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Europe defines astroparticle strategy

The APPEC report makes 21 recommendations for the astroparticle-physics community.

Multi-messenger astronomy, neutrino physics and dark matter are among several topics in astroparticle physics set to take priority in Europe in the coming years, according to a report by the Astroparticle Physics European Consortium (APPEC).

The APPEC strategy for 2017–2026, launched at an event in Brussels on 9 January, is the climax of two years of talks with the astroparticle and related communities. 20 agencies in 16 countries are involved and includes representation from the European Committee for Future Accelerators, CERN and the European Southern Observatory (ESO).

Lying at the intersection of astronomy, particle physics and cosmology, astroparticle physics is well placed to search for signs of physics beyond the standard models of particle physics and cosmology. As a relatively new field, however, European astroparticle physics does not have dedicated intergovernmental organisations such as CERN or ESO to help drive it. In 2001, European scientific agencies founded APPEC to promote cooperation and coordination, and specifically to formulate a strategy for the field.

Building on earlier strategies released in 2008 and 2011, APPEC’s latest roadmap presents 21 recommendations spanning scientific issues, organisational aspects and societal factors such as education and industry, helping Europe to exploit tantalising potential for new discoveries in the field.

There are plans to join forces with experiments in the US to build the next generation of NDBD detectors.

The recent detection of gravitational waves from the merger of two neutron stars (CERN Courier December 2017 p16) opens a new line of exploration based on the complementary power of charged cosmic rays, electromagnetic waves, neutrinos and gravitational waves for the study of extreme events such as supernovae, black-hole mergers and the Big Bang itself. “We need to look at cross-fertilisation between these modes to maximise the investment in facilities,” says APPEC chair Antonio Masiero of the INFN and the University of Padova. “This is really going to become big.”

APPEC strongly supports Europe’s next-generation ground-based gravitational interferometer, the Einstein Telescope, and the space-based LISA detector. In the neutrino sector, KM3NeT is being completed for high-energy cosmic neutrinos at its site in Sicily, as well as for precision studies of atmospheric neutrinos at its French site near Toulon. Europe is also heavily involved in the upgrade of the leading cosmic-ray facility the Pierre Auger Observatory in Argentina. Significant R&D work is taking place at CERN’s neutrino platform for the benefit of long- and short-baseline neutrino experiments in Japan and the US (CERN Courier July/August 2016 p21), and Europe is host to several important neutrino experiments. Among them are KATRIN at KIT in Germany, which is about to begin measurements of the neutrino absolute mass scale, and experiments searching for neutrinoless double-beta decay (NDBD) such as GERDA and CUORE at INFN’s Gran Sasso National Laboratory (CERN Courier December 2017 p8).

There are plans to join forces with experiments in the US to build the next generation of NDBD detectors. APPEC has a similar vision for dark matter, aiming to converge next year on plans for an “ultimate” 100-tonne scale detector based on xenon and argon via the DARWIN and Argo projects. APPEC also supports ESA’s Euclid mission, which will establish European leadership in dark-energy research, and encourages continued European participation in the US-led DES and LSST ground-based projects. Following from ESA’s successful Planck mission, APPEC strongly endorses a European-led satellite mission, such as COrE, to map the cosmic-microwave background and the consortium plans to enhance its interactions with its present observers ESO and CERN in areas of mutual interest.

“It is important at this time to put together the human forces,” says Masiero. “APPEC will exercise influence in the European Strategy for Particle Physics, and has a significant role to play in the next European Commission Framework Project, FP9.”

A substantial investment is needed to build the next generation of astroparticle-physics research, the report concedes. According to Masiero, European agencies within APPEC currently invest around €80 million per year in astroparticle-related activities, in addition to funding large research infrastructures. A major effort in Europe is necessary for it to keep its leading position. “Many young people are drawn into science by challenges like dark matter and, together with Europe’s existing research infrastructures in the field, we have a high technological level and are pushing industries to develop new technologies,” continues Masiero. “There are great opportunities ahead in European astroparticle physics.”

• View the full report at www.appec.org.

