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Posts Tagged ‘Fermi National Accelerator Laboratory’

Hadrons, the particles made of quarks, are almost unanimously produced in the two or three quark varieties in particle colliders. However, in the last decade or so, a new frontier has opened up in subatomic physics. Four-quark particles have begun to be observed, the most recent being announced last Thursday by a collaboration at Fermilab. These rare, fleetingly lived particles have the potential to shed some light on the Strong nuclear force and how it shapes our world.

The discovery of a new subatomic particle was announced last Thursday by the DØ (DZero) collaboration at Fermilab in Chicago. DØ researchers analysed data from the Tevatron, a proton-antiproton collider based at Fermilab. The new found particle sports the catchy name “X(5568)” (It’s labelled by the observed mass of 5,568 Megaelectron-volts or MeV. That’s about six times heavier than a proton.) X(5568) is a form of “tetraquark”, a rarer variety of the particles known as hadrons. Tetraquarks consist of two quarks and two antiquarks (rather than the usual three quarks or quark-antiquark pairs that make up hadrons particle physicists are familiar with). While similar tetraquark particles have been observed before, the new addition breaks the mould by consisting of four quarks of totally different flavours: bottom, strange, up and down.

[Regular readers and those familiar with the theory of QCD may wish to skip to the section marked ——]

a) An example of a quark-antiquark pair, known as Mesons. b) An example of a three-quark particle, known as Baryons. c) An example of a tetraquark (four quarks) Source: APS/Alan Stonebraker, via Physics Viewpoint, DOI: 10.1103/Physics.6.69

The particle’s decay is best explained Strong force, aptly named since it’s the strongest known force in the universe[1], which also acts to hold quarks together in more stable configurations such as inside the proton. The Strong force is described by a theory known as Quantum Chromodynamics (QCD for short), a crucial part of the Standard Model of particle physics. The properties of X(5568) will provide precision tests of the Standard Model, as well as improving our understanding of the nature of Confinement. This is a dimly understood process by which quarks are bound up together to form the particles (such as protons) that make up most of the visible matter in the universe.

Quarks are defined by the strong force, being the only particles known to physics that interact via QCD. They were originally conceived of in 1964 by two of the early pioneers of particle physics Murray Gell-Mann and George Zweig, who posited the idea of “quarks” to explain the properties of a plethora of particles that were discovered in the mid-twentieth century. After a series of experiments in the late ‘60s and ‘70s, the evidence in favour of the quark hypothesis grew much stronger[2] and it was accepted that many of the particles that interacted and decayed very quickly (due to the magnitude of the strong force) in detectors were in fact made up of these quarks, which are now known to come in six different varieties known as “flavours”. A more precise model of the strong force, which came to be known as QCD, was also verified in such experiments.

QCD is a very difficult theory to draw predictions from because unlike electromagnetism (the force responsible for holding atoms together and transmitting light between objects), the “force carriers” of QCD known as gluons are self-interacting. Whereas light, or photons, simply pass through one another, gluons pull on one another and quarks in complex ways that give rise to the phenomenon of confinement: quarks are never observed in isolation, only as part of a group of other quarks/antiquarks. These groups of quarks and anti-quarks are what we call Hadrons (hence the name Large Hadron Collider). This self interaction arises from the fact that, unlike light which simply couples to positive or negative charges, QCD has a more complicated structure based on three charges labelled as Red, Green and Blue (which confusingly, have nothing to do with real colours, but are instead based on a mathematical symmetry known as SU(3)).

The hadrons discovered in the twentieth century tended to come in pairs of three quarks or quark-antiquark pairs. Although we now know there is nothing in the theory of QCD that suggests you can’t have particles consisting of four, or even five quarks/antiquarks, such particles were never observed, and in fact even some of the finest minds in theoretical physics (Edward Witten and Sidney Coleman) once thought that QCD would not permit such particles to exist. Like clovers, however, although the fourfold or even fivefold variety would be much rarer to come by it turns out such states did, in fact, exist and could be observed.



