Posts Tagged ‘luminosity’

Can the LHC Run Too Well?

Friday, February 3rd, 2012

Seth Zenz

For CMS data analysis, winter is a time of multitasking. On the one hand, we are rushing to finish our analyses for the winter conferences in February and March, or to finalize the papers on analyses we presented in December. On the other, we are working to prepare to take data in 2012. Although the final decisions about the LHC running conditions for 2012 haven’t been made yet, we have to be prepared both for an increase in beam energy and an increase in luminosity. For example, the energy might go to 8 TeV center-of-mass, up from last year’s 7. That will make all our events a little more exciting. But it’s the luminosity that determines how many events we get, and thus how much physics we can do in a year. For example, if the Higgs boson exists, the number of Higgs-like events we’ll see will go up, and so will the statistical power with which we can claim to have observed it. If the hints we saw at 125 GeV in December are right, our ability to be sure of its existence this year depends on collecting several times more events in 2012 than we got in 2011.

We’d many more events over 2012 if the LHC simply kept running the way it already was at the end of the year. That’s because for most of the year, the luminosity was increasing over and over as the LHC folks added more proton bunches and focused them better. But we expect that the LHC will do better, starting close to last year’s peak, and then pushing to ever-higher luminosities. The worst-case we are preparing for is perhaps twice as much luminosity as we had at the end of last year.

But wait, why did I say “worst-case”?

Well, actually, it will give us the most interesting events we can get and the best shot at officially finding the Higgs this year. But increased luminosity also gives more events in every bunch crossing, most of which are boring, and most of which get in the way. This makes it a real challenge to prepare for 2012 if you’re working on the trigger, because have to sift quickly through events with more and more extra stuff (called “pileup”). As it happens, that’s exactly what I’m working on.

Let me explain a bit more of the challenge. One of the triggers I’m becoming responsible for is trying to find collisions containing a Higgs decaying to a bottom quark and anti-bottom quark and a W boson decaying to an electron and neutrino. If we just look for an electron — the easiest thing to trigger on — then we get too many events. The easy choice is to ask only for higher-energy electrons, but beyond a certain points we start missing the events we’re looking for! So instead, we ask for the other things in the event: the two jets from the Higgs, and the missing energy from the invisible neutrino. But now, with more and more extra collisions, we have random jets added in, and random fluctuations that contribute to the missing energy. We are more and more likely to get the extra jets and missing energy we ask for even though there isn’t much missing energy or a “Higgs-like” pair of jets in the core event! As a result, the event rate for the trigger we want can become too high.

How do we deal with this? Well, there are a few choices:

1. Increase the amount of momentum required for the electron (again!)
2. Increase the amount of missing energy required
3. Increase the minimum energy of the jets being required
4. Get smarter about how you count jets, by trying to be sure that they come from the main collision rather than one of the extras
5. Check specifically if the jets come from bottom quarks
6. Find some way to allocate more bandwidth to the trigger

There’s a cost for every option. Increasing energies means we lose some events we might have wanted to collect — which means that even though the LHC has produced more Higgs bosons, it’s counterbalanced by us seeing fewer of the ones that were there. Being “smarter” about the jets means more time spent by our trigger processing software on this trigger, when it has lots of other things to look at. Asking for bottom quarks not only takes more processing, it also means the trigger can’t be shared with as many other analyses. And allocating more bandwidth means we’d have to delay processing or cut elsewhere.

And for all the options, there’s simply more work. But we have to deal with the potential for extra collisions as well as we can. In the end, the LHC collecting much more data is really the best-case scenerio.

Tags: CMS, Higgs, LHC, luminosity, pileup, trigger
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One Recorded Inverse Femtobarn!!!

Tuesday, October 18th, 2011

Anna Phan

Last week I announced that LHC reached the 2011 milestone of delivering one inverse femtobarn of luminosity to LHCb. This week, LHCb reached the 2011 milestone of recording one inverse femtobarn of data.

There is a subtle difference between these two statements, which is better illustrated in the graph below, where it can see that the delivered and recorded integrated luminosities are different, and the difference seems to grow with time.

