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Archive for October, 2007

Data!

Tuesday, October 30th, 2007

No, I don’t mean that the LHC has miraculously turned on overnight and several months ahead of schedule. Not that kind of data. Cosmic data.

I love data! Even if it is only cosmic ray events and not beam. Just a little bit of data can go a long way. Take this event for example. The main frame is the cross-section of the detector. The lighter red cubes represent TileCal and the yellow cubes are cells within TileCal that record a significant energy deposit. The muon track can be clearly seen, entering at the top of the calorimeter, exiting through the bottom. The upper right inset shows the detector cross-section if you are looking down the beam pipe. Again TileCal is the lighter red cubes, the rectangles surrounding TileCal are the Muon system. The inset on the bottom right shows the photomultiplier tube pulse shapes for one of the yellow cells. The blue points are the data, the red curve is the reconstructed pulse shape. Beautiful.
TileMuon

Data, any type of data, is the ultimate reality check. Much of my day is spent running computer simulations to predict the detector’s response to certain types of physics events or testing front-end electronics and cables to verify that these elements are functioning and calibrated. You tend to focus on the details so much that it is easy to lose sight of the bigger picture. But data always brings that picture back into perspective. So much has to be right for this little event to take place. The high voltage, the low voltage, the front-end electronics, the electronic read-out, the event building, the event reconstruction, all of these things and more have to be working. If even one element of the chain is missing, then no yellow cubes. No data.

For the past week and continuing through next monday is ‘Milestone Week 5’ for ATLAS. aka M5. The purpose of the milestone weeks is to take cosmic data, integrating together as many sub-detector systems as possible. For M5, we have been running with the pixel detectors, the electromagnetic calorimeter (liquid Argon), TileCal, part of the muon system as well as the level-one trigger, the high level trigger, and the central trigger. Running with multiple sub-detectors means that even more elements in the chain have to be in place. And there have certainly been some hiccups this past week, but as seen in this lovely muon event, the data tells us we are on the right track.
Cosmic muon

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Branes Has A Flavor?

Sunday, October 28th, 2007

Naturally, many people are interested in the LHC physics program because of its promise to give a glimpse into whole new realms of physics: SUSY, extra dimensions, etc. A much smaller group is excited about the prospects of testing string theory in a completely different regime, via the proposed (and theoretically well-explored) duality between gauge theories and gravity. Leonard Susskind (everyone calls him Lenny, but I haven’t earned my stripes yet) has even been so bold as to call the physics which drives collisions at RHIC quantum gravity that is “blown up and slowed down by a factor of 10^20“. Lisa Randall was quoted in Seed last February saying that after her excitement about the LHC coming online in 2007 (ah, memories), exciting thing #2 was RHIC physics:

Also of interest is the recent application of string theory to the physics being done at the Relativistic Heavy Ion Collider (RHIC), where string theory permits some calculations that would otherwise be intractable. The idea at RHIC is to better understand the strong force that binds together the elements of a nucleon, and 2007 may see the theoretical advances of string theory inform the experimental results from RHIC.

From that perspective, it’s always boggled my mind why everyone hasn’t just dropped everything to understand both RHIC physics and string theory from this perspective. If the analogy really holds, then everyone wins: 1) heavy ion physics finally has a theoretical framework which is in principle better applicable than perturbative QCD (pQCD), which is only thought to work for asymptotically large momentum transfers between quarks and gluons (e.g. jets), and 2) string theory (although not necessarily a full “theory of everything” version) finally gets an experimental playground to propose and test observables. Even better, anything discussed at RHIC will have an immediate chance to be tested at 30x the energy at the LHC. People usually have to wait a generation for this kind of thing!

But reality is what it is and two factors work against this (what I thought would be inevitable) revolution. One, RHIC physics is still in its relative infancy and there is still lots of interesting debate about the meaning of various observables (although we all seem to agree that the “perfect fluid” we observe is deeply connected to the quark gluon plasma), and two, no-one is sure how to make a precise correspondence between the QCD we know, love, and use in real life, and the gravity dual that many thing should exist.

