• John
  • Felde
  • University of Maryland
  • USA

Latest Posts

  • James
  • Doherty
  • Open University
  • United Kingdom

Latest Posts

  • Andrea
  • Signori
  • Nikhef
  • Netherlands

Latest Posts

  • CERN
  • Geneva
  • Switzerland

Latest Posts

  • Aidan
  • Randle-Conde
  • Université Libre de Bruxelles
  • Belgium

Latest Posts

  • Sally
  • Shaw
  • University College London
  • UK

Latest Posts

  • Richard
  • Ruiz
  • Univ. of Pittsburgh
  • U.S.A.

Latest Posts

  • Laura
  • Gladstone
  • University of Wisconsin, Madison
  • USA

Latest Posts

  • Seth
  • Zenz
  • Imperial College London
  • UK

Latest Posts

  • Michael
  • DuVernois
  • Wisconsin IceCube Particle Astrophysics Center
  • USA

Latest Posts

  • Emily
  • Thompson
  • USLHC
  • Switzerland

Latest Posts

  • Ken
  • Bloom
  • USLHC
  • USA

Latest Posts

Posts Tagged ‘B-Physics’

Long-standing discrepancy put to rest

Saturday, July 20th, 2013

This morning at the European Physics Society conference in Stockholm, the LHCb experiment operating at the Large Hadron Collider (LHC) CERN brought one more argument to put to rest a long-standing discrepancy that had kept theorists puzzled for nearly two decades.

LHCb presented the most precise measurement to date of the b baryon lifetime. A baryon is a family of composite particles made of three quarks.  For example, protons and neutrons are made of a combination of u and d quarks.  What makes b baryons so special is that they contain a b quark, a much heavier type of quark. Composite particles containing b quarks like B mesons (made of a b and either a u or d quarks) and b baryons are unstable, meaning they have a short lifetime. About one picosecond after being created, they break down into smaller particles.

In theory, both B mesons and b baryons should have approximately the same lifetime. But in the 1990’s, when CERN operated with its previous accelerator called LEP (Large Electron Positron collider), all experiments measured a systematically shorter lifetime for b baryons than B mesons as can be seen on the plot below. Although the LEP experimental errors were quite large, the general trend of lower values was very puzzling since all four experiments (ALEPH, DELPHI, OPAL and L3) were working independently. Lb_lifetime_comparison

The various b baryon lifetime measurements over time from the oldest results at the bottom to the three latest results from the LHC experiments at the top. The measured value has now shifted toward a value of 1.5 picoseconds, as measured for the B mesons.

This prompted theorists to re-examine their calculations and to look for overlooked effects that could explain the difference. Despite all efforts, it was nearly impossible to reconcile the measured b baryon lifetime (somewhere between 1.1 to 1.3 picosecond) with the B meson lifetime at around 1.5 ps.

Nearly a decade later, D0 and CDF, the two experiments from another accelerator, the Tevatron near Chicago, started closing the gap. It took another decade for the LHC experiments to show that in fact, there is no large difference between b baryon and B meson lifetimes.

Already, earlier this year, ATLAS and CMS both reported values in line with the B meson lifetime. With this latest and most precise result from the LHCb experiment, there is now enough evidence to close the case on this two-decade-old discrepancy. LHCb measured the b baryon lifetime to be 1.482 ± 0.018 ± 0.012 ps. Hence, both lifetimes are now measured close to 1.5 picosecond and LHCb calculated their ratio to be 0.976 ± 0.012 ± 0.006, very close to unity as theoretically expected.

One possible explanation is that all LEP experiments were affected by a common but unknown systematic shift or simply, some statistical fluctuation (i.e. bad luck). The exact cause might never be found but at least, the problem is solved. This is a great achievement for theorists who can now rest assured that their calculations were right after all.

Pauline Gagnon

To be alerted of new postings, follow me on Twitter: @GagnonPauline
 or sign-up on this mailing list to receive and e-mail notification.

