Searching for hints of new physics in the subatomic world
Peer deeper into the heart of the atom than any microscope allows and scientists hypothesize that you will find a rich world of particles popping in and out of the vacuum, decaying into other particles, and adding to the weirdness of the visible world. These subatomic particles are governed by the quantum nature of the Universe and find tangible, physical form in experimental results.
Some subatomic particles were first discovered over a century ago with relatively simple experiments. More recently, however, the endeavor to understand these particles has spawned the largest, most ambitious and complex experiments in the world, including those at particle physics laboratories such as the European Organization for Nuclear Research (CERN) in Europe, Fermilab in Illinois, and the High Energy Accelerator Research Organization (KEK) in Japan.
These experiments have a mission to expand our understanding of the Universe, characterized most harmoniously in the Standard Model of particle physics; and to look beyond the Standard Model for as-yet-unknown physics.
“The Standard Model explains so much of what we observe in elementary particle and nuclear physics, but it leaves many questions unanswered,” said Steven Gottlieb, distinguished professor of Physics at Indiana University. “We are trying to unravel the mystery of what lies beyond the Standard Model.”
Ever since the beginning of the study of particle physics, experimental and theoretical approaches have complemented each other in the attempt to understand nature. In the past four to five decades, advanced computing has become an important part of both approaches. Great progress has been made in understanding the behavior of the zoo of subatomic particles, including bosons (especially the long sought and recently discovered Higgs boson), various flavors of quarks, gluons, muons, neutrinos and many states made from combinations of quarks or anti-quarks bound together.
Quantum field theory is the theoretical framework from which the Standard Model of particle physics is constructed. It combines classical field theory, special relativity and quantum mechanics, developed with contributions from Einstein, Dirac, Fermi, Feynman, and others. Within the Standard Model, quantum chromodynamics, or QCD, is the theory of the strong interaction between quarks and gluons, the fundamental particles that make up some of the larger composite particles such as the proton, neutron and pion.
Peering Through The Lattice
Carleton DeTar and Steven Gottlieb are two of the leading contemporary scholars of QCD research and practitioners of an approach known as lattice QCD. Lattice QCD represents continuous space as a discrete set of spacetime points (called the lattice). It uses supercomputers to study the interactions of quarks, and importantly, to determine more precisely several parameters of the Standard Model, thereby reducing the uncertainties in its predictions. It’s a slow and resource-intensive approach, but it has proven to have wide applicability, giving insight into parts of the theory inaccessible by other means, in particular the explicit forces acting between quarks and antiquarks.
DeTar and Gottlieb are part of the MIMD Lattice Computation (MILC) Collaboration and work very closely with the Fermilab Lattice Collaboration on the vast majority of their work. They also work with the High Precision QCD (HPQCD) Collaboration for the study of the muon anomalous magnetic moment. As part of these efforts, they use the fastest supercomputers in the world.
Since 2019, they have used Frontera at the Texas Advanced Computing Center (TACC)—the fastest academic supercomputer in the world and the 9th fastest overall—to propel their work. They are among the largest users of that resource, which is funded by the National Science Foundation. The team also uses Summit at the Oak Ridge National Laboratory (the #2 fastest supercomputer in the world); Cori at the National Energy Research Scientific Computing Center (#20), and Stampede2 (#25) at TACC, for the lattice calculations.
The efforts of the lattice QCD community over decades have brought greater accuracy to particle predictions through a combination of faster computers and improved algorithms and methodologies.
“We can do calculations and make predictions with high precision for how strong interactions work,” said DeTar, professor of Physics and Astronomy at the University of Utah. “When I started as a graduate student in the late 1960s, some of our best estimates were within 20 percent of experimental results. Now we can get answers with sub-percent accuracy.”
In particle physics, physical experiment and theory travel in tandem, informing each other, but sometimes producing different results. These differences suggest areas of further exploration or improvement.
“There are some tensions in these tests,” said Gottlieb, distinguished professor of Physics at Indiana University. “The tensions are not large enough to say that there is a problem here—the usual requirement is at least five standard deviations. But it means either you make the theory and experiment more precise and find that the agreement is better; or you do it and you find out, ‘Wait a minute, what was the three sigma tension is now a five standard deviation tension, and maybe we really have evidence for new physics.'”
DeTar calls these small discrepancies between theory and experiment ‘tantalizing.’ “They might be telling us something.”
Over the last several years, DeTar, Gottlieb and their collaborators have followed the paths of quarks and antiquarks with ever-greater resolution as they move through a background cloud of gluons and virtual quark-antiquark pairs, as prescribed precisely by QCD. The results of the calculation are used to determine physically meaningful quantities such as particle masses and decays.
One of the current state-of-the-art approaches that is applied by the researchers uses the so-called highly improved staggered quark (HISQ) formalism to simulate interactions of quarks with gluons. On Frontera, DeTar and Gottlieb are currently simulating at a lattice spacing of 0.06 femtometers (10-15 meters), but they are quickly approaching their ultimate goal of 0.03 femtometers, a distance where the lattice spacing is smaller than the wavelength of the heaviest quark, consequently removing a significant source of uncertainty from these calculations.
