## Archive for April, 2015

### Italian neutrino experiment to move to the US

Wednesday, April 22nd, 2015

The world’s largest liquid-argon neutrino detector will help with the search for sterile neutrinos at Fermilab. Photo: INFN

Mysterious particles called neutrinos seem to come in three varieties. However, peculiar findings in experiments over the past two decades make scientists wonder if a fourth is lurking just out of sight.

To help solve this mystery, a group of scientists spearheaded by Nobel laureate Carlo Rubbia plans to bring ICARUS, the world’s largest liquid-argon neutrino detector, across the Atlantic Ocean to the United States. The detector is currently being refurbished at CERN, where it is the first beneficiary of a new test facility for neutrino detectors.

Neutrinos are some of the most abundant and yet also most mysterious particles in the universe. They have tiny masses, but no one is sure why—or where those masses come from. They interact so rarely that they can pass through the entire Earth as if it weren’t there. They oscillate from one type to another, so that even if you start out with one kind of neutrino, it might change to another kind by the time you detect it.

Many theories in particle physics predict the existence of a sterile neutrino, which would behave differently from the three known types of neutrino.

“Finding a fourth type of neutrinos would change the whole picture we’re trying to address with current and future experiments,” says Peter Wilson, a scientist at Fermi National Accelerator Laboratory.

The Program Advisory Committee at Fermilab recently endorsed a plan, managed by Wilson, to place a suite of three detectors in a neutrino beam at the laboratory to study neutrinos—and determine whether sterile neutrinos exist.

Over the last 20 years, experiments have seen clues pointing to the possible existence of sterile neutrinos. Their influence may have caused two different types of unexpected neutrino behavior seen at the Liquid Scintillator Neutrino Detector experiment at Los Alamos National Laboratory in New Mexico and the MiniBooNE experiment at Fermilab.

Both experiments saw indications that a surprisingly large number of neutrinos may be morphing from one kind to another a short distance from a neutrino source. The existence of a fourth type of neutrino could encourage this fast transition.

The new three-detector formation at Fermilab could provide the answer to this mystery.

In the suite of experiments, a 260-ton detector called Short Baseline Neutrino Detector will sit closest to the source of the beam, so close that it will be able to detect the neutrinos before they’ve had a chance to change from one type into another. This will give scientists a baseline to compare with results from the other two detectors. SBND is under construction by a team of scientists and engineers from universities in the United Kingdom, the United States and Switzerland, working with several national laboratories in Europe and the US.

The SBND detector will be filled with liquid argon, which gives off flashes of light when other particles pass through it.

“Liquid argon is an extremely exciting technology to make precision measurements with neutrinos,” says University of Manchester physicist Stefan Soldner-Rembold, who leads the UK project building a large section of the detector. “It’s the technology we’ll be using for the next 20 to 30 years of neutrino research.”

Farther from the beam will be the existing 170-ton MicroBooNE detector, which is complete and will begin operation at Fermilab this year. The MicroBooNE detector was designed to find out whether the excess of particles seen by MiniBooNE was caused by a new type of neutrino or a new type of background. Identifying either would have major implications for future neutrino experiments.

Finally, farthest from the beam would be a liquid-argon detector more than four times the size of MicroBooNE. The 760-ton detector was used in the ICARUS experiment, which studied neutrino oscillations at Gran Sasso Laboratory in Italy using a beam of neutrinos produced at CERN from 2010 to 2014.

Its original beam at CERN is not optimized for the next stage of the sterile neutrino search. “The Fermilab beamline is the only game in town for this type of experiment,” says physicist Steve Brice, deputy head of Fermilab’s Neutrino Division.

And the ICARUS detector “is the best detector in the world to detect this kind of particle,” says Alberto Scaramelli, the former technical director of Gran Sasso National Laboratory. “We should use it.”

Rubbia, who initiated construction of ICARUS and leads the ICARUS collaboration, proposed bringing the detector to Fermilab in August 2013. Since then, the ICARUS, MicroBooNE and SBND groups have banded together to create the current proposal. The updated plan received approval from the Fermilab Program Advisory Committee in February.

“The end product was really great because it went through the full scrutiny of three different collaborations,” says MicroBooNE co-leader Sam Zeller. “The detectors all have complementary strengths.”

