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Posts Tagged ‘SUSY 2011’

Update: Section added to include LEP11 Results on Higgs Boson Exclusion (01 Sept 2011)

Expect bold claims at this week’s SUSY 2011 (#SUSY11 on Twitter, maybe) Conference at Fermilab, in Batavia, Illinois. No, I do not have any secret information about some analysis that undoubtedly proves Supersymmetry‘s existence; though, it would be pretty cool if such an analysis does exist. I say this because I came back from a short summer school/pre-conference that gave a very thorough introduction to the mathematical framework behind a theory that supposes that there exists a new and very powerful relationship between particles that make up matter, like electrons & quarks (fermions), and particles that mediate the forces in our universe, like photons & gluons (bosons). This theory is called “Supersymmetry”, or “SUSY” for short, and might explain many of the shortcomings of our current description of how Nature works.

At this summer school, appropriately called PreSUSY 2011, we were additionally shown the amount of data that the Large Hadron Collider is expected to collect before the end of this year and at the end of 2012. This is where the game changer appeared. Back in June 2011, CERN announced that it had collected 1 fb-1 (1 inverse femtobarn) worth of data – the equivalent of 70,000 billion proton-proton collisions – a whole six months ahead of schedule. Yes, the Large Hadron Collider generated a year’s worth of data in half a year’s time. What is more impressive is that the ATLAS and CMS experiments may each end up collecting upwards of 5 fb-1 before the end of this year, a benchmark number a large number of people said would be a “highly optimistic goal” for 2012. I cannot emphasize how crazy & surreal it is to be seriously discussing the possibility of having 10 fb-1, or even 15 fb-1, by the end of 2012.

Figure 1: Up-to-date record of the total number of protons collisions delivered to each of the Large Hadron Collider Detector Experiments. (Image: CERN)

What this means is that by the end of this year, not next year, we will definitely know whether or not the higgs boson, as predicted by the Standard Model, exists. It also means that by next year, experimentalists will be able to rule out the most basic versions of Supersymmetry which were already ruled out by previous, high-precision measurements of previously known (electroweak) physics. Were we to find Supersymmetry at the LHC now and not when the LHC is at designed specifications, which are expected to be reached in 2014, then many physicists would be at a loss trying to rectify why one set of measurements rule out SUSY but another set of measurements support its existence.

What we can expect this week, aside from the usual higgs boson and SUSY exclusion plots, are a set of updated predictions as to where we expect to be this time next year. Now that the LHC has given us more data than we had anticipated we can truly explore the unknown, so trust me when I say that the death of SUSY has been greatly exaggerated.

More on Higgs Boson Exclusion (Added 01 Sept 2011)

This morning a new BBC article came out on the possibility of the higgs being found by Christmas. So why not add some plots, shown at August’s Lepton-Photon 2011 Conference, that show this? These plots were taken from Vivek Sharma’s Higgs Searches at CMS talk.

If there is no Standard Model higgs boson, then the Compact Muon Solenoid Detector, one of the two general purpose LHC detectors, should be able to exclude the boson, singlehandedly, with a 95% Confidence Level. ATLAS, the second of the two general purpose detectors, is similarly capable of such an exclusion.

Figure A: The CMS Collaboration projected sensitivity to excluding the higgs boson with 5 fb-1 at √s = 7 TeV; the black line gives combined (total) sensitivity.

Things get less clear if there is a higgs boson because physical & statistical fluctuations adds to our uncertainty. If CMS does collect 5 fb-1 before the winter shutdown, then it is capable of claiming at least a 3σ (three-sigma) discovery for a higgs boson with a mass anywhere between mH≈ 120 GeV/c2 and mH ≈ 550 GeV/c2 . For a number of (statistical/systematic) reasons, the range might shrink or expand with 5 fb-1 worth of data but only by a few GeV/c2. In statistics, “σ” (sigma) is the Greek letter that represents a standard deviation; a “3σ result” implies that there is only a 0.3% chance of being a fluke. The threshold for discovery is set at 5σ, or a 0.000 06% of being a random fluke.

Figure B: The CMS Collaboration projected sensitivity to discovering the higgs boson with 1 (black), 2 (brown?), 5 (blue), and 10 (pink)  fb-1 at √s = 7 TeV.

By itself, the CMS detector is no longer sensitive. By combing their results, however, a joint ATLAS-CMS combined analysis can do the full 3σ discovery and a 5σ job down to 128 GeV/c2. The 114 GeV/c2 benchmark that physicists like to throw around is lower bound on the higgs boson mass set by CERN’s LEP Collider, which shutdown in 2000 to make room for the LHC.

Figure C: The projected sensitivity of a joint ATLAS-CMS analysis for SM higgs exclusion & discovery for various benchmark data sets.

