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Posts Tagged ‘Belle’

Testing theory…

Sunday, August 26th, 2012

As I discussed a BaBar result previously, it only seemed fair that I spend a post discussing a Belle one. For those of you who only associate the words BaBar and Belle to cartoon characters, they are also the names of two competing \(B\) physics experiments, both of which have finished data taking but are still producing results.

So which Belle result have I decided to discuss today? I’m going to talk about the updated measurement of the \(B^- \rightarrow \tau^- \overline{\nu}_\tau\) branching ratio that was first presented by Youngmin Yook in a parallel session at ICHEP and now can be found on arXiv.

Why would I choose this measurement you ask? Let’s have a look at the Feynmann diagram of the process on the right here. In the Standard Model, the decay can only proceed via the exchange of a \(W^-\) boson and so the branching ratio can be translated to a measurement of \(V_{ub}\), one of the CKM quark mixing matrix elements. However, new physics could significantly modify the branching ratio via the exchange of a new charged particle, like a charged Higgs boson.

An updated result of the branching ratio is even more interesting than that though, because the average of the previous consistent experimental measurements from Belle and BaBar, \((1.67 \pm 0.30)\times10^{-4}\), is higher than the prediction from CKM fit, \((0.733^{+0.121}_{-0.073})\times10^{-4}\) and the Standard Model \((1.2 \pm 0.25)\times10^{-4}\). This is what is shown on the left here, where the blue point is the average of the previous results, and the green area is the CKM fit prediction. Could this be due to new physics?

Experimentally, it is quite difficult to measure \(B^- \rightarrow \tau^- \overline{\nu}_\tau\) decays, due to the multiple undetectable neutrinos in the final state (as well as the one from the \(B^-\) decay, there is also at least one from the \(\tau^-\) decay). In fact, I’m pretty sure that we can’t perform this measurement at LHCb at all.

Belle and BaBar are able to as their \(B\) meson pairs are produced through the well defined process \(e^+e^− \rightarrow \Upsilon(4S) \rightarrow B\overline{B}\) and their detectors cover a larger solid angle, which allows them to make a fairly accurate estimate of neutrinos produced in decays. To the right, here is a plot of the extra detected energy in selected events, where the points are the data, the red dotted line shows the signal, the dashed blue line shows the background and the red solid line shows the total fit. They expect the signal to peak at zero, since neutrinos can’t be detected.

For the full details of the analysis, I encourage you all to look at the paper, here I’m only going quote the result: \([0.72^{+0.27}_{-0.25}(stat) \pm 0.11(syst)] \times 10^{−4}\) and then discuss the implications…

Firstly, does this new result bring the experimental average closer or further away from the predictions? As presented by Mikihiko Nakao in a plenary session at ICHEP, the plot below shows that the new Belle average (bottom blue point) and the new experimental average (red point) are both consistent with the CKM fit and Standard model predictions (pink and yellow bands respectively). So no hint for new physics here…

Secondly, since this result doesn’t seem to point to new physics, what does it say about \(V_{ub}\), the Standard Model parameter describing the mixing between the \(u\) and \(b\) quarks? As presented by Phillip Urquijo, also in a plenary session at ICHEP, below is a comparison of the various measurements of \(V_{ub}\), which has historically been an area of \(B\) physics which requires further investigation. This is because there are two different methods to measure \(V_{ub}\), called inclusive and exclusive, depending on what type of \(B\) decays are used, and there is currently a discrepancy between the two, which people have been trying to understand. And interestingly… the \(V_{ub}\) measurement from \(B^- \rightarrow \tau^- \overline{\nu}_\tau\) is in agreement with both methods…

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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.

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Hi All,

Exciting news came out the Japanese physics lab KEK (@KEK_jp, @KEK_en) last week about some pretty exotic combinations of quarks and anti-quarks. And yes, “exotic” is the new “tantalizing.” At any rate, I generally like assuming that people do not know much about hadrons so here is a quick explanation of what they are. On the other hand, click to jump pass “Hadrons 101” and straight to the news.

Hadrons 101: Meeting the Folks: The Baryons & Mesons

Hadrons are pretty cool stuff and are magnitudes more quirky than those quarky quarks. The two most famous hadrons, the name for any stable combination of quarks and anti-quarks, are undoubtedly the proton and the neutron:

According to our best description of hadrons (Quantum Chromodynamics), the proton is effectively* made up two up-type quarks, each with an electric charge of +2/3 elementary charges**; one down-type quark, which has an electric charge of -1/3 elementary charges; and all three quarks are held together by gluons, which are electrically neutral. Similarly, the neutron is effectively composed of two down-type quarks, one up-type quark, and all the quarks are held strongly together by gluons. Specifically, any combination of three quarks or anti-quarks is called a baryon. Now just toss an electron around the proton and you have hydrogen, the most abundant element in the Universe! Bringing together two protons, two neutrons, and two electrons makes helium. As they say, the rest is Chemistry.

