## Archive for July, 2015

### Antineutrinos and pentaquarks: it’s hot in Vienna!

Thursday, July 23rd, 2015

« We have been asking questions since 1365 ». This inspiring statement marking the 650th anniversary of the University of Vienna, is hanging near the imposing entrance of its main building. This sign welcomed this morning the 730 physicists who came to Vienna to participate to the main particle physics conference this year organised by the European Physics Society. For a week, the participants will have to choose among hundreds of presentations where the current status of knowledge in particle physics will be presented along with the newest avenues. and we are already making history: Vienna recorded yesterday its highest ever temperature with 39 ˚C.

And this first day brought recent and exciting results. As announced last week, the LHCb collaboration at CERN has discovered the first pentaquarks, composite objects made of five quarks. Quarks are some of the building blocks of matter. Physicists have observed for decades dozens of different particles made of two or three quarks. For example, many particles are made of a pairs of quark and antiquark, while others, like protons and neutrons, contain three quarks. However, in recent years, a few experimental groups also reported the discovery of tetraquarks, objects composed of four quarks. Finally, last week, thanks to the huge dataset made available by the Large Hadron Collider, scientists from the LHCb experiment achieved what many other groups had tried to do for decades without success, and proudly announced the discovery of pentaquarks. Such composite objects were expected but never observed before. It goes to show how much we still have to discover and understand.

Another nice piece of news: the T2K neutrino experiment, which takes place in Japan, may have detected the first signs for oscillations of antineutrinos. To this day, there are three known types of neutrinos, each one accompanying its own particle, namely the electron, the muon and the tau. The oscillation process describes how one type of neutrinos can change into another type. This phenomenon has already been observed for neutrinos, but it would be the first observation for antineutrinos. However, all is far from being set in concrete yet, quite the contrary. With only three events at hands, the T2K team still needs to verify if these events really involve antineutrinos and not just neutrinos. They therefore need to collect more date for another year or two before this issue can be settled. If it turns out to be indeed antineutrinos, we would learn more on the similarities or the differences between matter and antimatter.

Several experiments are also trying to establish if there could also be another type of neutrinos, called sterile neutrinos. Their spin would be the opposite of known neutrinos, meaning they would be spinning on themselves in the opposite direction. Clearly, any new type of particle is something worth watching for. The confirmation of the existence of sterile neutrinos would send a shock wave in particle physics since it would constitute an indisputable proof for the existence of a theory more encompassing than the current theoretical model, called the Standard Model. Everything would then have to be rethought. And who knows? Physicists could very well have enough to keep asking difficult questions for another 650 years…

Pauline Gagnon

### Nerds and Names

Thursday, July 16th, 2015

If there’s one thing that makes me jealous about planetary scientists, it’s how many things they get to name. They also seem to have an awful lot of fun with it. Consider these typical naming processes:

• Experimental particle physicists: “Jeff Weiss did an ‘availability search” of the Greek alphabet and found that the Greek letter Upsilon was not yet used”. [1]
• Planetary scientists: “Woooooooo, another mountain range! Let me get my copy of the Silmarillion!” [2]

They also seem to have snuck in a Marvel Cinematic Universe tie-in while naming one of Pluto’s newer moons.

But wait, you may ask, doesn’t particle physics have whimsical names?  A few, sure. But it was the theoretical physicists who named things like “quarks”; by the time we discover them, we already know what they’re supposed to be and don’t get to make up new names.  New particles with 5 quarks?  We’ll just be literal and call them “pentaquarks”; the specific states can be Pc(4450)+ and Pc(4380)+[3], names which give useful information about charge and mass but aren’t really any fun.  Really, the most fun we ever get to have is with tortured acronyms [4].  It’s just not fair at all.

But seriously, congratulations to everyone working on New Horizons.  Enjoy your fun — you’ve earned it. And maybe the next particle we discover, we’ll take a page from your playbook.

[1] J. Yoh (1998). “The Discovery of the b Quark at Fermilab in 1977: The Experiment Coordinator’s Story“. AIP Conference Proceedings 424: 29–42.

[2] Not an actual quote (as far as I know). But since yesterday, Pluto has a “Cthulhu” and a “Balrog” and Charon has a “Mordor”.

[4] ATLAS Collaboration (2008). “The ATLAS experiment at the Large Hadron Collider.” JINST 3 S08003. See the acronym list appendix.

