Quantum Diaries

Posts Tagged ‘Higgs’

Can the LHC Run Too Well?

Friday, February 3rd, 2012

Seth Zenz

For CMS data analysis, winter is a time of multitasking. On the one hand, we are rushing to finish our analyses for the winter conferences in February and March, or to finalize the papers on analyses we presented in December. On the other, we are working to prepare to take data in 2012. Although the final decisions about the LHC running conditions for 2012 haven’t been made yet, we have to be prepared both for an increase in beam energy and an increase in luminosity. For example, the energy might go to 8 TeV center-of-mass, up from last year’s 7. That will make all our events a little more exciting. But it’s the luminosity that determines how many events we get, and thus how much physics we can do in a year. For example, if the Higgs boson exists, the number of Higgs-like events we’ll see will go up, and so will the statistical power with which we can claim to have observed it. If the hints we saw at 125 GeV in December are right, our ability to be sure of its existence this year depends on collecting several times more events in 2012 than we got in 2011.

We’d many more events over 2012 if the LHC simply kept running the way it already was at the end of the year. That’s because for most of the year, the luminosity was increasing over and over as the LHC folks added more proton bunches and focused them better. But we expect that the LHC will do better, starting close to last year’s peak, and then pushing to ever-higher luminosities. The worst-case we are preparing for is perhaps twice as much luminosity as we had at the end of last year.

But wait, why did I say “worst-case”?

Well, actually, it will give us the most interesting events we can get and the best shot at officially finding the Higgs this year. But increased luminosity also gives more events in every bunch crossing, most of which are boring, and most of which get in the way. This makes it a real challenge to prepare for 2012 if you’re working on the trigger, because have to sift quickly through events with more and more extra stuff (called “pileup”). As it happens, that’s exactly what I’m working on.

Let me explain a bit more of the challenge. One of the triggers I’m becoming responsible for is trying to find collisions containing a Higgs decaying to a bottom quark and anti-bottom quark and a W boson decaying to an electron and neutrino. If we just look for an electron — the easiest thing to trigger on — then we get too many events. The easy choice is to ask only for higher-energy electrons, but beyond a certain points we start missing the events we’re looking for! So instead, we ask for the other things in the event: the two jets from the Higgs, and the missing energy from the invisible neutrino. But now, with more and more extra collisions, we have random jets added in, and random fluctuations that contribute to the missing energy. We are more and more likely to get the extra jets and missing energy we ask for even though there isn’t much missing energy or a “Higgs-like” pair of jets in the core event! As a result, the event rate for the trigger we want can become too high.

How do we deal with this? Well, there are a few choices:

1. Increase the amount of momentum required for the electron (again!)
2. Increase the amount of missing energy required
3. Increase the minimum energy of the jets being required
4. Get smarter about how you count jets, by trying to be sure that they come from the main collision rather than one of the extras
5. Check specifically if the jets come from bottom quarks
6. Find some way to allocate more bandwidth to the trigger

There’s a cost for every option. Increasing energies means we lose some events we might have wanted to collect — which means that even though the LHC has produced more Higgs bosons, it’s counterbalanced by us seeing fewer of the ones that were there. Being “smarter” about the jets means more time spent by our trigger processing software on this trigger, when it has lots of other things to look at. Asking for bottom quarks not only takes more processing, it also means the trigger can’t be shared with as many other analyses. And allocating more bandwidth means we’d have to delay processing or cut elsewhere.

And for all the options, there’s simply more work. But we have to deal with the potential for extra collisions as well as we can. In the end, the LHC collecting much more data is really the best-case scenerio.

Tags: CMS, Higgs, LHC, luminosity, pileup, trigger
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Higgs for the Holidays

Friday, December 23rd, 2011

TRIUMF

– By Theorist David Morrissey & Particle Physicist Anadi Canepa

Last week we hosted two particle physics workshops at TRIUMF – an ATLAS Canada collaboration meeting and a joint meeting for theorists and experimentalists to study new LHC results. Everything went smoothly, no participants were lost to the wilds of Vancouver, and we had some really great discussions and seminars. During one of these presentations, it occurred to me that these kinds of scientific meetings are not so different from a typical holiday gathering. In both situations, you frequently run into people you know but that you haven’t seen in a long time. You catch up, you gossip, and you eat too much food at the coffee breaks. There’s usually a large group dinner where you often meet new people and strike up conversations about future work. And every so often one of the participants has too much holiday cheer.

Despite these similarities, most scientific meetings don’t involve gifts. But this time around we were really lucky, and our workshops had a gift exchange of sorts as well. In this case, the gifts were the presentations by the ATLAS and CMS collaborations of exciting new results from their searches for the Higgs boson particle. On top of the live streaming presentations from CERN in the early hours of the morning, we were treated to a longer seminar in the afternoon at TRIUMF by Rob McPherson. His talk was standing-room only, and we had a great time bombarding him with questions about the ATLAS analysis.

The reason for all this excitement over a single particle is that the Higgs boson, first proposed nearly fifty years ago, is central to our current understanding of all known elementary particles, called the Standard Model. (See here, here, and here for more details.) In this theory, the Higgs is responsible for creating the masses of nearly all elementary particles and for making the weak force much weaker than electromagnetism. Even though we have not yet seen the Higgs directly, we have indirect evidence for it from precision measurements of the weak and electromagnetic forces. Discovering the Higgs boson would confirm the Standard Model, while not finding it would force us to drastically rethink our description of elementary particles and fundamental forces, which would perhaps be an even greater discovery.

