Thanks to Steve for the inspiration and Rozmin for her help with editing! And, of course, to the LHC team.
Archive for October, 2011
Pioneering Supercomputer QCDOC Retires, Regenerates in ‘Next-Generation’ QCDCQ
Monday, October 31st, 2011
On May 26, 2005, a new supercomputer, a pioneering giant of its time, was unveiled at Brookhaven National Laboratory at a dedication ceremony attended by physicists from around the world. That supercomputer was called QCDOC, for quantum chromodynamics (QCD) on a chip, capable of handling the complex calculations of QCD, the theory that describes the nature and interactions of the basic building blocks of the universe. Now, after a career of state-of-the-art physics calculations, QCDOC has been retired — and will soon be replaced by a new “next generation” machine. (more…)
For what it’s worth, neutrinos are weird. They are probably the strangest bits of matter in the Universe, and I do not mean in the quark sense either. Assuming that neutrinos are not actually trans-dimensional beings in search of a new home, there is probably no particle in Physics Past, Present, & Future that has bore more brunt of physicists’ creativity. On the other hand, as far as I know, there is no other particle that has solved as many problems in physics as neutrinos. The higgs boson is a good contender, but I still think neutrinos take the cake due to the fact that they have been around longer. Well, that and actually having been found to exist.
Figure 1: The (Left) Electron-, (Center) Muon-, and (Right) Tau-Neutrino, in plushie representation, brought to you by ParticleZoo. [Images: ParticleZoo]
I am sure by now you are wondering, “What are you talking about?”, and in all fairness, that is a very good question. In physics, neutrinos have a long history of being either the particle that broke the mold or the particle that saved physics. In doing so, neutrinos have developed this reputation for being the go-to particle for a new theory. In all fairness though, neutrinos are not doing themselves any favors if experiments keep finding contradictions with known laws of physics *cough*. I am sure for every flavor of ice cream at Baskin-Robbins or Ben & Jerry’s, there is a neutrino that has either been discovered or hypothesized.
Figure 2: The (Left) Electron-, (Center) Muon-, and (Right) Tau-Antineutrino, in plushie representation, also brought to you by ParticleZoo. [Images: ParticleZoo]
For today’s post, I though I would share with you a few of the many flavors of neutrinos. It is also my secret goal to mention “neutrinos” so often in this post that it will be at the top of Google’s queue. The table of contents is just below with the full list today’s neutrino flavors. Believe it or not, there are still plenty of types omitted. I suppose I have to write a future post to include these. 😀
Happy Halloween & Happy Colliding!
– richard (@bravelittlemuon)
Table of Contents
- The First Neutrino: Pauli’s Neutron
- Chadwick’s Neutrino: The Neutron
- Fermi’s Neutrino: The Key to the Weak Nuclear Force
- Majorana’s Neutrino
- The Super Massive Neutrino
- The Extra, Extra Neutrino
- The Sterile Neutrino: Type I
- The Sterile Neutrino: Type II
- The Tachyon Neutrino
1. The First Neutrino: Pauli’s Neutron
Back in the days when particle physics was still a young field in physics, about a decade before the discovery of Quantum Mechanics, experimentalists studying radioactive decay discovered something very startling: When a radioisotope decayed and emitted a high speed electron, then energy & momentum were not conserved. This was a very worrisome result because these conservation laws were, and still are, pillars of physics. In 1930, Wolfgang Pauli, after whom the famed Pauli-Exclusion Principle is named, made an audacious suggestion that perhaps radioactive decay involving electron emission also involved the production of an additional particle. Pauli’s stated that his neutrino, then named the neutron (different from today’s neutron), that was (1) electrically neutral and (2) massless, or nearly massless, (3) did not travel at the speed of light, and (4) virtually undetectable by contemporary, experimental standards.
Figure 3. The Nobel Foundation’s official portrait of Prof. Pauli (Nobel 1945). Yes, this is the man responsible for suggesting the existence of the neutrino. As father of all hypothetical particles, Pauli would later come to regret (mid-page) proposing an undetectable objects. [Image: Nobel Foundation]
At the end of the day Pauli was spot on with his suggestion. Radioactive decay involving electron emission does, indeed, require a very light, electrically neutral particle. In fact, the following generation of neutrino detectors were able to discover it without a problem. It turns out, all someone needed was a nuclear reactor and patience.
2. Chadwick’s Neutrino: The Neutron
Figure 4: The (real) neutron is composed of one up-flavor quark and two down-flavor quarks. [Image: Internet]
James Chadwick‘s discovery of the neutron proved one thing very, very well: that the Universe has an odd sense of humor and likes to confuse those to attempt to understand it. Having uses from nuclear power to cancer therapy, at the end of the day neutrons have been a boon for the scientific community and society as a whole. When first discovered, however, Chadwick initially misidentified it as Pauli’s neutron (a.k.a. the real neutrino). Today, the names we have for many particles are really artifacts of the confusion in particle physics through the 1930s & 40s. (For those of the physics history persuasion, this is just like the discovery of the “μ” meson.) Here is a time line the discovery of Chadwick’s neutrino (a.k.a. the fake neutrino):
- 1911 – The gold foil experiment is carried showing that the atom consists of a dense center. It is later found that an atom’s nucleus is too heavy to be composed only of protons. Fifty years later, gold foil is also discovered to be a source of unlimited amounts of chocolate.
