EPTUpdate on higgs search from CMS and ATLAS at stream here.
2.4 sigma (global) signal for a standard model Higgs at 126 GeV.
If you want a better writeup about the Higgs than mine below, I recommend wikipedia.
The new ATLAS results can be seen at their page.
Introduction
Do you know how when you read about starcraft in daily newspapers it often seems to send a message that doesnt feel accurate? That they somehow miss the point? That they easily focus on some part of it that they find sensational, motivating their article, and blow it out of proportions? And that's not even mentioning the comments the article will get, or what you will read on non-gamer forums.
Well, that is what I feel about particle physics sometimes.
Today, I will take the time to try to provide an unbiased inside view of one of the usual topics covered in newspapers and forums. With some luck, some of you will be able to understand some things a bit deeper, and some may even manage to look through some of the articles you will run into. 
I will try to do it an accurate manner, or at least be honest where I skim thinks over, while still keeping you awake. I also added lots of images!!! :o
I will start by writing a bit about the search for the Higgs particle. If I find the time, and if people seem interested, I may add more topics.
EDIT: I also just realized that I actually have a 10-15 page introduction to particle physics that I wrote for my thesis! So if anyone has the time and wants some background, here is the .pdf: http://www.mediafire.com/?c5eczxbalqm2o0f
![[image loading]](http://i.imgur.com/KFVpQ.jpg)
Ok, after having spent half an hour making the title image...
What you hear: mythbusting
If you ever have heard of particle physics, you have probably heard of the big accelerator they built in Geneva. If you have read an article about it, you will know that the name of the accelerator is "LHC": Large Hadron Collider, and is situatied in CERN: Centre Europe Research Nuclear, or however the french guys spell it. The article usually mention that they collide protons with each other.
So far, all this is accurate!!
It then diverges a bit depending on the angle of the article. Some of them (the more serious ones in general) continue and ask why this was built. A very interesting question for that matter, but I'll try to stay on track here.
They will in general end up spending a lot of time talking about the search for the Higgs particle.Also this is pretty accurate.
Of course, you could go on some economical tangent here and argue that it is built just to get money for all the useless particle physicists. While there may be some merit to that approach, I will ignoring that for now, and just leave say that I'm pretty sure the Higgs takes a big role in any official motivation they send to governments.
As it sounds fancy, you will often hear things along the lines of "recreating the big bang".
- It makes very little sense.
The one thing you could say is that "we accelerate particles to an energies that hasnt been around since the big bang". That would of course also be innacurate, but as the statement is a bit more specific, it is now wrong in two ways.- We get hit by particles with thousands times as much energy from cosmic rays on a daily basis.
- You shouldn't be talking about "big bang" here. Rather "the early universe". The big bang, the actual time = 0 start of universe divergence, is so full of infinities so it shouldn't even get close to a cute little particle accelerator. Also, of course, we have no idea if there were any time = 0 start of the universe divergence, as it is impossible to get any data from a divergent point. Not that it'd be a point either, but in lack of better words. We do have some pretty good astronomical observations giving us information about the early universe though. The article will go for the sexier "big bang" every time though.
- We get hit by particles with thousands times as much energy from cosmic rays on a daily basis.
Many articles and interview use the word "the god particle".
- This is not used in the scientific commuinity (as you may have guessed), or when it is used, it is for the lulz, and will cause smiles and giggles.
- Well, fine, call it what you want. >_>
An ambitious article will then go on and try to explain what the Higgs is, why it is so important, why we need this huge thing to find it, and why we hope to find it with this huge thing. At this point, the articles differ wideley depending on who they asked for information, and how the repporter chose to interpret the answer when he didnt understand it.
- Some are decently accurate, but very few manage to actually communicate any accurate information to the reader in a way that makes sense. The most accurate ones tend to make sense only to people who already know the field, while non-physicists will skim or skip that part.
- Others oversimplify, and make some analogy that maybe makes sense in one way, but that is extended far beyond the reach of the analogy, giving a very odd picture of things.
- Others yet (better not give examples, or what do you say new scientist?
) just dont bother with what is accurate, and write whatever sensational things they feel like that day and present it as truth to sell better.
So finally, let me try to give my explanation to what is actually going on with the Higgs at LHC.
