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Archive for May, 2010

My recent travels to CERN, and the long plane trips required, have finally allowed me to catch up on my back copies of Physics Today, the monthly magazine published by the American Institute of Physics. I particularly enjoyed the April issue. One feature article was about the origin of the term (and concept of) phase space. I should probably leave it for Flip to explain this, but phase space gives us a way to simultaneously visualize the spatial configuration and the momentum of a physical system. Just how it became a “space” and why there is a “phase” associated with it turns out to be an interesting story. There was also an article on spintronics, an emerging field that several of my departmental colleagues are working in, so I got to learn more about how that works.

However, what really caught my attention was the cover story on the design of three major physics research buildings that were opened in the late 1960’s, and what has become of them since. Perhaps unsurprisingly, the two most iconic buildings, designed by famous architects, have become less useful over time (one of them is in fact abandoned), while the third, a less-famous building at Brookhaven National Laboratory, is still going strong.

This is of particular interest to me right now because my department in the midst of the move to our totally new physics building. I have been on the department’s new-building committee since the project started almost four years ago, and it has been fascinating to see our building go through the design process and finally come to life. (This “new building” does have a tentative name, but I will wait until the Board of Regents approves it next month before I say what it is.) I think that the public spaces are really fabulous; they are going to be quite welcoming and will be a showplace for the department and the work that we do. The moving process itself has been, um, disruptive. It hasn’t been too bad for me personally, as I don’t have a lot of equipment to move (most of mine is at CERN), but for colleagues who have delicate equipment that needs to be dismantled, moved across campus and reconstructed, this is not going to be a fun summer. It will be very interesting to see how we bring our new home to life. We’re just beginning to learn the quirks of the building, not to mention figuring out how to find our colleagues in their new offices. Geography is destiny; we used to be spread across three buildings, and people’s offices and labs weren’t necessarily proximate to those of their closest collaborators. The new building is going to shake everything up, and we’ll see what new science results!

Our new building!

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Scoring Points!

Sunday, May 30th, 2010

In our collaboration (CMS), every institution involved is required to do a certain number of shifts watching the detector, making sure it runs smoothly while recording data.  The number of required shifts depends on the number of members in the institution’s group, and it’s up to each group to split up the work among their members as they wish.

For example, this means that if a professor doesn’t want to do shifts, their scientists, post-docs, or students must do them.

One complicating factor is that not every shift is worth the same.  The least popular shifts, or the ones “harder” to do – like overnight shifts – are worth more than others.

Here’s how many points each shift is worth in our collaboration:

  • Weekday morning shift (7am-3pm) is 0.75 points
  • Weekday afternoon shift (3pm-11pm) is 0.75 points, and
  • Weekday overnight shift (11pm-7am) is 1.5 points.
  • Weekend shifts add extra +0.5 points to above.

And since we are asked to do 24-points worth of shifts in a year, what kind of shift is most attractive to me, an unmarried, childless, young graduate student?

The weekend-overnight shifts, of course!  At 2-points each they’re pretty attractive.

Sometimes you have to take the shifts you can get, however, so I’ll be do weekday overnight shifts starting tomorrow.

Control room for the CMS detector.

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Hunting Background

Sunday, May 30th, 2010

Now that I am almost back home from my short trip to KEK, sitting on LH715 back to Munich I finally have a few minutes to look back at what has been going on. The primary goal of my trip was to participate in an experiment to study background at BELLE. Here is what this is about:

Two photon QED process, producing low-energy electrons and positrons that will hit our detector.

Two photon QED process, producing low-energy electrons and positrons that will hit our detector.

When we make measurements in physics, we are always interested in a signal. Background is what makes our life harder: Things that do not belong into the signal category, but still show up in your measurement. In the case of the new silicon pixel detector we are developing for BELLE-II, background are hits that we see in the detector, which come from particles that do not belong to an interesting physics event. Actually, most of the hits will be in that category.

Now, this is not really a problem as long as you can associate the right hits to particle tracks of interest, which works since we have detectors that see much less background than ours will, which then will help us decide which hits are good and which ones aren’t. However, this scheme works only as long as there is not too much of the background. And that is of course the key issue here: How much background do have to expect? After all, the accelerator for the BELLE-II experiment has not yet been built, so we can not measure it. We have to extrapolate from what we know from the present KEK-B collider, where the BELLE experiment operates. This is a complicated business: The new collider will have a factor 40 higher luminosity (interaction rates), so we have to extrapolate quite a bit. There are several types of background, for example beam-gas interactions (particles coming from collisions of beam particles with gas atoms in the beam pipe, due to the non-perfect vacuum), synchrotron radiation and Touschek Background (coming from electrons which stray of their nominal path in the collider due to intra-beam scattering). Those two strongly depend on the accelerator parameters, such as the beam currents, focusing, and so on. Usually these types of background are the dominant contributions. And by clever designs of the machine and its operational parameters, they can be reduced to tolerable levels. And most importantly: They don’t usually scale with the luminosity of the machine.

