The Higgs field giving mass to a particle has been likened to a crowd giving mass to a celebrity.

Higgs boson for beginners

What is the Higgs boson ?
What is the Higgs field ?
How can we find the Higgs boson ?
Who is looking for the Higgs boson ?
Where is the Higgs boson being searched for ?

Higgs boson for novices

What evidence is there for the Higgs boson ?
Why is the Higgs boson worth finding ?
Why is the Higgs boson so elusive ?
How close are we to finding the Higgs boson ?
What does it mean to exclude the Higgs boson ?
How much energy do you need to find the Higgs boson ?
How much data do you need to find the Higgs boson ?

Advanced Higgs boson

What does it mean to exclude the Higgs boson ?
What does "a limit" on the Higgs boson mean ?
How exactly do you produce a Higgs boson ?
How does a Higgs boson decay ?
How do you detect a Higgs boson ?
What are the experimental uncertainties ?
How do I interpret the Higgs limit plots ?

Still a skeptic ?

Is this all just a theory ?
What if no one ever finds the Higgs boson ?
What are the theoretical uncertainties ?
Are there alternate theories ?
Who cares if we find the Higgs boson ?

Particle Physics Basics

What is particle physics ?
What is the "Standard Model" of particle physics ?
How do you do particle physics ?


Higgs boson for beginners

What is the Higgs boson ?
The Higgs boson is a fundamental particle (meaning it is not made of anything) which is one of the building blocks of the universe. There are other fundamental particles, but this one is special in that when the fundamental particles which make up matter interact with the Higgs boson, they acquire mass. Without this interaction, these particles would have no mass, and would travel at the speed of light, as does the photon. The significance of the word "Higgs" is that Peter Higgs is one of the scientists who theorized this particle. In fact, there were three independent groups of scientists who theorized the Higgs mechanism in 1964 (Guralnik, Hagen, Kibble; Higgs; Englert, Brout). Below are excerpts from their original papers. Although the text probably won't make sense to the lay person, you might be able to pick out some similarities in their sentences. The significance of the word "boson" is that a boson is a force-carrying particle which is essentially a quantized unit of a field. One of the interesting unsolved mysteries about the Higgs boson is why it gives different particles different mass.
 

Excerpt of Hagen, Kibble, Guralnik paper

 

Excerpt of Higgs paper

 

Excerpt of Englert, Brout paper



It is important to note that the Higgs boson is a part of a successful model, but since it has not been directly observed, it is still just hypothetical. Since it is a very elegant simple solution to theoretical puzzles and satisfies all the data, it is general accepted by the particle physics community. Top

What is the Higgs field ?
  The Higgs field is a field that fills the universe like a water fills a pool. As particles move through the universe they acquire mass by interacting with the Higgs field. One way to imagine the Higgs field is to imagine trying to walk through a pool: The water pushes against you making you feel heavier, and making it harder for you to move. This effectively generates inertia or mass. Of course, one can climb out of the pool and walk normally. But particles can never escape the Higgs field since it is everywhere, including the vacuum of space.

 

Inertia walking through a pool

Now imagine that two people of the same size walk through the water, but one feels like they weigh 150 pounds, and one feels like they weigh 26 tons (52 million pounds). That is the situation in particle physics. The top quark has a mass of 350,000 times greater than the electron. That is the same difference in mass between a perch and a blue whale. This is because the top quark interacts much more strongly with the Higgs field than the electron. Physicists do not know why. This is a question for future particle physics research. Top

 

If the electron was a perch...

 

... the top quark would have the mass of a blue whale. On this scale, a muon would have the same mass as the diver in the photo of the whale.

How can we find the Higgs boson ?
  We can produce Higgs bosons in high energy collider accelerators like the Tevatron at Fermilab or the LHC at CERN. The basic idea is that if you produce two beams of high energy particles, like protons accelerated to trillions of electron volts of energy (as compared to electrons with 9 electron volts in a 9-volt battery), then bang the beams together, the energy from the particles gets converted into matter, and one can create new particles, more massive than the protons themselves. Specifically, E = m*c^2 is a famous equation where E is energy, m is mass, and c is a constant, the speed of light. We take the trillions of electron volts of energy E and can use it to make particles with masses hundreds of times more than that of protons, like what we expect the mass of the Higgs boson to be.

