BaBar celebrates its 25th anniversary
On 11 December 2018, 25 years after its inaugural meeting, the BaBar collaboration came together at the SLAC National Accelerator Laboratory in California to celebrate its many successes. David Hitlin, BaBar’s first spokesperson, described the inaugural meeting of what was then called the Detector Collaboration for the PEP-II “asymmetric” electron–positron collider, which took place at SLAC at the end of 1993. By May 1994 the collaboration had chosen the name BaBar in recognition of its primary goal to study CP violation in the neutral B-B̅ meson system. Jonathan Dorfan, PEP-II project director, recounted how PEP-II was constructed by SLAC, LBL and LLNL. Less than six years later, PEP-II and the BaBar detector were built and the first collision events were collected on 26 May 1999. Twenty-five years on, and BaBar has now chalked up more than 580 papers on CP violation and many other topics.
The “asymmetric” descriptor of the collider refers to Pier Oddone’s concept of using unequal electron and positron beam energies – tuned to 10.58 GeV, the mass of the ϒ(4S) meson and just above the threshold for producing a pair of B mesons. This relativistic boost enabled measurements of the distance between the points where the mesons decay, which is critical for the study of CP violation. Equally critical was the entanglement of the B meson and anti-B meson produced in the ϒ(4S) decay, as it marked whether it was the B0 or B̅0 that decayed to the same CP final state by tagging the flavour of the other meson.
By October 2000 PEP-II had achieved its design luminosity of 3 × 1033 cm–2 s–1 and less than a year later BaBar published its observation of CP violation in the B0 meson system based on a sample of 32 × 106 pairs of B0-B̅0 mesons – on the same day that Belle, its competitor at Japan’s KEK laboratory, published the same observation. These results led to Makoto Kobayashi and Toshihide Maskawa sharing the 2008 Nobel Prize in Physics. The ultimate luminosity achieved by PEP-II, in 2006, was 1.2 × 1034 cm–2s–1. BaBar continued to collect data on or near the ϒ(4S) meson until 2007 and in 2008 collected large samples of ϒ(2S) and ϒ(3S) mesons before PEP-II was shut down. In total, PEP-II produced 471 × 106 B-B̅ pairs for BaBar studies – as well as a myriad of other for other investigations.
The anniversary event also celebrated technical innovations, including “trickle injection” of beam particles into PEP-II, which provided a nearly 40% increase in integrated luminosity; BaBar’s impressive particle identification, made possible by the DIRC detector; and the implementation of a computing model – spurred by PEP-II delivering significantly more than design luminosity – whereby countries provided in-kind computing support via large “Tier-A” centres. This innovation paved the way for CERN’s Worldwide LHC Computing Grid.
Notable physics results from BaBar include the first observation in 2007 of D–D̅ mixing, while in 2008 the collaboration discovered the long-sought ηb, the lowest energy particle of the bottomonium family. The team also searched for lepton-flavour violation in tau–lepton decays, publishing in 2010 what remain the most stringent limits on τ → μγ and τ → eγ branching fractions. In 2012, making it onto Physics World’s top-ten physics results of the year, the BaBar collaboration made the first direct observation of time-reversal violation by measuring the rates at which the B0 meson changes quantum states. Also published in 2012 was evidence for an excess of B̅→ D(*)τ– ν̅τ decays, which challenges lepton universality and is an important part of the current Belle II and LHCb physics programmes. Several years after data-taking ended, it was recognised that BaBar’s data could also be mined for evidence of dark-sector objects such as dark photons, leading to the publication of two significant papers in 2014 and 2017. Another highlight, published last year, is a joint BaBar–Belle paper that resolved an ambiguity concerning the quark-mixing unitarity triangle.
Although BaBar stopped collecting data in 2008, this highly collegial team of researchers continues to publish impactful results. Moreover, BaBar alumni continue to bring their experience and expertise to subsequent experiments, ranging from ATLAS, CMS and LHCb at the LHC, Belle II at SuperKEKB, and long-baseline neutrino experiments (T2K, DUNE, HyperK) to dark-matter (LZ, SCDMS) and dark-energy (LSST) experiments in particle astrophysics.
J Michael Roney, University of Victoria and David MacFarlane, SLAC.