- 1911 – β-decay, the mechanism through which some radioisotopes decay, appears initially to violate the Law of Conservation of Energy.
- 1930 – Pauli proposes, in his famous “Dear Radioactive Ladies and Gentlemen” letter, the existence of a massless (0r near massless), electrically neutral particle, called the “neutron” (actually the electron-neutrino), to resolve the apparent energy non-conservation in radioactive β-decay.
- 1932 – Chadwick claims possible discovery of a massive, electrically neutral, particle within the nucleus of an atom. Believing it to be Pauli’s neutron (actually the electron-neutrino), he calls it the “neutron” (actually the real neutron).
- 1934 – Enrio Fermi, using the newly created framework of Quantum Field Theory, proposes a simple four-particle interaction to describe β-decay (See 3. Fermi’s Neutrino). With known experimental results, Fermi was able to determine that Chadwick’s neutron (real neutron) was much too heavy to be Pauli’s neutron (fake neutron; real neutrino) and renamed Pauli’s neutron the “neutrino,” which is Italian for “little neutral one.” The only thing more impressive than the accuracy to which this model actually describes Nature is how short the paper is.
- 1942 – Pauli’s neutrino is discovered. In full disclosure, the particle he proposed to solve the problems of β-decay and what was actually discovered first is really the anti-electron-neutrino.
The real neutron is not really a neutrino; it just stole the real neutrino’s name. That jerk (the neutron not Chadwick).
[Note: It is really hard to write “neutrino,” “neutron,” and embed hyperlinks, all while focusing on the historical context.]
3.Fermi’s Neutrino: The Key to the Weak Nuclear Force
The mathematical and physical description of radioactive decay is, by far, one of the most beautiful things I have every seen in either Mathematics or Physics. (The second is probably the metric structure in Special Relativity.) What is so amazing about it is how it changes at higher energies. On one end of the energy spectrum, you have everyday radioactive decay; somewhere near the middle, you have the restoration of electroweak symmetry and higgs boson production; and on the far end, you have the grand unification of all forces.
In attempt to explain a type of radioactive decay known as β-decay, Enrico Fermi, in 1934, supposed that during this process a radioisotope will decay into a more stable isotope, a high speed electron (β-particle), and a hypothetical particle predicted to exist by Pauli, called the neutrino (See 2. Chadwick’s Neutrino). They Feynman diagram that illustrates this interaction is just below. I should note now that what Pauli really predicted is a neutrino’s antimatter equivalent call the anti-neutrino.
Figure 5: Enrico Fermi’s 4-fermion interaction model to describe β-decay. n represents an incoming neutron, p represents an outgoing proton, e is an outgoing electron, and note the outgoing anti-electron-neutrino (νe). [Image: Mine]
Being a fermion, a neutrino has an antimatter partner called an anti-neutrino. Under the rules of Quantum Field Theory, one can then induce β-decay by directing a beam of neutrinos into a bunch of heavy nuclei, like a thick plate of steel. Such a process would be drawn like this:
Figure 6: Enrico Fermi’s 4-fermion interaction model to describe neutrino scattering. n represents an incoming neutron, p represents an outgoing proton, e is an outgoing electron, and note the incoming electron-neutrino (νe). [Image: Mine]
Though the probability of inducing β-decay is very small but it becomes larger with higher energy. If you extrapolate this to very high energies, you find out that eventually the probability of inducing β-decay becomes larger than 100%, which is total nonsense. You can never have a 101% of your interactions result in anything. In particle physics, the sum of all probabilities must add up to 100%; in such cases where they do not, we say that “unitarity has been violated.” This terminology originates from the fact that the matrix containing all possible interaction outcomes is a unitary matrix, implying that total probability is (1) conserved and (2) identically equal to 1 (or 100%).
How does Nature avoid breaking math at high energies? Well at around 100 GeV, rather than two particles smashing into each other to produce two different particles, a neutrino will radiate a W boson and become the high speed electron (β-particle). This W boson is then absorbed by a neutron (Chadwick’s neutron) and is turned into a proton, thereby transmuting one isotope into another isotope. Since producing a W boson (mW = 80.399 GeV/c2) is not cheap and requires a lot of energy, the probability of scattering a neutrino off a nucleus is driven down and prevents unitarity from being violated.
In summary, Fermi’s neutrino & Weak Nuclear Theory model is the foundation for the Electroweak component of the Standard Model.
