Physics Archives – Page 72 of 144

IceCube finds evidence for high-energy extra-terrestrial neutrinos

CCnew2_01_14 — Fig. 1. (a) The deposited-energy spectrum and (b) zenith angle distribution for the 28 signal events, compared to the expected backgrounds from atmospheric muons (red), conventional atmospheric neutrinos (blue, with uncertainties given by the hatched region), perturbative QCD predictions for prompt neutrinos and the 90% upper limit for prompt neutrinos (purple), plus the best-fit E^–2 astrophysical neutrino spectrum.

The IceCube collaboration has reported evidence, at the 4σ level, for a diffuse (i.e. isotropic) flux of high-energy extra-terrestrial neutrinos, mostly above 60 TeV (Aartsen et al. 2013). Using two years of data, the analysis selected 28 events – including the two events previously reported with energies above 1 PeV. This is substantially above the background estimate of 12.1 events.

In the energy range 60 TeV to 2 PeV, the data are well described by a neutrino energy spectrum that varies as E^–2, with a flux E_ν²φ <1.2±0.4 × 10^–8 GeV cm^–2 s^–1 sr^–1. This is near the Waxman–Bahcall bound – the flux expected if cosmic-ray nuclei undergoing acceleration interact strongly in their sources and transfer most of their energy to secondary particles (mainly π^± and K^±) whose decays produce neutrinos. For an E^–2 spectrum, the data indicate that there must be a cut-off at a few peta-electron-volts, otherwise more energetic events would have been seen. Alternatively, the energy spectrum might be somewhat softer: an E^–2.2 spectrum fits the data well.

The analysis combined multiple techniques to isolate the 28 events from a much larger background of downward-going cosmic-ray muons and atmospheric neutrinos. The event selection was simple. It involved choosing events that originated within the detector and produced more than 6000 observed photoelectrons. The origination criteria used the outer portion of the detector as a veto, therefore removing events with early light, which could be from entering tracks. The analysis estimated the muon backgrounds using two independent, nested veto regions around a smaller fiducial volume. Events tagged in the outer veto that missed the inner veto were used to determine the veto-miss fraction. The veto also eliminated energetic, downward-going atmospheric neutrinos, which should be accompanied by a cosmic-ray air shower with energetic muons that should trigger the veto.

The selection criteria were largely insensitive to the event topology, so the analysis selected ν_e, ν_μ and ν_τ interactions, providing they occurred inside the detector. The events fall into two classes: long tracks (muons) from ν_μ charged-current interactions, plus cascades, electromagnetic or hadronic showers from ν_e and most ν_τ charged-current interactions, and neutral-current interactions of any flavour. Most of the events that IceCube sees are atmospheric ν_μ charged-current interactions, but the requirement that the events originate within the detector, depositing 6000 photoelectrons, changes the fraction. Of the 28 events found, only seven are classed as track-like. While this is consistent with the 1:1:1 ratio of ν_e:ν_μ:ν_τ, it is a lower fraction of tracks than expected for atmospheric neutrinos, which are mostly ν_μ.

Figure 1(a) shows the deposited energy for the 28 events, together with the expected backgrounds for muons, conventional atmospheric neutrinos and prompt atmospheric neutrinos from the decay of charmed particles. The atmospheric neutrino fluxes include the effect of the downward-going veto. There is a substantial uncertainty for the prompt flux, which has not yet been observed – the range is based on theoretical estimates, with upper limits from previous IceCube studies. Although the two 1 PeV neutrinos are prominent, the signal rises above the background at energies above 60 TeV. The black line shows the best fit to an E^–2 astrophysical signal.

Figure 1(b) compares the zenith angle distribution of the data with the same background estimates. The muon background is entirely downward-going, while the atmospheric neutrino background is largely upward-going, owing to a combination of the downward-going veto and the absorption of high-energy neutrinos in the Earth. An isotropic extra-terrestrial signal would also be mostly downward-going because of this absorption. Of the 28 selected events, 24 are downward-going, which is more than expected from the background plus the astrophysical component from the fit. The excess is about 1.5σ. The angular agreement for a purely atmospheric neutrino flux is even worse.

This analysis shows that cosmic accelerators emit a significant fraction of their energies as neutrinos. The collaboration has also studied the arrival directions of the events, but observes no significant clusters. However, follow-up studies should further characterize the radiation and pin down its source. Already, some tantalizing hints have been presented at the 2013 International Cosmic Ray Conference.

Breakthrough of the Year

The first observations of high-energy cosmic neutrinos by IceCube was named 2013 Breakthrough of the Year by Physics World and also featured in Wired’s list of top scientific discoveries of 2013. Physics World highly commended nine other achievements, including the discovery of pear-shaped nuclei at CERN’s ISOLDE facility, the Planck space telescope’s most precise determination ever of the cosmic microwave background radiation and the South Pole Telescope’s measurement of B-mode polarization in the radiation. Wired also listed the dark-matter results from the LUX experiment.

