Shortly after the Higgs Seminar, Seth Zenz and I had a short, impromptu discussion about the results and what they mean for physics in the near future. Check out the video:
(Due to a technical problem, we lost the first two seconds of audio, so there is a slightly abrupt start.)
Real CMS proton-proton collision events in which 4 high energy electrons (green lines and red towers) are observed. The event shows characteristics expected from the decay of a Higgs boson but is also consistent with background Standard Model physics processes. Courtesy: CMS
Today physicists at CERN on the CMS and ATLAS experiments at the Large Hadron Collider announced an update on their search for the Higgs boson. That may make you wonder ( I hope) what is Fermilab’s role in this. Well, glad you asked.
Fermilab supports the 1,000 US LHC scientists and engineers by providing office and meeting space as well as the Remote Operation Center. Fermilab helped design the CMS detector, a portion of the LHC accelerator and is working on upgrades for both. About one-third of the members of each of the Tevatron’s experiments, CDF and DZero, are also members of the LHC experiments.
That means that a good portion of the LHC researchers are also looking for the Higgs boson with the Tevatron. Because the Tevatron and LHC accelerators collide different pairs of particles, the dominant way in which the experiments search for the Higgs at the two accelerators is different. Thus the two machines offer a complimentary search strategy.
If the Higgs exists and acts the way theorists expect, it is crucial to observe it in both types of decay patterns. Watch this video to learn how physicists search for the Higgs boson. These types of investigations might lead to the identification of new and unexpected physics.
Scientists from the CDF and DZero collaborations at Fermilab continue to analyze data collected before the September shutdown of the Tevatron in the search for the Higgs boson.
The two collaborations will announce their latest results for the Higgs boson search at an international particle physics conference in March 2012. This new updated analysis will have 20 to 40 percent more data than the July 2011 results as well as further improvements in analysis methods.
The Higgs particle is the last not-yet-observed piece of the theoretical framework known as the Standard Model of particles and forces. Watch this video to learn The nature of the Higgs boson and how it works. According to the Standard Model, the Higgs boson explains why some particles have mass and others do not. Higgs most likely has a mass between 114-137 GeV/c2, about 100 times the mass of a proton. This predicted mass range is based on stringent constraints established by earlier measurements made by Tevatron and other accelerators around the world, and confirmed by the searches of LHC experiments presented so far in 2011. This mass range is well within reach of the Tevatron Collider.
The Tevatron experiments already have demonstrated that they have the ability to ferret out the Higgs-decay pattern by applying well-established techniques used to search for the Higgs boson to observing extremely rare but firmly expected physics signature. This signature consists of pairs of heavy bosons (WW or WZ) that decay into a pair of b quarks, a process that closely mimics the main signature that the Tevatron experiments use to search for the Higgs particle, i.e. Higgs decaying to a pair of b quarks, which has by far the largest probability to happen in this mass range. Thus, if a Standard Model Higgs exists, the Tevatron experiments will see it.
If the Standard Model Higgs particle does not exist, Fermilab’s Tevatron experiments are on track to rule it out this winter. CDF and DZero experiments have excluded the existence of a Higgs particle in the 100-108 and the 156-177 GeV/c2 mass ranges and will have sufficient analysis sensitivity to rule out this winter the mass region between.
While today’s announcement shows the progress that the LHC experiments have made in the last few months, all eyes will be on the Tevatron and on the LHC in March 2012 to see what they have to say about the elusive Higgs Boson.
– Tona Kunz
Really difficult, and I mean really, really difficult. It is such an arduous job that even after 30 years worth of searching, by literally tens of thousands of physicists, it has yet to be found. However, that may all change Tuesday when spokespeople for the ATLAS and CMS experiments, the Large Hadron Collider‘s two general-purpose detector experiments, unveil the long-awaited results of their independent searches for the higgs boson.
