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