In December, scientists shipped the ICARUS detector from the Gran Sasso laboratory to CERN, where it is currently undergoing upgrades. The three-detector short-baseline neutrino program at Fermilab is scheduled to begin operation in 2018.

Kathryn Jepsen

### Mu2e breaks ground on experiment seeking new physics

Tuesday, April 21st, 2015

Fermilab’s Mu2e groundbreaking ceremony took place on Saturday, April 18. From left: Alan Stone (DOE Office of High Energy Physics), Nigel Lockyer (Fermilab director), Jim Siegrist (DOE Office of High Energy Physics director), Ron Ray (Mu2e project manager), Paul Philp (Mu2e federal project director at the Fermi Site Office), Jim Miller (Mu2e co-spokesperson), Doug Glenzinski (Mu2e co-spokesperson), Martha Michels (Fermilab ESH&Q head), Mike Shrader (Middough architecture firm), Julie Whitmore (Mu2e deputy project manager), Jason Whittaker (Whittaker Construction), Tom Lackowski (FESS). Photo: Reidar Hahn

This weekend, members of the Mu2e collaboration dug their shovels into the ground of Fermilab’s Muon Campus for the experiment that will search for the direct conversion of a muon into an electron in the hunt for new physics.

For decades, the Standard Model has stood as the best explanation of the subatomic world, describing the properties of the basic building blocks of matter and the forces that govern them. However, challenges remain, including that of unifying gravity with the other fundamental forces or explaining the matter-antimatter asymmetry that allows our universe to exist. Physicists have since developed new models, and detecting the direct conversion of a muon to an electron would provide evidence for many of these alternative theories.

“There’s a real possibility that we’ll see a signal because so many theories beyond the Standard Model naturally allow muon-to-electron conversion,” said Jim Miller, a co-spokesperson for Mu2e. “It’ll also be exciting if we don’t see anything, since it will greatly constrain the parameters of these models.”

Muons and electrons are two different flavors in the charged-lepton family. Muons are 200 times more massive than electrons and decay quickly into lighter particles, while electrons are stable and live forever. Most of the time, a muon decays into an electron and two neutrinos, but physicists have reason to believe that once in a blue moon, muons will convert directly into an electron without releasing any neutrinos. This is physics beyond the Standard Model.

Under the Standard Model, the muon-to-electron direct conversion happens too rarely to ever observe. In more sophisticated models, however, this occurs just frequently enough for an extremely sensitive machine to detect.

The Mu2e detector, when complete, will be the instrument to do this. The 92-foot-long apparatus will have three sections, each with its own superconducting magnet. Its unique S-shape was designed to capture as many slow muons as possible with an aluminum target. The direct conversion of a muon to an electron in an aluminum nucleus would release exactly 105 million electronvolts of energy, which means that if it occurs, the signal in the detector will be unmistakable. Scientists expect Mu2e to be 10,000 times more sensitive than previous attempts to see this process.

Construction will now begin on a new experimental hall for Mu2e. This hall will eventually house the detector and the infrastructure needed to conduct the experiment, such as the cryogenic systems to cool the superconducting magnets and the power systems to keep the machine running.

“What’s nice about the groundbreaking is that it becomes a real thing. It’s a long haul, but we’ll get there eventually, and this is a start,” said Julie Whitmore, deputy project manager for Mu2e.

The detector hall will be complete in late 2016. The experiment, funded mainly by the Department of Energy Office of Science, is expected to begin in 2020 and run for three years until peak sensitivity is reached.

“This is a project that will be moving along for many years. It won’t just be one shot,” said Stefano Miscetti, the leader of the Italian INFN group, Mu2e’s largest international collaborator. “If we observe something, we will want to measure it better. If we don’t, we will want to increase the sensitivity.”

Physicists around the world are working to extend the frontiers of the Standard Model. One hundred seventy-eight people from 31 institutions are coming together for Mu2e to make a significant impact on this venture.

“We’re sensitive to the same new physics that scientists are searching for at the Large Hadron Collider, we just look for it in a complementary way,” said Ron Ray, Mu2e project manager. “Even if the LHC doesn’t see new physics, we could see new physics here.”

