Higgs at 120Gev !

[CraigS]

Rumours are flying thick and fast following CERN's officially released Nov 18th combined ATLAS and CMS searches …

LHC Combination of Higgs Limits: MH<141 Gev

The scoop seems to be that they'll be announcing a likely candidate for the Higgs Boson to a three sigma level, at around 120GeV. It seems we'll have to wait until Dec 13th for the official announcement … and at only 3 sigma, they'll probably be calling for more data to confirm it … so, maybe, add another six months to that date for the confirmation announcement.

Cheers

[spacezebra (Petra)]

Craig

Is it possible to simplify the document attached, Im interested to know a little more about the Higgs Limits.

Cheers Petra d.

[CraigS]

Hi Petra;

Pretty convoluted graphs, eh ?

As I understand it, from the first graph, the intersection of black line and the red line indicates that if there were a Standard Model Higgs, (HB), boson at 140GeV, then the probability of getting a stronger signal than the one seen, would be 0.95.

Also, at the 140GeV point, there is almost a 3 sigma excess. What this means is that if there were not an HB at this point, then the probability of getting a weaker signal than the one observed, would be about 0.99. So, the signal indicating a Higgs boson at 140GeV is five times stronger than the one tending to exclude it.

The bottom plot of the second graph ('Best fit sigma/sigma SM' etc), takes into account past observations, (Bayesian analysis), and theoretical predictions, and assigns a higher probability that the HB will most likely be found at the lighter end of the mass scale. The presently found results are then re-plotted against the 'expected' range, in order to narrow down the mass search region. It shows the region from 110 to about 180 GeV, has the most chance of where theory and past observations would predict/expect finding the notorious HB.

The plot on top of this one has left me for dead .. I haven't got the time today to work out exactly what its saying … (sorry).

Hope that helps.

Cheers

[sjastro]

Originally Posted by CraigS

Hi Petra;

Pretty convoluted graphs, eh ?

As I understand it, from the first graph, the intersection of black line and the red line indicates that if there were a Standard Model Higgs, (HB), boson at 140GeV, then the probability of getting a stronger signal than the one seen, would be 0.95.

Also, at the 140GeV point, there is almost a 3 sigma excess. What this means is that if there were not an HB at this point, then the probability of getting a weaker signal than the one observed, would be about 0.99. So, the signal indicating a Higgs boson at 140GeV is five times stronger than the one tending to exclude it.

The bottom plot of the second graph ('Best fit sigma/sigma SM' etc), takes into account past observations, (Bayesian analysis), and theoretical predictions, and assigns a higher probability that the HB will most likely be found at the lighter end of the mass scale. The presently found results are then re-plotted against the 'expected' range, in order to narrow down the mass search region. It shows the region from 110 to about 180 GeV, has the most chance of where theory and past observations would predict/expect finding the notorious HB.

The plot on top of this one has left me for dead .. I haven't got the time today to work out exactly what its saying … (sorry).

Hope that helps.

Cheers

Petra and Craig,

Even though there is one LHC there are two experiments going on here, the Atlas and CMS searches. Both have the objective of locating the Higgs boson.

For the top plot in the second graph, each test produces it's own sample data. The data from Atlas and CMS has been combined.
While both tests separately should show the Higgs boson having the same mean energy value, sampling data from both experiments may indicate that the mean Higgs boson energy has deviated by some particular amount.
The P-value is the probability that this deviation is due to sampling errors in the tests.

The theoretical values are based on Monte Carlo experiments.

These are in effect mathematically generated virtual particle accelerator experiments.

Regards

Steven

[spacezebra (Petra)]

Thank you both for taking the time to reply.

Okay, I think that I have a handle on this. I do have another question - but need to think it through. You may have already answered it.

Thank you again - this is really getting interesting.

Cheers Petra d.

[Robh (Rob)]

Interesting final comment from this short article ...
http://www.wired.com/wiredscience/20...c-anticipated/

"A 125 GeV Higgs is lighter than predicted under the simplest models and would likely require more complex theories, such as supersymmetry, which posits the existence of a heavier partner to all known particles."

