From Fermi National Accelerator Lab: “Discovery of a new type of particle beam instability”

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From Fermi National Accelerator Lab , an enduring source of strength for the US contribution to scientific research world wide.

November 14, 2019
Alexey Burov

Accelerated, charged particle beams do what light does for microscopes: illuminate matter. The more intense the beams, the more easily scientists can examine the object they are looking at. But intensity comes with a cost: the more intense the beams, the more they become prone to instabilities.

One type of instability occurs when the average energy of accelerated particles traveling through a circular machine reaches its transition value. The transition point occurs when the particles revolve around the ring at the same rate, even though they do not all carry the same energy — in fact, they exhibit a range of energies. The specific motion of the particles near the transition energy makes them extremely prone to collective instabilities.

These particular instabilities were observed for decades, but they were not sufficiently understood. In fact, they were misinterpreted. In a paper published this year, I suggest a new theory about these instabilities. The application of this theory to the Fermilab Booster accelerator predicted the main features of the instability there at the transition crossing, suggesting better ways to suppress the instability. Recent measurements confirmed the predictions, and more detailed experimental beam studies are planned in the near future.

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Recent measurements at the Fermilab Booster accelerator confirmed existence of a certain kind of particle beam instability. More measurements are planned for the near future to examine new methods proposed to mitigate it.

Accelerating high-intensity beams is a crucial part of the Fermilab scientific program. A solid theoretical understanding of particle beam behavior equips experimentalists to better manipulate the accelerator parameters to suppress instability. This leads to the high-intensity beams needed for Fermilab’s experiments in fundamental physics. It is also useful for any experiment or institution operating circular accelerators.

Beam protons talk to each other by electromagnetic fields, which are of two kinds. One is called the Coulomb field. These fields are local and, by themselves, cannot drive instabilities. The second kind is the wake field. Wake fields are radiated by the particles and trail behind them, sometimes far behind.

When a particle strays from the beam path, the wake field translates this departure backward — in the wake left by the particle. Even a small departure from the path may not escape being carried backward by these electromagnetic fields. If the beams are intense enough, their wakes can destabilize them.

In the new theory, I suggested a compact mathematical model that effectively takes both sorts of fields into account, realizing that both of them are important when they are strong enough, as they typically are near transition energy.

This kind of huge amplification happens at CERN’s Proton Synchrotron, for example, as I showed in my more recent paper, submitted to Physical Review Accelerators and Beams. If not suppressed one way or another, this amplification may grow until the beam touches the vacuum chamber wall and becomes lost. Recent measurements at the Fermilab Booster confirmed existence of a similar instability there; more measurements are planned for the near future to examine new methods proposed to mitigate it.

These phenomena are called transverse convective instabilities, and the discoveries of how they arise open new doors to theoretical, numerical and experimental ways to better understanding and better dealing with the intense proton beams.

This work is supported by the DOE Office of Science.

Science paper:
Convective instabilities of bunched beams with space charge
Physical Review Accelerators and Beams

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Fermi National Accelerator Laboratory (Fermilab), located just outside Batavia, Illinois, near Chicago, is a US Department of Energy national laboratory specializing in high-energy particle physics. Fermilab is America’s premier laboratory for particle physics and accelerator research, funded by the U.S. Department of Energy. Thousands of scientists from universities and laboratories around the world
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From CERN: “LHCf gears up to probe birth of cosmic-ray showers”

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From CERN

11 November, 2019
Ana Lopes


CERN LHCf

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One of the LHCf experiment’s two detectors, LHCf Arm2, seen here during installation into a particle absorber that surrounds the LHC’s beam pipe. (Image: Lorenzo Bonechi)

Cosmic rays are particles from outer space, typically protons, travelling at almost the speed of light. When the most energetic of these particles strike the atmosphere of our planet, they interact with atomic nuclei in the atmosphere and produce cascades of secondary particles that shower down to the Earth’s surface. These extensive air showers, as they are known, are similar to the cascades of particles that are created in collisions inside particle colliders such as CERN’s Large Hadron Collider (LHC). In the next LHC, run starting in 2021, the smallest of the LHC experiments – the LHCf experiment – is set to probe the first interaction that triggers these cosmic showers.