Diana Kwon

See a two-minute video on the ceremony

### In Defense of Scientism and the Joys of Self-Publishing.

Friday, April 17th, 2015

As long-time readers of Quantum Diaries know I have been publishing here for a number of years and this is my 85th and last post[1]. A couple of years ago, I collected the then current collection, titled it “In Defense of Scientism,” after the title of one of the essays, and sent it off to a commercial publisher. Six months later, I got an e-mail from the editor complaining that he had lost the file and only found it by accident, and he somehow inferred that it was my fault. After that experience, it was no surprise he did not publish it.

With all the talk of self-publishing these days, I thought I would give it a try. It is easy, at least compared to finding the Higgs boson! There are a variety of options that give different levels of control, so one can pick and choose preferences – like off an á la carte menu. The simplest form of self-publishing is to go to a large commercial publisher.  The one I found would, for $50.00 USD up front and$12.00 a year, supply print on demand and e-books to a number of suppliers. Not sure that I could recover the costs from the revenue – and being a cheapskate – I decided not to go that route. There are also commissioned alternatives with no upfront costs, but I decided to interact directly with three (maybe four, if I can jump over the humps the fourth has put up) companies.  One of the companies treated their print-on-demand and digital distribution arms as distinct, even to the point of requiring different reimbursement methods. That is the disadvantage of doing it yourself, sorting it all out. The advantage of working directly with the suppliers is more control over the detailed formatting and distribution.

From then on things got fiddly[2], for example, reimbursement. Some companies would only allow payment by electronic fund transfer, others only by check. The weirdest example was one company that did electronic fund transfers unless the book was sold in Brazil or Mexico. In those cases, it is by check but only after \$100.00 has been accumulated. One company verified, during account setup, that the fund transfer worked by transferring a small amount, in my case 16 cents. And then of course there are special rules if you earn any money in the USA. For USA earnings there is a 30% withholding tax unless you can document that there is a tax treaty that allows you to get around it. The USA is the only country that requires this. Fine, being an academic, I am used to jumping through hoops.

Next was the question of an International Standard Book Number (ISBN). They are not required but are recommended. That is fine since in Canada you can get them for free. Just as well since each version of the book needs a different number. The paperback needs a different number from the electronic and each different electronic format requires its own number. As I said, it is a good thing it is free. Along with the ISBN, I got a reminder that the Library of Canada requires one copy of each book that sells more than four copies and two copies if it goes over a hundred and of course a separate electronic copy if you publish electronically. Fun, fun, fun[3]. There are other implications of getting you own ISBN number. Some of the publishers would supply an ISBN free of charge but then would put the book out under their own imprint and, in some cases, give wider distribution to those books. But again, getting your own number ultimately gives you more control.

With all this research in hand, it was time to create and format the content. I had the content from the four years’ worth of Quantum Diary posts and all I had to do was put it together and edit for consistency. Actually, Microsoft Word worked quite well with various formatting features to help. I then gave it to my wife to proofread. That was a mistake; she is still laughing at some of the typos. At least there is now an order of magnitude fewer errors. I should also acknowledge the many editorial comments from successive members of the TRIUMF communications team.

The next step was to design the book cover. There comes a point in every researcher’s career when they need support and talent outside of themselves. Originally, I had wanted to superimpose a picture of a model boat on a blackboard of equations. With that vision in mind, I set about the hallways to seek and enroll the talent of a few staff members who could make it happen. After normal working hours, of course. A co-op communication student suggested that the boat be drawn on the blackboard rather than a picture superimposed. The equations were already on a blackboard and are legitimate. The boat was hand drawn by a talented lady in accounting, drawing it first onto an overhead transparency[4] and then projecting it onto a blackboard. A co-op student in the communications team produced the final cover layout according to the various colour codes and margin bleeds dictated by each publisher. For both my own and your sanity, I won’t go into all the details. In the end, I rather like how the cover turned out.

Now, several months after I started the publishing process, I have jumped through all the hoops! All I have to do is lay back and let the money roll in so I can take early retirement. Well, at my age, early retirement is no longer a priori possible but at least I hope to get enough money to buy the people who helped me prepare the book a coffee. So everyone, please rush out and buy a copy. Come on, at least one of you.

As a final point, you may wonder why there is a drawing of a boat on the cover of a book about the scientific method. Sorry, to find out you will have to read the book. But I will give you a hint. It is not that I like to go sailing. I get seasick.

[1] I know, I have promised this before, but his time trust me. I am not like Lucy in the Charlie Brown cartoons pulling the football away.

