Physics Archives – Page 84 of 144

Neutrino production moves to an industrial scale

CCneu1_04_12 — Fig. 1. Diagram of the relationship between the mass eigenstates (labelled 1, 2 and 3) for neutrinos and the flavour eigenstates (ν_e, ν_μ and ν_τ). Neutrinos from the Sun have been used to show that m₂ > m₁; m₃ may be greater or less than m₁ and m₂ (the “normal hierarchy”) or smaller than m₁ and m₂ (the “inverted hierarchy”). The fractional contribution of each flavour to the mass eigenstates is indicted by the coloured bars. (After a figure by S Parke.)

The measurements of the electron- and muon-neutrino fluxes published by the Super-Kamiokande collaboration in 1998 marked a turning point in the history of particle physics. This team showed that fewer muon-neutrinos arrive at the surface of the Earth than are produced by cosmic-ray interactions in the upper atmosphere (atmospheric neutrinos). This in turn indicated evidence for neutrino oscillations, the phenomenon in which the flavour of the neutrino changes (oscillates) as the neutrino propagates through space and time. Since the publication of Super-Kamiokande’s seminal paper, the phenomenon of neutrino oscillations has been established through further measurements of atmospheric neutrinos, as well as of neutrinos and antineutrinos produced in the Sun, by nuclear reactors and by high-energy particle accelerators. It is arguably the most significant advance in particle physics of the past decade.

Extending the Standard Model

Neutrino oscillations imply that the Standard Model is incomplete and must be extended to include neutrino mass as well as mixing among the three neutrino flavours. The mechanism by which neutrino mass is generated is not known. An intriguing possibility is that the tiny neutrino mass is the result of physics at extremely high energy scales. Such a “see-saw” mechanism might also help to explain why neutrino mixing is so much stronger than the mixing among quarks. Mixing among three massive neutrinos admits the possibility that symmetry between matter and antimatter (CP-symmetry) is violated via the neutrino mixing matrix. Nonzero neutrino mass implies that lepton number must be used to distinguish a neutrino from an antineutrino. If lepton number is not conserved then a neutrino is indistinguishable from an antineutrino, i.e. the neutrino is a Majorana particle – a completely new state of matter. The determination of the properties of the neutrino, therefore, is fundamental to the development of particle physics.

CCneu2_04_12 — Oscillation processes at the Neutrino Factory.

These exciting new measurements imply that it may be possible to observe CP-violation in neutrino oscillations

Neutrino oscillations are readily described by extending the Standard Model to include three neutrino-mass eigenstates, ν₁, ν₂ and ν₃, such that the neutrino-flavour eigenstates, ν_e, ν_μ and ν_τ, are quantum-mechanical mixtures of the mass eigenstates (figure 1). Neutrino oscillations arise from the “beating” of the phase of the neutrino-mass eigenstates as a neutrino produced as an eigenstate of flavour propagates through space and time. The matrix by which the mass-basis is rotated into the flavour-basis is parameterized in terms of three mixing angles (θ₁₂, θ₂₃ and θ₁₃) and one phase parameter (δ). If δ is nonzero (and not equal to π), then CP-violation in the neutrino sector will occur so long as θ₁₃ > 0. Measurements of neutrino oscillations in vacuum can be used to determine the moduli of the mass-squared differences Δm²₃₁ = m²₃ – m²₁ and Δm²₂₁ = m²₂ – m²₁ and, with the aid of interactions with matter, also the sign.

The bulk of the measurements of neutrino oscillations to date have been collected using the dominant “disappearance” channels ν_e → ν_e and ν_μ → ν_μ. These data have yielded values for the three mixing angles, as well as for the magnitude of the mass-squared differences Δm²₃₁ and Δm²₂₁, and have shown that m₂ > m₁ (i.e. that Δm²₂₁ > 0). Last year, the T2K, MINOS and Double Chooz experiments presented evidence that θ₁₃ may be greater than zero. Then, in March this year, the Daya Bay collaboration reported that sin²2θ₁₃ = 0.092 ± 0.016 (stat.) ± 0.005 (syst.), i.e. that sin²2θ₁₃ = 0 is excluded at 5.2 σ. The announcement was soon followed by the report of a similar result from the RENO experiment. These exciting new measurements imply that it may be possible to observe CP-violation in neutrino oscillations. The challenge for the neutrino community, therefore, is to refine the measurement of θ₁₃ to determine the sign of Δm²₃₁ (the “mass hierarchy”), to discover CP-violation (if, indeed, it does occur) by measuring δ and to improve the accuracy with which θ₂₃ is known.

Over the next few years, several experiments – MINOS, T2K, NOνA, Double Chooz, Daya Bay and RENO – will exploit the ν_μ→ ν_e and ν_e → ν_x channels to improve significantly the precision with which θ₁₃ is known. The NOνA long-baseline experiment might also be able to determine the mass hierarchy. However, it is unlikely that either T2K or NOνA will be able to discover CP-violation, i.e. that δ ≠ 0 or π.

The Neutrino Factory

Neutrino oscillations also have implications well beyond the confines of particle physics. The possibility of CP-violation through the neutrino mixing matrix, combined with the possibility that the neutrino is a Majorana particle, makes it conceivable that the interactions of the neutrino led to the observed domination of matter over antimatter in the universe. The abundance of neutrinos in the universe is second only to that of photons. Even with a tiny mass, the neutrino may make a significant contribution to dark matter and thereby play an important role in determining the structure of the universe.

CCneu3_04_12 — Fig. 2. Left to right: The discovery potential at 3 σ for CP-violation, the mass hierarchy and sin²2θ₁₃. The discovery reach is plotted in terms of the “CP fraction” (the fraction of all possible values of the mixing phase, δ) as a function of sin²2θ₁₃. The red solid line shows the performance of the IDS-NF baseline Neutrino Factory, while the blue line refers to the “low-energy” option, optimized for large sin²2θ₁₃.

