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Posts Tagged ‘CMS’


Thursday, October 29th, 2015

Building on the success of rotating Twitter accounts like @realscientists, which I participated in last year, the CMS experiment has a new account: @CMSVoices.  The idea is that it’s an account for talking to CMS members and hearing about their day-to-day work, in contrast with the official news from the @CMSexperiment account.  Of course, you can already hear from many individual CMS physicists on Twitter (I’m normally @sethzenz), but the account gives you the chance to interact with a new person each month, and it might even help us get some new tweeters started!  I also tried to explain things in more detail and start some more general discussions, for example:

There weren’t too many discussions or too many followers so far, but we’re just getting started, and I’m looking forward to others taking the account over and seeing what the do with it.  The next holder of the CMSVoices account, starting in November, will be @matt_bellis.  Please welcome him next week, and let us know if you have any questions or ideas!


I know what you are thinking. The LHC is back in action, at the highest energies ever! Where are the results? Where are all the blog posts?

Back in action, yes, but restarting the LHC is a very measured process. For one thing, when running at the highest beam energies ever achieved, we have to be very careful about how we operate the machine, lest we inadvertently damage it with beams that are mis-steered for whatever reason. The intensity of the beams — how many particles are circulating — is being incrementally increased with successive fills of the machine. Remember that the beam is bunched — the proton beams aren’t continuous streams of protons, but collections that are just a few centimeters long, spaced out by at least 750 centimeters. The LHC started last week with only three proton bunches in each beam, only two of which were actually colliding at an interaction point. Since then, the LHC team has gone to 13 bunches per beam, and then 39 bunches per beam. Full-on operations will be more like 1380 bunches per beam. So at the moment, the beams are of very low intensity, meaning that there are not that many collisions happening, and not that much physics to do.

What’s more, the experiments have much to do also to prepare for the higher collision rates. In particular, there is the matter of “timing in” all the detectors. Information coming from each individual component of a large experiment such as CMS takes some time to reach the data acquisition system, and it’s important to understand how long that time is, and to get all of the components synchronized. If you don’t have this right, then you might not be getting the optimal information out of each component, or worse still, you could end up mixing up information from different bunch crossings, which would be disastrous. This, along with other calibration work, is an important focus during this period of low-intensity beams.

But even if all these things were working right out of the box, we’d still have a long way to go until we had some scientific results. As noted already, the beam intensities have been low, so there aren’t that many collisions to examine. There is much work to do yet in understanding the basics in a revised detector operating at a higher beam energy, such as how to identify electrons and muons once again. And even once that’s done, it will take a while to make measurements and fully vet them before they could be made public in any way.

So, be patient, everyone! The accelerator scientists and the experimenters are hard at work to bring you a great LHC run! Next week, the LHC takes a break for maintenance work, and that will be followed by a “scrubbing run”, the goal of which is to improve the vacuum in the LHC beam pipe. That will allow higher-intensity beams, and position us to take data that will get the science moving once again.


Ramping up to Run 2

Thursday, March 19th, 2015

When I have taught introductory electricity and magnetism for engineers and physics majors at the University of Nebraska-Lincoln, I have used a textbook by Young and Freedman. (Wow, look at the price of that book! But that’s a topic for another day.) The first page of Chapter 28, “Sources of Magnetic Field,” features this photo:


It shows the cryostat that contains the solenoid magnet for the Compact Muon Solenoid experiment. Yes, “solenoid” is part of the experiment’s name, as it is a key element in the design of the detector. There is no other magnet like it in the world. It can produce a 4 Tesla magnetic field, 100,000 times greater than that of the earth. (We actually run at 3.8 Tesla.) Charged particles that move through a magnetic field take curved paths, and the stronger the field, the stronger the curvature. The more the path curves, the more accurately we can measure it, and thus the more accurately we can measure the momentum of the particle.

The magnet is superconducting; it is kept inside a cryostat that is full of liquid helium. With a diameter of seven meters, it is the largest superconducting magnet ever built. When in its superconducting state, the magnet wire carries more than 18,000 amperes of current, and the energy stored is about 2.3 gigajoules, enough energy to melt 18 tons of gold. Should the temperature inadvertently rise and the magnet become normal conducting, all of that energy needs to go somewhere; there are some impressively large copper conduits that can carry the current to the surface and send it safely to ground. (Thanks to the CMS web pages for some of these fun facts.)

