Physics Archives – Page 65 of 145

Neutrons in full flight at CERN’s n_TOF facility

Fig. 1. Neutron-induced reaction cross-sections for a typical heavy nucleus as a function of the neutron kinetic energy, together with characteristic neutron fluxes present in stellar environments or in technological applications.
Image credit: n_TOF collaboration.

Accurate knowledge of the interaction probability of neutrons with nuclei is a key parameter in many fields of research. At CERN, pulsed bunches from the Proton Synchrotron (PS) hit the spallation target and produce beams of neutrons with unique characteristics. This allows scientists to perform high-resolution measurements, particularly on radioactive samples.

The story of the n_TOF facility goes back to 1998, when Carlo Rubbia and colleagues proposed the idea of building a neutron facility to measure neutron-reaction data needed for the development of an energy amplifier. The facility eventually became fully operational in 2001, with a scientific programme covering neutron-induced reactions relevant for nuclear astrophysics, nuclear technology and basic nuclear science. During the first major upgrade of the facility in 2009, the old spallation target was removed and replaced by a new target with an optimised design, which included a decoupled cooling and moderation circuit that allowed the use of borated water to reduce the background due to in-beam hydrogen-capture γ rays. A second improvement was the construction of a long-awaited “class-A” workplace, which made it possible to use unsealed radioactive isotopes in the first experimental area (EAR1) at 200 m from the spallation target. In 2014, n_TOF was completed with the construction of a second, vertical beamline and a new experimental area – EAR2.

Fig. 2. The neutron flux at EAR1 and EAR2 of the n_TOF facility.
Image credit: n_TOF collaboration.

One of the most striking features of neutron–nucleus interactions is the resonance structures observed in the reaction cross-sections at low-incident neutron energies. Because the electrically neutral neutron has no Coulomb barrier to overcome, and has a negligible interaction with the electrons in matter, it can directly penetrate and interact with the atomic nucleus, even at very low kinetic energies in the order of electron-volts. The cross-sections can show variations of several orders of magnitude on an energy scale of only a few eV. The origin of these resonances is related to the excitation of nuclear states in the compound nuclear system formed by the neutron and the target nucleus, at excitation energies lying above the neutron binding energy of typically several MeV. In figure 1, the main cross-sections for a typical heavy nucleus are shown as a function of energy. The position and extent of the resonance structures depend on the nucleus. Also shown on the same energy scale are Maxwellian neutron energy distributions for fully moderated neutrons by water at room temperature, for fission neutrons, and for typical neutron spectra in the region from 5 to 100 keV, corresponding to the temperatures in stellar environments of importance for nucleosynthesis.

The wide neutron energy range is one of the key features of the n_TOF facility.

In nuclear astrophysics, an intriguing topic is understanding the formation of nuclei present in the universe and the origin of chemical elements. Hydrogen and smaller amounts of He and Li were created in the early universe by primordial nucleosynthesis. Nuclear reactions in stars are at the origin of nearly all other nuclei, and most nuclei heavier than iron are produced by neutron capture in stellar nucleosynthesis. Neutron-induced reaction cross-sections also reveal the nuclear-level structure in the vicinity of the neutron binding energy of nuclei. Insight into the properties of these levels brings crucial input to nuclear-level density models. Finally, neutron-induced reaction cross-sections are a key ingredient in applications of nuclear technology, including future developments in medical applications and the transmutation of nuclear waste, accelerator-driven systems and nuclear-fuel-cycle investigations.

The wide neutron energy range is one of the key features of the n_TOF facility. The kinetic energy of the particles is directly related to their time-of-flight: the start time is given by the impact of the proton beam on the spallation target and the arrival time is measured in the EAR1 and EAR2 experimental areas. The high neutron energies are directly related to the 20 GeV/c proton-induced spallation reactions in the lead target. Neutrons are subsequently partially moderated to cover the full energy range. Energies as low as about 10 MeV corresponding to long times of flight can be exploited and measured at n_TOF because of its pulsed bunches spaced by multiples of 1.2 s, sent by the PS. This allows long times of flight to be measured without any overlap into the next neutron cycle.

Higher flux

Fig. 3. The horizontal neutron beamline in EAR1 (left) and the vertical neutron beamline in the newly constructed EAR2.
Image credit: n_TOF collaboration.

Another unique characteristic of n_TOF is the very high number of neutrons per proton burst, also called instantaneous neutron flux. In the case of research with radioactive samples irradiated with the neutron beam, the high flux results in a very favourable ratio between the number of signals due to neutron-induced reactions and those due to radioactive decay events, which contribute to the background. While the long flight path of EAR1 (200 m from the spallation target) results in a very high kinetic-energy resolution, the short flight path of EAR2 (20 m from the target) has a neutron flux that is higher than that of EAR1 by a factor of about 25. The neutron fluxes in EAR1 and EAR2 are shown in figure 2. The higher flux opens the possibility for measurements on nuclei with very low mass or low reaction cross-sections within a reasonable time. The shorter flight distance of about a factor 10 also ensures that the entire neutron energy region is measured in a 10 times shorter interval. For measurements of neutron-induced cross-sections on radioactive nuclei, this means 10 times less acquired detector signals due to radioactivity. Therefore the combination of the higher flux and the shorter time interval results in an increase of the signal-to-noise ratio of a factor 250 for radioactive samples. This characteristic of EAR2 was, for example, used in the first cross-section measurement in 2014, when the fission cross-section of the highly radioactive isotope ²⁴⁰Pu was successfully measured. An earlier attempt of this measurement in EAR1 was not conclusive. An example from 2015 is the measurement of the (n,α) cross-section of the also highly radioactive isotope ⁷Be, relevant for the cosmological Li problem in Big Bang nucleosynthesis.

