By Freeman J Dyson World Scientific
Hardback: £38
Paperback: £18
Birds and Frogs is a wonderful collection of essays and papers by Freeman Dyson from 1990 to 2014, and a sequel to a volume of earlier papers. It consists of a short introductory section followed by four more: “Talks about Science”, “Memoirs”, “Politics and History” and “Technical Papers”.
The book takes its title from one of the “Talks about Science”, in which Dyson classifies mathematicians – and, I would add, physicists – as either “birds” or “frogs”. He writes: “Birds fly high in the air and survey broad vistas of mathematics out to the far horizon. They delight in concepts that unify our thinking and bring together diverse problems from different parts of the landscape. Frogs live in the mud below and see only the flowers that grow nearby. They delight in the details of particular objects, and they solve problems one at a time. I happen to be a frog, but many of my best friends are birds.” This section contains a wealth of fascinating thoughts on, for example, the origins of life, resistance to new ideas in physics, and the nature of computation in the human brain.
Despite his claim to be a frog, much of the book is written with a bird’s-eye view. Dyson is perhaps uniquely placed among living scientists in having been privy to much that went on in the early days of quantum field theory, and to have met and be able to write about personal experiences with many of our modern-day heroes. In the “Memoirs” section, and elsewhere, he offers insights not only into their work, but also their lives and beliefs.
“Politics and History” ranges from science and religion to ethics, and education from the points of view of Tolstoy and Napoleon. His recollections and observations about the Second World War are as unique as they are fascinating. Ultimately, he shares spectacular and optimistic visions for our future as a species that can spread life throughout the universe.
It is the section on “Technical Papers” that shows Dyson the frog. Here, number theory, bounds on variation of the fine structure constant, detectability of gravitons and game theory all appear.
Whether you’re a frog or a bird or neither – Dyson has a penchant for classifying things into a small number of categories, often just two – you are certain to find much to delight you in this eclectic and yet somehow unified collection.
By Omar Tibolla et al. (eds) Elsevier Nuclear Physics B (Proc. Suppl.) 256–257 (2014)
Where do cosmic rays, discovered more than a century ago, come from? The standard model of their origin points to natural particle accelerators in the form of shock waves in supernova remnants, but there is mounting experimental evidence that there are other sources. This conference brought together a range of experts to examine the evidence and to consider some of the key questions. What other sources might there be in the Galaxy? What causes the knee? Where (in energy) is the transition to an extragalactic component? What extragalactic sources are conceivable?
In 1964, Murray Gell-Mann and George Zweig independently predicted a substructure for hadrons: baryons would be comprised of three quarks, mesons of a quark–antiquark pair. They also said that baryons with four quarks and one antiquark were possible, as were mesons with two quarks and two antiquarks – dubbed, respectively, pentaquarks and tetraquarks, after the number of constituents. Since then, the picture for baryons and mesons has been thoroughly established within QCD, the theory of the strong interaction. Claims of the sighting of pentaquarks, meanwhile, have been thoroughly debunked. Nevertheless, their existence could cast important new light on QCD.
Now, the LHCb collaboration has announced the observation of two pentaquark states, P+c, in analysis of data collected during Run 1 of the LHC at CERN. The discovery was made during the analysis of the decay Λb → J/ψ K– p, a decay mode used in the precision measurement of the Λb lifetime (CERN Courier July/August 2013 p8). There was, however, an apparent anomaly in the pattern of these decays. The Dalitz plot, in which only 5.4% is background, shows several expected Λ*→K– p resonances as vertical bands, but there is also a horizontal band, indicative of a resonance decaying into J/ψ p, which was completely unexpected (figure 1).
A resonance decaying into J/ψ p would be a pentaquark state (with quarks uudcc). So LHCb investigated more deeply, with a full six-dimensional amplitude analysis of the two interfering decay sequences: Λb → J/ψ Λ*, Λ* → K– p, and Λb → P+c K–, P+c→ J/ψ p. This analysis not only fit the invariant mass of the decay products, the angular distributions for the decays were also fit, along with the invariant mass – this was not a simple “bump hunt”.
