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Anticipating the first steps beyond the Standard Model

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.