The most important neutron-induced reactions that are measured at n_TOF are neutron-capture and neutron-fission reactions. Several detectors have been developed for this purpose. A 4π calorimeter consisting of 40 BaF₂ crystals has been in use for capture measurements since 2004. Several types of C₆D₆-based liquid-scintillator detectors are also used for measurements of capture γ rays. Different detectors have been developed for charged particles. For fission measurements, ionisation chambers, parallel-plate avalanche counters and the fission-fragment spectrometer STEFF have been operational. MicroMegas-based detectors have been used for fission and (n,α) measurements. Silicon detectors for measuring (n,α) and (n,p) reactions have been developed and used more recently, even for in-beam measurements.

The measurements at CERN’s neutron time-of-flight facility n_TOF, with its unique features, contribute substantially to our knowledge of neutron-induced reactions. This goes together with cutting-edge developments in detector technology and analysis techniques, the design of challenging experiments, and training a new generation of physicists working in neutron physics. This work has been actively supported since the beginning of n_TOF by the European Framework Programmes. A future development currently being studied is a possible upgrade of the spallation target, to optimise the characteristics of the neutron beam in EAR2. The n_TOF collaboration, consisting of about 150 researchers from 40 institutes, looks forward to another year of experiments from its scientific programme in both EAR1 and EAR2, continuing its 15 year history of measuring high-quality neutron-induced reaction data.

Further Reading
CERN-Proceedings-2015-001, p32

LUNA observes a rare nuclear reaction that occurs in giant red stars

Members of the LUNA collaboration pictured next to the facility.
Image credit: INFN.

In December, the Laboratory for Underground Nuclear Astrophysics (LUNA) experiment reported the first direct observation of sodium production in giant red stars, one of the nuclear reactions that are fundamental to the formation of the elements that make up the universe.

LUNA is a compact linear accelerator for light ions (maximum energy 400 keV). A unique facility, it is installed in a deep-underground laboratory and shielded from cosmic rays. The experiment aims to study the nuclear reactions that take place inside stars, where elements that make up matter are formed and then driven out by gigantic explosions and scattered as cosmic dust.

For the first time, LUNA has observed three low-energy resonances in the neon-sodium cycle, the ²²Ne(p,γ)²³Na reaction, responsible for sodium production in red giants and energy generation. LUNA recreates the energy ranges of nuclear reactions and, with its accelerator, goes back in time to one hundred million years after the Big Bang, when the first stars formed and the processes that gave rise to the huge variety of elements in the universe started.

This result is an important piece in the puzzle of the origin of the elements in the universe, which LUNA has been studying for 25 years. Stars assemble atoms through a complex system of nuclear reactions. A very small fraction of these reactions have been studied at the energies existing inside of the stars, and a large part of those few cases have been observed using LUNA.

A high-purity germanium detector with relative efficiency up to 130% was used for this particular experiment, together with a windowless gas target filled with enriched gas. The rock surrounding the underground facility at the Gran Sasso National Laboratory and additional passive shielding protected the experiment from cosmic rays and ambient radiation, making the direct observation of such a rare process possible.

Is there a ‘ninth planet’ after all?

Diagram of the orbit around the Sun of the predicted ninth planet (orange), maintaining the orbits of the six most distant Kuiper-belt objects on the opposite side (pink), and also forcing five known objects into elongated perpendicular orbits (blue). The diagram was created using WorldWide Telescope.
Image credit: Caltech/R. Hurt (IPAC)

Pluto was considered to be the ninth planet of the solar system, until it was relegated to a “dwarf planet” by the International Astronomical Union (IAU) in 2006. It was judged to be too small among many other trans-Neptunian objects to be considered a real planet. Almost 10 years later, two astronomers have now found indications of the presence of a very distant heavy planet orbiting the Sun. While it is still to be detected, it is already causing a great deal of excitement in the scientific community and beyond.

Pluto was discovered in 1930 by a young American astronomer, Clyde Tombaugh, who tediously looked at innumerable photographic plates to detect an elusive planet moving relative to background stars. With the progressive discovery – since the 1990s – of hundreds of objects orbiting beyond Neptune, Pluto is no longer alone in the outer solar system. It even lost its status of the heaviest trans-Neptunian object with the discovery of Eris in 2003. This forced the IAU to rethink the definition of a planet and led to the exclusion of Pluto from the strict circle of eight planets.