With the start of the LHC run just weeks away, CMS has turned the magnet back on by slowly ramping up the current. Here’s what that looked like today:


You can see that they took a break for lunch! It is only the second time since the shutdown started two years ago that the magnet has been ramped back up, and now we’re pretty much going to keep it on for at least the rest of the year. From the experiment’s perspective, the long shutdown is now over, and the run is beginning. CMS is now prepared to start recording cosmic rays in this configuration, as a way of exercising the detector and using the observed muon to improve our knowledge of the alignment of detector components. This is a very important milestone for the experiment as we prepare for operating the LHC at the highest collision energies ever achieved in the laboratory!


This article appeared in DOE Pulse on Nov. 10, 2014.

Fermilab's Oliver Gutsche leads worldwide computing operations for the CMS experiment. Photo: Reidar Hahn

Fermilab’s Oliver Gutsche leads worldwide computing operations for the CMS experiment. Photo: Reidar Hahn

Since he was a graduate student in Germany, Oliver Gutsche wanted to combine research in particle physics with computing for the large experiments that probe the building blocks of matter.

“When I started working on the physics data coming from one of the experiments at DESY, I was equally interested in everything that had to do with large-scale computing,” said Gutsche of his time at the German laboratory. Gutsche now works at DOE’s Fermi National Accelerator Laboratory. “So I also began working on the computing side of particle physics. For me that was always the combination I wanted to do.”

Gutsche’s desire to merge the two focuses has paid off. For the past four years Gutsche has been in charge of worldwide computing operations of the Large Hadron Collider’s CMS experiment, one of two experiments credited with the 2012 Higgs boson discovery. In December he was awarded the CMS Collaboration Award for his contributions to the global CMS computing system. And more recently, he has been promoted to assistant head of the Scientific Computing Division at Fermilab.

As head of CMS Computing Operations, Gutsche orchestrates data processing, simulations, data analysis and transfers and manages infrastructure and many more central tasks. Monte Carlo simulations of particle interactions, for example, are a key deliverable of the CMS Computing Operations group. Monte Carlo simulations employ randomness to simulate the collisions of the LHC and their products in a statistical way.

“You have to simulate the randomness of nature,” explained Gutsche. “We need Monte Carlo collisions to make sure we understand the data recorded by the CMS experiment and to compare them to the theory.”

When Gutsche received his Ph.D. from the University of Hamburg in 2005, he was looking for a job to combine LHC work, large-scale computing and a U.S. postdoc experience.

“Fermilab was an ideal place to do LHC physics research and LHC computing at the same time,” he said. His postdoc work led to his appointment as an application physicist at Fermilab and as the CMS Computing Operations lead.

Today Gutsche interacts regularly with people at universities and laboratories across the United States and at CERN, host laboratory of the LHC, often starting the day at 7 a.m. for transatlantic or transcontinental meetings.

“I try to talk physics and computing with everyone involved, even those in different time zones, from CERN to the west coast,” he said. Late afternoon in the United States is a good time for writing code. “That’s when everything quiets down and Europe is asleep.”

Gutsche expects to further enhance the cooperation between U.S. particle physicists and their international colleagues, mostly in Europe, by using the new premier U.S. Department of Energy’s Energy Sciences Network recently announced in anticipation of the LHC’s restart in spring 2015 at higher energy.

Helping connect the research done by particle physicists around the world, Gutsche finds excitement in all the work he does.

“Of course the Higgs boson discovery was very exciting,” Gutsche said. “But in CMS Computing Operations everything is exciting because we prepare the basis for hundreds of physics analyses so far and many more to come, not only for the major discoveries.”

Rich Blaustein


The World’s Largest Detector?