ATLAS and CMS observe Higgs-boson decays to fermions

Last year, the ATLAS and CMS collaborations confirmed that the new boson found in 2012 was indeed a Higgs boson with a mass around 125 GeV. The discovery relied on the observation of decays to pairs of bosons, namely photons, Ws and Zs – the carriers of the electromagnetic and weak forces – and provided strong support for the idea that the Brout–Englert–Higgs mechanism is responsible for the mass of the W and Z bosons, while leaving the photon massless. Now, with the full LHC data sets from 2011 and 2012, the two collaborations have turned their attention to testing whether the Higgs field also gives mass to fermions, i.e. quarks and leptons.

The CMS collaboration announced its first results on the coupling of the recently discovered Higgs boson to fermion pairs at the Rencontres de Moriond conference, in March 2013. At the time, the search for the Higgs-boson decays to b b and τ⁺τ^– had yielded evidence with a combined significance of 3.4σ for the Higgs coupling to third-generation fermions.

Now, the updated search for decays to τ⁺τ^–, based on an improved analysis of 4.9 fb⁻¹ of LHC data collected at a collision energy of 7 TeV in 2011 and 19.7 fb⁻¹ collected at 8 TeV in 2012, has revealed a 3.4σ excess at the mass of the Higgs boson in this channel alone. Taken together with the 2.1σ excess found in the earlier searches by CMS for b decays, this gives a combined significance of 4.0σ for the two channels, compared with an expectation of 4.2σ for a Standard Model Higgs boson. The observed rate for Higgs production with subsequent decay into b quarks or τ leptons divided by the expected rate for a Standard Model Higgs gives the ratio μ = 0.90±0.26, which suggests that the particle does indeed behave like a Standard Model Higgs boson.

CCnew6_01_14 — Fig. 1. Weighted mass spectrum for the τ⁺τ^– system in CMS. The inset shows a background-subtracted view of the spectrum with an excess around the mass of the discovered Higgs boson.

By contrast, the search for decays of the Higgs boson to μ⁺μ^– yields no signal, just as expected from the fact that the μ has nearly 17 times less mass than the τ, making the decays of the Higgs to μ⁺μ^– some 290 times less frequent than those to τ⁺τ^–. Likewise, the search for the Higgs-boson decay into a pair of electrons – which should be even more rare, given that the electron is 200 times lighter than the muon – returned empty handed. From this, it can be inferred that the couplings of the Higgs boson to leptons of different generations are not universal, in contrast to, for example, the couplings of the Z bosons, which do not distinguish between lepton flavours.

These measurements used the full capabilities of the CMS experiment, combining information from all of the detector’s components to reconstruct and measure the energy of each individual particle emerging from the proton–proton collision, via a “particle flow” algorithm. This technique allows τ decays to be identified efficiently, while rejecting the background from “jets” of particles originating from quarks and gluons. The precise charged-particle tracking of CMS helps identify jets coming from b quarks. In the case of Higgs-boson decays to τ⁺τ^–, the data have a strong peak at a τ⁺τ^– mass corresponding to that of the Z boson, which is produced much more copiously than Higgs bosons. The analysis was developed and optimized in a “blinded” way, i.e. not looking at the signal in the data, to avoid introducing a bias.

By carefully measuring and controlling this background, a clear signal from Higgs decays remains after subtracting the background (figure 1).

Armed with an array of techniques and decay modes with which to study the Higgs boson, CMS will continue to measure its properties ever more precisely – using present and future data – and to search for additional new particles, including possible cousins of the Higgs boson.

The ATLAS collaboration presented its first results on fermionic decays of the Higgs boson using the full 2012 data set in the di-muon and b b decay channels at the winter and summer conferences in 2013, respectively. However, the results did not yet establish the fermionic decays expected from a Standard Model Higgs boson. ATLAS has now found strong evidence for Higgs decays to fermions, using 20.3 fb⁻¹ of LHC data taken at a proton collision energy of 8 TeV in the centre of mass. This is an important test of the Standard Model, which predicts such decays.

Branching ratios for the Standard Model Higgs boson are predicted to scale with the mass-squared of the decay products. Hence the two fermionic final states expected to be most abundant are b b quark pairs and τ⁺τ^– lepton pairs, the most massive of the fermions to which the Higgs boson can decay.

CCnew7_01_14 — Fig. 2. The invariant mass of the τ⁺τ^– pair in ATLAS. The excess of data events (black dots) is consistent with the presence of a Higgs boson at 125 GeV, indicated by the red line. The main remaining background is Z → τ⁺τ^–. Background contributions with τ⁺τ^– invariant mass away from 125 GeV are suppressed by the analysis.