Eris is not the only massive trans-Neptunian object found by Mike Brown, an astronomer of the California Institute of Technology (Caltech), US, and colleagues. There are also Quaoar (2002), Sedna (2003), Haumea (2004) and Makemake (2005), all only slightly smaller than Pluto and Eris. Despite these discoveries, almost nobody during recent years would have thought that there could still be a much bigger real planet in the outskirts of our solar system. But this is what Mike Brown and one of his colleagues, the theorist Konstantin Batygin, now propose.

The evidence comes from an unexpected clustering of perihelion positions and orbital planes of a group of objects just outside of the orbit of Neptune

The two astronomers deduced the existence of a ninth planet through mathematical modelling and computer simulations, but have not yet observed the object directly. The evidence comes from an unexpected clustering of perihelion positions and orbital planes of a group of objects just outside of the orbit of Neptune, in the so-called Kuiper belt. All six objects with the most elongated orbits – with semi-major axes greater than 250 AU – share similar perihelion positions and pole orientations. The combined statistical significance of this clustering is 3.8σ, assuming that Sedna and the five other peculiar planetoids have the same observational bias as other known Kuiper-belt objects.

Batygin and Brown then show that a planet with more than about 10 times the mass of the Earth in a distant eccentric orbit anti-aligned with the six objects would maintain the peculiar configuration of their orbits. This possible ninth planet would rotate around the Sun about 20 times further out than Neptune, therefore completing one full orbit only approximately once every 10,000 years. Batygin’s simulations of the effect of this new planet further predict the existence of a population of small planetoids in orbits perpendicular to the plane of the main planets. When Brown realised that such peculiar objects exist and have indeed already been identified, he became convinced about the existence of Planet Nine.

Observers now know along which orbit they should look for Planet Nine. If it happens to be found, this would be a major discovery: the third planet to be discovered since ancient times after Uranus and Neptune and, as with the latter, it would have been first predicted to exist via calculations.

XENON opens a new era for dark-matter searches

Fig. 1. Members of the XENON1T collaboration at LNGS during the assembly of the TPC.
Image credit: XENON Collaboration.

In recent years, evidence for the existence of dark matter from astrophysical observations has become indisputable. Although the nature of dark matter remains unknown, many theoretically motivated candidates have been proposed. Among them, the most popular ones are Weakly Interacting Massive Particles (WIMPs) with predicted masses in the range from a few GeV/c² to TeV/c² and with interaction strengths roughly on the weak scale.

WIMPs are being searched for using three complementary techniques: indirectly, by detecting the secondary products of WIMP annihilation or decay in celestial bodies; by producing WIMPs at colliders, foremost the LHC; and by direct detection, by measuring the energy of recoiling nuclei produced by collisions with WIMPs in low-background detectors.

On 11 November 2015, the most sensitive detector for the direct detection of WIMPs, XENON1T, was inaugurated at the Italian Laboratori Nazionali del Gran Sasso (LNGS) – the largest underground laboratory in the world. XENON1T, led by Elena Aprile of Columbia University, was built and is operated by a collaboration of 21 research groups from France, Germany, Italy, Israel, the Netherlands, Portugal, Sweden, Switzerland, the United Arabic Emirates and the US. In total, about 130 physicists are involved.

XENON1T is the current culmination of the XENON programme of dark matter direct-detection experiments. Starting with the 25 kg XENON10 detector about 10 years ago, the second phase of the experiment, XENON100 (CERN Courier October 2013 p13) with 161 kg, has been tremendously successful: in the summer of 2012, the XENON collaboration published results from a search for spin-independent WIMP–nucleon interactions that provided the most stringent constraints on WIMP dark matter, until superseded by the LUX experiment (CERN Courier December 2013 p8) with a larger target.

Fig. 2. Predicted sensitivity of XENON1T including all known backgrounds. Results of other experiments are also shown.
Image credit: XENON Collaboration.

XENON100 has since then also provided a series of other important results, such as constraints on the spin-dependent WIMP nucleon cross-section, constraints on solar axions and galactic axion-like particles and, more recently, searches for annual rate modulations, which exclude WIMP–electron scattering that could have provided a dark-matter explanation of the signal observed by DAMA/LIBRA (CERN Courier November 2015 p10).

Low background is key

The new XENON1T detector has an estimated sensitivity that is a factor of 100 better than XENON100. This will be reached after about two years of data taking. With only one week of data-taking, XENON1T will be able to reach the current LUX limit, opening up a new phase in the search for dark matter in early 2016.

The XENON detectors are dual-phase time-projection chambers (TPCs) filled with liquid xenon (LXe) as the target material. Interactions of particles in the liquefied xenon give rise to prompt scintillation light and ionisation. The ionised electrons are drifted in a strong electric field and extracted into the gas above the liquid where a secondary scintillation signal is produced. Both scintillation signals are read out by arrays of photomultiplier tubes (PMTs) placed above and below the target volume. The position of the interaction vertex can be reconstructed in 3D by using the hit pattern on the upper PMT array and the time delay between the prompt and secondary scintillation signal. The position reconstruction facilitates self-shielding by only selecting events that interact with the inner “fiducial” volume of the detector. Because of their small cross-section, WIMPs will interact only once in the detector, so the background (e.g. from neutrons) can be reduced further by selecting single-scatter interactions. Beta and gamma backgrounds are reduced by selecting events with a ratio of secondary-to-prompt signal that is typical for nuclear recoils.