Excitement about finding the Higgs has been building since the summer, when it became clear that the LHC would be able to collect enough data by the end of the year to possibly find it. In the past few weeks the level has gone through the roof as rumours started to appear that the LHC experiments would soon release a significant result. What we learned this week is that these latest searches did not discover the Higgs boson, but that they do suggest that it might be there with a mass close to 133 times that of a proton (125 GeV). Finding a Higgs is hard work, and its delicate characteristic signal must be extracted from a huge amount of background noise. What we have at the moment is an intersting bump, as you can see in the figure above taken from the ATLAS search, where we see more signal events than would typically be expected from the background alone for a candidate Higgs mass of about 125 GeV. We just don’t have enough data right now to confirm that this bump is from a Higgs boson, and not just an especially unlucky spike in the background noise. Fortunately, the ATLAS and CMS collaborations will be taking much more data in the new year.

So, for this year all we get is a gift-wrapped box that we’re allowed to shake and prod. But if we’re good, we’ll get to open the box and find what’s inside at some point in 2012. Dear Santa…

Tags: Anadi Canepa, Canada, David Morrissey, Discovery, Higgs, New, Particle, physics, Theory, TRIUMF, Vancouver
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Brookhaven Lab and the Search for the Higgs

Tuesday, December 13th, 2011

Brookhaven

Today’s public seminar at CERN, where the ATLAS and CMS collaborations presented the preliminary results of their searches for the Standard Model (SM) Higgs boson with the full dataset collected during 2011, is a landmark for high-energy physics!

The Higgs boson is a still-hypothetical particle postulated in the mid-1960s to complete what is considered the SM of particle interactions. Its role within the SM is to provide other particles with mass. Specifically, the mass of elementary particles is the result of their interaction with the Higgs field. The Higgs boson’s properties are defined in the SM, apart from its mass, which is a free parameter of the theory.

Scientists are looking for signs of the Higgs boson by searching for the products of its decay. Two of the most prominent decay channels, or ways the Higgs can decay, are to form two photons or to form a pair of Z bosons, each of which subsequently decays to a pair of leptons (electrons or muons). Brookhaven National Laboratory (BNL) has played and continues to play a key role in the design, construction, and operation of the detectors of the ATLAS experiment that are used to observe electrons and photons (the liquid argon electromagnetic calorimeter) and muons (the muon spectrometer). Major contributions are also made in the data analysis, where Brookhaven scientists have leading roles. BNL also significantly contributes to the trigger — deciding which events to analyze in detail — and to computing.

Brookhaven physicist Denis Damazio controls the front end crate of the barrel liquid argon calorimeter in ATLAS with his laptop.

Owing to the excellent performance of the Large Hadron Collider (LHC) and the stable operation of the ATLAS and CMS detectors, the two collaborations have achieved a five-fold increase of the dataset presented during the summer conferences, only a few months ago. The new result excludes the vast majority of the range where the Higgs boson mass could potentially lie, and leaves very little hiding space for the elusive boson.

Furthermore, both experiments observed in several channels an intriguing upward fluctuation of the data. Is this the first glimpse of the Higgs boson or just a statistical fluctuation? Only improved analysis, and more data will tell!

Scientists at the LHC look eagerly forward to next year’s LHC run period starting in early spring 2012. If the LHC performance projections work out as expected — and the LHC crew has been very good in keeping promises — we should be able to double the available dataset in time for the summer conferences and have a conclusion on the existence or not of the last missing piece of the Standard Model of particle physics.

This post was written by Brookhaven physicist Kostas Nikolopoulos

Tags: ATLAS, Brookhaven National Laboratory, CERN, Higgs, LHC
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Higgs seminar discussion

Tuesday, December 13th, 2011

Aidan Randle-Conde

Shortly after the Higgs Seminar, Seth Zenz and I had a short, impromptu discussion about the results and what they mean for physics in the near future. Check out the video:

(Due to a technical problem, we lost the first two seconds of audio, so there is a slightly abrupt start.)

Tags: ATLAS, CERN, CMS, Higgs, LHC, seminar, video
Posted in Latest Posts | 7 Comments »

Fermilab hot on trail of Higgs boson with LHC, Tevatron

Tuesday, December 13th, 2011

Fermilab

Real CMS proton-proton collision events in which 4 high energy electrons (green lines and red towers) are observed. The event shows characteristics expected from the decay of a Higgs boson but is also consistent with background Standard Model physics processes. Courtesy: CMS

Today physicists at CERN on the CMS and ATLAS experiments at the Large Hadron Collider announced an update on their search for the Higgs boson. That may make you wonder ( I hope) what is Fermilab’s role in this. Well, glad you asked.

Fermilab supports the 1,000 US LHC scientists and engineers by providing office and meeting space as well as the Remote Operation Center. Fermilab helped design the CMS detector, a portion of the LHC accelerator and is working on upgrades for both. About one-third of the members of each of the Tevatron’s experiments, CDF and DZero, are also members of the LHC experiments.