Figure 7: Tree-level diagram of the neutrino scattering process in which (1) a neutrino will emit a W and become an electron, and is followed by (2) a down-type quark absorbing the W boson and becoming an up-type quark. The 4-fermion model is the low-energy approximation of this description. Color represents the QCD charge held by the quarks in a nuclei. Color also makes things look nicer. [Image: Mine]
4. Majorana’s Neutrino
Antimatter, the destroyer of basilicas, the stuff of warp drives, and just all around fascinating piece of science, was predicted to exist in 1928 by the great Paul Dirac, and discovered shortly thereafter (1932) by Caltech’s Carl Anderson. This is the same Anderson who is discovered the muon, and so he probably qualifies to be my hero. One way to describe antimatter is to imagine regular, ordinary matter, but for each charge a piece of matter has its antimatter partner has the opposite charge. For example, the top quark has a number of charges: +2/3 electric charge; it can have a red, blue, or green charge from the Strong Nuclear force (QCD); and it also has a “topness” (or “truthfulness”) charge under the Weak Nuclear force. An anti-topquark then must have: a -2/3 electric charge; an anti-red, anti-blue, or anti-green “color” charge; and has “anti-topness” (or “anti-truthfulness”… does that make anti-topquarks liars?).
Well, I suppose one has to wonder if it is possible for a particle to ever be its own anti-particle. The answer is yes. Such particles are called Majorana particles. Italian physicist Ettore Majorana speculated and determined a number of constraints, namely to conserve all the various types of charges (electric, color, weak) a Majorana particle must be neutral under all its charges. To get this right, I need an electrically neutral, colorfully neutral, and weakly neutral. To me, this sounds just like a neutrino! If it smells like a neutrino, looks like a neutrino, and tastes like a neutrino, then clearly it must be a duck neutrino.
What is the problem? Well, if neutrinos are their own antiparticle then physicists expect to see something called neutrino-less double β-decay (or 0νββ for short). In this process, a radioisotope will undergo β-decay and emit a high speed electron and an anti-electron neutrino. If neutrinos are indeed Majorana particles, then the anti-electron-neutrino is also an electron-neutrino and can force a second radioisotope to also emit a high speed electron.
To date, 0νββ has not been observed but that does not mean it does not exist. It is possible that 0νββ does exist, it must just be a really, really rare process.
Figure 8: Feynman diagram demonstrating how neutrino-less double β can occur if neutrinos are also Majorana particles. [Image: Wikipedia]
5. The Super Massive Neutrino
According to the Standard Model of Particle Physics, there are only three “light” neutrinos. “Light” is defined as less than 1/2 the mass of the Z boson, which mZ = 91.1876 GeV/c2. We have observed this empirically by producing Z bosons in copious amounts at the large electron positron collider and looking at all possible ways we can observe a Z boson can decay. The total number of observed Z decays is then used to calculate the Z boson’s average lifetime (or rate of decay). The observed decay rate is subtracted from the Standard Model’s prediction for the total decay rate. The difference between the theoretical prediction and the experimental observation is then compared to the situation where the Z boson were able to decay into 1, 2, 3, … different pairs of particles that could not be observed with our detectors. These sorts of decays are called “invisible decays” or “invisible decay modes.” From this data, all signs point to three different invisible decay modes, which correspond to the three neutrino flavors in the Standard Model (electron, muon, tau).
Time for caveat number 4,321: Z bosons can only decay into particles lighter than itself, otherwise all sorts of bad things would happen. By bad things, I mean violations of conservation laws. If any particle were to decay into two (almost) identical particles, then at most each daughter particle could weight half of the mother particle. This means, according to invisible decay searches of the Z boson, there are only three types of neutrinos with mass less than 1/2 the mass of the Z boson. It is fair game for neutrinos to be heavier than half the Z mass; in fact, it is possible for a neutrino to be as heavy as ten top quarks! (The top quark is currently the most heavy particle known to exist.)
The most recent experimental results have found that for a stable (non-decaying) neutrino, its mass must be at least 45.0 GeV/c2 (39.5 GeV/c2) for an ordinary (Majorana) neutrino. For a short-lived (decaying) neutrino, it must have a mass of at least 90.3 GeV/c2 (80.5 GeV/c2) for an ordinary (Majorana) neutrino.
6. The Extra, Extra Neutrino
Neutrinos can oscillate. What do I mean by that? Well, if you make a beam of neutrinos and look at the beam composition (% of electron-neutrinos v.s. % of muon-neutrinos, v.s. % of tau-neutrinos), as a function of distance, then one will notice that the relative composition changes.
For example: If I measure the beam to be made of 100% electron-neutrinos & 0% muon-neutrinos, and a few football pitches away I find that it is now 50% electron-neutrinos, 50% muon-neutrinos, then a few football pitches away from that I can expect to see 100% electron-neutrinos & 0% muon-neutrinos once again. I made up the exact numbers, but I hope you get the idea. It has only been recently (1,2) that all oscillation permutations have been observed.
