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Posts Tagged ‘ICHEP2012’

ICHEP 2012: Welcome

Wednesday, July 4th, 2012

Usually the welcome reception of a conference is a fairly low key affair. You register, you get your conference bag, you have a few drinks and nibbles while chatting to other conference participants. Not the ICHEP 2012 welcome reception though. This one included a broadcast of the LHC Higgs Seminar. This meant that people actually arrived before the reception and there was a fairly long line to register (though we didn’t have to wait as long as those at CERN). The main worry about the ICHEP broadcast was the connection to CERN. During the previous Higgs seminar in December, the webcast stopped working half way through. Luckily this time, everything went well, and we were able to listen and cheer at the announcements from CMS and ATLAS of the observation of a Higgs-like particle at 125-126 GeV.

It was a historic moment for particle physics, a triumph for the predictive power of the theory, not possible with without the hard work of many physicists, both on the LHC and the two experiments. Though, as was emphasised, this is only the beginning in Higgs boson studies. We now need to figure out what exactly this excess is… We certainly live in interesting times!


Today, in a special seminar held at CERN, the two collaborations operating large multipurpose detectors, CMS and ATLAS, presented new and convincing signals from the Large Hadron Collider (LHC) that could be coming from the Higgs boson.

This fundamental particle was predicted nearly fifty years ago in the framework of the Standard Model, the theoretical model that describes just about everything observed so far in the world of particle physics. Without the Higgs boson, this model however failed to explain how particles acquired their mass.

The seminar was webcasted live to the world and the ambiance was both festive and serious. Or should I say seriously festive.

To find this new particle, physicists sifted through billions of events, looking for more events having well defined characteristics than what is expected from other well-known processes described by the Standard Model. These we refer to as the background. An excess of events indicates something new is also present.

But particle physics follows statistical laws and what you get is never exactly what you expect, within a certain margin of error. Evaluating this margin is crucial to being able to make correct statements.

Imagine the following. In a large bag, mix a thousand blue marbles and a thousand red marbles. Then blindly draw ten marbles out of the bag, how many red ones will you get? Seven? Five? None? All these answers are probable, except 5 is more likely than 7 which is also more probable than none.

And if you draw 100 marbles, you are more likely to get closer to 50% red marbles. The same occurs in particle physics: the statistical fluctuations get smaller once you have a larger data sample. Hence, the error margin on the presence or not of the Higgs boson is smaller when adding the 6 inverse femtobarns (fb-1) of data collected in 2012 to the 5 fb-1 collected in 2011 (this is just how we measure the amount of data).

We cannot look for the Higgs directly since it decays into smaller, more stable particles. Hence, we find events potentially containing a Higgs boson by looking at its decay products.

The two decay channels that give the most precise measurements if a Higgs has a mass around 125 GeV are when a Higgs boson decays into two photons or when it decays into two Z bosons, each one breaking apart into two electrons or two muons. This is called the four-lepton channel, since both electrons and muons are leptons. As can be seen on the plot below, Higgs decays to a pair of b quarks or WW are more frequent but much less precise to find the exact mass and the background in these channels is also very large, making it harder to see anything.

The many different ways a Higgs boson can be produced and decay. At 125 GeV (shown by the vertical dashed line) many channels are possible. The vertical logarithmic scale shows which process occurs more often as a function of the Higgs boson mass (the horizontal axis). Any decay involving quarks (qq or bb), tau leptons τ or neutrinos ν is less precise since some of the fragments are totally or partially lost. Fewer events will happen where a Higgs boson decays to two photons γγ or four leptons l+ll+lbut they are easier to reconstruct since nothing is lost.

If the debris we find really comes from a Higgs boson breaking apart, once you recombine them, they will all cluster at the same mass and we will see an excess of events above the background at this particular mass value.

In the two-photon channel, CMS sees an excess of events that corresponds to 4.0 times the error margin on the expected number of events coming from the background. This is what we call a 4 sigma variation. For ATLAS, the excess corresponds to a 4.5 sigma deviation at a mass of 126.5 GeV.

Here are the 59059 events selected by ATLAS in the two photon channel. The small bump at 126.5 GeV corresponds to the 170 events that could be coming from the Higgs boson. The bottom insert shows what is left after subtracting the background, estimated in the upper plot by the red curve, making the Small excès more visible.

Then both experiments also see an excess of events in the four-lepton channel. It ranks at 3.4 sigma for ATLAS and 2.5 sigma for CMS. The most probable mass is 125 GeV for ATLAS and 125.5 for CMS.

Here we can see under the peak drawn in red the excess of events attributed to Higgs bosons decaying into

The other channels, although less sensitive, offer good cross-checks. When each experiment adds up the probabilities for all channels and data analyzed so far, CMS obtains a total probability of 4.9 sigma while ATLAS sees 5.0 sigma. Taking into account that statistical fluctuations are always possible if you look at all the mass values in the range of 110-150 GeV, then the global significance is slightly reduced to 4.7 sigma for ATLAS.  The most probable mass being measured at 126.5 GeV in ATLAS and 125.3 GeV in CMS.

The local probability that the observed excess of events found by ATLAS comes from a statistical fluctuation of the background is less than a chance in thirty million as shown by the solid curve. The dashed curve shows how strong the excess should be if it is produced by a Standard Model Higgs boson. Hence, the observed excess is slightly stronger than what is predicted by the Standard Model, but still within errors.

In particle physics, an experiment needs a 5 sigma excess to claim the evidence for a new particle. But since both experiments have it independently, it is clear that a new boson has just been discovered.

So welcome, little boson. But is it really the Higgs boson predicted by the Standard Model? That is too early to tell even though the chances are excellent. We must first check if this new particle is produced and decays exactly as the theory predicts for a Higgs boson in all possible channels.

Much work remains to be done and hopefully, other exciting discoveries will also come in due time as more data becomes available. Meanwhile, champagne is in order!

One event with two muons (tracks in red) and two electrons (tracks in green) found by CMS.

One of the four muon event selected by the ATLAS search. This could be coming from a Higgs boson decaying into four muons shown by the four red lines representing their tracks in the detector. There is no way to tell if this one in particular comes from a Higgs boson or some background event.

Pauline Gagnon

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What a discovery looks like

Wednesday, July 4th, 2012

What a remarkable day this has turned out to be, and how wonderful it has been to get to experience it here at ICHEP in Melbourne! As I remarked in the live blog, I knew coming in what the CMS results were — that we clearly had a strong signal for a new particle, and that as far as we could tell, it looked like the Higgs boson — but I didn’t have a clue about what ATLAS was going to present. And now we know: a totally independent experiment has made essentially the exact same observation. We can say without a doubt that we have discovered a new particle, and better still, it seems to be the one that we have been waiting for, for something like fifty years.

It was clear that everyone came into this seminar hoping that this would be the case. I must say that I did not expect that there would be a spontaneous round of applause as soon as “5σ” appeared on a slide. (Maybe I should start writing that on my slides, just to see what would happen during the presentation.) Five standard deviations is ultimately an arbitrary standard — what makes it better than six, or five and a half? — but it is the standard, and everyone was so eager for a discovery that it was embraced as soon as it appeared, even before all of the results were presented.

