Jobs – Page 156 of 527

SuperKEKB steps out at the intensity frontier

Celebrating first collisions in the Belle II control room. Image credit: KEK

On 26 April the SuperKEKB accelerator at the KEK laboratory in Japan collided its first beams of electrons and positrons, marking the start of an ambitious data-taking campaign that will allow ultraprecise measurements of Standard Model (SM) parameters.

These are the first particle collisions at KEK in eight years, following the closure in 2010 of the KEKB machine to prepare for its next phase. Many subsystems of the accelerator had to be upgraded, the most important involving the use of nanobeam technology to squeeze the vertical beam size at the interaction point to around 50 nm – 20 times smaller than it was at KEKB. This required a complicated system of superconducting final-focus magnets and low-emittance beams (CERN Courier September 2016 p32).

SuperKEKB will work at the so-called intensity frontier to produce copious amounts of B and D mesons and τ leptons, enabling precise measurements of rare decays that test the SM with unprecedented sensitivity. Since the first beams were stored over a month ago, KEK teams have worked to tune the two beams for first collisions at the centre of the Belle II detector – the “super-B factory” upgrade of its predecessor, Belle. When fully commissioned, Belle II will detect and reconstruct events at the much higher rates provided by the 40-fold higher design luminosity of SuperKEKB compared to KEKB. The Belle II outer detector is already in place, but the full inner detector will not be installed until the end of the year, and the first physics run with the complete detector is projected to start in February 2019.

A hadronic event spotted by the Belle II detector at 2.27 a.m. on 26 April.

In 2009 KEKB achieved a record instantaneous luminosity of 2.1 × 10³⁴ cm^–2 s^–1, but SuperKEKB is targeting 8 × 10³⁵ cm^–2 s^–1. The huge increase is projected to deliver to Belle II a dataset of about 50 billion BB meson pairs – 50 times larger than the entire data sample of the KEKB/Belle project – in about 10 years of operation.

According to Belle II spokesperson Tom Browder, it is not realistic to expect design luminosity straight away. “There will be a number of steps as the beam is progressively squeezed to smaller and smaller sizes, and we fight through each new technical challenge with nanobeams,” he explains. “Our luminosity profile assumes that we progressively resolve these problems at the same rate as KEKB or PEP-II [at SLAC]. In this sense, our programme resembles that of the LHC.”

Belle II has physics goals related to those of the LHCb experiment (CERN Courier April 2018 p23), set against the relatively cleaner environment of electron–positron collisions but with a lower production rate of heavy hadrons with respect to the LHC collisions. Examples include investigating whether there are new CP-violating phases in the quark sector, whether there are sources of lepton-flavour violation (LFV) beyond the SM, whether there is a dark sector of particle physics at the same mass scale as ordinary matter, and whether there are flavour-changing neutral currents beyond the SM. Browder says the Belle II collaboration expects to work on all of the goals, especially on LFV studies, and to catch up with LHCb on certain measurements as soon as a significant amount of luminosity is achieved. “Even with the very early data samples, the team should be able to have impactful results on the dark sector and new hadrons,” he says.

LHC physics soars ahead

The 2018 integrated-luminosity curve (green) shows an early and steep increase, but beware: planned technical stops, machine-development sessions and special physics runs will introduce plateaus as the year progresses.

On 28 April, 13 days ahead of schedule, operators at CERN’s Large Hadron Collider (LHC) successfully injected 1200 bunches of protons into the machine and brought them into collision – formally marking the beginning of the LHC’s 2018 physics season, and the final leg of Run 2.

Stable beams were first declared on 17 April, when the LHC experiments started to take data with three bunches per beam at very low luminosities. A stepwise increase of the number of bunches resulted in the maximum number of 2556 bunches per beam being reached on 5 May.

During the last steps of the intensity ramp-up, the average peak luminosity for ATLAS and CMS was close to 2.1 × 10³⁴ cm^–2 s^–1 – equalling or even surpassing the record peak luminosity with stable beams reached in 2017 – although the final calibration of the luminosity measurements still needs to be performed.

For the rest of the year, the LHC is dedicated to production mode for physics, with the operation of the machine being consolidated in parallel. As the Courier went to press in mid-May, the integrated luminosity for ATLAS and CMS had already surpassed 10 fb^–1 (with a target of 60 fb^–1 planned for 2018).

