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CMS sees first direct evidence for γγ→WW

In a small fraction of proton collisions at the LHC, the two colliding protons interact only electromagnetically, radiating high-energy photons that subsequently interact or “fuse” to produce a pair of heavy charged particles. Fully exclusive production of such pairs takes place when quasi-real photons are emitted coherently by the protons rather than by their quarks, which survive the interaction. The ability to select such events opens up the exciting possibility of transforming the LHC into a high-energy photon–photon collider and of performing complementary or unique studies of the Standard Model and its possible extensions.

CCnew4_06_13 — Fig. 1. Proton–proton collisions recorded by CMS at √s=7 TeV, featuring candidates for the exclusive two-photon production of a W⁺ W^– pair, where one W boson has decayed into an electron and a neutrino, the other into a muon and a neutrino.

The CMS collaboration has made use of this opportunity by employing a novel method to select “exclusive” events based only on tracking information. The selection is made by requesting that two – and only two – tracks originate from a candidate vertex for the exclusive two-photon production. The power of this method, which was first developed for the pioneering measurement of exclusive production of muon and electron pairs, lies in its effectiveness even in difficult high-luminosity conditions with large event pile-up at the LHC.

CCnew5_06_13 — Fig. 2. The p_T distribution of eμ pairs in events with no extra tracks compared with the Standard Model expectation (thick green line) and predictions for anomalous quartic gauge couplings (dashed green histograms).

The collaboration has recently used this approach to analyse the full data sample collected at √s=7 TeV and to obtain the first direct evidence of the γγ→WW process. Fully leptonic W-boson decays have been measured in final states characterized by opposite-sign and opposite-flavour lepton pairs where one W decays into an electron and a neutrino, the other into a muon and a neutrino (both neutrinos leave undetected). The leptons were required to have: transverse momenta p_T >20 GeV/c and pseudorapidity |η| < 2.1; no extra track associated with their vertex; and for the pair, a total p_T >30 GeV/c. After applying all selection criteria, only two events remained – compared with an expectation of 3.2 events: 2.2 from γγ→WW and 1 from background (figure 2).

CCnew6_06_13 — Fig. 3. Limits on anomalous quartic γγWW couplings.

The lack of events observed at large values of transverse momentum for the pair, which would be expected within the Standard Model, allows stringent limits on anomalous quartic γγWW couplings to be derived. These surpass the previous best limits, set at the Large Electron–Positron collider and at the Tevatron, by up to two orders of magnitude (figure 3).

ALICE through a gamma-ray looking glass

The ALICE experiment is optimized to perform in the environment of heavy-ion collisions at the LHC, which can produce thousands of particles. Its design combines an excellent vertex resolution with a minimal thickness of material. It has excellent performance for particle identification in a large range of momenta as it employs essentially all of the known relevant techniques. Accurate knowledge of the geometry and chemical composition of the detectors is particularly important for tracking charged particles, for the calculation of energy loss and efficiency corrections, as well as for various physics analyses such as those involving the antiproton–proton ratio, direct photons and electrons from semileptonic decays of heavy-flavour hadrons.

Fig. 1. Cross-section of the ALICE experiment in the X–Y plane as used in the 2012 run.

The γ-rays produced in proton–proton collisions at the LHC (mainly from π⁰ decays), which undergo pair production in the ALICE experiment, provide a precise 3D image of the detector, including the inaccessible innermost parts. The process is almost exactly the same as in 1895 when Wilhelm Röntgen produced an X-ray image of his wife’s hand – the inner parts of the body could be seen for the first time without surgery. The main difference lies in the energy of the radiation – of the order of 100 keV for Röntgen’s X-rays compared with more than 1.02 MeV for the γ-rays from pair conversions. Importantly for the ALICE experiment, it allows the implementation of the detector geometry in GEANT Monte Carlo simulations to be checked.

