The book is a tribute to the great experimental physicist Lord Patrick M S Blackett, written by one of his pupils at the Sphynx Observatory, Antonio Zichichi. Blackett is well known for his work on cloud chambers and cosmic rays, which earned him the Nobel Prize in Physics in 1948.
The author offers his personal testimony, from the first time he heard Blackett’s name to when he went to work with him, and then about the research he could be involved in. He provides a profile of his subject while giving an overview of Blackett’s work and, in particular, of his most significant discoveries, including the so-called vacuum-polarisation effect, the first example of “virtual physics”, and strange particles. The important implications of Blackett’s pioneering contribution to sub-nuclear physics are also discussed.
The book also presents a portrait of the world of physics during those times, and gives insights into life and research at CERN, as well as about Blackett’s ideas. He was very interested in the role of science in the culture of the time. He was convinced that physicists should be directly engaged with communicating to society, which should be informed about the contribution of science to the progress of our civilisation.
Rich in personal anecdotes, pictures and appendices, the book could appeal to physicists and students who are also interested in the history of science and in the human dimension of great scientists. As a final point, the layout and editing could be improved.
This is a detailed and up-to-date textbook on neutrino oscillations. After a short historical introduction (chapter 1), chapter 2 contains a concise, yet quite complete, presentation of neutrino theory in the Standard Model, including neutrino interactions and production in pion, muon and nuclear beta decay. The basic ideas of particle oscillation in quantum mechanics are introduced in chapter 3, and a detailed theory of neutrino oscillations is presented in chapter 4 – first in a two-neutrino approximation, then generalised to the three neutrino flavours – for oscillations both in vacuum and matter. In addition to the usual neutrino description in terms of plane waves, this chapter includes the mathematical treatment of a wave-packet oscillation, which helps in understanding neutrino oscillations over astronomical distances.
Chapter 5 contains a description of past and present oscillation experiments and of the results published prior to 2014, including the measurement of θ13. These results are again summarised in chapter 6, where the current knowledge of three-neutrino oscillation parameters is described. Future experiments to measure the remaining oscillation parameters (the so-called neutrino mass hierarchy and the CP-violation phase) are discussed in chapter 7, together with oscillation anomalies observed by a number of experiments (LSND, MiniBoone, Gallium and recent re-analyses of old reactor experiments). These anomalies, if confirmed, would imply the existence of at least one additional “sterile” neutrino with a mass in the order of 1 eV, requiring a mixing matrix of larger dimensions and more oscillation parameters. Chapter 7 also includes a discussion of the difference between Dirac and Majorana neutrinos, and the implications of direct measurements of the effective νe mass and of searches for neutrinoless double beta decay. Finally, chapter 8 contains a useful appendix summarising all the symbols, abbreviations and formulae used in the book.
The textbook contains all of the information that anybody interested in neutrino oscillations would like to know. Physicists involved in neutrino experiments should each have a copy in their private libraries.
Romania has become the 22nd Member State of CERN, having acceded to the Organization’s founding convention, which is deposited with UNESCO, on 17 July. The accession crowns a period of co-operation that stretches back 25 years. “This is a very special moment for Romania and its relationship with CERN,” says ambassador Adrian Vierita, Romania’s permanent representative to the United Nations in Geneva.
Bilateral discussions between the Romanian government and CERN began in 1991. Aspiring to become a Member State and therefore to contribute fully to the governance of the laboratory, Romania submitted its formal application to join CERN in April 2008.
Today, Romania has around 100 visiting scientists at CERN and a particularly strong presence in the LHC experiments ATLAS, ALICE and LHCb, in addition to the DIRAC, n_TOF and NA62 experiments. “The accession of Romania to full CERN membership underlines the importance of European research collaboration in the quest to understand nature at its most fundamental level,” says the president of CERN Council, Sijbrand de Jong. “United, we can do so much more than as individual countries.” The Romanian flag will be raised alongside 21 others at the CERN entrance on 5 September.
The 38th International Conference on High-Energy Physics (ICHEP 2016) took place on 3–10 August in Chicago, US. Among numerous results presented, the LHC experiments released their latest analyses of 13 TeV proton–proton collision data recorded in 2016.
Based on a data set of 12 fb–1, ATLAS released many new results including 50 conference notes. Highlights included new and highly precise measurements of WZ production that constrain anomalous boson couplings. ATLAS also searched in many final states for signs of direct production of supersymmetric and other new particles from beyond the Standard Model. No compelling evidence was found. In particular, the intriguing hint of a possible new state with a mass of 750 GeV decaying into photon pairs seen in the 2015 data has not reappeared. The larger data setalso allowed ATLAS to “rediscover” the Higgs boson with high statistical significance.