PBC initiative presents main findings
In a workshop held at CERN on 16–17 January, researchers presented the findings of the Physics Beyond Colliders (PBC) initiative, which was launched in 2016 to explore the opportunities at CERN via projects complementary to the LHC and future colliders (CERN Courier November 2016 p28). PBC members have weighed up the potential for such experiments to explore open questions in QCD and the existence of physics beyond the Standard Model (BSM), in particular including searches for signatures of hidden-sector models in which the conjectured dark matter does not couple directly to Standard Model particles.
The BSM and QCD groups of the PBC initiative have developed detailed studies of CERN’s options and compared them to other worldwide possibilities. The results show the international competitiveness of the PBC options.
The Super Proton Synchrotron (SPS) remains a clear attraction, offering the world’s highest-energy beams to fixed-target experiments in the North Area (see Fixed target, striking physics). The SPS high-intensity muon beam could allow a better understanding of the theoretical prediction of the muon anomalous magnetic moment (MUonE project), and a significant contribution to the resolution of the proton radius puzzle by COMPASS(Rp). The NA61 experiment could explore QCD in the interesting region of “criticality”, while upgrades of NA64 and a few months of NA62 operation in beam-dump mode (whereby a target absorbs most of the incident protons and contains most of the particles generated by the primary beam interactions) would explore the hidden-sector parameter space. In the longer term, the KLEVER experiment could probe rare decays of neutral kaons, and NA60 and DIRAC could enhance our understanding of QCD.
A novel North Area proposal is the SPS Beam Dump Facility (BDF). Such a facility could, in the first instance, serve the SHiP experiment, which would perform a comprehensive investigation of the hidden sector with discovery potential in the MeV–GeV mass range, and the TauFV experiment, which would search for forbidden τ decays. The BDF team has made excellent progress with the facility design and is preparing a comprehensive design study report. Options for more novel exploitation of the SPS have also been considered: proton-driven plasma- wakefield acceleration of electrons for a dark-matter experiment (AWAKE++); the acceleration and slow extraction of electrons to light–dark-matter experiments (eSPS); and the production of well-calibrated neutrinos via a muon decay ring (nuSTORM).
Fixed-target studies at the LHC are also considered within PBC, and these could improve our understanding of QCD in regions where it is relevant for new-physics searches at the high-luminosity LHC upgrade. The LHC could also be supplemented with new experiments to search for long-lived particles, and PBC support for a small experiment called FASER has helped pave the way for its installation in the ongoing long shutdown of CERN’s accelerator complex.
2018 was a notable year for the gamma factory, a novel concept that would use the LHC to produce intense gamma-ray beams for precision measurements and searches (CERN Courier November 2017 p7). The team has already demonstrated the acceleration of partially stripped ions in the LHC, and is now working towards a proof-of-principle experiment in the SPS. Meanwhile, the Electric Dipole Moment (CPEDM) collaboration has continued studies, supported by experiments at the COSY synchrotron in Germany (CERN Courier September 2016 p27), towards a prototype storage ring to measure the proton EDM.
The PBC technology team has also been working to leverage CERN’s skills base to novel experiments, for example by exploring synergies across experiments and collaboration in technologies – in particular, concerning light-shining-through-walls experiments and QED vacuum-birefringence measurements.
Finally, some PBC projects are likely to flourish outside CERN: the IAXO axion helioscope, now under consideration at DESY; the proton EDM ring, which could be prototyped at the Jülich laboratory, also in Germany; and the REDTOP experiment devoted to η meson rare decays, for which Fermilab in the US seems better suited.
The PBC groups have submitted their full findings to the European Particle Physics Strategy Update (http://pbc.web.cern.ch/).
Joerg Jaeckel Heidelberg University, Mike Lamont CERN and Claude Vallée CPPM Marseille.
Colombian students analyse LHC data
The annual Colombian national high-energy physics conference (COMHEP), held this year in the city of Cali, provided an opportunity for youngsters from the Tecnocentro “Somos Pacifico”, an out-of-school activity centre, to participate in a one-off masterclass analysing data from the LHC. The masterclass used the tools and setup from the International Particle Physics Outreach Group (IPPOG) international masterclasses, and was coordinated by ATLAS member Carlos Sandoval from the Universidad Antonio Nariño in Bogotá, and included a virtual visit to the ATLAS control room. The Tecnocentro is in a disadvantaged area of Cali, many of whose inhabitants have been displaced from the countryside by civil conflict.