Fig. 1. (Continued) Y-distribution versus X-distribution of the reconstructed photon conversion vertices for |η| < *0.9.*

To produce the γ-ray image, photons from pair conversions are reconstructed through the tracking of electron–positron pairs using a secondary vertex algorithm. Contamination from other secondary particles, such as K⁰_S, Λ and Λ, is reduced by exploiting ALICE’s excellent capabilities for particle identification. In this case, the analysis uses the specific energy-loss signal in the time-projection chamber (TPC) as well as the signal in the time-of-flight (TOF)detector. Photons from pair conversions provide an accurate position for the conversion vertex, directional information and a momentum resolution given by that for the charged particles – an advantage over calorimeter measurements at low transverse momentum.

Figure 1 shows the γ-ray picture of the ALICE experiment, i.e. the Y-distribution versus X-distribution of the reconstructed photon conversion vertices, compared with the actual arrangement used in the 2012 run. The different layers of the inner tracking system and the TPC, as well as their individual components (ladders, thermal shields, vessels, rods and drift gas), are clearly visible up to a radius of 180 cm. To obtain a quantitative comparison, the radial distribution of reconstructed photon conversion vertices normalized by the number of charged particles in the acceptance is plotted together with the Monte Carlo distributions in figure 2.

Fig. 2. Radial distribution of reconstructed photon conversion vertices (black) for |η| < 0.9 compared with Monte Carlo simulations done with Phojet (red). Distributions for true converted photons (yellow), physics contamination from π⁰ and η Dalitz decays (blue), random combinatorics (brown) and hadronic background (grey) are also shown.

This indicates an excellent knowledge of the material thickness of the ALICE experiment: up to a radius of 180 cm and in the pseudorapidity region |η| < 0.9, the thickness is 11.4±0.5(sys.)% of a radiation length. The systematic error is obtained from a quantitative comparison of the data with the Monte Carlo distributions, after taking into account the limited knowledge of the true photon sample, of the photon reconstruction efficiency and of the geometry and chemical composition of the detectors.

The accuracy achieved, as well as the full azimuthal acceptance of the central barrel, allows converted photons to be used in physics analyses. So far, photons from pair conversions have been used for the identification of neutral mesons in proton–proton collisions at 7 TeV down to a transverse momentum of 0.3 GeV/c – the first time in a collider experiment. Moreover, a direct photon signal observed in lead–lead collisions at √s_NN = 2.76 TeV has been measured with the photon-conversion method. The latter measurement demonstrates that the quark–gluon plasma formed at the LHC is the hottest matter ever made in the laboratory (CERN Courier December 2012 p6).

New precision measurement of the Λ_b lifetime

The value of the Λ_b lifetime has long been controversial but the situation has recently been clarified by a new measurement from the LHCb experiment.

There are many ways in which the decays of b quarks are used to search for physics beyond the Standard Model. One strategy, used by the CKMfitter and UTfit teams, is to compare the consistency of various sets of measurements and for this a knowledge of the elements |V_cb| and |V_ub| of the Cabibbo-Kobayashi-Maskawa (CKM) matrix is essential. One way of determining these from data is to use a theoretical framework called the heavy quark expansion (HQE).

An early prediction from this model was that the Λ_b lifetime was almost equal to that of the B⁰ meson but shorter by 1–2%. However, measurements from CERN’s Large Electron–Positron collider using the semileptonic decay Λ_b→ Λ_clν gave values of the ratio of the lifetimes, τ(Λ_b)/τ(B⁰), of around 0.8. This caused concern over the applicability of HQE and various attempts were made to explain the observations. More recent measurements of τ(Λ_b), from the CDF experiment at the Tevatron using Λ_b→Λ_c π^– (Aaltonen et al. 2002) and from ATLAS and CMS at the LHC using Λ_b→ J/ψΛ⁰ (Aad et al. 2013, Chatrchyan et al. 2013) have indicated larger values but with relatively large uncertainties.

CCnew12_06_13 — Invariant mass of J/ψ p K– combinations, showing the clear Λ_b signal.

