The inverse picobarn threshold has been crossed in ATLAS!

Another milestone has been passed in the long run of ATLAS toward new physics.
On Monday August 9, 2010 ATLAS has recorded the first inverse picobarn (pb^-1) of 7 TeV collisions. The trend is good and we recently reached the 0.1 pb^-1 per day of integrated luminosity (meaning that we can now collect in ~10 days the amount of data we have collected over the last 4 months).

The integrated luminosity delivered by LHC (green) and recorded by ATLAS (yellow)

This pb^-1 threshold has a psychological impact, as we now change units to evaluate our physics reach, and we’ll stay with these units for many months (the 1 fb^-1 (or 1000 pb^-1) threshold is for end-2011 in the current plans).
What is the meaning of “1 inverse picobarn” of data? It means that we have in our data 1 event if the cross-section of the process that gives birth to this event is of 1 picobarn (pb) and we are fully efficient to select it. As the ordinary collisions at 7 TeV have a cross-section of ~10¹¹ pb, it means that we start to “see” events as rare as 1 in 100 billions.

So, just to make one example, we have order of 10⁴ W–>lepton+neutrino and 10³ Z0–>lepton+lepton in the data and few tens of top decays should also show-up (some candidates already exist, see previous post).
The old particles “zoo” (including the heavier partners) all show-up in this first pb^-1 and will be studied in detail also using the other pb^-1‘s soon to come.

More we go into the many-pb^-1 domain, more details of the known physics we can study and more windows to the new physics will open-up.

A challenging and exciting time in front of us!

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ATLAS starting to get on Top of things

ATLAS is about to check one more particle off of its Standard Model (SM) checklist.  Namely the top quark.  This famous quark is perhaps one of the most complex of the SM particles.  Unlike other quarks the top will decay before it hadronizes into a jet (giving a single shower of particles).  The top is too heavy to bother with that.  Instead it will decay to the next heaviest quark (appropriately called the bottom quark) and a W boson.  The W will then decay to two other quarks, or to a charged lepton and a neutrino.  The bottom quark will then hadronize into a jet, but with the distinct displaced vertex (a convergence of tracks to a point that is not coincident with the actual collision point).  The top quark mass is about 173 GeV, while a proton is just under 1 GeV.  This little (but heavy) guy is so complex and interesting that entire armies of physicists are dedicated to looking at this one particle.  It is so young, and so complicated that there is still much to learn from it.  Top, which was discovered at the Fermilab Tevatron in 1995, is the latest of the non-neutrino-like particles of the SM to be discovered. (The tau neutrino was discovered in 2000 at Fermilab by the DONUT collaboration and is the last particle of the SM to be discovered, save the Higgs.)  The top quark is usually produced in pairs (a top anti-top pair) but sometimes only one top quark is produced (this is referred to as single top production).  Either way, the top quark has the smallest production cross-section of all of the SM particles.  The next particle to be found (as we progress in our investigation of smaller and smaller cross-sections) is going to be a new particle.  Could it be the Higgs?  Perhaps a SUSY particle?  Or even something never before dreamed of!

Event display of the e-mu dilepton candidate. The isolated muon track is shown in red, the isolated electron is shown as a green track pointing to a green calorimeter energy cluster. The b-tagged jet is marked as a blue circle in the eta-phi lego plot on the left side of the figure. The direction of the missing transverse energy is shown as a dashed line in the eta-phi lego plot.

On July 23, 2010 the ATLAS collaboration showed its first top quark candidate events to the public at the ICHEP 2010 Conference.  Now a particle of this sophistication cannot be “discovered” in just one event (or even nine, as the ATLAS Collaboration have found).  It took 17 of these distinct events for one of the Tevatron detectors to claim discovery.  Those 17 events took about 50 pb^-1 of data, which took nearly 2 years to collect and analyze.  So ATLAS isn’t going to do anything presumptuous and say it has found the top quark in its data, yet.  ATLAS will however demonstrate to the world that it is seeing very encouraging signs!

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More Data, One Step at a Time

Weekends and nights are a quiet time these days in the control room, but the lack of noise isn’t from a lack of collisions. Last weekend, in a single run, ATLAS collected almost as much data as in the previous months, thanks to a new peak luminosity record of 5 x 10²⁹ cm^-2 s^-1. Every crossing of the beams brought, on average, more than one pair of protons colliding!

