Live Blog: Webcast from Moriond Electroweak


Study of full ATLAS dataset for H->bb and H->tau tau is underway.


She shows all the channels – in none of them is a deviation observed from the background-only expectation in ATLAS data.


pp->bb production is a million times higher than production of Higgs and subsequence decay H -> bb. That’s a real challenge.


She will present new results from H->invisible decays and H->mu+ mu-. They use the full ATLAS data set in their results, compared to other results (bb, tau tau) last updated in 2012.


She reviews the motivation for looking at fermion decays of the Higgs.


The ATLAS talk is given by Dr. Victoria Martin (Univ. of Edinburgh).


Now it’s ATLAS’s turn to talk about decays of a Higgs-like particle to fermions.


Reminder: webcast here:


For the pure hadronic H -> tau tau final state, how much of the background is from real a1 -> 3pion decays?

For the background, you have an a1 or rho inside the tau decay from a Z decay. Most background we see is due to this effect. There are also combinatoric backgrounds from random triplets of tracks. However, they do try to reconstruct the a1 and rho inside the tau hadronic jet.


Bill Murray from ATLAS asks first question. There appears to be a deficit below the Z peak in tau tau and of course you are looking for an excess above the Z peak – how well is the tau energy scale understood?

She mentioned that they calibrate from the Z peak.




Results use 5+19/fb (7 + 8 TeV data). mu+hadronic tau channel is single most sensitive channel, but all channels combined to get clearer picture (with more weight given to more sensitive categories).

Data supports the existence of a signal contribution around 125 GeV, albeit with big uncertainties after background subtraction (background is huge).

Limits derived from data show observed excess over background-only expectation – observed sensitivity is 1.8 times the Standard Model. They measure a signal strength of 1.1 +/- 0.4.

Maximal local significance is 2.9 sigma at 120 GeV; at 125 GeV they have a signficance of 2.9 sigma as well.

Strong indications that new particle at 125 GeV does decay to taus.


Huge background from Z -> tau+ tau-; careful modeling is critical to success in this channel.


Now on to Higgs -> tau+ tau-!


They expected to set a limit around 5.2 times the Standard Model expectation; the observed limit is about 5.8 times the Standard Model, consistent with expectations from background-only. This also has a ways to go before it’s sensitive by itself to exactly the Standard Model prediction.


She now presents results from pp -> ttH, where the H->bb. This is an independent way of looking for the Higgs decaying to bottom quarks. This uses 5/fb each from 7 TeV and 8 TeV data.


They see a slight excess in data over pure background-only hypotheses, at around 125 GeV – but the resolution is not great in this channel (compared to ZZ or two-photon), so it’s hard to say exactly where the excess might be located. Signal strength is presently about 1.3 +/- 0.7 – still a ways to go to make this a strong channel all on its own.


The data is consistent with di-boson background (e.g. pp -> ZZ, WZ, WW, etc.), but can accommodate a Standard Model Higgs signal as well, though without much resolving power for now. They try to improve sensitivity using advanced multivariate techniques to take advantage of a suite of discriminating variables that each by themselves can separate signal from background, though perhaps not very well; when combined, the separation improves.


They take the “usual” strategy – look for Higgs produced in association with a boson, such as Z or W. This reduces the production rate of the Higgs, but cleans up a lot of the background that otherwise hides a possible signal. They are using 5/fb at 7 TeV and 12/fb of 2012 data – results are NOT UPDATED since November of last year.


She begins her results discussions with Higgs decays to a pair of bottom quarks – this is the largest branching fraction at low Higgs mass, but is challenging due to pollution by the copious quark and gluon jets produced by the LHC.


She is discussing the importance of good measurements of “missing energy” – energy that is assumed to arise from particles that cannot be detected, such as neutrinos. Good missing energy reconstruction is critical in flagship channels such as Higgs decay to a pair of tau leptons, where the tau leptons then also decay to longer-lived particles and neutrinos.


Now Valentina Dutta (MIT) is giving the status of CMS searches for Higgs decays to fermions. Fermions are particles that carry one-half-integer spin angular momentum (S=1/2, 3/2, 5/2, etc.). Examples are electrons, protons, and neutrons.


That’s it for now on experimental talks from ATLAS and CMS. Sorry I missed the CMS talk on the live stream. I’ll post notes on both the ATLAS and CMS results in the blog later, when the slides are available from the Moriond EW website:


Question: what is so different between ZZ and gamma-gamma that the masses measured from each appear to be in tension? There was concern about the energy scale of the ATLAS calorimeter.