 

The Tevatron gives particles the energy as if 100 billion (100,000,000,000) 9-volt batteries were strung together.

That is not the full story though. Higgs bosons are unstable, and if produced they quickly decay back into more stables particles with higher energy. The final result could be dozens or hundreds of low mass stable particles like protons, electrons, and photons. The physicists' job is to build a detector to observe all the final particles produced and then use software to reconstruct that a Higgs was produced. Some examples of detectors are CDF, D0, ATLAS and CMS.

The final product of producing a Higgs boson is a mess of high energy stable particles interacting with the detectors, but this is also the final product of just about every collision that happens in the detectors. Therefore it is with only a given probability that Higgs bosons can be identified. Only one out of 1,000,000,000 collisions produces a Higgs boson. The rest we call background. Even after doing our best analysis of a specific collision, we might decide that there was a 10% chance of it actually being a Higgs boson.

Therefore, the key to discovery is collecting enough data so that there are many collisions each with a small probability of having a Higgs boson in them. Then using a statistical analysis, we can determine that we found the Higgs boson ! Top

Who is looking for the Higgs boson ?
The CDF (Collider Detector at Fermilab) and D-Zero (the name is a coordinate) experiments at Fermilab are currently the only experiments which are producing, detecting, and reconstructing possible Higgs bosons. They are running 24 hours a day, collecting data, while their scientists perfect their algorithms for reconstructing Higgs bosons and statistically analyzing the data. These experiments have around 500 collaborators from around 20 countries around the world, representing around 100 universities and institutions. In addition to looking for the Higgs boson, they also each produce 40 publications per year on other particle physics measurements. Like what is the mass of the most massive particle in the universe to 1% precision ? Or is there a difference between matter and antimatter ? (called "CP violation") Or are there extra dimensions in our universe ?

Later in 2009, the ATLAS and CMS experiments at will run at a high energy and collects Higgs bosons at a higher rate than at Fermilab. So far, the machine has succeeded in circulating its beams at low energy around the accelerator. This year collisions at world record energies are planned. Then to find the Higgs boson, it is a matter of accumulating enough data. This will likely push chances for the Higgs boson to 2012 according to their most recent time table which has them running in 2009 and 2010, and then shutting down for renovations in 2011. Top

Where is the Higgs boson being searched for ?
CDF and D-Zero are located at Fermilab in Batavia, Illinois, USA, currently the highest energy collider in the world at 2,000,000,000,000 electron volts (2 TeV). These experiments are located on different sides of a 4-mile underground ring, which is a tunnel just about big enough for a golf cart to drive through. You can see the ring from satelite images. Do a "google maps" satellite view on "Batavia, IL" and see if you can spot the rings. The ring is inside the laboratory surrounded by a maintained wildlife prairie, next to fishing ponds and a herd of buffalo. Batavia is about 40 miles from downtown Chicago, and many physicists live there and commute.


CMS and ATLAS are located at CERN on the border of France and Geneva, Switzerland. The tunnel runs deep under the surrounding towns and city and is therefore not visible from space. The planned total energy will be 14 TeV, seven times higher than the Tevatron. To start off with, in winter 2009 its energy will be 0.45 TeV + 0.45 TeV, and soon after it will ramp up to 7 TeV (3.5 TeV per beam), and after a year or so, will ramp up to 14 TeV (7 TeV per beam). Top

Higgs boson for novices

What evidence is there for the Higgs boson ?
There is no direct evidence yet for a Higgs boson. The indirect evidence is based on theoretical arguments and experimentally observed data. The Higgs mechanism, including the Higgs boson, is the simplest explanation which accomplishes several things. It provides a way for particles to acquire mass, and it allows one to make calculations of production and decay of massive particles. Previous theories which gave particles mass failed because the formulas would break, and for instance calculate probabilities for things happening as infinite. Another reason to believe in a Higgs mechanism, is that it is a theory which allows the weak force to have heavy bosons (the W and Z bosons), while the electromagnetic force has a massless boson (the photon). It is also a theory which allows the electromagnetic and weak forces to unify at high energies, but for this electroweak symmetry to break at low energies. Since every measurement of the electroweak theory and of electroweak symmetry breaking has proven successful, in some cases to precisions much better than 1%, we have every reason to believe that the Higgs mechanism, and the Higgs boson it predicts, are real phenomena. Detecting the Higgs boson will be the only real proof though. Top

Why is the Higgs boson worth finding ?
If you've read this far, perhaps you already have an answer to that. Finding the Higgs boson is more than just the discovery of another particle. It is the proof of a mechanism which generates mass for fundamental particles, and it is also proof of the strange idea that the universe is filled with a field which generates mass.

