From Nautilus: “When Beauty Gets in the Way of Science”

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April 18, 2019
Sabine Hossenfelder

Insisting that new ideas must be beautiful blocks progress in particle physics.

When Beauty Gets in the Way of Science. Nautilus

The biggest news in particle physics is no news. In March, one of the most important conferences in the field, Rencontres de Moriond, took place. It is an annual meeting at which experimental collaborations present preliminary results. But the recent data from the Large Hadron Collider (LHC), currently the world’s largest particle collider, has not revealed anything new.

LHC

CERN map


CERN LHC Tunnel

CERN LHC particles

Forty years ago, particle physicists thought themselves close to a final theory for the structure of matter. At that time, they formulated the Standard Model of particle physics to describe the elementary constituents of matter and their interactions.

Standard Model of Particle Physics (LATHAM BOYLE AND MARDUS OF WIKIMEDIA COMMONS)

After that, they searched for the predicted, but still missing, particles of the Standard Model. In 2012, they confirmed the last missing particle, the Higgs boson.

CERN CMS Higgs Event

CERN ATLAS Higgs Event

The Higgs boson is necessary to make sense of the rest of the Standard Model. Without it, the other particles would not have masses, and probabilities would not properly add up to one. Now, with the Higgs in the bag, the Standard Model is complete; all Pokémon caught.

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HIGGS HANGOVER: After the Large Hadron Collider (above) confirmed the Higgs boson, which validated the Standard Model, it’s produced nothing newsworthy, and is unlikely to, says physicist Sabine Hossenfelder.Shutterstock

The Standard Model may be physicists’ best shot at the structure of fundamental matter, but it leaves them wanting. Many particle physicists think it is simply too ugly to be nature’s last word. The 25 particles of the Standard Model can be classified by three types of symmetries that correspond to three fundamental forces: The electromagnetic force, and the strong and weak nuclear forces. Physicists, however, would rather there was only one unified force. They would also like to see an entirely new type of symmetry, the so-called “supersymmetry,” because that would be more appealing.

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Supersymmetry builds on the Standard Model, with many new supersymmetric particles, represented here with a tilde (~) on them. ( From the movie “Particle fever” reproduced by Mark Levinson)

Oh, and additional dimensions of space would be pretty. And maybe also parallel universes. Their wish list is long.

It has become common practice among particle physicists to use arguments from beauty to select the theories they deem worthy of further study. These criteria of beauty are subjective and not evidence-based, but they are widely believed to be good guides to theory development. The most often used criteria of beauty in the foundations of physics are presently simplicity and naturalness.

By “simplicity,” I don’t mean relative simplicity, the idea that the simplest theory is the best (a.k.a. “Occam’s razor”). Relying on relative simplicity is good scientific practice. The desire that a theory be simple in absolute terms, in contrast, is a criterion from beauty: There is no deep reason that the laws of nature should be simple. In the foundations of physics, this desire for absolute simplicity presently shows in physicists’ hope for unification or, if you push it one level further, in the quest for a “Theory of Everything” that would merge the three forces of the Standard Model with gravity.

The other criterion of beauty, naturalness, requires that pure numbers that appear in a theory (i.e., those without units) should neither be very large nor very small; instead, these numbers should be close to one. Exactly how close these numbers should be to one is debatable, which is already an indicator of the non-scientific nature of this argument. Indeed, the inability of particle physicists to quantify just when a lack of naturalness becomes problematic highlights that the fact that an unnatural theory is utterly unproblematic. It is just not beautiful.

Anyone who has a look at the literature of the foundations of physics will see that relying on such arguments from beauty has been a major current in the field for decades. It has been propagated by big players in the field, including Steven Weinberg, Frank Wilczek, Edward Witten, Murray Gell-Mann, and Sheldon Glashow. Countless books popularized the idea that the laws of nature should be beautiful, written, among others, by Brian Greene, Dan Hooper, Gordon Kane, and Anthony Zee. Indeed, this talk about beauty has been going on for so long that at this point it seems likely most people presently in the field were attracted by it in the first place. Little surprise, then, they can’t seem to let go of it.

Trouble is, relying on beauty as a guide to new laws of nature is not working.

Since the 1980s, dozens of experiments looked for evidence of unified forces and supersymmetric particles, and other particles invented to beautify the Standard Model. Physicists have conjectured hundreds of hypothetical particles, from “gluinos” and “wimps” to “branons” and “cuscutons,” each of which they invented to remedy a perceived lack of beauty in the existing theories. These particles are supposed to aid beauty, for example, by increasing the amount of symmetries, by unifying forces, or by explaining why certain numbers are small. Unfortunately, not a single one of those particles has ever been seen. Measurements have merely confirmed the Standard Model over and over again. And a theory of everything, if it exists, is as elusive today as it was in the 1970s. The Large Hadron Collider is only the most recent in a long series of searches that failed to confirm those beauty-based predictions.