Now, what makes Tuesday’s announcement so different is that it will be the first time any higgs analysis will be publicly shown using 5.5 inverse femtobarns (fb-1), or a data set worth over 380 trillion proton collisions. To explain why 5.5 fb-1 is so special requires us to go back in time to late August, when this graph started making the rounds at conferences and summer schools:
Essentially, this graph tabulates how much data is needed for ATLAS and CMS to be sensitive to discovering the higgs boson. According to these numbers, with 5 fb-1 worth of data, ATLAS & CMS can either jointly rule out the existence of higgs boson as predicted by the Standard Model of Physics, or with equal excitement, claim evidence of its existence. Now I need to mention two important caveats: (1) this table assumes (1) benchmark parameters which are entirely worthless if there is any type of new physics (which is pretty likely, IMO); and (2) the numbers also assume that ATLAS and CMS combine their data sets. This last point is important because this is not the case tomorrow.
What will be seen live, from this link, are two 30-minute presentations by a spokesperson from each collaboration unveiling and announcing whatever conclusions that can justifiably be made considering the amount of data presently available. After that, there will be a 1 hour Q & A session with two spokespeople. My colleagues here at QD will definitely be live-blogging the event! I, on the other hand, will be teaching my undergraduates the importance of thermodynamics……
In summary, I am expecting three possible outcomes on Tuesday (Disclaimer! I am not a part of any experiment and currently am in Wisconsin, not CERN):
In light of results from last month using half the data (below), Tuesday will be very interesting.
Now that I built up the anticipation, here are some numbers I calculated to give an idea why discovering the higgs boson is such an incredible scientific feat. (Technical details as to how I generated these numbers can be found at the very bottom of this post.)
Okay, so suppose the higgs boson, as predicted by the Standard Model, were to exist. If we were to produce one at the LHC, then we would expect it to decay into something more familiar like photons or b-quarks. We physicists call the probability of this happening a “cross section,” and it is measured in barns.
As a concrete example, let us take a look at the first process where two protons (pp) collide and produce a higgs boson (h), which in turn decays into a b-quark and an anti-b-quark. The cross section (probability) is 16,320 femtobarns, or 0.00000000001632 barns. All you need to know is that 0.00000000001632 barns is a very small number and hence pp->h->bb is a very rare thing to happen. In 70 trillion proton-proton collisions (or 1 inverse femtobarn), our theory predicts we will have produced 16,320 higgs bosons. In 5.5 inverse femtobarns (or 380 trillion proton-proton collisions), our theory predicts we will have generated
16,320 fb x 5.5 fb-1 = 89,760 pp-> higgs -> bb Events.
89,000 higgs boson events may seem like a lot, but just wait until the next table. Here are some common ways a higgs is expected to decay and how many higgs events we expect to have produced this year. That is 102, 756 higgses in all!
Here is where things become absolutely unbearable. Let’s pretend now that the higgs boson does not exist. So ignoring the contribution from higgs bosons, we may calculate how many of these higgs-like events we expect to see. For example, let’s consider pp -> γγ (2 photons) and pp -> gg (2 gluons), then out of 380 trillion proton-proton collisions (5.5 fb-1) the Standard Model predicts almost 3 trillion gluon pairs and over 800,000 photon pairs. Trying to find the higgs with b-quarks requires us to sift through 2.6 trillion bb pairs in order to find almost 90,000 higgs -> bb events.
In other words, experimentalists are trying to find an excess of 0.0000034% more bb quarks than the Standard Model predicts, or 0.3% more ZZ events than the Standard Model predicts. Fortunately, it only means looking for an extra 0.014% photon pairs in 380 trillion protons-proton collisions.
So yeah, the higgs boson… it’s hard to find. Personally, I think finding a needle in a haystack would be easier.
At any rate, congratulations to all those who helped with the effort. I am just giddy with anticipation regarding tomorrow’s seminar, though that might also be my body telling me to go to sleep.
Happy Colliding!
