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The Higgs. Whaaaa? July 6, 2012

Posted by gordonwatts in ATLAS, CMS, Higgs, LHC, physics, press.
9 comments

Ok. This post is for all my non-physics friends who have been asking me… What just happened? Why is everyone talking about this Higgs thing!?

It does what!?

Actually, two things. It gives fundamental particles mass.  Not much help, eh? Smile Fundamental particles are, well, fundamental – the most basic things in nature. We are made out of arms & legs and a few other bits. Arms & legs and everything else are made out of cells. Cells are made out of molecules. Molecules are made out of atoms. Note we’ve not reached anything fundamental yet – we can keep peeling back the layers of the onion and peer inside. Inside the atom are electrons in a cloud around the nucleus. Yes! We’ve got a first fundamental particle: the electron! Everything we’ve done up to now says it stops with the electron. There is nothing inside it. It is a fundamental particle.

We aren’t done with the nucleus yet, however. Pop that open and you’ll find protons and neutrons. Not even those guys are fundamental, however – inside each of them you’ll find quarks – about 3 of them. Two “up” quarks and a “down” quark in the case of the proton and one “up” quark and two “down” quarks in the case of the neutron. Those quarks are fundamental particles.

The Higgs interacts with the electron and the quarks and gives them mass. You could say it “generates” the mass. I’m tempted to say that without the Higgs those fundamental particles wouldn’t have mass. So, there you have it. This is one of its roles. Without this Higgs, we would not understand at all how electrons and quarks have mass, and we wouldn’t understand how to correctly calculate the mass of an atom!

Now, any physicist who has made it this far is cringing with my last statement – as a quick reading of it implies that all the mass of an atom comes from the Higgs. It turns out that we know of several different ways that mass can be “generated” – and the Higgs is just one of them. It also happens to be the only one that, up until July 4th, we didn’t have any direct proof for. An atom, a proton, etc., has contributions from more than just the Higgs – indeed, most of a proton’s mass (and hence, an atom’s mass) comes from another mechanism. But this is a technical aside. And by reading this you know more than many reporters who are talking about the story!

The Higgs plays a second role. This is a little harder to explain, and I don’t see it discussed much in the press. And, to us physicists, this feels like the really important thing. “Electro-Weak Symmetry Breaking”. Oh yeah! It comes down to this: we want to tell a coherent, unified, story from the time of the big-bang to now. The thing about the big-bang is that was *really* hot. So hot, in fact, that the rules of physics that we see directly around us don’t seem to apply. Everything was symmetric back then – it all looked the same. We have quarks and electrons now, which gives us matter – but then it was so hot that they didn’t really exist – rather, we think, some single type of particle existed. Now, and the universe cooled down from the big bang, making its way towards present day, new particles froze out – perhaps the quarks froze out first, and then the electrons, etc. Let me see how far I can push this analogy… when water freezes, it does so into ice crystals. Say that an electron was one particular shape of ice crystal and a quark was a different shape. So you go from a liquid state where everything looks the same – heck – it is just water, to a solid state where the ice crystals have some set of shapes – and by their shape they become electrons or quarks.

Ok, big deal. It seems like the present day “froze” out of the Big Bang. Well, think about it. If our current particles evolved out of some previous state, then we had sure as hell be able to describe that freezing process. Even better – we had better be able to describe that original liquid – the Big Bang. In fact, you could argue, and we definitely do, that the rules that governed physics at the big bang would have to evolve to describe the rules that describe our present day particles. They should be connected. Unified!! Ha! See how I slipped that word in up above!?

We know about four forces in the universe: the strong (holds a proton together), weak (radioactive decay is an example), electro-magnetism (cell phones, etc. are examples), and gravity. The Higgs is a key player in the unification of the weak force and the electro-magnetic force. Finding it means we actually have a bead on how nature unifies those two forces. That is HUGE! This is a big step along the way to putting all the forces back together. We still have a lot of work to do!

Another technical aside. Smile We think of the first role – giving fundamental particles mass – a consequence of the second – they are not independent roles. The Higgs is key to the unification and in order to be that key, it must also be the source of the fundamental particle’s mass.

How long have you been searching for it?

A loooooong time. We are like archeologists. Nature is what nature is. Our job is to figure out how nature works. We have a mathematical model (called the Standard Model). We change it every time we find an experimental result that doesn’t agree with the calculation. The last time that happened was when we stumble upon the unexpected fact that neutrino’s have mass. The time before that was the addition of the Higgs, and that modification was first proposed in 1964 (it took a few years to become generally accepted). So, I suppose you could say in some sense we’ve been looking for it since 1964!