A visualisation of the production and decay of X(5568) to mesons in the Tevatron collider. Source: Fermilab http://news.fnal.gov/

The first hints of the existence of tetraquarks were at the Belle experiment, Japan in 2003, with the observation of a state called X(3872) (again, labelled by its mass of 3872 MeV). One of the most plausible explanations for this anomalous resonance[3] was a tetraquark model, which in 2013, an analysis by the LHCb experiment at CERN found to be a compatible explanation of the same resonance found in their detector. The same year, Belle and the BESIII experiment in China both found a resonance with the same characteristics, labelled Zc(3900), which is now believed to be the first independently, experimentally observed tetraquark. The most recent evidence for the existence of tetraquarks, prior to last Thursday’s announcement, was found by the LHCb experiment in 2014, the Z(4430). This verified an earlier result from Belle in 2007, with an astonishingly high statistical significance of 13.9σ (for comparison, one typically claims a discovery with a significance of 5σ). LHCb would also go on, unexpectedly, to find a pentaquark (four quarks and an antiquark) state in 2015, which could provide a greater understanding of QCD and even a window into the study of neutron stars.

Z(4430) was discovered from the analysis of its decay into mesons (hadrons consisting of quark-antiquark pairs), specifically the ψ’ and π mesons from the decay B0 → K + ψ’  π. In the analysis of the B0 decay, it was found that the Z(4430) was needed as an intermediate particle state to explain the resonant behaviour of the ψ’ and π. The LHCb detector, whose asymmetric design and high resolution makes it particularly well suited for the job, reconstructs these mesons and looks at their kinematic properties to determine the shape and properties of the resonance, which were found to be consistent with a tetraquark model. The recent discovery of X(5568) by the DØ collaboration involved a similar reconstruction from Bs and π mesons, which was used to infer its quark flavour structure (b, s, u, d, though which two are the particles and which two are the antiparticles remains to be determined).

X(5568) is found to have a large width (22 MeV) in the distribution of its decays, implying that it decays very quickly, best explained by QCD. Since quarks cannot change flavours in QCD interactions (while they can do so in weak nuclear interactions), this is what allowed DØ to determine its quark content. The other properties of this anomalous particle, such as its mass and its lack of spin (i.e. S = 0) are measured from the kinematics of the mesons it produces, and can help increase our understanding of how QCD combines the quarks in such an unfamiliar arrangement.

The two models for tetraquarks: Left, a single bound state of four quarks. Right, a pair of mesons bound to one another in orbit, resembing a four quark state. Source: Fermilab http://news.fnal.gov (Particle Physicists have a strange relationship with Comic Sans)

One of the long-standing controversies surrounding tetraquark states is whether the states are truly a joint four particle state or in fact a sort of molecule of two strongly bound mesons, which although they form a bound state of four particles in total, is actually analogous to two separate atoms in a molecule rather than a single, heavy atom. The analysis from DØ, based on X(5568)’s mass seems to imply that it’s the former, a single particle of four quarks tightly bound in an exotic hadron, though the jury is still out on the matter.

DØ’s discovery is based on an analysis of the historic data collected from the Tevatron from the 28 years it was operating, since the collider itself ceased operation 2011. Despite LHCb having found tetraquark candidates in the past and being suited to finding such a particle again, it has not yet independently verified the existence of X(5568). LHCb will now review their own data as well as future data that will recommence being collected later this year, to see if they too observe this unprecedented result and hopefully improve our understanding of its properties and whether they are consistent with the Standard Model. This is definitely a result to look out for later this year and should shed some light on one of the fundamental forces of nature and how it acts to create the particles, such as protons, that make up the world around us.

[1] That is, the dimensionless coupling of the force carrier particle interactions is greater than electromagnetism and the weak nuclear force, both of which in turn are stronger than gravity (consider how a tiny magnet can lift a paper clip against the gravity of the entire Earth). Many theories of Beyond the Standard Model physics predict new forces, and it may turn out that all the forces are unified into a single entity at high energies.

[2] For an excellent summary of the history of quarks and some of the motivations behind the quark model, check out this fantastic documentary featuring none other than the Nobel Prize wining physicists, Richard Feynman and Murray Gell-Mann themselves.

