Monday 23 July 2012

H-day: the morning after

No, I don't mean I've slept for almost 3 weeks ;-)  I mean this particular state of mind of waking up with a vague memory of a crazy party last night, but at the same time unwilling to open your eyes for the fear that the person lying next to you is really the one you think it is.   
  • Welcome.  There is no doubt that since the 4th of July we have a new particle, a boson with mass near 125 GeV.  There is little doubt that this particle is a Higgs boson. True, the discovery relies to a large extent on observing a resonance in the diphoton spectrum, which could also be produced by another spin-0 or even a spin-2 particle that has nothing to do with electroweak symmetry breaking. What convinces us of the higgsy nature of the new particle is the signal in the ZZ and WW final states. Indeed, the coupling [h V V], allegedly responsible for the decays to W and Z bosons, is a watermark signature of a Higgs boson, as it is central to its mission of giving mass to gauge bosons.
  • Farewell.  Welcoming the Higgs, we need to clean the room of some old toys we've got used to. First of all, Higgsless technicolor for obvious reasons goes into the trash bin of history. So does the unhiggs or the whole class of stealthy Higgs theories where the Higgs was supposed to escape detection by decaying into complicated final states. Quite robustly, the 4th generation of chiral  fermions is now excluded because, if it existed, the Higgs production rate would be many times larger than observed. Finally, a simple and neat theory of dark matter that annihilates or scatters via a Higgs exchange, the so-called Higgs portal dark matter, is getting disfavored because Higgs would have a large invisible branching fraction, and thus a suppressed rate of visible decays.  
  • Law. The other Pauli principle: that fermions are discovered in the US, while bosons are discovered  Europe has been spectacularly confirmed.  Note it was a very non-trivial prediction in this particular case. Higgs would have been discovered at the SSC if the US congress did not intervene to scrap the entire program. Furthermore, Higgs would have been discovered at the Tevatron if the Fermilab management didn't intervene to scrap some crucial Run-II detector upgrades, ensuring the Tevatron discovery potential stops just short of a 3 sigma significance.  This only shows how powerful the other Pauli principle is.  Don't you think it deserves, if not a Nobel prize, at least the ig-Nobel prize? ;-)  
  • Hope.   The Higgs data from ATLAS and CMS match well the Standard Model prediction with one exception: the diphoton event rate is 50-100% too large,  with the significance of about 2 sigma. These are most likely statistical fluctuations, but if the enhancement persists when more data is collected it may become the first clear evidence of new physics. If that is the case, the most plausible interpretation of the current data is that the enhancement is due a light 100 GeV-ish scalar or fermion that carries electric charge but no color. This way the loop contributions of that particle could affect the Higgs decays into photons without messing up the gluon fusion production mode.  Furthermore, the new scalar or fermion needs to have a large coupling to the Higgs boson, but its mass has to come dominantly from another source (otherwise it would actually decrease the diphoton rate).  If it were confirmed, it would  be a particle that apparently no one ordered. On the other hand,  theoretically cherished particles (stops, little Higgs top partners, staus) all require a serious tuning and some conspiracy to fit the available Higgs data.
  • Nightmare. Despite what I said above, one cannot help noticing that the data are indecently consistent with the simplest Higgs boson of the Standard Model. Overall, adding the 8 TeV data improved the consistency, eradicating some of the hints of non-standard  behavior we had last year. It's been  often stressed that the Higgs boson is the special one, a particle different from all  the others, a type of matter never observed before. Yet it appears in front of us exactly as described in detail over the last 40 years. This is a great triumph of particle theory, but at the same time it's very disappointing to those whose future existence depends on new physics, that is to a large majority of particle theorists.
In summary, Higgs hunting is over, the catch is now being skinned and prepared for grilling. Collider physics has achieved the most spectacular success in its history. At the same time, it came dangerously close to realizing Kelvin's nightmare, of science reduced to striving for the next decimal places of accuracy. Well, 100 years ago we avoided that fate, may be the history will repeat itself?

Wednesday 4 July 2012

H-day Live

10:58 The party's over now. It was a beautiful day, a historical day, the great triumph of science. Now I'm going to sleep the night off, and tonight we're all gonna celebrate, drink, and make out. Thank you.
10:57 Funny that nobody asks about the loose cable ;-) 
10:56 Higgs says: "I'm glad it happened in my lifetime".  
10:47 I got carried away, no underwear and bras on the stage, sadly. But the atmosphere in the auditorium is such that they might have been.  
10:46 Standing ovations, screams and shouts, the audience throwing bras and underwear at the stage. 
10:44 "I think we have it", concludes the DG. "We have a discovery of a Higgs boson, but which one"?
10:42 In summary, both ATLAS and CMS clearly see a Higgs boson in 2 channels: the diphoton and ZZ 4-lepton. Combining those two, the significance of the Higgs signal is  5.0 sigma in both experiments.

