The World after Moriond Electroweak

Rencontres de Moriond is a conference series taking place high in the Italian Alps where particle physics experiments like to present their latest analyses. This year the quality of snow was not quite satisfactory, but the quality of physics results somewhat made up for it. Some highlights have already been discussed on this and other blogs, but I think it's worth giving a short recap anyway.

• New SUSY searches from ATLAS and CMS.
  Searches in several new channels have been presented: jets+MET+b-tags (relevant for sbottom production), trileptons (relevant for gauge mediated models), e-mu resonance (relevant for certain R-parity violating scenarios), and so on. No excess has been seen, and the parameter space of SUSY has been further constrained. Later I may write something more about the significance of these results, but actually the most striking observation here is how both experiments illustrated their analyses. CMS showed a series of cartoons that pretty accurately summarizes the evolution of SUSY from the early 90s till today:
  
  
  ATLAS, on the other hand, for the illustration picked a screen from Impossible Mission, a computer game from the 80s. Those who grew up on ZX Spectrum remember well that the game was long, frustrating, and actually impossible to complete ;-)
• New Higgs combination from the Tevatron.
  In the last couple of years we got used to Tevatron shrinking the available range for the Higgs mass. This time the 95 percent exclusion range is actually slightly worse than in summer 2010 due to an upward background fluctuation. This may be a sign that the Tevatron Higgs searches are approaching the end of the line, and the full data set that will be analyzed next year may not bring significant improvements. See Tommaso's blog for more comments.
• First exciting Higgs results from the LHC.
  The ATLAS and CMS searches for Higgs-like scalars in the tau-tau final state chops off a new portion the MSSM Higgs parameter space. See this post for more details.
• LHC measurements of top quark properties.
  Top is at the moment the hottest issue in particle theory due to the anomaly in the forward-backward asymmetry of the top quark production measured by Tevatron's CDF. Most new physics explanations predict new phenomena that should affect various top quark properties measured at the LHC. CMS flashed some plots the very important distribution of the differential top pair production cross section as a function of the pair invariant mass.Nothing unusual can be seen there, except for a small glitch near 700 GeV. This measurement should severely constrain some explanations of the CDF anomaly, for example those involving the heavy gluon partner.

Two additional results are worth pointing out. Both are rather a show-off at this point, but they demonstrate the amazing potential of the LHC experiment and promise interesting physics in the coming year.

• CMS observation of single top.
  It took 8 years of the Tevatron Run-2 to pinpoint the single top production, and even now it cannot be extracted from the background without some neural network hocus pocus. On the other hand, CMS was able to observe that process with just 35 fb-1 of data. As new physics often modifies the coupling of the top quark to W and b, which in turn affects the cross section for the single top production, single top may provide important constraints or discoveries in the near future. See also this post on Symmetry Breaking.
• LHCb limits Bs → μμ.
  This is supposed to be the flag measurement of the LHCb experiment. The importance of this rare decay process stems from the fact that the branching fraction is very suppressed in the Standard Model whereas it can easily be enhanced in many theories beyond the Standard Model, in particular in the MSSM. The first LHCb limit is already close to that from the Tevatron, so that LHCb should take over already this summer. See Collider Blog for more details on the measurement.

This week is taking place the second part of Moriond'11 oriented more on QCD, so we are guaranteed another load of interesting results. As for me, I'm dying to see more of dijets and top quarks results.

LHC seriously into Higgs searches

As of today the Higgs search industry is still dominated by the Tevatron; for comments on the latest results based on 8.2 inverse femtobarn see Tommaso's blog. But the LHC is catching up faster than expected. Yesterday I saw the first interesting Higgs limits from the LHC. At Moriond, both ATLAS and CMS presented the results of the MSSM Higgs searches in the ττ final state. This search is of quite some interest because of earlier reports from the Tevatron: both CDF and D0 claimed a 2 sigma excess for MSSM Higgs searches in another channel with 3 b-quarks in the final state.