These decades of failure show that postulating new laws of nature just because they are beautiful according to human standards is not a good way to put forward scientific hypotheses. It’s not the first time this has happened. Historical precedents are not difficult to find. Relying on beauty did not work for Kepler’s Platonic solids, it did not work for Einstein’s idea of an eternally unchanging universe, and it did not work for the oh-so-pretty idea, popular at the end of the 19th century, that atoms are knots in an invisible ether. All of these theories were once considered beautiful, but are today known to be wrong. Physicists have repeatedly told me about beautiful ideas that didn’t turn out to be beautiful at all. Such hindsight is not evidence that arguments from beauty work, but rather that our perception of beauty changes over time.

That beauty is subjective is hardly a breakthrough insight, but physicists are slow to learn the lesson—and that has consequences. Experiments that test ill-motivated hypotheses are at high risk to only find null results; i.e., to confirm the existing theories and not see evidence of new effects. This is what has happened in the foundations of physics for 40 years now. And with the new LHC results, it happened once again.

The data analyzed so far shows no evidence for supersymmetric particles, extra dimensions, or any other physics that would not be compatible with the Standard Model. In the past two years, particle physicists were excited about an anomaly in the interaction rates of different leptons. The Standard Model predicts these rates should be identical, but the data demonstrates a slight difference. This “lepton anomaly” has persisted in the new data, but—against particle physicists’ hopes—it did not increase in significance, is hence not a sign for new particles. The LHC collaborations succeeded in measuring the violation of symmetry in the decay of composite particles called “D-mesons,” but the measured effect is, once again, consistent with the Standard Model. The data stubbornly repeat: Nothing new to see here.

Of course it’s possible there is something to find in the data yet to be analyzed. But at this point we already know that all previously made predictions for new physics were wrong, meaning that there is now no reason to expect anything new to appear.

Yes, null results—like the recent LHC measurements—are also results. They rule out some hypotheses. But null results are not very useful results if you want to develop a new theory. A null-result says: “Let’s not go this way.” A result says: “Let’s go that way.” If there are many ways to go, discarding some of them does not help much.

To find the way forward in the foundations of physics, we need results, not null-results. When testing new hypotheses takes decades of construction time and billions of dollars, we have to be careful what to invest in. Experiments have become too costly to rely on serendipitous discoveries. Beauty-based methods have historically not worked. They still don’t work. It’s time that physicists take note.

And it’s not like the lack of beauty is the only problem with the current theories in the foundations of physics. There are good reasons to think physics is not done. The Standard Model cannot be the last word, notably because it does not contain gravity and fails to account for the masses of neutrinos. It also describes neither dark matter nor dark energy, which are necessary to explain galactic structures.

So, clearly, the foundations of physics have problems that require answers. Physicists should focus on those. And we currently have no reason to think that colliding particles at the next higher energies will help solve any of the existing problems. New effects may not appear until energies are a billion times higher than what even the next larger collider could probe. To make progress, then, physicists must, first and foremost, learn from their failed predictions.

So far, they have not. In 2016, the particle physicists Howard Baer, Vernon Barger, and Jenny List wrote an essay for Scientific American arguing that we need a larger particle collider to “save physics.” The reason? A theory the authors had proposed themselves, that is natural (beautiful!) in a specific way, predicts such a larger collider should see new particles. This March, Kane, a particle physicist, used similar beauty-based arguments in an essay for Physics Today. And a recent comment in Nature Reviews Physics about a big, new particle collider planned in Japan once again drew on the same motivations from naturalness that have already not worked for the LHC. Even the particle physicists who have admitted their predictions failed do not want to give up beauty-based hypotheses. Instead, they have argued we need more experiments to test just how wrong they are.

Will this latest round of null-results finally convince particle physicists that they need new methods of theory-development? I certainly hope so.

As an ex-particle physicist myself, I understand very well the desire to have an all-encompassing theory for the structure of matter. I can also relate to the appeal of theories such a supersymmetry or string theory. And, yes, I quite like the idea that we live in one of infinitely many universes that together make up the “multiverse.” But, as the latest LHC results drive home once again, the laws of nature care heartily little about what humans find beautiful.

See the full article here .

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From Northwestern University: “Unprecedented look at electron brings us closer to understanding the universe”

Northwestern U bloc
From Northwestern University

October 17, 2018
Amanda Morris

Study supports Standard Model of particle physics, excludes alternative models.

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An artist’s representation of an electron orbiting an atom’s nucleus, spinning about its axis as a cloud of other subatomic particles pop in and out of existence.
No image caption or credit.

The scientific community can relax. The electron is still round.

At least for now.

In a new study, researchers at Northwestern, Harvard and Yale universities examined the shape of an electron’s charge with unprecedented precision to confirm that it is perfectly spherical. A slightly squashed charge could have indicated unknown, hard-to-detect heavy particles in the electron’s presence, a discovery that could have upended the global physics community.

“If we had discovered that the shape wasn’t round, that would be the biggest headline in physics for the past several decades,” said Gerald Gabrielse, who led the research at Northwestern. “But our finding is still just as scientifically significant because it strengthens the Standard Model of particle physics and excludes alternative models.”

The study will be published Oct. 18 in the journal Nature. In addition to Gabrielse, the research was led by John Doyle, the Henry B. Silsbee Professor of Physics at Harvard, and David DeMille, professor of physics at Yale. The trio leads the National Science Foundation (NSF)-funded Advanced Cold Molecule Electron (ACME) Electric Dipole Moment Search.

