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A summary Mw-Mt plot for Christmas 2006 January 5, 2007

Posted by dorigo in news, physics, science.

I suppose that the heavy machinery of global electroweak fits is at work at this very moment, to produce an updated blue band plot and a new estimate of the Higgs boson mass. I am unfortunately out of the loop, so I cannot broadcast here any first-hand information on what the new estimate of the Higgs boson mass is – but I guess it will not move much from the 85 GeV estimate of Summer 2006: probably the added information will kick it down by a couple GeV or so. You can always check the status of global fits at the LEP electroweak working group web page, http://lepewwg.web.cern.ch/LEPEWWG/ .

What I can do for you today is to show a couple of plots, which just summarize the status of top and W mass measurements together with the Standard Model fits predictions. Here is the by now famous “MtMw” plot:

As you see, the new CDF measurement does not add dramatically more information: indeed, the world average for Mw is more precise than CDF measurement alone, as the plot below shows clearly. However, the hints at a light higgs boson are growing.

Bear in mind that there is quite a bit of magic entering into this plot. For one thing, a indirect measurement of the W boson mass performed with neutrino scattering experiments computing the rate of neutral and charged current interactions comes out low, and inconsistent with all other determinations (except maybe the L3 one): that one result has been excluded from averages, but it is a somewhat arbitrary choice which, if reverted, would drive the blue ellipse in the plot above quite a bit down.

Anyway here is the new CDF measurement compared to previous determinations and to the current world average:

UPDATE: something’s wrong with the first plot above. See the first comment to this post.


1. dorigo - December 22, 2006

It makes little sense for me to comment on my own post, but actually, the first plot makes me frown, now that i look at it from a distance. The CDF measurement is 80413 MeV, so the star in the middle of the black line should lie at around halfway between the 80.4 mark and the next one (80.425). But it is much lower than that: if you look at it closely, it seems to lie at about 80.406 or so. The same applies if you look at the extremes of the error bar: looks like 80.455 and 80.360 or so, while you’d expect 80.461 and 80.365.

Hmmm. Will investigate.


2. Guess Who - January 6, 2007

Am I the only one to find it suggestive that the Higgs mass central value has been slowly creeping down to 2*m_top + small change?

3. dorigo - January 6, 2007

Hi GW,

most probably you meant M_top/2, right ? But in any case, I think it is much too early to play that kind of game with the -still unknown, and still too imprecisely predicted- higgs boson mass.

There do exist a number of coincidences in the mass spectrum of known elementary particles… Even after the advent of the quark model, when the club of really “elementary” bodies has shrunk considerably, there are a few puzzles left. One I would really like to see an answer to is the Yukawa coupling of the top quark being exactly unity (see https://dorigo.wordpress.com/2006/03/20/new-tevatron-average-of-top-quark-mass-13-total-error/ and comments therein).

Coincidences like that were indeed useful in the fifties, when from near-equality of masses or of mass differences between different hadrons whole new symmetries, approximated but none the less spectacularly puzzling, were invented. And those symmetries were instrumental to the deepening of our knowledge in the subatomic world.


4. Guess Who - January 6, 2007

Oops! If I can’t even get * and / right it’s evident that I desperately need to get off the eggnog now. 😉

But yes, I do wonder about the top’s Yukawa coupling too, and whether it may not be a(nother) sign that top and Higgs are more intimately related than currently understood.

5. dorigo - January 7, 2007

A further note about the first plot. The more I look at it the less I like it.
Indeed, since the red ellipse shows indirect measurements by LEP I and SLD, and since LEP II measured the W mass but NOT the top mass, putting together LEP II and Tevatron determinations into one single ellipse (the blue one) is deceiving, unfortunate, and unfair to the Tevatron.

Indeed, while the Tevatron experiments can draw an ellipse with direct determinations of BOTH the W and top quark masses, the LEP II experiments can only draw a horizontal band in that plot.

So it would be good to have a plot with:

– the odd-shaped ellipse of indirect measurements
– a band for LEP II measurements of the W mass
– an ellipse for Tevatron measurements of top and W masses
– another ellipse, centered at the same top mass value as the former, with the world average of direct measurements.

All labeled up properly, please!


6. Not Even Wrong » Blog Archive » Various Stuff - January 11, 2007

[…] coming out of the Tevatron, including a new, more accurate value of the W-mass. See for instance here, here, here, and here. About the new W-mass measurement, there’s also a Fermilab press […]

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