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Archive for July, 2009

In a previous post, I had described how we use photons to map the material in a detector. Here I will mention a complementary way using particles such protons, pions, neutrons, etc. (these particles are collectively known as hadrons).

Hadrons interact with matter differently than photons; the latter interact purely via the electromagnetic force, whereas the former do so mainly via the strong force. The likelihood of hadrons interacting in matter is quantified by a property called the “interaction length; more about this later.

Just as a photon can convert when it travels through material, a hadron can interact and produce what we call a “secondary interaction”. In a way, this is the same idea as when the two proton beams at the LHC collide. Let’s say I have a proton that was created in the primary collision. As it travels out through the detector, it can interact with another proton in a nucleus in, say, the silicon detector. At times, this secondary interaction will have two or more charged particles emerging from it; at other times, one may have only one charged particle coming, e.g., one pion and two neutrons, or, the initial proton may just suffer a small deflection, etc.

If the secondary interaction has two or more charged particles coming out of it, we can use our software to check if the daughter particles come from the same spatial point. If they do, we have a vertex describing the location of the secondary interaction. The spatial distribution of these secondary vertices will give us a map of the material in the detector. I am currently working on this project and preliminary results are very promising.

As I wrote in the previous post, the likelihood of photon conversions in a material can be quantified by a property called “radiation length”; this depends on the intrinsic properties of the material such as atomic number, i.e., number of protons in the atom, and also atomic mass, which is proportional to the number of protons and neutrons in the atom. Since photons interact via the electromagnetic force, “radiation length” has to depend on the charge of the nucleus, i.e., the atomic number. In contrast, the strong force makes no distinction between a proton and a neutron, thus, “interaction length” has no dependence on the atomic number, but only on the atomic mass. The latter length also has some dependence on the energy of the incident particle. Although, we can derive from one from the other, it can be tricky. Since every material in our simulation package has to be described with a radiation and an interaction length, material maps made using photons and hadrons serve as very good checks on our understanding.

— Vivek Jain, Indiana University

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Dr. Strangelove

Thursday, July 30th, 2009

tele_mitm26The last week we were working on getting our new pixel chip running in the test beam. It was tested in the lab by the developers in Strasbourg, but it was never in the beam and never read out with the telescope electronics. We had a few problems to solve during the week, but now everything is working!

On the picture you can see our telescope. In each of the boxes with the DESY labels are three planes with the original chip, a special CMOS sensor with 30 micrometer small pixel. In the middle you can see another box with three new sensors. They are produced in the same technology, but this new version has smaller pixel (18 micrometer), is much smarter and suppresses directly the readout of the non-hit pixels, and converts this information into a digital signal. Thus reducing the data size we get from our system dramatically to the really needed information. We have to upgrade our system to this new sensor as it will give us the possibility to run everything much faster. But first we want to test this three new sensors while using the telescope.

You know, particle physicists get rather excited when they see signals from particles in their detector, independent if this is a large experiment like ATLAS or a small device tested for the first time. So we were running between computers back and forth and cheering loudly when we saw the first reasonable plot, telling us that everything works as it should. Of course this is just the start. Now we have to take data the next couple of days, getting a minimum of 1 million events. Afterwards the data has to be analyzed.

We celebrated this success with a cake. But this cake was not only a celebration cake, but also a fare well cake. Toto, the main author of our analysis software, is leaving our team. He will start a new job outside high energy physics just next Monday. We are all sad that he is leaving as he is one of the core people in this extremely good team. But people have to move on and start new jobs. This sad part is also part of a scientist’s life. Even the best teams are split in the end because people get different contracts and have to leave.

BUT: We’ll meet again, don’t know where, don’t know when …. Thank you Toto!!

toto_cake

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The desperate remedy

Thursday, July 30th, 2009

Neutrinos, they are very small.
They have no charge and have no mass
And do not interact at all.
The earth is just a silly ball
To them, through which they simply pass,
Like dustmaids through a drafty hall
Or photons through a sheet of glass.
They snub the most exquisite gas,
Ignore the most substantial wall,
Cold-shoulder steel and sounding brass,
Insult the stallion in his stall,
And scorning barriers of class,
Infiltrate you and me! Like tall
And painless guillotines, they fall
Down through our heads into the grass.
At night, they enter at Nepal
And pierce the lover and his lass
From underneath the bed—you call
It wonderful; I call it crass.

“Cosmic Gall”, from Telephone Poles and Other Poems, John Updike, 1960.

Neutrinos, as Updike elegantly put it, are one of the most elusive particles in nature. They don’t only go through walls, planets, and humans; but also through the very instruments that are built to detect them, only occasionally leaving a trace behind them to evidence their pass.