Members from the Munich team, including my grad student Andreas, looking at first events of our background run in the BELLE control room.

Members from the Munich team, including my grad student Andreas, looking at first events of our background run in the BELLE control room.

But then there are other backgrounds which come from reactions of the colliding particles. Those are irreducible since they scale with luminosity, just as the signal we are interested in. And something that is totally irrelevant in the present BELLE detector might become a show stopper, or a serious headache in BELLE-II, once the luminosity (and the rate of these backgrounds) has increased by a factor of 40. The biggie here might be two photon QED background, which produces low energy electrons at high rate. In principle this can be calculated, but it is complicated. Predictions varying by a factor of 10 exist for BELLE-II. The high end of the predictions are a problem, while the low end are no concern whatsoever. So our goal is to measure the amount of QED background in BELLE, to figure out which of the predictions is right. The tough thing is that the QED background is swamped by beam gas and other things, so we have to work hard to tease it out.

Accelerator experts in the KEK-B control room, working their magic.

Accelerator experts in the KEK-B control room, working their magic.

What we did on Friday, in one monster effort lasting from 8 am in the morning to 1 am Saturday morning, was to vary the luminosity in BELLE in different ways which might affect the QED background and other backgrounds in a different fashion, allowing us to extract the contributions of the different things, or at least give us an upper limit. This required a lot from the machine operators, who had to operate the collider outside of its usual parameter range. And the experts really delivered in a way that still amazes me. They really understand their collider in all detail. For us, it was an exiting study, and one of the last chances to participate in the running of the BELLE experiment before it will be shut down and dismantled in the summer to make space for BELLE-II. Of course it was also a very tough day: at some point after 8 pm, without any breaks, all that kept me going was hot espresso out of a can… The wonders of Japanese vending machines!

Staying focused after 14 hours of work: Fast offline data analysis, fueld by espresse in a can.

Staying focused after 14 hours of work: Fast offline data analysis, fueled by espresso in a can.

Now, the analysis is just starting, we got some hints already during data taking that the worst-case scenario very likely is not the right one, and I put in a few hours on the laptop on the plane. But hard numbers will be hard work, and will take quite a while… As usual in high energy physics!

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One thing I have learned since starting my PhD is not to hold on to preconceptions. Of course we all work based on an idea of what the world is like. But as a scientist, you have to be ready to relinquish your theories in the blink of an eye if evidence shows otherwise. While in some professions, one can maybe get away without, it’s a lesson one learns by necessity when working in research.
There was a time during my PhD when my convictions about the problem I was working on got overturned about twice daily. All these radical changes in my ideas were kind of hard on me then, but I got used to this. It seems that even in everyday life, I now accept it much more easily if things turn out to be different from what I thought. I seem to be less disappointed.
I am not saying here that one should not have any ideas or assumptions, or call it theories, about the world. What I mean is that one should be able to let go of them without regrets the second they don’t fit anymore with new knowledge we have acquired.

Another important trait of a scientist is to be open to new ideas and solutions. Any ideas. Many problems were solved in ways that at first seemed very unconventional and maybe strange. By only sticking with what has worked in the past, one might miss important clues.
Some friends of mine take advantage of this, telling me some incredible stories and then laughing at me when I hesitate instead of immediately rejecting it as a lie. My brain somehow got used to admitting any idea to a thorough check (obviously I usually figure out after a few seconds that it was a hoax).
Looks like some aspects of the scientific open mind can also have their pitfalls in daily life ;-).

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Earlier today one of our University of Tennessee graduate students, Irakli, successfully defended his thesis proposal, officially becoming a doctoral candidate.  To do this, Irakli had to write a proposal for his thesis and give a presentation to his committee.  His written proposal was about 35 pages long and discussed relevant past results, proposed a measurement, argued for the relevance and value of the measurement, presented results from simulations demonstrating that the measurement he’s proposing will be feasible, presented results from test beam data demonstrating the performance of the detector, and laid out a time line with detailed steps in the analysis.  His presentation was about an hour long and he was grilled by the committee on various aspects of his proposal.