The faster-than-anticipated commissioning phase of the 2018 LHC restart has led to a revised machine schedule: the LHC will provide 131 days of physics operations with 25 ns-spaced proton beams, 17 days of special runs with protons, and 24 days of lead–lead collisions at the end of the year (the proton run will finish on 28 October). From December, the machine will enter Long Shutdown 2 in preparation for its high-luminosity upgrade.

KLOE-2 completes data-taking at Frascati Φ-factory

The KLOE/KLOE-2 and DAΦNE teams marked the end of an era on 30 March. Image credit: LNF

On 30 March the KLOE-2 experiment concluded its data-taking campaign at the electron–positron collider DAΦNE at the INFN National Laboratory of Frascati (LNF), Italy. This marks the conclusion of a two-decades long period of activity at the Frascati lab, which began with the first data collected by KLOE in 1999 and then continued with KLOE-2 since November 2014. In total, an integrated luminosity of around 8 fb⁻¹ (corresponding to around 24 billion φ-mesons) was acquired, representing the largest ever sample collected at the φ-resonance peak.

In terms of machine physics, the KLOE/DAΦNE programme has brought a wealth of results and a few world firsts. The KLOE-2 run saw the first application of the “crab-waist” concept – an interaction scheme developed in Frascati with the transverse dimensions of the beams and their crossing angle tuned to maximise the machine luminosity – in the presence of a high-field detector solenoid. The implementation of this innovative configuration by the DAΦNE team allowed KLOE-2 to collect an integrated luminosity of 5.5 fb⁻¹ in a period of just over three years.

Record performances in terms of peak luminosity (2.4 × 10³² cm^–2 s^–1) and maximum daily integrated luminosity (14 pb^–1/day) have been achieved for an electron–positron collider running at such centre-of-mass energies (approximately 1 GeV).

The general-purpose KLOE detector, comprising a 4 m-diameter drift chamber surrounded by a lead-scintillating-fibre electromagnetic calorimeter with very good energy and timing performance at low energies, underwent several upgrades including a cylindrical gas-electron-multiplier (GEM) detector for the inner tracker. To improve its vertex reconstruction capabilities near the interaction region, KLOE-2 was the first high-energy experiment using GEM technology with a cylindrical geometry – a novel idea that was developed at LNF.

Together with its predecessor KLOE, the KLOE-2 data sample is rich in physics. The analysis of KLOE data provided, and continues to provide, a variety of significant results on: neutral and charged kaon properties; tests of discrete symmetries; tests of the unitary of the quark mixing matrix; light-scalar-meson spectroscopy; η-meson decays; hadronic cross sections and the anomalous magnetic moment (g-2) of the muon; and searches for dark photons.

Analyses of the KLOE-2 data is ongoing, in particular extending the KLOE physics programme in precision tests of fundamental discrete symmetries and the quantum coherence of entangled neutral kaon pairs. The roughly 60-strong collaboration will also study rare K_S and η-meson decays and strong interactions in low-energy processes, in addition to γγ physics and the search for possible manifestations of dark matter.

Overall, the KLOE programme has involved hundreds of Italian and foreign physicists in a challenging human and scientific enterprise. But activities at the DAΦNE accelerator complex do not stop here. They are now continuing with the PADME and Siddharta-2 experiments, designed to search for dark photons and to study exotic atoms and strong interactions at low energies, respectively. Frascati Laboratory is also planning to revamp the DAΦNE complex, becoming a world-class test facility for R&D in accelerator physics, and is applying to host the future EuPRAXIA infrastructure for a European plasma-based free-electron Laser.

“The KLOE experiment has been a scientific milestone for the laboratory and for particle physics,” says LNF director, Pierluigi Campana. “DAΦNE will continue to produce physics for PADME and Siddharta-2, and we are thinking towards its future after 2020.”

OPERA concludes on tau appearance

The OPERA detector (left) and an event display showing a muon-to-tau neutrino conversion (right). Image credit: INFN

The OPERA experiment, located at the Gran Sasso Laboratory of the Italian National Institute for Nuclear Physics (INFN), was designed to conclusively prove that muon-neutrinos can oscillate into tau-neutrinos by studying beams of muons sent from CERN 730 km away.

In a paper published on 22 May, describing the very final results of the experiment on neutrino oscillations, the OPERA collaboration has reported the observation of a total of 10 candidate events for a muon- to tau-neutrino conversion. This result demonstrates unambiguously that muons morph into tau neutrinos on their way from CERN to Gran Sasso.