The sub-standard Standard Model

A longstanding theory, the Standard Model of particle physics describes most of the fundamental forces and particles in the universe.

The Standard Model of elementary particles (more schematic depiction), with the three generations of matter, gauge bosons in the fourth column, and the Higgs boson in the fifth.


Standard Model of Particle Physics from Symmetry Magazine

The model is a mathematical picture of reality, and no laboratory experiments yet performed have contradicted it.

This lack of contradiction has been puzzling physicists for decades.

“The Standard Model as it stands cannot possibly be right because it cannot predict why the universe exists,” said Gabrielse, the Board of Trustees Professor of Physics in Northwestern’s Weinberg College of Arts and Sciences. “That’s a pretty big loophole.”

Gabrielse and his ACME colleagues have spent their careers trying to close this loophole by examining the Standard Model’s predictions and then trying to confirm them through table-top experiments in the lab.

Attempting to “fix” the Standard Model, many alternative models predict that an electron’s seemingly uniform sphere is actually asymmetrically squished. One such model, called the Supersymmetric Model, posits that unknown, heavy subatomic particles influence the electron to alter its perfectly spherical shape — an unproven phenomenon called the “electric dipole moment.”

Standard model of Supersymmetry DESY

These undiscovered, heavier particles could be responsible for some of the universe’s most glaring mysteries and could possibly explain why the universe is made from matter instead of antimatter.

“Almost all of the alternative models say the electron charge may well be squished, but we just haven’t looked sensitively enough,” said Gabrielse, the founding director of Northwestern’s new Center for Fundamental Physics. “That’s why we decided to look there with a higher precision than ever realized before.”

Squashing the alternative theories

The ACME team probed this question by firing a beam of cold thorium-oxide molecules into a chamber the size of a large desk. Researchers then studied the light emitted from the molecules. Twisting light would indicate an electric dipole moment. When the light did not twist, the research team concluded that the electron’s shape was, in fact, round, confirming the Standard Model’s prediction. No evidence of an electric dipole moment means no evidence of those hypothetical heavier particles. If these particles do exist at all, their properties differ from those predicted by theorists.

“Our result tells the scientific community that we need to seriously rethink some of the alternative theories,” DeMille said.

In 2014, the ACME team performed the same measurement with a simpler apparatus. By using improved laser methods and different laser frequencies, the current experiment was an order of magnitude more sensitive than its predecessor.

“If an electron were the size of Earth, we could detect if the Earth’s center was off by a distance a million times smaller than a human hair,” Gabrielse explained. “That’s how sensitive our apparatus is.”

Gabrielse, DeMille, Doyle and their teams plan to keep tuning their instrument to make more and more precise measurements. Until researchers find evidence to the contrary, the electron’s round shape — and the universe’s mysteries — will remain.

“We know the Standard Model is wrong, but we can’t seem to find where it’s wrong. It’s like a huge mystery novel,” Gabrielse said. “We should be very careful about making assumptions that we’re getting closer to solving the mystery, but I do have considerable hope that we’re getting closer at this level of precision.”

See the full article here .

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On May 31, 1850, nine men gathered to begin planning a university that would serve the Northwest Territory.

Given that they had little money, no land and limited higher education experience, their vision was ambitious. But through a combination of creative financing, shrewd politicking, religious inspiration and an abundance of hard work, the founders of Northwestern University were able to make that dream a reality.

In 1853, the founders purchased a 379-acre tract of land on the shore of Lake Michigan 12 miles north of Chicago. They established a campus and developed the land near it, naming the surrounding town Evanston in honor of one of the University’s founders, John Evans. After completing its first building in 1855, Northwestern began classes that fall with two faculty members and 10 students.
Twenty-one presidents have presided over Northwestern in the years since. The University has grown to include 12 schools and colleges, with additional campuses in Chicago and Doha, Qatar.

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From Nature: “LHC physicists embrace brute-force approach to particle hunt”

Nature Mag
From Nature

14 August 2018
Davide Castelvecchi

The world’s most powerful particle collider has yet to turn up new physics [since Higgs] — now some physicists are turning to a different strategy.

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The ATLAS detector at the Large Hadron Collider near Geneva, Switzerland.Credit: Stefano Dal Pozzolo/Contrasto /eyevine

A once-controversial approach to particle physics has entered the mainstream at the Large Hadron Collider (LHC).

LHC

CERN map


CERN LHC Tunnel

CERN LHC particles

The LHC’s major ATLAS experiment has officially thrown its weight behind the method — an alternative way to hunt through the reams of data created by the machine — as the collider’s best hope for detecting behaviour that goes beyond the standard model of particle physics. Conventional techniques have so far come up empty-handed.

So far, almost all studies at the LHC — at CERN, Europe’s particle-physics laboratory near Geneva, Switzerland — have involved ‘targeted searches’ for signatures of favoured theories. The ATLAS collaboration now describes its first all-out ‘general’ search of the detector’s data, in a preprint posted on the arXiv server last month and submitted to European Physics Journal C. Another major LHC experiment, CMS, is working on a similar project.