Dr. Hubaut talked about how the calorimeter is calibrated and how to map from photons to electrons in energy measurements. These have all been extensively studied in the current data. More needs to be understood about material in front of the calorimeter and how to model it. Also, we can now use the new state itself to do calibration studies.


Mass from the two-photon channel is measured to be 126.8 +/- 0.2 (stat.) +/- 0.7 (syst.).

The signal strength for the ZZ channel (ratio of measured rate to Standard Model predicted rate) is 1.7 +/- 0.5, roughly. This is slightly larger than what was measured last year (1.4); but a lot has changed in the analysis since then, both additional data and improvements to the procedures. This is not an unexpected change.


Questions time.


The observation of the new state is at a significance greater than 6 standard deviations in both the $H^0\to\gamma\gamma$ and $H^0 \to ZZ^{(*)}$ channels separately. This is amazing. To go from combined discovery to this kind of strong individual channel discovery in just under a year is an amazing testament to the power of the LHC to deliver data and the power of ATLAS experimentalists to understand and present the data analysis.


Dr. Hubaut is now showing $H^0 \to Z \gamma$; SMU was central in making this a formal analysis within ATLAS after the discovery of the new state last summer, and with our many partner institutions in ATLAS we have completed the very first result from ATLAS on the search for this channel. Unfortunately, the channel is challenging and we are still far from being able to measure the Standard Model predicted rate for $H^0 \to Z \gamma$; but, we can at least say that there doesn’t appear to be an anomalously large coupling that produces this final state.


Dr. Hubaut has just shown the latest results from the ATLAS Experiment in the decay $H^0 \to ZZ^{(*)}$. ATLAS confirms again that the new particle indeed decays to this final state with good resolution, and reports the mass of the new particle to be measured in this channel to be still about 125 GeV.


Current, Dr. Fabrice Hubaut (CPPM) is presenting the results of Higgs decays to pairs of bosons. Bosons are particles with an integer unit of internal angular momentum, (S=0,1,2,…). Particles in nature that are boson are the familiar photon, the particle of light, and the less familiar Z and W particles that mediate the weak interaction (critical to nuclear decay, which makes the burning of the sun possible).


The webcast from the Moriond Electroweak Conference is live here:

This is the “Higgs Day” of the conference, where the latest studies of the new state at 125 GeV will be presented on behalf of the LHC and Tevatron experiments.


Postscript: Peter Higgs was MOBBED by the press on his way from the Main Auditorium to the press conference. I nearly got a black eye from a reporter running by me with his camera sticking out. This was quite an experience!



That’s a wrap! Thanks for tuning in!

More to come from the LHC!


Some of the members of the original theorists that conceived of the mechanism of electroweak symmetry breaking have passed away, and cannot be here. They would have loved this.


Four of the founders of the breaking of electroweak symmetry breaking are here! They are applauded when they stand.

They seem choked with emotion. Hard to get words out. Peter Higgs: “Really, it’s the most incredible thing that has happened in my lifetime.”


LHC: You (the experiments) have built outstanding scientific instruments. We should all be very proud.


More remarks: LHC 28 years in the making. Amazing that it has worked beyond expectations. A real testament to the accelerator and experimental communities.


Remarks: “another giant leap for mankind.”


Messages from ICHEP: happy to have taken part in this historic event, wishing all the best to those at CERN>



DG: this is a global effort, and it’s a global success for the Standard Model. Possible only with the exceptional performance of the accelerator and the experiments.

We have a DISCOVERY. We have observed a NEW PARTICLE consistent with the Higgs Boson.

But which one?

That remains open.

This is a historic milestone. This is only a beginning. There is a lot of work ahead of us.


A statement from the Directory General of CERN, Rolf-Dieter Heuer.

“I think we have it” says the DG.


Nature has been kind. A Higgs with this mass is easily studied at LHC in many channels.


Next steps on ATLAS: plan to submit a paper at the end of July, simultaneous with CMS in the same journal.

NEWS: ATLAS will add the H->WW channel.

We will embark on a great quest to understand this new land which we have discovered. Behold, a great mountain rises above the horizon of a wide and endless sea. Now, let us see if this is truly Mount Higgs.


Fabiola is giving the global picture of the progress of the data and the excesses in the data over time.


The results overall are fully compatible with the Standard Model expectaiton for the Higgs boson.

The best-fit mass from the combined channels is 125.6 GeV. This is very close to the CMS result, 125.3  +/- 4.9 GeV. Good agreement between two completely independent experiments with, in many cases, wildly different ways of collecting and analyzing the data. This is the process of science working its power. With one experiment alone, who can say? But with two? That’s powerful confirmation. This is why science works to obtain knowledge of the natural world.


