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Posts Tagged ‘LHC’

This article appeared in Fermilab Today on April 3, 2015.

This magnet recently achieved an important milestone, reaching its design field of 11.5 Tesla. It is the first successful niobium-3-tin, twin-aperture accelerator magnet in the world. Photo: Sean Johnson

This magnet recently achieved an important milestone, reaching its design field of 11.5 Tesla. It is the first successful niobium-3-tin, twin-aperture accelerator magnet in the world. Photo: Sean Johnson

Last month, a new superconducting magnet developed and fabricated at Fermilab reached its design field of 11.5 Tesla at a temperature nearly as cold as outer space. It is the first successful twin-aperture accelerator magnet made of niobium-3-tin in the world.

The advancements in niobium-3-tin, or Nb3Sn, magnet technology and the ongoing U.S. collaboration with CERN on the development of these and other Nb3Sn magnets are enabling the use of this innovative technology for future upgrades of the Large Hadron Collider (LHC). They may also provide the cornerstone for future circular machines of interest to the worldwide high-energy physics community. Because of the exceptional challenges — Nb3Sn is brittle and requires high-temperature processing — this important milestone was achieved at Fermilab after decades of worldwide R&D efforts both in the Nb3Sn conductor itself and in associated magnet technologies.

Superconducting magnets are at the heart of most particle accelerators for fundamental science as well as other scientific and technological applications. Superconductivity is also being explored for use in biosensors and quantum computing.

Thanks to Nb3Sn’s stronger superconducting properties, it enables magnets of larger field than any in current particle accelerators. As a comparison, the niobium-titanium dipole magnets built in the early 1980s for the Tevatron particle collider produced about 4 Tesla to bend the proton and antiproton beams around the ring. The most powerful niobium-titanium magnets used in the LHC operate at roughly 8 Tesla. The new niobium-3-tin magnet creates a significantly stronger field.

Because the Tevatron accelerated positively charged protons and negatively charged antiprotons, its magnets had only one aperture. By contrast, the LHC uses two proton beams. This requires two-aperture magnets with fields in opposite directions. And because the LHC collides beams at higher energies, it requires larger magnetic fields.

In the process of upgrading the LHC and in conceiving future particle accelerators and detectors, the high-energy physics community is investing as never before in high-field magnet technologies. This creative process involves the United States, Europe, Japan and other Asian countries. The latest strategic plan for U.S. high-energy physics, the 2014 report by the Particle Physics Project Prioritization Panel, endorses continued U.S. leadership in superconducting magnet technology for future particle physics programs. The U.S. LHC Accelerator Research Program (LARP), which comprises four DOE national laboratories — Berkeley Lab, Brookhaven Lab, Fermilab and SLAC — plays a key role in this strategy.

The 15-year investment in Nb3Sn technology places the Fermilab team led by scientist Alexander Zlobin at the forefront of this effort. The Fermilab High-Field Magnet Group, in collaboration with U.S. LARP and CERN, built the first reproducible series in the world of single-aperture 10- to 12-Tesla accelerator-quality dipoles and quadrupoles made of Nb3Sn, establishing a strong foundation for the LHC luminosity upgrade at CERN.

The laboratory has consistently carried out in parallel an assertive superconductor R&D program as key to the magnet success. Coordination with industry and universities has been critical to improve the performance of the next generation of high-field accelerator magnets.

The next step is to develop 15-Tesla Nb3Sn accelerator magnets for a future very high-energy proton-proton collider. The use of high-temperature superconductors is also becoming a realistic prospect for generating even larger magnetic fields. An ultimate goal is to develop magnet technologies based on combining high- and low-temperature superconductors for accelerator magnets above 20 Tesla.

The robust and versatile infrastructure that was developed at Fermilab, together with the expertise acquired by the magnet scientists and engineers in design and analysis tools for superconducting materials and magnets, makes Fermilab an ideal setting to look to the future of high-field magnet research.

Emanuela Barzi

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I don’t usually get to spill the beans on a big discovery like this, but this time, I DO!

CERN Had Dark Energy All Along!!

That’s right. That mysterious energy making up ~68% of the universe was being used all along at CERN! Being based at CERN now, I’ve had a first hand glimpse into the dark underside of Dark Energy. It all starts at the Crafted Refilling of Empty Mugs Area (CREMA), pictured below.

One CREMA station at CERN

 

Researchers and personnel seem to stumble up to these stations at almost all hours of the day, looking very dreary and dazed. They place a single cup below the spouts, and out comes a dark and eerie looking substance, which is then consumed. Some add a bit of milk for flavor, but all seem perkier and refreshed after consumption. Then they disappear from whence they came. These CREMA stations seem to be everywhere, from control rooms to offices, and are often found with groups of people huddled around them. In fact, they seem to exert a force on all who use them, keeping them in stable orbits about the stations.

In order to find out a little bit more about this mysterious substance and its dispersion, I asked a graduating student, who wished to remain unnamed, a little bit about their experiences:

Q. How much of this dark stuff do you consume on a daily basis?

A. At least one cup in the morning to fuel up, I don’t think I could manage to get to lunchtime without that one. Then multiple other cups distributed over the day, depending on the workload. It always feels like they help my thinking.

Q. Do you know where it comes from?

A. We have a machine in our office which takes capsules. I’m not 100% sure where those capsules are coming from, but they seem to restock automatically, so no one ever asked.