The LHCb collaboration has discovered a new decay mode Λ_b→ J/ψ pK^– that is ideally suited for measuring the lifetime, by determining it relative to that for the decays B⁰→ J/ψ K^*0, K^*0→K⁺π^–. In both the Λ_b and B⁰ decays, four charged tracks are produced at the position of the b-hadron’s decay. This minimizes systematic uncertainties in the ratio and provides excellent decay-time resolutions of around 40 fs in each mode. The figure shows the signal yield of more than 15,000 Λ_b decays in 1.0 fb^–1 of LHCb data. Use of ring-imaging Cherenkov detectors in the experiment removes most of the backgrounds from B_s→ J/ψ K⁺ K^– and B⁰→ J/ψ K⁺π^– decays.

The collaboration finds τ(Λ_b)/τ(B⁰) = 0.976±0.012±0.006, where the first uncertainty is statistical and the second is systematic (LHCb collaboration 2013). The result demonstrates consistency with the original HQE prediction and should help to resolve issues involving measurements of the CKM parameters |V_cb| and |V_ub|. Using the precisely measured value of τ(B⁰)=1519±7 fs from the Particle Data Group (Beringer et al. 2012) yields a value for τ(Λ_b)=1482±18±12 fs. This result is about twice as precise as the best previous measurement.

Much of the early work on the HQE was done by Nikolai Uraltsev, whose passing earlier this year is much lamented by the community.

Council updates European Strategy for Particle Physics

On 30 May, at a special meeting hosted by the European Commission in Brussels, the CERN Council formally adopted an update to the European Strategy for Particle Physics. Since the original European strategy was put into place seven years ago, the LHC has begun routine operation, producing its first major results at centre of mass energies of 7 TeV and 8 TeV, and the global particle-physics landscape has evolved with new neutrino and precision measurements. The updated strategy takes these changes into account and charts a leading role for Europe in a field that is increasingly globalized.

An important issue for the strategy is to ensure that Europe stays at the forefront of particle physics research, which pays dividends in terms of knowledge, innovation, education and training. CERN, in close collaboration with research institutions in its member states and under the guidance of the CERN Council, will co-ordinate future European engagement with global particle-physics projects in other regions.

The strategy emphasizes that Europe and the European particle-physics community should exploit the LHC to its full potential over many years via a series of planned upgrades. Alongside the LHC, the community should also be open to engaging in large particle-physics projects outside Europe and continuing to develop novel techniques for global future accelerator projects. European particle physics should maintain a healthy base in fundamental physics research, with universities and national laboratories contributing to a strong European focus through CERN. Last but not least, the community should continue to invest substantial effort in communication, education and outreach activities to engage global publics with science.

• The updated strategy is reproduced in full in a new brochure Accelerating science and innovation: societal benefits of European research in Particle Physics: http://cds.cern.ch/record/1551933. For the original statement, see http://council.web.cern.ch/council/en/EuropeanStrategy/ESParticlePhysics.html.

EC and CERN support major facility in Middle East

CCnew13_06_13 — Magnets of the SESAME booster synchrotron during installation in 2012. Commissioning of the booster will begin in late 2013.
Image credit: SESAME.

The European Commission (EC) and CERN have agreed to support the construction of the Synchrotron-light for Experimental Science and Applications in the Middle East (SESAME) facility, one of the most ambitious research initiatives in the Middle East. SESAME is a unique joint venture based in Jordan that brings together scientists from its members Bahrain, Cyprus, Egypt, Iran, Israel, Jordan, Pakistan, the Palestinian Authority and Turkey.

The SESAME synchrotron light source will allow researchers from the region to investigate the properties of advanced materials, biological processes and cultural artefacts. Alongside its scientific goals, the project aims to promote peace in the region through scientific co-operation.

Through an agreement that was announced on 28 May, the EC will contribute €5 million, allowing CERN – working with SESAME – to supply SESAME with magnets for a new electron storage ring at the heart of the facility. This will pave the way for commissioning to begin in 2015. In addition, the EC has already contributed more than €3 million to the project through the European Neighbourhood and Partnership Instrument and by supporting the SESAME networking, computing and data-handling systems.

Construction started in 2003. Like CERN, SESAME was established under the auspices of UNESCO. A key impetus to launch was the donation of components from the BESSY laboratory in Berlin. Since then, a growing community of local scientists has been working closely with partner facilities from around the world and several other laboratories have contributed to making the SESAME facility world-class.

