Photon-induced interactions have traditionally been studied with electron beams in fixed-target experiments and colliders, LEP (electron–positron) and HERA (electron–proton) in particular. However, photon–hadron and photon–photon interactions also occur when the electron beams are replaced by ultra-relativistic beams of other charged particles such as protons or heavy nuclei. In these cases, the maximum photon energies are restricted by the form factor of the projectile, but at the extremely high energies of the LHC they will be higher than at any other existing accelerator: up to a photon energy of around 4 TeV in the photon–proton centre-of-mass frame. Furthermore, since the intensity of the electromagnetic field – the number of photons in the “cloud” surrounding the charge of the beam particle – is proportional to the square of the particle’s charge Z, photonic interactions are enhanced by up to a factor of Z2, or around 104 for heavy ions. Indeed, the fields from heavy ions are strong enough that multiple photons may be exchanged in a single event. Figure 1 shows a schematic view of such an electromagnetic (or ultra-peripheral) nucleus–nucleus collision.
The study of photon-induced interactions at the LHC, as well as at existing hadron colliders such as RHIC at Brookhaven or the Tevatron at Fermilab, is challenging despite the high photon energies and fluxes. The interaction is always electromagnetic with an electron beam and the small contribution from the weak interaction can usually be neglected or easily separated. By contrast, the photonic interactions at hadron colliders must be separated from a dominant QCD background. The low multiplicity and mostly longitudinal kinematics of electromagnetic processes result in an event topology that is different from hadronic interactions. In particular, event triggering is a critical issue that depends much on instrumentation in the very forward direction, close to the beam line. The workshop on Photoproduction at collider energies: from RHIC and HERA to the LHC (held at ECT*-Trento in January), looked at how these issues have been addressed and solved in previous experiments, and considered the perspectives at the LHC. The workshop gathered around 40 physicists, equally divided between theorists and experimentalists.
Much of the workshop focused on the latest advances in the study of low-x parton densities in protons and nuclei probed by photons. Ultra-peripheral collisions at the LHC can probe the physics of parton saturation at Bjorken-x values as low as 10–5. Talks by SLAC’s Stan Brodsky, Mark Strikman of Pennsylvania and Leonid Frankfurt of Tel Aviv highlighted these theoretical aspects. HERA saw its last collisions at the end of June and has been an important machine for the field. Michael Klasen from Grenoble and DESY’s Sergey Levonian gave theoretical and experimental overviews, respectively, of the HERA results. At the Tevatron, the CDF collaboration has recently published its first analysis of two-photon interactions in proton–antiproton collisions. Andrew Hamilton of Geneva presented the results at the workshop. At RHIC, the STAR and PHENIX collaborations have studied ultra-peripheral gold–gold collisions. Yury Gorbunov of Creighton and David Silvermyr from Oak Ridge showed the latest results on vector meson photoproduction.
Looking to the future, Krzysztof Piotrzkowski from UC Louvain presented the group’s comprehensive study of various photon-induced electroweak and beyond-Standard Model processes that can be studied in proton–proton collisions at the LHC. These include associated W-Higgs and single-top photoproduction, as well as two-photon production of W boson pairs. To conclude the series of talks at the workshop, Otto Nachtmann of Heidelberg and Ute Dreyer of Basel covered the theory of anomalous gauge-boson couplings in γ– γ, γ–p and γ–A interactions.
The physics of photon–nucleus interactions in ultra-peripheral collisions is also the focus of a CERN Yellow Report, completed in June. This 230-page document, the joint effort of more than 20 contributors, summarizes results from the SPS at CERN and from RHIC. It examines planning for ultra-peripheral collisions at the ALICE, ATLAS, and CMS experiments at the LHC. The vitality of this research field was also evident in the number of contributions at the Photon 2007 conference held in Paris in July.
The conclusion is that the LHC has much to offer as a photon collider. Photon–hadron and photon–photon processes will reach energies an order of magnitude larger than at previous colliders. They will not only provide valuable information on the strong interaction – in particular of low-x parton densities and non-linear QCD phenomena – but will also open new windows on electroweak processes and physics beyond the Standard Model, which will complement the mainstream studies in proton–proton and nucleus–nucleus collisions.
Hans Jensen (1907–1973) is the only theorist among the three winners from Heidelberg University of the Nobel Prize for Physics. He shared the award with Maria Goeppert-Mayer in 1963 for the development of the nuclear shell model, which they published independently in 1949. The model offered the first coherent explanation for the variety of properties and structures of atomic nuclei. In particular, the “magic numbers” of protons and neutrons, which had been determined experimentally from the stability properties and observed abundances of chemical elements, found a natural explanation in terms of the spin-orbit coupling of the nucleons. These numbers play a decisive role in the synthesis of the elements in stars, as well as in the artificial synthesis of the heaviest elements at the borderline of the periodic table of elements.
Hans Jensen was born in Hamburg on 25 June 1907. He studied physics, mathematics, chemistry and philosophy in Hamburg and Freiburg, obtaining his PhD in 1932. After a short period in the German army’s weather service, he became professor of theoretical physics in Hannover in 1940. Jensen then accepted a new chair for theoretical physics in Heidelberg in 1949 on the initiative of Walther Bothe, who received the Nobel prize in 1954 for the development of the coincidence method. Apart from his work in nuclear and particle physics, Jensen became the driving force behind the rebuilding of physics research in Heidelberg after the Second World War. The Institute for Theoretical Physics obtained new chairs, particularly in theoretical particle physics. Together with Bothe, he expanded the experimental-physics department and convinced well-known experimentalists to come to Heidelberg, including his collaborator in the development of the shell model, Otto Haxel, in 1950 and Hans Kopfermann, a specialist on nuclear moments and hyperfine interactions, three years later.
The shell model past and present
To celebrate the centenary of Jensen’s birth, the Heidelberg Physics Faculty and the Institute for Theoretical Physics organized a symposium on Fundamental Physics and the Shell Model. A series of talks looked at Jensen’s life plus the role of the shell model in astrophysics and nuclear physics today. In keeping with Jensen’s interest in music, performances by the Heidelberg Canonical Ensemble complemented the talks. In the introductory talk on The Shell Model: Past and Present, former director at the Heidelberg Max Planck Institute, Hans Weidenmüller, gave an overall view of Jensen’s Nobel-prizewinning contribution to nuclear physics. The paper on the shell model by Haxel, Jensen and Hans Suess appeared in the same 1949 edition of Physical Review as Goeppert-Mayer’s work (Haxel, Jensen and Suess 1949 and Goeppert-Mayer 1949). It proved to be a surprising solution to the problem of nuclear-energy levels. Based on the picture of independent particle motion of protons and neutrons with strong spin-orbit coupling, the model yields the correct sequence of energy levels and explains the magic numbers in terms of energy gaps above full levels.
The apparent contradictions with the collective properties of nucleons in nuclei (evident from the rotational spectra) as well as with the chaotic properties of nuclei (evident in Niels Bohr’s compound nucleus picture) only found their explanations much later. Today, shell-model calculations in large configuration spaces can indeed explain rotational spectra, and within individual shells consistency with the random nuclear properties appears once the residual interaction is considered. However, a derivation of the shell model from the basic nucleon–nucleon interaction is still missing.
Berthold Stech, Jensen’s former colleague and long-time director of the Heidelberg theory institute, presented his recollections of Jensen with photographs and anecdotes. As a student representative after the war, Stech contributed to Jensen’s move to Heidelberg by writing a letter to the publisher of the local newspaper, who then went to the state government to ensure that the offer was made to Jensen. He talked about Jensen’s vital contributions to making Heidelberg a famous physics centre. With private rooms in the institute, Jensen often invited students and colleagues for discussions and to listen to music. Stech also quoted from a recent letter by Aage Bohr and Ben Mottelson, who emphasized Jensen’s inspiring personality.