[2] Epicurus, who made the lack of hassle the greatest good, would not have approved.

[3] Reminds me of an old Beach Boys song.

[4] An old overhead projector was found in a closet.

[5] Hey! We got through an entire conversation about formatting and word processing software without mentioning LaTeX despite me having been the TRIUMF LaTeX guru before I went over to the dark side and administration.

### Life Underground: Anything Anyone Would Teach Me

Friday, April 17th, 2015

Going underground most days for work is probably the weirdest-sounding this about this job. At Laboratori Nazionali del Gran Sasso, we use the lab to be underground because of the protection it affords us from cosmic rays, weather, and other disruptions, and with it we get a shorthand description of all the weirdness of lab life. It’s all just “underground.”

The last kilometer of road before reaching the above-ground labs of LNGS

Some labs for low background physics are in mines, like SURF where fellow Quantum Diariest Sally Shaw works. One of the great things about LNGS is that we’re located off a highway tunnel, so it’s relatively easy to reach the lab: we just drive in. There’s a regular shuttle schedule every day, even weekends. When there are snowstorms that close parts of the highway, the shuttle still goes, it just takes a longer route all the way to the next easy exit. The ride is a particularly good time to start drafting blog posts. On days when the shuttle schedule is inconvenient or our work is unpredictable, we can drive individual cars, provided they’ve passed emissions standards.

The guards underground keep a running list of all the people underground at any time, just like in a mine. So, each time I enter or leave, I give my name to the guards. This leads to some fun interactions where Italian speakers try to pronounce names from all over. I didn’t think too much of it before I got here, but in retrospect I had expected that any name of European etymology would be easy, and others somewhat more difficult. In fact, the difficult names are those that don’t end in vowels: “GladStone” become “Glad-eh-Stone-eh”. But longer vowel-filled names are fine, and easy to pronounce, even though they’re sometimes just waved off as “the long one” with a gesture.

There’s constantly water dripping in the tunnel. Every experiment has to be housed in something waterproof, and gutters line all the hallways, usually with algae growing in them. The walls are coated with waterproofing, more to keep any potential chemical spill from us from getting into the local groundwater than to keep the water off our experiments. When we walk from the tunnel entrance to the experimental halls, the cue for me to don a hardhat is the first drip on my head from the ceiling. Somehow, it’s always right next to the shuttle stop, no matter where the shuttle parks.

And, because this is Italy, the side room for emergencies has a bathroom and a coffee machine. There’s probably emergency air tanks too, but the important thing is the coffee machine, to stave off epic caffeine withdrawal headaches. And of course, “coffee” means “espresso” unless otherwise stated– but that’s another whole post right there.

When I meet people in the neighboring villages, at the gym or buying groceries or whatever, they always ask what an “American girl” is doing so far away from the cities, and “lavoro a Laboratorio Gran Sasso” is immediately understood. The lab is even the economic engine that’s kept the nearest village alive: it has restaurants, hotels, and rental apartments all catering to people from the lab (and the local ski lift), but no grocery stores, ATMs, gyms, or post offices that would make life more convenient for long-term residents.

Every once in a while, when someone mentions going underground, I can’t help thinking back to the song “Underground” from the movie Labyrinth that I saw too many times growing up. Labyrinth and The Princess Bride were the “Frozen” of my childhood (despite not passing the Bechtel test).

Just like Sarah, my adventures underground are alternately shocking and exactly what I expected from the stories, and filled with logic puzzles and funny characters. Even my first night here, when I was delirious with jetlag, I saw a black cat scamper across a deserted medieval street, and heard the clock tower strike 13 times. And just like Wesley, “it was a fine time for me, I was learning to fence, to fight–anything anyone would teach me–” (except that in my case it’s more soldering, cryogenics plumbing, and ping-pong, and less fighting). The day hasn’t arrived where the Dread Pirate Roberts calls me to his office and gives me a professorship.

And now the shuttle has arrived back to the office, so we’re done. Ciao, a dopo.

(ps the clock striking 13 times was because it has separate tones for the hour and the 15-minute chunks. The 13 was really 11+2 for 11:30.)

### Building a Neutrino Detector

Thursday, April 16th, 2015

Ever wanted to see all the steps necessary for building a neutrino detector? Well now you can, check out this awesome video of constructing the near detector for the Double Chooz reactor neutrino experiment in France.

