Quantum Diaries

Posts Tagged ‘ATLAS’

Location, Location, Location

Thursday, January 19th, 2012

Seth Zenz

If I had to pick one thing that’s definitely better on my old experiment, ATLAS, than on my new experiment, CMS — and especially if I had to pick something I could write publicly without getting into trouble — it would be this: the ATLAS detector is across the street from the rest of CERN. I’m not sure how that was decided, but once you know that, you know where CMS has to be: on the other side of the ring, 5 or 6 miles away. That’s because the detectors have the same goals and need the same beam conditions; two opposite points on the LHC are where a duplicate performance is easiest. The pre-existing caverns from the LEP collider, whose tunnel the LHC now uses, probably also helped determine where the detectors are.

In any case, it used to be that when I wanted to work on my detector, I had only to go across the street. Now I have to drive out of Switzerland and several miles into France. Except, I don’t like driving. So I’ve been working on alternate means of transportation. A few months ago I walked. Last night I had to go to downtown Geneva, so I took the bus. It’s actually pretty good, although the bus stop is a mile away from CMS. There’s also the shift shuttle, which runs from the main CERN site to CMS every 8 hours via a rather roundabout route. And I can bike, once the weather gets better and I get myself a little more road-worthy. To be honest, every option for getting here is much slower than driving, but I enjoy figuring out ways to get places enough that I’m going to keep trying for a while.

I have plenty of chances to try, because I’ll be here in the CMS control room a lot of the time over the next few weeks. Right now, I’m learning and helping with the pixel detector calibration effort. (We’re changing the operating temperature, so all the settings have to be checked.) Soon I’ll be learning to take on-call shifts. So the more I stay here, the more I learn. I got here this morning, and I won’t leave tonight until about 11 pm. I could take the shift shuttle back — or maybe I’ll just get a ride.

Tags: ATLAS, CMS, control room, LHC, pixel, shift, switching experiments, travel
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A new year, a new outlook

Saturday, December 31st, 2011

Aidan Randle-Conde

2011 has been a year of change and excitement. We’ve had plenty of good news and bad news to deal with. The new year doesn’t mean just another calendar on the wall, it means a new way of looking at physics. There’s no better way to bring in the new year than watching the fireworks in central London, surrounded by friends. There’s usually a fantastic display, because London is not only one of the most important cities in the world, but it’s also home of universal time. With the Greenwich Meridian running through the capital, we’re reminded of the role that timekeeping has played in the development our history and our science. But this year was even more special, since London is literally inviting the world to its streets this year for the Olympics. So I got caught up in the excitement of it all my thoughts turned to what we’ve seen in the world of physics, and where we’re going next.

New year fireworks in London (New York Times)

2011 got off to a start with ATLAS announcing a startling asymmetry in the jet momenta in heavy ion collisions. However, the joy was tainted by a leaked abstract from an internal document. That document never made it through internal review and should never have been made public. We were faced with several issues of confidentiality, ethics and biases, and how having several thousand people, all armed with the internet and with friends on competing experiments makes the work tough for all of us. In the end we followed the right course, subjected all the analyses to the rigors of internal and external review, and presented some wonderful papers.

There was more gossip over the CDF dijet anomaly presented at Blois. CDF saw a bump, and D0 didn’t. Before jumping to any conclusions it’s important to remember why we have two experiments at Tevatron in the first place! These kinds of double checks are exactly what we need and they represent the high standard of scientific research that we expect and demand. The big news for Tevatron was, of course, the end of running. We’re all sad that the shutdown had to happen and grateful for such a long, productive run, but lets look to the future in the intensity frontier.

Meanwhile both ATLAS and CMS closed in on the Higgs boson, excluding the vast majority of the allowed regions. The combinations and results just got better and better, until eventually on December 13th we saw the result of 5fb^-1 from each experiment. The world watched as the presentations were made and quite a few people were left feeling a little deflated. But that’s not the message we should take away. If the Higgs boson is there (and it probably is) then we’ll see by the end of the year. There’s no more of saying “Probably within a year, if we’re lucky”, or “Let’s not get ahead of ourselves”. This time we can be confident that this time next year we’ll have uncovered every reasonable stone. The strategies will change and we narrow the search. We may have new energies to explore, and we’ll tweak our analyses to get more discriminating power from the data. Now is the time to get excited! The game has changed and the end is definitely in sight.

