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Posts Tagged ‘conference’

My First Day at ICHEP (Again)

Thursday, July 3rd, 2014

ICHEPstartICHEP 2014 started today in Valencia, Spain. This is one of particle physics’s biggest conferences, held every two years. The last one, in 2012, coincided with the discovery of the Higgs boson. This year, we’re probably going to have more in the way of careful measurements than big new surprises. ATLAS and CMS have already released Higgs updates, and the pesky boson looks more and more like the Standard Model Higgs all the time.

This is the second ICHEP I’ve attended in person. I showed a poster at the first one, and wrote a blog post about it – which is a scary reminder of just how long I’ve been blogging. (I also still have my lanyard from that conference, which I’m wearing with my badge because it’s cooler than the boring black one we got this time.) This year, I’m here to give a parallel talk about the potential for even better measurements of the Higgs at the High-Luminosity LHC, which is a possible upgrade for the LHC that could take us well into the 2030s. By then, I suppose I should aspire to give an ICHEP plenary talk. ;)

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by Karen McNulty Walsh

With the discovery of the long-sought Higgs boson at the Large Hadron Collider (LHC), the world’s largest and most powerful particle collider, folks unfamiliar with the intricacies of particle physics might think the field has reached its end. But physicists gathered at the Large Hadron Collider Physics Conference in New York City June 2-7 say they are eager to move forward. Even amid discussions of tight budgets that make some proposed projects appear impossible, the general tenor, as expressed by leaders in the field, is that the future holds great potential for even more significant discoveries.

Physicist panel

Physicists joined New York Times science correspondent Dennis Overbye for a discussion on the future of the field.

At a session devoted to reflection and the future of the field, held Friday, June 6, Fabiola Gianotti, a particle physicist at Europe’s CERN laboratory (home of the LHC) and spokesperson for the LHC’s ATLAS experiment at the time of the Higgs discovery, said, “There is challenging work for everyone to make the impossible possible.” In fact, said James Siegrist, Associate Director of the Office of High Energy Physics within the U.S. Department of Energy’s (DOE) Office of Science, “I think the promise of the physics has never been greater.”

Co-sponsored by DOE’s Brookhaven National Laboratory and Columbia University, the week-long meeting featured updates on key findings from the LHC’s four experiments (including a possible hint of new physics), advances in theory, plans for future upgrades, and even future colliders—as well as apanel discussion moderated by Dennis Overbye, a science correspondent for the New York Times.

“We had a very successful conference with more than 300 participants discussing an impressive array of results from the recent LHC run,” said Brookhaven physicist Srini Rajagopalan, U.S. ATLAS Operations Program Manager and a co-organizer of the meeting. He also noted the extremely positive response to an open-to-the-public screening of Particle Fever, a documentary film that follows six scientists during the years leading up to the discovery of the Higgs boson. “I was simply amazed at the public interest in what we do. From young school students to senior citizens, people thronged to watch the movie and continued to ask questions late into the night.”

What keeps you up at night?

At Friday’s panel session, the Times’ Overbye had some questions of his own, perhaps more pointed that the public’s. He asked whether particle physicists’ streak of discoveries could be continued, whether the “glory days” for the U.S. were over, and what keeps physicists up at night. The panelists were realistic about challenges and the need for smart choices and greater globalization. But a spirit of optimism prevailed.

Natalie Roe, Director of the Physics Division at DOE’s Lawrence Berkeley National Laboratory—the first to respond—said, “I’m going to flip the question [of what keeps me up and night] and answer what gets me up in the morning.” Following a long period of experimental and theoretical successes, including the discovery of the Higgs, she said, “this is a very exciting time. There are still a few remaining details … dark matter and dark energy. And these are more than details; they are 95 percent of the universe!” With a long list of techniques available to get answers, she said, there is much work to be done.

University of California, Santa Cruz, physicist Steve Ritz, who recently chaired the Particle Physics Project Prioritization Panel (P5) and presented its recommendations for the future of the field, emphasized the importance of “telling our story,” staging and prioritizing future projects, and “aspiring to a greater program” that continues investments in crucial research and development to lay the foundation for future facilities.

