RECAST’ing light Higgs searches @ ALEPH

Itay Yavin

NYU

With James Beacham and Kyle Cranmer for the ALEPH collaboration 0901.0283

(with spiritual support from N. Weiner)

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Content

• Search for a light Higgs-boson

• RECAST - Request Efficiency Calculation for Alternative Signal Testing

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Matter and Mass“The Higgs boson is the only Standard Model particle that has not been observed. Experimental detection of the Higgs boson would help explain the origin of mass in the universe.”

Wikipedia - source of all knowledge…

Well…, not exactly, we actually don’t really know what is the origin of most of the mass in the universe.

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Matter and Mass“Oh, you are just being facetious, we mean the origin of mass for the 4% of the universe we are actually made of … “

Actually, most of the mass of the atom comes from the protons and neutrons in the nucleus. The mass of those is almost entirely the result of the potential energy of the strong force that binds the quarks together.

A true manifestation of Einstein’s M = E/c2

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Vector-Boson Scattering

The physical Higgs is responsible for taming the divergence in the vector-bosons scattering amplitude.

Where is Higgs?

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Higgs SearchesThe most extensive search was made in the Higgs to di-bottom channel,

LEP combined search - hep-ex/0306033v1

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Electroweak Precision Tests

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Higgs@98 ?

2.3 evidence for mh ~ 98 GeV. Also a 1.7 evidence for higgs at 115 GeV.

See Dermisek and Gunion, hep-ph/0510322. Also Drees hep-ph/0502075

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Choosing between two evils

From M. Chanowitz 0806.0890

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Supersymmetric HiggsIn the minimal Supersymmetric Standard Model (MSSM), at tree level the higgs is predicted to be lighter than the Z0 boson(1),

(1)Recall 2HDM and the MSSM has two Higgs field and the ratio of the vacuum expectation value is,

Higgs Hunter’s Guide, circa 1989. . .

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A Conundrum (a.k.a the LEP paradox)

There seems to be this tension between the direct production results which favor heavy Higgs, to the indirect probes/theory bias which favors a lighter Higgs …

Light HiggsHeavy Higgs

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Escaping the LEP BoundsIf Higgs decays in a non-standard way, it may have escaped detection. One of the most motivated examples is Higgs to four tau’s,

Dermisek and Gunion, hep-ph/0502105

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Higgs DecayThe Higgs boson likes to decay into Bottoms, because the Bottom’s Yukawa coupling is the largest among the accessible fermions. But, it’s pretty small and easy to overcome,

The decay into the pseudo-scalars can easily dominate,

See e.g. Chang et al. 0801.4554Dimensionless number related to the h-a-a coupling

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4 Fermion Final State

Kinematically accessible region which hasn’t been searched

Also, A. Haas at Tevatron constrained decays into muons! [hep-ex] 0905.3381

(see Lisanti and Wacker for theory [hep-ph] 0903.1377

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Closing the WindowHere is what J. Gunion thinks:

So we set out to discover the Higgs… or at least close this window.

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LEP and ALEPH

The ALEPH detector

ECAL: lead + proportional wire chambers, 22X0

HCAL: 23 layers of iron yolk + streamer tubes

muons identified via HCAL+2 muon chambers

Tracking: silicon + large time projection chamber (~31 hits)

Detector simulation based on Geant 3, analysis based on 10 year old fortran framework

1.5 T

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Expected Number of Events

Have enough events for discovery up to ~ 90 GeV (depending on a mass)

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Event Topology

Since the Higgs is rather heavy and the pseudo-scalar light, it is fairly boosted in the lab frame. Therefore, the resulting tau’s are very collimated and so are their decay products …

We designed three different searches depending on the decay of the Z boson. Before describing these in detail, let’s examine the resulting jets.

Simulated signal event

2 back-to-back electrons clearly distinguished from 2 back-to-back jets. not much else in the event (about 50 GeV of missing energy)

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JetsWe used the Jade algorithm to form jets with a given invariant mass.

J1

J2

Beam axis

Demand a beam-jet isolation cut, to make sure we are not missing tracks.

Each Jet contains 2,4, or 6 tracks from the 2 tau decays. This forms a powerful discreminant.

decays “1-prong” 85.36%, “3-prong” 14.56%

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Backgrounds

We formed several control samples, where no signal is expected in order to test our Monte-Carlo simulations.

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Control Samples

1. Exclude di-lepton invariant mass around mZ, kills the signal.2. #track < 1 (kills anything with more than one track).3. Z-> di-lepton, MJJ inv < 60 GeV 4. Z-> invisible, exclude on MET > 80 GeV5. #track > 6 in both jets AND MJJ < 60 GeV

We had to make sure that the MC is describing the background correctly. But, we wanted to keep a blind analysis so we defined a few control samples,

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Unboxing

Champagne, just in case…

We were aiming for discovery…

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Results

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Exclusion Limits

Ruling out a hidden Higgs?