Such a breadth of impact justifies an ambitious, far-reaching experimental programme. Determining the nature of the neutrino – whether Majorana or Dirac – through the search for neutrinoless double-beta decay (2β0ν) is an important part of this programme. The absolute neutrino mass must also be determined either through observations of 2β0ν decay or from the measurement of the end-point of the electron spectrum in beta decay. Equally important is the accurate determination of the parameters that determine the properties of the neutrino. This requires intense, high-energy neutrino and antineutrino beams – precisely what the Neutrino Factory is designed to produce.

In the Neutrino Factory, beams of ν_e and ν_μ (ν_e,ν_μ) are produced from the decays of μ⁺ (μ^–) circulating in a storage ring. High neutrino-energies can readily be achieved because the neutrinos carry away a substantial fraction of the energy of the muon. Time-dilation is beneficial, allowing sufficient time to produce a pure, collimated beam. The table above lists the oscillation channels that are available at the Neutrino Factory. Charged-current interactions induced by ν_e → ν_μ oscillations – the “gold channel” – produce muons that are opposite in charge to those produced by the ν_μ in the beam, so a magnetized detector is required. The additional capability to investigate the “silver” (ν_e → ν_τ) and “platinum” (ν_μ → ν_e) channels also makes the Neutrino Factory an excellent place to look for oscillation phenomena that are outside the standard three-neutrino mixing paradigm. It would be the ideal facility to serve the precision-era of neutrino-oscillation measurements.

In 2011, the International Design Study for the Neutrino Factory (the IDS-NF) collaboration presented two options for the facility in its Interim Design Report (IDR) (Choubey et al. 2011). The first, optimized for discovery reach at small θ₁₃ (sin²2θ₁₃ < 10^–2), calls for two distant detectors, with baselines of 2500–5000 km and 7000–8000 km, and a stored-muon energy of 25 GeV. The second option, optimized for sensitivity at large θ₁₃, requires a single detector at a distance of around 2000 km and a stored-muon beam with an energy of only 10 GeV. Figure 2 shows the discovery reach of the facility presented in terms of the fraction of all possible values of δ (the “CP fraction”) and plotted as a function of sin²2θ₁₃.

In the past few weeks, the Daya Bay and RENO collaborations have announced the first measurements of sin²2θ₁₃ with a value around 0.1. Figure 2 shows that at such a large value of θ₁₃, excellent performance can be achieved using the “low-energy” option. At such a large value of θ₁₃, the precision and discovery reach of a “low energy” Neutrino Factory is significantly better than the realistic alternatives (IDS-NF 2011).

Novel techniques

The IDS-NF baseline accelerator facility sketched in figure 3 provides a total of 10²¹ muon decays per year, split between the two distant neutrino detectors. The process of creating the muon beam begins with the bombardment of a pion-production target with a pulsed proton beam. The pions are captured in a solenoidal channel in which they decay to produce the muon beam. A sequence of accelerators is then used to manipulate and reduce (cool) the muon-beam phase space and to accelerate the muons to their final energy.

CCneu4_04_12 — Fig. 3. Schematic diagram of the IDS-NF baseline for the Neutrino Factory accelerator complex (Choubey *et al.* 2011). The various systems have been drawn to scale.

The muon’s short lifetime has required novel techniques to be developed to carry out these steps. Ionization cooling, the technique by which it is proposed to cool the muons, involves passing the beam through a material in which it loses energy through ionization and then re-accelerating it in the longitudinal direction to replace the lost energy. Muon acceleration will be carried out in a series of superconducting linear and recirculating linear accelerators. The final stage of acceleration, from 12.6 GeV to the stored-muon energy of 25 GeV, is provided by a fixed-field alternating-gradient (FFAG) accelerator. The baseline neutrino detector is a MINOS-like iron-scintillator sandwich calorimeter with a sampling fraction optimized for the Neutrino Factory beam. The baseline calls for a fiducial mass of 100 kilotonnes to be placed at the intermediate baseline and a detector of 50 kilotonnes at the magic baseline.

CCneu6_04_12 — Fig. 5. The MICE Hall in June 2011. The ISIS synchrotron lies behind the wall at the back. The downstream quadrupoles (blue) are visible in front of the concrete shielding (white), with a TOF hodoscope for particle identification immediately after the final quadrupole. This hodoscope is followed by the lead-scintillator section and the prototype electron-muon ranger. In the foreground are the prototype liquid-hydrogen delivery system and R&D cryostat.
Image credit: S Kill/STFC.

Much of the Neutrino Factory facility, the accelerator complex and the neutrino detectors exploit state-of-the-art technologies. To achieve the ultimate performance (10²¹ muon decays per year) the IDS-NF baseline calls for: a proton-beam power of 4 MW, delivered at a repetition rate of 50 Hz in short (around 2 ns) bunches; a pion-production target capable of accepting the high proton-beam power; an ionization-cooling channel that increases the useful muon flux by a factor of around 2; and an FFAG to boost the beam energy rapidly to 25 GeV. R&D programmes that address each of these issues are underway. CERN, along with other proton-accelerator laboratories, is actively developing the technologies necessary to deliver multimega-watt, pulsed proton beams. The principle of a mercury-jet pion-production target was demonstrated by the MERIT experiment in 2008 that ran in the beamline of n_TOF, the neutron time-of-flight facility at CERN. The nonscaling FFAG accelerator EMMA (the Electron Model of Muon Acceleration, also known as the Electron Model of Many Applications) has been commissioned at the Daresbury Laboratory in the UK and used to demonstrate the “serpentine acceleration” characteristic of the nonscaling FFAG. The international Muon Ionization Cooling Experiment (MICE) at the Rutherford Appleton Laboratory will provide the engineering demonstration of the ionization-cooling technique (see box, previous page).