### Finding a five-leafed clover

Wednesday, July 15th, 2015

Photo Credit: Cathy Händel, Published on http://www.suttonelms.org.uk/olla12.html

Sometimes when you’re looking for something else, you happen across an even more exciting result. That’s what’s happened at LHCb, illustrated in the paper “Observation of $$J/\psi p$$ resonances consistent with pentaquark states in $$\Lambda_b^0\to J/\psi K^-p$$ decays”, released on the arXiv on the 14th of July.

I say this is lucky because the analysts found these states while they were busy looking at another channel; they were measuring the branching fraction of $$B^0\to J/\psi K^+ K^-$$. As one of the analysts, Sheldon Stone, recalled to me, during the review of the $$B^0$$ analysis, one reviewer asked if there could be a background from the decay $$\Lambda_b^0\to J/\psi K^- p$$, where the proton was misidentified as a kaon. As this was a viable option, they looked at the PDG to see if the mode had been measured, and found that it had not. Without a certain knowledge of how large this contribution would be, the analysts looked. To their surprise, they found a rather large rate of the decay, allowing for a measurement of the lifetime of the $$\Lambda_b^0$$. At the same time, they noticed a peak in the $$J/\psi p$$ spectrum. After completing the above mentioned analysis of the $$B^0$$, they returned to the channel.

It’s nice to put yourself in the analysts shoes and see the result for yourself. Let’s start by looking at the decay $$\Lambda_b^0\to J/\psi p K^-$$. As this is a three body decay, we can look at the Dalitz Plots.

Dalitz plots from the decay $$\Lambda_b^0\to J/\psi K^- p$$. Compiled from http://arxiv.org/abs/1507.03414

The above Dalitz plots show all combinations of possible axes to test. In the one on the left, around $$m^2=2.3$$ GeV$$^2$$, running vertically, we see the $$\Lambda(1520)$$ resonance, which decays into a proton and a kaon. Running horizontally is a band which does not seem to correspond to a known resonance, but which would decay into a $$J/\psi$$ and a proton. If this is a strong decay, then the only option is to have a hadron whose minimum quark content is $$uud\bar{c}c$$. The same band is seen on the middle plot as a vertical band, and on the far right as the sloping diagonal band. To know for sure, one must perform a complete amplitude analysis of the system.

You might be saying to yourself “Who ordered that?” and think that something with five quarks hadn’t been postulated. This is not the case. Hadrons with quark content beyond the minimum were already thought about by Gell-Mann and Zweig in 1964 and quantitatively modeled by Jaffe in 1977  to 4 quarks and 5 quarks by Strottman in 1979. I urge you to go look at the articles if you haven’t before.

It appears as though a resonance has been found, and in order to be sure, a full amplitude analysis of the decay was performed. The distribution is first modeled without any such state, shown in the figures below.

Projections of the fits of the$$\Lambda_b^0\to J/\psi K^- p$$ spectrum without any additional components. Black is the data, and red is the fit. From http://arxiv.org/abs/1507.03414

Try as you might, the models are unable to explain the invariant mass distribution of the $$J/\psi p$$. Without going into too much jargon, they wrote down from a theoretical standpoint what type of effect a five quark particle would have on the Dalitz plot, then put this into their model. As it turns out, they were unable to successfully model the distribution without the addition of two such pentaquark states. By adding these states, the fits look much better, as shown below.

Mass projection onto the $$J/\psi p$$ axis of the total fit to the Dalitz plot. Again, Black is data, red is the fit. The inset image is for the kinematic range $$m(K p)>2 GeV$$.
From http://arxiv.org/abs/1507.03414

The states  are called the $$P_c$$ states. Now, as this is a full amplitude analysis, the fit also covers all angular information. This allows for determination of the total angular momentum and parity of the states. These are defined by the quantity $$J^P$$, with $$J$$ being the total angular momentum and $$P$$ being the parity. All values for both resonances are tried from 1/2 to 7/2, and the best fit values are found to be with one resonance having $$J=3/2$$ and the other with $$J=5/2$$, with each having the opposite parity as the other. No concrete distinction can be made between which state has which value.

Finally, the significance of the signal is described by under the assumption $$J^P=3/2^-,5/2^+$$ for the lower and higher mass states; the significances are 9 and 12 standard deviations, respectively.