The CMS collaboration presented more than 70 new results based on an integrated luminosity of 13 fb–1, also including a rediscovery of the Higgs. In line with the findings from ATLAS, an updated search for a 750 GeV diphoton resonance by CMS did not confirm the excess observed previously, setting a limit on its cross-section of 1.5 fb at the 95% CL. Searches for supersymmetric and exotic particles also showed no significant excesses, allowing mass limits to be increased by a few hundred GeV. New massive Z bosons up to 4 TeV and string resonances decaying into pairs of jets up to 7.4 TeV have now been excluded, while searches for dark matter exclude mediator masses up to 2 TeV in several standard scenarios.
LHCb presented many interesting new results in the domain of flavour physics. A particular highlight was the discovery of the decay mode B0→ K+K–, which is the rarest B-meson decay into a hadronic final state ever observed, as well as searches for CP violation in the charm system. Another first was a measurement of the photon polarisation in radiative decays of Bs mesons, and determinations of the production cross-sections of several key processes at a collision energy of 13 TeV – some of which at first sight are at variance with current predictions.
Based on lead–lead collisions with an energy of 5 TeV per nucleon pair, the ALICE collaboration presented new measurements of the properties of the quark–gluon plasma. These included fundamental measurements of the production of quarkonium at the highest collision energy ever reached at an accelerator. ALICE also measured the viscosity of the plasma at the new energy, showing that the system still behaves almost as an ideal liquid.
• CERN Courier went to press as ICHEP 2016 got under way. A full report will appear in the next issue.
The LHC relies on the injector complex to deliver proton beams with well-defined transverse and longitudinal characteristics, which fold directly into luminosity performance. On 16 July, LHC Operations made use of a new bunch-production scheme called Batch Compression Merging and Splitting (BCMS) which offers significantly lower transverse beam size. Although the LHC has already broken several performance records this year, the new scheme increased the peak luminosity of the LHC by around 20% and set a new record of 1.2 × 1034 cm–2s–1.
Proton beams emerging from Linac2 in the LHC’s accelerator chain are injected sequentially into each of the four rings of the Proton Synchrotron Booster (PSB) using a multiturn injection process. The total beam intensity per ring is controlled by varying the number of PSB turns during which beam is injected. The transverse emittance (a combined measure of the beam’s transverse size and angular divergence) is also determined by the multiturn nature of the injection process, and in general more injected protons translates to a larger transverse emittance. The eventual number of bunches and their temporal spacing are governed by radio-frequency (RF) gymnastics in the Proton Synchrotron (PS), which injects protons into the Super Proton Synchrotron (SPS) – from where they are fed into the LHC.
The nominal scheme for LHC beam production is based on injecting four and then two bunches from the PSB into the PS. The six PSB bunches are injected into the PS RF “harmonic 7”. The harmonic number is the number of bunch slots – or RF buckets – available in the full circumference of the ring, and this parameter is fully controlled by the RF system. Each of the six bunches is then split into three to reach harmonic 21, and then split into two twice more to result in 72 bunches spaced by 25 ns. To reduce the emittance, it is desirable to inject fewer turns from Linac2 into the PSB and to reduce the bunch-splitting factor in the PS, while still delivering nominal bunch population for 25 ns beam.
The new BCMS scheme makes maximal use of the PSB rings by taking eight bunches (four plus four) into the PS on harmonic 9. A batch compression is then performed by incrementing the harmonic number from harmonic 9 to 14, after which a bunch-merging puts the harmonic number back to 7 (see figure). From this point onwards, the RF gymnastics are similar to those used for the nominal beam but the number of bunches produced is different: eight bunches are merged into four, split by three, two and two again. This results in 48 bunches spaced by 25 ns being injected into the SPS, which is less than the nominal 72 bunches. The scheme wins by taking lower intensities and thus smaller bunches into the PS, and then establishing the basis for the nominal bunch population in the compression and merging process.
The Large Underground Xenon (LUX) experiment located at Sanford Underground Research Facility (SURF) in South Dakota, US, has released its latest results in the search for dark matter. Following the completion of its final 20 month-long run, during which the detector amassed a data set that is four times larger than before, no signal was found and the results were consistent with background expectations.
LUX is based on a 370 kg liquid-xenon dual-phase time-projection chamber that offers a high sensitivity to spin-independent nuclear-recoil interactions. It entered operation in 2013 to search directly for WIMPs in addition to other dark-matter candidates. The experiment’s latest results, which were presented on 21 July at the 2016 International Dark Matter conference in Sheffield, UK, carve out previously un-probed parameter space and exclude spin-independent WIMPS at the level of 0.22 zeptobarns. The collaboration plans further analyses of other dark-matter candidates, including axions.