The OPERA collaboration observed the first tau-neutrino event (evidence of muon-neutrino oscillation) in 2010, followed by four additional events reported between 2012 and 2015. A new analysis strategy applied to the full data sample collected between 2008 and 2012 led to the new total of 10 candidate events, with an extremely high level of significance. “We also report the first direct observation of the tau-neutrino lepton number, the parameter that discriminates neutrinos from antineutrinos,” says Giovanni de Lellis, OPERA spokesperson. “It is extremely gratifying to see today that our legacy results largely exceed the level of confidence we had envisaged in the experiment proposal.”

Beyond its contribution to neutrino physics, OPERA pioneered the use of large-scale emulsion films with fully automated and high-speed readout technologies with submicrometre accuracy. These technologies are now used in a wide range of other scientific areas, from dark-matter searches to investigations of volcanoes, and from the optimisation of hadron therapy for cancer treatment to the exploration of secret chambers in the Great Pyramid. The OPERA collaboration has also made its data public through the CERN open data portal, allowing researchers outside the collaboration to conduct novel research and offering tools such as a visualiser to help adapt the datasets for educational use.

US and India team up on neutrino physics

Inside the final structure of the protoDUNE module at CERN, which is soon to undergo its first beam tests. Image credit: J Ordan/CERN

On 16 April, US energy secretary Rick Perry and Indian Atomic Energy Secretary Sekhar Basu signed an agreement in New Delhi to expand the two countries’ collaboration in neutrino science. It opens the way for jointly advancing the Long-Baseline Neutrino Facility (LBNF) and the international Deep Underground Neutrino Experiment (DUNE) in the US and the India-based Neutrino Observatory (INO).

More than 1000 scientists from over 170 institutions in 31 countries work on LBNF/DUNE, construction for which got under way in July 2017. The project will direct the world’s most intense beams of neutrinos from Fermilab accelerators (driven by the new PIP-II machine) to detectors 1300 km away. INO scientists, meanwhile, will observe neutrinos that are produced in Earth’s atmosphere. Scientists from more than 20 institutions are working on INO, which is currently going through approval procedures.

The India–US agreement builds on one signed in 2013 authorising the joint development and construction of particle-accelerator components. Scientists from four institutions in India – BARC in Mumbai, IUAC in New Delhi, RRCAT in Indore and VECC in Kolkata – are contributing to the design and construction of magnets and superconducting particle-accelerator components for PIP-II at Fermilab and the next generation of particle accelerators in India.

Under the new agreement, US and Indian institutions will expand this to include neutrino research projects. DUNE, located about 1.5 km underground, will use almost 70,000 tonnes of liquid argon to detect neutrinos; and an additional detector will measure the neutrino beam at Fermilab as it leaves the accelerator complex. Prototype neutrino detectors are already under construction at CERN, which is also a partner in LBNF/DUNE. INO will use a different technology: an iron calorimeter. Its detector will feature what could be the world’s biggest magnet, allowing INO to be the first experiment able to distinguish signals produced by atmospheric neutrinos and antineutrinos produced when cosmic rays strike the atmosphere.

More than a dozen Indian institutions are involved in the collaboration on neutrino research. According to former INO spokesperson Naba Monda of the Saha Institute of Nuclear Physics, “this agreement is a positive step towards making INO a global centre for fundamental research. Students working at INO will get opportunities to interact with international experts.”

Higgs boson reaches the top

The CMS collaboration has published the first direct observation of the coupling between the Higgs boson and the top quark, offering an important probe of the consistency of the Standard Model (SM). In the SM, the Higgs boson interacts with fermions via a Yukawa coupling, the strength of which is proportional to the fermion mass. Since the top quark is the heaviest particle in the SM, its coupling to the Higgs boson is expected to be the largest and thus the dominant contribution to many loop processes, making it a sensitive probe of hypothetical new physics.

The associated production of a Higgs boson with a top quark–antiquark pair (ttH) is the best direct probe of the top-Higgs Yukawa coupling with minimal model dependence, and thus a crucial element to verify the SM nature of the Higgs boson. However, its small production rate – constituting only about 1% of the total Higgs production cross-section – makes the ttH measurement a considerable challenge.

The CMS and ATLAS collaborations reported first evidence for the process last year, based on LHC data collected at a centre-of-mass energy of 13 TeV (CERN Courier May 2017 p49 and December 2017 p12). The first observation, constituting statistical significance above five standard deviations, is based on an analysis of the full 2016 CMS dataset recorded at an energy of 13 TeV and by combining these results with those collected at lower energies.