Figure 9: To measure neutrino oscillations, a neutrino beam is typically shot into the Earth (right), measured by a detector close to the beam’s origin (near detector), and then detected by a detector on the opposite side of the planet (left). Yes, we literally shoot a beam a particles into the Earth and wait for them to come out the other side. PHYSICS. IS. AWESOME. [Image: Interactions]
Well, back in 2001 (that was over 10 years ago, weird…) a Los Alamos experiment LSND (Liquid Scintillator Neutrino Detector) saw a signal that could be explained if neutrinos were also oscillating into a fourth type of neutrino. The MiniBooNE experiment at Fermilab tried to verify this result and was unable to make a conclusive determination. In other words, the jury is still out on the existence of a 4th type of neutrino.
7. The Sterile Neutrino: Type I
I like sterile neutrinos; they are fun. According to the Standard Model, all observed neutrinos are (1) colorless (no interactions via the Strong Nuclear Force), (2) electrically neutral (no interactions via Electromagnetism), and (3) are left handed (Weak charge). This means that Standard Model neutrinos can only interact with the W bosons and sometimes with the Z boson. Well, suppose there were a right-handed neutrino (opposite Weak charge from left-handed neutrino). It is still invisible to the Strong Nuclear Force, the Electromagnetic Force, and the W± bosons (because all W‘s are left-handed). In principle right-handed neutrinos can interact with the Z boson, trying to separate the corresponding signal from background data is like trying to find a find a needle, in a haystack, at a fair. Did I mention this fair is a tri-state fair?
Right-handed neutrinos and other neutrinos that are invisible to the Standard Model forces are examples of what physicists call “sterile neutrinos.” (Personally, I like to qualify these sorts of little tykes with the title “Type I.” See 8. The Sterile Neutrino: Type II why I do so.) If right-handed neutrinos do exist, then there is no way to see detect them given our current understanding of physics. However, this does not mean they cannot interact through some new, undiscovered force.
To date, there is no confirmed evidence, direct or indirect, of the existence of a right-handed or any other type sterile neutrino. To date, there is no evidence for a new fundamental force either. Though interestingly enough, since sterile neutrinos, in principal, cannot be detected, then it is logical that there could be hundred or even thousands of slightly different sterile neutrinos. Alternatively, we can also a universe filled with a single type of neutrino and we would not be able to detect them outside of gravity (assuming they have mass), which brings me to mention that sterile neutrinos have even been proposed as a dark matter candidate. Neutrinos are resourceful, I will give them that.
Figure 10: A snow-covered hay bale at Fermilab. Imagine trying to find a needly in that field. [Image: FNAL]
8. The Sterile Neutrino: Type II
Sterile neutrino type II (again, I made up the “type” nomenclature) is very much like type I but with one glaring difference. Even if there are are new forces in the Universe, these types of neutrinos will still not interact with anything. The only possible forces through which these neutrinos might interact are gravity and whatever unified force that produced these oddballs.
9. The Tachyon Neutrino
In September, the Italian neutrino experiment OPERA (Oscillation Project with Emulsion-tRacking Apparatus) shocked the world when the collaboration announced it had observed neutrinos traveling at a speed faster than that at which light travels. My colleagues have blogged about it here, here, here, and more recently here. This is a huge deal because, according to Special Relativity, the speed of light (numerically c = 299,792, 458 m/s or 983, 571, 056 ft/s) is pretty much a cosmic speed limit that no real particle can surpass. So I am not sure which makes me happier, the fact that tachyons are seriously being floated as an explanation for this claim or that #FTLneutrinos is a thing. (“FTL” stands for “faster than light.”)
Metaphorically, tachyons are interesting sorts of creatures. I do not know too much about them beyond the fact that they have (in the mathematical sense) a purely imaginary mass. The last time I checked quantum mechanics, we cannot observe strictly imaginary quantities, but I digress. What I do know is that special relativity implies that having a purely imaginary mass should then enable tachyons to permanently travel at speeds faster than c. If neutrinos do travel at speeds faster than the speed of light, then they may also be tachyons. I think it is a perfectly reasonable argument. However, there is a very big elephant in the room that I have to address. Having imaginary mass means that all tachyons always travel at superluminal speeds. If some neutrinos are found to travel at subluminal speeds then the idea that neutrinos are tachyons is tossed out. End of story.
So in light of the considerable implications of any particle traveling faster than the speed of light, it is very appropriate to remain cautious and wait for OPERA to reproduce their results and independent verification, possibly by Fermilab’s MINOS Experiment or KEK’s T2K Experiment.