That means that a good portion of the LHC researchers are also looking for the Higgs boson with the Tevatron. Because the Tevatron and LHC accelerators collide different pairs of particles, the dominant way in which the experiments search for the Higgs at the two accelerators is different. Thus the two machines offer a complimentary search strategy.

If the Higgs exists and acts the way theorists expect, it is crucial to observe it in both types of decay patterns. Watch this video to learn how physicists search for the Higgs boson. These types of investigations might lead to the identification of new and unexpected physics.

Scientists from the CDF and DZero collaborations at Fermilab continue to analyze data collected before the September shutdown of the Tevatron in the search for the Higgs boson.

The two collaborations will announce their latest results for the Higgs boson search at an international particle physics conference in March 2012. This new updated analysis will have 20 to 40 percent more data than the July 2011 results as well as further improvements in analysis methods.

The Higgs particle is the last not-yet-observed piece of the theoretical framework known as the Standard Model of particles and forces. Watch this video to learn The nature of the Higgs boson and how it works. According to the Standard Model, the Higgs boson explains why some particles have mass and others do not. Higgs most likely has a mass between 114-137 GeV/c2, about 100 times the mass of a proton. This predicted mass range is based on stringent constraints established by earlier measurements made by Tevatron and other accelerators around the world, and confirmed by the searches of LHC experiments presented so far in 2011. This mass range is well within reach of the Tevatron Collider.

The Tevatron experiments already have demonstrated that they have the ability to ferret out the Higgs-decay pattern by applying well-established techniques used to search for the Higgs boson to observing extremely rare but firmly expected physics signature. This signature consists of pairs of heavy bosons (WW or WZ) that decay into a pair of b quarks, a process that closely mimics the main signature that the Tevatron experiments use to search for the Higgs particle, i.e. Higgs decaying to a pair of b quarks, which has by far the largest probability to happen in this mass range. Thus, if a Standard Model Higgs exists, the Tevatron experiments will see it.

If the Standard Model Higgs particle does not exist, Fermilab’s Tevatron experiments are on track to rule it out this winter. CDF and DZero experiments have excluded the existence of a Higgs particle in the 100-108 and the 156-177 GeV/c2 mass ranges and will have sufficient analysis sensitivity to rule out this winter the mass region between.

While today’s announcement shows the progress that the LHC experiments have made in the last few months, all eyes will be on the Tevatron and on the LHC in March 2012 to see what they have to say about the elusive Higgs Boson.

– Tona Kunz

Tags: ATLAS, CDF, CMS, DZero, Energy Frontier, Higgs, Higgs boson, Tevatron
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Higgs Liveblog

Tuesday, December 13th, 2011

Aidan Randle-Conde

Seminar / Webcast / My Twitter

Slides available

Go to the Seminar page for the slides that there presented today.

The liveblog

(For details about the seminar, see below. Some links to photos on the twitter feed.)

The most recent updates are at the top of the page. All times are CERN times.

More updates will be posted as they arrive. Thanks for reading!

Questions

15:49: Rolf says “No more questions, so final remarks”. Great to have first results, remember they are preliminary and with small numbers. “Keep in mind we’re also running next year.” “The window is getting smaller and smaller, but it’s still alive!” “Stayed tuned for next year”.

(G) means that Guido answered, (F) means that Fabiola answered. Thanks to Rozmin Daya for providing more detailed transcripts of the questions and answers. (Questions are ordered so that the most recent question is at the top of the post.)

Q: Regarding the crystal calorimeter, by how much has this improved your endcap H->gamma gamma mass resolution? Did I understand that you understand the scale to 0.1%?
A(G): No. for the scale, if you take a look at the plot on slide 33, this rms is what you’re able to acheive. In the endcap there’s a lot of progress, the scale does not evolve in big jumps. We have lots of room to improve. Up to now we were limited by this phenomena related to transparencies. Our tracker is fantastic but introduces extra material. Have to understand material and conversions more. We did this for barrel but must do more aggresively for barrel.

Q: For ATLAS, you have exclusion at 115.5 GeV. Is there a way to have some kind of Look Elsewhere effect for negative fluctuations? For CMS, you have excess everywhere between 115-135 GeV. Can it be that you’re simply misunderstimating background?
A(G): it could be, but we should be really precise. I cannot exclude but I’ll give small probability to this. Atlas: as far as I know we don’t have a LEE for this exclusion, but it’s an exclusion at the 95% confidence level.

Q: In the 4l final state analysis, how much would you lose if we count only 2/3 events (to ATLAS)
A(F): It would go down to 1.5sigma to 1.6sigma
Note: This question was motivated by fact that now there is discrepancy between ATLAS and CMS that wasn’t there at HCP time.

Q: What is the ultimate scale energy energy scale precision in the gamma gamma, because you were showing 0.5GeV from the Z.
A(F): Uncertainty on photon energy scale is more. It’s a few parts per million on the Z peak, but it’s below 1%. When we transfer to the photon, we have to take into account that we use the Monte Carlo simulation. We vary the material in the simulation, and we end up with a few parts per million to 1%.