- richard (@bravelittlemuon)
* Technical note: I calculated the higgs boson cross sections with MadGraph5 using the Higgs Effective Field Theory v4 model. To calculate the Standard Model background cross sections, I used MadGraph5 Standard Model v4. mh = 120 GeV. Additionally, I resorted to using the default parameter card for MadGraph4. Each calculation used 25, 000 proton-proton events at 7 TeV center of mass. Only basic (read: default) kinematic and fiducial cuts have been applied. Uncertainty was ignored for clarity. This ignores all acceptance cuts.
On Tuesday December 13th, there will be a seminar at CERN about the search for the Higgs boson using the 2011 dataset.
Physicists at ATLAS and CMS have been working very hard all year (and are still working) to show the results for 5fb-1 each. This means that we will have 5 times the amount of data available since the last update, and we can expect the exclusion of the Higgs to be even more impressive than what we saw in the summer.
Watch the video on youtube!
Since this an important milestone in the search for the Higgs boson, I will be liveblogging the event, from the main auditorium here at CERN. There will be a webcast available for those of us not at CERN. (The webcast details will appear on the seminar page on the day of the seminar.) So please join me on Tuesday, watch the webcast and follow the liveblog for minute by minute updates of the search for the Higgs boson.
If you want to know more about the Higgs boson I’d recommend you look at Flip’s recent post.
Check out the link to the Seminar page.
Follow the updates with the Twitter hashtag #higgsliveblog.
This post, originally published on 11/18/11 here, was written by Kétévi Adiklè Assamagan, a staff physicist at Brookhaven National Laboratory and the ATLAS contact person for the ATLAS-CMS combined Higgs analysis.
Today we witnessed a landmark LHC first: At the HCP conference in Paris, friendly rivals, the ATLAS and CMS collaborations, came together to present a joint result! This ATLAS-CMS combined Higgs search was motivated by the fact that pooling the dataset increases our chances of excluding or finding the Higgs boson over those of a single experiment. This is the first example of this kind of scientific collaboration at the LHC, and the success of the whole endeavor hinged on a whole host of thorny issues being tackled…
Discussions about combining our Higgs search results with CMS’s first started over a year ago, but before we could proceed with any kind of combined analysis, we had first to jointly outline how on earth we were going to go about doing it. This was no small undertaking; although we’re looking for the same physics, the ATLAS and CMS detectors are very different beasts materially, and use completely independent software to define and identify particles. How can we be certain that what passes for an electron in ATLAS would also be picked out as such in CMS? (more…)
It’s that moment when you realize something serious and exciting has happened, but it’s 5:45am and you have to wake somebody up to sort it out. As the LHC ramps up it’s my role to make sure that the trigger is ready. This means looking at the bunch structure in the LHC and checking that ATLAS knows what this structure looks like. It’s as simple as pressing a few buttons and updating a database, and if everything goes smoothly we have nothing to worry about.
This time it was a bit different, because the LHC used a bunch structure they had never used before. When I pressed the button I was actually telling ATLAS something new and witnessing one of those rare transitions in the normal running of the LHC! (Jim’s post gives a great explanation about what bunch structures are and how the LHC team design them.) Then I checked the instructions, and they told me I had to wake someone up and tell them about the change. Nobody likes to be woken up at 5:45am, especially if they have an important meeting the next day. To make matters worse, I know the guy on the other end of the line (although since he’s so sleepy I didn’t recognize his voice at first!) At that point I remembered what my flat mate had told me when he was on call and got woken up at night. He said “What we do would be easy if they just gave us two minutes to think about it. We need time to wake up!” So, feeling bad about waking up the expert I told him I’d call back in 5 minutes. There was a flurry of messages on the electronic logbook and short conversations in the Control Room, and then it was time to call again. This time the voice on the other end of the line was more alert and a bit happier! He said everything was fine. I could proceed as normal and as long as there are no serious problems we can take data as we usually do.
We have beams!
The LHC just declared stable beams. Now the fun begins…
Hi All! Great news: the CMS Experiment, just a moment ago, announced that the LHC delivered 5fb-1 today!
