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Zoe Louise Matthews | ASY-EOS | UK

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History in the making

Tuesday, December 15th, 2009

The past few days have been very successful for the LHC. Not only has it collided protons at 2.36 TeV (similar to TeVatron energies) for quite some time, but at 900 GeV the experiments have now gathered a large number of events – ALICE has seen over a quarter of a million, and with this much data the physics is starting to take shape. Data-taking will continue in the new year, and that’s when the really exciting stuff will come, but now, a few days from CERN’s winter shutdown, it is clear that not a second has been wasted.

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The prestigious "Castillo Ygay" wine: 1985 (left) and 1991 (right)

A few nights ago, to celebrate the historical achievement of LHC collisions, the ALICE Collaboration had a very joyous Christmas meal, but on the same night my supervisor, Orlando Villalobos Baillie, decided instead to gather the Birmingham ALICE group at CERN for a more intimate gathering. To explain, I need to take the story back a while.

Almost exactly 24 and a third years ago in Solihull, England, I decided to pop into the world, about 8 weeks earlier than intended. At around the same time, in Logroño, Rioja, Spain, a wine bottle that was roughly twice my size and almost exactly my weight was being filled with “Castillo Ygay”. This bottle was bought by Orlando in a London Airport in 1990, and tucked safely away in his cellar. At this time I was probably learning some interesting words to spell with my Grandfather. Years later, another bottle of “Castillo Ygay” was being brought into the world, at around the same time that I was carving “Zoe 1991” into some wet cement with a stick while my Dad was laying slabs in the garden.

In Spring 1993, the ALICE Letter of Intent was written, and inside it was a small passage about a trigger. This is how the Central Trigger Processor started life, and the Birmingham group began working on the project, writing a more detailed (and remarkably close to the modern day) design overview in the 1995 ALICE Technical Proposal, which was published exactly 14 years ago today. This was two years before my brother Connor was born.

In 1999, the second bottle of “Castillo Ygay” was bought by Orlando in Frankfurt and taken as a gift to Anton Jusko of Kosice’s ALICE group, to celebrate their joining the CTP project. Early the following year, CERN announced “compelling evidence” for a new state of matter in Pb-Pb collisions at the SPS. This was the year I first found out about CERN, and was one of only a handful of students in my school deciding to do Physics at A Level the following year. By the end of Sixth Form in 2003, there were only two of us taking the exams. I visited CERN a few times as a University of Birmingham undergraduate, and got to see ALICE close up, from the Silicon Pixel Detector in its clean room before it had been installed, to the huge solenoid and dipole magnets. In 2006 the CTP was finally built and moved to CERN. This was the year I wrote a critique on the findings of RHIC and the possibilities of ALICE, learning in more detail about the quark-gluon plasma and colour-deconfinement. It was also the year I worked with BABAR and learned about CP Violation, and decided that research was definitely for me.

In summer 2007 the CTP was moved down into the pit, just under the Muon Arm of the ALICE detector. It was a few months later that I joined the ALICE Collaboration as a Birmingham PhD student. The following summer, when the progress of the LHC had been so dramatic it seemed collisions could happen any day, I moved out to CERN to begin working with the “first physics” group. Orlando handed me a bag with a heavily bubble-wrapped bottle inside and asked me to move it to France and look after it. “To celebrate first collisions”, he said, mentioning that the wine was as old as me. Of course, 2008 brought great developments for the LHC but it also brought the terrible accident we know so well. Collisions never happened, and the wine had sat in my fridge on its side for a year and a half…until finally, this year, the LHC at last began colliding protons, and the CTP was finally selecting events.  It was time to celebrate.

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The Birmingham ALICE Group at CERN (missing a few on shift or abroad)

So that pretty much brings us up to speed. A few nights ago, the Birmingham ALICE group gathered to sip mature wine (surprisingly and lingeringly delicious!) and ponder the history gone by and yet to come. Some colleagues prepared Italian and Slovak food (more goulash than we could finish, even in the coming days!), I brought a home-baked chocolate cake, as is my own tradition. It was a wonderful night, and it inspired one member of the group to cook Mexican food for us tonight. I look forward to it, because I am moving out of my flat on Thursday, back to the UK, so I am otherwise currently surviving on the leftovers in my freezer!