The XENON1T detector is filled with about 3.5 tonnes of liquid xenon in total. Its TPC – 1 m high and 1 m in diameter in a cylindrical shape, laterally defined by highly reflective Teflon – is the largest liquid-xenon TPC ever built. Specially designed copper field-shaping electrodes ensure the uniformity of the drift field for the desired field strength of 1 kV/cm. The TPC’s active volume contains 2 tonnes of LXe viewed by two arrays of 3 inch PMTs – 121 at the bottom immersed in LXe and 127 on the top in the gaseous phase. The xenon gas is liquefied and kept at a temperature of about –95 °C by a system of pulse-tube refrigerators. The xenon gas is stored and can be recovered in liquid phase in a custom-designed stainless-steel sphere that can hold up to 7.6 tonnes of xenon in high-purity conditions. Figure 3 shows the XENON1T detector and service building situated in Hall B at LNGS. Figure 1 shows XENON collaborators active in assembling the TPC in a clean room above ground at LNGS.

Fig. 3. The XENON1T detector. Left: The water Cherenkov veto with cladding, showing the cryostat and its support structure inside. Right: the service building with cryogenics, purification, storage and recovery systems, as well data acquisition and slow control.
Image credit: XENON Collaboration.

The expected WIMP–nucleon interaction rate is less than 10 events in 1 tonne of xenon per year. Background rejection is therefore the key to success for direct-detection experiments. Externally induced backgrounds can be minimised by exploiting the self-shielding capabilities. In addition, the detector is surrounded by a cylindrical water vessel 10 m high and 9.6 m in diameter. It is equipped with PMTs to tag muons that could induce neutrons, with an efficiency of 99.9%.

For a detector the size of XENON1T, radioactive impurities in the detector materials and the xenon itself become the biggest challenge for background reduction. Extensive radiation-screening campaigns, using some of the world’s most sensitive germanium detectors, have been conducted, and high-purity PMTs have been specially developed by Hamamatsu in co-operation with the collaboration. Contamination of the xenon by radioactive radon (mainly ²²²Rn) and krypton (⁸⁵Kr), which dominate the target-intrinsic background, led to the development of cryogenic-distillation techniques to suppress the abundance of these isotopes to unprecedented low levels.

The best scenario

After about two years of data taking, XENON1T will be able to probe spin-independent WIMP–nucleon cross-sections of 1.6 × 10^–47 cm² (at a WIMP mass of 50 GeV/c²), see figure 2. In popular scenarios involving supersymmetry, XENON1T will either discover WIMPs or will exclude most of the theoretically relevant parameter space. Following the inauguration, the first physics run is envisaged to start early this year.

Most of the infrastructure, for example the outer cryostat, the Cherenkov muon veto, the xenon cryogenics, the purification and storage systems and the data-acquisition system, has been dimensioned for a larger experiment, named XENONnT, which is designed to contain more than 7 tonnes of LXe. A new TPC, about 40% larger in diameter and height and equipped with about 400 PMTs, will replace the XENON1T TPC. The goal for XENONnT is to achieve another order of magnitude improvement in sensitivity within a few years of data taking. XENONnT is scheduled to start data taking in 2018.

• For further details, see www.xenon1t.org/.

Zeroing in on Higgs boson properties

As Run 2 at the LHC gains momentum, a combined analysis of data sets from Run 1 by the ATLAS and CMS collaborations has provided the sharpest picture yet on the Higgs boson properties (ATLAS 2015, CMS 2015).Three years after the announcement in July 2012 of the discovery of a new boson, the two collaborations are closing the books on measurements of Higgs properties by performing a combined Run 1 analysis, which includes data collected in 2011 and 2012 at centre-of-mass energies of 7 and 8 TeV, respectively. This analysis follows hot on the heels of the combined measurement of the Higgs boson mass, m_H = 125.09±0.24 GeV, published in May by ATLAS and CMS (ATLAS and CMS 2015).

The new results are the culmination of one and a half years of joint work by the ATLAS and CMS collaborators involved in the activities of the LHC Higgs Combination Group. For this combined analysis, some of the original measurements dating back to 2013 were updated to account for the latest predictions from the Standard Model. A comprehensive review of all of the experimental systematic and theoretical uncertainties was also conducted to account properly for correlations. The analysis presented technical challenges, because the fits involve more than 4200 parameters that represent systematic uncertainties. The improvements that were made to overcome these challenges will now make their way into data-analysis tools, such as ROOT, that are widely used by the high-energy particle-physics community.

CCnew3_08_15 — Results of the analyses by the individual experiments (coloured) and of the combined analysis (black). The fit to the data is performed using coupling modification factors κ_V and κ_F that scale, respectively, the electroweak symmetry-breaking-related coupling to the weak bosons and the Yukawa-related coupling to all fermions, with respect to the Standard Model prediction. The black star at (κ_V, κ_F) = (1, 1) denotes the Standard Model expectation. In this fit, the total width is assumed to scale with the κ values and no allowance is made for invisible or undetected decay modes.

