• John
  • Felde
  • University of Maryland
  • USA

Latest Posts

  • James
  • Doherty
  • Open University
  • United Kingdom

Latest Posts

  • Andrea
  • Signori
  • Nikhef
  • Netherlands

Latest Posts

  • CERN
  • Geneva
  • Switzerland

Latest Posts

  • Aidan
  • Randle-Conde
  • Université Libre de Bruxelles
  • Belgium

Latest Posts

  • Sally
  • Shaw
  • University College London
  • UK

Latest Posts

  • Richard
  • Ruiz
  • Univ. of Pittsburgh
  • U.S.A.

Latest Posts

  • Laura
  • Gladstone
  • University of Wisconsin, Madison
  • USA

Latest Posts

  • Seth
  • Zenz
  • Imperial College London
  • UK

Latest Posts

  • Michael
  • DuVernois
  • Wisconsin IceCube Particle Astrophysics Center
  • USA

Latest Posts

  • Mandeep
  • Gill

  • Emily
  • Thompson
  • Switzerland

Latest Posts

  • Ken
  • Bloom
  • USA

Latest Posts

Posts Tagged ‘Higgs’

Liveblog: New ATLAS Higgs Results

Tuesday, October 7th, 2014

In a short while, starting at 11:00 CEST / 10:00 BST, ATLAS will announce some new Higgs results:

“New Higgs physics results from the ATLAS experiment using the full Run-1 LHC dataset, corresponding to an integrated luminosity of approximately 25 fb-1, of proton-proton collisions at 7 TeV and 8 TeV, will be presented.” [seminar link]

I don’t expect anything earth-shattering, because ATLAS already has preliminary analyses for all the major Higgs channels. They have also submitted final publications for LHC Run I on Higgs decaying to two photons, two b quarks, two Z bosons – so it’s reasonable to guess that Higgs decaying to taus or W’s is going to be covered today.

(Parenthetically, CMS has already published final results for all of the major Higgs decays, because we are faster, stronger, smarter, better looking, and more fun at parties.)

I know folks on ATLAS who are working on things that might be shown today, and they promise they have some new tricks, so I’m hoping things will be fairly interesting. But again, nothing earth-shattering.

I’ll update this very page during the seminar. You should also be able to watch it on the Webcast Service.

10:55 I have a front row seat in the CERN Council Chamber, which is smaller than the main auditorium that you might be more familiar with. Looks like it will be very, very full.

11:00 Here we go! (Now’s a good time to click the webcast, if you plan to.)

11:03 Yes, it turns out it will be taus and W’s.

11:06 As an entree, look how fabulously successful the Standard Model, including the Higgs, has been:

11:10 Good overview right now over overall Higgs production and decay and the framework we used to understand it. Have any questions I can answer during the seminar? Put them in the comments or write something at me on Twitter.

11:18 We’re learning about the already-released results for Higgs to photons and ZZ first.

11:24 Higgs to bb, the channel I worked on for CMS during Run I. These ATLAS results are quite new and have a lot of nice improvements from their preliminary analysis. Very pretty plot of improved Higgs mass resolution when corrections are made for muons produced inside b-jets.

11:30 Now to Higgs to tau tau, a new result!

11:35 Developments since preliminary analysis include detailed validation of techniques for estimating from data how isolated the taus should be from other things in the detector.

11:36 I hope that doesn’t sound too boring, but this stuff’s important. It’s what we do all day, not just counting sigmas.

11:37 4.5 sigma evidence (only 3.5 expected) for the Higgs coupling to the tau lepton!

11:39 Their signal is a bit bigger than the SM predicts, but still very consistent with it. And now on to WW, also new.

11:41 In other news, the Nobel Prize in Physics will be announced in 4 minutes: It’s very unlikely to be for anything in this talk.

11:44 Fixed last comment: “likely” –> “unlikely”. Heh.

11:48 When the W’s decay to a lepton and an invisible neutrino, you can’t measure a “Higgs peak” like we do when it decays to photons or Z’s. So you have to do very careful work to make sure that a misunderstanding of you background (i.e. non-Higgs processes) produces what looks like a Higgs signal.

11:50 Background-subtracted result does show a clear Higgs excess over the SM backgrounds. This will be a pretty strong result.

11:51 6.1 sigma for H –> WW –> lvlv. 3.2 sigma for VBF production mechanism. Very consistent with the SM again.

11:52 Lots of very nice, detailed work here. But the universe has no surprises for us today.

11:54 We can still look forward to the final ATLAS combination of all Higgs channels, but we know it’s going to look an awful lot like the Standard Model. Congratulations to my ATLAS colleagues on their hard work.

11:56 By the way, you can read the slides on the seminar link.