[3] Particles are discovered by the bumps or resonances they leave in the statistical distributions of particle decays/scattering events. See for example, one of the excesses of events that led to the discovery of the Higgs Boson.


“There it is — the world’s most beautiful physics experiment,” says physicist Chris Polly from a metal footbridge that crosses over the 14-meter blue steel ring of Brookhaven National Laboratory’s muon g – 2 experiment, now being disassembled. A haze of dust hangs in the air above Polly and a handful of other physicists and engineers who’ve gathered together to help resurrect the $20-million machine by transporting it hundreds of miles to Fermi National Accelerator Laboratory in Illinois. (more…)


As the time for our camera’s first light approaches, workload and excitement increase exponentially among the Dark Energy Survey collaborators and it is about time we start sharing the latter. Beginning today, you will find here at Quantum Diaries an insider’s account of our fast progress, frequently updated as we countdown to first light.

So, here we go. If you haven’t heard of us yet, DES is an experiment designed to investigate dark energy, one of the most trending topics of the last 30 years, featured among the top priorities in the world-wide scientific agenda despite a recent funding blow up. DES will image an area of 5,000-square degrees (nearly 1/8 of the sky) using five optical-bands, obtaining detailed measurements of about 300 million galaxies. With this data we can shed light on the mystery of cosmic acceleration by analyzing four complementary probes: supernovae, baryonic acoustic oscillations, galaxy clusters and weak lensing.

DES will use its own powerful new instrument, the Dark Energy Camera, or DECam, which is under construction at  Fermilab.  Building an entirely new system to answer a specific question is a growing trend in astrophysics, probably a consequence of developing close ties with the field of high energy physics.

This 570-megapixel, giant camera is being tested on a telescope simulator (the yellow and red rings that you see in the video) until the end of this month. As a Fermilab postdoc, I am heavily involved in these tests, together with a team that keeps up the fast pace even during the blizzard last week.

Check out this time-lapse video of the DECam construction:

We are now getting ready for a simulated observing run, a comprehensive integration test of the full system. We will use a star projector to simulate the sky and the goal is to take one night’s worth of data. The atmosphere here at the lab is of stress and excitement as this is our last test of the full system before we bring DECam down from the telescope simulator. The results of this test will be very important to guiding us through the next steps.

So here is where we stand nine months before first light. Stay tuned for more updates here or follow us on Facebook. Leave a message in the comments if you want to know more or would like to visit us while the camera is still up on the simulator.

–Marcelle Soares-Santos


Top left image shows SDSS-III's view of a small part of the sky, centered on the galaxy Messier 33. The middle top picture is a zoomed-in image on M33, showing the spiral arms of this galaxy, including the blue knots of intense star formation. The top right-hand image shows a further zoomed-in image of M33 highlighting one of the largest areas of intense star formation in that galaxy. Credit: SDSS

The world’s largest, digital, color image of the night sky became public this month. It provides a stunning image and research fodder for scientists and science enthusiasts, thanks to the Sloan Digital Sky Survey, which has a long connection to Fermilab.

Oh, yeah, and the image is  free.

The image, which would require 500,000 high-definition TVs to view in its full resolution, is comprised of data collected since the start of the survey in 1998.

“This image provides opportunities for many new scientific  discoveries in the years to come,” said Bob Nichol, SDSS-III scientific spokesperson and professor at University of Portsmouth.

Fermilab oversaw all image processing and distribution of data to researchers and the public from 1998 through 2008, for the first seven batches of data. These batches make up a large chunk of the ground-breaking more than a trillion-pixel image. The eighth batch of raw, reduced data, which was released along with the image at the 17th annual meeting of the American Astronomical Society in Seattle was processed by Lawrence Berkley National Laboratory. LBNL, New York University and Johns Hopkins University distributed that data. Fermilab’s SDSS collaboration members now focus solely on analysis.

“This is one of the biggest bounties in the history of science,” said Mike Blanton, professor from New York University and leader of the data archive work in SDSS-III, the third phase of SDSS.  “This data will be a legacy for the ages, as previous ambitious sky surveys like the Palomar Sky Survey of the 1950s are still being used today. We expect the SDSS data to have that sort of shelf life.”