Wolfgang Hillebrandt, director at the Max Planck Institute for Astrophysics in Munich-Garching, spoke about supernovae and the shell model. This active field of research represents a synthesis of astrophysics and nuclear physics. In Type Ia supernovae there is a high and almost identical fraction of nickel-56. Even though this is a doubly magic nucleus, it is not stable (its half-life is six days) and its decay through cobalt-56 to iron-56 is what makes these supernovae shine. Hence, the brightness of the supernova is proportional to the produced mass of nickel-56. For progenitor stars that are similar, this allows for very precise determination of distances, which since 1998 have been used to infer the accelerated expansion of the universe. Many physicists consider this to be the consequence of dark energy. Its origins are currently under investigation in many institutes, for example, at the Bonn–Heidelberg–Munich research centre “The Dark Universe”.
Core-collapse supernovae (Type II), such as SN1987A in the Large Magellanic Cloud, where a blue supergiant exploded in several seconds, allow the direct test of ideas about the synthesis of heavy elements. For example, observations of the characteristic gamma rays indicate the presence of the corresponding isotopes synthesized in the particular star or during the explosion. Elements beyond iron are, in particular, produced in a sequence of rapid neutron captures known as the r-process. It turns out that the element abundances are mainly determined by nuclear structure, and hence, by the shell model; the subtleties of the astrophysical processes prove to be comparatively unimportant.
In the final talk of the symposium, Peter Armbruster of the GSI in Darmstadt explained the synthesis of the heaviest elements using cold fusion (only one neutron emitted) up to and beyond roentgenium, symbol Rg and atomic number Z = 111. The relative stability of these elements, with mean lifetimes in the order of milli-seconds to seconds, is a consequence of the Goeppert–Jensen shell effects. Without these they would not exist. The element Z = 112, synthesized at GSI in 1996, is still unnamed. Meanwhile, Yuri Oganessian’s group at the Flerov Laboratory at JINR, Dubna, used radioactive targets in hot-fusion reactions with the emission of up to five neutrons, to create synthetically the elements 114, 116 and 118. Kosuke Morita and co-workers at RIKEN in Japan made element 113 in 2004.
Relativistic mean-field calculations indicate that the closed shell should occur at Z = 120 (the number of protons), with the magic neutron number of 184, as had appeared in the book of Jensen and Goeppert-Mayer about the shell model (Goeppert-Mayer and Jensen 1955). This means that this doubly magic superheavy nucleus should have 304 nucleons. It will, however, be extremely difficult to synthesize since its relatively low density of energy levels above the ground-state favours fission over neutron emission, as Armbruster emphasized. This would lead to a drastic reduction of the survival probability.
As a lasting tribute to Jensen, starting next year, the Jensen Guest Professorship will be created with the financial support of the Klaus Tschira Foundation, Heidelberg. During a five-year period, internationally renowned physicists will visit the Institute for Theoretical Physics in Heidelberg to conduct research, give seminars and one public lecture a year.
In nature, relatively few nuclei have a spherical shape in their ground state. Examples are 16O, 40Ca, 48Ca and 208Pb, which are “doubly magic”, with numbers of both protons and neutrons corresponding to closed shells in the nuclear shell model. By moving away from the closed shells and increasing the number of valence nucleons, both protons and neutrons, these nuclei can eventually acquire a permanent deformation in their ground state. Experiments reveal that sometimes – due to the complex interplay of single-particle and collective degrees of freedom – both a spherical and deformed shape occur in the same nucleus at low excitation energies. In the region around lead, for example, physicists in the 1970s first observed this “shape co-existence”, using optical spectroscopy at the ISOLDE facility at CERN (Bonn et al. 1972 and Dabkiewicz et al. 1979). Since then, an extensive amount of data has been collected throughout the chart of nuclei (Wood et al. 1992 and Julin et al. 2001).
Some of the best-known examples of shape co-existence are found in neutron-deficient lead nuclei (atomic number or number of protons, Z = 82). The uniqueness of this region is mainly due to three effects. First, the energy gap of 3.9 MeV above the Z = 82 closed proton shell forces the nuclei to adopt a spherical shape in their ground state. However, the energy difference is small enough for a second effect to occur: the creation of “extra” valence proton particles and holes as a result of proton-pair excitation across the gap. Third, a very large neutron valence space between the shell closures with the number of neutrons N = 82 and 126 results in a large number of possible valence neutrons as nuclei approach the neutron mid-shell at N = 104. The strong deformation -driving interaction between the “extra” valence protons and the valence neutrons produces unusually low-lying, deformed oblate (disc-like) and prolate (cigar-like) states in the vicinity of N = 104, where the number of valence neutrons is maximal (Wood et al. 1992). In some cases, the deformation-driving effect is so strong that the deformed state becomes the ground state, as happens near N = 104 in the light isotopes of mercury (Z = 80) and platinum (Z = 78).
Atomic spectroscopy provides direct and model-independent information on the properties of nuclear ground and isomeric states via a determination of hyperfine structure and the isotope shift. These are small effects on atomic energy levels due to the nuclear moments, masses, sizes and shapes of nuclear isotopes, allowing the spins, moments and changes in charge-radii of nuclei to be deduced. In particular, the changes in charge radii determined from the isotope shifts by optical spectroscopy in long isotopic chains have revealed collective nuclear properties clearly.
Figure 1 shows changes of mean-square charge radii (δ<r2>) of lead, mercury and platinum isotopes as a function of the number of neutrons. All the data for the nuclides furthest from stability were determined at ISOLDE by a variety of techniques (Otten 1989 and Kluge and Nörtershäuser 2003). In the 1970s, nuclear-radiation detected optical pumping and laser fluorescence spectroscopy were used, collinear spectroscopy in the 1980s and resonance ionization mass spectroscopy from the late 1980s onwards. Now laser spectroscopy in the laser ion source is used, as described below.
Figure 1 shows how the measured δ<r2> for platinum isotopes develop a distinct deviation from the smoothly decreasing trend expected from the spherical-droplet model. For mercury, a sudden and dramatic change in δ<r2> known as “shape staggering”, occurs between 187Hg and 185Hg (N = 107 and 105 respectively). A similar change occurs between the isomeric (I = 13/2) and ground (I = 1/2) states in 185Hg, in this case, “shape isomerism” or “shape co-existence” (Bonn et al. 1972 and Dabkiewicz et al. 1979). These effects are interpreted as a change from weakly deformed oblate to strongly deformed prolate shapes. The neutron-deficient lead isotopes are a particularly interesting example of shape co-existence. Theoretical calculations have long suggested the co-existence in these nuclei of three different shapes: spherical, prolate and oblate – hence triple co-existence. Recent particle (α, β) and in-beam studies have found strong evidence for this phenomenon in some of the isotopes from 182Pb to 208Pb.
One of the most spectacular examples is the mid-shell nucleus 186Pb, as indicated in figure 2. Here, studies of the α-decay of the parent nucleus 190Po have revealed a triplet of low-lying (E* < 650 keV)>+ states (Andreyev et al. 2000). These were assigned to co-existing spherical, oblate and prolate shapes, with the spherical state being the ground state. Subsequent in-beam studies identified excited bands built on top of these states. An important question arises, however, concerning the degree of mixing between different configurations. As the excited 0+ states decrease their energy when approaching N = 104 (186Pb), their mixing with the 0+ ground state could increase substantially, an effect that could possibly be seen in the value of the charge radii.
Therefore, the aim of experiment IS483 at ISOLDE was to measure for the first time the isotope shifts in the atomic spectra of the very neutron-deficient nuclei in the region 182Pb to 190Pb, deducing the mean-square charge radii in order to probe the ground state directly (De Witte et al. 2007 and Anderyev et al. 2002). However, the expected production rates were far too low (e.g. 1 ion/s for 182Pb) for the laser spectroscopy techniques used previously at ISOLDE. Instead, an extremely sensitive spectroscopic technique was employed: resonance ionization spectroscopy in the ion source, first developed at the Petersburg Nuclear Physics Institute in Gatchina for the investigation of rare-earth isotopes (Alkhazov et al. 1992).
