CERN experiments put Standard Model to stringent test

Jul 19, 2013

New results to be presented at the EPS-HEP conference in Stockholm, Sweden, this afternoon have put the Standard Model of particle physics to one of its most stringent tests to date. The CMS and LHCb experiments at CERN's Large Hadron Collider will present measurements of one of the rarest measureable processes in physics: the decay of a Bs (pronounced B-sub-s) particle into two muons.

The new measurements show that only a handful of Bs particles per billion decay into pairs of muons. Because the process is so rare, it is an extremely sensitive probe for new physics beyond the Standard Model2. Any divergence from the Standard Model prediction would be a clear sign of something new.

Both experiments will present results to a very high level of statistical significance (over 4 sigma for each experiment). These results are in good agreement with the Standard Model.

"This is a great result for LHCb," says the collaboration's spokesperson Pierluigi Campana. "It's precisely for measurements like this that LHCb was built. This result shows that we're really putting the Standard Model to the most stringent test yet at LHC energies, and so far it's coming through with flying colours."

The Standard Model has been pieced together over a period of over 40 years. It is a remarkably successful theory that accurately predicts the behaviour of the , and has been put to the test experimentally with great precision. But the Standard Model is not the end of the story: it does not account for gravity, for example, and it does not describe the so-called dark universe. Only around 5% of our universe consists of the kind of visible matter described by the Standard Model. The rest is made up of and energy, whose presence is deduced from the influence they have on the ordinary, .

"This is a process that have been trying to find for 25 years," says CMS spokesperson Joe Incandela. "It demonstrates the incredible capability of the LHC and experiments like CMS that are able to detect such a rare process involving a particle with a mass that is roughly 1000 times smaller than the masses of the heaviest particles we are searching for now."

Although these new results are a further feather in the cap of the Standard Model, the door is still very much open for new physics. One popular theory is known as supersymmetry, SUSY for short. It postulates the existence of a new type of particle for every Standard Model particle we know, and some of these particles would have just the right properties to make up a large part of the dark universe. There are many SUSY models in circulation, and SUSY is just one of many theoretical routes to physics beyond the Standard Model. Today's measurements allow physicists to sort between them. Many are incompatible with the new measurements, and so must be discarded, allowing the theory community to work on those that are still in the running.

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More information: eps-hep2013.eu/

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ValeriaT
3 / 5 (2) Jul 20, 2013
This is a victory for the Standard Model... discovery of rare decay narrows space for new physics
Less diplomatically speaking, this finding disproves the supersymmetry theory and superstring theory, which depends on it (well, again). That is because many supersymmetric models predict a higher rate of Bs meson to muon pair decay than suggested by the standard model, so these new observations count against those theories. LHCb and CMS detectors at CERN have seen more decays and are able to confirm that this is the rate of decay of a Bs meson. That is good news for CERN's high-precision instruments, but bad news for those looking for signs of supersymmetry, an extension of the standard model which says all particles have a heavier partner.
JIMBO
not rated yet Jul 23, 2013
Valeria, Nice comment ! You beat me to the punch.
As usual, physorg articles are out to lunch. A nearly identical experiment was done at Fermilab's Tevatron ~ 5yrs ago, & the press release indicated it was a Heavy blow against SUSY. The B_s switching rate was significantly off from the SUSY predix, yet the SM nailed it. We are witnessing the slow disintegration of the SUSY paradigm which has dominated particle physics for over 30 yrs ! Career-long efforts of theorists are in shambles, & the experimentalists have once again shown that Data Rule, not `beauty'.
JIMBO
not rated yet Jul 23, 2013
Valeria, Nice comment ! You beat me to the punch.
As usual, physorg articles are out to lunch. A nearly identical experiment was done at Fermilab's Tevatron ~ 5yrs ago, & the press release indicated it was a Heavy blow against SUSY. The B_s switching rate was significantly off from the SUSY predix, yet the SM nailed it. We are witnessing the slow disintegration of the SUSY paradigm which has dominated particle physics for over 30 yrs ! Career-long efforts of theorists are in shambles, & the experimentalists have once again shown that Data Rule, not `beauty'.
JIMBO
not rated yet Jul 23, 2013
Nice comment Valeria ! You beat me to the punch. A nearly identical expt. was done at Fermilab's Tevatron ~ 5 yrs ago, w/same result. SUSY predix were blatantly wrong for the B_s switching rate, but SM was right on.
The 30 yr long stranglehold of the SUSY paradigm is disintegrating w/each new expt. The SUSY community (& stringers) are in Total denial that their brainchild is on death row.
JIMBO
not rated yet Jul 23, 2013
Readers: Pls pardon the multiple comments. My mouse glitched.
shavera
5 / 5 (1) Jul 23, 2013
Believe it or not, particle physicists aren't married to any one theory, nor do they (generally) declare them to be "reality" until confirmed by experiment. SUSY just seemed to be one of the easier solutions to a number of problems for a while. Now the troubles it has makes it seem like it's not the right path to pursue. So we'll try new things. That's life in the world of science.
shavera
5 / 5 (1) Jul 23, 2013
And narrowing windows aren't particularly problematic in the first place. Remember all the fuss just before the Higgs about all the mass regions that were excluded. And the great hue and cry that the Higgs wouldn't be found. And then it was found? Closing some windows helps us to narrow our search for where the truth lies. Null results are an improvement in our knowledge by telling us what paths not to pursue further.
JIMBO
not rated yet Jul 24, 2013
Difference between Higgs & SUSY is that for over a decade, the Tevatron searched for & did not find sparticles at the energies predicted. LHC took over in 2009, & slowly purged the parameter space out to 800 Gev, leaving only a tiny window left. Established predictions failed & the grim reaper has taken note.