Irakli will measure heavy flavor production (meaning charm and beauty quarks) through the measurement of non-photonic electrons in proton-proton collisions in ALICE, focusing on measurements at high momenta using the electromagnetic calorimeter.  While Irakli is a heavy ion physicist, he’s doing a proton-proton measurement for a few different reasons.  We have proton-proton data already, so we are certain these data will be available.  Irakli will spend the next couple years working on data analysis, checking and cross-checking his results.  If he were trying to measure data that wouldn’t be available for a couple years, he’d have to do finish his analysis quickly or spend a long time in grad school.  The proton-proton result will be a way of checking theoretical calculations.  It’s also a good way to test our measurement technique, since proton-proton collisions are much simpler than heavy ion collisions.  Also, the proton-proton result will serve as a baseline measurement for heavy ion collisions.

Most universities in the US have some variation on this procedure.  There are two main goals – to ensure that the student is making sufficient progress and understands the measurement he’s trying to do and how it fits in to larger physics goals and to ensure that the advisor is directing the student towards a meaningful and feasible project which can be completed on an acceptable time line.  It is not only the student in the hot seat.  What these procedures tend to do is focus the student (and the advisor) on a clearly articulated goal.  Before the thesis proposal, students may try out various subjects, familiarize themselves with the experiment, and learn about possible thesis topics.  Irakli worked on the test beam calibration of our electromagnetic calorimeter, took shifts as a member of the team running the detector, tested front end electronics boards for the electromagnetic calorimeter, worked on the physics performance report for the electromagnetic calorimeter, and took classes.  These all provide valuable experience, but now that his goals are clearly articulated, his work will likely focus more on work directly related to his thesis.

So, congratulations Irakli!

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I often think about the stone I stole from the beach in Greece as a child that glowed bright orange in the sand. When I arrived back home, excitedly pulling it from the suitcase, it had lost its colour. It now sits in a box, a grey and sullen shadow of its former self. My mother told me it was because it grew sad, longing for its home. Years later, sitting in the sun on the coast of Buzios with fellow heavy-ion physicists, I relayed this story in the hope of a better answer and was asked, “Why would you want to know the truth, with a story like that? Don’t you want to keep the magic?”

If made to think of a magical story in my life, this is one of the few that hasn’t yet had its illusion broken. In truth, I haven’t tried very hard. My boyfriend is a chemist, I know he has access to equipment that would allow him to analyze the stone, tell me what’s in it. One day I will ask him to.

Most of the mysteries I came up against as a child instantly became challenges – understanding them was the primary goal. Just like magic tricks, once you have seen how they are done, you see them differently, but often the truth makes the phenomena in question all the more beautiful. I noticed last night during an advertisement for “Genius of Britain” on Channel 4 (starting this Sunday), Sir David Attenborough echoed this idea:

“The world is full of wonders, but they become more wonderful (not less) when science looks at them.” (See the full clip here)

Of the genuinely wondrous elements of nature this is surely true. However, we have all been disenchanted by the truth. I was very proud to have fathomed that my parents were responsible for the festive man in red, but that didn’t stop it hurting. That magic is gone forever.

I want to know your views. I know many of you don’t comment on here but please make an exception and let me know what you think:

Are answers snuffers or blowers of wonder?

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Inspired Direction

Wednesday, May 26th, 2010

I’m on shift again, and my usual streak has hit the ATLAS control room again with a bout of quench protection system problems and dumping of beams the like of which I’m starting to think might not happen when I’m not here. After all, if I’m analyzing data we must be taking it at some point. I should probably bring my rabbit’s foot with me for tomorrows graveyard shift. This brief hiatus from the LHC’s otherwise monumental first data taking run does provide me the opportunity to post though, for the first time in a while (sorry ’bout that!).

So what to say. My title refers to a visitor we had at CERN last week. Persis Drell, the director of SLAC, was at CERN for a couple of days with the purpose of catching up with the SLAC/Stanford group based out here. She tacked the CERN trip onto other official business she had in Europe but it was a very worthwhile couple of days. Persis had a pretty packed agenda planned for her including visits with the ATLAS hierarchy, a long chat with CERN’s own director Rolf Heuer and lunch with some of the ATLAS management. Into this day she also managed to squeeze some time with the staff and students who nominally work for her, but in many cases she may never have met. So, last Friday I found myself in a dusty basement, surrounded by boxes and broken equipment trying to explain the delicate inner workings of the ATLAS pixel detector, accompanied by one of the students in our group. Forgoing the setting up of any look-at-how-clever-we-are presentation we just had a chat and showed Persis around a bit. It was relaxed and informative. She asked questions and showed a genuine interest and it made me think about the last time someone had looked me in the eyes and showed a marked interest in the research I was doing? Dunno. Of course, you get good at these things when you’re high up the food chain. But there is a moment of inspiration in the recognition of your work as something beyond the ordinary.