Of course, we have to remember that we are just getting started, and that as result there is plenty that is unsettled. Does this “Higgs particle” have the right branching fractions? Right now, it seems to have trouble decaying to taus…or perhaps that’s just a downward fluctuation in the data. Does it have the correct spin and parity? We’re going to need more data to answer that. And ultimately, we have to remember that the Higgs is “just a standard model particle,” as a colleague said to me last year. Even if this is the Higgs we were looking for, it still leaves a lot unexplained, as Flip discussed (more elegantly than I would) in a recent post. We think that there must be something out there that helps it have the mass that it has.

After my visit to the Sydney Opera House yesterday, I went to see the Bienniale of Sydney, a contemporary art exhibition that is going on all over the city. While I was looking at one of the larger installations, a docent explained that it had taken a large team of people five weeks to install it. The artist had said that that effort in itself was a work of art. I would have to say the same of the observation of the Higgs boson — it has taken thousands of people, from all over the world and with a great variety of skills, many years to bring us to this day. And yes, that interaction too is a work of art. I’ll leave you with one of my favorite quotes, by Henry James: “We work in the dark — we do what we can — we give what we have. Our doubt is our passion, and our passion is our task. The rest is the madness of art.”

(with apologies to Ewa Partum)


11:44 pm PDT — Update from @Anadi, a TRIUMF physicist with the ATLAS experiment, live at CERN.

So situation is quite exciting! The only people that could enter the auditorium were there at 2AM (7 hours before the start time!). Most of the people then started lining up around 5AM and there is still a long line waiting outside the room. All other arranged areas are packed, people are discussing, excited, truly wondering what the other experiment has seen. It is a strong community feeling, I am meeting colleagues and friends I have not seen for years. Most of us are here today – to share the biggest achievement of particle physics in the past 30years.

12:02 am PDT (TM from TRIUMF)

The show starts with Director General Rolf Heuer.  Looks like he got a new haircut since his talk at @scienceworldca in early June in Vancouver.  Joe Incandella from CMS starts off…with more jokes!

Radiative corrections…square of the top mass.  This all has to do with what other measurements of other particles are used to indirectly measure the Higgs mass.

12:05 am PDT — update from @Anadi, live at CERN

When Peter Higgs entered the auditorium, big applause…two nights ago he was in CERN Restaurant 1 (the CERN ‘cantine’) and people clustered around him as he walked through – students running to get pictures.  Last night, CERN was full of students overnight, sitting on the lawn with candles, waiting for the big event. They all consider themselves extremely lucky to be here. Part of big family regardless if they worked on the higgs or not.  They are part of the biggest human endeavour…

12:11 am PDT (TM from TRIUMF)

JoeI points out the structure of the CMS detector.  It is barrel shaped and similar to ATLAS and all other collider-detectors.  Good calorimeter, meaning they can measure energy of electrons and photons very precisely. Lead-tungsten crystals.

12:14 am PDT (TM from TRIUMF)

The performance of the CMS detector actually degrades from radiation exposure but then the clever scientists can “correct” for this effect with good calibrations and modelling.  #Incandela also talks about Monte Carlo, which is the physics term for the heap of computer simulations used to benchmark and text understanding of the detector and the “known” physics.

12:18 am PDT (TM from TRIUMF)

#Incandela follows the sacred script of a particle-physics talk. State the physics, review the detector, show the performance of the detector, and then at the end…show the results.

12:20 am PDT (TM from TRIUMF)

Wow, Higgs to two photons event display!  Very elegant, very clean.  #Incandela then shows that CMS physicists “understand” the data, meaning they cross-check and compare how it works.  They also did a “blind” analysis for the 2012 new data, meaning that the physicists could not accidentally look at the “signal” until they were completely done with all the cross-checks.

He also discusses multi-variate techniques.  This means algorithms that use many different inputs that are combined to help assign weight or “value” to each event.

12:28 am PDT (TM from TRIUMF)

The first result…page 43 of the talk.  What a sweet signal plot of two photons being reconstructed to a consistent mass for a new particle!  Very elegant.

12:28 am PDT — from @Anadi, live at CERN

In the room where I am, people keep on coming sit on the floor, media are cornered. People clapped when Joe mentioned this is the work of 3,000 people for the past 10 years.

Signal just shown!  People are just astonished by the shape of the H –> gamma-gamma signal.   Quite impressive!

12:37 am PDT (TM from TRIUMF)

Interesting that CERN audience breaks into clapping for CMS result of 5.0 standard deviation significance for the combined Higgs signals.  Really an acknowledgement that is is real.  Results that are significant and bona fide.

12:37 am PDT — @Anadi, live from CERN

OK the 5 sigma has just been shown! People could not contain themselves. They just stood up, laughed, clapped…

12:48 am PDT (TM from TRIUMF)

125.3 +/- 0.6 GeV with 4.9 sigma

In five production channels, event yields in decay channels are roughly self-consistent.  Meaning that when you look across the different searches, the relative popularity of the decay modes is consistent with what you’d expect for a Higgs.

12:53 am PDT (TM from TRIUMF)

CMS concludes.

12:54 am PDT (TM from TRIUMF)

Canadians are at bat!  Or rather, ATLAS is up.  And that’s the experiment the Canadian team is part of.  Fabiola opens with requisite jokes and hints that ATLAS has got more data analyzed and understood than CMS.  Let’s see!

1:04 am PDT (TM from TRIUMF)

Three people leave our auditorium…they worked on the ATLAS analysis and know the results!  They stayed late at work tonight to hear what the “competition” (CMS) had accomplished.  Ahah!

1:05 am PDT (TM from TRIUMF)

The world lights up with press releases, comments, and new lab web pages as the full results are released! Wow…CERN even totally replaced their homepage.

1:22 am PDT (TM from TRIUMF)

Pretty careful analysis of backgrounds in the four-lepton decay channel of the Higgs.

1:31 am PDT (TM from TRIUMF)

Very attractive four-muon event and a four-electron event!  Candidates for Higgs, of course.  Clean tracks, good separation, good vertex. and then very nice exclusion plots!

1:35 am PDT (TM from TRIUMF)

Wow!  5,0 sigma and huge applause from combined gamma gamma and four lepton decay channels.  Additional checks show the results from the two separate channels are quite consistent.

1:40 am PDT (TM from TRIUMF)

Here comes the request for more data…time to study the heck out of this particle!

2:36 am PDT — @Anadi, live from CERN

Everyone is now thrilled, most of the people truly did not know about the other experiment’s results.  Lynn Evans made quite a strong statement, saying that it is the most important moment in his life. Peter higgs is here, he was just amazed by how quickly we could achieve a discovery. This is due to the excellent performance of the machine, detector, and the creativeness, dedication, enthusiasm,  ingenuity of the people in the collaboration (mainly the young collaborators).


N’oubliez pas de rafraichir votre fenêtre pour voir les derniers commentaires:

10:57 François Englert et Peter Higgs viennent témoigner de leur émotion et leur joie d’avoir assister à cet évènement. Ce sont deux des théoriciens qui ont prédit l’existence de cette particule il y a 48 ans. Et on aura pris 48 ans pour le trouver! Emouvant!