Observations of extensive air showers are generally interpreted using computer simulations that involve a model of how cosmic rays interact with atomic nuclei in the atmosphere. But different models exist and it’s unclear which one is the most appropriate. The LHCf experiment is in an ideal position to test these models and help shed light on cosmic-ray interactions.

In contrast to the main LHC experiments, which measure particles emitted at large angles from the collision line, the LHCf experiment measures particles that fly out in the “forward” direction, that is, at small angles from the collision line. These particles, which carry a large portion of the collision energy, can be used to probe the small angles and high energies at which the predictions from the different models don’t match.

Using data from proton–proton LHC collisions at an energy of 13 TeV, LHCf has recently measured how the number of forward photons and neutrons varies with particle energy at previously unexplored high energies. These measurements agree better with some models than others, and they are being factored in by modellers of extensive air showers.

In the next LHC run, LHCf should extend the range of particle energies probed, due to the planned higher collision energy. In addition, and thanks to ongoing upgrade work, the experiment should also increase the number and type of particles that are detected and studied.

What’s more, the experiment plans to measure forward particles emitted from collisions of protons with light ions, most likely oxygen ions. The first interactions that trigger extensive air showers in the atmosphere involve mainly light atomic nuclei such as oxygen and nitrogen. LHCf could therefore probe such an interaction in the next run, casting new light on cosmic-ray interaction models at high energies.

Find out more in the Experimental Physics newsletter article.

See the full article here.


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From CERN: “CERN Council appoints Fabiola Gianotti for second term of office as CERN Director General”

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From CERN

6 November, 2019

At its 195th Session today, the CERN Council selected Fabiola Gianotti, as the Organization’s next Director-General, for her second term of office.

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President of the CERN Council, Ursula Bassler and Director-General of CERN, Fabiola Gianotti (Image: CERN)

At its 195th Session today, the CERN Council selected Fabiola Gianotti, as the Organization’s next Director-General, for her second term of office. The appointment will be formalised at the December Session of the Council, and Gianotti’s new five-year term of office will begin on 1 January 2021. This is the first time in CERN’s history that a Director-General has been appointed for a full second term.

“I congratulate Fabiola Gianotti very warmly for her reappointment as Director-General for another five-year term of office. With her at the helm, CERN will continue to benefit from her strong leadership and experience, especially for important upcoming projects such as the High-Luminosity LHC, implementation of the European Strategy for Particle Physics, and the construction of the Science Gateway,” said President of the CERN Council, Ursula Bassler. “During her first term, she excelled in leading our diverse and international scientific organisation, becoming a role model, especially for women in science”.

“I am deeply grateful to the CERN Council for their renewed trust. It is a great privilege and a huge responsibility,” said CERN Director-General, Fabiola Gianotti. “The following years will be crucial for laying the foundations of CERN’s future projects and I am honoured to have the opportunity to work with the CERN Member States, Associate Member States, other international partners and the worldwide particle physics community.”

Gianotti has been CERN’s Director-General since 1 January 2016. She received her Ph.D. in experimental particle physics from the University of Milano in 1989 and has been a research physicist at CERN since 1994. She was the leader of the ATLAS experiment’s collaboration from March 2009 to February 2013, including the period in which the LHC experiments ATLAS and CMS announced the discovery of the Higgs boson. The discovery was recognised in 2013 with the Nobel Prize in Physics being awarded to theorists François Englert and Peter Higgs. Gianotti is a member of many international committees, and has received numerous prestigious awards. She was the first woman to become the Director-General of CERN.

See the full article here.


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From Symmetry: “Put it to the test beam”

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From Symmetry<

11/05/19
Lauren Biron

Before a detector component can head to its forever home, it has to pass the test.

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Photo by Reidar Hahn, Fermilab

If building a modern particle physics experiment is a marathon, then visiting a test beam facility is the 100-meter dash. Over the course of just a few weeks, small teams work non-stop to gather as much data as they can about a piece of equipment they are thinking of installing in an experiment.

