Physics Goals Research Articles

Hadron Spectroscopy at BESIII

The BESIII experiment, hosted at the IHEP of Beijing, has collected the world largest data sample in the charmonium energy region. One of the most important physics goals of BESIII is the investigation of the QCD prediction. QCD can be accessed in a unique way by means of hadron spectroscopy, which was extensively studied and many important progresses were achieved in the last years. Charmonium decays provide an excellent scenario for studying nucleons, hyperons and their excited states, XYZ resonances, as well as light hadrons. Some of the most recent results for hadron spectroscopy from BESIII will be reported.

The new experiment WAGASCI for water to hydrocarbon neutrino cross section measurement using the J-PARC beam

The T2K (Tokai-to-Kamioka) is a long baseline neutrino experiment designed to study various parameters that rule neutrino oscillations, with an intense beam of muon neutrinos. A near detector complex (ND280) is used to constrain non-oscillated flux and hence to predict the expected number of events in the far detector (Super-Kamiokande). The difference in the target material between the far (water) and near (scintillator, hydrocarbon) detectors leads to the main non-canceling systematic uncertainty for the oscillation analysis. In order to reduce this uncertainty a new water grid and scintillator detector, WAGASCI, has been proposed. The detector will be operated at the J-PARC neutrino beam line with the main physics goal to measure the charged current neutrino cross section ratio between water and hydrocarbon with a few percent accuracy. Further physics program may include high-precision measurements of different charged current neutrino interaction channels. The concept of the new detector will be covered together with the actual construction plan.

Vacuum laser acceleration of relativistic electrons using plasma mirror injectors

Accelerating particles to relativistic energies over very short distances using lasers has been a long standing goal in physics. Among the various schemes proposed for electrons, vacuum laser acceleration has attracted considerable interest and has been extensively studied theoretically because of its appealing simplicity: electrons interact with an intense laser field in vacuum and can be continuously accelerated, provided they remain at a given phase of the field until they escape the laser beam. But demonstrating this effect experimentally has proved extremely challenging, as it imposes stringent requirements on the conditions of injection of electrons in the laser field. Here, we solve this long-standing experimental problem for the first time by using a plasma mirror to inject electrons in an ultraintense laser field, and obtain clear evidence of vacuum laser acceleration. With the advent of PetaWatt class lasers, this scheme could provide a competitive source of very high charge (nC) and ultrashort relativistic electron beams.

Real-time flavour tagging selection in ATLAS

In high-energy physics experiments, online selection is crucial to the identification of the few interesting collisions from the large data volume processed. In the overall ATLAS trigger strategy, b-jet triggers are designed to identify heavy-flavor content in real-time and, in particular, provide the only option to efficiently record events with fully hadronic final states containing b-jets. In doing so, two different, but related, challenges are faced. The physics goal is to optimise as far as possible the rejection for light jets from QCD processes, while retaining a high efficiency on selecting jets from beauty, while maintaining affordable trigger rates without raising jet energy thresholds. This poses a challenging computing task, as charged tracks and their corresponding vertices must be reconstructed and analysed for each jet above the desired threshold, regardless of the increasingly harsh pile-up conditions. The performance of b-jet triggers during the LHC Run I data-taking campaigns is presented, together with an overview of the new online b-tagging strategy and algorithms, designed to face the above mentioned challenges, which will be adopted during Run II.

Performance of a Quintuple-GEM Based RICH Detector Prototype

Cerenkov technology is often the optimal choice for particle identification in high energy particle collision applications. Typically, the most challenging regime is at high pseudorapidity (forward) where particle identification must perform well at high high laboratory momenta. For the upcoming Electron Ion Collider (EIC), the physics goals require hadron ($\pi$, K, p) identification up to $\sim$~50 GeV/c. In this region Cerenkov Ring-Imaging is the most viable solution.\newline The speed of light in a radiator medium is inversely proportional to the refractive index. Hence, for PID reaching out to high momenta a small index of refraction is required. Unfortunately, the lowest indices of refraction also result in the lowest light yield ($\frac{dN_\gamma}{dx} \propto \sin^2{\left(\theta_C \right)}$) driving up the radiator length and thereby the overall detector cost. In this paper we report on a successful test of a compact RICH detector (1 meter radiator) capable of delivering in excess of 10 photoelectrons per ring with a low index radiator gas ($CF_4$). The detector concept is a natural extension of the PHENIX HBD detector achieved by adding focusing capability at low wavelength and adequate gain for high efficiency detection of single-electron induced avalanches. Our results indicate that this technology is indeed a viable choice in the forward direction of the EIC. The setup and results are described within.