The masses and widths turn out to be

$$m(P_c^+(4380))=4380\pm 8\pm 29 MeV$$

$$m(P_c^+(4450))=4449.8\pm 1.7\pm 2.5 MeV$$

With corresponding widths

Width$$(P_c^+(4380))=205\pm 18\pm 86 MeV$$

Width$$(P_c^+(4450))=39\pm 5\pm 19 MeV$$

Finally, we’ll look at the Argand Diagrams for the two resonances.

Argand diagrams for the two $$P_c$$ states.
From http://arxiv.org/abs/1507.03414

Now you may be saying “hold your horses, that Argand diagram on the right doesn’t look so great”, and you’re right. I’m not going to defend the plot, but only point out that the phase motion is in the correct direction, indicated by the arrows.

As pointed out on the LHCb public page, one of the next steps will be to try to understand whether the states shown are tightly bound 5 quark objects or rather loosely bound meson baryon molecule. Even before that, though, we’ll see if any of the other experiments have something to say about these states.

### Fermilab’s flagship accelerator sets world record

Thursday, July 9th, 2015

This Fermilab press release came out on July 8, 2015.

Fermilab’s Main Injector accelerator, one of the most powerful particle accelerators in the world, has just achieved a world record for high-energy beams for neutrino experiments. Photo: Fermilab

A key element in a particle-accelerator-based neutrino experiment is the power of the beam that gives birth to neutrinos: The more particles you can pack into that beam, the better your chance to see neutrinos interact in your detector. Today scientists announced that Fermilab has set a world record for the most powerful high-energy particle beam for neutrino experiments.

Scientists, engineers and technicians at the U.S. Department of Energy’s Fermi National Accelerator Laboratory have achieved for high-energy neutrino experiments a world record: a sustained 521-kilowatt beam generated by the Main Injector particle accelerator. More than 1,000 physicists from around the world will use this high-intensity beam to more closely study neutrinos and fleeting particles called muons, both fundamental building blocks of our universe.

The record beam power surpasses that of the 400-plus-kilowatt beam sent to neutrino experiments from particle accelerators at CERN.

Setting this world record is an initial step for the Fermilab accelerator complex as it will gradually increase beam power over the coming years. The next goal for the laboratory’s two-mile-around Main Injector accelerator — the final and most powerful in Fermilab’s accelerator chain — is to deliver 700-kilowatt beams to the laboratory’s various experiments. Ultimately, Fermilab plans to make additional upgrades to its accelerator complex over the next decade, achieving beam power in excess of 1,000 kilowatts, also referred to as 1 megawatt.

“We have the world’s highest-power beam for neutrinos, and we’re only going up from here,” said Ioanis Kourbanis, head of the Main Injector Department at Fermilab.

Laboratory-made neutrino experiments start by accelerating a beam of particles, typically protons, and then smashing them into a target to create neutrinos. Scientists then use particle detectors to “catch” as many of those neutrinos as possible and record their interactions. Neutrinos rarely engage with matter: Only one out of every trillion emerging from the proton beam will interact in an experiment’s detector. The more particles in that beam, the more opportunities researchers will have to study these rare interactions.

The amped-up particle beam provided by the Main Injector enriches the lab’s neutrino supply, positioning Fermilab to become the primary laboratory for accelerator-based neutrino research. Neutrinos are also made in stars and in the Earth’s core, and they pass through everything — people and planets alike.

“The idea is that if you build a more intense beam, neutrino scientists from around the world will beat a path to your door,” said Fermilab Deputy Director Joe Lykken. “This is exactly what’s happening.”

Fermilab currently operates four neutrino experiments: MicroBooNE, MINERvA, MINOS+ and the laboratory’s largest-to-date neutrino experiment, NOvA, which sends particles from Fermilab’s suburban Chicago location to a far detector 500 miles away in Ash River, Minnesota. The laboratory is working with scientists from around the world on expanding its short-baseline neutrino program and would also serve as host to the proposed flagship Long-Baseline Neutrino Facility and Deep Underground Neutrino Experiment, or DUNE. Scientists aim to address basic questions about the mass and properties of each kind of neutrino as well as the role neutrinos played in the evolution of the universe.

“Reaching this milestone is a fantastic achievement for Fermilab; beam power is everything in our field,” said DUNE co-spokesperson Mark Thomson of the University of Cambridge. “The ability for Fermilab to deliver, yet again, gives the international neutrino community huge confidence in the future U.S.-hosted neutrino program.”