The radioactive lead isotopes are produced at ISOLDE in a proton-induced spallation reaction, using protons at 1.4 GeV on a thick (50 g/cm2) target of uranium carbide (UCx). The reaction products diffuse out of the target toward the ionizer tube, which is heated to around 2050 °C. In the tube, a three-step laser ionization process selectively ionizes the lead isotopes. To determine the isotope shift of the appropriate optical spectral line, the laser for the first excitation step is set to a narrow linewidth of 1.2 GHz and its frequency is scanned over the resonance. After ionization and extraction, the radioactive ions are accelerated to 60 keV, mass separated and subsequently implanted in a carbon foil mounted on a rotating wheel at the focal plane of ISOLDE. A circular silicon detector (150 mm2 × 300 μm) placed behind the foil measures the α-radiation during a fixed implantation time, after which the laser frequency is changed and the implantation-measurement cycle repeated again. The implanted lead ions are counted via their characteristic α-decay.
Figure 3 shows the intensity of the α-lines as a function of laser frequency for a sequence of nuclei (with even N) from 188Pb to 182Pb. This reveals the optical isotope shift, which allows us to deduce the values of δ<r2> shown in figure 1. Similarly, the experiment also measured isotopes with an odd number of neutrons, 183,185,187Pb, all of them produced in the ground and isomeric states. Note that the “isomer separation” could be obtained by tuning the laser frequency to some specific values at which only one of the isomers is selectively ionized in the cavity and subsequently extracted and analysed.
Figure 1 compares the deduced values of δ<r2> with the predictions of the spherical-droplet model. The deviation from these predictions increases when moving away from the Z = 82 closed proton shell of lead. The large deviation observed for the ground state of the odd-mass mercury isotopes and the odd- and even-mass platinum isotopes around N = 104 has been interpreted as a result of the onset of strong prolate deformation. In the case of lead, from 190Pb downwards, the δ<r2> data show a distinct deviation from the spherical-droplet model. This suggests modest ground-state deformation, but comparisons of the data with model calculations show that δ<r2> is sensitive to correlations in the ground-state wave functions and that the lead isotopes essentially stay spherical in their ground state at – and even beyond – the N = 104 mid-shell region.
This experiment has shown that the extreme sensitivity of the combined in-source laser spectroscopy and α-detection allows us to explore the heavy-mass regions far from stability with isotopes produced at a rate of only a few ions a second (182Pb). An important development would be: to use the isomer shift in the case of odd-mass-number isotopes to ionize nuclei selectively in their ground or isomeric state; to post-accelerate these with the REX-ISOLDE facility; and use the isomerically pure beams of the 13/2+ and 3/2– isomers to investigate, for example, the influence of different spin states of the same incident particle on the reaction mechanism.
The International Conference on Strangeness in Quark Matter, SQM 2007, took place on 24–29 June in the charming old town of Levoc̆a, located in Spis̆ in north-eastern Slovakia. Organized by the Institute of Experimental Physics of the Slovak Academy of Sciences, Kos̆ice, it was the 12th in a well-established series of topical conferences that bring together experts working in particle physics, nuclear physics and cosmology. More than 100 scientists from 20 countries took part this year, and the contributions covered a wide range of issues, from the bulk properties of the partonic matter created in nucleus–nucleus collisions, to the energy loss of fast partons traversing the medium, with a particular emphasis on the perspectives for the future.
The SQM series is currently dedicated to understanding what the production of strange – and also charm and beauty – particles can reveal about the hot and dense partonic matter formed in a high-energy nucleus–nucleus collision. It could perhaps more appropriately be called Strangeness, Charm and Beauty in Quark Matter. However, because of tradition, the original name has stuck. The extension to flavours heavier than strangeness has occurred naturally over the years as the high energies available at RHIC (and expected at the LHC) have turned charm- and beauty-flavoured particles into practical and promising probes for exploring QCD matter. On the experimental side, the challenge of detecting strange, charm and beauty particles is similar – although more difficult with charm and beauty – as the complete identification of all of these types of particle relies on identifying their decay products and decay vertices. Hence the need for similar techniques with the three flavours, both for the apparatus (high-granularity vertex detectors) and for the analysis. The SQM conferences therefore provide an excellent forum for researchers in this field to exchange not only physics results, but also information on experimental techniques and analysis methods.
There were more than 70 theoretical and experimental contributions this year, including review talks and reports from all of the active experiments at Brookhaven’s RHIC (BRAHMS, PHENIX, PHOBOS and STAR), at CERN’s SPS (CERES, NA49, NA57 and NA60) and at GSI’s heavy-ion synchrotron, SIS (FOPI). As the start-up of the LHC is just around the corner, more contributions than ever illustrated the plans for physics at future facilities. There were presentations on ALICE, the LHC experiment dedicated to heavy-ion physics, and the heavy-ion programmes for ATLAS and CMS and on the experiment on compressed baryonic matter (CBM) that is planned at the Facility for Antiproton and Ion Research at GSI.
The first day was devoted to a symposium where graduate students and post-doctorates had the opportunity to present their research results. Before the summary talks on the last day, a brief commemoration took place in honour of Maurice Jacob. He was a leader in the theory of high-energy hadron physics, a strong supporter of heavy-ion physics and a friend to many of us. He passed away on 2 May and we are all sorry that he did not live to enjoy the LHC’s results.
Hadronization and fragmentation
The bulk of the observed hadrons with low transverse momenta (pT < 2 GeV/c) are produced from matter that seems to be well-equilibrated by the time it dresses up into hadrons. In other words, statistical hadronization models reproduce hadron yields and ratios well, and in terms of only a few fitted -parameters, such as temperature and chemical potentials. A robust collective flow accompanies this equilibration. In non-central collisions, the spatial azimuthal asymmetry of the initial state transfers very efficiently to a momentum asymmetry of the final state. In a hydro-dynamical description, an “elliptic flow” of this kind – generated at the early stages of the expansion – gives access to the equation of state of partonic matter. The combination of hydrodynamics and statistical hadronization leads to a reasonable parameterization of the low-pT hadronic spectra and elliptic flow.
Many of the theory presentations dealt with the understanding of relativistic hydrodynamics and of the quark matter equation of state. Among several new results on the experimental side, we note that the RHIC data on copper–copper collisions at 200 GeV show enhancements of the Λ, Ξ, anti-Λ, anti-Ξ and Φ-meson with respect to proton–proton collisions. These enhancements are similar to those found (at a given number of participant nucleons) in gold–gold collisions at the same energy and in lead–lead collisions at the energies of CERN’s SPS.
The presence of the medium appears to modify fragmentation functions, which describe the dressing up of partons into final state particles. At high pT, the fragmentation of the parent parton is the dominating process. At intermediate pT (2 < psub>T < 6 GeV/c), however, the valence quark recombination or coalescence seems to play an important role. As a result, hadron production cannot be considered to be either thermal or perturbative, since the medium interferes with the hadronization process. For example, if hadrons are formed by recombination, the features of the parton spectrum are shifted to higher pT in the hadron spectrum – and in a different way for mesons than for baryons.
In this context interesting new results on K* production were presented. The azimuthal asymmetry of these particles corresponds to that expected from the recombination of two valence quarks. This would occur if coalescence of a valence quark–antiquark pair forms the K*. This is in contrast to what would happen if the K* were produced in the hadronic phase by combining a K and a π, each formed from a valence quark–antiquark pair, therefore requiring the recombination of four valence quarks (figure 1).