So what exactly does “delivered” and “recorded” integrated luminsoity actually mean? Surprisingly for physics, they mean exactly how they sound. That is, delivered integrated luminosity refers to the integrated luminosity of proton-proton collisions which the LHC has delivered to LHCb while recorded luminosity refers to the amount of data we have recorded on disk. We obviously want these to be as close to each other as possible, but as I’ve mentioned before, this is not possible due to the detector hardware.

You may all be wondering why LHCb is celebrating one inverse femtobarn while ATLAS and CMS are celebrating five inverse femtobarns. This is due to the fact that the design instantaneous luminosities for LHCb is much lower than for ATLAS and CMS. In fact, at the beginning of the year, the milestone of one inverse femtobarn seemed almost unachievable. This remarkable accomplishment has only been possible due to the excellent performance of both the LHC team in implementing luminosity leveling and the LHCb team in running the detector at higher instantaneous luminosity than it was initial designed for.

In terms of physics, one inverse femtobarn of data corresponds to about seventy billion b quark pairs decaying in the LHCb detector. This huge amount of data allows us to significantly increase the accuracy of our results like \(\phi_s\). It also increases the statistics of various rare decays like \(B_{s}\rightarrow\mu\mu\). Stay tuned for more results!

Tags: LHCb, luminosity
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One Delivered Inverse Femtobarn!!!

Monday, October 3rd, 2011

Anna Phan

Last night, while most of the collaboration was sleeping, LHC reached the 2011 milestone of delivering one inverse femtobarn of luminosity to LHCb.

In the words of our spokesperson, Pierluigi Campana: This is a great achievement for us and for the scientific community, thanks to the excellent performance of the machine, to the skill and to the commitment of the whole LHC team.

This result is even more remarkable, considering that LHC was able to deliver high quality data at the same time, and with different luminosities, to the four LHC experiments.

This performance will allow us to push even further the search for new physics and for unexpected phenomena in the flavor sector.

I really have nothing to add to that except, congratulations to all involved.

Tags: LHCb, luminosity
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Coming attractions at the LHC

Friday, September 2nd, 2011

Ken Bloom

It’s Labor Day weekend here in the US, but over at CERN it’s the end of the August technical stop for the LHC. To rework a common saying, this is the first day of the rest of the 2011 run. We have two months left of proton-proton collisions, followed by one month of lead-lead collisions, and then in December we’ll have the holiday “extended technical stop” that will probably extend to the spring.

We’re expecting an important change in running conditions once we return from the technical stop, and that is a change in how the beams are focused. This will lead to an increased rate of collisions. Remember that the proton beams are “bunched”; the beam is not a continuous stream of particles but bunches with a large separation between them. The change in the focusing will help make the bunches more compact, and that in turn will mean that there will be more proton collisions every time a pair of bunches pass through each other. When our detectors record data, they record an entire bunch crossing as a single event. Thus, each individual event will be busier, with more collisions and more particles produced.

This is good news from a physics perspective — the more collisions happen, the greater the chance that there will be something interesting coming out. But it’s a challenge from an operational perspective. We try to record as many “interesting” events as possible, but we’re ultimately limited by how quickly we can read out the detector and how much space we have to store the data. Given that we’re going to have more data coming into fixed resources, we’re going to have to limit our definition of “interesting” a little further. The busier events are also a greater strain on the software and computing for the experiments (which I focus on). Each event takes more CPU time to process and requires more RAM. Previous experience and simulations give us some guidance as to how all of this will scale up from what we’ve seen so far, but we can’t know for sure without actually doing it. (The original plan for the machine development studies period before the technical stop was supposed to include a small-scale test of this, so that we could put the computing and everything else through its paces. But that got cancelled. I had originally planned to blog about that. Oh well.)

However, all of this will be worth the trouble. Remember all of the excitement of the EPS conference? That was at the end of July, just a little more than a month ago. There is now about twice as much data that can be analyzed. With the increases in collision rate, we might well be able to double the dataset once again just in these next two months. Or, we might do even better. This will have a critical impact on our searches for new phenomena, and could allow the LHC experiments to discover or rule out the standard-model Higgs boson by the end of this year. Coming soon, to a theater near you.

Tags: computing, Higgs, LHC, luminosity
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Congratulations LHCb!!!