It isn’t until recently, however (say in the late 90’s) that the machines we use have become powerful enough that we could honestly say we were “in the hunt for the Higgs.” The LHC, actually, had finding the Higgs as one of its major physics goals. There was no guarantee – no reason nature had to work like that – so when we built it we were all a little nervous and excited… ok. a lot nervous and excited.

So, why did it take so long!? The main reason is we hardly ever make it in our accelerators! It is very very massive!! So it is very hard to make. Even at the LHC we make one every 3 hours… The LHC works by colliding protons together at a very high speed (almost the speed of light). We do that more than 1,000,000 times a second… and we make a Higgs only once every 3 hours. The very definition of “needle in a haystack!”

Who made this discovery?

Two very large teams of physicists, and a whole bunch of people running the LHC accelerator at CERN. The two teams are the two experiments: ATLAS and CMS. I and my colleagues at UW are on ATLAS. If you hear someone say “I discovered the Higgs” they are using the royal-I. This is big science. Heck – the detector is half a (American) football field long, and about 8 or 9 stories tall and wide. This is the sort of work that is done by lots of people and countries working together. ATLAS currently has people from 38 countries – the USA being one of them.

What does a Cocktail Party have to do with it?

The cocktail party analogy is the answer to why some fundamental particles are more massive than particles (sadly, not why I have to keep letting my belt out year-after-year).

This is a cartoon of a cocktail party. Someone very famous has just entered the room. Note how everyone has clumped around them! If they are trying to get to the other side of the room, they are just not going to get there very fast!!

Now, lets say I enter the room. I don’t know that many people, so while some friends will come up and talk to me, it will be nothing like that famous person. So I will be able to get across the room very quickly.

The fact that I can move quickly because I interact with few people means I have little mass. The famous person has lots of interactions and can’t move quickly – and in this analogy they have lots of mass.

Ok. Bringing it back to the Higgs. The party and the people – that is the Higgs field. How much a particle interacts with the Higgs field determines its mass. The more it interacts, the more mass is “generated.”

And that is the analogy. You’ve been reading a long time. Isn’t this making you thirsty? Go get a drink!

Really, is this that big a deal?

Yes. This is a huge piece of the puzzle. This work is definitely worth a Nobel prize – look for them to award one to the people that first proposed it in 1960 (there are 6 of them, one has passed away – no idea how the committee will sort out the max of 3 they can give it to). We have confirmed a major piece of how nature works. In fact, this was the one particle that the Standard Model predicted that we hadn’t found. We’d gotten all the rest! We now have a complete picture of the Standard Model is it is time to start work on extending the Standard Model. For example, dark matter and dark energy are not yet in the Standard Model. We have no figured out how to fully unify everything we know about.

No. The economy won’t see an up-tick or a down-tick because of this. This is pure research – we do it to understand how nature and the universe around us works. There are sometimes, by-luck, spin-offs. And there are people that work with us who take it on as one of their tasks to find spin offs. But that isn’t the reason we do this.

What is next?

Ok. You had to ask that. So… First, we are sure we have found a new boson, but the real world – and data, is a bit messy. We have looked for it, and expect it to appear in several different places. It appeared in most of them – one place it seems to be playing hide and seek (where the Higgs decays to tau’s – a tau is very much like a heavy electron). Now, only one of the two experiments has presented results in the tau’s (CMS), so we have to wait for my experiment, ATLAS, to present its results before we get worried.

Second, and this is what we’d be doing no matter what happened to the tau’s, is… HEY! We have a shiny new particle! We are going to spend some years looking at it from every single angle possible, taking it out for a test drive, you know – kicking the tires. There is actually a scientific point to doing that – there are other possible theories out there that predict the existence of a Higgs that looks exactly like the Standard Model Higgs except for some subtle differences. So we will be looking at this new Higgs every-which way to see if we can see any of those subtle differences.

ATLAS and CMS also do a huge amount of other types of physics – none of which we are talking about right now – and we will continue working on those as well.

Why do you call it the God Particle!?

We don’t. (especially check out the Pulp Fiction mash-up picture).

What will you all discover next?

I’ll get back to you on that…

Whew. I’m spent!

The Way You Look at the World Will Change… Soon December 2, 2011

Posted by gordonwatts in ATLAS, CERN, CMS, Higgs, physics.
6 comments

We are coming up on one of those “lucky to be alive to see this” moments. Sometime in the next year we will all know, one way or the other, if the Standard Model Higgs exists. Or it does not exist. How we think fundamental physics will change. I can’t understate the importance of this. And the first strike along this path will occur on December 13th.