The current state of particle physics
Yes, I need to take a step back. Some context is needed to understand how things fit. It'll be brief, dont worry. + Show Spoiler +
That's what SHE said!!!!! ZINGGG!!!!!1111oneone
So, essentially, most things are sorted out. We can predict or at least explain all experiments (with very few exceptions) very well. Some with crazy high accuracy. Imagine firing a ball across the atlantic ocean with some cannon or catapult or whatever. Imagine some device measuring the speed of the ball as it leaves the firing device. Now the measuring device has a model for air resistance, wind, rotation of the earth and everything that can be used to predict where the ball will land. It can predict which house it will hit on the other side. Let's say its some kind of supermarket. The model can predict which row in the supermarket it will hit. Let say, it will hit the kitchen section. Lets say its a small ball. Then it can predict which thing it will hit. Like for example a cheese grater. Let's say its a REALLY small ball! it can then predict which hole in the cheese grater it will pass through. It will predict if it will bounce of the lower of upper end of the hole. From the other side of the atlantic. Thats the kind of accuracy we are taking.
![[image loading]](http://i.imgur.com/8eOcW.jpg)
An iPhone touch pad does not have much accuracy in the 8:th digit.
So how is this explained so well you ask? Actually im pretty sure you dont, but nm that. Ill answer anyways. It can actually be formulated very neatly in a way that bring the matter and the forces together. One principle to rule them both, one principle to find them. One principle to write them both, and in the Lagrangian bind them. In the land of CERN, where the LHC lie. + Show Spoiler +
![[image loading]](http://i.imgur.com/5XA14.gif)
+ Show Spoiler [Physics details] +
What I'm referring to here is Gauge Symmetry. When you build a quantum field theory theory (which is how you do a particle physics theory, if you want it to be consistent with quantum mechanics and special relativity), you do it by writing down a Lagrangian. A Lagrangian is essentially only describing how you calculate the action (the kinetic energy minus the potential energy) of a certain configuration of particles (or rather fields, as particles are described by fields).
A property of quantum mechanics (without special relativity) is that you can change the phase (that is multiply by a constant e^(i*phi)) of a wave function, and it does not change any observables. This is referred to as global gauge symmetry. In a Lagrangian, one would like to keep that property, and to further be consistent with the limited speed of light, one could make an argument for why the symmetry should be LOCAL. With local means that the phase depends on the position in space, ie multiplication by e^(i*phi(x)).
So, gauge symmetry is to require that the physics do not change when yuo multiply your particle fields with a phase e^(i*phi(x)). Assume that you first put in a fermionic field, that means a normal matter particle like an electron. Looking at http://en.wikipedia.org/wiki/Gauge_symmetry#An_example:_Electrodynamics, you see that by requiring gauge symmetry you HAVE TO add the new field "A", which is the photon, the carrier of electromagnetism. That is, just by requiring local gauge symmetry and assuming that there are matter particles, you PREDICT the existence of electromagnetism!
By requiring a more general gauge invariance you will get out also the weak and strong force. (from the rotation groups SU(2) and SU(3) for those that likes group theory).
A property of quantum mechanics (without special relativity) is that you can change the phase (that is multiply by a constant e^(i*phi)) of a wave function, and it does not change any observables. This is referred to as global gauge symmetry. In a Lagrangian, one would like to keep that property, and to further be consistent with the limited speed of light, one could make an argument for why the symmetry should be LOCAL. With local means that the phase depends on the position in space, ie multiplication by e^(i*phi(x)).
So, gauge symmetry is to require that the physics do not change when yuo multiply your particle fields with a phase e^(i*phi(x)). Assume that you first put in a fermionic field, that means a normal matter particle like an electron. Looking at http://en.wikipedia.org/wiki/Gauge_symmetry#An_example:_Electrodynamics, you see that by requiring gauge symmetry you HAVE TO add the new field "A", which is the photon, the carrier of electromagnetism. That is, just by requiring local gauge symmetry and assuming that there are matter particles, you PREDICT the existence of electromagnetism!
By requiring a more general gauge invariance you will get out also the weak and strong force. (from the rotation groups SU(2) and SU(3) for those that likes group theory).
A catch...
Well, of course, there is a catch. Namely that there is a measurement that blatantly violates that principle. Like imagine Flash playing your grandmother TvZ. On lost temple. Flash on 3, grandmother on 12. and your grandmother plays blind. Unless she already is blind, in which case she plays without sound. For you sc2 people, it is like a diamond player having a collosus in a PvZ vs anyone.
![[image loading]](http://i.imgur.com/2gl2s.jpg)
This is where you mom went last night...
