Fermilab – Scienceandsf -A Blog Published by Robert A. Lawler

Drexel Physics Seminar: Doctor Sowjanya Gollapinni on the current state of research on Neutrinos, the Ghost Particle of the Atom.

I took in a physics seminar at my old alma mater Drexel University on November 30th. I like to stop down once in a while to see what’s changed, a lot, as well as see who’s there that I still remember, seems like fewer each time.

The topic of the seminar was certainly one that interested me, Neutrinos; a kind of sub-atomic particle so difficult to detect it has been called a ghost particle. The German physicist Wolfgang Pauli first predicted the existence of neutrinos as a way of making the books balance in the radioactive process called beta (β) decay. Careful studies of the process showed that some energy was missing, and the angular momentum before and after didn’t match. Pauli suggested that if another particle was involved, one without electric charge and little or no rest mass, it could account for the differences while being very difficult to detect. The images below show the Nobel Laureate along with a diagram of the β decay process.

Beta Decay Process (Credit: Public Domain)

So difficult were neutrinos to detect that it took more than twenty years to prove that they existed. In fact neutrinos react with normal matter so rarely that while about ten billion (billion with a b) neutrinos are flying through your body every second only two or three will interact with a particle inside you during your entire life. Even today the way we study neutrinos is to arrange for zillions to fly through a detector so sensitive it can measure the properties of the one or two that interact.

Arranging that intense beam of neutrinos, and building that detector is the job of Doctor Sowjanya Gollapinni of the University of Tennessee at Knoxville. Dr. Gollapinni is one of the chief researchers of the MicroBooNE experiment currently running at Fermilab outside of Chicago along with being one of the chief designers of the future Deep Underground Neutrino Experiment (DUNE).

The MicroBooNE, BooNE stands for Boosting Neutrino Experiment by the way, is a new type of detector using a design known as a Liquid Argon Time Projection Chamber (LArTPC). In the detector scattering events (really just two particles bouncing off of each other) between neutrinos and Argon atoms occur inside a very large, uniform electric field. The electric field pulls the ionized atoms generated by the collision toward an incredibly fine mesh of detecting wires. The resulting data plots are then interpreted to determine the kind of neutrino (see below) as well as its energy. The images below show the first high-energy neutrino collision captured by MicroBooNE along with the first recorded cosmic neutrino event.

First Recorded Neutrino Event at MicroBooNE (Credit: MicroBooNE, Fermilab)

First Cosmic Neutrino event at MicroBooNE (Credit: MicroBooNE, Fermilab)

One of the reasons I like MicroBooNE so much is that it uses the Fermilab Tevatron as its source of high-energy neutrinos. The Tevatron was the world’s most powerful ‘atom smasher’ until the Large Hadron Collider (LHC) at CERN took the top spot in 2008. In the world of particle physics however being number two gets you nothing so the physicists at Fermilab have been working hard to reconfigure their equipment in order to continue to study new physics and MicroBooNE is a big part of that effort.

After talking about some of the results from MicroBooNE Dr. Gollapinni spent a little time talking about the next generation neutrino detector known as the Deep Underground Neutrino Experiment or DUNE. As shown in the figure below, DUNE will have two detectors, one just a short distance from the neutrino source at Fermilab while the second will be buried deep inside the Homestake Mine in South Dakota, a distance of 1300 kilometers away. When completed the DUNE detectors will be 400 times larger then MicroBooNE providing 400 time the data.

DUNE experimental setup (Credit: DUNE, Fermilab)

Now the reason for having a second detector a long distance away is to give the neutrinos produced at Fermilab time in order to change from one type or flavour of neutrino to another. You see one of the things we do know about neutrinos is that there are three flavours. One flavour is associated with the familiar electron, a second is associated with a particle called the muon who is like a heavy cousin of the electron while the third is associated with an even fatter cousin called the Tau particle. Even stranger is the fact that the three flavours will oscillate from one kind to another. Learning more about this oscillation process is one of the major goals of DUNE.

At the end of her discussion Dr. Gollapinni mentioned some preliminary but very exciting news. The results so far from MicroBooNE and several other neutrino experiments indicates, just indicates right now, the possible existence of a fourth flavour of neutrino, which would be a stunning result if proving to be true. Right now it’s just an indication, hopefully the DUNE experiment, scheduled to start collecting data in 2024, will give us the answer.

