Quantum Diaries

Posts Tagged ‘CERN’

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
Posted in Latest Posts | 1 Comment »

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 »

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 »

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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In the shadow of Shiva

Thursday, November 10th, 2011

Aidan Randle-Conde

In front of one of CERN’s most imposing and industrious buildings stands a statue of Dancing Shiva. During the day it’s a beautiful reminder of the friendship between CERN and India, celebrating the cosmic ballet that surrounds us all. By night it casts an ominous shadow over building 40, where discoveries are made, decisions are taken, results are shared and gossip is spread. But what does Shiva represent to us?

Working in the shadow of Shiva

The Dancing Shiva represents the changes in the universe around us, as matter and energy constantly bump into each other, create and destroy systems and keep renewing the world. I suppose we can attach any meaning we like to this, the constant chatter of culture, the renewal of our population as people die and children and born, the violent cosmological events that keep reorganizing the universe. Any and all of these interpretations are beautiful, powerful and majestic, but for me there is one interpretation which excites me more than any other and holds a very deep truth in it. This cosmic dance is the interaction of matter and antimatter.

Whenever we create new particles we create them in matter-antimatter pairs. They are literally equal and opposite components that make up everything we see. When they meet, they destroy each other in a burst of energy. If that was all there was to matter and antimatter, it would make a rather beautiful cosmic ballet, but not an interesting one. The fascinating part of the story is when we remember that we have more matter than antimatter, which means that this particular cosmic ballet is unbalanced, and the statue is a constant reminder of this fact.

The universe knows something we don't. And it acts on cosmic scales.

We don’t know why nature prefers to matter to antimatter, and until we know why we can’t really claim to understand how the universe works. We know how one mechanism has a preference (the weak force interacting with quarks) but this is much too small to explain the whole story. Whenever collide protons together at the LHC we have to live with the fact that we’re colliding matter with more matter in a detector made of matter. The particles that escape are not quite half matter half antimatter, as we might like. After a while, all the particles (except the neutrinos) slow down, decay and hit some rock. They join the rest of the stuff around them and either annihilate or get comfortable and settle in with their surroundings. All we’re really doing is moving matter around in a very complicated way; nature balances the books and every piece of antimatter we created (except the antineutrinos) gets removed from this small part of the universe. The cosmic dance continues, and if we’re lucky we get a small glimpse into how it really works. On the tiny, insignificant scales we work on we don’t see much of an imbalance at all. When we look up to the stars we see matter everywhere we look, across vast distances and far back in time.

Nature’s balance sheet has a few implications for our physics. For example, every time we produce a Higgs boson, we also produce a lot of noise in the detectors as well. In a matter-antimatter collider (such as LEP or Tevatron) this is less of a problem, since the Higgs boson is neither matter nor antimatter, it’s equal amounts of both. To create a Higgs boson we would need to create at least one antiparticle, and that takes a lot of energy. With this extra particle we get a lot more particles “for free”, leading to all kinds of noise!

So in the light of day, when CERN is teeming with life Shiva seems playful, reminding us that the universe is constantly shaking things up, remaking itself and is never static. But by night, when we have more time to contemplate the deeper questions Shiva literally casts a long shadow over our work, a bit like the shadows on Plato’s cave. Shiva reminds me that we still don’t know the answer to one of the biggest questions presented by the universe, and that every time we collide the beams we must take the cosmic balance sheet into account.

It’s rare that we get a symbol that inspires both clarity and beauty. It’s almost poetic. Why does Shiva prefer to destroy antimatter more than matter? The more data we gather the better chance we have of finding the answer to that question. I don’t think we’ll ever stop wondering about this question. It’s the reason there’s something instead of something and antisomething. It’s the reason atoms exist and stars can form. And yet the answer is still out of our grasp.

Tags: CERN, science
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Ever Wonder Why The Tevatron is Such a Big Deal?

Monday, September 19th, 2011

Richard Ruiz

Really though, have you? To date, it has not discovered the higgs boson, or Supersymmetry, or any kind of new physics. In fact, all the Tevatron has done since 1987 was find Standard Model physics. Though, that is my point.