Are We on the Brink of the Higgs Abyss?

One of the ambitious goals of particle physics is to elucidate the early history of the Universe and predict its distant future. Particle cosmologists examine whether the known laws of particle physics are consistent with the observed cosmological evolution and what future they might imply. Do such laws require some modifications to explain the present Universe? Do they suggest that the Universe is stable, or do they imply it is “metastable,” that is, temporarily stable on cosmological time scales but headed towards an inevitable, if distant, cataclysmic collapse? A new theoretical analysis by Alexander V. Bednyakov at the Joint Institute for Nuclear Research, Dubna, Russia, and co-workers connects these basic questions to the most recent discoveries obtained at the Large Hadron Collider (LHC) [1]. The authors conclude that if the standard model is correct, the measured values of certain quantities, such as the mass of the Higgs boson, imply the Universe is metastable. However, they also show that stability might be more likely than previous studies indicated. In quantum theory, a stable, “true” vacuum corresponds to the global minimum of the scalar potential, a function that depends on all the scalar fields associated with the fundamental forces of nature. A metastable or “false” vacuum is instead a local minimum. If the Universe lies in the only (or deepest) minimum of the potential, then its future is not threatened. However, it is also possible that the current minimum is “local” and a deeper minimum exists, or the potential has a bottomless abyss separated from the local minimum by a finite barrier. In these cases, the Universe will eventually tunnel out into some other state, in which life as we know it might be impossible. Of course, the probability of such a catastrophic event must be small, because the Universe has remained in its present state for over ten billion years. However, the mere possibility of an inevitable, albeit distant, “end of the world” is disquieting for all. The discovery of the Higgs boson confirmed that the FIG. 1: The ground state of the Universe depends on the potential of the Higgs field. If the Universe lies in the global minimum of the potential (a “true” vacuum state), then it is stable. But if the minimum is local and a deeper minimum exists, the vacuum is “false,” and the Universe might catastrophically tunnel out into the true vacuum state. (APS/Ala n Stonebraker)

Measurement of interaction between antiprotons.

One of the primary goals of nuclear physics is to understand the force between nucleons, which is a necessary step for understanding the structure of nuclei and how nuclei interact with each other. Rutherford discovered the atomic nucleus in 1911, and the large body of knowledge about the nuclear force that has since been acquired was derived from studies made on nucleons or nuclei. Although antinuclei up to antihelium-4 have been discovered and their masses measured, little is known directly about the nuclear force between antinucleons. Here, we study antiproton pair correlations among data collected by the STAR experiment at the Relativistic Heavy Ion Collider (RHIC), where gold ions are collided with a centre-of-mass energy of 200 gigaelectronvolts per nucleon pair. Antiprotons are abundantly produced in such collisions, thus making it feasible to study details of the antiproton-antiproton interaction. By applying a technique similar to Hanbury Brown and Twiss intensity interferometry, we show that the force between two antiprotons is attractive. In addition, we report two key parameters that characterize the corresponding strong interaction: the scattering length and the effective range of the interaction. Our measured parameters are consistent within errors with the corresponding values for proton-proton interactions. Our results provide direct information on the interaction between two antiprotons, one of the simplest systems of antinucleons, and so are fundamental to understanding the structure of more-complex antinuclei and their properties.