The Neutrino Factory is the facility of choice for the study of neutrino oscillations. It has excellent discovery reach and offers the best precision on the mixing parameters. The ability to vary the stored-muon energy and, perhaps the detector technology, gives the necessary flexibility to respond to developments in understanding neutrino physics and in the discovery of new phenomena. The R&D programme required to make the Neutrino Factory a reality will directly benefit the development of a muon collider and experiments that seek to discover charged lepton-flavour violation. The case for the Neutrino Factory as part of a comprehensive muon-physics programme is compelling indeed.

I gratefully acknowledge the help, advice, and support of my many colleagues within the IDS-NF, EUROnu and MICE collaborations and the Neutrino Factory community who have freely discussed their results with me and from whose work and results I have drawn freely.

BOX INSET

Cooling at MICE

CCneu5_04_12 — Fig. 4. Cutaway rendering of the international Muon Ionization Cooling Experiment. The muon beam enters from the bottom left of the figure.
Image credit: A DeMello/LBNL.

MICE is a single-particle experiment in which the position and momentum of each muon is measured before it enters the MICE cooling channel and is measured again after it has left (Gregoire et al. 2003 and 2005). Muons with momenta between 140 MeV/c and 240 MeV/c, with normalized emittance between 2 πmm and 10 πmm, will be provided by a purpose-built beamline at the 800 MeV proton synchrotron, ISIS, at the Rutherford Appleton Laboratory.

The MICE cooling channel, a single lattice cell, comprises three 20-l volumes of liquid hydrogen and two short linac modules each consisting of four 201 MHz cavities. Beam transport is achieved by a series of superconducting solenoids: the “focus coils” focus the beam into the liquid-hydrogen absorbers, while a “coupling coil” surrounds each of the linac modules. A particle-identification system, with scintillator time-of-flight (TOF) hodoscopes and threshold Cherenkov counters, upstream of the cooling channel allows a pure muon beam to be selected. Downstream of the cooling channel, a final hodoscope and a calorimeter system allow muon decays to be identified. The calorimeter is composed of a lead-scintillator section, of a similar design to that of the KLOE detector at DAΦNE but with thinner lead foils, followed by a fully active scintillator detector (the electron-muon ranger) in which the muons are brought to rest.

Charged-particle tracking in MICE is provided by two solenoidal spectrometers that together determine the relative change in transverse emittance of the beam, which is expected to be approximately 10%, with a precision of ±1% (i.e. a 0.1% measurement of the change in absolute emittance). The trackers themselves are required to have high track-finding efficiency in the presence of background that is induced by X-rays produced in the RF cavities.

In the first “step” of the experiment, the muon beam for MICE has been characterized using the beamline instrumentation and the TOF, Cherenkov and lead-scintillator systems (figure 5). The results, which are being prepared for publication, show that the muon beam can provide the range of momentum and emittance required by MICE. The trackers and a prototype of the electron-muon ranger have been tested and shown to perform to specification. The cavities that make up the two short linac sections have been manufactured by Lawrence Berkeley National Laboratory (LBNL). The superconducting magnets required for the cooling channel are all under construction. By the end of 2012, the collaboration will commission the two spectrometer modules and the first liquid-hydrogen absorber and focus-coil module. This will allow preliminary studies of the ionization-cooling effect to be performed. The full MICE cooling cell will be constructed once the initial cooling studies are complete.

Much food for thought at Moriond

CCnew1_03_12 — The Rencontres de Moriond have been running since 1966, approximately the same time as the preferred mechanism for electroweak symmetry breaking, which came under much scrutiny with the new results present during the first week of the 47th meeting.

The first week of the 47th Rencontres de Moriond, devoted to weak interactions and unified theories, came to a close on 10 March, leaving participants not only impressed but also puzzled by the new results presented at the conference held in La Thuile. The focus this year was to look at the results on searches for the Higgs boson, exclusion limits, searches for dark matter, precision measurements, flavour and neutrino physics, and to assess their impact on theoretical models, in particular those based on supersymmetry (SUSY) and extra dimensions.

The first excitement came from new measurements of the branching ratio for the decay B_s→ μμ from the LHCb, CMS and ATLAS experiments at CERN’s LHC. LHCb and CMS have a sensitivity within a factor of around two of the rate expected in the Standard Model for this extremely rare decay, where contributions from new physics could be detected. LHCb is setting the best limit to date, of less than 4.5 × 10^–9, barely above the Standard Model prediction of around 3.5 × 10^–9. This leaves little room for new physics. However, David Straub, a theorist affiliated with Scuola Normale Superiore and INFN in Pisa, showed that finding a branching ratio smaller than predicted by the Standard Model would also open the door to new physics, something that has previously received little attention but is now becoming possible with the increase in precision at the LHC.

The ATLAS and CMS collaborations showed updates to the results reported in December 2011. These include further analyses of the full 2011 data sample. In the low-mass Higgs region, the ranges not excluded at 95% confidence level (CL) have shrunk a little more. For ATLAS, all possible Higgs masses below 122.5 GeV (except at 118 GeV) are now excluded, together with those from 129 GeV up to 539 GeV; for CMS all masses between 127.5 and 600 GeV are excluded. This leaves only a small range where the Higgs boson could still be found.

The small excesses reported in December are still there, coming mostly from H → γγ for both experiments and also from Higgs → llll for ATLAS. Having analysed the whole 2011 data set and included new decay channels, CMS observes a 2.8 σ deviation at 125 GeV, while ATLAS has a 2.5 σ excess at 126 GeV. When the “look-elsewhere effect” is taken into account in the 110–145 GeV range, the significance of this excess goes down to about 2.1 σ.

Fermilab’s Tevatron experiments provided a surprise. Having analysed almost all of their data and greatly improved their analyses, the DØ collaboration sees a slight excess of events in the Higgs mass range of 115–145 GeV while CDF sees it for m_H < 150 GeV, coming mostly from the H → b-b and H → WW channels. The combined effect corresponds to a 2.2 σ excess above the predicted background. In addition, CDF and DØ greatly improved the precision on the masses of the W boson and the top quark. Both play an important role in determining the consistency of the Standard Model. In particular, CDF has measured the W mass to be 80.387 ± 0.019 GeV, while DØ finds a mass of 80.375 ± 0.023 GeV. These recent measurements now confine the Higgs mass to m_H = 94⁺²⁹_–24 GeV.