This is the second of two identical detectors near the Chooz nuclear power station in northern France. The experiment, along with competing experiments, already showed that the neutrino mixing angle, Theta_13, was non-zero. A second detector measuring the same flux of neutrinos from the two reactor cores will drastically reduce the final measurement uncertainty.

### Mapping the cosmos: Dark Energy Survey creates detailed guide to spotting dark matter

Monday, April 13th, 2015

This Fermilab press release came out on April 13, 2015.

This is the first Dark Energy Survey map to trace the detailed distribution of dark matter across a large area of sky. The color scale represents projected mass density: red and yellow represent regions with more dense matter. The dark matter maps reflect the current picture of mass distribution in the universe where large filaments of matter align with galaxies and clusters of galaxies. Clusters of galaxies are represented by gray dots on the map – bigger dots represent larger clusters. This map covers three percent of the area of sky that DES will eventually document over its five-year mission. Image: Dark Energy Survey

Scientists on the Dark Energy Survey have released the first in a series of dark matter maps of the cosmos. These maps, created with one of the world’s most powerful digital cameras, are the largest contiguous maps created at this level of detail and will improve our understanding of dark matter’s role in the formation of galaxies. Analysis of the clumpiness of the dark matter in the maps will also allow scientists to probe the nature of the mysterious dark energy, believed to be causing the expansion of the universe to speed up.

The new maps were released today at the April meeting of the American Physical Society in Baltimore, Maryland. They were created using data captured by the Dark Energy Camera, a 570-megapixel imaging device that is the primary instrument for the Dark Energy Survey (DES).

Dark matter, the mysterious substance that makes up roughly a quarter of the universe, is invisible to even the most sensitive astronomical instruments because it does not emit or block light. But its effects can be seen by studying a phenomenon called gravitational lensing – the distortion that occurs when the gravitational pull of dark matter bends light around distant galaxies. Understanding the role of dark matter is part of the research program to quantify the role of dark energy, which is the ultimate goal of the survey.

This analysis was led by Vinu Vikram of Argonne National Laboratory (then at the University of Pennsylvania) and Chihway Chang of ETH Zurich. Vikram, Chang and their collaborators at Penn, ETH Zurich, the University of Portsmouth, the University of Manchester and other DES institutions worked for more than a year to carefully validate the lensing maps.

“We measured the barely perceptible distortions in the shapes of about 2 million galaxies to construct these new maps,” Vikram said. “They are a testament not only to the sensitivity of the Dark Energy Camera, but also to the rigorous work by our lensing team to understand its sensitivity so well that we can get exacting results from it.”

The camera was constructed and tested at the U.S. Department of Energy’s Fermi National Accelerator Laboratory and is now mounted on the 4-meter Victor M. Blanco telescope at the National Optical Astronomy Observatory’s Cerro Tololo Inter-American Observatory in Chile. The data were processed at the National Center for Supercomputing Applications at the University of Illinois in Urbana-Champaign.

The dark matter map released today makes use of early DES observations and covers only about three percent of the area of sky DES will document over its five-year mission. The survey has just completed its second year. As scientists expand their search, they will be able to better test current cosmological theories by comparing the amounts of dark and visible matter.

Those theories suggest that, since there is much more dark matter in the universe than visible matter, galaxies will form where large concentrations of dark matter (and hence stronger gravity) are present. So far, the DES analysis backs this up: The maps show large filaments of matter along which visible galaxies and galaxy clusters lie and cosmic voids where very few galaxies reside. Follow-up studies of some of the enormous filaments and voids, and the enormous volume of data, collected throughout the survey will reveal more about this interplay of mass and light.

“Our analysis so far is in line with what the current picture of the universe predicts,” Chang said. “Zooming into the maps, we have measured how dark matter envelops galaxies of different types and how together they evolve over cosmic time. We are eager to use the new data coming in to make much stricter tests of theoretical models.”

The Dark Energy Survey is a collaboration of more than 300 scientists from 25 institutions in six countries. Its primary instrument, the Dark Energy Camera, is mounted on the 4-meter Blanco telescope at the National Optical Astronomy Observatory’s Cerro Tololo Inter-American Observatory in Chile, and its data is processed at the National Center for Supercomputing Applications at the University of Illinois at Urbana-Champaign.

