MATT STRASSLER
Of Particular Significance

Category: Higgs

Reviewing the Search for the Higgs

Since we’re now approaching the time when the preliminary results from December on the search for the Higgs particle at the Large Hadron Collider (LHC) will be presented in final form, possibly with small but important adjustments, and since there will be additional results based on the fall’s data in the next few weeks, it would be good to do a little review of where things stand and where they’re going.  I won’t do this all in one post but let’s get started.

Of course there is a lot of material on this website already and I’ll point you to it.  Perhaps my most concise and least technical discussion of the search for the Higgs appeared as a guest post on Cosmic Variance (thank you to Sean Carroll for the invitation.)  In that post I emphasized that the LHC’s Higgs program has two phases (broadly speaking.)  The first phase is to search for and either find or exclude the simplest possible form of Higgs particle, known as the Standard Model Higgs particle (or SM Higgs for short).  This is a finite task, so at a certain point (presumably this year) this phase comes to an end.  The second phase, which will take a better part of a decade, and (as I’ll describe over time) begins this year, depends on whether a particle that resembles an SM Higgs is discovered or not.  To explain the basic logic, I presented a figure in the Cosmic Variance post, which I am reproducing here:

Fig. 1: From my guest post in Cosmic Variance: The basic logic of the Higgs search, showing Phase 1 in which the simplest form of Higgs particle (the Standard Model Higgs) is found or excluded, and the logical questions that will follow in Phase 2. See Figure 3 for a more complete version.

If you’ve been noticing how slowly the discovery of the Higgs takes place — first there are hints, and only months later can one know whether those hints are real or not — then it will not surprise you to learn that the line between Phase 1 and Phase 2 isn’t sharp. The scientists at the LHC experiments ATLAS and CMS will be doing both of them this year, because they can, and because they should.

  • The search for the SM Higgs in 2011 was so successful that only a few possibilities remain (see Figure 2): a Higgs between about 115 and 128 GeV/c2 or above 600 GeV/c2 (though the latter is disfavored by precision measurements.)
  • We’ve got clear-enough hints for a Higgs particle with a mass of about 125 GeV/c2 (where c is the speed of light and GeV is defined here) to take seriously the possibility that it’s been found.
  • If it’s been found, it resembles (only roughly so far, but that’s perhaps because there’s not enough data yet) an SM Higgs particle.
  • All that remains to be done in Phase 1 is to close the window between 115 and 127 GeV/c2 , to confirm or refute the hints around 125, and push up the limits at 600 up as far as they can go, perhaps 800 and beyond.
Fig. 2: Phase 1 of the Higgs search now leaves only a tiny window at small masses and a window at large masses that is disfavored by two decades of precision measurements. Notice how much progress was made in 2011 alone!

So we can start looking ahead to Phase 2 — indeed we must.    In particular, if there is indeed a new particle with a mass of 125 GeV/c2 that resembles an SM Higgs, then (from Figure 1) we now have to verify whether it is or isn’t exactly what the Standard Model predicts.  Any deviation whatsoever from the precise predictions of the Standard Model would be a historic, game-changing, and Nobel Prize-deserving discovery!  So the stakes are very high.

Now, Figure 1 was actually a simplified version of the game plan for the Higgs search.  A more complete version, which was too elaborate for the short and sweet Cosmic Variance article, is shown in Figure 3.  I’ll go through this figure carefully (which itself isn’t entirely complete) over the coming days and weeks.  But suffice it to say that 2012 will involve a lot of work on the three research programs located at the far right of the figure:

Fig. 3: As Phase 1 of the Higgs search comes to a close, Phase 2 begins, with a series of logical questions and corresponding measurements. Even while the hints of a Higgs particle at 125 GeV are being checked with more data, studies of whether this might or might not be a Standard Model Higgs particle will be underway.

