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Posts Tagged ‘first results’

It’s an exciting time for your humble LHC blogger. She may just have a thesis topic… So what does that mean? (I often times wonder that myself).

With the recent success and in anticipation of high energy collisions (and therefore data), it’s time to figure out what can be found and what can’t given the projected amount of data. (We’re going to be running at ~7 TeV for the first part of the year, then ~10 TeV the latter half). Now lots of people are doing cross sections measurements – which is a different beast than searches (see below).  Cross section measurements take a particle that we know – Zs and Ws for example – and check to see if we measure what we predict. This is very important to do and I’m over simplifying but that’s the basic idea. Despite it’s importance, I personally feel like if I’m working on the highest energy accelerator in the world, I’d at least like to try to do a particle search.

The Cross Section Beast

The Cross Section Beast

This isn’t a completely trivial question because ever since the Tevatron turned on, theorists have been making predictions as to what was out of reach to our current experiments.  So what makes for a good early search? Lots of things, I’ll list some here:

  • Of interest…

Maybe this goes without saying, but I’m going to go ahead and say it anyway. A search has to be well defined and predicted. One doesn’t just look for the Higgs, or SUSY or Z’, they look for specific decay products that could come from the predicted particle and can not be explained by sometime else. Although we’re going to be 3.5x and 5x higher energy than the Tevatron, there has been years of data collected at the Fermilab experiments. Now there are some particles that are simply outside of their reach. For example just due to conservation of energy, nothing can be created >2TeV, but due to statistics (need that high fluctuation over the background again…) some limits are in the 100-200 GeV range. Increasing the energy will allow us, even with less data to raise limits.

  • High Signal:Background ratio

Since there is going to be a smaller data set (only 1 year of running), we simply won’t have enough statistics to say with confidence that we discovered certain particles. We need to say that the signal is an actual signal – not just a fluctuation of the background. I elaborate this in my Higgs post. This also means that there would be a distinct signature for example: something that would decay to 2 very high energy electrons and 2 very high energy jets. It could be di-boson production or W/Z+jets, but the electrons would come from a W/Z which was very far off mass shell – which is not impossible, but maybe not as probable.

  • Missing Energy (MET to be more specific)

This is a bit contentious, and maybe more a personal taste than anything. We won’t have a completely calibrated detector initially. The detector is calibrated by taking standard particles (Ws and Zs for example) and reconstructing them. We then convert the electrical signal out of the machine to energy and momentum. To do this, the more Z and W events the better – which like everything takes time. So the energy of the signal can be off. This isn’t a bad thing, but the way we calculate missing energy (say in the form of neutrinos) is by balancing the energy in the detector. For example if there is 40 GeV deposited in 1 part of the xy plane, then there has to be another 40 GeV in another part of the xy plane to balance it out. If we don’t really know if it’s 40 GeV or 45 GeV, then it’s hard to calculate missing energy. (I should also point out, it’s transverse energy, not just energy – which I can elaborate on if anyone is interested).

So these requirements gives us a whole range of particles to search for. I’m involved in a physics group called exotics. Exotics are a generic term for anything beyond the standard model and isn’t the Higgs or SUSY. This isn’t to say that Higgs/SUSY searches aren’t beyond the standard model… I guess they get their own groups since so many people are interested in them. It makes the exotics working group more intimate :-). My interests (and potential thesis) are in particles that would unite quarks and leptons (like how a W unites the family of quarks and the family of leptons). These generically are called leptoquarks.

So what’s wrong with the Higgs? It like the captain of the high school football team and head cheerleader all rolled into one particle to the high energy physics community. I don’t know… I’m just not that into it.

-Regina

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Local news

Friday, November 27th, 2009

Admittedly, it is a little harder to follow all the LHC excitement if you are here in the US rather than at CERN.  The announcement of first collisions on Monday came while I was teaching my class, and I’ve been trying to piece together the whole story by talking to our people over there and reading the slides from various meetings.  Of note was a public meeting at CERN yesterday (yes, Thanksgiving Day, another impediment if you are in the US) with presentations from Steve Meyers, CERN’s director for accelerators, and the four LHC experiments.  See the slides and video here.  As everyone else has been saying, the past week has been a thrill (or at least a vicarious one!) for the LHC, the four experiments on the ring, and really all of HEP.  Check out Meyers’s slides in particular, where he documents just how far we have come in the past fourteen months.  The experiments have turned around information from these first few collisions very quickly; some detectors are already able to reconstruct decays of the neutral pion, for instance.  We have huge expectations for the next set of collisions and then for the increases in collision energy that will follow.