During the day, the LHC performs tests and calibrations that allow a gradual increase in the number of collisions per second and continue the fantastic trend of doubling datasets. In the ATLAS control room, experts spend the day also testing, calibrating, improving, breaking, fixing, and tweaking small corners of our humming giant. At night, when the experts sleep and the tweaking pauses, the LHC and ATLAS run smoothly, collecting data, under the watch of dedicated and caffeinated night shifters.

Testing and tuning aren’t the only reasons for collisions to stop, however. Last week, a lightning bolt caused a power fluctuation which caused the beam to dump. A problem with a file server system, not developed by the LHC, also caused a delay.

These problems, however, are small and unimaginative compared to others in the past. The predecessor of the LHC, LEP, which was in the same tunnel, untangled some rather unexpected issues. The water in Lake Geneva compresses the ground in the region. When the water level changes due to seasonal variations, the nearby topology changes. The change is very slight, but the accelerator was so sensitive that even this tiny shift in the position of the magnets had to be accounted for.

The position of the moon and its gravitational field also had an effect on the energy of the LEP beams. Not all mysteries had natural origins, however. For months, LEP observed noise in their system that they couldn’t trace. Only when the French railway workers went on strike and the noise disappeared did they realize that the passing TGV was inducing unwanted current in a vacuum chamber. And finally, more locally, some difficulty in getting the beams to circulate more than 15 times was traced to a single quadrupole magnet. When the magnet was examined closely, two empty beer bottles were found actually inside the accelerator.

The LHC, at least so far, has had only one (at least, of which I am aware) bizarre externally-induced problem: the notorious day it was brought down for a short while by a bird with a baguette.

These sorts of problems might make the accelerator seem delicate. It is, after all, more than 26 km of some of the world’s most precise technology. Pushing the forefront of engineering and technology — the fastest humans have ever made something go, the smallest scales we have ever seen — requires building something crazily large and crazily sensitive. But its fragility is strengthened by adaptability, and by a team of experts working round the clock to solve problems bizarre and routine alike. As problems arise, solutions are found. It was not designed to be started one day with a big button and left running unobserved for years; unexpected problems are expected. As long as the LHC is kept safe from its own stored energy — and that is what many of the current tests are ensuring — baguettes and lightning bolts may cause a few minutes of excitement in the ATLAS control room and a period of hard work in the LHC one, but, after a pause, the collisions will continue.

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Sleepless Nights Lead to First Results of 2010 …

Do you hear that?  The incessant typing? The coffee machines vending cup after cup? If you go to Building 40, or Building 32, Building 188, or to any one of the many graduate student offices around the world, you will hear the tap of key boards, the whir of disk drives, and even the occasional heated civil discussions with “elevated” voices.

What is all of the work for? Well, in High Energy Physics (HEP) there are “seasons” of conferences, and this year the first conferences to see the first LHC physics results of 2010 will be the summer conferences.  The first big one being a conference called Physics at LHC (PLHC). The conference is being held at Hamburg Germany and physicists from all of the LHC experiments are working around the clock (and around the world) to look at the latest data and understand it before it gets presented at the conference.

Everyone is working to check, double check and even quadruple check their results. It will be the first time that results (https://twiki.cern.ch/twiki/bin/view/Atlas/AtlasResults) from the data taken this year will be shown to the public, and there are bunches of them!  The results are based upon the modest data sample that we have collected so far since March, (modest compared to what we will have in the next few months), however they will demonstrate that we are well on our way to understanding our detectors and understanding Standard Model physics at the new energy frontier of the LHC. Basically, it will be another set of exciting Firsts for ATLAS and the LHC and a demonstration that the LHC is progressing towards an understanding of the unknown.

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Conversations on Shift

When the detector is running smoothly, neighbors in the ATLAS control room sometimes get conversational. A few days back I was on shift, quietly looking at plots on the monitor in front of me, trying to decide if one small sensor was misbehaving or not. “I have a question,” the shifter next to me said. I thought he was going to ask me why the temperature was high in one place in the system. Or, maybe, in his lyrical northern Italian intonation, offer me a coffee.

“Do you believe in supersymmetry?”