Share

I know in my life at least, there are periods when all I want to do is talk to the public about physics, and then periods where all I would like to do is focus on my work and not talk to anyone. Unfortunately, the last 4 or so months falls into the latter category. Thank goodness, however, I am now able to take some time and write about some interesting physics which had been presented both this year and last. And while polar bears don’t really hibernate, I share the sentiments of this one.

Okay, I swear I'm up this time! Photo by Andy Rouse, 2011.

A little while ago, I posted on Dalitz Plots, with the intention of listing a result. Well, now is the time.

At the 7th International Workshop on the CKM Unitarity Triangle, LHCb presented preliminary results

Dalitz Plot Asymmetry for \(B^\pm\to\pi^\pm\pi\pi\)

Asymmetry of \(B^{\pm}\to\pi^{\pm}\pi^+\pi^-\) as a function of position in the Dalitz Plot. Asymmetry is mapped to the z-axis. From LHCb-CONF-2012-028

for CP asymmetry in the channels \(B\to hhh\), where \(h\) is either a \(K\) or \(\pi\). Specifically, the presentation was to report on searches for direct CP violation in the decays \(B^{\pm}\to \pi^{\pm} \pi^{+} \pi^{-}\) and \(B^{\pm}\to\pi^{\pm}K^{+}K^{-}\).  If CP was conserved in this decay, we would expect decays from \(B^+\) and \(B^-\) to occur in equal amounts. If, however, CP is violated, then we expect a difference in the number of times the final state comes from a \(B^+\) versus a \(B^-\). Searches of this type are effectively “direct” probes of the matter-antimatter asymmetry in the universe.

Asymmetry of \(B^\pm\to\pi^\pm K K\). From LHCb-CONF-2012-028

Asymmetry of \(B^\pm\to\pi^\pm K K\) as a function position in the Dalitz plot. Asymmetry is mapped onto the z-axis.From LHCb-CONF-2012-028

By performing a sophisticated counting of signal events, CP violation is found with a statistical significance of \(4.2\sigma\) for \(B^\pm\to\pi^\pm\pi^+\pi^-\) and \(3.0\sigma\) for \(B^\pm\to\pi^\pm K^+K^-\). This is indeed evidence for CP violation, which requires a statistical significance >3\(\sigma\).The puzzling part, however, comes when the Dalitz plot of the 3-body state is considered. It is possible to map the CP asymmetry as a function of position in the Dalitz plot, which is shown on the right. It’s important to note that these asymmetries are for both signal and background. Also, the binning looks funny in this plot because all bins are of approximately equal populations. In particular, notice red bins on the top left of the \(\pi\pi\pi\) Dalitz plot and the dark blue and purple section on the left of the \(\pi K K\) Dalitz plot. By zooming in on these regions, specifically \(m^2(\pi\pi_{high})>\)15 GeV/c\(^2\) and \(m^2(K K)<\)3 GeV/c\(^2\), and separating by \(B^+\) and \(B^-\), a clear and large asymmetry is shown (see plots below).

Now, I’d like to put these asymmetries in a little bit of perspective. Integrated over the Dalitz Plot, the resulting asymmetries are

\(A_{CP}(B^\pm\to\pi^\pm\pi^+\pi^-) = +0.120\pm 0.020(stat)\pm 0.019(syst)\pm 0.007(J/\psi K^\pm)\)

and

\(A_{CP}(B^\pm\to\pi^\pm K^+K^-) = -0.153\pm 0.046(stat)\pm 0.019(syst)\pm 0.007(J/\psi K^\pm)\).

Whereas, in the regions which stick out, we find:

\(A_{CP}(B^\pm\to\pi^\pm\pi^+\pi^-\text{region}) = +0.622\pm 0.075(stat)\pm 0.032(syst)\pm 0.007(J/\psi K^\pm)\)

and

\(A_{CP}(B^\pm\to\pi^\pm K^+K^-\text{region}) = -0.671\pm 0.067(stat)\pm 0.028(syst)\pm 0.007(J/\psi K^\pm)\).