+ Show Spoiler [Physics details] +
The problem is the mass of the W and the Z, the force carriers of the weak force. The photon and gluons are massless and cause no problem, but by putting in a mass term in the Lagrangian for the W and Z, you break the gauge symmetry. W and Z are measured to have very large masses in fact, just below 100GeV. Which is why I do the flash-analogy. Notice in the wiki link for gauge symmetry above that the mass terms for the matter particles (fermions) are fine. The problem is the mass of the force carriers.
Notice in the wiki link for gauge symmetry above that the mass terms for the matter particles (fermions) are fine. The problem is the mass of the force carriers.... with a solution!
Luckily, some smart guys some while back came up with a sneaky little way in which the principle can be saved. A hidden particle that can make it LOOK like the principle is broken, but that actually conserves it if you look close enough.
![[image loading]](http://wiki.teamliquid.net/starcraft/images2/7/7d/Dark_Templar.png)
Looking close enough?
+ Show Spoiler [Physics details] +
Look at how electromagnetism forms from the fermionic kinematic term again in the gauge symmetry wiki link. Notice that it is the derivative that is not gauge invariant on its own, because when you derive the e^(i*phi(x)) you get an extra term from the derivative of the product. You then insert the field A which you define to have a gauge transformation that exactly cancels out that extra term from the derivative.
This is essentially the same trick. You have a term that gives you extra terms when you transform it. So you introduce a new field (associated to a new particle) with a gauge transformation that exactly cancels out the problem. So in the same way that the fermion symmetry breaking "predicted" electromagnetism, you can say that the massive W and Z bosons predict the Higgs.
This is essentially the same trick. You have a term that gives you extra terms when you transform it. So you introduce a new field (associated to a new particle) with a gauge transformation that exactly cancels out the problem. So in the same way that the fermion symmetry breaking "predicted" electromagnetism, you can say that the massive W and Z bosons predict the Higgs.
Confirmation
Ok, cool! Well, to confirm that this actually is what is going on, let's look closer! How do you look closer then? Well, essentaily the answer to that is: build the LHC. This is related to the fact that small things can only be seen with high energies, but nevermind that here.
And when we build that stupid huge collider, what is it we expect to see that confirm the sneaky trick to restore the principle? Essentially, the answer is: the Higgs particle.
![[image loading]](http://i.imgur.com/OvedK.jpg)
Yo dawg, I put the key to decay in the car, k?
+ Show Spoiler [Physics details] +
Since the Higgs was not found at the LEP (a previous collider), it is know that it cannot have a mass below around 115GeV. Assuming it exists ofc. There are theoretical arguments you can make that makes it uncomfortable to have a higgs with a mass above around 180GeV. (because it would mess up the top mass through loop contributions or something, not sure about the details) So by building the LHC, which will cover masses far above that, we expect to find it if its there.
No Higgs?
All in all, Higgs has to be there for our neat explanation of the rest of particle physics to fit together. No Higgs means that we will have to revise how things are built up, and we may have to start questioning the master principle.
![[image loading]](http://i.imgur.com/GUDna.jpg)
+ Show Spoiler [Physics details] +
If the lower mass range closes, we will start seeing a flood of preprints trying to explain how to make the theory fit. There is for example a wide range of BSM (beyond standard model: "new" physics) models that can hide the higgs, ie make it very hard to detect for the LHC, even if it has a mass below 200GeV. There are other models that bypass the arguments for why the higgs shouldnt have a mass below 200GeV. And then I assume there is a zoo of models that skip the entire Higgs mechanism, and solve the gauge problem in another way, of maybe even gives up gauge theory, what do I know? The one thing that is sure it there will be a flood of preprints, and that no one will know which are correct.
Current state in the Higgs hunt
So, how are we doing? LHC has been running for some while now.
The answer can be summed up in this plot.
EDIT: updated version of this plot in edit at top of post.
![[image loading]](http://i.imgur.com/7Amjh.jpg)
This is how the (standard model) Higgs hunt results are shown within the scientific community. Let me explain the plot.
- The Higgs particle has a mass. It's refered to as m_H or M_H in the plot.
- The Higgs decays into other particles really quick. What we hope to see are the resulting particles it decays into.
- There are other "normal" collisions (ie without Higgs) that can produce the same particles that the Higgs decay into.
- This makes it harder to observe the Higgs.
- Higgs with different masses decay into different kinds of particles. Some are more easy to pick out that others.
- With some Higgs masses it is easier to find the Higgs than with other Higgs masses.
- The current state is described by the black thick line in the plot.
- If the black curve is below the dashed straight line, it means that the Higgs does not have that mass with 95% certainty.