10:50 beaucoup d’applaudissements, surtout au CERN. A Melbourne, on se sent un peu loin… seuls les kangourous sautent de joie. Déjà hier soir, les wallabis sautillaient partout et les petits pingouins se dandinaient et défilaient gaiement dans une belle parade.

10:40 Tout est clair. 5.0 sigma pour ATLAS quand tous les canaux sont combinés. Cela ne fait plus aucun doute. Bienvenue au nouveau petit boson. Il reste encore à en vérifier toutes les propriétés dans les mois à venir. Mais on devrait avoir encore deux fois de données encore cette année alors ce sera possible de le baptiser avant la fin de l’année. Bienvenue au nouveau petit boson.

10:27 Et enfin, voici le signal pour 4 leptons. L’excès est clair et confirme les autres, tout comme ceux de CMS. Elle montre aussi comment la collaboration a tout vérifié à basse masse et haute masse et que tout le reste est parfaitement consistent avec les bruits de fond sauf autour de 125 GeV. Au total 13 évènements entre 120 et 130 GeV alors que seulement 5 devraient venir du bruit de fond, et 5 autres en gros du signal. Donc clairment, il y a plus que ce que le bruit de fond prédit à lui tout seul.

10:15 Elle explique que dans le canal à deux photons, on devrait trouver environ 300 évènements venant de désintégrations du Higgs dans ce cana d’après la théorie si le boson de Higgs a une masse d’environ 126 GeV. Avec une efficacité de 40%, on en trouve environ 170, donc un peu plus que ce que le Modèle Standard prédit. Mais ce genre de variations est toujours possible et ce n’est pas particulièrement inquiétant. CMS en fait mesure un peu moins d’évènements que ce que la théorie prédit. Donc, entre les deux, on arrive à peu près à ce qui est prédit, ce qui est normal en physique avec les lois de la statistiques.

10:08 Elle refait le point sur les résultats de 2011 où on avait 2.9 sigma en combinant tous les canaux possible. C’était beau mais insuffisant pour conclure.

10:01 Elle explique qu’il aura fallu un énorme effort pour réussir à présenter ces résultats aussi vite. Elle parle maintenant sur comment les plusieurs améliorations ont permis d’augmenter la qualité et la quantité de données recueillies.

9:44: Il montre maintenant les canaux secondaires mais il commence à manquer de temps. Alors peu de détails sont expliqués… Il faut attendre son sommaire pour en tirer les bonnes conclusions. Ils ne voient rien avec Higgs en deux taus, mais c’est un canal très difficile. Donc à vérifier, il faut plus de données ici pour en dire plus.

9:37 Combinés, ils ont 5 sigma. Bienvenue au nouveau boson! Il ne fait plus aucun doute.

9:34 Et allez hop! un autre beau pic dans le canal des 4 leptons. Clair et inmanquable. On voit comment les données décrivent superbement le bruit de fond avec un autre signal, celui du Higgs, par-dessus. On ne peut pas demander mieux! 3.2 sigma  à 125.5 GeV, donc 125 fois plus lourd qu’un proton.

9:26: Bel évènement à deux photons est montré à l’écran. Mais c’est dans l’accumulation de plusieurs évènements qu’on peut dire si il y a quelque chose ou pas. Ils regardent dans plusieurs catégories de photons identifiés de façons différentes. Quand ils les combinent, on voit un beau gros pic qui ressort. 4 sigma: c’est énorme. Et ça promet!!!

9:25 Joe attaque enfin le canal à deux photons, le plus sensible de tous, expliquant comment ils ont utilisé de vraies données, et non pas le Monte Carlo, pour tout calibrer.

9:20 Maintenant, Joe nous montre comment CMS a aussi amélioré la qualité de leur identification des différentes particules reconstruites dans les détecteurs.

9:14 Mais le LHC fonctionnait si bien qu’en fait on obtenait plusieurs évènements superposés, ce qui a compliqué les analyses. Ils ont aussi produit 400 millions d’évènements Monte Carlo pour comparer avec les vrais données.

9:10 Joes devient plus sérieux. Il nous annonce que 2 canaux contribuent le plus: Higgs se désintégrant en 2 photons ou en 4 leptons (leptons sont electrons ou muons). C’est là que c’est le plus précis. Il montre aussi qu’on a un peu pls de données que l’an passé et que avec une plus grande énergie, c’est aussi plus fort, plus de chances de produire des vbosons de Higgs à 8 TeV qu’à 7 TeV comme l’an passé. Il montre aussi que le détecteur CMS fonctionnait super bien en 2012 et que les données recueillies sont excellentes.

9:05 Joe Incandela fait rire la salle avec une série de blagues, plus ou moins rigolotte mais il est clair que tout le monde est de bonne humeur et rit de bon coeur

9:00 La salle est complètement silencieuse, au CERN et à Melbourne quand le directeur général fait son entrée. Ici aussi. Il salue les participants de Melbourne. On applaudit comme des enfants. L’atmosphère est à la rigolade. Il dit qu’on est là à la recherche d’une particule dont il a oublié le nom…. CMS commence avec son porte-parole, Joe Incandela.

8:56 François Englert, un des théoriciens qui a proposé le Higgs, entre dans l’auditorium au CERN sous les applaudissements.

8:50 On nous dit que le séminaire durera un bon 2 heures… J’ai bien fait de manger une bouchée…

8:45 Plus que 15 minutes d’attente. L’auditorium est presque plein mais encore calme… et des journalistes un peu partout.

8:37: heure de Genève. Le séminaire donné au CERN sur les derniers résultats venant d’ATLAS et CMS débutera dans moins de 20 minutes. Vous pouvez suivre mes commentaires en direct pour vous aider à interpréter les résultats. N’oubliez pas de rafraichir votre écran à toutes les 5 minutes environ pour voir les derniers commentaires. Je suis dans un auditorium à Melbourne avec 900 autres physiciens et physiciennes victimes du décalage horaire s’apprêtant à participer à l’ouverture de la plus grande conférence de physique de l’année.

A tout de suite, Pauline



Higgs Seminar Liveblog from TRIUMF

Wednesday, July 4th, 2012

Hi there! This is TRIUMF’s liveblog of CERN’s Higgs seminar, coming to you all the way from Vancouver, Canada, where it is quickly approaching midnight. As someone who is not a scientist, I’ll be offering the layman’s point of view of the seminar. So, to reiterate, it is quite late and I’m not a scientist, meaning I should be making a fool of myself at least once tonight. I apologize in advance for that and for any technical difficulties I am sure to run into tonight. Enjoy. See you at midnight!

1:48 – Okay. It’s over now. Good night. I hope you all enjoyed my illuminating commentary.

1:46 – Rolf just killed it with the most succinct explanation of what was actually happening tonight. I understand the science was necessary, but still.

1:45 – Rolf is getting feedback on the mic! Noooo.

1:44 – “as a layman, I would now say, ‘I think we have it.'” This is why Rolf is the best.