Q. Have you been hiding this from the world on purpose?

A. Well our stock is important to our group, if we would just share it with everyone around we could run out. And no one of us can make it through the day without. We tried alternatives, but none are so effective.

Q. Do you remember the first time you tried it?

A. Yes, they hooked me on it in university. From then on nothing worked without!

Q. Where does CERN get so much of it?

A. I never thought about this question. I think I’m just happy that there is enough for everyone here, and physicist need quite a lot of it to work.

In order to gauge just how much of this Dark Energy is being consumed, I studied the flux of people from the cafeteria as a function of time with cups of Dark Energy. I’ve compiled the results into the Dark Energy Consumption As Flux (DECAF) plot below.

Dark Energy Consumption as Flux plot. Taken March 31, 2015. Time is given in 24h time. Errors are statistical.

 

As the DECAF plot shows, there is a large spike in consumption, particularly after lunch. There is a clear peak at times after 12:20 and before 13:10. Whether there is an even larger peak hiding above 13:10 is not known, as the study stopped due to my advisor asking “shouldn’t you be doing actual work?”

There is an irreducible background of Light Energy in the cups used for Dark Energy, particularly of the herbal variety. Fortunately, there is often a dangly tag hanging off of the cup  to indicate to others that they are not using the precious Dark Energy supply, and provide a clear signal for this study to eliminate the background.

While illuminating, this study still does not uncover the exact nature of Dark Energy, though it is clear that it is fueling research here and beyond.

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The LHC turns back on this year for Run II. What might we see day 1?

The highest-p_T jet event collected by the end of September 2012 (Event 37979867, Run 208781): the two central high-p_T jets have an invariant mass of 4.47 TeV, and the highest-p_T jet has a p_T of 2.34 TeV, and the subleading jet has a p_T of 2.10 TeV. The missing E_T and Sum E_T for this event are respectively 115 GeV and 4.97 TeV. Only tracks with p_T> 0.7 GeV are displayed. The event was collected on August 17th, 2012. Image and caption credit: ATLAS

The highest-p_T jet event collected by the end of September 2012 (Event 37979867, Run 208781): the two central high-p_T jets have an invariant mass of 4.47 TeV; the highest-p_T jet has a p_T of 2.34 TeV, and the subleading jet has a p_T of 2.10 TeV. The missing E_T and Sum E_T for this event are respectively 115 GeV and 4.97 TeV. Only tracks with p_T> 0.7 GeV are displayed. The event was collected on August 17th, 2012. Image and caption credit: ATLAS

In seven weeks CERN’s Large Hadron Collider (LHC), the largest and most energetic particle accelerator in history, is scheduled to turn back on. The LHC has been shutdown since December 2012 in order for experimentalists to repair and upgrade the different detector experiments as well as the collider itself. When recommissioning starts, the proton beams will be over 60% more energetic than before and probe a regime of physics we have yet to explore directly. With this in mind, today’s post is about a type of new physics that, if it exists, we can potentially see in the first days of LHC Run II: excited quarks.

Excited Quarks and Composite Quarks

Excited quarks are interesting little beasts and are analogous to excited atoms in atomic physics. When light (a photon) is shined onto an atom, electrons orbiting the nucleus will become energized and are pushed into higher, metastable orbits. This is called an excited atom.

excited_atom_NASA

After some estimable and often measurable period of time, an electron will radiate light (photon) and drop down to its original orbit. When this happens, we say that an excited atom has relaxed to its ground state.

emission_atom_NASA

In analogy, if quarks were bound states of something smaller, i.e., if they were composite particles, then we can pump energy into a quark, excite it, and then watch the excited quark relax back into its ground state.

Feynman diagram representing heavy excited quark (q*) production from quark (q)-gluon (g) scattering in proton collisions.

Feynman diagram representing heavy excited quark (q*) production from quark (q)-gluon (g) scattering in proton collisions.

Observing an excited quark would tell us that the quark model may not be the whole story after all. Presently, the quark model is the best description of protons and neutrons, and it certainly works very, very well, but this does not have to be the case. Nature may have something special in store for us. However, this is not why I think excited quarks are so odd and interesting. What is not obvious is that excited quarks, if they exist, could show up immediately after turning the LHC back on.

Early Dijet Discoveries at LHC Run II

Excited quarks participate in the strong nuclear force (QCD) just like ordinary quarks, which means they can absorb and radiate gluons with equal strength. This is key because protons at the LHC are just brimming with highly energetic quarks and gluons. Of particles in a proton carrying a small-to-medium fraction of the proton’s total energy, gluons are the most commonly found particle in a proton (red g curve below). Of those particles carrying a large fraction of the proton’s energy, the up and down quarks are the most common particles (blue u and green d curves below). Excited quarks, if they exist, are readily produced because their ingredients are the most commonly found particles in the proton.

