Quantum Diaries

Yesterday, the LHC collided its last particles for 2009. It has been an exciting end to the year, and the beginning of a new era in particle physics. The collider and all the experiments have proven that they work very well, and are ready for the first physics run.
This is also my last blog entry for the US LHC web site. It has been fun, and I appreciate all the interesting comments and questions to my posts over the last year and a half.
A few new bloggers are starting up, and I’ll be among the readers for an exciting 2010. Happy New Year!

I have been in the ATLAS control room on shift each of the last three weeks.  Each shift was more boring than the last.  The first collisions were on a Monday. I had shift that Tuesday.  Tonight we actually had a first while I was here.  We just had the “first time with two beams with 2 bunches per beam at 1.18 TeV.”  Okay, there are a lot of qualifiers in there.  But it was still exciting to watch the big screen in the control room that gives the status of the LHC as the beams were accelerated.

Two bunches of about 10 billion protons each were circulating in the LHC in each direction.  The energy of each beam was slowly increased starting from the injection energy of 0.45 TeV, and we oohed and aahed as it went up.  I took a photo of the big screen when both beams were up to 1.18 TeV (you can see the energy at the top in the middle, “1180 GeV”).  Hopefully some time in the next few weeks we will get collisions at this energy!

The excitement is building again here at CERN.  The LHC team is planning to collide protons in the LHC for several hours tomorrow, and the next few days after.  The goal is to provide the experiments with their first million collisions (a few weeks ago we had only a few hundred).  We need millions of collisions just to start to calibrate the detector, and to re-discover some of the well known particles (that will be created in the collisions via E=mc^2) and prove our detectors are working correctly.  There are a few differences between the upcoming collisions and the first collisions a few weeks ago.  First, there will be more protons in the LHC (probably about 4 billion protons in each “bunch” of protons, with 4 bunches simultaneously going in each direction around the LHC).  Second, the LHC teams have been carefully studying the beams in the LHC in the last few weeks so the bunches of protons should be better packed together.  This will decrease the number of protons straying away into the beam pipe, allowing the beams to stay in the machine for hours, and leading to more collisions.

This is not a chance to discover the Higgs boson (that will probably take many years), but it will be the first time we use the whole detector together to detect a significant number of collisions.  In the next few weeks, we will collect enough data to do quite a few checks and calibrations.

CERN closes December 18 for a few weeks, and we won’t have beams back in the LHC until at least February.  While we wait, we will analyze the data collected in the next few weeks.  Then the LHC will ramp up the energy of the beams and we will have billions and billions of collisions through the rest of 2010.

Since my last blog post, “Waiting for Collisions“, we’ve had them and plenty of excitement.  Each of the four big experiments recorded data in which they were able to see evidence of collisions in their detectors.  Last Thursday, there was a meeting where the LHC and the experiments showed some first results, and all of the presentations were very encouraging.  But what we have seen so far has only been the warmup.

We are now eagerly awaiting a significant number of collisions, and to collect data where we can see not just a few interesting events, but start analyzing data with large numbers of particles and testing the performance of the detector.

I am in the ATLAS control room today on shift.  The LHC has been doing tests with beams in the LHC in preparation for the first collisions with higher intensity beams.  They are testing all the safety systems, and doing everything they can to guarantee the proton beams are well controlled to protect the detectors from damage.

Hopefully we will have collisions in the next few days, and the next week should be even more interesting than the last!

It has been a very exciting weekend with proton beams in the LHC day and night, but it was still only the warmup.  Now we are eagerly awaiting the first collisions.  Then we will really have begun the LHC era.

But we are not just sitting and waiting for collisions.  I have spent most of the weekend looking at the data collected so far, and based on the email traffic so have many others.  My conclusion is that the ATLAS detector is performing extremely well.

Besides the quality, what has impressed me is how quickly the data has been made available around the world, and how well all the software to analyze it has worked.  This is not a big surprise to me since these are some of the things we have used the last year to improve, but it is nice to see it all work so well.

Just in case you haven’t seen them, teams have been scanning the data and making event displays.

Now that the LHC and the detectors have done everything that was done last year (and more), we can finally move on to what we have really been waiting for, collisions!

The first beam of 2009 has passed through ATLAS! Follow the events at http://twitter.com/cern.

CERN November 20, 2009

As you can see in the picture I took this morning, it is foggy here at CERN today as we await the first circulating beam of protons in the LHC since last year.  When this will happen exactly is a little foggy as well.  There will probably be protons put into the LHC sometime this evening, so perhaps overnight we will have a circulating beam.

At the ATLAS detector, we are excited for the first beam to ATLAS this year, which will happen first (the way the LHC is configured, the beam has to go almost all the way around the LHC’s ring from where it is injected to get to the ATLAS detector).

The beam will first be made to stop before ATLAS by moving the beam collimator in its way.  This will cause a huge cascade of particles to hit the ATLAS detector (similar to what was done recently at the CMS detector ), and it will be quite useful for us at ATLAS to detect all these particles and check our timing.

After that the collimator is removed and beam will pass through the ATLAS detector, at which point is has just about made one revolution around the LHC.  This will be repeated for the beam going in the other direction around the LHC.  Then in the coming days or weeks we will have two beams of protons in the LHC at the same time…and finally collisions!

Life is getting very exciting again at CERN.  In conversations about the future we are now using the words “days” and “weeks” instead of “months” and “years”.  Now that beam is back in the LHC, we are eagerly awaiting the first collisions.  The ATLAS detector has been operating 24 hours/7 days a week for a while now, and is ready.  The next milestone will be proton beams going all the way around the LHC ring.  Then the big milestone will be the first collisions of protons in the LHC.

While obviously we want collisions, the question is what can we do with the small amount of data we expect to collect this year?  We will not make any big discoveries in 2009.  We mainly want to establish that the detector is working well and to begin doing some basic checks. This starts with timing.  All of the different parts of the detector (that work pretty much independently of one another) have to be synchronized with each other down to the nanosecond level, something that is very hard to do without an absolute reference.  One really nice reference is a particle traveling through several different subdetectors.

The next thing we can do is find the first “jets” (sprays of particles formed from quarks and gluons), electrons, muons, and so on, the basic particles that our detector is built to detect. After that we can start to calibrate the detector.  This is mainly done by comparing measurements of particles from one part of the detector to measurements of similar particles from other parts of the detector, and also by checking that we measure basic things like the masses of well-known particles to be the same as measured many times by previous experiments.

After being so close last fall, it has been a long wait to get back to this point.  When the explosion happened last September, few people would have guessed it would take this long to get back to this point.  But the excitement of last fall has returned.  The line to get through the gates into CERN was a little longer this morning, parking has been much tougher to find the last couple of weeks, and (of course) the number of meetings is increasing.  After all the years of preparation, we are finally at the start of a wonderful new era of discovery.