42 zero. We know it's a boson.  We know it's not spin 1, because the young Landau theorem says an on-shell spin-1 particle cannot decay into massless vector bosons, and we do see a resonance in the diphoton channel. Formally it might be spin 2, but in practice it cannot be either. For spin 2, the new particle would couple to all matter via higher-dimensional non-renormalizable operator, and then  simple estimates suggest that the ratio of branching fractions in different channels, e.g. in ZZ vs gamma-gamma,  should be completely different than what is observed unless the couplings take highly unnatural values. Moreover, if Higgs were spin-2 the electroweak precision observables measured at LEP would be severely affected. Finally, spin-2 would be similar to a Higgsless theory in that the scattering amplitudes explode, and the theory would require new physics below 1 TeV (that we see absolutely no hint of) to make sense. Thus, the 125 GeV particle has to be a scalar, barring the biggest conspiracy since Roswell. If you don't buy that argument, bear in mind that you'll never be able to formally exclude the spin-2 possibility. Indeed, one can experimentally disfavor particular spin-2 benchmark models, but as there are numerous possible operators to couple  spin 2 to matter there will probably  always be a combination of the parameters that can fit the data. If you don't accept that argument either, note that Nima-Arkani Hamed said he would kill himself if it's spin 2, and most of us don't wish Nima dead.  

So we're left with a spin-0 particle as the only reasonable option. But zero spin is still not equivalent to a Higgs boson. In particular, a resonance in the diphoton channel -  where the detection remains most significant - is only a circumstantial evidence for the Higgs nature of the particle.  Indeed, any fool can couple to 2 photons or 2 gluons even if it has nothing to do with electroweak symmetry breaking.  So let's define here, once and for all,  what is a Higgs boson.  Sometimes the Higgs is described as the one that gives mass to other particles, or the one that has a vacuum expectation value,  but that cannot be unambiguously defined (one can always rewrite the theory such that the scalar has no vev at all, at the cost of the symmetries being non-linearly realized). Instead, the best definition is based on the couplings of the particle: a Higgs boson is a scalar that couples to mass. To be more specific, for the couplings to W and Z bosons in the lowest order of the effective theory expansion we can write the following interactions:

which couple the h scalar to the W and Z mass terms. These couplings are present for the standard model Higgs with cV=1. At the next order  of the effective theory expansion many more interactions are possible:

which are not couplings to mass terms. These are higher dimensional couplings than those in L1, so generically they should be less important. Nevertheless,  some symmetry reasons or selection rules may forbid the couplings in L1,  in which case the h scalar would be an impostor and not the Higgs boson. Any of the couplings in L1 or in L2 could mediate Higgs decays to W bosons and Z bosons (as observed at the LHC), or mediate Higgs production  in association with W/Z and in the vector-boson-fusion process (also kind of observed).  However, the difference between the couplings in L1 and L2 is that the latter play no role in electroweak symmetry breaking. In the technical jargon, they don't affect the amplitude of the longitudinal gauge boson scattering at high energies, therefore they are irrelevant for restoring unitarity of the theory with massive gauge bosons. 

So we the following definition. A Higgs boson is a scalar particle that couples to W and Z bosons as in L1, with cV > 0. Along the same vein, one can define the Higgs boson as a unique Higgs boson with the cV coupling close to one.

Thus,  to prove we are dealing with a Higgs and not an impostor it is enough to prove the coupling cV is non-zero. Actually, we already know that based on the current data. The shabby figure on the right shows the fit of the cV parameter to the LHC Higgs data (blue) and to LEP electroweak precision data (red), after marginalizing over the remaining parameters of the effective theory. Both of these data sets independently exclude cV=0 at a high confidence level, and even more so when they are combined (black). So, with very few assumptions, and independently on what the values of the other couplings are,  one can argue the particle discovered at the LHC is a Higgs boson. At 95% confidence level cV is within 15% of the standard model value cV=1, so it clearly smells like the Higgs boson. But of course that last statement is less robust as there's still a lot of room for other Higgs bosons being present in nature.
  