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Distributions of partons in a proton. The x-axis represents the fraction of the proton’s energy a parton has (x=1 means that the parton has 100% of the proton’s energy). The y-axis represents the likelihood of observing a parton. The left (right) plot corresponds to low (high) energy collisions. Credit: MSTW

When an excited quark decays, it will split back into quark and gluon pair. These two particles will be very energetic (each will have energy equal to half the mass of the excited quark due to energy conservation), will be back-to-back (by linear momentum conservation), and will each form jets (hadronization in QCD). Such collisions are called “dijet” events (pronounced: die-jet) and look like this

Words. Credit: CMS

Display for the event with the highest dijet mass (5.15 TeV) observed in CMS data. Image and caption credit: CMS

Although gluons and quarks in the Standard Model can mimic the signal, one can add up the energies of the two jets (which would equal the excited quark’s mass due to energy conservation) and expect to see a bump in the data centered about the mass of the excited quark. Unfortunately, the data (below) do not show such a bump, indicating that excited quarks with masses below a couple TeV do not exist.

cms_dijet_spectrum

Inclusive dijet mass spectrum from wide jets (points) compared to a fit (solid curve) and to predictions including detector simulation of multijet events and signal resonances. The predicted multijet shape (QCD MC) has been scaled to the data. The vertical error bars are statistical only and the horizontal error bars are the bin widths. For comparison, the signal distributions for a W resonance of mass 1.9 CMS.TeV and an excited quark of mass 3.6 CMS.TeV are shown. The bin-by-bin fit residuals scaled to the statistical uncertainty of the data are shown at the bottom and compared with the expected signal contributions. Image and caption credit: CMS

However, this does not mean that excited quarks do not or cannot exist at higher masses. If they do, and if their masses are within the energy reach of the LHC, then excited quarks are very much something we might see in just a few months from now.

Happy Colliding,

Richard Ruiz (@BraveLittleMuon)

Appreciation to Ms. Frost and her awesome physics classes at Whitney M. Young High School in Chicago, Illinois for motivating this post. Good luck on your AP exams!

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Doubly charged Higgs bosons and lepton number violation are wickedly cool.

Hi Folks,

The Standard Model (SM) of particle physics is presently the best description of matter and its interactions at small distances and high energies. It is constructed based on observed conservation laws of nature. However, not all conservation laws found in the SM are intentional, for example lepton number conservation. New physics models, such as those that introduce singly and doubly charged Higgs bosons, are flexible enough to reproduce previously observed data but can either conserve or violate these accidental conservation laws. Therefore, some of the best ways of testing if these types of laws are much more fundamental may be with the help of new physics.

Observed Conservation Laws of Nature and the Standard Model

Conservation laws, like the conservation of energy or the conservation of linear momentum, have the most remarkable impact on life and the universe. Conservation of energy, for example, tells us that cars need fuel to operate and perpetual motion machines can never exist. A football sailing across a pitch does not suddenly jerk to the left at 90º because conversation of linear momentum, unless acted upon by a player (a force). This is Newton’s First Law of Motion. In particle physics, conservation laws are not taken lightly; they dictate how particles are allowed to behave and forbid some processes from occurring. To see this in action, lets consider a top quark (t) decaying into a W boson and a bottom quark (b).

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asdasd

A top quark cannot radiate a W+ boson and remain a top quark because of conservation of electric charge. Top quarks have an electric charge of +2/3 e, whereas W+ bosons have an electric charge of +1e, and we know quite well that

(+2/3)e ≠ (+1)e + (+2/3)e.

For reference a proton has an electric charge of +1e and an electron has an electric charge of -1e. However, a top quark can radiate a W+ boson and become a bottom quark, which has electric charge of -1/3e. Since

(+2/3)e = (+1)e + (-1/3)e,

we see that electric charge is conserved.

Conservation of energy, angular momentum, electric charged, etc., are so well-established that the SM is constructed to automatically obey these laws. If we pick any mathematical term in the SM that describes how two or more particles interact (for example how the top quark, bottom quark, and W boson interact with each other) and then add up the electric charge of all the participating particles, we will find that the total electric charge is zero:

The top quark-bottom quark-W boson vertices in the Standard Model, and the net charge carried by each interaction.

The top quark-bottom quark-W boson interaction terms in the Standard Model. Bars above quarks indicate that the quark is an antiparticle and has opposite charges.

 

Accidental Conservation Laws

However, not all conservation laws that appear in the SM are intentional. Conservation of lepton number is an example of this. A lepton is any SM fermion that does not interact with the strong nuclear force. There are six leptons in total: the electron, muon, tau, electron-neutrino, muon-neutrino, and tau-neutrino. We assign lepton number

L=1 to all leptons (electron, muon, tau, and all three neutrinos),

L=-1 to all antileptons (positron, antimuon, antitau, and all three antineutrinos),

L=0 to all other particles.

With these quantum number assignments, we see that lepton number is a conserved in the SM. To clarify this important point: we get lepton number conservation for free due to our very rigid requirements when constructing the SM, namely the correct conservation laws (e.g., electric and color charge) and particle content. Since lepton number conservation was not intentional, we say that lepton number is accidentally conserved. Just as we counted the electric charge for the top-bottom-W interaction, we can count the net lepton number for the electron-neutrino-W interaction in the SM and see that lepton number really is zero:

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The W boson-neutrino-electron interaction terms in the Standard Model. Bars above leptons indicate that the lepton is an antiparticle and has opposite charges.

However, lepton number conservation is not required to explain data. At no point in constructing the SM did we require that it be conserved. Because of this, many physicists question whether lepton number is actually conserved. It may be, but we do not know. This is indeed one topic that is actively researched. An interesting example of a scenario in which lepton number conservation could be tested is the class of theories with singly and doubly charged Higgs boson. That is right, there are theories containing additional Higgs bosons that an electric charged equal or double the electric charge of the proton.

