Could DZero result point to multiple Higgses?

June 4, 2010 | 3:53 pm

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.

Tona Kunz

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Anticipating the first steps beyond the Standard Model

June 3, 2010 | 12:20 pm

The particle physics community has just entered a critical new era with the commissioning of the Large Hadron Collider. The LHC has broken through the energy frontier and the first results are beginning to come out. Ultimately, the LHC will let scientists probe the laws of physics in new ways and may potentially reveal mysterious new phenomena, indicating that our current understanding of nature is incomplete. Within the next year, the LHC could discover the first signatures of new particles, new symmetries or even extra dimensions of space-time. The data collected could solve outstanding puzzles in physics or reveal new mysteries.

Physicists’ knowledge of elementary particles is encapsulated in the Standard Model of particle physics, which currently describes almost everything we’ve seen. Yet there is compelling evidence that the Standard Model cannot be the complete description of nature. For example, despite all of its successes, the Standard Model describes only 20 percent of the mass of the Universe. Eighty percent of the mass is known as “dark matter,” which we have never directly observed and know next to nothing about.

Another mystery of the Standard Model is that it typically predicts that the masses of all particles are incredibly heavy. To get the observed particle masses, the parameters of the theory have to be carefully balanced—precise to one part in 1032—in order to add up many large contributions and get a small answer. The probability that this occurs randomly is like the chances of winning the California lottery four times in a row! We’ve seen such absurd tunings before in theories, and they almost always indicate that some physical mechanism is causing the delicate balance of parameters. The LHC is designed to discover the cause of this fine tuning of the Standard Model parameters. Of course, we don’t know what mechanism solves this fine tuning problem, but numerous theories have been developed over the past 30 years.

Chief among these new theories is supersymmetry, a theory that doubles the number of particles of nature and changes their spin from being an integer to half-integer or vice-versa. For instance, the gluon is a Standard Model particle that mediates the strong nuclear force and has spin of one. The gluon’s superpartner is the gluino which has spin of one half. Supersymmetric theories also include a natural particle that could be the dark matter of the universe and is typically the superpartner of the photon, sometimes called the photino. If real, the photino would be nearly invisible and stable. Photinos could pervade the Universe, with 1000 photinos passing through a person every second.

The LHC is currently running with an energy of 7000 GeV, only half of its design energy. However, this is still more than 3.5 times as energetic as the Tevatron at Fermilab. Pushing the energy frontier will suddenly enable the production of new particles that were previously hidden and, even with a modest amount of data, discoveries are possible. In the SLAC theory group, some of our work is to help estimate how effective the first year of the LHC will be at discovering new particles such as the gluino.

The gluino is one of the most spectacular particles that could be produced at the LHC. Gluinos can be produced in pairs when two gluons from the LHC’s colliding protons interact. After the gluinos are produced, most theories of supersymmetry predict that they will decay in a fleeting moment—10-24 seconds—producing quarks and a photino. The photinos will just fly out of the LHC undetected, but their presence can be inferred by determining that energy is missing in the aftermath of the collision. The quarks are expected to produce a spray of strongly interacting particles as they leave the interaction region and can be reconstructed as “jets.” Therefore, the signature of gluino production at the LHC would be anomalous events with multiple jets and missing energy.

The primary challenge in discovering the gluino is to avoid mistaking something in the Standard Model for the gluino and at the same time ensure we aren’t dismissing gluinos as unusual properties of the Standard Model. Discovering new particles is a challenge that will require close collaboration between experimentalists and theorists in the high energy physics community. Our preliminary studies indicate that this next year holds amazing promise. To set the stage, with data from the past 10 years of running of the Tevatron, physicists have been able to push the discovery potential for gluinos from masses of 280 GeV up to 440 GeV. The data from the next year of LHC running will be enough to discover gluinos with masses of up to 800 GeV. We’re studying other theories that have similar signatures to gluinos and we’ve found that the LHC doubles their discovery potential. All this indicates that in the next year the LHC will rewrite what we know about the physics that takes us beyond the Standard Model of particle physics.

by Eder Izaguirre and Jay Wacker

This story first appeared in SLAC Today on June 3, 2010.

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Listening to the sound of science

June 1, 2010 | 4:27 pm

Ever wondered what you would hear if you could shrink to the size of a proton, squeeze into the Large Hadron Collider beam pipe, and listen to particles colliding? Would there be sounds, noises, maybe even music? That’s what Lily Asquith, ATLAS scientist and head of the LHCsound project, believes.