ATLAS have excluded a SM Higgs boson for all masses up to 600 GeV EXCEPT around 125 GeV.

Combining the 2photon and the 4-lepton final states, the p-values again tell the tale. The combination is a 5.0 sigma excess over background only (local significance).  The global significance is 4.1-4.3 sigma, taking into account the wide search region.


Fabiola shows beautiful event displays of these Higgs candidates in the 4-lepton channel.

With more data, ATLAS has been able to exclude a Standard Model higgs boson at high masses . . . but not at low masses. Can’t exclude it.

The p-value plot tells the story. The excess over background-only expectation is 2.7 sigma (local significance). Taking into account the wide search region, the result is 2.5 sigma significant.

This is very compatible with Standard Model Higgs expectation.


Improvements to electron reconstruction lead to high, flat efficiency for reconstructing and identifying them. Beautiful. Sheer beauty to be able to map the lives and fates of subatomic particles so effectively in the ATLAS detector.

Resolution on Higgs mass is less than 2 GeV. Again, expect data to cluster in the mass of the four leptons.

She shows the mass data. The is a small excess at a mass of around 120-130 GeV.

In high mass region, there is a 30% excess in data over that expected from Standard Model background. This excess is in agreement with the ATLAS measurement of the ZZ(*) production cross-section. This is in general seen in ATLAS. Suggests theory calculation needs some additional attention to understand this.

Low mass region time! There is a clear peak at 90 GeV from “single-resonant” production and decay of Z bosons. We should see it and we do see it. A good sanity check of being able to reconstruct particles with low masses.

In the region below 160 GeV, 39 events are observed, 34 +/- 3 are expected. Around 125 GeV, 13 events are seen and 5.1 +/- 0.8 background events are expected.


Moving on to Higgs -> ZZ(*) -> 4 leptons.

THe mass can be fully reconstructed, as in 2photon. But, the “golden channel” is more pure than the 2photon channel. Low yields of events are expected. Expect events to “cluster” where a Higgs is decaying to 4 leptons.

Improvements have been made to electron reconstruction, leading to large gains in sensitivity in this channel.



Pernicious backgrounds from jets faking photons are shown to be under better control and well-characterized.

Photons are required to be isolated – a challenge in the presence of proton-proton pileup. We can even see the structure of the LHC bunches in the ATLAS photon isolation information! Quite cool!

She shows the fit to the data using background and signal. There is a clear excess in the data. She will try to quantify the excess we see.

2011 and 2012 data BOTH show an excess – ATLAS are unable to exclude a Standard Model Higgs. Combining the data shows and even clearer tension between achieved and expected limits. The p-value tells the story.

H->2 photon is observed at more than 4 sigma significance! the local significance of the result is 4.5 sigma. Taking into account the global search region, the significance is 3.6 sigma (wide search area means more chances of misinterpreting a fluctuation as a signal . . . reduces significance).

Signifiance is a factor of 2 larger than expected from Standard Model. THe value if 1.9 +/- 0.5 times the Standard Model. Still compatible with the Standard Model.


Begin with H->2 photon (“gamma gamma”). Signal expected to appear as a narrow peak on top of a large background. Data divided into ten categories depending on the topology and kinematics of the events. Some categories are more or less sensitive than others.

Since last time around, 15% gain in sensitivity was achieved. Expect about 170 signal events from Standard Model and more than 6000 background events. Big challenge.

She discusses how they confirm they can model and observe a two-photon signature that makes a bump in the mass spectrum.

Electromagnetic Calorimeter shown to be very stable over the operation of the ATLAS during the data taking. Its response is well-characterized and understood.


Results to be presented today: H->ZZ and H->2photon. Optimization was done using simulation. Then looked at 2012 data in control regions to verify understanding of backgrounds. Data-driven techniques are used to quantify the background expectation. Signal region only looked at once these other things are satisfied.

A new combination will be shown at the end


Fabiola begins by reviewing the state of 2011 ATLAS Higgs searches. With the 2011 data, ATLAS was unable to exclude a SM Higgs boson with a mass of around 125-126 GeV. This was last winter.

Only a few places left to scrutinize for a Higgs.


The ATLAS Standard Model searches for known processes all look good. Standard Model works well at 7-8 TeV with some little tensions (to be discussed later) – hinting at ATLAS Higgs results? :-)


Fabiola tips the hat to trigger and computing; these are crucial elements in making these results possible. Have to get the right events, and then process the data (and generate huge amounts of simulation to help develop research and understand backgrounds).


ATLAS results shown today will only be on H->ZZ->4 leptons and H->2 photons. The other channels have difficult challenges and will not be shown at today’s seminar.