Q: it’s interesting to understand what are the signal resolution for the few events in the 4 lepton final state.
A(F): The mass resolution is typically about 1.9 GeV for muons, and 1.7GeV for electrons.

Q: Question for CMS. Did you try to extract the 90% exclusion limit for gamma gamma? It’s a bit close to the ATLAS excess. Don’t understand the strategy for W->lnulnu.
A(G): We use continuous approach: cut based, and then the invariant mass of two leptons in a boosted decision tree.

Q: I’d like to understand looking at gam gam fit and the use of exponenetials to describe the background. Choice in CMS was not to do that. How confident are you to do that, knowing that QCD bg not well modeled by exponential?
A(F): You’ll find the slide in the spares. We tried several functions. We tried using background coming from Monte Carlo simulation, and also adopted a conservative estimation on background by taking as background uncertainty in a bin of 4GeV the difference between exponential and the expectation from Monte Carlo generator. We get consistent results using other functions.

Talks

15:36: Finish and applause. Rolf gives overview and opens up the floor to questions.

15:34: Two excesses at 119.5GeV and 124GeV. Both excesses seem compatible with a Standard Model Higgs. 95% confident limits include 127-600GeV. Some excess is present in all 5 channels. Statistical significance of 2.6sigma locally and 1.9sigma with Look Elsewhere Effect taken into account.

15:30: Modes split by resolution. Low and high resolution channels agree that something is happening around 125GeV. Maximum local significance is 2.6sigma. With look elsewhere effect it’s 1.9sigma in low mass region. Expect 2-3 sigma effect in region 115-127GeV. Look for best fit of Higgs cross section, shows best agreement at 124GeV.

15:29: CMS more sensitive than Tevatron experiments combined! Expected exclusion is 117-543GeV, observed is 127-600GeV. What is stopping CMS getting lower limit? There’s some kind of bump there in the low mass region…

15:26: First glance at invariant mass plot. Exclusion plot looks like it shows excess at 125GeV, but deficiency at around 128GeV. Interesting, given what ATLAS saw!

15:25: H->gamma gamma analysis. Improvements in the vertex identification, energy resolution. Vertex finding efficiency gives roughly 80% or better in all data periods. Resolution measured using Z, W decays and pi0 decays. Laser signals used to correct for transparency measurements. A lot of work has gone into understanding these issues!

15:20: Putting limits using H->ZZ* mode, one of the most powerful modes. Expected to exclude the ranges 130-160GeV, 182-420GeV, observed exclusion in 134-156GeV, 180-395GeV and 340 460GeV.

15:19: H->ZZ*(llll) and H->gammagamma have excellent resolution. H->ZZ* is the “golden channel”. Expect 67 events, observer 72 events in full mass range. In the low mass region (mH<160GeV) CMS has observed 13 events, expected 9.5 events.

15:13: H->bb mode. Very challenging! Huge background from QCD processes. Look for associated production of a boson with the Higgs boson. Better sensitivity, but lower efficiency. Require a very boosted W or Z produced in association. (pT of 100-160GeV) 5 sub channels of H->bb with associated production.

15:12: H->tautau mode. Slight hints of excess. Limit plot shows gentle excess across the low mass region (110-150GeV) compared to expectation.

15:08: Using multivariate analyses for the H->WW* state. Cut and count analysis shows most backgrounds removed by a handful of cuts. (Standard Model WW production dominates to the end. Expected exclusion is 129-236GeV, observed is 132-238GeV. Then using a boosted decision tree, split samples into different topologies. Look for discrepancies in the BDT spectrum Expected exclusion is 127-270GeV, observed is 129,-270GeV. Looks like a small excess just below 127GeV!

15:06: Now onto H->WW* analysis. Large non-resonant background from Standard Model WW processes. Angle between leptons can be used as a discriminant. Leptons emitted in small angle, so invariant mass of leptons not very large (it's all about spins of boson!) MET can be used to discriminate against background.

15:03: Monte Carlo simulation plots shown for events. Topological constraints useful for removing background. H->ZZ(ll,qq) mode extended to low mass region. Study at high mass includes H->ZZ(ll,tautau). 10.2 expected background events, 10 observed, so not sensitive in this mode yet.

14:58: 8 independent decay channels modes shown in a big table, with their sensitivities. 4.6-4.7fb^-1 of luminosity used for each of the 5 main modes (H->gamma gamma, ZZ*, WW*, tautau, bb) Resolution is 1-3% for gamma gamma and ZZ* final states. All 8 analyses made it to preliminary results to be shown today.

14:56: More than 90% data taking efficiency in each mode, and 91% overall. Impressive! Analysis requires good understanding of backgrounds and object reconstruction. Good agreement with data for identification efficiencies up to hundreds of GeV. Standard Model cross section plot shown. CMS agrees with data across all the processes, with a slight deficiency in ZZ production.

14:50: Guide starts, outlining the CMS collaboration and the detector. Overview of the Standard Model Higgs boson. Showing results up to 600GeV. Different production modes give different sensitivity.

14:52: Flashback to slides from a year ago, showing expectations. Expected sensitivity down to Standard Model across the whole range when combining channels. Projected significance decreases sharply in the low mass range. Sensitivity will come from combining channels.

14:48: Finish and applause! Guido takes the microphone. And goes through Fabiola's slides by mistake!