The first attempt was to fit the data without any P+c states, with the belief that the structure could be built up from Λ* interferences. This failed. The next attempt was with one P+c state, but the fit was deficient. Finally, a fit with two P+c states proved to be acceptable. The masses of the states are 4380±8±29 MeV and 4449.8±1.7±2.5 MeV, with widths of 205±18±86 MeV and 39±5±19 MeV, respectively. The states have opposite parities, with one state having spin 3/2 and the other spin 5/2. The final fitted J/ψ p mass spectra show the two states (figure 2). The significances of each state are more than 9σ.
LHCb has subjected the results to a great many systematic checks. These include ensuring that tracks were not “clones” or “ghosts”, splitting the data into different subsets, such as Λb versus Λb, data from 2011 versus 2012, magnetic field up versus down, etc. All of these tests have been passed.
One interesting fact is that these pentaquarks decay into J/ψ, as do candidate states for tetraquark mesons (CERN Courier June 2014 p12). This suggests that two heavy quarks may be needed to provide the binding for these exotic states.
The Islamic Republic of Pakistan became an associate member state of CERN on 31 July, following notification that Pakistan has ratified an agreement signed last December, granting this status to the country (CERN Courier January/February 2015 p6).
Pakistan’s new status will open a new era of co-operation that will strengthen the long-term partnership between CERN and the Pakistani scientific community. Associate membership will allow Pakistan to participate in the governance of CERN, through attending CERN Council meetings. Moreover, it will allow Pakistani scientists to become CERN staff members, and to participate in CERN’s training and career-development programmes. Finally, it will allow Pakistani industry to bid for CERN contracts, thus opening up opportunities for industrial collaboration in areas of advanced technology.
On 16 June, an 11 T superconducting dipole-magnet model manufactured at CERN for the High-Luminosity LHC project reached record performance levels in tests in hall SM18. Its magnetic-field intensity exceeded 11 T after just six quenches – a much faster increase than in previous models. In addition, it reached 12 T – corresponding to a current of 12,800 A – which is higher than in earlier models. The new magnets, based on a niobium-tin (Nb3Sn) superconductor, are being developed in a collaboration between Fermilab and CERN. Models constructed on both sides of the Atlantic have previously reached the required 11 T, but only after many quenches. The models are shorter than the final magnets – 2 m rather than 5.5 m – and have only a single bore, rather than two bores for the two LHC beams.
The Advanced European Infrastructures for Detectors and Accelerators (AIDA-2020) – the largest European-funded project for joint detector development – is making financial support available for small development teams to carry out experiments and tests at one of 10 participating European facilities. The project, which started on 1 May, will run for four years. Its main goal is to bring the community together and push detector technologies beyond current limits by sharing high-quality infrastructures provided by 57 partners from 34 countries, from Europe to Asia.
Building on the experience gained with the original AIDA project (CERN Courier April 2011 p6), the transnational access (TA) activities in AIDA-2020 are to enable financial support for teams to travel from one facility to another, to share existing infrastructures for efficient and reliable detector development. The support is organized around three different themes, providing access to a range of infrastructures: the Proton Synchrotron and Super Proton Synchrotron test beams, the IRRAD proton facility and the Gamma Irradiation Facility (GIF++) at CERN; the DESY II test beam; the TRIGA reactor at the Jožef Stefan Institute; the Karlsruhe Compact Cyclotron (KAZ); the Centre de Recherches du Cyclotron at the Université catholique de Louvain (UCLouvain); the MC40 Cyclotron at the University of Birmingham; the Rudjer Boskovic Institute Accelerator Facility (RBI-AF); and the electromagnetic compatibility facility (EMClab) at the Instituto Tecnológico de Aragón (ITAINNOVA).
Access to high-energy particle beams (TA1) at CERN and DESY enables the use of test beams free-of-charge. Here the main goal is to attract more researchers to participate in beam tests, in particular supporting PhD students and postdoctoral researchers to carry out beam tests of detectors.