Saturday, May 28th, 2011

Anna Phan

Just a quick post today to explain this LHC status from last night:

What was this about you ask? As I’ve mentioned previously, the target instantaneous luminosity for LHCb is \(2 \times 10^{32} cm^{-2} s^{-1}\) to \(3 \times 10^{32} cm^{-2} s^{-1}\).

LHCb started taking data within this target instantaneous luminosity on the 1^st of May with 756 colliding bunches corresponding to an instantaneous luminosity of \(2.15 \times 10^{32} cm^{-2} s^{-1}\). Last night the experiment moved into unknown territory, collecting data at an instantaneous luminosity of \(3 \times 10^{32} cm^{-2} s^{-1}\).

Experts have been carefully monitoring the detector behaviour and data quality, but so far it would seem that everything is performing well. Congratulations are indeed in order.

Tags: LHCb, luminosity
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The CERN Accelerator Complex

Sunday, April 24th, 2011

Brian Dorney

With all the buzz this past week regarding the breaking of the world instantaneous luminosity record, I thought it might be interesting for our readers to get an idea of how we as physicists achieved this goal.

Namely, how do we accelerate particles?

(This may be a review for some of our veteran readers due to this older post by Regina)

The Physics of Acceleration

Firstly, physicists rely on a principle many of us learn in our introductory physics courses, the Lorentz Force Law. This result, from classical electromagnetism, states that a charged particle in the presence of external electric and/or magnetic fields will experience a force. The direction and magnitude (how strong) of the force depends on the sign of the particle’s electric charge and its velocity (or direction its moving, and with what speed).

So how does this relate to accelerators? Accelerators use radio frequency cavities to accelerate particles. A cavity has several conductors that are hooked up to an alternating current source. Between conductors there is empty space, but this space is spanned by a uniform electric field. This field will accelerate a particle in a specific direction (again, depending on the sign of the particle’s electric charge). The trick is to flip this current source such that as a charged particle goes through a succession of cavities it continues to accelerate, rather than be slowed down at various points.

A cool Java Applet that will help you visualize this acceleration process via radio frequency cavities can be found here, courtesy of CERN.

Now that’s the electric field portion of the Lorentz Force Law, what about the magnetic? Well, magnetic fields are closed circular loops, as you get farther and farther away from their source the radii of these loops continually increases. Whereas electric fields are straight lines that extend out to infinity (and never intersect) in all directions from their source. This makes the physics of magnetic fields very different from that of electric fields. We can use magnetic fields to bend the track (or path) of charged particles. A nice demonstration of this can be found here (or any of the other thousands of hits I got for Googling “Cathode Ray Tube + YouTube”).

Imagine, if you will, a beam of light; you can focus the beam (make it smaller) by using a glass lens, you can also change the direction of the beam using a simple mirror. Now, the LHC ring uses what are called dipole and quadropole magnets to steer and focus the beam. If you combine the effects of these magnets you can make what is called a magnetic lens, or more broadly termed “Magnetic Optics.” In fact, the LHC’s magnetic optics currently focus the beam to a diameter of ~90 micro-meters (the diameter of a human hair is ~100 micro-meters, although it varies from person to person, and where on the body the hair is taken from). However, the magnetic optics system was designed to focus the beam to a diameter of ~33 micro-meters.

In fact, the LHC uses 1232 dipole magnets, and 506 quadrupole magnets. These magnets have a peak magnetic field of 8.3 Tesla, or 100,000 times stronger than Earth’s magnetic field. An example of the typical magnetic field emitted by the dipole magnets of the LHC ring is shown here [1]:

Image courtesy of CERN

The colored portions of the diagram indicate the magnetic flux, or the amount of magnetic field passing through a given area. Whereas the arrows indicate the direction of the magnetic field. The two circles (in blue) in the center of the diagram indicate the beam pipes for beams one and two. Notice how the arrows (direction of the magnetic field) point in opposite directions! This allows CERN Accelerator physicists to control two counter-rotating beams of protons in the same beam pipe (Excellent Question John Wells)!