Neutrons cooled for interrogation

A proton beamline at TRIUMF branches off to a tungsten spallation target, producing neutrons that are slowed to ultracold speeds in cryostats and directed to the UCN experimental area (right). The EDM cell is magnetically shielded by cylindrical layers. Image credit: TRIUMF

Researchers at TRIUMF in Canada have reported the first production of ultracold neutrons (UCN), marking an important step towards a future neutron electric dipole moment (nEDM) experiment at the Vancouver laboratory. Precision measurements of the nEDM are a sensitive probe of physics beyond the Standard Model: if a nonzero value were to be measured, it would suggest a new source of CP violation, possibly related to the baryon asymmetry of the universe.

The TUCAN collaboration (TRIUMF UltraCold Advanced Neutron source) aims to measure nEDM a factor 30 better than the present best measurement, which has a precision of 3 × 10^–26 e cm and is consistent with zero. For this to be possible, physicists need to provide the world’s highest density of ultracold neutrons. In 2010 a collaboration between Canada and Japan was established to realise such a facility and a prototype UCN source was shipped to Canada and installed at TRIUMF in early 2017.

The setup uses a unique combination of proton-induced spallation and a superfluid helium UCN source that was pioneered in Japan. A tungsten block stops a beam of protons, producing a stream of fast neutrons that are then slowed in moderators and converted to ultracold speeds (less than around 7 ms^–1) by phonon scattering in superfluid helium. The source is based on a non-thermal down-scattering process in superfluid helium below 1 K, which gives the neutrons an effective temperature of a few mK. The ultracold temperature is below the neutron optical potential for many materials, which means the neutrons are totally reflected for all angles of incidence and can be stored in bottles for periods of up to hundreds of seconds.

Tests late last year demonstrated the highest current operation of this particular source, resulting in the most UCNs it has ever produced (> 300,000) in a single 60-second-long irradiation at a 10 µA proton beam current. This is a record for TRIUMF, but the UCN source intensity is still two orders of magnitude below what is needed for the nEDM experiment.

Funding of C$15.7 million to upgrade the UCN facility, a large proportion of which was granted by the Canada Foundation for Innovation in October 2017, will enable the TUCAN team to increase the production of neutrons at higher beam current to levels competitive with other planned nEDM experiments worldwide. These include proposals at the Paul Scherrer Institute in Switzerland, Los Alamos National Laboratory in the US, the Institut Laue–Langevin in France and others in Germany and Russia. The neutron EDM is experiencing intense competition, with most projects differing principally in the way they propose to produce the ultracold neutrons (CERN Courier September 2016 p27).

The nEDM experimental campaign at TRIUMF is scheduled to start in 2021. “The TRIUMF UCN source is the only one combining a spallation source of neutrons with a superfluid helium production volume, providing the project its uniqueness and competitive edge,” says team member Beatrice Franke.

Lithuania formalises CERN membership

Lithuania president Dalia Grybauskaitė (left) and CERN Director-General Fabiola Gianotti, pictured at the 2018 World Economic Forum in Davos, signed the agreement in June last year. Image credit: Office of the President of the Republic of Lithuania

On 8 January the Republic of Lithuania formally became an associate Member State of CERN, following the completion of its internal approval procedures (CERN Courier July/August 2017 p7). Lithuania’s relationship with CERN dates back to an International Cooperation Agreement signed in 2004, with Lithuanian researchers contributing to the CMS experiment since 2007. Its new status strengthens the long-term partnership between CERN and the Lithuanian scientific community, makes Lithuanian scientists eligible for staff appointments, and entitles Lithuanian industry to bid for CERN contracts.

LHC prepares for final year of Run 2

Installation of a collimator at LHC point 1. Image credit: M Brice/CERN

Since 4 December, around 500 technicians and engineers have been working flat-out to maintain and upgrade the Large Hadron Collider (LHC) and other parts of the CERN accelerator complex. The current year-end technical stop will last until 9 March, and preparations for the machine and its infrastructure for the High Luminosity LHC (HL-LHC) have been a focus of activities.

Collimators are key to operating the HL-LHC, which will have roughly twice the stored energy (700 MJ) as the present machine. These devices control losses from the circulating proton beams so that they can be constrained to a small section of the machine’s circumference. Continuing work undertaken during last year’s extended year-end stop (CERN Courier March 2017 p9), two new collimators are being installed at point 1 containing a wire that generates an electromagnetic field to compensate for long-range beam–beam effects.

