Well it finally happened. My 13 year-old daughter has been pushing me for the last year to update my antiquated Nokia candy-bar cell phone. I think she was embarrassed to be seen with me and my phone. She, of course has the latest fashionable iPhone with a hot pink skin. I resisted because I did not want the novelty of a smartphone to become a semi-infinite time sink. In 20/20 hindsight perfectly explained by conservation of momentum, my Nokia and I fell into the ocean as I was hoisting an 8D battery from my dinghy into my lobster boat. If you don’t know, an 8D is the size used big diesel trucks and weights in the vicinity of 150 pounds (70 kg). About the same time, my digital camera which has served me well for many years started acting up by not recognizing the pictures stored on the smart card. To add insult to injury, my aging MacBook took a dump by deciding not to respond to any keys including the power on button. It seemed like a good time to get a smartphone while waiting for my replacement designed-by-Apple computer to arrive from China.
This turned out to be a good move. The UPS driver claimed he could not find Room 255 at 590 Commonwealth Avenue in Boston (go figure) and my new MacBook Air ended up in a warehouse in an obscure part of town, a not very nice part, at that. I was headed to CMS Physics week in Brussels without my computer which is rarely out of my reach. But thanks to a trip to Target with my daughter, I had my new iPhone!
Although the iPhone may be a fabulous toy in the right hands, it actually can be very useful. First of all, email is actually easier on the iPhone. Like most people, I use multiple accounts: Boston University for the most important items, but also a special gmail account for CMS hypernews where detector and physics analysis information is posted and can be accessed by subscription, another gmail account for personal use, a CERN account, and a Fermilab account. I can see all of these accounts at a glance on the iPhone and toggle between them, much easier than looking at multiple pages on my notebook. For sending longer messages or even typing this blog, I use the Apple wireless keyboard which works really well.
The first thing I did was access the CMS web pages, which have been recently upgraded. Thanks to Gilles Raymond and Lucas Taylor, we have a really nice set of public pages on CMS called “CMS TV”. Additional thanks to Tom McCauley who has made CMS TV especially useful for mobile devices.
Here is the LHC Page 1 broadcast on CMS TV. It shows fill number 2216 (this the number of times to date that protons have been injected into the LHC) and that we have “Stable Beams”. The red and blue traces show the intensities of beam-1 and beam-2 (number of protons in each ring) to be 1.8e+14. The graph on the right shows the instantaneous luminosity in units of protons per square cm per second, about 3000e+30, which we may write in the more compact form of 3/nb/s. More on what this number means and why we follow it so closely later in this blog. The beam status flags at the bottom show at a glance that everything is stable, when only the beam setup boxes are red.
Here is CMS page 1. It shows we are taking data. Yeah! The plot shows that the data taking started a bit before 08:00 and that data has been accumulated nearly linearly with time. The units are 1/pb. Note that using the instantaneous luminosity from LHC Page 1 (3/nb/s), that the conversion 1000/nb = 1/pb means that we are accumulating data at the rate of 1/pb every 1000 s. This means something rare with a cross section of 1 pb will be produced approximately every 15 minutes, on the average, sort of an Andy Warhol 15 minutes of fame. The red line tells us how many proton collisions were delivered and the blue line tells us what fraction of those collisions were actually processed by CMS electronics. Note that these two quantities can never be exactly exactly equal to each other even if nothing breaks because it takes time to read the data out even with our super-fast electronics; but as you can see the blue and red lines are very close to each other. The flags at the bottom show at a glance that all components of CMS are being read out (DAQ) and that the detector control systems (DCS) are all on. The DCS protects the detector from all kinds of bad things that might go wrong underground. The numbers at the bottom right are very important to us and that is why they are displayed on Page 1. First is the fill number followed by the CMS run number. This run number is how we will reference these data in the future. The data are labeled by “lumi sections” within the run so that if something goes wrong such as a voltage trip, we can go back and find those affected events quickly. Physics Bit Set ON means the data are tagged by the shift leader in the control room as being certified for physics analysis. The magnet is seen to be in its normal on state with a field of 3.8 tesla. The L1 rate of 77929 tells us how many events per second are being sent from the CMS hardware into the online computers for refined selection. More than 97 million events have been sent so far in this run. The last two numbers tell us the proton collision rate in luminosity units 2.4e+33 per square cm per second and the total number of collisions that have occurred 35.5/pb = 3.55e+37 per square cm. So thats a lot of useful information attainable at a glance and updated continuously in real time.