Fig. 1: Aerial view Fermilab‘s Tevatron Accelerator Complex. These images were taken around that big pool of water, in the center of the Tevatron Ring. (Photo: Symmetry Mag.)

The Tevatron, for the past 24 years, has done everything to prove that the idiotic, nonsensical, and just plain weird idea that all of matter is composed of quarks & leptons (plus some bosons) is actually correct. Of course CERN’s Large Electron Positron is due its respect for confirming the Standard Model first through precision measurements, however, the Tevatron set the thing in stone. Over the past decades, many, many clever physicists have tried to modify the Standard Model by introducing new particles, new interactions, new particles & new interactions, but one-by-one they have been shot down. In my opinion, the Tevatron will always be known as The Standard Model Factory.

The Tevatron: Past & Present

My history with the Tevatron dates back to the summer of 2007, when I was a physics undergraduate who was hired by my then advisor to do some summer research. Since then, I have spent quite a bit of time at Fermilab and have been present for quite a few events. So like many other physicists, I am saddened by the fact that the collider will be shutting down September 30th (next Friday!). Consequentially, I decided to put together a little grocery list of Tevatron discoveries. In full disclosure: below is really just summary of all of Fermilab’s press releases since 1995, which in its own right borderlines on being an encyclopedia of particle physics.