The night got me thinking about how life is just a series of interactions, and they are so fleeting but they mean more than we often realise. (This is much like a high energy collision but I really don’t want to make that pun! The “strong couplings with theorists” incident was bad enough!) I have been offered the opportunity to take part in a project next year (as has another QD blogger) called the3six5 project, in which I will be one of many making a small contribution to a sort of year long diary from the perspective of many. As an LHC physicist, I can appreciate that to be a small part of something huge, and to be able to see the big picture and where you fit into it, is a fantastic feeling. Not only that, but you really can’t tell where life is going to take you and that’s quite exciting. By the time I write my contribution, in August 2010, I will be planning the next steps of my career. I think it will be a really interesting blog to follow. Keep your eye out for it.

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Goulash - yum!

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“Day Of Colour” – my idea for the next subatomic particle action movie

Sunday, December 6th, 2009

This is a response to David’s post after seeing Angels and Demons and the recent 2012 movie.  What do you reckon to this?

A moving epic drama – “Day of colour: the chosen ions”. One quark family’s struggle to free themselves from the bindings of the strong force. A whole race of people(particles) imprisoned for all eternity, flashbacks to the dawn of time when they were scooped up from their soup and dragged into drab, colourless prisons. The quarks’ hopeless attempts to get out only lead to their bringing more and more children quarks/antiquarks into this desperate world. (Many of these children, called the lost ones, make suicide pacts and evaporate from the world forever). Two communities, for the first time in billions of years, are thrust into the throws of sudden freedom – they are the chosen ions, something much bigger has selected them to experience life as it once was. But their freedom is shortlived. It ends tragically, as they are dragged from their loved ones in this beautiful noise before being, all too soon, pulled back into colourless prisons, never to see their loved ones again. Philosophical postulation about the senselessness of their life. Lives are lost, passion and tragedy and excitement, gore and action, explosions – the story has everything. I claim all rights to this movie 😉

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Acronyms, Achievements and Accidents

Sunday, December 6th, 2009

Another day, another first for the ALICE detector! This morning, we began taking data from 900 GeV proton collisions again, with not just a handful but ALL of its detectors! This is an incredible achievement for ALICE, and once again the champagne was out. This time we have thousands of events. At some point, we will have to stop marking the “firsts” with champagne, or we will never get any work done! 😉

The progress at the moment is also marking out several “firsts” for the LHC, as more bunches are added to the beam and more protons are added to each bunch, taking us closer to optimal running conditions. There is still a long way to go before 14 TeV collisions every 25 ns of course, but the speed that these developments are coming along is impressive!

For ALICE newbies, just learning about the various detectors can seem daunting. When I first joined the group years ago I can remember being very confused for a while! To give you a feel for how many acronyms float around in a typical ALICE discussion, here’s a quick summary (I will post about their physics another time!):

“Central barrel” detectors (these collectively surround the beam pipe around the collision point, and are inside our large red 0.5 T solenoid). These are:

SPD – Silicon Pixel Detector,

SSD – Silicon Strip Detector,

SDD – Silicon Drift Detector;

These three detectors have two layers each and make up our Inner Tracking System (ITS), providing information about the path of particles passing through them.

TPC – Time Projection Chamber;

Our outer tracking detector – a huge gas filled chamber that helps us to identify particles by mapping their trajectory in three dimensions.

TRD – Transition Radiation Detector,

TOF – Time of Flight,

EMCAL – ElectroMagnetic CALorimeter,

PHOS – PHOton Spectrometer,

HMPID – High Momentum Particle IDentification;

These detectors work in various ways using various new technologies to identify a variety of particles with a range of momenta. I will detail them later, but collectively, ALICE has probably the most powerful particle identification in the world!

ACORDE – ALICE’s COsmic Ray DEtector;

This guy is pretty separate from the rest – it is a collection of scintillator detectors and it sits on top of the L3 magnet and looks for cosmic rays. It isn’t designed for collision measurements, but it has helped the detector alot over the past few years!

“Forward” detectors (Put simply, detectors placed such that they measure particles coming off more horizontally from the collision. They would be missed by the central barrel). These are:

FMD – Forward Multiplicity Detector,

PMD -Photon Multiplicity Detector,

Vo detectors,

To detectors,

ZDC – Zero Degree Calorimeters.

These provide various important information including timing to identify background events, extra triggers for event selection, and the particle density in this “forward” region (which is rather uncertain at new energies!)

“The ARM”. The muon arm is placed behind the “barrel”, and consists of the largest warm dipole magnet in the world, designed to bend the trajectory of “forward” muons that come from a collision, a thick absorber designed to block anything that isn’t muons, and these detectors:

MUON TRG – Muon TRiGgers,

MUON TRK – Muon TRacKers.

