Parton Distributions Prime LHC Users for Collisions
What's Hot in Physics, November/December 2010
By Dr. Simon Mitton
The Physics Top Ten this period includes for the first time a paper, #8, that provides a tool kit for the analysis of the collision data expected from CERN’s Large Hadron Collider (LHC). In September 2008, the LHC suffered a serious malfunction that halted the circulation of particle beams for over a year. As of now, however, the LHC, together with its enormous experimental detectors, is performing exceptionally well, with 3.5 TeV proton beams producing 7 TeV collisions. The LHC is closing in on the goal of the direct exploration of physics at the TeV scale.
Theorists at CERN are quietly confident that groundbreaking discoveries will be found in the early data, which would have profound implications for the future of particle physics. Indeed, particle physicists are already discussing the planning of accelerators beyond the LHC.
When Protons Collide
A technician walks under the core magnet of the CMS
experiment at the CERN in the village of Cessy. View larger image in tab
below.
REUTERS/Denis Balibouse.
In 1968 Richard Feynman proposed the parton model for hadron structure before it was known that the internal constituents of protons are point-like quarks (of which there are six types, or flavors) and gluons. In modern parlance a parton is a quark or a gluon. In deep-inelastic scattering and high-energy collisions, protons do not behave classically like rigid pool balls. When a proton collides, the outcome is determined by the distribution of its partons, and in reality only one of its partons is involved in the interaction.
For Science Watch, Alan Martin (University of Durham, U.K.) explains, "When two high-energy protons collide, as at the LHC, it is really a pair of constituent partons that initiate the interaction."
The head count of the partons in a colliding proton increases at higher collision energies. Martin adds, "The internal structure depends on how deeply you probe a fast-moving proton. The smaller the wavelength of the probe, the more quarks and gluons you find in the proton." A parton distribution function (PDF) describes the probability of finding a parton with a certain fraction of the proton’s momentum at a certain wavelength of the incoming probe. Parton distributions are essential ingredients for extracting the physics from experiments at colliders with proton beams.
At present it is not possible to calculate PDFs from first principles using quantum chromodynamics (QCD). Instead they have to be teased out of experimental data, and that’s what paper #8 is all about. The Durham group produced its first paper on parton distributions in 1988 (A.D. Martin, et al., Phys. Rev. D, 37[5]: 1161-73), following which they have repeatedly provided updates as new data have become available. The authors of #8—Martin, along with W.J. Stirling (University of Cambridge) and R.S. Thorne and G. Watt (both of University College London)—are known as the MSTW collaboration.
Universal PDFs
A Scientist looks at computer screens at the LHC control
center of the CERN in Geneva. View larger image in tab below.
REUTERS/Christian Hartmann.
To predict the rates of various collision processes anticipated at LHC, a set of universal PDFs is required. These are determined by global fits to data from deep-inelastic scattering and hard scattering processes involving protons. In #8 the MSTW quartet notes that there have recently been improvements in the precision and range of experimental data, as well as new types of data, and valuable theoretical developments.
The MSTW group emphasizes that they "use data on deep inelastic electron-proton scattering which were obtained at HERA (DESY, Hamburg) and elsewhere, together with data on the production of jets of hadrons, and on weak vector boson production from experiments at the Tevatron collider (Fermilab, Illinois)." All of these are put into the mix to perform the new global analyses, but the work is non-trivial. The research is based on applying perturbation theory in QCD.
This major update can justifiably claim to be the most complete and reliable set of PDFs at the dawn of the LHC era, which is why the paper is highly cited. They are required for calculating the production of very heavy particles, such as the Higgs boson or the supersymmetric particles, which many physicists believe to exist. At the LHC, data are now pouring out. But the processes that could produce new particles are extremely rare: one new particle in 108 collisions. The rate of collisions will be steadily ramped up, then the beam energy will be doubled in 2012.
Dr. Simon Mitton is a Fellow of St. Edmund’s College, University of Cambridge.
