• John
  • Felde
  • University of Maryland
  • USA

Latest Posts

  • USLHC
  • USLHC
  • USA

  • 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

  • TRIUMF
  • Vancouver, BC
  • Canada

Latest Posts

  • Laura
  • Gladstone
  • MIT
  • USA

Latest Posts

  • Steven
  • Goldfarb
  • University of Michigan

Latest Posts

  • Fermilab
  • Batavia, IL
  • USA

Latest Posts

  • Seth
  • Zenz
  • Imperial College London
  • UK

Latest Posts

  • Nhan
  • Tran
  • Fermilab
  • USA

Latest Posts

  • Alex
  • Millar
  • University of Melbourne
  • Australia

Latest Posts

  • Ken
  • Bloom
  • USLHC
  • USA

Latest Posts

Posts Tagged ‘Jets’

All those super low energy jets that the LHC cannot see? LHC can still see them.

Hi Folks,

Particle colliders like the Large Hadron Collider (LHC) are, in a sense, very powerful microscopes. The higher the collision energy, the smaller distances we can study. Using less than 0.01% of the total LHC energy (13 TeV), we see that the proton is really just a bag of smaller objects called quarks and gluons.

myproton_profmattstrassler

This means that when two protons collide things are sprayed about and get very messy.

atlas2009-collision-vp1-142308-482137-web

One of the most important processes that occurs in proton collisions is the Drell-Yan process. When a quark, e.g., a down quark d, from one proton and an antiquark, e.g., an down antiquark d, from an oncoming proton collide, they can annihilate into a virtual photon (γ) or Z boson if the net electric charge is zero (or a W boson if the net electric charge is one). After briefly propagating, the photon/Z can split into a lepton and its antiparticle partner, for example into a muon and antimuon or electronpositron pair! In pictures, quark-antiquark annihilation into a lepton-antilepton pair (Drell-Yan process) looks like this

feynmanDiagram_DrellYan_Simple

By the conservation of momentum, the sum of the muon and antimuon momenta will add up to the photon/Z boson  momentum. In experiments like ATLAS and CMS, this gives a very cool-looking distribution

cms_DY_7TeV

Plotted is the invariant mass distribution for any muon-antimuon pair produced in proton collisions at the 7 TeV LHC. The rightmost peak at about 90 GeV (about 90 times the proton’s mass!) is a peak corresponding to the production Z boson particles. The other peaks represent the production of similarly well-known particles in the particle zoo that have decayed into a muon-antimuon pair. The clarity of each peak and the fact that this plot uses only about 0.2% of the total data collected during the first LHC data collection period (Run I) means that the Drell-Yan process is a very useful for calibrating the experiments. If the experiments are able to see the Z boson, the rho meson, etc., at their correct energies, then we have confidence that the experiments are working well enough to study nature at energies never before explored in a laboratory.

However, in real life, the Drell-Yan process is not as simple as drawn above. Real collisions include the remnants of the scattered protons. Remember: the proton is bag filled with lots of quarks and gluons.

feynmanDiagram_DrellYan_wRad

Gluons are what holds quarks together to make protons; they mediate the strong nuclear force, also known as quantum chromodynamics (QCD). The strong force is accordingly named because it requires a lot of energy and effort to overcome. Before annihilating, the quark and antiquark pair that participate in the Drell-Yan process will have radiated lots of gluons. It is very easy for objects that experience the strong force to radiate gluons. In fact, the antiquark in the Drell-Yan process originates from an energetic gluon that split into a quark-antiquark pair. Though less common, every once in a while two or even three energetic quarks or gluons (collectively called jets) will be produced alongside a Z boson.

feynmanDiagram_DrellYan_3j

Here is a real life Drell-Yan (Z boson) event with three very energetic jets. The blue lines are the muons. The red, orange and green “sprays” of particles are jets.

atlas_158466_4174272_Zmumu3jets

 

