Monday 31 December 2012

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?

Thursday 13 December 2012

Twin Peaks in ATLAS

For the annual December CERN council meeting the ATLAS experiment provided an update of the Higgs searches in the γγ and ZZ→4 leptons  channels. The most interesting thing about the HCP update a month ago was why these most sensitive channels were *not*  updated (also CMS chose not to update γγ).  Now we can see why. The ATLAS analyses in these channels return the best fit Higgs masses that differ by more than 3 GeV: 123.5 GeV for ZZ and 126.6 GeV for γγ, which is much more than the estimated resolution of about 1 GeV. The tension between these 2 results is estimated to be 2.7σ.  Apparently, ATLAS used this last month to search for the systematic errors that might be responsible for the discrepancy but, having found nothing, they decided to go public.

One may be tempted to interpret the twin peaks as 2 separate Higgs-like particles. However in this case they most likely signal a systematic problem rather than interesting physics. First, it would be quite a coincidence to have two Higgs particles so close in mass (I'm not aware of a symmetry that could ensure it). Even if the coincidence occurs, it would be highly unusual that one Higgs decays dominantly to ZZ and the other dominantly to γγ, each mimicking pretty well the standard Higgs rate in the respective channel. Finally, and most importantly, CMS does not see anything like that; actually their measurements give a reverse picture. In the ZZ→4l channel CMS measures mh=126.2±0.6 GeV,  above  (but well within the resolution) the best fit mass they find in  the γγ channel which is 125.1±0.7 GeV GeV. That makes us certain that down-to-earth reasons are responsible for the double vision in ATLAS, the likely cause being  an ECAL calibration error, an unlucky background fluctuation, or alcohol abuse. 

The truly exciting thing about the new ATLAS results is that the diphoton rate continues to be  high. Recall that we are scared as fudge that the Higgs will turn out to be the boring one predicted by the standard model, and we're desperately looking out for some non-standard behavior. The measurements of Higgs decays to ZZ and WW do not bring any consolation: all rates measured by CMS and ATLAS so far are perfectly consistent with the standard model. Today's ATLAS update in the ZZ→4l channel continues the depressing trend, with the signal strength normalized to the standard model  one   measured at 1.0±0.4 (for mh=125 GeV). Currently our best hope is that the measured h→γγ cross section is consistently larger than the one predicted  by the standard model, both in ATLAS and CMS. If the enhancement is due to a statistical fluctuation one would expect it becomes less significant as more data is added. Instead, in ATLAS,  the central value of  has not moved since July, but the error has shrunk a bit! The current diphoton signal strength stands at  1.8 ± 0.4, roughly 2 sigma above the standard model. On the other hand, given there is something weird about the ATLAS Higgs data (be it miscalibration or fluctuation),  we should treat that excess with a grain of salt, at least until the double vision problem is resolved. And we're waiting for CMS to come out with what they have in the diphoton channel...

One more news today is that ATLAS also began studying some differential observables related to the Higgs boson, which usually goes by the name of "spin determination". In particular, they looked at the production and decay angles in the ZZ→4l channel  (similar to what CMS showed at HCP) and the Higgs production angle in the γγ channel (first measurement of this kind). For spin zero the production angle  should be isotropic (at the parton level, in the center-of-mass frame of the collision) while for higher spins some directions with respect to the beam axis could be preferred.  Not surprisingly, the measured Higgs production angle is perfectly consistent with the zero spin hypothesis   (ATLAS also quotes spin-2 being disfavored at 90% confidence level, although in reality they disfavor a particular spin-2 benchmark model).

Here are the links to the ATLAS diphoton, ZZ, and combination notes.

Monday 3 December 2012

WW puzzle

 The mood of this blog usually oscillates between  depressive and funereal, due to the lack of any serious hints of new physics near the electroweak scale. Today, for a change, I'm going to strike an over-optimistic tone. There is one, not very significant, but potentially interesting excess sitting in the LHC data. Given the dearth of anomalies these days, it's a bit surprising that the excess receives so little attention: I could find only 1 paper addressing it.

The LHC routinely measures cross sections of processes predicted by the standard model. Unlike the Higgs or new physics searches, these analyses are not in the spotlight, are completed at a more leisurely pace, and are forgotten minutes after publication. One such observable is the WW pair production cross section. Both CMS and ATLAS measured that cross section in the 7 TeV data using the dilepton decay channel, both obtaining the result slightly above the standard model prediction. The situation got more interesting last summer after CMS put out a measurement based on a small chunk of 8 TeV data. The CMS  result stands out more significantly, 2 sigma above the standard model, and the rumor is that in 8 TeV ATLAS it is also too high.

It is conceivable that new physics leads to an increase of the WW cross section at the LHC. This paper proposes SUSY chargino pair production as an explanation. If chargino decays dominantly  to a W boson and an invisible  particle - neutralino or gravitino,  the final state is almost the same as the one searched by the LHC. Moreover, if charginos are light the additional missing energy from the invisible SUSY particles is small, and would not significantly distort the WW cross section measurement.  A ~110 GeV wino would be pair-produced at the LHC with the cross section of a few pb - in the right ballpark to explain the excess.

Such light charginos are still marginally allowed. In the old days, the LEP experiments excluded new charged particles only up to ~100 GeV, LEP's kinematic reach for pair production. At the LHC, the kinematic reach is higher, however small production cross section of uncolored particles compared to the QCD junk the makes chargino searches challenging.  In some cases, charginos and neutralinos have been recently excluded up to several hundred GeV (see e.g. here), but these strong limits are not bullet proof as they  rely on trilepton signatures. If one can fiddle with the SUSY spectrum  so as to avoid decays leading to trilepton signatures (in particular, the decay χ1→ LSP Z* must be avoided in the 2nd diagram) then 100 GeV charginos can be safe.
Of course, the odds for the WW excess not being new physics are much higher. The excess at the LHC could simply be an upward fluctuation of the signal, or higher-order corrections to the WW cross section in the standard model may have been underestimated.  Still, it will be interesting to observe where the cross section will end up after the full 8 TeV dataset is analyzed. So, if you have a cool model that overproduces WW (but not WZ)  pairs, now may be the right moment to step out.