Then add this from wikipedia (on the Higgs boson) ...

"The Standard Model does not predict the mass of the Higgs boson. If that mass is between 115 and 180 GeV/c2, then the Standard Model can be valid at energy scales all the way up to the Planck scale (1016 TeV). Many theorists expect new physics beyond the Standard Model to emerge at the TeV-scale, based on unsatisfactory properties of the Standard Model."

I don't understand. If the standard model doesn't predict the mass of the Higgs boson, then why is a mass of 125 GeV a problem?

Regards, Rob

[CraigS]

Been thinking about your question here, Rob .. my pure guess would be that one (or both) the two pieces of information you presented may not be founded in accurate assumptions (??)

I'm not sure I understand the overall problem posed by the Standard Model (SM) not making an accurate prediction of the HB mass, either (??).

It looks to me as though it is expected (by the author) that the SM should be capable of being extrapolated both up and down the energy scale (ad infinitum ?). I'd question the rationale behind this expectation. The SM theory seems to have been constructed after the discovery of particles (from particle accelerator experiments) … who is to say that it is accurate beyond the range over which we know it to be useful ?

I think this aspect is why they're excited about 'new physics' emerging.

I can't see why 125 GeV would be seen as being outside the range of usefulness of the SM. If the HB is discovered at this mass range, to me, it would seem to be well within the applicability range of the SM ? Perhaps they're saying that the HB can only exist at this range if SUSY is found to be valid … (I'm not sure this rationale is logical, however).

Sorry I can't be of more help on this one.

Cheers

[sjastro]

Originally Posted by CraigS

Been thinking about your question here, Rob .. my pure guess would be that one (or both) the two pieces of information you presented may not be founded in accurate assumptions (??)

I'm not sure I understand the overall problem posed by the Standard Model (SM) not making an accurate prediction of the HB mass, either (??).

It looks to me as though it is expected (by the author) that the SM should be capable of being extrapolated both up and down the energy scale (ad infinitum ?). I'd question the rationale behind this expectation. The SM theory seems to have been constructed after the discovery of particles (from particle accelerator experiments) … who is to say that it is accurate beyond the range over which we know it to be useful ?

I think this aspect is why they're excited about 'new physics' emerging.

I can't see why 125 GeV would be seen as being outside the range of usefulness of the SM. If the HB is discovered at this mass range, to me, it would seem to be well within the applicability range of the SM ? Perhaps they're saying that the HB can only exist at this range if SUSY is found to be valid … (I'm not sure this rationale is logical, however).

Sorry I can't be of more help on this one.

Cheers

Rob and Craig,

The problem with a low Higgs boson mass is that it can create fine tuning problems.

It's difficult to explain without going into some very deep theory but one of the principles of Quantum field theory (QFT) is to define and sum all the momentum changes as a particle goes from an initial quantum state to a final quantum state. This can lead to divergences (the UV catastrophe) where the summed momentums become infinite.
To avoid such infinities the theory needs to be renormalizable.
Quantum electrodynamics and Quantum chromodynamics are examples of renormalizable theories.
Quantum gravity is not renormalizable which is why no one has been able to unify gravity to the other forces.

A problem with a low Higgs mass is that renormalization can add mass to the Higgs boson. To avoid this the theory needs to be very finely tuned.
If SUSY exists then the problem goes away as the infinite terms are cancelled out.

Unfortunately technical papers describing this issue are very complicated and require a knowledge of the mathematical aspects of QFT.
This is one of the "simpler" explanations (warning high maths content).
http://www.weizmann.ac.il/particle/n...e/chapter1.pdf

Regards

Steven

[Robh (Rob)]

Originally Posted by sjastro

Rob and Craig,

The problem with a low Higgs boson mass is that it can create fine tuning problems.

