Autumn 2018, Number 13 Department of Physics Newsletter BEECROFT BUILDING OFFICIAL OPENING BEAUTIFUL HIGGS BOSON DECAYS MAGNETIC PINWHEELS EXTREME BEAM CONTROL COSMIC SHOCKS IN THE LABORATORY PEOPLE, EVENTS AND MORE www.physics.ox.ac.uk PROUD WINNERS OF: SCIENCE NEWS SCIENCE NEWS www.physics.ox.ac.uk/research www.physics.ox.ac.uk/research BEAUTIFUL HIGGS BOSON EXTREME BEAM CONTROL An Oxford team has succeeded in stabilising the arrival time of a ‘relativistic’ beam of electrons, travelling at almost the speed of light, to 50 femtoseconds. This overcomes one of the major challenges facing the proposed DECAYS Compact Linear Collider (CLIC). On 28 August 2018 the ATLAS and Higgs from the background. These Standard Model, the prevailing theory CMS collaborations announced, with results were used in the final Tevatron of particle physics. If this prediction had a seminar at CERN, the observation of combination, which reached almost turned out to be incorrect, it would have the Higgs boson decaying into pairs of three standard deviations in 2012, not shaken the foundations of the Standard beauty (b) quarks. Both experiments enough for a discovery. Model and pointed to new physics. at the Large Hadron Collider (LHC) Instead, this is an important milestone had surpassed the five standard and a beautiful confirmation of the EXPERIMENTAL CONFIRMATION deviations (sigma) mark for this process, so-called 'Yukawa couplings', which in Prof Daniela Bortoletto which is the convention in particle Goethe said: ‘Not art and science the Standard Model give masses to all Head of Particle Physics physics to claim a discovery. Five- serve alone; patience must in the quarks and leptons, the building blocks sigma corresponds to a probability of work be shown.’ It was with patience, of matter. It also reminds us that this is -7 CERN © 3 x 10 that, if this process did not a new postdoc, and two students that I only the beginning of fingerprinting the take place, statistical fluctuations could continued to search for this challenging Higgs boson. We have only measured Researchers in the Department of Feedback On Nanosecond Timescales mimic it in our data. This is one of the decay with the ATLAS detector at the three of the nine Yukawa couplings, the Physics are partners on the Compact (FONT) group has risen to the challenge Synchronising the beam arrival most demanding analyses ever made LHC. I was delighted that this time ones to the third generation of quark Not art and Linear Collider (CLIC) study, an as reported in their paper, ‘Accelerators at the Compact Linear Collider at the LHC since pairs of b-quarks around we could contribute to the and leptons. We are also very far away international collaboration working and Beams’, published earlier this year (CLIC), a possible successor are produced in enormous quantities discovery of this important decay mode from measuring the Higgs self-coupling, science serve on a concept for a particle accelerator in Physical Review. to the Large Hadron Collider through other Standard Model processes of the Higgs boson. My postdoc Elisabeth which can be explored by studying alone; patience to collide electrons and positrons at CERN, is required at the in proton–proton collisions and extremely rare double-Higgs events. The team built a sophisticated ‘feed- Schopf and my students, Cecilia Tosciri (antielectrons) head-on at energies 50 femtosecond level. We are therefore is it is very difficult to isolate The complex analysis techniques that we must in the forward’ system for measuring the and Luca Ambroz, played significant The team, led by Prof Philip up to several Tera-electronvolts (TeV). proud to have shown that the Higgs-boson decay signal. roles in the measurement. Elisabeth have established for the measurement of work be Burrows (above), Interim This energy range is similar to the incoming arrival times of a sequence of electron bunches moving at almost this can be done and that optimised the sophisticated machine the Higgs decay into b-quarks, is critical Director of the John Adams Large Hadron Collider’s, but by using for double-Higgs searches, since they shown. the speed of light. The system, deployed one of the major challenges learning algorithms that allowed ATLAS Institute for Accelerator electrons and their antiparticles rather A LONG TIME COMING rely on at least one Higgs in the pair at the CLIC Test Facility beamline at to realising CLIC has now to increase the sensitivity to these events. — Goethe Science, is based in Oxford than protons, physicists will gain a decaying in this most probable way. CERN, calculated these times relative I started searching for the Higgs boson Luca established a new technique that different perspective on the underlying been overcome. Other types Particle Physics. The to the desired clock time and provided a and its decay into b quarks in 2005 with used simulated events in a clever way physics. of electron accelerators also project, with outstanding correction signal