Fast parton energy loss
Strong quenching of hadrons with large transverse momentum (pT > 6 GeV/c) is another striking phenomenon, first observed at RHIC. The high-pT partons generated in hard scatterings at the initial stages of the nucleus–nucleus collisions do not fly away and hadronize freely. Instead, the nearby matter seems to largely absorb them. High-pT photons instead remain essentially unaffected, leading to a picture of a dense medium that is opaque to partonic, coloured projectiles but relatively transparent to photons.
Vigorous theoretical and experimental efforts are under way to understand parton energy loss in terms of perturbative QCD (pQCD). Various groups have described the suppression of light hadrons in terms of radiative energy loss by gluon bremsstrahlung. According to such calculations, charm and beauty quarks should be absorbed significantly less than light quarks and gluons. However, data from the PHENIX and STAR experiments, which compare the production in nucleus–nucleus and proton–proton collisions of high-pT “non-photonic” electrons (thought to originate mainly in heavy-flavour decays), seem to indicate that heavy quarks lose energy as much as light quarks do.
There were many contributions devoted to this puzzle at SQM 2007. Attempts to reduce disagreement by including elastic-scattering losses in addition to the radiative ones are being considered. On the experimental side, participants stressed the need to separate out the fraction of electrons coming from the decay of beauty hadrons, since b quarks are expected to lose even less energy than c quarks. Another important experimental caveat concerns the distribution of heavy quarks among the different heavy-flavour hadron species. This could change when going from proton–proton to nucleus–nucleus collisions, leading to pT-dependent variations of the semi-electronic branching ratios. Such an effect should obviously be kept under control when comparing electron production in nucleus–nucleus and proton–proton collisions. Some groups are making useful attempts in these directions by identifying the charmed meson D0 from the reconstruction of its decay. However, vertex detectors such as those of the LHC experiments are necessary for pursuing these studies further.
The fate of the energy deposited by the partons along their path also turns out to be non-trivial. It appears as though the partons’ propagation gives rise to some collective hydro-dynamical motion. Among the contributions on this subject, there was an interesting study of the response of the medium to energy loss, by analysing two- and three-particle correlations. The results seem to indicate a peak in particle production on a cone at an angle of about one radian from the direction of the propagating parton. A possible explanation would be the generation of a shock wave in the medium. The answer to this and many other questions will probably have to wait for the LHC data. We hope that there will be some to discuss at the next two conferences in this series being held in Beijing (2008) and Rio de Janeiro (2009).
Researchers at the National Superconducting Cyclotron Laboratory (NSCL) at Michigan State University have made new observations in different regions of the isotopic landscape by examining the nuclear structure of 64Ge and 36Mg.
Nuclei with equal proton (Z) and neutron (N) numbers are important in unravelling nuclear structure, in particular in the context of the shell model. Between 56Ni and 100Sn they exhibit a variety of shapes, evolving from spherical to prolate (cigar-shaped) to oblate (pancake-shaped) as the mass increases. Studies of transition rates between excited states and ground states in these nuclei provide important information to test shell-model predictions.
One such experiment at NSCL has studied 64Ge (N = Z = 32), making use of the recoil distance method (RDM) to measure the lifetime of two excited states (Starosta et al. 2007.) This was only the second measurement of this kind conducted in this region of isotopes, and the first to use the RDM at a fast-fragmentation facility. The beam speed at NSCL, 10 times higher than in previous RDM studies, allows for greater precision and gives access to a range of previously unattainable isotopes.
The experiment used a variety of state-of-the-art techniques, including a plunger device developed at the University of Cologne for use with the RDM. The plunger device produced the 64Ge nuclei in reactions where a single neutron was knocked out of incident 65Ge nuclei in a beam that contained a mixture of rare isotopes. The RDM used high-resolution gamma-ray spectroscopy and the Doppler effect to determine the lifetime of the excited states. The results agree well with large-scale shell-model calculations for the two excited states studied, and show the promise of the techniques used.
Exotic nuclei far from N = Z, with too many neutrons, offer other possibilities for testing shell-model predictions. One area of interest is the “island of inversion” where around a dozen neutron-rich isotopes should exhibit shell orderings that differ from standard theoretical predictions.
Studies of magnesium isotopes have already placed 31–34Mg (Z = 12, N = 19–22) in the island. Now, for the first time, an experiment at NSCL has examined the shell structure of 36Mg which has as many as 24 neutrons (Gade et al. 2007). In this case, a secondary beam of 38Si collided with a beryllium target to create 36Mg on rare occasions: only 1 in 400,000 38Si nuclei yielded the desired 36Mg. Spectroscopic measurements of the first excited state confirmed shell-model predictions, placing 36Mg in the island of inversion as expected.
A continuing mystery in nuclear and particle physics is the large polarization observed in the production of Λ hyperons in high-energy, proton–proton interactions. These effects were first reported in the 1970s in reactions at incident proton momenta of several hundred GeV/c, where experiments measured surprisingly strong hyperon polarizations of around 30% (Heller 1997). Although the phenomenology of these reactions is now well known, the inability to distinguish between various competing theoretical models has hampered the field (Zuo-Tang and Boros 2000).
Two new measurements from the US Department of Energy’s Jefferson Lab in Virginia are now challenging existing ideas on quark spin dynamics through studies of beam-recoil spin transfer in the electro- and photoproduction of K+Λ final states from an unpolarized proton target. Analyses of the two experiments in Hall B at Jefferson Lab using the CLAS spectrometer (figure 1) have provided extensive results of spin transfer from the polarized incident photon (real or virtual) to the final state Λ hyperon.
The results indicate that the Λ polarization is predominantly in the direction of the spin of the incoming photon, independent of the centre-of-mass energy or the production angle of the K+. Moreover, the photoproduction data show that, even where the transferred Λ polarization component along the photon direction is less than unity, the total magnitude of the polarization vector is equal to unity. Since these observations are not required by the kinematics of the reaction (except at extreme forward and backward angles) there must be some underlying dynamical origin.
Both analyses have proposed simple quark-based models to explain the phenomenology, however they differ fundamentally in their description of the spin transfer mechanism. In the electroproduction analysis a simple model has been proposed from data using a 2.567 GeV longitudinally polarized electron beam (Carman et al. 2003). In this case a circularly polarized virtual photon (emitted by the polarized electron) strikes an oppositely polarized u quark inside the proton (figure 2a). The spin of the struck quark flips in direction according to helicity conservation and recoils from its neighbours, stretching a flux-tube of gluonic matter between them. When the stored energy in the flux-tube is sufficient, the tube is “broken” by the production of a strange quark–antiquark pair (the hadronization process).
In this simple model, the observed direction of the Λ polarization can be explained if it is assumed that the quark pair is produced with two spins in opposite directions – anti-aligned – with the spin of the s quark aligned opposite to the final u quark spin. The resulting Λ spin, which is essentially the same as the s quark spin, is predominantly in the direction of the spin of the incident virtual photon. The spin anti-alignment of the ss pair is unexpected, because according to the popular 3P0 model, the quark–antiquark pair should be produced with vacuum quantum numbers (J = 0, S = 1, L = 1, i.e. Jπ = 0+), which means that their spins should be aligned two-thirds of the time (Barnes 2002). This could imply that this model for hadronization may not be as widely applicable as previously thought.
The new photoproduction analysis, with data using a circularly polarized real photon beam in the 0.5–2.5 GeV range, introduces a different model that can also explain the Λ polarization data. In this hypothesis, shown in figure 2b, the strange quark–antiquark pair is created in a 3S1 configuration (J = 1, S = 1, L = 0, i.e. Jπ = 1–). Here, following the principle of vector-meson dominance, the real photon fluctuates into a virtual φ meson that carries the polarization of the incident photon. Therefore, the quark spins are in the direction of the spin of the photon before the hadronization interaction.