As likely or unlikely it may be for a Drell-Yan process or occur with additional energetic jets, the frequency at which they do occur appear to match very well with our theoretical predictions. The plot below show the likelihood (“Production cross section“) of a W or Z boson with at least 0, 1, 2, 3, or 4(!) very energetic jets. The blue bars are the theoretical predictions and the red circles are data. Producing a W or Z boson with more energetic jets is less likely than having fewer jets. The more jets identified, the smaller the production rate (“cross section”).

cms_StairwayHeaven_2014

How about low energy jets? These are difficult to observe because experiments have high thresholds for any part of a collision to be recorded. The ATLAS and CMS experiments, for example, are insensitive to very low energy objects, so not every piece of an LHC proton collision will be recorded. In short: sometimes a jet or a photon is too “dim” for us to detect it. But unlike high energy jets, it is very, very easy for Drell-Yan processes to be accompanied with low energy jets.

feynmanDiagram_DrellYan_wRadx6

There is a subtlety here. Our standard tools and tricks for calculating the probability of something happening in a proton collision (perturbation theory) assumes that we are studying objects with much higher energies than the proton at rest. Radiation of very low energy gluons is a special situation where our usual calculation methods do not work. The solution is rather cool.

As we said, the Z boson produced in the quark-antiquark annihilation has much more energy than any of the low energy gluons that are radiated, so emitting a low energy gluon should not affect the system much. This is like massive freight train pulling coal and dropping one or two pieces of coal. The train carries so much momentum and the coal is so light that dropping even a dozen pieces of coal will have only a negligible effect on the train’s motion. (Dropping all the coal, on the other hand, would not only drastically change the train’s motion but likely also be a terrible environmental hazard.) We can now make certain approximations in our calculation of a radiating a low energy gluon called “soft gluon factorization“. The result is remarkably simple, so simple we can generalize it to an arbitrary number of gluon emissions. This process is called “soft gluon resummation” and was formulated in 1985 by Collins, Soper, and Sterman.

Low energy gluons, even if they cannot be individually identified, still have an affect. They carry away energy, and by momentum conservation this will slightly push and kick the system in different directions.

feynmanDiagram_DrellYan_wRadx6_Text

 

If we look at Z bosons with low momentum from the CDF and DZero experiments, we see that the data and theory agree very well! In fact, in the DZero (lower) plot, the “pQCD” (perturbative QCD) prediction curve, which does not include resummation, disagrees with data. Thus, soft gluon resummation, which accounts for the emission of an arbitrary number of low energy radiations, is important and observable.

cdf_pTZ dzero_pTZ

In summary, Drell-Yan processes are a very important at high energy proton colliders like the Large Hadron Collider. They serve as a standard candle for experiments as well as a test of high precision predictions. The LHC Run II program has just begun and you can count on lots of rich physics in need of studying.

Happy Colliding,

Richard (@bravelittlemuon)

 

Share

BOOST!

Sunday, August 5th, 2012

A couple weeks ago, about 80 theorists and experimentalists descended on Valencia, Spain in order to attend the fourth annual BOOST conference (tag-line: “Giving physics a boost!”). On top of the fact that the organizers did a spectacular job of setting up the venue and program (and it didn’t hurt that there was much paella and sangria to be had) overall I’d have to say this was one of the best conferences I’ve attended.

so....much.....sangria......

Differing from larger events such as ICHEP where the physics program is so broad that speakers only have time to give a cursory overview of their topics, the BOOST conferences have more of a workshop feel and are centered specifically around the emerging sub-field of HEP called “boosted physics”. I’ll try to explain what that means and why it’s important below (and in a few subsequent posts).

Intro to top quark decay

In order to discuss boosted physics, something already nicely introduced in Flip’s post here, I’m going to use the decay of the top quark as an example.

Obligatory Particle Zoo plushie portraying the top quark in a happy state

The most massive of all known fundamental particles by far, weighing in at around 173 GeV/c2, the top quark has an extremely short lifetime….much shorter than the time scale of the strong interaction. Thus the top quark doesn’t have time to “hadronize” and form a jet…instead, it will almost always decay into a W boson and a b quark (more than 99% of the time), making it a particularly interesting particle to study. The W boson then decays into either a lepton and a neutrino or two lighter quarks, and the full top decay chain is colloquially called either “leptonic” or “hadronic”, respectively.