As you can see from the figure, these very large research programs needs to be carried out whether or not the current hints of a Higgs turn out to be ephemeral.  In other words, the experimenters can be working already on Phase 2 no matter what the outcome of Phase 1 turns out to be!  Very convenient, of course…  but it means we need to do some planning for Phase 2 now!  [And not of all of the most urgent issues have been addressed yet, which is why I’ve been very busy… more on that later…]

Picture of POSTED BY Matt Strassler

POSTED BY Matt Strassler

ON January 24, 2012

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Has LHC Found the Higgs? Good Reasons For/Against Confidence

Are the current hints of a signal in the search for the Standard Model Higgs particle (currently underway at the Large Hadron Collider experiments ATLAS and CMS) really reflective of a Higgs particle?  Should one be confident that they are, or not?  I’ve written an article that presents strong  arguments on both sides.  Where you go from there is up to you, of course.  Personally I tend to be conservative about these things, so if there’s a strong argument not to be confident, that’s how I lean. Others lean the other way — that’s fine with me, as long as they use a legitimate argument to get to that point of view.

Clearly if experienced scientists aren’t showing much agreement yet, a layperson can’t be too certain about what is going on.  Don’t worry — over time, with more data in 2012, the disagreements about the current hints will die away, and you’ll hear a more and more confident consensus emerge as we get more evidence.

Picture of POSTED BY Matt Strassler

POSTED BY Matt Strassler

ON December 19, 2011

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Theory/Experiment Discussion at CERN about Higgs

[This post is now out of date; for a discussion of the data showing strong arguments in favor of confidence, and against confidence, that the excesses in the data reflect the presence of a new particle, click here.]

At 2:30 CERN time today, there was a 90 minute discussion in a room packed with over 100 people, including many experimentalists from the LHC experiments ATLAS and CMS and particle theorists like myself.   (more…)

Picture of POSTED BY Matt Strassler

POSTED BY Matt Strassler

ON December 15, 2011

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A Truly Spectacular Year at the LHC

Well, yesterday’s update on the search at the Large Hadron Collider [LHC] for the Standard Model Higgs particle (the simplest possible type of Higgs particle) is past. It left us, as was widely expected, with an ambiguous situation about which, for now, intelligent people can (and do) disagree (compare my reaction with this one).  But it also left us with great hope for 2012 that all these ambiguities will be resolved, and that Phase 1 of the Higgs search will come to a definite close next year, with

  1. the discovery of a Standard Model Higgs particle (or a look-alike) between 115 and 127  GeV [with best bets at 125 or so] ; or
  2. confidence that there is no Standard Model Higgs, and the launch of the search for many other types of Higgs particles; or
  3. the discovery of something else along the way.

There’s still a lot more to say about yesterday’s results. Is what ATLAS and CMS are detecting a real signal, or not, and what might make us nervous or confident about it? What are the implications if it really is a signal of a new particle? How confident can we be that it is a Higgs particle? How much evidence is there that it is of Standard Model type? I will write an article about these issues soon. But there’s still a tremendous amount of work left (even for a theorist) to do to help plan for LHC in 2012, and to help assure analysis of the fall 2011 data is thorough.  During the rest of my stay at CERN I will be working hard on that!

However, I would be deeply remiss if I did not follow up on yesterday’s flurry of Higgs discussion by stepping back and reminding you what has happened here.

It has been an astonishing year. I find it challenging to remember how I thought about particle physics twelve months ago, after all these changes in our understanding brought about by the new results from the LHC.  This is especially true as far as the Higgs search, which has unambiguously excluded a huge part of the mass range for Standard Model (and also many non-Standard Model) Higgs particles — with implications that I am still coming to grips with. And this reshaping of the landscape of particle physics is due in large part to the extraordinary efforts of three groups of people:

  • The accelerator physics team at the LHC, the often unsung heroes who designed, built, and now operate this wonderful and incredibly reliable machine, overcoming obstacle after obstacle to bring its collision rate higher and higher, well beyond expectations for 2011, and delivering 150 times as much data as was produced last year, thereby making the achievements announced yesterday possible…
  • The experimental collaborations, the large teams of physicists who collectively built and now operate the giant and complex collision detectors, which work extraordinarily well, typically close to or even beyond their design parameters; who manage to cope with the incredibly challenging conditions at the LHC to obtain high quality data; and who have shown great ingenuity in finding ways to extract more information from that data — in particular, improving the methods for the Higgs particle search so much that they have  significantly exceeded original expectations for what one could learn from the amount of data currently available…
  • The theoretical physicists that you never hear about — the ones who work and sweat tirelessly on very difficult technical problems: combining high-precision theory and data carefully to provide a detailed understanding of the structure of the proton; or coming up with clever ways to calculate, with higher and higher precision, the rates for Higgs particle production, and for other processes that obscure the Higgs particle’s signals;  without these researchers the measurements that we heard about yesterday would have had much larger theoretical uncertainties and could not have led to such powerful conclusions…

I have nothing to say except:  Wow.  Wow.  You’re amazing.  I feel so privileged to be a witness to what you’ve done.

The rest of us — other particle physicists, scientists everywhere, and the public — owe you a long and deafening round of applause. May history always remember what you achieved this year.

And we also wish you the very best in 2012; may it be even more successful and historic than 2011.

The search by the ATLAS and CMS experiments for the Standard Model Higgs particle made astonishing progress in 2011, leaving all eyes on the tiny remaining gap (115.5 to about 128 GeV) and the hint of a signal around 125 GeV.

 

Picture of POSTED BY Matt Strassler

POSTED BY Matt Strassler

ON December 14, 2011

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What Did I Learn Today, During and After Presentation?

Ok, having heard the talks, had a lot of conversations with colleagues, and looked a bit more carefully at the plots from today’s talks at CERN, I have a few revised comments, somewhat more optimistic than my initial reaction, with more details to follow.

Exclusion in new regions:

  1. The CMS experiment has excluded a new chunk of the Higgs mass range, from 127 or 129 GeV (depending on how aggressive you want to be) to about 600 (previously the two experiments together excluded 141 to 476.  [Look to CMS for the official numbers]
  2. ATLAS did not change their result for high mass but excludes the lightweight Higgs down to 131.
  3. Both experiments are seeing effects in the 120-130 GeV range that are preventing them from excluding as much as they would have expected to do on average.

Hints from ATLAS:

  1. ATLAS, using Higgs –> 2 photons and Higgs –> ZZ –> two leptons and two antileptons, shows an excess at about 126 GeV, of a local significance of 3.6 sigma (but only 2.3 sigma with most conservative form of the look elsewhere effect; the more complete number lies somewhere between).
  2. If there were a Standard Model Higgs particle at 125-126 GeV, the expected local significance would be 2.4 sigma.  The signal seen is therefore compatible with a Higgs signal of this type.
  3. It is still compatible with a large fluctuation in the background.

Hints from CMS:

  1. CMS, using 5 different measurements — Higgs –> 2 photons, Higgs –> ZZ –> two leptons and two antileptons, Higgs –> WW –> lepton, antilepton, neutrino, antinuetrino, Higgs –> taus AND Higgs –> bottom quark/antiquark pairs — claims  an excess at about 120 GeV and another at 124 GeVwith a local significance of 2.6 sigma (but only 1.9  sigma with the most conservative form of the look elsewhere effect; the better number lies somewhere between).
  2. If there were a Standard Model Higgs particle at 125-126 GeV, the expected local significance would be 2.6 sigma.  The signal seen is therefore very compatible with a Higgs signal of this type.
  3. The claim CMS wants to make is that the ratios of the excesses in the 5 different channels is quite consistent with a Higgs signal at around 124 GeV.  I am not sure I believe this argument, but it is certainly a legitimate one to make.  To believe it, I would need a much more careful explanation of how the three more subtle analyses were done.
  4. It is still quite compatible with a moderate fluctuation in the background.

Are they compatible?