Figure 1: Proof. It happened. (Image: Mine)
This is terrific news and if you happen to see a member of CERN’s accelerator division, be sure to congratulate her or him.
Figure 2: Total (integrated) luminosity delivered to (red) and recorded by (blue) the CMS detector. (Image: CMS)
To give a little context, 1 fb-1 (pronounced: one inverse femtobarn) worth of data is measure of the number proton collisions (scaled by a bunch of physics and efficiency parameters) and is the equivalent of 70 trillion proton-proton collisions. So 5 fb-1 is 350 trillion proton-proton collisions, which is 3.5 × 1014 = 350,000,000,000,000 proton-proton collisions. Before the start of collisions this year, the LHC had only delivered about 35 pb-1 (0.035 fb-1), which is only about 2.45 trillion = 2,450,000,000,000 proton-proton collisions. In other words, 99.3% of the data generated by the LHC came between this past March and Today. How can you not be impressed by that? 
Figure 3: Total (integrated) luminosity recorded by ATLAS (black/behind green), CMS (green), LHCb (blue), and ALICE (red). (Image: CERN)
Figure 4: Log of total (integrated) luminosity recorded by ATLAS (black/behind green), CMS (green), LHCb (blue), and ALICE (red). (Image: CERN)
Due to detector efficiencies and such, not all the data generated is recorded. The above plot, generated & continuously updated by CERN, shows that ATLAS and CMS have a small bit before reaching 5 fb-1. However, it is very reasonable to suggest that both experiments will have recorded 5 fb-1 before the end of the third week of November October. (Thanks to Achintya & Dave for catching this mistake. I have “week 43” in my notes for this post, so I have no idea how I ended up with the November date.)
As always, happy colliding.
- richard (@bravelittlemuon)
PS. I refer you to a previous post about what the experiments can do with 5 fb-1.
My first impression, once I got myself properly into the CMS databases and joined the requisite forty or so mailing lists, was that CMS has a lot more acronyms than I was used to. Particularly jarring were the mysterious PVT (“Physics Validation Team”) meetings, and the many occurrences of “PU” (“pileup“) always looked to me like “Princeton University” until I realized that made no sense in context.
But then I remembered all the acronyms on ATLAS, and learned that “PU” has gotten more common there too now that the increasing pileup is a frequent subject of discussion. (I really wasn’t paying attention generally to either ATLAS or CMS for the year where I did my analysis and wrote my thesis.) So although the culture of acronym use may be a bit different, it’s really just a matter of translating from one experiment’s terms to another.
For example, I recently learned that a JSON (“JavaScript something something”) file indicates which LumiSections (not an acronym, oddly) are good in a set of runs — in other words, for which times are the recorded data for all parts of CMS in good shape? On ATLAS, it would have been a GRL (“good run list”) indicating which LumiBlocks were good.
I still think that acronyms are thrown around in conversation a bit more on CMS than on ATLAS. Fortunately, there is a public list of CMS acronyms to help me. I’m sure I’ll figure them out eventually.
What is like to be on shift in the ATLAS Control Room? It’s my second night on shift and we’re already off to an interesting start. As I walked through the door I saw that the LHC was ramping and my excitement grew. Soon we would have beams and data! But before I even sat down the beams got dumped, so the LHC had to try again. Since I don’t really have much to do this would be an excellent time to describe what I do on shift and why I don’t really have much to do right now.
There are a few words you need to know to understand what we do here.
Someone who is in the ATLAS Control Room and performing a routine task on shift is known as a Shifter. (There are often people there performing non-routine tasks who are not Shifters. They work with the Shifters to accomplish their tasks.)
Shifters work at a designated Desk. Each Desk has a label on it telling the other Shifters who works there, as well as its own computer system and telephone. There are 16 Desks in total, some which manage part of the detector, and some which manage different kinds of tasks that involve the whole detector.
The LHC provides beams of protons which can be in one of several states. Stable beams means that the beams parameters are very precisely tuned and we can use them to record data. Injection is when the LHC is preparing the beams to become stable. When the beams become unstable they get dumped (time for a toilet flush sound!) and the LHC also has periods of Ramp Up and Ramp Down before and after stable beams.