The release expands the sky coverage of SDSS to include a  sizable view of the south galactic pole. Previously, SDSS only imaged small, spread out slivers of the southern sky. Increasing coverage of the southern sky will aid the Dark Energy Survey and the Large Synoptic Survey Telescope both southern sky surveys that Fermilab participates in.

Comparing the two portions of the sky also will help astrophysicists pinpoint any asymmetries in the type or number of large structures, such as galaxies. Cosmic-scale solutions to Albert Einstein’s equations of general
relativity assume that the universe is spherically symmetric, meaning that on a large enough scale, the universe would look the same in every direction.

Finding asymmetry would mean the current understanding of the universe is wrong and turn the study of cosmology on its head, much as the discovery of particles not included in the Standard Model would do for collider physics.

“We would have to rethink our understanding of cosmology,” said Brian Yanny, Fermilab’s lead scientists on SDSS-III. So far the universe seems symmetric.

Whether the SDSS data reveals asymmetry or not it undoubtedly will continue to provide valuable insight into our universe and fascinate amateur astronomers and researchers.

Every year since the start of the survey, at least one paper about the SDSS has made it in the list of the top 10 astronomy papers of the year. More than 200,000 people have classified galaxies from their home computers using SDSS data and projects including Galaxy Zoo and Galaxy Zoo 2.

In the three months leading up to the image’s release a record number of queries, akin to click counts on a Web page,  occurred on the seventh batch of data. During that time, 90 terabytes of pictures and sky catalogues were down loaded by  scientists and the public. That equates to about 150,000 one-hour long CDs.

Scientists will continue to use the old data and produce papers from it for years to come. Early data also works as a check on the new data to make sure camera or processing flaws didn’t produce data anomalies.

“We still see, for instance, data release six gets considerable hits and papers still come out on that in 100s per year,” Yanny said.

So far, SDSS data has been used to discover nearly half a billion astronomical objects, including asteroids, stars, galaxies and distant quasars. This new  eighth batch of data promises even more discoveries.

Fermilab passed the job of data processing and distribution on to others in 2008. The eight batch of data was processed by Lawrence Berkley National Laboratory and distributed by LBNL, New York University and Johns Hopkins University.

Fermilab’s four remaining SDSS collaboration members now focuses solely

illustration of the concept of baryon acoustic oscillations, which are imprinted in the early universe and can still be seen today in galaxy surveys like BOSS. Credit: Chris Blake and Sam Moorfield and SDSS.

on analysis. They are expected to produce a couple dozen papers during the next few years. The group touches on all of SDSS-III’s four sky surveys but focus mainly on the Baryon Oscillation Spectroscopic Survey, or BOSS, which will map the 3-D distribution of 1.5 million luminous red galaxies.

“BOSS is closest to our scientists’ interests because its science goals are to understand dark energy and dark matter and the evolution of the universe,” Yanny said.

For more information see the following:

* Larger images of the SDSS maps in the northern and southern galactic hemispheres are available here and here.

*Sloan’s YouTube channel provides a 3-D visualization of the universe.

*Technical journal papers describing DR8
and the SDSS-III project can be found on the arXiv e-Print server.

*EarthSky has a good explanation of what the colors in the images represent and how SDSS part of an on-going tradition of sky surveys.

*The Guardian newspaper has a nice article explaining all the detail that can be seen in the image.

— Tona Kunz


CDF kicked off the new year with the paper “CDF Finds Evidence for a Mass Dependant Forward-Backward Production Asymmetry of Top Quarks”, which has drawn some interest in the blogosphere.

The analysis itself is intriguing. Physicists expect to see the number of top quarks and antitop quarks produced along the beam line to meet with Standard Model predictions of nearly equal amounts. But, CDF isn’t seeing that. The collaboration sees more top quarks along the proton direction and fewer top quarks produced along the antiproton direction. CDF co-spokesman Rob Roser explains the analysis nicely in the article below.