Wednesday, August 13th, 2014

This morning, the @CERN_JOBS twitter feed tells us that the ATLAS experiment is the world’s largest detector:

CERN_JOBS Tweet Largest Detector

Weighing over 7,000 tons, 46 meters long, and 25 meters high, ATLAS is without a doubt the particle detector with the greatest volume ever built at a collider. I should point out, though, that my experiment, the Compact Muon Solenoid, is almost twice as heavy at over 12,000 tons:


CMS is smaller but heavier — which may be why we call it “compact.” What’s the difference? Well, it’s tough to tell from the pictures, in which CMS is open for tours and ATLAS is under construction, but the big difference is in the muon systems. CMS has short gaps between muon-detecting chambers, while ATLAS has a lot of space in order to allow muons to travel further and get a better measurement. That means that a lot of the volume of ATLAS is actually empty air! ATLAS folks often say that if you could somehow make it watertight, it would float; as a CMS member, I heartily recommend attempting to do this and seeing if it works. 😉

But the truth is that all this cross-LHC rivalry is small potatoes compared to another sort of detector: the ones that search for neutrinos require absolutely enormous volumes of material to get those ghostlike particles to interact even occasionally! For example, here’s IceCube:

"Icecube-architecture-diagram2009" by Nasa-verve - IceCube Science Team - Francis Halzen, Department of Physics, University of Wisconsin. Licensed under Creative Commons Attribution 3.0 via Wikimedia Commons - https://commons.wikimedia.org/wiki/File:Icecube-architecture-diagram2009.PNG#mediaviewer/File:Icecube-architecture-diagram2009.PNG

Most of its detecting volume is actually antarctic ice! Does that count? If it does, there may be a far bigger detector still. To follow that story, check out this 2012 post by Michael Duvernois: The Largest Neutrino Detector.



Wednesday, August 6th, 2014

The particle with two names: The J/ψ Vector Meson. Again, under 500 words.


Trident decay of J/Psi Credit: SLAC/NOVA

Hi All,

The J/ψ (or J/psi) is a very special particle. Its discovery was announced in 1974 independently by two groups: one lead by Samuel Ting at Brookhaven National Laboratory (BNL) in New York and the second lead by Burton Richter at Standford Linear Accelerator Center (SLAC) in California. J/ψ is special because it established the quark model as a credible description of nature. Having been invented by Gell-Man and Zweig as a bookkeeping tool, it was not until Glashow, Iliopoulos and Maiani (GIM) that the concept of quarks as real particles was taken seriously. GIM predicted that if quarks were real, then they should come in pairs, like the  up and down quarks. Candidates for the up, down, and strange were identified, but there was no partner for the strange quark. J/ψ was the key.


Samuel Ting and his BNL team. Credit: BNL

Like the proton or an atom, the J/ψ is a composite particle. This means that J/ψ is made of smaller, more elementary particles. Specifically, it is a bound state of  one charm quark and one anticharm quark. Since it is made of quarks, it is a “hadron“. But since it is made of exactly one quark and one antiquark, it is specifically a “meson.” Experimentally, we have learned that the  J/ψ has an intrinsic angular momentum (spin) of 1ħ (same as the photon), and call it a “vector meson.” We infer that the charm and anticharm, which are both spin ½ħ, are aligned in the same direction (½ħ + ½ħ = 1ħ). The J/ψ must also be electrically neutral because charm and anticharm quarks have equal but opposite electric charges.


Burton Richter following the announcement of co-winning the 1976 Nobel Prize. Credit: SLAC

At 3.1 GeV/c², the J/ψ is a about three times heavier than the proton and about three-quarters the mass of the bottom quark. However, because so few hadrons are lighter than it, the J/ψ possesses a remarkable feature: it decays 10% of the time to charged leptons, like an electron-positron pair. By conservation of energy, it is forbidden to decay to heavier hadrons. Because there are so few  J/ψ decay modes, it is appears as a very narrow peak in experiments. In fact, the particle’s mass and width are so well-known that experiments like ATLAS and CMS use them as calibration markers.

Credit: CMS

Drell-Yan spectrum data at 7 TeV LHC Credit: CMS

The J/ψ meson is one of the coolest things in the particle zoo. It is a hadronic bound state that decays into charged leptons. It shares the same quantum numbers as the photon and Z boson, so it appears as a Drell-Yan processes. It established the quark model, and is critical to new discoveries because of its use as a calibration tool. In my opinion, not too shabby.

Happy colliding.

Richard (@BraveLittleMuon)


Two anomalies worth noticing

Monday, July 14th, 2014

The 37th International Conference on High Energy Physics just finished in Valencia, Spain. This year, no big surprises were announced: no new boson, no signs from new particles or clear phenomena revealing the nature of dark matter or new theories such as Supersymmetry. But as always, a few small anomalies were reported.