However, as the name implies, baryons are not the only type of hadrons in town. There also exists mesons, combinations of exactly one quark and one anti-quark. As an example, we have the pions (pronounced: pie-ons). The π+ (pronounced: pie-plus) has an electric charge of +1 elementary charges, and consists of an up-type quark & an anti-down-type quark. Its anti-particle partner, the π (pronounced: pie-minus), has a charge of -1, and is made up of an anti-up-type quark & a down-type quark.

 

If we now include heavier quarks, like strange-type quarks and bottom-type quarks, then we can construct all kinds of baryons, mesons, anti-baryons, and anti-mesons. Interactive lists of all known mesons and all known baryons are available from the Particle Data Group (PDG)***. That is it. There is nothing more to know about hadrons, nor has there been any recent discovery of additional types of hadrons. Thanks for reading and have a great day!

 

* By “effectively,” I mean to ignore and gloss over the fact that there are tons more things in a proton, like photons and heavier quarks, but their aggregate influences cancel out.

** Here, an elementary charge is the magnitude of an electron’s electron charge. In other words, the electric charge of an electron is (-1) elementary charges (that is, “negative one elementary charges”). Sometimes an elementary charge is defined as the electric charge of a proton, but that is entirely tautological for our present purpose.

*** If you are unfamiliar with the PDG, it is arguably the most useful site to high energy physicists aside from CERN’s ROOT user guides and Wikipedia’s Standard Model articles.

The News: That’s Belle with an e

So KEK operates a super-high intensity electron-positron collider in order to study super-rare physics phenomena. It’s kind of super. Well, guess what. While analyzing collisions with the Belle detector experiment, researchers discovered the existence of two new hadrons, each made of four quarks! That’s right, count them: 1, 2, 3, 4 quarks! In each case, one of the four quarks is a bottom-type quark and another is an anti-bottom quark. (Cool bottom-quark stuff.) The remaining two quarks are believed to be an up-type quark and an anti-down type quark.

The two exotic hadrons have been named Zb(10610) and Zb(10650). Here, the “Z” implies that our hadrons are “exotic,” i.e., not a baryon or meson, the subscript “b” indicates that it contains a bottom-quark, and the 10610/10650 tell us that our hadrons weigh 10,610 MeV/c2 and 10,650 MeV/c2, respectively. A proton’s mass is about 938 MeV/c2, so both hadrons are about 11 times heavier than the proton (that is pretty heavy). The Belle Collaboration presser is really great, so I will not add much more.

Other Exotic Hadrons: When Barry met Sally.

For those keeping track, the Belle Collaboration’s recent finding of two new 4-quark hadrons makes it the twelfth-or-so “tetra-quark” discovery. What makes this so special, however, is that all previous tetra-quarks have been limited to include a charm-type quark and an anti-charm-type quark. This is definitely the first case to include bottom-type quarks, and therefore offer more evidence that the formation of such states is not a unique property of particularly charming quarks but rather a naturally occurring phenomenon affecting all quarks.

Furthermore, it suggests the possibility of 5-quark hadrons, called penta-quarks. Now these things take the cake. They are a sort of grand link between elementary particle physics and nuclear physics. To be exact, we know 6-quark systems exist: it is called deuterium, a radioactive stable isotope of hydrogen (Thanks to @incognitoman for pointing out that deuterium is, in fact, stable.). 9-quark systems definitely exist too, e.g., He-3 and tritium. Etc. You get the idea. Discovering the existence of five-quark hadrons empirically establishes a very elegant and fundamental principle: That in order to produce a new nuclear isotope, so long as all Standard Model symmetries are conserved, one must simply tack on quarks and anti-quarks. Surprisingly straightforward, right? Though sadly, history is not on the side of 5-quark systems.

Now go discuss and ask questions! 🙂

Run-of-the-mill hadrons that are common to everyday interactions involving the Strong Nuclear Force (QCD) are colloquially called “standard hadrons.” They include mesons (quark-anti-quark pairs) and baryons (three-quark/anti-quark combinations). Quark combinations consisting of more than three quarks are called “exotic hadrons.”

 

 

 

 

Happy Colliding.

– richard (@bravelittlemuon)

 

PS, I am always happy to write about topics upon request. You know, QED, QCD, OED, etc.

http://en.wikipedia.org/wiki/Neutron
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