While all four collaborations – ATLAS, CMS, CDF and DØ – insisted that it was too early to jump to conclusions about the Higgs boson, theorists have already been checking the effects of the mass of the Higgs and find that the currently allowed range is already putting constraints on SUSY models.

Away from the colliders, the announcement during the conference of the measurement of the neutrino mixing angle θ₁₃ caused excitement (Daya Bay experiment measures θ₁₃). Another highlight concerned the 8 σ annual modulation observed by the DAMA/LIBRA dark-matter experiment, which the collaboration interprets as a signal of dark matter. It has been suggested that the effect could be caused by cosmic muons, but new calculations show that the data are inconsistent with the cosmic muon hypothesis at 99% CL.

Possible signs of a Higgs boson with production cross-sections and branching ratios compatible with the Standard Model coupled with no signs of new physics despite extremely precise tests, left all of the participants of this first week of “Moriond” rather puzzled. Perhaps it is time to go back to the drawing board.

Daya Bay experiment measures θ₁₃

CCnew4_03_12 — The sensitive photomultiplier tubes, which line the Daya Bay detector walls, are designed to amplify and record the faint flashes that signify an antineutrino interaction.
Image credit: Roy Kaltschmidt/Lawrence Berkeley National Laboratory.

The Daya Bay Reactor Neutrino Experiment, a multinational collaboration operating in the south of China, has reported its first results. The team has analysed tens of thousands of interactions of electron-antineutrinos caught by six massive detectors buried in the mountains adjacent to the powerful nuclear reactors of the China Guangdong Nuclear Power Group.

The copious data revealed for the first time a strong signal of the mixing angle θ₁₃, related to the type of neutrino oscillation in which electron-neutrinos morph into the other two flavours. This is the last mixing angle to be measured precisely and could reveal clues leading to an understanding of why matter predominates over antimatter in the universe. Once thought to be near zero, the first results indicate that sin²2θ₁₃ is equal to 0.092 ± 0.017.

The Daya Bay experiment counts the number of electron-antineutrinos detected in the halls nearest the Daya Bay and Ling Ao reactors and calculates how many would reach the detectors in the Far Hall if there were no oscillation. The number that apparently vanish on the way (by oscillating into other flavours) gives the value of θ₁₃. Because of the near-hall/far-hall arrangement, it is unnecessary to have a precise estimate of the antineutrino flux from the reactors.

The initial results will in the coming months and years be honed by collecting more data and reducing statistical and systematic errors. Refined results will open the door to further investigations and influence the design of future neutrino experiments, including how to determine which neutrino flavours are the most massive and whether there is a difference between neutrino and antineutrino oscillations.

¹¹⁰Pd: a new possibility for ββ0ν decay

CCnew5_03_12 — A charged particle is forced on the cyclotron orbit by a magnetic field, B, and stored in a Penning trap by applying a voltage, U, to hyperbola-shaped electrodes.

The search for the neutrinoless double-beta decay (ββ0ν decay) aims to solve a long-standing question concerning the nature of neutrinos. The decay, in which a nucleus decays by emitting two electrons but no neutrinos, can occur only if the neutrino is its own antiparticle, i.e. a Majorana particle. If it occurs, it must be extremely rare, with a half-life greater than 10²⁴ years. This poses an enormous experimental challenge regarding its unambiguous detection, with just a few nuclear isotopes offering a useful hunting ground. Now an experiment at the ISOLDE facility at CERN has identified a new potential candidate, the palladium isotope ¹¹⁰Pd.

The signature for ββ0ν decay appears in the sum of the energies of the two emitted electrons, which should have a single peak at the Q value for the decay, i.e. at the energy corresponding to the mass difference between the initial and final nuclide. (In double-beta decay with neutrinos (ββ2ν), the emitted electrons have a broad energy spectrum.) Calorimetric experiments searching for ββ0ν require detectors fabricated from sufficient quantities of the transmuting material to allow the detection of a decay within a reasonable amount of time. In addition, the energy of the decay peak must be known precisely if the detector is to have a high resolution at the correct energy.

With its high natural abundance, ¹¹⁰Pd offers a promising alternative for double-beta decay searches, now that its Q value has been measured directly with unprecedented accuracy. An experiment using the Penning-trap mass spectrometer ISOLTRAP at ISOLDE has determined the Q value from the cyclotron frequency ratio of ¹¹⁰Pd and its decay-product ¹¹⁰Cd by manipulating a few, singly charged ions in an isolated environment (Fink et al. 2012).

In a Penning trap, a charged particle is bound radially on the cyclotron orbit by a homogeneous magnetic field, while an electrostatic potential between the hyperbola-shaped electrodes provides axial confinement (see figure). Since the ions are trapped in three dimensions, they exhibit three eigenmotions (only one of which is shown in the figure for simplicity). An applied radio-frequency field can modify the energy stored in the eigenmotions, resonantly enhancing the energy transfer when it reaches the exact eigenfrequency. This can be measured using a technique known as time-of-flight ion-cyclotron-resonance. Usually, fewer than 10 ions of one species are excited in the trap and the cyclotron frequency is determined. The other species is then loaded into the trap and excited. This measurement cycle is repeated many times in order to collect statistics and minimize systematic effects.

In this experiment, the Q value was determined after roughly two days of measurement to be Q = 2017.85(64) keV. This value is shifted by 14 keV compared with previous results and is 17 times more precise. While the shift leads to a new value for the ¹¹⁰Pd half life, the lower uncertainty should enable future experiments on ββ0ν decay to have higher resolution.

The heavier B_s meson state lives longer

The LHCb collaboration has determined the sign of the width difference in the B_s system, ΔΓ_s, through the influence of quantum-mechanical interference. This shows for the first time that the heavier of the two B_s meson states has the longer lifetime, a result that is in agreement with the Standard Model expectation and similar to the situation in the kaon system.