1:41 – My boss just told me that everyone has been applauding because they’ve announced 5 sigma, which is like a “slam dunk” for confirmation in physics. It’s not perfect yet, but there’s no going back now. Finally, something I understood.

1:40 – “Need more data” is a recurring sentiment in science, I’ve found.

1:38 – No idea what that discovery was.

1:35 – A lot of clapping. Big discovery.

1:30 – 80% of this slide is graph.

1:27- On to the results!

1:24 – Um.

1:17 – “This part is perhaps a little too technical for this presentation” please, continue.

1:13 – The science is impenetrable.

1:06 – These slides have their own, inexplicable color schemes.

1:04 – Seriously, though, not a nice font.

1:02 – Second biggest challenge of 2012: the use of Comic-Sans on these slides.

12:59 – They are working beyond design. Impressive.

12:58 – Pile-up was the biggest challenge for them in 2012. Too much data!

12:56 – ATLAS was conceived 2 decades ago, built 1 decade ago

12:55- This data is really fresh.

12:54 – The events really are beautiful.

12:53 – ATLAS. No snack break.

12:53 – Way to go.

12:52 – 3,300 scientists on CMS.

12:50 – Just heard him say  “New boson” Woo!

12:49 – “In conclusion” graphs pop up all over the screen. Not helping.

12:48 – Rolf just coughed.

12:46 – “Do I have five minutes?”

12:44 – “Jumping to my results. Don’t want to do that” YES YOU DO

12:37 – People clapping. One person whistled.

12:33 – Anytime there is a longish gap in coverage, it means I literally understood nothing, my eyes became unfocused, and I slipped into a waking dream.

12:31 – The 2011 data was reoptimized blindly.

12:30 – Higgs to zz (as if I know what this means)

12:30 – Seriously, graphs.

12:28 –  There is a little bump in the comparison between the 2011 – 2012 data. “This is very significant” dramatic pause. Next slide. What!

12:27 – “It’s quite hard to see if anything is there” yep.

12:26 – Many graphs. Many, many graphs.

12:25 – He’s close to coming to results soon.

12:23 – “Well, these are technical things” (like everything else so far)

12:22 – How wild, though, is not within the scope of this talk.

12:21 – Blind analyses in 2012. Never looked in the band where the signal would be. Keeps people honest. It also gets “wild” when people look in the signal band.

12:20 – He’s talking about the Higgs now!

12:18 – I might be in the minority here, but I prefer the look of Fake Tau over Real Tau. Sorry!

12:16 – I’m looking at my computer like I understand what is happening, but I don’t.

12:13 – “I’m going to run over, Rolf”

12:13 – The CMS detector weighs 14000t. Weighty, indeed.

12:11 – This CMS diagram looks really good.

12:08 – They moved to 8TeV this year.

12:05 – Okay. Here comes the science. Picture looks nice and 50 interactions is impressive…I think.

12:04 – CMS progress on the Higgs search beginning.

12:03 – “For a certain particle. I forgot the name.” Rolf, getting the laughs, as per usual. Awesome speaker.

12:00 – Rolf is talking now. Let’s do this.


Written by Jordan Pitcher (Communications Assistant)


Don’t forget to refresh the screen periodically to see the new comments

10:57 Now Peter Higgs and François Englert, two of the theorists who predicted this boson, are there at CERN, telling how happy they are. And impressed we finally got it.

10:50 Lots of applause both at CERN and Melbourne, more subdued here though. I guess we feel slightly remote. But I saw many wallabis last night happily hopping in the forest in great anticipation of this event. Even the little pinguins organized a beautiful parade.

10:37 The two main excesses in 2 photon and 4 leptons channels are compatible with each other. The preferred combined value is at 126.5 GeV when all channels are combined, all of them for 2011 data and only the 2 most sensitive ones in 2012. This is also compatible with the strength of the signal a Higgs boson would give at this mass value. The signal also evolved over time nicely varying according to statistics. Nothing could be more convincing. Still we need to convince ourselves that this new boson is exactly what the Standard Model predicts in all aspects. Twice as much data still to come in 2012. Great possibilities still ahead.

10:34 Again, in the 4 lepton channel, nice excess found. So it looks great. A big 5.0 sigma when all is combined. The room breaks into a great applause. Fabiola is visibly both elated and exhausted.

10:24 Moving on to the 4 lepton channel. Again, great improvements in electron identification efficiency and stability, despite the large pile-up (when up to 30 events are all occurring at the same time). So the analysis has improved greatly.

10:11 Ok, she is moving on to the 2 photon channel in 2012. We should expect about 300 events in 2012 data sample if the Higgs has a mass around 126 GeV, according to theory. With an efficiency of about 40%, we find about 170 events, in excess from the background, so slightly more than what the theory predicts. But that’s normal with statistical laws. CMS saw slightly less so on average, we are fine. The excess is estimated at 4.5 sigmas locally (or less if we include the look-elsewhere effect, 2.5 sigma). So seen in 2011 and 2012, doing great.

10:08 With the 2011 data, we were able to find an excess of 2.9 sigma using not only the 2 most sensitive channels (2 photons and 4 leptons) but also with all the other channels. But that was not enough to be convinced.

10:05 Since in 2012 we ran at 8 TeV, we also got more chances of producing Higgs bosons. With the improved analyses, more efficiency in event reconstruction and data taking, we can expect better results too with the 2012 data than 2011.

10:01 She now exlains how much effort was needed to be able to present these results today. Many improvements went into the trigger and also in how fast we reconstructed these events. The GRID played a crucial role in this success.

9:54 It is now Fabiola Gionatti’s turn. She is ATLAS spokesperson. She reviews, like Joe did, the channels that contribute the most at low mass, in the vicinity of 125 GeV where we saw something interesting in last year’s data. The idea is that we want to have two independent samples giving the same results. Then we know something is there. And if she shows that not only CMS, but also ATLAS, sees it, bingo!

9:45 Joe showed many other less important channels, including the tau tau which reduces the overall sensitivity. In total, they have 4.9 sigma at 125.3 GeV when all is combined, fairly all consistent with Standard Model but more checks are in order of course.

9:37 There we go: 5 sigma when both are combined. Welcome to the new boson!

9:32 Now moving to the second most sensitive channel: 4 leptons. Again, new and improved analysis methods promising better results for 2012 data. And again, “a very nice peak” in Joe’s words, a beautiful sigma result 2.5 sigma at 125.5 GeV, so about 125 heavier than a proton.

9:30 At last, here is the 2 photon peak, nice and clear above background. No doubt about it. Gets them an applause. 4 sigma deviation, it’s huge!!! No doubt, we are getting there, slightly stronger even than what is predicted by Standard Model but within error.

9:25 Ok, getting on with the 2 photon canal, the most sensitive one, showing that they used the data directly to calibrate their results, and not only the Monte Carlo simulations. They also used sophisticated methods, (Boosted Decision Trees) to identify their photons. Background and Monte Carlo agree very well. Showing one event that could be from Higgs to 2 photons.

9:20 Joes also is showing that CMS improved their particle identification in 2012. All this to show that we should expect interesting results.

