Particle physicists at CERN have measured charge-parity (CP) violation in the D⁰ meson for the first time.  Announcing the finding today during the annual Rencontres de Moriond conference held in La Thuile, Italy, the result has a statistical significance of 5.3 σ – exceeding the 5σ “gold standard” for a discovery in particle physics. It is the first time that CP violation has been seen in charm mesons and opens up the possibility of searching for physics beyond the Standard Model.

According to conventional cosmological models, equal amounts of matter and antimatter should have been created in the aftermath of the Big Bang. Matter and antimatter should have then annihilated, leaving only photons. However, the clear domination of matter in our visible universe indicates that our understanding of the physics of the early universe is incomplete.

One possible explanation for the dearth of antimatter was proposed by Makoto Kobayashi and Toshihide Maskawa in 1973. They suggested that the weak nuclear force — responsible for some types of radioactive decay — could act differently on matter and antimatter. This asymmetry is thought to contribute to what physicists refer to as CP violation, which means that the laws of physics change slightly when a particle is replaced by its antiparticle and when all three directions in space are reversed.

CP violation was first observed in 1964 at the Brookhaven National Laboratory in the US in particles called neutral K mesons, which contain a strange quark. Experiments at the BaBar detector at the Stanford Linear Accelerator in the US and the KEK-B accelerator in Japan in 2001 also observed the phenomenon in neutral B mesons, which contain a bottom quark. Each finding led to a Nobel prize — one in 1980 for discovering CP violation in neutral K mesons and the other in 2008 for the theoretical framework of broken symmetry in particle physics that predicted CP violation in B mesons.

Taking the difference

One of the smaller experiments at the Large Hadron Collider (LHC) at CERN, the LHCb is designed to study the physics of B-mesons. But particle collisions taking place in such experiments also produce other mesons, including the D⁰  — a neutral meson that consists of a charm quark and an anti-up quark — and its antiparticle.

In late 2011, the LHCb team found the first hints of direct CP violation in D⁰ meson when searching for the decay of the D⁰ meson and its antiparticle into either a kaon/antikaon or pion/antipion pair. Using data from the first year the LHC was operational, the asymmetry value was reported at approximately -0.82% with a statistical significance of 3.5σ. However, further measurements brought that value closer to zero, therefore giving no significant CP asymmetry.

We’re very excited about this result

Tim Gershon

In the latest analysis, LHCb physicists used the LHC’s full dataset between 2011 and 2018, finding an asymmetry value of -0.154% with an uncertainty of 0.029% at a statistical significance of 5.3σ. “Looking for the two decay products gave us the required sensitivity to measure the tiny amount of CP violation expected for such decays,” says LHCb’s spokesperson Giovanni Passaleva. “Measuring the extent of the violation then boiled down to counting the D⁰ and anti-D⁰ decays and taking the difference.”

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Eckhard Elsen, CERN’s director for research and computing, calls the result a “milestone” in the history of particle physics. “Ever since the discovery of the D meson more than 40 years ago, particle physicists have suspected that CP violation also occurs in this system, but it was only now, using essentially the full data sample collected by the experiment, that the LHCb collaboration has finally been able to observe the effect,” he says.

While this latest measurement is consistent with Standard Model predictions, Tim Gershon from Warwick University, who works on the LHCb experiment, told Physics World that the findings “open up new possibilities” to expand the programme of CP violation in charm mesons to test if there is physics beyond the Standard Model. “We’re very excited about this result,” adds Gershon. “It is testament to the analysis that has been carried out and the ability to counter possible biases in the measurement that could impact the results.”

The stakes are high and, of course, extraordinary scientific claims require thorough verification

Thomas Browder

The LHCb experiment is currently undergoing an upgrade during the LHC’s two-year shutdown. Gershon adds that when it turns on in 2021 then the experiment will be able to record data at a rate 10 times greater than previously possible, allowing physicists to collect as much data in one year then they have in total to date. “This will make our measurements more precise to allow us to study additional processes,” adds Gershon. “For example, we will be able to measure the amount of CP violation in the mixing process when D⁰ mesons flip into their antiparticle, and vice versa, which has excellent sensitivity to physics beyond the Standard Model.”

Checking and verifying

Thomas Browder from the University of Hawaii who works on the Belle-II detector at the SuperKEKB particle accelerator in Japan told Physics World that the recently fully operational SuperKEKB facility will also produce large amounts of charm quarks and will be able to “check and hopefully verify the LHCb result on charm CP violation.”

“We congratulate LHCb and CERN on their new result on CP violation in the charm quark sector, which suggests an unexpected path to new physics” adds Browder “The stakes are high and, of course, extraordinary scientific claims require thorough verification”.