“My goal is to try to come up with a really new way to look for new physics” — one driven by the data rather than by theory, says Sascha Caron of Radboud University Nijmegen in the Netherlands, who has led the push for the approach at ATLAS. General searches are to the targeted ones what spell checking an entire text is to searching that text for a particular word. These broad searches could realize their full potential in the near future, when combined with increasingly sophisticated artificial-intelligence (AI) methods.

LHC researchers hope that the methods will lead them to their next big discovery — something that hasn’t happened since the detection of the Higgs boson in 2012, which put in place the final piece of the standard model. Developed in the 1960s and 1970s, the model describes all known subatomic particles, but physicists suspect that there is more to the story — the theory doesn’t account for dark matter, for instance. But big experiments such as the LHC have yet to find evidence for such behaviour. That means it’s important to try new things, including general searches, says Gian Giudice, who heads CERN’s theory department and is not involved in any of the experiments. “This is the right approach, at this point.”

Collision course

The LHC smashes together millions of protons per second at colossal energies to produce a profusion of decay particles, which are recorded by detectors such as ATLAS and CMS. Many different types of particle interaction can produce the same debris. For example, the decay of a Higgs might produce a pair of photons, but so do other, more common, processes. So, to search for the Higgs, physicists first ran simulations to predict how many of those ‘impostor’ pairs to expect. They then counted all photon pairs recorded in the detector and compared them to their simulations. The difference — a slight excess of photon pairs within a narrow range of energies — was evidence that the Higgs existed.

ATLAS and CMS have run hundreds more of these targeted searches to look for particles that do not appear in the standard model.

CERN/ATLAS detector


CERN/CMS Detector

The Standard Model of elementary particles (more schematic depiction), with the three generations of matter, gauge bosons in the fourth column, and the Higgs boson in the fifth.


Standard Model of Particle Physics from Symmetry Magazine

Many searches have looked for various flavours of supersymmetry, a theorized extension of the model that includes hypothesized particles such as the neutralino, a candidate for dark matter. But these searches have come up empty so far.

Standard model of Supersymmetry DESY

This leaves open the possibility that there are exotic particles that produce signatures no one has thought of — something that general searches have a better chance of finding. Physicists have yet to look, for example, events that produced three photons instead of two, Caron says. “We have hundreds of people looking at Higgs decay and supersymmetry, but maybe we are missing something nobody thought of,” says Arnd Meyer, a CMS member at Aachen University in Germany.

Whereas targeted searches typically look at only a handful of the many types of decay product, the latest study looked at more than 700 types at once. The study analysed data collected in 2015, the first year after an LHC upgrade raised the energy of proton collisions in the collider from 8 teraelectronvolts (TeV) to 13 TeV. At CMS, Meyer and a few collaborators have conducted a proof-of-principle study, which hasn’t been published, on a smaller set of data from the 8 TeV run.

Neither experiment has found significant deviations so far. This was not surprising, the teams say, because the data sets were relatively small. Both ATLAS and CMS are now searching the data collected in 2016 and 2017, a trove tens of times larger.

Statistical cons

The approach “has clear advantages, but also clear shortcomings”, says Markus Klute, a physicist at the Massachusetts Institute of Technology in Cambridge. Klute is part of CMS and has worked on general searches in at previous experiments, but he was not directly involved in the more recent studies. One limitation is statistical power. If a targeted search finds a positive result, there are standard procedures for calculating its significance; when casting a wide net, however, some false positives are bound to arise. That was one reason that general searches had not been favoured in the past: many physicists feared that they could lead down too many blind alleys. But the teams say they have put a lot of work into making their methods more solid. “I am excited this came forward,” says Klute.

Most of the people power and resources at the LHC experiments still go into targeted searches, and that might not change anytime soon. “Some people doubt the usefulness of such general searches, given that we have so many searches that exhaustively cover much of the parameter space,” says Tulika Bose of Boston University in Massachusetts, who helps to coordinate the research programme at CMS.

Many researchers who work on general searches say that they eventually want to use AI to do away with standard-model simulations altogether. Proponents of this approach hope to use machine learning to find patterns in the data without any theoretical bias. “We want to reverse the strategy — let the data tell us where to look next,” Caron says. Computer scientists are also pushing towards this type of ‘unsupervised’ machine learning — compared with the supervised type, in which the machine ‘learns’ from going through data that have been tagged previously by humans.

See the full article here .

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From Ethan Siegel: “What Was It Like When The Higgs Gave Mass To The Universe?”

From Ethan Siegel
Aug 8, 2018

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A candidate Higgs event in the ATLAS detector. Note how even with the clear signatures and transverse tracks, there is a shower of other particles; this is due to the fact that protons are composite particles. This is only the case because the Higgs gives mass to the fundamental constituents that compose these particles. (THE ATLAS COLLABORATION / CERN)

CERN CMS Higgs Event

CERN ATLAS Higgs Event

One moment, every particle in the Universe was massless. Then, they weren’t anymore. Here’s how it happened.