Looking for deviations from the theoretical predictions is precisely how experimentalists are trying to find a way to reveal “new physics”. It would help discover a more encompassing theory since everybody realises the current theoretical model, the Standard Model, has its limits and must be superseded by something else. However, all physicists know that small deviations often come and go. All measurements made in physics follow statistical laws. Therefore deviations from the expected value by one standard deviation occur in three measurements out of ten. Larger deviations are less common but still possible. A two standard deviation happens 5% of the time. Then there are systematic uncertainties that relate to the experimental equipment. These are not purely statistical, but can be improved with a better understanding of our detectors. The total experimental uncertainty quoted with each result corresponds to one standard variation. Here are two small anomalies reported at this conference that attracted attention this year.

The ATLAS Collaboration showed its preliminary result on the production of a pair of W bosons. Measuring this rate provides excellent checks of the Standard Model since theorists can predict how often pairs of W bosons are produced when protons collide in the Large Hadron Collider (LHC). The production rate depends on the energy released during these collisions. So far, two measurements can be made since the LHC operated at two different energies, namely 7 TeV and 8 TeV.

CMS and ATLAS had already released their results on their 7 TeV data. The measured rates exceeded slightly the theoretical prediction but were both well within their experimental error with a deviation of 1.0 and 1.4 standard deviation, respectively. CMS had also published results based on about 20% of all data collected at 8 TeV. It exceeded slightly the theoretical prediction by 1.7 standard deviation. The latest ATLAS result adds one more element to the picture. It is based on the full 8 TeV data sample. Now ATLAS reports a slightly stronger deviation for this rate at 8 TeV with 2.1 standard deviations from the theoretical prediction.


The four experimental measurements for the WW production rate (black dots) with the experimental uncertainty (horizontal bar) as well as the current theoretical prediction (blue triangle) with its own uncertainty (blue strip). One can see that all measurements are higher than the current prediction, indicating that the theoretical calculation fails to include everything.

The four individual measurements are each reasonably consistent with expectation, but the fact that all four measurements lie above the predictions becomes intriguing. Most likely, this means that theorists have not yet taken into account all the small corrections required by the Standard Model to precisely determine this rate. This would be like having forgotten a few small expenses in one’s budget, leading to an unexplained deficit at the end of the month. Moreover, there could be common factors in the experimental uncertainties, which would lower the overall significance of this anomaly. But if the theoretical predictions remain what they are even when adding all possible little corrections, it could indicate the existence of new phenomena, which would be exciting. It would then be something to watch for when the LHC resumes operation in 2015 at 13 TeV.

The CMS Collaboration presented another intriguing result. They found some events consistent with coming from a decay of a Higgs boson into a tau and a muon. Such decays are prohibited in the Standard Model since they violate lepton flavour conservation. There are three “flavours” or types of charged leptons (a category of fundamental particles): the electron, the muon and the tau. Each one comes with its own type of neutrinos. According to all observations made so far, leptons are always produced either with their own neutrino or with their antiparticle. Hence, the decay of a Higgs boson in leptons should always produce a charged lepton and its antiparticle, but never two charged leptons of different flavour. Violating a conservation laws in particle physics is simply not allowed.

This needs to be scrutinised with more data, which will be possible when the LHC resumes next year. Lepton flavour violation is allowed outside the Standard Model in various models such as models with more than one Higgs doublet or composite Higgs models or Randall-Sundrum models of extra dimensions for example. So if both ATLAS and CMS confirm this trend as a real effect, it would be a small revolution.

HtomutauThe results obtained by the CMS Collaboration showing that six different channels all give a non-zero value for the decay rate of Higgs boson into pairs of tau and muon.

Pauline Gagnon

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Even before my departure to La Thuile in Italy, results from the Rencontres de Moriond conference were already flooding the news feeds. This year’s Electroweak session from 15 to 22 March, started with the first “world measurement” of the top quark mass, from a combination of the measurements published by the Tevatron and LHC experiments so far. The week went on to include a spectacular CMS result on the Higgs width.