In more personal terms, the Higgs boson has been sought after for many years. At the Tevatron, hundreds of physicisists have been contributing to the effort to find it. Finding it - or alternatively, proving it didn't exist - would be a huge achievement for those searching. Top

Why is the Higgs boson so elusive ?
We think we know approximately what the Higgs mass is, and the Tevatron at Fermilab is capable of producing Higgs bosons of this mass. The CDF & D0 detectors are also capable of detecting a significant number of them. So why is it still hiding ?

There are two main issues, production rate and backgrounds. The standard model makes very specific predictions for how many Higgs bosons will be produced in the Tevatron data, and how these Higgs bosons will decay into other particles. These predictions vary according to the actual mass of the Higgs boson. It turns out that this rate is small relative to other processes that we have measured at the Tevatron. "Backgrounds" are other physics processes which produce events that look very similar to the Higgs boson. So for instance, one might expect to be able to detect Higgs bosons in events with one electron, one neutrino, and two b quarks. Well, it turns out that for every 10 Higgs events that have this "signature" there are thousands of backgrounds that have the same signature. A particular event in the data cannot be determined to be signal or background with 100% certainty. But if one collects enough events, one can study kinematic distributions like the energy and masses of the particles in the events, and use this to determine statistically whether there are Higgs bosons in a large dataset. So the name of the game is collecting lots of data, and studying kinematic distributions. Top

How close are we to finding the Higgs boson ?
We can quantify how close we are to finding the Higgs boson depending on its mass. This is done by dissecting the data according to the known physics processes and their possible variations, and then asking : if Higgs bosons were in the data, how much is the maximum amount that could be there. For instance, let's say there are 100 events in the data, and our standard model predicts that there should be 108 events of background physics processes (not Higgs), as well as 2 Higgs events. Based on a full analysis which includes statistical fluctuations as well as other uncertainties, we might conclude that we are 95% certain that there is not more than 20 Higgs events in the data. Since 20 events is 10 times larger than the 2 Higgs events the standard model predicts, we therefore set a limit on Higgs of 10*SM (read "ten times the standard model expectation"). Our goal is to get an analysis which could get this number below 1*SM. Once we are at 1*SM, we will either see an excess of events consistent with our hypothesis for the standard model Higgs boson, or we will see that the number of events is consistent with a standard model without a Higgs boson. Top

Advanced Higgs boson

What does it mean to exclude the Higgs boson ?
To exclude the Higgs boson means that we find that the data is inconsistent with the hypothesis that there is a Higgs boson. Now there are a few points here. First, we must quantify how confident we are that a Higgs boson is excluded. Typically, we quote 95% confidence level exclusion which means that there is a 95% chance that the Higgs boson is excluded, *but* there is also a 1 out of 20 chance that the Higgs boson does exist, and the data fluctuated such that we didn't see it. Therefore, we might seek to make this exclusion stronger. The other point is that by direct searches, we can only exclude specific hypotheses for the Higgs boson. The hypothesis in our case is its mass. We can test a range of masses of the Higgs boson, and find that the Higgs boson is excluded from having a mass of between 160 and 170 GeV/c^2 (1 GeV/c^2 is the mass of a proton) with 95% confidence level, but at the same we are unable to exclude or find evidence for masses less than 160 GeV/c^2 or greater than 170 GeV/c^2. There are four main reasons why we can exclude at some masses and not others : 1) the Higgs boson decays to different particles depending on its mass, 2) the Higgs is produced at different rates depending on its mass, 3) the physics processes which mimic the Higgs (backgrounds), vary depending on the assumed Higgs mass, 4) data fluctuates statistically, and sometimes you just get lucky or unlucky and five expected events fluctuates to 0 or 10 observed events. Top