Here is the CMS data acquisition (DAQ) status. The proton beams are colliding and the detector is in stable operation for physics data taking. The column to the left labeled “Data to Surface” shows the status of each detector component. The middle table labeled “SM streams” shows the status of the Storage Manager (SM) which is sorting the events into various categories for offline storage. The key rates from the SM are graphed below for quick visual inspection. The “Data Flow” column on the right contains a wealth of information about the health of CMS and the most important numbers are listed across the top of the page. The main data flow is through stream “A” which is seen to have a rate of 343 Hz (343 events per second are being sent to the offline computers). Other key numbers are the event size (478 kB), the dead time (2.3%) or percentage of time that the detector is insensitive to collisions, and the percentage load on the online computers (58.5%) which are selecting the events to be recorded. There is even a live event display on the upper left. This page is very busy but it can be zoomed on the iPhone to see all the numbers clearly.
Finally, we come to the event display which is fun because it gives you a quick visual of how the collisions are seen by CMS. The yellow lines are the trajectories of the charged particles that are created in the collision (only the electrically charged particles leave an ionization trail). All particles, however (except neutrinos and muons), are absorbed in the CMS calorimetry. There are two types of interactions: mostly electromagnetic in the crystals (ECAL) represented by the red boxes and mostly nuclear in the brass (HCAL) represented by the blue boxes. It is the combination of these three sets of measurements plus muons which are identified by penetration of the massive CMS iron if they are present that gives us the total picture of the collision. As expected, when the protons collide lots of particles fly forward along the original directions of the protons. Interesting things tend to pop out at large angles.
Now back to the luminosity. Particle physicists measure probability of occurrence as something called the event “cross section”. Mathematically, cross section is defined as the transition rate (number of times something that you have defined happens per second) divided by the incident flux (number of times per area per second that the protons cross each other). The time cancels out and the area goes upstairs so our cross section unit is that of area, or square meters. Intuitively, cross section is the EFFECTIVE size that the target presents for the thing to happen. The famous textbook example is crows in a tree. You can’t see them because of the leaves, but you pick up your gun and start firing bullets randomly into the tree at one shot per second. The tree has an area of 100 square meters and all your bullets hit the tree. Once every 3 hours you hit a crow and it falls to the ground. The transition rate is 1 per 10000 seconds (I have rounded off). The incident flux is 1e-2 per square meter per second. Dividing these two numbers, we get 1e-2 square meters as the cross section for the crow. Now if you were shooting the gun and wanted to hit a wood tick it would be much harder because the tic has a much smaller size, say 1e-6 square meters. If you wanted to hit a tic, how many times would you need to shoot the gun? You fire 1 shot per second per 100 square meters (into the tree), which equals 1e-2 per square meter per second. This is the instantaneous luminosity. After 100 s, you will have accumulated an integrated luminosity of 1/ square meter. After 1e+8 seconds, you have an integrated luminosity of 1e+6/ square meter and you could have expected on the average to have hit one tick. The tick is hard to hit and it will take 3 years of shooting to hit one, on the average. The tikc has a small cross section compared to the crow.
In particle physics things are very small, indeed the proton itself is about 1e-15 square meters in size. The cross section units that we use is the “barn” as in “big as barn” originally so named because of the observed relatively large nuclear cross sections. Uranium presents itself as a huge target to an incoming neutron. One barn (b) is 1e-28 square meters and on our scale this is indeed big. One nanobarn (nano means 1e-9) is equal to 1e-37 square meters, or 1e-33 square cm. One picobarn (pico means 1e-12) is 1e-36 square cm and one femtobarn (femto means 1e-15) is 1e-39 square cm. The total cross section for pp collisions, that is for protons to interact by the strong force is about 100 mb (milli means 1e-3) or 0.1 b. If we take the proton radius to be 1e-15 meters and estimate the cross section as the area of a circle containing two protons just touching each other, then we get the same order of magnitude, 0.1 b. Coincidence? No, this works because the proton is “strong charge” neutral (“colorless” as the jargon goes) made up of quarks and the wavelengths of the quarks define its size, just as the wave lengths of electrons define the atomic size. The wavelength, by the way, is inversely proportion to momentum and the momentum scale is set by strength of the pull, stronger for quarks in a proton compared to electrons in an atom, resulting in bigger momenta for quarks in protons compared to electrons in atoms, resulting in small protons compared to atoms. Protons interact when they touch each other, just as atoms also do to form molecules. Atoms are not mostly empty space just because the nucleus is tiny, in spite of the fact that you can find this statement in many school books. At atom is a ball of electron and photon waves. The proton is a ball of quark and gluon waves.