TOT measurement implemented in FPGA TDC**Supported by National Natural Science Foundation of China (11079003, 10979003)

Time measurement plays a crucial role for the purpose of particle identification in high energy physics experiments. With increasingly demanding physics goals and the development of electronics, modern time measurement systems need to meet the requirement of excellent resolution specification as well as high integrity. Based on Field Programmable Gate Arrays (FPGAs), FPGA time-to-digital converters (TDCs) have become one of the most mature and prominent time measurement methods in recent years. For correcting the time-walk effect caused by leading timing, a time-over-threshold (TOT) measurement should be added to the FPGA TDC. TOT can be obtained by measuring the interval between the signal leading and trailing edges. Unfortunately, a traditional TDC can recognize only one kind of signal edge, the leading or the trailing. Generally, to measure the interval, two TDC channels need to be used at the same time, one for leading, the other for trailing. However, this method unavoidably increases the amount of FPGA resources used and reduces the TDC's integrity.This paper presents one method of TOT measurement implemented in a Xilinx Virtex-5 FPGA. In this method, TOT measurement can be achieved using only one TDC input channel. The consumed resources and time resolution can both be guaranteed. Testing shows that this TDC can achieve resolution better than 15ps for leading edge measurement and 37 ps for TOT measurement. Furthermore, the TDC measurement dead time is about two clock cycles, which makes it good for applications with higher physics event rates.

Comparison of the calorimetric and kinematic methods of neutrino energy reconstruction in disappearance experiments

To be able to achieve their physics goals, future neutrino-oscillation experiments will need to reconstruct the neutrino energy with very high accuracy. In this work, we analyze how the energy reconstruction may be affected by realistic detection capabilities, such as energy resolutions, efficiencies, and thresholds. This allows us to estimate how well the detector performance needs to be determined a priori in order to avoid a sizable bias in the measurement of the relevant oscillation parameters. We compare the kinematic and calorimetric methods of energy reconstruction in the context of two muon-neutrino disappearance experiments operating in different energy regimes. For the calorimetric reconstruction method, we find that the detector performance has to be estimated with a ~10% accuracy to avoid a significant bias in the extracted oscillation parameters. On the other hand, in the case of kinematic energy reconstruction, we observe that the results exhibit less sensitivity to an overestimation of the detector capabilities.

Vetoing cosmogenic muons in a large liquid scintillator

At upcoming medium baseline reactor neutrino experiments the spallation Li-9 background will be somewhat larger than the inverse beta decay reactor neutrino signal. We use new FLUKA simulations of spallation backgrounds to optimize a class of veto strategies and find that surprisingly the optimal veto for the mass hierarchy determination has a rejection efficiency below 90%. The unrejected background has only a modest effect on the physics goals. For example Delta chi(2) for the hierarchy determination falls by 1.4 to 3 points depending on the muon tracking ability. The optimal veto strategy is essentially insensitive to the tracking ability, consisting of 2 meter radius, 1.1 second cylindrical vetoes of well tracked muons with showering energies above 3 to 4 GeV and 0.7 second full detector vetoes for poorly tracked muons above 15 to 18 GeV. On the other hand, as the uncertainty in 012 will be dominated by the uncertainty in the reactor neutrino spectrum and not statistical fluctuations, the optimal rejection efficiency for the measurement of theta(12) is 93% in the case of perfect tracking.

The LAr1-ND Experiment

LAr1-ND is a Liquid Argon Time Projection Chamber (LArTPC) based experiment which will operate in the Booster Neutrino Beam (BNB) at Fermilab, at a distance of 110m from the target. The 112t active volume detector will make precision measurements of the neutrino interaction cross section in argon, as well as forming the near detector for the short-baseline neutrino (SBN) programme at Fermilab. The 4m × 4m × 5m TPC consists of 2 anode planes, each composed of 3 wire planes, with a central cathode. In combination with the MicroBooNE and ICARUS-T600 detectors, this experiment provides a powerful opportunity to understand observed neutrino anomalies, and has the potential to make precision measurements of oscillations to sterile states.In addition to the physics goals, the detector technology developed and used in this experiment will form an important step towards the future detectors for long-baseline neutrino experiments. The detector design will be discussed in the context of its physics potential, and contribution to future technologies.