9:15 The LHC was working so well in fact that up to 30 events were piling up at the same time, which made it harder to study the data collected in 2012.

9:10 Showing that the 2012 data is more promising, more chances to produce Higgs at 8 TeV energy than 7 TeV like we did last year. Shows also the the CMS detector was working really well during all the data taking.

9:05 Joe is getting more serious, having gotten good laughs from a very relaxed audience. Everybody is in a good mood. He says he will talk about 5 decay modes but the best 2 are Higgs into 2 photons and Higgs into 4 leptons (leptons are electrons or muons).

9:00 The whole room at CERN quiets down in a flash as Director General Rolf Heuer enters and opens the meeting, greeting the participants in Melbourne. We cheers like school kids. Rolf Heuer, the DG, tells us we will be taking about a particle whose name he forgot… The audience laughs easily at all jokes. Joe Incandela, spokesperson of CMS, will give the first talk.

8:56 François Englert comes in the CERN auditorium under a thunderous applause. He was one of the first to propose the Higgs mecanism with Peter Higgs and Brout.

8:45 Only 15 minutes to go. The auditorium is nearly full. People are waiting with anticipation and the room is crawling with journalists. Everybody seems happy. Someone even has a sign saying: Ciao mamma!

8:30 CERN time – Seminar will start in 30 minutes

Hello everybody,

I am in Melbourne, Australia right now and the seminar is going to be broadcast live here to all participants of the largest physics conference of the year. About 900 jet-lagged physicists are about to be allowed in this auditorium to hear the latest results on the Higgs boson search by CMS and ATLAS.

Remember to refresh your screen periodically and you will be able to see my comments throughout the talk, to help you follow the main points and interpret the results.

Enjoy! Pauline



Good afternoon from the Melbourne Convention Centre, where we are all eagerly awaiting the start of the Higgs seminar that will be broadcast from CERN. I’ll be updating this post as we go along, so please stay with me and our other Quantum Diaries bloggers.

17:00: That’s a wrap here! I’ll try to post again soon-ish with some more thoughtful follow-up. Thanks everyone for joining us!

18:58: Not that Higgs is the only one who helped develop this theory…the others are getting their time to make congratulations too.

18:56: A big round of applause here and at CERN for the famous Peter Higgs.

18:52: No questions here! Rolf is right, we want to go to the reception!

18:48: Now questions. I’ll try to transcribe the ones that are coming from this room.

18:47: We’re watching the standing ovation at CERN over the video. Again, so nice to see lots of young people in the room. Then again, maybe they were the ones with the endurance to camp out in front of the auditorium!

18:44: Rolf summarizes by saying that this is a global effort and a global success. It’s only possible because of the extraordinary performance of the accelerators, experiments and computing grid. This is an observation of a new particle consistent of a Higgs boson…but we don’t know which one. It’s an historic milestone, but it is only the beginning, with global implications for the future.

18:41: The conclusion from ATLAS — an excess of events at about 126.5 GeV at 5.0 sigma significance. Fitted signal strength is 1.2 +- 0.3 of the SM expectation. Fabiola says that this is a very lucky mass to have, since it is easy to explore at the LHC in many decay channels.

18:39: Fabiola shows how the signal strength has evolved over time, in the past year — it is really quite striking how much we have advanced!

18:37: ATLAS overall signal strength is a bit higher than SM expectation for 125 GeV Higgs, whereas CMS was a little lower.

18:34: So here comes the combination of the new 2012 analyses with the 2011 results: another 5 sigma excess, and more applause!

18:30: ZZ also shows an excess in the 125 GeV range, 3.4 sigma here, when you’d expect 2.6 sigma from a standard-model Higgs.

18:28: Hmm, the standard-model ZZ rate is coming in bigger than expected. But it has now impact on the low-mass Higgs range.

18:23: Here comes ZZ, with 20-30% increase in sensitivity since December.

18:19: And now the results from the photons channel. Here too, an excess, at the 3.6 sigma level, after accounting for the look-elsewhere effect. 4.5 sigma local.

18:14: Thorough discussion of mass resolution in the photon channel, and the fact that it is independent of pileup. This is quite important, as the pileup is only going to get larger as the LHC luminosity increases.

18:10: Fabiola reviews what the changes are since December, here we go…..

18:09: Fermilab has a press release out too, http://www.fnal.gov/pub/presspass/press_releases/2012/Higgs-Search-LHC-20120704.html

18:07: The CERN press release is out! http://press.web.cern.ch/press/pressreleases/Releases2012/PR17.12E.html

18:04: I’ll admit that I’m finding less to say at the moment, as Fabiola is now covering similar ground to CMS in the preparation of the data, understanding of the detector etc. But it is still all important, of course!

17:57: I think I caught that ATLAS will show 5.9 fb-1 of data? More than CMS has certified.

17:55: Fabiola will show results from 2012 data in the ZZ and gamma gamma channels, but the other channels only for 2011 data. The 2012 results in those channels (which do have less mass resolution) aren’t quite ready yet.

17:53: Rolf congratulates everyone…and now let’s see what ATLAS has!

17:50: We have observed a new boson with a mass of 125.3 +- 0.6 GeV at 4.9 sigma significance. Enthusiastic applause heard on two continents. And then Joe acknowledges all of the theorists, machine physicists, and CMS experimenters who who have gotten us to this point.

17:49: Branching ratios are self-consistent across the decay channels (even with that tau tau result), but Joe emphasizes that it is early yet.

17:48: OK, it looks like there is something there, but is it consistent with a standard-model Higgs? Now we’ll get a first look at that.

17:44: Technical improvements in the tau tau channel have helped improve its sensitivity in the Higgs search. But uh oh, no evidence for a Higgs in that channel, and indeed you nearly exclude a 125 GeV SM Higgs here! It’s low statistics yet, we’ll need a lot more data to understand this.

17:41: Now on to the bb and tau tau channels. Not as much sensitivity to them, but they are important, as a) they actually have bigger branching fractions at ~125 GeV than the WW/ZZ/gammagamma, and b) they are fermions whereas all the others are bosons.

17:38: Unfortunately, the seminar is not over yet — let’s see if that 5.0 stands up with three more decay channels to look at.

17:37: The ZZ and gamma gamma joint significance is 5.0 standard deviations…Joe says that, and it gets applause! 5.0 is considered the threshold for a discovery….

17:33: Joe is now showing how event by event, we can compare the angular separations of the leptons to what would be expected from a Higgs signal or the ZZ background. I’ve always loved that approach, really gets at the physics.

17:30: Now the ZZ mode, with four leptons in the final state — this channel is about as clean as you can get, but very rare, so you need to be super-efficient in selecting the events.

17:28: Slide 43 — A mass bump! Thank goodness that there is a real mass bump. I’ve always said, if we make a discovery that is observed on a plot that runs from 0 to 1 with a slight enhancement at 1, I’ll jump out the window. Maybe I’m safe.

17:22: Now we’re getting into the meat of the Higgs search, with the search for a decay to two photons. CMS paid a lot of money for the lead tungstate crystals that give very good photon energy resolution.

17:17: In fact, Joe is carefully going through all the low-level ingredients that go into these analyses. It’s a lot of work, and that’s why we need thousands of people to get this science done.