But the really intriguing part is what this could all mean. It could signal a hint of new physics not predicted by the Standard Model.  Bloggers at Resonaances and Nature speculate on possible causes for the asymmetry. It could also be nothing more than an anomaly. In the patience-trying nature of particle physics, we will have to wait for more data to know for sure. Fortunately, CDF’s sister detector at the Tevatron, DZero, can do a complimentary analysis, widening the data set. Until that occurs, check out the analysis yourself below.

The article below ran in Fermilab Today January, 7.

CDF finds evidence for top quark production asymmetry

A new analysis that will be presented at a Wine and Cheese lecture at Fermilab today points to an asymmetry in top quark production. This analysis raises the asymmetry of forward and backward top quark production found in a 2008 analysis to a ~3 sigma level.

In nature, symmetry is seen as pleasing and balanced designs, such as the intricate pattern on a tortoise shell or the structure of a snowflake.

In elementary particle physics, symmetry is fundamental to the theories we use to describe the world in which we live. A discrepancy in the symmetry predicted by theories of the Standard Model can point to new types of physics, an anomaly in the data or that the current theories need revision.

CDF researchers have measured the symmetry of how top quarks emerge from collisions, forward or backward, and how they decay. This analysis was performed for the first time in 2006 at CDF. CDF and DZero both published their inclusive analysis of this asymmetry in 2008. These highly cited white papers already pointed to an anomaly that has generated much interest in the theoretical community. This latest result takes the 2008 publications a step further, by adding more data, and looking at the dependence on the mass of the system. It is this dependence that is most discrepant with the Standard Model.

Fermilab’s Tevatron produces collisions that create top quark and anti-top-quark pairs via the strong force. Simple theoretical calculations predict that the Tevatron detectors should observe symmetric distributions of both top and antitop quarks. However, more detailed calculations suggest that these oppositely charged particles should have a slight preference as to how they emerge from the collisions.

The origins of this symmetry are subtle, but CDF has the sensitivity to be able to observe the 6 percent imbalance, which is predicted by the Standard Model. This result shows that nature prefers an imbalance that is even larger than predicted.

It is important to determine whether the top quark we are observing  behaves the way we expect this Standard Model object to act. There are a number of Beyond-the-Standard-Model theories such as Z’ (pronounced Z-prime) and large extra dimensions that predicts much higher asymmetries. By measuring this asymmetry in top quark production, CDF physicists can compare it to theoretical expectations and probe for potentially undiscovered new physics.

The figures show the number of top events as a function of delta rapidity. The blue shape is that of the background, the green is the Standard Model prediction for top, and the points are our data. The plot on the left contains events in which the ttbar mass is less than 450 GeV/c2 and is very symmetric. The plot on the right is for a ttbar mass of greater than 450 GeV/c2 and illustrates the discrepancy between expected and observed.

Utilizing 5.3 inverse femtobarns of data, CDF measured the top forward backward asymmetry and observed significant asymmetries when studying the production and the production as related to the pairs’ center of mass energy, t-tbar.

When considering the pairs’ mass, the asymmetry is dependent on both the mass and the direction of the production. Scientists expect that the same number of top quarks and antitop quarks would be produced along the beam line, but CDF saw that more tops were produced along the proton direction of the beam and fewer tops produced along the antiproton direction of the beam. This effect is magnified when one looks at the mass dependence of the top-antitop system.

For Mttbar> 450 GeV/c2the asymmetry is measured to be 48 ± 11 percent, three standard deviations from Standard Model expectation (9 ± 1 percent). Some theories suggest that such a mass dependence could be evidence of a massive new particle just out of reach at the Tevatron’s collision energy.

The LHC, a machine with significantly higher energy, cannot easily study this phenomenon, since the LHC does not make protons and antiprotons collide. However a new particle would still be observable in energy spectrum at the LHC. If the result at CDF truly is a sign of new physics, it may be that both machines will be required to understand its nature.

This result may provide the first important clue that there is new physics beyond the Standard Model. There may still be other interesting results as scientists from both Fermilab experiments continue to analyze the Tevatron’s now nearly 10 inverse femtobarns of sample data.

— Rob Roser, CDF co-spokesperson