With the access to irradiation sources (TA2), the goal is to cover the range of particle sources needed for detector qualification for the High Luminosity LHC (HL-LHC) project. These include proton, neutron and mixed-field sources, as well as gamma irradiation. Through IRRAD, TRIGA, KAZ and MC40, it provides both the extreme fluences of up to 1017 neq/cm2 required for the forward region in HL-LHC experiments, and the lower fluences of 1015 neq/cm2 on 10 cm2 objects for the outer layers of trackers. GIF++ covers irradiation of large-scale objects such as muon chambers, while the Heavy Ion Irradiation Facility at UCLouvain is available for single-event-effects tests of electronics.
The third theme provides access to new detector-testing facilities (TA3). Semiconductor detectors will be one of the main challenges at the HL-LHC. Studying their behaviour with micro-ion beams at RBI will enhance the understanding of these detectors. Electromagnetic compatibility is a key issue when detectors have to be integrated in an experiment, and prior tests in a dedicated facility such as the EMClab at ITAINNOVA will make the commissioning of detectors more efficient.
The Japanese/German BASE collaboration at CERN’s Antiproton Decelerator (AD) has compared the charge-to-mass ratios of the antiproton and proton with a fractional precision of 69 parts in a trillion (ppt). This high-precision measurement was achieved by comparing the cyclotron frequencies of antiprotons and negatively charged hydrogen ions in a Penning trap. The result is consistent with charge–parity–time-reversal (CPT) invariance, which is one of the cornerstones of the Standard Model of particle physics, and constitutes the most precise test comparing baryons and antibaryons performed to date.
In their experiment, the BASE collaboration has profited from techniques pioneered in the 1990s by the TRAP collaboration at the Low Energy Antiproton Ring at CERN. The advanced cryogenic Penning-trap system used in BASE consists of four traps, two of which were used in this measurement – a measurement trap and a reservoir trap (figure 1). When the experiment receives a pulse of 5.3 MeV antiprotons from the AD, they strike the degrader structure, which is designed to slow them down, and release hydrogen. Negatively charged hydrogen ions (H–) can form in the process, producing a composite cloud with the antiprotons that is shuttled to the reservoir trap. BASE has developed techniques to extract single antiprotons and negative hydrogen ions from this cloud whenever needed. Moreover, the reservoir has a lifetime of more than a year, making the BASE experiment almost independent from AD cycles.
Using this extraction technique, and taking the timing from the AD cycle, BASE prepares a single antiproton in the measurement trap, while an H– ion is held in the downstream park electrode, as shown in figure 1. The cyclotron frequency of the antiproton is then measured in exactly 120 s, which corresponds to one AD cycle. The particles are subsequently exchanged by performing appropriate potential ramps, and the cyclotron frequency of the H– ion is measured. Thus, a single comparison of the charge-to-mass ratios takes only 240 s. This is much faster than in previous experiments, enabling BASE to perform about 6500 ratio comparisons in 35 days of measurement time (figure 2). The result is a value of the ratio-comparison: (q/m)p-/(q/m)p – 1 = 1(69) × 10–12, thus confirming CPT at the level of ppt.
The high sampling rate has also enabled the first high-resolution study of diurnal variations in a baryon/antibaryon comparison, which could be introduced by Lorentz-violating cosmic-background fields. The measurement sets constraints on such variations at the level of less than 720 ppt. In addition, by assuming that CPT invariance holds, the measurement can be interpreted as a test of the weak equivalence principle using baryonic antimatter. If matter respects weak equivalence while antimatter experiences an anomalous coupling to the gravitational field, this gravitational anomaly would contribute to a possible difference in the measured cyclotron frequencies. Thus, by following these assumptions, the result from BASE can be used to set a limit on the gravitational anomaly parameter, αg: |αg – 1| < 8.7 × 10–7.
The main goal for the BASE experiment, which was approved in June 2013, is to measure the magnetic moment of the antiproton with a precision of parts per billion. Using the double Penning trap system, the collaboration recently performed the most precise measurement of the magnetic moment of the proton.
The year 2015 began for the ATLAS experiment with an intense phase of commissioning using cosmic-ray data and first proton–proton collisions, allowing ATLAS physicists to test the trigger and detector systems as well as to align the tracking devices. Then the collection of physics data in LHC Run 2 started in June, with proton–proton collisions at a centre-of-mass energy of 13 TeV (CERN Courier July/August 2015 p25). Measurements at this new high-energy frontier were among the highlights of the many results presented by the ATLAS collaboration at EPS-HEP 2015.