It is a difficult question. More surprisingly, it is one not often asked so directly by physicists, to physicists. I don’t have a ready answer to this question, so I paused before responding, trying to disentangle in my mind the difference between my admiration for the theory’s elegance and my uncertainty over its plausibility.

Before I could start answering, he continued, “because I don’t.” “Really?” I returned, trying to decide if I was surprised or not. “Really. I know most of the theorists believe in it, and they’re really smart… so I must be stupid, but I don’t believe in it.”

But the thing is – believing in supersymmetry, or believing in any new theory we are chasing at the LHC, has little to do with what we usually define as intelligence. It takes intelligence to invent a new theory that fits the current puzzle without introducing bigger problems. It takes intelligence – or, perhaps, stubbornness – to understand the new theories and how they fit into the existing model of particle physics. But believing in one theory or another, before any of them have been tested, touches much more the intuitive side than the logical one.

Given a handful of theories, all of which are consistent with the experiments we’ve done in the last hundred years, only the experiment we’re currently running can separate the abstractly possible from the true. Until we have enough data to test the predictions each theory has made, the reasons people often cite for liking a particular theory are more often artistic than scientific. “It is beautiful,” people will say, or, “it’s the simplest way to fix the problems we have with the current model.” Which may be true. But the laws of the universe don’t have to match up with our intelligence or our idea of simplicity.

Then again, they might. To me, that is perhaps the most incredible thing. I’m too young to have paid attention to any major discoveries in particle physics while they were happening, and the major components of the standard model were in place before I began to learn them. The idea that we might discover something new here is amazing. The idea that we might discover something that we have actually predicted, with crazy mathematical arguments and complete leaps of faith, is totally astounding. And rather awesome.

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A new record run

In the evening of Saturday May 15, we have reached a new peak luminosity record of 6 10²⁸ cm^-2s^-1
This is about twice the peak luminosity of the last week-end and has been obtained colliding 3 bunches of 2 10¹⁰ protons in ATLAS. It proves that LHC is steadily progressing as expected.

Overnight ~3 nb^-1 of integrated luminosity have been recorded (and more is just coming…) with the usual high efficiency of the ATLAS data acquisition system.

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Putting the Squeeze on the Protons

It took a little bit of time, but the wait was worth it.  The LHC has successfully achieved its first physics run with “squeezed beams”!  An LHC proton beam consists of packets of protons regularly spaced along the accelerator’s ring.  Each packet of protons is referred to as a bunch.  When a bunch from beam 1 cross a bunch from beam 2, then collisions are possible.  Each packet can contain as much as 10¹⁰ protons (or more).  That’s 10 billion protons.  In that one bunch the density of the protons can be relatively low.  So when a bunch from beam 1 passes a bunch from beam 2 there may not be that many collisions, or not any at all!  Its kind of like a marching band performing during half time.  You see one group of the marching band marching in one direction and another group marching in another direction and it likes the two groups will collide but they are spaced out enough such that no one runs into each other.  So to increase the likelihood of getting a collision, the LHC squeezes the bunches.  This makes the proton density greater.  So instead of well spaced marching band members it is more like getting from one class to another during passing time at a crowded high school.  The likelihood of running into someone is high.

The current LHC proton store that is colliding now with squeezed beams has been colliding now for more than 24 hours!  This single store has single handedly doubled the integrated luminosity for ATLAS!  The instantaneous luminosity has increased by a factor of 10!  This is basically having all of our data set from the previous month delivered in one day!

The maximum instantaneous luminosity by day for ATLAS physics runs since March 29. As you can see there is an order of magnitude increase in the instaneous luminosity from the last few days. This large increase is due to the squeezed beems.

Now next week the LHC will have a planned technical stop to do some minor maintenance and tweak some things.  They also plan to do some finishing touches on the machine that will allow them to increase the amount of protons they can put in the machine.  So we expect in a matter of a few weeks we’ll have more factors of ten increases and more doubling of data sets!