These latter regions correspond to a statistical significance of >7\(\sigma\) and >9\(\sigma\), respectively. The interpretation of these results is a bit difficult: the asymmetries are four to five times that of the integrated asymmetries, and are not necessarily associated with a single resonance. We would expect in the \(\rho^0\) and \(f_0\) resonances to appear in the lowest region of \(\pi\pi\pi\) Dalitz plot, in the asymmetry. In the \(K K\pi\) Dalitz plot, there are really no scalar particles which we expect to give us an asymmetry of the kind we see. One possible answer to both these problems is that the quantum mechanical amplitudes are only partially interfering and giving the structure that we see. The only way to check this would be to do a more detailed analysis involving a fit to all of the possible resonances in these Dalitz plots. All I can say is that this result is certainly puzzling, and the explanation is not necessarily clear.

Zoom onto \(m^2(\pi\pi)\) lower axis.Zoom of \(m^2(K K)\)

Zoom onto \(m^2(\pi\pi)\) lower axis (left) and \(m^2(K K)\) axis (right) . Up triangles are \(B^+\), down are \(B^-\)

Share

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

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, mediated by a leptoquark

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

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)

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 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)

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)

Share

So in the last few posts, I’ve been talking about Jets.  I’ve also touched on ways to identify a specific type of jet: the b-Jet.  Recall, a b-Jet is a jet that is produced as a result of the hadronization of a b or anti-b quark (termed bbar or simply b).

I also outlined some properties of B-Hadrons (see the second link above).  So let’s start to put these properties to good use and flesh out one of the standard B-Taggers used by high energy physicists, namely the Track Counting (TC) Algorithm.

(Again, you may click on any of the below images to enlarge them further).

 

One Track, Two Track, Three Track, Four!?

In my previous post I stated that a B-Hadron will produce roughly five charged particles per decay.  These charged particles will then leave a track within the Silicon Tracker of CMS.  So if a jet is a b-jet, it will have more high impact parameter tracks then a jet produced from the hadronization of a light quark or gluon.

The Track Counting approach is actually rather simple, physicists require a jet to have at least N tracks (for some integer N).  In CMS we take N to usually be 3, but I will explain why in more detail below.  However, we don’t use all of a jet’s tracks in the TC Algorithm.  We require the tracks used to be of “high quality”.

But what does that mean?  How do you judge the “quality” of a track?  The answer is information, how much the detector knows about the particle that made that track.  This comes about in the form of how many hits a particle left within the silicon tracker as it traveled (these hits are then used to make the track).  If the particle left more hits in the tracker it means higher track quality.  Here’s an example of two tracks, the one on the left has more then 10 hits in the tracker, while the one on the right has only 5.  So, the left track is of higher quality.

 

An Example of two tracks left by charged particles recorded by the CMS Detector.  The higher quality track (at left) has more hits within the Silicon Tracker (Represented by the blue rectangles).

 

Here the blue dot at the start of each track represents the location  of the primary vertex (the point where the proton-proton collision occurred).  The track itself is represented by the green line.  The track’s hits in Silicon Tracker of CMS are represented by the blue rectangles (each rectangle is a piece of the Silicon Tracker).

 

A Measurement of Impact

Now that we have a collection of high quality tracks belonging to a jet, how do I use them to test if the jet is a b-Jet or not?  We look at something called the impact parameter, or the distance between the primary vertex and the closest approach to the track.  A visualization will help with understanding this:

 

Visualization of the impact parameter (IP, red line) for a trackVisualization of the Impact Parameter (IP, red line) of a track (Image courtesy of Jean-Roch Vlimant, of the CMS Collaboration).

 

The track is represented by the dotted blue line.  And this track belongs to a jet (with a direction given by the green arrow).  This Jet direction represents the direction of the jet’s cone within the detector (see the first link above to get an idea of what a jet cone is like in CMS).

The Impact Parameter, is represented by the red line, and is drawn from the primary vertex to the track.  Notice how the point where the IP touches the track, a right angle is formed, this is how the point of closest approach is identified.  Also, the location where the red line makes a right angle with the track is unique.  Meaning, the IP always makes a right angle with the track, and there is only one IP per track.