CLIC collaboration considers collider’s potential
The annual workshop of the Compact Linear Collider (CLIC), a proposed multi- TeV linear electron–positron collider at CERN, attracted more than 200 participants to CERN on 21–25 January. CLIC occupies a unique position in both the precision and energy frontiers, combining the benefits of electron–positron collisions with the possibility of multi-TeV collision energies. It uses a two-beam acceleration scheme based on novel, high-gradient X-band accelerating structures, and envisions a three-stage implementation with a collision energy stepping from 380 GeV to 3 TeV and a diverse physics programme spanning 30 years (CERN Courier November 2016 p21).
Key CLIC concepts such as drive-beam production and operation of high-efficiency radio-frequency cavities have all been demonstrated, reported Steinar Stapnes, CLIC project leader. “The CLIC project offers a cost-effective and innovative technology, and is ready to proceed towards a Technical Design Report, enabling the start of construction for the first stage by 2026 and realising electron–positron collisions at 380 GeV as soon as 2035,” he said.
A major focus for the CLIC collaboration during 2018 was the completion of a project implementation plan, as well as several comprehensive CERN Yellow Reports describing the accelerator, detector and detailed physics studies. A central point was an updated cost and power estimate, which for the 380 GeV stage amount to around 5.9 billion Swiss francs and 168 MW. Workshop participants also discussed the next important step for CLIC: a preparatory phase focusing on large-scale tests, industrial production and civil-engineering aspects including siting and infrastructure.
An overview of potential industrial involvement in CLIC’s core technologies is also being compiled. Several partner agreements support technical developments for smaller X-band accelerators, including the European Commission’s CompactLight study and the recently proposed eSPS project that would see a 3.5 GeV X-band electron linac feeding the Super Proton Synchrotron for further acceleration, followed by slow extraction to study dark-sector physics. The linear tunnel of CLIC also provides a natural infrastructure for long-term future projects based on plasma-wakefield and other acceleration techniques.
Concerning CLIC detector R&D, the latest test-beam analysis and simulation results show promise for meeting the challenging CLIC vertex and tracker requirements. The next generation of detector assemblies will be tested at DESY in Hamburg, where the CLICdp vertex and tracker group will be welcomed for several weeks during the current long shutdown of CERN’s accelerators.
CLIC’s physics programme generated rich discussions at the January workshop. In particular the Higgs self-coupling, which determines the shape of the Higgs potential, can be directly accessed at the multi-TeV collisions at CLIC via double-Higgs production. A dedicated mini-workshop jointly set up by theorists and experimentalists covered CLIC’s potential to extend our knowledge of physics beyond the Standard Model, including possible compositeness of the Higgs boson and dark-matter candidates such as the thermal Higgsino and axion-like particles. The full physics case, designs, costs and timescales for CLIC were submitted to the European Strategy for Particle Physics Update in December.
Rickard Ström CERN.
Supporting future female physicists
The Conferences for Undergraduate Women in Physics (CUWiP) are three-day regional events for undergraduate physics students run by the American Physical Society and many volunteers. Their goal is to provide female students with resources and motivation to support their pursuit of a degree and a career in physics, and is addressed in many ways, including inspirational talks by female physicists and workshops and panel discussions on graduate school and physics careers. This year’s CUWiP took place on 18–20 January at 12 universities across the US and Canada, which collectively hosted a record number of attendees. The keynote speaker, CERN Director-General Fabiola Gianotti, delivered her talk from CERN titled “Why a Professional Life in Physics?” She gave details of her path to becoming a physicist – which included her study of humanities, constant curiosity as a child, and continuous thirst for knowledge – and expressed that it does not matter if you had a late start in physics as long as you have a passion.
Kai Wright and Theodore Hodapp American Physical Society.
Higgs workshop returns to Tokyo
The seventh in the series of Higgs Couplings workshops, which began in Tokyo in 2012, returned to the Japanese capital on 26–30 November 2018. Lively discussions between experimentalists and theorists have been a strength of the meeting since the beginning. The 2018 workshop attracted more than 130 participants and included 49 plenary talks, 35 parallel talks and 18 talks by young scientists, covering a broad range of Higgs-boson physics.