An important early goal for ATLAS was to record roughly 200 million inelastic proton–proton collisions with a very low level of secondary collisions within the same event (“pile-up”). This data sample allowed ATLAS physicists to perform detailed studies of the tracking system, which features a new detector, the “Insertable B-layer” (IBL). The IBL consists of a layer of millions of tiny silicon pixels mounted in the innermost heart of ATLAS at a distance of 3.3 cm from the proton beam (CERN Courier October 2013 p28). Together with the other tracking layers of the overall detector, the IBL allows ATLAS to measure the origin of charged particles with up to two times better precision than during the previous run. Figure 1 shows the resolution achieved for the longitudinal impact parameter of the beam.
ATLAS exploited the early data sample at 13 TeV for important physics measurements. It allowed the collaboration to characterize inelastic proton–proton collisions in terms of charged-particle production and the structure of the “underlying event” – collision remnants that are not directly related to the colliding partons in the proton. This characterization is important for validating the simulation of the high-luminosity LHC collisions, which contain up to 40 inelastic proton–proton collisions in a given event (one event involves the crossing of two proton bunches with more than 100 billion protons each). Figure 2 shows the evolution of the charged-particle multiplicity with centre-of-mass energy.
ATLAS also measured the angular correlation among pairs of the produced charged particles, confirming the appearance of a so-called “ridge” phenomenon in events with large particle multiplicity at a centre-of-mass energy of 13 TeV. The “ridge” (figure 3) consists of long-range particle–particle correlations not predicted by any of the established theoretical models describing inelastic proton–proton collisions.
After the low-luminosity phase, the LHC operators began to increase the intensity of the beams. By the time of EPS-HEP 2015, ATLAS had recorded a total luminosity of 100 pb–1, of which up to 85 pb–1 could be exploited for physics and performance studies. ATLAS physicists measured the performance of electron, muon and τ-lepton reconstruction, the reconstruction and energy calibration of jets, and the reconstruction of “displaced” decays of long-lived particles, such as weakly decaying hadrons containing a bottom quark. The precision of the position measurements of displaced decay locations (vertices) is significantly improved by the new IBL detector.
ATLAS used these data to classify the production of J/ψ particles at 13 TeV in terms of their immediate (“prompt”) and delayed (“non-prompt”) origin. While non-prompt J/ψ production is believed to be well understood via the decay of b hadrons, prompt production continues to be mysterious in some aspects.
ATLAS also performed a first study of the production of energetic, isolated photons and a first cross-section measurement of inclusive jet production in 13 TeV proton–proton collisions. Both are correctly described by state-of-the-art theory.
The data samples at high collision energy contain copious numbers of Z and W bosons, the mediators of the weak interaction, whose leptonic decays provide a clean signature in the detector that can be exploited for calibration purposes. ATLAS has studied the kinematic properties of these bosons, also in association with jet production. Their abundance in 13 TeV proton–proton collisions is found to be consistent with the expectation from theory. ATLAS has also observed some rare di-boson (ZZ) events, which – with a hundred times more data – should allow the direct detection of Higgs bosons. Figure 4 shows a candidate ZZ event.
In higher-energy proton collisions, the rate of particle production for many heavier particles for a given luminosity increases. The heaviest known particle, the top quark – with a mass approximately 170 times that of a proton – is predominantly produced in pairs at the LHC, and the cross-section for the production of top-quark pairs is expected to increase by a factor of 3.3 at 13 TeV, compared with the 8 TeV collisions of Run 1. ATLAS has performed an early measurement of the top-pair production cross-section in the cleanest channels where one top quark decays to an electron, an electron-neutrino and a jet containing a b-hadron (“b-jet”), while the other top-quark decays to a muon, a muon-neutrino and a b-jet. The small backgrounds from other processes in this channel allow a robust measurement with small systematic uncertainties. The measured cross-section agrees with the predicted increase of a factor of 3.3. The precision of the measurement is limited by the 9% uncertainty in luminosity, which is expected to improve significantly during the year. Figure 5 shows the evolution of the top-pair production cross-section.
