Result – Page 3 – Going Up Alleys

May 29, 2009May 28, 2009

To CERN: messages from today, messages from 1980

Next week (June 2-6), I will be at CERN for the first time in many years (and to kickoff my many visits to come!). I am attending the ATLAS Collaboration Physics Week, a four-day event centering on ATLAS physics analysis. I am excited to be heading to the new Mecca of particle physics, the new global hub that will soon define the frontier of collider physics.

In the spirit of that new frontier, I had a very interesting conversation today at lunch. A senior SLAC theoretical physicist started discussing the very interesting evidence that he thinks is mounting for significant contributions to the proton from charm and bottom quarks. Called “intrinsic charm” or “intrinsic bottom”, these contributions mark the ultimate in quantum mechanics. In the classical physics view of the proton, the quarks rattle around inside glued together by the strong force. Quantum mechanics, which accurately described the proton and crushes the classical picture, instead tells us that the proton is mostly NOT the three quarks that define it. It’s mostly gluons, and in fact those gluons are constantly producing pairs of virtual particles that flit in and our of existence and contribute significant structure to the proton. Even in that latter picture, the basis of all computation, there are assumptions about how much you can ignore charm and bottom that may be proving to be wrong.

That more common view sees the contribution of intrinsic charm and bottom as negligible. But, in the high energy environment of leading hadron colliders – the Tevatron and soon the LHC – that picture may be incorrect (c.f. [1,2]). In fact, a recent D0 preliminary result [3] suggest that the proton/anti-proton collision debris close to the beamline (at large rapidity) exhibit behavior that greatly differs from the leading calculations of these collisions. These calculations are provided in the Coordinated Theoretical-Experimental QCD (CTEQ) parton distribution functions (PDFs) [4]. The D0 authors state that the version of CTEQ PDFs used in their comparison do not include the Tevatron Run II data results, so they represent a pre-Run-II picture of QCD at hgh energy. Their data suggests that picture needs updating.

The picture painted at lunch today was of this rich universe of phenomena lurking just inside this deceptively simple picture of the proton. Looking back at some papers on this, I also realize that we are entering an era where these ideas that seemed, perhaps, so far off in 1980 (ala [1]) are on our doorstep now, demanding attention. In the fight to understand the Standard Model, so that we can see beyond it, such issues will become more and more critical.

[1] S.J. Brodsky, P. Hoyer, C. Peterson and N. Sakai. Phys. Lett. B 93 (1980)

[2] http://arxiv.org/abs/hep-ph/0508126

[3] http://www-d0.fnal.gov/Run2Physics/WWW/results/prelim/QCD/Q14/

[4] http://www.phys.psu.edu/~cteq/

February 15, 2009February 15, 2009

Let’s go Higgs hunting

When we proposed the taking of the world’s largest sample of Upsilon(3S) mesons (which then led also to the taking of the world’s largest sample of Upsilon(2S) mesons), a key component of the proposal was coverage of low-mass Higgs boson scenarios. Such scenarios can arise in extensions of the minimal supersymmetric Standard Model, and could have eluded detection even at the most famous electron-positron “Higgs factory” – LEP at CERN.

Our friends at the CLEO experiment released their results in a search for a low-mass Higgs boson, decaying into muons or tau leptons, just months after we concluded our data-taking on the Upsilon(3S) and Upsilon(2S) [1]. It was a bit frustrating to see CLEO reach this milestone before we had analyzed the data. But, they had at their disposal just 21.5M Upsilon(1S) decays. In contrast, BaBar collected 122 million Upsilon(3S) and 99 million Upsilon(2S); contained in the two of those samples is a rich subsample of 24 million Upsilon(1S) produced by the charged two-pion transition from the parent Upsilon(3S) or Upsilon(2S) mesons. Taken together, BaBar possesses the world’s most powerful sample of mesons to throw at the question of whether or not there is a low-mass Higgs boson.

This past week, at the Aspen Conference on Particle Physics, BaBar unleashed its first results in the search for the low-mass Higgs decaying into leptons. Presented by Kevin Flood from the University of Wisconsin-Madison, BaBar has performed an extensive search for the process of an Upsilon(3S) decaying into a low-mass Higgs by emitting a photon [2]. This means of searching for the Higgs was first suggested over thirty years ago by Frank Wilczek [3].

In contrast to the CLEO result, which only looked for a Higgs up to a mass of about 3.6 GeV, BaBar went all the way up above the Upsilon(1S) mass at 9.46 GeV. This was important for several reasons. First, simply covering the mass range is critical to conclusively ruling in or out the Higgs. In addition, there are several places in the mass spectrum of specific interest, most importantly the mass region of the bottomonium ground state [4]. Why? Since the bottomonium ground state and certain hypothetical low-mass Higgses share the same quantum numbers, nature lets them mix one into the other. It could be that the state we thought was the bottomonium ground state is mixing with a Higgs, and when it does so it can then decay readily into leptons. We were anxious to see if a “bump” in the muon spectum appeared at that region.

CLEO excluded the rate at which this decay to a Higgs occurred down to the level of a few parts per million, the best limits to date on such a process. BaBar, with just its Upsilon(3S) sample, has improved on the CLEO results in most of their mass range by a factor of two, and above their mass range has set the most stringent limits to date (down to a few parts per million) on a low-mass Higgs boson produced in this way. We see no evidence that the bottomonium ground state decays to leptons, and from this data we conclude that the rate at which the ground state decays to muons is less than 0.8%. The limit curves in the full mass region, and in several zoomed regions, are shown below.

Mining this rich, unique sample of mesons will continue, as we press deeper into space of exotic physics that can be achieved with a precision experiment at the luminosity frontier.

[1] http://arxiv.org/abs/0807.1427v1

[2] http://indico.cern.ch/getFile.py/access?contribId=63&sessionId=15&resId=1&materialId=slides&confId=38534 and available on the arXiv at http://arxiv.org/abs/0902.2176v1

[3] Phys.Rev.Lett.39:1304,1977. http://www.slac.stanford.edu/spires/find/hep/www?irn=241814

[4] http://steve.cooleysekula.net/goingupalleys/2008/07/07/behold-the-elusive-ground-state-of-bottomonium/

December 7, 2008

Confirming the eta_b

Just shy of 6 months after the presentation of the discovery of the bottomonium ground state, BaBar has presented confirmation of the discovery using Y(2S) decays to the ground state instead of Y(3S) decays. The results were presented by on of the core analysts, Peter Kim, at the QWG workshop in Nara, Japan. His slides are available online [1].

The compatibility of the results is excellent, with variations only at the level of a single statistical deviation. You can see a comparison of the (non-peaking) background-subtracted data and the overlays of the fitted peaking backgrounds (left two peaks) and the signal (right-hand peak). The previous result was a 10-sigma result; the current one is just over 3 sigma, enough for a clear confirmation but not, by itself, an independent discovery. That’s OK – we were going for confirmation, so this worked out as well as we could have hoped!

Every analysis on BaBar is led by a core team of researchers, but reflects the hard work of dozens of people in guiding and reviewing the result. As with the original bottomonium ground state work, I continue to find it remarkable what discoveries lay in this Upsilon data, and how the hard work of the BaBar collaboration continues to pay off.

A paper will soon follow.

[1] http://www-conf.kek.jp/qwg08/session1_1/kim.pdf