If it does not exist that will force us to tear down and rebuild – in some totally unknown way – our model of physics. Our model that we’ve had for 40+ years now. Imagine that – 40 years and now that it finally meets data… poof! Gone. Or, we will find the Higgs, and we’ll have a mass. Knowing the mass will be in itself interesting, and finding the Higgs won’t change the fact that we still need something more than the Standard Model to complete our description of the universe. But now every single beyond-the-standard model theory will have to incorporate not only electrons, muons, quarks, W’s, Z’s, photons, gluons – at their measured masses, but a Higgs too with the appropriate masses we measure!

So, how do I know this is going to happen? Look at this plot that was released during the recent HCP conference (deepzoom version Smile) in Paris.

Ok, this takes a second to explain. First, when we look for the Higgs we do it as a function of its mass – the theory does not predict exactly how massive it will be. Second, the y-axis is the rate at which the Higgs is produced. When we look for it at a certain mass we make a statement “if the Higgs exists at mass 200 GeV/c2, then it must be happening at a rate less than 0.6 or we would have seen it.” I read the 0.6 off the plot by looking at the placement of the solid black line with the square points – the observed upper limit. The rate, the y-axis, is in funny units. Basically, the red line is the rate you’d expect if it was a standard model Higgs. The solid black line with the square points on it is the combined LHC exclusion line. Combined means ATLAS + CMS results. So, anywhere the solid black line dips below the red horizontal line means that we are fairly confident that the Standard Model Higgs doesn’t exist (BTW – even fairly confident has a very specific meaning here: we are 95% confident). The hatched areas are the areas where the Higgs has already been ruled out. Note the hatched areas at low mass (100 GeV or so) – those are from other experiments like LEP.

Now that is done. A fair question is where would we expect to find the Higgs. As it turns out, a Standard Model Higgs will mostly likely occur at low masses – exactly that region between 114 GeV/c2 and 140 GeV/c2. There isn’t a lot of room left for the Higgs to hide there!! These plots are with 2 fb-1 of data. Both experiments now have about 5 fb-1 of data recorded. And everyone wants to know exactly what they see. Heck, while in each experiment we basically know what we see, we desperately want to know what the other experiment sees. The first unveiling will occur at a joint seminar at 2pm on December 13th. I really hope it will be streamed on the web, as I’ll be up in Whistler for my winder ski vacation!

So what should you look for during that seminar (or in the talks that will be uploaded when the seminar is given)? The above plot will be a quick summary of what the status of the experiments. Each experiment will have an individual one. The key thing to look for is where the dashed line and the solid line deviate significantly. The solid line I’ve already explained – that says that for the HIggs of a particular mass if it is there, it must be at a rate less than what is shown. Now, the dashed line is what we expect – given everything was right – and the Higgs didn’t exist at that mass – that is how good we expect to be. So, for example, right around the 280 GeV/C2 level we expect to be able to see a rate of about 0.6, and that is almost exactly what we measure. Now look down around 120-130 GeV/c2. There you’ll notice that the observed line is well above the solid line. How much – well, it is just along the edge of the yellow band – which means 2 sigma. 2 sigma isn’t very much – so this plot has nothing to get very interested yet. But if one of the plots shown over the next year has a more significant excursion, and you see it in both experiments… then you have my permission to get a little excited. The real test will be if we can get to a 5 sigma excursion.

This seminar is the first step in this final chapter of the old realm of particle physics. We are about to start a new chapter. I, for one, can’t wait!

N.B. I’m totally glossing over the fact that if we do find something in the next year that looks like a Higgs, it will take us sometime to make sure it is a Standard Model Higgs, rather than some other type of Higgs! 2nd order effect, as they say. Also, in that last long paragraph, the sigma’s I’m talking about on the plot and the 5 sigma discovery aren’t the same – so I glossed over some real details there too (and this latter one is a detail I sometimes forget, much to my embarrassment at a meeting the other day!).

Update: Matt Strassler posted a great post detailing the ifs/ands/ors behind seeing or not seeing – basically a giant flow-chart. Check it out!

16,000 Physics Plots January 12, 2011

Posted by gordonwatts in ATLAS, CDF, CMS, computers, D0, DeepTalk, physics life, Pivot Physics Plots.
4 comments

Google has 20% time. I have Christmas break. If you work at Google you are supposed to have 20% of your time to work on your own little side project rather than the work you are nominally supposed to be doing. Lots of little projects are started this way (I think GMail, for example, started this way).