10:40 "This is just the beginning"
10:38 The CMS and ATLAS preferred Higgs mass differ by more than 1 GeV, there will surely be questions about that. 
10:35 5.0 sigma combined excess with the maximum significance mh=126.5 GeV.

Higgs discovered by both experiments!    


10:33 Going to the combination (ATLAS won't show any more channels today). 
10:30 Excess near m4l=125 GeV, although by eye less beautiful peak than in CMS. 3.4 sigma excess vs 2.6 expected in the SM.

10:26 Press release is out. The discovery officially blessed.
10:22 Now the ZZ 4-lepton channel. 
10:21 The measured rate in the diphoton channel is almost twice that predicted in the SM, with the SM rate about 1.5 sigma away. Interesting! So both experiments continue to see to much signal in the Higgs diphoton channel.   
10:20 4.5 sigma excess in the Higgs diphoton channel! (who cares about the look elsewhere effect anymore).  
10:11 Diphoton channel, finally. 
10:11 Boooring.... yet another particle being discovered....

10:05 Both speakers today felt compelled to devote the first 15min to irrelevant bla-bla. Probably because the main subject doesn't appear that exciting.
9:53 Fabiola Gianotti on the stage. Time for ATLAS. 
9:50 In summary, CMS observes a Higgs boson with mass 125.3±0.6 GeV at 4.9 sigma significance. Some funny glitches in the data (a slightly too large diphoton signal,  no excess in the di-tau channel) but overall good consistency with the Standard Model predictions.

9:47 All channels combined, 4.9 sigma significance, vs 5.9 expected.
9:41 Some excess, but not signifcant, also observed in the WW dilepton channel, and b-bbar associated with W/Z. No excess at all in the tau-tau channel, although there should be. 
9:38 Combining diphoton and 4-lepton channels the significance of the Higgs signal is 5.0 sigma

Higgs discovered!!!

9:34 Beautiful peak in the 4-lepton channel. Higgs observed with 3.2 sigma significance in this channel, vs 3.5 sigma expected in the SM.
9:32 Now the ZZ 4-lepton channel
9:31 CMS sees a Higgs in the diphoton channel with the rate about 50% larger than predicted by the Standard Model (but barely one sigma above the SM).
9:30 Over 4 sigma signal in the diphoton channel

9:29 "That's pretty significant"

9:22 Finally, Higgs to diphotons.
9:18 It's not that I stopped blogging, it's that Joe is boring. We want the meat!  
9:11 5.2 inverse femtobarn  of 2012 data, 5.6 in the muon channel. 
9:06 "One page for theorists, that's all they deserve" :-)  
9:04 Joe Incandela on the stage, the CMS talk start.
9:02 C'est parti! "Today is a special day" says DG.   
8:56 Yes! Higgs is here!!! Everything ready for the discovery.
8:50 10 minutes to the seminar. Still no Higgs. But the other Nobel prize winner this year is already inside.
8:43 By the way, if you come across  a press article today about the god particle that's a perfect gauge the author is an idiot and has no idea what he's talking about. 

8:38 The audience is a funny mix. One half are 60+ big shots who could get themselves a sit reservation, the other half are 20-something Higgs groupies who had a strength to queue all night.  
8:25  The title of the seminar is Higgs Search Update. Reminds me of  A Model of Leptons.

8:15 The first accurate prediction of the Higgs mass was formulated in this video.  It has gone unnoticed, however, because Jim Morrison  was stoned and reading it backwards.
8:05 There's a still a wild crowd in front of the auditorium, looks like Walmart on Black Friday... hope there will be no riot today.  
7:45 While waiting for the announcement it may worth checking this page. There is a theory that Higgs influences our present from the future, so as to avoid being discovered. At this point, destroying the whole universe might be his only chance...  
7:35 The door are open, people flowing in, but miraculously no stampede. 
7:25 People have been have been camping all night in front of the auditorium door  to get inside and see the discovery live. These are pictures from 3am last night.

7:20 This is the day. The most important day for particle physics in this century, and probably  ever.

Tuesday 3 July 2012

H-day -1: vigil

It's the evening of the last day of the old B.H. era, tomorrow we start counting from zero. I'm at CERN to attend the historical seminar starting at an ungodly hour tomorrow. On a night like this no way I can write anything semi-intelligent, so instead let me give just a bunch of personal, chaotic remarks.  

