Hot Topics
Blogging from this Summer’s Conferences
Summertime means travelling to conferences. This year, Quantum Diarists are journeying all over the map and blogging about the discoveries and discussions shared. Conferences range from charm physics in balmy Hawaii to physics computing in New York City. Two bloggers, Aidan Randle-Conde and Richard Ruiz, will be posting regularly from the Eleventh Conference on the Intersections of Particle and Nuclear Physics and Neutrino 2012, respectively. So stay tuned and follow Quantum Diaries for the latest!
Neutrino 2012: Day 1
By Richard Ruiz | June 3, 2012It is Day 1 of Neutrino 2012, an annual conference dedicated to all things neutrino, and today’s talks about about to begin shortly with a welcome from, count them: two Nobel laureates. The first is by Jack Steinberger, co-discoverer of the muon neutrino along with science education advocate Leon Lederman, on the present state of neutrinos, what we know about them, and what we definitely do not know. It is a highly appropriate talk to kick off such an important conference. The second talk is by Makoto Kobayashi, “K” of the famed CKM matrix, and is on the existence of neutrino masses and how that discovery has defined a generation of on-going research.
Higgs update (CIPANP 2012)
By Aidan Randle-Conde | June 1, 2012Now that we’re in the conference season we’re treated to the latest results from the LHC and Tevatron. For now we focus on squeezing as much as we can from the 2011 data, so it’s a great time to look at the status of the Higgs searches. We’ll see some of the 2012 results at ICHEP in July (as summer abruptly turns into winter, with ICHEP being held in Australia.) Until then we must be content with what we can see with the data up to the end of 2011.
Computing for particle physics in perspective
By Ken Bloom | May 27, 2012Last week: 19th CHEP held in New York City, and I was there. There were five hundred people registered, all eager to talk about the latest advances and future directions in software and computing for particle and nuclear physics, and also to explore one of the world’s great cities. (As a native of the New York area, I was happy to play tour guide, although I didn’t expect that I’d end up escorting 17 people to Katz’s over the course of four days.) It was a good opportunity to think about the impact that advances in computing have made on physics.
Latest Posts
This article first appeared in Fermilab Today on May 22.
Tengming Shen was awarded a DOE Early Career Award to develop a high-performance superconducting material for accelerator technology. <em>Photo: Reidar Hahn</em>
Over the years, engineers have found ways to cram more and more transistors onto a single integrated circuit. As a result of these improvements, they have been able to pack more computing power into smaller machines.
In much the same way, the key to developing better high-energy particle accelerators has been building increasingly powerful magnets to put inside them.
The Department of Energy recently presented an Early Career Research Award to Fermilab scientist Tengming Shen, a 2010 Peoples Fellow working to spur the next magnet revolution.
DOE awarded Shen $500,000 per year for five years for his research into engineering high-field superconducting materials for advanced accelerator technology. If his team succeeds, the work could pave the way for the construction of high-field superconducting magnets for future accelerators such as Fermilab’s proposed muon collider, for energy upgrades of the Large Hadron Collider and for the development of new medical imaging devices.
Shen’s strategy is to search for a better magnet-making material. Scientists currently use two niobium-based materials, NbTi and Nb3Sn.
“You have to go into a territory that’s new,” he said.
Shen works with superconducting magnets, which conduct electricity without resistance when cooled below a certain temperature. This reduces the amount of energy required to power them and allows them to achieve higher magnetic fields.
To reach this point in his research, Shen has collaborated with other scientists in the Very High Field Superconducting Magnet Collaboration, a partnership among U.S. national laboratories, universities and members of superconductor industry.
Fermilab’s Tevatron was the first particle accelerator to use niobium-titanium superconducting magnets. Before superconducting magnets, scientists had used iron or copper magnets, which required large amounts of electricity and, when used with insufficient cooling, tended to melt.
Fermilab founder Bob Wilson purchased as much niobium-titanium as he could, and Fermilab scientists developed a process for building large superconducting magnets. Members of industry eventually adopted the technology to mass-produce magnets used in MRI machines, now found in most hospitals. The major particle accelerators that have followed – the LHC at CERN, HERA at DESY and RHIC at Brookhaven National Laboratory – all depend on this technology.