ILC Technical Design Report is published

A five-volume report containing the blueprint for the International Linear Collider (ILC) was published on 12 June. The authors of the Technical Design Report handed it over to the International Committee for Future Accelerators in three consecutive ceremonies in Tokyo, CERN and Fermilab, representing Asia, Europe and the Americas. Its publication marks the completion of many years of globally co-ordinated R&D and completes the mandate of the Global Design Effort for the ILC.

The ILC – a 31-km electron–positron collider with a total collision energy of 500 GeV – was designed to complement and advance LHC physics. The report contains all of the elements needed to propose the collider to collaborating governments, including the latest, most technologically advanced design and implementation plan optimized for performance, cost and risk.

Some 16,000 superconducting cavities will be needed to drive the particle beams. At the height of operation, bunches of 2 × 10¹⁰ electrons and positrons will collide roughly 7000 times a second. The report also includes details of two state-of-the-art detectors to record the collisions and an extensive outline of the geological and civil-engineering studies conducted for siting the ILC.

The design effort continues in the Linear Collider Collaboration. This combines the two most mature future particle-physics projects at the energy frontier – the ILC and the Compact Linear Collider (CLIC) – in an organizational partnership to co-ordinate and advance global development work for a linear collider. Some 2000 scientists worldwide – particle physicists, accelerator physicists and engineers – are involved in the ILC or in CLIC and often in both projects.

• For the report, see www.linearcollider.org/ILC/Publications/Technical-Design-Report.

Data centre opens

CCnew14_06_13 — Image credit: Wigner RCP.

CERN and the Wigner Research Centre for Physics inaugurated the CERN Tier-0 data-centre extension in Budapest on 13 June, marking the completion of the facility. CERN’s director-general, Rolf Heuer, far left, joined József Pálinkás, president of the Hungarian Academy of Sciences, and Viktor Orbán, prime minister of Hungary, in the ceremonial “cutting the ribbon”, in the company of Péter József Lévai, far right, general director of the Wigner Research Centre for Physics (RCP). This extension adds up to 2.5 MW capacity to the 3.5 MW load of the data centre at CERN’s Meyrin site, which has already reached its limit. The dedicated and redundant 100 Gbit/s circuits connecting the two sites have been functional since February and about 20,000 computing cores, 500 servers and 5.5 PB of storage are already operational at the new facility.

Giant magnet ring makes epic journey

CCnew15_06_13 — The 15-m magnet on the move from Brookhaven.
Image credit: BNL.

Muon g-2 is a new experiment to measure the anomalous magnetic moment of the muon – in other words, the difference of the value of its magnetic moment g from the simplest expectation of 2. However, before the experiment begins, its centrepiece – a complex electromagnet spanning more than 15 m in diameter – had to embark on a long, careful journey from Brookhaven National Laboratory (BNL) in New York to Fermilab in Illinois.

The magnet ring was built at Brookhaven in the 1990s for the E821 experiment, which ran there from 1997 until 2001. Its measurement of g-2 is still one of the few hints for new physics beyond the Standard Model. Constructed of aluminium and steel with superconducting coils inside, the magnet cannot be taken apart or twisted more than a few millimetres without irreparably damaging the coils. As a result, the Muon g-2 team devised a plan for a five-week journey of 5150 km over land and sea.

The journey began when the ring left Brookhaven on 22 June. It was loaded onto a specially prepared barge to be taken down the East Coast of the US, around the tip of Florida and up a series of rivers to Illinois. The ring was then attached to a truck built specially for the move and driven to Fermilab to arrive there in late July.

• For more information about Muon g-2, see http://muon-g-2.fnal.gov.

DESY inaugurates prototype for the Cherenkov Telescope Array

The prototype of the medium-sized telescope (MST). In the foreground is a camera housing, now installed on the telescope. Image credit: DESY.

The first prototype telescope for the planned Cherenkov Telescope Array (CTA) has been inaugurated in Berlin. The prototype was designed and built at DESY in an international collaboration involving more than 1000 scientists and engineers from 27 countries.