On the experiments side, measurements of the Higgs-boson mass, width, couplings, spin and parity were discussed. In particular, the recent confirmation of the top and bottom Yukawa couplings was presented, and effective field theory was described as the next phenomenological framework for parametrising measurements of the couplings. Additional Higgs-boson searches including dark-matter aspects were discussed, as were new data-analysis techniques such as boosted topology and machine learning. On the theory side, recent advances in higher-order calculations in precision Higgs phenomenology were presented.
The importance of future colliders for the measurement of Higgs-boson properties, including its self-coupling, was also highlighted. The high-luminosity LHC, the high-energy LHC, linear colliders such as the ILC, as well as future circular colliders, were discussed. Many prospective results from these colliders were documented in CERN Yellow Reports submitted to the European Strategy for Particle Physics Update, and were reported for the first time at the workshop.
There was also a special session on “beyond collider” studies, which underlined the synergies with collider experiments and featured recent results from gravitational-wave observatories and cosmic-microwave-background experiments.
The conference also included a young scientists’ forum, which saw many excellent talks. Finally, the Japanese traditional ceremony of Kagami biraki, in which a cask of sake is opened at a party or ceremony, celebrated the measurement of the top and bottom Yukawa couplings and of a new era in Higgs measurements.
The next Higgs Couplings workshop will be held in Oxford in the UK from 30 September to 4 October 2019.
Yuji Enari chair of the local organising committee of Higgs Couplings 2018.
Paris event reflects on the history of the neutrino
Neutrinos, discovered in 1956, play an exceptional role in particle and nuclear physics, as well as astrophysics, and their study has led to the award of several Nobel prizes. In recognition of their importance, the first International
Conference on the History of the Neutrino took place at the Université Paris Diderot in Paris on 5–7 September 2018.
The purpose of the conference, which drew 120 participants, was to cover the main steps in the history of the neutrino since 1930, when Wolfgang Pauli postulated its existence to explain the continuous energy spectrum of the electrons emitted in beta decay. Specifically, for each topic in neutrino physics, the aim was to pursue an historical approach and follow as closely as possible the discovery or pioneering papers. Speakers were chosen as much as possible for their roles as authors or direct witnesses, or as players in the main events.
The first session, “Invention of a new particle”, started with the prehistory of the neutrino – that is, the establishment of the continuous energy spectrum in beta decay – before moving into the discoveries of the three flavour neutrinos. The second session, “Neutrinos in nature”, was devoted to solar and atmospheric neutrinos, as well as neutrinos from supernovae and Earth. The third session covered neutrinos from reactors and beams including the discovery of neutral-current neutrino interactions, in which the neutrino is not transformed into another particle like a muon or an electron. This discovery was made in 1973 by the Gargamelle bubble chamber team at CERN after a race with the HPWF experiment team at Fermilab.
The major theme of neutrino oscillations from the first theoretical ideas of Bruno Pontecorvo (1957) to the Mikheyev–Smirnov–Wolfenstein effect (1985), which can modify the oscillations when neutrinos travel through matter, was complemented by talks on the discovery of neutrino oscillations by Nobel laureates Takaaki Kajita and Art McDonald. In 1998, the Super-Kamiokande experiment, led by Kajita, observed the oscillation of atmospheric neutrinos, and in 2001 the Sudbury Neutrino Observatory experiment, led by McDonald, observed the oscillation of solar neutrinos.
The role of the neutrino in the Standard Model was discussed, as was its intrinsic nature. Although physicists have observed the rare process of double beta decay with neutrinos in the final state, neutrinoless double beta decay with no neutrinos produced has been searched for for more than 30 years because its observation would prove that the neutrino is Majorana-type (its own antiparticle) and not Dirac-type.
To complete the panorama, the conference discussed neutrinos as messengers from the wider universe, from the Big Bang to violent phenomena such as
gamma-ray bursts or active galactic nuclei. Delegates also discussed wrong hints and tracks, which play a positive role in the development of science, and the peculiar sociological aspects that are common to particle physics and astrophysics.
Following the conference, a website dedicated to the history of this fascinating particle was created: https://neutrino-history.in2p3.fr.
Michel Cribier APC Laboratory, Paris and CEA-Saclay, and Daniel Vignaud APC Laboratory, Paris.