COSMOLOGY WITH MASSIVE NEUTRINOS: CHALLENGES TO THE STANDARD ΛCDM PARADIGM

Determining the absolute neutrino mass scale and the neutrino mass hierarchy are central goals in particle physics, with important implications for the Standard Model. However, the nal answer may come from cosmology, as laboratory experiments provide measurements for two of the squared mass differences and a stringent lower bound on the total neutrino mass { but the upper bound is still poorly constrained, even when considering forecasted results from future probes. Cosmological tracers are very sensitive to neutrino properties and their total mass, because massive neutrinos produce a specic redshiftand scale-dependent signature in the power spectrum of the matter and galaxy distributions. Stringent upper limits on P m will be essential for understanding the neutrino sector, and will nicely complement particle physics results. To this end, we describe here a series of cosmological hydrodynamical simulations which include massive neutrinos, specically designed to meet the requirements of the Baryon Acoustic Spectroscopic Survey (BOSS) and focused on the Lyman- (Ly ) forest { also a useful theoretical ground for upcoming surveys such as SDSS-IV/eBOSS and DESI. We then briey highlight the remarkable constraining power of the Ly forest in terms of the total neutrino mass, when combined with other state-of-the-art cosmological probes, leading to a stringent upper bound on P m .

Contexts, Systems and Modalities: A New Ontology for Quantum Mechanics

In this article we present a possible way to make usual quantum mechanics fully compatible with physical realism, defined as the statement that the goal of physics is to study entities of the natural world, existing independently from any particular observer's perception, and obeying universal and intelligible rules. Rather than elaborating on the quantum formalism itself, we propose to modify the quantum ontology, by requiring that physical properties are attributed jointly to the system, and to the context in which it is embedded. In combination with a quantization principle, this non-classical definition of physical reality sheds new light on counter-intuitive features of quantum mechanics such as the origin of probabilities, non-locality, and the quantum-classical boundary.

Radio detection of high-energy cosmic rays with the Auger Engineering Radio Array

The Auger Engineering Radio Array (AERA) is an enhancement of the Pierre Auger Observatory in Argentina. Covering about 17km2, AERA is the world-largest antenna array for cosmic-ray observation. It consists of more than 150 antenna stations detecting the radio signal emitted by air showers, i.e., cascades of secondary particles caused by primary cosmic rays hitting the atmosphere. At the beginning, technical goals had been in focus: first of all, the successful demonstration that a large-scale antenna array consisting of autonomous stations is feasible. Moreover, techniques for calibration of the antennas and time calibration of the array have been developed, as well as special software for the data analysis. Meanwhile physics goals come into focus. At the Pierre Auger Observatory air showers are simultaneously detected by several detector systems, in particular water-Cherenkov detectors at the surface, underground muon detectors, and fluorescence telescopes, which enables cross-calibration of different detection techniques. For the direction and energy of air showers, the precision achieved by AERA is already competitive; for the type of primary particle, several methods are tested and optimized. By combining AERA with the particle detectors we aim for a better understanding of cosmic rays in the energy range from approximately 0.3 to 10EeV, i.e., significantly higher energies than preceding radio arrays.

Enhancing t t ¯ h h $$ t\overline{t}hh $$ production through CP-violating top-Higgs interaction at the LHC and future colliders

The measurement of Higgs self-coupling is one of the most crucial physics goals at the future colliders. At the LHC, the di-Higgs production is a main way to measure the Higgs trilinear coupling. As a complementary to the di-Higgs production, $$ t\overline{t}hh $$ process may open a new avenue to measure di-Higgs physics at the LHC and a future 100 TeV pp collider or a high energy e + e − collider since the extra $$ t\overline{t} $$ in the final states may efficiently suppress the backgrounds. However, such a kind of process is also controlled by the top-Higgs coupling. In this work, we investigate the impact of CP-violating top-Higgs coupling on $$ t\overline{t}hh $$ production at the LHC, e + e − and a 100 TeV hadron collider under the current Higgs data. Within 2σ Higgs data allowed parameter region, we find that the cross section of $$ t\overline{t}hh $$ at the LHC-14 TeV, e + e −-1 TeV and VHE-LHC/SPPC-100 TeV can be enhanced up to 2.1 times the SM predictions. The future precise measurement of Higgs coupling will reveal the nature of top-Higgs interaction and improve the sensitivity of the determination of Higgs self-coupling through $$ t\overline{t}hh $$ production.