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Models with scalar SU(2) triplets contain additional neutral Higgs bosons as well as singly and doubly charged Higgs bosons.

Doubly charged Higgs bosons have an electric charge that is twice as large as a proton (2e), which leads to rather peculiar properties. As discussed above, every interaction between two or more particles must respect the SM conservation laws, such as conservation of electric charge. Because of this, a doubly charged Higgs (+2e) cannot decay into a top quark (+2/3 e) and an antibottom quark (+1/3 e),

(+2)e ≠ (+2/3)e + (+1/3)e.

However, a doubly charged Higgs (+2e) can decay into two W bosons (+1e) or two antileptons (+1e) with the same electric charge,

(+2)e = (+1)e + (+1)e.

but that is it. A doubly charged Higgs boson cannot decay into any other pair of SM particles because it would violate electric charge conservation. For these two types of interactions, we can also check whether or not lepton number is conserved:

For the decay into same-sign W boson pairs, the total lepton number is 0L + 0L + 0L = 0L. In this case, lepton number is conserved!

For the decay into same-sign leptons pairs, the total lepton number is 0L + (-1)L + (-1)L = -2L. In this case, lepton number is violated!

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Doubly charged Higgs boson interactions for same-sign W boson pairs and same-sign electron pairs. Bars indicate antiparticles. C’s indicate charge flipping.

Therefore if we observe a doubly charged Higgs decaying into a pair of same-sign leptons, then we have evidence that lepton number is violated. If we only observe doubly charged Higgs decaying into same-sign W bosons, then one may speculate that lepton number is conserved in the SM.

Doubly Charged Higgs Factories

Doubly charged Higgs bosons do not interact with quarks (otherwise it would violate electric charge conservation), so we have to rely on vector boson fusion (VBF) to produce them. VBF is when two bosons from on-coming quarks are radiated and then scatter off each other, as seen in the diagram below.

Figure 2: Diagram depicting the process known as WW Scattering, where two quarks from two protons each radiate a W boson that then elastically interact with one another.

Diagram depicting the process known as WW Scattering, where two quarks from two protons each radiate a W boson that then elastically interact with one another.

If two down quarks, one from each oncoming proton, radiate a W- boson (-1e) and become up quarks, the two W- bosons can fuse into a negatively, doubly charged Higgs (-2e). If lepton number is violated, the Higgs boson can decay into a pair of same-sign electrons (2x -1e). Counting lepton number at the beginning of the process (L = 0 – 0 = 0) and at the end (L = 0 – 2 = -2!), we see that it changes by two units!

Same-sign W- pairs fusing into a doubly charged Higgs boson that decays into same-sign electrons.

Same-sign W- pairs fusing into a doubly charged Higgs boson that decays into same-sign electrons.

If lepton number is not violated, we will never see this decay and only see decays to two very, very energetic W- boson (-1e). Searching for vector boson fusion as well as lepton number violation are important components of the overarching Large Hadron Collider (LHC) research program at CERN. Unfortunately, there is no evidence for the existence of doubly charged scalars. On the other hand, we do have evidence for vector boson scattering (VBS) of the same-sign W bosons! Additional plots can be found on ATLAS’ website.  Reaching this tremendous milestone is a triumph for the LHC experiments. Vector boson fusion is a very, very, very, very, very rare process in the Standard Model and difficult to separate from other SM processes. Finding evidence for it is a first step in using the VBF process as a probe of new physics.

Words. Credit: Junjie Zhu (Michigan)

Same-sign W boson scattering candidate event at the LHC ATLAS experiment. Slide credit: Junjie Zhu (Michigan)

We have observed that some quantities, like momentum and electric charge, are conserved in nature. Conservation laws are few and far between, but are powerful. The modern framework of particle physics has these laws built into them, but has also been found to accidentally conserve other quantities, like lepton number. However, as lepton number is not required to reproduce data, it may be the case that these accidental laws are not, in fact, conserved. Theories that introduce charged Higgs bosons can reproduce data but also predict new interactions, such as doubly charged Higgs bosons decaying to same-sign W boson pairs and, if lepton number is violated, to same-sign charged lepton pairs. These new, exotic particles can be produced through vector boson fusion of two same-sign W boson pairs. VBF is a rare process in the SM and can greatly increase if new particles exist. At last, there is evidence for vector boson scattering of same-sign W bosons, and may be the next step to discovering new particles and new laws of nature!

Happy Colliding

– Richard (@BraveLittleMuon)

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The Ties That Bind

Sunday, January 18th, 2015
Cleaning the ATLAS Experiment

Beneath the ATLAS detector – note the well-placed cable ties. IMAGE: Claudia Marcelloni, ATLAS Experiment © 2014 CERN.

A few weeks ago, I found myself in one of the most beautiful places on earth: wedged between a metallic cable tray and a row of dusty cooling pipes at the bottom of Sector 13 of the ATLAS Detector at CERN. My wrists were scratched from hard plastic cable ties, I had an industrial vacuum strapped to my back, and my only light came from a battery powered LED fastened to the front of my helmet. It was beautiful.