How much energy do you need to find the Higgs boson ?
The energy needed to find the Higgs boson depends on a few things. First, it depends on the Higgs mass. This is an example of E = mc^2. The Energy (E) produced in a collision must be enough to produce a particle with mass m. Second, the energy needed depends on how you produce the Higgs. At an accelerator called LEP, they attempted to produce Higgs bosons with Z bosons, so the energy they needed was equal to the mass of the Higgs boson *plus* the mass of the Z boson. LEP kept increasing their energy until their magnets could no longer yield the magnetic field necessary to keep the beams moving in a circle. They were able to exclude (see above) a Higgs boson with mass up to 114.4 GeV/c^2. Third, it depends on what particles you are colliding. At the Tevtron, we collide protons and antiprotons, each with 1000 GeV/c^2. But to produce a Higgs boson, we don't collide the actual protons and antiprotons, but instead we collide the quarks or the gluons inside them. These quarks and gluons on average only have only a fraction of the energy of the energy of the protons. So, although the Tevatron produces collisions of 2 TeV, it is extremely rare for even half of that energy to get used in the collision. That said, since we expect the Higgs boson to have a mass between about 114 and 200 GeV/c^2, the Tevatron has enough energy to find the Higgs boson - it just needs enough data to pull out the signal. Top

How much data do you need to find the Higgs boson ?
The amount of data necessary to find the Higgs boson depends basically on how many Higgs signal events we can reconstruct from our detector, how many mundane background events are produced with the same "signature", and how well we can separate the Higgs signal events from the mundane (non-Higgs) background events. The analyses done at CDF and D0 are good enough to observe (if it exists) or exclude (if it doesn't) the Higgs boson if we had enough data. We can determine how much data we need by using simulations to see what our upper limit on the amount of Higgs bosons in the data is based on the expectations of the standard model, and then extrapolating these results, by seeing how much extra data we would need for the upper limit to be equal to the actual expected Higgs signal. See How close are we above for more information.

There is some statistical variation of these numbers, since 90 events could easily fluctuate to 100, and 2 could fluctuate down to 1 or 0 or fluctuate up. Also, we try to take into account uncertainties in these predictions (called systematic uncertainties) which can change our expected number of background or signal events. For instance, lets say we measure our electron finding efficiency in our detector to be 50 +/- 5% (read "50 plus or minus 5 %"), this means that there is a reasonable chance that we may only find 45% of electrons or 55% of electrons. If the Higgs events we are looking for have electrons, then we have to take into account the uncertainty on electron finding when we evaluate how many Higgs events we expect there to be in the data. There are typically a dozen or so such uncertainties in a particular Higgs search, ranging from identification efficiencies, to uncertainties on energy measurements, to uncertainties in rates of background processes. Top

Advanced

What does "a limit" on the Higgs boson mean ?
A "limit" on the Higgs boson is a limit on how many Higgs bosons are produced in a certain amount of data. The standard model predicts a certain number of produced Higgs bosons, and the goal is to get the limit low enough so that one can test if the the prediction is correct or not. See How close are we to finding the Higgs boson ?
for more information. Top

How exactly do you produce a Higgs boson ?
There are a few ways to produce Higgs bosons at the Tevatron at Fermilab which collides protons and antiprotons together. The primary way is when a gluon, which is the particle that holds the quarks together inside a proton, begins to collide with a different gluon from the antiproton. Some of the time, when these gluons have enough energy, and when there is a quantum fluctuation like a roll of a dice choosing a particular number, the gluons will exchange a top quark, and the top quarks will merge, and transform into a Higgs boson. This might seem strange, but this is the kind of thing that is going on all the time in the sub-atomic world. Forces allow particles to transform, as long as momentum and energy are conserved at the start and finish of the process, and the standard model is obeyed in between. There are several other ways Higgs bosons can be produced: they can be emitted from a high energy Z boson or W boson, or they can be formed when W bosons are emitted from a quark and antiquark in the collision, and the W's merge into a Higgs boson like the top quarks did in the above case. At CDF and D0, we simulate all possible modes of Higgs boson production, and we develop algorithms to search for each mode of production. Top

