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Posts Tagged ‘top quark’

Top quark still raising questions

Wednesday, October 15th, 2014

This article appeared in symmetry on Oct. 15, 2014.

Why are scientists still interested in the heaviest fundamental particle nearly 20 years after its discovery? Photo: Reidar Hahn, Fermilab

Why are scientists still interested in the heaviest fundamental particle nearly 20 years after its discovery? Photo: Reidar Hahn, Fermilab

“What happens to a quark deferred?” the poet Langston Hughes may have asked, had he been a physicist. If scientists lost interest in a particle after its discovery, much of what it could show us about the universe would remain hidden. A niche of scientists, therefore, stay dedicated to intimately understanding its properties.

Case in point: Top 2014, an annual workshop on top quark physics, recently convened in Cannes, France, to address the latest questions and scientific results surrounding the heavyweight particle discovered in 1995 (early top quark event pictured above).

Top and Higgs: a dynamic duo?
A major question addressed at the workshop, held from September 29 to October 3, was whether top quarks have a special connection with Higgs bosons. The two particles, weighing in at about 173 and 125 billion electronvolts, respectively, dwarf other fundamental particles (the bottom quark, for example, has a mass of about 4 billion electronvolts and a whole proton sits at just below 1 billion electronvolts).

Prevailing theory dictates that particles gain mass through interactions with the Higgs field, so why do top quarks interact so much more with the Higgs than do any other known particles?

Direct measurements of top-Higgs interactions depend on recording collisions that produce the two side-by-side. This hasn’t happened yet at high enough rates to be seen; these events theoretically require higher energies than the Tevatron or even the LHC’s initial run could supply. But scientists are hopeful for results from the next run at the LHC.

“We are already seeing a few tantalizing hints,” says Martijn Mulders, staff scientist at CERN. “After a year of data-taking at the higher energy, we expect to see a clear signal.” No one knows for sure until it happens, though, so Mulders and the rest of the top quark community are waiting anxiously.

A sensitive probe to new physics

Top and antitop quark production at colliders, measured very precisely, started to reveal some deviations from expected values. But in the last year, theorists have responded by calculating an unprecedented layer of mathematical corrections, which refined the expectation and promise to realigned the slightly rogue numbers.

Precision is an important, ongoing effort. If researchers aren’t able to reconcile such deviations, the logical conclusion is that the difference represents something they don’t know about — new particles, new interactions, new physics beyond the Standard Model.

The challenge of extremely precise measurements can also drive the formation of new research alliances. Earlier this year, the first Fermilab-CERN joint announcement of collaborative results set a world standard for the mass of the top quark.

Such accuracy hones methods applied to other questions in physics, too, the same way that research on W bosons, discovered in 1983, led to the methods Mulders began using to measure the top quark mass in 2005. In fact, top quark production is now so well controlled that it has become a tool itself to study detectors.

Forward-backward synergy

With the upcoming restart in 2015, the LHC will produce millions of top quarks, giving researchers troves of data to further physics. But scientists will still need to factor in the background noise and data-skewing inherent in the instruments themselves, called systematic uncertainty.

“The CDF and DZero experiments at the Tevatron are mature,” says Andreas Jung, senior postdoc at Fermilab. “It’s shut down, so the understanding of the detectors is very good, and thus the control of systematic uncertainties is also very good.”

Jung has been combing through the old data with his colleagues and publishing new results, even though the Tevatron hasn’t collided particles since 2011. The two labs combined their respective strengths to produce their joint results, but scientists still have much to learn about the top quark, and a new arsenal of tools to accomplish it.

“DZero published a paper in Nature in 2004 about the measurement of the top quark mass that was based on 22 events,” Mulders says. “And now we are working with millions of events. It’s incredible to see how things have evolved over the years.”

Troy Rummler

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Top Quarks… So Many Top Quarks

Wednesday, April 30th, 2014

Thousands of paper on top quarks exist. Why?

