Quantum Diaries

… and I am not really talking about the Higgs status presentations yesterday at CERN, even though I have to admit that this has us all abuzz with excitement and speculations about the possibilities. And, the excitement is spreading to the general public: Today, the biggest newspaper in Munich, the “Sueddeutsche Zeitung”, has an LHC event display on the front page, above the fold.

Simulated Higgs decay into 2 muons at a 3 TeV CLIC collider, for an assumed Higgs mass of 120 GeV.

Although nothing really has changed (yet), the world feels a bit different today. Not a bad starting point for a colloquium on a possible future project in particle physics, a high-energy linear e+e- collider, which I’ll be giving in Prague this afternoon. Higgs physics is high on the list of things to do at such a machine, which promises to provide very precise measurements of its properties, and its coupling to other particles, which will show us its connection to particle masses. Some of these measurements have been studied in some detail for the CLIC conceptual design report just recently, which shows that, given enough luminosity and running time, even the very rare decay of a Higgs particle going into two muons might be measurable, with a statistical precision of a bit better than 25%.

Lets see how the LHC results develop, they might give us a whole new field to study…

Planning the week in a nice setting: The IEEE NSS conference booklet, at the beach. It promises to be an interesting conference!

Fall is here, and with it the annual cold and flu season. This year, I got a good taste of that: A rather nasty cold, and, to make it perfect, in a week where I had to give a two hour lecture on Monday in Munich, attend the CLIC CDR Review in Manchester and give a one hour talk Tuesday and Wednesday, and teach a whole-day short course on calorimetry at the IEEE Nuclear Science Symposium in Valencia in Saturday (that last one together with two other speakers).

But with a bit of chemical help, and by consuming of what feels like a ton of those well-known Swiss herbal drops, I got through it all, without completely losing my voice or feeling close to asphyxiation half of the time. And there is that universal law that such things take 7 days no matter what you do anyway, so an end was in sight right from the beginning. Yesterday, with a day off in Valencia before the start of the main conference, I got the chance to go to the beach for a few hours, again able to smell the salt air, and with some vocal trance from the iPod, to just breathe…

At the moment, LHC is setting the pace of particle physics. OK, there are intriguing, unexpected things about neutrinos, which might or might not be real, hints for possible direct signs for Dark Matter, just to give two examples. But clearly, we are all watching the LHC, and with each additional collected inverse femtobarn, hopes are rising higher for hints for New Physics, and we expect to finally get more insight into the mechanism of electroweak symmetry breaking by the discovery or non-discovery of the Higgs particle over the next year or so.

Amid the flood of papers coming out of the LHC experiments at the moment, it is important to remember that the spectacular results build on a quarter of a century of intense efforts, with first ideas for LHC in the eighties, detector concepts and technology development in the nineties and beyond, and construction over the last decade. Given these long time scales for large experiments (which are getting even substantially longer today!), the particle physics community has to plan for new accelerators and new experiments already before results from the present generation comes in. One such project is CLIC, the Compact Linear Collider, an accelerator that is, by design, capable of colliding electrons and positrons at energies up to 3 TeV. The accelerator concept is currently being developed into maturity, and over the last two years, detector designs for experiments at such a machine have been developed from the already well established proposals for the International Linear Collider ILC.

For this project, a Conceptual Design Report (CDR) is currently being written, providing a complete overview over the accelerator technology as well as over the physics case and the detectors. The second volume, summarizing physics and detectors, has been recently released and was reviewed earlier this week by an internal committee in an intense three day meeting in Manchester, UK. This draft of the report is available on the web, and the team is inviting signatures from particle physicists who “wish to express support to the physics case and the study of a multi-TeV Linear Collider based on the CLIC technology, and its detector concepts”.

Over the next few posts, I will try to discuss some of the ideas and results presented in the report, so stay tuned for a glimpse at a possible future of collider-based particle physics at the energy frontier!

The SPS North Area test beam hall at CERN: Is there a better place to spend the weekend than in an industrial setting under artificial lights?