The B_s meson, made up of a b quark and s antiquark, has some fascinating properties. Because it is neutral, it can mix with its antiparticle (which has a b antiquark and s quark), and this quantum-mechanical effect leads to the B_s system having two states with well defined mass, m_H and m_L (for “heavy” and “light” respectively). The B_s oscillates from its particle to antiparticle state, with a frequency that is proportional to the difference in those masses, Δm_s = m_H – m_L, a frequency that is now well measured. However, the two states are also expected to have different lifetimes, so that their widths (defined as the inverse of their lifetimes) should differ by ΔΓ_s ≡ Γ_L – Γ_H. Until now, the sign of ΔΓ_s was not known.

CCnew7_03_12 — Constraints on the mixing parameters of the B_s meson, shown as contours in the ΔΓ_s vs φ_s plane. The results from the DØ and CDF experiments are shown in red and green respectively, and the previous result from LHCb in blue. For the latest result from LHCb the ambiguity in the solution has been resolved, leaving only the single solution shown in yellow. This is still consistent with the Standard Model expectation indicated by the black point.

This parameter is intimately involved in the study of CP-violation in B_s mixing, where the phase of the B_s oscillations, φ_s, is measured. In the Standard Model, the phase is expected to be small but accurately predicted, φ_s = –0.036 ± 0.002 rad. It has been studied using the decays of the B_s to two lighter mesons, B_s → J/ψ φ. Because of correlations between ΔΓ_s and φ_s, the experimental searches have been presented until now as contours in the ΔΓ_s vs φ_s plane, as shown in the figure, which is an update of a previous measurement. However, because the sign of ΔΓ_s was unknown, there was an ambiguity in the solution, seen as two selected regions in the ΔΓ_s vs φ_s plane.

The new analysis from LHCb uses the fact that, when the spin-1 φ meson is reconstructed in its decay to K⁺K^– , a small admixture of spin-0 kaon pairs is also included in the selected events, because – at any given K⁺K^– mass value – the two possible spin states are quantum-mechanically indistinguishable, interference effects can be observed in the data. The relative phase of these two components varies as a function of the reconstructed mass and the trend of that variation is to increase or decrease depending on the sign of ΔΓ_s. The experimental data show clearly a decreasing trend, with 4.7 σ significance, demonstrating that ΔΓ_s is positive (LHCb 2012a). As a consequence, in the latest update of the CP-violation study there is only a single solution in the plane.

This new result was presented at the Moriond conference (Much food for thought at Moriond) and uses the full data set collected by LHCb so far (LHCb 2012b). When combined with another channel (J/ψ f₀), the result is φ_s = –0.002 ± 0.083 (stat.) ± 0.027 (syst.) rad. While this is consistent with the Standard Model prediction, there is still room for contributions from new physics to this phase. Another exciting step forward is expected with the further doubling of the LHCb data set, expected this year.

Supersymmetry in the third generation

Despite the current absence of direct experimental evidence, supersymmetry (SUSY) at the weak scale remains among the most motivated and studied extensions of the Standard Model.

CCnew9_03_12 — Results of the ATLAS search for direct b^˜ production showing the observed (red curve) and expected (dashed blue curve) limits at 95% confidence level as a function of the b^˜ and LSP (χ^˜⁰) masses. The area below the red curve is excluded by this analysis. Limits from Tevatron experiments are shown as solid areas.

A common feature of many models is that third-generation SUSY particles – the stop (t~), sbottom (b~) and stau (τ~), which are the partners of the third-generation quarks and of the τ lepton – are lighter than the partners of the first two generations. Hence, they can be produced at large rates via pair production or in the decay of gluinos, the scalar partners of gluons. Furthermore, they should decay to heavy quarks (t, b) or τ leptons, providing characteristic and striking experimental signatures. The ATLAS collaboration recently presented the results from several searches for third-generation SUSY particles based on 2 fb^–1 of data (ATLAS 2012). Different strategies are used in each of these analyses, which rely on signatures with one or two hadronic taus in τ~ searches, b-jets with a lepton veto in b~ searches and two same-sign leptons or b-jets with a lepton in t~ searches. In the models considered, each SUSY decay-chain ends with the production of a stable, lightest supersymmetric particle (LSP), which is only weakly interacting and escapes detection. Therefore, high missing transverse-momentum is also required in all of these analyses.

The searches found no significant excess over the Standard Model background and provide the most stringent limits to date on models that are characterized by the decay of third-generation SUSY particles. The figure shows, as an example, that the exclusion limits obtained in the search for scalar bottom pair-production – using events with exactly two b-jets – extend the existing limits on the b~ mass by about 150 GeV.

In the coming months and with increasing amounts of data, these analyses will probe unexplored regions, corresponding for example to high-LSP or gluino masses, and so may shed light on the existence of third-generation SUSY particles.

Shedding light on dark matter

Dark matter may constitute 83% of the particles in the universe, but so far there has been no direct observation of its presence in experiments. With its high-energy collisions, the LHC is a promising hunting ground for this elusive form of “matter”, either by producing dark-matter particles directly or new particles that decay into dark matter. Recently, the CMS collaboration completed a search for dark matter, sifting through the full 2011 data set of proton collisions at a centre-of-mass energy of 7 TeV.

Dark-matter particles produced at the LHC would presumably escape undetected, yielding “missing momentum” in the event. However, they could be accompanied by a jet or a photon, or some other particle. CMS has looked for evidence of these visible companions by studying “monojet” and “monophoton” data. Within the framework of a simple model for the production of dark matter, the CMS analysis significantly extends the sensitivity of direct searches, which look for tiny interactions of dark-matter particles in very sensitive detectors.

CCnew11_03_12 — The 90% CL upper limits from CMS on the χ-nucleon cross-section as a function of χ mass for spin-independent (left) and spin-dependent scattering (right). Also shown are limits from other experiments.