There are literally thousands of papers, collaboration notes, and conference notes with the words “Top” and “Quark” in the title. As of this post, there are 3,477 since 1979 listed on inSpires. There are many, many more that omit the word “quark”. And sure, this is meager compared to the 5,114 papers with the words “Higgs Boson” written since ’74, but that is over 50,000 pages of top quarks (estimating 15 pages/paper). To be fair, there are also many, many more that omit the word “boson”. But for further comparison, there are only 395 papers with a title including the words “Bottom Quark“, 211 with “Bottomonium“, and 125 with “Bottom Hadron“. So why are there so many papers written about the top quark? The answer is that the top quark is weird special.

http://www.symmetrymagazine.org/breaking/2009/09/02/top-quark-chefs

A single top quark candidate event at the Collider Detector experiment at Fermilab. Credit: CDF Collaboration

The top quark is very heavy, about 185 times heavier than the proton and ranks as the heaviest known elementary particle in all the particle kingdom. The second heaviest quark, the bottom quark, is only 4 or 5 times heavier than the proton. If you or I were a proton, then a medium-to-large school bus (without any people) would be a top quark. In fact, the top quark is so heavy it can decay into a real (on-shell) W boson, which is roughly half its mass. The only other particle that can do this is the Higgs. Though it is rare, exceedingly rare, the top quark can decay into real Z  and Higgs bosons as well. Not even the Higgs can top that last feat.

Top quark decaying into real, on-shell W boson and bottom quark. Credit: DZero Collaboration

However, the top quark is still a quark. It has an electric charge that is 2/3 as large as the proton. It has an intrinsic angular momentum (spin) equal to the proton’s or electron’s spin. The top quark is also colored, meaning that is interacts with gluons and is influenced by the strong nuclear force (QCD). When colored objects (quarks and gluons) are produced at collider and fixed target experiments, they undergo a process called hadronization. Hadronization is when two colored objects are far away from one another and the strong nuclear attraction between the two becomes so strong that a pair of colored objects will spontaneously be produced in the space between them. These new colored particles will then form bound states with the old colored states. However, the process hadronization means that we only observe the bound states of colored objects and not the colored objects themselves. Physicists have to infer their properties from the physics of bound states…. or do we?

jets

Colored objects before (L), during (Center L and Center R), and after (R) hadronization.

The onset of hadronization is typically occurs about 10-24 seconds after the creation of a colored object. Yes, that is 0.000000000000000000000001 seconds. That is incredibly fast and well beyond anything that can be done at an experiment. The mean lifetime of the top quark on the other hand is about 10-25 seconds. In other words, the top quark is much more likely to decay in to a W boson, its principle decay mode, than hadronize. By looking at the decays of the W boson, for example to an electron and an electron-neutrino, their angular distributions, and other kinematic properties, we can measure directly the top quark’s quantum numbers. The top quark is special because it is the only quark whose spin and charge quantum numbers we can measure directly.

feynman_t_decay_ljetsqq_pink

Top quark decaying into real, on-shell W boson and bottom quark. The W boson can subsequently decay into a charged lepton and a neutrino or into a quark and anti-quark. Credit: DZero Collaboration

The top quark tells us much about the Standard Model of particle physics, but it also may be a window to new physics. Presently, no one has any idea why the top quark is so much heavier than the bottom quark, or why both are orders of magnitude heavier than the electron and muon. This is called the “Mass Hierarchy Problem” of the Standard Model and stems from the fact that the quark and lepton masses in the theory are not predicted but are taken as input parameters. This does not mean that the Standard Model is “wrong”. On the contrary, the model works very, very well; it is simply incomplete. Of course there are new models and hypotheses that offer explanations, but none have been verified by data.

However, thanks to the 2012 discovery of the Higgs boson, there is a new avenue that may shed light upon the mass hierarchy problem. We now know that quarks and leptons interact with the Higgs boson proportionally to their masses. Since the top quark is ~40 times more massive than the bottom quark, it will interact with Higgs boson 40 times more strongly. There is suspicion that since the Higgs boson is sensitive to the different quark and lepton masses, it may somehow play a role in how masses are assigned.

Happy Colliding

– richard (@BraveLittleMuon)

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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

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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.

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This Fermilab press release was published on February 24.