• February 1995 – Discovery of the top quark. Not exactly sure where to begin with this one. I mean, the top quarks existence is evidence of several things: (1) the quark structure of matter; (2) the universality of the Weak force, meaning all quarks & leptons have partners under Weak nuclear charge, e.g., up & down, charm & strange, top & bottom; (3) and also provides a tidy way of explaining some of the differences between matter & antimatter in something called the CKM matrix. The then head of the Dept. of Energy had this to say about the top quark, “This discovery serves as a powerful validation of federal support for science.” Below is the top quark, as imagined by the Particle Zoo.
• March 1999 – Direct measurement that matter and antimatter behave differently (CP violation). The Kaons at the Tevatron (KTeV) experiment diverted protons from the Tevatron accelerator to produce a well-known particle called a Kaon, in order to measure its lifetime. The significantly larger-than-expected measurement of CP violation implied (1) CP violation was not negligibly small and (2) all particle theories have to accommodate this fact. An attractive and popular theory at the time, called the Superweak Theory, nicely explained a number of different phenomena but implied zero CP violation. You can guess why no one talks about that theory today.
• March 2001 – Tevatron Run II begins. From this day on, the Tevatron began colliding protons & antiprotons at an impressive 1.96 TeV. It took the remainder of the decade for that record to be topped.
• November 2001 – The Neutrinos at the Tevatron (NuTeV) Experiment discover a worrisome discrepancy between theoretical predictions and experiment measurements of the quantity sin²θ_W, which can be thought of as the ratio between the mass of the W boson and the mass of the Z boson. The NuTeV Experiment, like KTeV, diverted Tevatron protons to produce a different particle. In this case, neutrinos were produced and then were sequentially fired into 700 tons of steel. This anomaly had less than a 0.25% chance of being a random, statistical fluctuation (~3σ), and is now believed to related to the superstructure of protons & neutrons in a nucleus.
• June 2004 – Tevatron results set the first “modern” constraints on the higgs boson. Thanks to the top quark, the DZero Experiment was able to set a best estimate of the higgs boson’s mass (117 GeV/c²) and a definite upper bound (251 GeV/c²). Of course these numbers exclude new physics, but so began Today’s hunt for the higgs boson.
• April 2005 – Tevatron analyses go global. In order to cope with the huge amount of data being generated, the Tevatron detector experiments decide to connect their networks to The Grid, a global network of computers with the sole purpose of acting like one, giant computer, not unlike Deep Thought or planet Earth. This computing model is the heart and soul of the way CERN processes the LHC’s 15 petabytes a year.
• September 2006 – Oscillations in the recently famed B_s meson are discovered! A B_s (pronounced: B-sub-s) is a bound state that occurs when a b-quark and a s-quark begin to orbit around each other, like an electron and a proton in a hydrogen atom. The “oscillations” refer to how often the two quarks exchange a W boson. This high precision measurement is considered a benchmark tests of the Standard Model due to its sensitivity to new physics. They B_s mixing Feynman diagram is below (pulled from the QD image library).
• October 2006 – The “Period Table of Particles” is fleshed out. Just like how the theory of electrons, neutrons, & protons implies the existence of the period table of elements, the theory of quarks implies the existence of a gigantic number of combinations. This is the point of no return: The Standard Model works. It may be incomplete, it may be missing attachments, but from here on out no one can say that it is wrong.
• December 2006 – The production of individual top quarks is identified. Okay, this needs a bit of explanation. Top quarks are heavy, like really heavy. We are talking over 40 times heavier than the second heaviest quark and well over 300,000 times heavier than the electron; it weighs as much as 180 hydrogen atoms. According to the Standard Model, it is actually easier to produce a top quark and anti-top quark at the same time than individually. This is because individual top quark production involves the Weak nuclear force and just shrinks the chances of producing one. Like B_s, single top quark production is a Standard Model benchmark because it is very sensitive to new physics. Interestingly enough, single top quark production also provides a mean for testing Supersymmetry, Technicolor, and different higgs boson models.
• July 2008 – Diboson production is at long last discovered. The Standard Model predicts that it is possible to produce two Z bosons, simultaneously, from collisions. It is a very rare thing to see and most every addition to the Standard Model affects the rate two Z bosons are produced. There are plenty of ways to modify the oscillation rate of B_s or the rate of single top quark production and still maintain consistency with the Standard Model; modifying diboson production rates is a whole different behemoth… good luck with that. I was actually at the talk when this was announced; I remember that week very well because it was rumored that the higgs boson had been found.
• August 2008 – “Tevatron Experiments Double-Team Higgs Boson.” The CDF & DZero Experiments combine their powers to call Captain Pla… I mean, for the first time, combine their independent higgs boson searches and begin directly excluding possible mass values for the boson. This juggernaut of an analysis (plot below) was quickly recognized for its level of sophistication and set expectations for the LHC experiments.
• May 2010 – The infamous dimuon asymmetry is discovered. Remember how in “September 2006″ I mentioned that B mesons, like Bs, are sensitive to new physics? Well, B mesons can decay into two muons or two anti-muons, plus some other things. When the number of muon pairs & anti-muon pairs were measured, it was discovered that more muon pairs were produced than anti-muon pairs. The LHC experiments still need more data to be as sensitive to confirm this high precision measurement but this might actually be the first detection of physics beyond the Standard Model at a collider. If a reader knows of an earlier collider experiment signal that hints at Beyond the Standard Model physics, I am happy to pass the title on to that.
• August 2011 – The Tevatron’s updates its higgs boson mass exclusion with over 8 fb^-1. (Below)

The Standard Model Factory

You know, when I started writing this post I had an idea how impressive the Tevatron is/was. Having systematically gone through each of Fermilab’s press releases in search for major milestones, and trust me I omitted a fair number, I do not really know what else to say. I am a bit star-struck. Yes, the Tevatron has been running since 1987 and I happily acknowledge that it just simply cannot compete with the LHC beyond 2012 projections. Just recently, the LHC reached the 3 fb^-1 threshold, which translates to generating 1/3^rd of the entire Tevatron data set in about 9 months; but really Really, the LHC has some pretty big shoes to fill.

Congratulations to the Tevatron Experiments, past & present, for undeniably establishing the Standard Model of Particle Physics.

More importantly, congratulations to the Tevatron Accelerator Division, for having repeatedly done the impossible because you could.