During the question period one of the students who were attending asked Dr. Gollapinni how many flavours of neutrino she thought there were and she answered ‘Well if we find a fourth it’ll be a Nobel Prize and that’s enough for me’.

I certainly wish her luck.

The Deep Underground Neutrino Experiment (DUNE) and why are Neutrinos so Important Anyway.

Over the past month or so I’ve published a series of posts describing what I see as the decline of science in America (28June to 12July2017). Well today I have some good news. Just this week construction has begun on the Deep Underground Neutrino Experiment or DUNE.

DUNE is a collaboration between two already existing physics labouratories. The Sanford Underground Research Facility (SURF) which is buried 2km deep in the old Homestake gold mine outside of Lead, South Dakota along with Fermilab outside of Chicago, the site of America’s most powerful particle accelerator, second only to the LHC at CERN in Europe.

By the way the Homestake mine first became a physics labouratory back in the 1960s when the first Neutrino telescope was build there to measure the flux of neutrinos coming from the Sun. An experiment that provided the first direct evidence that the Sun gets its energy from hydrogen fusion reactions.

The idea behind the DUNE experiment is that Fermilab will use its accelerator to generate an intense beam of the sub-atomic particles called Neutrinos, a particle that has been called the ghost particle because they only interact very rarely with other particles. To give you an idea of how rarely an interaction occurs, every second thousands of Neutrinos are going right through your body but over your entire lifespan only a handful will interact with a particle inside you.

That beam of Neutrinos from Fermilab will be aimed very precisely at SURF where the world’s largest Neutrino Detectors are now being installed. The 2000km trip underground will mean almost nothing to the Neutrinos; a few may be absorbed but only very a few. There will also be an identical detector array right at the output of Fermilab’s accelerator so that scientists can study what happens to the Neutrinos during their 2000km journey. The picture below shows a diagram of the planned setup of the DUNE experiment.

DUNE Experimental Layout (Credit: Fermilab)

You may ask, if only a few Neutrinos are absorbed in 2000km of rock won’t even fewer be captured by the detectors in South Dakota. Yes, absolutely, but the scientists will be able to measure precisely every characteristic of every single Neutrino that is detected.

So, what do the scientists hope to learn from DUNE, why are Neutrinos so important anyway? Well, first of all there is increasingly strong evidence that Neutrinos are actually far more numerous than the electrons and quarks that make up what we think of as matter. In a sense scientists simply don’t enjoy knowing so little about such an important particle.

There are some more well defined problems that we hope DUNE can help to solve. For one, the there’s the question why the Universe, or at least our part of it, is so dominated by matter with so little anti-matter. From all of our experiments at places like Fermilab the Universe should be composed of equal parts matter and anti-matter and Neutrinos may hold the key to understanding the imbalance.

Physicists also hope that a greater understanding of Neutrinos will give us greater insight into fundamental forces, gravity, electromagnetism and the nuclear forces. Understanding Neutrinos is also important because they play a large role in some of the most energetic events in the Universe, everything from supernova to black hole formation.

Despite the recent lack of support for science from our government it’s still true that America’s scientists are second to none and the DUNE project demonstrates how they will always find a way to do new and important work. If you’d like to read more about the DUNE experiment the links below will take you to the SURF and Fermilab WebPages for DUNE.

http://www.dunescience.org/

http://lbnf.fnal.gov/

America’s Science Decline: Part 3, Our Forgotten Atom Smashers

This is the third in a series of posts discussing what I see as the decline of Science in the United States. In part two of this series I talked about how for more than a century the United States built ever larger and larger telescopes, the largest in the World. I spoke of how those instruments made some of the most important discoveries in the history of science. I ended that post by pointing out that America no longer possessed the World’s largest telescope. I described how our largest scopes now had been built back in the 1990s and that while Europe and the rest of the World were planning to build the next generation of telescopes the United States was not.

This week I’m going to tell a very similar story about the scientific instruments that allow scientists to see the smallest objects in the Universe. I’m talking about the particle accelerators, the Atom Smashers with which we study the fundamental building blocks of creation.

The first scientist to smash one kind of particle into another was the Englishman Ernest Rutherford, who aimed the alpha particles from radioactive Uranium at a thin film of gold atoms. The scattering pattern from those alpha particles revealed the basic structure of the atom as a dense nucleus surrounded by a cloud of electrons.