By selecting events when muons pass through the triggers and tracking their trajectory through the dipole magnet with the 5 tracking points, ALICE can spot particles decaying to muons (like the J/Psi).

Multiply that lot of detectors by the challenge each sub-group faces in ensuring their detector is functioning correctly on time for data-taking, and you can see how much of an achievement today is for ALICE.

Just after this great achievement however, the LHC beam was dumped. I have it from my insider knowledge (that could just be rumour so don’t take this as Gospel) that the LHC operator saw that some screens had crashed so they rebooted the system…which simultanously triggered the dump. Oops! As silly as this sounds, it is a fantastic safety feature – I find it reassuring that if for any reason control is lost, the beam will be safely dumped within a few turns of a near-light-speed proton  🙂

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The First Physics Paper using LHC Collisions: ALICE has its first paper accepted

Wednesday, December 2nd, 2009

I am back on shift again, and after a few minor tests, hiccups and changes, some ALICE detectors are now taking data with LHC beam dumps and all is relatively quiet, so I finally have 5 minutes here to bring you some news.

A few days ago, the first physics paper to use LHC collision data – an ALICE study using 284 events from the LHC’s first 900 GeV collisions of 23rd November – was approved by CERN, and only yesterday (1st December) it was accepted by European Physics Journal C. Because of this, I feel confident telling you about it at last 🙂 The paper, which can be found here, describes the distribution of particles in the collisions, and was submitted on 28th November – just days after the collisions took place. I was lucky enough to be working in the group that deals with ALICE’s early studies, so for the past few weeks  my colleagues and I have been very busy!

The story began on 23rd November, when our Silicon Pixel Detector (SPD), a cylindrical detector surrounding the beam pipe,  selected the collision events (looking for hits in its two layers indicating particles, whenever bunches of protons were passing through our detector from both sides). Once an event was triggered, the Central Trigger Processor (CTP) instructed the SPD, and the other detectors in our central “Inner Tracking System” (ITS), the Silicon “Drift” and “Strip” detectors, to record what they saw.

Also observing these events were our two “V0” detectors – scintillator counters placed further along the beam (what we call “forward” as apposed to “central”), either side of the ITS. The timing of the signals from these detectors could determine whether the event they came from was a collision within the ITS or an interaction with gas further down the beam pipe.

The High Level Trigger (HLT) system used information from the detectors to reconstruct the particles’ paths and locate the “vertex” or interaction point of the events, which, for such preliminary collisions, was surprisingly centered. This helped us get rid of the last of the background, including one or two cosmic ray events that had timed themselves perfectly with the proton bunch crossings! Finally, after careful analysis, and thanks to injection tests and cosmic ray data taking leading up to collisions that helped us to better understand our detectors, the “pseudorapidity density of charged primary particles in the central region” was measured.

So what does this mean? Roughly speaking, a pseudorapidity distribution is the angular distribution of particles in collisions (angular with respect to the beam, that is). In this picture, pseudorapidity density is a measure of the number of particles in a given “angular” range – in this case, looking in the very central region – particles coming out at right angles to the beam, straight into the SPD. Later, when more of our detectors are available (in particular, our three Forward Multiplicity Detectors, which again are positioned on either side of the ITS, further along the beam pipe) and we have more statistics, we will be able to see this picture more clearly – how particles distribute themselves in more “forward” regions.

We will also be able to make more interesting physics observations as time goes on – with our Time Projection Chamber and magnetic field, and eventually many more detectors, we will be able to reconstruct a large number of tracks, including particle decays, in collisions of even higher energy, and identify the particles causing the tracks in the first place. For now though, it is baby steps, and to see simply that this result is consistent compared with p-antip collisions of the same energy is a nice cross-check that the world still makes sense.

The atmosphere has really changed over the past month, and not just for ALICE. People are more on edge. Coffee is being consumed at a higher rate and the jumpiness it causes is evident. Sleep feels like a weakness. Tiny, insignificant details seem like potential disasters. Big decisions are needed at short notice. As a result, big changes to plan are occurring at short notice.  People are prone to panic. (As a person who has always been somewhat prone to panic, I have found the experience strangely helpful – I am, for the first time, finding myself able to put on a calm face to myself as well as to others.) Competition between rival experiments is rearing its head. (Expression ranging from jovial to bitter. With the healthy middle ground of constructive.) It has all become very real. The fact that collisions are scheduled in the next week doesn’t seem like a crazy dream. People are talking about “data”. Tiny, insignificant steps are breaking brand new ground. Meetings have become twice, even thrice daily. (Significant updates are still reported in each one.) Poorly timed obstacles such as having children to look after, being stranded in another country with failing communication technology, having been awake for days on end or being confined to a hospital bed with no visiting rights are no longer enough to hold people back. Seeing what can be achieved during this exciting, stressful time has really impressed me. I hope this head-bursting, stomach-flipping, maddening and satisfying atmosphere doesn’t fade too soon, because we are thriving on it.