In the MSSM, the Higgs sector is extended as compared to the Standard Model. Apart from the Higgs boson there are 2 additional electrically neutral scalar particles. The production cross section of Higgs and its partners can be largely enhanced for large values of the parameter tanβ. Once produced, part of the time the Higgses decay into a pair of τ-leptons. Lacking any observable excess in the ττ channel, CMS and ATLAS thus produced the limits in the tanβ-mA plane:












See that they beat the corresponding Tevatron limits. It is now clear that the Tevatron 3-b excess is unlikely to be explained within the MSSM. However, the excess can still be a hint of a more general extended Higgs sector, for example a non-susy 2-Higgs doublet model.

One more interesting result was presented by ATLAS. This time the search was for a light particle in the 10 GeV mass ballpark decaying to a pair of muons. This could be for example a pseudoscalar Higgs in the NMSSM, a dark Higgs in the hidden-valley scenario, etc. The ATLAS limits display an intriguing bump near 7 GeV. It's probably nothing but a harmless fluke, but it's worth keeping an eye on. Especially if you can keep an eye over a shoulder of an experimentalist ;-)

NEUTEL11

Contrary to what the name suggests, NEUTEL is not a telecom company but a series of conferences focused on neutrino physics. This year's edition is currently taking place in Venice. For me, the most expected talk is the one from the Xenon100 collaboration who may or may not present their new results of dark matter direct detection searches. For this and other highlights, check out the conference blog that Tommaso is running.

Update: It's confirmed that Xenon100 will not present new results at NEUTEL. We have to bite our fingernails for a few more weeks.

CDF: curiouser and curiouser

The Tevatron may be a dead man walking but it continues to kick ass. The CDF collaboration just posted a new measurement of the forward-backward asymmetry of the top pair production. Recall that earlier this year CDF made a surprising claim about that asymmetry. Restricting to the top pairs with the invariant mass larger than 450 GeV (about 30% of all t-tbar events) the asymmetry is stunning 48±11 percent, which is 3.4 sigma away from the Standard Model prediction of 9 percent. This is completely crazy: 3 times more often top quarks choose to shoot forward rather than backward (with respect to the direction of the proton beam), even though in the first approximation they should not prefer any direction. Even fancy new physics model have a hard time to predict such a huge asymmetry.

The previous CDF measurement was dealing with semileptonic top decays when one top or anti-top quark decays leptonically to an electron/muon + neutrino + b-quark, while the other decays to 3 quarks. The new measurement that I'm reviewing here is focused on dileptonic decays when both the top and the anti-top decay leptonically. This type of event is more rare; only about 5% of the top pairs decay in this manner. Nevertheless, the top stash at the Tevatron is now large enough. In 5.1 inverse femtobarn that went into the new analysis one expects over 200 dileptonic top events. This is enough to study differential distributions and asymmetries. The CDF note gives the result for the inclusive forward-backward asymmetry, that is for the entire dileptonic sample regardless of the invariant mass of the reconstructed top pair. The measured inclusive asymmetry is 14± 5% which, after unfolding the background and instrumental effects, corresponds to the parton level asymmetry of 42 ± 16 percent. The Standard Model predicts meager 5% so the discrepancy is 2.3 sigma. Much as in the semileptonic sample, the asymmetry is larger at higher t-tbar invariant masses (see the pictures), however poor statistics precludes any firm conclusions. One should also compare that result to the inclusive asymmetry of the semileptonic sample. The latter is much smaller, 16±7%, nevertheless the two results are consistent within 2 sigma.

Formally, the new CDF result is merely a 2 sigma deviation from the Standard Model. However, when combined with the previous 3 sigma anomaly, it has a much stronger psychological impact. One could worry that the CDF measurement of the asymmetry suffers from some unaccounted for systematic effects. In fact, the semileptonic sample has a quirky trait that the entire asymmetry comes from the events featuring a muon, while the events containing an electron do not show a significant asymmetry. Until very recently the following explanations of the anomaly seemed equally plausible:

• a cat got stranded in the CDF muon chambers,
• the QCD contribution to the asymmetry has been underestimated,
• the asymmetry is a manifestation of new physics.

The new result makes the cat hypothesis less likely. In the dileptonic sample the asymmetry is actually the largest for the dielectron events. The systematic effects are quite different for the two measurements, yet both consistently show a large positive asymmetry, which is reassuring.