The ATLAS Detector is one of the largest, most complex scientific instruments ever constructed. It is 46 meters long, 26 meters high, and sits 80 metres underground, completely surrounding one of four points on the Large Hadron Collider (LHC), where proton beams are brought together to collide at high energies.  It is designed to capture remnants of the collisions, which appear in the form of particle tracks and energy deposits in its active components. Information from these remnants allows us to reconstruct properties of the collisions and, in doing so, to improve our understanding of the basic building blocks and forces of nature.

On that particular day, a few dozen of my colleagues and I were weaving our way through the detector, removing dirt and stray objects that had accumulated during the previous two years. The LHC had been shut down during that time, in order to upgrade the accelerator and prepare its detectors for proton collisions at higher energy. ATLAS is constructed around a set of very large, powerful magnets, designed to curve charged particles coming from the collisions, allowing us to precisely measure their momenta. Any metallic objects left in the detector risk turning into fast-moving projectiles when the magnets are powered up, so it was important for us to do a good job.

ATLAS Big Wheel

ATLAS is divided into 16 phi sectors with #13 at the bottom. IMAGE: Steven Goldfarb, ATLAS Experiment © 2014 CERN

The significance of the task, however, did not prevent my eyes from taking in the wonder of the beauty around me. ATLAS is shaped somewhat like a large barrel. For reference in construction, software, and physics analysis, we divide the angle around the beam axis, phi, into 16 sectors. Sector 13 is the lucky sector at the very bottom of the detector, which is where I found myself that morning. And I was right at ground zero, directly under the point of collision.

To get to that spot, I had to pass through a myriad of detector hardware, electronics, cables, and cooling pipes. One of the most striking aspects of the scenery is the ironic juxtaposition of construction-grade machinery, including built-in ladders and scaffolding, with delicate, highly sensitive detector components, some of which make positional measurements to micron (thousandth of a millimetre) precision. All of this is held in place by kilometres of cable trays, fixings, and what appear to be millions of plastic (sometimes sharp) cable ties.

Inside the ATLAS Detector

Scaffolding and ladder mounted inside the precision muon spectrometer. IMAGE: Steven Goldfarb, ATLAS Experiment © 2014 CERN.

The real beauty lies not in the parts themselves, but rather in the magnificent stories of international cooperation and collaboration that they tell. The cable tie that scratched my wrist secures a cable that was installed by an Iranian student from a Canadian university. Its purpose is to carry data from electronics designed in Germany, attached to a detector built in the USA and installed by a Russian technician.  On the other end, a Japanese readout system brings the data to a trigger designed in Australia, following the plans of a Moroccan scientist. The filtered data is processed by software written in Sweden following the plans of a French physicist at a Dutch laboratory, and then distributed by grid middleware designed by a Brazilian student at CERN. This allows the data to be analyzed by a Chinese physicist in Argentina working in a group chaired by an Israeli researcher and overseen by a British coordinator.  And what about the cable tie?  No idea, but that doesn’t take away from its beauty.

There are 178 institutions from 38 different countries participating in the ATLAS Experiment, which is only the beginning.  When one considers the international make-up of each of the institutions, it would be safe to claim that well over 100 countries from all corners of the globe are represented in the collaboration.  While this rich diversity is a wonderful story, the real beauty lies in the commonality.

All of the scientists, with their diverse social, cultural and linguistic backgrounds, share a common goal: a commitment to the success of the experiment. The plastic cable tie might scratch, but it is tight and well placed; its cable is held correctly and the data are delivered, as expected. This enormous, complex enterprise works because the researchers who built it are driven by the essential nature of the mission: to improve our understanding of the world we live in. We share a common dedication to the future, we know it depends on research like this, and we are thrilled to be a part of it.

ATLAS Collaboration Members

ATLAS Collaboration members in discussion. What discoveries are in store this year? IMAGE: Claudia Marcelloni, ATLAS Experiment © 2008 CERN.

This spring, the LHC will restart at an energy level higher than any accelerator has ever achieved before. This will allow the researchers from ATLAS, as well as the thousands of other physicists from partner experiments sharing the accelerator, to explore the fundamental components of our universe in more detail than ever before. These scientists share a common dream of discovery that will manifest itself in the excitement of the coming months. Whether or not that discovery comes this year or some time in the future, Sector 13 of the ATLAS detector reflects all the beauty of that dream.

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2015: The LHC returns

Saturday, January 10th, 2015

I’m really not one for New Year’s resolutions, but one that I ought to make is to do more writing for the US LHC blog.  Fortunately, this is the right year to be making that resolution, as we will have quite a lot to say in 2015 — the year that the Large Hadron Collider returns!  After two years of maintenance and improvements, everyone is raring to go for the restart of the machine this coming March.  There is still a lot to do to get ready.  But once we get going, we will be entering a dramatic period for particle physics — one that could make the discovery of the Higgs seem humdrum.

The most important physics consideration for the new run is the increase of the proton collision energy from 8 TeV to 13 TeV.  Remember that the original design energy of the LHC is 14 TeV — 8 TeV was just an opening step.  As we near the 14 TeV point, we will be able to do the physics that the LHC was meant to do all along.  And it is important to remember that we have no feasible plan to build an accelerator that can reach a higher energy on any near time horizon.  While we will continue to learn more as we record more and more data, through pursuits like precision measurements of the properties of the Higgs boson, it is increases in energy that open the door to the discovery of heavy particles, and there is no major energy increase coming any time soon.  If there is any major breakthrough to be made in the next decade, it will probably come within the first few years of it, as we get our first look at 13 TeV proton collisions.