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One in a Few Millions

ATLAS has been designed to detect rare events in high energy proton-proton collisions (see the last Post “It’s all about the Lumi!”, by Jim).
ATLAS ultimate goal is to measure events as rare as one in several thousand billions, but we are modest (for the time being) waiting for the luminosity to rise.
While over the last weeks priority has gone to LHC tuning in view of the luminosity increase, still some data have been taken in stable conditions and we have reached the sensitivity to see the production of W bosons (the charged mediator of the electroweak force).
The predicted production rate of this particle is shown in the previous post (the sigma_W curve) indicating that one W is produced every few million ordinary collisions at 7 TeV.
As the W is quite massive (~80 times heavier than the proton) and decays instantly in energetic particles it has a very peculiar signature which makes it pop-up out of the background of the ordinary collisions. This is particularly true when the two decay products are leptons. As the W is charged, one lepton should be charged (an electron, a muon or a tau) while the other should be neutral (a neutrino). An event where a W is produced should, for instance, contain a very energetic muon and a large missing energy recoiling against the muon. The neutrino can indeed travel kilometres in rock without interacting, it therefore “brings out of ATLAS” energy which can only be measured indirectly (as energy unbalance in the collision event or “missing energy”).
This pattern (an energetic muon and a missing energy recoiling against it) is exactly what has been seen in the event shown below, which is the first of few candidates W recorded by ATLAS.
The era of the “high mass objects” in ATLAS has started!

First W candidate observed in ATLAS. It decays to a muon and a neutrino.

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Its All About The Lumi!

Now that the LHC has established colliding stable beams at a center of mass energy of 7 TeV, the next step to maximize its physics reach is to provide the most luminosity possible.  As Leo posted, http://pdg2.lbl.gov/atlasblog/?p=329, we need to increase the number of proton – proton collisions to make sure we have a chance of seeing the physics that we are looking for.  The reason for that is because different physics processes have different probabilities.  These probabilities are referred to as cross-sections (in a vague reference to the particle’s size).  If one multiplies a cross section by a luminosity than what you get is a number of events.

Cross Sections for Specific Physics Processes, from the ATLAS TDR (2003). The dotted lines show the energies of two hadron collider (The Tevatron at 1.96 TeV, and The LHC at 14 TeV). We'll have to wait a little longer for the 14 TeV. So just move that line over from 14 TeV to 7 TeV for now.

Luminosity (or Lumi for those in the know) at particle colliders is usually cited in two different ways depending on the context.  The first is in units of inverse barns. This is the integrated luminosity or the luminosity collected over an extended period of time. What the LHC has provided so far (after barely a week, actually they JUST finished one of their longest fills so far … more than 20 consecutive hours of stable beam collisions!) is on the order of hundred inverse micro-barns.  This means that processes charcterized by production cross-section of ~10 nano-barn (nb) start to become accessible (see the left scale of the figure above to evaluate which processes we are speaking of).

Another way of discussing luminosity is in instantaneous luminosity.  This is in units of interactions per square cm per second (cm^-2s^-1) which is the units on the right side of the plot.  So that plot shows the number of specific physics events that occur every second, assuming that the instantaneous luminosity is 10³⁴cm^-2s^-1, the LHC design luminosity. Right now we are seeing something more like 10²⁷cm^-2s^-1.

But don’t worry.  The LHC is still brand spanking new and they are still trying to understand the nuances of the machine.  Soon enough they will be making leaps and bounds in the luminosity (by leaps and bounds I mean many orders of magnitude).

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Life Imitating Reality

I was home sick today, probably from the stress of getting ready for “M-Day” (aka Media Day), more likely though I finally succumbed to the cold that had been spreading through the Control Room.  As it so happened, my laptop had been in the shop because it experience an “incident”  (actually I just dropped it) last Monday (the week before Media Day), and I just picked it up yesterday. 

 I couldn’t refrain from thinking of my laptop as an analogy for the LHC.  The laptop though, is much cheaper than the LHC and can be replaced.  Non-the less for something that is so common these days they still break down, or accident’s happen.  Well the LHC is a one of a kind machine.  If the LHC “breaks” or has an incident, it cannot be replaced.  We can go back to using the Tevatron, but that is like replacing your laptop with an Apple IIe  (ok maybe not THAT drastic, but close).   So even after my laptop was returned to my possession I started to run the software update on it, and then it stopped working again!  I just managed to get it back up and running, but it isn’t at its optimal performance by no means.  So the next time you hear someone complain about the LHC, just remind them that it is the cutting edge of technology and if we can’t expect all of our common technological tools to all operate perfectly, we shouldn’t expect something as advanced and complicated as the LHC to operate any better.

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