Matteo Cremonesi, left, of the University of Oxford and the CDF collaboration and Reinhard Schwienhorst of Michigan State University and the DZero collaboration present their joint discovery at a forum at Fermilab on Friday, Feb. 21. The two collaborations have observed the production of single top quarks in the s-channel, as seen in data collected from the Tevatron. Photo: Cindy Arnold

Matteo Cremonesi, left, of the University of Oxford and the CDF collaboration and Reinhard Schweinhorst of Michigan State University and the DZero collaboration present their joint discovery at a forum at Fermilab on Friday, Feb. 21. The two collaborations have observed the production of single top quarks in the s-channel, as seen in data collected from the Tevatron. Photo: Cindy Arnold

Scientists on the CDF and DZero experiments at the U.S. Department of Energy’s Fermi National Accelerator Laboratory have announced that they have found the final predicted way of creating a top quark, completing a picture of this particle nearly 20 years in the making.

The two collaborations jointly announced on Friday, Feb. 21, that they had observed one of the rarest methods of producing the elementary particle – creating a single top quark through the weak nuclear force, in what is called the s-channel. For this analysis, scientists from the CDF and DZero collaborations sifted through data from more than 500 trillion proton-antiproton collisions produced by the Tevatron from 2001 to 2011. They identified about 40 particle collisions in which the weak nuclear force produced single top quarks in conjunction with single bottom quarks.

Top quarks are the heaviest and among the most puzzling elementary particles. They weigh even more than the Higgs boson – as much as an atom of gold – and only two machines have ever produced them: Fermilab’s Tevatron and the Large Hadron Collider at CERN. There are several ways to produce them, as predicted by the theoretical framework known as the Standard Model, and the most common one was the first one discovered: a collision in which the strong nuclear force creates a pair consisting of a top quark and its antimatter cousin, the anti-top quark.

Collisions that produce a single top quark through the weak nuclear force are rarer, and the process scientists on the Tevatron experiments have just announced is the most challenging of these to detect. This method of producing single top quarks is among the rarest interactions allowed by the laws of physics. The detection of this process was one of the ultimate goals of the Tevatron, which for 25 years was the most powerful particle collider in the world.

“This is an important discovery that provides a valuable addition to the picture of the Standard Model universe,” said James Siegrist, DOE associate director of science for high energy physics. “It completes a portrait of one of the fundamental particles of our universe by showing us one of the rarest ways to create them.”

Searching for single top quarks is like looking for a needle in billions of haystacks. Only one in every 50 billion Tevatron collisions produced a single s-channel top quark, and the CDF and DZero collaborations only selected a small fraction of those to separate them from background, which is why the number of observed occurrences of this particular channel is so small. However, the statistical significance of the CDF and DZero data exceeds that required to claim a discovery.

“Kudos to the CDF and DZero collaborations for their work in discovering this process,” said Saul Gonzalez, program director for the National Science Foundation. “Researchers from around the world, including dozens of universities in the United States, contributed to this important find.”

The CDF and DZero experiments first observed particle collisions that created single top quarks through a different process of the weak nuclear force in 2009. This observation was later confirmed by scientists using the Large Hadron Collider.

Scientists from 27 countries collaborated on the Tevatron CDF and DZero experiments and continue to study the reams of data produced during the collider’s run, using ever more sophisticated techniques and computing methods.

“I’m pleased that the CDF and DZero collaborations have brought their study of the top quark full circle,” said Fermilab Director Nigel Lockyer. “The legacy of the Tevatron is indelible, and this discovery makes the breadth of that research even more remarkable.”

Fermilab is America’s national laboratory for particle physics research. A U.S. Department of Energy Office of Science laboratory, Fermilab is located near Chicago, Illinois, and operated under contract by the Fermi Research Alliance, LLC. Visit Fermilab’s website at www.fnal.gov and follow us on Twitter at @FermilabToday.

The DOE Office of Science is the single largest supporter of basic research in the physical sciences in the United States, and is working to address some of the most pressing challenges of our time. For more information, please visit science.energy.gov.

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BOOST!

Sunday, August 5th, 2012

A couple weeks ago, about 80 theorists and experimentalists descended on Valencia, Spain in order to attend the fourth annual BOOST conference (tag-line: “Giving physics a boost!”). On top of the fact that the organizers did a spectacular job of setting up the venue and program (and it didn’t hurt that there was much paella and sangria to be had) overall I’d have to say this was one of the best conferences I’ve attended.

so....much.....sangria......