Figure 11: A real life tachyon. [Image: ParticleZoo]
In a couple of hour’s time the clocks go back an hour in Geneva, as European daylight savings time ends. Usually this isn’t a big deal. We adjust our watches, the nights get darker earlier, and some of us turn up an hour early for work while the rest enjoy the extra hour of rest! But what happens in the Control Room? It’s not as trivial as you’d think…
The timing of the detector will be okay, as the passage of “normal” time is full of leap seconds and minor corrections here and there. Protons don’t care about that kind of thing, they just care about the amount of absolute time that has elapsed. But when humans get involved it gets more complicated, because we work with the time of day. Our plots show the hour along the x-axis for the past day or so. If we’re going to keep these plots online we’ve got to make a decision, either to double-count the results for that hour (ugh) or repeat the hour on the plots (and see some protons turn up 60 minutes earlier than expected!) It’ll be made even worse when people try to use the log books to recreate the events of the evening. They’ll see that some experts forgot to change their watches, and some didn’t. For those that did change their watches they’ll have 2am occur twice. Does this means we’ll have 2am(A) and 2am(B)? Will my replacement turn up at 6am or 7am? (I hope he turns up at 6am so I can send him to get some coffee while I sit through my extra hour!)
It’s tempting to write something like this in the electronic log book:
02:00: Some neutrinos arrive. Where did they come from?!
02:59: We have beams! We’ll be getting some data soon.
03:01: Wait a minute.
If only ATLAS could detect neutrinos…
The blind men and the elephant is a common tale in India with many variants. Below is the one from Saxe:
| The Blind Men and the Elephant John Godfery Saxe (1816 — 1887) |
||
| I. It was six men of Indostan To learning much inclined, Who went to see the Elephant (Though all of them were blind), That each by observation Might satisfy his mind. |
IV. The Third approached the animal, And happening to take The squirming trunk within his hands, Thus boldly up and spake: “I see,” quoth he, “the Elephant Is very like a snake!” |
VII. The Sixth no sooner had begun About the beast to grope, Than, seizing on the swinging tail That fell within his scope, “I see,” quoth he, “the Elephant Is very like a rope!” |
| II. The First approached the Elephant, And happening to fall Against his broad and sturdy side, At once began to bawl: “God bless me!-but the Elephant Is very like a wall!” |
V. The Fourth reached out his eager hand, And felt about the knee. “What most this wondrous beast is like Is mighty plain,” quoth he; “‘Tis clear enough the Elephant Is very like a tree!” |
VIII. And so these men of Indostan Disputed loud and long, Each in his own opinion Exceeding stiff and strong, Though each was partly in the right, And all were in the wrong! |
| III. The Second, feeling of the tusk, Cried: “Ho!-what have we here So very round and smooth and sharp? To me’t is mighty clear This wonder of an Elephant Is very like a spear!” |
VI. The Fifth, who chanced to touch the ear, Said: “E’en the blindest man Can tell what this resembles most; Deny the fact who can, This marvel of an Elephant Is very like a fan!” |
MORAL. So, oft in theologic wars The disputants, I ween, Rail on in utter ignorance Of what each other mean, And prate about an Elephant Not one of them has seen! |
In many ways this poem is an excellent metaphor for science and demonstrates how science works. The blind men want to learn about the elephant so they make observations. Good so far—in science, observations rule. They then make models to describe what they have seen, or in this case, felt. Again, good scientific methodology: the tusk is indeed well modeled as a spear. Since each has chanced upon a different part of the elephant, the models differ. This not a problem. In science we always model different parts of reality differently; we do not use the standard model of particle physics to describe planetary orbits or cell division.
But the blind men then make the classic error and assume theirs is the global model that describes the entire elephant. The scientific method would have required them to make predictions based on their models and test them against additional observations. This would have revealed the problem—and the elephant. Instead, they argued. This, of course, bears no resemblance to real scientists who never argue heatedly on the basis of partial data. Nope, never, has not happened. (OK, stop thinking of your colleague.)
The moral is also interesting (not just for theologic war but also scientific ones): Rail on in utter ignorance of what each other mean. This seems like a foreshadowing of Kuhn’s incommensurability of paradigms. Their ideas and frames of reference are so different they cannot understand each other.
And prate about an Elephant Not one of them has seen! Here we come to the nub. To a large extent science is precisely this; prating about an elephant we have not seen. We can learn a lot about the elephant we have not seen by making repeated observations of the beast and learning how the various models fit together and interlock with each other. We may never, by feel or even by sight, be able to construct the ultimate model of the elephant but we can obtain a lot of useful information and construct quite accurate models of at least some aspects of the elephant. The trick is not to make the mistake of assuming one’s partial model is the whole truth. This mistake has been made repeatedly: Newton’s laws of motion, Maxwell’s equation, the fixed continents, … . There is now a talk of a theory of everything (TOE); the same mistake being made again.
The blind men’s error has been common in the philosophy of science as well. Different models for the scientific method capture different aspects of the problem: induction, hypothesis, logical positivism (verification), paradigms, falsification, and even no method. The philosophers do indeed resemble the blind men disputing loud and long. The real task is to see how the various models fit together to give a coherent whole. Like the models of the elephant, the different approaches to the scientific method are not so much wrong as incomplete. The approach I advocate of models competing against each other in making successful predictions is an attempt at a more unified approach; one can see how the various precursor models are aspects of it. But as always, this will probably be just one aspect of a more complete approach that can never be completely known and will always be debated. But for most practical purposes, the scientific method can be, and probably is known well enough. Blind men, even those with a philosophical bent, can learn a lot about an elephant by groping in the dark.
