A big jump

I heard of this jump weeks ago. But I did not play dice on it, anyway I feel it may not hold up, but it might. What will be the result ? NO body knows for the moment. There is indeed a churr in the community, as described in this news [http://www.sciencemag.org/content/332/6027/296.full]:

Particle physicists haven't discovered anything truly surprising in 35 years, so a mere hint of something odd works them up in a hurry. So it was last week, when, aided by press reports, news spread that scientists in the United States may have spotted a bit of matter unlike any seen before. But even as they contemplate the implications, physicists are taking the result with a grain of salt. The supposed signal could be an experimental artifact, caution the researchers who found it. And if a new particle is there, physicists may have to perform theoretical contortions to explain why they didn't spot it before. “I think the result is rather inconclusive,” says Christopher Hill, an experimenter at Ohio State University in Columbus, who was not involved in the work.

The finding comes from the 700-member team working with the CDF particle detector at Fermi National Accelerator Laboratory (Fermilab) in Batavia, Illinois. The team analyzed the billions of collisions of protons and antiprotons produced by Fermilab's atom smasher, the 25-year-old Tevatron, which will shut down this year. Those high-energy collisions can blast into fleeting existence massive subatomic particles not seen in the everyday world. Physicists try to identify those particles by studying the combinations of familiar particles into which they decay.

In this case, experimenters searched for collisions that produced a particle called a W boson, which weighs about 86 times as much as a proton, along with some other particle that disintegrates into two sprays of particles called “jets.” A jet arises when a collision or decay kicks out a particle called a quark. A quark cannot exist on its own but must be bound to other quarks or an antiquark. So the energetic quark quickly rips more quarks and antiquarks out of the vacuum of empty space, and they instantaneously form particles called mesons, each containing a quark and an antiquark. From the energies and momenta of the two jets, researchers can infer the mass of the particle that produced them.

CDF researchers see about 250 events in which the jets seem to come from a particle weighing about 155 times as much as proton. Those events show up as an unexpected peak in a data plot (see diagram). The chances that random jets or jet pairs from other sources would produce a fake signal that strong are 1 in 1300, the physicists estimate. “We've been struggling for 6 months to make this peak go away, and we haven't been able to do it,” says Robert Roser, a physicist at Fermilab and co-spokesperson for the CDF team. Still, he says, the signal is “not even close” to strong enough to claim a discovery.

Experimenters have several reasons to be cautious. The analysis depends critically on physicists' understanding of jets. CDF does not measure every particle in a jet, so researchers must make a 25% upward correction to a jet's measured energy. If the uncertainty in that fudge factor is bigger than they estimate, “then maybe the excess isn't so significant,” says Shahram Rahatlou of Sapienza University of Rome.

CDF physicists must also take care that they haven't mistaken random pairs of jets for new particles. To see the peak, they must subtract out a huge “background” produced by events containing a W and random jets. If that subtraction isn't just right, it could produce a fake signal. “The real question is how well do we understand that [background],” says Joseph Lykken, a theorist at Fermilab.

But those caveats have not stopped theorists from trying to explain the curious bump in terms of new particles. Felix Yu, a theorist at the University of California, Irvine, suggests that the new particle could be one known as a Z′ (pronounced Z-prime), which would convey a new force much like a very short-range electromagnetic force. Estia Eichten, a theorist at Fermilab, and colleagues say the particle could be a “technipion,” a particle predicted by a type of theory called “technicolor,” which posits a new kind of strong nuclear force.

To have escaped notice until now, however, a particle would have to have some weird properties. Generally, a Z′ ought to decay into an electron and an antielectron. In fact, experimenters have already searched for and failed to find that decay. So Yu's Z′ must not decay that way for some reason. The technipion may face similar problems. CDF researchers are searching for the long-sought Higgs boson, the key to physicists' understanding of mass, by looking for events in which it is produced with a W boson and in which the Higgs decays into two jets specifically triggered by particles called bottom quarks. The hypothetical new particle hasn't shown up in those Higgs searches, so it must not often decay into bottom quarks, as one would expect a technipion to do.

For those reasons, some physicists say such explanations of the bump seem contrived or “unnatural.” “Yesterday, these models weren't popular,” Hill says. Yu counters that “having a theory that looks pretty but doesn't fit the data isn't natural.”

The supposed signal should be confirmed or ruled out in short order. The CDF team has analyzed only half of the data it has already collected. And the Tevatron's other large particle detector, D0, has a data set as big as CDF's. If the particle is there, D0 should see it, too. “We hope that within a few weeks you'll be hearing from us,” says Dmitri Denisov, a physicist at Fermilab and co-spokesperson for the D0 team. In the meantime, physicists will enjoy the buzz.