12:02 The most significant result here might actually be the single-channel observation of the Vector Boson Fusion production mechanism. The Higgs boson really is behaving the way the Standard Model says it should! Signing off here, time for lunch


My First Day at ICHEP (Again)

Thursday, July 3rd, 2014

ICHEPstartICHEP 2014 started today in Valencia, Spain. This is one of particle physics’s biggest conferences, held every two years. The last one, in 2012, coincided with the discovery of the Higgs boson. This year, we’re probably going to have more in the way of careful measurements than big new surprises. ATLAS and CMS have already released Higgs updates, and the pesky boson looks more and more like the Standard Model Higgs all the time.

This is the second ICHEP I’ve attended in person. I showed a poster at the first one, and wrote a blog post about it – which is a scary reminder of just how long I’ve been blogging. (I also still have my lanyard from that conference, which I’m wearing with my badge because it’s cooler than the boring black one we got this time.) This year, I’m here to give a parallel talk about the potential for even better measurements of the Higgs at the High-Luminosity LHC, which is a possible upgrade for the LHC that could take us well into the 2030s. By then, I suppose I should aspire to give an ICHEP plenary talk. ;)


Top Quarks… So Many Top Quarks

Wednesday, April 30th, 2014

Thousands of paper on top quarks exist. Why?

There are literally thousands of papers, collaboration notes, and conference notes with the words “Top” and “Quark” in the title. As of this post, there are 3,477 since 1979 listed on inSpires. There are many, many more that omit the word “quark”. And sure, this is meager compared to the 5,114 papers with the words “Higgs Boson” written since ’74, but that is over 50,000 pages of top quarks (estimating 15 pages/paper). To be fair, there are also many, many more that omit the word “boson”. But for further comparison, there are only 395 papers with a title including the words “Bottom Quark“, 211 with “Bottomonium“, and 125 with “Bottom Hadron“. So why are there so many papers written about the top quark? The answer is that the top quark is weird special.


A single top quark candidate event at the Collider Detector experiment at Fermilab. Credit: CDF Collaboration

The top quark is very heavy, about 185 times heavier than the proton and ranks as the heaviest known elementary particle in all the particle kingdom. The second heaviest quark, the bottom quark, is only 4 or 5 times heavier than the proton. If you or I were a proton, then a medium-to-large school bus (without any people) would be a top quark. In fact, the top quark is so heavy it can decay into a real (on-shell) W boson, which is roughly half its mass. The only other particle that can do this is the Higgs. Though it is rare, exceedingly rare, the top quark can decay into real Z  and Higgs bosons as well. Not even the Higgs can top that last feat.

Top quark decaying into real, on-shell W boson and bottom quark. Credit: DZero Collaboration

However, the top quark is still a quark. It has an electric charge that is 2/3 as large as the proton. It has an intrinsic angular momentum (spin) equal to the proton’s or electron’s spin. The top quark is also colored, meaning that is interacts with gluons and is influenced by the strong nuclear force (QCD). When colored objects (quarks and gluons) are produced at collider and fixed target experiments, they undergo a process called hadronization. Hadronization is when two colored objects are far away from one another and the strong nuclear attraction between the two becomes so strong that a pair of colored objects will spontaneously be produced in the space between them. These new colored particles will then form bound states with the old colored states. However, the process hadronization means that we only observe the bound states of colored objects and not the colored objects themselves. Physicists have to infer their properties from the physics of bound states…. or do we?


Colored objects before (L), during (Center L and Center R), and after (R) hadronization.

The onset of hadronization is typically occurs about 10-24 seconds after the creation of a colored object. Yes, that is 0.000000000000000000000001 seconds. That is incredibly fast and well beyond anything that can be done at an experiment. The mean lifetime of the top quark on the other hand is about 10-25 seconds. In other words, the top quark is much more likely to decay in to a W boson, its principle decay mode, than hadronize. By looking at the decays of the W boson, for example to an electron and an electron-neutrino, their angular distributions, and other kinematic properties, we can measure directly the top quark’s quantum numbers. The top quark is special because it is the only quark whose spin and charge quantum numbers we can measure directly.


Top quark decaying into real, on-shell W boson and bottom quark. The W boson can subsequently decay into a charged lepton and a neutrino or into a quark and anti-quark. Credit: DZero Collaboration

The top quark tells us much about the Standard Model of particle physics, but it also may be a window to new physics. Presently, no one has any idea why the top quark is so much heavier than the bottom quark, or why both are orders of magnitude heavier than the electron and muon. This is called the “Mass Hierarchy Problem” of the Standard Model and stems from the fact that the quark and lepton masses in the theory are not predicted but are taken as input parameters. This does not mean that the Standard Model is “wrong”. On the contrary, the model works very, very well; it is simply incomplete. Of course there are new models and hypotheses that offer explanations, but none have been verified by data.