Great technology progress, great challenges

In an overview talk that preceded the panel discussion, Gianotti presented a range of such future projects, including two possible linear accelerators, one in Japan the other at CERN, and two possible circular colliders, one in China and one at CERN. The latter, dubbed FCC, would be a proton-proton collider 80-100 kilometers in circumference—on the scale of the Superconducting Supercollider (SSC) once planned for and later cancelled in the U.S. Such a machine would push beyond the research limits of even the most ambitious upgrades proposed for the LHC.

Those upgrades, planned for data taking in Phase I in 2020 and Phase II in 2025, will begin the exploration of the coupling of the Higgs with other particles to explore the mechanism by which the Higgs generates mass, “electroweak symmetry breaking,” and searches for new physics beyond the standard model and into the realm of dark matter.

But, to really get at the heart of those questions and possibly reveal unknown physics, the scientists say the need for even higher precision and higher energy is clear.

Journey to the dark side

“Our elders had it easy compared to our students,” said Siegrist, describing the physics challenges now open to exploration. He likened this moment in time to the end of a video game his son had played where, “at the end of the game, you end up on ‘the dark side’ and have to start again.” In physics, he said, the dark sector—exploring dark matter and dark energy—is going to be equally challenging to everything that has come before.

To those who say building the future machines needed for this journey is impossible, Gianotti says, “didn’t the LHC also look close to impossible in the 1980s?” The path forward, she emphasized, is to innovate.

“Accelerator R&D is very important,” said Ritz, noting that, “anything we can do to design these machines to cost less” in terms of construction and operation should be done. “We need to be impatient about this,” he said. “We need to ask more and jump in more.”

Panelist Nima Arkani-Hamed, a theorist at the Institute of Advanced Study at Princeton University and Director of the Center for Future High Energy Physics in Beijing, China, likely agrees. He acknowledges the difficult task facing U.S. leadership in high-energy physics. “They are trying to make due with a budget that’s two or three times less than what our vision and this country deserves, and they are doing a good job,” he said. “But I worry that our generation will be viewed as the one that dropped the ball.”

“The sequence of steps for the next few decades is possible,” he added later. “It’s just a matter of will, not technology.”

But because of the scale and cost of future projects, he, like others, emphasized that “we will need the whole world and new pockets of resources and talent.”

The value of collaboration, competition, and globalization

Sergio Bertolucci, Director for Research and Computing at CERN, agreed. “We have been international, but we need to be truly global.”

Such cooperation and competition among nations is good for the field, Ritz emphasized. “We are intensely competitive. We want to be the ones to discover [something new.] But we are also cooperative because we can’t do it alone.”

Panelist Jerry Blazey, Assistant Director for Physical Sciences in the
Office of Science and Technology Policy, DOE’s Siegrist, and others agreed that the LHC is a great model for the field to stand behind and emulate for future collaborative projects. Blazey and Siegrist said OSTP and DOE would work together to discuss ways to smooth the process for such future multinational collaborations and to implement the recommendations of the P5 report.

These include future U.S. work at the LHC, an internationalized Long Baseline Neutrino Facility located at Fermi National Accelerator Laboratory, and a role in Japan’s proposed linear collider, as well as continued investments in the technologies needed for future experiments. Said University of California, Irvine, physicist Andrew Lankford, chair of the High Energy Physics Advisory Panel (HEPAP) to whom the report was delivered, the P5 report describes a field optimized for scientific progress. “It’s a ten year strategic plan—way more than a collection of cool experiments,” he said.

And it emphasizes the value of international competition and cooperation—perhaps one of the biggest successes of particle physics, aside from the breathtaking discoveries. Turning again to the example of the LHC collaborations, Ritz said, “50 years ago some of these people were in countries that were trying to kill one another. Now we don’t even think about what country they are from.”

As Brookhaven’s Rajagopalan summed up, “It is an exciting time for our field as we plan to move forward with ambitious global projects to address the fundamental questions of nature.”

Brookhaven Lab’s particle physics research is supported by the DOE Office of Science.

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Karen McNulty Walsh is a science writer in the Media & Communications Office at Brookhaven National Laboratory.

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Higgs Hunting in Progress

Wednesday, August 28th, 2013

ParishiggshuntingLast month I was at the annual Higgs Hunting workshop, in Orsay and  Paris, France.  Starting less than a week after EPS, it didn’t have much in the way of new results.  What it did give us is an opportunity to talk through where we are and where we’re going.  What do we know about the Higgs so far?  What do we still need to find out, and how do we go about it?  Why aren’t the coffees stronger, or at least larger?

