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June 11, 2004

Higgs Up

[via Sean Carroll] Now, I know I’m not supposed to believe any physics papers published in Nature, but this one — if true — is pretty interesting. It seems that DØ has been reanalyzing their data on the top quark, and their new analysis pushes the central value for the mass up by 4 GeV. The previous world average value was m t=174.3±5.14m_t = 174.3 \pm 5.14 GeV. The new value is 1784.3178 \mp 4.3 GeV.

This may not sound like much of a change, but if you recall, the Higgs mass in the MSSM depends rather sensitively on m tm_t. The one-loop contribution, up to logarithmic corrections, goes like m t 4m_t^4,

(1)m h 2<m Z 2cos 2(2β)+6|λ t| 2m t 24π 2log(m t˜/m t) m_h^2 \lt m_Z^2 \cos^2(2\beta)+\frac{6|\lambda_t|^2 m_t^2}{4\pi^2}\log(m_{\tilde{t}}/m_t)

where λ t=m t/H\lambda_t= m_t/\langle H\rangle is the top Yukawa coupling, m t˜m_{\tilde{t}} is the stop mass, and tan(β)=H/H˜\tan(\beta)= \langle H\rangle/\langle\tilde{H}\rangle.

To accommodate the current experimental lower bound, m h>114m_h\gt 114 GeV, one needs a very heavy stop, m t˜>850m_{\tilde{t}} \gt 850 GeV. And, even so, one has an upper limit of something like m h<135m_h \lt 135 GeV.

Shift the top mass up by 44 GeV, and the upper limit on m hm_h also shifts up to m h<140m_h \lt 140 GeV. But, more significantly, the central “best-fit” value goes up from the now-experimentally excluded 9696 GeV to 117117 GeV. Just out of range of what would have been seen at LEP, but about the first thing they’ll see at the LHC.

Plot of Higgs mass versus tan beta, for old and new values of top quark mass

The mass of the lightest Higgs boson in the minimal supersymmetric standard model (MSSM). The predicted value is shown as a function of the parameter tanβ\tan\beta (the other MSSM parameters are chosen such that they maximize the resulting value of the Higgs mass). The predicted Higgs mass is sensitive to the value of the top-quark mass used in the calculation. The solid line indicates the prediction using the new measurement of the top-quark mass from the DØ Collaboration; the white band indicates the uncertainty of the prediction that results from the error on the top-quark mass. The dashed line shows the situation before the new measurement (the previous experimental error of 5.1 GeV/c2 is not shown). Based on the new value of the top-quark mass, an upper bound on the mass of the lightest MSSM Higgs boson of about 140 GeV/c2 is established. [Nature 429 (10 June 2004) p. 614.]

Posted by distler at June 11, 2004 12:06 AM

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more links

As Nature is not a free link, perhaps it is worth to point readers also towards hep-ex/0406012, from the D0 and CDF collaborations, as well as to the website of theTop D0 group, where some recent slides contain similar results. The more recent world averages were presented in hep-ex/0312023; I believe they already include the new evaluation of Top.

Said that, let me to bring into attention hep-ex/0105057, a preliminary result of L3 which was very against the minimal SUSY, because it hinted of some possible charged higs bosson with mass lower than Z0. I am inclined to give some credit to it, even when final L3 results are smoother.

Posted by: alejandro rivero on June 13, 2004 2:48 PM | Permalink | Reply to this

Re: Top-up, Higgs Up?

Since the old and new values are within each others’ uncertainty, this shouldn’t really be a surprise.

Or rather, it should be a surprise that people didn’t take the top mass to its 2 sigma limits when forecasting problems for the MSSM. Maybe they were squeamish about forcing the data in the direction the MSSM “wanted” - or people had a residual prejudice against a heavy top. The top is always heavier than you think!

This may mean that people will have to redo analyses of t-b-tau unification… although if they took 2 sigma in their previous papers they shouldn’t have to write a new one. (Some hope!)

I hadn’t seen that L3 claim (hep-ex/0105057), but since there hasn’t been any followup - the result was called PRELIMINARY in 2001 (their capitals!) - I doubt anything worldshaking is going to occur. Recall that it was L3 who got most of the 115 GeV “signal” - based on what are now thought to be optimistic assumptions about background.

Posted by: Thomas Dent on June 14, 2004 5:05 AM | Permalink | Reply to this

Re: Top-up, Higgs Up?

Well, to be precise the ALEPH got the highest 115 GeV, and the L3 supported the finding. In any case it is good to have these results in the memory if just to see why the “95% confidence level” is not a “99% confidence level”. Damn stats. (on other hand, I ack that the 68GeV signal is very good for my own excentric modelling, so be aware I can be biased here).

Ah, an update I take from Woit’s log: the Nature paper is already XXXed, hep-ex/0406031

Posted by: alejandro rivero on June 14, 2004 10:22 AM | Permalink | Reply to this

Link updated

Thanks. I’ve updated my link to point to the version on the Arxivs.

Posted by: Jacques Distler on June 14, 2004 10:51 AM | Permalink | PGP Sig | Reply to this

Re: Top-up, Higgs Up?

My mistake, it was indeed ALEPH not L3 that had much the largest number of claimed 115GeV events.

Legend has it that ALEPH is always eager to claim deviations from the SM, new physics, etc…

Posted by: Thomas Dent on June 14, 2004 2:12 PM | Permalink | Reply to this

Re: Top-up, Higgs Up?

I guess the legend was born at the same time than the 115 GeV announcement.

It is true that other collaborations are more cautious. This week I have seem an old preliminary plot from OPAL having the same 68 GeV micro-peaks that L3 had, but it seems they never announced “over background” there (By the way, here is an scoop: I keep seeing it in my exotic estimate of W boson mass from beta decay.)

Posted by: alejandro rivero on June 19, 2004 4:02 PM | Permalink | Reply to this

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