However, the error on the IP measurement could sometimes be large.  To account for this physicists divide the IP by its error, and this new value is called the IP-Significance (IP-Sig).

We also have a sign convention for this IP-Sig value.  If the cosine of the angle between the track and the jet axis is positive (marked as θ in above diagram), the IP-Sig is a positive number (the track is said to be “downstream” of the jet axis).  If the cosine of this angle is negative, the IP-Sig is negative (and this is said to be “upstream” of the jet axis).

 

Discriminating against non-b-Jets

The goal of all B-Tagging algorithms is to create what is called a discriminator.  A discriminator is some number that is calculated from a jet’s properties.  As the value of a jet’s discriminator increases, the likelihood that the jet is a b-Jet also increases.  It’s a very simplistic approach, and works beautifully.

In the TC Algorithm, the discriminator is the signed IP-Sig value mentioned above.  The reason we use the signed IP-Sig value is best summarized as:

Prompt tracks from the primary vertex have small IP values while tracks from decays of B hadrons have rather large IP values because of the B hadron lifetime [1].

So b-jets will have several tracks with large IP values.  But as I mentioned above, we convert these IP values to signed IP-Sig values to minimize the impact of the measurement’s error on our discriminator.  In summary,  if a jet has many tracks with small signed IP-Sig values, it is most likely not from the hadronization of a b quark/anti-quark.  While a jet originating from a b quark/anti-quark will have tracks with large IP values, because they will be “displaced” from the primary vertex.

This again ties back to my previous post which outlined the properties of B Hadrons (second link at the start of this post).  And it was these B-Hadron properties that motivated the creation of the TC Algorithm years ago.

But this raises a new question.  Each of the high quality tracks within a jet has a signed IP-Sig value, so which of these of these IP-Sig values do we use in B-Tagging?

To answer this we first order all of a jet’s high quality tracks by decreasing IP-Sig value.  We then choose to look at the Nth track in this listing for all of our jets under study (remember how I said N was usually 3 above?).  The Nth track has a signed IP-Sig value greater then some number Y; and thus the jet has a chance X of being b-Jet.  As the number Y increases, the chance to be a b-Jet, X, also increases.  Here are some plots that will let us get a better understanding of this:

 

From left to right: signed IP-Sig for all selected tracks, 1st, and 3rd track, in selected jets found in proton-proton collisions recorded by the CMS Detector in 2010 [2].

 

In the above plots, CMS physicists have plotted the signed IP-Sig values for: (from left to right) all high quality tracks within all jets, the first high quality track within a jet, and the third high quality track within a jet.  The x-axis in each case is the value of the signed IP-Sig of the jet’s track(s).  The y-axis represents the number of jets found with tracks/a track with that signed IP-Sig value.

The black dots in each of the colored distributions represent the signed IP-Sig values of jets found in actual collision data. Whereas the colored distributions represent the values found in simulation for light jets (blue), c-jets (green), and b-jets (red).  Recall that when I say a jet is a light-jet or a c-jet, I mean the jet was created by the hadronization of a light quark/anti-quark (or gluon), or a c quark/anti-quark.

The distributions below the colored distributions represent how well the simulation compares to actual data.  If the simulation matches data, the black points there should be at one, or close to one.  For the most part, the simulation describes the data well, and we are constantly improving our simulation so that the agreement becomes better and better.

What’s interesting to note is what happens when we look at a jet’s high quality track that has the third highest IP-Sig value.  We see that as this value increases positively, the distribution (far right) is completely dominated by b-jets.  Whereas in the other two distributions, there is still a reasonable contribution of light jets at all values of signed IP-Sig.

This far right distribution is known as the Track Counting High Purity (TCHP) Algorithm.  And CMS Physicists use this algorithm to search for b-Jets in many different research areas; from top quark physics, precision QCD measurements, to supersymmetric searches, this algorithm is one of the major tools employed by high energy physicists as a whole.

