The Standard Model — shaken but not stirred: New study confirms old puzzle
In a hurriedly arranged conference on March 10, 2020, Dr. Eluned Anne Smith, on behalf of
the LHCb (Large Hadron Collider ‘beauty’) collaboration, dramatically revealed ‘mildly
increased tension’ with the Standard Model as compared to the earlier study. This
discrepancy could imply indication of new, undiscovered physics beyond the Standard
Model. This may open the door for the discovery of an entirely new type of matter such as
a chameleon, a hypothetical particle of dark energy and axions hypothetical particles of
dark matter.
Standard Model is the precise but incomplete equations that have governed particle
physics for over 40 years. Like Newton’s laws, before the advent of Einstein’s theory of relativity is used to detail all known fundamental particles, describe their interaction and explain three out of four fundamental forces, electromagnetic, weak nuclear forces and strong nuclear forces.
Large Hadron Collider’s ‘beauty’ experiment (LHCb) studies the rare decay products of
exotic particles such as B meson (a boson containing the B, or beauty/bottom, quark). The
actual observed rate of rare decay and the angular distribution of their decay products help
resolve the conundrum of Standard Model theories.
The current experiment, an update of a previous one, analysed twice as much data of rare
decay of B0 meson. The inference of this new study is enough to cause eyebrows to rise,
but not compelling to conclude the demise of the Standard Model.
Incidentally, an Indian scientist, Rahul Sinha of the Institute of Mathematical Sciences,
Chennai and his collaborators proposed the B-meson decay experiment to look for cracks
in the armour of the Standard Model.
CERN was initially slated to make this announcement at the annual ‘Rencontres de
Moriond’ conference, held at La Thuile, Italy. However, due to coronavirus shutdown in
Italy, the meeting was hastily arranged at CERN itself.
Standard Model
Thousand years ago, we speculated that all material things are made up of five elements –
Pancha Bhoota, bhūmi (earth), jala (water), agni (fire), vayu (air) and akash (space).
Chemical revolution during the 17th and 18th century showed water, for example, H2O is a
mixture of basic chemical elements Hydrogen and Oxygen. Identification of other chemical
elements resulted in the atomic theory. The discovery of electrons, protons and neutrons in
the early decades of the twentieth century let to the notion of sub-atomic particles as the
foundation of matter. During the 1950s and 60s quarks, fundamental particles, constitutive
of subatomic particles like proton and neutron were discovered. The Standard Model is the
theory that explains all the known fundamental particles and three, electromagnetic, weak,
and strong, out of the four known fundamental forces.
Only the gravity is outside the purview of the Standard Model. The successful discovery of hitherto unknown Higgs boson, popularly known as ‘god particle’, predicted by the Standard Model, by the CERN in 2012 was its crowning glory. Despite its accomplishment, there is a lurking feeling among physicist that the Standard Model is neither a complete description of the microscopic world of elementary particles nor the last word. Are quarks and the Standard Model particles the ultimate building blocks of everything around us?
The impasse
Standard Model is a remarkable theory to describe the interactions between fundamental
particles and fundamental forces. It has been remarkably successful at predicting the
outcome of particle physics experiments. Yet this theory is at a deadlock.
The Standard Model explains well the ‘visible universe’, that is matter that interacts with
electromagnetism. However, this type of matter that constitute atoms, stars and all the
object we see around is estimated to be just 4% of the entire universe. Rest of the 96% is
made up of still elusive ‘dark matter’ and ‘dark energy’. The Standard Model is of no help.
What is dark matter made up of? Standard Model has no clue.
Gravity is another enigma. Despite its sway over the motion of galactic clusters and stars
even at the very edge of our universe, gravity is weakest of all the known four forces. It is
about 10^24 (one followed by 24 zeros) times frailer than the weak nuclear force. 10^40
times less potent than the magnetism. That is why even a small magnet could pull an iron
needle against gravity.
Called hierarchy problem the large discrepancy between gravity and other three forces is a
riddle. Theoretical solutions within standard Model like supersymmetry, where the space
has more three dimensions that we experience, and possibly as many as 10, tries to
explain the enigmas. According to this supersymmetry string theory, the gravity is so weak
because, unlike the other forces, it leaks in and out of these extra dimensions. While the
theoretical solutions are cute until now, we have no experimental evidence for such
audacious claims. Even after decades of LHC experiments, no supersymmetric particles
have emerged in the detectors.
Revolution or reform
History of science shows there have always been two types of resolutions out of the
dilemma. The makeover of an earlier theory or fundamental transformation of old theory. For example, the perturbations observed in the path of Uranus after its discovery was
inexplicable. One had to make a choice between Newton’s theory is fundamentally wrong,
or there was a planet beyond the orbit of Uranus, which was giving a gravitational nudge.
Holding on to Newton’s laws, astronomers computed the position of the mystery planet.
Turning the telescope to that location in the heavens, astronomers discovered the planet
Neptune. Newtons laws were saved by adding one more planet into the solar system.
In contrast, the story of aberration in the orbit of Mercury is dissimilar. Taking a cue from
the discovery of Neptune a planet, Vulcan was hypothesised between Sun and Mercury.