How much is our reach for new physics extended with the increase in energy?  One interesting way to look at it is through a tool called Collider Reach that was devised by theorists Gavin Salam and Andreas Weiler.  (My apologies to them if I make any errors in my description of their work.)  This tool makes a rough estimate of the mass scale of new physics that we could have access to at a new LHC energy given previous studies at an old LHC energy, based on our understanding of how the momentum distributions of the quarks and gluons inside the proton evolve to the new beam energy.  There are many assumptions made for this estimate — in particular, that the old data analysis will work just as well under new conditions.  This might not be the case, as the LHC will be running not just at a higher energy, but also a higher collision rate (luminosity), which will make the collisions more complicated and harder to interpret.  But the tool at least gives us an estimate of the improved reach for new physics.

During the 2008 LHC run at 8 TeV, each experiment collected about 20 fb-1 of proton collision data.  In the upcoming “Run 2″ of the LHC at 13 TeV, which starts this year and is expected to run through the middle of 2018, we expect to record about 100 fb-1 of data, a factor of five increase.  (This is still a fairly rough estimate of the future total dataset size.)  Imagine that in 2008, you were looking for a particular model of physics that predicted a new particle, and you found that if that particle actually existed, it would have to have a mass of at least 3 TeV — a mass 24 times that of the Higgs boson.  How far in mass reach could your same analysis go with the Run 2 data?  The Collider Reach tool tells you:

100fb

Using the horizontal axis to find the 3 TeV point, we then look at the height of the green curve to tell us what to expect in Run 2.  That’s a bit more than 5 TeV — a 70% increase in the mass scale that your data analysis would have sensitivity to.

But you are impatient — how well could we do in 2015, the first year of the run?  We hope to get about 10 fb-1 this year. Here’s the revised plot:

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The reach of the analysis is about 4 TeV. That is, with only 10% of the data, you get 50% of the increase in sensitivity that you would hope to achieve in the entire run.  So this first year counts!  One year from now, we will know a lot about what physics we have an opportunity to look at in the next few years — and if nature is kind to us, it will be something new and unexpected.

So what might this new physics be?  What are the challenges that we face in getting there?  How are physicists preparing to meet them?  You’ll be hearing a lot more about this in the year to come — and if I can keep to my New Year’s resolution, some of it you’ll hear from me.

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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

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This Fermilab press release came out on Oct. 20, 2014.

ESnet to build high-speed extension for faster data exchange between United States and Europe. Image: ESnet

ESnet to build high-speed extension for faster data exchange between United States and Europe. Image: ESnet

Scientists across the United States will soon have access to new, ultra-high-speed network links spanning the Atlantic Ocean thanks to a project currently under way to extend ESnet (the U.S. Department of Energy’s Energy Sciences Network) to Amsterdam, Geneva and London. Although the project is designed to benefit data-intensive science throughout the U.S. national laboratory complex, heaviest users of the new links will be particle physicists conducting research at the Large Hadron Collider (LHC), the world’s largest and most powerful particle collider. The high capacity of this new connection will provide U.S. scientists with enhanced access to data at the LHC and other European-based experiments by accelerating the exchange of data sets between institutions in the United States and computing facilities in Europe.

DOE’s Brookhaven National Laboratory and Fermi National Accelerator Laboratory—the primary computing centers for U.S. collaborators on the LHC’s ATLAS and CMS experiments, respectively—will make immediate use of the new network infrastructure once it is rigorously tested and commissioned. Because ESnet, based at DOE’s Lawrence Berkeley National Laboratory, interconnects all national laboratories and a number of university-based projects in the United States, tens of thousands of researchers from all disciplines will benefit as well.

The ESnet extension will be in place before the LHC at CERN in Switzerland—currently shut down for maintenance and upgrades—is up and running again in the spring of 2015. Because the accelerator will be colliding protons at much higher energy, the data output from the detectors will expand considerably—to approximately 40 petabytes of raw data per year compared with 20 petabytes for all of the previous lower-energy collisions produced over the three years of the LHC first run between 2010 and 2012.

The cross-Atlantic connectivity during the first successful run for the LHC experiments, which culminated in the discovery of the Higgs boson, was provided by the US LHCNet network, managed by the California Institute of Technology. In recent years, major research and education networks around the world—including ESnet, Internet2, California’s CENIC, and European networks such as DANTE, SURFnet and NORDUnet—have increased their backbone capacity by a factor of 10, using sophisticated new optical networking and digital signal processing technologies. Until recently, however, higher-speed links were not deployed for production purposes across the Atlantic Ocean—creating a network “impedance mismatch” that can harm large, intercontinental data flows.

An evolving data model
This upgrade coincides with a shift in the data model for LHC science. Previously, data moved in a more predictable and hierarchical pattern strongly influenced by geographical proximity, but network upgrades around the world have now made it possible for data to be fetched and exchanged more flexibly and dynamically. This change enables faster science outcomes and more efficient use of storage and computational power, but it requires networks around the world to perform flawlessly together.