However, thanks to the 2012 discovery of the Higgs boson, there is a new avenue that may shed light upon the mass hierarchy problem. We now know that quarks and leptons interact with the Higgs boson proportionally to their masses. Since the top quark is ~40 times more massive than the bottom quark, it will interact with Higgs boson 40 times more strongly. There is suspicion that since the Higgs boson is sensitive to the different quark and lepton masses, it may somehow play a role in how masses are assigned.

Happy Colliding

- richard (@BraveLittleMuon)


Massive thoughts

Thursday, April 24th, 2014

This article appeared in symmetry on April 24, 2014.

The Higgs boson and the neutrino fascinate the general public and particle physicists alike. Why is that?

The Higgs boson and the neutrino fascinate the general public and particle physicists alike. Why is that?

If there are two particles that everyone has read about in the news lately, it’s the Higgs boson and the neutrino. Why do we continue to be fascinated by these two particles?

As just about everyone now knows, the Higgs boson is integrally connected to the field that gives particles their mass. But the excitement of this discovery isn’t over; now we need to figure out how this actually works and whether it explains everything about how particles get their mass. With time, this knowledge is likely to affect daily life.

One way it could possibly bridge the gap between fundamental research and the commercial market, I believe, is in batteries. The ultimate battery in nature is mass. The expression E=mc2 is a statement of that fact. During the early moments of the universe, all particles were massless and traveling at the speed of light. Once the Higgs mechanism turned on, particles suddenly began interacting with the field and, in this process, converted their energy into what we now refer to as mass. In a recent address to the Canadian Nuclear Society, I suggested that if engineers of the future could learn how to manipulate the Higgs field (to “turn it on and off”), then we would have developed the ultimate energy source and the best battery nature has created. This idea definitely belongs in the science-fiction category, but remember that progress in science is driven by thinking “outside the box!”

This sort of thinking comes from looking at the Higgs from another angle. According to the Standard Model, many particles come in left-handed and right-handed versions (in the former, the particle’s direction of spin matches its direction of motion, while in the latter, they are opposite).

Keeping this fact in mind, let’s look at the mass of the familiar electron as an example. When we say that the mass of the electron is created by interactions with the Higgs field, we can think of this as the Higgs field rapidly changing a left-handed electron into a right-handed electron, and vice versa. This switching back and forth is energy and, through E=mc2, energy is mass. A heavier particle like the top quark would experience this flipping at a much higher frequency than a lighter particle like the electron. As we learn more about how this process works, I encourage physicists to also seek applications of that knowledge.

And what about neutrinos? Do they get their mass from the Higgs field or in a completely different way? Once thought to be massless, neutrinos are now known to have a tiny mass. If the Higgs mechanism is responsible for that mass, there must exist both a left-handed and a right-handed neutrino. A good number of physicists think that both are out there, but we do not yet know. That knowledge may help us understand why the neutrino mass is tiny, as well as why there is more matter than antimatter in the universe—one of the most important questions facing our field of particle physics.

But since the neutrino is a neutral particle, the story gets more interesting. It may instead be possible that there is another type of mass. Referred to as a Majorana mass, it is not a mass described by the flipping of left- and right-handed neutrinos back and forth, but it is “intrinsic,” not derived from any kind of “motional energy.” I expect that the efforts by our field of particle physics, in the collective sense, will pursue the questions associated with both the Higgs boson and the neutrino with enthusiasm, and that the results will lead to advancements we can’t even imagine today.

Nigel Lockyer, Fermilab director


Getting to the Bottom of the Higgs

Thursday, January 30th, 2014

Updated Friday, January 31, 2014: Candidate event of Higgs boson decaying to bottom quarks has been added at the bottom.

CMS has announced direct evidence of the Higgs coupling to bottom quarks. This is special.

Last week, the Compact Muon Solenoid Experiment, one of the two general purpose experiments at the CERN Large Hadron Collider (LHC), submitted two papers to the arXiv. The first claims the first evidence for the Higgs boson decaying directly to tau lepton pairs and the second summarizes the evidence for the Higgs boson decaying directly to bottom quarks and tau leptons. (As an aside: The summary paper is targeted for Nature Physics, so it is shorter and more broadly accessible than other ATLAS and CMS publications.) These results are special, and why they are important is the topic of today’s post. For more information about the evidence was obtained, CERN posted a nice QD post last month.

Event display of a candidate Higgs boson decaying into a tau lepton and anti-tau lepton in the CMS detector.

Fig 1. Event display of a candidate Higgs boson decaying into a tau lepton and anti-tau lepton in the ATLAS detector.

There is a litany of results from ATLAS and CMS regarding the measured properties of the Higgs boson. However, these previous observations rely on the Higgs decaying to photons, Z bosons, or W bosons, as well as the Higgs being produced from annihilating gluons or being radiated off a W or Z. Though the top quark does contribute to the Higgs-photon and Higgs-gluon interactions, none of these previous measurements directly probe how fermions (i.e., quarks and leptons) interact with the Higgs boson. Until now, suggestions that the Higgs boson couples to fermions (i) proportionally to their masses and (ii) that the couplings possess no other scaling factor were untested hypotheses. In fact, this second hypothesis remains untested.