To catch up on things, members of both camps met up on Friday in one of the big lecture halls at Pupin Hall at Columbia University for what was called “AdS Strings Intersect with Nuclear Beams at Columbia“. It was unsurprisingly well attended by about 50 physicists, most from the NY-area institutions (Columbia, BNL, Stony Brook, Princeton, Yale) but a few from farther afield (e.g. MIT). The topics ranged widely, from overviews of the RHIC and LHC experimental programs (by Bill Zajc and John Harris), a discussion of QCD energy loss from a string theory perspective (by Steve Gubser, working from the blackboard and handwritten notes), applications of string theory to energy loss phenomenology (Will Horowitz, Hong Liu, and Derek Teaney) and an interesting discussion of bulk viscosity in QCD, which controls the transition to scale invariance (conformal symmetry) by Dima Kharzeev of BNL. A marathon afternoon to be sure, and not without its share of excitement and frustration, but everyone I chatted with agreed that the cross-currents between the nuclear physics at RHIC and the LHC and the string theory communities can only be a good thing, pushing both sides in directions they never expected.

Afterwards, about 20 of us ventured via NYC subway to a Turkish place in midtown for a post-workshop dinner. Good fun to let people finally hang out and discuss the open issues of the day (which had multiplied since that morning…you know what they say about the more you know…). We all certainly appreciated the Columbia Physics Department’s hospitality and Will Horowitz’ tireless work to pull the day together.

And, yes, the photos are up on my Flickr page. And as a bonus, I tried to write down a few of the good lines from the day:

  1. On the choice of r vs z for the 5th dimension (which are, duh, inversely proportional to each other making the “real” world sit near zero in one variable, and infinity in the other), Hong Liu suggested “always ask”
  2. “Wherever there’s tickling, there’s dragging” – Derek
  3. And while it’s not a “line” per se, the idea of branes carrying a “flavor” (i.e. using them to implement SU(N) symmetries) put me in mind of Return of the Living Dead and Halloween.

OK, so not that funny out of context, but it was that kind of afternoon.

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TileCal BBQs

Friday, October 26th, 2007

I think if there was contest on which ATLAS sub-detector had the most parties, TileCal would be a serious contender, if not the outright winner. The people of TileCal are experts at finding an excuse to have a BBQ. Past party reasons have included, ‘100 power supplies installed’, ‘128 power supplies installed’, ‘ATLAS week has started’, ‘ATLAS week has ended’, ‘ATLAS week ended 10 days ago’, ’10 weeks until the next ATLAS week’. In other words, we need no reason. Oddly enough it has been nearly three weeks since we have had a BBQ and honestly I am experiencing some serious withdrawal.

Most of the BBQs take place at building 171 which is our surface assembly building. TileCal is a ‘sampling calorimeter’ meaning that it is composed of tiles of scintillator sandwiched between tiles of lead. This scintillator/lead pattern repeats many, many times along the detector. When particles pass through the lead, they are slowed down. When they pass through the scintillator they produce light. We measure the light using photomultiplier tubes. The amount of light measured is proportional how much energy the particle deposited in the scintillator. Since the detector was built on surface but had to be transported underground, 256 pie shaped wedges were assembled on surface. The first wedges were transported underground in 2004, the last coming down in 2006. 64 of these wedges can be barely seen in the following picture. At the outer ring, there are these dark blue boxes. One of those boxes holds the power supplies for one wedge. The active detector (the scintillator and lead tiles) extended back behind the supply but are completely hidden at this point.
Calorimeters_innerdetector

Since all the wedges are now down in the pit, the assembly building is largely empty making it the perfect place for a little BBQ. The advantage to working with a large international group is that the food is never just burgers and fries. The Spanish for example clearly have an infinity of ways to prepare pork. They show up every time with something different. The Brazilians are always arriving with some exotic type of meat. One day they came with at least 100 seasoned chicken hearts in a bowl. The Russians pretty much have cornered the sausage and hard liquor department. Occasionally the Lebanese bring a deep fryer to make falafel, which is to die for. And no one cannot beat the Italians at desserts.

Three weeks and no BBQ though? Clearly we are going to have to conjure up reason. How about ‘256 wedges still installed underground’? Why not?