The results of the combination present a picture that is consistent with the individual results. The combined signal yield relative to the Standard Model expectation is measured to be 1.09±0.11, and the combination of the two experiments leads to an observation of the H → τ⁺τ^– decay at the level of about 5.5σ – the first observation of the direct decay of the Higgs boson to fermions. Thanks to the combined power of the data sets from ATLAS and CMS, the analysis yields unprecedented measurements of the properties of the Higgs boson, with a precision that enables the search for physics beyond the Standard Model in possible deviations of the measurements from the model’s predictions. The figure shows clearly the increased precision obtained when combining the ATLAS and CMS analyses.

The combined analysis is performed for many benchmark models that the LHC Higgs Cross-Section Working Group proposed, so as to be able to explore the various different effects of physics models that go beyond the Standard Model. As Run 2 gains momentum, the two collaborations are looking forward to reaping the benefits of the increase in centre-of-mass energy to 13 TeV, which will make some of the most interesting processes, such as the production of Higgs bosons in association with top quarks, more accessible than ever. However, even with the first results from Run 2, this set of combined results from 7 and 8 TeV collisions in Run 1 will continue to provide the sharpest picture of the Higgs boson’s properties for some time to come.

The most precise picture of the proton

Fig. 1. Diagrams of neutral-current (top) and charged-current (bottom) deep-inelastic electron–proton scattering processes.
Image credit: DESY.

After 15 years of measurement and another eight years of scrutinizing and calculations, the H1 and ZEUS collaborations have published the most precise results to date about the innermost structure and behaviour of the proton. The two collaborations, which took data at DESY’s electron–proton collider, HERA, from 1992 to 2007, have combined nearly 3000 measurements of inclusive deep-inelastic cross-sections (H1, ZEUS 2015). With its completion, the paper secures the legacy of the HERA data.

Within the framework of perturbative QCD, the proton is described in terms of parton-density functions, which provide the probability of scattering from a parton, either a gluon or a quark. The H1 and ZEUS collaborations have also produced the first QCD analysis of the data, encompassed in the HERAPDF2.0 sets of parton-distribution functions (PDFs), which form a significant part of the paper. The combined data presented in the new publication will be the basis of all analyses of the structure of the proton for years to come.

As figure 1 depicts, in deep-inelastic scattering, a boson – γ, Z⁰ or W^± – acts as a probe of the structure of the proton by interacting with its constituents, through neutral-current (γ, Z⁰) or charged-current (W^±) reactions. Of course, this picture is simplified: the proton is a dynamic structure of quarks and gluons, but by measuring deep-inelastic scattering over a wide kinematic range, this internal structure can be mapped precisely. The variables used to do this are the squared four-momentum, Q², of the exchanged boson, and Bjorken x, x_Bj, the fraction of the proton’s momentum carried by the struck quark.

A wealth of data

The data, taken over the 15-year lifetime of the HERA accelerator, correspond to a total luminosity of about 1 fb^–1 of deep-inelastic electron–proton and positron–proton scattering. All of the data used were taken with an electron/positron beam energy of 27.5 GeV, with roughly equal amounts of data for electron–proton and positron–proton scattering being recorded. HERA initially operated with a proton-beam energy of 820 GeV, which was increased subsequently to 920 GeV; these data constitute the bulk of the combined measurements. Towards the end of HERA’s run, special data samples with a proton-beam energy of 575 GeV and 460 GeV were taken and are also included. The data were combined separately for the e⁺p and e^–p runs and for the different centre-of-mass energies. Overall, 41 separate data sets were used in the combination, spanning 0.045 < Q² < 50,000 GeV² and 6 × 10^–7< x_Bj< 0.65, i.e. six orders of magnitude in each variable. The initial measurements consisted of 2937 published cross-sections in total, which were combined to produce 1307 final combined cross-section measurements. These results supersede the previous paper with combined measurements of deep-inelastic scattering cross-sections in which only data up to the year 2000 were combined (CERN Courier January/February 2008 p30).

Fig. 2. The newly combined HERA data, together with original data points from H1 and ZEUS from the two running periods, HERA I and HERA II, showing the cross-section for inclusive deep-inelastic scattering versus the photon virtuality, Q², at selected values of the parton-momentum fraction, x_Bj. These results are for e⁺p collisions; a similar sample exists for e^–p collisions. The close-up of a few points clearly reveals the power of the combination procedure.

The procedure for combining the data involved a careful treatment of the various uncertainties between all of the data sets. In particular, the correlations of the various sources were assessed, and those uncertainties deemed to be point-to-point correlated were accounted for as such in the averaging of the data based on a χ² minimization method. The resulting χ² is 1687 for 1620 degrees of freedom, demonstrating excellent compatibility of the multitude of data sets. Figure 2 illustrates the power of the data combination. It displays a selection of the data in bins of the photon virtuality, Q², and for fixed values of x_Bj, showing separately individual data sets from several different analyses. A combined data point can be the combination of up to eight individual measurements. The improvement in precision is striking, as is seen more clearly in the close-up on some of the points. An indication of the precision of the combined data is that the total uncertainties are close to 1% for the bulk region of 3 < Q² < 500 GeV².

