### @CMSVoices

Thursday, October 29th, 2015

Building on the success of rotating Twitter accounts like @realscientists, which I participated in last year, the CMS experiment has a new account: @CMSVoices.  The idea is that it’s an account for talking to CMS members and hearing about their day-to-day work, in contrast with the official news from the @CMSexperiment account.  Of course, you can already hear from many individual CMS physicists on Twitter (I’m normally @sethzenz), but the account gives you the chance to interact with a new person each month, and it might even help us get some new tweeters started!  I also tried to explain things in more detail and start some more general discussions, for example:

There weren’t too many discussions or too many followers so far, but we’re just getting started, and I’m looking forward to others taking the account over and seeing what the do with it.  The next holder of the CMSVoices account, starting in November, will be @matt_bellis.  Please welcome him next week, and let us know if you have any questions or ideas!

### My Week as a Real Scientist

Thursday, March 6th, 2014

For a week at the end of January, I was a real scientist. Actually, I’m always a real scientist, but only for that week was I tweeting from the @realscientists Twitter account, which has a new scientist each week typing about his or her life and work. I tweeted a lot. I tweeted about the conference I was at. I tweeted about the philosophy of science and religion. I tweeted about how my wife, @CuratorPolly, wasn’t a big fan of me being called the “curator” of the account for the week. I tweeted about airplanes and very possibly bagels. But most of all I tweeted the answers to questions about particle physics and the LHC.

Real Scientists wrote posts for the start and end of my week, and all my tweets for the week are at this Storify page. My regular twitter account, by the way, is @sethzenz.

I was surprised by how many questions people had when I they were told that a real physicist at a relatively high-profile Twitter account was open for questions. A lot of the questions had answers that can already be found, often right here on Quantum Diaries! It got me thinking a bit about different ways to communicate to the public about physics. People really seem to value personal interaction, rather than just looking things up, and they interact a lot with an account that they know is tweeting in “real time.” (I almost never do a tweet per minute with my regular account, because I assume it will annoy people, but it’s what people expect stylistically from the @realscientists account.) So maybe we should do special tweet sessions from one of the CERN-related accounts, like @CMSexperiment, where we get four physicists around one computer for an hour and answer questions. (A lot of museums did a similar thing with #AskACurator day last September.) We’ve also discussed the possibility of doing a AMA on Reddit. And the Hangout with CERN series will be starting again soon!

But while you’re waiting for all that, let me tell you a secret: there are lots of physicists on Twitter. (Lists here and here and here, four-part Symmetry Magazine series here and here and here and here.) And I can’t speak for everyone, but an awful lot of us would answer questions if you had any. Anytime. No special events. Just because we like talking about our work. So leave us comments. Tweet at us. Your odds of getting an answer are pretty good.

In other news, Real Scientists is a finalist for the Shorty Award for social media’s best science. We’ll have to wait and see how they — we? — do in a head-to-head matchup with giants like NASA and Neil deGrasse Tyson. But I think it’s clear that people value hearing directly from researchers, and social media seems to give us more and more ways to communicate every year.

### Tweeting the Higgs

Wednesday, January 23rd, 2013

Back in July two seminars took place that discussed searches for the Higgs boson at the Tevatron and the LHC. After nearly 50 years of waiting an announcement of a $$5\sigma$$ signal, enough to claim discovery, was made and all of a sudden the twitter world went crazy. New Scientist presented an analysis of the tweets by Domenico et al. relating to the Higgs in their Short Sharp Scient article Twitter reveals how Higgs gossip reached fever pitch. I don’t want to repeat what is written in the article, so please take a few minutes to read it and watch the video featured in the article.

The distribution of tweets around the July 2nd and July 4th announcements (note the log scale)

Instead of focusing on the impressive number of tweets and how many people were interested in the news I think it’s more useful for me as a blogger to focus on how this gossip was shared with the world. The Higgs discovery was certainly not the only exciting physics news to come out of 2012, and the main reason for this is the jargon that was used. People were already familiar with acronyms such as CERN and LHC. The name “Higgs” was easy to remember (for some reason many struggled with “boson”, calling it “bosun”, or worse) and, much to physicists’ chagrin, “God particle” made quite a few appearances too. It seems that the public awareness was primed and ready to receive the message. There were many fellow bloggers who chose to write live blogs and live tweet the event (I like to think that I started bit of a trend there, with the OPERA faster than light neutrinos result, but that’s probably just wishful thinking!) Following the experiences of December 2011, when the webcast failed to broadcast properly for many users had twitter on standby, with tweets already composed, hungry for numbers. The hashtags were decided in advance and after a little jostling for the top spot it was clear which ones were going to be the most popular. Despite all the preparation we still saw huge numbers of #comicsans tweets. Ah well, we can’t win them all!

The point is that while the world learned about the Higgs results I think it’s just as important that we (the physicists) learn about the world and how to communicate effectively. This time we got it right, and I’m glad to see that it got out of our control as well. Our tweets went out, some questions were asked and points clarified and the news spread. I’m not particularly fond of the phrase “God particle” , but I’m very happy that it made a huge impact, carrying the message further and reaching more people than the less sensational phrase “Higgs boson”. Everyone knows who God is, but who is Higgs? I think that this was a triumph in public communication, something we should be building on. Social media technologies are changing more quickly each year, so we need to keep up.

A map of retweets on July 4th, showing the global spread.

But moving back to the main point, the Higgs tweets went global and viral because they were well prepared and the names were simple. Other news included things like the search for the $$B_s$$ meson decaying to two muons and the limits that places on SUSY, but how does one make a hashtag for that? I would not want to put the hashtag #bs on my life’s work. It’s always more exciting to announce a discovery than an exclusion too. The measurement of $$\theta_{13}$$ was just as exciting in my opinion, but that also suffered the same problem. How is the general public supposed to interpret a Greek character and two numbers? I should probably point out that this is all to do with finding the right jargon for the public, and not about the public’s capacity to understand abstract concepts (a capacity which is frequently underestimated.) Understanding how $$\theta_{13}$$ fits in the PMNS mixing matrix is no more difficult than understanding the Higgs mechanism (in fact it’s easier!) It’s just that there’s no nice nomenclature to help spread the news, and that’s something that we need to fix as soon as possible.

As a side note, $$\theta_{13}$$ is important because it tells us about how the neutrinos mix. Neutrino mixing is beyond the Standard Model physics, so we should be getting more excited about it! If $$\theta_{13}$$ is non-zero then that means that we can put another term into the matrix and this fourth term is what gives us matter-antimatter asymmetry in the lepton sector, helping to explain why we still have matter hanging around in the universe, why we have solid things instead of just heat and light. Put like that is sounds more interesting and newsworthy, but that can’t be squeezed into a tweet, let alone a hashtag. It’s a shame that result didn’t get more attention.

It’s great fun and a fine challenge to be part of this whole process. We are co-creators, exploring the new media together. Nobody knows what will work in the near future, but we can look back what has already worked, and see how people passed on the news. Making news no longer stops once I hit “Publish”, it echoes around the world, through your tweets, and reblogs, and we can see its journey. If we’re lucky it gets passed on enough to go viral, and then it’s out of our control. It’s this kind of interactivity that it so rewarding and engaging.

You can read the New Scientist article or the original paper on the arXiV.

### Everytime a Belle Rings, A Hadron Gets Its Wings

Friday, January 27th, 2012

Fun post for everyone today. In response to last week’s post on describing KEK Laboratory’s discovery of additional exotic hadrons, I got an absolutely terrific question from a QD reader:

Surprisingly, the answer to “How does an electron-positron collider produce quarks if neither particle contains any?” all begins with the inconspicuous photon.

## No Firefox, I Swear “Hadronization” is a Real Word.

As far as the history of quantum physics is concerned, the discovery that all light is fundamentally composed of very small particles called photons is a pretty big deal. The discovery allows us to have a very real and tangible description of how light and electrons actually interact, i.e., through the absorption or emission of photon by electrons.

Figure 1: Feynman diagrams demonstrating how electrons (denoted by e) can accelerate (change direction of motion) by (a) absorbing or (b) emitting a photon (denoted by the Greek letter gamma: γ).

The usefulness of recognizing light as being made up many, many photons is kicked up a few notches with the discovery of anti-particles during the 1930s, and in particular the anti-electron, or positron as it is popularly called. In summary, a particle’s anti-particle partner is an identical copy of the particle but all of its charges (like electric, weak, & color!) are the opposite. Consequentially, since positrons (e+) are so similar to electrons (e) their interactions with light are described just as easily.

Figure 2: Feynman diagrams demonstrating how positrons (e+) can accelerate (change direction of motion) by (a) absorbing or (b) emitting a photon (γ). Note: positrons are moving from left to right; the arrow’s direction simply implies that the positron is an anti-particle.

Then came Quantum Electrodynamics, a.k.a. QED, which gives us the rules for flipping, twisting, and combining these diagrams in order to describe all kinds of other real, physical phenomena. Instead of electrons interacting with photons (or positrons with photons), what if we wanted to describe electrons interacting with positrons? Well, one way is if an electron exchanges a photon with a positron.

Figure 3: A Feynman diagram demonstrating the exchange of a photon (γ) between an electrons (e)  and a positron (e+). Both the electron and positron are traveling from the left to the right. Additionally, not explicitly distinguishing between whether the electron is emitting or absorbing is intentional.

And now for the grand process that is the basis of all particle colliders throughout the entire brief* history of the Universe. According to electrodynamics, there is another way electrons and positrons can both interact with a photon. Namely, an electron and positron can annihilate into a photon and the photon can then pair-produce into a new electron and positron pair!

Figure 4: A Feynman diagram demonstrating  an annihilation of an electrons (e)  and a positron (e+) into a photon (γ) that then produces an e+e pair. Note: All particles depicted travel from left to right.

However, electrons and positrons is not the only particle-anti-particle pair that can annihilate into photons, and hence be pair-produced by photons. You also have muons, which are identical to electrons in every way except that it is 200 times heavier than the electron. Given enough energy, a photon can pair-produce a muon and anti-muon just as easily as it can an electron and positron.

Figure 5: A Feynman diagram demonstrating  an annihilation of an electrons (e)  and a positron (e+) into a photon (γ) that then produces a muon (μ) and anti-muon(μ+) pair.

But there is no reason why we need to limit ourselves only to particles that have no color charge, i.e., not charged under the Strong nuclear force. Take a bottom-type quark for example. A bottom quark has an electric charge of -1/3 elementary units; a weak (isospin) charge of -1/2; and its color charge can be red, blue, or green. The anti-bottom quark therefore has an electric charge of +1/3 elementary units; a weak (isospin) charge of +1/2; and its color charge can be anti-red, anti-blue, or anti-green. Since the two have non-zero electric charges, it can be pair-produced by a photon, too.

Figure 6: A Feynman diagram demonstrating  an annihilation of an electrons (e)  and a positron (e+) into a photon (γ) that then produces a bottom quark (b) and anti-bottom quark (b) pair.

On top of that, since the Strong nuclear force is, well, really strong, either the bottom quark or the anti-bottom quark can very easily emit or absorb a gluon!

Figure 7: A Feynman diagram demonstrating  an annihilation of an electrons (e)  and a positron (e+) into a photon (γ) that produces a bottom quark (b) and anti-bottom quark (b) pair, which then radiate gluons (blue).

In electrodynamics, photons (γ) are emitted or absorbed whenever an electrically charged particle changes it direction of motion. And since the gluon in chromodynamics plays the same role as the photon in electrodynamics, a gluon is emitted or absorbed whenever  a “colorfully” charged particle changes its direction of motion. We can absolutely take this analogy a step further: gluons are able to pair-produce, just like photons.

Figure 8: A Feynman diagram demonstrating  an annihilation of an electrons (e)  and a positron (e+) into a photon (γ) that produces a bottom quark (b) and anti-bottom quark (b) pair. These quarks then radiate gluons (blue), which finally pair-produce into quarks.

At the end of the day, however, we have to include the effects of the Weak nuclear force. This is because electrons and quarks have what are called “weak (isospin) charges”. Firstly, there is the massive Z boson (Z), which acts and behaves much like the photon; that is to say, an electron and positron can annihilate into a Z boson. Secondly, there is the slightly lighter but still very massive W boson (W), which can be radiated from quarks much like gluons, just to a lesser extent. Phenomenally, both Weak bosons can decay into quarks and form semi-stable, multi-quark systems called hadrons. The formation of hadrons is, unsurprisingly, called hadronization. Two such examples are the the π meson (pronounced: pie mez-on)  or the J/ψ meson (pronounced: jay-sigh mezon). (See this other QD article for more about hadrons.)

Figure 9: A Feynman diagram demonstrating  an annihilation of an electrons (e)  and a positron (e+) into a photon (γ) or a Z boson (Z) that produces a bottom quark (b) and anti-bottom quark (b) pair. These quarks then radiate gluons (blue) and a W boson (W), both of which finally pair-produce into semi-stable multi-quark systems known as hadrons (J/ψ and π).

In summary, when electrons and positrons annihilate, they will produce a photon or a Z boson. In either case, the resultant particle is allowed to decay into quarks, which can radiate additional gluons and W bosons. The gluons and W boson will then form hadrons. My friend Geoffry, that is how how you can produce quarks and hadrons from electron-positron colliders.

Now go! Discuss and ask questions.

Happy Colliding

– richard (@bravelittlemuon)

* The Universe’s age is measured to be about 13.69 billion years. The mean life of a proton is longer than 2.1 x 1029 years, which is more than 15,000,000,000,000,000,000 times the age of the Universe. Yeah, I know it sounds absurd but it is true.

### Neutrinos: The Great Asterisk of the Standard Model

Monday, October 31st, 2011

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)

## 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.
• 1911β-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).
• 1934Enrio 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]

### DPF 2011, tweet tweet!

Friday, August 5th, 2011

I know, I know, everyone has been focusing on the EPS and Lepton-Photon conferences (not to mention repeatedly putting in hyperlinks to their Web sites), but let’s not forget that the 2011 Meeting of the Division of Particles and Fields of the American Physical Society (DPF 2011, for short) starts this coming Tuesday. This will be the largest conference exclusively focused on particle physics in the United States this year, and it’s organized by the nation’s grass-roots membership organization of physicists, the APS. There are currently more than 450 people registered, so a large slice of the US particle-physics community will be there. This will be the fourth time that I’ve been to a DPF meeting, and I really do enjoy them — they are large enough to cover a broad range of topics, yet still small enough that you don’t get lost in the crowd.

For the first time ever, I find myself giving two presentations at the same conference — one on behalf of the CMS Collaboration (on the status of our distributed computing operation) and one on behalf of the D0 Collaboration at the Tevatron (on measurements of spin correlations in top-antitop production). On top of that, I’m also co-organizing a lunchtime panel discussion on “physics and the modern media.” What you are reading right now is a form of modern media, of course. We’re going to be talking with science journalists, communicators and bloggers about where communication about science is going…and what these sorts of people think of each other!

Since we’re going to talk about modern media, we figured that we should jump in with both feet, and that means Twitter. I must admit that I haven’t done much Twitter (although I do now have an account), but it seems to be all the rage. So, we’re encouraging Twitter users who will be attending the conference, and those who aren’t but want to keep up with what’s going on, to tweet away using the hash tag #DPF2011. If you are interested in the modern-media panel, feel free to tweet to us on Tuesday at 12:30 PM Eastern time; we’ll be keeping an eye on the feed and relaying interesting comments and questions to the panel.

More next week from fabulous Providence, Rhode Island!

### Weighing Antimatter

Thursday, July 28th, 2011

How much does antimatter weigh?

It is a great question and to be honest physicists don’t know. In fact, it is a great question precisely because we don’t know. To clarify: I am talking about “weight,” not “mass.” I wrote a few words at the bottom of this post about the difference between the two. For now I will just say that mass is what makes pushing or pulling something in a new direction harder; weight is that pull, by a planet’s gravity, on things that have mass. In the Universe, there are some kinds of matter that do not have mass, like photons (packets of light), and thus are also weightless. Other kinds of matter, like protons & electrons, do have mass and consequentially weigh something.

Figure 1: CERN’s Atomic Spectroscopy And Collisions Using Slow Antiprotons (ASACUSA) Experiment. (Photo: CERN)

Okay, so here is where things get interesting. Back in the 1920’s a guy named Paul Dirac discovered the theory of antimatter.  The theory not only predicted that each piece of matter has an “antimatter partner” but also that the two partners have the same mass. This morning, the ASACUSA Experiment (Fig. 1) at CERN announced that the anti-proton has the same mass as its partner, the proton. Well, at least up to experiment’s capabilities of resolving the two. Anyone keeping track of CERN’s anti-matter physics program, or has watched the first 15 minutes of “Demons & Angles,” might know that the lab has been making significant progress trapping and collecting anti-hydrogen. While the amount being produced at CERN may not be enough to make a small city-state disappear, it is close to the amount needed to determine the weight of anti-hydrogen. This is good news for physicists at Fermilab who are working on the Antimatter Gravity Experiment (AGE), the goal of which is to measure anti-hydrogen’s weight. Interesting, no?

Figure 2: A hydrogen atom consists of an electron and a proton orbiting around one another, and are kept together because of their mutual electric attraction. Similarly, an anti-hydrogen atom consists of a positron (anti-electron) and an anti-proton. (Image: Wikipedia)

Now for the exciting part. Our theories, e.g. the time-tested Standard Model of Physics, only say that matter-antimatter partners should have the same mass. There is NO reason whatsoever, other than helping one sleep at night, that the partners should have the same weight. Since weight is innately related to gravity, any measurement of an individual anti-particle’s weight is inherently a measurement of gravity at the quantum scale. Additionally, any description of the behavior of antimatter acting under gravity is at the very least a stepping stone to Quantum Gravity. Quantum Gravity, by the way, is the theory of gravity at the microscopic scale; it does not really exist, yet; and is preventing physicists from constructing a full description (theory) of our universe. Determining that the proton and anti-proton have the same mass makes it easier to spot any differences in their weight. On top of that, if there is a difference in the weight of hydrogen & anti-hydrogen, then it might also explain why there is so much more matter in the universe than antimatter.

If you are not excited by now, I give up. 🙂 Note: A big thanks to @symmetrymag for bringing this news to my attention.

### A Few Words on Mass vs. Weight

Physically, “inertia” is the natural resistance to a change in movement; a measurement of inertia is called “mass.” One way to think about mass is if you & I were running down a football pitch, side-by-side, and you tried pushing me over. Mass is that bit of resistance you feel when you try pushing me over. If I were twice as tall, it would be harder to push me over. If I were half as tall, it would be easier to push me over. Next time you are playing football, like right after you read this Quantum Diaries post, try it out. “Weight” is that specific, attractive pull (force) a planet has on an object. The big difference is that mass is universal property of an object whereas weight can vary. A single electron will always have the same mass but a human will weigh less and less the further away he/she is from the Earth. Since this rock I like to call home is approximately a sphere, the gravitational pull it has at its surface is approximately constant. Consequentially, the difference between 1 lb (a unit of force) and 1 kg (a unit of mass) is a numerical constant. I hope this helped.

Happy Colliding.

– richard (@bravelittlemuon)

### Science in Space: When an End is not The End.

Tuesday, July 12th, 2011

Hi, All.

It’s less than two weeks old but July has been a very eventful month for American science and the beginning of a very busy month for me. Those following my Twitter account (@bravelittlemuon) this past weekend learned pretty quickly that I was live-tweeting the Space Shuttle Atlantis’ final launch from the Kennedy Space Center (KSC) as a part of NASA’s phenomenal #NasaTweetup program. In summary, NASA invited 150 followers of its @NasaTweetup account to get a once-in-a-lifetime opportunity to visit KSC and get the VIP treatment on the condition that for 48 hours all we did was tweet. Seeing the space shuttle from about 1500 feet and talking with an astronaut on board the International Space Station (ISS) about the Alpha Magnetic Spectrometer (AMS) was really, really, cool. Like really cool… and all in the name of public outreach†. I tip my many hats to NASA for a job well done.

The Space Shuttle Atlantis is just about to break the sound barrier (Photo mine). Click for the high-res version.

One thing that caught me off guard this weekend was how many times I was asked, “As a scientist, are you worried that the shuttle fleet’s retirement means the end of science in space?” I grin whenever I hear that question because if anything NASA is just getting started. The AMS detector, for example, is an honest-to-goodness particle detector that was built at CERN and installed on the ISS during a previous shuttle mission (STS-134). Its purpose is to measure the relative abundances of matter & antimatter, as well as test dark matter models. The new SUV-sized Mars rover, Curiosity, is expected to launch later this year and will be able to measure the composition of Martian rocks and boulders thanks its shoulder-mounted laser. (Personally, I say  we rename it “Johnny V.”) By knowing the precise composition of Martian soil we will learn if the ground was (still) able to support vegetation. Long gone are the days of experimenting with ants in micro-gravity considering that vegetables are now grown on the space station. I was told by NASA science coordinators about the half dozen ISS projects currently in the pipeline (read: proposals not publicly available, yet), one of which included an artificial gravity experiment.

NASA is getting out of the ferrying business, so what? Consider this: these are the people who stuck a couple of humans on the moon because some guy dared them††. After that, these same people (and their international counterparts!) built a space station. A SPACE STATION! With all due respect, I think NASA’s time is better spent sending people to Mars or another star system. FTL drives, anyone? So if anyone tells you that the Space Administration is past its prime, just send them over to its Current Missions web page. By the way, there is a telescope (Kepler) currently looking for habitable planets outside our solar system. I will not even begin to go into all the practical applications that have resulted from space research. Additionally to our American readers, if you feel NASA should doing more science tell your representatives in Congress; I’ve done it.

A picture of the Space Shuttle Atlantis I took fewer than 24 hours before its launch. Click for the high-res version.

As I mentioned at the top, July is a very busy month for me. I actually wrote the draft of this post somewhere over Kentucky/Tennessee on my way back to Madison to attend the “Coordinated Theoretical-Experimental Project on QCD Summer School on QCD Analysis,” or CTEQ for short. Quantum Chromodynamics (QCD) is what we call the theory of the Strong Nuclear Force; it explains why protons and neutrons behave the way they do. Expect something soon about the fact that particle physicists like to spend their summers indoors, or in Aspen.

† You can read more about Science Outreach in a previous QD post, here.

†† Okay, this guy may have also been the President of The United States.