The s quark of the pair merges with the unpolarized di-quark within the target proton to form the Λ baryon. The s quark merges with the remnant u quark of the proton to form a spinless K+ meson. In this model, the strong force, which rearranges the s and s quarks into the Λ and K+, respectively, can precess the spin of the s quark away from the beam direction, but the s quark, and therefore the Λ, remains 100% polarized. This provides a natural explanation for the observed unit magnitude of the Λ polarization vector seen for the first time in the measurements by CLAS.
The model interpretations presented from the two viewpoints do not necessarily contradict each other. Both assume that the mechanism of spin transfer to the Λ hyperon involves a spectator Jπ = 0+ di-quark system. The difference is in the role of the third quark. Neither model specifies a dynamical mechanism for the process, namely the detailed mechanism for quark-pair creation in the first case or for quark spin precession in the second. If we take the gluonic degrees of freedom into consideration, the model proposed in the electroproduction paper (Carman et al. 2003) can be realized in terms of a possible mechanism in which a colourless Jπ = 0– two-gluon subsystem is emitted from the spectator di-quark system and produces the ss pair (figure 2a). This is in conflict with the 3P0 model, which requires a Jπ= 0+ exchange. To the same order of gluon coupling, the model interpretation proposed by the photoproduction analysis (Schumacher 2007) is the quark-exchange mechanism, which is again mediated by a two-gluon current. The amplitudes corresponding to these models may both be present in the production, in principle, and contribute at different levels depending on the reaction kinematics.
Extending these studies to the K*+Λ exclusive final state should be revealing. In the electroproduction model, the spin of the u quark is unchanged when switching from a pseudoscalar K+ to a vector K*+. If the ss quark pair is produced with anti-aligned spins, the spin direction of the Λ should flip. On the other hand, in the photoproduction model the u quark in the kaon is only a spectator. Changing its spin direction – changing the K+ to a K*+ – should not change the Λ spin direction. Thus, there are ways to disentangle the relative contributions and to understand better the reaction mechanism and dynamics underlying the associated strangeness-production reaction. Analyses at CLAS are underway to extract the polarization transfer to the hyperon in the K*+Λ final state.
Beyond the studies of hyperon production, understanding the dynamics in a process of this sort can shed light on quark–gluon dynamics in a domain thought to be dominated by traditional meson and baryon degrees of freedom. These issues are relevant for a better understanding of strong interactions and hadroproduction in general, owing to the non-perturbative nature of QCD at these energies. We eagerly await further experimental studies and new theoretical efforts to understand which multi-gluonic degrees of freedom dominate in quark pair creation and their role in strangeness production, as well as the appropriate mechanism(s) for the dynamics of spin transfer in hyperon production.
Researchers at the National Superconducting Cyclotron Laboratory (NSCL) at Michigan State University have produced the heaviest silicon isotope ever observed. The recent identification of 44Si expands the chart of known isotopes and lays the groundwork for the future study of rare, neutron-rich nuclei.
Beyond a certain range of combinations of protons and neutrons, nuclei cannot form at all, and additional nucleons will immediately leave the nucleus owing to zero binding energies. Pursuit of this limit, known as the drip line, has proved to be a scientific and technical challenge – particularly when it comes to neutron-rich nuclei. While the proton drip line has been mapped out for much of the chart of nuclei, the neutron drip line is known only up to oxygen (Z = 8). Producing isotopes at or near the neutron drip line remains a long-standing goal in experimental nuclear physics. For example, 43Si was detected for the first time at Japan’s Institute of Physical and Chemical Research (RIKEN) in 2002 (Notani et al. 2002). That same year, researchers at the GANIL laboratory in France detected the neutron-rich isotopes 34Ne and 37Na (Lukyanov et al. 2002).
In the 44Si experiment conducted at the NSCL Coupled Cyclotron Facility in January, a primary beam of 48Ca was accelerated to 142 MeV/u and directed at a tungsten target. Downstream from the target, the beam was filtered through NSCL’s A1900 fragment separator. Eventually, some 20 different isotopes (including three nuclei of 44Si) hit a set of detectors that could identify each ion as it arrived (Tarasov et al. 2007).
The study was intended to document the yield of isotopes containing 28 neutrons that lie between 48Ca (the nuclei in the beam) and 40Mg to extrapolate the expected yields in this region. 40Mg is yet to be observed, and according to some theories should be on the drip line. Knocking out only protons from 48Ca could create these isotopes, although this is a difficult feat because of the larger number of neutrons in the beam nuclei. The production of 44Si is therefore an even greater feat, given that the collision must also transfer two neutrons from the tungsten target to the beam nucleus as it speeds past. The observation of 44Si in the A1900 fragment separator stretches the limits of its single-stage separation. The excessive number of particles that come along with the rare nuclei can swamp the detectors used to identify the beam in the separator. The next-generation technique will use two-stage separation, delivering fewer particles to the detectors as more are filtered out travelling down the beamline.
Researchers are developing new two-stage separators that could run experiments with higher initial beam intensities, which offer a better chance of generating the sought-after, near-dripline nuclei. Preliminary testing on a new two-stage separator at NSCL has delivered promising results. Also, a new device has just been constructed at RIKEN in Japan, and one is planned for GSI in Germany. Nuclear scientists at NSCL hope that two-stage separation will help uncover the next generation of rare isotopes.
When the LHC starts up at CERN, it will provide proton collisions at higher energies than any previous accelerator and at high collision rates. While these conditions should reveal new high-energy phenomena, such as the Higgs mechanism and supersymmetry, they will at the same time open a different window onto new physics through the study of rare processes among existing particles in the Standard Model. This is the territory that the Large Hadron Collider beauty (LHCb) experiment will explore.
By undertaking precision studies of the decays of particles that contain heavy flavours of quarks (charm and beauty), LHCb will stringently test our knowledge of the Standard Model. In addition, these studies will search for new particles beyond the Standard Model through their virtual effects – just as the mass of the top quark was known well before it was directly observed. The results will provide a profound understanding of the physics of flavour and will cast more light on the subtle difference between matter and antimatter that is manifest in CP violation.
Good particle identification is a fundamental requirement
The LHCb detector looks very different from the average hadron collider detector – indeed, it looks more like a fixed-target detector (figure 1) – because of its focus on heavy flavour particles. This choice of detector geometry is motivated by the fact that, at high energies, both B(D) and B(D) hadrons are produced at predominantly low angles and in the same “forward” cone. The detector geometry is optimized to detect these forward events efficiently.
LHCb’s physics programme depends on being able to distinguish between the particle species produced so good particle identification is a fundamental requirement. The LHCb detector contains calorimeters and muon chambers to identify electrons, photons and muons. But to separate pions, kaons and protons in selected decays, a powerful different technique comes into play. This is the ring imaging Cherenkov (RICH) detector, first proposed at CERN in 1977 by Jacques Séguinot and Tom Ypsilantis, who was a member of the LHCb collaboration until his death in 2000.
The basic idea is that when a charged particle passes through a medium faster than the speed of light in that medium, it will emit Cherenkov radiation (named after the 1958 Nobel prize winner Pavel Cherenkov, who was the first to characterize the radiation rigorously). The effect is like a shock wave of light similar to the sonic boom of an aircraft travelling faster than the speed of sound.
This radiation is emitted at an angle to the direction of motion of the particle, forming a cone of light around the particle’s track. The angle of emission, θ, depends on the velocity of the particle but not on its mass, with cosθ = 1/nβ, where n is the refractive index of the medium and β is the velocity relative to the velocity of light in free space, c. Combining this velocity information with a measurement of the momentum of the particle (using tracking detectors and a known magnetic field), yields the mass of the particle and therefore its identity.
The simplest Cherenkov detectors are threshold devices that only produce a signal if the velocity of a charged particle exceeds the minimum necessary to produce Cherenkov radiation in a particular medium, or “radiator”. Taken together with a momentum measurement, this allows particles that are heavier than a certain mass to be separated from lighter ones. Such detectors have been employed in many experiments since the 1950s, for example in the classic detection of the antiproton at Berkeley – an experiment in which the young Ypsilantis participated.
