05.06.2013

Back to the Future with HERA

Charm quarks point the way for science at LHC

Over a time span of 15 years, DESY’s particle accelerator HERA served the international particle physics community as the world’s most precise electron microscope for studies of the proton’s inner structure. Although HERA experiments ended in 2007, ongoing data analyses continue to point the way for future particle physics experiments.

Example of a “charm event” recorded by the ZEUS detector. The colored lines are traces of particles emerging from a positron-proton collision at HERA. The red and gray traces are identified as the fragments of a charm particle. The red rectangles visualize the energy loss of the collision fragments in the detector. From 1992 to 2007, DESY’s HERA accelerator used electrons and their antiparticles, positrons, to probe the inner structure of protons.

Researchers from H1 and ZEUS, two former HERA experiments, have sifted through approximately one billion collision events at HERA and determined the “optimal mass” of an elementary particle known as the charm quark. The scientists found that using this mass in predicting collision events at the state-of-the-art accelerator LHC (Large Hadron Collider, CERN, Switzerland) greatly improves the precision of their forecasts. The study was published in the "European Physical Journal C" and is one of the research highlights in DESY’s latest Particle Physics Annual Report.

Revealing fireballs

Inside LHC’s particle racetrack, proton beams travel at nearly the speed of light. In powerful collisions, the protons release their energy into fireballs – clouds of particle debris that scientists detect. Similar to assembling a jigsaw puzzle from its many pieces, the researchers analyze the collision products to derive information about the properties of the universe’s fundamental particles and forces.

Solving the collision puzzle is not trivial. Protons are particles with a complex inner structure and, hence, a large variety of stable and unstable particles emerge from the fireballs. “However, if we know the proton’s composition, we can determine the probabilities of various collision events,” says DESY scientist Stefan Schmitt, spokesperson of the H1 collaboration.

Research at HERA has led to the most precise description of the proton’s structure yet. The proton is made of various components known as partons. These include three valence quarks which are held together by exchanging gluons, the carrier particles of the strong nuclear force. In addition, sea quarks frequently appear and disappear inside the proton’s confinement. “HERA was like a magnifier glass for protons,” Schmitt says. “It allowed us to look at the partons and determine their distribution inside protons.” Researchers call this distribution the proton’s parton density function and use it to predict proton-proton collision events, for instance at the LHC.

A prime objective of LHC research is to identify and characterize the Higgs boson – an elementary particle whose existence can explain how other particles get their mass. One of several Higgs boson production pathways is the fusion of two W or Z bosons, which are produced in LHC’s proton-proton collisions. “Using HERA’s data, we can make precise forecasts for the production of W and Z bosons at the LHC and expect to contribute to a detailed understanding of the Higgs mechanism,” Schmitt says.

The proton’s inner charm

It turns out that the charm quark, a sea quark that was abundantly produced at HERA, has a large influence on the researchers’ predictions. “In our recent analysis, we combined all available H1 and ZEUS data and determined more precisely than ever how often and how many charm quarks were produced at HERA,” explains Achim Geiser, head of DESY’s ZEUS group. “From these data, we calculated the charm quark mass and its influence on the parton density function for a variety of theoretical schemes.”

Since researchers cannot simply weigh charm quarks on a balance, they have to use theoretical approaches to infer the quark mass from the quark production data. Depending on the theoretical scheme chosen for the calculation, the charm quark mass comes out slightly differently. The mass uncertainty translates into differences in the parton density functions and, consequently, into a considerable uncertainty in the predictions for W and Z boson productions at the LHC.

When using the same charm quark mass for each scheme, the scientists’ predictions varied by approximately six per cent. To increase the accuracy of their forecasts, the researchers fitted the combined H1 and ZEUS data using various theoretical approaches and determined an optimal charm quark mass for each of them. A scheme’s optimal mass corresponds to the best fit.

Improved predictions

With HERA's optimal charm quark masses, the researchers reduced the corresponding uncertainty of their predictions by more than a factor of three.In fact, their forecasts became practically independent on the theoretical scheme. “This large impact of the charm mass on the W and Z production, which we quantified for the first time, was unexpected,” Geiser points out.

The researchers will continue to use HERA’s data to refine their predictions of LHC events. “The improved understanding of the proton will also lead to constraints for other processes, including the production of top quarks and Higgs bosons,” says Geiser. HERA’s past will continue to point the way for future particle physics research.

The results discussed here regarding the charm quark production at HERA and its implications for LHC predictions were in a large part created by the DESY group of Katerina Lipka and funded by the Helmholtz Association (VH-NG-401).

Original Publication

The H1 and ZEUS Collaborations, Combination and QCD analysis of charm production cross section measurements in deep-inelastic ep scattering at HERA, Eur. Phys. J. C (2013) 73:2311, DOI: 10.1140/epjc/s10052-013-2311-3.