Differing from larger events such as ICHEP where the physics program is so broad that speakers only have time to give a cursory overview of their topics, the BOOST conferences have more of a workshop feel and are centered specifically around the emerging sub-field of HEP called “boosted physics”. I’ll try to explain what that means and why it’s important below (and in a few subsequent posts).

Intro to top quark decay

In order to discuss boosted physics, something already nicely introduced in Flip’s post here, I’m going to use the decay of the top quark as an example.

Obligatory Particle Zoo plushie portraying the top quark in a happy state

The most massive of all known fundamental particles by far, weighing in at around 173 GeV/c2, the top quark has an extremely short lifetime….much shorter than the time scale of the strong interaction. Thus the top quark doesn’t have time to “hadronize” and form a jet…instead, it will almost always decay into a W boson and a b quark (more than 99% of the time), making it a particularly interesting particle to study. The W boson then decays into either a lepton and a neutrino or two lighter quarks, and the full top decay chain is colloquially called either “leptonic” or “hadronic”, respectively.

From the experimental point of view, top quarks will look like three jets (one from the b and two from the light quarks) about 70% of the time, due to the branching fraction of the W boson to decay hadronically. Only 20% of tops will decay in the leptonic channel with a jet, a muon or electron, and missing energy. (I’m ignoring the tau lepton for the moment which has it’s own peculiar decay modes)

In colliders, top quarks are mostly produced in top/anti-top (or “t-tbar”) pairs….in fact, the top-pair production cross section at the LHC is about 177 pb (running at sqrt(s)=7 TeV), roughly 25 times more than at the Tevatron!! Certainly plenty of tops to study here. Doing some combinatorics and still ignoring decay modes with a tau lepton, the whole system will look:

  1. “Fully hadronic”: two hadronically-decaying tops (about 44% of the time)
  2. “Semi-leptonic”: one leptonically-decaying and one hadronically-decaying top (about 30% of the time)
  3. “Fully leptonic”: two leptonically-decaying tops (only about 4% of the time)

Branching fractions of different decay modes in t-tbar events (from Nature)

 

The point: if a t-tbar event is produced in the detector, it’s fairly likely that at least one (if not both) of the tops will decay into jets! Unfortunately compared to the leptonic mode, it turns out this is a pretty tough channel to deal with experimentally, where at the LHC we’re dominated by a huge multi-jets background.

What does “boost” mean?

If a t-tbar pair was produced with just enough energy needed to create the two top masses, there wouldn’t be energy left over and the tops would be produced almost at rest. This was fairly typical at the Tevatron. With the energies at the LHC, however, the tops are given a “boost” in momentum when produced. This means that in the lab frame (ie: our point of view) we see the decay products with momentum in the same direction as the momentum of the top.

This would be especially conspicuous if, for example, we were able to produce some kind of new physics interaction with a really heavy mediator, such as a Z’ (a beyond-the-Standard-Model heavy equivalent of the Z boson), the mass of which would have to be converted into energy somewhere.

Generally we reconstruct the energy and mass of a hadronically-decaying top by combining the three jets it decays into. But what if the top was so boosted that the three jets merged to a point where you couldn’t distinguish them, and it just looked like one big jet? This makes detecting it even more difficult, and a fully-hadronic t-tbar event is almost impossible to see.

At what point does this happen?

It turns out that this happens quite often already, where at ATLAS we’ve been producing events with jets having a transverse momentum (pT) of almost 2 TeV!

A typical jet used in analyses in ATLAS has a cone-radius of roughly R=0.4. (ok ok, the experts will say that technically it’s not a “cone,” let alone something defined by a “radius,” as R is a “distance parameter used by the jet reconstruction algorithm,” but it gives a general idea.) With enough boost on the top quark, we won’t be able to discern the edge of one of the three jets from the next in the detector. Looking at the decay products’ separation as a function of the top momentum, you can see that above 500 GeV or so, the W boson and the b quark are almost always within R < 0.8. At that momentum, individual R=0.4 jets are hard to tell apart already.

The opening angle between the W and b in top decays as a function of the top pT in simulated PYTHIA Z'->ttbar (m_Z' =1.6 TeV) events.