ATLAS has looked for all possible τ⁺τ^– decay channels, namely to two leptons, to one lepton plus hadrons and to hadrons only. The analysis was optimized to test whether a 125 GeV Higgs boson decays to a τ⁺τ^– pair and suppresses contributions that have a τ⁺τ^– invariant mass away from 125 GeV, including the Z → τ⁺τ^– background, although the main remaining background is still Z → τ⁺τ^–. Thanks to the detector’s powerful lepton identification capabilities and the application of sophisticated analysis methods, the contamination from backgrounds to the τ⁺τ^– final state could be reduced greatly. Remaining backgrounds were estimated directly from the data, or taken from simulation with their rates normalized to data in signal-free control regions. To avoid any possible bias, the analysis was developed and optimized without looking at the signal in the data (blinded).

After “unblinding”, ATLAS observes an excess of events (figure 2) in a region consistent with the previously measured mass of the Higgs boson (125 GeV) and a statistical significance of 4.1σ, against 3.2σ expected. The ratio of the observed signal to that expected for a Standard Model Higgs decaying to a τ⁺τ^– pair yields μ = 1.4^+0.4_–0.5, which is compatible with one.

ATLAS also searched for the decay of a Higgs boson to a muon pair. Because the Higgs boson couples to mass and the mass of the muon is 17 times lower than that of the τ, the H → μ⁺μ^– rate is expected to be around 290 times lower than that of H → τ⁺τ^–. The absence of a signal in the ATLAS μ⁺μ^– search, setting an upper limit of 9.8 times the H → μ⁺μ^– rate predicted by the Standard Model, therefore provides strong evidence that the Higgs boson does not decay to leptons in a flavour-blind way, but favours decays to heavy leptons, as predicted by the Standard Model.

These results, which are derived from the LHC’s first run, are so far compatible with the Standard Model predictions. The broad physics programme of ATLAS, which includes precision measurements of the properties of the Higgs boson, will continue to test the Standard Model in the years to come.

These results from the two collaborations now establish the coupling of the Higgs bosons to fermions. More data will allow testing the Higgs couplings at a deeper level, where other models for physics beyond the Standard Model predict differences. The next LHC run, which begins in 2015, is expected to produce several times the existing data sample. In addition, the proton collisions will be at higher energies, producing Higgs bosons at higher rates.

How long can beauty and charm live together?

The LHCb collaboration has recently made the world’s most precise measurement of the lifetime of the B⁺_c meson – a fascinating particle that has both beauty and charm.

The heavy flavours of beauty and charm are produced in proton–proton collisions at the LHC in quark–antiquark pairs. The resulting hadrons usually contain the original pair, as in the case of quarkonia, or a single heavy quark bound to the abundantly produced light quarks. However, in rare cases, a c quark and a b antiquark combine into a B⁺_c. Since the top quark, t, decays too quickly to form hadrons, this is the only meson composed of two particles carrying different heavy flavours. As such, it offers a unique laboratory to test theoretical models of both the strong interaction, which accounts for its production, and the weak interaction, via which the meson has to decay. Indeed, the lifetime of the B⁺_c meson is one of the key parameters that provide a test-bench for theoretical models. Knowledge of the lifetime is also essential to develop selection algorithms and to improve the accuracy of the branching-fraction determination for most B⁺_c decay modes.

CCnew9_01_14 — Lifetime distribution of the B⁺_c candidates, with the fitted components indicated.

Following initial investigation at the Tevatron, the B⁺_c meson is being studied extensively at the LHC. In particular, the LHCb collaboration has already published several observations of new decay channels and the world’s most precise determination of the B⁺_c mass. Now, the collaboration has achieved the world’s most precise measurement of the lifetime by studying the semileptonic decays B⁺_c → J/ψμν, with the subsequent decay J/ψ → μ⁺ μ^–. The particle identification capabilities of LHCb allow a high-purity sample to be selected for these three-muon decays without any requirement on the decay time, therefore not biasing the measured lifetime. Using the data sample collected in 2012, about 10,000 signal decays were selected – the largest sample of reconstructed B⁺_c decays to have ever been reported.

The challenge with semileptonic decays is that the B⁺_c kinematics is not completely reconstructed, because of the impossibility of detecting the neutrino. This effect can be corrected on a statistical basis, although at the cost of introducing an uncertainty owing to the theoretical model of the decay used for the correction. LHCb developed a technique to constrain this model-dependence using data and found that the corresponding systematic uncertainty is small.

The result for the lifetime is 509±15 fs. This is twice as precise as the current world-average from the Particle Data Group, obtained combining measurements by the CDF and D0 experiments at the Tevatron, and opens the door for a new era of precision B⁺_c studies.