Fig. 2: Event display of a candidate Higgs boson decaying into a tau lepton and anti-tau lepton in the CMS detector.

As it stands, CMS claims “strong evidence for the direct coupling of the 125 GeV Higgs boson” to bottom quarks and tau leptons. ATLAS has comparable evidence but only for tau leptons. The CMS experiment’s statistical significance of the signal versus the “no Higgs-to-fermion couplings” hypothesis is 3.8 standard deviations, so no rigorous discovery yet (5 standard deviations is required). For ATLAS, it is 4.1 standard deviations. The collaborations still need to collect more data to satisfactorily validate such an incredible claim. However, this should not detract from that fact that we are witnessing phenomena never before seen in nature. This is new physics as far as I am concerned, and both ATLAS and CMS should be congratulated on discovering it.

Event display of a candidate Higgs boson decaying into a tau lepton and anti-tau lepton in the CMS detector.

Fig. 3: Event display of a candidate Higgs boson decaying into a bottom quark and anti-bottom quark in the ATLAS detector. HT to Jon Butterworth for the link.

The Next Step

Once enough data has been collected to firmly and undoubtedly demonstrate that quarks and leptons directly interact with the Higgs, the real tests of the Standard Model of particle physics start up. In the Standard Model, the strength at which a fermion interacts with the Higgs is proportional to the fermion mass and inversely proportional to the ground state energy of the Higgs field. There is no other factor involved. This is definitively not the case for a plethora of new physics models, including scenarios with multiple Higgs bosons, like supersymmetry, as well as scenarios with new, heavy fermions (heavy bottom quark and tau lepton partners). This is definitely a case of using newly discovered physics to find more new physics.

Happy Colliding.

- Richard (@bravelittlemuon)

PS I was unable to find an event display of a Higgs boson candidate decaying into a pair of bottom quarks. If anyone knows where I can find one, I would be very grateful.

PSS Much gratitude toward Jon Butterworth for providing a link to Higgs-bbar candidate events.


Les collaborations ATLAS et CMS du CERN ont maintenant l’évidence que la nouvelle particule découverte en juillet 2012 se comporte de plus en plus comme le boson de Higgs. Les deux expériences viennent en fait de démontrer que le boson de Higgs se désintègre aussi en particules tau, des particules semblables aux électrons mais beaucoup plus lourdes.

Pourquoi est-ce si important? CMS et l’ATLAS avaient déjà établi que ce nouveau boson était bien un type de boson de Higgs. Si tel est le cas, la théorie prévoit qu’il doit se désintégrer en plusieurs types de particules. Jusqu’ici, seules les désintégrations en bosons W et Z de même qu’en photons étaient confirmées. Pour la première fois, les deux expériences ont maintenant la preuve qu’il se désintègre aussi en particules tau.

La désintégration d’une particule s’apparente beaucoup à faire de la monnaie pour une pièce. Si le boson de Higgs était une pièce d’un euro, il pourrait se briser en différentes pièces de monnaie plus petites. Jusqu’à présent, le distributeur de monnaie semblait seulement donner la monnaie en quelques façons particulières. On a maintenant démontré qu‘il existe une façon supplémentaire.

Il y a deux classes de particules fondamentales, appelées fermions et bosons selon la valeur de quantité de mouvement angulaire. Les particules de matière comme les taus, les électrons et les quarks appartiennent tous à la famille des fermions. Par contre, les particules associées aux diverses forces qui agissent sur ces fermions sont des bosons, comme les photons et les bosons W et Z.

L”été dernier, l’expérience CMS avait déjà apporté la preuve avec un signal de 3.4 sigma que le boson de Higgs se désintégrait en fermions en combinant leurs résultats pour deux types de fermions, les taus et les quarks b. Un sigma correspond à un écart-type, la taille des fluctuations statistiques potentielles. Trois sigma sont nécessaires pour revendiquer une évidence tandis que cinq sigma sont nécessaires pour clamer une découverte.

Pour la première fois, il y a maintenant évidence pour un nouveau canal de désintégration (les taus) – et deux expériences l’ont produit indépendamment. La collaboration ATLAS a montré la preuve pour le canal des taus avec un signal de 4.1 sigma, tandis que CMS a obtenu 3.4 sigma, deux résultats forts prouvant que ce type de désintégrations se produit effectivement.

En combinant leurs résultats les plus récents pour les taus et les quarks b, CMS a maintenant une évidence pour des désintégrations en fermions avec 4.0 sigma.