Higher performing injectors that can produce more intense particle beams are another demand of the HL-LHC, and this aspect is being managed by the LHC Injector Upgrade (LIU) project (CERN Courier October 2017 p32). An upgraded kicker magnet, one of eight fast-pulsed magnets that inject particle beams coming from the Super Proton Synchrotron (SPS) into the LHC, will be installed at point 8. A special coating applied to the inner wall of the ceramic pipe of the magnet is one of several techniques developed to reduce the heating of components in the harsher HL-LHC environment.

While work steps up on the LHC, which has been temporarily emptied of its 120 tonnes of helium coolant, brand-new accelerator technology that will help the HL-LHC achieve its unprecedented luminosities is being prepared for tests in the SPS. Two prototype radiofrequency crab cavities – designed to tilt particle bunches before they collide to maximise the overlapping of the beams and increase the probability of collisions – have been installed for testing during 2018. In around five years from now, during Long Shutdown 3 (LS3), the full system will be installed in the LHC.

Further down the accelerator chain, a major de-cabling campaign is taking place in the Proton Synchrotron (PS) to create space for the deployment of the LIU project during Long Shutdown 2 (LS2) beginning next year. The transfer line linking the PS to the SPS is also having all of its 43 quadrupole magnets replaced, among numerous other works. The whole CERN injector chain is undergoing an annual check-up, in particular concerning the cooling, ventilation, cryogenics and electrical supply systems. Other important activities are taking place to consolidate the infrastructure, such as the installation of a new lift at LHC point 8, and to update the beam control systems.

During the 2018 LHC performance workshop, held in Chamonix from 29 January to 2 February, the performance of the LHC during 2017 was reviewed and operational scenarios for 2018 were discussed. A particular focus of the workshop was on the status of the LIU and HL-LHC projects, which will be rolled-out in LS2 and LS3, respectively. There was lively discussion about the organisation and planning of activities for LS2, and the final session of the workshop covered the full energy exploitation of the LHC. Until LS2 the machine will run at a centre-of-mass energy of 13 TeV, but prospects for running at 14 TeV after LS2 and eventuallly even 15 TeV were also discussed.

Rare hyperon-decay anomaly under the spotlight

The invariant mass distribution of Σ⁺ → pµ⁺µ^– candidates and, in the inset, the background-subtracted distribution of the dimuon invariant mass.

The LHCb collaboration has shed light on a long-standing anomaly in the very rare hyperon decay Σ⁺ → pµ⁺µ^– first observed in 2005 by Fermilab’s HyperCP experiment. The HyperCP team found that the branching fraction for this process is consistent with Standard Model (SM) predictions, but that the three signal events observed exhibited an interesting feature: all muon pairs had invariant masses very close to each other, instead of following a scattered distribution.

This suggested the existence of a new light particle, X⁰, with a mass of about 214 MeV/c², which would be produced in the Σ⁺ decay along with the proton and would decay subsequently to two muons. Although this particle has been long sought in various other decays and at several experiments, no experiment other than HyperCP has so far been able to perform searches using the same Σ⁺ decay mode.

The large rate of hyperon production in proton–proton collisions at the LHC has recently allowed the LHCb collaboration to search for the Σ⁺ → pµ⁺µ^– decay. Given the modest transverse momentum of the final-state particles, the probability that such a decay is able to pass the LHCb trigger requirements is very small. Consequently, events where the trigger is activated by particles produced in the collisions other than those in the decay under study are also employed.

This search was performed using the full Run 1 dataset, corresponding to an integrated luminosity of 3 fb^–1 and about 10¹⁴ Σ⁺ hyperons. An excess of about 13 signal events is found with respect to the background-only expectation, with a significance of four standard deviations. The dimuon invariant- mass distribution of these events was examined and found to be consistent with the SM expectation, with no evidence of a cluster around 214 eV/c². The signal yield was converted to a branching fraction of (2.1^+1.6_–1.2) × 10^–8 using the known Σ⁺ → pπ⁰ decay as a normalisation channel, in excellent agreement with the SM prediction. When restricting the sample explicitly to the case of a decay with the putative X⁰ particle as an intermediate state, no excess was found. This sets an upper limit on the branching fraction at 9.5 × 10^–9 at 90% CL, to be compared with the HyperCP result (3.1^+2.4_–1.9 ± 1.5) × 10^–8.