The RICH detector is a far more sophisticated development. In a RICH device, the cone of Cherenkov light emitted in the radiator is detected on a position-sensitive photon detector. This allows the reconstruction of a ring or disc, the radius of which depends on the emission angle, θ, and hence on the velocity of the particle. In the RICH used by LHCb, the photons are collected by a spherical mirror and focused onto an array of photon detectors at the focal plane (figure 2 shows the principal in LHCb’s RICH1 detector). By focusing the radiation, the photons will form a ring with a radius that depends on the emission angle, θ, but not on where the light is emitted along the particle track.
The choice of which radiator to use is crucial, as every medium has a restricted velocity range over which it can usefully identify particles. Too low a velocity, and the particle will produce no light; too high, and the Cherenkov angle for all particle species will saturate to a common value, making identification impossible. It was therefore important for LHCb to choose a medium, or combination of different media, that would be effective over the full momentum range of interest – from around 1 GeV/c, up to and beyond 100 GeV/c. To achieve this coverage, the experiment uses a combination of three radiators – aerogel, perfluoro-n-butane (C4F10) and carbon tetrafluoride (CF4).
Silica aerogel is a colloidal form of quartz solid, but with an extremely low density and a high refractive index (1.01–1.10), which makes it perfect for the lowest-momentum particles (order of a few GeV/c). One of the key design issues for LHCb was the use of aerogel in ring-imaging mode. This was a new idea, inspired by the development of much higher-quality, very clear aerogel (figure 3). Previously, the material had only been used in threshold counters. To cover the regions of medium and high momentum, LHCb uses a combination of C4F10 and CF4 radiators for momenta from around 10 GeV/c to around 65 GeV/c, and from around 15 GeV/c to more than 100 GeV/c, respectively.
The early design of the system had three separate detectors, one for each radiator, but for a variety of reasons it proved more practical to combine the aerogel and C4F10 radiators into a single device with wide acceptance. This is the RICH1 detector, which is located upstream to detect the low-momentum particles (figure 1). The CF4 radiator is housed in RICH2, downstream of the tracking system and the LHCb magnet. This has an acceptance that is limited to the low-angle region where there are mostly high-momentum particles.
One challenge in both cases, was to minimize the amount of material within the detector acceptance. Therefore, the designs were changed at an early stage to tilt the focusing mirrors slightly and to introduce secondary flat mirrors that bring the Cherenkov radiation right out of the detector acceptance. This allows for a smaller photon-detector area and a more compact system.
A more radical redesign took place later, as the engineering designs for the various subdetectors became more realistic. It became clear that LHCb had too much material and needed re-designing. The challenge was also to improve the trigger performance by increasing the precision of the momentum measurement, and this required increasing the magnetic field in the region of the VErtex LOcator (VELO) and the trigger tracker (TT) between RICH1 and the dipole magnet (see figure 1).
While RICH2 remained relatively unaffected, RICH1 underwent a major redesign. To protect the sensitive photon detectors from the greatly increased magnetic field, extremely heavy iron shielding had to be added to the apparatus. Accommodating these shields in the very congested region of LHCb’s experimental area near RICH1 was a major challenge.
Seeing the light
Particles produced in the collisions in LHCb will travel through the mirrors of RICH1 prior to reaching measurement components further downstream. To reduce the amount of scattering, RICH1 uses special lightweight spherical mirrors constructed from a carbon-fibre reinforced polymer (CFRP), rather than glass. There are four of these mirrors, each made from two CFRP sheets moulded into a spherical surface with a radius of 2700 mm and separated by a reinforcing matrix of CFRP cylinders. The overall structure contributes about 1.5% of a radiation length to the material budget of RICH1. As RICH2 is located downstream of the tracking system and magnet, glass could be used for its spherical mirrors, which in this case are composed of hexagonal elements (see cover).
Perhaps surprisingly, the “flat” secondary mirrors in the RICH detectors are not truly flat. Producing completely flat, but thin, mirrors is a difficult technological challenge because it is hard to maintain their rigidity over a long period of time. Instead, giving the mirrors a small amount of curvature (a radius of curvature greater than 600 m in RICH1 and around 80 m in RICH2), increases their structural integrity. The small distortions that this curvature introduces to the images of the Cherenkov ring can be corrected with software during data analysis, and therefore do not degrade the final performance of the system.
The experiment requires 484 tubes in total
Both RICH detectors use hybrid photon detectors (HPDs) to measure the positions of the emitted Cherenkov photons. The HPD is a vacuum photon detector in which a photoelectron, released when an incident photon converts within a photocathode, is accelerated by a high voltage of typically 10–20 kV onto a reverse-biased silicon detector. The tube focuses the photoelectron electrostatically – with a demagnification factor of around five – onto a small silicon detector array.
The LHCb collaboration has developed a novel dedicated pixel–HPD for the RICH detectors, working in close co-operation with industry. Here, the silicon detector is segmented into 1024 “super” pixels, each 500 μm × 500 μm in area and arranged as a matrix of 32 rows and 32 columns. When a photoelectron loses energy in silicon, it creates electron-hole pairs at an average yield of one for every 3.6 eV of deposited energy. The nominal operating voltage of LHCb’s HPDs is –20 kV, corresponding to around 5000 electron-hole pairs released in the silicon. Careful design of read-out electronics and interconnects to the silicon detector results in a high efficiency for detecting single photoelectrons. The experiment requires 484 tubes in total – 196 for RICH1 and 288 for RICH2 – to cover the four detection surfaces.
Testing times
To verify the quality of the HPDs and the associated components in the low-level data acquisition (DAQ), the LHCb collaboration has conducted a series of RICH test-beam exercises, most recently during September 2006 in the North Area at CERN’s Prévessin site. In the test beam, the apparatus consisted of a gas vessel filled with either nitrogen (N2) or C4F10 as the radiator medium, together with a housing for the photo-detectors that was separated from the gas enclosure by a transparent quartz window. The test beam from the SPS consisted mainly of pions, with small contributions from electrons, kaons and protons, and had a 25 ns bunch-structure; the same as will be provided by the LHC.
Columns of 16 HPDs observed the Cherenkov radiation emitted by the particles as they traversed the gas enclosure. The ring of Cherenkov light illuminated either one HPD, when using N2 as radiator, or up to four neighbouring HPDs with the C4F10 radiator (figure 4). The resulting data were recorded using final versions of the DAQ electronics and pre-production releases of the LHCb online software environment. An early version of the LHCb online-monitoring kept a check on the status of the test set-up and the quality of recorded data.
The analysis of the recorded test-beam data using the full LHCb reconstruction and analysis software involved a significant effort, but the results made it worthwhile. The tests verified the design specifications of the HPDs in a “real life” environment, with the measurement of properties such as the photoelectron yield and the resolution of the Cherenkov angle reconstructed from the data. Using the official LHCb software framework for the analyses also allowed the quality of the software to be verified with real data, so the team could spot any issues not seen in earlier simulation studies. The evaluation of the beam-tests indicates so far that all the hardware and software components involved in the tests match – or exceed – expectations, successfully passing an important milestone on the way to the start-up of the LHCb experiment.
The full LHCb detector has been extensively modelled in a detailed simulation, based on the Geant4 software package, taking into account all important aspects of the geometry and materials together with a full description of the optics of the RICH detectors. This has provided a platform for the development of sophisticated analysis software to reconstruct the events and provide excellent particle identification. Figure 5 shows an example of the complex event environment that LHCb will face in collisions at the LHC. To disentangle the event, the analysis performs a combined likelihood-fit to all known tracks in the event. By considering all tracks and radiators in a single fit, the algorithm naturally accounts for the most predominant background to a given ring, namely the neighbouring rings.
