Quantum Diaries

Fermilab recently launched a new web site describing the idea of a muon collider. Why think about a future collider when we haven’t even started using the LHC yet? Because it takes a really long time to design and build one. The LHC was conceived a few decades ago, and the LHC project was approved in 1994, fifteen years ago. Also, Fermilab is soon going to shut down its Tevatron collider that has operated for almost 30 years, and is planning for its future.

I find the idea of a muon collider very intriguing. The idea is to collide positive and negative muons together, as opposed to colliding protons together as will be done at the LHC. A muon collider would be well suited to precise measurements, whereas the LHC is more of a discovery machine. This is because the protons colliding at the LHC are made of other particles called quarks and gluons, and it is those particles that actually collide in the LHC. So a collision of protons is really a collision of many quarks and gluons, which can be quite a messy thing to try to understand. A muon collider would collide muons only, so the collsions would be “cleaner” in the sense that there are fewer particles colliding at once. This makes understanding what happened in a collision easier to understand.

A muon collider would be a great tool for precisely studying whatever new particles are found at the LHC. Another option for a precise instrument is an electron collider, and there are proposals to build electron colliders as well. A muon collider has the advantage of being much smaller (a circle 2 km across instead of the proposed 30 or 50 km in a straight line proposed for electron colliders).

However, the technology necessary to build a muon collider is still being developed, whereas the technology needed to build an electron collider is already well advanced. But assuming we discover some interesting new particles at the LHC, building an electron or muon collider to follow up would be a great idea. So when will this happen? In the case of the muon collider, the schedule estimates I’ve seen currently put first collisions around 2028.

I’ve written about working in the ATLAS control room before, but I haven’t spent too much time there recently.  On Friday, I was there all day and the new visitor’s center was in full swing. The visitor’s area at ATLAS has a glass wall (see picture here) that allows visitors to see into the control room from behind the glass, avoiding disturbing the workers inside. The only problem is that I was sitting 3 feet from the glass window on Friday.
There were people coming through all day, and flash bulbs constantly going off. This was a bit distracting, but not as distracting as the people who tapped on the glass. The other nice feature of the ATLAS visitor’s center is the simulated disaster scenario. There is a big red button that activates a flashing red light, and everyone loves this button. A flashing red light is a bit distracting as well, as you might imagine. I have no idea who designed this, but I have to ask, why?
The afternoon brought another interesting event. I knew something was up when I saw a couple of chairs being set up, and a few flags as well. There was one flag I didn’t recognize. Then I started seeing well-dressed people mulling around, then security people. The head of ATLAS was there, and some prominent physicists from the lab, so I knew a big shot was coming to visit. The stairs out in front of the building were swept, the garbage cans in the control room were emptied, and the banisters on the stairwell were polished! Finally I got the word that the President of Slovenia was coming. Of course we stopped work at this point and found him on wikipedia. A little later the whole entourage including the CERN Director arrived, reporters in tow.  They toured the visitor’s center, but I think they abstained from the big red button for some reason.  Then they came through the control room.  Obviously big shots don’t have to stay behind the glass.  They spoke with someone at the ID (inner detector) desk before hustling off, presumably to go down to see the real tourist attraction, the detector.

So now we know that the LHC will be colliding beams at an energy of 7 TeV instead of 14 TeV, at least for a few months.  Does this change anything from the point of view of the experiments?  Yes!

We have been preparing for collisions at 14 TeV for over a decade, and in fact the final plans of 2,610 ATLAS physicists for what to do with the data just arrived here at my office (some of my work here at CERN has been a contribution to these studies).  The 1,828 page, three volume set is a (very) comprehensive final evaluation of what we think we can do with the ATLAS detector and a plan for what physics topics we will study.  This was all done with very sophisticated software simulations that took thousands of person-years to code and test.  After all those years of preparation for collisions at 14 TeV, there are some changes that we have to make for working with real collision data at 7 TeV.

First, the laws of physics predict different rates of particle production at different energies.  For example, our best tool for calibration is probably the particle called the Z boson.  It is relatively easy to pick out from the rest of the data and it has been very well studied.  So we can find it and measure its properties, then compare what we measure to the already known numbers, see if our detector is working as expected, and make any necessary changes to calibrate it.  At 14 TeV, in 50/pb of data (perhaps a few months of data taking), we expected to have about 25,000 Z bosons that would decay into 2 electrons.  In the same amount of data at 7 TeV, we expect about half the number of Z bosons.  So it will take twice as long to accumulate enough data to achieve the same level of precision in our calibration.

There are also some differences in the behavior of particles produced at different collision energies.  In higher energy collisions, particles tend to be produced with more kinetic energy (which means they start out moving faster) and this causes them to go flying off into different parts of our detector than particles produced in lower energy collisions.  Also, having less energy means fewer particles overall are produced per collision.  All of this is important because the simulations that I mentioned above, done at 14 TeV, led us to develop software reconstruction algorithms that would work at 14 TeV.  So now we will have to produce new simulations corresponding to 7 TeV and re-do many studies.