Now, it remains to make sure that higher order QCD corrections are not playing a dirty trick on us. If not, there will be one option left on the table....

More SUSY limits

Here in France this time of the year is known as winter holidays among schoolkids, or winter conferences among particle physicists, which amounts to the same thing (unless you're a PhD student working 24h/day to meet the deadline). These days new experimental results pop up like mushrooms with the peak expected middle March at the Moriond conference. Last week new results from LHC SUSY searches were presented at the Aspen conference, both by CMS and ATLAS. The latest additions are the jets+MET search from ATLAS, and the photons+MET and dileptons+MET searches from CMS. The new ATLAS search provides the current best limits on the mSUGRA parameter spaceUnfortunately, for the moment ATLAS and CMS present their theoretical interpretations only in this obscure and contrived way. So it might be worthwhile to discuss the physics behind the above plot and its more general consequences.

At the current stage, SUSY searches are in fact searches for squarks and gluinos, the superpartners of the Standard Model quarks and gluons. That's because only superpartners carrying the QCD charge have had a chance to be produced at the LHC in reasonable quantities. With 45pb-1 of luminosity acquired so far, the LHC is sensitive to cross sections of order a picobarn. As can be read off the plot on the right, this roughly translates to a sensitivity to 600 GeV gluinos and squarks, slightly less than what you might naively guess from the mSUGRA plot. However, if squarks and gluinos have comparable masses one can profit from the squark+gluino associated production which has the largest cross section of all the production channels. The proximity of squark and gluino masses occurs in a large portion of the mSUGRA parameter space, that's why the exclusion limits extend up to 800 GeV masses in this case.

Thus the production processes are relatively straightforward, it's the decay where the supersymmetric hell breaks loose. In the popular SUSY scenarios squarks and gluinos decay to the lightest superpartner, usually a neutralino, who is an electrically neutral stable particle that escapes the detector. In the simplest case, the squark decays to 1 quark + 1 neutralino, while gluino decays to 2 quarks + 1 neutralino (via an off-shell squark). Thus the experimental signature of both squarks and gluinos is a number of energetic jets accompanied by missing energy carried off by the neutralino. This is precisely what is targeted by the jet+MET search in ATLAS and CMS, and it is the most robust signature of supersymmetry. However things can get infinitely more complicated. For example, if charginos (superpartners of W bosons and charged Higgs fields) are lighter than squarks, as is always the case in mSUGRA, a squark may choose to first decay to a chargino who then decays down the lightest neutralino. These cascade decays may spit electrons or muons on the way. Thus, ATLAS and CMS also search for jets and missing energy associated with one, two, or three leptons. This is slightly less robust, as the presence of light charginos is not guaranteed, but at the same time the leptons in the final state help to reduce the Standard Model background. Comparing the recent ATLAS 0-lepton and 1-lepton searches one finds that the former gives slightly better limits on mSUGRA, but that might be different in other SUSY scenarios.

So the cord tightens. With several final states already covered, and more to appear soon, it's getting harder to avoid the stringent LHC limits in most popular SUSY scenario. Nevertheless, the possibility of sub-TeV superpartners has not been completely excluded yet. Firstly, uncolored superpartners are not constrained by the LHC. Furthermore, gluinos and/or squarks with masses 500 GeV or less are still allowed as long as the mass splitting with the lightest neutralino is small enough, such that the supersymmetric events fail the missing energy cuts. Stops, that is the scalar superpartners of the top quark, are even less constrained due to the smaller production cross section and the pesky t-tbar background. As a last resort one can turn to R-parity violating scenarios which are not constrained by the current LHC searches.

To conclude this post and fulfill my weekly quota of malice and scoff, here is a picture shown at the end of the ATLAS talk in Aspen:
Hmmm. Sorry to disappoint you guys but it's not the "stop", it's the "wrong way"; a symbolic mistake in this context. Let's hope future discovery claims from ATLAS will be more carefully scrutinized ;-)

See also Peter's comments.