In summary, for theorists the answer to the question in the title is "since several months". For the LHC experimenters the coming Moriond conference would be a good opportunity to make that bold step forward, were it not for the fact that the H-word is banned in there ;-)

How many neutrinos in the sky?

Just before Christmas, the WMAP collaboration posted the 9-years update of their Cosmic Microwave Background results. Before going into details it's worth to stop for a second and admire the picture that I pasted here. Thanks to the observations of the WMAP satellite (black) and the terrestrial telescopes SPT (blue) and ACT (orange) we know the spectrum of the CMB fluctuations down to the scales of 0.1 degree. All these peaks and wiggles are predicted by the standard cosmological model, the so-called ΛCDM model (black line). In particular, the positions and the relative sizes of the peaks  provide the strongest argument to date for the existence of dark matter in the Universe: a non-relativistic matter component that, unlike baryons, does not interact with photons. At present, the alternatives to dark matter cannot even dream of quantitatively explaining the observed features of the CMB.  

That late in the game one would not expect any spectacular turns of the action. Indeed, compared to the 7-years WMAP data, the update typically brings a 20-30% reduction of already tiny errors on the composition of the Universe. There is however one number that changed visibly. The effective number of relativistic degrees of freedom at the time of CMB decoupling, the so-called Neff parameter, is now Neff = 3.26 ± 0.35, compared to Neff = 4.34 ± 0.87 quoted in the 7-years analysis.  For the fans and groupies of this observable it was like finding a lump of coal under the christmas tree...  

So, what is this mysterious Neff parameter? According to the standard cosmological model, at the temperatures above 10 000 Kelvin the energy density of the universe was dominated by a plasma made of neutrinos (40%) and photons (60%). The photons today make the  CMB  about which we know everything.  The neutrinos should also be around, but for the moment we cannot  study them directly. However we can indirectly infer their presence in the early universe via other observables. First of all, the neutrinos affect the energy density stored in radiation:  

which controls the expansion of the Universe during the epoch of radiation domination. The standard model predicts Neff equal to the number of known neutrinos species, that is Neff = 3  (in reality 3.05, due to finite temperature and decoupling effects). Thus, by measuring how quickly the early Universe was expanding, we can determine Neff.  If we find Neff ≈ 3  we confirm the standard model and close the store.  On the other hand, if we measured that Neff is significantly larger than 3, that would mean a discovery of  additional light  degrees of freedom in the early plasma that are unaccounted for in the standard model. Note that these new hypothetical  particles don't have to be similar to neutrinos, in particular they could be bosons, and/or have a different temperature (in which case they would correspond to non-integer increase of  Neff ). All that is required from them is that they are weakly interacting and light enough to be relativistic at the time of CMB decoupling.   Theorists have dreamed up many viable candidates that could show up in Neff : additional light neutrinos species, axions, dark photons, etc.

One way to measure Neff  is via nucleosynthesis (in principle it's not the same observable as in that case one measures the number of relativistic degrees of freedom at a much earlier epoch, but in most models Neff at the time of nucleosynthesis and CMB decoupling are similar). Here the physics is rather straightforward.   The larger Neff, the faster the universe expands, and the earlier the weak interactions transforming neutrons into protons fall out of equilibrium. Almost all the neutrons that survive the thermal bath end up bound into helium atoms, thus by measuring the amount of helium-4 in the universe one can infer Neff. A recent analysis of the nucleosynthesis constraints on Neff  is summarized in the plot. The upper panel shows the standard model prediction (red band) of the primordial helium mass fraction Yp as a function of Neff, confronted with experimental constraints. (Notice that different observations of the helium abundance are not quite consistent with each other, but that's normal in astrophysics;  the rule of thumb is that 3 sigma uncertainty in astrophysics is equivalent to 2 sigma in conventional physics).  Neff ≈ 4 seems to be preferred, although, given the uncertainties, any  Neff between 3 and 5 is consistent with the data.