In the earliest stages of the hot Big Bang, the Universe was filled with all the particles, antiparticles, and quanta of radiation it had the energy to create. As the Universe expanded, it cooled: the stretching fabric of space also stretched the wavelengths of all the radiation within it to longer wavelengths, which equates to lower energies.

If there are any particles (and antiparticles) that exist at higher energies that are yet to be discovered, they were likely created in the hot Big Bang, so long as there was enough energy (E) available to create a massive (m) particle via Einstein’s E = mc². It’s possible that a slew of puzzles about our Universe, including the origin of the matter-antimatter asymmetry and the creation of dark matter, are solved by new physics at these early times. But the massive particles we know today are foreign to us. At these early stages, they have no mass.

The particles and antiparticles of the Standard Model are easy to create, even as the Universe cools and the fractions-of-a-second ticked by. The Universe might start of at energies as large as 10¹⁵ or 10¹⁶ GeV; even by time it’s dropped to 1000 (10³) GeV, no Standard Model particle is threatened. At the energies achievable by the LHC, we can create the full suite of particle-antiparticle pairs that are known to physics.

But at this point, unlike today, they’re all massless. If they have no rest mass, they have no choice but to move at the speed of light. The reason particles are in this strange, bizarre state that’s so different from how they exist today? It’s because the fundamental symmetry that gives rise to the Higgs boson — the electroweak symmetry — has not yet broken in the Universe.

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The particles and antiparticles of the Standard Model have now all been directly detected, with the last holdout, the Higgs Boson, falling at the LHC earlier this decade. Today, only the gluons and photons are massless; everything else has a non-zero rest mass. (E. SIEGEL / BEYOND THE GALAXY)

When we look at the Standard Model today, it’s arranged as follows:

six quarks, each of which come in three colors, and their antiquark counterparts,
three charged leptons (e, μ, τ) and three neutral ones (ν_e, ν_μ, ν_τ), and their antimatter counterparts,
the eight massless gluons that mediate the strong force between the quarks,
the three heavy, weak bosons (W+, W-, and Z_0) that mediate the weak nuclear force,
and the photon (γ), the massless mediator of the electromagnetic force.

But there’s a symmetry that’s broken at today’s low-energy scale: the electroweak symmetry. This symmetry was restored in the early days of the Universe. And when it’s restored versus when it’s broken, it fundamentally changes the Standard Model picture.

The Standard Model of elementary particles (more schematic depiction), with the three generations of matter, gauge bosons in the fourth column, and the Higgs boson in the fifth.


Standard Model of Particle Physics from Symmetry Magazine

Instead of the weak and electromagnetic bosons (W+, W-, Z_0, γ), where the first three are very massive and the last is massless, we have four new bosons for the electroweak force (W_1, W_2, W_3, B), and all of them have no mass at all. The other particles are all the same, except for the fact that they, too, have no mass yet. This is what’s floating around in the early Universe, colliding, annihilating, and spontaneously being created, all in motion at the speed of light.

As the Universe expands and cools, all of this continues. So long as the energy of your Universe is above a certain value, you can think about the Higgs field as floating atop the liquid in a soda (or wine) bottle. As the level of the liquid drops, the Higgs field remains atop the liquid, and everything stays massless. This is what we call a restored-symmetry state.

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When a wine bottle is either completely or partially filled, a drop of oil or a ping pong ball will float on the wine’s surface inside the bottle. At any location, the wine-level, and hence what’s floating atop it, will remain at the same level. This corresponds to a restored-symmetry state. (EVAN SWIGART FROM CHICAGO, USA)

But below a certain liquid level, the bottom of the container starts to show itself. And the field can no longer remain in the center; more generally, it can’t take on simply any old value. It has to go to where the liquid level is, and that means down into the divot(s) at the bottom of the bottle. This is what we call a broken-symmetry state.

When this symmetry breaks, the Higgs field settles into the bottom, lowest-energy, equilibrium state. But that energy state isn’t quite zero: it has a finite, non-zero value known as its vacuum expectation value. Whereas the restored-symmetry state yielded only massless particles, the broken symmetry state changes everything.

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When a wine bottle is completely empty, any ball or drop of oil inside will slide all the way down to the lowest-level ‘ring’ at the bottom. This corresponds to a broken symmetry state, since all values (i.e., locations) are no longer equivalent. (PATRICK HEUSSER, X8ING.COM)

Once the symmetry breaks, the Higgs field has four mass-containing consequences: two are charged (one positive and one negative) and two are neutral. Then, the following things all happen at once:

The W_1 and W_2 particles “eat” the charged, broken-symmetry consequences of the Higgs, becoming the W+ and W- particles.
The W_3 and B particles mix together, with one combination eating the uncharged broken-symmetry consequence of the Higgs, becoming the Z_0, and with the other combination eating nothing, to remain the massless photon (γ).
The last neutral broken-symmetry consequence of the Higgs gains mass, and becomes the Higgs boson.
At last, the Higgs boson couples to all the other particles of the Standard Model, giving mass to the Universe.

This is the origin of mass in the Universe.