Happy Colliding.
- richard (@bravelittlemuon)

http://en.wikipedia.org/wiki/Beyond_the_Standard_Model

Tags: CERN, Fermilab, FNAL, LHC, ppbar, Tevatron
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PreSUSY 2011 & Nature’s Little Secrets

Monday, August 29th, 2011

Richard Ruiz

Update: Section added to include LEP11 Results on Higgs Boson Exclusion (01 Sept 2011)

Expect bold claims at this week’s SUSY 2011 (#SUSY11 on Twitter, maybe) Conference at Fermilab, in Batavia, Illinois. No, I do not have any secret information about some analysis that undoubtedly proves Supersymmetry‘s existence; though, it would be pretty cool if such an analysis does exist. I say this because I came back from a short summer school/pre-conference that gave a very thorough introduction to the mathematical framework behind a theory that supposes that there exists a new and very powerful relationship between particles that make up matter, like electrons & quarks (fermions), and particles that mediate the forces in our universe, like photons & gluons (bosons). This theory is called “Supersymmetry”, or “SUSY” for short, and might explain many of the shortcomings of our current description of how Nature works.

At this summer school, appropriately called PreSUSY 2011, we were additionally shown the amount of data that the Large Hadron Collider is expected to collect before the end of this year and at the end of 2012. This is where the game changer appeared. Back in June 2011, CERN announced that it had collected 1 fb^-1 (1 inverse femtobarn) worth of data – the equivalent of 70,000 billion proton-proton collisions – a whole six months ahead of schedule. Yes, the Large Hadron Collider generated a year’s worth of data in half a year’s time. What is more impressive is that the ATLAS and CMS experiments may each end up collecting upwards of 5 fb^-1 before the end of this year, a benchmark number a large number of people said would be a “highly optimistic goal” for 2012. I cannot emphasize how crazy & surreal it is to be seriously discussing the possibility of having 10 fb^-1, or even 15 fb^-1, by the end of 2012.

Figure 1: Up-to-date record of the total number of protons collisions delivered to each of the Large Hadron Collider Detector Experiments. (Image: CERN)

What this means is that by the end of this year, not next year, we will definitely know whether or not the higgs boson, as predicted by the Standard Model, exists. It also means that by next year, experimentalists will be able to rule out the most basic versions of Supersymmetry which were already ruled out by previous, high-precision measurements of previously known (electroweak) physics. Were we to find Supersymmetry at the LHC now and not when the LHC is at designed specifications, which are expected to be reached in 2014, then many physicists would be at a loss trying to rectify why one set of measurements rule out SUSY but another set of measurements support its existence.

What we can expect this week, aside from the usual higgs boson and SUSY exclusion plots, are a set of updated predictions as to where we expect to be this time next year. Now that the LHC has given us more data than we had anticipated we can truly explore the unknown, so trust me when I say that the death of SUSY has been greatly exaggerated.

More on Higgs Boson Exclusion (Added 01 Sept 2011)

This morning a new BBC article came out on the possibility of the higgs being found by Christmas. So why not add some plots, shown at August’s Lepton-Photon 2011 Conference, that show this? These plots were taken from Vivek Sharma’s Higgs Searches at CMS talk.

If there is no Standard Model higgs boson, then the Compact Muon Solenoid Detector, one of the two general purpose LHC detectors, should be able to exclude the boson, singlehandedly, with a 95% Confidence Level. ATLAS, the second of the two general purpose detectors, is similarly capable of such an exclusion.

Figure A: The CMS Collaboration projected sensitivity to excluding the higgs boson with 5 fb^-1 at √s = 7 TeV; the black line gives combined (total) sensitivity.

Things get less clear if there is a higgs boson because physical & statistical fluctuations adds to our uncertainty. If CMS does collect 5 fb^-1 before the winter shutdown, then it is capable of claiming at least a 3σ (three-sigma) discovery for a higgs boson with a mass anywhere between m_H≈ 120 GeV/c² and m_H ≈ 550 GeV/c² . For a number of (statistical/systematic) reasons, the range might shrink or expand with 5 fb^-1 worth of data but only by a few GeV/c². In statistics, “σ” (sigma) is the Greek letter that represents a standard deviation; a “3σ result” implies that there is only a 0.3% chance of being a fluke. The threshold for discovery is set at 5σ, or a 0.000 06% of being a random fluke.

