No particle physicist received a phone call from Stockholm today. There had been some expectations for an award honoring the Higgs discovery. Well, it was maybe naive but not completely unrealistic to think that the Nobel committee might want to reestablish some connection with the original Nobel's will (which, anecdotally, awarded prizes for discoveries made during the preceding year). To ease my disappointment, let me write about a purely probabilistic but potentially gruesome aspect of today's decision. Warning: the discussion below is a really bad taste; don't even start reading unless Borat is among your favorite movies!
Peter Higgs is 83, and François Englert is almost 80. Taking the US data on lifetime expectancy as the reference, they have respectively 9% and 6% probability to pass away within a year from now. Thus, the probability of at least one of them being gone by the time of the next announcement is approximately 14%! To give an everyday analogy, it's only a tad safer than playing Russian Roulette with 1 bullet in a 6-shot colt revolver. The probability grows to stunning 27% if one includes Philip Anderson among the potential recipients (nearly 89, 15%). Obviously, the probability curve is steeply rising as a function of t, and approaches 100% for the typical Nobel recognition time lag.
Well, the Nobel for the Higgs discovery will be awarded sooner or later. Even if one of the crucial actors does not make it, the prestige of the physics Nobel prize won't be hurt too much (it has survived far more serious embarrassments). But, that would be just sad and unjust, even more so than the Cabibbo story. So why not make it rather sooner than later?
Here's is more on the dangers of playing Russian Roulette:
Particle Physics Blog
Tuesday, 9 October 2012
Tuesday, 2 October 2012
What's the deal with vacuum stability?
This year we learned that the Higgs mass is 125.5 GeV, give or take 1 GeV. As a consequence, we learned that God plays not only dice but also russian roulette. In other words, that life is futile because everything we cherish and hold dear will decay. In other words, that the vacuum of the standard model is not stable.
Before we continue, keep in mind the important disclaimer:
The stability of our vacuum depends on the sign of the quartic coupling in the λ |H|^4 term in the Higgs potential: for negative λ the potential is unbounded from below and therefore unstable. We know exactly the value of λ at the weak scale: from the Higgs mass 125 GeV and the expectation value 246 GeV it follows that λ = 0.13, positive of course. But panta rhei and λ is no exception. At large values of |H|, the Higgs potential in the standard model is, to a good approximation, given by λ(|H|) |H|^4 where λ(|H|) is the running coupling evaluated at the scale |H|. If Higgs were decoupled from the rest of matter then λ would grow with the energy scale and would eventually explode into a Landau pole. However, the Yukawa couplings of the Higgs boson to fermions provide another contribution to the evolution equations that works toward decreasing λ at large energies. In the standard model the top Yukawa coupling is large, of order 1, while the Higgs self-coupling is moderate, so Yukawa wins.
In the plot showing the evolution of λ in the standard model (borrowed from the latest state-of-the-art paper) one can see that at the scale
of about 10
million TeV the Higgs self-coupling becomes
negative. That sounds like a catastrophe as it naively means that the Higgs potential is unbounded from below. However,
we can reliably use quantum field theory only up to the Planck scale,
and one can assume that some unspecified physics near the Planck scale
(for example, |H|^6 and higher terms in the potential) restore the
boundedness of the Higgs potential. Still, between 10^10 and 10^19 GeV the
potential is negative and therefore it has a global minimum at large |H| that
is much deeper than the vacuum we live in. As a consequence, the path integral will
receive contributions from the field configurations interpolating between
the two vacua, leading to a non-zero probability of tunneling into the
other vacuum.
Fortunately for us, the tunneling probability is proportional to Exp[-1/λ], and λ gets only slightly
negative in the
standard model. Thus, no reason to panic, our vacuum is meta-stable, meaning its average lifetime extends beyond December 2012. Nevertheless,
there is something intriguing here. We happen to occupy a
very special patch of the standard model parameter space. First of all
there's the good old hierarchy problem: the mass term of the Higgs field
takes a very special (fine-tuned?) value such that we live extremely
close to the boundary between the broken (v > 0) and the unbroken
(v=0) phases. Now we realized the potential is even more special: the
quartic coupling is such that two vacua coexist, one at low |H| of order TeV and the
other at large |H| of order the Planck scale. Moreover, not only λ but also it's beta
functions is nearly zero near the Planck scale, meaning that λ evolves
very slowly at high scales. Who sets these boundary conditions? Is that yet another remarkable coincidence, or is there a physical reason? Something to do with quantum gravity? Something to do with inflation? I think it's fair to say that so far nobody has presented a compelling proposal explaining these boundary conditions satisfied by λ.