It's difficult to explain without going into some very deep theory but one of the principles of Quantum field theory (QFT) is to define and sum all the momentum changes as a particle goes from an initial quantum state to a final quantum state. This can lead to divergences (the UV catastrophe) where the summed momentums become infinite.
To avoid such infinities the theory needs to be renormalizable.
Quantum electrodynamics and Quantum chromodynamics are examples of renormalizable theories.
Quantum gravity is not renormalizable which is why no one has been able to unify gravity to the other forces.

A problem with a low Higgs mass is that renormalization can add mass to the Higgs boson. To avoid this the theory needs to be very finely tuned.
If SUSY exists then the problem goes away as the infinite terms are cancelled out.

Unfortunately technical papers describing this issue are very complicated and require a knowledge of the mathematical aspects of QFT.
This is one of the "simpler" explanations (warning high maths content).
http://www.weizmann.ac.il/particle/n...e/chapter1.pdf

Regards

Steven

Steven, thanks for the info.

I actually get some of that. Renormalization is used to remove infinities and make the theory conform to reality. A low Higgs mass is more sensitive to renormalization. I don't pretend to know how they derived those equations but I understand how the function can diverge to infinity.

So, are you saying with SUSY that renormalisation isn't needed?

Regards, Rob

[sjastro]

Originally Posted by Robh

Steven, thanks for the info.

I actually get some of that. Renormalization is used to remove infinities and make the theory conform to reality. A low Higgs mass is more sensitive to renormalization. I don't pretend to know how they derived those equations but I understand how the function can diverge to infinity.

So, are you saying with SUSY that renormalisation isn't needed?

Regards, Rob

Hi Rob,

SUSY doesn't eliminate the need for renormalization.
There are two types of divergences, logarithmic and quadratic divergences.
Renormalization handles logarithmic divergences quite well, quadratic divergences are a different story.

SUSY eliminates the quadratic divergences.

http://en.wikipedia.org/wiki/Hierarchy_problem

Regards

Steven

[Robh (Rob)]

Originally Posted by sjastro

Hi Rob,

SUSY doesn't eliminate the need for renormalization.
There are two types of divergences, logarithmic and quadratic divergences.
Renormalization handles logarithmic divergences quite well, quadratic divergences are a different story.

SUSY eliminates the quadratic divergences.

http://en.wikipedia.org/wiki/Hierarchy_problem

Regards

Steven

Nice clarification! Thanks, Steven.

Regards, Rob

[CraigS]

Thanks Steven ... very interesting.
I'm starting to see why the 120 GeV mass range might represent a kind of interesting prospect for particle physicists.

I read somewhere, (can't remember who said it), words to the effect that: 'possibly the most boring result which could come from the LHC, would be if the Higgs is found ... and nothing else'.

If the Higgs is found at 125 GeV however ... and nothing else ... then the fine tuning issue may drive some particle physicists to drink !

Cheers

[sjastro]

Originally Posted by CraigS

Thanks Steven ... very interesting.
I'm starting to see why the 125 GeV mass range might represent a kind of interesting prospect for particle physicists.

I read somewhere, (can't remember who said it), words to the effect that: 'possibly the most boring result which could come from the LHC, would be if the Higgs is found ... and nothing else'.

If the Higgs is found at 125GeV however ... and nothing else ... then the fine tuning issue may drive some particle physicists to drink !

Cheers

Hi Craig,

That remark came from none other then Steven Weinberg, the winner of the 1974 Nobel Prize for the development of Electroweak theory.

I find the remark a trifle strange as Electroweak theory as developed by Weinberg assumes the existence of the Higgs boson.

Regards

Steven

[CraigS]

Originally Posted by sjastro

Hi Craig,

That remark came from none other then Steven Weinberg, the winner of the 1974 Nobel Prize for the development of Electroweak theory.

I find the remark a trifle strange as Electroweak theory as developed by Weinberg assumes the existence of the Higgs boson.

Well, from what you've pointed out, perhaps he knows that more than a mere particle 'discovery' will be needed to answer the truly nagging issues iconified by the Higgs ?
Cheers

[sjastro]

Not sure about this, but 5.0 sigma is the trigger for announcing the discovery.

http://physicsworld.com/blog/2011/12...meeting_a.html

Regards

Steven

[CraigS]

If you are really optimistic, I believe you can add these two results together in quadrature to get an overall result with a significance of 4.3σ.