Between given lectures, attending meetings, organizing workshops, supervising analyses and writing notes, I managed to squeeze in some real experimental particle physics time: A weekend of shifts at the CALICE test beam at the CERN SPS. OK, admittedly my wife complained bitterly that I take the weekend for that, but things have been really a bit crazy lately, so that was the only time I could squeeze in.

And yes, it was worth the three day trip to CERN (combined with a meeting this afternoon, some habits are just too hard to break): Of course there is the obligation part: I’m hoping to do a little bit of analysis on the data myself, and two of my students have a special setup running in that beam, so I felt I have to do my share. But then, test beam is something I also really enjoy: This is really experimental particle physics at its best! Of course it is not hunting the Higgs or trying to observe rare reactions showing hints for new physics, but you are never so close to a complicated high energy physics detector and to the details of operating particle beams as during test beam campaigns. Plus, things get really experimental: To measure muons for calibration also in the lower edges of our calorimeter, we installed some dedicated trigger scintillators: A quick access into the beam enclosure, armed with a power drill to fix a rail to the back of our detector, and a scintillator held in place by black scotch tape and a chain fixed by cable ties – real HEP style.

Our calorimeter setup, seen from the back... During the preparations for the installation of the trigger scintillator, seen in the lower right of the picture.

And since the data taking in the past week was going really well, we even had the chance to play a bit last night: We took a sample of charged kaons, in addition to the pions we usually use to test our calorimeter. The charged kaon consists of an up and an anti-strange quark, and decays after a few (in case of the high energies we have, after some one hundred) meters in flight. Mostly into a muon and a neutrino or a charged and a neutral pion. But with 5% probability also into three charged pions. Those will hit our calorimeter close together, and maybe, just maybe, we can separate them into three particle showers using the high granularity that we have. No idea if this will work, but I’m eagerly waiting for the data to run through the processing to take a first look. But that will have to wait for tomorrow. Now, after landing at Munich with the last flight back from Geneva, I’m just a short train and cab ride away from home, and some really much needed rest.

Scintillator installation, test beam style: Black scotch, and cable ties to hold everything in place.

Technology is a key driver for advances in particle physics, as with many other disciplines as well. In recognition of this importance, there is a conference series dedicated to this topic, Technology and Instrumentation in Particle Physics, TIPP, which started two years ago in Tsukuba, Japan. The second conference in this series is taking place at the moment in Chicago.

A slide from Marcel Demarteau's opening talk at TIPP11, illustrating a well-known example of quantum leaps in innovation.

A key theme of the conference is innovation and the development of new technologies as the enabler of discoveries. Very often, we go in small steps: Take a proven technology, something we’ve been working with for quite a while, and push the limits a bit further. But once in a while, you need something different: A radical solution, a big step forward that changes performance parameters by large amounts. We’ve seen such leaps in the past: A prominent example is the invention of the multi-wire proportional chamber by George Charpak in 1968, for which he was awarded the 1992 Nobel Price of Physics. That technology has enabled modern particle physics, by providing the possibility for good spatial resolution and high trigger rates. In many forms, this technology is still with us today in small and large experiments. George Charpak passed away late last year, and was remembered in a dedicated session here in Chicago.

Current transformative technologies often have something to do with semiconductors. Here, I just give two examples:
Tiny photon sensors on silicon basis which provide the possibility for single photon detection – This, together with the advances in electronics allows to increase the channel counts in modern calorimeters by a factor of thousand (I’ve written a lot about these things here…). 3D-technology for silicon detectors, which promises to deliver ultra-thin, fast detectors with low power consumption that even provide “intelligence” right on the detector by combining different semiconductor technologies for signal generation and processing in one piece of silicon. With these technologies, we see close collaboration with industry. The Silicon Photomultipliers originated from basic research, and now are the coming thing in many applications related to light detection, such as medical diagnostics. 3D Silicon technology is driven by the semiconductor industry to provide ever more powerful processors, and will very likely also transform the way we do tracking in our detectors.

