Fifth force

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In physics, there are four observed

nuclear forces) with a range of anywhere from less than a millimeter to cosmological scales. Another proposal is a new weak force mediated by W′ and Z′ bosons
.

The search for a fifth force has increased in recent decades due to two discoveries in cosmology which are not explained by current theories. It has been discovered that most of the mass of the universe is accounted for by an unknown form of matter called dark matter. Most physicists believe that dark matter consists of undiscovered subatomic particles[1] that are difficult to detect, but some believe that it could be related to an unknown fundamental force. Second, it has also recently been discovered that the expansion of the universe is accelerating, which has been attributed to a form of energy called dark energy. Some physicists speculate that a form of dark energy called quintessence could be a fifth force.[2][3]

History

The term fifth force originates in a 1986 paper by Fischbach et al. who reanalyzed the data from the

inverse square law. [4][5]
: 57  The reanalysis was sparked by theoretical work in 1971 by Fujii [6][7]: 3  proposing a model that changes distance dependence with a Yukawa potential-like term:

The parameter characterizes the strength and the range of the interaction.[5] Fischbach's paper found a strength around 1% of gravity and a range of a few hundred meters.[8]: 26  The effect of this potential can be described equivalently as exchange of vector and/or scalar bosons, that is a predicting as yet undetected new particles.[5] However, many subsequent attempts to reproduce the deviations have failed.[9]

Theory

Theoretical proposals in the fifth-force category are driven by two inconsistencies between the current models of general relativity and quantum field theory, the hierarchy problem and the cosmological constant problem. Both issues suggest the possibility of corrections to the gravitational potential around .[5]: 58 

Experimental approaches

There are at least three kinds of searches that can be undertaken, which depend on the kind of force being considered, and its range.

Equivalence principle

One way to search for a fifth force is with tests of the strong

bending of light rays
.

The fifth force would manifest itself in an effect on solar system orbits, called the

Lunar Laser Ranging experiment[10] and very-long-baseline interferometry
.

Extra dimensions

Another kind of fifth force, which arises in

supersymmetric large extra dimensions — dimensions with size slightly less than a millimeter — has prompted an experimental effort to test gravity on very small scales. This requires extremely sensitive experiments which search for a deviation from the inverse-square law of gravity over a range of distances.[11]
Essentially, they are looking for signs that the Yukawa interaction is engaging at a certain length.

Australian researchers, attempting to measure the gravitational constant deep in a mine shaft, found a discrepancy between the predicted and measured value, with the measured value being two percent too small. They concluded that the results may be explained by a repulsive fifth force with a range from a few centimetres to a kilometre. Similar experiments have been carried out on board a submarine, USS Dolphin (AGSS-555), while deeply submerged. A further experiment measuring the gravitational constant in a deep borehole in the Greenland ice sheet found discrepancies of a few percent, but it was not possible to eliminate a geological source for the observed signal.[12][13]

Earth's mantle

Another experiment uses the Earth's mantle as a giant particle detector, focusing on geoelectrons.[14]

Cepheid variables

Jain et al. (2012)[15] examined existing data on the rate of pulsation of over a thousand cepheid variable stars in 25 galaxies. Theory suggests that the rate of cepheid pulsation in galaxies screened from a hypothetical fifth force by neighbouring clusters, would follow a different pattern from cepheids that are not screened. They were unable to find any variation from Einstein's theory of gravity.

Other approaches

Some experiments used a lake plus a tower that is 320 meters high.[16] A comprehensive review by Ephraim Fischbach and Carrick Talmadge suggested there is no compelling evidence for the fifth force,[17] though scientists still search for it. The Fischbach–Talmadge article was written in 1992, and since then, other evidence has come to light that may indicate a fifth force.[18]

The above experiments search for a fifth force that is, like gravity, independent of the composition of an object, so all objects experience the force in proportion to their masses. Forces that depend on the composition of an object can be very sensitively tested by

torsion balance experiments of a type invented by Loránd Eötvös. Such forces may depend, for example, on the ratio of protons to neutrons in an atomic nucleus, nuclear spin,[19] or the relative amount of different kinds of binding energy in a nucleus (see the semi-empirical mass formula). Searches have been done from very short ranges, to municipal scales, to the scale of the Earth, the Sun, and dark matter
at the center of the galaxy.

Claims of new particles

Main article: X17 particle

In 2015, Attila Krasznahorkay at

lithium-7, which created unstable beryllium-8
nuclei that then decayed and ejected pairs of electrons and positrons. Excess decays were observed at an opening angle of 140° between the e+ and e−, and a combined energy of 17 MeV, which indicated that a small fraction of beryllium-8 will shed excess energy in the form of a new particle.

In November 2019, Krasznahorkay announced that he and his team at ATOMKI had successfully observed the same anomalies in the decay of stable helium atoms as had been observed in beryllium-8, strengthening the case for the X17 particle's existence.[21]

Feng et al. (2016)

femtometer range could explain the data.[23] The force may explain the muon g − 2 anomaly and provide a dark matter candidate. Several research experiments are underway to attempt to validate or refute these results.[20][22]

See also

References

  1. ^ Chown, Marcus (17 August 2011). "Really dark matter: Is the universe made of holes?". New Scientist. Pretty much everyone thinks that this so-called dark matter is made of hitherto undiscovered subatomic particles.
  2. ^ Wetterich, C. "Quintessence – a fifth force from variation of the fundamental scale" (PDF). Heidelberg University.
  3. S2CID 118461474
    .
  4. PMID 10032514
    .
  5. ^
    ISSN 0034-6861
    .
  6. ISSN 0300-8746
    .
  7. ISBN 978-1-4612-7144-4
    .
  8. PMID 28179848
    .
  9. ISBN 978-3-319-28412-5
    .
  10. ^ "Lunar laser ranging". Archived from the original on 28 November 2016. Retrieved 7 May 2005.
  11. ^ "Satellite Energy Exchange (SEE)". Archived from the original on 7 May 2005. Retrieved 7 May 2005., which is set to test for a fifth force in space, where it is possible to achieve greater sensitivity.
  12. PMID 10040395
    .
  13. doi:10.1029/JB095iB10p15483
    .
  14. ^ Aron, Jacob (2013). "Earth's mantle helps hunt for fifth force of nature". New Scientist.
  15. S2CID 119260435
    . 39.
  16. doi:10.1016/0375-9601(92)90582-7
    .
  17. S2CID 21255315
    .
  18. S2CID 119113836
    .
  19. ^ Hall, A. M.; Armbruster, H.; Fischbach, E.; Talmadge, C. (1991). "Is the Eötvös experiment sensitive to spin?". In Hwang, W.-Y. Pauchy; et al. (eds.). Progress in High Energy Physics. New York: Elsevier. pp. 325–339.
  20. ^
    S2CID 124347962
    .
  21. ^ "Scientists may have discovered fifth force of nature, laboratory announces". The Independent. London, UK. Retrieved 26 November 2019.
  22. ^
    S2CID 206279817
    .
  23. ^ "New boson claim faces scrutiny". Quanta Magazine. Retrieved 24 November 2019.


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