Whaaaa ????
Talk about cookin' the numbers !??!

At the very best, this has to be the worst kept announcement of all time !
The way its going, there's no need for the official annoucement ... everyone already knows it all !

cheers

[sjastro]

Originally Posted by CraigS

Whaaaa ????
Talk about cookin' the numbers !??!

At the very best, this has to be the worst kept announcement of all time !
The way its going, there's no need for the official annoucement ... everyone already knows it all !

cheers

I think we need the input of a statistician on this.

Incidentally I hear the announcement will be webcast.
https://indico.cern.ch/conferenceDis...?confId=164890

Steven

[Robh (Rob)]

Originally Posted by CraigS

Whaaaa ????
Talk about cookin' the numbers !??!

cheers

Craig, I agree with you.

If you have two random variables x and y with mean1 and mean2, and then you add the variables x + y, the new mean = mean1 + mean2.
The standard deviations add in quadrature i.e. new sd = sqr(sd1^2+sd2^2).
In the article, given 2.5 sigma and 3.5 sigma,
then he proposes that the new sd = sqr(2.5^2+3.5^2) = 4.3 or 4.3 sigma.

However, this is different to just combining two lots of data together.
Here the mean will be (m*mean1+n*mean2)/(m+n) where m and n are the size of each population.
Similar calculations for the new sd will involve sd1 and sd2 with elements that reflect the population size in each experiment,
new sd = sqr((m*sd1^2+n*sd2^2)/(m+n)).

The author is mistakingly using the sum of two variables x + y.

Regards, Rob.

[CraigS]

Originally Posted by Robh

Craig, I agree with you.

If you have two random variables x and y with mean1 and mean2, and then you add the variables x + y, the new mean = mean1 + mean2.
The standard deviations add in quadrature i.e. new sd = sqr(sd1^2+sd2^2).
In the article, given 2.5 sigma and 3.5 sigma,
then he proposes that the new sd = sqr(2.5^2+3.5^2) = 4.3 or 4.3 sigma.

However, this is different to just combining two lots of data together.
Here the mean will be (m*mean1+n*mean2)/(m+n) where m and n are the size of each population.
Similar calculations for the new sd will involve sd1 and sd2 with elements that reflect the population size in each experiment.

The author is mistakingly using the x + y scenario.

Regards, Rob.

Cool explanation, Rob.

Thanks for confirming what I must admit is now, was just a remnant recollection from a past life which required some involved data analysis.
There are many other treatments which may end up being applied to the datasets before combining them also (eg: normalisations, weighting factors, skewing corrections ... and stuff I have no idea about). These treatments can easily rule out seemingly 'obvious' handling of derived distribution statistics. (As an aside, I've lost track of how many times I've seen people attempting to average averages, arithmetically).

Still, it all depends on which process step of the two datasets they attempt to combine. Until they present it all, we should view it with a 'steely eye', eh ?
I'm sure the folk dealing with the information know exactly what they're doing ... perhaps it was just the journo at work this time eh ?

Cheers

[sjastro]

There is a conspiracy theory going around that since the Dec 13th announcement is being presented by the ATLAS and CMS project leaders instead of the usual subordinates, this is a sign of a major discovery.

Some interesting comments, some very strange, from physicists speculating on the 13th December announcement.

Shelly Glashow, Boston University. Nobel prize in physics, 1979

"They said when the collider goes on
Soon they'd see that elusive boson
Very soon we shall hear
Whether Cern finds it this year
But it's something I won't bet very much on."

Frank Wilczek, MIT, Nobel prize in physics, 2004

"The Higgs mechanism for generating masses is extremely attractive and has no real competition. Beyond that there's little certainty. A near-minimal implementation of supersymmetry, perhaps augmented with ultra-weakly interacting particles, is the prettiest possibility. So I'd like several Higgs particles, Higgsinos, some ghostly stuff, and a pony."
[Note: A Higgsino is a supersymmetric partner of a Higgs boson].