Figure 6 illustrates just how powerful this technique is. Here, using the detailed Geant4 simulations, the mass peak for the decay BS → KK is shown, together with the background contributions from other two-body decays. Without the kaon identification capabilities provided by the RICH detectors, the BS signal is swamped by background. Such efficient hadron identification will be a crucial component in the successful analysis of LHCb data.
Currently, the RICH group is fully focused on the commissioning of the RICH detectors at the experimental area at Point 8 on the LHC ring. The RICH2 detector is completely installed and the HPDs and readout systems are being commissioned. The magnetic shielding and radiator enclosure for RICH1 is in place and installation of all HPDs and optics will be completed later this year. Commissioning of the detector control and safety systems, together with the readout DAQ systems is also progressing at full speed. Everything is on track to have the system fully functional and ready for action for first data in 2008.
When the LHC starts up, heavy-ion physics will enter an era where high transverse-momentum (pT) processes contribute significantly to the nucleus–nucleus cross-section. The LHC will produce very hard, strongly interacting probes – the attenuation of which can be used to study the properties of the quark–gluon plasma (QGP) – at sufficiently high rates to make detailed measurements. At the LHC, high rates are for the first time expected at energies at which jets can be fully reconstructed against the high background from the underlying nucleus–nucleus event.
To prepare for the new high pT and jet analysis challenges, the physics department at the University of Jyväskylä, Finland, organized the five-day Workshop on High pT Heavy-Ion Physics at the LHC. More than 60 participants attended the workshop, ranging from senior experts in heavy-ion physics to doctoral students. It brought together physicists from operating facilities – mainly RHIC at the Brookhaven National Laboratory (BNL) – as well as from future LHC experiments (ALICE, ATLAS and CMS), and included valuable contributions from theorists. Jyväskylä in early spring, coupled with reindeer-meat dinners and animated student lectures in the evening, created a superb atmosphere for many discussions of physics, even outside of the official programme.
Mike Tannenbaum of BNL gave an opening colloquium which looked back to the 1970s. He listed old results that raised the same questions that are the focus of today’s discussions. Many recent questions in high-pT physics can be traced back to the 1970s at CERN, with proton–proton (pp) collisions at the ISR, which were followed in the early 1980s by proton–antiproton collisions at the SPS. This was when jet physics was born and the first methods of jet analysis were developed. It was reassuring to learn that many CERN results remain valid and that recent thinking is really based on those early understandings. On the other hand, many ideas still remain in a premature state. Only the high-luminosity experiments at RHIC and the LHC are – or will be – able to investigate certain phenomena and measure their effects more precisely. These pp data are therefore very important, not merely because they serve as a baseline for understanding results in heavy-ion collisions.
Striking gold at RHIC
Several presentations at the workshop reviewed results from RHIC on single-particle spectra and two-particle correlations at high pT. Striking effects have been observed in central gold–gold collisions. Among the most prominent are the suppression of high-pT particles and the suppression of back-to-back correlations. These results show that the jet structure is strongly modified in dense matter consistent with perturbative QCD calculations of partonic energy loss via induced gluon radiation.
One of the striking effects observed at RHIC: strong suppression of high-pT pions (π) and etas (η) in gold–gold collisions [PHENIX 2006 Phys. Rev. Lett. 96 202301] and no or moderate suppression of direct photons.The first photon data have shown no nuclear effects up to 10–12 GeV/c, in line with the general expectation that photons (with no colour charge) have no final-state interaction with the deconfined matter that is produced. However, the recent measurement by the PHENIX experiment indicates unexpected suppression, by a factor of two, of photon production in the region above 15 GeV/c – this is almost as large as in the case of light mesons (figure 1). This surprising observation ignited great excitement at the workshop, leading to further discussion of what the possible consequences for LHC physics might be. Any detailed study, however, should await the release of the final data.
Data on heavy flavours from RHIC experiments have also provided puzzles. The measured suppression of heavy-flavour pT spectra, which is close to that of light flavours, cannot be explained by radiative energy loss alone and requires a contribution from elastic scattering. Further issues can be addressed by analysis of dijet topology or by the use of two- or multi-particle correlation techniques. Several experimental and theoretical presentations given at the meeting examined the possibility of using multi-particle correlation and photon–hadron correlations to study the partonic pT distributions, fragmentation functions, jet shape and other parton properties sensitive to the details of parton interactions with excited nuclear matter.
Another series of talks investigated the features of the parton coalescence process, which is supported by a large amount of experimental data on particle spectra and asymmetrical flow production. On the other hand, jet-orientated analysis of different data (for example, Ω–-charged hadron correlations) does not show the behaviour expected from quark coalescence. Therefore, further work is needed to understand this puzzling situation before the new experiments begin at the LHC.
Among the four large LHC experiments, ALICE is the one that is optimized for heavy-ion physics. The CMS and ATLAS collaborations have also established a heavy-ion programme, which will certainly strengthen the field. The workshop heard about the capabilities of the three experiments for jet reconstruction and analysis of jet structure. The large background from the underlying event is a challenge for all experiments, requiring the development of new techniques for background subtraction. The strength of the ATLAS and CMS experiments is their full calorimetric coverage, and therefore large measured jet rates, which will allow them to measure jets in central lead–lead collisions up to 350 GeV and to perform Z0-jet correlation studies. ALICE will use the combination of its central tracking system and an electromagnetic calorimeter to measure jets. The smaller acceptance of the detector will limit the energy range to about 200 GeV. The strength of ALICE lies in its low-pT and particle identification capabilities. These allow ALICE to measure fragmentation functions down to small momentum fractions and to determine the particle composition of jets (figure 2).
A consistent theoretical approach to describe jet measurements in heavy-ion collisions can only be obtained through detailed Monte Carlo studies of jet production and in-medium modifications. They are needed to optimize the data analysis and to discriminate between different models. Some new event-generators adopted for the challenges of LHC physics (PyQuench, HydJet, HIJING-2) were also discussed during the meeting.
The workshop also examined the recent interest in understanding strongly interacting particles using conjectures from string and higher-dimensional physics. Stan Brodsky of SLAC gave a summary of his understanding of the many QCD effects that appear in kinematical regions not testable by perturbative QCD, where anti-de Sitter space/conformal field-theory models could come into consideration. In the duality picture, due to Juan Maldacena, the intensively interacting quark and gluon fields produced in heavy-ion collisions can be treated as a projection into the higher dimensional black-hole horizon. The equation of motion on the black-hole horizon could become analytically solvable in contrast to the vastly complicated numerical (lattice) approach in non-perturbative QCD theory. The new experiments at LHC energies May shed more light on the role of extra dimensions in curved space and could initiate a revolution in the description of strongly interacting matter.
The next workshop on this topic will be in Budapest in March 2008 and will offer the opportunity to display the latest theoretical results before the LHC is running with pp collisions at 14 TeV.
Exceptionally beautiful weather, Munich’s Holiday Inn hotel and the Gasteig, a modern cultural centre, combined to provide a pleasant and stimulating atmosphere for DIS 2007, the 15th International Workshop on Deep-Inelastic Scattering (DIS) and Related Subjects. Held on 16–20 April, the workshop united more than 300 physicists from around the world, including an encouraging number of students. The programme contained reviews of progress in DIS and QCD, as well as presentations of the latest results from HERA, the Tevatron, Jefferson Lab, RHIC and fixed-target experiments. It also covered related theoretical topics and future experimental opportunities.
With two full days of plenary sessions and six streams of parallel sessions on the other three days, the meeting followed the traditional style of DIS workshops. The parallel sessions covered structure functions and low-x physics, electroweak measurements and physics beyond the Standard Model, heavy flavours, hadronic final states, diffraction and spin physics. A special session that looked to the future of DIS was particularly topical in view of the shutdown at DESY of HERA, the world’s only electron–proton collider, at the end of June.
