OPERA Catches Its First Tau-Neutrino

NEWS N e u t r i N o S OPERA catches its first tau-neutrino The OPERA collaboration has announced the 10 mm CS 5 100 μm observation of the first candidate tau-neutrino 1 1 (ντ) in the muon-neutrino (νμ) beam sent 2 mm 7 5 3 through the Earth from CERN to the INFN’s Gran 3 2 Sasso Laboratory 730 km away. The result is 4 parent an important final piece in a puzzle that has γ2 γ1 challenged science for almost half a century. 4 1 2 8 daughter γ The puzzle surrounding neutrinos originated 6 6 8 in the 1960s when the pioneering experiment daughter by Ray Davis detected fewer neutrinos arriving at the Earth from the Sun than solar models OPERA’s τ– candidate event. The longitudinal view (left) shows the tracks of charged particles detected predicted (CERN Courier December 2002 in the emulsion layers, together with the showers indicative of two photons (γ1 and γ2). A closer p15). A possible solution, proposed in 1969 transverse view (right) reveals the kink characteristic of a decay with track 4 (the parent) leading to by Bruno Pontecorvo and Vladimir Gribov, was track 8 (the daughter product). Track 4 is about 1.3 mm long. Both the kink angle and the path length that oscillatory changes between different satisfy the selection criteria for the decay of a τ–. types of neutrinos could be responsible for the apparent neutrino deficit. Conclusive demanding particle tracking at micrometre The collaboration has identified the first evidence that electron-neutrinos, νe, from the resolution to reconstruct the topology: candidate ντ in a sample of events from Sun change type en route to the Earth came either a kink – a sharp change (>20 mrad) data taken in 2008–2009, corresponding to from the Sudbury Neutrino Observatory in in direction occurring after about 1 mm – as 1.89 × 1019 protons on the target at CERN. 2002, a few years after the Super-Kamiokande the original τ decays into a charged particle The sample contains 1088 events, including experiment found the first evidence for together with one or more neutrinos, or the 901 that appear to be charged-current oscillations in νμ created by cosmic rays in the vertex for the decay mode into three charged interactions. The search through these has atmosphere. Accelerator-based experiments particles plus a neutrino. yielded one candidate with the characteristics – have since observed the disappearance of νμ, The OPERA apparatus has two identical expected for the decay of a τ into a charged – 0 confirming the oscillation hypothesis, but until Super Modules, each containing a target hadron (h ), neutral pions (π ) and a ντ. now there have been no observations of the section and a large-aperture muon Indeed, the kinematical analysis suggests the – – appearance of a ντ in a νμ beam. spectrometer. The target consists of decay τ → ρ ντ. The event has a significance OPERA’s result follows seven years of alternate walls of lead/emulsion bricks of 2.36 σ of not being a background – – 0 preparation and more than three years of – 150 000 bricks in total – and modules fluctuation for theτ decay to h (π )ντ. beam provided by CERN (CERN Courier of scintillator strips for the target tracker. This candidate event is an important first November 2006 p24 and p27). The neutrinos The nuclear-emulsion technique allows step towards the observation of ντ → νμ are generated at CERN when a proton beam the collaboration to measure the neutrino- oscillations through the direct appearance of from the Super Proton Synchrotron strikes interaction vertices with high precision. the ντ. That claim will require the detection of a target, producing pions and kaons. These The scintillators provide an electronic a few more events, but so far the collaboration quickly decay, giving rise mainly to νμ that pass trigger for neutrino interactions, localize the has analysed only 35% of the data taken in unhindered through the Earth’s crust towards particular brick in which the neutrino has 2008 and 2009 and ultimately should have Gran Sasso. The appearance and subsequent interacted, and perform a first tracking of five times as much data than as at present. decay of a τ in the OPERA experiment would muons within the target. The relevant bricks provide the telltale sign of νμ to ντ oscillation are then extracted from the walls so that the Further reading through a charged-current interaction. film can be developed and scanned using N Agafonova et al. OPERA collaboration 2010 Detecting the τ decay is a challenging task, computer-controlled scanning microscopes. arXiv:1006.1623v1, submitted Phys. Letts. B. Sommaire OPERA capture son premier neutrino du tau 5 Communication des premiers résultats du LHC à Hambourg 9 DØ observe une asymétrie