  • There are some weird features in the data that could be due to low statistics, but roughly speaking the answer seems to be that the hints at ATLAS and CMS are reasonably compatible with each other.

Given this,

  • What we saw today is probably compatible with a Standard Model-like Higgs at about 125 GeV.
  • What we saw today is also probably compatible with a large but not extraordinary fluctuation in the backgrounds, perhaps combined with a subtle technical problem in one or another analysis.
  • And the only way to find out which of these two is the truth is to gather a lot more data in 2012.  Period.
Picture of POSTED BY Matt Strassler

POSTED BY Matt Strassler

ON December 13, 2011

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Higgs Update Today: Inconclusive, As Expected

This post is now superseded by this one.

Picture of POSTED BY Matt Strassler

POSTED BY Matt Strassler

ON December 13, 2011

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At CERN, waiting. Here’s what to watch for on Tuesday.

This week I’m at CERN, home of the Large Hadron Collider [LHC] (and have been assigned the office of none other than John Ellis, who perhaps surprisingly is out of town.) Tomorrow, some new results on the Higgs particle search at ATLAS and CMS, the two general-purpose experiments at the LHC, will be presented.  In today’s post, I’ll first summarize the  situation, and then I’ll explain what I’ll be paying closest attention to in tomorrow’s presentation.

First, a summary of basics, with links.

Now, about the presentations on Tuesday.  Despite various rumors, I do not expect (and most experts I talk to do not expect, and CERN itself warns us not to expect) any evidence presented tomorrow to be convincing yet; here’s why.  But we might (or might not) see some evidence that is moderately compelling.  How should we evaluate it?  What should we be watching for?

[My reasoning here may be opaque to you unless you’ve read through the Standard Model Higgs trilogy (at least the first half of part 3, and also the older article it links to) or you’re already somewhat knowledgeable about the subject.]

First, how compelling are the hints of the Higgs particle shown by ATLAS and CMS, treating the experiments separately?

  • More precisely, how much of the signal for each experiment comes from the clean searches for Higgs –> two-photons and Higgs –> ZZ –> two lepton-antilepton pairs, which have low systematic and theoretical errors and give a sharp measurement of the Higgs mass, and how much comes from the messy WW –> lepton/anti-neutrino anti-lepton/neutrino signal, which has much higher systematic and theoretical errors and gives a vague estimate of the Higgs mass?
  • In particular, if you were to drop the Higgs –> WW measurement entirely, are the signals of the Higgs in two-photons and in two lepton-antilepton pairs convincing by themselves? (This tests whether the result’s significance relies most heavily on a tricky and controversial measurement or on the simpler non-controversial ones.)
  • If Higgs –> two lepton-antilepton pairs plays an important role, this will be because of one or two collisions that suggest the same Higgs mass as a signal in Higgs –> two photons. How much less convincing would the result become if you were to remove one of those collisions? (This checks sensitivity of the result to an individual chance event.)
  • How much of the region near to the signal has actually been excluded? Should we believe evidence in favor of, say, a 125 GeV Higgs if a Higgs at 128-135 or 118-122 has not yet been excluded? (This helps clarify whether the search is really close to completion.)
  • If the WW measurement plays a large role in the evidence, how confident do the theory experts on Higgs –> WW, and on the various backgrounds to this search, seem to be with the theoretical errors stated by the experiments? Do the experiments have detailed evidence that they understand their systematic errors on this measurement? How well under control is their ability to measure the “missing momentum” signal of the neutrinos with sufficient accuracy and precision? (Until we have convincing answers to these questions, I personally think we should be very cautious trusting the WW contribution to the significance of the result.)

Next, we should view the experiments together.  Are they really seeing evidence for the same thing?

  • Are the ATLAS and CMS signals for a Higgs particle really sitting at the same Higgs mass? or do the preferred masses differ by more than 2%, which would be inconsistent with a Higgs particle’s signal?
  • Does the evidence really become much more compelling when you combine the evidence from the two experiments (which we’ll have to do by eye, since the statistically valid, officially authorized combination of the two will probably not be carried out for quite some time)?