Figure B: The CMS Collaboration projected sensitivity to discovering the higgs boson with 1 (black), 2 (brown?), 5 (blue), and 10 (pink) fb-1 at √s = 7 TeV.

By itself, the CMS detector is no longer sensitive. By combing their results, however, a joint ATLAS-CMS combined analysis can do the full 3σ discovery and a 5σ job down to 128 GeV/c². The 114 GeV/c² benchmark that physicists like to throw around is lower bound on the higgs boson mass set by CERN’s LEP Collider, which shutdown in 2000 to make room for the LHC.

Figure C: The projected sensitivity of a joint ATLAS-CMS analysis for SM higgs exclusion & discovery for various benchmark data sets.

However, there are two caveat in all of this. The smaller one is that these results depend on another 2.5 fb^-1 being delivered by the upcoming winter shutdown; if there are any more major halts in data collection, then the mark will be missed. The second, and more serious, caveat is that this whole time I have been talking about the Standard Model higgs boson, which has a pretty rigid set of assumptions. If there is new physics, then all these discovery/exclusion bets are off.

Nature’s Little Secrets

On my way to PreSUSY, a good colleague of mine & I decided to stop by Fermilab to visit a friend and explore the little secret nooks that makes Fermilab, in my opinion, one of the most beautiful places in the world (keep in mind, I really love the Musée d’Orsay). What makes Fermilab such an gorgeous place is that is doubles as a federally sanctioned nature preserve! From bison to butterflies, the lab protects endangered or near-endangered habitats while simultaneously reaching back to the dawn of the Universe. Here is a little photographic tour of some of Nature’s best kept secrets. All the photos can be enlarged by clicking on them. Enjoy!

Figure 2: The main entrance to the Enrico Fermi National Accelerator Laboratory, U.S. Dept. of Energy Laboratory Designation: FNAL, nicknamed Fermilab. The three-way arch that does not connect evenly at the top is called Broken Symmetry and appropriately represents the a huge triumph of Theoretical (Solid State & High Energy) Physics: Spontaneous Symmetry Breaking. Wilson Hall, nicknamed “The High-Rise” can be see in the background. (Image: Mine).

Figure 3: Wilson Hall, named after FNAL’s first director and Manhattan Project Scientist Robert Wilson, is where half of Fermilab’s magic happens. Aside from housing all the theorists & being attached to the Tevatron Control Room, it also houses a second control room for the CMS Detector called the Remote Operations Center. Yes, the CMS Detector can be fully controlled from Fermilab. The photo was taken from the center of the Tevatron ring. (Image: Mine)

Figure 4: A wetlands preserve located at the center of the Tevatron accelerator ring. The preservation has been so successful at restoring local fish that people with an Illinois fishing license (See FAQ) are actually allowed to fish. From what I have been told, the fish are exceptionally delicious the closer you get to the Main Ring. I wonder if it has anything to do with all that background neutrino rad… never mind.
Disclaimer: The previous line was a joke; the radiation levels at Fermilab are well within safety limits! (Image: Mine)

Figure 5: The Feynman Computing Center (left) and BZero (right), a.k.a., The CDF Detector Collision Hall. The Computing Center, named after the late Prof. Richard Feynman, cannot be justly compared to any other data center, except with maybe CERN‘s computing center. Really, there is so much experimental computer research, custom built electronics, and such huge processing power that there are no benchmarks that allows for it to be compared. Places like Fermilab and CERN set the benchmarks. The Collider Detector at Fermilab, or CDF for short, is one of two general purpose detectors at Fermilab that collects and analyzes the decay products of proton & anti-proton collisions. Magic really does happen in that collision hall. (Image: Mine)

Figure 6: The DZero Detector Collision Hall (blue building, back), Tevatron Colling River (center) , and Collision Hall Access Road (foreground). Like CDF (Figure 5), DZero is one of two general-purpose detectors at Fermilab that collects and analyzes the decay products of proton & anti-proton collisions. There is no question that the Tevatron generates a lot of heat. It was determined long ago that by taking advantage of the area’s annual rainfall and temperature the operating costs of running the collider could be drastically cut by using naturally replenishable source of water to cool the collider. If there were ever a reason to invest in a renewable energy source, this would be it. The access road doubles as a running/biking track for employees and site visitors. If you run, one question that is often asked by other scientists is if you are a proton or anti-proton. The anti-protons travel clockwise in the Main Ring and hence you are called an anti-proton if you bike/run with the anti-protons; the protons travel counter-clockwise. FYI: I am an anti-proton. (Image: Mine)