Although nearing its 50th anniversary, Moriond has kept its edge. Despite the growing numbers of must-attend HEP conferences, Moriond retains a prime spot in the community. This is in part due to historic reasons: it’s been around since 1966, making a name for itself as the place where theorists and experimentalists come to see and be seen. Let’s take a look at what the LHC experiments had in store for us this year…

New Results­­­

Stealing the show at this year’s Moriond was, of course, the announcement of the best constraint yet of the Higgs width at < 17 MeV with 95% confidence reported in both Moriond sessions by the CMS experiment. Using a new analysis method based on Higgs decays into two Z particles, the new measurement is some 200 times better than previous results. Discussions surrounding the constraint focussed heavily on the new methodology used in the analysis. What assumptions were needed? Could the same technique be applied to Higgs to WW bosons? How would this new width influence theoretical models for New Physics? We’ll be sure to find out at next year’s Moriond…

The announcement of the first global combination of the top quark mass also generated a lot of buzz. Bringing together Tevatron and LHC data, the result is the world’s best value yet at 173.34 ± 0.76 GeV/c2.  Before the dust had settled, at the Moriond QCD session, CMS announced a new preliminary result based on the full data set collected at 7 and 8 TeV. The precision of this result alone rivals the world average, clearly demonstrating that we have yet to see the ultimate attainable precision on the top mass.

ot0172hThis graphic shows the four individual top quark mass measurements published by the ATLAS, CDF, CMS and DZero collaborations, together with the most precise measurement obtained in a joint analysis.

Other news of the top quark included new LHC precision measurements of its spin and polarisation, as well as new ATLAS results of the single top-quark cross section in the t-channel presented by Kate Shaw on Tuesday 25 March. Run II of the LHC is set to further improve our understanding of this

A fundamental and challenging measurement that probes the nature of electroweak symmetry breaking mediated by the Brout–Englert–Higgs mechanism is the scattering of two massive vector bosons against each other. Although rare, in the absence of the Higgs boson, the rate of this process would strongly rise with the collision energy, eventually breaking physical law. Evidence for electroweak vector boson scattering was detected for the first time by ATLAS in events with two leptons of the same charge and two jets exhibiting large difference in rapidity.

With the rise of statistics and increasing understanding of their data, the LHC experiments are attacking rare and difficult multi-body final states involving the Higgs boson. ATLAS presented a prime example of this, with a new result in the search for Higgs production in association with two top quarks, and decaying into a pair of b-quarks. With an expected limit of 2.6 times the Standard Model expectation in this channel alone, and an observed relative signal strength of 1.7 ± 1.4, the expectations are high for the forthcoming high-energy run of the LHC, where the rate of this process is enhanced.

Meanwhile, over in the heavy flavour world, the LHCb experiment presented further analyses of the unique exotic state X(3872). The experiment provided unambiguous confirmation of its quantum numbers JPC to be 1++, as well as evidence for its decay into ψ(2S)γ.

Explorations of the Quark-Gluon Plasma continue in the ALICE experiment, with results from the LHC’s lead-proton (p-Pb) run dominating discussions. In particular, the newly observed “double-ridge” in p-Pb is being studied in depth, with explorations of its jet peak, mass distribution and charge dependence presented.

New explorations

Taking advantage of our new understanding of the Higgs boson, the era of precision Higgs physics is now in full swing at the LHC. As well as improving our knowledge of Higgs properties – for example, measuring its spin and width – precise measurements of the Higgs’ interactions and decays are well underway. Results for searches for Beyond Standard Model (BSM) physics were also presented, as the LHC experiments continue to strongly invest in searches for Supersymmetry.

In the Higgs sector, many researchers hope to detect the supersymmetric cousins of the Higgs and electroweak bosons, so-called neutralinos and charginos, via electroweak processes. ATLAS presented two new papers summarising extensive searches for these particles. The absence of a significant signal was used to set limits excluding charginos and neutralinos up to a mass of 700 GeV – if they decay through intermediate supersymmetric partners of leptons – and up to a mass of 420 GeV – when decaying through Standard Model bosons only.

Furthermore, for the first time, a sensitive search for the most challenging electroweak mode producing pairs of charginos that decay through W bosons was conducted by ATLAS. Such a mode resembles that of Standard Model pair production of Ws, for which the currently measured rates appear a bit higher than expected.