“It is stressful, but I think it’s super fun,” says Jessica Metcalfe, a researcher at Argonne working on upgrades for the innermost part of the ATLAS detector, one of the two major detectors at CERN that co-discovered the Higgs boson.

CERN ATLAS Credit CERN SCIENCE PHOTO LIBRARY

“You’re all there squeezed together in the tiny control room, problem solving, all very focused on a very specific goal, and you learn a lot—really fast.”

Test beams generally sit to the side of full-on accelerators, sipping beam and passing it to the reconfigurable spaces housing temporary experiments. Scientists bring pieces of their detectors—sensors, chips, electronics or other material—and blast them with the well understood beam to see if things work how they expect, and if their software performs as expected. If things check out, they’re one step closer to being installed in a detector, and if not, it’s a chance to do some R&D, tinker and make things work.

“We’re typically testing pieces that are going into a larger experiment, but you’re also doing research on the detector technology, which is a form of research in itself,” Metcalfe says. “We’re not just getting ready to build something, we’re also learning a lot about the devices. There’s often many iterations of design and redevelopment.”

Test beam visits are typically short, and getting time can be competitive because there are only a handful of places around the world that have high-energy particle beams available for testing. When it comes to hadrons—particles made of quarks—there are really just two: the Department of Energy’s Fermilab in the United States and CERN in Europe.

Other test beams specialize in different particles, for example, electrons (at Germany’s DESY or California’s SLAC National Accelerator Laboratory) and photons (like at the Research Center for Electron Photon Science in Japan).

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“It’s part of the lifecycle of any detector you build,” says Mandy Rominsky, who manages the Fermilab Test Beam Facility. “You start on a bench with an idea, and before you put something into a running experiment, you always put it in a test beam. You need to be able to characterize it, change it and go back and forth—and you can’t do that on a bench.”

Groups come with components of all shapes and sizes to the test beam. At the Fermilab test beam alone, researchers have tested teeny, tiny pieces of scintillator (the material that captures particles of light) and detector panels taller than people. Researchers come from many scientific fields, including nuclear physics, neutrino physics, collider physics, dark matter physics and astronomy. There are people working on general research and development without a specific experiment in mind, and ultra-specific tasks, like the crew working on turning smartphones into cosmic ray detectors. Still others are interested in learning how the materials they plan to put in a detector will change over time, especially in the harsh environment surrounding particle collisions.

Test-beam facilities try to keep useful experimental infrastructure on hand for visiting researchers: There are movable tables to pull equipment in and out, cooling systems and electronics, cables, different kinds of gas, cranes, and, of course, the beam itself, which often comes in many flavors of particles and energies. But some experiments need to bring in a little something extra, creating odd requests for facility managers—like when a visiting group from the IceCube experiment needed about 1000 gallons of deionized water to test their modules before similar detectors were shipped to the South Pole and entombed in the ice.

“It is surprisingly difficult to get that much deionized water,” Rominsky notes. “We couldn’t use a tanker and had to ship it in from Indiana in 55-gallon drums.”

And while most components will have only a short stay in the test beam, some facilities do have areas for longer-term experiments. For example, the LArIAT (Liquid Argon in a Test beam) detector lived its full existence in the test beam, collecting data for three years at Fermilab.

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Its goal was to better understand how particles interact with argon, the material now being used in massive neutrino detectors such as MicroBooNE and the international Deep Underground Neutrino Experiment hosted by Fermilab.

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FNAL LBNF/DUNE from FNAL to SURF, Lead, South Dakota, USA

“I like that we help everybody,” Rominsky says. “It doesn’t matter which groups come to us. Our policy is to be very helpful to everyone.”

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IceCube PMT beam test at Fermilab Test Beam Facility. Photo by Reidar Hahn, Fermilab.

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Meson Test Beam Facility with LArIAT Detector. Photo by Reidar Hahn, Fermilab

Research crucible

Test beams are not only important for detectors themselves—the test beam experience is also formative for the researchers who come to do the hands-on work. Teams work together over long hours, sharing both shifts and meals, ups and downs.