Fermilab is also preparing to operate two experiments for studying muons, short-lived particles that could reveal secrets about the earliest moments of the universe. The increased beam power will also benefit the Fermilab Test Beam Facility, one of the few facilities in the world that provides muons, pions and other particles that researchers can use to test their particle detectors.

Since 2011, Fermilab has made significant upgrades to its accelerators and reconfigured the complex to provide the best possible particle beams for neutrino and muon experiments. With the dedicated work of the Fermilab Accelerator Division, the Main Injector is on track to nearly double its Tevatron-era beam power by 2016.

“Fermilab’s beamline has been a tremendous driver of neutrino science for many years, and the continued improvements to the intensity mean that it will remain a driver for many years to come,” said Indiana University’s Mark Messier, co-spokesperson for the NOvA experiment.

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.

### Getting teachers back on TRAC

Wednesday, July 8th, 2015

Kerbie Reader, a high school math teacher, works at the Muon g-2 ring as part of Fermilab’s TRAC program. Photo: Ali Sundermier

Bonnie Weiberg sits down in front of a small monitor in the Proton Assembly Building at Fermilab. Her job is to test the signal strength of the liquid-argon purification monitors for the proposed DUNE experiment. But Weiberg isn’t your average particle physicist. In fact she isn’t a physicist at all: She’s a physics and chemistry teacher at Niles North High School in Skokie, Illinois.

Weiberg is here this summer as part of the Fermilab TRAC program, which is funded by the Particle Physics Division. Harry Cheung, an associate head for the CMS Department who has been head of the TRAC program since 2010, said that this year, seven teachers were selected from a pool of 33 applicants to be matched with a mentor and work on cutting-edge physics.

The TRAC program gives middle school and high school teachers of science, math, computer science and engineering an opportunity to come to Fermilab, work with a scientist or an engineer for eight weeks, and experience what Fermilab research is like.

This summer the teachers, most of whom are from Illinois, are working on projects such as building and testing photodetectors, reconstructing the Muon g-2 ring and controlling high-voltage supplies for the MINERvA neutrino experiment.

“Many of us haven’t done any research since college,” Weiberg said. “It’s nice to come back and be in a research environment to see what’s happening on the cutting edge.”

Kerbie Reader, a high school math teacher at Forest Ridge School of the Sacred Heart in Bellevue, Washington, said that TRAC is the only program she could find in the country that enables teachers to participate in this sort of research. She appreciates the opportunity to remember what it’s like to be a student and to gain experience that will help her relate to her own students.

“We’re seeing the same material year after year. We forget what it’s like to be the person who’s learning,” Reader said. “Instead of saying it’s been 10 or 20 years since I felt that way, I can say, ‘I felt that way last summer. I get that it’s hard, and this is how we’re going to work through it.'”

Weiberg and Reader agreed that the most valuable aspect of this program is being able to gain real-life experiences that they can bring back to their schools and share with their students. Weiberg is even working on a unit about particle physics to incorporate into her curriculum.

“It’ll help us engage our students more,” Weiberg said. “The more real-world things you can bring into your classroom, the better.”

Reader added that the TRAC program gives her a chance to participate in difficult research: to be challenged and learn the value of getting things wrong.

“I want to teach my students not to give up on something because they think it’s hard, to be able to tell them: making a mistake is not the problem,” Reader said. “Everybody that works on all these fantastic things have been making mistakes their entire lives. The day you figure out what your mistakes are, that’s the day you celebrate.”

Ali Sundermier

### Why Dark Matter Exists: Believing Without Seeing

Saturday, July 4th, 2015

The Milky Way rises over the Cerro Tololo Inter-American Observatory in northern Chile. The Dark Energy Survey operates from the largest telescope at the observatory, the 4-meter Victor M. Blanco Telescope (left). Photo courtesy of Andreas Papadopoulos

For decades physicists have been convinced that most of our universe is invisible, but how do we know that if we can’t see it? I want to explain the thought process that leads one to believe in a theory via indirect evidence. For those who want to see a nice summary of the evidence, check this out. So this post isn’t 3000 words, I will simply say that either our theories of gravity are wrong, or the vast majority of the matter in our universe is invisible. That most of the matter in the universe is invisible, or “dark”, is actually well supported. Dark matter as a theory fits the data much better than modifications to gravity (with a couple of possible exceptions like mimetic dark matter). This isn’t necessarily surprising; frankly it would be a bit arrogant to assume that only matter similar to us exists. Particle physicists have known for a long time that not all particles are affected by all the fundamental forces. For example, the neutrino is invisible as it doesn’t interact with the electromagnetic force (or strong force, for that matter). So the neutrino is actually a form of dark matter, though it is much too quick and light to make up most of what we see.

