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Monica Dunford | USLHC | USA

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Doing Physics

Wednesday, January 16th, 2008

Call me a geek but I do believe that anyone who thinks that doing physics isn’t cool has never actually seen physics being done.

The purpose of the Tile Calorimeter in ATLAS is simple: estimate the energy of particles entering the calorimeter. These energies are then used to identify the potentially interesting particles (such as the Higgs) produced at the beam’s collision point. It is such a simple statement and yet so hard to achieve in reality. For this purpose, the Tile Calorimeter is designed as a ‘sampling calorimeter’ composed of scintillating tiles sandwiched between plates of iron. In some ways a sampling calorimeter is a lot like trying to watch a dance through a series of photographic snapshots. The dancers move from one picture to the next and although you didn’t directly observe the movement you can derive what the movements were. The faster the snapshots, the more lifelike the dance.

tiles_backlit

TileCal is similar except we try to measure a particle’s energy not a dancer’s movement and we do that by measuring the amount of light in the scintillating tiles. The scintillator/iron structure is clearly shown in this picture. For a sense of scale, this is only part of one wedge of the calorimeter and there are a total of 256 wedges. The dark lines between the backlit scintillating tiles are layers of iron. Energetic particles coming from beam’s collision will be slowed down in the iron and will produce light in the scintillator. Without the iron, the particles would not slow down very quickly and the calorimeter would have to be even more humungous than it already is. The small circles at the bottom are where the photomultiplier tubes (PMT) sit. Optical fibers are run from each scintillator to a PMT and the PMT measures the total light.

tile_mod_fibers

A wedge with all the fibers connected looks like this. The upper row of fibers is just a template for the technicians to know which fiber goes to which PMT. If the fibers are run to the wrong location, we will be confused as to which scintillator tile is being lit by the particles passing through.

When asked what it means for me to ‘do physics’, I could say, ‘we estimate the energy of particles entering the calorimeter’ or I could show these pictures. They both have the same meaning but I think they leave very different impressions. To me the former sounds pretty boring but the latter looks very cool.

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Coffee and Conversation

Friday, January 11th, 2008

CERN has come back to life. The quiet vacantness of the week before the CERN closure is gone, replaced by the soft clicking of coffee cups and saucers and the not-so-soft babble of physicists in the coffee area outside my office door.

In my building (which is where a good fraction of the ATLAS and CMS offices are located), the coffee bar is literally right outside my door. This is a good and bad thing. Good because it takes no time and effort to get coffee. Bad because it takes no time and effort to get coffee and therefore I get coffee all the time. As a result, my weekly coffee consumption is probably…. On second thought, let’s not try to estimate that. Too depressing.

But the coffee area right outside my door also means that most of the ATLAS collaboration is also often times right outside my door. This is a good and bad thing too. Bad because physicists have a tendency to speak very loudly especially when they are excited about something (Which is pretty much all the time. Physicists can be accused of many things, lack of passion is not one of them). Good because I can be privy to all the latest detector and beam rumors (which like most rumors are unreliable but at least entertaining). Thus far the new year has not brought any new rumors about the beam schedule. Ah, but give it time. 2008 is CERN’s ‘year of the beam’. And I predict there will be more rumors about the schedule than the number of physicists.

You need spend only one hour at CERN before realizing that what motor oil is to cars, coffee is to physicists. I once tried to convince my office mates to put an espresso machine inside the office, arguing why walk 20 steps to pay for coffee when you can walk zero steps and get some for free. This was shot down. The general reasoning being: right now our colleagues are just outside the door, with a free espresso machine they would then all be inside the office. I do have to agree with this logic. An espresso machine in the office would be too much of a good thing. That ‘good thing’ being either coffee or collaborators. You can choose which.

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Silence

Wednesday, December 19th, 2007

‘Twas the week before the CERN closure…

This is one of my favorite weeks to be at CERN. CERN officially closes for two weeks at the end of every year. During this time, I suppose it is possible to enter CERN, but bring a heavy coat because they turn the heat off to most of the buildings. This is the last week before the closure and already things are starting to wind down.