Each doubling of resolution, however, requires about two orders of magnitude more computing power, putting a 0.03 femtometers lattice spacing firmly in the quickly-approaching ‘exascale’ regime.
“The costs of calculations keeps rising as you make the lattice spacing smaller,” DeTar said. “For smaller lattice spacing, we’re thinking of future Department of Energy machines and the Leadership Class Computing Facility [TACC’s future system in planning]. But we can make do with extrapolations now.”
The Anomalous Magnetic Moment Of The Muon And Other Outstanding Mysteries
Among the phenomena that DeTar and Gottlieb are tackling is the anomalous magnetic moment of the muon (essentially a heavy electron) – which, in quantum field theory, arises from a weak cloud of elementary particles that surrounds the muon. The same sort of cloud affects particle decays. Theorists believe yet-undiscovered elementary particles could potentially be in that cloud.
A large international collaboration called the Muon g-2 Theory Initiative recently reviewed the present status of the Standard Model calculation of the muon’s anomalous magnetic moment. Their review appeared in Physics Reports in December 2020. DeTar, Gottlieb and several of their Fermilab Lattice, HPQCD and MILC collaborators are among the coauthors. They find a 3.7 standard deviation difference between experiment and theory.
“… the processes that were important in the earliest instance of the Universe involve the same interactions that we’re working with here. So, the mysteries we’re trying to solve in the microcosm may very well provide answers to the mysteries on the cosmological scale as well.”
Carleton DeTar, Professor of Physics, University of UtahWhile some parts of the theoretical contributions can be calculated with extreme accuracy, the hadronic contributions (the class of subatomic particles that are composed of two or three quarks and participate in strong interactions) are the most difficult to calculate and are responsible for almost all of the theoretical uncertainty. Lattice QCD is one of two ways to calculate these contributions.
“The experimental uncertainty will soon be reduced by up to a factor of four by the new experiment currently running at Fermilab, and also by the future J-PARC experiment,” they wrote. “This and the prospects to further reduce the theoretical uncertainty in the near future… make this quantity one of the most promising places to look for evidence of new physics.”
Gottlieb, DeTar and collaborators have calculated the hadronic contribution to the anomalous magnetic moment with a precision of 2.2 percent. “This give us confidence that our short-term goal of achieving a precision of 1 percent on the hadronic contribution to the muon anomalous magnetic moment is now a realistic one,” Gottlieb said. They hope to achieve a precision of 0.5 percent a few years later.
Other ‘tantalizing’ hints of new physics involve measurements of the decay of B mesons. There, various experimental methods arrive at different results. “The decay properties and mixings of the D and B mesons are critical to a more accurate determination of several of the least well-known parameters of the Standard Model,” Gottlieb said. “Our work is improving the determinations of the masses of the up, down, strange, charm and bottom quarks and how they mix under weak decays.” The mixing is described by the so-called CKM mixing matrix for which Kobayashi and Maskawa won the 2008 Nobel Prize in Physics.
The answers DeTar and Gottlieb seek are the most fundamental in science: What is matter made of? And where did it come from?
“The Universe is very connected in many ways,” said DeTar. “We want to understand how the Universe began. The current understanding is that it began with the Big Bang. And the processes that were important in the earliest instance of the Universe involve the same interactions that we’re working with here. So, the mysteries we’re trying to solve in the microcosm may very well provide answers to the mysteries on the cosmological scale as well.”More information: T. Aoyama et al, The anomalous magnetic moment of the muon in the Standard Model, Physics Reports (2020). DOI: 10.1016/j.physrep.2020.07.006
Citation: Searching for hints of new physics in the subatomic world (2021, March 24) retrieved 25 March 2021 from https://phys.org/news/2021-03-hints-physics-subatomic-world.html
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Has the Large Hadron Collider finally challenged the laws of physics?
By Richard Webb
The LHCb experiment is looking for new physics
Brice, Maximilien; Ordan, Julien Marius/CERN
There has been a buzz of excitement surrounding what has been described as “tantalising hints of new physics” emanating from the LHCb experiment at the CERN particle physics lab, but just how excited should we be? In short: a little, but anyone holding their breath is in for an uncomfortable time.
LHCb is one of four big experiments at CERN’s Large Hadron Collider (LHC) near Geneva, Switzerland. As the “b” in the name indicates, it is intended to analyse decays of particles containing one of the six known flavours of quark, the “bottom” or alternatively “beauty” quark.
Bottom quarks are much heavier than the up and down quarks that make up protons and neutrons of conventional atomic matter, meaning particles containing them have lots of ways they can decay into lighter particles. Particles containing b quarks are also unusually long-lived, and these two properties combined make them very useful to physicists looking for physics beyond the standard model – our current best understanding of all particle interactions.
Particle physicists are desperate for any hints at expanding the standard model, which is supremely well tested but also woefully lacking, saying nothing about gravity, one of the four fundamental forces, or dark matter and dark energy, which seem to make up over 95 per cent of the cosmos.
Those are pretty key gaps, but when the standard model works, it really works, producing extremely precise predictions. LHCb seems to have found a deviation from these predictions in the rates at which a certain type of b-quark-containing particle, the B+, decays into the electron and its heavier cousin, the muon.