Researchers are looking ahead to the next-generation LUX-ZEPLIN (LZ) detector, also located at SURF, which is scheduled to start operations by the end of the decade. With an active mass of seven tonnes, LZ will be sensitive to WIMP masses ranging from a few GeV to hundreds of TeV, and will therefore probe even deeper into dark-matter parameter space.
The LHCb collaboration has reported the observation of three new exotic hadrons and confirmed the existence of a fourth by analysing the full data sample from LHC Run 1. Although the theoretical interpretation of the new states is still under study, the particles each appear to be formed by two quarks and two antiquarks. They also do not seem to contain the lightest up and down quarks, which means they could be more tightly bound than other exotic particles discovered so far.
Until recently, all observed hadrons were formed either by a quark–antiquark pair (mesons) or by three quarks only (baryons). The underlying reason has remained a mystery, but during the last decade several experiments have found evidence for particles formed by more than three quarks. For example, in 2009 the CDF collaboration at Fermilab in the US observed evidence for a tetraquark candidate dubbed X(4140), which was later confirmed by the CMS and D0 collaborations (the latest LHCb analysis yields a clear observation of this state, although finds a slightly larger width than the other experiments). Then, in July 2015, LHCb announced the first observation of two pentaquark particles, which are hadrons composed of five quarks.
Each of the four states observed by LHCb – dubbed X(4274), X(4500) and X(4700), in addition to the X(4140) – has a statistical significance above five standard deviations. Sophisticated analysis of the angular distribution of B+ meson decays into J/ψ, φ and K+ mesons also allowed the collaboration to determine the quantum numbers of the exotic states with high precision. Alas, the data could not be described by a model that contains only ordinary mesons and baryons.
The binding mechanism of the new states could involve tightly bound tetraquarks or strange charmed meson pairs bouncing off each other and rearranging their quark content to emerge as a J/ψφ system. The high statistics of the LHCb data set and the sophisticated techniques exploited in the analysis will help to shed further light on the production mechanisms of these particles.
LHCb has made several other important contributions to the investigation of exotic particles. In February 2013, the quantum numbers of the X(3872) particle discovered in 2003 by the Belle experiment in Japan were determined, and in April 2014 the collaboration showed that the Z(4430) particle (also discovered at Belle) is composed of four quarks: ccdu. The latest exotic results from LHCb, which were first presented in June at the Meson 2016 workshop in Cracow, Poland, have been submitted for publication.
When heavy nuclei collide at high energies, a novel state of hot and dense matter – the quark–gluon plasma (QGP) – is expected to form. As a region of QGP expands and cools, it dissociates into a very large number of particles. Earlier findings at Brookhaven’s RHIC and CERN’s LHC revealed that the QGP exhibits strong collective behaviour comparable to a fluid. One of the key experimental pieces of evidence for this state is the observation of long-range anisotropic azimuthal correlations between particles emitted over a wide rapidity range, often referred to as the “ridge phenomenon”. In a typical proton–proton (pp) collision, a ridge correlation is not expected because the system is too dilute to produce a fluid-like state.
In 2010, however, CMS reported an unexpected observation of a ridge phenomenon in pp events with high particle multiplicities. This finding was regarded as a hint for possible collective effects that might occur in rare pp interactions with similar high particle densities as those in heavy-ion collisions. Due to statistical limitations of the LHC Run 1 data set, detailed studies of the ridge phenomenon in pp collisions, especially concerning its connection to collectivity, remained inconclusive. With the increased collision energy of the LHC Run 2, high-multiplicity pp events are produced at significantly higher rates. This makes it possible to perform a detailed examination of ridge and collective phenomena in pp collisions.
The CMS silicon tracking system is ideally suited for measurements of multi-particle correlations and therefore for studies of the collective nature of the ridge. Recently, the CMS collaboration measured the “elliptic-flow” harmonic v2 in pp events from LHC Run 2 as a function of particle multiplicity (see figure above). This is the first extraction of v2 – an observable that can be used to quantify the ridge signal – in pp collisions using a multi-particle correlation technique.
Strikingly, the observed v2 signals show nearly no dependence on the number of correlated particles, providing direct evidence for a collective origin for the long-range ridge observations. Similar observations were previously made in proton–lead and lead–lead collisions (see figure, middle and right, respectively), providing a consistent picture for the collective nature of the medium in all three collision systems. Furthermore, CMS studied long-range correlations using identified K0s and Λ hadrons, observing a clear dependence of the v2 signal on the particle species in high-multiplicity events. Such “mass ordering” provides further evidence for collective behaviour.
These intriguing observations, revealed through the study of small hadronic systems, therefore open up unexpected territory in the understanding and characterisation of dense quark–gluon matter. With the larger data samples of pp and pPb collisions to be collected in the future, CMS is poised for additional exciting physics discoveries in the coming years.