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What’s next for the LHC?

Monday, November 23rd, 2009

So, after 20 years of hard work, the LHC is finally a Large Hadron Collider, officially. The experiments will all get a fair few more of these lovely events before the year is out, to give us a first look at our detectors’ potential. So what comes next, after this momentous threshold has been passed? Of course, the incredible machine is capable of much more powerful things. 900 GeV collisions are relatively puny compared with where it is headed.

Now, there’s the testing challenge of repeating what was done last year – the careful raising of acceleration, the “ramping up” of the supercool magnets, to keep much much faster protons contained in the beam and under control. A lot of work is still to be done before we all go home for Christmas. If things go well, we may have collisions at over a TeV.

The next big step for the LHC will be to collide protons at energies never before seen. This will really make history, and those of us lucky enough to work on the experiments, the simulation, or on the theoretical side, will have the opportunity to analyse and interpret the absolutely brand new. Unseen by the human eye.You can’t get more exciting than that!

As the months go by, the LHC will edge closer and closer to its full potential. The ultimate goal, of course, is 14 TeV collisions. With energy this high, the accelerator will be pushed to its limits to show us an area of the world that challenges our current understanding. Science as we know it will be rewritten with those collisions.

And as if that wasn’t enough, the LHC’s progress will eventually continue, to make the same gradual push into the unknown with collisions of Lead ions. The behavior of matter at trillions of degrees, as far as we can tell so far, is not only incredibly difficult to produce, but highly unusual to say the least. Because our universe began as a small blob of the molten, broken-down soup we call “quark-gluon plasma”, we feel it is our duty as scientists to properly unravel the mysteries of this exciting phase transition. The LHC will take us, for an unfathomably short length of time, much hotter and much closer to the conditions of the early “soupy” universe than ever before. With the ALICE detector’s powerful particle identification, we should be able to make impressive sense of such a brief glimpse at the earliest point in our universe’s history.

The LHC has made history already, but over the coming years, it will help scientists learn so much about our world, and the baffling puzzles in it. The brilliantly powerful “Standard Model” of fundamental particles and forces, as it currently stands, may have some of the few missing puzzle pieces fitted, or perhaps illuminate some ill-placed ones…in any case, we will have a better clue to the whole picture – no matter what it finds, the LHC will change our understanding of the universe forever.

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Pre Christmas LHC – ALICE Night Shifts – 3…LHC HAS COLLIDED PROTONS!!!

Monday, November 23rd, 2009

OK, so my last update was a little vague because we weren’t allowed to make an announcement before CERN. However, I am now allowed to say that yes, LHC collided protons this evening ! 😀

ALICE caught some of these events, some time around 5pm, and they were reconstructed and put up on the big screen in the control room – after the rising noise of discussions from the sardine-like density of people, as soon as the first collision was seen, a silence fell. Followed by a united awe-struck gasp, followed by lots and lots of cheering. Then the Champagne was popped and the tense atmosphere turned to one of celebration. Every single face was smiling intensely. Even me, as I managed, after a night shift and 25 hours of being awake, to stick around long enough to watch it.

This is a fantastic day for everyone – a historical moment we are all glad we caught. There will be a gathering of photos and as soon as they are on the web I will post the link.

Now, however, after my brief sleep, it is time to return to the control room for a third shift. I can’t wait!

UPDATE, 0:20am:

ALICE has already put some videos on youtube – the visualisation of the first event:

http://www.youtube.com/watch?v=UlhV7LFjloQ

and a video just after it, showing the atmosphere of the control room:

http://www.youtube.com/watch?v=ZOJvgatf2VY

I’m the girl with the red cardigan near the back!

Now on with the usual shift leader business 🙂 Things are a little quieter here tonight, and luckily the coffee machine is working again so I think I’ll survive!

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First Collisions – keep your eyes peeled!

Monday, November 23rd, 2009

First collisions in the LHC are looking very very imminent. If you want constant updates keep your eye on the comments of this blog!