Now Rutherford only aimed his alpha particles, collimated is the technical term. He couldn’t increase their energy in any way but other scientists soon began looking for techniques to do just that. Attempting to build instruments that would accelerate sub-atomic particles and use those particles to probe deeper and deeper into the atom.

The first really practical such atom smasher was the cyclotron, developed by Ernest Lawrence at the University of California at Berkeley in 1932. To understand the operation of the cyclotron, and particle accelerators in general, refer to the picture below.

In a cyclotron charged particles, usually protons, are confined to move in circular orbits by a large external magnetic field. The size of the orbit is determined by the velocity / energy of the charged particle. The particles orbit inside two metal “D”s that are connected to a high voltage oscillator that gives one of the “D”s a positive voltage and the other a negative voltage with the voltages flipping back and forth at very high frequency.

The positively charged protons are repelled by the positive “D” and attracted to the negative “D”, but by the time they get to the correct side the voltage has flipped causing the protons to fly back and forth, gaining energy with each orbit. The increasing energy increases the size of the orbit until the protons reach the outer edge of the “D”s where they are extracted and fired at a target being studied.

The “D”s in Lawrence’s first instrument measured only 11 inches (28cm) across and could only accelerate the protons to an energy of 1.2MeV. (An eV is an electron volt, it stands for the amount of energy that an electron will gain as it crosses a potential of 1 Volt. an MeV is a million eVs, GeV is a billion eVs and TeV is a trillion)

In the years that followed Lawrence built progressively more powerful instruments including a 184 inch (467cm) device that was used during the development of the atomic bomb to study the separation of uranium isotopes.

In the 1950s a new design of accelerators was developed where the strength of the confining magnetic field was synchronized to the energy of the accelerated particles. These accelerators were christened synchrotrons and they continued to grow in size and energy. The Bevatron, still at UC Berkeley succeeded in producing the first the anti-protons and anti-neutrons while the Cosmotron at Brookhaven National Laboratory on Long Island discovered the Delta particle and produced the first artificial mesons. The picture below shows the Bevatron at Berkeley.

The Bevatron Particle Accelerator (Public Domain)

The rest of the world just couldn’t keep up. The US just kept building the most powerful instruments and making all the discoveries. In 1960 Brookhaven got a new 33Gev machine called the Alternating Gradient Synchrotron, which is still making important discoveries today. In 1983 a brand new facility was opened outside Chicago called Fermilab with an accelerator ring over one and a third mile (2.2km) in diameter. The instrument named the Tevatron because it not only accelerated protons to over a TeV but it also accelerated anti-protons in the opposite direction and studied the collisions between them. The discoveries made by American Atom Smashers formed the basis of what physicists call ‘The Standard Model’. In 1995 the Top quark was discovered at Fermilab, the last elementary particle to be discovered at an American facility.

At almost the same time the US congress cancelled the next great American accelerator, the Superconducting Super Collider or SSC, whose ring would have been over 17 miles (27.7km) in diameter and whose total energy would have reached 40TeV.

Instead the Europeans have taken the lead with their Large Hadron Collider (LHC) at CERN. This is the instrument that finally discovered the Higgs boson in 2013 with its 8.6 km ring (5.4miles) and energy of 13Tev. It is worth keeping in mind that America’s SSC would have been completed earlier than Europe’s LHC and still been more powerful if the politicians had not fought over a deficit that they’ve pretty much ignored since then anyway. And now even the Tevatron at Fermilab has been shut down over budget concerns.

Europe meanwhile is pressing on. There are plans under development at CERN for an even bigger, more powerful machine. Called the Future Circular Collider it will have a ring 32 km (20 miles) in diameter and a top energy of 100Tev. So therefore it will be Europe that in the next decades will lead the search for physics beyond the standard model.

In my next post I’ll conclude my discussion of how the United States is losing its once predominant position in Science.

Post Script: Even as I was writing this post the Physicists at CERN have announced the discovery of a new particle! Now this is not a new fundamental particle but rather the first composite particle with two heavy quarks. Worse yet, Fermilab had published data over ten years ago indicating the possible existence of this particle but the Tevatron was not quite powerful enough to meet the tight requirements needed to officially announce a discovery.