Because of B-Hadron properties, physicists have come up with a way to identify b-Jets, require the jet to have tracks with high IP-Sig values.

Recall that this TC Algorithm made use of the fact that B-Hadrons decay into many charged particles, and the long life-time of B-Hadrons.  This long-lifetime ensures that particles produced by decaying B-Hadrons will have tracks with large IP values (this then translates into large IP-Sig values).  All of these things are illustrated in the three distributions shown above.

 

Until next time,

-Brian

 

 

References:

[1]  CMS Collaboration, “Performance of track and vertex reconstruction and b-tagging studies with CMS in pp collisions at sqrt(s) = 7 TeV,” Proceedings of Science, Kruger National Park, Mpumalanga, South Africa, December 2010.

[2] CMS Collaboration, “Commissioning of b-jet identification with pp collisions at sqrt(s) = 7 TeV,” CMS Physics Analysis Summary, CMS-PAS-BTV-10-001, http://cdsweb.cern.ch/record/1279144?ln=en.

Share

Well its been longer then usual since my last post.  This past weekend was commencement at my host institution; and I spent it with my friends who graduated with their Master’s degrees (myself included) and family who came down to visit.

But today I’d like to talk about something that is crucial to my own research, B-Tagging.  Or, the experimental tools we (as high energy physicists) use to identify b-jets.  Here b stands for the “beauty” or “bottom” quark; these are two interchangeable terms for the same particle.

Importance of B-Tagging

But first, what’s the big deal about B-Tagging?

Well, many current Standard Model physics process under study at the LHC have a b and an anti-b quark (termed bbar or simply b) in the final state.

To understand what “final state” means, let’s look at an example.  Suppose in a proton-proton collision a top quark, t, and an anti-top quark, t, are produced (this is called top-quark pair, tt, production).  Well the top quark decays to a W boson, and another quark, q, ~10% of the time [1]. i.e.

t → W + ql+v q

Here: the W+ boson has decayed leptonically into a lepton, l, a neutrino, v; and a quark, q. The quark may be either a d, s, or b quark.  The case for anti-top quark decay, t, is shown here:

t → W - ql-v q

Where q = d, s, or b.

Then the final state for this event is two oppositely charged leptons, two neutrinos, a quark, and an anti-quark (These quarks will turn into jets as they cannot exist freely, more information on this below).  But when looking for top-quark pair production events in a collider, it’s easiest to find them if you look for events containing two b-jets along with the charge leptons and neutrinos.

In addition to top-quark pair production, the Higgs Boson is theorized to decay into a bb final state; which in turn form two b-jets (see one of my older posts here, or one of Flip Tanedo’s posts here for more details regarding the Higgs Boson).

But b-jets don’t just don’t come from Standard Model processes, many new physics searches (such as supersymmetric searches) have bb final states.

With this in mind, it is of paramount importance to be able to find and identify b-jets.  But to do this we first need to understand the properties of the b quark itself; or more importantly b-Hadrons as these are what we actually observe.

Beautiful Hadrons

B-Hadrons are rather unique in elementary particle physics.  They offer us a chance to study so much; we are able to use them to investigate topics from Quantum Chromodynamics, to CP-Violation, and even physics beyond the Standard Model.  Simply put, B-Hadrons have got it going on!

They are very heavy particles, with rest masses of approximately 5-10 GeV/c2; or roughly five to ten times the proton’s rest mass [2].

B-Hadrons are also very “long lived” particles, with mean life-times of approximately 1.6 pico-seconds, or 1.6·10-12 seconds.  For comparison, the π0 meson has a lifetime of roughly 8·10-17 seconds [2]; and the top quark has an even shorter lifetime of ~10-23 seconds.  So B-Hadrons are very long lived indeed.  In fact, because of this long life-time a B-Hadron has a of approximately 480-500 micro-meters (τ being the proper time, or the B-Hadron’s mean life-time; and c is the speed of light).  Putting this into more tangible terms, a B-Hadron will travel roughly half a millimeter before it decays.  For comparison, the π0 meson has a of only 25 nano-meters (2.5·10-8 meters).