From the experimental point of view, top quarks will look like three jets (one from the b and two from the light quarks) about 70% of the time, due to the branching fraction of the W boson to decay hadronically. Only 20% of tops will decay in the leptonic channel with a jet, a muon or electron, and missing energy. (I’m ignoring the tau lepton for the moment which has it’s own peculiar decay modes)

In colliders, top quarks are mostly produced in top/anti-top (or “t-tbar”) pairs….in fact, the top-pair production cross section at the LHC is about 177 pb (running at sqrt(s)=7 TeV), roughly 25 times more than at the Tevatron!! Certainly plenty of tops to study here. Doing some combinatorics and still ignoring decay modes with a tau lepton, the whole system will look:

  1. “Fully hadronic”: two hadronically-decaying tops (about 44% of the time)
  2. “Semi-leptonic”: one leptonically-decaying and one hadronically-decaying top (about 30% of the time)
  3. “Fully leptonic”: two leptonically-decaying tops (only about 4% of the time)

Branching fractions of different decay modes in t-tbar events (from Nature)

 

The point: if a t-tbar event is produced in the detector, it’s fairly likely that at least one (if not both) of the tops will decay into jets! Unfortunately compared to the leptonic mode, it turns out this is a pretty tough channel to deal with experimentally, where at the LHC we’re dominated by a huge multi-jets background.

What does “boost” mean?

If a t-tbar pair was produced with just enough energy needed to create the two top masses, there wouldn’t be energy left over and the tops would be produced almost at rest. This was fairly typical at the Tevatron. With the energies at the LHC, however, the tops are given a “boost” in momentum when produced. This means that in the lab frame (ie: our point of view) we see the decay products with momentum in the same direction as the momentum of the top.

This would be especially conspicuous if, for example, we were able to produce some kind of new physics interaction with a really heavy mediator, such as a Z’ (a beyond-the-Standard-Model heavy equivalent of the Z boson), the mass of which would have to be converted into energy somewhere.

Generally we reconstruct the energy and mass of a hadronically-decaying top by combining the three jets it decays into. But what if the top was so boosted that the three jets merged to a point where you couldn’t distinguish them, and it just looked like one big jet? This makes detecting it even more difficult, and a fully-hadronic t-tbar event is almost impossible to see.

At what point does this happen?

It turns out that this happens quite often already, where at ATLAS we’ve been producing events with jets having a transverse momentum (pT) of almost 2 TeV!

A typical jet used in analyses in ATLAS has a cone-radius of roughly R=0.4. (ok ok, the experts will say that technically it’s not a “cone,” let alone something defined by a “radius,” as R is a “distance parameter used by the jet reconstruction algorithm,” but it gives a general idea.) With enough boost on the top quark, we won’t be able to discern the edge of one of the three jets from the next in the detector. Looking at the decay products’ separation as a function of the top momentum, you can see that above 500 GeV or so, the W boson and the b quark are almost always within R < 0.8. At that momentum, individual R=0.4 jets are hard to tell apart already.

The opening angle between the W and b in top decays as a function of the top pT in simulated PYTHIA Z'->ttbar (m_Z' =1.6 TeV) events.

 

We’ll definitely want to develop tools to identify tops over the whole momentum range, not just stopping at 500 GeV. The same goes for other boosted decay channels, such as the imminently important Higgs boson decay to b-quark pairs channel, or boosted hadronically-decaying W and Z bosons. So how can we detect these merged jets over a giant background? That’s what the study of boosted physics is all about.

Next: Finding boosted objects using jet “mass” and looking for jet substructure

Next next: Pileup at the LHC….a jet measurement nightmare.

