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New Higgs predictions from EW fits: M(h)=80 GeV! January 19, 2007

Posted by dorigo in news, physics, science.

An improved version of the Mw-Mt plot has been produced for winter conferences. I am still unaware of the numbers entering in this new global fit, but here is the January 2007 result anyhow: the most likely higgs mass sits at 80 GeV, with an uncertainty of +36-26 GeV. This supersedes the former result of 85+39-28 GeV.  

The plot shows the dependence between the W mass and the top quark mass in the Standard Model, as a function of the unknown mass of the Higgs. Lines belonging to the green band represent the functional dependence between W and top mass for a fixed value of Higgs mass. The red ellipse constrains the three particle masses using electroweak measurements at LEP and SLD, while the blue ellipse encrypts all direct measurements of the top and W masses. The fact that the intersection between the blue ellipse and the red one is very small, and does not include any green region, implies that there is a tension between the lower limit on the Higgs mass obtained at LEP II (114 GeV) and the favored higgs mass, now at 80 GeV and thus about one sigma away from the allowed region.  

This is becoming a story of the incredibly shrinking higgs. But wait until my team blesses an improved jet energy scale measurement in CDF, using the Z->bb peak: I expect it to change things a bit, since most of CDF top quark mass determinations will be affected. If the authors of the analyses include the new scale measurement into their results, that is.


1. Thomas Larsson - January 19, 2007

OK, I’m totally confused. In your last post you claimed evidence for a 160 GeV MSSM Higgs (or did you?). I thought SUSY was disproven if M(H) > 130 GeV. Now you say M(H) = 80 GeV. Didn’t LEP rule out M(H)

2. Thomas Larsson - January 19, 2007

Hm, the end of my previous comment disappeared. It ended with Didn’t LEP rule out M(H) less than 115 GeV?

3. Tommaso - January 19, 2007

Hi Thomas,
I deny having claimed evidence of anything 🙂
Indeed, I should have been more farecul (sic) because while physicists do not take seriously a 2.1sigma effect, people outside our field could.
If you measure a bump in a mass spectrum, and you did not know beforehand where it would show up (i.e. if you did not make a prior hypothesis on the mass), the significance of the bump is way overestimated. That is to say, _somewhere_ a disagreement with background predictions is very likely to occur. The real significance of the 160 GeV excess is very, very small.

SUSY does not get disproven if a Higgs is found at 160. Only, it would then probably not be the lightest of the quintuplet (in the MSSM five higgs bosons are predicted, in other less minimal models you can have as many as you like).

Mh=80 is a prediction, based on indirect information. Mh>114 found at LEP II was a direct search limit. These two numbers are still not totally incompatible.

Hope that helps… Maybe I will post more about this later.

4. Kea - January 20, 2007

WOW! This just gets better and better. But it’s very frustrating not having anything really definite ….

5. Guess Who - January 20, 2007

Let’s see if I can keep * and / straight this time. 80*2 = 160, right? So your 160 GeV bump is two-Higgs production, with an 80 GeV Higgs. 😉

6. dorigo - January 21, 2007

Hmmm GW sorry to dissolve your theory, but we are not at the point where we have to resort to numerology – yet 🙂

Kea, in what sense is this getting better ? I mean, you like the ellipse shrinking or the fact that the fit Mh is getting farther and farther from 114 ?

7. Kea - January 22, 2007

Hi Tommaso. Both are pretty neat. Cheers.

8. Tony Smith - January 23, 2007

Tommaso said “… here is the January 2007 result anyhow: the most likely higgs mass sits at 80 GeV, with an uncertainty of +36-26 GeV…there is a tension between the lower limit on the Higgs mass obtained at LEP II (114 GeV) and the favored higgs mass, now at 80 GeV and thus about one sigma away from the allowed region. …”.

Although one sigma is far from enough to say that the Electroweak Standard Model is refuted by LEP II HIggs search, it does raise the question:
If the Higgs is really at least 114 GeV, then what parts of the Electroweak Standard Model are most likely to be changed in making the global fit to data match the experimental Higgs mass.