Ah, and don't forget the disclaimer:
Before we continue, keep in mind the important disclaimer:
All this discussion is valid assuming the standard model is the correct theory all the way up to the Planck scale, which is unlikely.Indeed, while it's very likely that the standard model is an adequate description of physics at the energies probed by the LHC, we have no compelling reasons to assume it works at, say, 100 TeV. On the contrary, we know there should be some new particles somewhere, at least to account for dark matter and the baryon asymmetry in the universe, and those degrees of freedom may well affect the discussion of vacuum stability. But for the time being let's assume there's no new particles beyond the standard model with a significant coupling to the Higgs field.
The stability of our vacuum depends on the sign of the quartic coupling in the λ |H|^4 term in the Higgs potential: for negative λ the potential is unbounded from below and therefore unstable. We know exactly the value of λ at the weak scale: from the Higgs mass 125 GeV and the expectation value 246 GeV it follows that λ = 0.13, positive of course. But panta rhei and λ is no exception. At large values of |H|, the Higgs potential in the standard model is, to a good approximation, given by λ(|H|) |H|^4 where λ(|H|) is the running coupling evaluated at the scale |H|. If Higgs were decoupled from the rest of matter then λ would grow with the energy scale and would eventually explode into a Landau pole. However, the Yukawa couplings of the Higgs boson to fermions provide another contribution to the evolution equations that works toward decreasing λ at large energies. In the standard model the top Yukawa coupling is large, of order 1, while the Higgs self-coupling is moderate, so Yukawa wins.
In the plot showing the evolution of λ in the standard model (borrowed from the latest state-of-the-art paper) one can see that at the scale
of about 10
million TeV the Higgs self-coupling becomes
negative. That sounds like a catastrophe as it naively means that the Higgs potential is unbounded from below. However,
we can reliably use quantum field theory only up to the Planck scale,
and one can assume that some unspecified physics near the Planck scale
(for example, |H|^6 and higher terms in the potential) restore the
boundedness of the Higgs potential. Still, between 10^10 and 10^19 GeV the
potential is negative and therefore it has a global minimum at large |H| that
is much deeper than the vacuum we live in. As a consequence, the path integral will
receive contributions from the field configurations interpolating between
the two vacua, leading to a non-zero probability of tunneling into the
other vacuum. Fortunately for us, the tunneling probability is proportional to Exp[-1/λ], and λ gets only slightly
negative in the
standard model. Thus, no reason to panic, our vacuum is meta-stable, meaning its average lifetime extends beyond December 2012. Nevertheless,
there is something intriguing here. We happen to occupy a
very special patch of the standard model parameter space. First of all
there's the good old hierarchy problem: the mass term of the Higgs field
takes a very special (fine-tuned?) value such that we live extremely
close to the boundary between the broken (v > 0) and the unbroken
(v=0) phases. Now we realized the potential is even more special: the
quartic coupling is such that two vacua coexist, one at low |H| of order TeV and the
other at large |H| of order the Planck scale. Moreover, not only λ but also it's beta
functions is nearly zero near the Planck scale, meaning that λ evolves
very slowly at high scales. Who sets these boundary conditions? Is that yet another remarkable coincidence, or is there a physical reason? Something to do with quantum gravity? Something to do with inflation? I think it's fair to say that so far nobody has presented a compelling proposal explaining these boundary conditions satisfied by λ. Ah, and don't forget the disclaimer:
All this discussion is valid assuming the standard model is the correct theory all the way up to the Planck scale, which is unlikely.
Tuesday, 25 September 2012
Fermi line cont'd
A look at hep-ph listing tells you that what excites particle theorists these days is the Fermi line. Recall, that an independent analysis of gamma-ray data from the Fermi telescope discovered the monochromatic emission from the center of our galaxy at the energy of approximately 130 GeV. The signal is so strong that it's unlikely a fluctuation, and no known astrophysical processes are expected to produce monochromatic lines. The line may be a weird instrumental effect, or it may be the signal of dark matter annihilating into a pair photons with the cross section of few*10^-27 cm^3/sec. If the latter is true, it would dwarf the Higgs boson discovery...