The CTA is an international €200-million project to observe cosmic γ rays (CERN Courier July/August 2012 p28). Designed to achieve a sensitivity that is 10-times better than existing installations, it will combine three types of telescope, each optimized for its own energy range between a few tens of giga-electron-volts and 300 TeV. The prototype is a full-scale version of the medium-sized telescope (MST) with a tessellated 12-m mirror. Forty MSTs will form the central part of the CTA.

Fully functional mechanically, the prototypes will be used to test many aspects including the drive and safety systems, the understanding of vibrations and deformations, the mirror alignment, telescope pointing and the array control. The results will allow the design of the MSTs to be optimized before their production begins.

Construction of the CTA is expected to commence in 2015 at two sites in the southern and northern hemispheres. The larger southern observatory will include 70–100 telescopes that will be spread over 10 km² and the smaller observatory in the north will have 20–30 telescopes that will be distributed over 1 km². The sites will be chosen at the end of this year.

For further information, see www.cta-observatory.org/ and www.desy.de/cta.

Galaxy clusters shed new light on dark matter

The Subaru Telescope on the left and the two domes of the W M Keck Observatory at sunset on Mauna Kea, Hawai’i.
Credit: Mailseth.

What is the actual distribution of dark matter? Does it follow the expectations for cold dark matter (CDM)? “Yes, it does,” according to a team of astronomers that has measured the average distribution of dark matter derived from gravitational lensing in 50 clusters of galaxies. The density is observed to decrease outwards from the centre of these cosmic giants in excellent agreement with the predictions of CDM models.

While dark matter still eludes the scrutiny of particle physicists it is becoming more substantial to astrophysicists. Its gravitational role is essential to hold galaxies together in clusters, to account for the high rotational velocity of stars in the outer regions of galaxies and more generally to explain the formation of large-scale structures in the universe. The relatively recent ability to observe the distribution of dark matter in space makes it even more concrete (CERN Courier January/February 2007 p11). The method consists of measuring the slight distortion of background galaxies that is induced by the gravitational deformation of space–time along the line of sight. This so-called weak-lensing effect allows astronomers to locate and measure the amount of dark matter, even though it is transparent on the image.

The fluctuations of the cosmic microwave background as observed by the Planck satellite imply that CDM constitutes about 27% of the energy content of the universe (CERN Courier May 2013 p12). However, it is important to check whether the distribution of mass in galaxy clusters follows the expectations of a medium of dark-matter particles that only interact with each other via gravity and do not emit or absorb photons. An international team led by Nobuhiro Okabe of Academia Sinica, Taiwan, and Graham Smith of Birmingham University used a sample of 50 massive clusters of galaxies to test this. The clusters were selected from an X-ray catalogue in a narrow redshift range of around z= 0.2 and observed by the Prime Focus Camera of the Japanese 8.2-m Subaru Telescope located next to the two Keck Telescopes on Mauna Kea, Hawai’i. For each cluster, the team derived the weak-lensing signal imprinted in the shape of background galaxies and stacked the results to obtain the average density profile.

The obtained map of the mean dark-matter distribution is remarkably symmetrical with a pronounced central peak. The radial profile is found to be consistent with the Navarro-Frenk-White model at the sub-10% precision level. Measurement of the concentration parameter of the mass in the cluster gives c₂₀₀ = 4.22 + 0.40–0.36, which is slightly above but still broadly in line with theoretical predictions for CDM. This result confirms that dark matter behaves in clusters of galaxies as expected by the CDM scenario. It solves a tension between previous observations of individual clusters that found a high central concentration and other studies that included the dynamics of the galaxies in the clusters and suggested a shallower distribution.

In the future, the team would like to improve this analysis by measuring the dark-matter density on even smaller scales right in the centre of these galaxy clusters. This will be possible with the installation of a new camera called the Hyper-Suprime-Cam on the Subaru Telescope, which will also allow astronomers to study smaller galaxy clusters, which are predicted to have a slightly higher dark-matter concentration. The team notes, however, that significant advances on the precision achieved in the current study on massive low-redshift galaxies are only expected with future facilities such as the Large Synoptic Survey Telescope or ESA’s Euclid Satellite.

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