 

The ways to produce a Higgs boson. The highest production rate at the Tevatron is from "gluon fusion" (denoted as "gg fusion") where gluons from the proton and antiproton interact through a quark loop to produce a Higgs boson. The top quark is so massive that it dominates the quark loop and the other quarks can be ignored. The next highest production rate at the Tevatron is from Higg-Strahlung, otherwise known as "associated production" because the Higgs boson is produced in association with a W or a Z boson. In this mode, a quark and an anti-quark collide, producing an off-shell W or Z boson. "Off-shell" means that the W or Z boson is virtual, and can be thought of to have its energy converted into extra mass for a moment before decaying back in to a real "on-shell" W or Z boson after emitting a Higgs boson.



How does a Higgs boson decay ?
A Higgs boson, once produced, prefers to transform or "decay" into the heaviest particles it can. Depending on what is the mass of the Higgs boson, it will decay in different ways. For instance, if the Higgs boson is 120 GeV/c^2, it will decay 68% of the time to a b quark and an anti-b quark, it will decay to a W+ boson and a W- boson 13% of the time, 7% of the time to a tau+ and a tau- lepton, 7% of the time to two gluons, and 3% of the time to charm quarks. But, if the Higgs boson mass is 160 GeV, it will decay to a b quark and an anti-b quark 4% of the time, and to W+ boson and a W- boson 90% of the time. These percentages mean that one can never predict exactly what a Higgs boson will decay into, only the chances that it will decay into them - this is because of the nature of quantum mechanics where although there are accurate predictions, there is always a fuzziness about what actually happens in any given situation. Our analyses at CDF and D0 take into account the possible ways Higgs bosons can decay and the statistical fluctuations that can happen in our data. Top

 

This plot shows how a Higgs decays as a function of its mass. Because of quantum mechanics, we do not know exactly what will happen. We only know the probability of a Higgs decay. To read this, first pick a mass. Second, draw a vertical line up from that mass. The top curve has the highest probability of occurring, or the highest branching ratio. For instance, at a mass of 120 GeV/c^2, the curve for H decaying to a b quark and anti-b quark has a value of 0.7 (careful, its a log scale on the y-axis!), so a Higgs boson decays to b quarks 70% of the time. The branching ratio to W+ and W- is about 0.15, or 15%. But at a mass of 150 GeV/c^2, we see the opposite: 70% decay to W+ and W- and, 15% decay to b quarks. This means that we have different search strategies depending on the mass of the Higgs boson.



How do you detect a Higgs boson ?
You can't see a Higgs boson with your eyes, or use a microscope to see a Higgs boson. And if you produce one, it instantly decays into other particles in a time that is too short to observe. So how do you detect them ?

Many PhD theses and publications are dedicated to credibly explaining this question. But here is the short answer. First you do a computer simulation of Higgs bosons being produced and decaying according to calculations of the standard model. Next, you develop an algorithm to select all the particles that our produced in these decays. Next, you consider all the other physics processes or "backgrounds" that can produce these same particles, and you simulate them as well, and select the particles. Then you develop a method for telling the difference between the signal and the background by comparing energies, momenta, angles, masses, and other quantities that you can measure in the detector. Then you compare the data to your model of signal and background processes, and calculate how much signal is in the data. Also, see How can we find the Higgs boson ? and Why is the Higgs boson so elusive ? . Top

What are the experimental uncertainties in detecting Higgs bosons ?
There are quite a few experimental uncertainties involved in detecting Higgs bosons. For instance, lets take the case where a Higgs boson decays to a b quark and an anti-b quark in our detector. The b quarks will undergo some decays and transformations that turn them into stable particles, and then a spray of these particles called a "jet" collides with the CDF calorimeters. The calorimeters measure the light deposited when the jet traverses the detector. We also have tracking chambers that can reconstruct the direction that charged particles traversed the detector, as well as the spot of "vertex" in which the particles were produced from the quark decay in order to determine if it was a b quark or not. From these measurements, we determine the energy of the jet, as well as the distance that the b quark traveled before it decayed into the charged particles. We can get the jet energy correct to within 15% of the true value for any particular event, and on average we might be able to get the jet energy correct to within a few % of the average true value. Our algorithm to identify b quarks might find 50% of them, but there is a 5% uncertainty, so it might be 45% or 55%. This algorithm might falsely identify non b quarks as b quarks. We might determine that it does this 1 +- 0.1 % of the time. All of these uncertainties are propagated through the analysis in order to determine the effect on our ablity to determine how many Higgs events are in the data. Top