By now most of the Americans have left. It is very expensive to fly this coming weekend so most people have already headed back to the States for the break. A lot of the Europeans have left for the same reason. It is easier to travel now then over the weekend. There are no big meetings at CERN because everyone else is traveling. No one wants to start any serious operations or tests on the detector because we have to power off the full system tomorrow anyway. So all that is left at CERN is basically a skeleton crew.

And I love it. One week to just sit in my office, with no meetings, no phone calls, no detector crises to attend to, no interruptions. Just work.

It is not that I don’t like the normal hustle and bustle of CERN. I enjoy the silence of this week so much only because it is such a stark contrast to all other weeks. But I think one of the most commonly spoken phrases in all of particle physics is probably, ‘if I just had two days with no interruptions I could get [insert name of your analysis here] done’.

So that is how I see this week. A week where I can start to tackle some of those if-I-just-had-two-days tasks that I have been putting off for so long. Let’s see how successful I actually am. Thus far my success rate is zero-point-zero. But I have grand ambitions.

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An Apology to CMS

Wednesday, December 12th, 2007

I feel I must apologize to my CMS colleagues. In a previous posting lamenting the number of ATLAS meetings, I quoted that CMS had ONLY 947 meetings this past year. It seems there was a problem with the conference web server at the time. This is what the web site lists now

cms_meeting2

2905 meetings in this past year. My apologies to CMS for suggesting they had so few meetings.

I am simultaneously relieved and disappointed by these new statistics. Relieved in the sense that maybe ATLAS isn’t as verbally inefficient as I thought. ATLAS has had 4992 meeting so far this year. This is only approximately two times CMS’ number. But disappointed in the sense that maybe all this means is that we are just BOTH verbally inefficient. It really just mystifies me that 5000 meetings a year is the price of doing physics in a large collaboration.

But never say that we doesn’t plan ahead. CMS already has a meeting scheduled for 2019!

Okay. Seriously. I am done obsessing about this. No more on this subject.

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The Fear of Pursuit

Thursday, December 6th, 2007

When I was in college competing for the university rowing team, my coach, Carrie, was a mastermind at inventing new and creative torturous practices. I remember one land practice in particular. Our boathouse was located in Back Bay of Newport Beach, CA, nestled into the bluffs. We’d had a heavy rain the previous day and the water had cut trails down the bluffs. Carrie led us out to relatively shallow-sloped bluff and told us to sprint up the bluff, along one of the water cut paths. Then repeat that 30 times. And just to spice the practice up a bit, a few seconds after one person started up the hill, she started a second person. The purpose was the first person must reach the top without being caught; the second person must catch the first. And the person who fails must do N number of push-ups, squats, sit-ups, etc. where N is a large number. By construction one person must fail.

To this day, I can still remember every detail of that path up the bluff. I can remember sprinting up that hill, trying not to trip on the rugged terrain, focusing on this little patch of bushes that marked the end of the hill and thus salvation, hearing the pounding of my teammate’s footsteps behind me. The ears are remarkably bad at estimating the speed of an object behind you. In this case, it was impossible to judge if those pounding footsteps were approaching or retreating. And you dared not look back for fear of stumbling and losing precious seconds. All you could do was run as fast as you possibly could, knowing that you were being pursued but having no idea where your pursuer was.

So, how does this story relate to particle physics? Well, every time someone asks me about the interactions between ATLAS and CMS, the memory of this practice always inadvertently comes to my mind. Although it is rarely in the open, the fear of pursuit is always in the background. ATLAS and CMS both have the same experimental goals in mind: discovery of the Higgs, discovery of new physics. And they both want to be the first experiment to make those discoveries. It is not that as individuals people on ATLAS don’t associate or like people on CMS. There is plenty of interaction between the two experiments. But on the other hand, as an experiment there is the goal to be the first to discover new physics. And there is the fear that the other experiment is closer to making that discovery. But how much closer? That is the unknown.

This leads to the question, ‘Will the fear of pursuit, the pressure to be the first cause either ATLAS or CMS to publish before they are ready?’