14:45 With current data set ATLAS has excluded 112.7-115.5GeV and 131-453GeV (except for 237-251GeV) ATLAS is now competing with LEP's low mass results! There is a large deviation in p0-values at 126GeV. 1.9e-4, or an excess of 3.6sigma (gamma gamma 2.8 has sigma, ZZ* has 2.1, WW* has 1.4sigma)

Updating all other analyses for full data set. We need more data in 2012 in order to confirm if this is the Higgs. 126GeV is a nice mass for the Higgs- it can be probed with lots of modes (gamma gamma, ZZ*, WW*, bb, tautau).

14:40: Apologies, connectivity issues.

Now discussing H->ZZ* analysis. Statistics limited background studies for SM ZZ processes. Electron identification efficiency comes from J/psi, W and Z decays. Covers wide range of transvere energy (up to 50GeV). Monte Carlo simulation tracks particle identification well as pileup increases- we understand the detector very well. Isolated muons selected, isolation performance looks impressive, even as pileup increases.

Simultation gives mass resolution of about 2GeV, 85% of signal falls within two standard deviations of mass point. 71 events seen in the full range, expected background is 62 events. In the low mass region (gamma gamma at 126GeV is 2.8sigma! If it's due to background only, it's a very large fluctuation. There are nine categories of photon, with the background modeled with an exponential function, and Crystal ball+Gaussian for signal. Excess shown at 126GeV

14:27: Discussion of the angle measurement. Need to know position performance in the calorimeter. Resolution of position of primary vertex is ~1.5cm. Potentially large background from jets and hadrons. The faking is rare, but the rate of production of jets is orders of magnitude larger than the rate of Higgs boson production.

14:26: Sensitive at lower energies. Different from previous channel, need good resolution of photon measurements. Irreducible background from Standard Model gamma gamma, also some fake gammas from jets. Mass resolution and positive robust against pileup. About 5GeV width in the invariant gamma gamma mass (in simulation, based on knowledge of detector.) Energy scale known to 0.5%, about 1% for linearity and uniformity. Z->ee mass shown, good performance there. Knowledge of how electrons interact inform energy scale for photons.

14:19: Discussion of Standard Model backgrounds for WW* channel. Turn on of ttbar background for this mode at Missing Energy (MET)>50GeV. MET strongly affected by pileup.

Expected background: 76 events, Data seen: 94 events, Expected signal :19 events. Cannot improve limit with this mode alone.

14:17: There are lots of backgrounds to consider! Concentrating on the gamma gamma, WW* and ZZ* modes. Backgrounds are jets, photons and W/Z bosons.

14:16: Huge efforts go into understanding the detector. As the regions of the Higgs search change, the requirements of the analyses change.

14:12: Outline of Higgs search motivation. The two photon sample is most sensitive at low mass ranges. Massive vector bosons sensitive at higher masses. Theorists have been working hard to update their expectations. The allowed region is small. We'll make it even more smaller today, and maybe see something very interesting in there as we do!

14:10: "The Standard Model works at 7TeV. Very Good." Good performance of Standard Model processes. We must understand these to understand the backgrounds, and also to calbirate measurements.

14:08: Discussion of pileup, the price we pay for high luminosity. We increase the number of events we record at once by having several interactions per beam crossing. A big challenge at working at the LHC, and a challenge we meet. Triggers are closely monitored to pick out the most interesting results.

14:06: Understanding of the search is "well advanced". Fabiola expresses thanks to the LHC team. Data taking efficiency is 93.5%. Good quality data fraction is greater than 90% for all analyses.

14:05: First two slides. Fabiola explains the importance and difficult of Higgs searches. The first slide shows plots from several different analyses with data and Monte Carlo simulation.

14:01: Rolf introducing Fabiola and Guido. Huge round of applause for all the experts and LHC team. Building up the moment with a great sense of community. In spite of the competition between ATLAS and CMS, we're here together to present and see the results together.

Before the talks

13:59: One minute to go. Both talks and both speakers ready.

13:54: Just spotted Guido Tonelli, the Spokesperson for CMS and the second speaker today! Both he and Fabiola are looking smart, and ready to give us the facts.

13:45: Fabiola's talk has been copied and it is ready for her. She taking a sip of water and chatting with Rolf and technical support.

13:41: Experts from the LHC are here too. They have worked very hard to make sure the machine works for us, and we've had fantastic running this year. We must not forget their role in this work!

13:40: Fabiola Gianotti, the ATLAS Spokesperson is here. She will give the first talk in about 20 minute's time. She's chatting to the Director General of CERN, Rolf Heuer. She's smiling, but if I was in her position I'd be quite nervous right now! She's taking off her coat, looking at the microphone and so on. She's given many talks before, so she knows what's she's doing. Still, it must be nerve-wracking for her!

13:32: The webcast is now available! http://webcast.web.cern.ch/webcast/

13:30: A “delegation” of very smartly dressed people are arriving in the front rows! They’re more smartly dressed than most physicists, and they have reserved seats, so they are probably management, dignitaries etc.

13:27: Of course big names from CMS are here as well, including Albert de Roeck, Jim Virdee, and Gigi Rolandi! (Being an ATLAS member, it’s easier to recognize other ATLAS members!)