Walking through Cambridge one Friday evening, I saw a strange sight: a man praying to a Federal Express drop box. At first it seemed odd, but then I could hear the prayer in Hebrew. I looked at my watch and saw that it was 6:04 PM, the moment of sunset for Cambridge. His orientation was facing east, so clearly he was a Hasidic man praying toward Jerusalem. The initial oddity wore off as I unraveled the mystery. He must have been en route to some destination when the sunset overtook him.
I teach a course called Primitive Navigation. The subject is how navigational strategies emerged in many cultures across the earth prior to the Scientific Revolution. As part of the course, I have students identify stars and planets, and become accustomed to their motion in the sky. I track the positions of the stars and planets as a matter of course, and can even anticipate events like the backward motion of planets called retrograde motion. Currently Jupiter is executing a retrograde motion.
Now, let’s try an experiment here. Google the words “Jupiter” and “retrograde” and look at the how many hits you have to go through before you find an astronomy, as opposed to an astrology website. You’ll go through five pages of astrology hits. That’s not all. If you read one of the websites, it will tell you that Jupiter is retrograde in Taurus. But, if I look up in the sky, I don’t see Jupiter in Taurus, I see it in Aries – quite some distance away.
What’s going on?
Most people don’t care if Jupiter is executing a retrograde motion unless it tells them whether they should buy stocks or wear heels on a blind date. The scheme used by western astrologers is based on a version of the sky that was frozen in time when the ancient Greeks made observations of the sky. Since that time, the earth’s axis had wobbled to a new orientation, but, due to tradition, the astrologers use the old locations of the zodiac as a basis for the augury.
What does any of this have to do with particle physics? Clearly the two stories – the man praying at the Fed Ex drop box, and the belief in Jupiter executing retrograde motion in Taurus are matters of faith. We might scoff at these as relics from a bygone era, but we should be cautious on two accounts: 1.) descriptions of the universe at one time saw astronomy and astrology as somewhat inseparable and 2.) even modern science rests on many assumptions that to many of us have become articles of faith.
Now, you might say, “but…science has a right to examine any and all assumptions, but things like religion and astrology take all assumptions as sacred.” The problem is ‘which assumptions?’. There are so many. When I was a graduate student, I studied something called the Dirac equation. Paul Dirac was a theoretical physicist who is best known for his famous equation describing the motion of particles like electrons. He successfully combined two pillars of modern theory: quantum mechanics, which describes particles as a wave with special relativity, which takes the speed of light to be a fundamental constant of nature.
When he put these two factors together, his equation had some strange properties. At first blush, you find that electrons have this strange motion that’s called ‘zitterbewegung’; German for ‘trembling motion’. This is a very rapid oscillation of electrons at the speed of light, which doesn’t seem to really happen and would lead to odd results. This was more or less brushed under the rug.
The second problem is that there are negative energy solutions, which should be forbidden because they would violate our cherished principle of the conservation of energy. One of my friends at graduate school laughingly called these “Dirac fairy tales.” But an odd thing happened. Dirac tried to plug up the negative energy states by saying that nature somehow filled up all the negative energy solutions, leaving behind the possibility that ‘bubbles’ in the sea could be created. These bubbles would have the same mass, but opposite charge of the electron. It was an inevitable consequence of his equations, but had never been seen and he was ready to dismiss this as he had done with the zitterbewegung. He speculated that the positively charged electron might be the proton, but was dissuaded and published a prediction of a positively charged electron. In 1932, Carl Anderson discovered the positron.
So, the zitterbewegung was discarded as meaningless, but the problem of the negative energies ended up predicting the existence of antimatter.
More articles of faith: infinite energies. You might recall that the strength of the electric field gets larger as you get closer to a charged particle. In particle physics, we like to talk about ‘point-like’ particles, meaning that they have no size. In practice, this creates a problem: right at the electron, the electric field is infinite. Energy gets tied up in the electric fields, so, in principle, the energy of the electric field of the electron is infinite. This seems like a problem, since the electron seems to move quite happily around. How do physicists deal with this?
We have a process that removes the infinite energies and renders them finite. It’s called renormalization. The main idea is that we decide, somewhat arbitrarily, that our rules of physics change at very high energy scales or very small distance scales. The scale where everything changes is so remote from the scales that we’re testing that it won’t make a difference in our calculations. We ‘kill’ the infinite energies this way and make our calculations finite. This process also provides a look at particles we have yet to see that might exert a hidden influence on the strength of forces and values of masses. This process, however contrived it might seem, has given amazingly solid predictions. Yet we don’t know what it means for the physics to change at very, very short distances. It’s a matter of faith.