That’s a lesson that everybody (even some of my fellow co-bloggers) working with this particle has learned the hard way. Detecting a neutrino always involves putting on its way a huge amount of matter to force a couple of them to “show up”, to reveal their existence. But why should we care about building such huge detectors if the neutrino is so indifferent to our efforts?

Well, the fact that its interaction with matter is so small makes it a perfect probe to observe the places where some of the most violent astrophysical phenomena in the universe are taking place. After its production, the neutrino will leave the place quietly, going through huge amounts of matter almost without being attenuated, bringing us the news, only if we have cared enough to put a detector in its way. As they have no electrical charge, they travel cosmological distances without being affected by magnetic fields so, at the time of their detection, they still point to their source. They also have huge decay times (if they decay at all) so they can travel for a long while without “breaking apart”. A neutrino is, in a sense, “our man in Havana.”

I’m tempted to say that neutrinos have such a weird personality because their father didn’t love them from the moment they were born, and called them just “a desperate remedy.” 🙂

Coming soon: the description of a full-fledged neutrino telescope. By the way, since the time that Updike wrote his poem it has been proved that neutrinos have a tiny but non-zero mass.

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Hello!

Sorry for my absence, I have been on my lovely holidays. Thanks to the Slovak-UK connection in the ALICE group, and the experiences of my supervisor and his family, my boyfriend and I decided to take ourselves to the eastern part of Europe for a while, starting in Kosice. We did so much with our time, seeing things that would otherwise be tricky, with great thanks to one of my colleagues, Slavo, who organised visits, bought tickets, taxied us around Slovakia in his car…many thanks indeed!

As a result I have a few very cool experiences to share. The first was a tour of the university and academy of physical sciences in Kosice, renowned for its low temperature physics, so please pardon my pun. Throughout its history, the Low Temperature Physics group has broken boundaries, and in June 2005 they achieved the lowest temperature in central Europe, 50 mK. The lab had an area of wall unpainted and preserved behind glass indicating its first low-temperature achievements, to 1.7 K and so on (the years I cannot recall) and they seemed to find it inspiring to see how far they had come. And it has been a long way – their equipment is very advanced. The lab is unique to Slovakia, something they are proud of but also saddened by – other labs have lost funding, or the heads of group have taken their research elsewhere. They are, like most low temperature physicists, looking at superconductivity, a phenomenon very useful to particle physicists because it means zero resistance, which makes for very easy to maintain powerful magnets, provided you can keep them cold enough! They are exploring many different compounds, investigating their properties and trying to achieve superconductivity at warmer temperatures, which would make our lives even easier! However, the group are also doing some very interesting studies at extremely low (microKelvin) temperatures. There is a lot to learn from superfluid Helium (He4 as a Bose Einstein Condensate, He3 as a fermionic condensate). In this state, the superfluid has zero viscosity, and some very strange behaviour! The properties, (magnetic, electrical, thermal etc) are really quite percquliar, not to mention tricky to measure. Tiny wires on handmade equipment are immersed in the strange substances as probes. This is the kind of science I love!

We also met with the group working on a phenomenon called sonoluminescence, discovered in the 30’s and still poorly understood. This involves creating a bubble in a fluid (apparently it works best with water and air) and applying a standing sound wave causing the bubble to expand and contract in the centre of the beaker. With high enough intensity, the bubble collapses so fast, it produces phenomenal temperatures (albeit very briefly) of the order of tens of thousands of Kelvin, releasing a shock wave of around 10 million photons before expanding again. You can watch the beaker glow a like a tiny blue pixel. If this sounds familiar, it was the phenomenon that was linked to “cold”, or “tabletop” fusion by some scientists in 2002, the idea being that much higher temperatures might be reached, and was claimed to have produced cold fusion in 2006 by the Rensselaer Polytechnic Institute, but is yet to be reproduced (I am saying nothing on that one!). However, it is being used by Pistol shrimp under the sea, who click their claws and create a pressure wave, releasing a bubble from the water that collapses at speed due to the pressure (I think that’s how it works) and knocks the life out of its prey! I have to say, I hadn’t expected my holiday to leave me postulating so much about phase transitions and bubbles and extreme temperatures – it felt like a normal day at work (in a good way)!

There are many more adventures I want to share with you, but I am saving them for another time, so I can get back to work. After all, now that I am back from holidays I find myself with far too many things to do, what with shifts for cosmic ray events starting soon, and LHC collisions due before the year is out!

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Security in Science

Thursday, July 30th, 2009

SLAC is different from many other DOE facilities in that the public (with ID) can enter. There is a radiation fence that the public is not allowed past (without proper escort) but SLAC is open – from a security point of view – since weapons and other “top secret” type government work is not done there.