The Higgs boson was always everywhere particle physicists look, so it was easy to forget it  was a hypothetical concept. Superficially, tomorrow we'll simply learn, to a 1 GeV precision, the value of the last free parameter of the Standard Model. But if you stop and  think about it for a while, it really blows your mind. Almost a 50 years a shy guy writing what was then a fringe paper to shut up the referee adds in the revised version a mention of the scalar particle excitation predicted by his toy model. Within a few years the importance of the particle is generally recognized, and papa Weinberg incorporates it  in the  Standard Model, to this day the valid theory of fundamental interactions. With  time, the indirect evidence for its existence has been mounting. But only 48 years and many colliders later the search will come to an end. Even though the prediction is highly non-trivial (theoretically,  it is based on a weird concept of a scalar field obtaining a uniform vacuum expectation value throughout the spacetime; phenomenologically, never before have we seen a fundamental spin-0 particle, etc.),  the particle shows up in the final states where it was predicted to show up, and up to a factor of 2 within the predicted rate. This is a perfect moment to shout  "Physics works, bitches".   

I'll be blogging live from the CERN auditorium, you can tune to mine or one of a dozen of competing relations.

Monday 2 July 2012

H-day -2: the warm-up act

Every decent rock concert features a support band whose role to warm you up before the main gig or, alternatively, give you time to buy a beer and chat up a blonde. The support band at the Higgs concert -- the Tevatron from Fermilab, Illinois -- is worth giving an ear to  because it offers slightly different qualities than the star of the evening.

The Tevatron collider has been shot down last September so the amount of data has not increased since the last Higgs update at the Moriond conference in March.  Nevertheless, the collaborations are still able to make adiabatic improvements in the analysis, especially now when they know where the Higgs is. At Moriond, the Higgs-like excess was observed mostly in the b-bbar final state by the CDF collaboration; what changed today is that D0 observes a (somewhat smaller) excess in the same channel, making the claim more credible. All in all, the combined (local) significance of the Higgs excess at the Tevatron reaches the maximum of 3 sigma for mh=120 GeV, although it's more like 2.7 sigma at the true value of mh=125 GeV.

However, there is an aspect of the data presented today that is more interesting than the  sigma pissing contest. The Tevatron experiments are most sensitive to the Higgs boson decaying into a pair of b-quarks and produced in association with a W or Z boson. What they're testing is thus the Higgs couplings to electroweak gauge bosons and to b-quarks, both of which are central to establishing the higgsy nature of the newly discovered particle. In particular, the Tevatron data are suggesting that the particle indeed  decays frequently into b-quarks (which, according to the Standard Model,  should happen about 60% of the times). Thus, the Tevatron provides an important piece of the puzzle that, at the moment, is not available from the LHC. Actually, the rate observed in the VH→bb channel is 2±0.7 larger than predicted by the Standard Model, adding up to other intriguing hints of a non-standard Higgs behavior.

By the end of the year the LHC experiments should reach a comparable sensitivity in the same channel, clarifying whether the Tevatron excess was the real thing, or a classic look-here effect...  

Sunday 1 July 2012

H-day -3: How to pump up the Higgs-to-gamma-gamma rate?

The 2011 LHC data featured a bump in the diphoton invariant mass spectrum, readily interpreted as a manifestation of the Higgs boson decaying to 2 photons. Actually, the bump was about almost 50% larger than expected assuming the Standard Model Higgs production and decay rates. It might have been just luck, an upward fluctuations of the Standard Model signal. Or it may be a sign of something interesting going on.

Photons are massless, therefore, in the Standard Model, the Higgs boson does not couple directly to photons. Instead, the decay of the Higgs into 2 photons is a loop mediated process. It happens to be dominated by two contributions: one, larger, from the W boson, and the other,  5 times smaller, from the top quark. Incidentally, these two contributions enter with opposite signs (in fact it's not quite an accident but a consequence of the theorem that links these contributions to the electromagnetic beta function). The other known charged particles are expected to contribute much less because their coupling to the Higgs is much smaller.

Now it is clear what tricks can be played to pump up the Higgs decay width into photons: 
  • Increase the Higgs couplings to W bosons. But then one needs to explain why no excess in the WW channel has been observed. 
  • Decrease the Higgs coupling to the top quark. But then the Higgs production rate would be decreased as well, and one needs to cook up new production channels. 
  • Introduce one (or more) new charged particle that contributes as much as the top quark, but with the opposite sign. 
 The last possibility is very attractive. It implies new physics the weak scale that could soon be discovered (the new particle couples to photons, thus it has a non-zero production rate at the LHC). Anything that has a charge and a large enough coupling to the Higgs could do the work: the new particle could be a scalar, a fermion, or a vector boson. 

Does the Higgs diphoton rate continues to be enhanced above the SM prediction also in the 2012 data? This is probably the most interesting question that we'll obtain an answer to on the 4th of July. If yes, it means that, maybe, maybe, new physics is really just behind the corner.