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Peter Steinberg | USLHC | USA

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The First Discovery at the LHC

Friday, August 8th, 2008

In the recent mini-wave of “What Will We Find at the LHC” posts, no-one mentioned that the first actual measurements at the LHC will certainly not be of anything as exotic as the Higgs boson, supersymmetry, or large extra dimensions. This is not for any reason as prosaic as the fact that it’ll take time to get to the design energy and luminosity, which is true. If we define the very notion of “finding” something as being “measuring a quantity that we could not predict with current tools”, then the very first measurements at the LHC will count as discoveries of great interest to those not just focussed on what typically counts as exotic phenomena.


To boil it down to something concrete, consider the number of particles produced in a typical collision at the LHC. And to make things more straightforward, only consider the number of charged particles, the ones that leave curling tracks when moving through a magnetic field.
These are neat looking events, and they happen each and every time protons collide, at every energy, since the dawn of the accelerator era (you need a few GeV to even make a bunch of pions!). Thus, these particles are the “grass” that one sees in lego plots showing two huge jets, or the steel wool amidst which the two high energy muons emerge after a Higgs particle decays. For most people, this part of the collision is a background that needs to be cut away to see the interesting physics.

Here’s the rub: while it’s moderately easy to count the number of particles in each event, no-one has ever managed to come up with a bottom-up theoretical scheme by which one can predict this number. This is mainly due to the somewhat-scandalous situation that we know what protons “do” when they get close to each other (and can propagate that information into very precise predictions for the production of high pT particles, etc — the bread and butter of the LHC), but we don’t really get “why” they do it. Thus, we don’t have a very solid means to extrapolate our current knowledge into the LHC era, even if the Tevatron is only a factor of 7 lower in energy.

Thus, I promise that the first things you will see coming out of the LHC program are a bunch of measurements pertaining to “minimum bias” events, i.e. the events 99% of the experiment want to throw away so they can look for the needle in the haystack. Some of us (which include many of us directly interested in the heavy ion program at the LHC) want to see how the grass grows when those first collisions appear. We’ll count the particles emerging near 90 degrees, turn it into a “particle density” (by restricting the angle over which we count them), and put it on a plot with the rest of the data — probably with a few curves reflecting our favorite predictions. And everyone wants the first paper from the LHC, so it’ll be a real race, and so the results will appear almost as soon as real collision data is written to tape (which may well be this fall!)

For entertainment value, here’s my take.

This is a pretty straightforward application of the ancient (and bizarre) Landau hydrodynamical model to p+p minimum bias collisions and heavy ion collisions. It assumes that the two protons dump all of their energy into the collision as they overlap, and the system expands collectively like a relativistic fluid after that (sound familiar?). It describes the multiplicity (linear with the entropy) weirdly well for heavy ions (the top curve) above 20 GeV or so, and predicts a very high density at the LHC. It is pretty scratchy for p+p (the bottom curve — which may or may not be related to trigger bias issues — we’ll have to discuss that soon, too) but at least predicts something quite a bit higher than most popular models. But if this model has anything non-trivial to say about proton-proton collisions (something suggested by Landau and Fermi in the 1950’s, but which became controversial and even “heretical” during the rise of QCD, something I wrote about a few years ago), then we may have to start to take seriously the possibility that even small systems have “medium”-like aspects similar to what people already say about the QGP at RHIC. And how fun would that be?

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Large Hadron Rapper

Wednesday, July 30th, 2008

The NY Times is now hip to the work of ATLAS’s own alpinekat (a writer/editor for the ATLAS e-News). She was also our editor on this blog for a while, so we can say we knew her before she got internet famous. Go Katie!

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Where We’re At (Wordle-style)

Tuesday, July 22nd, 2008

Cosmic Variance pointed me to it, and now I can’t stop using Wordle to analyze everything I am working on.  Here’t the current version of this page, and the message seems to be loud and clear:

“ATLAS beam good LHC now point really ready still work”

or maybe

“beam now ready”

Sounds good to me, but one thing still mystifies me: where are the other experiments?

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Behind Enemy Lines

Tuesday, July 15th, 2008

OK, OK, all I meant is that I took a lightning fast tour of CMS yesterday, hosted by a RHIC colleague of mine (thanks, Dave!). But who do I run into but, not one, but two US LHC bloggers — voila:

Anyway, this was my first time seeing CMS, and impressive it was. Impossibly dense, compared to the general ATLAS impression of being impossibly huge. And this may be the last time many of us ever get a chance to see it (ironically, the better the LHC does in the first couple of years, that chance will just get smaller). So enjoy my photos (which admittedly aren’t much better/different than many others) here.

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ATLAS vs. ATLAS

Thursday, July 10th, 2008

While CMS folks are off scuba diving in Cyprus, ATLAS is entertaining itself in a somewhat more strenuous way back here in Bern — playing an intramural soccer game in the Stade de Suisse: The ATLAS Lions vs. the ATLAS Warriors. OK, most of us are goofing off watching, and rattling yellow noisemakers every time something happens to either team (I mean we’re all collaborators in the end), but we certainly wouldn’t all fit on the field. I’ll let you know the score, when it’s over (too tired to liveblog, given that I gave a talk today).

UPDATE: Lions 5, Warrors 3 — ATLAS wins! Um.

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ATLAS in Bern

Wednesday, July 9th, 2008

Believe it or not, this is a photo just outside the ATLAS meeting in Bern, Switzerland that Monica and I are both attending this week. We’d never met before, but I found myself sitting behind her, so introduced myself by pointing out that ATLAS has put us both on their “Blogs” outreach page. Gee, I have to get posting more.