Thus, accelerator physicists at CERN use electric fields to accelerate the LHC proton/lead-ion beams and the magnetic fields to steer and squeeze these beams (Also, these “magnetic optics” systems are responsible for “Lumi Leveling” discussed by Anna Phan earlier this week).

However, this isn’t the complete story, things like length contraction, and synchrotron radiation affect the acceleration process, and design of our accelerators. But these are stories best left for another time.

The Accelerator Complex

But where does this process start? Well, to answer this let’s start off with the schematic of this system:

Image courtesy of CERN

One of our readers (thanks GP!) has given us this helpful link that visualizes the acceleration process at the LHC (however, when this video was made, the LHC was going to be operating at design specifications…but more on that later).

A proton’s journey starts in a tank of research grade hydrogen gas (impurities are measured in parts per million, or parts per billion). We first take molecular hydrogen (a diatomic molecule for those of you keeping track) and break it down into atomic hydrogen (individual atoms). Next, we strip hydrogen’s lone electron from the atom (0:00 in the video linked above). We are now left with a sample of pure protons. These protons are then passed into the LINear ACcelerator 2 (LINAC2, 0:50 in the video linked above), which is the tiny purple line in the bottom middle of the above figure.

The LINAC 2 then accelerates these protons to an energy of 50 MeV, or to a 31.4% percent of the speed of light [2]. The “M” stands for mega-, or times one million. The “eV” stands for electron-volts, which is the conventional unit of high energy physics. But what is an electron-volt, and how does it relate to everyday life? Well, for that answer, Christine Nattrass has done such a good job comparing the electron-volt to a chocolate bar, that any description I could give pales in comparison to hers.

Moving right along, now thanks to special relativity, we know that as objects approach the speed of light, they “gain mass.” This is because energy and mass are equivalent currencies in physics. An object at rest has a specific mass, and a specific energy. But when the object is in motion, it has a kinetic energy associated with it. The faster the object is moving, the more kinetic energy, and thus the more mass it has. At 31.4% the speed of light, a proton’s mass is ~1.05 times its rest mass (or the proton’s mass when it is not moving).

So this is a cruel fact of nature. As objects increase in speed, it becomes increasingly more difficult to accelerate them further! This is a direct result of Newton’s Second Law. If a force is applied to a light object (one with little mass) it will accelerate very rapidly; however, the same force applied to a massive object will cause a very small acceleration.

Now at an energy of 50 MeV, travelling at 31.4% the speed of light, and with a mass of 1.05 times its rest mass, the protons are injected into the Proton Synchrotron (PS) Booster (1:07 in the video). This is the ellipse, labeled BOOSTER, in the diagram above. The PS Booster then accelerates the protons to an energy of 1.4 GeV (where the “G” stands for giga- or a billion times!), and a velocity that is 91.6% the speed of light [2]. The proton’s mass is now ~2.49 times its rest mass.

The PS Booster then feeds into the Proton Synchrotron (labeled as PS above, see 2:03 in video), which was CERN’s first synchrotron (and was brought online in November of 1959). The PS then further accelerates the protons to an energy of 25 GeV, and a velocity that is 99.93% the speed of light [2]. The proton’s mass is now ~26.73 times its rest mass! Wait, WHAT!?

At 31.4% the speed of light, the proton’s mass has barely changed from its rest mass. Then at 91.6% the speed of light (roughly three times the previous speed), the proton’s mass was only two and a half times its rest mass. Now, we increased speed by barely 8%, and the proton’s mass was increase by a factor of 10!?

This comes back to the statement earlier, objects become increasingly more difficult to accelerate the faster they are moving. But this is clearly a non-linear affect. To get an idea of what this looks like mathematically, take a look at this link here [3]. In this plot, the Y-axis is in multiples of rest mass (or Energy), and the x-axis is velocity, in multiples of the speed of light, c. The red line is this relativistic effect that we are seeing, as we go from ~91% to 99% the speed of light, the mass increases gigantically!