13:22: ATLAS Higgs Group Conveners, including Bill Murray and Eilam Gross, arriving now. These are the people in charge of the various Higgs searches at ATLAS. Some of the analysts are here as well. Lots of big names arriving.

13:14: Some thoughts about the media, science, and what we can expect to see today. The physicist sitting next to me asked about my blog and twitter feed, and we started discussing how pressure from the media can affect what scientists do. While it’s true that we love the media to be informed, we don’t change our results or interpretation based on public opinion. The results we see today are going to be exciting, but we need to be careful and do a proper job. If it’s not 5σ yet, it’s not a discovery yet. We’ve been searching for the Higgs boson for decades, so we want to get it right and we don’t want to sacrifice our standards for the sake of getting in a few months early. If we see the Higgs in the summer (and we probably will, if the rumors are to be believed) it will be the most important discovery in high energy physics since the W was discovered in 1983!

(If we don’t see the Higgs, that will also be an important observation, as it will tell us there is something else out there . Convincing bumps have been known to disappear when we add more data.)

12:53: The projector is being tested. As soon as the webcast is available I’ll update to let you know.

12:51: Katie snapped a photo of me and Pauline! It’s on yfrog.

12:28: I’ve just heard that the security staff at the door are no longer allowing more people to enter.

12:27: This seminar is one that should not be missed. Looking through the audience I’m glad to see most of my closest colleagues have found somewhere to sit, including @marktibbetts, Matt, Tina, Rozmin, Catrin and Sudan. We are normal physicists, fighting for whatever seats we can get. The first three rows have been reserved for special guests, representatives etc. There are people sitting on all the stairs. We’ve all heard the rumors. None of us has heard the official results form both experiments yet.

11:57: There are lot of people from the blogging community here, including Pauline Gagnon (to my right), Anna Phan (to my left) and Seth Zenz (in front of me)! You can follow Seth’s tweets, @sethzenz

11:42: Seating is at a premium, I just someone bring their own chair in!

11:31: I’ve been here for an hour now, and the auditorium is nearly full. There are roughly 10 seats left (none of them have desks, or power supplies.) Nearly everyone here has their laptop with them, it’s like a commercial for Apple! Staff are checking IDs at the door. There is a lot of chatter here.

Details about the seminar

Today sees CERN’s seminar on the update of the Higgs search. I’ll be updating this page as the information comes in. Refresh this page to get the updates! The most important points will be also be tweeted.

Time

The seminar will begin at 14:00 CERN time. (08:00 East Coast, 05:00 Pacific)

There will be a talk from the ATLAS spokesperson 14:00-14:40 giving updates on the ATLAS search, and then a talk from the CMS spokesperson 14:40-15:20. There will then be questions and answers to both speakers until 16:00.

Links

Here are the links to:
Seminar page

Webcast video

Follow the updates on twitter using the hashtag #higgsliveblog (my account there is @aidanatcern.)

The seminar page also has a chat room!

Shortlink for this page: http://bit.ly/u0wALv

Shortlink for the seminar page: http://bit.ly/s5X4Zm

Shortlink for the webcast: http://bit.ly/q2QB

Connectivity

We’re expecting a lot of internet traffic at CERN today, so there is a small possibility the network may get jammed for a few seconds from time to time. Thank you to Kevin for allowing caching of this page so that it can still be accessible in case of any problems.

Please report any errors in the transcript of this blog post in the comments.

Aidan Randle-Conde

Tags: CERN, Higgs, liveblog, seminar
Posted in Latest Posts | 19 Comments »

How Difficult is it to Find the Higgs?

Monday, December 12th, 2011

Richard Ruiz

Really difficult, and I mean really, really difficult. It is such an arduous job that even after 30 years worth of searching, by literally tens of thousands of physicists, it has yet to be found. However, that may all change Tuesday when spokespeople for the ATLAS and CMS experiments, the Large Hadron Collider‘s two general-purpose detector experiments, unveil the long-awaited results of their independent searches for the higgs boson.

Now, what makes Tuesday’s announcement so different is that it will be the first time any higgs analysis will be publicly shown using 5.5 inverse femtobarns (fb^-1), or a data set worth over 380 trillion proton collisions. To explain why 5.5 fb^-1 is so special requires us to go back in time to late August, when this graph started making the rounds at conferences and summer schools:

Essentially, this graph tabulates how much data is needed for ATLAS and CMS to be sensitive to discovering the higgs boson. According to these numbers, with 5 fb^-1 worth of data, ATLAS & CMS can either jointly rule out the existence of higgs boson as predicted by the Standard Model of Physics, or with equal excitement, claim evidence of its existence. Now I need to mention two important caveats: (1) this table assumes (1) benchmark parameters which are entirely worthless if there is any type of new physics (which is pretty likely, IMO); and (2) the numbers also assume that ATLAS and CMS combine their data sets. This last point is important because this is not the case tomorrow.