17:15: Joe takes a moment to point out the hard work done by the CMS software and computing teams. Yes, we’ve got a fabulous detector, but we can’t get this done without S&C…so I do appreciate the shout-out!

17:11: CMS will show 5.2 fb-1 of certified data today. And again, I’m curious to see how much data ATLAS will show!

17:09: CMS will be showing results from five Higgs decay modes — WW, ZZ, photons, bb, taus. I’m curious to see how many of those ATLAS also has.

17:06: Joe Incandela, for CMS, also starts with standup jokes. And then, on to a discussion of the experimental support for the standard model…except that we haven’t seen the Higgs. Yet.

17:03: Rolf notes that ICHEP is opening with a talk from a different continent — a symbol of how well we collaborate across the world. Then he starts with a standup routine….

17:02: Big round of applause here when Rolf Heuer greets ICHEP!

17:00: Here we go! Watching a very quiet CERN auditorium on video….

16:58: On the video feed from CERN, we can see a lot of young people. I’m glad they were able to get seats, as they are the ones who really make these experiments go.

16:54: Everyone in the hall just laughed at the video feed, which showed someone holding a “Ciao Mamma!” sign.

16:51: Geoff Taylor, the chair of ICHEP2012, is telling us about the timetable. The talks will be running longer than we originally envisioned (45 minutes each), followed by 30 minutes of questions. (We will be able to ask questions from here to CERN.) Hmm, hope everyone can hold out for the reception, which will be at 7 PM now!

16:47: I should say that as a member of CMS, I know the CMS results. Thus, I’m deeply curious about the ATLAS results. Will they agree with CMS or contradict? I’ve been hearing rumors, of course, but it will be interesting to see if they are true.

16:44: While we’re waiting for things to start, let me wish a happy Independence Day to everyone in the US, and also a happy wedding anniversary to my parents and a happy birthday to my colleague Aaron Dominguez, who is watching the proceedings from CERN Filtration Plant conference room.

16:42: I’m now sitting in the auditorium where we will be hearing the talk. Sitting to my left is Joel Butler, the manager of the US CMS operations program (I am also in the program management, as deputy manager of software and computing), and to my right is a reporter from the Australian Associated Press. On the big screen above the stage we can see the video feed from the CERN auditorium, and another panel which I presume will be the slides.

16:24: Just spoke to Pier Oddone and Young-Kee Kim, the director and deputy director of Fermilab. Yes, all of the spotlight is on CERN right now, but the Tevatron experiments have been very important for getting the Higgs search started, and it is also quite possible that CMS and ATLAS will not be able to beat CDF/D0 on the Higgs to bb decay mode anytime soon.

16:16: The registration line is still very long! I have been walking along it and chatting with friends in the hope of entertaining them a little. We’ll see if we can get everyone through in time for the seminar….

15:46: I’ve now picked up my registration materials. There is a long line at the registration desk — no one wants to be late for the start. Right now I’m at a table in the foyer with some of the CMS leadership: Greg Landsberg, the CMS physics coordinator; Chris Hill, a CMS deputy coordinator, and Christoph Paus, one of the leaders of the CMS Higgs group. On the way in, I discovered that the Melbourne Boat Show is also happening in the convention centre. I wonder if we have any buyers at ICHEP….


CERN are holding a seminar for the latest results for the ATLAS and CMS Higgs searches. This is the first such update since December 2011, and there is a reasonable chance that at least one of the experiments could show a 5 sigma excess. This is my liveblog, follow along for live updates!

“Observation of a new particle consistent with a Higgs Boson (but which one…?)”

Thank you to all who joined me on this liveblog and on twitter!

The seminar is webcast live so that you can watch from anywhere in the world. The link is http://cern.ch/webcast. The seminar will begin at 09:00 CERN time (00:00 US West Coast, 03:00 US East Coast, 08:00 UK, 17:00 Melbourne.)

This is my liveblog and I will be providing updates as the seminar proceeds. Most recent updates at the top of the page. Also follow me on twitter (@aidanatcern) and Seth Zenz (@sethzenz). Ken Bloom is also liveblogging from ICHEP, and my boss, @drsekula is liveblogging for SMU.

The liveblog

10:59: Rolf: We can all be proud of this day. Enjoy it! (Applause)

Questions, answers, and comments

10:55: Any comments from the theorists? (Applause) Many congratulations!

10:50: Many thanks offered from the front row.

10:48: Any questions from Melbourne? Any applause from Melbourne?! (Applause from Melbourne.) Any remarks? A: Grateful to take part in this historic event and wish you the best.

Overview (Rolf Heuer)

10:44: “Speaking as a layman: I think we have it.” We have a discovery consistent with a Higgs boson (but which one?) This is the beginning. “Global implications for future. Standing applause!

ATLAS talk (Fabiola Gianotti)

10:42: Local excess of 5.0 sigma, dominated by gamma gamma and ZZ* final states.

10:41: Only recorded on third of 2012 data. More data to come. The LHC is working beyond expectation. Theorists: please be patient!

10:40: Next steps: publish paper, then gather more data.

10:38: Evolution of excess with time. December saw 3.5 sigma peak. Seeing a nice 5 sigma peak today!

10:37: Excess compatible with Standard Model Higgs boson.

10:34: Excluded all points in the Higgs mass spectrum now, except around 125GeV and at very high mass.

10:33: Observe 3.4 local (2.5 global) sigma excess at 125GeV.

10:30: Slight excess above background + Standard Model signal at 125Gev. (Expect 10.4 +- 1.1 total, observe 13)

10:29: Z->4 leptons seen in the spectrum.

10:28: 1.3 times more ZZ events in data at higher masses.

10:26: Total reconstruction efficiency for electrons 98% flat in eta, pt and pileup. Required for low transverse momentum objects. 60% gain in acceptance times efficiency electrons. 45% gain for muons.

10:24: H->ZZ*->4 leptons final state. Backgrounds suppressed using isolation requirements. High efficiency needed, down to low transverse momentum objects. Gain in sensitivity of 20-30% since 2011.

10:21: 4.5 local (3.6 global) sigma excess in gamma gamma. Signal strength is 1.9 +/- 0.5. Cross section seems a little high, but consistent with Standard Model within 2 sigma.

10:19: Background model taken from data, using sidebands. Both 2011 and 2012 exclusions show compatible shapes.

10:18: Isolation of photon used to reject jets. Subtraction algorithm used to remove some pileup dependent effects.

10:17: Rejection of jets is 1 part in 10^4, at 90% signal efficiency.

10:15: Need to know the position of the vertex to get the angle of the photons and the mass. Do not use tracking information, in order to be insensitive to pileup. Use longitudinal and lateral segmentation of the electromagnetic calorimeter to point the photons.

10:14: Important to have powerful gamma identification to reject jet backgrounds. Energy scale known to 0.3% at the mass of the Z. Linearity known to better than 1% up to a few 100 GeV. Mass resolution not seriously affected by pileup.