Unfortunately, there was a recent act of vandalism at a SLAC facility – the Stanford Synchrotron Radiation Lightsource. More unfortunately, this vandalism was not just someone spray-painting something offensive on the wall. A few thousand samples of crystallized protein were removed from their liquid nitrogen storage, hence destroyed. While these samples are replaceable, it will require time and money. The samples are estimated to be worth half a million dollars.

This was a shock to the SLAC community. I wondered who could do such a thing. Were they a disgruntled (former?) graduate student, a competing experimenter, or someone who is anti-science (and a bit crazy)? This may cause security to significantly increase at SLAC, which would be regrettable if it means that the Stanford (and wider) community can not come to SLAC for talks. I wonder if CERN has increased security measures in fear of people trying to somehow stop the LHC “from destroying the world”. There have been violent attacks upon scientific facilities before, and there are people out there who genuinely think the LHC will somehow cause harm. I’m sure Dan Brown’s books don’t help people trust CERN either.

The FBI investigated the incident and has made an arrest. It turns out that she is a former employee who felt overworked and was fired from the group about a month ago. I’m somewhat glad this is the answer – it doesn’t point to an anti-science terrorist group or a maliciously competitive experimental culture. Hopefully this does not change the open culture of SLAC. I must also add that while groups and advisors vary, I find that SLAC groups seem to expect more reasonable workloads than many University groups out there. This act is a reflection of her, and not the SLAC work environment.

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A day in the life of a proton

Thursday, July 30th, 2009

Being a member of ATLAS, I’ve spent a fair amount of time in the ATLAS control room. I realized when I was talking with some of my colleagues who work on accelerator operations that I’ve never seen the actual LHC control room.
Yesterday, while at lunch, I asked if my friend if she would give me a tour (since you need  proper access to get in), so I thought I’d share the experience.

Picture taken from Proceedings of ICALEPCS07,Knoxville,Tennessee

Picture taken from Proceedings of ICALEPCS07,Knoxville,Tennessee

The main CERN control room is not just the for LHC, it is the heart and brain of all the accelerators at CERN. It’s brand new facility, completed in 2006 in preparation for the start of the LHC. I had taken an accelerator physics course before, but needless to say, it’s quite an experience to witness it first hand.
Our little proton friends have quite a journey before they make it into our interaction point and get smashed into bits. Here’s how it goes:
There are 4 accelerators that are used to inject the initial beam into the LHC. The magic begins in “Linac 2” (Linac – Linear Accelerator) which creates the protons and then injects them into the Proton Synchrotron Booster (PSB, or “Booster”). This accelerator provides beam to the ISOLDE (Isotope Separator On-Line) experiment. (There are many physics experiments at CERN other than the ones located on the LHC ring). Every 1.2 seconds a decision is made as to where to send the beam. When ISOLDE doesn’t need it, the Booster can inject into the Proton Synchrotron (PS) where is gets accelerated up to 25 GeV. The PS accelerated it’s first protons in 1959 and continues to work to this day. There is a physics complex called the East Area which utilizes this beam when it’s not being sent elsewhere. Once the protons are up to energy, they are injected into the Super Proton Synchrotron (SPS). Off of this ring, CNGS, a fixed target experiment which sends neutrinos off to Gran Sasso, Italy, and the North Area physics facility are located.
Now for the big one. Once the protons are accelerated to 450 GeV, they can then be injected into the main LHC ring. It will take several injections of the SPS to fill the LHC (~2000) which initially will take around 20 min. All of these injections will have to be in phase, and steered properly because losing the beam at this energy isn’t an option.

The CERN Accelerator Complex

The CERN Accelerator Complex

This is my rudimentary understanding of an amazingly complex system, that sometimes as a particle physicist I don’t appreciate enough. So hats off to the accelerator physicists, the next time you run into one buy him or her a drink. 🙂

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iris scanner

Thursday, July 30th, 2009

eye scan
I was recently underground on a tour of the LHCb detector. It was the last one of the four main LHC detectors I hadn’t seen in person. I also got to see the old DELPHI detector which is still down there. I will post some pictures soon.
On my way down, I took a picture of one of the infamous iris scanners mentioned in Angels and Demons. In the book, someone at CERN is murdered and their eye taken in order to get through this security feature. In reality, a dead eye cannot be used with this technology.
Everyone who visits the underground areas at CERN is fascinated by this technology. I think CERN should put one of these scanners in the visitor’s center just for tourists.