“For me, it was really communal,” says Clara Nellist, an ATLAS researcher at Radboud University in the Netherlands and former co-organizer of the annual Beam Telescope and Test Beams Workshop. “I learned so much from other people, even though we were from different universities and, in essence, we were competing.”

Nellist did her PhD thesis on proposed technology for the ATLAS pixel detector and spent many night shifts at the CERN test beam facility. Sometimes, groups working on a different proposed sensor intended for the same slot in the detector would share the same experimental setup. When the competing team didn’t have enough people to run their shifts, she volunteered to take data for them. A few months later, she unexpectedly found herself on their research paper for contributing to their data.

“We needed each other’s expertise,” Nellist says. “There are friends I made in the first week of my PhD who, 10 years later, I’m still friends with and check up on.”

The diverse nature of projects also means researchers from all different stages of their careers make their way through the test beam facility doors.

“You get people who are legends in the detector R&D community, and they need beam time like everyone else, and then there are undergrads having their first lab experience,” explains Aria Soha, an engineering physicist at Fermilab who managed the test beam facility until 2013. For those new to hardware testing—and even the more seasoned pros—it’s a thrill to watch those first particle tracks splash across the detector.

“I remember knowing when the beam was coming and watching the particles show up, and thinking, ‘This is cool, this is why I went to school for physics,’” Soha says.

Those moments of triumph often come after a stressful period of testing and debugging.

“You test everything in the lab before you go. Everything works perfectly and then you go [to the test beam] and nothing works,” Metcalfe says. “Checking the cables and turning things on and off solves about 80% of the problems.”

The granular, hands-on experience can make a big difference in understanding the results coming out of the detector later on. Visiting a test beam teaches researchers how particles are going through their detector, how they interact, how the data looks when it comes out, and much more. If researchers see a problem in their data analysis, they can recognize the potential causes more quickly, Metcalfe says. These are skills for the future of physics.

“There’s going to be a next generation of experiments, and people need to know how to design them and how to make that design motivated by the physics you want to do,” Metcalfe says. “It’s part of the training.”

See the full article here .


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From CERN Courier: “European strategy [for HEP] enters next phase”


From CERN Courier

2 October 2019

Matthew Chalmers, editor

European strategy enters next phase

Physicists in Europe have published a 250-page “briefing book” to help map out the next major paths in fundamental exploration. Compiled by an expert physics-preparatory group set up by the CERN Council, the document is the result of an intense effort to capture the status and prospects for experiment, theory, accelerators, computing and other vital machinery of high-energy physics.

Last year, the European Strategy Group (ESG) — which includes scientific delegates from CERN’s member and associate-member states, directors and representatives of major European laboratories and organisations and invitees from outside Europe — was tasked with formulating the next update of the European strategy for particle physics. Following a call for input in September 2018, which attracted 160 submissions, an open symposium was held in Granada, Spain, on 13-16 May at which more than 600 delegates discussed the potential merits and challenges of the proposed research programmes. The ESG briefing book distills input from the working groups and the Granada symposium to provide an objective scientific summary.

“This document is the result of months of work by hundreds of people, and every effort has been made to objectively analyse the submitted inputs,” says ESG chair Halina Abramowicz of Tel Aviv University. “It does not take a position on the strategy process itself, or on individual projects, but rather is intended to represent the forward thinking of the community and be the main input to the drafting session in Germany in January.”

Collider considerations

An important element of the European strategy update is to consider which major collider should follow the LHC. The Granada symposium revealed there is clear support for an electron–positron collider to study the Higgs boson in greater detail, but four possible options at different stages of maturity exist: an International Linear Collider (ILC) in Japan, a Compact Linear Collider (CLIC) or Future Circular Collider (FCC-ee) at CERN, and a Circular Electron Positron Collider (CEPC) in China. The briefing book states that, in a global context, CLIC and FCC-ee are competing with the ILC and with CEPC. As Higgs factories, however, the report finds all four to have similar reach, albeit with different time schedules and with differing potentials for the study of physics topics at other energies.