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Weddings and Proposals

Wednesday, October 24th, 2007

And boom — I’m back. I’ve been telling myself not to use this excuse for my disappearance (not just from this blog, but from everything), but just this One Last Time: since I last wrote, I’ve gotten married, gone on honeymoon, and returned. And while no stranger to jet lag, this last round was a doozy, and I’m only crawling out of it now. Still, no complaints here: both wedding and honeymoon were spectacular and I can’t begin to express how great people have been about giving me the space and time to do both.

That said, my future at the LHC beckons. So while I’ve managed to make good on one proposal (to my wife Kate), another is already in the pipeline. Those of you who read biographies carefully may have noticed that I never quite say that I’m part of the ATLAS collaboration. In fact, while I have been going to CERN fairly regularly for the last three years, have given ATLAS talks at major international conferences, and feel fully invested in the experiment and collaboration, it’s still not a fully fledged marriage yet (although with 1500+ collaborators, maybe I should drop this marriage analogy now…). While all signs have been quite encouraging, the Department of Energy has not fully committed to supporting all three big LHC experiments for Heavy Ion research. Thus, my bio lists that I am current working on a “proposal” to do this work at the LHC, and this is the thing taking up most of my energy and time in the last few months, and will do so for the next few. More on this later: just wanted to pipe up after my few weeks of silence!

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Halloween approacheth

Sunday, October 21st, 2007

Ah, a fine Sunday in the Boston area – bizzare, in that it was ~75 F (25 C) and sunny in late October, but no one is complaining. It was a good day to carve pumpkins, something we didn’t get to do last year in Geneve. Halloween over there was quite uneventful – the boys’ school had a “trunk or treat” where a bunch of expatriate parents put candy in their trunks (“boots” to the British, and “couche” in French) and met at the school parking lot, where the kids went around visiting other people’s trunks. It was weird. No kids were wandering around our neighborhood. Turkey day too is a different affair- over there you tend to have to hunt down a butcher to supply the bird, and the day itself is of no importance. As a consequence, people tend to spread their parties out over the week and abutting weekends, meaning you can often have a Thanksgiving meal twice or three times that week – and then you don’t have to eat until Christmas.

What the Genevois do celebrate is “Escalade”, dating back to the day in early December (night of the 4th?) back in 1602 when the people of Geneve joined with the soldiers to repel the dreaded Dukes of Savoie from taking the city. Legend has it one particular lady (the “Mere Royaume”) dumped a pot of hot soup on the bad guys coming up the ramparts on a ladder, thus saving the city. Today the Escalade is a big party, with lots of soup and hot wine, roasted chestnuts, and a big running race which I think half the city partakes in. Last year I ran, as did my wife and one of my kids, and fellow blogger Pam and her husband, plus a bunch of other friends. The race is a few times up the hill into the old town and back, and the biggest problem is the narrow cobblestone streets and wall to wall crowd of runners- it’s more of a mobile game of human bumper cars than a race, but it is fun. The final heat is the “fun run” where people dress up in crazy costumes, sometimes multiple people per costume! Sad to say it doesn’t look like I’ll make it to CERN for Escalade this year, even though it is right before a collaboration meeting. Good luck runners!

Ah, now my wife tells me we did carve pumpkins after all, although it is still true that no one came to our door. I stand corrected. Better quit while I’m not too far behind…

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Lowering again!

Friday, October 19th, 2007

Told you it would go faster. CMS lowered YB-2 on Wednesday. Must be getting crowded in the cavern!

By the way, Geneve isn’t always beautiful mountains, as my niece Giovanna found out when she visited. We tried to go to the mountains, ended up at Lac d’Emosson eating sandwiches in the back of the station wagon in the rain. Her view was roughly like:

Giovanna’s view of the alps

Very cloudy.

Gotta go teach!

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How Time Passes

Thursday, October 18th, 2007

Like many others in this field, I am often asked the question by friends, ‘What is it that you do exactly?’ This is my cue to launch into my standard spiel about discovering the origins of mass, etc. But usually I don’t get that far before being interrupted with, ‘No, no, no. You go to work… Time passes… You come home… Something happens in the middle there.’