The interest of particle physicists in Neff  come from the fact that, until recently, the CMB data also pointed at Neff≈4 with a comparable error.  The impact of Neff  on the CMB is much more contrived, and there are many separate effects one needs to take into account. For example, larger Neff delays the moment of matter-radiation equality, which affects the relative strength and positions of the peaks. Furthermore, Neff affects how the perturbations grow during the radiation era, which may show up in the CMB spectrum  at l ≥ 100. Finally, the larger Neff,  the larger is the effect of Silk damping at l ≥ 1000. Each single observable has a large degeneracy with other input parameters (matter density, Hubble constant, etc.) but, once the CMB spectrum is measured over a large range of angular scales, these degeneracies are broken and stringent constraints on Neff can be derived. That is what happened recently, thanks to the high-l CMB measurements from the ACT and SPT telescopes, and some  input from other astrophysical observations. The net result is that from the CMB data alone one finds  Neff = 3.89 ± 0.67, while using in addition an input from Baryon Acoustic Oscillations and Hubble constant measurements brings it down Neff = 3.26 ± 0.35.  All in all, the measured effective number of relativistic degrees of freedom in the early Universe can be well accounted for by the three boring neutrinos of the standard model. Well, life's a bitch.  The next update on Neff is expected in March when Planck releases its cosmological results,  but the rumor is that it will do nothing to cheer us up.

Update: as pointed out by a commenter, there's a rumor that the WMAP-9 analysis has a bug, and when it's corrected Neff increases significantly. So don't throw your sterile neutrinos models into a fire yet. 
Update #2: the bug was fixed in v2. The new number is Neff = 3.84 ± 0.40, consistent within 2 sigma with the standard model, but leaving some room for hope.     

What's in store in 2013

When you think about it, the end of the world in 2012 would have made perfect sense. Last year we found the Higgs boson -- the last of the particles predicted by the standard model. It may well be that humans have already discovered all elementary particles, in which case all that's left is looking in the sixth place of decimals. Alas, the armageddon didn't happen, so we have to drag on. What will the year 2013 bring to particle physicists?

Well, this year is surely going to be depressing because the LHC will come to a long halt, after a brief period of shit-on-shit collisions in January and February*. During the next 2 years the machine will undergo necessary repairs upgrades so that it can restart with the collision energy of about 13 TeV. Nevertheless this year should be entertaining, as the analyses of the full 8 TeV dataset will be flowing in. First of all, we're waiting for the Higgs update expected around the time of the  Moriond conference in March. The most important question is whether the measured rate in the diphoton decay channel  will continue to show an excess over the standard model, as currently hinted by ATLAS, or whether it will drift towards the standard model value, as hinted by CMS. Other Higgs search channels are unlikely to show a major departure from the standard model, given the existing data... but one never knows.  Besides, there will be of course hundreds of new physics searches; as long as there is data there's hope  that a new exciting phenomenon  may pop up somewhere...    

On the other side of the Atlantic two important experiments will kick off in 2013. Between Fermilab and Minnesota, the NOvA neutrino experiment will carry the first attack on the CP violating phase in the neutrino mixing matrix. Over in South Dakota, the LUX dark matter experiment will join Xenon100 on the frontier of WIMP detection.  However, neither of the above is likely to deliver anything groundbreaking as early as this year.

What else? When times are tough  we turn our eyes to heaven. The Planck satellite, that has performed precise measurements of the Cosmic Microwave Background, will release the cosmological results in March. Disappointingly, the release will not include the CMB polarization data, which are supposed the meat of the whole mission. Nevertheless, the new CMB temperature maps should give us a better grip on cosmological parameters and improve on what we learnt from WMAP. Elsewhere in the sky the AMS-02 experiment,  glued to the International Space Station since 1 and half year now, is supposed to release first results this year. Although we don't expect anything spectacular, a confirmation of the positron excess claimed a few years ago by the PAMELA satellite would already be something. Back to the Earth, an upgraded version of the HESS gamma-ray telescope has been operating in Namibia since last summer. The previous HESS data has been used to put limits on the monochromatic gamma-ray emission from the galactic center at very high energies, 0.5-25 TeV. According to some reports, HESS-II should be able to go down in energy and quickly refute the presence of the line at 135 GeV that seems to be present in the data from the  Fermi gamma-ray satellite.