Les données rassemblées par l’expérience ATLAS (les points noirs) sont en accord avec la somme de tous les évènements venant du bruit de fond (histogrammes en couleur) en plus des contributions venant d’un boson de Higgs se désintégrant en une paire de taus (la ligne rouge). En dessous, le bruit de fond est soustrait des données pour révéler la masse la plus probable du boson de Higgs, à savoir 125 GeV (la courbe rouge).

CMS commence aussi à voir des désintégrations en paires de quarks b avec un signal de 2.0 sigma. Bien que ceci ne soit toujours pas très significatif, c’est la première indication pour cette désintégration jusqu’ici au Grand collisionneur de hadrons (LHC). Les expériences du Tevatron avaient rapporté l’observation de telles désintégrations à 2.8 sigma. Bien que le boson de Higgs se désintègre en quarks b environ 60 % du temps, il y a tant de bruit de fond qu’il est extrêmement difficile de mesurer ces désintégrations au LHC.

Non seulement les expériences ont la preuve que le boson de Higgs se désintègre en paires de taus, mais elles mesurent aussi combien de fois ceci arrive. Le Modèle Standard, la théorie qui décrit à peu près tout ce qui a été observé jusqu’à maintenant en physique des particules, stipule qu’un boson de Higgs devrait se désintégrer en une paire de taus environ 8 % du temps. CMS a mesuré une valeur correspondant à 0.87 ± 0.29 fois ce taux, c’est-à-dire une valeur compatible avec 1.0 comme prévu pour le boson de Higgs du Modèle Standard. ATLAS obtient 1.4 +0.5-0.4, ce qui est aussi consistent avec la valeur de 1.0 à l‘intérieur des marges d’erreur.


Un des événements captés par la collaboration CMS ayant les caractéristiques attendues pour les désintégrations du boson de Higgs du Modèle Standard en une paire de taus. Un des taus se désintègre en un muon (ligne rouge) et en neutrinos (non visibles dans le détecteur), tandis que l’autre tau se désintègre en  hadrons (particules composées de quarks) (tours bleues) et un neutrino. Il y a aussi deux jets de particules vers l’avant (tours vertes).

Avec ces nouveaux résultats, les expériences ont établi une propriété de plus prédite pour le boson de Higgs du Modèle Standard. Reste encore à clarifier le type exact de boson de Higgs que nous avons. Est-ce bien le plus simple des bosons, celui associé au Modèle Standard? Ou avons nous découvert un autre type de boson de Higgs, le plus léger des cinq bosons de Higgs prévus par une autre théorie appelée la supersymétrie.

Il est encore trop tôt pour écarter cette deuxième hypothèse. Tandis que le boson de Higgs se comporte jusqu’ici exactement comme ce à quoi on s’attend pour le boson de Higgs du Modèle Standard, les mesures manquent encore de précision pour exclure qu’il soit de type supersymétrique. Une réponse définitive exige plus de données. Ceci arrivera une fois que le LHC reprendra du service à presque deux fois l’énergie actuelle en 2015 après l’arrêt actuel pour maintenance et consolidation.

En attendant, ces nouveaux résultats seront affinés et finalisés. Déjà ils représentent un petit pas pour les expériences et un bond de géant pour le boson de Higgs.

Pour tous les détails (en anglais seulement)

Présentation donnée par la collaboration ATLAS le 28 novembre 2013

Présentation donnée par la collaboration CMS le 3 décembre 2013

Pauline Gagnon

Pour être averti-e lors de la parution de nouveaux blogs, suivez-moi sur Twitter: @GagnonPauline ou par e-mail en ajoutant votre nom à cette liste de distribution.


Higgs Convert

Friday, November 29th, 2013

Since 4th July 2012, the physicists at CERN have had a new boson to play with. This new boson was first seen in the searches that were optimised to find the world famous Higgs boson, and the experiments went as far as to call it a “Higgs-like” boson. Since then there has been an intense program to study its spin, width, decay modes and couplings and so far it’s passed every test of Higgs-ness. Whether or not the new boson is the Standard Model Higgs boson is one of the most pressing questions facing us today, as there is still room for anomalous couplings. Whatever the answer is, a lot of physicists will be pleased. If we find that the properties match those of a Standard Model Higgs boson exactly then we will hail it as a triumph of science and a fitting end to the quest for the Standard Model which has taken the work of thousands of physicists over many decades. If we find some anomaly in the couplings this would be a hint to new physics hiding “just around the corner” and tease is with what we may see at higher energies when the LHC turns on again in 2015.