Share

Anatomy of a Jet in CMS

Wednesday, June 1st, 2011

We talk often about Jets here at US LHC.  We talk about ways to identify them, their structure, and we even mention some crazy phenomenon involving them.  But one thing we don’t always talk about is what a jet looks like.  And this is what I would like to show today, in gory detail.  So this post is about pictures, lots and lots of pictures.

We can’t see jets with our eyes.  The particles that make up a jet are just to small.  But using a device as large as the CMS detector, we can take a “snap-shot” of a jet created in a proton-proton collision at the LHC.  So it behooves us to start with a brief illustration of CMS.

(Clicking on any of the images below allows you to blow them up in another window, just in case you need a bigger picture)

The CMS Detector

It’s a gigantic cylinder, twenty-one meters long and fifteen meters in diameter!  For comparison, the average height of all American women & men over 20 years old is 1.62 & 1.76 meters (5′ 4″, 5′ 9″), respectively.  But being gigantic isn’t the only thing CMS has in common with an ogre, CMS also has layers; and each layer is a different sub-detector responsible for identifying a different class of particles.

But Here’s computer generated image of CMS, with a cut-away section showing some of these layers:

 

Cut away view of CMS, Hadronic & Electromagnetic Calorimeters not shown.  The camera guide in the bottom right shows CMS’s coordinate axis, (x,y,z) for (red,green,blue).

 

The blue disks and red rectangles on the outside of the detector are part of CMS’s muon chambers (the sub-detector responsible for picking up muons).

The inner cylinder represents CMS’s silicon tracker, this sub-detector is a rather complex instrument.  Its made of silicon strips, and is essentially a giant CCD camera (with over ten million pixels).  The silicon tracker is responsible for reconstructing the trajectories of charged particles as they pass through CMS; this is done by basically playing a giant game of connect the dots.  A close-up of the silicon tracker is shown here:

 

Close-up of the silicon tracker

 

The green and yellow portions are the silicon tracker.  The grey/silver part is what’s called the silicon pixel detector.  It is less then an inch away from where the proton-proton collisions occur in CMS, and thus the closest detection element.

While not shown in the image above, CMS’s calorimeters and superconducting magnet are located between the silicon tracker and the muon system.  They can be seen in this interactive applet on particle detection, of which I’ve taken a screenshot of and shown below:

 

 

The calorimeters are responsible for measuring the energy and momentum of charged & neutral particles (the tracker plans a role in this as well).  They are critical to jet identification & reconstruction…without them we would not be able to do any jet physics in CMS.

Basically what happens is calorimeters are designed so that a particle loses all of its energy as it travels through the calorimeter.  From the energy deposited, and the location of where the deposit occurs we can determine the direction and momentum of charged particles (again, the tracker also plays a role in this).

CMS has two types of calorimeters: an electromagnetic calorimeter (ECAL) for detecting electrons and photons; and a hadronic calorimeter (HCAL) for detecting heavy particles that can pass through the ECAL.  The HCAL is also the only place in CMS where we can detect neutral particles with non-zero mass (such as a neutron).

I should mention that neutrino’s escape detection, and we have to infer their presence by looking for “missing energy.”

But now that I’ve introduced you to CMS, let’s get down to business and talk about Jets.

 

Jets in CMS

I use jets extensively in my own research and its sometimes hard to get a handle on what a jet really is.  I like to think of it like a shotgun blast of particles slamming into the detector.  Jets arise from the hadronization of colored particles, and because of this they are made up of many particles.  Jets can be made of leptons, hadrons and even bosons (specifically the photon).  These particles are usually collimated in a given direction, and you can kinda draw a cone around them (like a shotgun blast!).