Fabiola reviews the challenges in 2012. Machine operating beyond design; ATLAS must cope. Painstaking work.


Time for the ATLAS results. Fabiola Gianotti takes the stage.



Incandela: “These results are now shared by all mankind.”


CMS claims observation of a new boson with mass of 125.3 +/- 0.6 at 4.9 sigma significance.


Results are all consistent with the Standard Model Higgs hypothesis, looking across all channels and in 2011 and 2012 data.  They have also scanned the couplings to bosons and fermions, and so far is consistent with the Standard Model.



Higgs mass current measurement: 125.3 +/- 0.6 GeV from combining all channels. Fresh off the press.


Then Higgs to tau tau. They are nearly sensitive to the Standard Model – big surprise for 2012! They don’t see an excess in the data – in fact, they nearly exclude 125 GeV Higgs in this channel. But, they have so few events and probably a downward fluctuation. NEED MORE DATA to shed light on this channel.


They also look at H->bb. Data consistent with either background or signal+background. Need more data.

They combine the H->ZZ, WW, and 2-photon and achieve a 5.1 sigma significance result overall.


So far, for the H->WW analysis, they have only had time to pursue a cut-based approach.. They are looking at WW->ee, mu mu, mu e (+ neutrinos).

They see a little excess in the 0-jet category. No obvious excess in 1-jet. Statistics are low.

If they combine the 2011 and 2012 data, they see a small excess over the expectation from background only. There is tension with the expected 95% confidence level exclusion plots. They tried seeing what it would look like if a signal was there. Data are seemingly consistent with a signal, but the statistics are low.



Now onto Higgs to WW.

Room was ELECTRIC when he showed the ZZ+2photon combination. So much applause. So much energy! People are sighing with emotion and relief.


If they combine the ZZ and 2photon, they achieve a 5 sigma excess above background-only excess




Onto the ZZ(*) punchline: they see a peak in the data not modeled by the background. The peak is around 125 GeV.

They then show the data using the angular analysis. THere is a clustering of events in mass and the angular variables that is interesting.

The low statistics makes subdividing these events very hard. So far consistent with signal.

After cutting on the angular discriminant, there is still an excess remaining around 125 GeV.

They see a clear excess in their 95% confidence level exclusion plot – they cannot exclude a SSM Higgs as that mass. Looking at the p-values, they see a 3.2 sigma global excess for H->>ZZ(*)


They look at H->4 leptons through intermediate ZZ(*). They look at electron and muon final states. They expect to be MORE than sensitive to a SM expected decay of the higgs to this final state.

This is the so-called “golden channel” because it is so clean. Very few background events survive all selection criteria, unlike in the 2photon channel.

He shows the background modeling.

Ah . . . they are also looking at the final-state angles of the leptons. This is used to further separate background and signal.


Now onto H->ZZ(*)


Joe goes on: the result is about 1.56 +/- 0.43 times the Standard Model – consistent with expectation of SM Higgs. The global significance of the H->2photon result is 3.2 sigma in the whole search region.


Onto significance. CMS 95% confidence level exclusion for SM Higgs shows clear excess around 125 GeV. Now he moves to p-values – probability that background could fluctuate to this extent. Combining both years, the result is 4.1 sigma significant.


He’s showing the mass distributions now. First for 2011. Demonstrates how the data jitters around and they have to be careful interpreting it. This is the old data (last year).

He shows 2012. Look similar to 2011. Hard to see what is there.

Now they combine them, weighting them by signal-to-background: CLEAR SIGNAL APPEARS.


The H->2 photons search uses multi-variate classifiers to reject backgrounds, and they cross-check their method with an alternate background model extraction process and a cut-based analysis. Multiple cross-checks.

All CMS analyses were blinded in 2012, to avoid researcher bias. They exclude the place where they think signal might be lurking and freeze the analysis development before “opening the box.”

Monte Carlo simulation only used to optimize the selection. Data-driven background estimates used for real signal extraction from data.

They spend a lot of effort getting the Higgs mass reconstruction right. They also check carefully their efficiency.

They split the event into categories using the output of the multi-variate classifier. In each category, they fit the mass distribution to look for signal.




First Higgs search channel CMS talk will discuss:Higgs decays to two photons.


Joe is reviewing all of the reconstruction that is done by CMS on these proton collsisions.


Wifi here is not good (too many physicists). Trying to update as quick as I can…


Joe outlines the challenges that so many pileup (proton-proton) collisions brought to data size and processing time. Computing experts found massive gains in speed to cope with this.


CMS boasts and all-silicon charged particle tracker and a huge Lead-Tungstate crystal calorimeter. Crucial for tracking and energy measurement.