Possible futures of electroweak precision: ILC, FCC-ee, and CEPC

The future of high-precision electroweak physics lies in e + e − collider measurements of properties of the Z boson, the W boson, the Higgs boson, and the top quark. We estimate the expected performance of three possible future colliders: the ILC, FCC-ee (formerly known as TLEP), and CEPC. In particular, we present the first estimates of the possible reach of CEPC, China’s proposed Circular Electron-Positron Collider, for the oblique parameters S and T and for seven-parameter fits of Higgs couplings. These results allow the physics potential for CEPC to be compared with that of the ILC and FCC-ee. We also show how the constraints on S and T would evolve as the uncertainties on each of the most important input measurements change separately. This clarifies the basic physics goals for future colliders. To improve on the current precision, the highest priorities are improving the uncertainties on m W and sin2 θ eff . At the same time, improved measurements of the top mass, the Z mass, the running of α, and the Z width will offer further improvement which will determine the ultimate reach. Each of the possible future colliders we consider has strong prospects for probing TeV-scale electroweak physics.

Neutrino Physics with SNO+

SNO+ is the successor to the Sudbury Neutrino Observatory (SNO) for which SNO's heavy water target is replaced by approximately 780 T of liquid scintillator (LAB). The combination of the 2 km underground location, the use of ultra-clean materials and the high light-yield of the liquid scintillator means that a low background level and a low energy threshold can be achieved. This creates a new multipurpose neutrino detector with the potential to address a diverse set of physics goals, including the detection of reactor, solar, geo- and supernova neutrinos. The main physics goal of SNO+ is the search for neutrinoless double beta decay. By loading the liquid scintillator with 0.3% of natural Tellurium, resulting in about 800 kg of 130Te (isotopic abundance is slightly over 34%), a competitive sensitivity to the effective neutrino mass can be reached.

New results from RENO and prospects with RENO-50

RENO (Reactor Experiment for Neutrino Oscillation) has made a definitive measurement of the smallest mixing angle θ13 in 2012, based on the disappearance of electron antineutrinos. More precise measurements have been obtained and presented on the mixing angle and the reactor neutrino spectrum, using ∼800 days of data. We have observed an excess of inverse-beta-decay (IBD) prompt spectrum near 5 MeV with respect to the most commonly used models. The excess is found to be consistent with coming from reactors. We have also successfully measured the reactor neutrino flux with a delayed signal of neutron capture on hydrogen. A future reactor neutrino experiment of RENO-50 is proposed to determine the neutrino mass hierarchy and to make highly precise measurements of θ12, Δm212, and Δm312. Physics goals and sensitivities of RENO-50 are presented with a strategy of obtaining an extremely good energy resolution toward the neutrino mass hierarchy.

IceCube Results and PINGU Perspectives

The last three years of IceCube operation with the completed detector have resulted in a plethora of results, including the first observation of high energy astrophysical neutrinos, tests of a possible neutrino flux from atmospheric charm meson decay, and competitive results of neutrino oscillation from atmospheric muon neutrino disappearance. Based on the success of IceCube, a new low energy in-fill, known as the Precision IceCube Next Generation Upgrade, is being proposed with the primary physics goal of resolving the ordering of the neutrino mass hierarchy.

Precision reconstruction of the cold dark matter-neutrino relative velocity fromN-body simulations

Discovering the mass of neutrinos is a principle goal in high energy physics and cosmology. In addition to cosmological measurements based on two-point statistics, the neutrino mass can also be estimated by observations of neutrino wakes resulting from the relative motion between dark matter and neutrinos. Such a detection relies on an accurate reconstruction of the dark matter-neutrino relative velocity which is affected by non-linear structure growth and galaxy bias. We investigate our ability to reconstruct this relative velocity using large N-body simulations where we evolve neutrinos as distinct particles alongside the dark matter. We find that the dark matter velocity power spectrum is overpredicted by linear theory whereas the neutrino velocity power spectrum is underpredicted. The magnitude of the relative velocity observed in the simulations is found to be lower than what is predicted in linear theory. Since neither the dark matter nor the neutrino velocity fields are directly observable from galaxy or 21 cm surveys, we test the accuracy of a reconstruction algorithm based on halo density fields and linear theory. Assuming prior knowledge of the halo bias, we find that the reconstructed relative velocities are highly correlated with the simulated ones with correlation coefficients of 0.94, 0.93, 0.91 and 0.88 for neutrinos of mass 0.05, 0.1, 0.2 and 0.4 eV. We confirm that the relative velocity field reconstructed from large scale structure observations such as galaxy or 21 cm surveys can be accurate in direction and, with appropriate scaling, magnitude.