Scientists cooled magnets in the Tevatron with liquid helium to 4.2 Kelvin; they reached a magnetic field strength of 4.3 Tesla. The scientists who built the Large Hadron Collider cooled their magnets with superfluid liquid helium to an even colder 1.9 Kelvin and almost doubled that performance to 8.3 Tesla. Fermilab and other U.S. laboratories have recently developed new technology using niobium-tin, Nb3Sn, which scientists hope will help them make the jump to 12- to 13-Tesla magnets.
The next step, according to Shen, is to push the limit of superconducting magnet technology by exploring new materials beyond the niobium family. This would allow scientists to more than double the energy reach of the LHC without increasing the size of the accelerator, he said.
Shen plans to study a group of high-field superconductors, in particular Bi2Sr2CaCu2Ox. He expects he could use this material to build magnets with a reach of up to 50 Tesla.
Even better, the new material could be used to construct 1- to 5-Tesla magnets that operate at higher temperatures. Whereas current superconducting magnets must be cooled with liquid helium, Shen’s magnets could potentially be cooled with a simpler refrigeration unit.
“Helium is very expensive,” Shen said. “There are many places like Africa, India and China that would like to develop cryogen-free devices.”
The development of high-temperature superconductors could eventually lead to better power lines, faster computers and more energy-efficient transportation, Shen said.
“There are many superconducting materials and many more to be discovered,” he said. “The whole world could be superconducting.”
—Kathryn Grim
Greetings from Kyoto! The sun is high and the solar neutrino rate is brimming.
Conference Poster for Neutrino 2012 in Kyoto, Japan (http://neu2012.kek.jp/)
It is Day 1 of Neutrino 2012, an annual conference dedicated to all things neutrino, and today’s talks about about to begin shortly with a welcome from, count them: two Nobel laureates. The first is by Jack Steinberger, co-discoverer of the muon neutrino along with science education advocate Leon Lederman, on the present state of neutrinos, what we know about them, and what we definitely do not know. It is a highly appropriate talk to kick off such an important conference. The second talk is by Makoto Kobayashi, “K” of the famed CKM matrix, and is on the existence of neutrino masses and how that discovery has defined a generation of on-going research.
Okay, time for the bad news. There is no internet in the main lecture hall and, as a consequence, I cannot physically live-blog this week. This is a bit of a disappointment but check back here often for regular updates through the week. After an interesting conversation on the flight over here, I am expecting to hear some very interesting and very new results.
Happy Colliding
- richard (@bravelittlemuon)
When I was in elementary school we read a story that went something like this. The protagonist’s grandmother was baking a cake for his tenth birthday. Before the big day she told him that she had a surprise: she was going to serve a cake prepared by a thousand people! The kid could hardly wait to see this enormous cake, baked by all those people in honor of his coming of age. When the family arrived grandma brought out the cake – a nine inch layer cake with ten candles. Disappointed, the kid protested that this was not the thousand-baker cake he was led to expect. Then comes the moral of the story, where grandma explains that indeed a thousand people contributed to the making of this cake. Someone had to grow the wheat, another had to mill it into flour. Then there was the milk and the butter and the sugar. Then the people who built the mixer and the oven and the cookware. You get the idea.
This is partly the reason why high energy physics collaborations’ publications have author lists with as many as thousands of people. There is usually over a decade’s worth of work by hundreds of people from conception to the beginning of an experiment. Then these experiments run anywhere from a few to tens of years. Generations of graduate students complete their dissertation work on an experiment. Each has a few people in the control room 24/7 monitoring the operation of every component. Countless technicians maintain the hardware. There are vast computing resources at labs and universities. Each of the groups and people I listed who contributes to the experiment is an author on publications; he gets to comment on what said publications state and how it’s phrased. (This last part is no fun. Perhaps more on that later when the paper I’m writing is submitted.)
The tradition of high energy physics, you’ll say, is very admirable since it credits all of these individuals who collaborate on the science. True, HEP (and science in general) has a very strict understanding of giving credit where it’s due and respecting intellectual property. However, we also have a very strong sense of possession when it comes to our data. We, the thousand some collaborators, designed, built, ran this experiment. We will analyze every last bit of data and publish the results with our names on it. We might even calculate what not to publish so that other scientists don’t read our journal article and proceed to do an analysis that we haven’t yet done but intend to do in the future.