Raise a glass as we say farewell to a great year of physics, and welcome another

It’s been a good year for heavy flavor physics as well. LHCb has gone from strength to strength, probing deeper and deeper into the data. We’ve seen the first new particle at the LHC, a state of bottomonium. Precision measurements of heavy flavor physics give some of the most sensitive tests of new physics models, and it’s easy to forget the vital role they play in discover.

ALICE has been busy exploring different questions about our origins, and they’ve studied the quark gluon plasma in great detail. The findings have told us that the plasma acts like a fluid, while showing unexpected suppression of excited bottomonium states. With even more data from 2011 being crunched we can expect even more from ALICE in 2012.

The result that came completely out of left field was the faster than light neutrinos from OPERA. After seeing neutrinos break the cosmic speed limit, OPERA repeated the measurements with finer proton bursts and got the same result. Something interesting is definitely happening with that result. Either it’s a subtle mistake that has eluded all the OPERA physicists and their colleagues across the world, or our worldview is about to be overturned. I don’t think we’ll get the answers in the immediate future, so let’s keep an eye out for results from MINOS and OPERA.

Finally it’s been an incredible year for public involvement. It’s been a pleasure to have such a responsive audience and to see how many people all across the world have been watching CERN and the LHC. A couple of years ago I would not have thought that the LHC and Higgs boson would get so much attention, and it’s been a of huge benefit to everyone. The discoveries we share with the world are not only captivating us all, they’re also inspiring the next generation of physicists. We need a constant supply of fresh ideas and new students to keep the cutting edge research going. If we can reach out to teenagers in schools and inspire some of them to choose careers in science then we’ll continue to answer the most fascinating, far reaching and beautiful questions about our origins.

So when you a raise a glass to the new year, don’t forget that we’ve had an incredible 2011 for physics, and that 2012 is going to deliver even more. We don’t even know what’s out there, but it’s going to be amazing. To physics!

Tags: ALICE, ATLAS, CERN, CMS, Intensity Frontier, LHC, LHCb, Tevatron
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Science with ties to Fermilab top year-end ‘best of’ lists

Wednesday, December 28th, 2011

Fermilab

Christmas time brings not only presents and pretty cookies but an outpouring of media lists proffering the best science stories of the year and predicting those that will top the list in 2012.

While the lists varied wildly everyone seemed excited by a few of the same things: upsetting Einstein’s theory of special relativity, a hint of the ‘god particle’ and finding planets like our own.

Several of the stories that made nearly every media outlet’s list, though in various rankings, have a connection, directly or indirectly, to Fermilab. Here’s a sampling with the rankings from the publications.

Discover magazine had the largest list, picking the top 100 science stories.

1: A claim by researchers at the OPERA experiment at CERN that they had measured neutrinos traveling faster than the speed of light, something disallowed by Einstein’s Theory of Special Relativity. Now the scientific community is looking for another experiment to cross-check OPERA’s claim.

That brought renewed interest to a 2007 measurement by the MINOS experiment based at Fermilab that found neutrinos skirting the cosmic speed limit, but only slightly. The MINOS collaboration always planned to study this further when it upgrades its detector in early 2012 but the OPERA result added new urgency.

Look in 2012 for MINOS to update the time of flight of neutrinos debate in three stages. First, MINOS is analyzing the data collected since its 2007 result to look for this phenomena. Results should be ready in early 2012. This likely will improve the MINOS precision in this area by a factor of three from its 2007 result. Second, MINOS is in the process of upgrading its timing system within the next few months using a system of atomic clocks to detect when the neutrinos arrive at the detector. The atomic clock system will progressively improve resolution, which is needed to make the MINOS analysis comparable to the OPERA result and improve precision from the 2007 MINOS result by as much as a factor of 10. That will tell us if OPERA was on the right track or not, but may not be the definitive answer. That answer will come with the upgrades to the MINOS experiment and a more powerful neutrino beam, producing a larger quantity of neutrino events to study. The upgraded MINOS experiment will be in many ways a more precise system than OPERA’s and could produce a result comparable with OPERA’s precision likely by January 2014.

