Strangeness at LHCb

New results were published from LHCb last week which will help physicists to simulate proton-proton collisions at the Large Hadron Collider.  Members of the Imperial HEP group have measured two ratios of strange particles which give clues about how hadrons are produced.

The huge energy of LHC collisions allow physicists to look deep inside the protons to see interactions between the constituent quarks and gluons.  Good predictions can be made for these high energy interactions using the theory of Quantum Chromodynamics (QCD).

Sometimes, the interactions of partons can produce "resonances", heavy particles like Z bosons which can decay to produce a shower of quarks and gluons: 

Illustration of an LHC proton-proton collision.

These many-particle events are extremely hard to predict because of a surprising property of QCD: low energy interactions occur with more strength.  If gravity behaved like this, you could make yourself heavier by moving more slowly, or lighter by running very fast -- sort of like a fat couch potato compared with a trim Olympic athlete.

As more and more quarks and gluons are produced their share of the available energy becomes less and less and the interactions get stronger and stronger until the quarks become "trapped" in groups of two (called mesons) or three (called baryons), just like the partons originally inside the colliding protons.  This process is called hadronisation.

Hadronisation involves so many interactions that we cannot use QCD theory to predict what will happen.  Instead we use approximate models:

“The predictions of the model are reasonable enough physically that we expect it may be close enough to reality to be useful in designing future experiments and to serve as a reasonable approximation to compare to data. We do not think of the model as a sound physical theory . . . ” – Richard Feynman and Rick Field, 1978

A popular model connects up all the partons with a "string" which snaps to produce mesons and baryons:

Hadronisation of a parton shower.

These models need to be tested against real experimental results.  LHCb's strange particle results are useful because the strange quarks sit in a Goldilocks zone where they are light enough to be produced by the hadronisation process and yet do not provide a net contribution to the structure of the colliding protons.

The first ratio anti-Λ/Ks compares how often strange quarks end up in groups of 3 (the anti-Λ baryon) or in groups of 2 (the Ks meson):

This ratio is much higher in data than predicted by hadronisation models, so the models must be underestimating how often strange quarks group into 3s. And this underestimate gets worse with higher particle momentum (perpendicular to the proton beams).

The second ratio anti-Λ/Λ, compares how many times anti-strange quarks group in 3s compared to strange quarks.  Protons are made of quarks, not anti-quarks (really less anti-quarks), so it should be easier to make Λ than anti-Λ.  This behaviour changes with the angle to the proton beam, or the "rapidity" -- think of large rapidity as a small angle to the proton beam and small rapidity as a large angle.

LHCb is unique amount the LHC experiments with a view of the high rapidity (small angle) region.  The anti-baryon/baryon ratio shows a significant change in behaviour across this region. At small rapidity data matches models which have already been validated at the Tevatron but at high rapidity the best match is PerugiaNOCR, a model with localised hadronisation, which uses shorter strings that don't connect all the partons together.

These results will be of great use to future developments of hadronisation models.  It is very important to have accurate predictions at the LHC in order to test the Standard Model and search for new physics.
If you want to read more, you can get a copy of the paper for free.  You may know that this is not generally the case for scientific publications. CERN has made special arrangements for all LHC results to be made freely available to the general public, in line with the spirit of its founding charter:

“The Organization shall provide for collaboration among European States in nuclear research of a pure scientific and fundamental character, and in research essentially related thereto. The Organization shall have no concern with work for military requirements and the results of its experimental and theoretical work shall be published or otherwise made generally available.” – Convention for the establishment of a European organization for nuclear research, Article II, Section 1, Paris, 1 July 1953

Imperial High Energy Physics Group Open Day

Happy New Year!

We will be holding a Group Visit day on the 19th of January, for anyone who is considering joining our group as a PhD student. If you would like to come and talk to us, please email my colleague, Dr David Colling!

We're all going on... LTA

Since the end of our coursework and final presentations there has been a steady stream of my fellow first year students leaving for exotic places and now it's my turn - off to CERN and Geneva on LTA.