This is not entirely analogous to the cake story above, but I’ll try to draw the parallel that I find interesting. In HEP, the thousand collaborators are the one baking grandma. And they all get credit, always. The unnamed contributors whose roles are indirect and difficult to quantify are the rest of us. Universities are knowledge and research hubs that enhance science as a whole, independent of the actual number of professors and students working on one particular experiment. Without the rest of the university community, the handful of people in one field in one department, such as high energy physics, wouldn’t really exist. Same goes for national labs. It further applies to smaller universities overseas who produce quite a lot of the researchers that work on these large experiments. We’re a large community in the knowledge-making business whose boundaries are blurry. So whose is the vast amount of data we generate everyday?
This is a somewhat controversial subject (one which an un-tenured scientist wiser than myself might avoid), but I find it necessary to debate the ownership of data. All of these experiments are funded by the government, therefore by the taxpayer. Science benefits from scrutiny and from transparency. On the other hand, science values being the first to discover something above all else. And science needs expertise, something which those who designed and ran an experiment for years will have a lot more of than a distant colleague looking at an unfamiliar set of data.
What do we do then? Do we want to be the best baker in town with the most sought after cake at all cost? Do we take no interest in who bakes the cake as long as it’s the best cake possible? Is a compromise possible? I think the metric should be the quality of science itself and the speed with which it progresses, and while familiarity with and expertise of an experiment are highly important, ownership shouldn’t be.
The biggest news at CIPANP 2012 for particle physicists seems to be coming from the “low” energy frontier, at energies in the ballpark of 10GeV and lower. This may come as a surprise to some people, after all we’ve had experiments working at these energies for a few decades now, and there’s a tendency to think that higher energies mean more potential for discovery. The lower energy experiments have a great advantage over the giants at LHC and Tevatron, and this is richer collection of analyses.
There’s a big difference between discovering a new phenomenon and discovering new physics, which is something that most people (including physicists!) don’t appreciate enough. Whenever a claim of new physics is made we need to look at the wider implications of the idea. For example, let’s say that we see the decay of a \(\tau\) lepton to an proton and a \(\pi^0\) meson. The Feynman diagram would look something like this:
tau lepton decay to a proton and a neutral pion, mediated by a leptoquark
The “X” particle is a leptoquark, and it turns leptons into quarks and vice versa. Now for this decay to happen at an observable rate we need something like this leptoquark to exist. There is no Standard Model process for \(\tau\to p\pi^0\) since it violates baryon number (a process which is only allowed under very special conditions). So suppose someone claims to see this decay, does this mean that they’ve discovered new physics? The answer is a resounding “No”, because if they make a claim of new physics they need to look elsewhere for similar effects. For example, if the leptoquark existed the proton could decay with this process:
proton decay to an electron and neutral pion, mediated by a leptoquark
We have very stringent tests on the lifetime of the proton, and the lower limits are currently about 20 orders of magnitude longer than the age the universe. Just take a second to appreciate the size of that limit on the lifetime. The proton lasts for at least 20 orders of magnitude longer than the age of the universe itself. So if someone is going to claim that they have proven the leptoquark exists we need to check that what they have seen is consistent with the proton lifetime measurements. A claim of new physics is stronger than a claim of a new phenomena, because it must be consistent with all the current data, not just the part we’re working.
How does all this relate to CIPANP 2012 and the low energy experiments? Well it turns out that there are a handful of large disagreements in this regime that all tend to involve the same particles. The \(B\) meson can decay to several lighter particles and the BaBar experiment has seen the decays to the \(\tau\) lepton are higher than they should be. The disagreement is more than \(3\sigma\) disagreement with the Standard Model predictions for \(B\to D^{(*)}\tau\nu\), which is interesting because it involves the heaviest quarks in bound states, and the heaviest lepton. It suggests that if there is a new particle or process, that it favors coupling to heavy particles.
Standard model decays of the B mesons to τν, Dτν, and D*τν final states
In another area of \(B\) physics we find that the branching fraction \(\mathcal{B}(B\to\tau\nu)\) is about twice as large as we expect from the Standard Model. You can see the disagreement in the following plot, which compares two measurements (\(\mathcal{B}(B\to\tau\nu)\) and \(\sin 2\beta\)) to what we expect given everything else. The distance between the data point and the most favored region (center of the colored region) is very large, about \(3\sigma\) in total!