The way that the dark-matter particles (χ) are produced and interact depends on their spin. With respect to direct searches, CMS is sensitive in the low-mass region below 3.5 GeV if the spin of the produced particles is ignored, and it can set the world’s best limits at all masses in the spin-dependent case.

The monophoton search looks for single, isolated photons (γ) with transverse energy greater than 145 GeV and more than 130 GeV of missing transverse energy. Events with excessive hadronic activity (jets) are vetoed. After the application of selection criteria, 73 events remain, where 71.9 ± 9.1 would be expected in the absence of dark-matter particles. Standard Model background-events are expected mainly from pp → Zγ – where the Z decays to two neutrinos – and from events with misidentified jets or electrons, or from instrumental sources.

The monojet search requires at least one jet with transverse momentum greater than 110 GeV and more than 350 GeV of missing transverse momentum. Events with isolated leptons or more than two jets are vetoed. After event selection, 1142 events are found in data with an expectation from Standard Model processes of 1224 ± 101 events. Again, a contribution from “invisible” decays to neutrinos dominate this expectation, either from pp → Z+jets with the Z decaying to two neutrinos, or from pp → W+jets where the W escapes detection. There seem to be no signs of a new production mechanism for the two “mono-object” signatures analysed, so CMS can use the null results to place limits on the cross-section for dark matter. The limits depend on the presumed mass of the dark-matter particles and are presented as regions in the plane of cross section vs mass in the figures.

J/Ψ production in high-multiplicity pp collisions

The J/Ψ meson, a bound state of a charm and an anticharm quark, has always been an important testing ground for quantum chromodynamics (QCD). However, understanding J/Ψ production in proton–proton (pp) collisions remains a challenge. While the production of unbound c–c pairs via partonic (quark and gluon) hard-scattering processes can be described within perturbative QCD, the subsequent formation of a colourless bound state, such as the J/Ψ, leaves much room for theoretical modelling. Recently, a new measurement by the ALICE collaboration at the LHC adds yet another challenge for theoretical models.

ALICE, which among the LHC experiments has the unique capability to identify J/Ψ at low transverse momenta, measured J/Ψ production in pp collisions at √s = 7 TeV as a function of the charged-particle multiplicity (dN_ch/dη) of the underlying event. As the figure shows, this study revealed a remarkably linear increase of the J/Ψ yield with dN_ch/dη (Abelev et al. 2012). Even though the charged-particle multiplicity is measured only at central rapidity, this increase is seen in not only this region but also at forward rapidities.

CCnew13_03_12 — The J/Ψ yield (dN_J/Ψ/dy) as a function of the charged-particle multiplicity densities at mid-rapidity (dN_ch/dη) (Abelev *et al.* 2012). Both are divided by the corresponding averaged value for minimum-bias pp collisions (<dN_J/Ψ/dη>, <dN_ch/dη>). Measurements are shown at forward rapidities (2.5 < y < 4) and at mid-rapidity (|y| < 0.9).

The charged-particle multiplicity is mostly the result of processes happening with a small momentum transfer, i.e. so-called soft processes. J/Ψ mesons, on the other hand, are expected to be produced in hard processes, as described above. The assumption has been that the two are not necessarily correlated. However, the measurement by ALICE shows that the yields of the heavy J/Ψ scale just like those of any other light hadron. Regardless of its high rest mass (3.097 GeV/c²) the J/Ψ behaves just like a “light” particle.

The event multiplicity dN_ch/dη could be directly related to the impact parameter of a given pp collision. This would be analogous to heavy-ion physics where, depending on the centrality of the reaction, different numbers of binary collisions occur between the nucleons. A similar situation could arise in pp collisions at LHC energies because the probability is high that many collisions take place between the quarks and gluons inside the protons. Most of these multiparton interactions are usually assumed to have a relatively low momentum-transfer and therefore to affect mainly soft-particle production. That J/Ψ production behaves similarly to the production of other charged particles might indicate that harder processes are also affected by multiparton interactions.

This measurement is an example of the new experimental opportunities that the LHC has opened up, allowing the correlation of observables on soft and hard scales. Further studies on the charged-particle multiplicity dependence of other hard processes, such as the production of Υ, open charm and beauty, should shed more light on the nature of this effect.

Interactions with André Petermann

CCpet1_03_12 — The seminal paper of 1953 by Stueckelberg and Petermann.

The origin of this conceptual revolution was the work in which these two theoretical physicists discovered that all quantities such as the gauge couplings (α_i ) and the masses (m_j) must “run” with q², the invariant four-momentum of a process (Stueckelberg and Petermann 1951). It took many years to realize that this “running” allows not only the existence of a grand unification and opens the way to supersymmetry but also finally produces the need for a non-point-like description of physics processes – the relativistic quantum-string theory – that should produce the much-needed quantization of gravity.

It is interesting to recall the reasons that this paper attracted so much attention. The radiative corrections to any electromagnetic process had been found to be logarithmically divergent. Fortunately, all divergencies could be grouped into two classes: one had the property of a mass; the other had the property of an electric charge. If these divergent integrals were substituted with the experimentally measured mass and charge of the electron, then all theoretical predictions could be made to be “finite”. This procedure was called “mass” and “charge” renormalization.

Stueckelberg and Petermann discovered that if the mass and the charge are made finite, then they must run with energy. However, the freedom remains to choose the renormalization subtraction points. Petermann and Stueckelberg proposed that this freedom had to obey the rules of an invariance group, which they called the “renormalization group” (Stueckelberg and Petermann 1953). This is the origin of what we now call the renormalization group equations, which – as mentioned – imply that all gauge couplings and masses must run with energy. It was remarkable many years later to find that the three gauge couplings could converge, even if not well, towards the same value. This means that all gauge forces could have the same origin; in other words, grand unification. A difficulty in the unification was the new supersymmetry that my old friend Bruno Zumino was proposing with Julius Wess. Bruno told me that he was working with a young fellow, Sergio Ferrara, to construct non-Abelian Lagrangian theories simultaneously invariant under supergauge transformations, without destroying asymptotic freedom. During a nighttime discussion with André, in the experimental hall to search for quarks at the Intersecting Storage Rings in 1977, I told him that two gifts were in front of us: asymptotic freedom and supersymmetry. The first was essential for the experiment being implemented, the second to make the convergence of the gauge couplings “perfect” for our work on the unification. We will see later that this was the first time that we realized how to make the unification “perfect”.