An LTA (Long Term Attachment) is intended to get the students closer to our experiments as we try to make the transition from being clueless to useful in our adoptive teams - hopefully.

I joined LHCb and have already found it very useful being out here. CERN is the hub of activity for the (close to 60) institutions working on the project, whether based here or visiting, and I've had the oportunity to learn a lot in just a few short weeks.

Yesterday, I went underground to visit the experiment for the first, and maybe the last, time as it will be closed at the end of this week in preparation for the start-up of the collider. Many people have invested a lot of years and effort in the project and it was very impressive to see in all it's detail - even if it was difficult to reconcile the simplified diagrams that I knew with the mass of metal supports and cabling I saw.

As a UK-funded student, I'm living in a studio apartment arranged for me by STFC. It's in Geneva, close to the Old Town and about an hour's commute to the office at CERN. I recently got married and my wife has moved out here with me. She's finding plenty to do in the city, enjoying the relaxed atmosphere, shopping in outdoor markets and getting to grips with the local language.

French isn't essential here, with Geneva's large ex-pat community and English the common languge at CERN, but it helps to get the most out of what seems, so far, to be an interesting place to live.

Ask me... I'm an expert

Remember the posts about the Big Bang exhibition at the Science Museum? Well, if you haven't seen it yet hurry up. It's on until the 7th of October, after which it will be replaced by [not allowed to tell ;-) ].

To add a little more action to the displays the Science Museum set up a little interactive event last week where they would show some kit and have a couple of scientists explaining it. The plan was to have a cosmic ray detector and some Geiger counters. We provided the latter but they were quite useless in the end as nobody was able to bring anything even remotely radioactive to make them tick. So we concentrated on cosmic rays, which was not a bad idea anyway. Our colleagues from Bristol provided a scintillator and a small spark chamber, definitively the star of the show. It's much smaller than the huge one Imperial and RAL built long ago and we could not get to work again. At least the Bristol chamber worked nicely. Don't know if there's a lesson here...

It was a lot of fun explaining cosmic muons to the general public (although the general public found at the Science Museum is a quite biased sample) and answering the most unexpected questions. How do we know they are muons? Where do the cosmic rays come from? How does it work? Could they have triggered life? What's this black thing? (the trigger). Wouldn't the muons have all decayed before reaching the ground? (This guy knew more than expected). How do you get a satellite on orbit? Hmmm, somewhat unrelated, but I had a tag saying "asking me... I'm an expert" so I just had to know.

Many thanks to the Bristol guys David, Clare, Ben and John for providing the nice kit and to Will and Tom for representing Imperial.

Finally I agree with Gavin: we should try to build a similar chamber and display it on level 5.

Rector supports blog

Last Friday Imperial's rector Sir Richard Sykes visited CERN - in particular the two experiments with IC involvement CMS and LHCb.

The visit was supervised by CERN's VIP service, a guarantee that nothing can go wrong: 9:00 pick up at the hotel, 9:25 arrive at LHCb, visit. 9:45 go to CMS, photographer is waiting... and so on until the signature of the guest book at 14:15 (involving another photographer). A quite impressive organization that has not much in common with the usual "private" visits of LHCb or CMS.

So early in the morning Jim and the rector met us (Tatsuya, our spokesman, Will and myself) at point 8 for a visit of the LHCb detector. Tatsuya showed every interesting detail and let us enter any usually forbidden door. A quite interesting tour, even for LHCb members!

Of course we had a long stop in front of RICH1 where I tried to explain how it works (remember Cherenkov radiation?) and why we need such a device and CMS don't. I also took the opportunity to ask him if I could make a picture for the blog. That's when he said it was a great idea. One needs to be modern...

After a brief visit of the LHC tunnel we left Tatsuya and Will and continued to point 5 for a CMS visit. Since I hadn't seen CMS for quite some time (especially not since there's something in the cavern) I joined the CMS tour - which was really impressive. Not only because of CMS, but mostly because Jim knows every detail of it and seems to remember an anecdote about every piece of equipment. He also seems to remember each price tag...