Norman Graf from SLAC, quoting Daniel Burnham at TIPP11.

Pushing the limits of understanding in particle physics is not easy: As experiments, in particular at the energy frontier of particle physics, get more and more complex and more expensive, the time scales get longer. And funding is increasingly uncertain. In his talk about the SiD detector, one of the two detector concepts being developed for the International Linear Collider, Norman Graf nicely showed it in his final slide: We have to think big and make daring plans for the future, to go further in unravelling the secrets of our universe.

There is something every experimental particle physicist has to learn sooner or later: Even if there is a detailed schedule, always stay flexible! This is especially true when you are working at test beam facilities. I learned this during my very first weeks as a particle physicists more than a decade ago, while I was still an undergrad and just started to work in a research group a bit besides my studies. The time slot for the first beam time I participated in shifted several times, due to schedule changes and accelerator problems. Back then, I did not care: When things were “go”, we loaded our stuff into the car and drove to CERN.

But of course the resulting life style, the unwillingness (or even inability) to plan things like vacations and days off is something that annoys in particular partners and family to no end. Here, my wife is no exception. In particular since she has to plan her vacations six to twelve months in advance. She is working as a pharmacist, and there always has to be one in the pharmacy whenever it is open, requiring quite rigid personnel planning. That very often is incompatible with my schedule (or lack of schedule, for that matter).

My students are now also learning this: Last week, we were supposed to take data at the CERN SPS. But due to the failure of motors on so-called TAX (Target Absorbers) blocks, the start of beam operations in the test areas at the SPS is delayed by more than a month, completely derailing our well thought-out test beam plans.

The TAX blocks are massive pieces of iron, aluminum and copper, which can be moved in and out of the particle beams. They are important components, since they can block beam to enter certain areas of the experimental hall, and are used to control the size of the particle beams. They are key pieces for the overall accelerator safety system, so without them working properly, no experiments can take place. And repairing the motors is a tricky procedure: Since the TAX blocks absorb high fluxes of beam particles, they are highly radioactive, requiring careful planning of any work being carried out to limit the exposure of the technicians performing the work. So everything has to be diagnosed and understood before people go in and do something. It is also important that the real reason for the failure is understood, to avoid it from happening again. If a motor fails again after beam has started, repairs will not be possible for several months, which would completely stop test beam experiments for this year.

Lars, one of my PhD students, during the installation of our timing experiment in the CALICE Tungsten HCAL at the CERN SPS: The only missing piece now is beam.

At the moment it is unclear how the experimental schedule will develop – Also the plans for the later part of the year will probably be rearranged, but before the accelerator is back in operation, everything is speculation. The CALICE Tungsten HCAL is fully ready for beam already, and last week, right before the CALICE collaboration meeting at CERN, my students also installed our timing experiment in the experimental hall. So all we now need is beam time… And that might come at short notice, and at other times than originally anticipated. I guess quite a bit of flexibility will be needed over the summer!

I’m at Geneva Airport, the second time today: I’m now on the way back from giving a talk at the CMS Upgrade Week, where I discussed the technology and results of the CALICE calorimeters with the team planning the upgrade of the forward calorimeters of CMS. A very interesting exchange, which also gave me quite a few ideas of what to study next.

That is often how it goes: If you step outside of your usual circles (Linear Colliders in my case), you get confronted with new problems and new ideas. And that is really what pushes science forward. You discover common questions: How much light do you get out of a piece of plastic, and into a photon sensor? And start to think more about new problems: What do slow neutrons do to your silicon photomultiplier? They themselves don’t do all that much, but they bump into protons in your plastic scintillator, and can kick those into your silicon photomultiplier. And those guys can do quite some damage… Something I’ve not thought about before, since these kind of things are not really an issue at a Linear Collider, but are extremely important at the LHC.