What's Hot in Physics |
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Rank | Paper |
Cites This Period May-Jun 10 |
Rank Last Period Mar-Apr 10 |
1 | E. Komatsu, et al., "Five-year Wilkinson Microwave Anisotropy Probe observations: Cosmological interpretation," Astrophys. J. Suppl. Ser., 180(2): 330-76, February 2009. [14 institutions worldwide] *406EI | 173 | 1 |
2 | J. Dunkley, et al., "Five-year Wilkinson Microwave Anisotropy Probe observations: Likelihoods and parameters from the WMAP data," Astrophys. J. Suppl. Ser., 180(2): 306-29, February 2009. [14 U.S. and Canadian institutions] *406EI | 72 | 3 |
3 | S.H. Park, et al., "Bulk heterojunction solar cells with internal quantum efficiency approaching 100%," Nature Photonics, 3(5): 297-302, May 2009. [U. Calif., Santa Barbara; Gwangju Inst. Sci. & Tech., S. Korea; U. Laval, Quebec City, Canada] *447UY | 59 | 2 |
4 | O. Adriani, et al., "An anomalous positron abundance in cosmic rays with energies 1.5-100 GeV," Nature, 458(7238): 607-9, 2 April 2009. [17 institutions worldwide] *427RK | 51 | 6 |
5 | F.-C. Hsu, et al., "Superconductivity in the PbO-type structure alpha-FeSe," PNAS, 105(38): 14262-4, 23 September 2008. [Acad. Sinica, Taipei, Taiwan; Natl. Tsing Hua U., Hsinchu, Taiwan; Duke U., Durham, NC] *353TY | 42 | 5 |
6 | M. Kowalski, et al., "Improved cosmological constraints from new, old, and combined supernova data sets," Astrophys. J., 686(2): 749-78, 20 October 2008. [41 institutions worldwide] *364YB | 34 | + |
7 | J.K. Adelman-McCarthy, et al., "The Sixth Data Release of the Sloan Digital Sky Survey," Astrophys. J. Suppl. Ser., 175(2): 297-313, April 2008. [84 institutions worldwide] *327WN | 33 | 10 |
8 | A.D. Martin, et al., "Parton distributions for the LHC," Eur. Phys. J., 63(2): 189-285, September 2009. [U. Durham, U.K.; U. Cambridge, U.K.; U. Coll. London, U.K.] *495BC | 30 | + |
9 | K.K. Ni, et al., "A high phase-space-density gas of polar molecules," Science, 322(5899): 231-5. 10 October 2008. [U. Colorado, Boulder; Temple U., Phila., PA; NIST and U. Maryland, Gaithersburg] *358FK 29 + | 29 | + |
10 | M. Hicken, et al., "Improved dark energy constraints from ~ 100 CfA supernova type Ia light curves," Astrophys. J., 700(2): 1097-1140, 1 August 2009. [7 institutions worldwide] *471ZU | 29 | + |
SOURCE: Thomson Reuters Hot Papers Database. Read the Legend. |
Photo 1:
Photo 1: A technician walks under the core magnet of the CMS experiment at the CERN in the village of Cessy. REUTERS/Denis Balibouse.
Photo 2:
Photo 1: A Scientist looks at computer screens at the LHC control center of the CERN in Geneva. REUTERS/Christian Hartmann.
Special Topic of Hadron Colliders
The features of this Special Topic (November 2010) represent distinct slices of citation data. By approaching citation data from multiple angles, we can observe trends and anomalies across categories—leading to more rich and nuanced stories behind the data.
The baseline time span for this database is (publication years) January 1, 2000-June 30, 2010 (third bimonthly period 2010). This analysis was created using the Web of Science® from Clarivate. The resulting database contained 15,317 (10 years) and 4,328 (2 years) papers; 28,915 authors; 82 nations; 626 journals; and 2,901 institutions. See additional information below in the overview & methodology sections.
KEYWORDS: Large Hadron Collider, LHC, CERN, partons, parton distribution function, PDFs, Alan D. Martin, MSTW group.