CMS expects to see a Standard Model Higgs boson, if it exists, with 5-6 sigma significance.


CMS will show results in 5 Higgs search modes: H -> gamma gamma, ZZ(*), WW(*), tau tau, bb.


Joe tips his hat to the entire CMS collaboration.


Joe Incandela takes the stage and begins his talk. He starts with discussing the massive pileup that had to be handled this year, with over 2 dozen proton-proton collisions per crossing. Time to hear about the CMS hunt for the Higgs.


I just heard that CERN will do a press release at 10 am CET (3am US Central).


Applause for Peter Higgs as he takes his seat!


About ten minutes from the start here.


Professor Jodi Cooley at ICHEP! We can see her on the feed. She’s the purple blob on the left side of my grainy photo. :-)


A panorama of the room from a little while ago.



OK, wireless internet connection has gotten very dodgy in here. I will not be able to post any more photos before the event starts. From now on, it’s probably all text!


CERN Director General Rolf-Dieter Heuer is here and socializing with the distinguished guests in the reserved section.


Just to prove Aidan and I are *actually* here in the Main Auditorium…


People are chatting and milling about. Still just under an hour to go. Lively conversations happening everywhere.


There is a live link to Melbourne; we can see them, and they can see us. HELLO, ICHEP!


Here is the line just before we entered the Main Auditorium. CRAZY.



They moved us into the main auditorium very promptly at 07:30 CET. They were counting people as we entered. Some of the specially reserved seats at the front were already taken by their owners; the section above that was open to all of us, so we picked seats. Aidan was interviewed by an Israeli TV crew; him mom is here for this event (complete with CERN vistor pass) and they were very interested in what a non-physicist found so interesting about this event. The atmosphere here is ELECTRIC.

Earlier, when the Spokesperson of CMS arrived (Joe Incandela), he was joking with some CMS people at the front of the line. A few moments later, as they were laughing together, a chant was started by some of the summer students at the very front of the line. “ATLAS! ATLAS! ATLAS!” This rippled back through the line. It was strange and funny all at the same time.


Keep seeing people I haven’t seen in years. It is amazing how big and how small our community is, and all at once.


Fire alarm was switched off by the fire brigade after about 15  minutes. Crowd undaunted by shrill mind-liquifying alarm. Queue stretches from front door of auditorium, around past the CERN council chambers, along the wall, dow the stairs, and into Restaurant 1.


Fire alarm started going off 5 minutes ago. Nobody is budging. Not sure how it was tripped, but security is also not evacuating the building. People are just waitiing it out.


The scene outside the main CERN auditorium circa 04:30 CET

The line stretches 4-5 people thick and all the way from the main entrance of the auditorium to the CERN Council Chambers.  People are showing up saying things like, “See, I got here early!” Which then prompts everybody around them to quip, “You’re NOT here early.” Atmosphere is still jovial (and sleepy). Aidan and I made coffee a little while ago and brought it back to the line. We’re perking up for the seminar. Doors open in about 2 hours. Whew. TWO HOURS.


I'm queuing for the Higgs seminars, and proud to be a Mustang! :-)

Aidan snaps a photo of me queuing with the rest of the “99%” . . . of people who do NOT have Nobel Prizes. :-)


A growing number of people collect outside the main CERN auditorium.

This was the scene about 45 minutes ago; the number of people waiting here for the doors to open (in about 6 more hours) is more than double now. It’s a positive atmosphere overall; some people are napping, most are either reading, socializing, or watching movies. I saw at least one Higgs Convener (one of the organizers of the overall Higgs physics programs in one of the experiments) here earlier, though I think they left. Mostly, it’s younger people.

I’m listening to some podcasts to pass the time, and chatting with people from time to time. I am SURE glad Aidan and I decided to get here when we did; by 5am, it would have been too late to get a seat. Luckily, we are near the head of the queue.


People begin to queue for the seminars.

As I suspected, we are NOT the only ones who had the idea of getting here early. There are about a dozen or more people here, camping out and waiting for the doors to the auditorium to open. That should happen at 07:30 or so.


I slept for about 4 hours and just woke up a little while ago. Aidan and I are going to head to CERN tonight to make sure we can get in a good position in line in the morning for getting into the main auditorium. Let’s hope 7000 other people haven’t had the same idea.

I am seeing buzz all over the internet about a video being leaked ahead of the formal scientific seminars in the morning. I am trying to be a good scientist, and focus on the fact that I am excited for the seminars and not just the results. I have not watched the video, and don’t intend to until after the real seminars have concluded.

Assuming I am still awake at that point…!


About Stephen Sekula

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