According to the 2006 PDG review “10. Electroweak Model …” revised September 2005 by Erler and Langacker:
“… The global fit to all data, including the CDF/D0 average, m_t = 172.7 +/- 3.0 GeV, yields M_H = 89 +38 -28 GeV …
the goodness of the fit to all data is very good … Only g_L^2 from NuTeV and A^(0,b)_FB from LEP are currently showing large (2.7 sigma and 2.4 sigma) deviations. …

The strongest individual pulls toward smaller M_H are from M_W and A^0_LR …
A_LR was measured precisely by the SLD collaboration …

while A^(0,b)_FB and the NuTeV results favor high values …
the Z-pole forward-backward asymmetries at LEP … are given by A^(0,f)_FB … where f = e, mu, tau, b, c, s …
LEP 2 … Measurements were made … including … A_FB for b and c …

it appears that at least some of the problem in A^(0,b)_FB is experimental …
the uncertainty in A^(0,b)_FB is strongly statistics dominated. …
for … A^(b,c)_FB + M_Z …[gives]… M_H … 349 +250 -148 …[GeV]…
for … All data …[gives]… M_H … 89 +38 -28 …[GeV]…”.

If the value of A^(0,b)_FB (pull toward high Higgs mass) were to be given more weight
the value of A^0_LR (pull toward low Higgs mass) were to be given less weight
would that give a Higgs mass more nearly in line with LEP II search results?

Since I am very naive about all these things, I will ask
whether the fact that LEP measured A^(0,b)_FB whose value is a pull that “favor[s] high values” of M_H
is related to the line in table 10.7 of the 2006 PDG Review:
“for … LEP 1 (no m_t) …[gives]… M_H … 168 +232 -91 …[GeV]…” ?
whether the fact that SLD measured A_LR whose value is one of the “strongest individual pulls toward smaller M_H”
is related to the line in table 10.7 of the 2006 PDG Review:
“for … SLD + M_Z …[gives]… M_H … 28 +26 -16 …[GeV]…” ?

Also, I note that the 2006 PDG Review says:
“… The forward-backward asymmetry, A_FB, for e+ e- final states in ppbar collisions has been measured by CDF and a value of sbar_l^2 extracted … A^0,b)_FB … and the hadronic asymmetries are mainly sensitive to sbar_l^2 …”,
another very naive question I have is
whether CDF results support the LEP result (pull toward high Higgs mass) ?

Tony Smith

9. dorigo - January 24, 2007

Hi Tony –

sorry for answering this comment so late. I am in no position to discuss the subtleties of the A_fb measurements, of which I am sure there are readers of this post who are more knowledgeable than me. However, I can certainly say you are right about those two pulls in opposite directions by LEP and SLD are measured in strength by the fact that if used alone in the fits they produce quite different values of the higgs mass. The relative strength of these pulls is, very simply, measurable by the distance between the higgs mass they predict with that of global fits (eg. 28 vs 80, 168 vs 80) and their uncertainty. The SLD measurement is indeed suspicious if exposed bare as the PDG does in the line you mention.

As for Nutev, their measurement is indeed quite off the mark, but I think that taking it away from the soup “because it is unlikely” subtracts information illegitimately. If included, the higgs mass magically grows by quite a bit… but the inclusions pays the price of a bad agreement with other measurements, so the “global” chisquare worsens significantly. I do not know by how much, unfortunately, since the blue band plot hides that information as far as I recall.

I will add that CDF measures the W mass high, but the pull in the global fit is not large. It is much larger the pull due to small top masses, and here let me be a bit suspicious, because my insider information happens to indicate that our jets are measured slightly low – by maybe a couple of GeV in the top decay range – and once that is taken into account, the top mass measurements increase by a little bit, enough to make the higgs rise also a bit. I cannot be more precise, since the b-jet energy scale measurement is not blessed yet, but it will be so in a few weeks, so stay tuned!

As for ppbar measurements of A_fb, I need to look those up!

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