For this reason in CMS we like to think of jets as cones, like in this image:

 

A jet cone in CMS

 

This is a jet cone created in a single proton-proton collision recorded by CMS detector in 2010.  In this image I’ve turned the silicon tracker’s graphics off, along with everything else that happened in this event (it can get really messy anyway).  A zoom in of this jet cone can be seen here:

 

Zoom in of the jet cone

 

Now this jet cone may look small in comparison to the entire CMS Detector; but don’t be fooled, I choose a very energetic jet for this post.  This jet’s component of momentum in the xy-plane (green & red axis above) is 115 GeV/c.  Most jets created in proton-proton collisions have xy-momentum components of less then 30 GeV/c.  In fact, if you plot the number of jets detected against their xy-momentum components, you get a distribution that looks similar to an decaying exponential.  So, 115 GeV/c jet is rather energetic.

But what makes up this jet!?  Simple answer A LOT!  This one jet shown above is made up of over 20 different particles, all of which are conveniently hidden at the moment.  So let’s go about fleshing this jet out, piece by piece.

 

Jet Anatomy

So what did CMS see as this jet hit the detector?  Let’s start with the silicon tracker:

 

A jet & its tracks in CMS

 

So here I’ve turned off the view of the muon chambers, and just shown the jet cone and the tracks (the green lines) reconstructed in the silicon tracker.

A few things to note, the green lines appear to be coming from the same point in space (for the most part).  This point is called the primary vertex, its the point at which the proton-proton collision actually occurred.  Another interesting feature is that these tracks go outside of the jet’s cone!  What’s up with that!?

Well the answer is two-fold.  First, these tracks where made by charged particles; and thus their trajectories are bent in the presebce of CMS’s magnetic field.  In other words, we forced the tracks to go outside the jet cone by having a superconducting magnet in our detector (this allows us to make more precise momentum measurements).  Second, treating jets as cones is just a model (which works well).  It often happens that these tracks are indeed all inside the cone, but I purposefully chose a jet with tracks that had large curvature for this demonstration.

In fact, jets are created by using different algorithms; and not all algorithms use a cone geometry!  There are many different algorithms that you can use, they all have subtle differences…but that is really a story for another day.  I just want to show  you what makes up a jet, and what it looks like to the detector.

So this is all the information that the silicon tracker gave us about the jet.  It’s time to ask the calorimeters what they saw, starting with ECAL:

A Jet, its tracks, and its energy deposits in CMS’s electromagnetic calorimeter

 

These yellow-ish squares represent the energy deposited in ECAL.  How far these squares protrude from that wire-frame represents how much energy has been deposited (for those of you who are keeping track, the scale is 10 GeV per meter).  I’ll show some images illustrating this protrusion later on; right now I want to talk about the relation between the tracks, the jet cone and these ECAL energy deposits.

It looks like the jet cone is centered on the bulk of the energy deposits in ECAL.  We actually intended this to happen because of something called clustering.  We group pieces of the calorimeters (both ECAL & HCAL) into clusters, and then clusters into superclusters.  And it is these clusters/superclusters that we go to when beginning to construct a jet.

It also looks like some of the tracks in our silicon tracker match up with these ECAL energy deposits.  But, there are clearly some energy deposits that don’t match up to any track!  What has happened here!?

The answer, is photons and other neutral particles!

The silicon tracker is incapable of detecting particles without an electric charge (like the photon).  But the ECAL was designed specifically to capture electrons & photons (which is why it is called the electromagnetic calorimeter).  Adding “photon candidates” to the picture gives us this result:

 

A Jet, its tracks, its “photon candidates”, and its energy deposits in CMS’s electromagnetic calorimeter

 

These dotted purple-ish lines are the trajectories of the “photon candidates” in this jet.  I say candidates because they might not actually be photons.  To be an actual photon the candidate must past very stringent quality requirements which only a real photon will satisfy.  I haven’t enforced any quality requirements here, so all bets are off (remember I’m just trying to show what made up this jet!).

Again all these photon candidates appear to be coming from the primary vertex, most of them are within the jet’s cone (not all); and almost every photon candidate is linked to an energy deposit in the ECAL.  These photon candidates are also linked to ECAL energy deposits that are not linked to tracks identified by the silicon tracker.