Funding for the DES Projects has been provided by the U.S. Department of Energy Office of Science, the U.S. National Science Foundation, the Ministry of Science and Education of Spain, the Science and Technology Facilities Council of the United Kingdom, the Higher Education Funding Council for England, ETH Zurich for Switzerland, the National Center for Supercomputing Applications at the University of Illinois at Urbana-Champaign, the Kavli Institute of Cosmological Physics at the University of Chicago, Financiadora de Estudos e Projetos, Fundação Carlos Chagas Filho de Amparo à Pesquisa do Estado do Rio de Janeiro, Conselho Nacional de Desenvolvimento Científico e Tecnológico and the Ministério da Ciência e Tecnologia, the Deutsche Forschungsgemeinschaft and the collaborating institutions in the Dark Energy Survey. The DES participants from Spanish institutions are partially supported by MINECO under grants AYA2012-39559, ESP2013-48274, FPA2013-47986 and Centro de Excelencia Severo Ochoa SEV-2012-0234, some of which include ERDF funds from the European Union.

Fermilab is America’s premier national laboratory for particle physics and accelerator research. A U.S. Department of Energy Office of Science laboratory, Fermilab is located near Chicago, Illinois, and operated under contract by the Fermi Research Alliance, LLC. Visit Fermilab’s website at www.fnal.gov and follow us on Twitter at @Fermilab.

The DOE Office of Science is the single largest supporter of basic research in the physical sciences in the United States and is working to address some of the most pressing challenges of our time. For more information, please visit science.energy.gov.

### CUORE-0 Results Tour Kicks Off

Thursday, April 9th, 2015

The CUORE-0 collaboration just announced a result: a new limit of 2.7 x1024 years (90%C.L.) on the halflife of neutrinoless double beta decay in 130Te. Or, if you combine it with the data from Cuorecino, 4.0×1024 years. A paper has been posted to the arXiv preprint server and submitted to the journal Physical Review Letters.

Bottom: Energy spectrum of 0νββ decay candidates in CUORE-0 (data points) and the best-fit model from the UEML analysis (solid blue line). The peak at ∼2507 keV is attributed to 60Co; the dotted black line shows the continuum background component of the best-fit model. Top: The nor-369 malized residuals of the best-fit model and the binned data.370 The vertical dot-dashed black line indicates the position of371 Qββ. From arXiv.

CUORE-0 is an intermediate step between the upcoming full CUORE detector and its prototype, Cuoricino. The limit from Cuoricino was 2.8×1024 years**, but this was limited by background contamination in the detector, and it took a long time to get to that result. For CUORE, the collaboration developed new and better methods (which are described in detail in an upcoming detector paper) for keeping everything clean and uniform, plus increased the amount of tellurium by a factor of 19. The results coming out now test and verify all of that except the increased mass: CUORE-0 uses all the same cleaning and assembly procedures as CUORE, but with only the first of 19 towers of crystals. It took data while the rest of the towers were being built. We stopped taking CUORE-0 data when the sensitivity was slightly better than Cuoricino, which only took half the exposure time of the Cuoricino run. The resulting background was 6 times lower in the continuum parts of the spectrum, and all the energy resolutions (which were calibrated individually for each crystal each month) were more uniform. So this is a result to be proud of: even before the CUORE detector starts taking data, we have this result to herald its success.

The energy spectra measured in both Cuoricino and CUORE-0, displaying the factor of 6 improvement in the background rates. From the seminar slides of L. Canonica.

The result was announced in the first seminar in a grand tour of talks about the new result. I got to see the announcement at Gran Sasso today–perhaps you, dear reader, can see one of the talks too! (and if not, there’s video available from the seminar today) Statistically speaking, out of these presentations you’re probably closest to the April APS meeting if you’re reading this, but any of them would be worth the effort to see. There was also a press release today and coverage in the Yale News and Berkley Labs news, because of which I’m making this post pretty short.

The Upcoming Talks:

There are also two more papers in preparation, which I’ll post about when they’re submitted. One describes the background model, and the other describes the technical details of the detector. The most comprehensive coverage of this result will be in a handful of PhD theses that are currently being written.

(post has been revised to include links with the arXiv post number: 1504.02454)

**Comparing the two limits to each other is not as straightforward as one might hope, because there were different statistical methods used to obtain them, which will be covered in detail in the papers. The two limits are roughly similar no matter how you look, and still the new result has better (=lower) backgrounds and took less time to achieve. A rigorous, apples-to-apples comparison of the two datasets would require me to quote internal collaboration numbers.