But back to the proton’s journey, the PS injects the protons into the Super Proton Synchrotron (names in high energy physics are either very generic, and bland, or very outlandish, e.g. matter can be charming). The Super Proton Synchrotron (SPS, also labeled as such in above diagram, 3:10 in video above) came online in 1976, and it was in 1983 that the W and Z bosons (mediators of the weak nuclear force) were discovered when the SPS was colliding protons with anti-protons. In today’s world however, the SPS accelerates protons to an energy of 450 GeV, with a velocity of 99.9998% the speed of light [2]. The mass of the proton is now ~500 times its rest mass.

The SPS then injects the proton beams directly into the Large Hadron Collider. This occurs at 3:35 in video linked above, however, when this video was recorded the LHC was operating at design energy, with each proton having an energy of 7 TeV (“T” for tera-, a million million times). However, presently the LHC accelerates the proton to half of the design energy, and a velocity of 99.9999964% the speed of light. The protons are then made to collide in the heart of the detectors. At this point the protons have a mass that is ~3730 times their rest mass!

So, the breaking of the world instantaneous luminosity record was not the result of one single instrument, but the combined might of CERN’s full accelerator complex, and in no small part by the magnetic optics systems in these accelerators (I realize I haven’t gone into much detail regarding this, my goal was simply to introduce you to the acceleration process that our beams undergo before collisions).

Until next time,

-Brian

References:

[1] CERN, “LHC Design Report,” https://ab-div.web.cern.ch/ab-div/Publications/LHC-DesignReport.html

[2] CERN, “CERN faq: The LHC Guide,” http://cdsweb.cern.ch/record/1165534/files/CERN-Brochure-2009-003-Eng.pdf

[3] School of Physics, University of Southern Wales, Sydney Australia, http://www.phys.unsw.edu.au/einsteinlight/jw/module5_equations.htm

Tags: accelerator, luminosity
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LUMI LEVELING: What, Why and How?

Sunday, April 24th, 2011

Anna Phan

I’m interrupting my descriptions of LHCb to discuss something more relevant to the current status of the LHC. Namely this LHC status from just after midnight the other day:

Ken has already discussed the luminosity record in this post, and today I’ll be discussing luminosity leveling (LUMI LEVELING). You may be wondering what this has got to do with LHCb? Well, interaction point 8 (IP8) is where LHCb is located as can be seen in this image:

Aidan has timely discussed what luminosity is in this post where he said that larger instantaneous luminosity means having more events, we want to do everything we can to increase instantaneous luminosity. However, if you’ve been looking at the LHC luminosity plots for 2011, like the one for peak instantaneous luminosity below, you might have noticed that the instantaneous luminosities of ALICE and LHCb are lower than those of ATLAS and CMS.

The reason for the difference between the experiments is that the design instantaneous luminosities for LHCb and ALICE are much lower than for ATLAS and CMS. The target instantaneous luminosity for LHCb is \(2 \times 10^{32} cm^{-2} s^{-1} \) to \(3 \times 10^{32} cm^{-2} s^{-1}\) and for ALICE is \(5 \times 10^{29} cm^{-2} s^{-1} \) to \(5 \times 10^{30} cm^{-2} s^{-1}\) while ATLAS and CMS are designed for an instantaneous luminosity of \(10^{34} cm^{-2} s^{-1}\).

This means that while the LHC operators are trying to maximise instantaneous luminosity at ATLAS and CMS, they are also trying to provide LHCb and ALICE with their appropriate luminosities.

As Aidan mentioned in his post, there are a couple of different ways to modify instantaneous luminosity: you can change the number of proton bunches in the beam or you can change the area of the proton bunches that collide.

Last year the LHC operators optimised the collision conditions and this year have been increasing instantaneous luminosity by increasing the number of proton bunches.

The varying instantaneous luminosity requirements of the experiments have so far been handled by having a different number of proton bunches colliding at each of the interaction points. For example, last week there were 228 proton bunches in the beam, 214 of which were colliding in ATLAS and CMS, 12 of which were colliding in ALICE and 180 of which were colliding in LHCb.

However as more and more proton bunches are injected into the beam, it is not possible to continue to limit the instantaneous luminosity at ALICE and LHCb by limiting the number of colliding bunches. Instead, the LHC operators need to modify the collision conditions. This is what luminosity leveling refers to.