Each Christmas break I tend to hack on some project that interests me – but is often not directly related to something that I’m working on. Usually by the end of the break the project is useful enough that I can start to get something out of it. I then steadily improve it over the next months as I figure out what I really wanted. Sometimes they never get used again after that initial hacking time (you know: fail often, and fail early). My deeptalk project came out of this, as did my ROOT.NET libraries. I’m not sure others have gotten a lot of use out of these projects, but I certainly have. The one I tackled this year has turned out to be a total disaster. Interesting, but still a disaster. This plot post is about the project I started a year ago.  This was a fun one. Check this out:

image

Each of those little rectangles represents a plot released last year by DZERO, CDF, ATLAS, or CMS (the Tevatron and LHC general purpose collider experiments) as a preliminary result. That huge spike is July – 3600 plots (click to enlarge the image) -  is everyone preparing for the ICHEP conference. In all the 4 experiments put out about 6000 preliminary plots last year.

I don’t know about you – but there is no way I can keep up with what the four experiments are doing – let alone the two I’m a member of! That is an awful lot of web pages to check – especially since the experiments, though modern, aren’t modern enough to be using something like an Atom/RSS feed! So my hack project was to write a massive web scraper and a Silverlight front-end to display it. The front-end is based on the Pivot project originally from MSR, which means you can really dig into the data.

For example, I can explode December by clicking on “December”:

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and that brings up the two halves of December. Clicking in the same way on the second half of December I can see:

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From that it looks like 4 notes were released – so we can organize things by notes that were released:

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Note the two funny icons – those allow you to switch between a grid layout of the plots and a histogram layout. And after selecting that we see that it was actually 6 notes:

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That left note is title “Z+Jets Inclusive Cross Section” – something I want to see more of, so I can select that to see all the plots at once for that note:

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And say I want to look at one plot – I just click on it (or use my mouse scroll wheel) and I see:

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I can actually zoom way into the plot if I wish using my mouse scroll wheel (or typical touch-screen gestures, or on the Mac the typical zoom gesture). Note the info-bar that shows up on the right hand side. That includes information about the plot (a caption, for example) as well as a link to the web page where it was pulled from. You can click on that link (see caveat below!) and bring up the web page. Even a link to a PDF note is there if the web scrapper could discover one.

Along the left hand side you’ll see a vertical bar (which I’ve rotated for display purposes here):

image

You can click on any of the years to get the plots from that year. Recent will give you the last 4 months of plots. Be default, this is where the viewer starts up – seems like a nice compromise between speed and breadth when you want to quickly check what has recently happened. The “FS” button (yeah, I’m not a user-interface guy) is short for “Full Screen”. I definitely recommend viewing this on a large monitor! “BK” and “FW” are like the back and forward buttons on your browser and enable you to undo a selection. The info bar on the left allows you do do some of this if you want too.

Want to play? Go to http://deeptalk.phys.washington.edu/ColliderPlots/… but first read the following. Smile And feel free to leave suggestions! And let me know what you think about the idea behind this (and perhaps a better way to do this).

  • Currently works only on Windows and a Mac. Linux will happen when Moonlight supports v4.0 of Silverlight. For Windows and the Mac you will have to have the Silverlight plug-in installed (if you are on Windows you almost certainly already have it).
  • This thing needs a good network connection and a good CPU/GPU. There is some heavy graphics lifting that goes on (wait till you see the graphics animations – very cool). I can run it on my netbook, but it isn’t that great. And loading when my DSL line is not doing well can take upwards of a minute (when loading from a decent connection it takes about 10 seconds for the first load).
  • You can’t open a link to a physics note or webpage unless you install this so it is running locally. This is a security feature (cross site scripting). The install is lightweight – just right click and select install (control-click on the Mac, if I remember correctly). And I’ve signed it with a certificate, so it won’t get messed up behind your back.
  • The data is only as good as its source. Free-form web pages are a mess. I’ve done my best without investing an inordinate amount of time on the project. Keep that in mind when you find some data that makes no sense. Heck, this is open source, so feel free to contribute! Updating happens about once a day. If an experiment removes a plot from their web pages, then it will disappear from here as well at the next update.
  • Only public web pages are scanned!!
  • The biggest hole is the lack of published papers/plots. This is intentional because I would like to get them from arxiv. But the problem is that my scrapper isn’t intelligent enough when it hits a website – it grabs everything it needs all at once (don’t worry, the second time through it asks only for headers to see if anything has changed). As a result it is bound to set off arxiv’s robot sensor. And the thought of parsing TeX files for captions is just… not appealing. But this is the most obvious big hole that I would like to fix some point soon.
  • This depends on public web pages. That means if an experiment changes its web pages or where they are located, all the plots will disappear from the display! I do my best to fix this as soon as I notice it. Fortunately, these are public facing web pages so this doesn’t happen very often!

Ok, now for some fun. Who has the most broken links on their public pages? CDF by a long shot. Smile Who has the pages that are most machine readable? CMS and DZERO. But while they are that, the images have no captions (which makes searching the image database for text words less useful than it should be). ATLAS is a happy medium – their preliminary results are in a nice automatically produced grid that includes captions.

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