Category: Result

The chimera is a mythical beast,

. . . a monstrous fire-breathing female creature  . . . composed of the parts of multiple animals: upon the body of a lioness with a tail that ended in a snake’s head, the head of a goat arose on her back at the center of her spine. [1]

Composed from the parts of multiple animals, the chimera plays a central role in parts of Greek mythology. Recently, my first contribution to research using the ATLAS experiment was made public. A team of 17 physicists, including myself and SMU post-doc Aidan Randle-Conde, concluded a small study that will set the stage for a much larger and more focused search this summer. What were we looking for? A subatomic chimera.

The discovery of the Higgs particle is the central quest of the Large Hadron Collider. Believed to provide mass for all known subatomic particles, the Higgs is strongly expected to exist. But it has never been observed, and while experiments at Fermilab and the CERN are racing to find this particle, so far it has escaped detection.

The Higgs that is often discussed is known as the Standard Model Higgs. There is expected to only be one, it should have no spin angular momentum (“zero spin”), no electric charge, and from that we can make predictions about how often and with whom it interacts in the subatomic world. These predictions are the guide posts for experiment; whether or not the Higgs, as envisioned in the Standard Model, is there is up to Nature.

My colleagues and I are interested in another kind of Higgs, one which naturally would arise in the situation when the Standard Model is not the be-all and end-all of theories of quantum physics and relativity. In that situation, more Higgs particles would be needed to provide mass to new subatomic particles. One new Higgs that seems to commonly arise in even modest expansions of the Standard Model is a Higgs that still has zero spin, but carries electric charge. This is the “charged Higgs boson,” and this is what we started looking for this winter.

The 17 ATLAS physicists I mentioned above were just one of a few teams looking for this chimera; there are many ways to hunt for such a rare and lovely beast as a particle that gives rise to mass, and couples through the weak force, and couples through the electromagnetic force. The charged Higgs is also predicted to behave in certain ways, and a bunch of us have been looking for this particle decaying into a tau lepton and a tau neutrino. This is challenging search, but the rewards of the study have been worth it.

In the process of looking for this particle, I was able to learn something about top quarks and how to reconstruct them in an experiment like ATLAS. I’ve never worked with so heavy a quark before, and certainly not at a hadron collider, so for me this was a really novel experience. It was like being in graduate school all over again, albeit with more experience. Top quarks may decay to this charged Higgs, and we’re working to see if that is true. By the end of the summer, we expect to have a really solid and potentially field-leading first search for this process.

Meanwhile, enjoy the fruits of our labor from the 2010 data. We’ve begun to understand processes that could fake the signature of this charged Higgs boson, and armed with this information we’re gearing up for a real search for the summer.

• Data-driven estimation of the background to charged Higgs boson searches using hadronically-decaying τ final states in ATLAS.
• Study of discriminating variables for charged Higgs boson searches in tt events with leptons, using 35 pb−1 of data from the ATLAS detector

[1] http://en.wikipedia.org/wiki/Chimera_(mythology)

It took over a year-and-a-half, but my colleagues and I have completed an extremely sensitive search for low-mass dark matter [1]. This search was a key part of the case for taking the Upsilon(3S) and Upsilon(2S) data back in 2007-2008, before the BaBar experiment was shut down.

The strategy was one that evolved out of a series of conversations back in 2005 with my colleague, Bob McElrath, which he published and posted to the Physics Archive [2].  In short, since we don’t know anything about dark matter it may be as complex as the matter in the Standard Model; it may have both heavy and light components. We can search for a low-mass component using the decay of quarkonium, such as the Upsilon, the J/psi, and the eta meson; if such states can decay into dark matter, we have the challenge of looking for something turning into nothing.

So back in 2005 and 2006, lacking Upsilon(3S) data, I instead pursued a sample of Upsilon(3S) mesons derived from the huge sample of Upsilon(4S) mesons. The challenges of this sample were significant, and I was expecting then to be sensitive down only to a decay rate to dark matter of a few percent. That’s not stunning, but at the time it would have been the only such search. I got blown out of the water by the Belle and CLEO experiments [3,4]; Belle took three days of data at the Upsilon(3S) resonsance and beat my anticipated sensitivity by a factor of 10.

Realizing that pursuing this measurement using Upsilon(4S) data was a dead end, I abandoned Upsilon physics and concentrated on my old B physics haunts, as well as some tau physics. When the opportunity to take a few weeks of Upsilon(3S) data was pitched, I  started working with my old Upsilon colleagues on some ideas to motivate the data. Since it was only a few weeks of data, probably to be taken in June 2008, I wasn’t sweating it. I wanted to do the dark matter search, but it wasn’t a high priority.