New charged charmonium-like states observed at BESIII

CCnew10_01_14 — Sum of simultaneous fits to the π^±h_c mass distributions for e⁺e^– → π⁺π^–h_c events at centre-of-mass energies of 4.23 GeV, 4.26 GeV, and 4.36 GeV; the inset shows the fit to the π⁺h_c distributions at 4.23 GeV and 4.26 GeV with the Z_c(3900) and Z_c(4020) clearly visible. Dots are data; shaded histograms are normalized sideband background; the solid curves show the total fit, and the dotted curves the backgrounds from the fit.

In studies at the Beijing Electron–Positron Collider (BEPCII), the international team that operates the Beijing Spectrometer (BESIII) experiment has found evidence for a family of what could well be four-quark states. The new results follow the discovery of the electrically charged Z_c(3900) in March last year.

These breakthroughs are the result of a dedicated study by the BESIII collaboration of the decays of the puzzling Y(4260) state. Discovered by the BaBar collaboration at SLAC in 2005, this state has a well-established mass that is inconsistent with the interpretation that it consists only of a charm quark, c, and an anti-charm quark, c. Moreover, it tends to decay to charmonium (cc states) plus conventional mesons rather than to a pair of charmed particles, as expected for a particle of this mass. So, more complicated models for its composition need to be considered, such as the addition of more quarks to the system, the existence of excited gluons binding the cc system, or even more exotic scenarios. The problem has been to find a way to distinguish experimentally between the different theoretical possibilities.

By tuning the energy at which electrons and positrons annihilate at BEPCII to the mass of the Y(4260), the BESIII collaboration has been able to produce the state directly and collect large samples of its decays. The first surprising result was the discovery of the Z_c(3900), a charged state that decays π^± J/ψ. To decay this way, the Z_c(3900) must contain a charm quark and an anticharm quark (to form the neutral J/ψ), together with something else that is charged, i.e., additional lighter quarks. Hence, it must be (at least) a four-quark object.

Since then, the BESIII collaboration has discovered a partner to the Z_c(3900) – the Z_c(4020). The new state appeared in the decay π^±h_c (BESIII collaboration 2013a). Like the Z_c(3900), the Z_c(4020) is electrically charged and decays to a particle consisting of a cc – in this case, the h_c – so the interpretation is the same: it must also be a four-quark object. It appears, therefore, that the BESIII collaboration has begun to unveil a whole family of four-quark objects.

One possible clue for the interpretation of the Z_c(3900) and Z_c(4020) is that they appear near the minimum masses required to allow decays to pairs of D mesons (each consisting of a charm quark and an anti-up or anti-down quark). The Z_c(3900) has a mass just above the combined mass of the D and D^* and the Z_c(4020) has a mass just more than twice that of the D^*. So one idea is that the Z_c(3900) is a four-quark bound state consisting of a D and a D^*, each composed of two quarks. Similarly, the Z_c(4020) could be a D^*D^* bound state. BESIII has explored this piece of evidence further by studying experimentally the charged D^*D and D^*D^* systems, both of which show clear enhancements with properties similar to those of the Z_c(3900) and Z_c(4020) (BESIII collaboration 2013b and 2013c).

Another clue to the nature of all of these states came with the discovery of what appears to be a Y(4260) decaying to a photon and another particle, designated the X(3872) (BESIII collaboration 2013d). Unlike the Z_c(3900) and the Z_c(4020), the X(3872) is electrically neutral and has been experimentally established for more than 10 years. It has long been suspected of being a four-quark object, but it has been difficult to distinguish this interpretation from others as it has no electric charge. Now that BESIII has observed it alongside the Z_c(3900) and Z_c(4020), it seems that a definitive theoretical interpretation must be closer at hand.

2013 Nobel Prize in Physics goes to Englert and Higgs

CCnew1_10_13 — Top: François Englert (left), at the ATLAS experiment, and Peter Higgs, at CMS. Bottom: A jubilant crowd at CERN watches the announcement of the Nobel prize live on the screen visible to the right.

Champagne corks popped at CERN on 8 October, to celebrate the award of the 2013 Nobel Prize in Physics to François Englert, professor emeritus at the Université libre de Bruxelles, and Peter Higgs, professor emeritus at the University of Edinburgh. They received the honour “for the theoretical discovery of a mechanism that contributes to our understanding of the origin of mass of subatomic particles, and which recently was confirmed through the discovery of the predicted fundamental particle, by the ATLAS and CMS experiments at CERN’s Large Hadron Collider”. The announcement of the discovery by the ATLAS and CMS collaborations took place at CERN on 4 July last year.

“I’m thrilled that this year’s Nobel prize has gone to particle physics,” said CERN’s director-general, Rolf Heuer. “The discovery of the Higgs boson at CERN last year, which validates the Brout–Englert–Higgs mechanism, marked the culmination of decades of intellectual effort by many people around the world.”