As usual, the most popular game is to fit the signal into every possible model, including those that firmly resist. There's been some interesting developments on this front, but I'll keep that for another post. For now, I'll restrict to the properties of the signal and astrophysical constraints.
In summary, there are 2 main arguments against the Fermi line being a signal of dark matter. One is the presence of the line in the Earth limb photon sample. The other is that it's good to be true. Based on that, it's probably worth staying excited for a little longer, until there are better reasons to stop the fun.
As usual, the most popular game is to fit the signal into every possible model, including those that firmly resist. There's been some interesting developments on this front, but I'll keep that for another post. For now, I'll restrict to the properties of the signal and astrophysical constraints.
- The statistical significance of the line is large, the precise number depending on how the data are cut and cooked. In the original paper the significance was 4.6σ (before taking into account the trial factor), but for example in this paper the numbers 5.0σ or even 5.5σ are bandied around. That paper also claims that a slightly better fit to the data is with 2 lines, one at 129 GeV and another at 111 GeV, and that the center of the emission is off by 1.5 degree from the galactic centre. The former may be a good news for dark matter, as most models predict 2 separate lines, from annihilation into γγ and into γZ. The latter doesn't have to be a bad news, in view of the recent simulations of dark matter distribution.
- Two groups were recently scanning the Fermi data for suspicious features that could suggest
hat the line is an instrumental artifact. They may have found one: a 130 GeV line in the Earth limb sample. Cosmic rays hitting the atmosphere produce gamma-rays that sometimes fall into Fermi's field of view. This provides a sort of calibration sample where no signal is expected. Instead, there seems to be a 3σ line in the Earth limb photons that can be made even more prominent with specific cuts on the photon incidence angle. Is that an unlucky fluctuation? On the other hand, it's difficult to imagine an instrumental effects or a software bug that could be responsible for both the galactic center and the Earth limb lines. - There are 2 more places in the sky where the presence of the 130 GeV line was claimed. The line was observed in the nearby galaxy clusters, which may be a good news. Also, the line was observed in the unassociated gamma-ray sources, which is probably a bad news given those were later claimed to be AGNs. No line was detected from the dwarf satellite galaxies of the Milky Way, which is probably not a problem, and no line emission was found in the galactic plane, which is good.
- In most models of dark matter a gamma-ray line would be accompanied by a 1000 times more intense continuum photon signal, just because dark matter annihilation into other final states
(that later emit photons) would be dominant. However, the observed photon spectrum from the galactic center - the same one that displays the monochromatic signal - puts very strong constraints on the continuum emission. Typically, the cross section for dark matter annihilation into other final states can be at most 10 times larger than the cross section for the annihilation into 2 photons. For example, this paper claims the limits on the annihilation rate of 130 GeV dark matter into most final states is comparable to the thermal cross section 3*10^-26 cm^3/sec (the one that guarantees the correct relic abundance if dark matter is of thermal origin), and even stronger with less conservative assumptions about the dark matter density profile. This is a severe constraint on theory, such that the models explaining the Fermi line have to be tailor-made to satisfy it.
In summary, there are 2 main arguments against the Fermi line being a signal of dark matter. One is the presence of the line in the Earth limb photon sample. The other is that it's good to be true. Based on that, it's probably worth staying excited for a little longer, until there are better reasons to stop the fun.
sorry :-{
Tja, 2 months without writing a post is my personal best since I started this blog. It cannot be just laziness. I blame it on the frantic atmosphere surrounding the Higgs discovery, which resulted in post-coital tristesse. Indeed, a face-to-face with a genuine discovery only makes you realize the day-to-day misery of high-energy physics today. Now it's much harder to get excited about setting limits on new physics or even about seeing hints of new physics that will surely go away before you blink. New limits on SUSY from the 8 TeV LHC run? Yawn. First robust limits on superpartners of the top quark? Phew. Best ever limits on direct detection of dark matter? Boooring. Another smoking-gun signal of dark matter? Wait...
Well, it's time go back to the daily grind because, in the long run, that may be the only life we have :-)
Well, it's time go back to the daily grind because, in the long run, that may be the only life we have :-)
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.
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.
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.
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