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When the electroweak symmetry is broken, the W+ gets its mass by eating the positively charged Higgs, the W- by eating the negatively charged Higgs, and the Z_0 by eating the neutral Higgs. The other neutral Higgs becomes the Higgs boson, detected and discovered earlier this decade at the LHC. The photon, the other combination of the W3 and the B boson, remains massless. (FLIP TANEDO / QUANTUM DIARIES)

This whole process is called spontaneous symmetry breaking. And for the quarks and leptons in the standard model, when this Higgs symmetry is broken, every particle gets a mass due to two things:

The expectation value of the Higgs field, and
A coupling constant.

And this is kind of the problem. The expectation value of the Higgs field is the same for all of these particles, and not too difficult to determine. But that coupling constant? Not only is it different for every particle, but — in the standard model — it’s arbitrary.

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The Higgs boson, now with mass, couples to the quarks, leptons, and W-and-Z bosons of the Standard Model, which gives them mass. That it doesn’t couple to the photon and gluons means those particles remain massless. (TRITERTBUTOXY AT ENGLISH WIKIPEDIA)

We know that the particles have mass; we know how they get mass; we’ve discovered the particles responsible for mass. But we still have no idea why the particles have the values of the masses they do. We have no idea why the coupling constants have the couplings that they do. The Higgs boson is real; the gauge bosons are real; the quarks and leptons are real. We can create, detect, and measure their properties exquisitely. Yet, when it comes to understanding why they have the values that they do, that’s a puzzle we cannot yet solve. We do not have the answer.

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The masses of the fundamental particles in the Universe, once the electroweak symmetry is broken, spans many orders of magnitude, withe the neutrinos being the lightest massive particles and the top quark being the heaviest. We do not understand why the coupling constants have the values they do, and hence, why the particles have the masses they do. (FIG. 15–04A FROM UNIVERSE-REVIEW.CA)

Before the breaking of the electroweak symmetry, everything that is known to exist in the Universe today is massless, and moves at the speed of light. Once the Higgs symmetry breaks, it gives mass to the quarks and leptons of the Universe, the W and Z bosons, and the Higgs boson itself. Suddenly, with huge mass differences between light particles and heavy ones, the heavy ones spontaneously decay into the lighter ones on very short timescales, especially when the energy (E) of the Universe drops below the mass equivalent (m) needed to create these unstable particles via E = mc².

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A visual history of the expanding Universe includes the hot, dense state known as the Big Bang and the growth and formation of structure subsequently. Without the Higgs giving mass to the particles in the Universe at a very early, hot stage, none of this would have been possible. (NASA / CXC / M. WEISS)

Without this critical gauge symmetry associated with electroweak symmetry breaking, existence wouldn’t be possible, as we do not have stable, bound states made purely of massless particles. But with fundamental masses to the quarks and charged leptons, the Universe can now do something it’s never done before. It can cool and create bound states like protons and neutrons. It can cool further and create atomic nuclei and, eventually, neutral atoms. And when enough time goes by, it can give rise to stars, galaxies, planets, and human beings. Without the Higgs to give mass to the Universe, none of this would be possible. The Higgs, despite the fact that it took 50 years to discover, has been making the Universe possible for 13.8 billion years.

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“Starts With A Bang! is a blog/video blog about cosmology, physics, astronomy, and anything else I find interesting enough to write about. I am a firm believer that the highest good in life is learning, and the greatest evil is willful ignorance. The goal of everything on this site is to help inform you about our world, how we came to be here, and to understand how it all works. As I write these pages for you, I hope to not only explain to you what we know, think, and believe, but how we know it, and why we draw the conclusions we do. It is my hope that you find this interesting, informative, and accessible,” says Ethan

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From CERN ATLAS: “Could a new type of quark fix the “unnaturalness” of the Standard Model?”

CERN ATLAS Higgs Event

CERN/ATLAS
From CERN ATLAS

8th August 2018
ATLAS Collaboration

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Figure 1: One of the Feynman diagrams for T pair production at the LHC. (Image: ATLAS Collaboration © CERN 2018)

While the discovery of the Higgs boson at the Large Hadron Collider (LHC) in 2012 confirmed many Standard Model predictions, it has raised as many questions as it has answered. For example, interactions at the quantum level between the Higgs boson and the top quark ought to lead to a huge Higgs boson mass, possibly as large as the Planck mass (>1018 GeV). So why is it only 125 GeV? Is there a mechanism at play to cancel these large quantum corrections caused by the top quark (t)? Finding a way to explain the lightness of the Higgs boson is one of the top (no pun intended) questions in particle physics.

A wide range of solutions have been proposed and a common feature in many of them is the existence of vector-like quarks – in particular, a vector-like top quark (T). Like other quarks, vector-like quarks would be spin-½ particles that interact via the strong force. While all spin-½ particles have left- and right-handed components, the weak force only interacts with the left-handed components of Standard Model particles. However, vector-like quarks would have “ambidextrous” interactions with the weak force, giving them a bit more leeway in how they decay. While the Standard Model top quark always decays to a bottom quark (b) by emitting a W boson (t→Wb), a vector-like top can decay three different ways: T→Wb, T→Zt or T→Ht (Figure 1).