Finally, the experimental signatures of new particles we may create, and of background processes that mimic these signatures that we have to know about to account for, will be different at 7 TeV and 14 TeV.  The potential for discovery depends on the energy of collisions.  At 7 TeV, we do have a chance at producing new particles that have never been seen by previous experiments, although not as good a chance as at 14 TeV.  Again, we will have to re-do some studies to know quantitatively what our chances are, but preliminary studies show that we have a chance to find new particles even with just a few months of data at 7 TeV.

Even though Regina and Ken have already commented, I wanted to add my two cents on the LHC start-up plan.

If you don’t know already, the director of CERN has announced the plan for starting up the LHC.  If all goes well, there will be protons circulating in the LHC in November, and first collisions shortly thereafter.  Next, probably in December or early in 2010, the energy of the collisions will be increased to 7 TeV and the LHC will become the world’s highest energy collider.

The articles cited by Ken in his post gave the impression that the LHC project is on the verge of failure, while this is simply not the case.  As Ken urged, “let’s try to be optimistic”!  While the accelerator will not run at the full energy it is designed for right away, this plan allows operators to gain their first experience colliding beams in the LHC, and allows the detector groups to see particles from collisions in the LHC for the first time as well.  The data we collect next year will allow us to produce our first physics results.  If all goes well at 7 TeV, the energy will then be raised again to about 10 TeV, and we would have a chance at discoveries next year.

But even 7 TeV would be quite an achievement.  Remember that the Tevatron collider at Fermilab currently holds the world record for energy at just under 2 TeV.  7 TeV is not exactly total failure!

I think the announced plan is good news for everyone in the particle physics community.  Last year’s accident hit the community very hard, especially after being so close to having collisions.   Collisions last year would have brought pure joy.  Collisions this year will bring joy, but first probably relief. Relief at not having to answer questions about the LHC not working, and relief for graduate students who would have data they could analyze in order to graduate.  Many of us will be holding our breath for the next few months.  After we see some collisions we can experience that joy, and then start down the long path towards answering some of the fundamental questions we have about the universe.

As you can read about in this article in the New York Times, the decision about the plan for starting up the LHC is to be decided later this week.  The first collisions will not be at the design energy of the LHC (center-of-mass energy of 14 TeV), but at something lower.  This has been known for a while, but now we are close to finding out, given the results of tests that have been made during the last few months, what energy the accelerator experts think is achievable.

We hoped to get 10 TeV after finding out that 14 TeV was not achievable.  And in fact many physics simulations have been carried out in the last months in order to get ready for collisions at 10 TeV.  But now we know that 10 TeV is also not going to happen, and we have to prepare for something lower.

Why does the center-of-mass energy matter?  Well, according to e=mc^2, the energy in the collision is converted into the mass of new particles that we are trying to create.  So the more energy we have in the collisions, the more massive particles we could potentially discover.  For example, to discover a dark matter particle, the energy of the collision is converted into the mass of the new particle.  Right now, we don’t know exactly what mass the dark matter particle has, so the higher the collision energy, the more massive particle we could potentially make.  Our potential to discover something new depends on the energy of the collisions.

The other factor that matters is called luminosity, which is basically how many collisions we have.  This is something else that will be decided this week.  How long, probably some number of months, will we run at whatever energy we have?  This is a difficult question because a shorter amount of running means starting the repairs of the LHC sooner, and getting to 14 TeV sooner.  But a longer amount of running now would give us more data to analyze in the meantime, even though it is at a lower energy, and that would be very useful to have right now to calibrate the detector and find any problems.

My opinion is that the LHC should run at the highest energy it can as soon as possible, and run until we have a reasonable amount of data collected, enough to do some calibration and maybe even some first studies.  Then shut the machine down, and we will do those studies while the repairs are done.  Repairs often take much longer than expected, so getting data now, even if it is not the data we need to find the Higgs boson, is the way to go.

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.

I just returned from a vacation to the United States. It was nice to see my family and friends and get out of the CERN bubble for a little while. Of course, I got the usual questions:

• 1) Did that accelerator start yet/when will it start?
• 2) What good will it do us?

My answer to #1 was that I have no idea, hopefully in 2009.

My answer to #2 changes, probably depending on who I am talking to, or maybe just my mood. Sometimes I say something about Higgs bosons and supersymmetry (I try to avoid black holes since then I have to talk about the end-of-the-world fearmongering). Sometimes I say something about the practical benefits of past projects and that this project will also have as-yet-unknown benefits. And sometimes I say that there probably will be no direct benefit to the average person from the LHC.  Each of these is partially true, but whatever answer I give, I usually feel like I could have done better.  After being asked this question so many times over the years, I should have settled on an answer by now.

From now on, I think the answer should just be my personal reason and not have anything to do with the benefits to society overall.  For me and I think most people that work on LHC physics, I think the answer is the same: curiosity. Humans are curious, and science is the systematic way of satisfying curiosity. And I think that anyone that takes the time to learn about LHC physics would be curious to know more.  70,000 people showed up at CERN for the open day last year which makes me sure that people are curious about the things we study here.