Theorists say that any viable theory of particles and their interactions must be of a sort that allows us to remove the infinite energies. This requirement guides theorists to root out non-conforming theories and accept theories that avoid the infinite energies. In essence, this is the role of the Higgs boson — by being the thing that makes mass, it allows our beloved Standard Model to avoid the infinite energies and does.
Physicists rarely questions these articles of faith. Sweeping zitterbewegung under the rug is fine, but what does it mean? Is there any meaning to the process that we use to kill the infinite energies, or is it just a mathematical trick that appears to work? Although they may seem like ‘fairy tales’ as my friend said, they have become articles of faith and are rarely questioned. The problem is: we don’t know whether we will be forced to reexamine some of these, or if so, which of these we have to reexamine. Until then, we have our own Fed Ex drop box where we worship.
So if your days have been anything like mine in recent weeks anytime I talk to anyone with even a vague semblance of what particle physics is and that I am an experimentalist (in training of course) the question comes….
“So what about CERN proving Einstein wrong with those things going faster than light?”
To which I respond politely, “Crazy stuff…but anytime someone says they see something going faster than light I put my hand on my wallet because something is fishy”.
If the person is nice/interested enough to want a further explanation I try to explain what the OPERA measurement is along with loads of caveats that I don’t work on this experiment, as scientists they did hundreds of cross-checks, and that they wouldn’t release this result if they weren’t convinced something is strange here…etc…etc…
If someone is daring enough to push and ask what I think about it my response has been simple: “Science is about repeatability and accuracy so I’ll wait till the next group of experimentalists weighs in”.
Today on the BBC I saw the news announcement that “Faster-than-light neutrino experiment to run again“. Aside from the obvious things wrong with the title of the argument (this wasn’t an experiment to search for faster-than-light neutrinos) the article explains that during this next run they are going to attempt to remove on of the largest possible sources of systematic errors in the OPERA measurment, namely the length of the length of the proton bunch widths being sent towards Gran Sasso from 10 microseconds to ~ 1 nanosecond with ~ 500 nanoseconds between pulses.
While you still can’t measure exactly which neutrino is from which proton the way you would like to in a perfect measurement, this should allow them to be more accurate on average than before and take away a source of error many people I would consider experts have said is of greatest concern.
While I’m sure this is only one of many improvements that will be made to this measurement to address all the…shall we say…”constructive criticism” the OPERA 
experiment has received since their result. The bad news is that if they end up with a null measurement and find that neutrinos don’t in fact go faster than the speed of light the news and fan fare will be much less…because while for scientists a null result is still a result…for the rest of the world a null result is not news.
So I think we have some interesting times in experimental physics coming in the very near future!
A talk about how a helicopter can advance high-energy physics was part of my initiation to my first collaboration meeting for the new muon g- minus 2 experiment at Fermilab.
A similar helicopter will carry the ring, just as the tank is carried here, from Brookhaven National Laboratory a portion of the way to Fermilab.
The meeting was a very exciting (and exhausting!) experience.
And let’s be honest any collaboration meeting with a talk devoted to helicopters is awesome. From the way we talk about this thing, it’s going to be our mascot. We need a helicopter because we are going to use the muon storage ring from the previous muon g-2 measurement at Brookhaven National Laboratory, which took a similar measurement to what we will look for. The helicopter will take the ring from Brookhaven, drop it on a barge that sends it to Illinois and then a helicopter will take it to Fermilab. I think it’s going to be very cool to see that happen.
This was our first meeting after receiving Stage 1 approval from Fermilab, meaning Lab management thinks this experiment it worth doing, although, there is no funding attached to it yet. The meeting took place in March over a Friday and Saturday and we needed every second of that time. There was much to discuss and it was all interesting, especially to me, as a newbie.
What is the muon g-2 experiment? Okay, some jargon, just to sound cool. The g-2 experiment’s goal is to measure the difference between the gryomagnetic ratio (spin/angular momentum) and the Bohr magneton.
Fermilab’s planned muon g-2 experiment will use the storage ring that was used in a previous muon g-2 experiment at Brookhaven National Laboratory.
And what does that mean? It’s basically measuring intrinsic properties (such as spin and angular momentum) of a particle. Experimentally, we are measuring the precession of the muon due to a magnetic field. You can imagine a top, just as it’s about to topple over. That motion is called precession. We measure the frequency of that (how many times the muon goes around before decaying, or in the case of the top, toppling over). Theorists can calculate the frequency of this very, very precisely and experimentalists can measure it very, very precisely. Because of this level of preciseness, we are sensitive to physics beyond the Standard Model. The Standard Model is incorporates what we know now about particles and interactions, but does have some holes. The results from the new muon g minus 2 experiment will help us plug those holes by pointing to which theories beyond the Standard Model are most likely.
So that’s a brief summary about the physics for the muon g-2 experiment.
The muon g-2 collaboration at Fermilab during a March meeting.