In this context, CMS has presented new results on the search for the electroweak pair production of higgsinos through their decay into a Higgs (at 125 GeV) and a nearly massless gravitino. The final state sports a distinctive signature of 4 b-quark jets compatible with a double Higgs decay kinematics. A slight excess of candidate events means the experiment cannot exclude a higgsino signal. Upper limits on the signal strength at the level of twice the theoretical prediction are set for higgsino masses between 350 and 450 GeV.

In several Supersymmetry scenarios, charginos can be metastable and could potentially be detected as a long-lived particle. CMS has presented an innovative search for generic long-lived charged particles by mapping their detection efficiency in function of the particle kinematics and energy loss in the tracking system. This study not only allows to set stringent limits for a variety of Supersymmetric models predicting chargino proper lifetime (c*tau) greater than 50cm, but also gives a powerful tool to the theory community to independently test new models foreseeing long lived charged particles.

In the quest to be as general as possible in the search for Supersymmetry, CMS has also presented new results where a large subset of the Supersymmetry parameters, such as the gluino and squark masses, are tested for their statistical compatibility with different experimental measurements. The outcome is a probability map in a 19-dimension space. Notable observations in this map are that models predicting gluino masses below 1.2 TeV and sbottom and stop masses below 700 GeV are strongly disfavoured.

… but no New Physics

Despite careful searches, the most heard phrase at Moriond was unquestionably: “No excess observed – consistent with the Standard Model”. Hope now lies with the next run of the LHC at 13 TeV. If you want to find out more about the possibilities of the LHC’s second run, check out the CERN Bulletin article: “Life is good at 13 TeV“.

In addition to the diverse LHC experiment results presented, Tevatron experiments, BICEP, RHIC and other experiments also reported their breaking news at Moriond. Visit the Moriond EW and Moriond QCD conference websites to find out more.

Katarina Anthony-Kittelsen


This article appeared in symmetry on March 19, 2014.

An international team of scientists from Fermilab’s Tevatron and CERN’s Large Hadron Collider has produced the world’s best value for the mass of the top quark.

An international team of scientists from Fermilab’s Tevatron and CERN’s Large Hadron Collider has produced the world’s best value for the mass of the top quark.

Scientists working on the world’s leading particle collider experiments have joined forces, combined their data and produced the first joint result from Fermilab’s Tevatron and CERN’s Large Hadron Collider. These machines are the past and current holders of the record for most powerful particle collider on Earth.

Scientists from the four experiments involved—ATLAS, CDF, CMS and DZero—announced their joint findings on the mass of the top quark today at the Rencontres de Moriond international physics conference in Italy.

Together the four experiments pooled their data analysis power to arrive at a new world’s best value for the mass of the top quark of 173.34 ± 0.76 GeV/c2.

Experiments at the LHC at the CERN laboratory in Geneva, Switzerland and the Tevatron collider at Fermilab in Illinois, USA are the only ones that have ever seen top quarks—the heaviest elementary particles ever observed. The top quark’s huge mass (more than 100 times that of the proton) makes it one of the most important tools in the physicists’ quest to understand the nature of the universe.

The new precise value of the top-quark mass will allow scientists to test further the mathematical framework that describes the quantum connections between the top quark, the Higgs particle and the carrier of the electroweak force, the W boson. Theorists will explore how the new, more precise value will change predictions regarding the stability of the Higgs field and its effects on the evolution of the universe. It will also allow scientists to look for inconsistencies in the Standard Model of particle physics—searching for hints of new physics that will lead to a better understanding of the nature of the universe.

“The combining together of data from CERN and Fermilab to make a precision top quark mass result is a strong indication of its importance to understanding nature,” says Fermilab director Nigel Lockyer. “It’s a great example of the international collaboration in our field.”

Courtesy of: Fermilab and CERN

Courtesy of: Fermilab and CERN

A total of more than six thousand scientists from more than 50 countries participate in the four experimental collaborations. The CDF and DZero experiments discovered the top quark in 1995, and the Tevatron produced about 300,000 top quark events during its 25-year lifetime, completed in 2011. Since it started collider physics operations in 2009, the LHC has produced close to 18 million events with top quarks, making it the world’s leading top quark factory.