Yuri Dokshitzer, of the University of Paris VI and VII, opened the scientific programme with a review of recent developments in perturbative QCD (pQCD). He explained his motto “1-loop drill, 2-loop thrill, 3-loop chill” and expressed the hope that higher-order corrections can be calculated with the help of N = 4 super-Yang–Mills quantum field theory.
Appetizing glimpses of the many new results from the two collider experiments at HERA featured in talks by Christinel Diaconu from the Centre for Particle Physics in Marseille, and by Massimo Corradi of INFN Bologna, for the H1 and ZEUS experiments, respectively. Both experiments have accumulated a total of 0.5 fb–1 at a proton beam energy of 920 GeV, and analyses of the entire data sample are in full swing. The first H1 and ZEUS combined analysis of xF3 was a clear highlight of the conference (figure 1). This is the structure function that is dominated by photon–Z interference and is sensitive to the valence quarks at low Bjorken-x.
Further highlighted results included new data on neutral-current and charged-current inclusive scattering, jets and heavy-flavour production. These data will serve as input for the next generation of more precise fits for parton distribution functions (PDFs) for the proton – essential for studying physics at the LHC at CERN.
Since mid-March the proton beam energy at HERA has been lowered to 460 GeV to enable, in conjunction with the high-energy data at 920 GeV, a model-free determination of the longitudinal structure function FL. This measurement is essential for a direct extraction of the gluon distribution within the proton and as a consistency check of DIS theory. Beyond the Standard Model, H1 continues to see, with the full statistics at high energy, the production of isolated leptons at a level of 3 σ above the expectation. In contrast, ZEUS sees no deviation from the Standard Model.
With the Tevatron proton–antiproton collider at Fermilab performing well, Giorgio Chiarelli of INFN Pisa was able to show a sample of beautiful new results from the CDF and DØ experiments. For this conference, he presented data corresponding to up to
2 fb–1, covering neutral B-meson oscillations, electroweak physics, jets, searches and results on the production of the top quark, with a new world average for its mass of 170.9 ± 1.8 GeV/c2. This new (low) value is interesting since, together with the mass of the W particle, it favours the minimal supersymmetric model.
William Zajc from Columbia University addressed current understanding of particle production in heavy-ion collisions, as studied at RHIC at Brookhaven National Laboratory (BNL). He highlighted several interesting experimental observations, such as “away-side” jet suppression, that cannot be described within current models, but which May be interpreted as a signal for the production of a nearly perfect, highly viscous quark–gluon fluid.
Turning to spin physics, Jörg Pretz from the University of Bonn gave an overview with emphasis on the nucleon spin puzzle. He presented recent data on helicity distributions for quarks (Δq) and gluons (ΔG), from the HERMES experiment at DESY and COMPASS at CERN, respectively, as well as direct measurements of ΔG from RHIC. He also showed the first combined results on transversity using data from both HERMES and COMPASS as well as from the BELLE experiment at KEK. In a related overview of the rich programme at Jefferson Lab, Zein-Eddine Mezani of Temple University in Philadelphia covered measurements of unpolarized and polarized structure functions and transversity, as well as deeply virtual Compton scattering and generalized parton distributions.
Theoretical input
Andreas Vogt of Liverpool University spoke about progress and challenges in determining and understanding the PDFs of the proton in next-to-leading order (NLO) and next-to-NLO. An important improvement in the extraction of PDFs, implemented by the Coordinated Theoretical–Experimental Project on QCD (CTEQ), is the inclusion of the effects of charm-mass suppression in DIS, which results in an increase in the PDFs for the u and d quark. A dramatic consequence is an increase by about 8% of the W/Z cross-sections expected at the LHC. Rates of W/Z events are foreseen to serve as precision “luminosity meters” for the LHC data-taking.
Gustav Kramer of Hamburg University discussed recent developments in heavy-flavour production and explained the various heavy-flavour schemes used for pQCD calculations. He stressed the importance of interpolating schemes with variable-flavour number and massive heavy quarks (like the general-mass variable-flavour-number scheme) and showed successful comparisons of calculations with data from HERA and the Tevatron.
To allow comparison with experiment, pQCD calculations usually need to be implemented in Monte Carlo generators. Zoltan Nagy from CERN covered this important subject and critically reviewed the various approximations of current implementations of parton showers and their matching to leading order or NLO matrix elements. Nagy expressed concern that current Monte Carlo tools might fail at the LHC and he argued for the development of a new shower concept that allows the shower to be matched to Born and NLO matrix elements.
Raju Venugopalan from BNL covered small-x physics and the expected non-linear effects beyond the conventional Dokshitzer–Gribov–Lipatov–Altarelli–Parisi evolution. He discussed the question of saturation in the context of various models (e.g. colour glass condensate) and data from HERA and RHIC. He also pointed to excellent opportunities at a possible future electron–ion collider (EIC) or even at a “super-HERA” collider such as a large hadron–electron collider (LHeC).
Peter Weisz and Johanna Erdmenger, both from MPI Munich, discussed non-perturbative aspects of QCD. Weisz presented recent algorithmic advances and various results in lattice QCD, indicating progress in the simulation of dynamical quarks beyond the quenched approximation. Erdmenger looked at new approaches that connect string theory and QCD by establishing a connection between a strong coupling (non-perturbative) theory, such as N = 4 SYM (“QCD”), and a “dual” weak coupling theory, such as supergravity. Such a relation – the anti-de Sitter/conformal field theory correspondence – can provide new tools to address problems within QCD.
The seven threads of parallel sessions contained a total of 260 talks. Despite the wonderful weather, the sessions had very good attendance, with many lively and fruitful discussions. The spontaneous formation of two additional topical sessions was very much in the spirit of the workshop. One of these was on αS measurements from HERA and LEP, and one was on the complications involved when dealing with a variable number of quark flavours in QCD fits. On the last day the convenors, usually a theorist and an experimentalist for each working group, summarized the parallel sessions.
Life after HERA
Concluding a special session on the future of DIS, Joel Feltesse of DAPNIA gave a detailed and critical view of future opportunities in DIS. In his opinion DIS will not stop with the end of data-taking at HERA. There is Jefferson Lab with its upgrade to 12 GeV and new machines, such as the EICs at Jefferson Lab and BNL, are on the horizon. An LHeC at CERN would offer an attractive physics programme, particularly if the LHC provides an additional physics case for it. The workshop itself concluded with a talk from Graham Ross from Oxford University. He discussed open questions beyond the Standard Model, which provide motivation for the next round of high-energy physics experiments at the LHC.
For the coming years, much careful analysis remains to be done with the data from HERA to achieve the best possible precision. This is expected to yield valuable information for the understanding of QCD and of the data to be produced at the LHC. HERA’s final legacy will be an important asset to high-energy physics. Although the LHC will, we hope, find the Higgs boson and “explain” the mass of gauge bosons, quarks and leptons, it remains the case that the mass of hadronic matter – about 99% of the mass of the visible universe – is entirely dominated by effects due to the strong interaction between gluons and quarks. Deep-inelastic scattering is the tool to study these interactions. It remains to be seen how much progress will be achieved in the future without new data from an electron–hadron collider.
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The technical storage or access is strictly necessary for the legitimate purpose of enabling the use of a specific service explicitly requested by the subscriber or user, or for the sole purpose of carrying out the transmission of a communication over an electronic communications network.
Preferences
The technical storage or access is necessary for the legitimate purpose of storing preferences that are not requested by the subscriber or user.
Statistics
The technical storage or access that is used exclusively for statistical purposes.The technical storage or access that is used exclusively for anonymous statistical purposes. Without a subpoena, voluntary compliance on the part of your Internet Service Provider, or additional records from a third party, information stored or retrieved for this purpose alone cannot usually be used to identify you.
Marketing
The technical storage or access is required to create user profiles to send advertising, or to track the user on a website or across several websites for similar marketing purposes.