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Figure 2: Lower limit (scale on right axis) on the mass of a vector-like top as a function of the branching ratio to Wb and Ht (bottom and left axes). (Image: ATLAS Collaboration © CERN 2018)

The ATLAS collaboration uses a custom-built programme to search for vector-like top pairs in LHC data. It utilizes data from several dedicated analyses, each of them sensitive to various experimental signatures (involving leptons, boosted objects and/or large missing transverse momentum). This allows ATLAS to look for all of possible decays, increasing the chance of discovery.

ATLAS has now gone one step further by performing a combination of all of the individual searches. While individual analyses are designed to study a particular sets of decays, combined results provide sensitivity to all possible sets of decays. These have allowed ATLAS to search for vector-like tops with masses over 1200 GeV. It appears, however, that vector-like tops are so far nowhere to be found. On the bright side, the combination allows ATLAS to set the most stringent lower limits on the mass of a vector-like top for arbitrary sets of branching ratios to the three decay modes (Figure 2).

Between these limits on vector-like top quarks and those on other theories that could offer a solution (like supersymmetry), the case for a naturally light Higgs boson is not looking good… but Nature probably still has a few tricks up its sleeve for us to uncover.

The Standard Model of elementary particles (more schematic depiction), with the three generations of matter, gauge bosons in the fourth column, and the Higgs boson in the fifth.


Standard Model of Particle Physics from Symmetry Magazine

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From FNAL: “The MiniBooNE search for dark matter”

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FNAL Art Image by Angela Gonzales

Fermilab is an enduring source of strength for the US contribution to scientific research world wide.

July 18, 2017
Ranjan Dharmapalan
Tyler Thornton

FNAL/MiniBooNE

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This schematic shows the experimental setup for the dark matter search. Protons (blue arrow on the left) generated by the Fermilab accelerator chain strike a thick steel block. This interaction produces secondary particles, some of which are absorbed by the block. Others, including photons and perhaps dark-sector photons, symbolized by V, are unaffected. These dark photons decay into dark matter, shown as χ, and travel to the MiniBooNE detector, depicted as the sphere on the right.

Particle physicists are in a quandary. On one hand, the Standard Model accurately describes most of the known particles and forces of interaction between them.

The Standard Model of elementary particles (more schematic depiction), with the three generations of matter, gauge bosons in the fourth column, and the Higgs boson in the fifth.

On the other, we know that the Standard Model accounts for less than 5 percent of the universe. About 26 percent of the universe is composed of mysterious dark matter, and the remaining 68 percent of even more mysterious dark energy.

Some theorists speculate that dark matter particles could belong to a “hidden sector” and that there may be portals to this hidden sector from the Standard Model. The portals allow hidden-sector particles to trickle into Standard Model interactions. A large sensitive particle detector, placed in an intense particle beam and equipped with a mechanism to suppress the Standard Model interactions, could unveil these new particles.

Fermilab is home to a number of proton beams and large, extremely sensitive detectors, initially built to detect neutrinos. These devices, such as the MiniBooNE detector, are ideal places to search for hidden-sector particles.

In 2012, the MiniBooNE-DM collaboration teamed up with theorists who proposed new ways to search for dark matter particles. One of these proposals [FNAL PAC Oct 15 2012] involved the reconfiguration of the existing neutrino experiment. This was a pioneering effort that involved close coordination between the experimentalists, accelerator scientists, beam alignment experts and numerous technicians.

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Results of this MiniBooNE-DM search for dark matter scattering off of nucleons. The plot shows the confidence limits and sensitivities with 1, 2σ errors resulting from this analysis compared to other experimental results, as a function of Y (a parameter describing the dark photon mass, dark matter mass and the couplings to the Standard Model) and Mχ (the dark matter mass). For details see the Physical Review Letters paper.

For the neutrino experiment, the 8-GeV proton beam from the Fermilab Booster hit a beryllium target to produce a secondary beam of charged particles that decayed further downstream, in a decay pipe, into neutrinos. MiniBooNE ran in this mode for about a decade to measure neutrino oscillations and interactions.

In the dark matter search mode, however, the proton beam was steered past the beryllium target. The beam instead struck a thick steel block at the end of the decay pipe. The resulting charged secondary particles (mostly particles called pions) are absorbed in the steel block, reducing the number of subsequent neutrinos, while the neutral secondary particles remained unaffected. The photons resulting from the decay of neutral pions may have transformed into hidden-sector photons that in turn might have decayed into dark matter, which would travel to the MiniBooNE detector 450 meters away. The experiment ran in this mode for nine months for a dedicated dark matter search.

Using the previous 10 years’ worth of data as a baseline, MiniBooNE-DM looked for scattered protons and neutrons in the detector. If they found more scattered protons or neutrons than predicted, the excess could indicate a new particle, maybe dark matter, being produced in the steel block. Scientists analyzed multiple types of neutrino interactions at the same time, reducing the error on the signal data set by more than half.