It’s true, the last question isn’t about the Higgs, but it does reflect that a lot of the learning and discussion went on during the coffee breaks.  (I should stress in case the organizing committee reads this that the drinks and snacks at the coffee breaks were, on the whole, quite excellent.)  But of course we had talks too, and you can see both the slides and videos here.  I should warn you, though, that the talks are very technical — even more technical than might be usual for a Higgs conference, because it was generally assumed that participants already know the strategy for hunting the Higgs.

My talk was about the CMS search for Higgs decays to bottom quark pairs.  It covered four analyses, which are different from each other not because of what the Higgs decays into but because of what it’s produced in association with.  Without extra particles, we can’t see the Higgs in this decay channel because of all the bottom quark pairs from QCD.  But this direction of looking at different production mechanisms is also where Higgs searches as a whole are going, because ultimately Higgs production tells us as much about what the Higgs interacts with as Higgs decay.  And what we really hope to find is some difference from the Standard Model in those interactions.

From what we’ve seen so far, it looks like we’re hunting precisely the Standard Model Higgs.  But we are far from an exact answer; we haven’t even officially established evidence for the Higgs to bottom quark pair decay at all, yet.  So we’ll keep hunting, and hope the Higgs Beast turns out to be subtly different from the one we’re expecting.

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Are the Higgs Rumors True?

Monday, October 22nd, 2012

What Higgs rumors, you may ask? Well, there aren’t any that I know of, yet. But there might be soon…

There might be rumors soon because we are about to do another round of updates, for the 2012 Hadron Collider Physics Symposium (HCP). There aren’t any yet because our results (at least on CMS) are still “blinded,” which means that we haven’t actually looked at the “places” in the data where we see signs of our new boson. What we’re doing instead is looking at simulated data to see how much our results might improve when we add in the collisions we’ve recorded since ICHEP. We’re also putting in a few new analysis techniques, and checking them in the same way. And of course we are looking at data in other “places,” and we’re comparing it to simulations to make sure they’re doing a good job.

There will be several weeks between the moment we “unblind” — that is, look for the first time at what our signal looks like with the new data — and when results are shown at HCP. This is just as things were at ICHEP, and during those few weeks there were a lot of rumors around. It’s not possible to confirm or deny rumors when you know the status of ongoing work but haven’t yet agreed with your colleagues that it’s finished and ready to talk about publicly. So this time, I’m going to get in some general comments about rumors before I know anything at all about actual results. These comments will apply just as well to future updates.

What are we doing during the gap between unblinding and the conference? We’re checking our results, and putting them in a final presentable form. This is already compressed into a very short, hectic time, as I’ve written about before.

Are the rumors true? They are definitely not our official results, but they might turn out to be close. Or they might not. Specifically, the possibilities are:

  • A rumor is pretty much right. It’s no secret that particle physicists are bad at keeping secrets, and we really don’t want to be good at it. If one in 3,000 physicists decides to tell the Internet what our first-pass internal results look like, we can’t really stop them. Of course they’re breaking the rules, and we wish they wouldn’t, because it’s a collaborative effort and we’d prefer to agree together that we’re finished before announcing our results — because we want to make sure we did everything as well as we can. But still, our first-pass results are usually pretty close to final.
  • A rumor isn’t quite right. This could happen if we do find small mistakes or make refinements in the last few weeks of analyzing the data. This changes the answer by a bit, so the rumor is out of date. You could also make up a “not-quite-right” rumor just by making an educated guess based on our last results and how much new data we’ve taken!
  • A rumor is just plain wrong. Nobody says rumors have to be based on anything. Or they could be based on a misunderstanding of far-from-complete internal results.

We physicists working on this stuff don’t find it easy to wait for the answers either, and as Jon Butterworth has pointed out, rumors of other experiments’ results are actually dangerous for our work! For everybody else who’s tempted to indulge in rumors, just remember: you might be getting part of the picture early, or you might not. The only way to be sure is to wait for the next real update.

