...we studied the γγ invariant mass distribution over the range of 80 to 150 GeV/c2. With 37.5 pb−1 data from 2010 and 26.0 pb−1 from 2011, we observe a γγ resonance around 115 GeV with a significance of 4σ. The event rate for this resonance is about thirty times larger than the expectation from Higgs to γγ in the standard model...So, the note claims no less than a firm evidence for a 115 GeV Higgs boson decaying into 2
photons. This decay occurs in the standard model with the amplitude dominated by a W-boson loop. However, this decay is very rare - only about 0.2 percent of Higgses decay this way. It's absolutely impossible that ATLAS is seeing the standard model Higgs - the rate is way too small. However, one can imagine theories beyond the standard model where the production cross section of the Higgs and/or its branching fraction into 2 photons is enhanced.Well, it's past 4am, I really need to sleep. So just a few hasty remarks
- The H to γγ rate 30 times larger than the standard model one appears to be in tension with the existing Tevatron bounds. Both CDF and D0 in this mass range place a limit of order 20 times the standard model cross section. Actually, D0's limits shown at Moriond display an intriguing 2-sigma excess around 115 GeV...
- ATLAS also showed their first H to γγ results at Moriond. At that time, their limits were about 40 times the standard model rate. Actually, a bumpy feature near 115 GeV can already be discerned, especially if one assumes that the background rate is overestimated...
- Enhancement of the H to γγ rate by 20-30 times sounds humongous. This is certainly not a feature in any mainstream model, and I'm not aware of a single theory paper that predicts it. A recent paper argues that in the MSSM the rate is usually suppressed; in the NMSSM an enhancement is possible, however by a factor of 2 rather than 20. One ancient paper studies this issue in a more general manner. Large enhancement of the branching fraction to 2 photons is possible if the Higgs couples only to up-type quark (h0_u), or if it does not couple to fermions at all, the so-called fermiophobic Higgs (h0_bh). But this is still not enough; one would need in addition new charged particles with a large coupling to the Higgs (4th generation?, composite fermions?) in order to pump up the production rate.
- Strictly speaking, ATLAS observes a particle decaying to 2 photons, likely a scalar, and
likely produced in gluon fusion. A coupling to photons or gluons is not a defining property of the Higgs boson. To demonstrate the Higgsesness of the new particle one would have to pinpoint its coupling to electroweak gauge bosons, for example by measuring its associated production with W and Z bosons. Until that is done, alternative options are on the table.
Update: Tommaso points to newer and more stringent CDF limits than I linked to above. For a 115 GeV Higgs the γγ rate must be less than 15 times the standard model rate, which further disfavors the ATLAS signal. On the other hand, CDF has some excess near 120 GeV...
Taken at face value this finding (if real) reports a bump at approx. 115 GeV in the di-photon channel. Nothing else. As Jester points out, the exceedingly large production rate most likely rules out SUSY or SM Higgs.
ReplyDeleteAnd this is where the excitment begins...
Ervin
If real, you should not be calling something like this a Higgs. The standard terminology is very misleading. By the way, a mass of 113 GeV is equal to sqrt(2)M_W.
ReplyDelete... and how do we know that you are not a part of a hoax?
ReplyDeleteHi Jester, as you must know, 115 GeV is the vastly preferred Higgs mass in SUSY.
ReplyDeleteIf you were a bit scared by the news and if you need to replace your underwear right now, you may buy new underwear and up to $10 may be subtracted from those $10,000 you will pay later.
Cheers
LM
Given your post of the beginning of this month, this may well turn out to be a fundon ;)
ReplyDeleteThis is not the abstract of an internal ATLAS Note, its the abstract of something that is unchecked, undiscussed and unofficial within the collaboration. So, it is not ATLAS that has found the Higgs, but 4 private persons claiming to have found something while using data of the ATLAS detector - probably without fully understanding what they are doing. This might turn out to be interesting, but it might very well turn out to be a series of mistakes.
ReplyDeleteWhile it wouldn't suprise anyone that there is a Higgs at 115 GeV. The fact that its enhanced 30 fold is simply BS and more or less ruled out by Tevatron. Most likely the individuals involved have not applied the cuts properly.
ReplyDeleteIt's interesting to realize that the author of the internal note had some scores to settle with a 115-GeV Higgs... http://innovators.vassar.edu/innovator.html?id=72
ReplyDelete"But this is still not enough; one would need in addition new charged particles with a large coupling to the Higgs (4th generation?"
ReplyDeleteAm I correct in thinking that this would work via a t' or b' triangle, and that the enhancement is due to the large Yukawa coupling of the t' or b'? The t' or b' mass is proportional to its Yukawa, and the enhancement in the decay rate is proportional to the square of the Yukawa. You said the SM rate is dominated by a W loop, but, not knowing the SM rate for the top loop, and assuming for simplicity it is comparable to the W loop, would an enhancement factor of 30 in the decay rate then indicate a t' with mass around sqrt(30) x 174 GeV \sim 950 GeV, or a b' with mass 16 times that to compensate for the factor (1/2)^4 coming from the smaller electric charge? Please correct me if I've got any of this wrong.
Sorry, it should have been a b' with mass 4 times that.
ReplyDeleteSorry again, the amplitude could have 3 denominator powers of m_{t'} from the propagators, but the diagram is linearly UV divergent by power counting, so it's not directly clear how it depends on m_{t'}.