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Still hiding?

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By lowering the branching ratio to taus, one can still hide the Higgs in the mixed decay channel. That’s what we are going after next,

This makes things more difficult, but by using jet substructure we are hopeful we can reduce the background and exclude this possibility as well.

More Analysis• Mixed channels

• Higgs to gluons/charms?

• Higgs to Lepton Jets? (Falkowski, Ruderman, Volansky, Zupan)

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Provocative…

From M. Chanowitz 0806.0890 From 1005.2757

RECAST

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Request Efficiency Calculation for Alternative Signal Testing

BSM PhysicsMany new models of beyond the Standard Model physics have been suggested:

• SUSY – SUGRA,GMSB,AMSB,…

• RS

• UED

• Little Higgs

Many powerful tools were created to allow fast incorporation and simulation of new particle physics,

• Madgraph/Madevent

• Calchep/Comphep

• PYTHIA,HERWIG

• LHE format

Can we use all these new tools to ‘quickly’ limit the new theories?

(This is, if you like, a complementary question to the one N. Toro posed yesterday, that of characterization of new physics in the case of discovery)

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Alice

Experimentalist

Signal independent

Signal dependent

Alice is searching for some signal and reports an exclusion plot based on that signal. The cuts and procedure she employs leads to some signal efficiency which she quotes.

Reporting an Experimental Search

Sometimes in the Future…Bob

Experimentalist

Bob wants to search for a different signal. But, maybe Alice’s search already covers his signal in certain regions of the parameter space. If that is true Bob’s job is made much simpler, he can concentrate on these regions which are not already excluded by Alice’s analysis…

But how will Bob know? He needs to know what is the efficiency of Alice’s search for his new signal!

Carol

Theorist

Carol just thought of a new particle that can explain all sorts of things. But, she realizes that this particle may result in a signal which, while not the same as Alice’s, does have some overlap with it. Maybe it’s already excluded by Alice’s analysis. . .

How will Carol know? She needs to know what is the efficiency of Alice’s search for her new signal!

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RECASTRequest Efficiency Calculation for Alternative Signal Testing

LHENew signal

Alice’s analysis code including all the cuts, detector effects and etc.

Efficiencyfor new signal

Seen only by Alice

Can be submitted by everyone

Seen by everyone, no data, no code, only efficiency

Carol

Theorist

Alice

Bob

Experimentalist

Everyone Benefits!!!• More impact for Alice’s search!!!

• Alice does not have to worry about interpreting her results under many different signal assumptions.

• Bob can use Alice’s results to make sure the new signal he is planning to search for is not already excluded.

• Maybe some regions of his new signal are excluded, so concentrate and optimize his analysis to those which are not!

• Carol can confidently estimate the coverage of Alice’s analysis on her new model.

• Help to direct the theorist thinking into these regions not already excluded even when considering new models which have not been explicitly searched for.

Examples1. The buried/charmed Higgs scenarios of Csaki et al.

(0906.3026, 0910.3210) could have been easily constrained by RECASTing existing Higgs to 2 jets flavor independent analyses.

2. Meade, Reece, and Shih (0911.4130) derived limits on prompt decays of general neutralino NLSPs at Tevatron using the limited existing analysis available. Their efforts could have been greatly reduced with RECAST.

3. Falkwoski et al. suggested hiding the Higgs boson through Higgs to lepton-jets. Again, RECASTing existing analyses could have helped in placing better limits on this scenario.

The End

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What is a?If other scalar fields aside from the Higgs field are present, then the pseudo-scalar component can be naturally light (pseudo-Nambu-Goldstone boson). This happens in extensions of the SM, such as MSSM, and/or NMSSM…

This Lagrangian contains a vertex allowing the Higgs to decay into two a’s

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a DecayThe pseudo-scalar decays back to SM fermions through its mixing with the Higgs and hence it will decay to the heaviest fermions kinematically accessible.

Below the bottom meson threshold, the pseudo-scalar likes to decay to tau’s

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OPAL Search

OPAL collaboration, hep-ex/0209068

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Higgs Production

Track Multiplicity

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signal

backgrounds

coded track multiplicity in

Each Jet contains 2,4, or 6 tracks from the 2 tau decays. This forms a powerful discreminant.

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Analysis

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Analysis

Both the muon and electron channel tend to involve extra photons in the final state which must be taken into account to correctly reconstruct the Z.

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Analysis

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Expected SignficanceThe lower reach at higher a masses is due to lower signal efficiency. We are presently correcting this and the reach should improve.

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Event Counts