Precise measurements of final states containing multiple electroweak bosons (W, Z or γ) offer a powerful probe of the gauge structure of the Standard Model (SM), and are therefore a promising avenue to search for new physics. Using proton–proton collision data from the LHC collected at centre-of-mass energies of 7, 8 and 13 TeV, the ATLAS collaboration has recently measured the cross-section for boson-pair final states with unprecedented precision, challenging calculations from quantum chromodynamics (QCD).
For example, the uncertainty on the total production cross-section measurement of the WZ final state at 7 TeV is approximately 10%, which is large enough to cover the differences between the next-leading-order (NLO) and next-to-next-to-leading-order (NNLO) QCD predictions. At 8 and 13 TeV, the uncertainties on the ATLAS measurement reveal a tension with the NLO prediction, although this has been mitigated by recent NNLO QCD calculations.
The large size of the 8 TeV data setallowed ATLAS, for the first time, to become sensitive to tri-boson production and also to boson pairs produced through vector-boson scattering. The production cross-section of such processes in the fiducial volume where the measurement is performed is of the order of 1 fb or less, which is very difficult to measure. ATLAS performed significant cross-section measurements of the Zγγ, Wγγ, and W±W±+2j final states, while limits were set on the production cross-section of the WWW and WZ+2j final states.
These measurements probe the gauge-boson self-couplings, which are sensitive to contributions from new physics – especially at high energies. To parameterise possible anomalous gauge couplings, an effective-field-theory (EFT) approach was used. The ATLAS collaboration has searched for deviations in the coupling of three gauge bosons in the WZ final state using data collected at collision energies of 8 TeV and 13 TeV. By including the 13 TeV data, the previous sensitivity on the EFT coefficients (CWWW/Λ2, CW/Λ2, and CB/Λ2) has improved by 40%. No evidence for new physics has been found and new limits have been derived.
The large data set from LHC Run 2 will provide sensitivity to rare processes that have not been observed so far, and the expected high accuracy on multi-boson cross-sections will allow higher-order QCD and electroweak corrections to be probed.
In a collision between two nuclei that exhibits a large impact parameter, the initial spatial anisotropy of the overlap region was conjectured to be smooth and almond shaped. In the past few years, however, experimental measurements and hydrodynamical calculations have changed this paradigm. We now know that the overlap region has an irregular shape originating from the initial random distribution of the gluons and nucleons in the nuclei, which fluctuates from one event to the next. These fluctuations appear as azimuthal correlations between final-state particles relative to the system’s symmetry plane. These correlations are quantified by a Fourier series of the azimuthal distribution of particle production relative to the system’s symmetry plane.
The second harmonic – v2, which is also known as the elliptic-flow coefficient – has recently been the main focus of the heavy-ion community. Indeed, elliptic-flow measurements and hydrodynamical calculations led to the revelation that the quark–gluon plasma (QGP) generated in heavy-ion collisions behaves as an almost perfect liquid. The ratio of shear viscosity to entropy density (η/s) in the QGP, which is a measure of its fluidity, is very close to the lower bound of ħ/4π kB – as conjectured by the anti-de-Sitter/conformal field theory (AdS/CFT) correspondence.
However, there are also higher-order contributions to the particle correlations and these are more sensitive probes of the QGP state than elliptic flow. Indeed, by carrying out such studies for identified particles rather than unidentified ones, it is also possible to probe the effect of the dissipative, late-stage hadronic re-scattering on the flow coefficients.
Profiting from its excellent particle-identification capabilities, the ALICE collaboration has recently used lead–lead collisions recorded in 2011 at a collision energy of 2.76 TeV to measure the elliptic (v2), triangular (v3), quadrangular (v4) and pentagonal (v5) flow coefficients of charged π and K mesons, protons and antiprotons for different centrality intervals (see figure). For ultra-central collisions, in which the evolution of the system is predominantly driven by the initial-state fluctuations, one observes significant nonzero values for all harmonics and particle species. In addition, v3 and v4 become progressively dominant with increasing transverse momentum, while even v5 for pT > 4 GeV/c is comparable to v2. For mid-central collisions, v2 is the dominant flow harmonic and has a significantly larger value. Higher harmonics also have significant nonzero values but do not seem to change significantly with centrality.
These observations confirm that elliptic flow is driven mainly by the anisotropy in the collision geometry, whereas the initial-state fluctuations are the main driving force behind higher harmonics. A mass ordering expected in hydrodynamical calculations is seen that describes the QGP as a nearly perfect liquid. In addition, in the intermediate-pT region, the flow harmonics of different hadrons show a baryon–meson grouping. The analysis of lead–lead collisions collected in 2015 will allow for a higher-precision measurement of such effects and therefore place more stringent limits on η/s and the initial conditions of a heavy-ion collision.
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