The muon g-2

The second occasion for me to know about André came in 1960, when I was engaged in measuring the anomalous magnetic moment (g-2) of the muon. He had made the most accurate theoretical prediction, but there was no high-precision measurement of this quantity because technical problems remained to be solved. For example, a magnet had to be built that could produce a set of high-precision polynomial magnetic fields throughout as long a path as possible. This is how the biggest (6-m long) “flat magnet” came to be built at CERN with the invention of a new technology now in use the world over. André worked only at night and because he was interested in the experimental difficulties he spent nights with me working in the SC-Experimental Hall. It was a great help for me to interact with the theorist who had made the most accurate theoretical prediction for the anomalous magnetic moment of a particle 200 times heavier than the electron. The muon must surely reveal a difference in a fundamental property like its g-value. Otherwise, why is its mass 200 times greater than that of the electron? (Even now, five decades later, no one knows why.)

When the experiment at CERN proved that, at the level of 2.5 parts in a million for the g-value, the muon behaves as a perfect electromagnetic object, the problem changed focus to ask why are there so many muons around? The answer lay in the incredible value of the mass difference between the muon and its parent, the π. Could another “heavy electron” – a “third lepton” – exist with a mass in the range of giga-electron-volts? Had a search ever been done for this third “lepton”? The answer was no. Only strongly interacting particles had been studied. This is how the search for a new heavy lepton, called HL, was implemented at CERN, with the Proton AntiProton into LEpton Pairs (PAPLEP) project, where the production process was proton–antiproton annihilation. André and I discussed these topics in the CERN Experimental Hall during the night shifts he spent with me.

The results of the PAPLEP experiment gave an unexpected and extremely strong value for the (time-like) electromagnetic form-factor of the proton, whose consequence was a factor 500 below the point-like cross-section for PAPLEP. This is how, during another series of night discussions with André , we decided that the “ideal” production process for a third “lepton” was (e⁺e^–) annihilation. However, there was no such collider at CERN. The only one being built was at Frascati, by Bruno Touschek, who was a good friend of Bruno Ferretti and another physicist who preferred to work at night. I had the great privilege of knowing Touschek when I was in Rome. He also became a strong supporter of the search for a “third lepton” with the new e⁺e^– collider, ADONE. Unfortunately the top energy of ADONE was 3 GeV and the only result that we could achieve was a limit of 1 GeV for the mass of the much desired “third lepton”.

Towards supersymmetry

Another topic talked about with André has its roots in the famous work with Stueckelberg – the running with energy of the fundamental couplings of the three interactions: electromagnetic, weak and strong. The crucial point here was at the European Physical Society (EPS) conferences in York (1978) and Geneva (1979). In my closing lecture at EPS-Geneva, I said: “Unification of all forces needs first a supersymmetry. This can be broken later, thus generating the sequence of the various forces of nature as we observe them.” This statement was based on work with André where in 1977 we studied – as mentioned before – the renormalization-group running of the couplings and introduced a new degree of freedom: supersymmetry. The result was that the convergence of the three couplings improved a great deal. This work was not published, but known to a few, and it led to the Erice Schools Superworld I, Superworld II and Superworld III.

This is how we arrived at 1991 when it was announced that the search for supersymmetry had to wait until the multi-tera-electron-volt energy threshold would become available. At the time, a group of 50 young physicists was engaged with me on the search for the lightest supersymmetric particle in the L3 experiment at CERN’s Large Electron Positron (LEP) collider. If the new theoretical “predictions” were true then there was no point in spending so much effort in looking for supersymmetry-breaking in the LEP energy region. Reading the relevant papers, André and I realized that no one had ever considered the evolution of the gaugino mass (EGM). During many nights of work we improved the unpublished result of 1977 mentioned above: the effect of the EGM was to bring down the energy threshold for supersymmetry-breaking by nearly three orders of magnitude. Thanks to this series of works I could assure my collaborators that the “theoretical” predictions on the energy-level where supersymmetry-breaking could occur were perfectly compatible with LEP energies (and now with LHC energies).

Finally, in the field of scientific culture, I would like to pay tribute to André Petermann for having been a strong supporter for the establishment of the Ettore Majorana Centre for Scientific Culture in Erice. In the old days, before anyone knew of Ettore Majorana, André was one of the few people who knew about Majorana neutrinos and that relativistic invariance does not give any privilege to spin-½ particles, such as the privilege of having antiparticles, all spin values having the same privilege. In all of my projects André was of great help, encouraging me to go on, no matter what the opposition could present in terms of arguments that often he found to be far from being of rigorous validity.

Saul Perlmutter: from light into darkness

CCsau1_03_12 — Saul Perlmutter surrounded by the press on the day that he learnt he had received a share in the 2011 Nobel Prize in Physics.
Image credit: LBNL.

Paradoxically, work on “light candles” led to the discovery that the universe is much darker than anyone thought. Arnaud Marsollier caught up with Saul Perlmutter recently to find out more about this Nobel breakthrough.

Saul Perlmutter admits that measuring an acceleration of the expansion of the universe – work for which he was awarded the 2011 Nobel Prize in Physics together with Brian Schmidt and Adam Riess – came as a complete surprise. Indeed, it is exactly the opposite of what Perlmutter’s team was trying to measure: the decelerating expansion of the universe. “My very first reaction was the reaction of any physicist in such a situation: I wondered which part of the chain of the analysis needed a new calibration,” he recalls. After the team had checked and rechecked over several weeks, Perlmutter, who is based at Lawrence Berkeley National Laboratory and the University of California, Berkeley, still wondered what could be wrong: “If we were going to present this, then we would have to make sure that everybody understood each of the checks.” Then, after a few months, the team began to make public its result in the autumn of 1997, inviting scrutiny from the broader cosmology community.

