The DIS 2005 workshop reviewed progress in deep inelastic scattering and quantum chromodynamics, and provided the chance to plan for the future, reports Wesley Smith.
DIS 2005 was the 13th in the series of annual workshops on deep inelastic scattering (DIS) and quantum chromodynamics (QCD). Hosted by the Physics Department of the University of Wisconsin-Madison, the workshop was held on 27 April – 1 May at the Monona Terrace Community and Convention Center in Madison.
The workshop, which brought together 280 experimentalists and theorists, began with plenary sessions that featured review talks. Parallel working group sessions followed, and the workshop ended with plenary sessions that included reports from the working groups and a conference summary. The topics of the working groups were: structure functions and low-x, diffraction and vector mesons, electroweak physics and beyond the Standard Model, hadronic final states, heavy flavours, spin physics, and the future of DIS. There were 240 talks in total, replete with many exciting new results.
The working group on structure functions focused on the future. Final measurements from the first period of data-taking at the Hadron Electron Ring Accelerator (HERA) at DESY were shown, alongside the first electroweak measurements from the new HERA data. Attention was paid to new extraction techniques for determining the parton distribution functions (PDFs) and to improving the standard methods. The goal is to improve PDF uncertainties, which play a crucial role for measurements not only at the Large Hadron Collider (LHC) at CERN, but also at Fermilab’s Tevatron and in neutrino-oscillation experiments.
New results from the Relativistic Heavy Ion Collider (RHIC) at Brookhaven sparked much discussion of parton evolution and saturation at very low proton momentum fraction x. Strong particle suppression in forward rapidities in deuteron-gold collisions, reported by the BRAHMS, PHENIX and STAR collaborations, hint at the possible mechanism behind parton saturation. At the other end of the x spectrum, new results in the high-x resonance region from Jefferson Lab suggest that future data from there will significantly improve our understanding of proton structure.
The working group on diffraction surveyed the abundance of data over an extended kinematic range from the HERA experiments, which has enabled precise measurements of the diffractive structure functions and extraction of the diffractive parton distribution functions (DPDFs). Several new, independent next-to-leading-order (NLO) QCD fits suggest that the DPDFs are gluon-dominated. Recent results on deeply virtual Compton scattering and exclusive meson production from HERA experiments, and from the Common Muon and Proton Apparatus for Structure and Spectroscopy (COMPASS) experiment at CERN and the CEBAF Large Acceptance Spectrometer (CLAS) at Jefferson Lab, are sensitive to the generalized parton distribution functions (GPDFs). These provide information on correlations between partons, their transverse momentum, and the contribution of the quark angular momentum to the proton spin. A new window on diffractive processes will open at the LHC with the TOTEM detector, integrated with CMS. The FP420 proposal to equip a region 420 m from the ATLAS and/or the CMS interaction point would add to this.
Preparations for searches and precise electroweak measurements at the LHC highlight the machine’s vast discovery potential
The working group on electroweak physics examined the first measurements from HERA of the cross-sections for charged and neutral-current DIS with polarized leptons, confirming the V-A structure of the electroweak interaction. Participants discussed the impact on the Standard Model Higgs mass of the latest high-precision top-quark mass measurement from the Tevatron. High-precision measurements of the W mass and the top mass need a good understanding of the structure of the proton, in particular nonperturbative effects, from HERA data. The discovery of single-top events is expected with the increasing integrated luminosity of Run II at the Tevatron, and measurements of the production cross-section could constrain new physics models that modify the coupling of the top quark to gauge bosons. The excess of events with high-pT isolated leptons reported by the H1 collaboration at HERA could be attributed to the anomalous coupling of top quarks to up quarks. Many recent searches at HERA and the Tevatron have produced inconclusive evidence of new physics, but the substantial increases in luminosity at both colliders make a discovery more likely. Preparations for searches and precise electroweak measurements at the LHC highlight the machine’s vast discovery potential.
The working group on hadronic final states studied the perturbative QCD calculations of jet cross-sections that have been understood with unprecedented accuracy at HERA.
These determine the strong coupling constant with a precision that is comparable to the most accurate value obtained in e+e– interactions. These achievements pave the way for an understanding of jet production at the LHC. Large theoretical uncertainties (of order 100%) remain for the production of hadrons at small (forward) angles to the incoming proton’s momentum, which pro collisions with small momentum fractions x and momentum transfers Q. This is where new dynamical mechanisms associated with scattering at asymptotically high collision energies may turn on.
