A pentaquark is found
1 August 2015
The Large Hadron Collider experiment LHCb revealed with more than 9 σ significance a pentaquark, i.e., a c(anti-c)uud particle consisting of four quarks and one antiquark. The pentaquarks were identified as intermediate resonant states in Λb0 → J/ψK-p decays of baryons Λb0 born in 7 and 8 TeV pp collisions with center-of-mass energies. That the existence of pentaquarks is possible in principle was first pointed out by D. Strottman in 1979, and the properties of positive-strangeness pentaquarks were predicted in 1997 papers by D.I. Dyakonov, M.V. Polyakov, and V.Yu. Petrov. Some evidence of the production of pentaquarks with S=+1 had been obtained before, but the data turned out to be contradictory. The pentaquark c(anti-c)uud which refers to another type (with hidden charm and S=0) was found on LHCb in two states with masses of 4380 MeV and 4450 MeV and with most probable spin-parity values JP=3/2- and 5/2+, respectively.
Source: arXiv:1507.03414 [hep-ex]
Network with quantum cryptography
1 August 2015
The use of quantum cryptography in distributed networks (that is, in data packet transmission without preferred communication channels) was impeded by a very low operation rate of the secret-key distribution protocol. The team of researchers headed by S. Pirandola (York University, Great Britain) managed to solve this problem through adapting the continuous quantum-variable method to the case of a network. Within this approach, the sender and the recipient create quantum states, modulate them by a signal with Gaussian distribution and send to an intermediate station where signal correlations are measured. Making use of their notes of quantum states and open classical data on correlations, the sender and the recipient decipher the signals, while it is inaccessible for an alien. The authors proposed this scheme theoretically and realized it in their experiment in which, as distinct from quantum entanglement schemes, linear optics appeared to suffice. The information was transmitted by the “quantum network” through 25 km at a rate three orders of magnitude higher than was achieved in previous experiments. With a further increase in the distance the transmission efficiency falls, but a distance of 25 km is actually enough to exchange quantum-encoded data within a city.
Source: Nature Photonics 9 397 (2015)
Beresinskii – Kosterlitz – Thouless phase transition in an ultracold gas
1 August 2015
P.A. Murthy (Heidelberg University, Ruperto Carola, Germany) with colleagues investigated a quasi-two-dimensional ultracold gas of #Li atoms in the BEC – BCS crossover regime (transition between the Bose – Einstein condensate and the Bardeen-Cooper-Schrieffer state) to reveal that the crossover has the form of the Beresinskii – Kosterlitz – Thouless phase transition. This became clear from the scale-invariant behavior of the first-order correlation functions in the distribution of atoms. In the experiment, the interatomic interaction force (the scattering length) was regulated by the Feshbach resonance method. The correlation functions turned out to have a power-law form with the exponent depending on temperature only. The increase of the exponent with rising temperature corresponds qualitatively to the Beresinskii – Kosterlitz – Thouless theory. Coherence of the superfluid state was investigated in the fermion region — such studies had been conducted before in 3D systems only. In the presence of density inhomogeneity the role of phase fluctuations was shown to be significant.
Source: Phys. Rev. Lett. 115 010401 (2015)
Quantum dot spectrometer
1 August 2015
J. Bao (Tsinghua University, China) and M.G. Bawendi (Massachusetts Institute of Technology, USA) designed a simple compact spectrometer based on quantum dots (microscopic single crystals in this case). The spectrometer operation principle rests on selective response of quantum dots to waves of different frequencies, which allows them to serve as filters. 195 quantum dots differing in size or composition and, accordingly, in the spectral properties constituted an array. Each filter of this array covered several CCD-camera pixels that worked as detectors. The spectrometer works by the multiplexing scheme when several spectral regions are measured simultaneously, and then the original signal undergoes computer reconstruction. The spectrometer prototype demonstrated in the experiment overlaps the range of 390 to 690 nm with a resolution up to ≈ 1 nm. Owing to compactness and low price of the device the new method of spectral measurement may find application in portative microspectrometers.
Source: Nature 523 67 (2015)
Supernova of record brightness
1 August 2015
S. Dong (Peking University, China) with colleagues discovered a supernova ASASSN-15lh which is the brightest of all previously observed ones. This supernova was first registered by an ASAS-SN automated system of small telescopes and was then observed by large optical telescopes and in the UV range by the cosmic telescope Swift. The supernova has a red shift z=0.2326 and its total (bolometric) luminosity makes up 2.2 × 1045 erg s-1, which is about 2.5 times higher than that of the previously brightest supernovae. The spectrum of ASASSN-15lh is close to that of superbright SLSNe-I type supernovae. These supernovae are about 100 times brighter than the typical supernovae, and their bursts are three orders of magnitude rarer. However, an exact classification of supernova ASASSN-15lh is hampered, and the energy release mechanism is unknown. Within the two months of observations the energy released by the supernova was 7.5 × 1051 erg. Such a high value can hardly be explained in the magnetar model. The alternative hypothesis of tidal star destruction by a black hole is also practically excluded because of the absence of hydrogen and helium emission lines.
Source: arXiv:1507.03010 [astro-ph.HE]
The Extracts from the Internet is a section of Uspekhi Fizicheskih Nauk (Physics Uspekhi) the monthly rewiew journal of the current state of the most topical problems in physics and in associated fields. The presented News is devoted to the fundamental discoveries of physics and astrophysics.
Permanent editor is Yu.N. Eroshenko.
It is compiled from a multitude of Internet sources.