Analysts concluded that the data was consistent with the Standard Model prediction, enabling the experimenters to set a limit on a specific model of dark matter, called vector portal dark matter. To set the limit, scientists developed a detailed simulation that estimated the predicted proton or neutron response in the detector from scattered dark matter particles. The new limit extends from the low-mass edge of direct-detection experiments down to masses about 1,000 times smaller. Additionally, the result rules out this particular model as a description of the anomalous behavior of the muon seen in the Muon g-2 experiment at Brookhaven, which was one of the goals of the MiniBooNE-DM proposal. Incidentally, researchers at Fermilab will make a more precise measurement of the muon — and verify the Brookhaven result — in an experiment that started up this year.

This result from MiniBooNE, a dedicated proton beam dump search for dark matter, was published in Physical Review Letters and was highlighted as an “Editor’s suggestion.”

What’s next? The experiment will continue to analyze the collected data set. It is possible that the dark matter or hidden-sector particles may prefer to scatter off of the lepton family of particles, which includes electrons, rather than off of quarks, which are the constituent of protons and neutrons. Different interaction channels probe different possibilities.

If the portals to the hidden sector are narrow — that is, if they are weakly coupled — researchers will need to collect more data or implement new ideas to suppress the Standard Model interactions.

The first results from MiniBooNE-DM show that Fermilab could be at the forefront of searching for hidden-sector particles. Upcoming experiments in Fermilab’s Short-Baseline Neutrino program will use higher-resolution detectors — specifically, liquid-argon time projection chamber technology — expanding the search regions and possibly leading to discovery.

Ranjan Dharmapalan is a postdoc at Argonne National Laboratory. Tyler Thornton is a graduate student at Indiana University Bloomington.

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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
collaborate at Fermilab on experiments at the frontiers of discovery.

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From ATLAS: “New insight into the Standard Model”

CERN ATLAS Higgs Event

CERN/ATLAS
ATLAS

9th May 2017
ATLAS Collaboration

ATLAS releases the first study of a pair of neutral bosons produced in association with a high-mass dijet system.

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Figure 1: Distribution of (a) the centrality of the Z boson-photon (Zγ) system and (b) the transverse energy of the photon. These studies show data collected by ATLAS in 2012 (black points) compared to Standard Model predictions (coloured histograms). The signal that is looked for is displayed as the dark red histogram and the main background is shown as the light blue one. The bottom panels show the ratio of the data to the sum of all the predictions. The error band (blue) shows the total uncertainty on these predictions. A sign of new physics could appear as an enhancement at large momentum, as shown by the dotted blue line in (b). (Image: ATLAS Collaboration/CERN)

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Figure 2: Feynman diagram of the signal process, the Electroweak production of a Z boson, photon (γ) and two high-energy jets. (Image: ATLAS Collaboration/CERN)

Ever since the LHC collided its first protons in 2009, the ATLAS Collaboration has been persistently studying their interactions with increasing precision. To this day, it has always observed them to be as expected by the Standard Model.

The Standard Model of elementary particles (more schematic depiction), with the three generations of matter, gauge bosons in the fourth column, and the Higgs boson in the fifth.

Though it remains unrefuted, physicists are convinced that a better theory must exist to explain certain fundamental questions: What is the nature of the dark matter? Why is the gravitational force so weak compared to the other forces?

Answers may be found by looking at a very rare process that had previously never been studied by ATLAS: the interaction of four bosons, whose signature is the presence of a Z boson, a photon and two high-energy jets. This is an excellent probe of the electroweak sector of the Standard Model and is very sensitive to new physics models. However, this process is very difficult to detect, given its rarity and the large number of different processes that can mimic its signature (known as “background”). The main background comes from the production of a Z boson and a photon accompanied by two jets, which, unlike the electroweak process we are interested in, is produced via strong interactions.

This leads to differences in the kinematics of the observed jets, which are described in a recently-submitted paper to the Journal of High Energy Physics [no link found], where ATLAS presents a search for such events using 8 TeV data. Utilizing the knowledge that the recoiling quarks (see Figure 2) will produce jets that have a very large invariant mass and are widely separated in the detector, ATLAS has been able to reduce the background and mitigate the large experimental uncertainties in order to extract the signal.

The background is suppressed by selecting events where the two jets have an invariant mass larger than 500 GeV. The signal and main background are further separated by quantifying the centrality of the Z-photon system with respect to the two jets. Events with low centrality are more likely to be produced via the electroweak signal process while those with high centrality are more likely to come from strong interactions. This is illustrated in Figure 1(a), where a small excess of events above the predicted background is observed, with a statistical significance of 2σ.

The centrality is used to measure the event rate (cross section) of the signal alone, and of the sum of the signal and the major background. Both were found to be in agreement with Standard Model predictions within the large statistical uncertainty. Anomalies on the coupling of four bosons have also been searched for, by looking at the tails of the photon transverse energy spectrum that may be enhanced by new physics contributions (blue dotted line in Figure 1(b)). No deviation from the Standard Model has been seen and stringent limits are set on the presence of new physics in this region.

The Standard Model will continue to keep its secrets… until the next set of results!

Links:

Studies of Zγ electroweak production in association with a high-mass dijet system in pp collisions at 8 TeV with the ATLAS detector(arXiv: 1705.01966, see figures)
See also the full lists of ATLAS Conference Notes and ATLAS Physics Papers.

See the full article here .

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