Recent results from HERA suggest that further theoretical improvements are needed to describe small-x scattering. These may come from developments in higher-order computations, resummation and parton shower models. The latest cross-sections for jet production in Run II at the Tevatron help to constrain the gluon density in the proton, while a comparison of the rates for pion and photon production at RHIC independently confirms the formation of an extended dense quark-gluon medium in the aftermath of gold-gold collisions.
The experimental status of pentaquarks remains ambiguous, but new high-statistics measurements from Jefferson Lab should soon provide a more definite answer. The ZEUS and HERMES experiments at HERA reported observations of a Θ+ state at around 1520 MeV, whereas H1 at HERA and BaBar at SLAC see nothing. On the other hand, H1 remains unique in reporting the observation of a charmed pentaquark. The CLAS experiment at Jefferson Lab has now accumulated a large sample of photoproduction events from dedicated runs, and with only 1% of the data analysed there is no sign of a Θ+.
The heavy-flavour working group heard that the new heavy-quark PDFs from Martin-Roberts-Stirling-Thorne (MRST) and the Coordinated Theoretical-Experimental Project on QCD (CTEQ) now describe the HERA data on charm structure functions quite well. Recent progress on soft resummation for heavy quarks in DIS should allow its inclusion in PDFs and the extraction of resummed parton densities. New calculations describing the production of D-mesons at the Tevatron can be further extended to DIS processes. A new model for heavy quarkonium production agrees with data from RHIC and the Tevatron, in particular with the J/ψ polarization measurements from the Collider Detector at Fermilab (CDF) at the Tevatron, and PHENIX at RHIC. NLO corrections were shown to improve the description of charmonium production in two-photon collisions. New measurements of the charm and beauty contribution to the proton structure function show good agreement with the predictions based on NLO QCD and gluon densities obtained from global PDF analyses.
New heavy-flavour results are moving beyond the production of single heavy mesons to measure fragmentation parameters, heavy-quark correlations, heavy-quark-jet characteristics and unexplored kinematic regions. While NLO QCD describes charm well, the situation for beauty is less clear. Precise measurements of b-quark production at high pT or large Q2 agree with theory, but measurements over the full pT and Q2 range are a factor of two higher. In another puzzle, the final measurement of charm production in neutrino-nucleon scattering by the Neutrinos at the Tevatron (NuTeV) experiment excludes a strange sea asymmetry large enough to explain their anomaly on sin2θW.
The spin physics working group basked in a wealth of new high-precision data from the HERMES, COMPASS and Jefferson Lab experiments on the spin structure functions, which extend the coverage at both low- and high-x and into the transition from the partonic to the hadronic regime. Since the contribution to the proton spin from the longitudinal spin of the quarks is now well established and small, recent measurements and global analyses focused on understanding other spin contributions. New data on transversity distributions were presented by the above-mentioned experiments, and from BELLE at KEK, and STAR and PHENIX at RHIC.
DIS 2005 featured a plenary session devoted to the future of DIS studies. Although HERA is expected to close in two years’ time, much of its integrated luminosity is still in the future. This is particularly true for the measurement of the helicity dependence of the charged-current cross-section. There is interest in running HERA for a while at lower energy to extract the longitudinal structure function FL. There was discussion of the physics potential of continuing HERA beyond 2007 with new injectors, or combining the LHC with a future linear electron collider to produce DIS collisions at the tera-electon-volt scale.
Another proposal, eRHIC, combines an electron accelerator with RHIC to produce an electron-proton and electron-nucleus collider with polarized beams at a centre-of-mass energy in the range 30-100
GeV. There is also a proposal to upgrade the DIS programme at Jefferson Lab from 6 GeV to 12 GeV, featuring DIS at large x and the use of what is effectively a target of free neutrons. Ideas also exist for DIS experiments at fixed targets, particularly at CERN; for neutrino experiments with Minerva at Fermilab; and future neutrino projects based on the Fermilab Proton Driver. These proposals often look at the GPDFs that can be accessed using deeply virtual Compton scattering and that illuminate the structure of hadrons in transverse space.
The workshop attendees emerged with a renewed sense of the importance of DIS and QCD measurements and theory to the future of particle and nuclear physics. They also gained an enhanced appreciation for the range of exciting developments in the field, and a determination to pursue experimental and theoretical opportunities.
• The workshop was sponsored by Argonne, the US Department of Energy, DESY, the US National Science Foundation and the University of Wisconsin-Madison.