This result, together with the recent search for the rare decay K_S → μ⁺μ^– shows the potential of LHCb in performing challenging measurements with strange hadrons. As with a number of results in other areas reported recently, LHCb is demonstrating its power not only as a b-physics experiment but as a general-purpose one in the forward region. With current data, and in particular with the upgraded detector thanks to the software trigger from Run 3 onwards, LHCb will be the dominant experiment for the study of both hyperons and K_S mesons, exploiting their rare decays to provide a new perspective in the quest for physics beyond the SM.

ESO

The new ExTrA facility Image credit: ESO

A new national facility at La Silla Observatory in Chile, operated by the European Southern Observatory (ESO), made its first observations at the beginning of the year. ExTrA (Exoplanets in Transits and their Atmospheres) will search for Earth-sized planets orbiting nearby red dwarf stars, its three 0.6 m-diameter near-infrared telescopes (pictured) increasing the sensitivity compared to previous searches. ExTrA is a French project also funded by the European Research Council and the telescopes will be operated remotely from Grenoble.

ATLAS measures rare top plus boson production

(left) Multivariate discriminator output distribution for the events selected by the tZ analysis, with data in black, signal simulation in pink and backgrounds in other colours. (right) Normalised differential tW cross-sections as a function of the mass of the two charged leptons and the b-jet unfolded from data, compared with a Monte Carlo prediction.

Measuring the production of the top quark with vector bosons can provide fresh insight into the Standard Model (SM), in particular by testing the top quark and heavy vector boson vertices, which may be modified by extensions to the SM. In two new results, ATLAS presents strong evidence for the production of a single top quark in association with a Z boson (tZ) and has for the first time extracted differential cross-sections for the production of a top quark in association with a W boson (tW). While tW production was already measured during LHC Run 1, the next in line, the tZ process, is much harder to observe because its production rate is about one hundredth lower.

For both the tZ and tW processes, separating them from background events is critical. ATLAS searched for events containing leptons (electrons or muons), jets and transverse momentum imbalance. All the information from the measured particles is condensed into one multivariate discriminator (MVA) trained to separate the signal from the background.

The new ATLAS results use data collected in 2015 and 2016, corresponding to an integrated luminosity of 36.1 fb^–1. For the tZ analysis, 25 signal events are found after selection, together with 120 background events. Applying the MVA allows the signal and background to be better separated (see figure, left), leading to a signal significance of 4.2 standard deviations. This constitutes strong evidence that the associated production of a single top quark and a Z boson has been seen, and the observed production rate agrees with that predicted by the SM.

The extraction of differential cross-sections for tW is particularly challenging, as top quarks almost always decay into a b quark and a W boson, leaving two W bosons in the final state. The dominant background from the production of a top quark with a top antiquark has an 11 times larger inclusive production rate. Applying the MVA it is possible to select events with a signal to background ratio of about 1:2, which allows the signal cross-section to be extracted as a function of kinematic observables. Differential cross-sections have been measured as a function of several variables and measured and compared to predictions implemented in different Monte Carlo programmes (see figure). The uncertainty on the measurements is at the 20–50% level, dominated by statistical effects. While the analysis was not able to exclude particular models, the data tend to have more events with high-momentum particles than predicted.

With the additional data to be collected over the next years, ATLAS will study both tW and tZ production in more detail, and improve its searches for the even rarer and more elusive production of a (single) top quark in association with a Higgs boson.

CMS hunts for heavy neutral leptons

(Left) Diagram showing the production and decay of heavy neutral leptons. (Right) Exclusion region at 95% CL in the |V_µN|² m_N planes. The dashed black curve is the expected upper limit (with one and two sigma bands in green and yellow) and the solid black curve is the observed upper limit. The dotted black curve is the observed limit in the approximation of prompt N decays. Also shown are the upper limits from other direct searches at DELPHI and ATLAS.