“Having the new infrastructure in place will meet the increased need for dealing with LHC data and provide more agile access to that data in a much more dynamic fashion than LHC collaborators have had in the past,” said physicist Michael Ernst of DOE’s Brookhaven National Laboratory, a key member of the team laying out the new and more flexible framework for exchanging data between the Worldwide LHC Computing Grid centers.

Ernst directs a computing facility at Brookhaven Lab that was originally set up as a central hub for U.S. collaborators on the LHC’s ATLAS experiment. A similar facility at Fermi National Accelerator Laboratory has played this role for the LHC’s U.S. collaborators on the CMS experiment. These computing resources, dubbed Tier 1 centers, have direct links to the LHC at the European laboratory CERN (Tier 0).  The experts who run them will continue to serve scientists under the new structure. But instead of serving as hubs for data storage and distribution only among U.S.-based collaborators at Tier 2 and 3 research centers, the dedicated facilities at Brookhaven and Fermilab will be able to serve data needs of the entire ATLAS and CMS collaborations throughout the world. And likewise, U.S. Tier 2 and Tier 3 research centers will have higher-speed access to Tier 1 and Tier 2 centers in Europe.

“This new infrastructure will offer LHC researchers at laboratories and universities around the world faster access to important data,” said Fermilab’s Lothar Bauerdick, head of software and computing for the U.S. CMS group. “As the LHC experiments continue to produce exciting results, this important upgrade will let collaborators see and analyze those results better than ever before.”

Ernst added, “As centralized hubs for handling LHC data, our reliability, performance and expertise have been in demand by the whole collaboration, and now we will be better able to serve the scientists’ needs.”

An investment in science
ESnet is funded by DOE’s Office of Science to meet networking needs of DOE labs and science projects. The transatlantic extension represents a financial collaboration, with partial support coming from DOE’s Office of High Energy Physics (HEP) for the next three years. Although LHC scientists will get a dedicated portion of the new network once it is in place, all science programs that make use of ESnet will now have access to faster network links for their data transfers.

“We are eagerly awaiting the start of commissioning for the new infrastructure,” said Oliver Gutsche, Fermilab scientist and member of the CMS Offline and Computing Management Board. “After the Higgs discovery, the next big LHC milestones will come in 2015, and this network will be indispensable for the success of the LHC Run 2 physics program.”

This work was supported by the DOE Office of Science.
Fermilab is America’s premier national laboratory for particle physics and accelerator 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.

Brookhaven National Laboratory is supported by the Office of Science of the U.S. Department of Energy.  The 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.

One of ten national laboratories overseen and primarily funded by the Office of Science of the U.S. Department of Energy (DOE), Brookhaven National Laboratory conducts research in the physical, biomedical, and environmental sciences, as well as in energy technologies and national security. Brookhaven Lab also builds and operates major scientific facilities available to university, industry and government researchers. Brookhaven is operated and managed for DOE’s Office of Science by Brookhaven Science Associates, a limited-liability company founded by the Research Foundation for the State University of New York on behalf of Stony Brook University, the largest academic user of Laboratory facilities, and Battelle, a nonprofit applied science and technology organization.

Visit Brookhaven Lab’s electronic newsroom for links, news archives, graphics, and more at http://www.bnl.gov/newsroom, follow Brookhaven Lab on Twitter, http://twitter.com/BrookhavenLab, or find us on Facebook, http://www.facebook.com/BrookhavenLab/.

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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by Karen McNulty Walsh

With the discovery of the long-sought Higgs boson at the Large Hadron Collider (LHC), the world’s largest and most powerful particle collider, folks unfamiliar with the intricacies of particle physics might think the field has reached its end. But physicists gathered at the Large Hadron Collider Physics Conference in New York City June 2-7 say they are eager to move forward. Even amid discussions of tight budgets that make some proposed projects appear impossible, the general tenor, as expressed by leaders in the field, is that the future holds great potential for even more significant discoveries.

Physicist panel

Physicists joined New York Times science correspondent Dennis Overbye for a discussion on the future of the field.

At a session devoted to reflection and the future of the field, held Friday, June 6, Fabiola Gianotti, a particle physicist at Europe’s CERN laboratory (home of the LHC) and spokesperson for the LHC’s ATLAS experiment at the time of the Higgs discovery, said, “There is challenging work for everyone to make the impossible possible.” In fact, said James Siegrist, Associate Director of the Office of High Energy Physics within the U.S. Department of Energy’s (DOE) Office of Science, “I think the promise of the physics has never been greater.”

Co-sponsored by DOE’s Brookhaven National Laboratory and Columbia University, the week-long meeting featured updates on key findings from the LHC’s four experiments (including a possible hint of new physics), advances in theory, plans for future upgrades, and even future colliders—as well as apanel discussion moderated by Dennis Overbye, a science correspondent for the New York Times.

“We had a very successful conference with more than 300 participants discussing an impressive array of results from the recent LHC run,” said Brookhaven physicist Srini Rajagopalan, U.S. ATLAS Operations Program Manager and a co-organizer of the meeting. He also noted the extremely positive response to an open-to-the-public screening of Particle Fever, a documentary film that follows six scientists during the years leading up to the discovery of the Higgs boson. “I was simply amazed at the public interest in what we do. From young school students to senior citizens, people thronged to watch the movie and continued to ask questions late into the night.”