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How I Spend My Summer Vacation

Wednesday, July 29th, 2009

I have returned to the San Francisco Bay Area for two weeks for a friend’s wedding.  Fortuitously, that means I can visit other friends and also see my parents — it’s nice to have things in one place!  Oh, and one other thing is here: Lawrence Berkeley National Laboratory, where my group is based, where I work when I’m not working at CERN.  Yesterday, I got up at 5 AM to make the train trip from my parents’ house to Berkeley in time for an 8 AM meeting with my advisor, and I ended up working the whole day.  Yes, I’m on vacation, but I didn’t have any friends who were free during the day, and I thought it would be useful to work with and talk to the various group members who I don’t see so much of because we work on separate continents.  I like my job; I don’t take vacations to get away from it, but because there are other things I want to do.  So, if I have a free moment, well, why not get a few things done for work?

For the Facebook crowd: I’m Seth, my page is http://blogs.uslhc.us/?author=9

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DPF in Detroit (part I)

Wednesday, July 29th, 2009

On Saturday I flew to Detroit where I am participating in the APS Division of Particle and Fields conference at Wayne State University. This was exciting for me as I both get to present an EXO talk and I get to return to my “homeland”. This is the first time I’ve been back to Michigan in over 5 years. I’m spending my time outside of the conference catching up with family, friends, and seeing the area.

The conference has been full of exciting physics, and I’m not yet to the end of the 2nd day! There are 4 parallel sessions, and each one has a neutrino session. In addition to the big accelerator based experiments (like MINOS and NOvA) there have also been theory talks, astrophysical neutrino talks, and representation from the smaller experiments. Unfortunately some of the neutrino talks are in other parallel sessions, like the particle-astro or “Low energy searches for physics beyond the standard model”, and I’ve had to choose between two really interesting talks. I’ve heard about an new analysis method that could be applicable to EXO, hierarchy tests with supernova neutrinos, and tests for Lorentz violations with neutrinos. I’ve always thought that neutrinos were an exciting and fast developing field, but I am still surprised by everything they can be related to!

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Back to the Future

Wednesday, July 29th, 2009

A project I recently joined is the Insertable B-Layer (IBL) for the ATLAS Pixel Detector. This is an upgrade of the ATLAS Pixel Detector. Why do we already need an upgrade for the LHC detectors when they are actually just completed?
In case of the pixel detector, made of silicon and sitting only a few centimeters from the beam pipe, one reason is the radiation damage. The particles passing through the material not only create a signal giving us the possibility to measure their tracks, they also can displace silicon atoms from their lattice position. Depending on the type of radiation, this leads to point defects or damage clusters. The more damaged the silicon gets, the more the electrical properties are changed up to the point when the detector stops operating.

This properties and the subsequent detector operation were studied in great detail during the last decades by all LHC experiments and the systems designed are the best what was on the “market” when the detector productions started. Nevertheless it was clear from the beginning that the innermost layer of the ATLAS Pixel Detector called B-Layer* cannot survive 10 years of LHC operation. Therefore a replacement of this layer surviving longer has to be prepared. The idea is, to put in an additional layer inside of the current ATLAS pixel detector to run in parallel to the other layers until the radiation damage takes its toll.

Even so the insertion of this new layer will not be before 2014, it is now time to work on the details. Sensors have to be developed which are more radiation hard. The technology advanced quite a bit in the recent years. Many properties are understood much better and this knowledge helps to design the silicon devices such a way that they survive longer. But many tests have to be done before we are sure that this is good enough. Devices will be irradiated at different facilities and then put in the test beam to see how the detector operation is affected (here our telescope comes in).

Another challenge is the mechanical integration. As already mentioned, there is a detector in place and will not be removed before the installation of this new layer. Therefore the new layer has to be squeezed in a very tight gap between the beam-pipe and the current detector, while not damaging anything. One gains more space by reducing the beam pipe diameter, meaning that the current beam pipe has to be removed without touching anything. To be honest, I am rather happy that I am not one of the mechanical engineers.

While people working on the analysis of the LHC data are waiting for LHC to be turned on, detector developers are already thinking of the future…

*Why is this layer called B-Layer if there is no A or C Layer. The naming was chosen because this innermost layer of the Pixel Detector is used for b-tagging. Quarks cannot be directly measured in a detector, they hadronize (find other quarks to make a particle) directly after they are created at the interaction point. The newly created particles are creating so-called jets. The measured jet leaving the interaction point are called primary vertex.

Some particles (b-or c-hadrons, and τ-leptons) can travel a considerable distance before decaying. The jets are only starting a little outside the interaction point. The jets created by these particles are called secondary vertices. Measuring the starting points of these vertices is called vertexing. A detector with good vertexing ability can separate primary and secondary vertices, and thus perform identification of b/c/τ-particles. Measuring the distance between the interaction point and the start of the secondary vertex is called b tagging.

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