The morning was quite challenging - the rector - a biologist - had many interesting questions and seemed very interested about the goals of the research and the technical challenges. I am also not used to being followed by a professional photographer all the time!

We then went to CERN for a lunch with Lyn Evans - head of the LHC project - and Geoff (who would later show the CMS tracker). Of course I tried to get a few insider information about when the LHC will start, but the official statement remains: there will be an announcement by the end of the month. (But feel free to drop by my office I you want to hear some unofficial statements).

First LHCb week at CERN

I'm out at CERN for my first LHCb week. Four times a year, everyone working on the LHCb gets together to discuss how things are going with the experiment. For me, it's a great opportunity to meet lots of people, some of which I've already been working with, and some who I will work with in the future. I've been here for 5 days already, and have found my time very productive.

I have been working on a software project called Ganga, a neat little Grid front end tool used developed by members of the LHCb and ATLAS collaborations. It’s been really good to meet other people working on the project and discuss ideas face to face.

There are lots of interesting meetings scheduled this week, and today I'm off to find out about the RICH detector and the "online" software (used during data taking). Later in the week, there are meetings on Physics and computing, which I am looking forward too, and a lecture by Michael Frayn, a well known British playwright. I’ll try to post a follow up to let everyone know how the week went.

Fieldwork for the Science Museum

This week I've been trying to fix a big spark chamber for the Science Museum with the precious help of Prof. Websdale. On the picture you can see him installing a scintillator on top of the chamber. I'm happy he accepted to help me because he used to build such devices in the past. I belong to the generation of particle physicist who remotely log on to a detector when it doesn't work and debug it by looking at the logfiles. In this case we use screwdrivers, a voltmeter and an oscilloscope. Something quite unusual for me...

I'm doing this because the I am LHCb-UK Outreach coordinator. As such I am supposed to coordinate the outreach activities in the UK of the LHCb experiment, i.e. explain to the average UK citizen what LHCb is doing. Since LHCb has very little manpower (to be taken as an euphemism) available for such activities there's hardly anything done. Please don't visit the public LHCb web-site, it's such a shame.

Yet as a CERN experiment we should devote part of our efforts explaining to the public what we are doing and why. CERN itself is an open laboratory with a quite decent public web-site.

What is more important in my activity as outreach coordinator is that I am part of the larger group of people who try to promote the LHC in the UK. What for? might you think, we have nothing to sell so why bother making an advertising campaign. That's the subtle difference between marketing and outreach. The UK taxpayers pay about £60M a year as a contribution to CERN and that's not counting what is paid to the research groups in the UK (my salary for instance). Some consider this as a contribution to culture: it's the price for increasing our understanding of Nature. Some might consider it as a long term investment in the future. Our economy is presently growing on the grounds laid during the first half of the past century, when quantum mechanics was developed (think of computers, mobile phones, microwave ovens, lasers...). We might be discovering now what the economy will be based on in 50 years (... or not).

In any case we owe the British public an explanation of what we do with the £1 each taxpayer spends every year to fund our research. We try to have contact with the media, we prepare material for teachers, we have set up a UK website about the LHC (here)... But we first had to find out what to tell the public. The buzzword is Big-Bang: the key message is that we are recreating the conditions of the Big-Bang in the laboratory. If you are interested in the details look at the result of the formative evaluation.

This message will also be central at the upcoming exhibition about the LHC at the Science Museum, for which I am part of the advisory panel. The exhibition will start in April and I am of course not supposed to tell you what it will show. Come and see it. There will for sure be a post here about its opening.

In order to show particles to the public the Science Museum wanted to have a "Cosmic Ray Detector" on display, a device that would show the path of cosmic rays. After some research I found out that they already had one, which Imperial College and Rutherford laboratory built for them some time ago, but that it was probably labeled "Spark Chamber" in their stores. They indeed managed to find it and now we are trying to get it operational.

Stay tuned, hopefully I'll soon be able to add another post about the successful outcome of this fieldwork.