ReplyDeleteChris, for the 4th generation the enhancement of the Higgs production cross section is almost independent of the masses of the 4th generation particles (the coupling to the Higgs in the numerator cancels against the mass in the denominator). Thus, each new degree of freedom gives roughly the same contribution to the gg->h amplitude as the SM top, and the cross section goes up by a factor of 9. However, one should take into account that the 4th generation would contribute to the h->gamma gamma BF. I haven't checked it yet, but people on facebook say that the 4th generation interferes *destructively* with the SM W loop, in which case the enhancement of the gamma gamma rate would be smaller than the factor of nine.
ReplyDeleteby people on facebook you probably mean people on arxiv...
ReplyDeletehttp://arxiv.org/pdf/0706.3718
"On the other hand, CDF has some excess near 120 GeV"
ReplyDeleteBut if I'm right 115 is different from 120 ?
Yes they differ more than the quoted resolution. This CDF search is a hint against the ATLAS signal. Furthermore, looks like every search so far has had some excess somewhere, which may suggest that the backgrounds are not well modeled by Monte Carlo.
ReplyDeleteFYI: The CDF analysis uses a sideband fit to the data for the background estimate. No MC involved.
ReplyDelete-CDFer
"115 GeV is the vastly preferred Higgs mass in SUSY."
ReplyDeleteTo be more precise, 115 GeV became the vastly preferred Higgs mass in SUSY after LEP II ruled out all the previous vastly preferred lower values...
@-2, you're right, sorry, I should have said "...which may suggest that the backgrounds are not well modeled point" ;-)
ReplyDeleteJester, thanks. Anon @ 13:33, thanks for the ref. From pages 5 to 6 it seems that for the 2 gamma final state with a 4th generation the enhancement factor of 9 in the Higgs production rate is approximately cancelled by a factor \sim 1/9 from the reduction in the H -> 2 gamma decay rate, so a 4th generation can't explain the enhancement found by Wu et al.
ReplyDeleteThis histogram shows the number of known nuclear excited states for a given N,Z; and thus for a given mass A, if you look at it in diagonal. Given that abrupt transitions happen at about 81 and 115 GeV (look here for the diagonal sum), I have from time to time wondered if calibration errors can happen in the measurement systems for precisely these values of mass.
ReplyDeleteSorry to say this, but the group who did the analysis has a long history of discovering non-existent exotic particles, going back to LEP, UA1 and beyond...
ReplyDeleteHmmm. I didn't mean to sound that negative. We should wait for confirmation by other analysts within ATLAS.
ReplyDeleteObviously, from the plot the background is not modeled at all, neither for the slope nor for normalization. A peak can be artificially introduced if they have applied wrong sliding cuts trying to optimize signal over background for a certain higgs mass hypothesis.
ReplyDeleteSibab
Yet another "first definitive physics beyond the Standard Model". When I was in graduate school the Standard Model contained no neutrino mass, and no Majorana particles or fermions without gauge couplings. So there is already physics beyond the Standard Model, unless you just redefine the SM to be whatever is consistent with the data as of today (Shelley Glashow, by the way, said neutrino mass would be BSM). And of course there is DM, which whatever it is, certainly isn't SM. So for those of us who know something about physics that isn't done at colliders, "BSM" is already old hat. But of course I realize that the only physics which counts as particle physics is done at colliders, where all sorts of unexpected new stuff has been discovered recently like the, uhm, er, like the, ah, oh yes, like the tau lepton.
ReplyDeleteI think Anonymous makes an excellent point.
ReplyDeleteI don't. Classing neutrino mass as BSM is debatable, as the SM can easily accomodate this - just give the neutrino a small Yukawa coupling. Sure, it's a puzzle why it's so small, but we already have this problem with the quarks.
ReplyDeleteDark matter is a big puzzle, but... it hasn't actually been observed. We know it's there, but we don't know what it is. If a DM candidate is observed in a collider experiment that would be a massive result.
First of all Anonymous, you failed to get the joke, which is there's like 5 Anonymous's all saying different things.
ReplyDeleteBut anyway *that* anonymous is plainly right. Theorists regularly moan that they can't move forward because there are no results inconsistent with the Standard Model, but meanwhile the Standard Model keeps changing little by little. Neutrino masses. Dark matter. Inflatons.
There's all sorts of stuff out there unaccounted for.
The result is four sigma from no Higgs, which sounds like a signal. But, query how many sigma it is from SM Higgs or MSSM Higgs at 115-120 GeV? It might well be within a couple of sigma of that.
ReplyDeleteThus, while it might be a fluke that it is 30x or what have you from expectation, it might not be a fluke that there is a signal.
Is anyone else even concerned that, in light of the low probablility of Higgs particle even being discovered, Wu has claimed to have observed it at least once before? It concerns me. Despite her being a tenured professor at the University of Wisconsin at Madison, and again, despite the probabilty factors involved, in 1999, before the LEP closing, she claimed to have discovered 'the strong possibility of discovery' of the Higgs boson. See (http://www-wisconsin.cern.ch/~wus/talks/mumutalk.pdf)
ReplyDeleteI know her credibility has certainly dropped into the basement after seeing her name appear on this COM note from Atlas...
I'm not too concerned about who did the analysis, I'm only interested whether the signal is really there. As far as I know, nobody in ATLAS says that the analysis is junk.
ReplyDeleteYou know, Rubbia before getting famous for discovering the W boson was famous for discovering alternating neutral currents ;-)
Let's say If higgs boson was discovered how significant is it going to be besides able to explain how particles get mass?
ReplyDelete