A candidate for a Higgs boson decaying to two tau leptons (ATLAS)

A candidate for a Higgs boson decaying to two tau leptons (ATLAS)

For those who have read my blog for a long time, you may remember that I wrote a post saying how I was skeptical that we would find the Standard Model Higgs boson. In fact I even bet a friend $20 that we wouldn’t find the Standard Model Higgs boson by 2020, and until today I’ve been holding on to my money. This week I found that ATLAS announced the results of their search for the Higgs boson decaying to two tau leptons, and the results agree with predictions. When we take this result alongside the decays to bosons, and the spin measurements it’s seems obvious that this is the Higgs boson that we were looking for. It’s not fermiophobic, and now we have direct evidence of this. We have see the ratio of the direct ferimonic couplings to direct bosonic couplings, and they agree very well. We’d had indirect evidence of fermionic couplings from the gluon fusion production, but it’s always reassuring to see the direct decays as well. (As a side note I’d like to point out that the study of the Higgs boson decaying to two tau leptons has been the result of a huge amount of very hard work. This is one of the most difficult channels to study, requiring a huge amount of knowledge and a wide variety of final states.)

Now the reason for my skepticism was not because I thought the Standard Model was wrong. In fact the Standard Model is annoyingly accurate in its predictions, making unexpected discoveries very difficult. What I objected to was the hyperbole that people were throwing around despite the sheer lack of evidence. If we’re going to be scientists we need to rely on the data to tell us what is real about the universe and not what some particular model says. If we consider an argument of naturalness (by which I mean how few new free terms we need to add to the existing edifice of data) then the Higgs boson is the best candidate for a new discovery. However that’s only an argument about plausibility and does not count as evidence in favour of the Higgs boson. Some people would say things like “We need a Higgs boson because we need a Brout-Englert-Higgs mechanism to break the electroweak symmetry.” It’s true that this symmetry needs to be broken, but if there’s no Higgs boson then this is not a problem with nature, it’s a problem with our models!

The fact that we’ve seen the Higgs boson actually makes me sad to a certain extent. The most natural and likely prediction has been fulfilled, and this has been a wonderful accomplishment, but it is possible that this will be the LHC’s only new discovery. As we move into LHC Run II will we see something new? Nobody knows, of course, but I would not be surprised if we just see more of the Standard Model. At least this time we’ll probably be more cautious about what we say in the absence of evidence. If someone says “Of course we’ll see strong evidence of supersymmetry in the LHC Run II dataset.” then I’ll bet them $20 we won’t, and this time I’ll probably collect some winnings!


Today the 2013 Nobel Prize in Physics was awarded to François Englert (Université Libre de Bruxelles, Belgium) and Peter W. Higgs (University of Edinburgh, UK). The official citation is “for the theoretical discovery of a mechanism that contributes to our understanding of the origin of mass of subatomic particles, and which recently was confirmed through the discovery of the predicted fundamental particle, by the ATLAS and CMS experiments at CERN’s Large Hadron Collider.” What did they do almost 50 years ago that warranted their Nobel Prize today? Let’s see (for the simple analogy see my previous post from yesterday).

The overriding principle of building a theory of elementary particle interactions is symmetry. A theory must be invariant under a set of space-time symmetries (such as rotations, boosts), as well as under a set of “internal” symmetries, the ones that are specified by the model builder. This set of symmetries restrict how particles interact and also puts constraints on the properties of those particles. In particular, the symmetries of the Standard Model of particle physics require that W and Z bosons (particles that mediate weak interactions) must be massless. Since we know they must be massive, a new mechanism that generates those masses (i.e. breaks the symmetry) must be put in place. Note that a theory with massive W’s and Z that are “put in theory by hand” is not consistent (renormalizable).

The appropriate mechanism was known in the beginning of the 1960’s. It goes under the name of spontaneous symmetry breaking. In one variant it involves a spin-zero field whose self-interactions are governed by a “Mexican hat”-shaped potential


It is postulated that the theory ends up in vacuum state that “breaks” the original symmetries of the model (like the valley in the picture above). One problem with this idea was that a theorem by G. Goldstone required a presence of a massless spin-zero particle, which was not experimentally observed. It was Robert Brout, François Englert, Peter Higgs, and somewhat later (but independently), by Gerry Guralnik, C. R. Hagen, Tom Kibble who showed a loophole in a version of Goldstone theorem when it is applied to relativistic gauge theories. In the proposed mechanism massless spin-zero particle does not show up, but gets “eaten” by the massless vector bosons giving them a mass. Precisely as needed for the electroweak bosons W and Z to get their masses!  A massive particle, the Higgs boson, is a consequence of this (BEH or Englert-Brout-Higgs-Guralnik-Hagen-Kibble) mechanism and represents excitation of the Higgs field about its new vacuum state.

It took about 50 years to experimentally confirm the idea by finding the Higgs boson! Tracking the historic timeline, the first paper by Englert and Brout, was sent to Physical Review Letter on 26 June 1964 and published in the issue dated 31 August 1964. Higgs’ paper, received by Physical Review Letters on 31 August 1964 (on the same day Englert and Brout’s paper was published)  and published in the issue dated 19 October 1964. What is interesting is that the original version of the paper by Higgs, submitted to the journal Physics Letters, was rejected (on the grounds that it did not warrant rapid publication). Higgs revised the paper and resubmitted it to Physical Review Letters, where it was published after another revision in which he actually pointed out the possibility of the spin-zero particle — the one that now carries his name. CERN’s announcement of Higgs boson discovery came 4 July 2012.