How do I interpret the Higgs limit plots ?
Okay, now you know everything you need to know. You just need a guide to understand how to interpret our results and the figures we show. The main plot we show is the limit plot. The x-axis is the mass of the Higgs boson. At about 20 different masses along the axis, we calculate limits. See What does "a limit" on the Higgs boson mean ?. The y-axis is the limit in terms of its ratio to the standard model predicted cross-section for Higgs production. So achieving 1*SM implies that we would be able to identify a Higgs boson with 95% Confidence Level, 2*SM means that we would be able to identify a Higgs boson if it was produced with twice the expected cross-section. At each mass point, there may be more than one analysis contributing, and so we do a simultaneous fit of each analysis, where we vary the uncertainties, and extract the maximum amount of Higgs that is consistent at the 95% confidence level with all the analyses. The expected limit demonstrates how much Higgs production we would expect to find based on our model of all the backgrounds (non-Higgs physics processes) in the data. We get the expected limits from evaluating the limit in 100,000s of simulated reproductions of our dataset assuming no Higgs boson, each "pseudo-experiment" with variations in the model to account for experimental and theoretical uncertainties. There is also a colored band around the expected limit which shows what the range is we would expect for about 2/3 of our observed limits when we check the data. The other curve is the observed limit which is what we actually see in the data. In general, we expect the observed limit to fall within the range of values colored by the expected uncertainty band, which demonstrates that the data agrees with the model. However, the presence of a signal can also alter the agreement between expectations and observations. If the observed limit is lower than the expected limit, we would say the data fluctuated low, and we would therefore have a better limit than what we expected. If the observed limit is higher than the expected limit, we would say that the data fluctuated high, and we would have a worse limit than expected. However, if there is signal, we would also see the observed limit higher than the expected one. Once we are down to SM*1 sensitivity, if there were a signal, we would expect to see an excess. If we instead see that the observed limit is lower than the expected limit, or the same, then we can exclude a range of hypothetical masses that the Higgs boson can have. Top

 

Here is a Higgs limit plot from March 2009 from the Tevatron. We typically update these once or twice a year, so maybe this isn't the newest one. This plot says "Tevatron", which means it includes both CDF and D0, and about 10 separate analyses.

 

Here is a CDF limit plot from August 2009, so this uses only half the Tevatron data since D0 is not added.

 

Here is the same CDF limit plot from August 2009, but this shows all the analyses that are contributing. It is very busy to look at. The bottom red curves are the same as the limit on the CDF limit plot to the left.

 

A limit plot from an individual channel, in this case a search for Higgs produced by associated production with a W boson, and decaying to a b quark and an anti-b quark. Note that this analysis is mainly sensitive to low mass Higgs, providing its best sensitivity for masses close to 100 GeV/c^2.



Still a skeptic ?

Is this all just a theory ?
"Theory" is a word that is tossed around a lot these days. The standard model of particle physics is a working model that has been precision tested. Many elements of the Higgs mechanism are already well tested, precisely because the Higgs mechanism is able to explain real observations. The facts are that the photon is massless, while the W boson and Z boson have precisely determined masses. Quarks and leptons have mass. We predict how often quarks, leptons, and bosons are produced through electroweak and strong interactions, and how often they decay, and into what particles they decay, and we have verified these predictions to high precision. We have made measurements of all other parameters of the standard model and everything fits together precisely. (How many times have I said the word "precise" so far ?) We know that something like the Higgs mechanism has to be there in order to explain our observations, and also to solve some theoretical dilemnas that would prevent the standard model from making proper calculations. The Higgs mechanism just happens to be the simplest theory which explains everything. Top