As well as showing the precision of the data and power of the combination, the cross-section dependence for the different values of x_Bj demonstrates the dynamic structure of the proton in a striking way. For x_Bj = 0.08, the cross-section dependence is reasonably flat as a function of Q². This is known as Bjorken scaling, and is expected from the simple parton model in which inelastic electron–proton scattering is viewed as a sum of elastic electron–parton scattering, where the partons are free point-like objects. At lower values of x_Bj, the cross-section rises increasingly more steeply with increasing Q² and decreasing x_Bj. This effect is known as scaling violation, and is indicative of the density of gluons in the proton increasing.

Fig. 3. The proton-structure function extracted from measured cross-sections, shown at four selected values of Q². Predictions, with their associated uncertainties, using HERAPDF2.0 at next-to-leading order (NLO) in QCD, are also shown.

The increased density and rise of the cross-section can also be observed by considering the proton-structure function F₂ (which is closely related to the cross-section) plotted versus x_Bj at fixed Q², as in figure 3. The strong rise of F₂ with decreasing x_Bj was one of the most important discoveries at HERA. Previous experiments, which were with fixed targets, could not constrain this behaviour, because the data were at low values of Q² and high values of x_Bj. The figure also shows how the rise towards low x_Bj is steeper with increasing Q². At higher Q², the exchanged boson effectively probes smaller distances, and so can see more of the inner structure of the proton and hence resolves more and more gluons.

Parton distributions

The proton structure of quarks and gluons is often parameterized in terms of the PDFs, which correspond to the probability of finding a gluon or a quark of a given flavour with momentum fraction x in the proton, given the scale μ of the hard interaction. The behaviour of the PDFs with scale is predicted by QCD, but the absolute values need to be determined from fits to data. Using the HERA data, the PDFs can be extracted, while at the same time the evolution as a function of the scale is tested. This analysis is performed at leading order, next-to-leading order (NLO) and next-to-next-to-leading order, yielding the HERAPDF2.0 family of PDFs.

Figure 3 compares the predictions of the PDF analysis at NLO with the measurements of the structure functions. In general, the QCD predictions describe the data well, although this becomes poorer at low Q², indicating inadequacies in the theory used at these low scales. Such precise knowledge of the PDFs is also of highest importance for physics at the LHC at CERN, because the uncertainties stemming from the knowledge of the PDFs are increased for proton–proton collisions compared with deep-inelastic scattering.

Fig. 4. The HERA combined differential cross-section versus scale, Q², for neutral current (NC) and charged current (CC) processes for e⁺p and e^–p collisions. Predictions, with their associated uncertainties, using HERAPDF2.0 at NLO in QCD are also shown. Unification of the electromagnetic and weak forces can be seen at high values of Q² where the NC and CC cross-sections become similar in magnitude.

The QCD analysis can also be extended to include data from the production of charm quarks and jets at HERA. Charm production is measured again as a function of x_Bj and Q², however with the condition of detecting a charm meson in the final state. Jet production is measured in the Breit frame, where jets with non-zero transverse momentum are expected from hard QCD processes only. By including the charm and jet data, the analysis becomes particularly sensitive to the strong-coupling constant, α_s(M_Z), whereas without jet data the coupling constant is strongly correlated with the normalization of the gluon density. The combined analysis of inclusive data, charm data and jet data at NLO results in an experimentally very precise measurement of the strong-coupling constant, α_s(M_Z) = 0.1183±0.0009 (exp.), with significantly larger uncertainties of ^+0.0039_–0.0033 related to the model and theory.

It is also interesting to look at data from HERA on neutral-current (NC) and charged-current (CC) scattering that is differential in Q² but integrated over x_Bj, as shown in figure 4 both for e⁺p and e^–p. At small Q², the cross-sections for NC are much larger than for CC, whereas at large Q², in the order of the vector-boson mass squared, they become similar in size. This is a direct visualization of the electroweak unification: the CC process is mediated by weak forces, whereas photon exchange dominates the NC cross-section. Looking in more detail, the NC cross-sections for e⁺p and e^–p are almost identical at small Q² but start to diverge as Q² grows. This is owing to γ–Z⁰ interference, which has the opposite effect on the e⁺p and e^–p cross-sections. The CC cross-sections also differ between e⁺p and e^–p scattering, with two effects contributing: the helicity structure of the W^± exchange and the fact that CC e^–p scattering probes the u-valence quarks, whereas d-valence quarks are accessed in CC e⁺p.

In summary, the HERA collider experiments H1 and ZEUS have combined their precision data on deep-inelastic scattering, reaching a precision of almost 1% in the double-differential cross-section measurements. It is the largest coherent data set on proton structure, spanning six orders of magnitude in the kinematic variables x_Bj and Q². A QCD analysis of the HERA data alone results in a set of parton-density functions, HERAPDF2.0, without the need for data from other experiments. Also, using HERA jet and charm data, the strong-coupling constant is measured together with proton PDFs. QCD and electroweak effects are probed at high precision in the same data set, providing beautiful demonstrations of the validity of the Standard Model.