So, the new year holds some promises, although it's unlikely to match the fabulous 2012. Actually, I'm already worried about 2014, when there'll be so little new data.  Most likely, Résonaances will then  have to turn into a tabloid blog publishing topless pictures of physicists on Caribbean beaches.  But let's not think about that for a moment, and let's enjoy 2013 with the avalanche of LHC data soon to be released. 

*) As correctly pointed out by a commenter, this year we'll have proton-on-shit rather than shit-on-shit collisions. Apologies for the inaccuracy.

2012 Highlights

One can safely assume nothing else important will happen this year... so let's wrap up.  Here are the greatest moments of the year 2012, from the point of view of an obscure particle physics blog.

•  Higgs boson discovery
  This one is obvious: the Higgs tops the ranking not only on Résonaances, but also on BBC and National Enquirer.  So much has been said about the boson, but let me point out one amusing though rarely discussed aspect: as of this year we have one more fundamental force. The 5 currently known fundamental forces are 1) gravitational, 2) electromagnetic, 3) weak, 4) strong, and 5) Higgs interactions. The Higgs force is attractive and  proportional to the masses of interacting particles (much like gravity) but manifests itself only at very short distances of order 10^-18 meters. From the microscopical point of view, the Higgs force is different from all the others in that it is mediated by a spinless particle. Résonaances offers a signed T-shirt to the first experimental group that will directly measure this new force.  
• The Higgs diphoton rate
  Somewhat disappointingly, the Higgs boson turned out to look very much as predicted by the current theory. The only glitch so far is the rate in which it decays to photon pairs. Currently, the ATLAS experiment measures the value 80% larger than the standard model prediction,  while CMS also finds it a bit too large, at least officially. If this were true, the most likely explanation would be a new charged particle with the mass of order 100 GeV and a large coupling to the Higgs. At least until the next Higgs update in March we can keep a glimmer of hope that the standard model is not a complete theory of the weak scale...
• Theta-1-3
  Actually, the year 2012 was so kind as to present us not with one but with two fundamental parameters. Except the Higgs boson mass, we also learned about one entry in the neutrino mixing matrix, the so-called θ_13 mixing angle. This parameter controls, among other things, how often the electron neutrino transforms into other neutrino species. It was pinpointed this year by the neutrino reactor experiment Daya Bay who measured θ_13 to be about 9 degrees -  a rather uninspired value. The sign of the times:  the first prize was snatched by the Chinese (Daya Bay), winning by a hair before the Koreans (RENO), and leaving far behind the Japanese (T2K), the Americans (MINOS), and the French (Double-CHOOZ). The center of gravity might be shifting...
• Fermi line
  Dark matter is there in our galaxy, but it's very difficult to see its manifestations other than the gravitational attraction.  One smoking-gun signature would be a monochromatic gamma-ray line from the process of dark matter annihilation into photon pairs. And, lo and behold,  a  narrow spectral feature near 130 GeV was found in the data collected by the Fermi gamma-ray observatory.  This was first pointed out by an independent analysis, and later confirmed (although using a less optimistic wording) by the collaboration itself.  If this was truly a signal of dark matter, it would be even  more important than the Higgs discovery. However past experience has taught us to be pessimistic, and we'd rather suspect a nasty  instrumental effect to be responsible for the observed feature. Time will tell...  
• Bs-to-μμ
  This year the LHCb experiment finally pinpointed the super-rare process of the Bs meson decaying into a muon pair. The measured branching fraction is about 3 in a billion, close to what was predicted. The impact of this result on theory was a bit overhyped, but it's anyway an impressive precision test.  Even if "The standard model works, bitches" is not really the message we wanted to hear...
• Pioneer anomaly
  A little something for dessert: one long standing  mystery was ultimately solved this year.  We knew all along that the thermal emission from Pioneer's reactors could easily be responsible for the anomalous deceleration of the spacecraft, but this was cleanly demonstrated only this year.  So, one less  mystery, and  no blatant violation of Einstein's gravity in our solar system...

So much for now, happy new year, maybe it won't be much worse?