The quest to search for new physics inspires searches in CMS for very rare processes, which, if discovered, could open the door to a new understanding of particle physics.

One such process is the production and decay of heavy sterile Majorana neutrinos, a type of heavy neutral lepton (HNL) introduced to describe the very small neutrino masses via the so-called seesaw mechanism. Two further fundamental puzzles of particle physics can be solved by adding three HNLs to the Standard Model (SM) particle spectrum: the lightest (with a mass of a few keV) can serve as a dark-matter candidate; the two heavier ones (heavier than about a GeV) could, when mass-degenerate, be responsible for a sizable amount of CP violation and thus help explain the cosmological matter–antimatter asymmetry.

Through their mixing with the SM neutrinos (see figure, left), the heavier HNLs could be produced at the LHC in leptonic W-boson decays. Subsequently, the HNL can decay to another W boson and a lepton, leading to a signal containing three isolated leptons. Depending on how weakly the new particles couple to the SM neutrinos, characterised by the parameters |V_eN|², |V_μ_N|² and |V_τ_N|², they can either decay shortly after production, or after flying some distance in the detector.

A new search performed with data collected in 2016 by CMS focuses on prompt trilepton (electrons or muons) signatures of HNL production. It explores a mass range from 1 GeV to 1.2 TeV, more than doubling the scope of LHC results so far. It also probes a mass regime that was unexplored since the days of the Large Electron-Positron collider (LEP), indicating that eventually the LHC will supersede these results with more data.

The trilepton final state does not lead to a sharp peak in an invariant mass spectrum, and therefore the search has to employ various kinematic properties of the events to be able to detect a possible presence of HNLs. To be sensitive to very low HNL masses, the search uses soft muons (with p_T > 5 GeV) and electrons (p_T > 10 GeV). While no signs of HNL have been found so far (see figure, right), the constraints on |V_μ_N|² (|V_eN|² is similar) in the high-mass region are the strongest to date. In the low mass region, the analysis has comparable sensitivity to previous searches.

Using dedicated analysis techniques, it is foreseen to extend this search to explore the parameter space where HNLs have longer lifetimes and so travel large distances in the detector before they decay. Together with more data this will enable CMS to significantly improve the sensitivity at low masses and eventually probe unexplored territory in this important region of HNL parameter space.

ALICE investigates charm-quark hadronisation

(Left) The Λ⁺_c/D⁰ baryon-to-meson ratio measured in pp and p–Pb collisions as a function of transverse momentum, compared with different event generators for pp collisions. (Right) The ratio of the *p_T* differential cross-sections of Ξ⁰_c baryons (multiplied by the branching ratio into e⁺ ν_e Ξ^–) as a function of transverse momentum, showing the large uncertainty on the Ξ⁰_c → e⁺ ν_e Ξ^– branching ratio (shaded bands).

In two publications submitted to the Journal of High Energy Physics and Physics Letters B in December, the ALICE collaboration reports new production cross-section measurements of the charmed baryons Λ⁺_c and Ξ⁰_c in proton–proton collisions at an energy of 7 TeV and in proton–lead collisions at a collision energy of 5.02 TeV per nucleon–nucleon pair. The Λ⁺_c were reconstructed in the hadronic decay modes Λ⁺_c → pK⁻ π⁺ and Λ⁺_c → p K⁰_S, and in the semileptonic channel Λ⁺_c → e⁺ ν_e Λ (and charge conjugates). For the Ξ⁰_c analysis, the semi-leptonic channel Ξ⁰_c → e⁺ ν_e Ξ^– was used.

The comparison of charm baryon and meson cross-sections provides information on c-quark hadronisation. Surprisingly, the measured values of the Λ⁺_c/D⁰ baryon-to-meson ratio were significantly larger than those previously measured in other experiments in collisions involving electron beams at different centre-of-mass energies, rapidity and p_T intervals.

The results (see figure) are compared with the expectations obtained from perturbative QCD calculations and Monte Carlo event generators. None of the models reproduce the data, indicating that the fragmentation of charm quarks is not well understood. A similar pattern is seen when comparing the Ξ⁰_c/D⁰ baryon-to-meson ratio with predicted values (see figure, right), where the latter have a sizable uncertainty due to the unknown branching ratio of the decay.