CCJune18_News-CMS — Top: distribution of all events as a function of log₁₀ S/B, where S and B are the expected signal and background yields, respectively, evaluated for each bin in the combined likelihood analysis. The signal contribution is shown for both the SM expectation (μ = 1.0) and the value measured in data (μ = 1.26). Bottom: the likelihood-ratio test statistic q as a function of the tt̅H signal-strength modifier, μ, at 7+8 TeV and at 13 TeV, separately, and their combination. The expected SM result for the combination is also shown. The horizontal dashed lines indicate the p-values for the background-only hypothesis expressed in the number of standard deviations.

The ttH process gives rise to a wide variety of final states, and the new CMS analysis combines results from a number of them. Top quarks decay almost exclusively to a bottom quark (b) and a W boson, the latter subsequently decaying either to a quark and an antiquark or to a charged lepton and its associated neutrino. The Higgs-boson decay channels include the decay to a bb quark pair, a τ⁺τ^– lepton pair, a photon pair, and combinations of quarks and leptons from the decay of intermediate on- or off-shell W and Z bosons. These five Higgs-boson decay channels were analysed by CMS using sophisticated methods, such as multivariate techniques, to separate signal from background events. Each channel poses different experimental challenges: the bb channel has the largest rate but suffers from a large background of events containing a top-quark pair and jets, while the photon and Z-boson pair channels offer the highest signal-to-background ratio at a very small rate.

CMS observed an excess of events with respect to the background-only hypothesis at a significance of 5.2 standard deviations. The measured values of the signal strength in the considered channels are consistent with each other, and a combined value of 1.26 +0.31/–0.26 times the SM expectation is obtained (see figure). The measured production rate is thus consistent with the SM prediction within one standard deviation. The result establishes the direct Yukawa coupling of the Higgs boson to the top quark, marking an important milestone in our understanding of the properties of the Higgs boson.

ALICE probes partons inside lead nuclei

Top: invariant yield of dimuons from Z-boson production in the rapidity range 2.5 < y < 4.0 divided by the average nuclear overlap function in the 0–90% centrality class in PbPb collisions at 5.02 TeV. The horizontal bar represents the statistical uncertainty, while the yellow band represents the systematic uncertainty, and the result is compared to theoretical calculations with and without nuclear modification of the PDFs. Bottom: nuclear modification factor as a function of the collision centrality: vertical error bars are statistical, while boxes represent systematic uncertainties.

The large centre-of-mass energy and luminosity of the LHC have made possible the first measurements of electroweak-boson production in ultrarelativistic heavy-ion collisions. The production cross sections for such processes in proton–proton (pp) collisions are known with high precision, and indeed have been suggested as “standard candles” for luminosity measurements at the LHC. Since the electroweak bosons and their leptonic decay products do not interact strongly with the hot and dense quark–gluon matter produced in heavy-ion collisions, they can also be used as a reference in this environment. Here, the production rates of W and Z bosons directly probe initial-state effects, such as the u- and d-quark density (isospin) and the difference between the parton density functions (PDFs) of nucleons that are bound in nuclei and those that are free. These effects are studied by comparing the measurements in lead–lead (PbPb) collisions to the results from pp collisions, taking their different collision-impact parameters into account.

The ALICE experiment has measured, for the first time, Z-boson production at large rapidity in PbPb collisions at a centre-of-mass energy of 5.02 TeV per nucleon pair (figure, top). The measurement was compared with theoretical calculations at next-to-leading order, considering a combination of proton and neutron PDFs to account for the isospin of the lead nucleus. Those calculations that include a nuclear modification of the PDFs (through three different parameterisations) describe the data well, while the calculation using free proton and neutron PDFs overestimates the data by 2.3 standard deviations.

The Z production rate was studied as a function of rapidity and collision centrality. To study the radial dependence of the nuclear effects, the data sample was divided into two different centrality classes and the nuclear modification factor R_AA was evaluated by dividing the normalised yields by the theoretical pp cross-section reference (figure, bottom). The measurements are well described by the calculations using an impact-parameter-dependent nuclear PDF, and the data point in the 0–20% most central collisions deviates from the predictions with free PDFs by three standard deviations.

The Z-boson measurements at large rapidity in PbPb collisions at LHC energies are well-described by calculations that include nuclear modifications of the PDFs. These have been inferred mostly from deep-inelastic scattering experiments at lower energies, while the predictions using free PDFs deviate from data. The data from the upcoming PbPb data-taking period in November will allow ALICE to improve the precision of the electroweak-boson measurements and provide more precise information on the modification of PDFs in nuclei.