I’ll try to get most or all these questions answered for you, to the best of my ability, by tomorrow evening or Wednesday morning. Whether I’ll be able to get you any information during and/or right after the presentation tomorrow may depend on how much of a zoo it is at CERN tomorrow, and how well the wireless network can handle the inevitable massive flood of data going in and out of CERN!

Picture of POSTED BY Matt Strassler

POSTED BY Matt Strassler

ON December 12, 2011

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3 Articles on Standard Model Higgs: #1 Production; #2 Decays; #3 Search and Study

To get you all set for the presentations on the Higgs particle search coming up on Tuesday, December 13th, I’ve written a sequence of three articles about the Standard Model Higgs particle, the simplest form of Higgs particle that might be present in nature, and the one that is the main target, during Phase 1 of the search for the Higgs particle, that the Large Hadron Collider (LHC) experiments are aiming to discover or rule out.  Links to the three articles now appear at the bottom of this page, which provides context by describing the Standard Model Higgs particle in some detail.

  • Article 1 is about producing the Standard Model Higgs particle,
  • Article 2 is about how the (lightweight) Standard Model Higgs particle decays, and
  • Article 3 combines the previous two into a search strategy, and into a program for studying the Standard Model Higgs in detail if/when it is found.  Article 3 also points to a couple of older articles that cover important details highly relevant, in my judgment, to Tuesday’s presentation.

Re: Article 3, which is new today — I should warn you that I’m suffering from jet lag, so … well, you know what that might mean.  I hope there’s nothing amiss, and that the prose is readable!  Also, article 3 is long, and you might want to read it in two sittings, divided at what will be an obvious point.

One reason I’m very tired today is that last night’s flight to Geneva was shortened by extraordinary 200 mile-per-hour (320 kilometer-per-hour) tail winds for the first couple of hours.  We had a ground speed over 700 miles per hour for a while (1150 km per hour)!   The pilots told me they later had to slow the plane’s air speed down significantly in order that we wouldn’t arrive before the customs officials at the Geneva airport start their day.  At normal air speed we apparently would have made the trip in less than 5 1/2 hours!

Picture of POSTED BY Matt Strassler

POSTED BY Matt Strassler

ON December 11, 2011

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Articles on Standard Model Higgs: #1 How it is Produced; #2 How It Decays

To tell you what you need to know for the presentations on the Higgs particle search coming up on December 13th, I’m writing a sequence of three articles about the Standard Model Higgs particle, the simplest form of Higgs particle that might be present in nature, and the one that is the main target, during Phase 1 of the search for the Higgs particle, that the Large Hadron Collider (LHC) experiments are aiming to discover or rule out.

Click here for Article # 1, which is about how these particles are made. There are five major processes by which the Standard Model Higgs particle may be produced at the LHC.  In this article I’ll tell you what they are, and what we’d learn if we measured each of them precisely.

Click here for Article #2: how the Standard Model Higgs particle decays, and what we can learn by measuring those decays precisely.

Sunday or Monday, Article #3: how we search for the Standard Model Higgs particle, combining what we know about its production and decay with what we know about other processes that will mimic its presence in an LHC detector.  And also [probably to be extended later in the week] how we will go about making precise measurements of the properties of any candidate Higgs particle that we observe, to confirm that it really is a Standard Model Higgs particle, and not something more complicated.

Enjoy!

Picture of POSTED BY Matt Strassler

POSTED BY Matt Strassler

ON December 10, 2011

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Article #1 on Standard Model Higgs: How It Is Produced at the LHC

To tell you what you need to know for the presentations on the Higgs particle search coming up on December 13th, I’m writing a sequence of three articles…

[OUT OF DATE: SEE THIS NEW POST]

Picture of POSTED BY Matt Strassler

POSTED BY Matt Strassler

ON December 9, 2011

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