My particular contribution to CMS has been in computing, and I’m happy to say that all of that has gone quite smoothly so far.  The prompt reconstruction of events went off without a hitch, and data was flowing very quickly out of CERN to the Tier-1 and Tier-2 sites.  We soon lost track of how many sites had copies of the collision data, and now we’re seeing plenty of people use the distributed computing system to analyze it.  When the next round of collisions comes, we’ll be ready to do it all again.

So while it’s hard to follow the news up to the minute, I’m still connected to the start of a great particle physics adventure.  I’m trying to drag the rest of Nebraska along with me — we managed to get a release placed in the local paper, and if you read this post soon enough, you can hear me at 8:30 AM Central time on Saturday 11/28 on KZUM, Lincoln’s community radio station.  I’ve already taped the interview; let’s hope I didn’t sound incoherent!  (At least when I type the blog posts, there is a backspace key….).

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First ATLAS Pixel Tracks!

Sunday, September 14th, 2008

I’m on my 18th hour on training shift since Saturday morning, getting in as much time in the control room as I can, and it’s been a very exciting time. One of my colleagues has just discovered that, last night, we recorded the first cosmic ray tracks in the ATLAS pixel detector!

First ATLAS Pixel Detector Track!

This is very exciting news for us; we’re working right up to the wire to make sure our pixel detector is able to run stably along with the rest of the detector. Collisions are coming soon soon soon!

Update (Sept 15): In response to two excellent questions in the comments, I wrote in a little more detail what you’re looking at in the picture. I figure the explanations might as well go in the entry:

1. What’s the perspective? Where’s the LHC?

You’re looking at the inner part of the ATLAS detector, which is wrapped around one of the collision points of the LHC. The large image in the upper left is a cross-section of the detector; the white dot in the very center is where the LHC beam pipe is. The image along the bottom shows the same tracks from the side; the LHC beam pipe isn’t shown, but it would run horizontally (along the Y’ = 0 cm line).

2. What do the dot colors mean? What’s the line?

All the dots are the actual points at which we have a signal from our detector. The red dots represent the signal that we think was left by a charged particle when it passed through, and the red line is the path we think that particle took (i.e. the “track”). The green dots are also signals in the detector, but we think they’re random firings in our electronics, because we can’t make any tracks out of them.

It may look like a lot of electronic noise, because there are more hits from random firings than from the track. But remember that there were only one or two tracks to be found, whereas we have over eighty million pixels in our detector. Thus the fraction of noisy pixels was actually quite small, and certainly didn’t interfere with finding the track. We also have a list of especially noisy pixels that we can “mask” (i.e. ignore), which will bring down the noise by quite a lot but which we haven’t begun to use yet.

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After First Beam

Wednesday, August 27th, 2008

If you haven’t already, check out Peter’s posting on the first findings of the LHC. He does a nice job of discussing the basic foundations we need to establish first before we can focus on the ‘sexy’ physics; supersymmetry, the higgs, extra dimensions, etc. But before we can even do the studies that Peter mentions, we have to first calibrate and understand the detector. This in itself is no easy task.

Now that Sept 10th has been set as the day of first beam, the most frequent question I get these days is, ‘So, when are you going to see the higgs?’

I wish that I knew. But it is really impossible to put a timeline on something like this. So the answer is ‘I don’t know’. And if you are annoyed by physicists refusing to estimate when results will be ready, then you are not alone. I was speaking with a journalist earlier this week, who was clearly exasperated with me on this point. This was the gist of our conversation.

Journalist: What is the next milestone for ATLAS after first collisions?
Me: Once there are collisions, our next steps will be in the understanding of and the final calibration of the detector.
Journalist: And how long until that is finished and there are first results? A few hours?
Me: ATLAS has roughly 100 million electronics channels and nine different detector technologies. Calibration of that full system is incredibly complex.
Journalist: Two days?
Me: When we are satisfied that any detector-induced effects in the data are understood, we will confirm that we can observe the particles that we already know exist. Particles like the W and Z bosons.
Journalist: One week?
Me: Then we can be in a position to search for physics beyond the standard model.
Journalist: Two weeks?

In the mist of the first beam excitement, I hate to sound like a killjoy about the timeline for new physics results. But I think the focus is wrong. The next milestones for ATLAS might not be Higgs discovery but they are very exciting. Right now, even the background to the Higgs search is unknown to us. And as Peter mentions this is extremely interesting in its own right. So, who knows maybe by the time we are ready to search for the Higgs, it won’t be the most exciting particle in physics anymore….

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