Despite great astonishment, acceptance of the result was swift. “Maybe in science’s history, it’s the fastest acceptance of a big surprise,” says Perlmutter. In a colloquium that he presented in November 1997, he remembers how cosmologist Joel Primack stood up and instead of talking to Perlmutter he addressed the audience, declaring: “You may not realize this, but this is a very big problem. This is an outstanding result you should be worried about.” Of course, some colleagues were sceptical at first. “There must be something wrong, it is just too crazy to have such a small cosmological constant,” said cosmologist Rocky Kolb in a later conference in early 1998.

According to Perlmutter, one of the main reasons for the quick acceptance by the community of the accelerating expansion of the universe is that two teams reported the same result at almost the same time: Perlmutter’s Supernova Cosmology Project and the High-z Supernova Search Team of Schmidt and Riess. Thus, there was no need to wait a long time for confirmation from another team. “It was known that the two teams were furious competitors and that each of them would be very glad to prove the other one wrong,” he adds. By the spring of 1998, a symposium was organized at Fermilab that gathered many cosmologists and particle physicists specifically to look at these results. At the end of the meeting, after subjecting the two teams to hard questioning, some three quarters of the people in the room raised their hands in a vote to say that they believed the results.

What could be responsible for such an acceleration of the expanding universe? Dark energy, a hypothetical “repulsive energy” present throughout the universe, was the prime suspect. The concept of dark energy was also welcomed because it solves some delicate theoretical problems. “There were questions in cosmology that did not work so well, but with a cosmological constant they are solved,” explains Perlmutter. Albert Einstein had at first included a cosmological constant in his equations of general relativity. The aim was to introduce a counterpart to gravity in order to have a model describing a static universe. However, with evidence for the expansion of the universe and the Big Bang theory, the cosmological constant had been abandoned by most cosmologists. According to George Gamow, even Einstein thought that it was his “biggest blunder” (Gamow 1970). Today, with the discovery of the acceleration of the expansion of the universe, the cosmological constant “is back”.

Since the discovery, other kinds of measurements – for example on the cosmic microwave background radiation (CMB), first by the MAXIMA and BOOMERANG balloon experiments, and then by the Wilkinson Microwave Anisotropy Probe satellite – have proved consistent with, and even made stronger, the idea of an accelerating expansion of the universe. However, it all leads to a big question: what could be the nature of dark energy? In the 20th century, physicists were already busy with dark matter, the mysterious invisible matter that can only be inferred through observations of its gravitational effects on other structures in the universe. Although they still do not know what dark matter is, physicists are increasingly confident that they are close to finding out, with many different kinds of experiments that can shed light on it, from telescopes to underground experiments to the LHC. In the case of dark energy, however, the community is far from agreeing on a consistent explanation.

When asked what dark energy could be, Perlmutter’s eyes light up and his broad smile shows how excited he is by this challenging question. “Theorists have been doing a very good job and we have a whole landscape of possibilities. Over the past 12 years there was an average of one paper a day from the theorists. This is remarkable,” he says. Indeed, this question has now become really important as it seems that physicists know about a mere 5% of the whole mass-energy of the universe, the rest being in the form of dark matter or, in the case of more than 70%, the enigmatic, repulsive stuff known as dark energy or a vacuum energy density.

Including a cosmological constant in Einstein’s equations of general relativity is a simple solution to explain the acceleration of the expansion of the universe. However, there are other possibilities. For example, a decaying scalar field of the kind that could have caused the first acceleration at the beginning of the universe or the existence of extra dimensions could save the standard cosmological model. “We might even have to modify Einstein’s general relativity,” Perlmutter says. Indeed, all that is known is that the expansion of the universe is accelerating, but there is no clue as to why. The ball is in the court of experimentalists, who will have to provide theorists with more data and refined measurements to show precisely how the expansion rate changes over time. New observations by different means will be crucial, as they could show the way forward and decide between the different available theoretical models.

“We have improved the supernova technique and we know what we need to make a measurement that is 20 times more accurate,” he says. There are also two other precision techniques currently being developed to probe dark energy either in space or from the ground. One uses baryon acoustic-oscillations, which can be seen as “standard rulers” in the same way that supernovae are used as standard candles (see box, previous page). These oscillations leave imprints on the structure of the universe at all ages. By studying these imprints relative to the CMB, the earliest “picture of the universe” available, it is possible to measure the rate at which the expansion of the universe is accelerating. The second technique is based on gravitational lensing, a deflection of light by massive structures, which allows cosmologists to study the history of the clumping of matter in the universe, with the attraction of gravity contesting with the accelerating expansion. “We think we can use all of these techniques together,” says Perlmutter. Among the projects he mentions, are the US-led ground-based experiments BigBOSS and the Large Synoptic Survey Telescope and ESA’s Euclid satellite, all of which are under preparation.

However, the answer to this obscure mystery – or at least part of it – could come from elsewhere. The full results from ESA’s Planck satellite, for instance, are eagerly awaited because they should provide unprecedented precision on measurements of the CMB. “The Planck satellite is an ingredient in all of these analyses,” explains Perlmutter. In addition, cosmology and particle physics are increasingly linked. In particular, the LHC could bring some input into the story quite soon. “It is an exciting time for physics,” he says. “If we just get one of these breakthroughs through the LHC, it would help a lot. We are really hoping that we will see the Higgs and maybe we will see some supersymmetric particles. If we are able to pin down the nature of dark matter, that can help a lot as well.” Not that Perlmutter thinks that the mystery of dark energy is related to dark matter, considering that they are two separate sectors of physics, but as he says, “until you find out, it is still possible”.

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