Well…it probably wouldn’t be the first time something like that has happened in particle physics. Right now, though, we are all (ATLAS and CMS) focused more on just making it over the next hill—being ready to take data when the beam turns on. But who is to say what will happen when we have that almost-convincing Higgs plot in our hands? A peak in a mass plot that just might be evidence for supersymmetry, but might also be the miscalibration of our energy scale? And then you hear whispers in the CERN corridors that your rival is seeing “something” in their data. You hear your pursuer’s footsteps behind you and wonder if they are getting closer or further away.

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Another Tragic Loss

Tuesday, December 4th, 2007

It has been a very tragic past few weeks for the high-energy physics community. First we lost Michael Schmidt and now we lost six members of our community to the tragic plane crash in Turkey.

It is with deep sorrow that I have to inform you of the tragic death of some of our Turkish colleagues in the recent plane crash in Turkey. Professor Engin Arik, Engin Abat and Berkol Dogan from Bogazici University Istanbul perished in the accident, as well as three CAST colleagues from Dogus University travelling with them. They were on their way to a workshop for the design and planning of a Turkish accelerator complex.

Engin Arik pioneered the Turkish involvement in ATLAS, and she has motivated generations of young people to work with us in ATLAS, the TRT and DAQ. Engin Abat was with us this summer as a young MSc student. Engin Arik had worked at CERN since a long time in various experiments, having been a strong supporter of Turkish HEP at CERN and ATLAS.

In this dark moment our thoughts are with the families, colleagues and friends of the victims. In the name of the Collaboration I would like to express to our Turkish friends our sincere sympathy and condolences.

Peter Jenni
On behalf of the ATLAS Collaboration

I knew Engin Arik by her good reputation only. She was an active member of the ATLAS women’s group. Within that group she was known for being an endless source of inspiration, a great leader and a pioneer for promoting women in science as well as forwarding science in Turkey.

A fellow member of the women’s group, Christine Kourkoumelis, best eulogizes Engin’s impact on this field and the people in it.

Engin was a very dear friend of mine (actually I was the one who introduced her to the women’s group of ATLAS and she was eager to participate from the very beginning).

Me being Greek and Engin being Turkish was a good example how friendship can dissolve all nationalist prejudices. Together with Engin we participated in the first IUPAP Women in Physics Conference in Paris 2002 and tried to push the recognition of women scientists in the Balkan scene, as well. Engin inspired the research ideals in a large number of young girls and unfortunately, as you know, one of them died with her. She was also an ideal example of the “twin carrier “, being married to a physicist and pushing her career together with him in parallel routes. She also had two beautiful children and two grandchildren.

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Control Rooms

Thursday, November 29th, 2007

As control rooms go, I think the ATLAS control room has style. The visitors like it because there are lots of flat screens. The physicists like it because there are lots of flat screens.

atlas_control_room

The control room is set-up so that each sub-detector or sub-group has a desk. There are 15 stations. When the beam is running, there will be at least one person for each sub-detector on shift. 24 hours a day, 7 days a week. That will be the situation when we get into the swing of data taking. Right now, when we try to take cosmic data with multiple sub-detectors, there are usually 20 people at each station.

This picture was not taken on a day where we are trying to run a combined cosmic run. During those days, this room is packed. Standing room only, with people sitting on the floors, typing frantically on laptops. Mobs of people at each station. People yelling over the background noise in every language imaginable. The roaring cheers and clapping when everything is working. The massive sighs and moaning when everything isn’t working. If there is a heart to any experiment, it is the control room.

There is, however, one very strange aspect about the ATLAS control room that I have never really become comfortable with. It is way too clean. Where is the clutter? Where are all the random papers thrown on the desks? Where are remnants from last night’s 2am coffee run? A little dust even? The place is spotless. Always. It is not like it is usually a mess and then gets cleaned up for press tours. It is always clean. Not that I am complaining. I am all for less entropy in the workplace. But it is disconcerting all the same.

But as control rooms go, the LHC control room…. Now that’s style. Still too clean though.

lhc_control_room

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SNO vs. TileCal

Wednesday, November 21st, 2007

Last week I received the following comment from Chris and thought I might address it in more detail.