What if no one ever finds the Higgs boson ?
Physicists agree that if there is no Higgs boson, there must be something in its place. So no matter what, we expect there to be new physics to explain how particles acquire mass. In other words, the standard model without the Higgs boson will eventually break and fail to make calculations that work. So if we prove a Higgs doesn't exist, it is actually quite exciting, since it implies an even more exotic solution. Top

What are the theoretical uncertainties ?
The biggest theory uncertainty is the mass of the Higgs boson - the theory provides no prediction for the mass. However, by measuring the mass of the heaviest particle, the top quark, we can provide a prediction for the Higgs boson mass. Likewise, there are approximately 20 measurements we can make that each give an estimate of the Higgs boson mass. The reason this works is because the Higgs boson mass shows up as a term in equations that relate quantities in the standard model. Since we don't measure any of these 20 quantities with infinite precision, we can't predict the exact mass of the Higgs boson. As of 2009, taking into account all of these ~20 measurements, and assuming that these measurements are all related as specified by the standard model of particle physics, we determine that the Higgs boson mass is 87 +35 -26 GeV/c2.

Of course, there are other theoretical uncertainties related to how exact the equations are. For instance, in order to predict how often Higgs bosons are produced at a given mass, theorists must perform very complicated computations with many parts. As each part is computed, approximations are made and corrections are incorporated.

Think of these corrections and approximations like this. Lets say your job was to determine how many people go from New York to California per year. To first approximation, you might count up all the flights from New York to California from all the New York airports to all the California airports, then find out how many passengers each plane holds, and then add it all up to get your answer. However, you realize that there are some corrections to this. For one, you would be missing all the people who made connections at other airports along the way. This you might have to approximate since you might not have a way of determining what fraction of the people who flew from New York to Colorado, then flew to California. You would also not be taking into account how many passengers were on the planes. What if the only way to check this was to measure it by flying back and forth to California and seeing how many seats were filled. Then you take your measurement, and the uncertainty of that measurement, and apply it to your calculation. Next, you must take consider the people who drove from New York to California. Some factors are negligible, and you ignore them completely. For instance, the number of people who bicycle this distance might be non-zero but it makes such little difference that it doesn't matter. In the end, once you do all the work, you might come up with a prediction and an uncertainty. You publish the result, and then others read it, and determine that you have not considered all of the highways that people use to make the trip, and that your result is wrong by a few percent. This goes on, and over time, you converge on the correct answer.

This is the nature of theoretical uncertainties. But of course, the calculations are much less familiar than the example above. Top

Are there alternate theories ?
There are alternate theories which are not part of the standard model of particle physics, and there are plenty of them. These theories typically attempt to solve some additional problems. Supersymmetry is the most popular alternate theory. It doubles the number of quarks and leptons, and predicts that there are five Higgs bosons instead of one. Although the extra number of particles (none of which have been observed) make it more complicated than the standard model, supersymmetry or "susy" has some nice features. It adds a symmetry between matter and force particles, and it makes it possible to unify the forces at high energies, meaning that it is on the right track for creating a grand unified theory. At the Tevatron, we search for supersymmetric Higgs bosons as well as standard model Higgs bosons. In fact, some of our standard model Higgs boson searches could also reveal supersymmetric Higgs bosons.

One theory holds that there might be a higgs boson that only interacts with quarks and leptons (called "fermions") and not with the bosons. We search for this "fermio-phobic" Higgs boson at the Tevatron also. Another theory called "technicolor" holds that there is no Higgs boson, but instead there are new interactions that generate the masses. These technicolor models predict new particles which can be observed in similar ways to how we observe the Higgs boson, and CDF and D0 search for these particles as well. Top

Who cares if we find the Higgs boson ?
Knowledge is so important because it can be built upon. Until we find out how particles acquire mass, we cannot take the next step in knowledge.

If gravity was not understood, we could not launch satelites, or build tall buildings, or fly planes. If electricity was not understood, we'd have no electrical devices.

The frontier of knowledge about the smallest objects in the universe, the highest energies of the universe, and the origins of the forces that control objects at these energies has brought mankind to a point in time where we will soon be able to understand how particles can gain mass. This is a huge feat of knowledge for humanity, and the full implications of it may not be understood for many years, until we have built upon this knowledge many times over in order to learn and create new knowledge and technology.