So, a worthwhile, although long day trip: Since I arranged everything at short notice, direct flights were unaffordable. Now I’m connecting in Zurich both ways, meaning I’m taking 4 hops of 200 miles each. Feels kind of weird to spend more time in the plane on the ground than in the air. That happened in Munich this morning – big airport, morning rush, means lot of taxiing. Lets see how the return trip goes, two flights to go…

I know, it has been a while since my last post. I also realize that I’m writing openings like this all too frequently lately: Too much work to do, in particular many things that involve writing of some document or other, but of course also a few trips here and there. Right now, I’m on my way to Hamburg for two days, and last week, I spent two days at CERN. As usual, two days packed with various meetings, but I also had the time to see how the preparations for this years CALICE Tungsten HCAL test beams are going.

After a first, very successful beam period last year, this year we are going for high energies at the CERN SPS. First beam is less than two weeks away, so we are busy with preparations. These tests are absolutely crucial: The Tungsten HCAL (for a very brief answer to the obvious question: Why Tungsten? See here… ) we are testing is, after all, a prototype for a calorimeter at CLIC, where we expect very high-energetic particles and jets. These high energies are actually the motivation for using expensive Tungsten, so we better also test our detectors with highly energetic particles.

Puting things back together: Installing Tungsten plates for the CALICE Tungsten HCAL test beam due to start in less than two weeks.

Last year, our first commissioning test was performed at the CERN PS, where we got various particles at energies up to 10 GeV. There are several good reasons to start there: Testing at low energies is interesting, in particular from the point of view of understanding the shower physics and comparing to simulation models. Also, testing at the PS is more relaxed: While the test beam time at the SPS is notoriously oversubscribed, causing very tight schedules, things are a bit easier at the PS, allowing for more time to understand complex systems that get operated for the first time.

Now, we are getting ready for the SPS, with energies all the way up to several hundred GeV, which will show us how big the advantage of using Tungsten instead of Steel really is. A first step of of that I could see last week: The Tungsten absorber structure is getting reassembled, with a few more layers compared to last year. Each of the absorber layers weighs more than 100 kg, so they are lifted in place by crane. What I was looking at in particular was the last layer, clearly visible in the picture: This is just an empty aluminum frame, which will hold a special little experiment run by my group, which already took data last year. From that, we have first results, and I promise to write a post about that, too… This time, I hope you will not have to wait too long.

Dim lecture halls: Nobu Toge giving an update of the situation at KEK in Japan.

My spring travel season is in full swing, and brings a lot of work with it: New results to finalize, talks to prepare and to give, discussions, … which means I’ve again been neglecting Quantum Diaries. I am in Eugene, Oregon at the moment, at the Linear Collider Workshop of the Americas. Things are now winding down, after almost five days packed with new results and intense discussions.

Of course one of the recurring themes were the LHC results which are now coming in, and which are starting to seriously eat into the space for models for new physics that could be explored at a new collider. The Higgs might be just around the corner, and other discoveries are hoped for. What that is… Who knows? It could be Supersymmetry, a number of other models floating around, or, perhaps most exciting, something completely unexpected. Needless to say, that would give the whole field a new push, and show us where to head with a new large scale experimental facility.

News from Japan are also omnipresent, with a large number of our colleagues from Japanese labs and universities absent for understandable reason. Just this morning, Nobu Toge gave a report from KEK, including a video shot in the accelerator test facility at KEK during the earthquake. That showed everyone once more what a powerful natural disaster has hit Japan less than two weeks ago. Damage to experimental facilities is still being assessed, but the really good news is that as far as we know now, none of our colleagues has suffered bodily harm.

The pacific coast, west of Eugene, Oregon, on a rainy Sunday morning.

Of course, five days in dim lecture halls without getting out at all is too much, so I took the opportunity to clear my head by visiting the stunningly beautiful pacific coast. Weather was not perfect, but nothing beats some fresh air. Right here in Eugene, there are first signs of spring, with flowers and trees in bloom, during a rare moment of sunshine. Today, the rain is back, with bad weather reported from San Francisco which has me worried about my connection for my trip back home tonight… Keep your fingers crossed, but then, there are worse places to get stranded than San Francisco…

Signs of sping in Eugene, Oregon.