The Brout–Englert–Higgs (BEH) mechanism was first proposed in 1964 in two papers published independently – the first by Belgian physicists Robert Brout (now deceased) and his colleague Englert, the second by British physicist Higgs. It explains how the force responsible for β decay turns out to be much weaker than electromagnetism, but it is better known as the mechanism that endows fundamental particles with mass. A third paper, published by Americans Gerald Guralnik and Carl Hagen with their British colleague Tom Kibble, further contributed to the development of the new idea, which now forms an essential part of the Standard Model of particle physics. A key prediction of the idea, as was pointed out by Higgs, is the existence of a massive boson of a new type. After searches in earlier experiments, mainly at CERN and Fermilab, the particle was finally discovered by the ATLAS and CMS experiments at the LHC in 2012.

The Standard Model describes both the fundamental particles from which all the visible matter in the universe is made and the interactions that govern their behaviour. It is a remarkably successful theory that has been thoroughly tested by experiment over many years. Until last year, the BEH mechanism was the last remaining piece of the model to be experimentally verified. Now that it has been found, experiments at CERN are searching assiduously for physics beyond the Standard Model.

LHCb and theorists chart a course for discovery

Comparison of LHCb data for P₅´ with the Standard Model prediction using two different treatments of the theoretical uncertainty.

Experimenters from LHCb and theorists recently met at CERN to discuss the best ways to obtain the most out of the rich harvest of data from the LHC.

Despite the large size of modern particle-physics collaborations, every experiment faces issues that arise because resources do not match ambitions. For LHCb, the 70 journal papers the collaboration has submitted this year are only the tip of the iceberg of the interesting and often unique measurements that could be achieved. Because this iceberg might be able to fracture the hull of the Standard Model, it is essential to maximize the output of physics. This makes close communication with theorists crucial.

To facilitate such discussions, on 14–16 October LHCb held a workshop at CERN on “Implications of LHCb measurements and future prospects”. Following the tradition of two earlier meetings in the series, approximately 50 theorists from around the world joined members of the LHCb collaboration for intense discussions. Sessions covered charm mixing and CP violation, B mixing and CP violation, rare decays and “forward exotica”, including topics such as the production of top quarks and Higgs bosons in the LHCb acceptance. There was also a session dedicated to the interplay of LHCb results with, for example, studies of the Higgs boson and searches for supersymmetry at ATLAS and CMS.

One of the hottest topics concerned recent measurements by LHCb of the angular distribution of the decay products of B⁰ → K^*0μμ transitions that have revealed tension with the prediction of the Standard Model (LHCb collaboration 2013). The observable known as P₅´ – shown in the figure as a function of the dimuon invariant-mass squared (q²) – is particularly interesting. This parameter is sensitive to the modulation of the angular distribution that depends on the interaction between different operators contributing to the decay. It is therefore sensitive to the effects of physics beyond the Standard Model. Additionally, P₅´ is insensitive at leading order to theoretical uncertainties related to the K^* hadronic form factor. However, corrections from higher-order terms introduce residual uncertainty. The tension in the data can be reduced if the uncertainty is allowed to be larger than original estimates – an observation that sparked a debate about the best estimate of the size of the so-called “power corrections”.

Several key points emerged from the discussion. On the theory side, further studies can help to understand the uncertainties in the form factor. Experimentally, improved analyses with the full LHCb data sample of 3 fb^–1 are keenly anticipated: with this large data sample and exploiting the power of the LHCb particle identification system, it might be possible for the first time to perform a full angular analysis that also separates the subtle Kπ S-wave component from the K^* signal. Moreover, continuing discussions between theorists and experimenters will be needed to understand which of several different approaches to control the uncertainties is the most sensitive to physics beyond the Standard Model.

For more about the workshop, see http://indico.cern.ch/conferenceDisplay.py?ovw=True&confId=255380.

First results from LUX on dark matter

CCnew9_10_13 — The titanium cryostat that encloses the LUX dark-matter detector, seen here suspended in its water tank.
Image credit: Matt Kapust/Sandford Underground Research Facility.

The collaboration that built and runs the Large Underground Xenon (LUX) experiment, operating in the Sanford Underground Research Laboratory, has released its first results in the search for weakly interacting massive particles (WIMPs) – a favoured candidate for dark matter.

The LUX detector holds 370 kg of liquid xenon, with 250 kg actively monitored in a dual-phase (liquid–gas) time-projection chamber measuring 47 cm in diameter and 48 cm in height (cathode-to-gate). If a WIMP strikes a xenon atom it recoils from other xenon atoms and emits photons and electrons. The electrons are drawn upwards by an electrical field and interact with a thin layer of xenon gas at the top of the tank, releasing more photons. Light detectors in the top and bottom of the tank can detect single photons and so the two photon signals – one at the interaction point, the other at the top of the tank – can be pinpointed to within a few millimetres. The energy of the interaction can be measured precisely from the brightness of the signals.