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Well, this is a bit of a late posting for me, but it’s been a crazy few months with a house sale, house purchase, daughters starting high school, and a frantic build of two stations for the ARA (Askaryan Radio Array) experiment that we’ll deploy at the South Pole this Austral Summer. More on most of those topics soon, but a little report on the PACIFIC 2012 conference and some biology that I learned while I was there. (Any and all biology in this article is as reported by myself, an astroparticle physicist, so apologies to actual biologists, this is posted out of “hey, that’s neat” intellectual curiosity and not some sort of physicist supremacy theory.)

So, the PACIFIC 2012 conference was held in early September in beautiful Moorea in French Polynesia. I felt pretty guilty heading to a meeting in paradise, so I did my best to bring back some gifts… It’s an annual, small meeting on particle astrophysics and cosmology, held at the Richard B. Gump South Pacific Research Station run by the University of California-Berkeley. The meeting was especially interesting to me with the small group of scientists there, in a relatively isolated environment, and with a good mix of experimentalists and theorists. I gave a talk on the IceCube experiment (talk is post here, NB: large file and somewhat technical), including some recent results on the gamma-ray bursts and also a few of the first bright (PeV) events seen by the telescope.

I did manage to find some time to go snorkeling during the meeting. There were a lot of fish and sea creatures in the protected reef waters of the lagoon, and in fact Moorea featured in a recent National Geographic magazine article exploring the living content of a cubic foot of the coral reef. It’s a ridiculous variety and density of life, teeming life in great numbers of species and individuals, in every corner, every niche of the Earth. But, if you read why Moorea was chosen as one of the locations (in work performed at the Gump Center) you find it’s because the isolated French Polynesian islands (it’s a really long flight, and look at it on Google Earth, far from the US, Australia, New Zealand, Asia, and Hawaii) have the least biodiversity on Earth.

Basically, it’s a really long way from everything else, and relatively few species (comparatively!) got there, established themselves, and evolved in place. So, in a manner that a physicist would approve of, Moorea can be used as a simple model of an ecosystem for systematic study. It’s far simpler than most other ecosystems, and many of the species present have their time and method of introduction to the island known. Some arrived with the Polynesian voyageurs, others with colonial masters, and others as accidental tourists in the jet age.

Okay, so you have simple system that you’re going to use as a template for more complex systems later, and maybe one of the first things that you want to do is catalog the species present on the island, in the soil, and in the lagoon. The Biocode project at Gump tries to do just this. Identify all of the species. Since I last took a biology class of course this is not just a process of identifying the species by its characteristics (Wikipedia background on taxonomy) but also via the DNA characterization of the species. (I must put a call out here to the brilliant DNA analysis work on restaurant sushi. Check it out if you haven’t before.) In fact, increasingly the species being discovered aren’t being formally named, or even identified in the sense of “here’s a canary, you can tell by its song, and this brilliant plumage” but rather by the existence of a unique DNA sequence.

For example, you can identify all of the critters you can (in Moorea, down to about a couple of mm in length), record their DNA, and then take the contents of a fish stomach and sequence that DNA. In the stomach you find the DNA of species that you had not previously identified. In fact, it seems that many, or most, species do not have a catalog entry, a sample pinned onto a board in the basement of the natural history museum, they just have some DNA in the instrumentation in the laboratory. These are the dark taxa, genetic information without the classical context of the detailed, properly named, taxonomy entry. The name is nicely analogous to the Dark Matter. Most of the mass of the universe is not in ordinary, observed matter, but in a “dark matter” which interacts gravitationally but is otherwise not observed directly (yet).

The Dark Taxa was introduced by Roderic Page who was considering the entries in species catalogs and noting the explosion in the number of species identified only genetically, with no classical taxonomy. Estimates seem to vary, but perhaps 90% of the animals (let’s not even think about the bacteria) are unknown. The already plenty-amazing world is that much richer still. Most species are unknown, and in fact the definition of species and broader classifications are moving rapidly with the ability of genetic tools to make a more reasonable “a is closely related to b” based on evolutionary distance rather than appearance. The Biocode effort on Moorea was making use of large scale databases, sequencing everything biological that the researchers could find on the island, and working through the classifications via software. It’s a very different biology than I recall from class, more analytic and less descriptive. I hesitate to mention the perhaps somewhat similar division between classic descriptive astronomy (“twelve barred spiral galaxies”) and astrophysics (“we modeled the bar formation with a 3-D hydrodynamics code”) but it does have some of that feel.

Okay, so I went to a tropical paradise, talked physics, and got pretty excited about biology, in particular biological classification. It was a good trip.