Luminosity leveling is performed by moving the proton beams relative to each other to modify the area available for interactions as the bunches pass through each other. This concept is much easier to explain diagrammatically: if the centres of the beams are aligned like on the left, there are more interactions than if they are offset from each other like on the right.

This luminosity leveling process can be seen in action in the graph below from the nice long LHC fill from last night. You can see the ATLAS and CMS luminosities slowly decreasing due to collisions, while the LHCb luminosity stays roughly constant at \(1.3 \times 10^{32} cm^{-2} s^{-1} \), where the vertical red lines are when the beam adjustments were made.

Tags: LHCb, luminosity
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LHC sets world record for instantaneous luminosity; wins Pulitzer Prize

Thursday, April 21st, 2011

Ken Bloom

OK, the second part of the title isn’t actually true, but more on that in a moment….

The fill that is currently in the LHC started at an instantaneous luminosity over 4E32:

Not only is this the highest collision rate ever achieved at the LHC, it’s also the highest ever at a hadron collider, exceeding the largest instantaneous luminosity ever recorded by Fermilab’s venerable Tevatron collider. As has been discussed by many of the US LHC bloggers, luminosity is key at this point — the larger it is, the more collisions we record, and the greater the chance that we can observe something truly new. In the four hours since the fill started, CMS has already recorded about one sixth of the useful data that was recorded in all of 2010!

As for the Pulitzer, this week Mike Keefe of the Denver Post won the 2011 Pulitzer for editorial cartooning for a portfolio of twenty cartoons that included this one about the LHC. (I’d rather not actually run the cartoon here, as I’m not sure we have the rights to it.) Good to see that we are part of journalism history!

Tags: LHC, luminosity
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What the L?!

Tuesday, April 19th, 2011

Aidan Randle-Conde

There are few things that particle physicists like to talk about more than luminosity (know affectionately as “L”). We measure it obsessively, we boast about it shamelessly and we never forget to mention it in our papers, plots and talks. So what’s the big deal? What is luminosity and why is it important?

The concept of instantaneous luminosity is borrowed from the field of astrophysics, and in that field it’s used to describe how much energy a star gives off. To calculate the instantaneous luminosity, simple measure how much energy flows through a surface in an interval of time. To get the instantaneous luminosity in particle physics simply swap energy for the number of particles and the definition is the same!

The instantaneous luminosity is a measure of how many particles (blue) pass through a surface of unit area (yellow) in unit time (not shown.)

Well, not quite. If you take a quick look at any of the experiments at the LHC you’ll notice that there are two beams, so to get any meaningful measurement of luminosity you’ll have to take the flows of particles in both beams, a task which doesn’t seem easy! In order to use the concept of instantaneous luminosity we need to apply some knowledge of special relativity. We imagine that the protons in one of the beams are all at rest, and see how many protons from the other beam pass through per unit area and unit time. (The instantaneous luminosity makes more sense for fixed target experiments, where there is only one beam and the other matter is kept at rest. This is how most early experiments operated, and we’ve been stuck using luminosity ever since!)

In itself, the instantaneous luminosity is useless to us, and to make any real use of it we must combine it with a cross section. A cross section used to describe how often some process happens, and the analogy is very simple! Imagine placing lots of targets in front of the beam of particles, each one representing a different process. The larger targets will be hit by more protons, so we’ll see those processes more often. A larger cross section means a higher rate of process! To get the number of events where that process happens (per unit time) we just multiply the cross section by the luminosity, and that tells us how many “hits” we can expect. Simple!

Since having a larger instantaneous luminosity means having more events, we want to do everything we can to increase instantaneous luminosity. We can do that in quite a few ways, and the most obvious way is to increase the number of protons in the beam. After all, each proton has its own tiny (very very tiny) targets, and since the cross section of a given process is the same for each proton, you can increase the total size of a given target by increasing the numbers of protons. Another way to increase the instantaneous luminosity is to cram the same number of protons into a narrower beam, and this is called squeezing. After a while we start to reach physical limits of what we can achieve (this is due to phase space factors, beam shape parameters and all sorts of fascinating properties of the beam that would make for another blog post!) so we need to resort to simpler methods. One of the most effective methods is to increase the number of bunches in the LHC ring, and this means that instead of cramming more protons into the same part of the ring at the LHC, we put more protons in the empty regions of the ring.