ILC schematic, being planned for the Kitakami highland, in the Iwate prefecture of northern Japan



CLIC Collider annotated

CERN FCC Future Circular Collider details of proposed 100km-diameter successor to LHC

China Circular Electron Positron Collider (CEPC) map

Also considered in depth are design studies in Europe for colliders that push the energy frontier, including a 3 TeV CLIC and a 100 TeV circular hadron collider (FCC-hh). The briefing book details the estimated timescales to develop some of these technologies, observing that the development of 16 T dipole magnets for FCC-hh will take a comparable time (about 20 years) to that projected for novel acceleration technologies such as plasma-wakefield techniques to reach conceptual designs.

“The Granada symposium and the briefing book mention the urgent need for intensifying accelerator R&D, including that for muon colliders,” says Lenny Rivkin of Paul Scherrer Institut, who was co-convener of the chapter on accelerator science and technology. “Another important aspect of the strategy update is to recognize the potential impact of the development of accelerator and associated technology on the progress in other branches of science, such as astroparticle physics, cosmology and nuclear physics.”

The bulk of the briefing book details the current physics landscape and prospects for progress, with chapters devoted to electroweak physics, strong interactions, flavour physics, neutrinos, cosmic messengers, physics beyond the Standard Model, and dark-sector exploration. A preceding chapter about theory emphasises the importance of keeping theoretical research in fundamental physics “free and diverse” and “not only limited to the goals of ongoing experimental projects”. It points to historical success stories such as Peter Higgs’ celebrated 1964 paper, which had the purely theoretical aim to show that Gilbert’s theorem is invalid for gauge theories at a time when applications to electroweak interactions were well beyond the horizon.

“While an amazing amount of progress has been made in the past seven years since the Higgs boson discovery, our knowledge of the couplings of the Higgs-boson to the W and Z and to third-generation charged fermions is quite imprecise, and the couplings of the Higgs boson to the other charged fermions and to itself are unmeasured,” says Beate Heinemann of DESY, who co-convened the report’s electroweak chapter. “The imperative to study this unique particle further derives from its special properties and the special role it might play in resolving some of the current puzzles of the universe, for example dark matter, the matter-antimatter asymmetry or the hierarchy problem.”

Readers are reminded that the discovery of neutrino oscillations constitutes a “laboratory” proof of physics beyond the Standard Model. The briefing book also notes the significant role played by Europe, via CERN, in neutrino-experiment R&D since the last strategy update concluded in 2013. Flavour physics too should remain at the forefront of the European strategy, it argues, noting that the search for flavour and CP violation in the quark and lepton sectors at different energy frontiers “has a great potential to lead to new physics at moderate cost”. An independent determination of the proton structure is needed if present and future hadron colliders are to be turned into precision machines, reports the chapter on strong interactions, and a diverse global programme based on fixed-target experiments as well as dedicated electron-proton colliders is in place.

Europe also has the opportunity to play a leading role in the searches for dark matter “by fully exploiting the opportunities offered by the CERN facilities, such as the SPS, the potential Beam Dump Facility, and the LHC itself, and by supporting the programme of searches for axions to be hosted at other European institutions”. The briefing book notes the strong complementarity between accelerator and astrophysical searches for dark matter, and the demand for deeper technology sharing between particle and astroparticle physics.

Scientific diversity

The diversity of the experimental physics programme is a strong feature of the strategy update. The briefing book lists outstanding puzzles that did not change in the post-Run 2 LHC era – such as the origin of electroweak symmetry breaking, the nature of the Higgs boson, the pattern of quark and lepton masses and the neutrino’s nature – that can also be investigated by smaller scale experiments at lower energies, as explored by CERN’s dedicated Physics Beyond Colliders initiative.

Finally, in addressing the vital roles of detector & accelerator development, computing and instrumentation, the report acknowledges both the growing importance of energy efficiency and the risks posed by “the limited amount of success in attracting, developing and retaining instrumentation and computing experts”, urging that such activities be recognized correctly as fundamental research activities. The strong support in computing and infrastructure is also key to the success of the high-luminosity LHC which, the report states, will see “a very dynamic programme occupying a large fraction of the community” during the next two decades – including a determination of the couplings between the Higgs boson and Standard Model particles “at the percent level”.