BASE compares charge-to-mass ratios of proton and antiproton to high precision

Fig.1. Schematic of the measurement (MT) and the reservoir (RT) Penning traps. Radio-frequency drives for particle manipulation are applied to the upstream correction electrode of the measurement trap. The upstream and downstream park electrodes are used for the particle shuttling scheme. The electron gun allows for electron cooling of antiprotons. The entire assembly is mounted in a cryogenic vacuum chamber.

The Japanese/German BASE collaboration at CERN’s Antiproton Decelerator (AD) has compared the charge-to-mass ratios of the antiproton and proton with a fractional precision of 69 parts in a trillion (ppt). This high-precision measurement was achieved by comparing the cyclotron frequencies of antiprotons and negatively charged hydrogen ions in a Penning trap. The result is consistent with charge–parity–time-reversal (CPT) invariance, which is one of the cornerstones of the Standard Model of particle physics, and constitutes the most precise test comparing baryons and antibaryons performed to date.

In their experiment, the BASE collaboration has profited from techniques pioneered in the 1990s by the TRAP collaboration at the Low Energy Antiproton Ring at CERN. The advanced cryogenic Penning-trap system used in BASE consists of four traps, two of which were used in this measurement – a measurement trap and a reservoir trap (figure 1). When the experiment receives a pulse of 5.3 MeV antiprotons from the AD, they strike the degrader structure, which is designed to slow them down, and release hydrogen. Negatively charged hydrogen ions (H^–) can form in the process, producing a composite cloud with the antiprotons that is shuttled to the reservoir trap. BASE has developed techniques to extract single antiprotons and negative hydrogen ions from this cloud whenever needed. Moreover, the reservoir has a lifetime of more than a year, making the BASE experiment almost independent from AD cycles.

Fig. 2. (a) All 6521 values of the p̅-to-H^– cyclotron frequency ratios measured in 35 days as a function of time. (b) The measured ratios projected onto a histogram.

Using this extraction technique, and taking the timing from the AD cycle, BASE prepares a single antiproton in the measurement trap, while an H^– ion is held in the downstream park electrode, as shown in figure 1. The cyclotron frequency of the antiproton is then measured in exactly 120 s, which corresponds to one AD cycle. The particles are subsequently exchanged by performing appropriate potential ramps, and the cyclotron frequency of the H^– ion is measured. Thus, a single comparison of the charge-to-mass ratios takes only 240 s. This is much faster than in previous experiments, enabling BASE to perform about 6500 ratio comparisons in 35 days of measurement time (figure 2). The result is a value of the ratio-comparison: (q/m)_p-/(q/m)_p – 1 = 1(69) × 10^–12, thus confirming CPT at the level of ppt.

The high sampling rate has also enabled the first high-resolution study of diurnal variations in a baryon/antibaryon comparison, which could be introduced by Lorentz-violating cosmic-background fields. The measurement sets constraints on such variations at the level of less than 720 ppt. In addition, by assuming that CPT invariance holds, the measurement can be interpreted as a test of the weak equivalence principle using baryonic antimatter. If matter respects weak equivalence while antimatter experiences an anomalous coupling to the gravitational field, this gravitational anomaly would contribute to a possible difference in the measured cyclotron frequencies. Thus, by following these assumptions, the result from BASE can be used to set a limit on the gravitational anomaly parameter, α_g: |α_g – 1| < 8.7 × 10^–7.

The main goal for the BASE experiment, which was approved in June 2013, is to measure the magnetic moment of the antiproton with a precision of parts per billion. Using the double Penning trap system, the collaboration recently performed the most precise measurement of the magnetic moment of the proton.

STAR helps to pin down a key phenomenon in gold collisions

The STAR detector at RHIC.
Image credit: BNL.

The STAR collaboration at the Brookhaven National Laboratory (BNL) has published new evidence indicative of a “chiral magnetic wave” rippling through the quark–gluon plasma created in high-energy gold–gold collisions at the Relativistic Heavy Ion Collider (RHIC).

Heavy-ion collisions at RHIC and the LHC involve many spectators – nucleons that are not involved in any direct collision. The charged spectators – protons – have an important influence because they can produce a magnetic field of some 10¹⁴ T. In principle, this can lead to a collective excitation in the hot dense matter produced, the chiral magnetic wave. It results from the separation both of electric charge and of chiral charge, that is, right or left “handedness”, but only in a chirally symmetric phase. The phenomenon is predicted to manifest itself as an electric quadrupole moment of the collision system, where the “poles” and “equator” of the system acquire, respectively, additional positive and negative particles. This in turn influences differently the elliptic flow of positive and negative particles, decreasing the former and increasing the latter.