Is this the last Nobel Prize for particle physics? I think not. There are still many unanswered questions — and the answers would warrant Nobel Prizes. Theory of strong interactions (which ARE responsible for masses of all luminous matter in the Universe) is not yet solved analytically, the nature of dark matter is not known, the picture of how the Universe came to have baryon asymmetry is not cleared. Is there new physics beyond what we already know? And if yes, what is it? These are very interesting questions that need answers.


A Nobel Prize most appreciated at CERN

Tuesday, October 8th, 2013

The whole of CERN was elated today to learn that the Nobel Prize for Physics had been awarded this year to Professors François Englert and Peter Higgs for their theoretical work on what is now known as the Brout-Englert-Higgs mechanism. This mechanism explains how all elementary particles get their masses.


CERN had good reason to celebrate, since last year on 4 July, scientists working on LHC experiments proudly announced the discovery of a new particle, which was later confirmed to be a Higgs boson. This particle proves that the theory Robert Brout, François Englert and Peter Higgs developed, along with others, in 1964 was indeed correct.

The Higgs boson discovery was essential to establish their theory so we are all happy to see their work (and to some extent, our work) acknowledged with this prestigious award.

It took another decade before Steve Weinberg, co-recipient of the Nobel Prize in 1979, saw the full implication of their work while unifying two fundamental forces, the electromagnetic and weak forces, as Peter Higgs explained in July at the European Physical Society meeting of the Particle Physics division, where he gave a highly appreciated presentation. There he detailed the work of all those who preceded him, including Englert and Brout, in bringing key elements that enabled him to conceive his own work.

Peter Higgs recalled how it all began with pioneering work on “spontaneous symmetry breaking” done by Yoichiro Nambu in 1960 (for which he shared the Nobel Prize in 2008). Nambu himself was inspired by Robert Schrieffer, a condensed matter physicist who had developed similar concepts for the theory of superconductivity with John Bardeen and Leon Cooper (1972 Nobel Prize).

Spontaneous symmetry breaking is central in the Brout-Englert-Higgs mechanism rewarded today by the Swedish Academy of Science.

Jeffrey Goldstone then introduced a scalar field model often referred to as the “Mexican hat” potential while another condensed matter theorist, Philip Anderson (Nobel Prize in 1977) showed how to circumvent some problems pointed out by Goldstone.

Then, Englert and Brout published their paper, where the mechanism was finally laid out. Peter Higgs, who was working entirely independently from Brout and Englert, had his own paper out a month later with a specific mention of an associated boson. Tom Kibble, Gerald Guralnik and Carl Hagen soon after contributed additional key elements to complete this theory.

“I had to mention this boson specifically because my paper was first rejected for lack of concrete predictions”, Peter Higgs explained good-heartedly in his address last summer. This explicit mention of a boson is partly why his name got associated with the now famous boson.

The history of the Brout-Englert-Higgs mechanism just goes to show how in theory just like in experimental physics, it takes lots of people contributing good ideas, a bit of luck but mostly great collaboration to make ground-breaking discoveries.

The thousands of physicists, engineers and technicians who made the discovery of the Higgs boson possible at the LHC are also all celebrating today.

Pauline Gagnon

To find out more about the Higgs boson, here is a 25-minute recorded lecture I gave at CERN on Open Days

To be alerted of new postings, follow me on Twitter: @GagnonPauline
 or sign-up on this mailing list to receive and e-mail notification.




Nobel Dreams

Friday, October 4th, 2013

The liveblog

Greeting from Brussels! This is my liveblog of the Nobel Prize Announcement Ceremony, bringing you the facts and the retweets as they happen.

14:14: Press Conference ongoing. “This is a great day for young people.”

13:56: A moving statement from Kibble (source):

I am glad to see that the Swedish Academy has recognized the importance of the mass-generating mechanism for gauge theories and the prediction of the Higgs boson, recently verified at CERN. My two collaborators, Gerald Guralnik and Carl Richard Hagen, and I contributed to that discovery, but our paper was unquestionably the last of the three to be published in Physical Review Letters in 1964 (though we naturally regard our treatment as the most thorough and complete) and it is therefore no surprise that the Swedish Academy felt unable to include us, constrained as they are by a self-imposed rule that the Prize cannot be shared by more than three people. My sincere congratulations go to the two Prize winners, François Englert and Peter Higgs. A sad omission from the list was Englert’s collaborator Robert Brout, now deceased.