My pitch was over and I embarked on a relatively successful but not particularly bloggable afternoon of work before our Friday group meeting started. 5pm every Friday, mostly to accommodate our colleagues back in the bay area, is a bad time to have a meeting. Unless, that is, you have wine with the meeting. Labs in the US aren’t allowed alcohol on site. One occasionally gets the impression that CERN may grind to a halt if such a rule were imposed here. We get the uneasy looks from our head as she realizes the wine bottles aren’t just ‘for show’. She politely declines the offer of a plastic cup half filled with cote du rhone (or half emptied of air I suppose) and we go through our presentations and updates in the usual manner.

We had a rather enjoyable work dinner that evening. I’m sometimes skeptical of going out to work related functions, often robbing you of your free time only to force you into conversation with someone who wants to talk solely about what they did at work that week. This was one of the good ones though. Of course there’s the physics talk, but it’s interspersed with all types of topics, anecdotes, even jokes.
It’s great to see all members of a group getting along, regardless of position. The lab director and one of the students in heated argument.

Being geographically separated from the place where you work has it’s pitfalls, mostly from an administration standpoint but also in that it makes you forget the potential support network that exists. Out of sight, out of mind in a sense. But when the boss, the big boss, puts in the time and effort to make you realize that there are people in your corner, it inspires you to do that little bit more in return…..and sometimes a little bit more is all it takes.

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Equipment Failure

Wednesday, May 26th, 2010

Sometimes travel can be a curse. After barely 9 days at home in Munich, I’m again back in Asia, Japan this time. And getting here proved to be more annoying than usual, due to technical problems with the aircraft, something that would be called equipment failure in industry terms, I guess. So instead of getting on board, settling in and taking off, by the time we were already supposed to be in Russian airspace we were still sitting at the gate, with technicians all over the plane trying to fix a broken fuel computer. In the end, they gave up and we had to wait for another aircraft. Probably a smart choice, I can imagine better things than running out of kerosene 36 000 feet above eastern Sibera. But annoying none the less: Adding 3 hours at the gate to a flight that already takes more then 11 hours is not something you usually wish for. In the end, things turned out not so bad due to favorable winds, so only 2 hours later than expected I made it finally to my hotel close to KEK in Tsukuba, Japan, a bit more than 21 hours after leaving my office in Munich.

So, what is going on here at KEK? Most importantly, I’m here for a special run of the KEKB accelerator and the BELLE experiment, where we want to learn more about the background (meaning unwanted particles not related to interesting physics) we have to expect for the new pixel detector we are constructing for the BELLE-II experiment. The current word is that we’ll be on tomorrow, with 16 hours scheduled for us. Depending how things go, we might get a bit more time, even. Since this study requires extremely close collaboration with the accelerator operators, we’ll have a preparatory meeting with them in a bit more than an hour, to make plans for tomorrow.

In addition to the experiment, I’m also here to give a seminar at KEK this afternoon, about Particle Flow and Imaging Calorimeters, the jet reconstruction and detector technologies we are developing for a future Linear Collider. So, for once, this trip really covers my two main areas of research, all withing the space of a few hours.

It’s going to be a few intense days, and I hope I’ll have some interesting impressions from our experiment to talk about soon.

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I felt like a giddy child when I found out about these results! See below for the article I wrote for Physics World (unedited version).

Muons could explain missing antimatter in the universe

Particle physicists at Fermilab’s Tevatron accelerator in the US have found an exciting new result that could explain one of the big mysteries of cosmology – why there is matter in our universe.

Guennadi Borissov of Lancaster University in the UK led an international team of researchers in the analysis of proton-antiproton collisions at the DO experiment. In these collisions, they were searching for B_d and B_s mesons decaying to muons.

New Physics

CP violation is a fundamental difference between the behaviour of a particle and its antiparticle. Without it, matter would not have survived in the universe after the Big Bang, because matter and antimatter were created in equal amounts and should have annihilated completely.

To date, precision B-factory experiments BELLE, Japan and BABAR, US have accurately measured CP violation manifesting in the decay of kaons and B_d mesons, and the results have been consistent with predictions from the Standard Model of particle physics. However, this is not nearly enough to explain the matter-antimatter asymmetry in the universe, so scientists are thrilled to finally see signs of CP violation beyond the theory’s expectations. B-physicist Tim Gershon of Warwick University, who has worked on these experiments, explains:

“Measurements from the B factories have placed stringent limits on many of the possible deviations from the Standard Model. The B_s system has long been thought a good place to look for the extra CP violation that we know exists in nature.  Results from both CDF and D0 have hinted at new physics effects before, causing great excitement in the community.”