In addition to their long life-time and large mass, a B-Hadron will produce five charged particles per decay (on average)!  In comparison a Δ++ (a light baryon made of u quarks) will produce only 3 charge particles per decay; and a π+ (a light meson made of a u and d) will produce merely a single charged particle per decay.

Also, when a B-Hadrons decays, there is a 10% chance that a lepton will be produced during the decay process.

In summary, B-Hadrons have:

  1. Large mass
  2. A long life-time
  3. Large 
  4. High number of charged particles per decay
  5. Chance of leptonic decay

 

But what does this have to do with B-Tagging?  For this we must ask ourselves how these properties listed above would show themselves within our detectors.

Experimental Signatures: B-Tagging

Hadrons will exist in clustered collimated groups within a detector, known as jets.  Thus we say jets are due to “hadronic activity.”

But what causes this clustering/jet structure to occure?  Well when protons collide in the LHC, a quark or gluon may “escape” the proton it was originally found in.  But quarks/gluons cannot exist freely in nature!  Thus quarks/anti-quarks use some of their kinetic energy to pull other quarks/antiquarks out of the vacuum to form hadrons (this is called hadronization, see one of Flip’s old posts here for more info).  And gluons “split” into a quark an anti-quark pair.  The produce quark and anti-quark in this “gluon-splitting” will then in turn undergo hadronization process.

But this all occurs within a jet!

So these B-Hadrons that we have been talking about are going to be found within jets.  So b-jets must have B-Hadrons inside them (hence the name)!

Now here is where our B-Hadron properties start to come into play.

Since a B-Hadron has a long lifetime, and travels some distance before decaying, we are able to look for what’s called a secondary vertex (SV).  A SV is a spot that new particles spew from because of the decay of a heavier particle (creating  tracks if these new particles have an electric charge).  So if a jet has a SV it is much more likely to be a b-jet.

To get an idea what an SV might look like in our detectors take a look at this image uploaded by Anna Phan.  Here a B-Hadron (Bs) has decayed into a charmed hadron (D+) and a muon (μ); the charmed hadron has then decayed into three other particles (hence a total of five charged particles were produced due to the B-Hadron’s decay!).

Notice how these particles are collimated and clustered together, i.e. this is a jet; and in this case it is a b-jet!

Also, because many charged particles are produced by a decaying B-Hadron, a jet that has a large number of tracks within it is more likely to be a b-jet.  So these four tracks that are produced after the SV (image I linked above) gave experimentalists in the LHCb Collaboration the ability to determine that this was a b-jet.  Had this jet come from say, a pion or a Δ++, there may have only been one or two tracks, and it would not be tagged as a b-jet (and rightly so!).

Finally, this picture has also shown the B-Hadron having a lepton (the muon) in its decay chain.  As a result, experimentalists look for jets that have nearby leptons when looking for b-jets.

So in summary, we as experimentalists in our B-Tagging efforts combine as much of this information as possible when searching for b-jets.  When we want to determine if a jet is a b-jet, we look to see if it has one or more of the following things:

  1. A secondary vertex
  2. A large number of tracks within them
  3. A nearby lepton

 

So from our list of B-Hadron properties we were able to construct a list of what to look for when attempting to perform B-Tagging.

 

But that wraps up our discussion for today.  There are many more levels to this then I’ve illustrated here.  If you are interested in more details, simply post below and I’ll try to answer your questions!

One last piece of information, creating the tools to perform B-Tagging is not something that one person alone can do.  For example, CMS has a very large staff of researchers (over 100) who focus directly on developing techniques these  techniques that I spoke on briefly.  Needless to say, without the researchers who develop these B-Tagging techniques, my own research would be impossible!

But until next time,

-Brian

 

References

[1] Particle Data Group, http://pdg.lbl.gov/2010/tables/rpp2010-sum-quarks.pdf, May 11th 2011.

[2] Particle Data Group, http://pdg.lbl.gov/2010/tables/rpp2010-sum-mesons.pdf, Math 11th 2011.

Share