What will be seen live, from this link, are two 30-minute presentations by a spokesperson from each collaboration unveiling and announcing whatever conclusions that can justifiably be made considering the amount of data presently available. After that, there will be a 1 hour Q & A session with two spokespeople. My colleagues here at QD will definitely be live-blogging the event! I, on the other hand, will be teaching my undergraduates the importance of thermodynamics……

In summary, I am expecting three possible outcomes on Tuesday (Disclaimer! I am not a part of any experiment and currently am in Wisconsin, not CERN):

The higgs boson is discovered and we all dance around in merriment while enjoying waterfalls of champagne. Twitter is credited with breaking the news. Wagers between physicists are also paid off.
The higgs boson, as predicted by the Standard Model, is definitively ruled out. This, of course, would be a terrible disappointment. However, the higgs boson is a very wonderfully rich piece of physics; if one of the slickest things in all of physics does not exist… I cannot even fathom what does. (See this post!)
The higgs boson is not “discovered” but it is definitely not ruled out; there remains a mass window in which the higgs boson may still lie; and an elephant-shaped couch appears in the room near 120 GeV. This is still pretty satisfying because it gives us an idea what to expect from a fully combined analysis. Personally, I think this is the most likely outcome.

In light of results from last month using half the data (below), Tuesday will be very interesting.

The Proverbial Needle in the Proverbial Haystack

Now that I built up the anticipation, here are some numbers I calculated to give an idea why discovering the higgs boson is such an incredible scientific feat. (Technical details as to how I generated these numbers can be found at the very bottom of this post.)

Okay, so suppose the higgs boson, as predicted by the Standard Model, were to exist. If we were to produce one at the LHC, then we would expect it to decay into something more familiar like photons or b-quarks. We physicists call the probability of this happening a “cross section,” and it is measured in barns.

As a concrete example, let us take a look at the first process where two protons (pp) collide and produce a higgs boson (h), which in turn decays into a b-quark and an anti-b-quark. The cross section (probability) is 16,320 femtobarns, or 0.00000000001632 barns. All you need to know is that 0.00000000001632 barns is a very small number and hence pp->h->bb is a very rare thing to happen. In 70 trillion proton-proton collisions (or 1 inverse femtobarn), our theory predicts we will have produced 16,320 higgs bosons. In 5.5 inverse femtobarns (or 380 trillion proton-proton collisions), our theory predicts we will have generated

16,320 fb x 5.5 fb^-1 = 89,760 pp-> higgs -> bb Events.

89,000 higgs boson events may seem like a lot, but just wait until the next table. Here are some common ways a higgs is expected to decay and how many higgs events we expect to have produced this year. That is 102, 756 higgses in all!

Here is where things become absolutely unbearable. Let’s pretend now that the higgs boson does not exist. So ignoring the contribution from higgs bosons, we may calculate how many of these higgs-like events we expect to see. For example, let’s consider pp -> γγ (2 photons) and pp -> gg (2 gluons), then out of 380 trillion proton-proton collisions (5.5 fb^-1) the Standard Model predicts almost 3 trillion gluon pairs and over 800,000 photon pairs. Trying to find the higgs with b-quarks requires us to sift through 2.6 trillion bb pairs in order to find almost 90,000 higgs -> bb events.

In other words, experimentalists are trying to find an excess of 0.0000034% more bb quarks than the Standard Model predicts, or 0.3% more ZZ events than the Standard Model predicts. Fortunately, it only means looking for an extra 0.014% photon pairs in 380 trillion protons-proton collisions.

So yeah, the higgs boson… it’s hard to find. Personally, I think finding a needle in a haystack would be easier.

At any rate, congratulations to all those who helped with the effort. I am just giddy with anticipation regarding tomorrow’s seminar, though that might also be my body telling me to go to sleep.

Happy Colliding!

- richard (@bravelittlemuon)

* Technical note: I calculated the higgs boson cross sections with MadGraph5 using the Higgs Effective Field Theory v4 model. To calculate the Standard Model background cross sections, I used MadGraph5 Standard Model v4. mh = 120 GeV. Additionally, I resorted to using the default parameter card for MadGraph4. Each calculation used 25, 000 proton-proton events at 7 TeV center of mass. Only basic (read: default) kinematic and fiducial cuts have been applied. Uncertainty was ignored for clarity. This ignores all acceptance cuts.

Tags: 5.5 fb-1, @bravelittlemuon, ATLAS, CMS, cross section, Higgs, LHC
Posted in Latest Posts | 4 Comments »

CERN Higgs seminar liveblog!

Wednesday, December 7th, 2011

Aidan Randle-Conde

Follow the liveblog here!

On Tuesday December 13th, there will be a seminar at CERN about the search for the Higgs boson using the 2011 dataset.

Physicists at ATLAS and CMS have been working very hard all year (and are still working) to show the results for 5fb^-1 each. This means that we will have 5 times the amount of data available since the last update, and we can expect the exclusion of the Higgs to be even more impressive than what we saw in the summer.

Watch the video on youtube!

Since this an important milestone in the search for the Higgs boson, I will be liveblogging the event, from the main auditorium here at CERN. There will be a webcast available for those of us not at CERN. (The webcast details will appear on the seminar page on the day of the seminar.) So please join me on Tuesday, watch the webcast and follow the liveblog for minute by minute updates of the search for the Higgs boson.

If you want to know more about the Higgs boson I’d recommend you look at Flip’s recent post.

Check out the link to the Seminar page.

Follow the updates with the Twitter hashtag #higgsliveblog.