### Absence of gravitational-wave signal extends limit on knowable universe

Thursday, April 9th, 2015

The Holometer is sensitive to high-frequency gravitational waves, allowing it to look for events such as cosmic strings. Photo: Reidar Hahn

Imagine an instrument that can measure motions a billion times smaller than an atom that last a millionth of a second. Fermilab’s Holometer is currently the only machine with the ability to take these very precise measurements of space and time, and recently collected data has improved the limits on theories about exotic objects from the early universe.

Our universe is as mysterious as it is vast. According to Albert Einstein’s theory of general relativity, anything that accelerates creates gravitational waves, which are disturbances in the fabric of space and time that travel at the speed of light and continue infinitely into space. Scientists are trying to measure these possible sources all the way to the beginning of the universe.

The Holometer experiment, based at the Department of Energy’s Fermilab, is sensitive to gravitational waves at frequencies in the range of a million cycles per second. Thus it addresses a spectrum not covered by experiments such as the Laser Interferometer Gravitational-Wave Observatory, which searches for lower-frequency waves to detect massive cosmic events such as colliding black holes and merging neutron stars.

“It’s a huge advance in sensitivity compared to what anyone had done before,” said Craig Hogan, director of the Center for Particle Astrophysics at Fermilab.

This unique sensitivity allows the Holometer to look for exotic sources that could not otherwise be found. These include tiny black holes and cosmic strings, both possible phenomena from the early universe that scientists expect to produce high-frequency gravitational waves. Tiny black holes could be less than a meter across and orbit each other a million times per second; cosmic strings are loops in space-time that vibrate at the speed of light.

The Holometer is composed of two Michelson interferometers that each split a laser beam down two 40-meter arms. The beams reflect off the mirrors at the ends of the arms and travel back to reunite. Passing gravitational waves alter the lengths of the beams’ paths, causing fluctuations in the laser light’s brightness, which physicists can detect.

The Holometer team spent five years building the apparatus and minimizing noise sources to prepare for experimentation. Now the Holometer is taking data continuously, and with an hour’s worth of data, physicists were able to confirm that there are no high-frequency gravitational waves at the magnitude where they were searching.

The absence of a signal provides valuable information about our universe. Although this result does not prove whether the exotic objects exist, it has eliminated the region of the universe where they could be present.

“It means that if there are primordial cosmic string loops or tiny black hole binaries, they have to be far away,” Hogan said. “It puts a limit on how much of that stuff can be out there.”

Detecting these high-frequency gravitational waves is a secondary goal of the Holometer. Its main purpose is to determine whether our universe acts like a 2-D hologram, where information is coded into two-dimensional bits at the Planck scale, a length around ten trillion trillion times smaller than an atom. That investigation is still in progress.

“For me, it’s gratifying to be able to contribute something new to science,” said researcher Bobby Lanza, who recently earned his Ph.D. conducting research on the Holometer. He is the lead author on an upcoming paper about the result. “It’s part of chipping away at the whole picture of the universe.”

Diana Kwon

### [email protected]: Going beyond the collider

Tuesday, April 7th, 2015

While everybody is excited by the coming “phase 2” of the LHC, someone else is already looking beyond it, thinking: “what are the possible future scenarios for our beloved Large Hadron Collider?”

The community of “phenomenologists”, theoreticians who like to play with data, closely collaborate with experimentalists to plan new experiments. We are hoping to get the most out of a set-up and think about future stages and improvements.

In the last months there has been a lot of interest around a proposal for a new experiment at the LHC: “[email protected]”, namely A Fixed Target ExpeRiment at the LHC. This means that we do not have particles running in opposite directions within two rings (the collider setting), crashing head-on; rather, there is just one ring where particles run coherently and are then extracted by means of a crystal and smashed against a fixed target, like hitting a wall.

You may actually wonder: “Why should I prefer this instead of the super nice and Nobel-prize-generator collider?”

In the LHC protons are accelerated at approximately the speed of light and collide along the ring. The protons are made out of quarks and gluons, so each proton-proton collision can be interpreted as a smashing among their elementary constituents. In particular, since gluons are the most relevant elementary constituents at the LHC energy, the latter can be thought as a collider of gluons.