At the moment, discovering the origins of mass and new physics is more like the light at the end of the tunnel than a daily reality. These discoveries are the things we all strive to make; the reason we endeavor to build a massive 6-story detector. But a detector that big means there are billions of little pieces that must be put together and tested to verify that they are functioning properly. The majority of my days are spent making sure that my little corner of ATLAS is doing just that: functioning properly. So that when the beam turns on, we’ll be ready to start answering the big questions.

Just like my fellow blogger, Pam, my corner of ATLAS involves a lot of cable. Actually all of ATLAS involves cables. Real estate on the detector itself is hard to come by because any type of read-out electronics, power supplies, or structural support, is dead material. As the goal is to measure very energetic particles flying out from the beam’s interaction in the detector’s center, if these particles pass through dead material, they cannot be measured accurately. Therefore anything that doesn’t absolutely HAVE to be on the detector isn’t. Hence the cable.

A short way off from the detector is a series of rooms, collectively called the ‘counting room’ which is the destination of the miles of cables flowing off the detector to the remainder of ATLAS’ power supplies and electronics. And I do mean miles. As seen in this picture, beneath the floors of the counting room is cable tray after tray. I have only had to crawl under the floor laying cable once and I emerged from the experience sore from the acrobatics required to move around the trays and looking like a human impersonation of a dust ball.
The counting room floor

LVL1 CablesThe cables that I am most intimately familiar with are those coming from the Tile Calorimeter to the Level-one trigger. (I will leave the description of TileCal and level-one trigger for another day.) The cables shown here carry the analog trigger signals from the calorimeter to the counting room. Don’t be deceived by the apparent innocence of these cables. It is a serious triceps workout to wrestle those cables into the connectors.

Before we can be ready for the beam, there are a series of tests that need to be done. First, we have to confirm that every cable and pin within that cable is going to the correct place in the counting room electronics. Second, we have to confirm that the electronics producing the trigger signals on the detector are functioning properly. Third, we have to calibrate both the electronics on the detector and in the counting room. The calibration is essential for us to be able to convert the signal we actually measure in the electronics (which is in milli-volts) to the quantity we actually care about (which is the particle’s energy as it passed through the detector).

While eventually one day we will be answering fundamental questions about the origin of mass, today I can only tell you which TileCal sectors are fully cabled and ready to go. On a day-to-day basis, getting those sectors tested and functioning is how time passes. And it passes quickly.

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Cable testing 100 m Underground

Tuesday, October 16th, 2007

Tomorrow I have another full day underground, probably friday as well. We are in the process of testing and installing cables. The cables were tested for continuity at the vendor that produced them but after we install them we test them again to make sure the data comes over the cable from the calorimeters’ boards to our RCT’s boards.
A lot can go wrong. At our end, the RCT, we use VGA connectors to make the connections to our boards. I don’t know if you have ever taken a look at how your monitor connects to your PC, but if you have a VGA or even a newer version of a monitor connection, they often have pins. These can get bent if you put the connector in badly. Or, as we found out, it is possible to put the connector in the wrong way entirely. We only use a few of the VGA connectors pins, so we can reverse the connector (the shell bends easily) without much effort. So we are working on figuring out if: 1o26 data cables are plugged in correctly and the boards at both ends work. We are doing this bit by bit to start, with an eventual goal of automating it. But first it is cable by cable, and it goes slow.

When we are down there, there are no visual clues as to what time of day it is, so I just go on two things, my computer – which has a display of the time, and my stomach, which seems to tell me I am hungry with astounding regularity.

To be able to work underground, we are required to watch a DVD about safety, take a test about safety on the web, and wear the appropriate attire. This includes two special pieces of equipment: a hard hat

Hard hat
and a pair of very flattering steel-toed safety shoes.
shoes

The hard hat is a nice one, with a quick-adjust so that I can put it on and
off without tweaking my ponytail that I keep my long hair in (the environment is not long-hair friendly – fans keep the air moving in the racks and hair gets tangled in things). It also has a headlamp, which is supposed to help if we have a power outage (imagine total blackness) and I find it useful for dark corners underground. The shoes, well, the less said the better. No special clothing, but it is better to have a warm fleece and wear (dark-colored) washables, as the floor is pretty dirty, and nothing that is too new. I have found grease on my clothes from goodness-knows-what. It is definitely a “dress-down” type of environment.