But about that soccer field, the meeting itself is being held at the Stade de Suisse, just outside downtown Bern. The UEFA EURO2008 tournament just ended of course (which I believe inspired the locale), and just in time for an ATLAS-on-ATLAS soccer match tomorrow. Stay tuned for photos posted to my Flickr set. You can also see some photos in and around Bern, which is a lovely place. And some nice mountains nearby, that my wife and I hiked in and on and over last week.

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Speaking of Cosmics…

Saturday, June 21st, 2008

As reported in the Times (which generally makes things “real”, at least in the US, right?), the CERN-appointed LHC Safety Assessment Group (LSAG) has finally come out with its report.

The Large Hadron Collider (LHC) can achieve an energy that no other particle accelerators have reached before, but Nature routinely produces higher energies in cosmic-ray collisions. Concerns about the safety of whatever may be created in such high-energy particle collisions have been addressed for many years. In the light of new experimental data and theoretical understanding, the LHC Safety Assessment Group (LSAG) has updated a review of the analysis made in 2003 by the LHC Safety Study Group, a group of independent scientists.

LSAG reaffirms and extends the conclusions of the 2003 report that LHC collisions present no danger and that there are no reasons for concern. Whatever the LHC will do, Nature has already done many times over during the lifetime of the Earth and other astronomical bodies. The LSAG report has been reviewed and endorsed by CERN’s Scientific Policy Committee, a group of external scientists that advises CERN’s governing body, its Council.

“Nature has already generated on Earth as many collisions as about a million LHC experiments – and the planet still exists”. It almost makes the LHC sound boring since it’s been done over and over again and nothing “happened”. Fine, no one was looking in the right place, and individual microscopic collisions generally have no macroscopic effects (right?) but still. In any case, I think it’s great that CERN has really rolled up their sleeves and addressed this issue head-on. I’ll report back after having a chance to read it more carefully — but I’m already psyched finally to see the inclusion of actual RHIC data!

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Countdown Down?

Wednesday, June 11th, 2008

Why do I  learn about things like the LHC Countdown web page from Gizmodo, of all places? Yes, I like gadgety things like the next physicist (can I get a witness here?), but I’m always surprised when I learn things relevant to, like, science, from, like, you know, nerds.

But why is the webpage, um, down for the count?  Gizmodo effect?  Google cache has a recent copy, and it doesn’t look like an official announcement page (as I expected).  Nor does it look like a doomsday-fringe page, as I expected next.  Seems to be a genuinely-excited person, but one who seems to be attracting a lot of excited, um, commenters.  Blogs are what they are, folks.

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Long-distance Performance Anxiety

Monday, June 9th, 2008

It’s kind of mind-bending how fast things are ramping up, but no-one knows when “it” will happen, and how “it” will go — “it” being the initial collisions and the data analysis to follow. You get the same sense from everyone you talk to about the LHC, and especially from the younger people i know (students and postdocs) whose, well, *lives* depend on things getting going soon. I remember this feeling well, just as RHIC was starting — although things felt smaller in those days, or at least my 50-person experiment did.

This week is a Physics and Performance Workshop concerned with how ATLAS will deal with early data. I have a particular interest in this from the standpoint of a guy interested in heavy ions since this is precisely the data we will use to compare with lead-lead collisions when they eventually arrive. And yet, the last few months finishing up our proposal (more on that later, i’m serious) have shielded me from the outrageous amount of work going into so many different aspects of the detector and analysis. I’m scrambling to catch up long-distance, from my office, reading slides, and keeping up with emails, and we even have a heavy ion videoconference tomorrow (for which I’m assembling a talk now…). But there’s a lot to keep up with, and then I still have to get ready for the big ATLAS week in July! (and let’s not mention we have a short Alpine vacation to plan as well…)

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On the Volga

Friday, May 23rd, 2008

For all the non-blogging I’ve been doing, I can’t say that I haven’t been giving most of my life to the upcoming LHC run. The main distraction was a recent trip to an ATLAS workshop in Dubna, Russia, on the Volga River, next to a huge reservoir (which someone there called the “Moscow Sea”). While I’ve heard of Dubna for years, as I’ve had collaborators on previous experiments hailing from there, I had never been there, much less to Russia in general. Can’t say that anymore.

The workshop (“Heavy Ion Physics with the ATLAS Detector”) was early last week, and took place in a conference center on Veksler Street, well outside the lab itself. It turns out that just as it’s getting harder and harder to get our non-US colleagues into our national labs, it’s getting equally as laborious to get us into foreign labs. So while I didn’t get to see their facilities, we did hear a nice talk about their planned new low energy heavy ion collider facility (NICA). And the workshop participants (half local, half international) presented a nice set of talks both on ATLAS capabilities for heavy ion physics, but also on Russian involvement the other heavy ion efforts at the LHC in CMS & ALICE. My talk, on bulk observables at RHIC, can be found here — for your enjoyment.

Finally, when the workshop was over we took a half day trip to Sergiyev Posad, home of the Troitse-Sergiyeva Lavra monatery — the spiritual center of the Russian Orthodox Church. Fascinating — especially the private tour of their collection of icons.

And if you’re really curious, you can check out my photos of the Dubna & Sergei Posad parts of my journey on my flickr page. I also spent a day in Moscow on either end, and that was amazing as well — more on that on my personal page.

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