4: Kepler’s search for Earth-like planets that could sustain life produces a bounty of cosmic surprises, fueled, in part, by the computing skills of a Fermilab astrophysicist.
32: The on-again, off-again rumor of finding the Higgs boson particle. Physicists working with experiments at Fermilab’s Tevatron experiments and CERN’s Large Hadron Collider expect to answer the question of whether a Standard Model version of the Higgs exists in 2012.
65: The shutdown of the Tevatron at Fermilab after 28 years and numerous scientific and technological achievements.
82: Fermilab physicist Jason Steffen’s frustration with slow airplane boarding drives him to figure out a formula to speed up the aisle crawl.

Nature’s year in review didn’t rank stories but started off by mentioning the Tevatron’s shutdown after 28 years and following up shortly with the puzzling particle news of potentially FTL neutrinos and a Higgs sighting.

For science — as for politics and economics — 2011 was a year of upheaval, the effects of which will reverberate for decades. The United States lost three venerable symbols of its scientific might: the space-shuttle programme, the Tevatron particle collider and blockbuster profits from the world’s best-selling drug all came to an end.

Cosmos magazine rankings:

The MINOS far detector in the Soudan Mine in Minnesota. Credit: Fermilab

1: Kepler’s exoplanet findings
2: FTL neutrinos
3: Higgs

Scientific American‘s choices:

3: FTL neutrinos
5: Higgs

ABC News asked science radio and TV host physicist Michio Kaku for his top 10 picks. They include:

3: Hint of Higgs
5: Kepler’s exoplanet findings
10: Nobel Prize for the discovery that the expansion of the universe is accelerating, which laid the groundwork for the today’s search for dark energy. Fermilab has several connections to to this work. The latest tool in dark energy survey experiments, the Dark Energy Camera, was constructed at Fermilab in 2011. One of the three prize winners, Saul Perlmutter, is a member of the group that will use the camera, the Dark Energy Survey collaboration. Adam Riess, another of the winners, is a member of the SDSS-II experiment, a predecessor to DES that Fermilab was key in building and later operating its computing system.

Live Science

5: FTL neutrinos
4: Kepler’s exoplanet findings
2: Higgs

If the Higgs boson’s mass is high, it is expected to decay predominantly into two W bosons. Plushies images from the Particle Zoo.

To make the Ars Technica list stories had to be awe inspiring in 2011 AND have a chance of making the 2012 list as well.

1: FTL neutrinos
2: Kepler’s exoplanet findings
6: Higgs hunt

Science magazine chose the best scientific breakthrough of the year. Kepler’s exoplanet hunt made it into the runner up list.

Tell us who you agree with or, better, yet give us your own top 10 science stories of the year.

— Tona Kunz

Tags: ATLAS, CDF, CMS, Cosmic Frontier, DZero, Energy Frontier, Higgs boson, Intensity Frontier, MINOS, neutrinos
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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
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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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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
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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
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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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The best and worst moment on shift…

Tuesday, October 25th, 2011

Aidan Randle-Conde

It’s that moment when you realize something serious and exciting has happened, but it’s 5:45am and you have to wake somebody up to sort it out. As the LHC ramps up it’s my role to make sure that the trigger is ready. This means looking at the bunch structure in the LHC and checking that ATLAS knows what this structure looks like. It’s as simple as pressing a few buttons and updating a database, and if everything goes smoothly we have nothing to worry about.

This time it was a bit different, because the LHC used a bunch structure they had never used before. When I pressed the button I was actually telling ATLAS something new and witnessing one of those rare transitions in the normal running of the LHC! (Jim’s post gives a great explanation about what bunch structures are and how the LHC team design them.) Then I checked the instructions, and they told me I had to wake someone up and tell them about the change. Nobody likes to be woken up at 5:45am, especially if they have an important meeting the next day. To make matters worse, I know the guy on the other end of the line (although since he’s so sleepy I didn’t recognize his voice at first!) At that point I remembered what my flat mate had told me when he was on call and got woken up at night. He said “What we do would be easy if they just gave us two minutes to think about it. We need time to wake up!” So, feeling bad about waking up the expert I told him I’d call back in 5 minutes. There was a flurry of messages on the electronic logbook and short conversations in the Control Room, and then it was time to call again. This time the voice on the other end of the line was more alert and a bit happier! He said everything was fine. I could proceed as normal and as long as there are no serious problems we can take data as we usually do.

We have beams!

The LHC just declared stable beams. Now the fun begins…

Tags: ATLAS, control room, shift
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