Now, D0 has shown that B_s decays may indeed hold the key to understanding our existence. The recent measurement suggests a comparatively large asymmetry that could overthrow Standard Model predictions and help to explain the universe’s matter dominance. This, Borissov says, is “the most important implication of our result.”

Measuring asymmetry

Neutral B mesons can oscillate between their particle and antiparticle, which means that spotting which one is which to measure any asymmetry can be tricky. One way to do this is to look for semileptonic decays, such as to muons. In this case, a W boson carries the charge of the flavour-changing bottom quark to the muon. The meson can then be identified as B0 or anti-B0 by the muon’s charge.

In this measurement, D0 looked for two muons of the same sign coming from the same B anti-B pair, meaning that one oscillated to its antiparticle before decaying. “Each one could decay into a muon, a neutrino and, say, a charm-flavour meson”, Borissov explains. Asymmetry between the B and anti-B is then measured as an overall preferred charge for the measured muon-pairs.

However, he is keen to point out that it isn’t as easy as it sounds, warning that many muons in the proton anti-proton collisions come from kaon decays. This background is serious because kaons have an artificial preference over anti-kaons for decaying in the D0 detector, so that if they were mistaken for B mesons a fake asymmetry would be seen. To get around this problem, the asymmetry in control samples of kaon decays was measured and removed.

Considering the challenges, the result is remarkable and has exciting implications. The final measured asymmetry deviates from the Standard Model prediction by 3.2 s. “This means that the probability of the result being simply a statistical fluctuation is around 1 in 1000”, says Prof Terry Wyatt of University of Manchester, former spokesperson of D0. Clearly the measurement is striking, but more work needs to be done before scientists can be certain the deviation is real.

Outlook for new physics

The largest uncertainty limiting D0s measurement is from low statistics, so they are continuing to gather data. Wyatt continues, “We hope to increase the collected data set by about a factor of 2. In addition, we can hope for improvements in the analysis techniques that could reduce the uncertainty further.” An agreement from D0’s sister experiment, CDF, would also be a promising test of the measurement.

Possibly the most excited by this result are those at the LHCb experiment at the LHC. Gershon, who is now working on the experiment, remarks, “The LHCb’s data samples will soon become the envy of the global B physics community.”  Guy Wilkinson, LHCb’s physics coordinator, explains why.

“LHCb can record up to 2000 of the interesting decays every second, and our instrumentation is optimised for reconstructing these decays. These two factors mean that for many studies LHCb expects to surpass the sensitivity of the Tevatron experiments rather early in this, the first LHC run, of 2010-11.”

Measurements of muon asymmetry are already underway to compare with D0s result. The potential changes to the Standard Model also have exciting implications for another potential CP-violation measurement, which LHCb are calling their “golden channel”. LHCb B-physicist and CERN fellow Rob Lambert gushes, “The release of that paper was like Christmas come early for me. I stayed up into the small hours getting overly excited by the great things we can do at LHCb”.

You can find the published version here.

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I was going out to dinner last night with some of my colleagues and the topic of new physics results at the Tevatron (specifically DZero) came up. I’m always happy to hear about new physics happenings  because it’s too easy to get so caught up in your work that you don’t notice other work being done around you. I know a couple of other bloggers have already posted it, but I’m still excited about it. And maybe a little out of the loop since it’s my last week at CERN and I’ve been really focused on getting a functioning analysis together :).

So for those of you that haven’t heard/read… there’s some interesting stuff happening with CP violation at the Tevatron. Here’s some background:
The Tevatron produces copious amounts of particles called B mesons. B mesons (quark-antiquark b – d or s pairs) oscillate between themselves and their antiparticles (Something I’m happy to elaborate on if anyone is interested). This oscillation can violate CP (charge conjugation and parity symmetry) thus preferring one state in the matter/antimatter system vs. the other.  (This also happens in the kaon system, and was one of the first pieces of experimental evidence that CP was violated). According to the standard model, this asymmetry is supposed to be very small (on the order of 10-4) – so small the Tevatron experiments can not measure it accurately. However that’s not what they found…
There is evidence that the asymmetry is about 100x larger than expected or 3.2 sigma (or 3.2 standard deviations). Now that’s not enough away from the predicted value to claim discovery just yet (need 5 sigma to be really sure). But it’s a very interesting result. Here’s a link to the arXiv paper

The New York Times also published a nice article.  You know you’ve hit the big time, when the NYT has an article about it ;). Although I dislike it when physicists talk about finding faces or toes of god.

-Regina

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