The detector was filled with liquid xenon in February and the first results, for data taken during April to August, represent the analysis of 85.3 live days of data with a fiducial volume of 118 kg. The data are consistent with a background-only hypothesis, allowing 90% confidence limits to be set on spin-independent WIMP–nucleon elastic scattering with a minimum upper limit on the cross-section of 7.6 ×10^–46 cm² at a WIMP mass of 33 GeV/c². The data are in strong disagreement with low-mass WIMP signal interpretations of the results from several recent direct-detection experiments.

Neutrinos head off again to Minnesota

When completed, the NOvA far detector will comprise 28 detector blocks, each measuring about 15 m tall, 15 m wide and nearly 2 m deep.
Image credit: Fermilab.

In August, after a 16-month shutdown, Fermilab resumed operation of its Neutrinos at the Main Injector (NuMI) beamline and sent the first muon neutrinos to three neutrino experiments: MINERvA, MINOS+ and the new NOvA experiment. Numerous upgrades to the Fermilab accelerator complex have laid the groundwork for increasing the beam power of the NuMI beamline from about 350 kW to 700 kW. In addition, Fermilab has changed the NuMI horn and target configurations to deliver a higher-energy neutrino beam compared with pre-shutdown operation.

The NOvA experiment – still under construction – will study the properties of neutrinos, especially the elusive transition of muon neutrinos into electron neutrinos. The results will help to answer questions about the neutrino-mass hierarchy, neutrino oscillations and the role that neutrinos might have played in the evolution of the universe. The construction of the NOvA near and far detectors, both located 14 milliradians off the NuMI beam axis, is advancing quickly.

The near detector – located 100 m underground in a new cavern that has been excavated at Fermilab – has more than a quarter of its structure in place. Meanwhile, 810 km away in northern Minnesota, technicians have installed more than three quarters of the plastic structure that is the skeleton of the huge, 14,000 tonne far detector. More than 70% of the far detector’s plastic modules have been filled with 5.7 million litres of liquid scintillator and the first modules are recording data. The first part of the near detector will turn on before the end of the year.

The MINOS+ experiment uses the existing MINOS near and far detectors and takes advantage of the fact that the post-shutdown NuMI neutrino beam differs from earlier operation. The new beam, which is optimized for the NOvA experiment, yields higher-energy neutrinos at the location of the MINOS detector and should not show measurable oscillations. This means that MINOS+ can look for surprises. New types of neutrino interactions could deform the spectrum at the far detector’s distance of 735 km and the observation of additional neutrinos would indicate new physics. The experiment can even search for extra dimensions.

MINERvA – located in front of the MINOS near detector – is a dedicated neutrino-interaction experiment designed to study a range of nuclei. These measurements will not only improve understanding of the nucleus but will also be important inputs to neutrino-oscillation experiments. The MINERvA detector has several targets including helium, carbon, scintillator, water, steel and lead, followed by precise tracking and calorimetry. Previously, MINERvA took data in a beam around 3 GeV, where quasi-elastic, resonance and deep-inelastic scattering processes contribute roughly equally to the event rates. With the new, higher-energy neutrino beam, the event rate is much higher and the events are dominated by deep-inelastic scattering. While MINERvA will study all processes at higher energy, the huge increase in deep-inelastic scattering events in particular will allow precise measurements of the nuclear structure-functions.

CLOUD shines new light on aerosol formation in atmosphere

CCnew3_09_13 — Fig. 1. Atmospheric, experimental and theoretical nucleation rates as a function of sulphuric acid (H₂SO₄) concentration, where 10⁷ cm^–3 is equivalent to a mixing ratio of 0.4 parts per trillion by volume (pptv). The mixing ratios of ammonia (NH₃) or dimethylamine (DMA) in the CLOUD experiment are indicated by the colour scale. Theoretical expectations (the Helsinki ACDC model) are indicated for H₂SO₄ nucleation with 10 pptv NH₃ (dashed blue line), and with 10 pptv DMA plus 10 pptv NH₃ (dashed red line, assuming 0.5 neutral–neutral sticking probability). The bands represent the theoretical uncertainty.

The CLOUD experiment at CERN, which is studying whether cosmic rays have a climatically significant effect on aerosols and clouds, is also tackling one of the most challenging and long-standing problems in atmospheric science – understanding how new aerosol particles are formed in the atmosphere and the effect that these particles have on climate. In a major step forward, the CLOUD collaboration has made the first measurements – either in the laboratory or in the atmosphere – of the formation rates of atmospheric aerosol particles that have been identified with clusters of precisely known molecular composition.