As I partly discussed in a previous post, we can study the structure of the proton with 3D probability distributions (transverse-momentum-dependent distributions, TMDs) which allow you to access all the possible spin and momentum configurations of the constituents. For example, quark and gluons can be investigated with and without their spin state, and the proton where they live in can be polarized or not. There are several of these combinations and each one represents a fundamental piece in the puzzle of the proton structure.

The LHC is currently running with beam of unpolarized protons only. Meaning we do not consider their spin in analyses. For those who want to investigate the puzzle of a proton’s structure, this is a limitation. We are able to access only two out of the eight (under certain assumptions) configurations of polarizations, namely the unpolarized and the linearly polarized gluons. So there are six options we don’t get to study!

In this table the eight available TMD distributions shaping the physics of (un)polarized gluons inside (un)polarized protons are listed. At the LHC we can access the first row only, at AFTER more combinations will be investigated.

And here is the answer to our question. The fixed target at AFTER could be easily polarized, allowing us to study the physics of gluons inside polarized protons, which would be impossible at the present collider! There is only another machine in the world where hadrons can be polarized: the Relativistic Heavy Ion Collider – RHIC at Brookhaven National Lab.

For this reason, AFTER could access novel phenomena intrinsically related to the polarization of hadrons and, at the same time, allow us to study processes already available at the LHC but in different physical regions. For example, there is the possibility of accessing the simple 1D probability distributions in a region where they are still poorly known.

A particularly interesting observable which AFTER could look at is the so-called “Sivers” distribution for gluons, namely the probability of extracting unpolarized gluons from a proton whose spin is transverse with respect to the direction of the beam. Part of its core features cannot be calculated from first principles in the theory, so a good way to explore it would be extraction from experimental data. In the past years physicists got indications that the Sivers effect for gluons could be small, but an experimental insight at AFTER would be really important.

As you can see, there could be a lot of cool physics going on. We are in the early stages, where all the possible (including economic) constraints need to be taken into account and where a good scientific motivational plan is fundamental.

When you try to give birth to an experiment you face a lot of problems, like “What’s a realistic estimate of its scientific impact? Do we really need a new machine or not?” Some of these questions have already been addressed and the answers are collected in scientific publications, which you can partly find here.

If everything goes according to plan and desires, [email protected] will bring very good insight and contributions to the study of the proton structure: stay tuned for updates!

### Choose as Many as You Like

Tuesday, April 7th, 2015

I want to understand the universe.

I want to understand how the universe works.

I want to build models of how the universe works that predict the results of experiments.

I want to build models of how the universe works that predict the results of experiments, because I believe those models get closer and closer to the truth.

I want to build models of how the universe works that predict the results of experiments, because I believe those models get closer and closer to the true rules of reality.

I want to build models of how the universe works that predict the results of experiments, because I believe that although it’s unknowable whether reality has “true” rules, building better and better models is the closest we can get.

I want to build models of how the universe works that predict the results of experiments, because I believe that understanding the true rules of reality will help us understand why the universe exists.

I want to build models of how the universe works that predict the results of experiments, because I believe that understanding the true rules of reality will shed light on the Creator of the universe.

I want to build models of how the universe works that predict the results of experiments, because I believe that the more we can explain without religion the less people will rely on it.

I want to build models of how the universe works that are simple and beautiful.

I want to build models of how the universe works that are simple and beautiful, because these models have the best track record of predicting the broadest range of experimental results.

I want to build models of how the universe works that are simple and beautiful, because I believe the true rules of reality are simple and beautiful.

I want to understand enough of how the universe works that I can build machines to improve people’s lives.

I want to understand enough of how the universe works that I can find new ways to save lives and heal the sick and injured.

I want to understand enough of how the universe works that I can help us stop endangering the climate of our planet.

I want to understand how the universe works so that other people can someday find new practical ways to improve and save lives, even if I don’t quite know what they are and probably won’t work on them myself.

I want to build machines for studying how the universe works, because I find working on those machines to be challenging and fulfilling.

I want to write programs for analyzing data from experiments on how the universe works, because I find analyzing data to be challenging and fulfilling.

Choose as many as you like. If other people want to hear about it, tell them – or, if you prefer, don’t. And if you have more you’d like to add, leave them in the comments!