13:37: CERN are holding a press conference at 14:00 (CET) link

13:22: Commentary continues at the Nobel Prize page. Currently discussing why the boson was so hard to find. “This particle has been looked for at every accelerator that has existed.”

13:20: As expected, so many news sites have been created: CMS, ATLAS, ULB, Edinburgh

13:14: I think my twitter account has exploded with tweets. Also, some Belgian news pages are down, probably due to high traffic. Wow!

13:11: Wow, what a great announcement. Too short though!

13:08: Find out more about the physics at Brussels, where the Brout-Englert-Higgs mechanism was born! The IIHE and the Nobel Prize

13:01: Englert is on the phone. Good to hear from him :)

12:59: Animation of the boson appearing, cool!

12:57: We just opened the champagne here at ULB!

12:52: Text for the announcement:

“For the theoretical discovery of a mechanism that contributes to our understanding of the origin of mass of subatomic particles, and which recently was confirmed through the discovery of the predicted fundamental particle, by the ATLAS and CMS experiments at CERN’s Large Hadron Collider”

12:48: The award goes to Englert and Higgs!

12:44: One minute to go!

12:39: We all know what the Brout-Englert-Higgs mechanism is and what the boson discovery means, so let’s instead take a look at the other likely awards. The prize could go to the discovery of extra solar planets. 51 Pegasi b was an extra solar planet discovered in 1995, orbiting a sun-like star. This discovery could have far reaching implications. What would happen if we saw spectral lines suggesting the presence amino acids coming from the planet? (I’m not sure such a phenomenon is even possible, but if it is it would be a very strong indicator of RNA-like life from another planet.) That discovery took place 18 years ago, and the Brout-Englert-Higgs boson was discovered only one year ago. Either discovery would certainly be worthy of the prize.

12:33: A quantum approach to the delay problem:

Someone go observe the academy and make them leave this terrible superposition. (@lievenscheire)

12:32: Another possible reason for the delay:

There’ll be a new hunt for the #Higgs. He’s gone to the Highlands to avoid the fuss if he wins #nobelprize. Maybe reason for delay. (@BBCPallab)

12:31: The Nobel Prize committee are stalling by suggesting we look at previous awards. At least they are trying to keep up amused while we wait :)

12:29: Around the world people are patiently waiting. People from the US have been awake since 5:00am. In Marakech the ATLAS Collaboration looks on. Here are ULB/IIHE the cafeteria seem deserted. (I’m glad there’s a coffee machine on the desk next time mine.) I’m starting to think this is a plot to get some more media attention for what is bound to be a controversial year for physics. There are many good choices of topic this year, and even some of the topics have controversial choices of Laureates.

12:21: Some humourous speculation about the delay:

The Academy only has 3 #sigma evidence of more votes for than against, waiting for more data (@SethZenz)

They can’t get Comic Sans installed on the Academy’s computer (@orzelc)

The committee were mobbed trying to get across a cocktail party. (@AstroKatie)

12:07: The announcement is delayed until 12:45 CET. People are complaining about the background music!

11:58: The announcement is delayed until 12:30 CET.

11:44: According to the Guardian (source) there will be a delay of 30 minutes.

11:42: Just over two minutes to go. This could be a very exciting year for Belgium.

11:33: See the livecast.

Other info

On Tuesday October 8th the recipient(s) of the 2013 Nobel Prize in Physics will be announced. There has already been a lot of speculation about who might be the Nobel Laureates this year, and there is a lot of interest in the likely contenders! Each year Thomson Reuters publishes predictions of who might receive the Nobel Prizes, and this year they have narrowed the scope down to three likely awards in physics:

  • ‣ Francois Englert and Peter Higgs, for their prediction of the Brout-Englert-Higgs mechanism. (Brout is deceased and the Nobel Prize is not awarded posthumously.)
  • ‣ Hideo Hosono, for his discovery of iron-based superconductors.
  • ‣ Geoffrey Marcy, Michel Mayor, and Didier Queloz, for their discoveries of extrasolar planets.
The 2012 Nobel Prize Award Ceremony (Copyright © Nobel Media AB 2012 Photo: Alexander Mahmoud)

The 2012 Nobel Prize Award Ceremony (Copyright © Nobel Media AB 2012 Photo: Alexander Mahmoud)

There has also been speculation that either Anderson or Nambu may receive a second Nobel Prize for their work related to spontaneous symmetry breaking.

With so many different predictions and so many opinions it can be hard to keep up with all the latest news and blogs! I know that a lot of people plan to share their views and experiences of the day, so I’ll be keep a list of bloggers and tweeters that you can follow.

Seth Zenz:

See Seth’s excellent post about the Nobel Prize, Englert and Higgs, and CERN. You can also follow his twitter account: @SethZenz

James Doherty:

See James’s great post about the Nobel Prize, He’s on twitter too: @JimmyDocco

Guardian liveblog

Other twitter accounts to follow: