## Posts Tagged ‘CKM’

### How to discover new physics

Saturday, June 2nd, 2012

The biggest news at CIPANP 2012 for particle physicists seems to be coming from the “low” energy frontier, at energies in the ballpark of 10GeV and lower. This may come as a surprise to some people, after all we’ve had experiments working at these energies for a few decades now, and there’s a tendency to think that higher energies mean more potential for discovery. The lower energy experiments have a great advantage over the giants at LHC and Tevatron, and this is richer collection of analyses.

There’s a big difference between discovering a new phenomenon and discovering new physics, which is something that most people (including physicists!) don’t appreciate enough. Whenever a claim of new physics is made we need to look at the wider implications of the idea. For example, let’s say that we see the decay of a $$\tau$$ lepton to an proton and a $$\pi^0$$ meson. The Feynman diagram would look something like this:

tau lepton decay to a proton and a neutral pion, mediated by a leptoquark

The “X” particle is a leptoquark, and it turns leptons into quarks and vice versa. Now for this decay to happen at an observable rate we need something like this leptoquark to exist. There is no Standard Model process for $$\tau\to p\pi^0$$ since it violates baryon number (a process which is only allowed under very special conditions). So suppose someone claims to see this decay, does this mean that they’ve discovered new physics? The answer is a resounding “No”, because if they make a claim of new physics they need to look elsewhere for similar effects. For example, if the leptoquark existed the proton could decay with this process:

proton decay to an electron and neutral pion, mediated by a leptoquark

We have very stringent tests on the lifetime of the proton, and the lower limits are currently about 20 orders of magnitude longer than the age the universe. Just take a second to appreciate the size of that limit on the lifetime. The proton lasts for at least 20 orders of magnitude longer than the age of the universe itself. So if someone is going to claim that they have proven the leptoquark exists we need to check that what they have seen is consistent with the proton lifetime measurements. A claim of new physics is stronger than a claim of a new phenomena, because it must be consistent with all the current data, not just the part we’re working.

How does all this relate to CIPANP 2012 and the low energy experiments? Well it turns out that there are a handful of large disagreements in this regime that all tend to involve the same particles. The $$B$$ meson can decay to several lighter particles and the BaBar experiment has seen the decays to the $$\tau$$ lepton are higher than they should be. The disagreement is more than $$3\sigma$$ disagreement with the Standard Model predictions for $$B\to D^{(*)}\tau\nu$$, which is interesting because it involves the heaviest quarks in bound states, and the heaviest lepton. It suggests that if there is a new particle or process, that it favors coupling to heavy particles.

Standard model decays of the B mesons to τν, Dτν, and D*τν final states

In another area of $$B$$ physics we find that the branching fraction $$\mathcal{B}(B\to\tau\nu)$$ is about twice as large as we expect from the Standard Model. You can see the disagreement in the following plot, which compares two measurements ($$\mathcal{B}(B\to\tau\nu)$$ and $$\sin 2\beta$$) to what we expect given everything else. The distance between the data point and the most favored region (center of the colored region) is very large, about $$3\sigma$$ in total!

The disagreement between B→τν, sin2β and the rest of the unitary triangle measurements (CKMFitter)

Theorists love to combine these measurements using colorful diagrams, and the best known example is the unitary triangle. If the CKM mechanism describes all the quark mixing processes then all of the measurements should agree, and they should converge on a single apex of the triangle (at the angle labeled $$\alpha$$). Each colored band corresponds to a different kind of process, and if you look closely you can see some small disagreements between the various measurements:

The unitary triangle after Moriond 2012 (CKMFitter)

The blue $$\sin 2\beta$$ measurement is pulling the apex down slightly, and green $$|V_{ub}|$$ measurement is pulling it in the other direction. This tension shows some interesting properties when we try to investigate it further. If we remove the $$\sin 2\beta$$ measurement and then work out what we expect based on the other measurements, we find that the new “derived” value of $$\sin 2\beta$$ is far off what is actually measured. The channel used for analysis of $$\sin 2\beta$$ is often called the golden channel, and it has been the main focus of both BaBar and Belle experiments since their creation. The results for $$\sin2\beta$$ are some of the best in the world and they have been checked and rechecked, so maybe the problem is not associated with $$\sin 2\beta$$.

Moving our attention to $$|V_{ub}|$$ the theorists at CKMFitter decided to split up the contributions based on the semileptonic inclusive and exclusive decays, and from $$\mathcal{B}(B\to\tau\nu)$$. When this happens we find that the biggest disagreement comes from $$\mathcal{B}(B\to\tau\nu)$$ compared to the rest. The uncertainties get smaller when $$\mathcal{B}(B\to\tau\nu)$$ is combined with the $$B$$ mixing parameter, $$\Delta m_d$$, which is well understood in terms of top quark interactions, but these results still disagree with everything else!:

Disagreement between B→τν, Δmd and the rest of the unitary triangle measurments (CKMFitter)

What this is seeming to tell us is that there could be a new process that affects $$B$$ meson interactions, enhancing decays with $$\tau$$ leptons in the final state. If this is the case then we need to look at other processes that could be affected by these kinds of processes. The most obvious signal to look for at the LHC is something like production of $$b$$ quarks and $$\tau$$ leptons. Third generation leptoquarks would be a good candidate, as long as they cannot mediate proton decay in any way. Searching for a new particle of a new effect is the job of the experimentalist, but creating a model that accommodates the discoveries we make is the job of a theorist.

That, in a nutshell is the difference between discovering a new phenomenon and discovering new physics. Anyone can find a bump in a spectrum, or even discover a new particle, but forming a consistent model of new physics takes a long time and a lot of input from all different kinds of experiments. The latest news from BaBar, Belle, CLEO and LHCb are giving us hints that there is something new lurking in the data. I can’t wait to see wait to see what our theorist colleagues do with these measurements. If they can create a model which explains anomalously high branching fractions $$\mathcal{B}(B\to\tau\nu)$$, $$\mathcal{B}(B\to D\tau\nu)$$, and $$\mathcal{B}(B\to D^*\tau\nu)$$, which tells us where else to look then we’re in for an exciting year at LHC. We could see something more exciting than the Higgs in our data!

(CKMFitter images kindly provided by the CKMfitter Group (J. Charles et al.), Eur. Phys. J. C41, 1-131 (2005) [hep-ph/0406184], updated results and plots available at: http://ckmfitter.in2p3.fr)

Sunday, October 2nd, 2011

Update I: Included Medicine Award (Oct 03)

Update II: Included Physics Award (Oct 04)

… it’s Nobel Week! October means three things: Halloween (duh), Fall, and Nobel Week, the week during which the famed prizes are awarded to those who have “conferred the greatest benefit on mankind” [1]. Okay, before I get comments about the subjectivity of those who award the prizes, I gladly admit that the history of the prize is not without controversy relating to those who have & have not won, in both the science and non-science categories.

I am just going to ignore all of that and talk about why everyone should be excited about this week. Though before I talk about this week’s Nobels, I feel I should probably give the SparkNotes version of the prizes’ history.

Figure 1: The 2008 Chemistry Prize was awarded for the discovery and development of green fluorescent protein (GFP), which when inserted into a soon-to-be parent is passed onto an offspring who can then glow green. Glowing cat!
(Image: The Nobel Foundation)

## A Brief History of Alfred Nobel

Figure 2: Alfred Nobel. (Image: The Nobel Foundation)

The year is 1866, the Second Industrial Revolution is raging, innovation is surging, and the US Civil War over.

Insert Alfred Nobel: A son of a successful engineer who developed controlled explosives for the demolition and mining industries. The younger Nobel, unsurprisingly, decided be a chemist after playing with nitroglycerin in a French laboratory. As a public service announcement, I should probably mention that nitroglycerin is very dangerous and is a principle ingredient in dynamite. In fact, Nobel was so convinced that nitroglycerine had useful application in construction that he decided to invent dynamite. Needless to say, dynamite made Nobel a very, very, very rich man. At the end of his life, he decided to endow, with the bulk of his fortune, a set of prizes to recognize those who have contributed greatest in the Fields of Medicine, Physics, Chemistry, Literature, and Peace. Economics, though not stipulated in the original will, was added later and is funded separately.

Figure 2: The chemical structure of nitroglycerin. This stuff is wicked; the physical chemistry behind its structure worth a gander. Consider this an advertisement to go earn a chemistry degree. (Image: Wikipedia)

## What Makes a Prize

The Nobels has come a long way since they were first instituted. Most notably, they no longer are awarded for the greatest discovery or invention from the past year; the prizes now award those results with the most lasting influence and impact. Take last year for example. The 2011 award for Physiology or Medicine went solely to Sir Robert Edwards for having developed in vitro fertilization. You would think something that is, in every sense of the word, responsible for the existence of millions of people would have been awarded long, long ago. I mean, that is what went through my mind last October. Therein lies the novelty of the Nobel Prizes: These days, the awards are given to what seem like common knowledge, because in some sense they are. What one has to realize though is that prior a laureate’s discovery or invention, these ideas and concepts just did not exist. Imagine a world in which no one knew of insulin (Nobel 1923). Weird, no?

This brings me to why Nobel Week is so much fun. Sometimes you know quite a bit about the award-winning discovery and so you get to spend the day reading news articles and science blogs learning all about the topic’s history. Werner Forssmann’s invention of the cardiac catheter (Nobel 1953) has a hysterical history that is well worth a read. At other times, you have no idea what the award citation even means, but you just know it is worth spending a few minutes or even a few hours learning. I mean, why else would a Nobel be awarded? Take, as another example, 2008’s Physics prize. The award citation reads:

“… for the discovery of the mechanism of spontaneous broken symmetry in subatomic physics,” [2]

and

“for the discovery of the origin of the broken symmetry
which predicts the existence of at least three families of quarks in nature
.” [2]

Yup, it is a mouthful and probably seems a bit obtuse. That is, until you start looking up Wikipedia or news articles (or Quantum Diaries!), and realize how amazingly awesome these discoveries are. I mean, sure discovering spontaneous symmetry breaking (SSB) sounds nice and fancy but did you know that is why the bosons in the Standard Model of Physics have the masses they do?!? SSB, when applied specifically to the Electroweak bosons (photon, W, & Z) is the Higgs Mechanism, and when applied to fermions, is what generates the higgs boson. SSB is an established scientific fact and is also the driving force behind superconductivity (Nobel 1972) Whether or not the higgs boson exists, however, is completely different story.

Figure 3: The quark sector of the Standard Model of Particle Physics and their discovery dates. (Image: Nobel Foundation)

So back in 1977 a Fermilab team, led by Leon Lederman, discovered the bottom quark (Nobel 1988), and in 1995, the CDF & DZero Tevatron experiments discovered the top quark. Ever wonder how we knew to look for them in the first place? It was because of something called the CKM matrix. It was introduced as a way of organizing the the different ways particles in the Standard Model could interact and decay. However, as gorgeous as this new organization was, in order to work the CKM matrix required the existence of two new quarks. Well guess what, Fermilab found those two quarks and set the Standard Model in stone.

The 2009 Nobel Prizes are equally impressive. Half the prize was awarded for the development of fiber optics, which is the foundation of modern telecommunications, and something called Charged-Coupled Devices (CCD). What took me a few hours to learn is that if you take this sensor, attach a flashbulb, a battery, and maybe a memory card, you get a digital camera. In other words, half the 2009 prize was awarded for inventing the digital camera. The prize winners were simply trying to develop a better way of storing data and inadvertently created an entire industry. A fun fact: the first transistor (Nobel 1967) was made of paperclips. If you are curious about what makes transistors so important, take apart your computer and take a peek. (Please, make sure the computer is unplugged before opening it.)

## Does Every Major Discovery/Invention Get a Prize?

No. First off, Nobel Prizes are no longer awarded posthumously. Secondly, from my discussions about this issue, there seems to be a consensus there may be a limit to what is & is not awarded when it comes to the sciences. Now the Swedish Academies always reserve the right to set a new precedent, however, it is unlikely that any organizations will be awarded a Nobel in science categories anytime soon. (This is the complete opposite for the Peace Prize, of course.) What does this all mean? Well, the top quark was a pretty heavy discovery and is well worth its weight in gold, at least in my opinion. However, to whom would you award the prize? No single person at the CDF experiment can justly say she or he discovered the quark; it was a team effort and all CDF personnel can proudly state she or he helped discover the quark.

“Which of the Gang of Six, if the higgs boson is discovered, should get the Nobel, if at all?” is an honest, open question and is well above my pay grade. A similar statement could be made about Supersymmetry.

## Turning Nobel Week into Fun-bel Week

Now for the fun part. So during this week, pick your favorite subject, which of course is physics, and go figure out what the whole big hubbub is. Depending on your timezone, this may either be with your morning coffee or afternoon tea. In any case, it is an excuse to learn something new! 🙂

Alternatively, you can check back here Tuesday afternoon (Madison/Chicago time) because I am sure many of us will be commenting on the latest news.

## This Week’s Schedule

Live Video Player here.

Physiology or Medicine – Awarded for the discovery of the innate and adaptive immune systems! Okay, really this is great. The human body has evolved to be inherently immune to certain pathogens. The human body, in its resourcefulness, can also adapt and become immune to pathogens. The end result is that when the two are combined and wait a few hundred thousand years,  you get us!

Physics – Awarded for discovering that expansion rate of the universe, is itself increasing. The universe expands, Edwin Hubble discovered that decades ago. Today’s award winners discovered that the universe expands at an accelerating rate! Bravo!

Chemistry – The prize will be announced on Wednesday 5 October, 11:45 a.m. CET [5:45 am  CDT/Chicago].

Peace – The prize will be announced on Friday 7 October, 11:00 a.m. CET [5:00 am  CDT/Chicago].

Economics – The prize will be announced on Monday 10 October, 1:00 p.m. CET [7:00 am  CDT/Chicago].

Literature – To Be Announced

Regardless of the outcome, I would love to read everyone’s thoughts and speculations before and after the awards!

Happy Colliding

– richard (@bravelittlemuon)

### Symmetry in Physics, Pt. 3: Internal Symmetries

Monday, October 12th, 2009

Now time for another installment of “symmetry in physics.” For those of you tuning in late (or who have forgotten what we’ve been discussing), we started out in part 1 with a very general discussion of the symmetries of spacetime and how this constrains the form of our theories. Next, in part 2 we looked at discrete symmetries and how they relate the notion of antimatter to charge and parity conjugation. We’ll be using some of the jargon of part 2, so make sure you brush up and remember what “CP” means. Now we’d like to address another mystery of the Standard Model: why is there so much repetition?

Family Symmetry

Let’s review the matter content of the Standard Model:

The top two rows are quarks, the bottom two are leptons (charged leptons and neutrinos). Each row has a different electric charge. The top row has charge +2/3, the 2nd row has charge -1/3, the third row has charge -1, and the last has charge 0. As discussed in part 2, there are also the corresponding anti-particles with opposite charges [note 1]. Just about all of the matter that you’re used to is made up of only the first column. All atoms and everything they’re made of are more-or-less completely composed of up and down quarks and electrons (the neutrinos haven’t done much since early in the universe).

The replication of the structure of the first column is known as family symmetry. For each particle in the first column, there are two other particles with nearly the same properties. In fact, they would have exactly the same properties, except that they are sensitive to the Higgs field in different ways so that the copies end up having heavier masses. Technically the Higgs discriminates between different generations and breaks this symmetry, but we are still left with the question: why are there two other families of matter?