When the budget cuts of December 2007 hit, we were already ready with a fairly fleshed-out case for taking this data. As it became clear that this case was THE case for taking any data in 2008, we dropped everything and focused on this data. It had to be taken,  with no or few problems, and analyzed as quickly as possible. It was a huge team effort, and I am proud to see our work in the search for low-mass dark matter add to the legacy of the BaBar Upsilon data.

So what did we do? We start with our Upsilon(3S) mesons. We look for them to decay to the Upsilon(1S) state by emitting a pair of electrically-charged pions, pi+ pi-. This “dipion transition” happens about 5% of the time, and has a huge advantage used by both Belle and CLEO in their searches: the mass recoiling against the pions, regardless of how the Upsilon(1S) then decays, is the Upsilon(1S) mass. We expect to see a big bump in the recoil mass spectrum at 9.46 GeV if a real Upsilon(1S) is recoiling against the pions.

We then require that there are no other particles in the event – that is, that the event contains pions and apparently nothing else. This is what the dark matter signature would look like in BaBar. The result is shown below – a big bump in the spectrum. The catch is that there are a number of things that can fake the dark matter signature, most notably when perfectly detectable particles from the Upsilon(1S)  miss the detector (only about 80-90%) of the space around the interaction point is covered by the detector). We found that all of this bump is caused by these backgrounds. Most of our time was spent understanding this background, insuring the reliability of our estimate and the uncertainty on that estimate.

Still, the result is stunning. With only 7 times more Upsilon(3S) data than Belle, we improved the sensitivity of this search by more than a factor of 8. Our approach to background rejection seems to have given us an advantage, and this advantage translates into strong sensitivity to invisible Upsilon(1S) decay. If the Upsilon(1S) decayed to dark matter even as rarely as 0.05% of the time, we definitely would have seen it.

We learned a lot while doing this search, and we intend to feed that back into future efforts. Meanwhile, we invite you to read the paper and provide comments and criticism and suggestions. We hope to see this published in Physical Review Letters soon.

[1] http://arxiv.org/abs/0908.2840

[2] Phys.Rev. D72 (2005) 103508 and http://arxiv.org/abs/0712.0016v2

[3] Phys. Rev. Lett. 98, 132001 (2007)

[4] Phys.Rev. D75 (2007) 031104

The past year has been a remarkable one in the search for the Higgs. Not only have the Tevatron experiments made strides to actually rule out a Standard Model Higgs Boson in certain mass regions, but progress has been made in filling in an important gap in our understanding of the Higgs sector. In certain extensions of the Standard Model, supersymmetry is introduced as a way to solve problems of the Standard Model at high energy. While solving some problems, supersymmetry creates some new ones. There have been attempts to solve these problems that do so by adding extra Higgs bosons to the theory. In doing this, they create the possibility for one of the Higgs bosons to be very light, perhaps even accessible at BaBar and Belle.

This possibility was part of the motivation for taking the world’s largest Y(3S) and Y(2S) data samples in 2008 at BaBar. In the past year, we have made remarkable progress as a community toward squeezing the possibility of this light Higgs. The CLEO collaboration fired the first volley shortly after BaBar started taking the Y(2S) data. I discussed those results, and the first results from BaBar, in an earlier post [1]. In the past few weeks, the D0 collaboration and BaBar have presented even more stringent results to the physics community.

D0 did what we always wanted LEP to do: look for h -> a a -> 4leptons, where “h” is the Standard Model higgs boson and “a” is the light Higgs boson. This process was not searched for at LEP, and if the light Higgs boson has a mass below the threshold for producing b-quarks it would have eluded detection at LEP.  So D0 went for the gap [2]. They don’t see any evidence for this process, a key prediction of the light Higgs boson scenario in extensions of the Standard Model and minimal supersymmetry.

Attacking the problem from the bottom up (pun intended), BaBar has just released two searches. One is brand new, and searches for Y(3S) -> photon + Higgs [3], where the Higgs decays to a pair of tau leptons. The other is an update of a previous preliminary result (discussed in [1]) looking for the same Higgs decaying to a pair of muons [4]. This update uses both the Y(3S) and Y(2S) meson samples, and makes significant improvements in the results. Both see no evidence for the low mass Higgs boson.

The Higgs Boson has eluded us for decades. As the Tevatron continues to push and the LHC comes online, I wonder how much longer the Higgs can withstand the onslaught of brilliant particle physicists. I guess the only escape for the Higgs is if it doesn’t exist at all. And wouldn’t THAT be interesting.

[1] http://steve.cooleysekula.net/goingupalleys/2009/02/15/lets-go-higgs-hunting/

[2] V Abazov et al. (D0 Collaboration), “Search for NMSSM Higgs bosons in the h->aa->mumu mumu, mumu tautau channels using ppbar collisions at sqrt{s}=1.96 TeV,” 0905.3381 (May 20, 2009), http://arxiv.org/abs/0905.3381.

[3] http://arxiv.org/abs/0906.2219

[4] http://arxiv.org/abs/0905.4539