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VERTEX 2012

Friday, October 5th, 2012

Seth talking at the VERTEX2012 conferenceNever mind my complaints about travel, VERTEX 2012 was a very nice conference. There were a lot of interesting people there, mostly much more expert than me on the subject of vertex detectors. (I’ve written before about how tracking works and how a pixel detector works. In general, a vertex detector is a high-precision tracker designed to measure exactly where tracks come from; a pixel detector is one type of vertex detector.) My talk was about the current operations of the CMS pixel detector; you can see me giving the talk at right, and the (very technical) slides are here. Other talks were about future development in on-detector chip and sensor technology; this work is likely to affect the next detectors we build, and the upgrades of our current detectors as well.

VERTEX 2012 Conference attendees at Sunrise Peak, JejuThe location of the conference — Jeju, Korea — was also very nice, and we got an afternoon off to see some of the island. The whole island is volcanic. The central mountain dominates the landscape, and there are lots of grass-covered craters. Sunrise peak, at left, erupted as recently as 5,000 years ago, but it seemed pretty quiet when we were there.

Overall, the conference was a great opportunity to meet people from all over the world and learn from them. And that’s really why we have to travel so far for these things, because good people work everywhere.

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I don’t really like flying, but…

Saturday, September 15th, 2012

An airplane wing over Jeju, Korea You wouldn’t think so, given how much time I spend on airplanes, but I don’t like flying at all. I like seeing new places, but I think I’d be just as happy exploring every stop on the New York Subway as flying to new countries and exotic locales. But then it turned out that the science I wanted to do, and also the love of my life, happened to be on another continent. (Luckily, the same one!) Being a physicist is a travel-intensive business. So here I am, on my first trip to Asia, about to be run over by a typhoon.

Look forward to an entry from me sometime this week on the VERTEX 2012 conference. The conference doesn’t have a hash tag, but I might tweet about it anyway, if you’re terribly curious how it’s going.

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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)

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 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:

Limits for Higgs decaying to b quarks (B LaForge, CIPANP2012)

ATLAS limits for Higgs decaying to b quarks (B LaForge, CIPANP2012)

CMS limits for Higgs decaying to b quarks (C Palmer, 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)

ATLAS limits for Higgs decaying to tau leptons (B LaForge, CIPANP2012)

CMS limits for Higgs decaying to tau leptons (C Palmer, 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)

ATLAS limits for Higgs decaying to photons (B LaForge, CIPANP2012)

CMS limits for Higgs decaying to photons (C Palmer, 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)

ATLAS limits for combined Higgs channels (B LaForge, CIPANP2012)

CMS limits for combined Higgs channels (C Palmer, 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.

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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

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.

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

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!

Dolphins!

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The summer conference season may be winding down, but that doesn’t mean we are quite done yet.  Today was the first day of the Lepton Photon 2011 (LP2011) Conference; which is taking place in Mumbai, India all this week.  The proceedings of LP2011 are available via webcast from CERN (although Mumbai is ~10 hours ahead if you are in the Eastern Standard Timezone).  But if you’re a bit of a night owl and wish to participate in the excitement, then this is the link for the webcast.

The complete schedule for the conference can be found here.

But what was shown today?  Today was a day of Higgs & QCD Physics.  I’ll try to point out some of the highlights of the day in this post.  So let’s get to it.

The Hunt for the Higgs

Today’s update on the CMS Collaboration’s search for the ever elusive Higgs boson made use of ~110-170 trillion proton-proton collisions (1.1-1.7 fb -1); covering eight separate decay channels and a Higgs mass range of 110-600 GeV.   The specific channels studied and the corresponding amount of data used for each are shown in the table at left.  Here l represents a charged lepton and v represents a neutrino.

The CMS Collaboration has not reported a significant excess of events in the 110-600 GeV range at LP2011.  However, the exclusion limits for the Higgs boson mass range were updated from our previously reported values at EPS2011.  By combining the results of the eight analyses mentioned above the CMS Collaboration produced the following plot summarizing the current state of Higgs exclusion (which I have taken from the Official CMS Press Release, Ref. 1; and CMS PAS HIG-11-022, Ref. 2.  Please see the PAS for full analysis details):

 

Standard Model Higgs boson combined confidence levels showing current exclusion regions, image courtesy of the CMS Collaboration (Ref 1 & 2).

