Sabtu, 09 Maret 2013

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 :-)    

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