The protons presents many different processes, and each process has its own cross section. This diagram is not at all to scale, and the QCD cross section is much larger than the other cross section shown!

As usual, things aren’t quite as simple as this. There are many different processes and each with its own cross section. Some of them are much, much larger than others, and most of the larger cross sections are boring to us, so if we want to get to the interesting physics we need a way to artificially reduce the sizes of the boring cross sections. (It would be nice if we could increase the sizes of the interesting cross sections instead, but that’s not physically possible at the LHC!) The notoriously large cross section at the LHC is the quantum chromodynamical (QCD) cross section, which dominates everything we see and for most people it’s an annoyance that makes it harder to find the interesting physics. To reduce the cross sections of these processes we use a prescale, which is very simple. We only record events that fire the trigger, and the trigger looks for different kinds of events. A prescale tells the trigger to ignore a proportion of a specific kind of decay, and that way we can record fewer boring events and save our precious resources for the most interesting ones.

Now if you see a plot from a collaboration you’ll often see the luminosity written on the plot, but this is not the instantaneous luminosity, it’s the integrated luminosity. To get the integrated luminosity we multiply the instantaneous luminosity by the time interval when the instantaneous luminosity was delivered. This means that it has units of inverse area, and when we multiply it by a cross section we get a number of events. This is why the integrated luminosity is so important to us- if we know the cross section for a process, and we know the integrated luminosity we can work out how many events we expect to see, and compare that to how many we actually see. This tells us when to expect a discovery, and when we find something truly new and interesting!

A typical mass spectrum plot, proudly declaring the integrated luminosity for all to see. arXiv:1103.6218v1 hep-ex

It seems elegant and simple, but personally I find the whole thing is spoiled by the choice of units and converting things ever so slightly baffling (probably not something I should admit to in public!) Instantaneous luminosity is usually measured in cm²s^-1, which is an odd choice. In these units a typical value is 10³³, which is an unimaginably large number! This is almost inevitable because luminosity varies so widely between experiments and as new technologies become available. If we choose new units now to make the numbers more manageable, they’ll still become ridiculously large in the future. To confuse things further the integrated luminosity is usually measured in inverse barns (as in “You can’t hit a barn with that!”). A barn is 10^-28m², so this makes the integrated luminosity a little bit easier to express in terms that don’t make my head spin. But even after that, our integrated luminosities need prefixes to make the numbers nice, so you’ll often see integrated luminosities written in inverse picobarns (pb^-1) or inverse femtobarns (fb^-1) and then the smaller the prefix, the large the amount of integrated luminosity! I find that the easiest way to remember whether I need to multiply or divide by 1,000 to convert the units is to just go with what feels wrong and it’ll be right. Smaller inverse areas mean larger numbers of events. If that isn’t a crazy choice of units, I don’t know what is!

To get an idea of a typical integrated luminosity, let’s think about how much data we’d need to see a standard model Higgs boson of mass 200GeV. Let’s imagine we see 100 events which are not consistent with known backgrounds. To make our job easier, let’s think about the “gold plated” decay of H→ZZ and Z→ll, where l is a charged lepton. The branching fraction for this decay is about 25% for H→ZZ and about 7% for Z→ll, and let’s assume we are 50% efficient at reconstructing a Z. Altogether we’d need to produce about 80,000 Higgs bosons to see 100 events of this type. Dividing by the cross section of Higgs production at 200GeV gives us an integrated luminosity of 16ab^-1. That’s a lot of events! Luckily, there are many more final states we can explore, and when we add it all up, it turns out we’ll have enough data to be sensitive to a standard model Higgs before too long.

That’s all very impressive, but the punchline comes from the world of “low high energy physics”, for example the BaBar experiment. Whenever I want to tease my friends at the LHC, I remind them that my previous experiment had 550fb^-1 of data, about 5,000 times what we have right now, and a number the LHC will not reach any time soon!

You can usually tell what kind of physicist you’re talking to immediately by asking them what the luminosity is at the LHC. An experimental physicist will tell you in terms of data (ie inverse barns) where as an accelerator physicist will tell you in terms of beams (ie cm^-2s^-1.) I find it quite amusing that the accelerator physicists generally find everything up to the point of collision deeply fascinating, and everything after that a frightful bore that makes their work even more complicated, whereas the experimental physicists thinks the other way around!