Atmospheric aerosol particles cool the climate by reflecting sunlight and by forming smaller but more numerous cloud droplets, which makes clouds brighter and extends their lifetimes. By current estimates, about half of all cloud drops are formed on aerosol particles that were “nucleated” – that is, produced from the clustering of tiny concentrations of atmospheric molecules rather than being emitted directly into the atmosphere, as happens with sea-spray particles. Nucleation is therefore likely to be a key process in climate regulation. However, the physical mechanisms of nucleation are not understood, nor is it known which molecules participate in nucleation and whether they derive from natural sources or are emitted by human activities.

CLOUD has studied the formation of new atmospheric particles in a specially designed chamber under extremely well controlled laboratory conditions of temperature, humidity and concentrations of nucleating vapours. This chamber is the first to reach the challenging technical requirements on ultra-low levels of contaminants that are necessary to carry out these experiments in the laboratory. Using state-of-the-art instruments that are connected to the chamber, the experiment can measure extremely low concentrations of atmospheric vapours. It can also study the precise molecular make-up and growth of newly formed molecular clusters from single molecules up to stable aerosol particles.

This has enabled CLOUD to measure the formation of particles that are caused by sulphuric acid and tiny concentrations of dimethylamine near the level of 1 molecule per trillon (10¹² ) air molecules. The measurements, made at 278 K and 38% relative humidity, involved different combinations of sulphuric acid (H₂SO₄) and water (H₂O), with ammonia (NH₃) or dimethylamine (DMA). The figure shows the results from CLOUD together with various atmospheric measurements and theoretical expectations based on quantum chemical calculations of cluster binding energies. The results indicate that amines at typical atmospheric concentrations of only a few parts per trillion by volume combine with sulphuric acid to form highly stable aerosol particles at rates that are similar to those observed in the lower atmosphere. The figure also shows that these highly detailed measurements allow a fundamental understanding of the nucleation process at the molecular level because they can be reproduced by the theoretical calculations of molecular clustering.

Amines are atmospheric vapours that are closely related to ammonia. Derived largely from anthropogenic activities – mainly animal husbandry – they are also emitted by the oceans, the soil and from biomass burning. The results from CLOUD suggest that natural and anthropogenic sources of amines could influence climate. CLOUD has also found that ionization by cosmic rays has only a small effect on the formation rate of amine–sulphuric-acid particles, suggesting that cosmic rays are unimportant for the generation of these particular aerosol particles in the atmosphere.

• The CLOUD collaboration consists of the California Institute of Technology, Carnegie Mellon University, CERN, Finnish Meteorological Institute, Helsinki Institute of Physics, Johann Wolfgang Goethe University Frankfurt, Karlsruhe Institute of Technology, Lebedev Physical Institute, Leibniz Institute for Tropospheric Research, Paul Scherrer Institute, University of Beira Interior, University of Eastern Finland, University of Helsinki, University of Innsbruck, University of Leeds, University of Lisbon, University of Manchester, University of Stockholm and University of Vienna.

ICRC 2013: from Earth to the Galaxy and beyond

At the traditional dinner party they danced to samba music while holding caipirinhas. During the day, the more than 700 physicists who attended the 33rd Cosmic Ray Conference (ICRC 2013) in Rio de Janeiro listened carefully during the 400 scheduled talks in a variety of plenary and parallel sessions on 2–9 July. Instead of caipirinhas, they held laptops and notepads as they focused on the important findings and data presented at the first ICRC to be held in South America.

Organized under the auspices of the International Union of Pure and Applied Physics (IUPAP) and its C4 Commission on Cosmic Rays, ICRC 2013 was hosted by the Centro Brasileiro de Pesquisas Físicas – an institute of the ministry of science, technology and innovation – the Federal University of Rio de Janeiro and the Brazilian Physical Society. It was sponsored by the National Council for Scientific and Technological Development (CNPq), the Coordination for Improvement of Higher Education Personnel (CAPES) and the Research Support Foundation of the state of Rio de Janeiro (FAPERJ).

The location in South America was not the only “first”. The organization of the 33rd ICRC had a scientific programme committee for the first time, consisting of leading experts in solar and heliospheric physics, cosmic-ray physics, gamma-ray astronomy, neutrino astronomy and dark-matter physics. Also for the first time, ICRC included research on dark matter as a main branch of the programme. For this reason, ICRC 2013 adopted the subtitle “The Astroparticle Physics Conference”. This might also become the C4 Commission’s new name, as Johannes Knapp, the commission’s chair, announced during the closing session. The commission organized a poll during the nine days of the conference in which all registered participants could vote on changing the name from “Cosmic Rays” to “Astroparticle Physics”. The majority voted for the change and the commission is now consulting IUPAP on the matter. To maintain tradition, the conference’s main title – ICRC – will remain unchanged.