Years of the search was in vain. Einstein’s theory of relativity overthrowing Newton’s
ultimately resolved the Mercury anomaly. Are the paucity of Standard Model be set right
with the reformation, or is it a fundamental flaw pressing for a revolution?
CERN experiments
CERN, the European Organisation for Nuclear Research, is one of the extensive science experiment in the world situated on the Franco-Swiss border near Geneva, Switzerland. It
hosts Large Hadron Collider (LHC), world’s largest and most powerful particle accelerator.
Circular in shape it is 27-kilometre in length. The accelerator sits in a tunnel 100 metres
underground.
Adorned with a ring of superconducting magnets the energy of the particles is boosted
while going around the ring. Fast-moving particles, primarily protons travelling in opposite
directions, are made to collide at four points around the machine.
It first started up on September 10, 2008. The first operational run was between 2009 to
2013. Higgs boson was discovered during this run. The machine was shut down for up-
gradation between 2013 to 2015 and the second operational run lasted from 2015 to 2018.
In the second upgraded run, colliding protons together had combined energy of 13 TeV.
Currently, the LHC has shut down again for enhancement.
When protons collide at near the speed of light in LHC, they break apart into quarks and
gluons. These fundamental particles interact, eventually forming a shower of fast-moving
particles. Several detectors placed around the LHC record the interactions. ALICE (A
large ion collider experiment), CMS (Compact muon solenoid), ATLAS (A toroidal LHC
apparatus) and the LHCb (Large hadron collider beauty) are four experiments that collect
data from the collisions of particles. Thousands of experimentalists and theorists use the
data to infer about the microcosm and the universe we live in.
B0 meson decay
The present study focused on a particular type of B meson called B0. Although B mesons
do not exist naturally, they can be created easily in high-energy particle collisions such as
LHC. A peculiar trait of some B mesons is that they spontaneously transform into their own
antiparticles and back before decaying into new particles.
“The B0 meson is made up of a bottom quark and a down quark. Occasionally B0 meson
decays into K* meson (containing a strange quark and down quark) and a pair of muons.
This is very rare decay. The Standard Model predicts only one such decay for every million
B0 decays. Any fluctuation would be evidence for new physics” says Rahul. Significant
deviations between the theory and experimental results suggest undiscovered particle or
force at play.
The Standard Model gives exact predictions for such decays. Further, all the various
variants of the Standard Model also render a specific rate. The angular distribution of the
decay particles is distinct in all the alternatives of Standard Model. Hence computing the
rate and angular distribution provides a testing ground for new physics. Rahul says
“incidentally in 1996 and 1999 we were the first to propose angular analysis in this mode to search for the new physics”.
The LHCb particle detector is a highly specialised instrument specifically designed to study
short-lived B mesons and is systematically investigating the rarest decays of these
particles. It can count these rare decays.
Anomaly persist
Using the LHCb data from the first run, during 2011 and 2012, one parameter known as P5
was analysed in 2017. It was found to be deviating from the various Standard Model predictions. Today’s result includes LHCb data from the machine’s second run during
2016. Deviation still persists, hinting at physics beyond the Standard Model.
Rahul says, “Although this data is not adequate to completely dismiss all variants of the
Standard Model nor enough to guess the layout of the new physics lurking behind. We can
still speculate, with other data the global parameter fit may indicate the presence of new
gauge bosons.” In particle physics, gauge bosons are associated with fundamental forces,
like photon with electromagnetic force and W and Z boson with weak nuclear force and
gluons with strong nuclear force.
Although the results from the Run 1 and Run 2 are smiler, the statistical significance of the
deviation observed is only about 3 sigma. At least 4 sigma results are necessary; the
Higgs discovery for comparison had five sigma statistical significance. Hence not all
physicist are convinced that it is time to write an obituary for Standard Model.
This result is at best an ‘indication’ that physics is beyond Standard Model but cannot be
classified as a ‘discovery’. “Here and in other related analyses, we keep seeing moderate
tensions with the Standard Model. We still don’t know how this mystery will turn out –
nothing has yet reached the level of solid proof” says Mat Charles, LHCb’s Physics Coordinator.
Way forward
About the lack of conclusive evidence, Prof. Sinha says “we must await the data from Belle II experiments as well as full data from the LHCb experiments in 2022.” While LHCb at
CERN is experimenting with B mesons obtained from the proton-proton collision, at the
High Energy Accelerator Research Organisation (KEK) located in Tsukuba, Japan, highly
intense electron-positron beams are made to collide, and a considerable number of B-
meson obtained.
Named Belle-II, the facility has an Indian connection. The fourth layer of the vertex
a detector in this facility was built by the physicists and engineers from the Tata Institute of
Fundamental Research, Mumbai.
The aim of Belle-II is similar to LHCb experiment at CERN, study the decay of the short-
lived B-mesons, and unearth clues to ‘new physics’. The initial results from Belle II on a similar study is expected around 2022. Rahul is hopeful “we can compare the data from both the LHCb and Belle II and come to a more sturdy conclusion.”
(TV Venkateswaran is a science communicator with Vigyan Prasar, New Delhi.)