- If it WOULD have that mass, it wouldve been seen by now.
- In the other masses, where the black line is above the dashed straight line, we still do not know.
- As LHC continues to run, more and more masses gets excluded. ie masses where we WOULD HAVE found it by now, if it would be there.
- If the there is no Higgs, we will have closed this entire mass range down by the end of next year. If LHC runs as planned that is.
- People are mainly looking at the range below 155, to the far left in the plot.
- If there is a Higgs (in this mass range), it will be found before that.
Let me try to clarify a bit. At any given point on the x-axis, he further down the black curve is, the more sure we are that there is no Higgs with the corresponding mass. The horizontal line marks where we are 95% certain. So below 155 GeV we still do not know very well (actually the limit is down to 147 GeV now i heard on a meeting two weeks ago). It may be there, it may not. We are pretty sure that there is no higgs between 155 and 190, and not between 295 and 450. It may still be between 190 and 295 (for the experimentalists. most theory people dont expect to find it there).
The dotted curve with the error band shows where we EXPECT the black curve to be, assuming there is no Higgs. As LHC takes more and more statistics, the dotted curve with the error band will move down, as we will get more and more certain that there is no Higgs (if there is none).
If there is no Higgs, the black curve will follow decently inside the error bands (as it is now) and eventually will be below the horizontal line for all masses. At that point we can 95% exclude the Higgs, and there will be articles all over the news about it.
If there is a Higgs at a certain mass, the black curve will not follow the error bands down at that mass as statistics increase. The green and yellow area mean 1sigma and 2sigma deviation from the expected line. If it hits 5sigma above the expected dotted line, it will officially be labeled a discovery, and there will also be articles all over. With some imagination, you can see hints of that to happen around 135 GeV, 250 GeV and 600 GeV, but it can also just be a statistical fluctuation.
The moral of the story: In a year we will know!
QnA:
+ Show Spoiler +
On October 13 2011 20:12 arbitrageur wrote:
Q: Why is LHC taking so long. What's the constraint(s) that stop(s) it from figuring out if it exists in a shorter time scale?
Q: Why is LHC taking so long. What's the constraint(s) that stop(s) it from figuring out if it exists in a shorter time scale?
+ Show Spoiler [A] +
On October 13 2011 20:21 Cascade wrote:
A: It is because of the protons the collide being a mess to disentangle. Actually you can get a hint if you read the part about the strong force in the .pdf i edited in just now.
Briefly, it is due to the proton-proton collisions creating A LOT of events that look very much like an event with a higgs. So there is a huge background to the signal in most cases. They record billions and billions of events, and only a few of them actually contains Higgs (if it exists). It is crazy hard to determine, among the billions of events, if some of them actully are Higgs events or not.
An analogy would be looking for a nail in a haystack, where some of the straws look damn similar to nails.
A: It is because of the protons the collide being a mess to disentangle. Actually you can get a hint if you read the part about the strong force in the .pdf i edited in just now.
Briefly, it is due to the proton-proton collisions creating A LOT of events that look very much like an event with a higgs. So there is a huge background to the signal in most cases. They record billions and billions of events, and only a few of them actually contains Higgs (if it exists). It is crazy hard to determine, among the billions of events, if some of them actully are Higgs events or not.
An analogy would be looking for a nail in a haystack, where some of the straws look damn similar to nails.
Soooo, that said, place your bets! Can TL predict the correct Higgs mass?
Poll: What is the Higgs mass?
No, I will not give "over 9000" as an option.... (50)
38%
No Higgs (31)
24%
over 200 GeV (16)
12%
120-130 GeV (12)
9%
150-200 GeV (8)
6%
140-150 GeV (5)
4%
less than 120 GeV (4)
3%
130-140 GeV (4)
3%
130 total votes
No Higgs (31)
over 200 GeV (16)
120-130 GeV (12)
150-200 GeV (8)
140-150 GeV (5)
less than 120 GeV (4)
130-140 GeV (4)
130 total votes
Your vote: What is the Higgs mass?
(Vote): less than 120 GeV
(Vote): 120-130 GeV
(Vote): 130-140 GeV
(Vote): 140-150 GeV
(Vote): 150-200 GeV
(Vote): over 200 GeV
(Vote): No, I will not give "over 9000" as an option....
(Vote): No Higgs
tldr: + Show Spoiler +
If you dont feel like reading, this is not the thread for you right now. Go play this flash game until your concentration is back: http://not.vlambeer.com/luftrauser/