Tags: luminosity
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Balancing the budget, one collision at a time

Thursday, April 7th, 2011

Aidan Randle-Conde

Imagine you’re in charge of a budget for a large organization of a few thousand people who are experts in their field. Imagine that if you don’t spend some of the money in the budget that you can’t keep what you’ve saved- it will be lost forever. Now imagine that there’s another group of a few thousand experts with exactly the same budget, right down the last penny.

That’s the kind of scenario that we face at the LHC, except the budget is in time and not money. We count proton collisions and not dollars. The LHC is delivering world record luminosities right now, and the different experiments are getting as much data as they can. For LHCb and ALICE there is pressure to perform, but between ATLAS and CMS the competition is cut throat. They’re literally looking at the same protons and racing for the same discoveries. Any slight advantage one side can get in terms of data is crucial.

What does any of this have to do with my work at ATLAS? Well I’m one of the trigger rates experts for pileup. When we take data we can’t record every proton collision, there are simple too many. Instead, we pick the interesting events out and save those. To find the interesting events we use the trigger, and we only record events when the trigger fires. Even when we exclude most of the uninteresting events we still have more data than we can handle! To get around this problem we have prescales, which is where we only keep a certain fraction of events. The trigger is composed of a range of trigger lines, which can be independent of one another, and each trigger line has its own prescale.

High pileup scenarios. Can you count the vertices? (ATLAS Collaboration)

The term “pileup” refers to the number of proton collisions per bunch crossing (roughly how many interactions we can expect to see when we record an event.) When I came to ATLAS from BaBar I had to get used to a whole new environment and terminology. The huge lists of trigger lines alone made my head spin, and so far pileup has been the strangest concept I’ve had to deal with. Why take a scenario that is already overwhelmingly complicated, with one of the most intricate machines the world, and make it even harder to understand, for the sake of a few more events? Because we’re in competition with CMS, that’s why, and everything counts. The image on the right shows a typical event with multiple interactions. Even counting the number of vertices is difficult!

Balancing the different prescales is where things get interesting, because we have to decide how we’re going to prescale each trigger. We have to make sure that we take as much data as possible, but also that we don’t over-burden our data taking system. It’s a fine balancing act and it’s hard to predict. Our choice of trigger prescales is informed by what the physicists want from the dataset, and what range of types of events will maximize our output. The details of what kinds of events we want is a very hotly debated topic and one that is best left to a separate blog post! For now, we’ll assume that the physicists can come up with a set of prescales that match the demands of their desired dataset. What usually happens then is that the trigger menu experts ask what would happen if things were a little different, if we increased or decreased a certain prescale.

The effects of proton burning on luminosity. (LHC)

We need to pick the right times to change the prescales, and it turns out that as we keep taking data, the luminosity decreases because we lose protons when they interact. This is known as proton burning and you can see the small but noticeable effect of this the image above. As we burn more protons we can change the prescales to keep the rate of data-taking high, and that’s where my work comes in. The rates for different trigger lines depend on pileup in different ways, so understanding how they act in different scenarios allows us to change the prescales in just the right way. We can make our trigger very versatile, picking up the slack by changing prescales on interesting trigger lines, and pushing our systems to the limit. My job is to investigate the best way to make these predictions, and use the latest data to do this. The pileup scenarios change quite rapidly, so keeping up to date is a full time job! And every second spent working on this means more protons have been burned and more collisions have taken place.

It’s not an easy task, it forces me to think about things I’ve never considered before, and keeps the competition at the forefront of my mind. I knew I’d be in a race for discovery when I joined ATLAS, but I never realized just how intense it would be. It’s exciting and a little nerve-wracking. I don’t want to think about how many protons pass by in the time it takes to write a blog post. Did we record enough of them? Probably. Can we do better? Almost certainly. There’s always more space in this budget, and always pressure to stretch it that little bit further.

Tags: ATLAS, luminosity, pileup, trigger
Posted in Uncategorized | 2 Comments »

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