To look for this effect, STAR measured the elliptic flow, v₂, of π⁺ and π^– produced in gold–gold collisions at mid-rapidity, as a function of the event-by-event charge asymmetry, A_CH, over a range of energies. The team found that v₂ increased linearly with A_CH for π^–, but decreased for π⁺. At the highest energy, √s_NN = 200 GeV, the slope of the difference in v₂ between the π⁺ and π^– as a function of A_CH depends on the centrality of the collision in a manner consistent with calculations that incorporate the chiral magnetic wave. The team also found a similar result for energies down to √s_NN = 27 GeV, with no obvious dependence on beam energy. The researchers note that none of the conventional models they have considered appear to explain the observations.

B̅⁰ decay reveals an intriguing anomaly

One-dimensional projections of the 3D fit for R(D*) in each of the three fit variables: the invariant-mass squared of the neutrino system m2miss, the energy of the muon in the B0 rest frame E*μ, and the invariant square of the momentum transfer q2.

At the Flavor Physics and CP violation (FPCP) conference in Nagoya, the LHCb collaboration presented a measurement of the rate of B⁰ → D^*+τ^–ν_τ relative to the related decay B⁰→ D^*+μ^–ν_μ. The first measurement of any B → τX decay at a hadron collider, it also indicates a tantalizing anomaly.

In the Standard Model, the ratio of these two branching fractions differs from unity only as a result of effects related to the mass of the much heavier τ lepton. The ratio R(D^*) = BR(B⁰ → D^*+τ^–ν_τ)/BR(B⁰ → D^*+μ^–ν_μ) is therefore precisely calculable in the Standard Model as equal to 0.252±0.003.

Lepton universality dictates that the electroweak coupling strength of the electron, muon and tau are identical, with the three flavours distinguished only by their respective masses. So the observation of decays with differing rates to each lepton flavour, after accounting for mass effects, would be a clear sign of physics beyond the Standard Model. Owing to the large τ mass, the semitauonic B⁰ → D^*+τ^–ν_τ decay rate is particularly sensitive to the charged Higgs bosons predicted by many extensions of the Standard Model. Previous measurements have consistently been above predictions, making new results hotly anticipated.

LHCb has analysed 3 fb^–1 of data from Run 1 of the LHC to measure R(D^*) using the τ → μν_μν_τ decay, which allows both the semitauonic and semimuonic mode to be reconstructed in the same final state. The two decays are distinguished via a fit to the decay kinematics, reconstructed using the visible decay products and an approximation for the rest frame of the B (see figure). In addition to the B⁰ → D^*+τ^–ν_τ and B⁰ → D^*+μ^–ν_μ decays, the D^*+μ^–X final state also receives large contributions from several background processes. The modelling of these backgrounds in LHCb is constrained using control samples in data, strongly controlling uncertainties due to theoretical models. The result presented of 0.336±0.027±0.033 is in close agreement with a result from BaBar in 2012, and is 2.1σ away from the Standard Model prediction.

Between the results from LHCb, BaBar and the Belle collaboration – which also presented updated results at the conference – a tantalizing picture is emerging in this channel. LHCb already has plans for complementary measurements in the decays B^– → D⁰τ^–ν_τ and Λ⁰_b → Λ⁺_c τ^–ν_τ with the LHC Run 1 data set, and data from Run 2 is expected to allow for exciting improvements.

LHCb observes top production in the forward region

Production of top observed by LHCb in association with Wb.

Studies of the production of top quarks in the forward region at the LHC are potentially of great interest in terms of new physics. Not only does the process have an enhanced sensitivity to physics beyond the Standard Model (owing to sizable contributions from quark–antiquark and gluon–quark scattering), but measurements of the forward production of top-quark pairs (tt) can be used to constrain the gluon parton distribution function (PDF) at large momentum fraction. Reducing the uncertainty on this PDF will increase the precision of many Standard Model predictions, especially those that serve as backgrounds to searches for new high-mass particles.

Top quarks decay almost exclusively to a W boson and a b-quark jet. The LHCb collaboration has already made high-precision measurements of W-boson production, and recently demonstrated the ability to identify, or tag, jets originating from b and c quarks (LHCb 2015a). Now, the collaboration had combined these two abilities in a study of W-boson production in association with b and c jets (LHCb 2015b), using a subset of these data samples to observe top-quark production for the first time in the forward region (LHCb 2015c). The data show a large excess of events compared with the Standard Model’s W+b-jet prediction in the absence of top-quark production (see figure).

LHCb measured the top-quark production cross-sections in a reduced fiducial region chosen to enhance the relative top-quark content of the W+b-jet final state. Within this region, the inclusive top-quark production cross-sections, which include contributions from both tt and single-top production, are σ(top) [7 TeV] = 239±53(stat.)±38(syst.) fb and σ(top) [8 TeV] = 289±43(stat.)±46(syst.) fb. These values are in agreement with the Standard Model predictions of 180⁺⁵¹_–41 (312⁺⁸³_–68) fb at 7(8) TeV obtained at next-to-leading order using MCFM, the Monte Carlo programme for femtobarn processes.

In the LHC’s Run 2, the higher beam energy should lead to a greatly increased cross-section and acceptance for top-quark production. This will allow LHCb to measure precisely both tt and single-top production, and so provide important constraints on the gluon PDF as well as potential signs for physics beyond the Standard Model.

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