Data taking is split up into long periods of stable beams called a Run. Each run is split up into short time intervals of about 1 minute called Lumiblocks. A lumiblock is a small period of time with roughly the same kinds of conditions.
I’m sitting at the Trigger Desk and that means that I’m responsible for making sure the trigger is behaving well. Here is a picture of my desk:
My Trigger Desk
There’s a lot of stuff there! Going from left to right we have the Red Folder Of Answers (RFOA? That would be a difficult acronym!) This holds a copy of nearly all the training material we need to be familiar with, and also some simple notes about how to handle a few common situations. Every desk should have one of these. I’m not just talking about in the ATLAS Control Room, I mean every desk in the world.
Next we have the Ominous Telephone Of Panic. Well, not really. Most of the time this is just used to exchange simple information. But there’s always the possibility that it will ring and there will be an expert on the other end of the line who has some difficult questions. If there’s one thing we don’t like on shift it’s surprises.
Then my laptop. I’d be lost without this. On here I have all the most useful websites and talks I could find that relate to the trigger. It’s quicker and easier to look up information on here, I can’t break anything by accidentally clicking the wrong button, and it’s shinier than the computers that comes with the desk. And of course, this is where I write my blog posts! And next to that is my ID. I need that to get in and out.
Towards the back of the desk we have the monitors that give me all the information I need to be able to be a good Shifter. Each monitor has about four applications open, and each one of those has about 20 tabs. There are literally hundreds of plots and tables that are available, and I can’t look at them all for all of the time! That is what makes this role difficult. To help things, I reserve the left most monitor for the most important plots (how quickly we are recording data) and the right most monitor for essential reference material (the plan for the day, the normal instructions and the instructions for special tasks.) The other two monitors are where most of the work takes place. Looking at plots, reading error messages and “spying” on the other Shifters.
Then we have the two pads of paper. The big one is mine and that’s for notes, doodles and anything else that comes to mind. (I think I have some cocktail recipes in there somewhere!) The smaller pad is for sending short messages to other Shifters in the room. For each set of “prescale keys” I have a different sheet of paper. It’s just a matter of picking the right one.
Then there’s my food. Since we’re not allowed to leave the Control Room for very long we need to bring all our food with us for the next 8 hours. I’ve got some bread and cheese, some fruit, and some yoghurt. Very healthy! Unfortunately I’ve also got a can of coke and some chocolate. Oops. Also note the coffee cup. This is the night shift, after all.
In the background we see the projectors. These are one of the ways the Shifters can share information between each other and see what is happening. The projector immediately in front of me shows the data as they flow through the system (from left to right.) If there’s a problem, those boxes turn red and it’s my job to work out what is happening. Usually when that happens it sorts itself out after about a minute. The other projectors show the trigger rates, the state of the various parts of the machine, the Daily Run Plan, the status of the LHC and then some event displays.
Although you can’t see it, there’s also a monitor to my left that shows the LHC Operation Page and the LHC Page 1. This is essential to the Trigger Shifter as we need to know the state of the LHC for our job.
(And for those who are observant, yes I do have a Higgs boson with me! Who knew it would be so easy to find? And in ATLAS Control Room of all places.)
We can get bored easily on shift...
So what do I actually do? In principle it’s actually quite straightforward. When we are taking data I look through the plots to make sure that they look okay. This is not a particularly well defined task though, since we don’t know exactly what the plots will look like for a given run (if we did then we wouldn’t need a human to compare them.) The advice we get about the plots is usually very good, so most of the time we can be confident that the plots look okay. If they look a little odd and it’s a Day Shift we usually phone the expert on call, whereas if it’s a night shift it’s usually good to get a second opinion before waking up a poor sleepy civilian. If the plot is obviously wrong, then a call is made pretty much immediately (with a few seconds spent getting some vital information about what happened immediately before the problem.) Once I’ve looked at all the plots I take a break and come back to them again a little while later.