On a more personal level, I’m involved with research and development for the tracking detector, which is used to find out where the decay positrons go, among other measurements. Our current plan is to use straws. They look pretty much like you would expect from the name. They are tubes made of a lightweight material that is usually coated with some sort of metal and a super thin wire runs through the center, and they are filled with a gas. When a charged particle passes through them, the gas ionizes and we collect the resulting signal. We aren’t sure what type of materials we are going to use for the straws, which is part of the fun. We are trying to figure out the best detector we can build on a reasonable budget.
–Mandy Rominsky
This story first appeared in Fermilab Today Oct. 10.
The 1970s were a thriving time in the world of physics, heralding such milestones as the development of the Standard Model and the discovery of the bottom quark. Now scientists at Fermilab are bringing some experimental pieces of that era back – bubble chambers and fixed-target physics.
Mike Crisler, a Fermilab scientist working on COUPP, is building the chambers for the CITRE experiment. Photo: Reidar Hahn
Peter Cooper, a Fermilab physicist, is heading a new experiment calibrating the classic bubble chamber technology, which is used today to search for dark matter.
The Chicagoland Observatory for Underground Particle Physics (COUPP) collaboration looks for bubbles in chambers filled with a compound containing carbon, fluorine and iodine. The fluid is superheated beyond the boiling point but has no rough surface to form bubbles. When a specific type of particle interacts in the chamber, it can deposit enough energy to boil the fluid and make a bubble. Electrons do not produce bubbles, while a dark matter particle interacting with a nucleus can – making this the key for dark matter detection.
“When a bubble forms, we notice it in the pictures,” Cooper said of the chamber technology. “The bubbles in the fluid are slow enough that high-speed cameras will capture the changes through continuous still shots. We’re making the world’s most boring movie.”
However, scientists are uncertain about the energy it takes to create a bubble in the chamber, which directly influences the sensitivity of the experiment.
The new experiment, named COUPP Iodine Recoil Threshold Experiment (CIRTE), will calibrate the energy threshold of the COUPP bubble chambers so that the COUPP dark matter results are on a firmer foundation. Scientists will fire pions, the lightest meson, in the Fermilab Test Beam Facility at a tiny pen-sized bubble chamber to measure how much energy needs to be deposited in the chamber to form a bubble.
The CITRE collaborators will use a fixed-target technique called elastic scattering of pions. The pions interact with iodine, the target nucleus in the COUPP bubble chambers with the most sensitivity to the most popular dark matter candidates. The pions are surrogates for dark matter – the bubble chamber sees them both in the same way by observing the bubble from the recoiling iodine.
Unlike dark matter, however, pions can be easily observed with other detectors on both sides of the bubble chamber, allowing COUPP scientists to know how much energy the pion gave to a scattered iodine nucleus.
Cooper and his team are currently running preliminary beam tests on solid carbon, fluorine and iodine targets to ensure that they understand how the experiment will work, in preparation for putting an actual bubble chamber in the beam. By watching how the pions interact with each target, they can determine how the pions should behave once the bubble chamber is in place.
However, the bubble chamber will only be able to produce one measurable bubble per beam spill, or one cycle of the accelerator. After the one bubble appears, the entire chamber needs to be recompressed in order to reset the contents.
The current COUPP chamber operates at the underground SNOLAB in Canada. Being deep below the surface allows scientists to suppress background events, such as those from cosmic rays. That bubble chamber is already setting limits on dark matter interactions that approach the best in the world. But they are hampered by the uncertainty on the energy threshold. With a little help from CIRTE, the COUPP experiment will be on a solid foundation as its search for dark matter increases.
—Brad Hooker
It’s that moment when you realize something serious and exciting has happened, but it’s 5:45am and you have to wake somebody up to sort it out. As the LHC ramps up it’s my role to make sure that the trigger is ready. This means looking at the bunch structure in the LHC and checking that ATLAS knows what this structure looks like. It’s as simple as pressing a few buttons and updating a database, and if everything goes smoothly we have nothing to worry about.
This time it was a bit different, because the LHC used a bunch structure they had never used before. When I pressed the button I was actually telling ATLAS something new and witnessing one of those rare transitions in the normal running of the LHC! (Jim’s post gives a great explanation about what bunch structures are and how the LHC team design them.) Then I checked the instructions, and they told me I had to wake someone up and tell them about the change. Nobody likes to be woken up at 5:45am, especially if they have an important meeting the next day. To make matters worse, I know the guy on the other end of the line (although since he’s so sleepy I didn’t recognize his voice at first!) At that point I remembered what my flat mate had told me when he was on call and got woken up at night. He said “What we do would be easy if they just gave us two minutes to think about it. We need time to wake up!” So, feeling bad about waking up the expert I told him I’d call back in 5 minutes. There was a flurry of messages on the electronic logbook and short conversations in the Control Room, and then it was time to call again. This time the voice on the other end of the line was more alert and a bit happier! He said everything was fine. I could proceed as normal and as long as there are no serious problems we can take data as we usually do.
We have beams!
The LHC just declared stable beams. Now the fun begins…