 

We’ll definitely want to develop tools to identify tops over the whole momentum range, not just stopping at 500 GeV. The same goes for other boosted decay channels, such as the imminently important Higgs boson decay to b-quark pairs channel, or boosted hadronically-decaying W and Z bosons. So how can we detect these merged jets over a giant background? That’s what the study of boosted physics is all about.

Next: Finding boosted objects using jet “mass” and looking for jet substructure

Next next: Pileup at the LHC….a jet measurement nightmare.

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Update I: Included Medicine Award (Oct 03)

Update II: Included Physics Award (Oct 04)

… it’s Nobel Week! October means three things: Halloween (duh), Fall, and Nobel Week, the week during which the famed prizes are awarded to those who have “conferred the greatest benefit on mankind” [1]. Okay, before I get comments about the subjectivity of those who award the prizes, I gladly admit that the history of the prize is not without controversy relating to those who have & have not won, in both the science and non-science categories.

I am just going to ignore all of that and talk about why everyone should be excited about this week. Though before I talk about this week’s Nobels, I feel I should probably give the SparkNotes version of the prizes’ history.

Figure 1: The 2008 Chemistry Prize was awarded for the discovery and development of green fluorescent protein (GFP), which when inserted into a soon-to-be parent is passed onto an offspring who can then glow green. Glowing cat!
(Image: The Nobel Foundation)

[1] http://www.nobelprize.org/alfred_nobel/will/will-full.html

A Brief History of Alfred Nobel

Figure 2: Alfred Nobel. (Image: The Nobel Foundation)

The year is 1866, the Second Industrial Revolution is raging, innovation is surging, and the US Civil War over.

Insert Alfred Nobel: A son of a successful engineer who developed controlled explosives for the demolition and mining industries. The younger Nobel, unsurprisingly, decided be a chemist after playing with nitroglycerin in a French laboratory. As a public service announcement, I should probably mention that nitroglycerin is very dangerous and is a principle ingredient in dynamite. In fact, Nobel was so convinced that nitroglycerine had useful application in construction that he decided to invent dynamite. Needless to say, dynamite made Nobel a very, very, very rich man. At the end of his life, he decided to endow, with the bulk of his fortune, a set of prizes to recognize those who have contributed greatest in the Fields of Medicine, Physics, Chemistry, Literature, and Peace. Economics, though not stipulated in the original will, was added later and is funded separately.

Figure 2: The chemical structure of nitroglycerin. This stuff is wicked; the physical chemistry behind its structure worth a gander. Consider this an advertisement to go earn a chemistry degree. (Image: Wikipedia)

What Makes a Prize

The Nobels has come a long way since they were first instituted. Most notably, they no longer are awarded for the greatest discovery or invention from the past year; the prizes now award those results with the most lasting influence and impact. Take last year for example. The 2011 award for Physiology or Medicine went solely to Sir Robert Edwards for having developed in vitro fertilization. You would think something that is, in every sense of the word, responsible for the existence of millions of people would have been awarded long, long ago. I mean, that is what went through my mind last October. Therein lies the novelty of the Nobel Prizes: These days, the awards are given to what seem like common knowledge, because in some sense they are. What one has to realize though is that prior a laureate’s discovery or invention, these ideas and concepts just did not exist. Imagine a world in which no one knew of insulin (Nobel 1923). Weird, no?

This brings me to why Nobel Week is so much fun. Sometimes you know quite a bit about the award-winning discovery and so you get to spend the day reading news articles and science blogs learning all about the topic’s history. Werner Forssmann’s invention of the cardiac catheter (Nobel 1953) has a hysterical history that is well worth a read. At other times, you have no idea what the award citation even means, but you just know it is worth spending a few minutes or even a few hours learning. I mean, why else would a Nobel be awarded? Take, as another example, 2008’s Physics prize. The award citation reads:

“… for the discovery of the mechanism of spontaneous broken symmetry in subatomic physics,” [2]

and

“for the discovery of the origin of the broken symmetry
which predicts the existence of at least three families of quarks in nature
.” [2]

Yup, it is a mouthful and probably seems a bit obtuse. That is, until you start looking up Wikipedia or news articles (or Quantum Diaries!), and realize how amazingly awesome these discoveries are. I mean, sure discovering spontaneous symmetry breaking (SSB) sounds nice and fancy but did you know that is why the bosons in the Standard Model of Physics have the masses they do?!? SSB, when applied specifically to the Electroweak bosons (photon, W, & Z) is the Higgs Mechanism, and when applied to fermions, is what generates the higgs boson. SSB is an established scientific fact and is also the driving force behind superconductivity (Nobel 1972) Whether or not the higgs boson exists, however, is completely different story.