 

But how do you interpret this plot?  Rather than re-inventing the wheel, I suggest you take a quick look at Aidan‘s nice set of instructions in this post here.

Now then, from the above plot we can see that the Standard Model Higgs boson has been excluded at 95% confidence level (C.L.) in the ranges of 145-216, 226-288 and 310-400 GeV [1,2].  At a lower CL of 90%, the Collaboration has excluded the SM Higgs boson for a mass window of 144-440 GeV [1,2].

These limits shown at LP2011 improve the previous limits shown at EPS2011 (using 1.1 fb-1).  The previous exclusion limits were 149-206 and 300-440 GeV at 95% C.L., or 145-480 GeV at 90% C.L.

While the LP2011 results did not show a Higgs discovery, the CMS Collaboration is removing places for this elusive boson to hide.

QCD Physics

Today’s other talks focused on quantum chromodynamics (QCD).  With the CMS Collaboration’s results shown for a variety of QCD related measurements.

One of the highlights of these results is the measurement of the inclusive jet production cross section.  The measurement was made for a jet transverse momentum over a range of ~20-1100 GeV.  The range in cross-section covers roughly ten orders of magnitude!

Measurement of the inclusive jet cross-section made with the CMS Collaboration, here data are the black points, the theoretical prediction is given by the red line. Image courtesy of the CMS Collaboration (Ref. 3).

In this plot above each of the data series are “binned” by what is known as a jet’s rapidity (denoted by the letter y). Or in this case the absolute value of the jets rapidity.  Rapidity is a measure of where a jet is located in space.

The CMS detector is a giant cylinder, with the collisions taking place in the center of the cylinder.  If I bisect the detector at the center with a plane (perpendicular to the cylinder’s axis), objects with lower rapidities make a small angle with this plane.  Whereas objects with higher rapidities make a large angle with this plane.

As we can see from the above plot, the theoretical prediction of QCD matches the experimental data rather well.

Another highlight of CMS Collaboration’s results shown at LP2011 is the measurement of di-jet production cross-section

Measurement of the dijet production cross-section made with the CMS Collaboration.  Again, data are the black points, the theoretical prediction is given by the red line.  Image courtesy of the CMS Collaboration (Ref. 3).

Here the CMS results shown cover an invariant dijet mass of up to ~4 TeV, that’s over half the CoM collision energy!  Again, the theory is in good agreement with the experimental data!

And the last highlight I’d like to show is the production cross section of isolated photons as recorded by the CMS Detector (this is a conference about leptons and photons after all!).

Measurement of the isolated photon production cross-section made with the CMS Collaboration. Again, data are the black points, the theoretical prediction is given by the red line.  Image courtesy of the CMS Collaboration (Ref. 3).

What happens in isolated photon production is a quark in one proton interacts with a gluon in the other proton.  This interaction is mediated by a quark propogrator (which is a virtual quark).  The outgoing particles are a quark and photon.  Essentially this process is a joining of QCD and QED, an example of the Feynman Diagram for isolated photon production is shown below (with time running vertically):

From the above plot, the theoretical predictions for isolated photon production are, again, in good agreement with the experimental data!

These and other experimental tests of QCD shown at LP2011 (and other conferences) are illustrating that the theory is in good agreement with the data, even at the LHC’s unprecedented energy level.  Some tweaks are still needed, but the theorists really deserve a round of applause.

 

 

But I encourage anyone with the time or interest to tune into the live webcast all this week!  Perhaps I’ll be able to provide an update on the other talks/poster sessions in the coming days (If not check out the above links!).

Until Next Time,

-Brian

 

References

[1] CMS Collaboration, “New CMS Higgs Search Results for the Lepton Photon 2011 Conference,” http://cms.web.cern.ch/cms/News/2011/LP11/, August 22nd 2011.

[2] CMS Collaboration, “Combination of Higgs Searches,” CMS Physics Analysis Summary, CMS-PAS-HIG-11-022, http://cdsweb.cern.ch/record/1376643/, August 22nd 2011.

[3] James Pilcher, “QCD Results from Hadron Colliders,” Proceedings of the Lepton Photon 2011 Conference, http://www.ino.tifr.res.in/MaKaC/contributionDisplay.py?contribId=122&sessionId=7&confId=79, August 22nd 2011.

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