10:11: Gamma gamma final state. Large backgrounds, split signal into 10 categories, depending on the kinematics and conversion variables. Expect gain in sensitivity by 15%. Signal to background ratio is very small. (170 signal events for 6340 background events.)

10:09: Use experience with the detector from 2011 to inform analyses in 2012. Improved reconstruction and identification of physics objects.

10:07: Previous results show exclusions except near 116GeV and 125GeV.

10:06: As center of mass energy changes from 7TeV to 8TeV, cross section increases by a factor of 1.3. Irreducible background cross sections increase by a factor of 1.2-1.25, whereas reducible backgrounds increase by a factor of 1.4-1.5. This gives an increase of sensitivity of 10%.

10:05: Many electroweak results , with cross sections of rare and rarer processes. Small amounts of tension in measurements.

10:04: Analysis not possible without dedicated computing resources. Usually 100,000 jobs in parallel at a time.

10:02: Trigger thresholds rise and luminosity rises. This keeps the good physics events for lower mass objects. Efficiency of electron trigger is flat and 94%. Stable performance required with respect to changes in pileup. Pileup changes as the run progresses.

10:00: Pileup showing big challenges for the continued analysis of data. Missing transverse energy resolution rises linearly with pileup, but is fine and flat after pileup suppression using information from the detector.

09:58: Pileup is increasing quickly. Average of 30 collisions per bunch crossing (with 50ns bunch spacing, rather than 25ns which is design performance.)

09:56: Integrated luminosity of 6.3fb^-1. 94% efficiency. 90% of delivered luminosityy is recorder to disk, in spite of very fresh data and harsher conditions.

09:55: Results are preliminary, data taking stopped two weeks ago. Pileup increased, harsher conditions. Present the highest sensitivity and best resolution modes (gamma gamma and ZZ*.) Other channels contains missing energy, poorer mass resolution and sensitive to pileup.

CMS talk (Joe Incandela)

09:51: Following lots of applause, acknowledgements. Lots of people to thank.

09:49: Event yields are self consistent across the topologies. Ratio of WW* and ZZ* states consistent. Couplings consistent with Standard Model at 95% confidence, we need more data. “We have observed a new boson with a mass of 125.3 +/- 0.6 GeV at 4.9sigma significance.”

09:48: Combined mass is 125.3 +/- 0.6 GeV. Now we need to see if it is compatible with Standard Model Higgs boson. Signal strength is 0.8+/-0.2.

09:46: Observed limit 1.06 x Standard Model cross section. Low statistics may cause some slight bias. Needs investigation. “Very interesting channel.” (Nice to hear open and candid discussion about results. Responsible science.)

09:44: tau tau channel. Challenging, lots of sub modes. 2 times improvement in sensitivity since 2011. “Use a very fancy fit that I won’t explain in detail…”

09:42: Current limits are compatible with signal or background.

09:42: Now bb, large branching fraction but huge background. Look for associated production mode. (W+H, Z+H; H->bb)

09:41: Still working on combination.

09:39: WW* analysis. Very difficult channel at low mass. DeltaPhi between leptons and invariant mass of two leptons used as discriminators.

09:37: Combined result for gamma gamma and ZZ* is 5.0 sigma. That’s a discovery!

09:35: Broader distribution for mass of Z bosons. Needs to be watched in the future…

09:34: Z->4l peak seen in the final mass spectrum! Also a bump at 126GeV.

09:32: Moving to ZZ* search. 20% improvement since 2011. Using all four (light) lepton final states. Backgrounds estimated from data. Angular analysis of leptons performed. 8 degrees of freedom in this angular analysis.

09:30: 4.2 sigma local significance, 3.2 sigma global. 1.56 +/- 0.43 x Standard Model cross section.

09:28: Peak clearly visible at 125GeV at the 2.3 sigma leve.

09:28: Classes combined weighted by signal to background ratio. Impressive bump appears!

09:27: Background model comes from data. Bias must be less than 20% of statistical error in the data.

09:25: Multivariate analysis used with kinematic variables, identification and per event mass resolution and vertex probability. Classes arranged in decreasing order of purity.

09:24: Photons selected using kinematic variables (transverse energy and mass of diphoton system.) Mass reconstruction depends on the vertex position. Aim to be within 1cm of the correct vertex. Correct to 83%(80%) in 2011 (2012).

09:23: Different algorithms for electron reconstruction, including brem recovery. Slightly better performance in Monte Carlo compared to data, so smear the data.

09:22: Analysis performed blind in 2012. Most studies are data driven.

09:21: Multivariate analysis used, using boosted decision trees. Classify different kinds of events, end up with four event classes. Crosschecked using an alternate background model, using sideband subtraction. Also a cut based crosscheck.

09:20: Standard Model cross sections well measured, including ttbar.

09:19: Jets a challenging but performing well. Shape differences are evident for pileup jets. Jet resolution good to within 15% up to the TeV scale.

09:18: Muon efficiency appears flat a function of pileup, as does isolation. 2012 has lower fake rates for electrons than 2011 for the same efficiency. Tau identification is ~70% with very low fake rates.

09:16: Particle flow used to great effect at CMS. Sophisticated electron reconstructed. Electron and photon calibrations show excellent performance. Gaining in sensitivity with identification algorithms.

09:15: Data recording and Monte Carlo production shown impressive performance and improvements.

09:14: Laser monitored correction for light loss in ECAL crystals. Resolution good to 1% using Z lineshape for calibration.

09:13: CMS detector, silicon tracker with 200m2 and 10M channels. Huge 3.8T solenoid (which is what CMS is named after.) Very fine granularity. Electromagnetic calorimeter a first for hadron experiment, using PbW04 75,000 crystals. Close to 100% up time for subsystems.

09:11: Luminosity increasing appreciably in 2012. 5.2fb^-1 collected so far in 2012.

09:10: Discovery potential: expect 5 to 6 sigma sensitivity for a Standard Model Higgs around 125GeV.

09:07: Constraints come from masses of top quark and W boson. Great exclusions coming from Tevatron.

09:08: In 2012 LHC moved from 7TeV to 8TeV. Dominant production mechanism is gluon gluon fusion. (Others include vector boson fusions, top radiation and associated produciton.

09:09: Main decay modes: WW, ZZ, bb, tautau, gammgamma.

09:05: “A lot of effort to combine all the work of thousands of people… it’s very tricky.”

09:06: Big challenge from pileup, about 50 interactions per event. Very rare particle, lots of sleepless nights.

Before the talks

09:02: Rolf Heuer: “Good morning everybody at CERN. Good afternoon everybody at Melbourne.” The seminar is about to begin. “Today is a special day.”

08:59: It is time. May the announcements begin.

08:56: Peter Higgs just arrived! Applause.

08:48: Why the Higgs boson is the “God particle”: It gives us mass. Mass is the fundamental unit of Catholicism.

08:46: Less than 15 minutes to go. I hope my typing is good enough and fast enough! Apologies for any typos.

08:40: We can see our colleagues in Melbourne and they can see us. Jon Ellis just arrived. There are many cameras here. I’m waiting for Peter Higgs to show up…

08:29: ATLAS Spokesperson, Fabiola Gianotti has arrived. As far as I know CMS will present first, and ATLAS will present second. (Last time ATLAS presented first.)