How does the work dynamic compare to your previous work on SNO? Is the TileCal working group your new “collaboration”? Do you feel lost in a sea of meetings? Does working with that many people mean there is enough organization such that it almost feels like *gasp* an industry job?

INCO_mine Ah, SNO! How I miss it! I had no idea what emptiness or the phrase ‘as far as the eye could see’ really meant until I had spent a winter in Sudbury, Ontario. When looking out over the landscape I was always simultaneously awed by the vastness of space and depressed by the physical scarring of the landscape from more than a century of mining. This picture perhaps shows a little of this. In the background is the head-shaft for shaft number 7. The Creighton mine, which is where SNO was located, has 11 shafts in total but only one, shaft number 9 is still used for passenger, equipment and ore transport. The only trees that can survive in the soil near the mines are these skinny white birch as seen in the picture’s foreground. But my advisor, Gene Beier used to tell me that 20 years ago there were zero trees. So life has been making a comeback in the area.

In the high energy physics world, the SNO collaboration was minuscule. Our author list of roughly 150 people could fit on a single page. That is compared to the ATLAS author list which last time I checked was 11 pages of names and 7 pages of institutions. But there are many similarities in terms of work dynamic between these two experiments.

Although ATLAS has an order of magnitude more people than SNO, TileCal is about the same size. In terms of experimental complexities, TileCal is also comparable with SNO. TileCal has roughly 12,000 electronics channels whereas SNO had 10,000 channels. But the major difference between SNO and TileCal, or SNO and ATLAS is that SNO was designed with one major goal in mind: measurement of the solar neutrino flux. And although SNO published other results such as a search for antineutrinos, the measurement of the solar neutrino flux was its main measurement. For the most part, everyone in the collaboration was focused on that goal.

In the TileCal community, people are largely interested in the calibration and studies of ‘jets’ (collimated showers of energetic particles which deposit energy in the Liquid Argon and Tile calorimeters). But there is a huge range of physics measurements to focus on such as measurements of the Top quark mass, discovery of the Higgs particle, or searches for new physics such as SUSY. Within a single physics group, there are members from every sub-detector, each bringing to the discussion expertise from his/her sub-system. In that sense I think it is easier to ‘get lost’ within ATLAS compared to SNO. Because the people working with you on TileCal might have totally different physics interests. There are so many different things to work on it is hard to decide what to work on.

Considering how many different institutions, sub-detectors, and physics groups there are, ATLAS is surprisingly unorganized (though not necessarily disorganized) and yet still functional. ATLAS can never be organized like an industrial company because there is no real power of ‘purse or sword’ within the administrative hierarchy. For example, most institutions are individually funded. So if institution A is unhappy with someone at institution B, institution A can appeal to the ATLAS administration but that administrative body has no power to remove anyone at institution B. Therefore to get what they want and need, the institutions, sub-detectors, physics groups have to negotiate with each other, trading the only resources they have: their people’s time, electronics/construction facilities, expertise, etc. And the elected ATLAS hierarchy has the very difficult task of trying to channel all of these independent networks in one general direction.

It is a thankless task, which is why I am glad that as a post-doc I am near the bottom of the chain where I have the luxury of being able to focus on the detector and the physics.

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ATLAS Spam Filter

Thursday, November 15th, 2007

I’m beginning to think that the phrase ‘combined TileCal/Level-one trigger tests’ is synonymous with the phrase ‘TileCal is experiencing cooling failures’. The TileCal cooling system is designed to keep the electronics at an acceptable operating temperature. If the cooling fails, then the power to the system must be shut down to prevent damage to the electronics. It is not that TileCal experiences cooling failures often, but it seem we always experience cooling failures when doing combined tests with the level-one. And no cooling means no power which means no combined testing for you today. Seriously, you can set your clock by it. The first time it happens, you’re frustrated. The second time you’re laughing. The third time you’re suspicious. The fourth time, you’re convinced you are the butt of some cosmic joke.

I am exaggerating of course. Usually we recover from the failure rather quickly and are able to continue the tests. But the correlation is uncanny….