Tags: ATLAS, CERN, CMS, Higgs, LHC
Posted in Latest Posts | 14 Comments »

Joining forces in the search for the Higgs

Monday, November 21st, 2011

Brookhaven

This post, originally published on 11/18/11 here, was written by Kétévi Adiklè Assamagan, a staff physicist at Brookhaven National Laboratory and the ATLAS contact person for the ATLAS-CMS combined Higgs analysis.

Today we witnessed a landmark LHC first: At the HCP conference in Paris, friendly rivals, the ATLAS and CMS collaborations, came together to present a joint result! This ATLAS-CMS combined Higgs search was motivated by the fact that pooling the dataset increases our chances of excluding or finding the Higgs boson over those of a single experiment. This is the first example of this kind of scientific collaboration at the LHC, and the success of the whole endeavor hinged on a whole host of thorny issues being tackled…

Discussions about combining our Higgs search results with CMS’s first started over a year ago, but before we could proceed with any kind of combined analysis, we had first to jointly outline how on earth we were going to go about doing it. This was no small undertaking; although we’re looking for the same physics, the ATLAS and CMS detectors are very different beasts materially, and use completely independent software to define and identify particles. How can we be certain that what passes for an electron in ATLAS would also be picked out as such in CMS? Not only that – the Higgs working groups from ATLAS and CMS are made up of several hundred people apiece, making the challenge of combining results not only a technical one but also a sociological one.

Experimental Higgs boson exclusion limits for combined ATLAS and CMS data. *Full explanation of this plot underneath the blog!

From the start of the year, experts from both experiments started meeting regularly to try to converge on the combination procedure. First up, crucially, we had to ensure that we were both using consistent theoretical estimates of the rate we expected the Higgs to be produced (its ‘production cross section’) and of the relative probabilities of it decaying to each of the various signature collections of particles we use to spot it (so-called ‘branching ratios’). In anticipation of this, the pre-existing LHC Higgs cross-section group, including members of ATLAS, CMS, and the theory community, had already put a huge amount of work into providing common tools to compute Higgs cross-sections, decay branching ratios, and their uncertainties. With them, we also discussed ways to separate genuine Higgs signals from the sea of similar-looking background processes.

Defining the systematic uncertainties – those that affect our theoretical computations or experimental measurements, often due to our limited understanding of- and ability to model the proton at the minutest level, the complexity of the computations, and/or the precision of our measurements – and correlating them between the two experiments was another important thing to tackle early on.

Of course, we had to agree on common ways to handle every part of the analysis – such as how to set confidence limits and quantify any excesses – but we also had to convince ourselves that we were implementing and interpreting our agreed procedure in exactly the same way. To achieve this validation, each experiment began working in individual private areas of a shared information-exchange platform known as the WorkSpace. Both prepared their data by building individual WorkSpaces, which were then shared with the other group. Each then built their own version of the combined WorkSpaces, and statistical calculations were performed on them. The two groups then met to compare results and, in all cases, they were in excellent agreement, giving us confidence to finally go ahead and prepare the main physics results and submit them to the collaborations for review and approval.

Almost ten months in the making, this first ATLAS and CMS combined Higgs result was presented publically today in Paris. Together we can say that no evidence of the Standard Model Higgs boson has yet been found. The results exclude the Standard Model Higgs boson in the mass range of 141-476 GeV at the 95 percent confidence level. The ATLAS and CMS collaborations have now each collected more than double the data used for the current combined results, meaning that the search for the Higgs will only intensify from here on.

* The plot shows experimental limits from the LHC on Standard Model Higgs production in the mass range 100-600 GeV. The solid curve reflects the observed experimental limits for the production of Higgs of each possible mass value (horizontal axis). The region for which the solid curve dips below the horizontal line at the value of 1 is excluded with a 95% confidence level (CL). The dashed curve shows the expected limit in the absence of the Higgs boson, based on simulations. The green and yellow bands correspond (respectively) to 68%, and 95% confidence level regions from the expected limits. The hatched regions show the exclusions from the searches at the different colliders. See here for deeper explanation of how to interpret Higgs boson exclusion plots.

Tags: ATLAS, Brookhaven National Laboratory, CERN, CMS, Higgs, LHC, physics
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Have we Found the Higgs Yet?

Monday, November 21st, 2011

Seth Zenz

Along with a bunch of important people who actually know how to give interviews, I answer that question in this video:

The video goes along with this Nature News article. You may also be interested in the recent combined ATLAS and CMS Higgs result, which uses only the first half of this year’s data.

By the way, when I talk about a “minimal Higgs, that only does the part we know that something like the Higgs has to do,” I’m referring to the so-called fermiphobic Higgs. It plays the usual role of the Standard Model Higgs boson in breaking electroweak symmetry, but doesn’t couple to quarks and leptons (i.e. fermions). We already know from the way the weak and electromagnetic forces work that the relationship between them has its origins in something like the Higgs — but we have less reason to be certain that the same particle takes care of quark and lepton masses too. This version of the Higgs boson is more difficult to find, but perfectly sensible, and we’ll probably hear a lot more about it in coming years if we don’t have a big discovery this year or next.

Tags: Higgs, publicity
Posted in Latest Posts | 5 Comments »

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