However, there are two caveat in all of this. The smaller one is that these results depend on another 2.5 fb-1 being delivered by the upcoming winter shutdown; if there are any more major halts in data collection, then the mark will be missed. The second, and more serious, caveat is that this whole time I have been talking about the Standard Model higgs boson, which has a pretty rigid set of assumptions. If there is new physics, then all these discovery/exclusion bets are off. 🙂

Nature’s Little Secrets

On my way to PreSUSY, a good colleague of mine & I decided to stop by Fermilab to visit a friend and explore the little secret nooks that makes Fermilab, in my opinion, one of the most beautiful places in the world (keep in mind, I really love the Musée d’Orsay). What makes Fermilab such an gorgeous place is that is doubles as a federally sanctioned nature preserve! From bison to butterflies, the lab protects endangered or near-endangered habitats while simultaneously reaching back to the dawn of the Universe. Here is a little photographic tour of some of Nature’s best kept secrets. All the photos can be enlarged by clicking on them. Enjoy!

Figure 2: The main entrance to the Enrico Fermi National Accelerator Laboratory, U.S. Dept. of Energy Laboratory Designation: FNAL, nicknamed Fermilab. The three-way arch that does not connect evenly at the top is called Broken Symmetry and appropriately represents the a huge triumph of Theoretical (Solid State & High Energy) Physics: Spontaneous Symmetry Breaking. Wilson Hall, nicknamed “The High-Rise” can be see in the background. (Image: Mine).

Figure 3: Wilson Hall, named after FNAL’s first director and Manhattan Project Scientist Robert Wilson, is where half of Fermilab’s magic happens. Aside from housing all the theorists & being attached to the Tevatron Control Room, it also houses a second control room for the CMS Detector called the Remote Operations Center. Yes, the CMS Detector can be fully controlled from Fermilab. The photo was taken from the center of the Tevatron ring. (Image: Mine)

Figure 4: A wetlands preserve located at the center of the Tevatron accelerator ring. The preservation has been so successful at restoring local fish that people with an Illinois fishing license (See FAQ) are actually allowed to fish. From what I have been told, the fish are exceptionally delicious the closer you get to the Main Ring. I wonder if it has anything to do with all that background neutrino rad… never mind. 🙂
Disclaimer: The previous line was a joke; the radiation levels at Fermilab are well within safety limits! (Image: Mine)

Figure 5: The Feynman Computing Center (left) and BZero (right), a.k.a., The CDF Detector Collision Hall. The Computing Center, named after the late Prof. Richard Feynman, cannot be justly compared to any other data center, except with maybe CERN‘s computing center. Really, there is so much experimental computer research, custom built electronics, and such huge processing power that there are no benchmarks that allows for it to be compared. Places like Fermilab and CERN set the benchmarks. The Collider Detector at Fermilab, or CDF for short, is one of two general purpose detectors at Fermilab that collects and analyzes the decay products of proton & anti-proton collisions. Magic really does happen in that collision hall. (Image: Mine)

Figure 6: The DZero Detector Collision Hall (blue building, back), Tevatron Colling River (center) , and Collision Hall Access Road (foreground). Like CDF (Figure 5), DZero is one of two general-purpose detectors at Fermilab that collects and analyzes the decay products of proton & anti-proton collisions. There is no question that the Tevatron generates a lot of heat. It was determined long ago that by taking advantage of the area’s annual rainfall and temperature the operating costs of running the collider could be drastically cut by using naturally replenishable source of water to cool the collider. If there were ever a reason to invest in a renewable energy source, this would be it. The access road doubles as a running/biking track for employees and site visitors. If you run, one question that is often asked by other scientists is if you are a proton or anti-proton. The anti-protons travel clockwise in the Main Ring and hence you are called an anti-proton if you bike/run with the anti-protons; the protons travel counter-clockwise. FYI: I am an anti-proton. (Image: Mine)

Figure 7: The Barn (red barn, right) and American bison pen (fence, foreground). Fermilab was built on prairie land and so I find it every bit appropriate that the laboratory does all it can to preserve an important part of America’s history, i.e., forging the Great American Frontier. Such a legacy of expanding to the unknown drives Fermilab’s mantra of being an “Ongoing Pioneer of Exploring the Frontier of Discovery.” (Image: Mine)

Figure 8: American bison (bison bison) in the far background (click to enlarge). At the time of the photo, a few calves had just recently been born. (Image: Mine)

 

Happy Colliding.

 

– richard (@bravelittlemuon)

 

 

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Paper vs. Protons (Pt. 2)

Tuesday, August 9th, 2011

Yup, it’s still summer conference season here in the Wonderful World of Physics. My fellow QD bloggers rocked at covering the European Physics Society meeting back in July, so check it out. Aside from the summer conferences, it is also summer school season for plenty of people (like me!). To clarify, I am not talking about repeating a class during the summer. Actually, it is quite the opposite: these are classes that are at most offered once a year and are taught in different countries, depending on the year.