The level-one trigger is like ATLAS’ spam filter. The beam will collide inside ATLAS approximately 40 million times per second. We can’t possibly store all of that data to disk, nor would we want to. Most of those 40 million per second events aren’t very interesting. Rather I should say, they aren’t AS interesting. They are ‘old physics’, physics we have studied before. We are interested in ‘new physics’, physics we have never seen before. Here is an example:

MC Susy Event

This is a simulation of what a supersymmetry (SUSY) event might look like in the detector. (Much more complicated then the cosmic data we are taking now.) SUSY, like other theories of new physics, predicts certain types of events that will be produced at the LHC. Typically these events involve lots of particles, with lots of energy, flying everywhere as seen in the picture. The goal of the level-one trigger is to sift through those 40 million events per second, find the interesting one like SUSY and ditch the not-so-interesting ones. And it has to cut the event rate down by a factor of 500, meaning for every event that the level-one accepts, it has rejected 500 events. There are additional layers to the trigger. Once an event passes the level-one, it must also pass the ‘high level triggers’ before being written to disk. The final rate of events being stored for analysis is approximately 100-200 events per second. Imagine that. For every email in your inbox, there are 200,000 deleted as spam.

The Tile Calorimeter as well as the electromagnetic calorimeter (called the Liquid Argon Calorimeter) plays a critical role in the level-one trigger decision. In this picture, TileCal is the orange sections (the upper and lower row of orange) and the liquid argon is the gray sections (plus the two orange sections in the center on the left and right). The level-one makes a decision based on the amount of energy deposited in small regions of the calorimeters. SUSY events are predicted to have very large energy deposits. The purpose of combined tests between TileCal and the level-one is to calibrate the electronics’ signal in voltage and convert that to the amount of energy deposited in the calorimeter. If the electronics aren’t working properly or the calibration is incorrect, the trigger might delete as spam the very events that you are interested in studying. The calibration is a long process and will take many months but it is crucial to get right. Because once the events are rejected, we can’t get them back. There will be more tests on Monday. The cooling gods allowing of course.

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Meetings, meetings, meetings

Saturday, November 10th, 2007

Yet another big ‘week’ has passed us by. A few weeks ago it was ‘ATLAS week’, a week of meetings about the detector’s status. This week was ‘Trigger and Physics week’, another week of meetings about all the physics studies being done in preparation for the beam turn-on.

Some friends and I were discussing the volume of meetings within ATLAS. And I thought I might support this discussion with some statistics. This is the 13 year summary of the number of ATLAS meetings registered on our main scheduling website:

atlas_meetings

Really? Really guys? Did we really have 4531 meetings in the past year? I would have guessed like 1000 per year.

Actually what is shown here is the number of ‘events’ in the past year. For example, Trigger and Physics week which was five full days of meetings is listed as one event in this figure (which makes this figure all that more depressing). Say there are approximately 250 working days at CERN, this would be approximately 18 meetings per day. It baffles me that we actually have that much to talk about!

And since I just couldn’t resist I decided to look at the number of CMS events in the past few years.

cms_meetings

947 to ATLAS’ 4531. Hmm.

I think there are two possible explanations here. CMS uses a different scheduling/conference website. This is entirely possible. They might want to avoid having random ATLAS bloggers comb through their meeting statistics. Or. CMS is just more verbally efficient. They say in one word what ATLAS says in four.

It would be interesting to see the monthly statistics but the website doesn’t generate those. This is probably for the greater good of the experiment. People can really get into plotting all the various statistics. And knowing ATLAS, we would probably have to schedule a meeting to discuss the results.

If you were to ask me (and I feel represent the population well for this question), ‘Do you spend too much time in meetings?’ I would say, yes. But if the next question was, ‘Which meetings do you think ATLAS could afford to get rid of?’. I would say, none.

Take Trigger and Physics week. Of the talks that I attended, the information presented was useful and relevant. Meaning that for the most part it was information that I needed to know. Information that I need for the continuation of my own work. I can not point to a single talk that was not worth having. Nor are there many talks that are redundant. Certainly there is some overlap, where one talk might be using as input some work presented elsewhere. But I didn’t feel like I was being told the same thing twice. So maybe 4500 meetings per year is the reality of doing physics in an experiment with 2000 people.

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