So our geometric picture is that the protons interact if they touch each other. The cross section is 0.1 b. Let’s take our LHC instantaneous luminosity of earlier this morning of 2.4e+33 per square cm per second which we may write as 2.4/nb/s. Remember cross section is interaction rate divided by luminosity so interaction rate is cross section times luminosity, or (0.1 b) (2.4/nb/s) = 2.4e+8 per second. (Remember nano is 1e-9 and it is in the denominator of the luminosity). So that is a lot of proton interactions, 240 million per second. We love this as long as our electronics can handle it because we are getting a high rate of proton collisions. We need this to observe something rare. To put these rarer collisions in perspective, the W and Z have cross sections around 100 nb, the top around 1 nb and the higgs if it exists around 30 pb or so, roughly speaking. So right now we can expect to be creating a couple hundred W/Z per second, 2 top events per second and a higgs particle every 15 seconds. I got these numbers by multiplying 2.4/nb/s times the known particle cross sections. The proton is the leaf on the tree- easy to hit- the crow is our W and Z, a smaller birdie the top, and the higgs is the wood tick.
It was recently announced that CMS had recorded 5/fb. This is becoming an interesting number. Let’s estimate how many interesting events have been produced. (Remember femto is 1e-15 and it is in the denominator). We get 1e+8 W/Z, 1e+6 top events, and 3e+4 standard model higgs. This does not mean that we have recorded 30 thousand higgs particles (if it even exists), because we are only sensitive to certain branching fractions that distinguish themselves from the enormous number of ordinary collisions (remember the 240 million per second). The same goes for the W, Z and top. It may be that the big discovery to be made at the LHC is sitting at a cross section in the fb range. If so, we will need to observe a LOT of collisions to see it. The LHC design luminosity is 10/nb/s. This would give us an inverse femtobarn every 1e+5 seconds, or 28 hours. The super-LHC upgrade would push that number to 0.1/pb/s, or an inverse femtobarn every 3 hours!
Now back to the iPhone. For the princely sum of $9.99 (CMS TV being free would seem to be a bargain by comparison), I purchased the iSSH app, allowing my iPhone to become a secure remote terminal. I can log into any machine (for which I have an authorized account, of course) from anywhere in the world (needing either wireless or an ATT signal)… Adding the Apple wireless keyboard makes the iPhone very useful for many physics-type geeky tasks.
I can even run Root from my phone (Root is the powerful object-orented data analysis framework used in high energy physics). You can even follow Root on Twitter to get the latest development news!
Oh, I almost forgot, I wanted to report on my recent fishing trip. Fishing is great. It gets you outside in the saltwater air, relaxes you so you can think about physics (or not), and provides fantastic vistas and the rythym of the sea. Also, I love seafood, and everybody knows that nothing tastes better than the fish you catch yourself. Each year in New England as the water reaches a certain temperature, the Atlantic bonito (Sarda Sarda) and the false albacore (Euthynnus alletteratus, or “little tunny”) both members of the tuna family, peel off the jet stream and venture inshore. They are extremely fast swimmers and notoriously hard to catch earning them the collective name amongst local anglers as “funny fish”. A few years ago, I decided to become something of an expert at catching these fish. This past week, I ventured to the back side of Martha’s Vineyard and hooked this rather large bonito:
which became my favorite meal… sashimi!
Before departing for home, I searched for and downloaded a nifty free app called iSailor. To make proper use of it, you do have to purchase a set of nautical charts for $4.99 (always a catch right?). Anyway it works really well. After leaving Oak Bluffs harbor to cross Nantucket Sound, we came happened across a large school of albies on a nearby shoal. The iPhone allowed us to criss-cross this school multiple times (worked better than the more expensive boat GPS) and I hooked 5 of these beautiful fish in short order. I only kept one, but I will be flush with sashimi for a while.
So the iPhone is more than just a toy in the right hands. That’s my story anyway and I am sticking by it.