The disagreement between B→τν, sin2β and the rest of the unitary triangle measurements (CKMFitter)
Theorists love to combine these measurements using colorful diagrams, and the best known example is the unitary triangle. If the CKM mechanism describes all the quark mixing processes then all of the measurements should agree, and they should converge on a single apex of the triangle (at the angle labeled \(\alpha\)). Each colored band corresponds to a different kind of process, and if you look closely you can see some small disagreements between the various measurements:
The unitary triangle after Moriond 2012 (CKMFitter)
The blue \(\sin 2\beta\) measurement is pulling the apex down slightly, and green \(|V_{ub}|\) measurement is pulling it in the other direction. This tension shows some interesting properties when we try to investigate it further. If we remove the \(\sin 2\beta\) measurement and then work out what we expect based on the other measurements, we find that the new “derived” value of \(\sin 2\beta\) is far off what is actually measured. The channel used for analysis of \(\sin 2\beta\) is often called the golden channel, and it has been the main focus of both BaBar and Belle experiments since their creation. The results for \(\sin2\beta\) are some of the best in the world and they have been checked and rechecked, so maybe the problem is not associated with \(\sin 2\beta\).
Moving our attention to \(|V_{ub}|\) the theorists at CKMFitter decided to split up the contributions based on the semileptonic inclusive and exclusive decays, and from \(\mathcal{B}(B\to\tau\nu)\). When this happens we find that the biggest disagreement comes from \(\mathcal{B}(B\to\tau\nu)\) compared to the rest. The uncertainties get smaller when \(\mathcal{B}(B\to\tau\nu)\) is combined with the \(B\) mixing parameter, \(\Delta m_d\), which is well understood in terms of top quark interactions, but these results still disagree with everything else!:
Disagreement between B→τν, Δmd and the rest of the unitary triangle measurments (CKMFitter)
What this is seeming to tell us is that there could be a new process that affects \(B\) meson interactions, enhancing decays with \(\tau\) leptons in the final state. If this is the case then we need to look at other processes that could be affected by these kinds of processes. The most obvious signal to look for at the LHC is something like production of \(b\) quarks and \(\tau\) leptons. Third generation leptoquarks would be a good candidate, as long as they cannot mediate proton decay in any way. Searching for a new particle of a new effect is the job of the experimentalist, but creating a model that accommodates the discoveries we make is the job of a theorist.
That, in a nutshell is the difference between discovering a new phenomenon and discovering new physics. Anyone can find a bump in a spectrum, or even discover a new particle, but forming a consistent model of new physics takes a long time and a lot of input from all different kinds of experiments. The latest news from BaBar, Belle, CLEO and LHCb are giving us hints that there is something new lurking in the data. I can’t wait to see wait to see what our theorist colleagues do with these measurements. If they can create a model which explains anomalously high branching fractions \(\mathcal{B}(B\to\tau\nu)\), \(\mathcal{B}(B\to D\tau\nu)\), and \(\mathcal{B}(B\to D^*\tau\nu)\), which tells us where else to look then we’re in for an exciting year at LHC. We could see something more exciting than the Higgs in our data!
(CKMFitter images kindly provided by the CKMfitter Group (J. Charles et al.), Eur. Phys. J. C41, 1-131 (2005) [hep-ph/0406184], updated results and plots available at: http://ckmfitter.in2p3.fr)
“Give it to me—the real news”
“So I will”
“Well, Dadamashay, let me see what skill you have. Tell me the big new news of these days, making it ever so small.”
“Listen”[1]
When, I was a graduate student, somewhat after the time of the Vikings in long boats, my thesis supervisor, Prof. Bhaduri[2], took me with him when he went on sabbatical to Copenhagen, a Mecca for nuclear physics at that time. When we were leaving there, his officemate gave him a small Mickey Mouse figurine so he would know what kind of physics to work on. Well another man might have been angry, And another man might have been hurt, But another man never would have[3] stressed during his seminar that he was using a Mickey Mouse model. A yes, Mickey Mouse science, the simple model or calculation that brings out salient features that are all too often lost or obscured in the complete calculation.