Figure 3: The quark sector of the Standard Model of Particle Physics and their discovery dates. (Image: Nobel Foundation)

So back in 1977 a Fermilab team, led by Leon Lederman, discovered the bottom quark (Nobel 1988), and in 1995, the CDF & DZero Tevatron experiments discovered the top quark. Ever wonder how we knew to look for them in the first place? It was because of something called the CKM matrix. It was introduced as a way of organizing the the different ways particles in the Standard Model could interact and decay. However, as gorgeous as this new organization was, in order to work the CKM matrix required the existence of two new quarks. Well guess what, Fermilab found those two quarks and set the Standard Model in stone.

The 2009 Nobel Prizes are equally impressive. Half the prize was awarded for the development of fiber optics, which is the foundation of modern telecommunications, and something called Charged-Coupled Devices (CCD). What took me a few hours to learn is that if you take this sensor, attach a flashbulb, a battery, and maybe a memory card, you get a digital camera. In other words, half the 2009 prize was awarded for inventing the digital camera. The prize winners were simply trying to develop a better way of storing data and inadvertently created an entire industry. A fun fact: the first transistor (Nobel 1967) was made of paperclips. If you are curious about what makes transistors so important, take apart your computer and take a peek. (Please, make sure the computer is unplugged before opening it.)

[2] http://www.nobelprize.org/nobel_prizes/physics/laureates/2008/

Does Every Major Discovery/Invention Get a Prize?

No. First off, Nobel Prizes are no longer awarded posthumously. Secondly, from my discussions about this issue, there seems to be a consensus there may be a limit to what is & is not awarded when it comes to the sciences. Now the Swedish Academies always reserve the right to set a new precedent, however, it is unlikely that any organizations will be awarded a Nobel in science categories anytime soon. (This is the complete opposite for the Peace Prize, of course.) What does this all mean? Well, the top quark was a pretty heavy discovery and is well worth its weight in gold, at least in my opinion. However, to whom would you award the prize? No single person at the CDF experiment can justly say she or he discovered the quark; it was a team effort and all CDF personnel can proudly state she or he helped discover the quark.

“Which of the Gang of Six, if the higgs boson is discovered, should get the Nobel, if at all?” is an honest, open question and is well above my pay grade. A similar statement could be made about Supersymmetry.

Turning Nobel Week into Fun-bel Week

Now for the fun part. So during this week, pick your favorite subject, which of course is physics, and go figure out what the whole big hubbub is. Depending on your timezone, this may either be with your morning coffee or afternoon tea. In any case, it is an excuse to learn something new! 🙂

Alternatively, you can check back here Tuesday afternoon (Madison/Chicago time) because I am sure many of us will be commenting on the latest news.

This Week’s Schedule

Live Video Player here.

Physiology or Medicine – Awarded for the discovery of the innate and adaptive immune systems! Okay, really this is great. The human body has evolved to be inherently immune to certain pathogens. The human body, in its resourcefulness, can also adapt and become immune to pathogens. The end result is that when the two are combined and wait a few hundred thousand years,  you get us!

Physics – Awarded for discovering that expansion rate of the universe, is itself increasing. The universe expands, Edwin Hubble discovered that decades ago. Today’s award winners discovered that the universe expands at an accelerating rate! Bravo!

Chemistry – The prize will be announced on Wednesday 5 October, 11:45 a.m. CET [5:45 am  CDT/Chicago].

Peace – The prize will be announced on Friday 7 October, 11:00 a.m. CET [5:00 am  CDT/Chicago].

Economics – The prize will be announced on Monday 10 October, 1:00 p.m. CET [7:00 am  CDT/Chicago].

Literature – To Be Announced

 

 

 

 

Regardless of the outcome, I would love to read everyone’s thoughts and speculations before and after the awards!

Happy Colliding

– richard (@bravelittlemuon)

 

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