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

The LHC inaugration and the LHCFest

The official ceremony of inaugration of the Large Hadron Collider (LHC) took place on 21.10.2008 which was attended by quite a few important people, especially from the CERN member states. There were also delegations from other countries from all over the world, the delegations typically headed by education or science ministers of the respective countries. There were also the guests invited by the Director General (DG) of CERN, which were mainly physicists from CERN and all over the world, and heads of laboratories and funding agencies.
The ceremony took place in the SM18, which is the testing facility for the LHC magnets. In one of its halls, expositions from all the four major detectors were set up, along with posters and displays of parts of the LHC magnets. So the guests were to visit this hall, where, apart from the science, they were also served the "molecular cuisine", which was an assortment of chef Ettore Bocchia's creations including the famous ice creams cooled with liquid nitrogen to maintain a velvety texture. I had the good luck of being there as one of the presenters at the LHCb exposition, and given my sweet tooth, I had a ball. And you can find me stirring the ice cream while liquid nitrogen is being poured!
Going back to the ceremony, the guests were then taken to another hall called SMA18 where they were to be seated for the rest of the evening. There was also an exhibition titled "Accelerating Nobels", which is a series of photographs of Nobel laureates who were asked to make a drawing of their Nobel discovery and hold it while posing for the photograph. Another interesting thing about the exhibition was the photographer's comments for each of the photograph, and the ones for Anthony Leggett (nobel prize 2003) were
"It’s quite a twist when atoms form pairs to allow for superfluidity, but Tony Leggett wanted to show exactly how they do it – with his hands! “But Professor, how can you hold up your drawing to the camera when you need both of your hands for your presentation of paired atoms?”, I ask. “Just tape the drawing onto me!”, he replies."
This is ofcourse my favourite picture....

The ceremony began with a speech from the DG followed by speeches by the distinguished delegates.
Then there was a concert named "Origins: concert for a collider", a celebration of the wonder of the cosmos and the glory of life on Earth, as expressed through the imagery of a nature photographer Frans Lanting and the music by Philip Glass.
This was the end of the formal ceremony and the guests left by 1800.
The LHC Fest, which was open to all CERN users, included a buffet and a concert, was to start from 1830. Both the exposition hall and the ceremony hall were open so one could look at the displays by the four experiments and the Accelerating Nobels exhibition. There was a speech by the DG and also a performance of "Origins" for the fest. After which, the buffet was served at 2030 and a concert followed, which was in full swing even as I left around 2300. It was a very enjoyable evening overall and I am glad I was able to experience this unique day.

LHC start up

Amazingly nobody from the group has posted anything about the LHC startup. I'd hoped (and in fact promised Yoshi) to blog from the CMS control room yesterday but in all the excitement didn't manage it.

For those few who have no idea what I'm talking about yesterday the Large Hadron Collider at CERN was finally switched on after decades of planning and construction. Our group works on the CMS experiment, one of the two larger experiments at the LHC.

So what happened yesterday? Well the LHC is a 27 km long circular accelerator buried under the Swiss and French countryside near Geneva. Yesterday for the first time beams of protons were circulated all the way around the ring. The media coverage of the event was astonishing. Radio 4 covered it live (pleasing my mother very much) and journalists from all over the world were here at CERN all day. I think the best account from the perspective of our experiment can be found here. Plenty of nice pictures of events and a video. Perhaps a little technical in places, but I'd be happy to answer any questions on it. Hats off to Lyn Evans and company, if anything they made it look a little too easy!

So what does it mean? From a scientific perspective it's the start of a journey to exciting discoveries we hope. In the next weeks and months we'll be working hard to calibrate some of the largest and most complex scientific instruments ever built. From a personal point of view all the publicity will make it much easier to explain what I do for a living to people in the pub.

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.

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.