What keeps you up at night?

At Friday’s panel session, the Times’ Overbye had some questions of his own, perhaps more pointed that the public’s. He asked whether particle physicists’ streak of discoveries could be continued, whether the “glory days” for the U.S. were over, and what keeps physicists up at night. The panelists were realistic about challenges and the need for smart choices and greater globalization. But a spirit of optimism prevailed.

Natalie Roe, Director of the Physics Division at DOE’s Lawrence Berkeley National Laboratory—the first to respond—said, “I’m going to flip the question [of what keeps me up and night] and answer what gets me up in the morning.” Following a long period of experimental and theoretical successes, including the discovery of the Higgs, she said, “this is a very exciting time. There are still a few remaining details … dark matter and dark energy. And these are more than details; they are 95 percent of the universe!” With a long list of techniques available to get answers, she said, there is much work to be done.

University of California, Santa Cruz, physicist Steve Ritz, who recently chaired the Particle Physics Project Prioritization Panel (P5) and presented its recommendations for the future of the field, emphasized the importance of “telling our story,” staging and prioritizing future projects, and “aspiring to a greater program” that continues investments in crucial research and development to lay the foundation for future facilities.

Great technology progress, great challenges

In an overview talk that preceded the panel discussion, Gianotti presented a range of such future projects, including two possible linear accelerators, one in Japan the other at CERN, and two possible circular colliders, one in China and one at CERN. The latter, dubbed FCC, would be a proton-proton collider 80-100 kilometers in circumference—on the scale of the Superconducting Supercollider (SSC) once planned for and later cancelled in the U.S. Such a machine would push beyond the research limits of even the most ambitious upgrades proposed for the LHC.

Those upgrades, planned for data taking in Phase I in 2020 and Phase II in 2025, will begin the exploration of the coupling of the Higgs with other particles to explore the mechanism by which the Higgs generates mass, “electroweak symmetry breaking,” and searches for new physics beyond the standard model and into the realm of dark matter.

But, to really get at the heart of those questions and possibly reveal unknown physics, the scientists say the need for even higher precision and higher energy is clear.

Journey to the dark side

“Our elders had it easy compared to our students,” said Siegrist, describing the physics challenges now open to exploration. He likened this moment in time to the end of a video game his son had played where, “at the end of the game, you end up on ‘the dark side’ and have to start again.” In physics, he said, the dark sector—exploring dark matter and dark energy—is going to be equally challenging to everything that has come before.

To those who say building the future machines needed for this journey is impossible, Gianotti says, “didn’t the LHC also look close to impossible in the 1980s?” The path forward, she emphasized, is to innovate.

“Accelerator R&D is very important,” said Ritz, noting that, “anything we can do to design these machines to cost less” in terms of construction and operation should be done. “We need to be impatient about this,” he said. “We need to ask more and jump in more.”

Panelist Nima Arkani-Hamed, a theorist at the Institute of Advanced Study at Princeton University and Director of the Center for Future High Energy Physics in Beijing, China, likely agrees. He acknowledges the difficult task facing U.S. leadership in high-energy physics. “They are trying to make due with a budget that’s two or three times less than what our vision and this country deserves, and they are doing a good job,” he said. “But I worry that our generation will be viewed as the one that dropped the ball.”

“The sequence of steps for the next few decades is possible,” he added later. “It’s just a matter of will, not technology.”

But because of the scale and cost of future projects, he, like others, emphasized that “we will need the whole world and new pockets of resources and talent.”

The value of collaboration, competition, and globalization

Sergio Bertolucci, Director for Research and Computing at CERN, agreed. “We have been international, but we need to be truly global.”

Such cooperation and competition among nations is good for the field, Ritz emphasized. “We are intensely competitive. We want to be the ones to discover [something new.] But we are also cooperative because we can’t do it alone.”

Panelist Jerry Blazey, Assistant Director for Physical Sciences in the
Office of Science and Technology Policy, DOE’s Siegrist, and others agreed that the LHC is a great model for the field to stand behind and emulate for future collaborative projects. Blazey and Siegrist said OSTP and DOE would work together to discuss ways to smooth the process for such future multinational collaborations and to implement the recommendations of the P5 report.

These include future U.S. work at the LHC, an internationalized Long Baseline Neutrino Facility located at Fermi National Accelerator Laboratory, and a role in Japan’s proposed linear collider, as well as continued investments in the technologies needed for future experiments. Said University of California, Irvine, physicist Andrew Lankford, chair of the High Energy Physics Advisory Panel (HEPAP) to whom the report was delivered, the P5 report describes a field optimized for scientific progress. “It’s a ten year strategic plan—way more than a collection of cool experiments,” he said.

And it emphasizes the value of international competition and cooperation—perhaps one of the biggest successes of particle physics, aside from the breathtaking discoveries. Turning again to the example of the LHC collaborations, Ritz said, “50 years ago some of these people were in countries that were trying to kill one another. Now we don’t even think about what country they are from.”

As Brookhaven’s Rajagopalan summed up, “It is an exciting time for our field as we plan to move forward with ambitious global projects to address the fundamental questions of nature.”

Brookhaven Lab’s particle physics research is supported by the DOE Office of Science.

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Karen McNulty Walsh is a science writer in the Media & Communications Office at Brookhaven National Laboratory.

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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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