Figure 7: The Barn (red barn, right) and American bison pen (fence, foreground). Fermilab was built on prairie land and so I find it every bit appropriate that the laboratory does all it can to preserve an important part of America’s history, i.e., forging the Great American Frontier. Such a legacy of expanding to the unknown drives Fermilab’s mantra of being an “Ongoing Pioneer of Exploring the Frontier of Discovery.” (Image: Mine)

Figure 8: American bison (bison bison) in the far background (click to enlarge). At the time of the photo, a few calves had just recently been born. (Image: Mine)

Happy Colliding.

- richard (@bravelittlemuon)

Tags: 3σ, 5σ, Animals, ATLAS, ATLAS-CMS, CERN, CMS, data, Fermilab, Higgs, Higgs boson, Higgs Discovery, Higgs Exclusion, LHC, PreSUSY 2011, Statistics, susy, SUSY 2011, SUSY11
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What If It’s Not The Higgs?

Sunday, August 21st, 2011

Richard Ruiz

Updated: Monday, 2011 August 29, to clarify shape of angular distribution plots.

It’s the $10 billion question: If experimentalists do discover a bump at the Large Hadron Collider, does it have to be the infamous higgs boson? Not. One. Bit. Plainly and simply, if the ATLAS & CMS collaborations find something at the end of this year it will take a little more data to know we are definitely dealing with a higgs boson. Okay, I suppose I should back up a little an add some context.

The Standard Model of Particle Physics (or SM for short) is the name for the very well established theory that explains how almost everything in the Universe works, from a physics perspective at least. The fundamental particles that make up the SM, and hence our Universe, are shown in figure 1 and you can learn all about them by clicking on the hyperlink a sentence back. Additionally, this short Guardian article does a great job explaining fermions & bosons.

Fig 1. The Standard Model is composed of elementary particles, which are the fundamental building blocks of the Universe, and rules dictating how the particles interact. The fundamental building blocks are known as fermions and the particles which mediate interactions between fermions are called bosons. (Image: AAAS)

As great as the Standard Model is, it is not perfect. In fact, the best way to describe the theory is to say that it is incomplete. Three phenomena that are not fully explained, among many, are: (1) how do fermions (blue & green boxes in figure 1) obtain their mass; (2) why is there so little antimatter (or so much matter) in the Universe; and (3) how does gravity work at the nanoscopic scale? These are pretty big questions and over the years theorists have come up with some pretty good ideas.

The leading explanation for how fermions (blue & green boxes in figure 1) have mass is called the Higgs Mechanism and it predicts that there should be a new particle called the higgs boson (red box at bottom of figure 1). Physicist believe that the Higgs Mechanism may explain the fermion masses is because this same mechanism very accurately predicts the masses for the other bosons (red boxes in figure 1). It is worth nothing that when using the Higgs Mechanism to explain the masses of the bosons, no new particle is predicted.

Unfortunately, the leading explanations for the huge disparity between matter & antimatter, as well as a theory of gravity at the quantum level, have not been as successful. Interestingly, all three types of theories (the Higgs Mechanism, matter/antimatter, and quantum gravity) generally predict the existence of a new boson, namely, the higgs boson, the Z’ boson (pronounced: zee prime), and the graviton. A key property that distinguishes each type of boson from the others is the intrinsic angular momentum they each carry. The higgs boson does not carry any, so we call it a “spin 0″ boson; the Z’ boson carries a specific amount, so it is called a “spin 1″ boson; and the graviton carries precisely twice as much angular momenta as the Z’ boson, so the graviton is called a “spin 2″ boson. This will be really important in a few paragraphs but quickly let’s jump back to the higgs story.