Following a drafting session to take place in Bad Honnef, Germany, on 20-24 January, the ESG is due to submit its recommendations for the approval of the CERN Council in May 2020 in Budapest, Hungary.

“Now comes the most challenging part of the strategy update process: how to turn the exciting and well-motivated scientific proposals of the community into a viable and coherent strategy which will ensure progress and a bright future for particle physics in Europe,” says Abramowicz. “Its importance cannot be overestimated, coming at a time when the field faces several crossroads and decisions about how best to maintain progress in fundamental exploration, potentially for generations to come.”

See the full article here .


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LHCb
CERN LHCb New II

LHC

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CERN LHC Grand Tunnel

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From CERN CMS: “Watching the top quark mass run”

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From CERN CMS

10.7.19
CMS Collaboration

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A candidate event for a top quark–antiquark pair recorded by the CMS detector. Such an event is expected to produce an electron (green), a muon (red) of opposite charge, two high-energy “jets” of particles (orange) and a large amount of missing energy (purple) (Image: CMS/CERN)

For the first time, CMS physicists have investigated an effect called the “running” of the top quark mass, a fundamental quantum effect predicted by the Standard Model.

Mass is one of the most complex concepts in fundamental physics, which went through a long history of conceptual developments. Mass was first understood in classical mechanics as a measure of inertia and was later interpreted in the theory of special relativity as a form of energy. Mass has a similar meaning in modern quantum field theories that describe the subatomic world. The Standard Model of particle physics is such a quantum field theory, and it can describe the interaction of all known fundamental particles at the energies of the Large Hadron Collider.

Quantum Chromodynamics is the part of the Standard Model that describes the interactions of fundamental constituents of nuclear matter: quarks and gluons. The strength of the interaction between these particles depends on a fundamental parameter called the strong coupling constant. According to Quantum Chromodynamics, the strong coupling constant rapidly decreases at higher energy scales. This effect is called asymptotic freedom, and the scale evolution is referred to as the “running of the coupling constant.” The same is also true for the masses of the quarks, which can themselves be understood as fundamental couplings, for example, in connection with the interaction with the Higgs field. In Quantum Chromodynamics, the running of the strong coupling constant and of the quark masses can be predicted, and these predictions can be experimentally tested.

The experimental verification of the running mass is an essential test of the validity of Quantum Chromodynamics. At the energies probed by the Large Hadron Collider, the effects of physics beyond the Standard Model could lead to modifications of the running of mass. Therefore, a measurement of this effect is also a search for unknown physics. Over the past decades, the running of the strong coupling constant has been experimentally verified for a wide range of scales. Also, evidence was found for the running of the masses of the charm and beauty quarks.

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Figure 1: Display of an LHC collision detected by the CMS detector that contains a reconstructed top quark-antiquark pair. The display shows an electron (green) and a muon (red) of opposite charge, two highly energetic jets (orange) and a large amount of missing energy (purple).

With a new measurement, the CMS Collaboration investigates for the first time the running of the mass of the heaviest of the quarks: the top quark. The production rate of top quark pairs (a quantity that depends on the top quark mass) was measured at different energy scales. From this measurement, the top quark mass is extracted at those energy scales using theory predictions that predict the rate at which top quark-antiquark pairs are produced.

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Figure 2: The running of the top quark mass determined from the data (black points) compared to the theoretical prediction (red line). As the absolute scale of the top quark mass is not relevant for this measurement, the values have been normalised to the second data point.

Experimentally, interesting top quark pair collisions are selected by searching for the specific decay products of a top quark-antiquark pair. In the overwhelming majority of cases, top quarks decay into an energetic jet and a W boson, which in turn can decay into a lepton and a neutrino. Jets and leptons can be identified and measured with high precision by the CMS detector, while neutrinos escape undetected and reveal themselves as missing energy. A collision that is likely the production of a top quark-antiquark pair as it is seen in the CMS detector is shown in Figure 1. Such a collision is expected to contain an electron, a muon, two energetic jets, and a large amount of missing energy.