It is noisy, cool (all the water cooling and fans in there), and kind of cramped, but we get all done that we need to, and I will be glad to have finished it. But there is always more to be done.

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Another ATLAS Week

Saturday, October 13th, 2007

“ATLAS week” is over. Hallelujah.

I notice that my ability to stay mentally focused through lectures or talks has diminished seriously with age. At the beginning of college I could go through three straight hours of lectures with no problem. By my 4th year it was a good day if 20 minutes of the lecture had passed and I hadn’t yet reached REM sleep. Back then, I had this great sweatshirt. It was incredibly warm and had this really thick, puffy hood. The hood could be rolled just at the back of the neck. And if I slouched down it made the perfect pillow against back of the chair. Eventually I had to get rid of the sweatshirt because it was having such a negative effect on my grades.

During this ATLAS week I managed each day to follow the first two or three talks with vigor, the next two or three with attentive yet forced comprehension, but the two or three after that I was longing for that puffy, warm sweatshirt.

The goal of ATLAS week is for people who are not full time at CERN to come and hear a full status report of ATLAS. It is also for people at CERN to hear the status of the other sub-detectors outside their own area of work. The week is organized first with each sub-detector giving a status report of their system, followed by status of the LHC, the status of ATLAS installation and overall commissioning (such as the commissioning of the magnetic fields and gas systems), software developments, data preparation developments and finally ended with discussions about the strategies of the physics studies. It is five days packed with talks which all take place in CERN’s biggest lecture hall. And still during some talks people have to press in the aisles and doorways.

In a previous posting, I was lamenting about the lack of hit-me-with-the-raw-truth in the detector overview talks. And I have to admit I was pleasantly surprised during this past week. I thought Leonardo Rossi’s talk on the status of the inner detector was particularly good. He just told it like it was. Didn’t make things sound better or worse. And made no excuses. Lyn Evan’s talk on the LHC beam status was also straight to the point and very reassuring. Wild rumors about the beam’s schedule have been plaguing CERN. All of which he claimed were exaggerated and blown out of proportion. But there were a few overview talks (that will remain nameless) where I had the distinct feeling that I was being persuaded to buy a used car that I didn’t really want.

It also occurs to me in these meetings that wireless internet access has actually done a disservice to the high energy physics community. There are indeed many people attending the talks but how many are actually listening? I would scan room and see a room full of people, their faces lit by a soft, blue glow from their laptop screens. Checking their email, browsing the web, frantically putting their own talk together. And I have no justification to take the moral high ground. I am certainly an offender too. When I asked a friend of mine how her talk went, her response went something like ‘It went great. I gave my talk to numerous attentive Dell’s, Vaio’s, Macs and a few human beings’.

Maybe the laptop is the modern version of a warm, hooded sweatshirt.

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Lowering again!

Saturday, October 13th, 2007

I need to break through bloggers block….and in the interest of having someone other than my relatives read this, I’ll be brief.

The most exciting news from CMS is that lowering has recommenced! This is the part where we take the big chunks of our experiment and lower them 90 m down to the cavern floor. To see how this goes, time lapse videos are available for Endcap 2 (alternative view ) Endcap 1 with the funny looking nose, Barrel 2, and the piece de resistance, the central barrel, aka YB0, which holds the most powerful magnet ever built, and only had a few tens of centimeters of clearance over a 13 meter length. YB0 weighs about 1200 tons, and its touchdown on Feb 28 released a flood of media attention as well as a flood of work. When YB0 was placed, work could begin installing all the cables and pipes needed to drive all the detectors inside the magnet bore, namely the Tracker, Electromagnetic Calorimeter, and Hadronic Calorimeter Barrel. For the last 7 months that has been going on at a furious pace, and has only now abated enough that the lowering of the other side (the “minus” side) can commence. This starts with the first barrel piece “YB-1”, which touched down yesterday! Here it is on the cavern floor:

.YB Minus 1 on the Cavern floor

Fortunately the rest of the pieces still to be lowered are mostly precabled, so the duration between subsequent lowerings should be considerably less than 7 months from here on out (indeed, the 5 “plus” side pieces went down all in about 3 months).

By the way, my Dad is visiting, but he doesn’t get the wrench until he hands over the Boson.

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