Although the available data sample does not yet allow a significant increase in the sensitivity to the most prominent new physics phenomena, ATLAS has exploited the data to perform important early measurements. The excellent detector performance has allowed the confirmation of theoretical expectations with 13 TeV proton–proton collision energies.
The highlight of EPS-HEP 2015 for the CMS collaboration was the publication of the first physics result exploring the new territory at the LHC energy of 13 TeV: the measurement of the charged-hadron multiplicity (dN/dη), where η, the pseudorapidity, is a measure for the direction of the particle track. When protons collide at the LHC, more than one of their constituents (quarks or gluons) can interact with another one, so every collision produces an underlying spray of charged hadrons, such as pions and kaons, and the greater the energy, the higher the number of produced particles. Knowing precisely how many charged hadrons are created at the new collision energy is important for ensuring that the theoretical models used in the simulations employed in the physics analyses describe these underlying processes accurately. The publication from CMS at 13 TeV reports the differential multiplicity distribution for values of η < 2, and a measured density for central charged hadrons (with |η| < 0.5) of 5.49±0.01 (stat.)±0.17 (syst.). Figure 1 shows the differential distribution and the energy dependence of the new measurement compared with earlier data at lower energies.
CMS has, in addition, produced a full suite of performance plots covering a range of physics objects and final states, using up to 43/pb of 13 TeV data. Figure 2 shows the dimuon mass spectrum obtained from multiple trigger paths, where several resonances from the ω meson to the Z boson can be seen clearly. The B physics group in CMS has studied this spectrum in detail from the J/ψ to the Υ masses, and also the decay-time distributions for events with J/ψ or B+ mesons. Dedicated performance plots were presented at the conference for various muon, electron and photon kinematic and identification variables, as well as the measured reconstruction and identification efficiencies. The reconstruction of several low-mass states, including Ks, Λ, D0, D*, B+, B0 and Bs, demonstrate the good performance of the CMS tracker. In addition, the position of the beam spot has been measured in all three dimensions. Simulations are already found to reproduce these physics-object data well at this early stage.
The physics groups in CMS have also started to study several processes at 13 TeV in some detail. One highlight is a first look for searches in the dijet invariant-mass spectrum, which so far reaches up to approximately 5 TeV (figure 3). Results of the same analysis on Run 1 data were released only in spring, but CMS is already continuing the search where it ended at 8 TeV up to 13 TeV, thus demonstrating the collaboration’s readiness for discovery physics in the new energy regime. The TOP group has studied top–antitop (tt) events in the dilepton and lepton+jet channels, in addition to taking a first look at events consistent with the production of single top quarks.
While eagerly jumping on the new data, CMS continues to produce world-class physics results on the Run 1 data collected at 7 and 8 TeV. The collaboration has recently approved more than 30 new results, which were shown at the conference. These include searches for new physics as well as precision Standard Model measurements. The results presented include measurements of the two-photon production of W-boson pairs through the interaction of two photons, the electroweak production of a W boson accompanied by two jets, production rates for particle jets at 2.76 TeV compared with 8 TeV, as well as the production of two photons along with jets.
Discovered more than two decades ago, the top quark continues to play a vital role in physics analyses for both measurements and searches, because it is the heaviest elementary particle known so far. New CMS results with this special type of quark include measurements of the tt production rates in the fully hadronic sample, and a measurement of the tt+bb process as well as the tt production in conjuction with a Z or W boson. In addition, searches for signs of new physics continue, most recently in the process where top decays to a charm quark and a Higgs boson, t → cH, and the Higgs boson transforms to photons.
On the Higgs front itself, CMS has performed three new searches for non-Standard Model Higgs bosons containing τ leptons in the decay products, while on the supersymmetry front, analyses have looked for dark-matter candidates and other supersymmetric particles. Heavy-ion results from Run 1, using proton–proton, proton–lead and lead–lead collisions, include Υ polarization as a function of charged-particle multiplicity in proton–proton collisions, Z-boson production, jet-fragmentation functions in proton–lead collisions, and nuclear modification of Υ states in lead–lead collisions.