Fig 2. Feynman Diagrams representing a higgs boson (left), Z’ boson (center), and graviton (right)
decaying into a b quark (b) & anti-b quark (b).

In July, at the European Physics Society conference, the CDF & DZero Experiments, associated with the Tevatron Collider in Illinois, USA, and the CMS & ATLAS Experiments, associated with the Large Hadron Collider, in Geneva, Switzerland, reported their latest results in the search for the higgs boson. The surprising news was that it might have been found but we will not know for sure until the end of 2011/beginning of 2012.

This brings us all the way back to our $10/€7 billion question: If the experiments have found something, how do we know that it is the higgs boson and not a Z’ boson or a graviton? Now I want to be clear: It is insanely unlikely that the new discovery is a Z’ or a graviton, if there is a new discovery at all. If something has been been discovered, chances are it is the higgs boson but how do we know?

Now, here is where awesome things happen.

The Solution.

In all three cases, the predicted boson can decay into a b quark (b) & anti-b quark (b) pair, which you can see in the Feynman diagrams in figure 2. Thanks to the Law of Conservation of Momentum, we can calculate the angle between each quark and the boson. Thanks to the well-constructed detectors at the Large Hadron Collider and the Tevatron, we can measure the angle between each quark and the boson. The point is that the angular distribution (the number of quarks observed per angle) is different for spin 0 (higgs), spin 1 (Z’), and spin 2 (graviton) bosons!

To show this, I decided to use a computer program to simulate how we expect angular distributions for a higgs → bb, a Z’→ bb, and a graviton → bb to look. Below are three pairs of plots: the ones to the left show the percentage of b (or b) quarks we expect at a particular angle, with respect to the decaying boson; the ones on the right show the percentage of quarks we expect at the cosine (yes, the trigonometric cosine) of the particular angle.

Figure 3. The angular distribution (left) and cosine of the angular distribution (right) for the higgs (spin-0) boson, m_H = 140 GeV/c². 50K* events generated using PYTHIA MSUB(3).

Figure 4. The angular distribution (left) and cosine of the angular distribution (right) for a Z’ (spin-1) boson, m_Z’ = 140 GeV/c². 50K* events generated using PYTHIA MSUB(141).

Figure 5. The angular distribution (left) and cosine of the angular distribution (right) for a graviton (spin-2) boson, m_G = 140 GeV/c². 40K* events generated using PYTHIA MSUB(391), i.e., RS Graviton.

Thanks to the Law of Conservation of Angular Momentum, the intrinsic angular momenta held by the spin 0 (higgs), spin 1 (Z’), and spin 2 (graviton) force the quarks to decay preferentially at some angles and almost forbid other angles. Consequentially, the angular distribution for the higgs boson (spin 0) will give one giant hump around 90°; for the Z’ boson will have two humps at 60° and 120°; and the graviton (spin 2) will have three humps at 30°, 90°, and 150°. Similarly in the cosine distribution: the spin-0 higgs boson has no defining peak; the spin-1 Z’ boson has two peaks; and the spin-2 graviton has three peaks!

In other words, if it smells like a higgs, looks like a higgs, spins like a higgs, then my money is on the higgs.

A Few Words About The Plots

I have been asked by a reader if I could comment a bit on the shape and apparent symmetry in the angular distribution plots, both of which are extremely well understood. When writing the post, I admittedly glossed over these really important features because I was pressed to finish the post before traveling down to Chicago for a short summer school/conference, so I am really excited that I was asked about this.

At the Large Hadron Collider, we collide protons head-on. Since the protons are nicely aligned (thanks to the amazing people who actually operate the collider), we can consistently and uniformly label the direction through which the protons travel. In our case, let’s have a proton that come from the left be proton A and a proton that comes from the right be proton B. With this convention, proton A is traveling along what I call the “z-axis”; if proton A were to shoot vertically up toward the top of this page it would be traveling along the “x-axis”; and if it were to travel out of the computer screen toward you, the reader, the proton would be traveling in the “y direction” (or along the “y-axis”). The angle between the z-axis and the x-axis (or z-axis and the y-axis) is called θ (pronounced: theta). You can take a look at figure 6 for a nice little cartoon of the coordinate system I just described to you.