to a ‘kicker’. The kicker the Collider Detector at Fermilab (CDF) that led to a higher than expected require beam synchronisation technical contributions Next-generation subatomic particle deflected ‘early’ bunches onto longer experiment at the Tevatron, a proton significance for the analysis. Cecilia at this level, and our technique from Colin Perry and Glenn accelerators will require exquisite paths, and ‘late’ bunches onto shorter anti-proton collider which produced upgraded a method to improve the Christian, provided the control of the particle beams in both paths, through a magnetic chicane. has potential applications to collisions with a centre of mass energy experimental mass resolution of the basis of the doctoral thesis space and time. Since the beams travel These corrections ensured that the future free-electron lasers. of almost 2 TeV, about 6.5 times lower Higgs boson decaying into b-quarks. of Jack Roberts, and was at almost the speed of light, controlling outgoing bunches were synchronised than the LHC. The Tevatron operated performed in collaboration them requires advanced feedback to the correct clock time to within 50 PROF PHILIP BURROWS at Fermilab, near Chicago, and was the Left: Mural of a THE WAY AHEAD with colleagues from CERN. systems based on novel, extremely fast femtoseconds, as measured by special highest energy collider in the world collision within a electronic devices. The Oxford Physics beam ‘phase monitors’. between 1983 and 2011. At the Tevatron, The ATLAS and CMS measurements particle detector on like at the LHC, we studied Higgs bosons determine that the Higgs boson decays the exterior of the produced together with a W or Z boson. building hosting the to b-quarks about 60 per cent of the Measured distribution of the beam ‘phase’ This is a subset of all the Higgs bosons ATLAS experiment. time, in accord with the prediction of the © S HEZLET (expressed in degrees for a sine-wave of produced, but with a much better signal frequency 12 GHz) without (blue) and to noise. My group focused on events with (red) the operation of the feed- where the W and Z bosons are not Left: A candidate forward system. The red histogram has directly identified in the detector but event display for the an rms width of 50 femtoseconds, which are inferred from a large transverse production of a Higgs meets the CLIC goal. energy imbalance, for example where boson decaying the Z boson decays to neutrinos, which to two b-quarks do not interact with the detector. My (blue cones), in first student graduated in 2007 with association with a an analysis that used simple ‘cut and Z boson decaying count’ techniques and just 10% of the to neutrinos. The data that eventually would be delivered neutrinos leave the by the Tevatron. Three other students detector unseen, and continued seeking the Higgs at CDF, are reconstructed Schematic of the feed-forward system showing the beam time including more data, when it became through the missing monitors (phi) on the incoming and outging electron beams, and the available, and introducing machine transverse energy kickers (K) in the magnetic chicane. learning techniques to distinguish the © ATLAS COLLABORATION/CERN (dashed line). 2 | Department of Physics Newsletter | Autumn 2018 Department of Physics Newsletter | Autumn 2018 | 3 SCIENCE NEWS SCIENCE NEWS www.physics.ox.ac.uk/research www.physics.ox.ac.uk/research within the thermal plasma, implying Fig. 2: The image of 10 10 6 that these had been accelerated directly the turbulent plasma 4 a) b) from the thermal background. is obtained by firing 8 8 3 CREATING ASTROPHYSICAL a beam of protons 4 Both magnetohydrodynamic (MHD) through it. The 6 6 2 and ‘particle-in-cell’ simulations protons are focused 4 4 Distance (cm) can be used to aid our experimental into concentrated Distance (cm) 2 SHOCKS IN THE LABORATORY 1 2 2 understanding and reproduce the regions by magnetic Normalised proton flux relevant physical processes occuring Path-integrated magnetic field (kG cm) fields. (a) This proton 0 0 0 0 0246810 0246810 in the experiment. In analogy to the radiograph image Distance (cm) Distance (cm) shows strong, cometary case, these simulations of sharp features, the experiment demonstrate that the wind. This energy transfer is mediated where proton MAGNETISED TWO-STREAM X-rays produced in the laboratory are, by lower-hybrid waves, resulting in the concentrations INSTABILITY in fact, due to a hot electron distribution, acceleration of electrons to near a keV greatly exceed their generated from the magnetised two- Another astrophysical phenomenon so that they can produce the observed Prof Gianluca Gregori average value. (b) By stream instability. involving particle energisation which Atomic & Laser Physics analysing the number soft X-ray emission. we have recently investigated in the of protons passing laboratory, is the interaction of a comet An experiment demonstrating this through the plasma, EXPLAINING THE INJECTION with the Solar wind.