We all know what big science is: the big detectors at the Large Hadron Collider (CMS has a 12,500 ton steel yoke) or the Super-Kamiokande (50,000 tons of water). That is big science. Even theoretical physics does big science: the massive calculations of lattice quantum chromodynamics (QCD) or the nuclear shell model. Now, there have been attacks on big science, either the LHC or lattice QCD, as being inherently evil because they are so big. Would you believe, even books written on the topic? I strongly disagree with that view. Large science is an essential part of science. Big is needed to answer the questions we want answers to. However, there is more to science than that. We need the little to complement the big, the simple to complement the complex. As a post-doc, I was returning from a somewhat annoying conference with Gerry Brown[4] (b. 1926), one of leading nuclear physicists of that generation, when he turned to me in exasperation and said that people did not realize how many hours of computer time went into his simple estimates. There is an interesting concept: using computer time to justify simple estimates, simple complementing the complex. The purpose of computing is insight, not numbers[5] and the simple Mickey Mouse models are essential in generating that insight—even when they are justified by complex calculations.
The simple models are useful in a number of ways. First, they are useful in checking the results of complex computer calculations. I have learnt through bitter experience never to believe the result of a computer calculation until I have “understood” them (and not always then). That is, until using some simple model or estimates, either explicitly or implicitly, I can reproduce the main trends of the results. In trying to do that, I have frequently found errors. Never trust a number you do not understand.
Second, we want to understand what aspects of the model are important in reproducing the results and which are coincidental. Scientific models are designed to predict future observations, but which aspects of the model are crucial to that endeavour. It is through the use of simple models that we can most easily explore the dependencies of the results on the assumptions. We calculate some nuclear cross-section. Is that bump significant? What, if anything, does the location of the bump tell us? What about the turn up near threshold? Is that an artifact? We want to know more than merely if the calculation fits the data. It is here that the simple models come in. They give us the insight into how the models can be improved and what assumptions are not necessary and can be eliminated.
Finally, and most importantly, it is the simple models that allow us, as people, to understand the results. It is not just for the layman that we need the simple models, but for the expert as well. A prime example would be the non-relativistic quark model. Its success calculating the properties of the excited states of the proton was touted as proof of the quark model but all it tested was the symmetries built into the calculations. The simple approximations to the non-relativistic quark model revealed it pretentions. But as a Mickey Mouse model, the non-relativistic quark model gave us insight into QCD that would have been difficult if not impossible to obtain otherwise.
I suppose one could hook up the computers directly to the experiments and have them generate models, test the models against new observations and then modify the experimental apparatus without any human intervention. However, I am not sure that would be science. Science is ultimately a human activity and the models we produce are products of the human mind. It is not enough that the computer knows the answer. We want to have some feeling for the results, to understand them. Without the simple models, Mickey Mouse science, that would not be possible: the big news made ever so small.
To receive a notice of future posts follow me on Twitter: @musquod.
Now that we’re in the conference season we’re treated to the latest results from the LHC and Tevatron. For now we focus on squeezing as much as we can from the 2011 data, so it’s a great time to look at the status of the Higgs searches. We’ll see some of the 2012 results at ICHEP in July (as summer abruptly turns into winter, with ICHEP being held in Australia.) Until then we must be content with what we can see with the data up to the end of 2011.
Both CMS and ATLAS are still searching for the Higgs boson, and that means that if it exists, it must exist in the difficult low mass region. This is something that Standard Model advocates have “known” all along, since the global fit to electroweak data all point to a Higgs mass around 95GeV. The further away the mass of the Higgs is from 95GeV the more we need to explain why it has the mass that it does. The diagram below shows the electroweak fit and the right hand axis shows how many sigmas away the point is from what we expect. (I explained about sigmas in a previous post. About one third of all results are more than \(1\sigma\) away from expectation. For 2, 3, 4 and 5\(sigma\) these numbers are about 5% , 0.25%, 1 in 15,000, and 1 in 1.7 million respectively.) As we can see, moving up to about 160GeV the probability for discovering the Higgs is already as low as a few percent.
The electroweak fit ( arXiv:1107.0975v1 hep-ph)
It gets very tricky to reconcile a very high mass Higgs boson with existing constraints, so a high mass Higgs suggests physics beyond the Standard Model. The high mass region is cleaner, it’s easier to study, and it’s more exciting if there is a discovery. By contrast the lower mass region is takes much longer to see any evidence, the final states are more complicated and take more time to analyze. If we discover the Higgs bosons and only the Higgs boson then all that happens is we confirm that the Standard Model is an accurate description of reality. It looks like nature is teasing us with a low mass scenario.