What LHC tells about SUSY

The first SUSY searches using the full 2010 LHC dataset are out now, one from CMS and one from ATLAS. The nagging question is what are their implications for low-energy supersymmetry. Some answers can be found in this talk of Alessandro Strumia who provides a cute visualization of the impact of the latest LHC results:
Let me explain what's on this plot. It assumes the so-called mSUGRA scenario which is cherished by experimentalist because it parametrizes the multitude of the MSSM parameters in terms of just 5 variables, thus creating an illusion of order in the Universe. 2 of these variables, the A-term and tanβ, are fixed above to a specific value, the same as the one assumed by CMS and ATLAS in their theoretical interpretations. This leaves 3 variables: the universal scalar mass m_0, the universal gaugino mass M_1/2, and the μ-term (the first two are defined at the GUT scale and related to the physical masses of the SUSY particles by complicated differential equations; this is one of the curious idiosyncrasies of SUSY phenomenology). Quite generally, in the MSSM one can compute the weak scale, that is the Higgs vacuum expectation value, in terms of the parameters of the lagrangian. In the case at hand, to reproduce the correct weak scale or equivalently the correct Z boson mass, the 3 remaining variables need to satisfy the constraint of the form
This constraint divides the mSUGRA parameter space into 3 regions:

• If m_0 and M_1/2 are too small one cannot solve the above constraint for any μ. This corresponds to the "vev=0" region on the left-hand side of the plot.
• For large SUSY breaking parameters the Higgs potential may not have a stable minimum. This corresponds to the "vev = ∞" region on the right-hand side of the plot.
• In the remaining parameter space one can always choose μ such that the above constraint is satisfied. Nature could in principle choose one particular point in this region.

Unfortunately, most of this available parameter space has already been excluded by the LEP experiment back in the 90s. The failure to observe any superpartners of the Standard Model at LEP left only a narrow sliver of the parameter space close to the "vev=0". Now, the latest LHC results excluded a part of the remaining sliver, which is marked in the plot as the darker red region.

You may want to zoom in to fully appreciate the impact of the LHC searches:
Recall that blue is theoretically unavailable, light red is excluded by LEP, dark red is excluded by the LHC, and white is allowed. The breathtaking endeavor of the LHC for the next few years will be to further shrink the white stripe.

Of course, the way it is presented here is a bit tendentious, and in a larger picture things might look less bleak. For example, the plot refers to a very specific constrained SUSY scenario; in more favorable scenarios the unexcluded parameter space may be twice as large. Furthermore, the ATLAS and to some extent also the CMS search are not completely robust. Thus, one can easily design SUSY scenarios that are less constrained by the LHC, at least for another month until Moriond. For example, SUSY models without light charginos would be missed by the ATLAS search. As a last resort, one can always present the results such that the allowed parameter space is better visible:Here, the x-axis corresponds to a relative fine-tuning of a given point in the mSUGRA parameter space: small fine-tuning on the right (when SUSY parameters are or order the Z boson mass), big fine-tuning on the left. The allowed parameter space is the green chimney close to the left edge. The breathtaking endeavor of the LHC for the next few years will be to move the red region further up the chimney.

See also Alessandro's paper for more details. For more pedagogical and less malicious comments on LHC SUSY searches see this post on US/LHC Blog.

2 more years at 7

It's official now. Here is an excerpt from DG's email:

...The main decisions we have taken are that the LHC will run through 2012 before a long shutdown, we'll keep the energy at 3.5 TeV during 2011, and we'll work hard to increase the luminosity steadily...

Seems reasonable enough to me. With the pressure from the Tevatron gone, it is not critical to squeeze every bit out of the machine. Hence the decision to remain at the 7 TeV center-of-mass energy of collisions which increases the chances that the thing will not explode in our hands again. More importantly, the run-1 is going to continue until the end of 2012. By that time, the LHC should have acquired some 5 inverse femtobarns of data, maybe more. This will be enough to see glimpses of new physics, provided there is anything below a TeV. But the most solid advantage of extending the run-1 is that the Higgs will be discovered earlier than in the alternative scenario with the 2012 shutdown. Thus, the minimum plan should be accomplished by the early 2013, if only Higgs is where we expect him to be. The disadvantage is that for 5 more years, rather than 4, I'll have to listen to talks about supersymmetry.

For a more illuminating analysis of the Higgs prospects, see Tommaso's blog.