The efficiency of the trigger needs to be kept high (we are competing with CMS, after all) so we keep adjusting it. This is done by changing the “Trigger Menu”, which is a list of different kinds of triggers and how often we use them to record data. The different trigger menus are specified using prescale keys, so when the conditions change enough to justify changing the menu, I get a new set of keys (which is just a fancy word for “four digit number”.) I write these down on my little pad and pass them to the Run Control Shifter whose job it is to update the menu. Why is that not the job of the Trigger Shifter? I’m not really sure. I suppose two heads are better than one.
Hurrah, data!
When we have no beams the Trigger Shifter usually has nothing useful to do. I can prepare the prescale keys for the next run, but otherwise I can’t really do much until the next time the LHC ramps up. As it ramps up I need to watch the Bunch Groups (this tells ATLAS how many bunches of protons are moving through the LHC.) If they change, I create new bunch group information. In fact, that’s the most useful part of the Red Folder Of Answers. It’s something I will have to later today for the first time!
From time to time we have special running conditions, and we have special prescale keys for that. Right now I have to keep track of seven different sets of prescale keys (for normal running, for when the machine is in Standby mode, for when we take calibrations and for different special tasks, such as when the LHC tries a new configuration.) Each one is written on its own tiny sheet of paper!
Apart from the tasks listed above, I have be able to answer general questions about the trigger, such as making comments about its performance, finding out what a specific part of the trigger is doing and working out problems. I also need to be in constant communication with the other Shifters, usually the Shift Leader (the boss!) and the Run Control Shifter who needs to know what changes to make to the trigger menu. My Desk allows me to “spy” on the Run Control Shifter to make sure the trigger settings are correct there. Right now the prescale keys are exactly what we expect. Unless there is a serious breakdown of communication, they should always match. From time to time the Run Control Shifter gets a message popping up on their screen about background events, and then ask the Trigger Shifter to make a decision. The answer is usually “No”.
At the moment the LHC is injecting beams. That means that I can’t make any changes. The prescale keys are ready for the next run. I’ve updated myself with all the information for the day. I’ve even written up a summary for the Shift Leader and the next Trigger Shifter about the day’s plans. There are currently some plots flashing on the monitor before me. When the LHC starts to ramp up I need to watch these plots carefully. If they change even by a little bit I’ll need to update the bunch group information. If not, then I can just sit back until we start recording data again.
It’s quite an odd job when you think about it.
“So, you’ve turned to the dark side?” I’ve heard it surprisingly often, usually from my new colleagues on CMS. “Yes,” I reply. “My hate makes me powerful.”
We’re just kidding, of course.
I’ve been asked more seriously, on a number of occasions, why I switched from working with ATLAS to working with CMS. There are several ways I can answer that one:
1. Why not? ATLAS and CMS both look for the same exciting things at the LHC: the Higgs boson, supersymmetry, and all sorts of other new physics. They have roughly similar capabilities and, for the most part, conceptually similar designs. So I should be happy to work on either one.
2. It came with the job. Being happy to work on either experiment means I applied to some groups working on ATLAS and some on CMS. The job I ended up with is with Princeton, and they have a CMS group, so…
3. It’s good for our field to exchange techniques and expertise between experiments.
4. It’s good for me to know people from both collaborations and learn different ways of doing things, and good to be forced into doing something completely different than what I did as a graduate student.
So why would switching be a bad idea? Well, mostly, it’s harder. There is more logistics to deal with to get started as a postdoc — on top of the logistics of starting a job — and a lot of time spent learning new software and new organization. And it will take me quite a bit longer to be in a position where I know enough and people have enough confidence in my work to give me significant responsibilities. But all of this, I hope, is transitory.
In the end, neither experiment is the dark side. They do compete with each other — as intended, to keep everyone working hard — but they’re more like opposing sports teams than opposite sides of the Force. You may despise the team across town much of the time, but without them you couldn’t play baseball. And once in a while, players get traded.