The standard cosmological model, the ΛCDM model, has had tremendous success explaining the evolution of our universe. This is what most people refer to when they think of dark matter: the CDM stands for “cold dark matter”, and it is this consistency that allows us to explain observations from almost every cosmological epoch that is so compelling about dark matter. We see the effect of dark matter across the sky in the CMB, in the helium formed in primordial nucleosynthesis, in the very structure of the galaxies. We see dark matter a minute after the big bang, a million years, a billion years, and even today. Simply put, when you add in dark matter (and dark energy) almost the entirety of cosmological history makes sense.  While there some elements that seem to be lacking in the ΛCDM model (small scale structure formation, core vs cusp, etc), these are all relatively small details that seem to have solutions in either simulating normal matter more accurately, or small changes to the exact nature of dark matter.

Dark matter is essentially like a bank robber: the money is gone, but no-one saw the theft. Not knowing exactly who stole the money doesn’t mean that someone isn’t living it up in the Bahamas right now. The ΛCDM model doesn’t really care about the fine details of dark matter: things like its mass, exact interactions and formation are mostly irrelevant. To the astrophysicist, there are really two features that they require: dark matter cannot have strong interactions with normal matter (electromagnetic or strong forces), and dark matter must be moving relatively slowly (or “cold”). Anything that has these properties is called a dark matter “candidate” as it could potentially be the main constituent of dark matter. Particle physicists try to come up with these candidates, and hopefully find ways to test them. Ruling out a candidate is not the same as ruling out the idea of dark matter itself, it is just removing one of a hundred suspects.

Being hard to find is a crucial property of dark matter. We know dark matter must be a slippery bastard, as it doesn’t interact via the electromagnetic or strong forces. In one sense, assuming we can discover dark matter in our lifetime is presumptuous: we are assuming that it has interactions beyond gravity. This is one of a cosmologist’s fondest hopes as without additional interactions we are screwed. This is because gravity is by far the weakest force. You can test this yourself – go to the fridge, and get a magnet. With a simple fridge magnet, weighing only a few grams, you can pick up a paperclip, overpowering the 6*10^24 kg of gravitational mass the earth possesses. Trying to get a single particle, weighing about the same as an atom, to show an appreciable effect only through gravity is ludicrous. That being said, the vast quantities of dark matter strewn throughout our universe have had a huge and very detectable gravitational impact. This gravitational impact has led to very successful and accurate predictions. As there are so many possibilities for dark matter, we try to focus on the theories that link into other unsolved problems in physics to kill two birds with one stone. While this would be great, and is well motivated, nature doesn’t have to take pity on us.

So what do we look for in indirect evidence? Essentially, you want an observation that is predicted by your theory, but is very hard to explain without it. If you see an elephant shaped hole in your wall, and elephant shaped foot prints leading outside, and all your peanuts gone, you are pretty well justified in thinking that an elephant ate your peanuts. A great example of this is the acoustic oscillations in the CMB. These are huge sound waves, the echo of the big bang in the primordial plasma. The exact frequency of this is related to the amount of matter in the universe, and how this matter interacts. Dark matter makes very specific predictions about these frequencies, which have been confirmed by measurements of the CMB. This is a key observation that modified gravity theories tend to have trouble explaining.

The combination of the strong indirect evidence for dark matter, the relative simplicity of the theory and the lack of serious alternatives means that research into dark matter theories is the most logical path. That is not to say that alternatives should not be looked into, but to disregard the successes of dark matter is simply foolish. Any alternative must match the predictive power and observational success of dark matter, and preferably have a compelling reason for being ‘simpler’ or philosophically nicer then dark matter. While I spoke about dark matter, this is actually something that occurs all the time in science: natural selection, atomic theory and the quark model are all theories that have all been in the same position at one time or another. A direct discovery of dark matter would be fantastic, but is not necessary to form a serious scientific consensus. Dark matter is certainly mysterious, but ultimately not a particularly strange idea.

Disclaimer: In writing this for a general audience, of course I have to make sacrifices. Technical details like the model dependent nature of cosmological observations are important, but really require an entire blog post to themselves to answer fully.