Central exclusive production at LHCb

Photoproduction cross section for J/ψ mesons as a function of the proton–photon centre-of-mass energy from various experiments.

Four years ago, LHCb measured the central exclusive production (CEP) of J/ψ and ψ(2S) mesons at a centre-of-mass energy of 7 TeV (CERN Courier March 2014 p7). In CEP, two incoming protons emerge intact from the collision, with a central system created by the fusion of two propagators that do not contain colour (e.g. photons or pomerons). This results in an unusual final state for hadron collisions with just a few particles detected: in the case of J/ψ and ψ(2S) mesons, it leads to the characteristic signature of two muons from the meson decay and no other observed activity in the event.

A major background to this process is due to collisions where the protons dissociate but the remnants travel close to the beamline and thus remain undetected. To address this, the LHCb collaboration designed and built a new detector called HeRSCheL, which was installed at the beginning of 2015 in the LHC tunnel. It consists of 20 square plastic scintillators approximately 30 cm wide placed just outside the vacuum pipe at distances up to 114 m from the interaction point. Whilst LHCb is fully instrumented in the pseudorapidity region 2 < η < 5, HeRSCheL significantly extends the sensitivity to 5 < | η | < 10 and therefore improves the precision with which the experiment can observe CEP processes.

LHCb has now taken advantage of the extra reach to measure J/ψ and ψ(2S) CEP in 13 TeV proton–proton collisions. By including HeRSCheL, backgrounds have been reduced by a factor of two compared to the measurement at 7 TeV. Furthermore, by comparing events with and without activity in HeRSCheL, a much better understanding of those backgrounds has been achieved, resulting in an improved precision.

The figure shows the derived photoproduction cross section for J/ψ mesons as a function of the proton–photon centre-of-mass energy for LHCb data at 7 and 13 TeV, with good agreement observed compared to theoretical predictions. Also shown are ALICE results in proton–lead collisions and HERA (H1 and Zeus) results at lower energies. The shaded band is a power-law extrapolation of the HERA data, which is seen to be inconsistent with the data at the highest energies.

Measurements of the CEP process can be used to test perturbative QCD predictions as well as to improve our understanding of the distribution of gluons inside the proton. This new measurement paves the way to future CEP analyses at LHCb and beyond, not only using proton–proton but also heavy-ion collisions.

Trigger-level searches for low-mass dijet resonances

The compact TLA data format records approximately 1% of the nominal event size at a much higher rate than standard data-taking. As a result, more data are recorded at lower dijet invariant mass (left). The large TLA dataset allows ATLAS to set its strongest limits on resonances decaying to quarks in the mass range between 450 GeV and 1 TeV (right).

The LHC is not only the highest-energy collider ever built, it also delivers proton–proton collisions at a much higher rate than any machine before. The LHC detectors measure each of these events in unprecedented detail, generating enormous volumes of data. To cope, the experiments apply tight online filters (triggers) that identify events of interest for subsequent analysis. Despite careful trigger design, however, it is inevitable that some potentially interesting events are discarded.

The LHC-experiment collaborations have devised strategies to get around this, allowing them to record much larger event samples for certain physics channels. One such strategy is the ATLAS trigger-object level analysis (TLA), which consists of a search for new particles with masses below the TeV scale decaying to a pair of quarks or gluons. The analysis uses selective readout to reduce the event size and therefore allow more events to be recorded, increasing the sensitivity to new physics in domains where rates of Standard Model (SM) background processes are very large.

Dijet searches look for a resonance in the two-jet invariant mass spectrum. The strong-interaction multi-jet background is expected to be smoothly falling, thus a bump-like structure would be a clear sign of a deviation from the SM prediction. As the invariant mass decreases, the rate of multi-jet events increases steeply – to the point where, in the sub-TeV mass range, the data-taking system of ATLAS cannot handle the full rate due to limited data-storage resources. Instead, the ATLAS trigger system discards most of the events in this mass range, reducing the sensitivity to low-mass dijet resonances.

By recording only the final-state objects used to make the trigger decision, however, this limitation can be bypassed. For a dijet-resonance search, the only necessary ATLAS detector signals are the calorimeter information used to reconstruct the jets. This compact data format records far less information for each event, about 1% of the usual amount, allowing ATLAS to record dijet events at a rate 20 times larger than what is possible with standard data-taking (figure, left).