Lisa Randall, author of Knocking on Heaven's Door, Harvard

"It is difficult to think of alternatives that are consistent theoretically and with everything observed to date that don't involve the Higgs mechanism – the process of essentially distributing a 'charge' throughout the vacuum. Elementary particles interact with this 'charge' and acquire mass. It is not necessarily clear, however, what is responsible for that charge in the first place and that is what determines what experiments will see.

"I still think the most likely answer is a conventional light Higgs boson. But when asked what I thought the odds were in a popular lecture, I surprised myself by saying 70%. I've even bet chocolate based on those odds. If not true, I think a heavier composite Higgs boson made up of more fundamental components might be the answer."

John Terning, University of California, Davis

"We know that strong interactions of quarks and gluons provide the bulk of the proton's mass; I suspect that there are some new – very strongly interacting – particles that provide the masses for the fundamental particles. The most spectacular possibility is that these new particles are the magnetic monopoles that Paul Dirac predicted."

Martinus Veltman, Universities of Michigan and Utrecht. Nobel prize in physics, 1999

"You are mistaken about the Higgs search at Cern. The machine runs at half energy so far, and no one expects relevant (for the Higgs particle) results. After the shutdown [in 2013] the machine will gradually go up in energy, and if all goes well (this is non-trivial) then in about half a year the machine energy might reach design value and there might be Higgs-relevant results. So if you are thinking next week then you are mistaken. Of course, we never know what surprises nature has in store for us … It is my opinion that there is no Higgs."

Philip Anderson, Princeton University. Nobel prize in physics, 1977

"I doubt if the opinions of one who thinks about these problems perhaps every 30 years or so will carry much weight. I've been busy. But the last time I thought, I realised a) that the Higgs(-A) mechanism fits the facts too beautifully not to be true, but b) it must be incomplete, because there's no proper accounting of the vacuum energy."
[Note: Anderson essentially described the Higgs mechanism in 1962, two years before Higgs and five other physicists published the theory.]

David Kaplan, University of Washington, Seattle

"I expect some variant of the Standard Model is correct, such as a two-Higgs doublet theory, although later one could well discover the Higgs bosons to be composite particles. Discovery of neutrino masses has opened a window onto physics beyond the Standard Model, and discovery of the mass-generation mechanism for quarks and leptons will open it wider."
[Note: the two Higgs doublet model calls for five Higgs bosons]

David Curtin, Stony Brook University

"It could be the Standard Model Higgs, but I sincerely hope not. Only data will reveal what nature chose, but two of my favourite alternatives are extra dimensions and supersymmetry – their discovery would tell us incredibly exciting things about several fundamental questions, including (but not limited to) the nature of space-time itself."

Gerard 't Hooft, Utrecht University, Nobel prize in physics 1999

"The whole idea that something should give mass to the fundamental particles is a hype that resulted from over-commercialisation of the Higgs theory, which actually might backfire on us. Fact is that in our present theoretical descriptions, most of the mass terms in the equations for the fundamental particles appear to violate an important symmetry (chiral symmetry) unless they can be connected to an additional field, the Higgs field, which would also require the existence of a not yet discovered particle, the Higgs particle …

"However, since chiral symmetry is unavoidable for the inner consistency of our description of the fundamental particles, the beautiful theoretical prediction of a Standard Model Higgs particle still stands out, and I still consider the near discovery of such a particle very likely. Alternative descriptions, such as many Higgs particles, each of which are more difficult to detect, or some altogether different mechanism, are much less attractive theoretically. As we know from the history of science, this argument does not suffice to rule out the existence of such alternatives, but I consider them much less probable."

David Miller, University of Glasgow

"Technicolor models use a new force of nature to generate particle masses. This new force is very strong, confining particles in bound states, and the binding energy gives the mass of the state. This is directly analogous to the generation of mass for the proton by the strong nuclear force."

Regards

Steven