Taking a look into the low mass regime (less than about 150GeV) we can see why there is such a challenge. The dominant decays of the Higgs boson are \(b\bar{b}\) quarks, \(\tau^+\tau^-\) pairs, and other quark and gluon processes. There are rarer decays too, and the most important is the \(\gamma\gamma\) final state. The branching fractions are shown in the plot below. A branching fraction is the fraction of Higgs bosons which will decay into each final state:
The Standard Model Higgs boson branching fractions (arXiv:1101.0593v3 hep-ph)
The analyses from ATLAS and CMS are closing in on the Standard Model Higgs boson now. The limits are a few times the Standard Model, and once the yellow and green bands (“Brazil band plots”, as one speaker called them) pass below the line \(1\times\)Standard Model we can exclude the Higgs boson. If the Higgs boson exists then one point will stay far above the \(1\times\)Standard Model line, and that’s the location of the Higgs boson. If you want a primer on how to read these plots see my previous post on the topic.
There are three main ways to produce a Higgs boson:
- • from gluon gluon fusion, which is the dominant process. In this case we get a Higgs boson, some jets from QCD and not much else. It’s a higher statistics sample, but there is nothing remarkable about the events.
- • with associated production, which is about a factor of ten smaller. Higgs bosons love to couple of massive vector bosons, so whenever we have a massive vector boson there’s a small but significant chance we’ll also see a Higgs boson. We can use the massive vector boson to “tag” these extraordinary events, making the search with lower statistics, but cleaner.
- • from vector boson fusion, a weird process that has a similar rate to associated production. In this mode the quarks from the protons exchange some massive bosons, which create a Higgs, and then the protons scatter off each other, leaving two jets at shallow angles. These events can be hard to reconstruct, but they are cool to look at.
The size of the background for \(b\bar{b}\) quarks is about 50 million times larger than the Higgs processes, so any analysis using a \(b\bar{b}\) final state must be very crafty. Generally we require that the Higgs is produced in association with a massive vector boson. When this happens the two bosons usually move back to back in the lab frame, so we can look for a high momentum Higgs boson. This makes things easier for the \(b\bar{b}\) final state because the two b-jets should be on the same side of the detector, and look like a “fat” jet. Even so, there are still large backgrounds from QCD processes. Since December 2011 physicists have been busy working to get as much discrimination between the Higgs and the background processes as possible, so its no surprise that we see more use of multivariate analyses in these searches. With a more dedicated study we can split up our searches based on the final states and tailor each final state accordingly. This “divide and conquer” method has lead to improved limits. The current exclusion for \(H\to b\bar{b}\) is already a few times the Standard Model:
ATLAS limits for Higgs decaying to b quarks (B LaForge, CIPANP2012)
CMS limits for Higgs decaying to b quarks (C Palmer, CIPANP2012)
For the next dominant mode, the \(\tau^+\tau^-\) final state, we have a different set of challenges. \(tau\) leptons produce neutrinos, which carry away some of the momentum, making it harder for us to reconstruct the event. To make things worse, the \(\tau\) can decay to leptons or to hadrons, so we need to split up our analyses and treat each case separately. And if that wasn’t enough, we also have a large background from decays of the Z boson, which have exactly the same final state. Given all this it’s a wonder we can use this channel at all. Unfazed by the challenges, both ATLAS and CMS have shown great improvements in this channel:
ATLAS limits for Higgs decaying to tau leptons (B LaForge, CIPANP2012)
CMS limits for Higgs decaying to tau leptons (C Palmer, CIPANP2012)
The next dominant processes are \(c\bar{c}\) and \(gg\), which are of no use to us at all. Backgrounds from QCD processes are just too high for these modes to be useful. So that leaves the \(\gamma\gamma\) final state, and this is the cleanest mode for the lower mass scenarios. To decay \(\gamma\gamma\) the Higgs boson must go through some intermediate particles in a loop. The challenges presented by the \(\gamma\gamma\) final states are mostly associated with the detectors. How do we know when we see a photon in the detector, and not a jet? What control samples can we use to calibrate our energy scale? These are tough questions to answer, and since the backgrounds for this channel are so high we need to have confidence in our abilities to recognize and reconstruct photons. (I’m actually a bit skeptical that we have seen hints of a Higgs based on these kinds of questions. Our most sensitive channel is the one with some of the biggest questions.) Even so, the limits are looking encouraging:
ATLAS limits for Higgs decaying to photons (B LaForge, CIPANP2012)
CMS limits for Higgs decaying to photons (C Palmer, CIPANP2012)
I’ve skipped the massive vector boson final states (\(ZZ^*\) and \(WW^*\)), although these are sensitive to some of the range too. As we look to lower and lower mass ranges the contributions from these final states diminish rapidly, and the kinematic constraints get worse and worse. (At high mass the Higgs boson would produce real \(WW\) and \(ZZ\) pairs, giving us fantastically clean mass peaks. At lower masses one of the bosons must be virtual, and we lose one of our most useful constraints.)
