Category Archives: Physics

CDF/D0 Combined Top Mass Result

For those of you interested in HEP in general and Higgs searches in specific, you may have already heard of the Combined CDF/D0 top mass preliminary release. They’ve changed from 170.9^+_- 1.1(sta)^+_- 1.5(sys) last year to 172.6^+_- 0.8(sta)^+_- 1.1(sys) which is a total uncertainty of 0.8%. Tommaso Dorigo has a really nice post on the individual measurements and combined uncertainties that went in to the result. There’s also a bit of general discussion on how to combine measurements and an interesting point on how top mass measurements may-or-may-not be effected by jet multiplicities in the final state. He suggests that the full luminoisty this might improve to 0.5% .

For those of you who don’t get all giggly at slightly lower error bars for their own sake, the top quark is the heaviest of the six flavors of quarks, and was first measured with certainty around 1994-5. The mass of the top is an important parameter in estimating properties of the yet-to-be-seen Higgs Boson, as well as an important input to describing the plethora of other physics processes at the LHC. If you want to know where to look for the Higgs, and you want to know what backround stuff isn’t new physics, then you want to know the top mass really really well.

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New Physics in Heavy Flavors: The plot thickens

So, yesterday’s announcement by the UTfit collaboration is not the only new potential signal of new physics.
This paper came out a few days ago, and they seem to see an excess in D_s to leptons + missing pt. This significance is 3.8 sigma, which is comparable to the UTfit difference, although in a totally different process.

The D_s is the lightest bound state of c\bar{s}, one charm, and one anti strange quark. This is an unstable state, and c\bar{s} likes to annihilate into a virtual W boson, which likes to decay into a lepton and a neutrino. So, if were seeing more leptons and missing pt than expected coming from D_s decays, maybe its because they have some other intermediate decay channel, like a charged Higgs, or something totally weird like a half-quark/half-lepton thing called a leptoquark. This can’t say what were seeing, only that its probably not SM like. 3.8 sigma is big, but when combining results between lots of experiments, its not as easy as averaging. there’s room for mistakes.

How does this relate to yesterday’s UTfit paper? They were looking at lots of processes and doing a global fit to a fundamental parameter, so the message isn’t as clear (to me) as “new particle/coupling, but many of those processes are dominated by something called a penguin diagram These types of diagrams are likely candidates for new physics, with the W line replaced with a new boson, or the quark loop replaced with new fermions. This is basically why people build dedicated machines to make B mesons. So far though, the B factories have measured everything relating to their sector 60 ways to Sunday, but have just discovered that the SM works really really ridiculously well in describing our world.

I don’t know about you, but I’m stoked.

Addendum:  I just got a reply from a friend at D0, and his statement is that the D0 measurement was only limited by statistics.  Since they should have double the luminosity anylized by the summer conferences, either they will be able to verify the UTfit with an individual experiment, or wash it off as an unchecked systematic.  Since the UTfit paper doesn’t use any B-factory data, I’ve been asking folks what they can measure at B-factories to test this, but I haven’t gotten any serious suggestions yet.  Looks like lots of the UTfit people are members of BaBar, though,  so they’re no doubt planning this as we speak.

3-sigma signal of new physics in b to s transitions?

Wow.

The Unitarity Triangle Fit Collaboration Just released a report on the arXiv that combined CDF/D0 data yeilds a 3\sigma deviation from the SM determination of B_s mixing phase.