The measured running of the top quark mass is shown in Figure 2. The markers correspond to the measured points, while the red line represents the theoretical prediction according to Quantum Chromodynamics. The result provides the first indication of the validity of the fundamental quantum effect of the running of the top quark mass and opens a new window to test our understanding of the strong interaction. While a lot more data will be collected in the future LHC runs starting with Run 3 in 2021, this particular CMS result is mostly sensitive to uncertainties coming from the theoretical knowledge of the top quark in Quantum Chromodynamics. To witness the top quark mass running with even higher precision and maybe unveil signs of new physics, theory developments and experimental efforts will both be necessary. In the meantime, watch the top quark run!

See the full article here.


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From Stanford University: “Stanford physicists funded to pursue ‘tabletop’ physics experiments”

Stanford University Name
From Stanford University

September 25, 2019
Ker Than
(650) 723-9820
kerthan@stanford.edu

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Peter Graham and Savas Dimopoulos are among Stanford physicists working on smaller-scale devices to answer large questions. (Image credit: L.A. Cicero)

With the future of large particle accelerators uncertain, Stanford theorists are exploring the use of smaller, more precise “tabletop” experiments to investigate fundamental questions in physics.

The history of particle accelerators is one of seemingly constant one-upmanship. Ever since the 1920s, the machines – which spur charged particles to near light speeds before crashing them together – have grown ever larger, more complex and more powerful.

Consider: When the 2-mile-long linear accelerator at SLAC National Accelerator Laboratory opened for business in 1966, it could boost electrons to energies of about 19 gigaelectronvolts. The Large Hadron Collider (LHC) at CERN, which finished construction in 2008, can boost protons to more than 700 times higher energy levels and resides in a massive elliptical tunnel wide enough to encircle a small town. Future supercolliders being planned by CERN, China and Japan promise to be even more immense and energetic (and also more expensive).

CERN FCC Future Circular Collider map

China Circular Electron Positron Collider (CEPC) map

J-PARC Facility Japan Proton Accelerator Research Complex , located in Tokai village, Ibaraki prefecture, on the east coast of Japan

The strategy has paid off handsomely with discoveries that have helped confirm the soundness of the Standard Model, our current best understanding of how nature’s fundamental forces and subatomic matter interact.

As successful as particle accelerators have been, however, Stanford theorists Savas Dimopoulos and Peter Graham are betting that scientific treasures await discovery in the other direction as well. For years, the pair have argued that smaller and less expensive, but more sensitive, instruments could help answer stubborn mysteries in physics that have resisted the efforts of even the largest atom smashers – questions like “What is dark matter?” and “Do extra spatial dimensions exist?”

“Peter and I and our group have been thinking about this for 15 years,” said Dimopoulos, who is the Hamamoto Family Professor at Stanford’s School of Humanities and Sciences. “We were sort of lonely but very happy because we were exploring new territory all the time and it was a lot of fun. We felt like eternal graduate students.”

Scalpel vs. hammer

But their ideas have been slowly gaining traction among physicists, and last fall the Gordon and Betty Moore Foundation awarded Stanford and SLAC researchers three grants totaling roughly $15 million to use quantum technologies to explore new fundamental physics. Key to these efforts are the kinds of small-scale, “tabletop” experiments (so-called because most of them would fit on a lab bench or in a modest-sized room) that Dimopoulos and Graham have long advocated for. “Everything is smaller, except for the ideas,” Dimopoulos quipped. “These types of experiments could help solve some very important problems in physics.”

The instruments Dimopoulos and Graham have in mind exploit the weird properties of quantum mechanics – such as wave-particle duality and the seemingly telepathic link between entangled particles – to detect and measure minute signals and effects that particle accelerators are simply not attuned to.

Tabletop experiments are considered high-risk, high-reward projects because they are generally cheaper to build and operate than colliders, said Asimina Arvanitaki, a theoretical physicist at the Perimeter Institute. “If you’re pitching a project that costs several billion dollars, you better have a very good reason for its existence and be reasonably sure you’re going to succeed,” added Arvanitaki, a former Stanford postdoc in Dimopoulos’ lab. “But the cost of tabletop experiments is so low, and the timescales for producing results is so short, that it takes some of that pressure off.”

