A joint Fermilab/SLAC publication

Could DZero result point to multiple Higgses?

06/04/10
The DZero detector at Fermilab.
The DZero detector at Fermilab.

As if potentially helping explain why the universe is made of matter were not enough, a trio of  Fermilab theoretical physicists say a new DZero result could give weight to the belief that the story of matter has a sequel beyond the Standard Model.

The DZero collaboration at the lab's Tevatron collider found that when particles called B mesons decay, they give rise to pairs of muons significantly more often than to pairs of antimuons.  Although the difference was just 1 percent, it was a much greater preference for the creation of matter versus antimatter than previous experiments had found -- and one too big for the Standard Model of particles and forces to explain.  This imbalance, known as asymmetry, is important because it explains why matter and antimatter, created in equal amounts in the big bang, didn't simply annihilate each other;  instead matter came to dominate, allowing people and planets to exist.

What caused the DZero result's large deviation from Standard Model predictions is just as earth-shaking a mystery. The answer could point to the completion of the Standard Model, missing only the theorized Higgs boson particle, or the creation of a new story line for a host of new particles in the saga of how matter in the universe behaves.

In their quest for a full explanation, scientists debate whether they are simply missing a chapter in the Standard Model or if they need a sequel that goes beyond the model, potentially including extra dimensions or a theory called supersymmetry that would double the number of known particles.

For those who believe the Standard Model is nearly complete, the discovery of the Higgs boson--a theoretical particle that imparts mass to all the other particles- would close out the final chapter.

But for others who think that undiscovered physics properties exist- so-called new physics--a sequel to the Standard Model is needed. Bogdan A. Dobrescu, Patrick J. Fox, and Adam Martin fall into this camp. They say the DZero data hint that not just one, but five Higgs bosons may exist, and that those Higgses interact with other particles more strongly than previously predicted.

Dobrescu, Fox, and Martin published the paper CP violation in Bs mixing from heavy Higgs exchange in arXiv, the particle physics repository of preprint publications.

While a single Higgs particle can exist within the framework of the Standard Model, the existence of whole families of Higgs bosons requires a Supersymmetric extension of the Standard Model, in which every particle has a yet undiscovered superpartner of a heavier mass, requires the existence of whole families of Higgs bosons.

The fact that DZero found 1 percent more muons than antimuons produced in high-energy collisions between protons and antiprotons could be attributed to new particles, Martin says, particularly to five Higgs particles with similar masses -- three of neutral charge and one each of positive and negative charge. This configuration is called the two-Higgs doublet model.

It’s too early to tell definitively whether new physics beyond the Standard Model is at play, but the implications are there.

Fox equates the DZero data to a man in a dark room finding a box of matches; he can look around,  but still can’t see as clearly as if he had found the light switch.

To test their theory of why the DZero data shows such a large deviation from Standard Model predictions, physicists need to predict how the existence of a five-Higgs-boson world would affect other particles and then conduct experiments to find those effects.

Indirect evidence, such as refined measurements of the DZero data or searches at the Tevatron and the Large Hadron Collider for a meson decaying to a pair of muons or to a tau, could add more light to the room and reveal whether the next volume in the saga of the understanding of matter does indeed involve multiple Higgs bosons.

While the Tevatron can perform these indirect searches, it is too early to tell yet if the Higgs bosons would have masses the Tevatron can detect or would only be within reach of the higher-energy LHC.

You can hear more about the DZero asymmetry result and its possible implications for the universe and for research at the LHC during the one-hour science radio program "The World Revealed"  on Carnegie-Mellon University's WRCT 88.3 FM. About six minutes into the WAV file of the station's hourly programing begins the interview with Stefan Söldner-Rembold, a University of Manchester professor and co-spokesman of DZero; Manfred Paulini, a Carnegie-Mellon University professor and member of CDF and CMS; and Patrick Fox and Adam Martin, Fermilab theorists.

Update, June 18, 2010:

The DZero asymmetry result and how its sister experiment, CDF, will respond to it has been the talk of the blogosphere.

The study of the difference in the production of pairs of muons and pairs of antimuons in the decay of B mesons requires an extremely complicated analysis. In essence, physicists must sort through hundreds of millions of collisions daily to analyze two very large data sets of particle decays and determine the difference to an incredible level of precision. Because of the analysis’ complexity, cross checking the DZero analysis requires a nearly duplicate analysis, not something that simply looks at the same underlying physics process.

Complexity also breeds debate, and sometimes rumors. In particular, two rumors seem to have gained traction: that CDF already did a comparable study to DZero’s that negates the DZero claim of a 1 percent predisposition of muons over antimuons, and that CDF  for technical reasons cannot conduct an analysis that could confirm or deny the result.

First, did CDF already rule out the DZero result? No

As mentioned in the blogs Not Even Wrong and Resonaances, the CDF collaboration did release a sin(2beta_s) result a few weeks ago that looked at CP violation states and found the discrepancy between the amount of particles that decayed to matter versus antimatter in line with the Standard Model, the opposite of what DZero’s study found.  The CDF study is an update of a 2007 meson decay analysis that both CDF and DZero conducted where both found discrepancies in the preference for decays to matter and antimatter. The recent CDF update of this analysis, which is being submitted to the scientific journal Physical Review Letters, found a smaller discrepancy than in 2007. DZero has plans to conduct its own update.

However, comparing the updated CDF study and the current DZero asymmetry result is like comparing apples and oranges. True, both studies look at CP violation in meson decays, which can be used to search for “new physics” beyond the Standard Model. However, while the studies look at the same underlying physics, they use vastly different approaches and analysis methods that renders it impossible to draw a conclusion from the CDF study that would deny or confirm the DZero asysmmetry result, according to CDF leaders. Also, the uncertainties of both measurements more than “cover” the disagreement--so neither rules the other one out.

What the CDF study does do is highlight the need for CDF to conduct an apple-to-apple type study of the current DZero result that would have the potential to validate or invalidate it.

This brings up the second rumor: Can CDF cross check the DZero result? Yes

While there was some initial discussion, even among CDF collaborators, about whether CDF could perform the same search because the magnet construction in its detector differs from DZero's, CDF collaborators believe they can reach the same level of sensitivity as DZero to the decays.

CDF conducted a similar apple-to-apple type study several years ago, but with a much smaller dataset, not enough to make a judgment either way, according to collaboration leaders. The DZero result is based on more than 6 inverse femtobarns in total integrated luminosity, corresponding to hundreds of trillions of collisions between protons and antiprotons in the Tevatron collider.

However, the success of CDF’s past study, leads CDF collaborators to believe they can conduct a larger, comparable analysis, although in a slightly different fashion, than the past CDF study and the current DZero study.

The CDF collaboration announced at the Fermilab Users' Meeting in early June that it will perform this analysis with its full data set. The duration of the study will depend on the amount of preliminary work conducted, but collaborators estimate a result in time for a presentation at the winter 2011 physics conferences if the CDF search turns out to be as competitive as DZero's.

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