Figure 6: A coordinate system in which proton A (pA) is traveling along the z-axis and proton B (pB) in the negative z direction. The angle θ is measure as the angle between the z-axis and the x-axis, or equally, between the z-axis and the y-axis.

When the quarks (spin 1/2) inside a proton collide to become a higgs (spin 0), Z’ (spin 1), or graviton (spin 2), angular momentum must always be conserved. The simplest way for a quark in proton A and a quark in proton B to make a higgs boson is for the quarks to spin opposite directions, while still traveling along the z-axis, so that their spins cancel out, i.e., spin 1/2 – spin 1/2 = spin 0. This means that the higgs boson (spin 0) does not have any angular momentum constraints when decaying into two b-quarks and thus the cosine of the angle between the two b-quarks should be roughly flat and uniform. This is a little hard to see in figure 3 (right) because, as my colleague pointed out, the resolution in my plots are too small. (Thanks, Zhen!)

Turning to the Z’ boson (spin 1) case, protons A & B can generate a spin 1 particle most easily when their quarks, again while traveling along the z-axis, are spinning in the same direction, i.e., spin 1/2 + spin 1/2 = spin 1. Consequentially, the spin 1 Z’ boson and its decay products, unlike the higgs boson (spin 0), are required to conserve 1 unit of angular momentum. This happens most prominently when the two b-quarks (1) push against each other in opposite directions or (2) travel in the same direction. Therefore, the cosine of the angle made by the b-quarks is dominantly -1 or +1. If we allow for quantum mechanical fluctuations, caused by Heisenberg’s Uncertainty Principle, then we should also expect b-quarks to sometimes decay with a cosine greater than -1 and less than +1. See figure 4 (right).

The spin 2 graviton can similarly be explained but with a key difference. The spin 2 graviton is special because like the Z’ boson (spin 1) it can have 1 unit of angular momentum, but unlike Z’ boson (spin 1) it can also have 2 units of angular momenta. To produce a graviton with 2 units of angular momenta, rarer processes that involve the W & Z bosons (red W & Z in figure 1) must occur. This allows the final-state b-quarks to decay with a cosine of 0, which explains the slight enhancement in figure 5 (right).

It is worth noting that the reason why I have been discussing the cosine of the angle between the the quarks and not the angle itself is because the cosine is what we physicists calculate and measure. The cosine of an angle, or equally sine of an angle, amplify subtle differences between particle interactions and can at times be easier to calculate & measure.

The final thing I want to say about the angular distributions is probably the coolest thing ever, better than figuring out the spin of a particle. Back in the 1920s, when Quantum Mechanics was first proposed, people were unsure about a keystone of the theory, namely the simultaneous particle and wave nature of matter. We know bosons definitely behave like particles because they can collide and decay. That wavy/oscillatory behavior you see in the plots are exactly that: wavy/oscillatory behavior. No classical object will decay into particles with a continuous distribution; no classical has ever been found to do so nor do we expect to find one, at least according to our laws of classical physics. This wave/particle/warticle behavior is a purely quantum physics effect and would be an indicator that Quantum Mechanics is correct at the energy scale being probed by the Large Hadron Collider.

Happy Colliding.

- richard (@bravelittlemuon)

PS I apologize if some things are a little unclear or confusing. I traveling this weekend and have not had time to fully edit this post. If you have a question or want me to clarify something, please, feel free to write a comment.

PPS If you are going to be at the PreSUSY Summer School in Chicago next week, feel free to say hi!

*A note on the plots: I simulated several tens of thousands of events for clarity. According to my calculations, it would take four centuries to generate 40,000 gravitons, assuming the parameters I chose. In reality, the physicists can make the same determination as we did with fewer than four years worth of data.

Tags: @bravelittlemuon, b quarks, CERN, EPS, graviton, Higgs, Higgs boson, LHC, LP11, LP2011, No Higgs, PreSUSY, Tevatron, Z prime, Z'
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