But we still have some ECAL deposits that are clearly not linked to either tracks identified by the silicon tracker or photon candidates!  We need to bring up the rest of the neutral particles:

 

A Jet, its tracks, its “photon candidates”, its neutral hadrons, and its energy deposits in CMS’s electromagnetic calorimeter

 

The dotted blue lines are the “neutral hadron candidates” within this jet.  Similar observations as before can be made.  But since neither a photon or a neutral hadron leaves a track in the silicon tracker, how do I distinguish between them?   This is were HCAL comes into play.

 

A Jet, its tracks, its “photon candidates”, its neutral hadrons, and its energy deposits in CMS’s hadronic calorimeter

 

Now I’ve turned off the energy deposits in ECAL and turned on the deposits in HCAL (teal rectangles).  I’ve also drawn a crude circle around one of the jet’s photon candidates.  It clearly has no HCAL energy deposit around it, but all the dotted blue lines are linked to HCAL deposits.  Now we can clearly see the difference between neutral hadrons and photon candidates; one has energy deposited in HCAL, the other doesn’t.

It might be interesting to note that some photon candidates appear to have HCAL deposits.  There are two reasons for this: 1) the photon candidate isn’t a real photon (it would fail the quality requirements I mentioned above), or 2) there is a nearby hadronic particle that is actually responsible for the HCAL energy deposit.

Now Let’s add the charged hadrons into the picture as well:

 

A Jet, its tracks, its “photon candidates”, its neutral hadrons, its charged hadrons, and its energy deposits in CMS’s hadronic calorimeter

 

These bright blue lines represent the trajectories of charged hadrons.  Notice that they are also coincident with the tracks in the silicon tracker (as they should be!).  These charged hadrons also link to energy deposits in HCAL.  In addition, they also have a chance of depositing energy in ECAL as well:

 

A Jet and its components in CMS

 

I’ve turned the ECAL deposits back on in the image above.

Notice the ECAL and HCAL deposits are stacked on top of each other (with ECAL appearing first).  We like to do this because this gives us the full idea of the direction of energy deposition in the CMS detector.  Let’s turn our view around so we can see the differences in this jet’s energy deposition:

 

A jet’s calorimeter deposits in CMS (also note the curvature of the jet’s tracks!!)

 

Rotated view of a jet’s calorimeter deposits in CMS

 

Final view of a jet’s calorimeter deposits in CMS

 

So from these views we can see the amount of energy deposited around the jet’s cone.

Again, height above the wire frame corresponds to the amount of total energy deposited in a region.  In some cases this energy was deposited in both the ECAL (yellow-ish rectangles) and the HCAL (teal rectangles).  The height of these different rectangles corresponds to the amount of energy deposited in their respective calorimeters.

I’ve also now colored all of the jet’s constituents blue, this now is the complete jet, its a spray of particles that goes along a specific direction (shown by the black shaded cone).

For those of you wondering, this is a specific type of jet called an anti-kT particle flow jet.  The algorithm, anti-kT particle flow, used to “reconstruct” this jet made use of the energy deposited in the calorimeters, the tracks in the silicon tracker, and the primary vertex (for determining tracks of neutral particles).

Some algorithms make use of only the calorimeters and the primary vertex (these are called calo-jets).  But discussing the different jet algorithms is a story for another day.

Remember how I said that the scale of those calorimeter deposits was 10 GeV per meter.  Let’s put that into perspective now:

 

The complete jet recorded by CMS, note the intense curvature of some of the jet’s tracks!

 

Remember the diameter of CMS is 15 meters, so from the primary vertex to the edge of the red muon system (near my coordinate axis guide at the bottom right) is 7.5 meters.  Hopefully, this gives you an idea of the amount of energy deposited in each of the calorimeter clusters.

So this is what a jet looks like!  All in all this jet had 29 different particles that were used in its construction.

So when we talk about Jets here at US LHC (and the rest of Quantum Diaries) I hope you will have a much better idea of what a jet really is.

 

Until next time,

-Brian

For some further reading on Jets, I suggest taking a look at these older posts:

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 qlv 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