To give you context, graduate students normally run out of courses to take in our second or third of our PhD program; and although the purpose of a PhD is to learn how to conduct research, there will always be an information gap between our courses and our research. There is nothing wrong with that, but sometimes that learning curve is pretty big. In order to alleviate this unavoidable issue, university professors often will teach a one-time-only “topics” course on their research to an audience of three or four students during the regular academic year. Obviously, this is not always sustainable for departments, large or small, because of fixed costs required to teach a course. The solution? Split the cost by inviting a hundred or so students from around the world to a university and cram an entire term’s worth of information into a 1- to 4-week lecture series, which, by the way, are taught by expert faculty from everywhere else in the world. 🙂

To be honest, it is like learning all about black holes & dark matter from the people who coined the names “black holes” & “dark matter.” So not only do graduate students get to learn about the latest & greatest from the people who discovered the latest & greatest, but we also get to hear all the anecdotal triumphs and setbacks that lead to the discoveries.

Fig. 1: Wisconsin’s state capitol in Madison, Wi., taken from one of the bike paths
that wrap around the city’s many lakes. (Photo: Mine)

This brings us to the point of my post. Back in July, I had the great opportunity to attend the 2011 CTEQ Summer School in Madison, Wi., where for 10 days we talked about this equation:

Now, this is not just any ordinary equation, it is arguably the most important equation for any physicist working at the Large Hadron Collider, the Tevatron, or any of the other half-dozen atom smashers on this planet. In fact, this equation is precisely what inspired the name Paper vs. Protons.

Since quantum physics is inherently statistical most calculations result in computing probabilities of things happening. The formula above allows you to compute the probability of what happens when you collide protons, something experimentalists can measure, by simply calculating the probability of something happening when you collide quarks, something undergraduates can do! Physicists love quarks very much because they are elementary particles and are not made of anything smaller, at least that is what we think. Protons are these messy balls of quarks, gluons, photons, virtual particles, elephant-anti-elephant pairs, and are just horrible. Those researchers studying the proton’s structure with something called “lattice QCD” have the eternal gratitude of physicists like me, who only deal with quarks and their kookiness.

Despite being so important the equation only has three parts, which are pretty straightforward. The first part, is that tail end of the second line:

which is just probability of this happening:

Fig. 2: Feynman diagram representing the qq-bar → γ → e+e- process.

If you read Paper vs. Protons (Pt. 1) you might recognize it. This Feynman diagram represents a quark (q) & an antiquark (q with a bar over it) combine to become a photon (that squiggly line in the center), which then decays into an electron (e-) & its antimatter partner, the positron (e+). Believe it or not, the probability of this “qq-bar → γ → e+e-” process happening (or cross section as we call it) is something that advanced college students and lower level graduate students learn to calculate in a standard particle physics course. Trust me when I say that every particle physicist has calculated it, or at the very least a slight variation that involves muons. By coincidence, I actually calculated it (for the nth time) yesterday.

Okay, time for the second part of the equation. To help explain it, I am using a great image (below) from the LHC experiment ALICE. So you & I know that all matter is made from atoms (left). Atoms, in turn, consist of a nucleus of protons & neutrons (center) that are being orbited by electrons (white dots, left). A proton (right) is made up of three quarks (three fuzzy, white dots, right) that bathe in a sea of gluons (red-blue-green fuzziness, right). About 45% of a proton’s energy at the LHC is shared by the three quarks; the remaining 55% of the proton’s energy is shared by the gluons.

Fig. 3: An atom (left), an atom’s nucleus (center), and a free proton (right). (Image: ALICE Expt)

How do we know those numbers? Easy, with something called a “parton distribution function”, or p.d.f. for short! A p.d.f. gives us back the probability of finding, for example, a quark in a proton with 15% of the proton’s energy. Since we want to know the probability of finding a quark (q) in the first proton (with momentum x1) and the probability of finding an anti-quark (q with a bar over its head) in the second proton (with momentum x2) we need to use our p.d.f. (which we will call “f”) twice. Additionally, since the quark and anti-quark can come from either of the two protons we need to use “f” a total of four times. Part 2 of our wonderful equation encapsulates the entire likelihood of finding the quarks we want to smash together:

Now the third (and final!) part is the simple to understand because all it tells us to do is to add: add together all the different ways a quark can share a proton’s energy. For example, a quark could have 5% or 55% of a proton’s energy, and even though either case might be unlikely to happen we still add together the probability of each situation happening. This the third part of our wonderful equation:

Putting everything together, we find that the probability of producing an electron (e-) and a positron (e+) when smashing together two protons is actually just the sum (part 3) of all the different ways (part 2) two quarks can produce an e+e- pair (part 1). Hopefully that made sense.

Though it gets better. When we plug our values into the formula, we get a number. This number is literally what we try to measure that the Large Hadron Collider; this is how we discover new physics! If theory “A” predicts a number and we measure a number that is way different, beyond any statistical uncertainty, it means that theory “A” is wrong. This is the infamous Battle of Paper vs Protons. To date, paper and protons agree with one another. However, at the end of this year, when the LHC shuts down for routine winter maintenance, we will have enough data to know definitively if the paper predictions for the higgs boson match what the protons say. Do you see why I think this equation is so important now? This is equation is how we determine whether or not we have discovered new physics. :p

Happy Colliding.

– richard (@bravelittlemuon)

PS. If you will be at the PreSUSY Summer School at the end of August, be sure to say hi.

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