At EPS-HEP2015, the LHCb collaboration presented the first measurement of the J/ψ production cross-section in proton–proton (pp) collisions at 13 TeV. Using this measurement, they also determined the b-quark cross-section at this new, higher energy.
J/ψ mesons can be produced both “promptly”, in the pp collision, and as a product of decays of B hadrons, dubbed “J/ψ-from-b”. The two components are visible in figure 1, which shows the J/ψ decay-time distribution with respect to the pp collision time. The black points with error bars show the data, the solid red line indicates the best fit to the data, and the prompt J/ψ contribution is shown in blue. The black line indicates the J/ψ-from-b contribution, which falls exponentially with a time constant characteristic of the lifetime of B hadrons.
While the prompt J/ψ cross-section is interesting for constraining QCD models, the J/ψ-from-b cross-section is used to compute the b-quark pair total cross-section. The data at 13 TeV confirm the expected rise of the B-particle production rate of about a factor two with respect to 7 TeV. This increase will enable LHCb to obtain even more precise, interesting and, hopefully, surprising results in LHC Run 2.
This analysis was the first to benefit from a new scheme for the LHCb software trigger that was introduced for Run 2. Splitting the event selection into two stages, it allows alignment and calibration to be performed in real time after the first stage of the software trigger and then used directly in the second stage. The same alignment and calibration information is propagated to the offline reconstruction, to ensure consistent and high-quality particle-identification information in the trigger and offline. The identical performance of the online and offline reconstruction achieved in this way offers the opportunity to perform physics analyses directly using candidates reconstructed in the trigger – the online reconstruction is used, for example, in the J/ψ cross-section measurement. The storage of only the triggered candidates leads to a reduction in the event size of an order of magnitude, permitting an increased event rate with higher efficiency.
LHCb also presented the determination based on Run 1 data of the Cabibbo–Kobayashi–Maskawa (CKM) matrix element |Vub|, which describes the transition of a b quark to a u quark. The measurement – published during the conference in Nature Physics – was made by studying a decay Λ0b baryon, Λ0b → pμ–νμ (LHCb 2015a). The measurement of decays involving a neutrino is very challenging at a proton collider, and it was quite a surprise that this measurement could be done.
Measurements of |Vub| by previous experiments had returned two sets of inconsistent results, depending on the method used. Inclusive determination using all b → ulν transitions where l is either a muon or an electron give values of |Vub| above 0.004, while exclusive determinations, mainly from B → πlν, yield values around 0.003. This could be explained by a new particle, in addition to the W boson, contributing to the quark transition with a right-handed current. LHCb’s new measurement is of the exclusive category, but is the first to involve a baryon decay and hence a spin-1/2 particle. The result is |Vub| = (3.27±0.15±0.16±0.06) × 10–3, where the uncertainties are experimental, related to the theoretical calculation, and to the value of |Vcb|, respectively. This number agrees with previous exclusive determinations and is inconsistent with the hypothesis of new right-handed currents. So it still leaves the puzzle of why the inclusive and exclusive measurements do not agree. Further intensive research, both at the experimental and theoretical level, will continue to try to understand this disagreement.
While the above measurement constrains one side of the CKM unitarity triangle, the other (the third being unity) is best constrained by the B-meson oscillation frequency. LHCb presented the most precise measurement to date at the conference, using semileptonic B0 decays. The result of (503.6±2.0±1.3) ns–1 is consistent with, but more precise than, the world average (LHCb 2015b).
In other highlights from Run 1, the collaboration reported new results on long-range correlations in proton–lead collisions. LHCb’s latest measurements show that the so-called “ridges” seen in the most violent collisions span across even larger longitudinal distances, as figure 2 shows at Δφ = 0 below the (truncated) peak at (0,0). This is the first time that the effect has been seen in the forward direction (LHCb 2015c). Moreover, because of its acceptance, the LHCb experiment distinguishes between configurations where the lead-ion enters from the front and those where it is the proton. Somewhat unexpectedly, the ridge is seen in both cases.
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