Combining the results gives better exclusions. As we can see there is not much space left for the Higgs boson!
ATLAS limits for combined Higgs channels (B LaForge, CIPANP2012)
CMS limits for combined Higgs channels (C Palmer, CIPANP2012)
Most people’s money is on the region 124-126GeV. All we need to do now is collect the 2012 data and see if it shows the same bump. The waiting is the hardest part.
Summer is a productive time for us and tends to involve lots of traveling.
Fig. 1: My 2010 PDG booklet and my Japan Rail pass. I am not sure which is more important.
Hi All,
As fellow QDer Aidan posted this morning, it is conference season, again! Lots and lots of conferences for all the different sub-sub-fields in physics. Two big ones on my plate are Neutrino 2012, which is about ALL things that begin with the letters n-e-u-t-r-i-n-o and end in the letter -s; and ICHEP 2012, which is the mother-of-all high energy physics conferences. (Much more on ICHEP in a few weeks seeing that I have been invited to be a panelist on the “Social Media in Science Communication” session. Trust me, it will be good.)
Neutrinos are all the rage these days: from #FTLneutrinos to θ13, we are determined to know precisely how neutrinos work. Fortunate for us, there is a huge international conference, imaginatively called “Neutrino,” next week in the gorgeous, ancient city of Kyoto, Japan, and you can definitely count on there be a Quantum Diaries presence. QDer Zeynep Isvan will be around, and, with the suggestion from my chief editor, Daisy, I will be live-blogging the plenary sessions when I can. The programme is also already online, so feel free to check out the topics.
After the conference, however, is when things get kicked into high gear for me. A few months ago I won a NSF summer fellowship to research dark matter in Japan. It is now summer, so for the next three months I will be a visitor at University of Tokyo’s prestigious Institute for the Physics and Mathematics of the Universe, or IPMU for short. I still have plots to make for a meeting today and my first flight is (literally) 24 hours from now. At least I have my trusty messenger bag already packed with two of the more important things: a Japan Rail pass and my 2010 PDG booklet!
See you in Kyoto!
Happy Colliding
- richard (@bravelittlemuon)
PS While adding links and sources to the post, I found my IPMU host on Twitter.
PPS More than 3.6 fb-1 worth of data has already been collected by the collider experiments.
Fig. 2: Conference Poster for Neutrino 2012 in Kyoto, Japan (http://neu2012.kek.jp/)
Greetings from Florida! The summer conference season has just started, and on both sides of the Atlantic, in Florida and France, physicists are meeting to share the latest news from the LHC and the Tevatron. I’m at the Eleventh Conference on the Intersections of Particle and Nuclear Physics (CIPANP 2012), and with 70 parallel sessions, 10 plenary sessions, and 64 posters there’s a lot to explore here! While the Higgs boson is a hot topic, it’s not the main focus of the conference, topics include neutrino physics, cosmology, nuclear physics, dark matter and hadronic structure. Physicists are chatting over coffee, catching up on gossip and rumors, and trying to find the time to fit in the most interesting talks.
I delivered my talk yesterday (a whirlwind tour of Higgs bosons decaying to final states with tau leptons) so I can now relax and enjoy the rest of the conference. Given the diverse nature of CIPANP this is a great opportunity to find out about the other areas of physics. In the very low mass region there are extremely stringent tests of the Standard Model which keep getting better. It’s easy to forget that the most precise tests are not found at the high energy frontier, so hearing from colleagues who work with muons and neutrinos is vital.
Presenting my talk
So far I’ve mostly limited myself to the Higgs sessions and the plenary talks. We’ve seen ATLAS, CMS, CDF, and D0 squeeze as much as they can out of their datasets, looking in much more detail at the decay channels, splitting analyses into ever finer categories in order to improve the techniques. Even so, we’re going to have to wait for ICHEP in July to see some substantially improved exclusion limits.