“Collaborative competition is the name of the game,” says CERN’s Director General Rolf Heuer. “Competition between experimental collaborations and labs spurs us on, but collaboration such as this underpins the global particle physics endeavor and is essential in advancing our knowledge of the universe we live in.”

Each of the four collaborations previously released their individual top-quark mass measurements. Combining them together required close collaboration between the four experiments, understanding in detail each other’s techniques and uncertainties. Each experiment measured the top-quark mass using several different methods by analyzing different top quark decay channels, using sophisticated analysis techniques developed and improved over more than 20 years of top quark research beginning at the Tevatron and continuing at the LHC. The joint measurement has been submitted to the arXiv.

A version of this article was originally issued by Fermilab and CERN as a press release.


In August I moved away from CERN, and I’ve been back and forth between CERN and Brussels quite a lot since then. In fact right now I’m sitting in the building 40 where people go to drink coffee and have meetings, and I can see the ATLAS Higgs Convener sitting on the next table. All this leaves me feeling a little detached from what is really happening at CERN, as if it’s not “my” lab anymore, and that actually sums up how many people think about particle physics at the moment. With LHC Run I we found the Higgs boson. It was what most people expected to see, and by a large margin it was the most probable thing we would have discovered. Things will be different for Run II. Nobody has a good idea about what to expect in terms of new particles (and if they say they do have a good idea, they’re lying.) In that sense it’s not “our” dataset, it’s whatever nature decides it should be. All we can do is say what is possible, not what is probable. (Although we can probably say one scenario is more probable than another.)

The problem we now face is that there is no longer an obvious piece that’s missing, but there are still many unanswered questions, which means we have to move from an era of a well constrained search to an era of phenomenology, or looking for new effects in the data. That’s not a transition I’m entirely comfortable with for several reasons. It’s often said that nature is not spiteful, but it is subtle and indifferent to our expectations. There’s no reason to think that there “should” be new physics for us to discover as we increase the energy of the LHC, and we could be unlucky enough to not find anything new in the Run II dataset. A phenomenological search also means that we’d be overly sensitive to statistical bumps and dips in the data. Every time there’s a new peak that we don’t expect we have to exercise caution and skepticism, almost to the point where it stops being fun. Suppose we find an excess in a dijet spectrum. We may conclude that this is due a new particle, but if we’re going to be phenomenologists about it we must remain open minded, so we can’t necessarily expect to see the same particle in a dimuon final state. It would then be prudent to ask if such a peak comes from a poorly understood effect, such as jet energy scales, and those kinds of effects can be hard to untangle if we don’t have a good control sample in data. At least with the discovery of the Higgs boson, the top quark, and the W and Z bosons we knew what final states to expect and what ratios they should exhibit. There’s also something a little unsettling about not having a roadmap of what to expect. When asked to pick between several alternative scenarios that are neither favoured by evidence nor disfavoured by lack of evidence it’s hard to decide what to prioritise.

Take your pick of new physics!  Each scenario will have new phase space to explore in LHC Run II [CMS]

Take your pick of new physics! Each scenario will have new phase space to explore in LHC Run II [CMS]

On the other hand there is reason to be excited. Since we don’t know what to expect in LHC Run II, anything we do discover will change our views considerably, and will lead to a paradigm shift. If we do discover a new particle, or even better, a new sector of particles, it could help frame the Standard Model as a subset of something more elegant and unified. If that’s the case then we can look forward to decades of intense and exciting research, that would make the Higgs discovery look like small potatoes. So the next few years at the LHC could be either the most boring or the most exciting time in the history of particle physics, and we won’t know until we look at the data. Will nature tantalise us with hints of something novel, will it give us irrefutable evidence of a new resonance, or will it leave us with nothing new at all? For my part I’m taking on the dilepton final states. These are quick, clean, simple, and versatile signatures of something new that are not tied down to a specific model. That’s the best search I can perform in an environment of such uncertainty and with a lack of coherent direction. Let’s hope it pays off, and paves the way for even more discoveries.

What's happening at 325GeV at CDF?  Only more data can tell us! (CDF)

What’s happening at 325GeV at CDF? Only more data can tell us. Based on what the LHC has seen, this is probably a statistical fluctuation. (CDF)