The standard model says that electrons and muons should be produced at roughly the same rate in these decays, but LHCb’s result suggests that they aren’t – and that is just the sort of hint of physics beyond the standard model that researchers are desperate to see.
Heady stuff. The fact is, though, that rumours of this anomaly have been around for a while – this one at LHCb for the best part of a decade. The news reports this week are based on a paper released by the collaboration that the anomaly has passed the “3-sigma” level of statistical significance, conventionally seen as the threshold for being “interesting” by particle physicists.
A 3-sigma result amounts to a probability of about 1 in 1000 that you would see a pattern of data like this if the standard model were correct. That might sound like a pretty solid indication that there’s something new here.
The problem is, however, that these sorts of decays are incredibly rare, and in looking for them physicists have to sift through a whole load of statistical noise, scanning widely. That leads to a seemingly paradoxical effect – the wider you cast your gaze, the more likely you are to see something that seems statistically significant. Gather more data, and these anomalies disappear again.
Particle physics is littered with 3-sigma effects that have come and gone, so researchers have settled on a much higher test threshold for discovery – “5-sigma”, corresponding to a probability of about 1 in 3.5 million that a pattern of data like this is a statistical fluke.
That is the bar the ATLAS and CMS experiments reached in 2012 with the Higgs boson – with the added security that two independent collaborations were seeing the same thing. LHCb has much further to go. Judging by the rate of data analysis – and the fact that the LHC has been switched off for an upgrade for the past two years – it is going to be a good while before they have anything more definite. Breathe out.
It is likely that this anomaly will fade away like the many others before it. On the other hand, if there is physics beyond the standard model accessible to like the LHC, our knowledge of it will start with an anomaly like this.
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Scientists Might Have Just Stumbled Upon a New Kind of Physics
Refinery29Here’s The Fun New Way Republican Men Are Threatening Public Safety
According to the strict rules of masculinity, manly men should not recycle, lean in a certain direction, or wear a face mask. Now, evidently, the latest unnecessarily gendered action is receiving the life-saving COVID-19 vaccine and becoming immune to a contagious illness that has killed nearly 3 million people worldwide. With each day, more Americans become eligible for vaccination, but certain demographics are more hesitant to take advantage of the shot. According to a new NPR/Marist study, 41% of self-identified Republicans, 34% of Independents, and 11% of Democrats say they do not plan on becoming vaccinated. Americans were also broken down by race, generation, education level, and voting history, and Republican men comprise the most anti-vaccine group. Compared to 34% of Republican women, 14% of Democrat women, and only 6% of Democrat men, 49% of Republican men say they will not get the vaccine. According to Nigel Barber, PhD, men have always been more likely to take life-threatening, “deliberate risks” than women. This can explain why men were more hesitant to mask up, too. “Men were more likely to say masks make them feel not cool. Mask-wearing represents a stigma for men,” Barber wrote in Psychology Today. “Wearing a mask expresses vulnerability. As a sign of risk aversion, it is perceived as unmanly.” He also wrote that men believe themselves to be lower-risk for COVID-19 than women, which is factually inaccurate. Melissa Deckman, a Washington College politics professor who specializes in gender, told The Lily that some men just don’t find vaccines “manly” and that succumbing to vaccination might mean admitting they are not invincible. Lots to unpack here! Republicans have also refused the vaccine for a variety of reasons, including distrust of Joe Biden’s administration, fears that the vaccine was “rushed,” and the belief that the virus was never life-threatening in the first place. According to Dr. Peter Hotez, a vaccine scientist, some Republicans feel that by turning down the vaccine, they’re supporting their political party. “Being against vaccines has been seen now as a badge or as a sign of loyalty to the Republican Party,” Hotez told PBS News Hour. This is also very publicly apparent. Conservative pundits like Tucker Carlson have expressed doubts about the shot. Donald Trump — whose voters are overwhelmingly uninterested in inoculation, according to the NPR poll — got his vaccine in January, although he declined to do so publicly, and didn’t even share that he had received it until this month. “I would recommend it to a lot of people that don’t want to get it. And a lot of those people voted for me, frankly,” he recently said on Fox News. “But again, we have our freedoms, and we have to live by that. And I agree with that also.” (One might think Trump’s supporters would be clamoring to receive the vaccine, seeing as he’s repeatedly stated it was his doing. Still, 47% of his supporters don’t want the “beautiful” shot.) Public health officials say that between 70 and 85% of the population must take the vaccine in order to reach herd immunity, and Dr. Anthony Fauci has warned that the vaccine needs bipartisan support. “The numbers you gave are so disturbing, how such a large proportion of a certain group of people would not want to get vaccinated merely because of political consideration,” Fauci told Meet the Press. “We’ve got to dissociate political persuasion from what’s common sense, no-brainer, public health things.” In other words, toxic masculinity is now a public health crisis. Literally. Like what you see? How about some more R29 goodness, right here?Republicans Vote Down Violence Against Women ActStacey Dash Is Sorry For Being A RepublicanRepublicans Criticize Biden For Saying “Nance”