While the TLA technique gives access to physics at lower thresholds, the ATLAS detector information for these events is incomplete. Dedicated reconstruction and calibration techniques had to be developed to deal with the partial event information and, as a result, the invariant mass computed from TLA jets is comparable to that using jets reconstructed from the full detector readout within 0.05%.

The data recorded by ATLAS in 2015 and 2016 at a centre-of-mass energy of 13 TeV did not reveal any bump-like structure in the TLA dijet spectrum. The unprecedented statistical precision allowed ATLAS to set its strongest limits on resonances decaying to quarks in the mass range between 450 GeV and 1 TeV (figure, right). The analysis is sensitive to new particles that could mediate interactions between the SM particles and a dark sector, and to other new resonances at the electroweak scale. This analysis probes an important mass region that could not otherwise be explored in this final state with comparable sensitivity.

ATLAS joins CMS and LHCb with an analysis technique that requires fewer storage resources to collect more LHC data. The technique will be extended in the future, with upgraded trigger farms and detectors making tracking information available at early trigger levels. It will thus play an important role at LHC Run 3 and at the high-luminosity LHC upgrade.

HESS proves the power of TeV astronomy

Surface-brightness maps of the three supernova-remnant candidates detected by HESS in our galactic plane. The green ellipse denotes the radio counterpart of HESS J1534-571. Image credit: HESS Collaboration

For hundreds of years, discoveries in astronomy were all made in the visible part of the electromagnetic spectrum. This changed in the past century when new objects started being discovered at both longer wavelengths, such as radio, and shorter wavelengths, up to gamma-ray wavelengths corresponding to GeV energies. The 21st century then saw another extension of the range of astronomical observations with the birth of TeV astronomy.

The High Energy Stereoscopic System (HESS) – an array of five telescopes located in Namibia in operation since 2002 – was the first large ground-based telescope capable of measuring TeV photons (followed shortly afterwards by the MAGIC observatory in the Canary Islands and, later, VERITAS in Arizona). To celebrate its 15th anniversary, the HESS collaboration has published its largest set of scientific results to date in a special edition of Astronomy and Astrophysics. Among them is the detection of three new candidates for supernova remnants that, despite being almost the size of the full Moon on the sky, had thus far escaped detection.

Supernova remnants are what’s left after massive stars die. They are the prime suspect for producing the bulk of cosmic rays in the Milky Way and are the means by which chemical elements produced by supernovae are spread in the interstellar medium. They are therefore of great interest for different fields in astrophysics.

HESS observes the Milky Way in the energy range 0.03–100 TeV, but its telescopes do not directly detect TeV photons. Rather, they measure the Cherenkov radiation produced by showers of particles generated when these photons enter Earth’s atmosphere. The energy and direction of the primary TeV photons can then be determined from the shape and direction of the Cherenkov radiation.

Detections by HESS demonstrate the power of TeV astronomy to identify new objects

Using the characteristics of known TeV-emitting supernova remnants, such as their shell-like shape, the HESS search revealed three new objects at gamma-ray wavelengths, prompting the team to search for counterparts of these objects in other wavelengths. Only one, called HESS J1534-571 (figure, left), could be connected to a radio source and thus be classified as a supernova remnant. For the two other sources, HESS J1614-518 and HESS J1912+101, no clear counterparts were found. These objects thus remain candidates for supernova remnants.

The lack of an X-ray counterpart to these sources could have implications for cosmic-ray acceleration mechanisms. The cosmic rays thought to originate from supernova remnants should be directly connected to the production of high-energy photons. If the emission of TeV photons is a result of low-energy photons being scattered by high-energy cosmic-ray electrons originating from a supernova remnant (as described by leptonic emission models), soft X-rays would also be produced while such electrons travelled through magnetic fields around the remnant. The lack of detection of such X-rays could therefore indicate that the TeV photons are not linked to such scattering but are instead associated with the decay of high-energy cosmic-ray pions produced around the remnant, as described by hadronic emission models. Searches in the X-ray band with more sensitive instruments than those available today are required to confirm this possibility and bring deeper insight into the link between supernova remnants and cosmic rays.

The new supernova-remnant detections by HESS demonstrate the power of TeV astronomy to identify new objects. The latest findings increase the anticipation for a range of discoveries from the future Cherenkov Telescope Array (CTA). With more than 100 telescopes, CTA will be more sensitive to TeV photons than HESS, and it is expected to substantially increase the number of detected supernova remnants in the Milky Way.

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