anormale dans les désintégrations Un linac à récupération d’énergie à deux orbites au laser à de mésons B 6 électrons libres de Novossibirsk 10 Le Conseil du CERN opte pour davantage d’intégration 7 De la vie artificielle créée pour la première fois en laboratoire 12 Poursuite de la mise en service: amélioration de la luminosité 8 Les rayons X détectent de la « matière manquante » dans des gaz 14 CERN Courier July/August 2010 5 CCJulAug10News.indd 5 05/07/2010 15:41 NEWS FERMILAB DØ sees anomalous asymmetry in decays of B mesons The DØ collaboration at Fermilab has reported evidence of a violation of matter–antimatter DØ, 6.1 fb–1 symmetry (“CP symmetry”) in the behaviour of neutral mesons containing b quarks. 0.01 Studying collisions where B mesons decay semi-leptonically into muons, the team fi nds sl 0 s about 1% more collisions when two negatively a charged muons are produced than collisions with two positively charged muons. The –0.01 DØ → Ab 68% collisions in the DØ detector occur through a sl 95% symmetric proton–antiproton state and the standard model 99% C.L. –0.02 expected CP asymmetry from the Standard B factory W.A. Model is predicted to be much smaller than DØ Bs→Ds μ X what has been observed. An asymmetry of 1% –0.03 preliminary is therefore completely unexpected. combination The properties of B mesons, created in – collisions where a bb quark pair is produced, –0.04 –0.02 0 d are assumed to be responsible for this asl asymmetry. Mesons containing b quarks are known to oscillate between their particle The semileptonic dimuon B meson charge – – – – – b (B=bd or bs) and anti-particle (B=bd or bs) asymmetry, Asl, obtained by the DØ collaboration, state before they decay into a positively compared with the Standard Model prediction and s d charged muon (for the B) or a negatively with the measured muon asymmetries asl and asl – 0 0 charged muon (for the B). If a B meson for Bs and Bd mesons. oscillates before its decay, its decay muon has the “wrong sign”, i.e. its charge is contains strange quarks, than to the K+, identical to the charge of the muon from the which contain strange antiquarks. In detailed other b decay. Having 1% more negatively studies the collaboration has derived the charged muon pairs therefore implies that the contribution of this effect almost entirely B meson decays slightly more often into its from the data, making the measurement of matter state than into its antimatter state. the asymmetry in B-meson decays largely The DØ detector has two magnets, independent of external assumptions and a central solenoid and a muon-system simulation. toroid, which determine the curvature and The fi nal result is 3.2 σ from the Standard charge of muons. By regularly reversing the Model expectation, corresponding to a polarities of these magnets the collaboration probability of less than 0.1 per cent that can eliminate most effects coming from this measurement is consistent with any asymmetries in the detection of positively known effect. The analysis was based on and negatively charged muons. This feature is an integrated luminosity of 6.1 fb–1 and crucial for reducing systematic effects in this the plan is to increase the accuracy of this measurement. measurement by adding signifi cantly more Another known source of asymmetry arises data and improving future analysis methods. in muons produced in the decays of charged kaons. Kaons contain strange quarks and the Further reading interaction cross-sections of positively and V M Abazov et al. 2010 DØ collaboration, Mega Industries, LLC negatively charged kaons with the matter arxiv:1005.2757, submitted to Phys. Rev. D.; 28 Sanford Drive, Gorham, ME 04038 USA making up the DØ detector differ signifi cantly: www-D0.fnal.gov/Run2Physics/WWW/ Phone: 207-854-1700 Fax: 207-854-2287 more interactions are open to the K–, which results/fi nal/B/B10A/. E-Mail: [email protected] www.megaind.com 6 CERN Courier July/August 2010 CCJulAug10News.indd 6 05/07/2010 15:41 NEWS CERN CERN Council opens the door to greater integration At its 155th session, on 18 June, the CERN through a period of fi nancial austerity, and document CERN/2918, which is available at Council opened the door to greater integration endorsed CERN’s new code of conduct. http://indico.cern.ch/getFile.py/access?resI in particle physics when it unanimously first_draft.qxpFull details of the new 01/06/2010membership 15:06 d=1&materialId=0&contribId=35&sessionIdPage 1 adopted the recommendations of a working arrangements can be found in Council =0&subContId=0&confId=96020. group that was set up in 2008 to examine the role of the organization in the light of increasing globalization in particle physics.

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