These two results suggest that charmed baryon formation might not be universal, and that the baryon/meson ratio depends on the collision system. Hints of non-universality of the fragmentation functions are also seen when comparing beauty-baryon production measurements at the Tevatron and LHC with those at LEP. The ratios measured in pPb collisions are similar to the result in pp collisions.

The statistical precision of the Λ⁺_c and Ξ⁰_c measurements is expected to be improved with data collected during the LHC Run 2, and with data from Run 3 and Run 4 following a major upgrade of the ALICE apparatus. This set of measurements also provides a reference for future investigation of Λ⁺_c and Ξ⁰_c production in lead–lead collisions, where the formation and kinematic properties of charm baryons are expected to be affected by the presence of the quark–gluon plasma.

Ancient black hole lights up early universe

The spectrum from J1342+0928, with the red line showing the modelled original spectrum before absorption by both neutral and ionised hydrogen.

Many questions remain about what happened in the first billion years of the universe. At around 100 million years old, the universe was a dark place consisting of mostly neutral hydrogen without many objects emitting detectable radiation. This situation changed as stars and galaxies formed, leading to a phase transition known as reionisation where the neutral hydrogen was ionised. Exactly when reionisation started and how long it took is still not fully clear, but a recent discovery of the oldest massive black hole ever found can help answer this important question.

Up to about 300,000 years after the Big Bang, the universe was hot and dense, and electrons and protons were fully separated. As the universe started to expand, it cooled down and underwent a first phase transition where electrons and protons formed neutral gases such as hydrogen. The following period is known as the cosmic dark ages. During this period, protons and electrons were mostly combined into neutral hydrogen, but the universe had to cool much further before matter could condense to the level where light-producing objects such as stars could form. These new objects started to emit both the radiation we can now detect to study the early universe and also the radiation responsible for the last phase transition – the reionisation of the universe. Some of the brightest and therefore easiest-to-detect objects are quasars: massive black holes surrounded by discs of hot accreting matter that emit radiation over a wide but distinctive spectrum.

Using data from a range of large-area surveys by different telescopes, a group led by Eduardo Bañados from the Carnegie Institution for Science has discovered a distant quasar called J1342+0928, with the black hole at its centre found to be eight million solar masses. After the radiation was emitted by J1342+0928, it travelled through the expanding universe, increasing its wavelength or “red shifting” in proportion to its travel time. Using known spectral features of quasars, the redshift (and therefore the moment at which the radiation was emitted) can be calculated.

The spectrum of J1342+0928, shown in the figure, demonstrates that the universe was only 690 million years old – just 5% of its current age – at the time we see J1342+0928. The spectrum also shows a second interesting feature: the absorption of a part of the spectrum by neutral hydrogen, which implies that at the time we are observing the black hole, the universe was not fully ionised yet. By modelling the emission and absorption, Bañados and co-workers found that the spectrum from J1342+0928 is compatible with emission in a universe where half the hydrogen was ionised, putting the time of emission right in the middle of the epoch of reionisation.

The next mystery is to explain how a black hole weighing eight million solar masses could form so early in the universe. Black holes grow as they accrete mass surrounding them, but the accreting mass radiates and this radiation pushes other accreting mass away from the black hole. As a result, there is a theoretical limit on the amount of matter a black hole can accrete. Forming a black hole the size of J1342+0928 with such accretion limits would require black holes in the very early universe with sizes that challenge current theoretical models. One possible explanation, however, is that this particular black hole is a peculiar case and was formed by a merger of several smaller black holes.

Thanks to continuous data taking from a range of existing telescopes and upcoming new instrumentation, we can expect more objects like J1342+0928 or even older to be discovered, offering a probe of the universe at even earlier stages. The discovery of further objects would allow a more exact date for the period of reionisation, which can be compared with indirect measurements coming from the cosmic microwave background. At the same time, more measurements will show if black holes of this size in the early universe are just an anomaly or if there are more. In either case, such observations would provide important input for research on early black hole formation.

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