Perhaps the best part of traveling to conferences is the change of scenery and break from the usual habits. I don’t want to give the impression that it’s like a vacation- nearly everyone is still working very hard while they’re here. Instead the travel breathes new life into our approach to physics, giving us a chance to think a bit differently about what we do.
A popular plenary session.
As I sit in talks I find my mind wandering to the public understanding of physics, because I struggle to understand a lot of the presentations from theorists. We tend to skip over a lot of information when we present our work, so it would be useful to be able to take things more slowly when explaining the more important areas. Unfortunately we need to get permission to present plots using data, so for now we are stuck with the plots that have been approved. They are often busy, pragmatic, and try to condense as much information as possible in as little space as possible. Putting in a few more steps could make the ideas much more accessible to the wider public, so if I get time in the next few months I want to explore making it easier to get more suitable plots approved for the public.
A physicist takes a break between sessions
I’ll focus more on the physics results in a different blog post. For now I just want to say that it’s great to be back in the USA again and (tedious border control aside) it’s been a very pleasant experience to be on this side of the Atlantic for a week. At these conferences there are always social events and receptions, so imagine how happy I was to see that there was a dolphin watching cruise on the schedule!
Dolphins!
The past couple of weeks I’ve been busy juggling research with an opportunity I couldn’t pass up: the chance to give lectures about the Standard Model to Cornell’s undergraduate summer students working on CMS.
The local group here has a fantastic program which draws motivated undergrads from the freshman honors physics sequence. The students take a one credit “research in particle physics course” and spend the summer learning programming and analysis tools to eventually do CMS projects. Since the students are all local, some subset of them stay on and continue to work with CMS during their entire undergraduate careers. Needless to say, those students end up with fantastic training in physics and are on a trajectory to be superstar graduate students.
Anyway, I spent some time adapting my Feynman diagram blog posts into a series of lectures. In case anyone is interested, I’m posting them publicly here, along with some really nice references at the appropriate level.
There are no formal prerequisites except for familiarity with particle physics at the popular science/Wikipedia level, though they’re geared towards enthusiastic students who have been doing a lot of outside [pop-sci level] reading and have some sophistication with freshman level math and physics ideas.
The whole thing is an experiment for me, but the first lecture earlier today seems to have gone well.
1985: First Computing in High Energy Physics (CHEP) conference is held in Amsterdam.
1991 or 1992: I encounter the World Wide Web for the first time. There is no graphical browser for it yet, so I am underwhelmed and not sure what it would ever be good for.
1998: CHEP to be held in Chicago. First time I had heard of the conference, and the thought that popped into my head was, “shoot me if I ever go to that.”
2005: I start to work on computing for the CMS experiment at the LHC.
2007: I attend CHEP in Victoria, Canada. No one shot me.
Last week: 19th CHEP held in New York City, and I was there. There were five hundred people registered, all eager to talk about the latest advances and future directions in software and computing for particle and nuclear physics, and also to explore one of the world’s great cities. (As a native of the New York area, I was happy to play tour guide, although I didn’t expect that I’d end up escorting 17 people to Katz’s over the course of four days.) It was a good opportunity to think about the impact that advances in computing have made on physics.
It’s worth looking at the keynote talk by Glen Crawford of the Department of Energy, who described the role of computing as a key enabling technology for our field. Here is a slide of his that I particularly liked:
On the right is what has become the meme (I guess) that we have been using in the US to illustrate how we need the interplay of scientific explorations in three scientific frontiers — energy, intensity, and cosmic — to understand critical problems in particle physics. But I hadn’t previously seen the diagram in the lower left, which shows the required interplay of advanced technologies to achieve these goals. (It certainly hadn’t occurred to me to put computing on the same footing as, say, the LHC accelerator itself.) Glen goes on to describe how particle physics has long been an early adopter of computing technologies, from networks to grids to the World Wide Web (yes, invented by particle physicists). And, in turn, these technologies have been absolutely necessary to handle the huge amounts of data produced by particle-physics experiments that need to be shared among thousands of researchers all over the world.
Other items that caught my attention:
2013: Next CHEP to be held in Amsterdam. Having survived this one without undue violence, maybe I’ll go to that one too. It will be interesting to see which predictions of this CHEP will have come true by then!