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Pre Christmas LHC – ALICE Night Shifts: 2

Monday, November 23rd, 2009

Tonight has seen further achievement for the LHC, and the anticlockwise beam seems to have been their main focus through the past 24 hours. Our Silicon Pixel detector triggered on a few beam-gas events from it, and there have been a few important timing measurements made for ALICE. Mostly though, we are waiting with baited breath for collisions. Which, looking at the incredible progress, could come very soon indeed…a Champagne bottle is waiting in the coffee room. We are all very excited! Watch this space…

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Pre Christmas LHC – ALICE night shifts: 1

Saturday, November 21st, 2009

I’ve been here since 23.45 and everything is running smoothly – we have had protons passing through our detector, clockwise round the LHC ring, more than 11,000 times a second. We are finding, usefully, that there are not very many beam-gas interactions (the events where protons collide with gas particles in the beam pipe). This shows us that the vacuum in the beam pipe is very good – not very much in there to interact with! Once we are colliding, any beam-gas events that we get while waiting for a proton-proton interaction will have to be thrown away, so it is positive that this background is low. However, the few signals we are receiving are being used as we speak to check, for example, timing between the different triggering detectors (the detectors whose signals will tell us whether or not to keep an event). As the Central Trigger Processor shifter, my job for the moment is to continue monitoring, making sure detectors are taking data normally. I am impressed with the number of people here at 2am with fresh and excited faces, working hard and feeling positive.

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Beam circulation and beyond: pre-Christmas LHC activities

Friday, November 20th, 2009

Hi!

You will have seen various blogs tonight (20th Nov 2009) celebrating the first circulation of the proton beams this year. It is a great achievement for the LHC, and a very exciting time for all of us. I am one of the lucky people here at CERN to witness history unfolding and it has to be the best Christmas present of my life 🙂 As of tomorrow midnight (21st – 22nd Nov) I am on shift for 6 days 0:00-8:00 and will be posting once a night to update you on what’s going on – keep glued to our blogs! I will once again be shift leader, so it will have to be brief and with a few updates through the night, because I will be rather busy! Photos will be uploaded after my 9am meeting, because I can’t upload pictures on shift unfortunately.

From the developments of today, with a bit of luck, all the LHC experiments will be able to make use of what they see while beam passes through their detectors – of course this is not centre of mass collisions yet, but simply protons from the beam colliding with the very low levels of gas in the beam pipe. It is still very exciting! By now, the ALICE detector is used to catching cosmic rays in its detector, often one at a time, so when the SPS beam was being injected into the LHC earlier this year and “dumped” downstream of our detector, and similarly ATLAS had their own beam splash events today (see previous post) it was really something to celebrate – hundreds of thousands of tracks in our detector, much more than we expect from a typical LHC collision, even at high energy, and even with heavy ions!

The reason for this is because the whole beam, with something like 100,000,000,000 protons coming all at once in a “bunch”, is fired into a concrete block, these particles then interacting and showering in the material to lose energy and be absorbed. The term “bunch” always makes me laugh when you consider how many it’s talking about! In a typical colliding of two “bunches” of particles, though, even though the LHC works hard to squeeze them close together (to a few microns across, less than the width of a human hair), it is most likely there will only be a handful of these protons colliding at a time. Now, with beam travelling through the beam-pipe, we get to look at something not too dissimilar from the collisions we have been waiting for – but as you can imagine, there is still a lot of work to do before then. Colliding these beams will take alot of precision! (Think of trying to use magnets to collide two pins point-first…and at nearly the speed of light too!)

And the question on everyone’s lips is “When do we get collisions?” Not least me, as I am eagerly awaiting some data to play with! Unfortunately, we have to be very open to change at a time like this, and I am not about to tell anyone a date because apart from anything else, I don’t have one. 😉 However, I can tell you that I am on shift for the next 6 days, and again for a week at the start of December, and I can mention that it is my guess, and hope, that one of these days sees collisions! If it does, you will be informed 😀

Questions are welcome, and I will answer as soon as I can.

I want to finish on this: a friend of mine is working on the coldest physics in the universe. Often LHC steals that line, on account of its 1.9K superconducting magnets. Now, whilst the ALICE detector, when we collide heavy ions in the next year or so, will be quite possibly the hottest place in the universe since the big bang, (for about 10^-30 of a second)…we can’t claim low temperature. Various supercold labs around the world are beating us (including the Kosice Physical Sciences Lab, see my previous blogs) down to temperatures of the order nK and less. And yet, tonight this friend was nice enough to tell me that, in many ways, I am “one of the coolest people in the world at the moment”. Thanks for that Matt, and I wouldn’t say coolest (I do play dungeons and dragons on occasion, let’s not get carried away!) but I am certainly one of the lucky ones and there’s no denying that.

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