The price of knowlege can probably be judged by how much you, and all of humanity for all future generations, would be be willing to charge to have that knowledge taken away for all time. For instance, if someone said "who cares about the principles of electricity?", and decided that this knowledge was not important, how much would they charge someone to take away their t.v., computer, cell phone, fridge, medical equipment, and everything that electricity helped to produce for them. Now, take that price, and add it up for each person in the world, and then add it up for all their future kids, and all future generations.

Pure knowledge at the boundaries of creative thought and observation drives us to become something our ancestors could never have imagined. The truth is that until we ask the questions, and learn the answers, we don't know what we will gain. Top

Particle Physics Basics

What is particle physics ?
Particle physics is research that answers a few questions :
  • What are the smallest building blocks of the univese ?

  • What are the forces that govern the universe at a quantum (particle) level ?

  • What are the equations that can predict the behavior of particles and forces ?

  • What are the patterns and symmetries of the universe ?

  • What are the interactions that happen at the highest achievable energies ?

  • What did the universe look like when it started ?

  • How did the universe evolve from its beginning to the state it is in today ?
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    What is the "Standard Model" of particle physics ?
    The standard model of particle physics is a well-tested theory that explains what the fundamental building blocks of the universe are, and how they interact with each other. The particles are divided into fermions and bosons. Fermions, such as electrons, are the constituents of matter. Bosons, such as photons, are force carrying particles, and can transform matter from one particle to another or transform its properties. All fields, such as electric fields, are recognized as being composed of individual particles or quanta. The fields and forces that we have a quantum-level understanding of are the electric field, the weak field, the and the strong field. We also have a unified theory of the electric and weak fields called the electroweak field theory. The Higgs field is unverified so far, but is part of the standard model. Finding the Higgs boson will provide that evidence. And finally, we do not have a quantum field theory for gravity. We know very well how gravity works by warping space and time at large scales, but we have not detected its quantum which would be called "gravitons", and we do not have a predictive theory for gravity at the quantum level. Part of the reason for this is that gravity is so weak compared to every other force. Also, see Is this all just a theory ?
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    How do you do particle physics ?
    There are many different ways to do particle physics depending on what you want to study. Here are some examples.

  • We accelerate a beam of particles to high energies using electric fields or electromagnetic waves, and then we collide this beam in to a target. We then use detectors to study the collisions. This type of particle physics was responsible for most of the early advances that identified a zoo of particles.

  • We accelerate a beam of particles to high energies, and then collide it with another beam of particles that has high energies. This has the advantage that there is much more energy available to produce new particles with higher mass. This is how we search for the Higgs boson, and is also how measurements of the heaviest particles, including the W and Z bosons, the top quark are done.

  • We accelerate a beam of particles, collide these particles into a target to produce other more exotic particles, then we use magnets to select out the particles we want, and then we steer these particles, and collide them with other beams of particles or other targets.

  • Cosmic ray particle physics : The earth is being bombarded with cosmic rays (or nuclei emitted from stars) of varying energies. We can study the nuclei before they interact with the atmosphere by using detectors in satellites or balloon experiments, or after they hit the atmosphere and decay into other particles such as muons. Humans (and everything else on earth) are being constantly bombarded with muons. The highest energy particle collisions are studied by arrays of ground detectors that take up hundreds of square miles. These experiments measure collisions that have thousands of times more energy than the Tevatron or LHC. But there are very few of these collisions, and they cannot be precisely studied as we do in a controlled LHC environment.

  • Out in space, we can also study gamma rays, or high energy photons, that are emitted from various sources in the universe.

  • On the ground, we can study neutrinos coming from sources throughout the universe. The advantage of neutrinos is that they interact at very low rates and they are neutral, so they are our best way of pointing back into the universe.

  • Nuclear reactors produce a huge number of neutrinos, and we use neutrino detectors to study them.

  • We try to detect dark matter (matter which contributes to the mass of our galaxies but is otherwise undetectable) by trying to detect it colliding with matter, or from its decay products. One way to do this is to shield a very accurate detector from background radiation, and then count how often nuclei in the detector get hit by unexplained sources.
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