8:13: Famous faces arriving. Rolf Heuer, Director General of CERN. Guido Tonelli, the former CMS Spokesperson. Eilam Gross, the ATLAS Higgs Convener and Bill Murray (not the actor, the former ATLAS Higgs Convener).

08:02: I waited in the lobby since 11pm last night, with food and blankets and books. There was a very communal atmosphere and people tweeted their experience (search for the #occupyCERN tag!) Now we reap the benefits of the wait.

08:01: A short while ago me and my mother were interviewed by an Israeli TV station!

07:45: I waited 8 hours to get a seat, and I have a wonderful view! I should be able to hear the speakers well, all questions being asked, and the answers. I’m sitting here with my mother to my right (she flew all the way from the UK to attend!) and my boss to my left.


The upcoming Higgs seminar could be the biggest announcement in particle physics for nearly 30 years. There have been several excellent blog posts and videos explaining what the Higgs is and what it does, so I’ll link to those at the bottom of the page. What I want to do here is give you the overview of what you really need to know to get the best from the talk.

Of course you should follow along with the liveblog as well!

What’s happening with the webcast?

CERN have put in a lot of resources for the webcast. General users can get to the webcast at http://cern.ch/webcast. If you have a CERN login you can use a second webcast at http://cern.ch/webcast/cern_users.

The webcast will start around 09:00 CST (that’s 00:00 US West Coast, 03:00 US East Coast, 08:00 UK, and 17:00 Melbourne.

What is the Higgs boson? What does it do?

The Higgs boson is part of the Standard Model of particle physics. The Standard Model includes the quarks and leptons (which make up all the matter see around us) and the photon, gluons, and \(W\) and \(Z\) boson (which carry all the forces in nature, except for gravity.) Three of these particles, the \(W^+\), \(W^-\) and \(Z\) bosons, have mass, but according to our framework of physics, they should not have mass, unless the Higgs boson exists. The Standard Model of physics predicts that the \(W\), \(Z\), photon and Higgs all come as a package and they are all related to each other. If we don’t see a Higgs boson, we don’t understand the world around us.

People say that the Higgs boson gives particles mass, but this isn’t quite what happens. The Higgs boson allows some particles to have mass. The Higgs boson does not explain the mass that comes from binding energies (for example, most of the mass of the proton) and it does not explain the mass associated with dark matter. If the Higgs boson is discovered it will complete the Standard Model of physics, but it will not complete our picture of the universe. There will still be many unanswered questions.

What would a discovery look like?

In order to claim a discovery an experiment would need to see a 5 sigma excess over the expected background. A sigma is a measure of uncertainty, and the chance of seeing a 5 sigma excess due to statistical fluctuations is about 1 in 3 million. If both experiments see an excess of 5 sigma in the same region the chances that this is due to a fluctuation is 1 in 9 million million!

The experiments produce “Brazil plots”, which show what they expect to see if there is no Higgs, and compare it to what they actually see. The green band shows 1 sigma deviations, the yellow bands show 2 sigma deviations, and then you have to use your imagination to see the remaining bands, and colors. When the green and yellow bands pass below the SM=1 line, and the central black line does too, then the Higgs is excluded in that region to 95% confidence. If the black line stays above the SM=1 line then we haven’t excluded the Higgs boson in that region yet. So when the green and yellow bands fall far below the SM=1 line, but the black line stays above or at the SM=1 line then we accumulate evidence for a Higgs boson.

How do we search for the Higgs boson?

The search for the Higgs boson depends on its mass. At high mass it can decay to heavy particles with clean signatures, so the high mass region was the first region to see an exclusion. At very high mass the width of the Higgs boson is large, so the events get spread out over a large range, so the searches take a little longer. At low mass the decays get very messy, so we have to pick our decay modes carefully. The cleanest modes are the two photon mode (often called gamma gamma), the ZZ* mode and the WW* mode. Of these three, the gamma gamma and ZZ* modes are the most sensitive, so we can expect to see these presented tomorrow.

The data are collected that the detectors and stored to disk, and the physicists spend their time analyzing the data. This is a slow process, full of potential pitfalls, so the internal review process is long and stringent. This is one of the reasons why we need two experiments, so that they can check each other’s findings. The experiments at Tevatron have already presented their results and they see an excess in the same region. This is vital because they are sensitive to different final states, so between the Tevatron and the LHC we have all the analyses covered.

For each analysis there are two kinds of background, the “reducible” backgrounds where particles fake the particles we are looking for (for example, a high energy electron can look just like a high energy photon) and the “irreducible” backgrounds where particles are the same kind as the ones we are looking for. So when you see plots showing the gamma gamma searches, you can expect to see four categories: gamma gamma (irreducible Standard Model background), jet gamma, jet jet, and “other”. As we make more and more stringent requirements to eliminate these backgrounds we also lose signal events, so we have trade off background rejection against signal acceptance.

On top of all these problems we also have to take reconstruction and acceptance into account. We cannot record every event, so we pick and choose events based on how interesting they look. Does an event have two high energy photon candidates? If so, record it. Does an event have four leptons in the signal state? If so, record it. These trigger decisions are affected by definitions of “high energy”, by the algorithms we use, and by the coverage of the detectors. We have to take all of these biases into account with systematic uncertainties, and these can dominate for some of the searches.

When we put all this together we end up asking some simple questions: “How many background events do we expect?” “How many events do we see in data?” “What is the total uncertainty on the background and signal?” “How many signal events do we think we see?” “How much larger is this than the uncertainty?” This then gives us the “n sigma” for that mode across the mass range. We combine these sigmas within a single experiment, taking correlated uncertainties into account, and that’s how we get our Brazil plots.

How likely is a discovery?

In 2011 we had about \(5fb^{-1}\) of luminosity and we saw about 3 sigma for each experiment. In 2012 we had about \(6.5fb^{-1}\) of luminosity at slightly higher energy (giving a factor of 1.25). So we can work out what to expect for 2012 sensitivity- just take the 3 sigma and add it in quadrature to \((\sqrt{1.25\times 6.5/5})\times 3\) sigma and that comes out at 4.9 sigma. If we’re lucky one or more experiments might see more than 5 sigma, meaning we could have a discovery!

What next for the Higgs?

If we make a discovery, either now or in the coming weeks, then we need to measure the properties of the new particle. We can’t claim to have discovered the Standard Model Higgs boson until we’ve measured its branching fractions and spin. Fortunately, if the Higgs boson is at 125GeV then we have a rich variety of decay modes, and this could give us insights into all kinds of interesting measurements, such as the quark masses.

Now go and enjoy the seminar!

Learn more about the Higgs

What comes next? (Richard Ruiz)

How difficult is it find the Higgs? (Richard Ruiz)

Why do we expect to find a Higgs boson? Part I Electroweak Symmetry Breaking (Flip Tanedo)

Why do we expect a Higgs boson? Part II Unitarization of Vector Boson Scattering (Flip Tanedo)

(Video) What is a Higgs boson? (Dom Lincoln)

(Video) Higgs boson – Latest update (Dom Lincoln)