In neutrino research, the IceCube experiment has some thrilling results

ICRC 2013 was certainly a success. During the plenary session on results from the Pierre Auger Observatory, Antoine Letessier-Selvon of CNRS and Université Pierre et Marie Curie presented evidence of what could be called “the muon problem”. It concerns the conflict between the prediction from Monte Carlo simulations of the number of muons in the surface Cherenkov detectors and the value extracted from the experimental data, which is about a factor of 1.5 higher. Letessier-Selvon argued that a change in composition at higher energies is not sufficient to explain the discrepancy.

The ground-based gamma-ray experiments HESS, MAGIC and VERITAS have added new gamma-ray sources – both in the Galaxy and beyond it – to the catalogue, which now totals about 150 sources. Teams at the northern-hemisphere observatories reported flaring of the blazar Mkn 421 in April this year, while MAGIC registered another flare in November 2012, in IC 310 – an extra-galactic source that it had previously discovered. Miguel Mostafa of Colorado State University presented the results of the “first light” – in fact, gamma rays – in the High Altitude Water Cherenkov Observatory installed at an altitude of 4150 m in Mexico. It is designed to detect ultra-high-energy gamma rays and is sensitive to energies above 300 GeV. With approximately only one third of the detector in operation, the collaboration was still able to present their view of the Mkn 421 flare of April.

Data from Voyager 1 show a clear “wall” characterizing the heliosheath – and indicating that after travelling for 36 years, the spacecraft is at the edge of the solar system.

In neutrino research, the IceCube experiment has some thrilling results. Spencer Klein of the Lawrence Berkeley National Laboratory and the University of California, Berkeley presented the 28 events that were detected with energies above 50 TeV, which include the previously revealed events above 1 PeV (CERN Courier July/August 2013 p5). Klein also spoke of the observation of another very-high-energy event in the ongoing analysis of 2012 data – but its characteristics remain “top secret”.

Another highlight of ICRC 2013 was the presentations by Nobel laureate Sam Ting and the Alpha Magnetic Spectrometer (AMS) collaboration of the first results from two years of AMS-02 operation on the International Space Station (ISS). The main goal is to perform a high-precision, large-statistics and long-duration study of cosmic nuclei, elementary charged particles and gamma rays. At the conference the collaboration presented high-precision measurements of the fluxes, ratios and anisotropies of electrons and positrons, as well as first results on proton and helium fluxes (CERN Courier October 2013 p22).

Moving further out in space, Ed Stone from Caltech presented the saga of the Voyager 1 spacecraft, launched in 1977, which is now at the edge of the solar system. The data clearly show a “wall” characterizing the heliosheath. It is astonishing that Voyager 1 is still collecting data after all these years – with an on-board computer of the 1970s and a power source that is still very much alive having passed through the harsh environment of Jupiter and Saturn. Stone was seen not only by the conference participants but also by the 40 million viewers who watched an interview with him during a popular programme on Brazilian TV.

The parallel sessions included presentations on a plethora of new projects ranging from next-generation imaging air-Cherenkov telescopes, represented by the Cherenkov Telescope Array, to the Extreme Universe Space Observatory onboard the Japanese Experiment Module (JEM-EUSO). To be installed on the ISS, JEM-EUSO is designed to measure ultra-high-energy cosmic rays through the fluorescence of the extensive air showers that they produce – an expression of optimism in the future of the field.

The 34th ICRC meeting will be held at The Hague, the Netherlands, in July 2015 and will be followed two years later by the 35th meeting in Busan, Korea. Although there will be no samba or caipirinhas, there will surely be the same level of results and commitment from astroparticle physicists worldwide.

Awards for astroparticle physics

Besides the announcements of important findings and experiments, the conference was the occasion for the traditional awards for outstanding contributions in astroparticle physics. Six people were honoured, from more than 30 nominations.

Aya Ishihara, from Shiba University, received an IUPAP Young Scientist Award for her outstanding work on the search for ultra-high-energy neutrinos and the detection of the two neutrino events at >1 PeV with the IceCube detector. A second Young Scientist Award went to Daniel Mazin, from IFAE Barcelona, for his outstanding work on gamma-ray blazars and extragalactic background light, using the MAGIC Cherenkov telescopes.

Rolf Bühler, from DESY Zeuthen, received the Shakti Duggal Award for his outstanding work on the variability of the emission from the Crab nebula and extragalactic background light, using the HESS and Fermi telescopes. The O’Ceallaigh Medal was awarded to Edward Stone, from Caltech, for his contributions to cosmic-ray physics and speciﬁcally his leading role in the Voyager mission.

Motohiko Nagano, from ICRR Tokyo and Fukui University, received the Yodh Prize for his pioneering leadership in the experimental study of the highest-energy cosmic rays. Sunil Gupta, from TIFR Mumbai, was awarded the Homi Bhabha Medal and Prize for his contributions to non-thermal astrophysics and his leading role in the development of gamma-ray astronomy.

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Breakthrough of the Year

Awards for astroparticle physics