To really answer questions about whether the LHC is worth the cost from a practical point of view, it would probably be necessary to put a dollar amount on the knowledge and other benefits we gain from it, and on the things that we then don’t do because the money is spent on the LHC. I have no idea how to do this, it probably isn’t even possible, but if you try to say whether the LHC is worth its cost any other way, you are just guessing or justifying your own beliefs.

So I think the reason to build the LHC (and to do it now as opposed to later, after we solve the problems of world hunger and disease and all the other things some people say the money would be better spent on) is that the LHC project is right now the next logical step in a series of questions and answers that started a long time ago. Scientists asked some questions and got answers through experimentation, which raised more questions, which led to more experiments, and machines got bigger and bigger until we ended up with a 17 mile long machine. Nothing else on Earth but human civilization could express its defining trait, curiosity, as the LHC. And what other point is there to civilization than coming together to collectively do what we cannot do individually, and what is more important than our defining characteristic, curiosity?

This week I am in Foz do Arelho, Portugal for the ATLAS Hadronic Calibration Workshop. As you can see, it is beautiful here, but the 100 or so of us that are here aren’t here for a nice vacation (We have meetings all day every day, so it is a bit unfortunate that we see the beach all day but we can’t go sit on it). We are here to work, and prepare for the data we hope to get in a few months.
This workshop is a chance to review where we stand, and then make plans for the early phase of data taking, the next 18 months or so. Specifically, we are talking mainly about “jets” and “missing energy”, two major topics of particle detection.
“Jets” are what we call the huge number of particles that we detect after a quark or gluon is created in a collision in the center of our detector. It flies away from the center and quickly decays into other particles which all crash into our detector and create many other particles in the collisions.
“Missing energy”, what I mainly work on, is a topic that relies on the conservation of energy in a particle collision. The idea is that the energy held by two particles before a collision is equal to the energy held by particles created in that collision, and to the hundreds of particles that result from the subsequent collisions that happen as those particles travel through our detector, hit it, and in turn decay into more and more particles. Since no energy is missing before the collision, no energy ought to be missing after. If we detect all the resultant particles in our detector, and add up all their energies, we should get zero. The one major exception is if particles called neutrinos are produced, which we are not capable of detecting, so they fly away and take their energy with them. Other than that case, checking whether we actually get zero when we add up the energies of all the particles we detect is a really useful check of the performance of our detector.
What unifies the topics of “jets” and “missing energy” is that both rely on the hadronic calibration of the ATLAS detector, which is the subject of this workshop.
Hadronic calibration is the process of turning many of the signals we measure with our detector into the final measurements of the particles we use for physics studies. This process has many steps and takes a huge amount of work by many people, which is why we are here all week.

I was having a conversation at lunch recently with some people who are anxiously awaiting LHC data.  Okay, everyone wants data to analyze so we can discover the secrets of the universe, but the course of some people’s lives are determined by when, or whether or not, the LHC starts colliding particles, something completely outside their control.

Graduate students in physics typically take about 6 years to get a PhD.  If you take much longer than that, people may ask “what took you so long?” and overlook you for other positions later.  There is also the slight issue of living on a graduate student salary for longer than six years.  So with the LHC delay, many US graduate students (including one from the Stony Brook group I am a part of) who were doing research at the LHC and expecting to use data from the LHC are heading back to Fermilab in Illinois to do research using the Tevatron collider, which is recording data right now.  Most of these people preferred the physics of the LHC, but because of the LHC delays are choosing what is likely to be a quicker way out of graduate school.

Similar problems confront postdocs like me, who are waiting for data to improve our chances of landing another position in the field.  Some postdocs I’ve known, after waiting long enough, have decided to leave the field rather than wait any longer.  And of course there are plenty of non-tenured faculty waiting for data so they can get tenure at universities.

One question we spent quite a lot of time discussing at lunch was “do American students need data to graduate”?  It might seem obvious that you need data to do research but universities in Europe allow their PhD students to graduate using simulated data if no real data ia available.  They can develop calibration or other analysis strategies on simulated data, for example, that are later applied to real data.

Another question is whether this “real data” requirement of American universities will survive as experiments get bigger and bigger and timescales for them get longer and longer.  We didn’t come up with any answers, just more questions to ponder while waiting for data.

CERN has issued a press release on the progress of LHC repairs.  Most importantly, it says:

Director General Rolf Heuer confirmed that the Large Hadron Collider (LHC) remains on schedule for a restart this autumn, albeit about 2-3 weeks later than originally foreseen.

So when will we have collisions?  The release goes on to say that further tests are needed:

to determine the start-up date and initial operating energy of the LHC

The center-of-mass energy of collisions will be between 8 and 10 TeV.  The higher the energy that the LHC can achieve, the more new physics we will be able to do next year.  The center-of-mass energy goal of the LHC is 14 TeV.

Update: For those interested in more information, Symmetry Breaking has posted links to reports and presentations by two external committees that reviewed the systems put into place to prevent future incidents.