Asquith started thinking about an “audio event display” while chatting with some musician friends. “I was trying to make the noises I hear in my mind’s ear when I think of different particles moving through the detector,” she recalls. The project gradually took shape while Asquith finished her Ph.D. dissertation at University College London on the search for a low mass Higgs boson. In January, Asquith was awarded a grant from the UK’s Science and Technology Facilities Council for the sonification of ATLAS data, and she’s now leading a small team of two musicians/programmers, Archer Endrich and Richard Dobson from the Composer’s Desktop Project (CDP), and one illustrator, Toya Walker. The core development team is flanked by Ed Chocolate, DJ and producer, and Sir Eddie Real, percussionist, who are starting to incorporate sonified ATLAS data into their live performances.

Sonification is a technique that turns data into sounds while retaining all the original information contained in the data. A simple example is the beeping sound of a car parking sensor: the frequency of the beeps is related to the distance between your car and the obstacle, so that the closer you get, the higher the frequency. Sonification is also used in helping blind people to see, predicting earthquakes and identifying micrometeoroids impacting the Voyager II spacecraft.

The challenge for LHCsound is to find a coherent framework to map into sound the wide variety of abstract processes and variables involved in LHC experiments, such as the distribution in space and time of the particles emerging from a collision, the evolution of particles traveling through and interacting with matter, or even Feynman diagrams. A typical working session for the team includes a lively exchange of ideas (as well as computer programs and sound files) between the physicist and the musicians; whenever they are at a loss for words, diagram-drawing and strange vocalizations come into play.

“I bombard Dobson with crazy ideas for things like deconstructing a jet [of particles],” explains Asquith, “and he somehow understands what I am wittering on about and tells me what he needs to make the sound. Then I hack up bits of ATLAS software, stick them together and run over data to produce files of the format he needs.”

While Dobson created software to transform ATLAS data directly into sounds, Archer worked on converting these data in a format that can be used by the CDP software and thus be available for composers. Just three columns of numbers extracted by Asquith from the ATLAS reconstruction program gave birth to a wealth of sonifications of the quark jets generated from a decaying Higgs boson, with sound bites up to 90 seconds long.

The LHCsound website’s artwork is inspired by a similar brainstorming between Asquith and Walker, who discuss basic concepts like what makes particles different from one another. One discussion about the philosophical implications of finding the Higgs boson led to a portrait of the elusive particle as the Golden Snitch from the Harry Potter books.

Asquith is convinced that sonification could be useful for physics analyses. Our hearing system is a sophisticated neural network, capable of extracting a wealth of information from the sounds we listen to, with amazing precision – just the qualities that physicists expect from their analysis software. So, although the project was born as an outreach and musical composition endeavor, it might turn out to also have an impact on particle physics research.

Physicists’ curiosity about what particles sound like isn’t confined to the LHC. Across the Atlantic, physicists Agnes Mocsy from the Pratt Institute in New York and Paul Sorenson from Brookhaven National Laboratory teamed up with Alex Doig, senior illustration major at the Pratt Institute for “The Sound of the Little Bang” project. The “little bang” is what some physicists call the collisions between heavy nuclei inside particle accelerators, since they reproduce the condition of matter as it existed only millionths of seconds after the Big Bang. Brookhaven’s Relativistic Heavy Ion Collider smashes gold ions at nearly the speed of light to produce a peculiar type of matter called quark-gluon plasma. The plasma behaves like a liquid, and therefore sound waves should be able to propagate into it. Mocsy and Sorenson analysed the available RHIC data and obtained the sound produced by the quark-gluon plasma as it expands and cools down. The group posted a YouTube video describing the project and accompanying the sound with animation.

Both the LHC and RHIC projects aim at bringing science closer to the public in an innovative way, through music. Music is also the medium chosen by musician and producer John Boswell for his “Symphony of Science” – a series of inspiring digital mashups featuring famous scientists and science communicators like Carl Sagan, Richard Feynman and Brian Greene. Audios and videos from television programs and interviews are sampled and mixed, voices are pitch corrected and Boswell’s original compositions are added as soundtracks. The first video has collected almost 4 million hits on YouTube since its release in Fall 2009, and four more have since been produced. With the wealth of sounds now becoming available via sonification projects, perhaps a future Symphony of Science video will feature sounds from real RHIC or LHC data!

– Manuela Cirilli

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