Building on existing technologies

The Moore Foundation grants will fund three projects: Two are experimental and will focus on developing new technologies for detecting dark matter and measuring gravitational waves. But the third, worth about $2.5 million and awarded to Dimopoulos and Graham, will be used to further develop the theoretical underpinnings that will enable future experiments.

“There’s been a history of particle accelerators discovering new physics and finding new particles, but it’s not clear that that can go on forever, so it’s important to think of other complementary ways to get at these underlying questions about nature,” said Ernie Glover, the Moore Foundation’s science program officer.

Crucially, the experiments Dimopoulos and Graham are proposing rely on relatively mature, high-precision technologies that, for the most part, were developed with other uses in mind and for other fields, such as medicine and applied physics. “That’s what got us really excited,” Dimopoulos said. “We realized there were all these possibilities out there that particle theorists weren’t really thinking about.”

A good example is nuclear magnetic resonance, or NMR, imaging, which forms the basis of magnetic resonance imaging, or MRI, a common medical scanning technique.

A few years ago, Graham and others theorized that a proposed ultralightweight dark matter candidate called an axion could influence the nuclear spin of normal matter. Dark matter is thought to make up the bulk of the matter in the universe, but it has evaded every attempt so far at characterization. Excited, Graham contacted an atomic physicist at the University of California, Berkeley, named Dmitry Budker to discuss designing a dark matter detector based on this effect – only to discover that the technology already exists.

“He said it’s going to work because what we were describing was basically NMR,” said Graham, a theoretical physicist at the Stanford Institute for Theoretical Physics.

Graham and Budker teamed up with other physicists to design the Cosmic Axion Spin Precession Experiment, or “CASPEr,” which uses NMR (nuclear magnetic resonance) to detect axion and axion-like particles. These particles are predicted to have such weak interactions and low masses that they would never show up in a collider, which are better equipped to search for massive dark matter candidates such as WIMPs (weakly interacting massive particles).

Similarly, another Moore Foundation-funded tabletop experiment called MAGIS-100 relies on atom interferometry technology initially developed in the 1990s as a general-purpose tool for making precise measurements. The project, a collaboration between Stanford’s Mark Kasevich and Jason Hogan and researchers at Fermilab and other universities, could potentially detect ripples in spacetime known as gravitational waves around 1 hertz, a frequency range beyond the sensitivity of most existing or even proposed detectors.

Current gravitational wave detectors like LIGO are sensitive to the very final moments of the black hole collisions that generate the spacetime ripples, but MAGIS-100 could provide scientists with a much longer viewing window.

“LIGO saw just a fraction of a second of the event, but the black holes were twirling around each other and generating gravitational waves for millions or billions of years before that. Those waves were just in lower frequency bands,” Graham said. “By looking at other frequencies, we could observe the black holes for longer and perhaps discover new gravitational wave sources.”

Intuition

Dimopoulos and Graham plan to use the Moore Foundation-funding to continue devising new schemes for co-opting technologies like NMR and atom interferometry in the service of fundamental physics research.

“It’s that connection that’s hard,” Graham said. “The experimental physicists and engineers who develop the technologies aren’t necessarily thinking about what other deep, fundamental questions could be tested, and the theorists are often unaware that tools for testing their ideas already exist.”

But Dimopoulos and Graham are now old hands at making such connections. “In principle, you have to know all possible technologies,” Graham said. “In practice, you just have to know the right ones, but it takes a nontrivial intuition to realize something like ‘Oh, wait a minute, it looks like this technique might actually be able to observe extra dimensions or some other new physics.’”

In one sense, what Dimopoulos and Graham are advocating for is a return to the way physics was done before colliders came to play such an important role in physics and the division of physicists into primarily theoretical and experimental camps.

“Before World War II, physics was just like what we’re doing right now,” Dimopoulos said. “Felix Bloch was both a theorist and an experimentalist, and so was Enrico Fermi. Even Einstein did experiments. There wasn’t a ready group of experimentalists that you could outsource your ideas to. You had to invent the techniques and look around at emerging technologies.”

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