# Technicolor Explanation for the CDF Excess

###### Abstract

We propose that the excess at in the dijet mass spectrum of reported by CDF is the technipion of low-scale technicolor. Its relatively large cross section is due to production of a narrow resonance, the technirho, which decays to . We discuss ways to enhance and strengthen the technicolor hypothesis and suggest companion searches at the Tevatron and LHC.

^{†}

^{†}preprint: FERMILAB-Pub-11-165-T

1. Introduction The CDF Collaboration has reported a surprising excess at in the dijet mass distribution of events. Fitting the excess to a Gaussian, CDF estimated its production rate to be . This is 300 times the standard model Higgs rate . The Gaussian fit is consistent with a zero-width resonance. Its significance, for a search window of 120– and including systematic uncertainties, is Aaltonen:2011mk .

In our view the most plausible new-physics explanation of this excess is resonant production and decay of bound states of technicolor (TC), a new strong interaction at several of massless technifermions Weinberg:1979bn ; Susskind:1978ms ; Hill:2002ap ; Lane:2002wv . These technifermions are assumed to belong to complex representations of the TC gauge group and transform as quarks and leptons do under electroweak (EW) . Then, the spontaneous breaking of their chiral symmetry breaks EW symmetry down to electromagnetic with a massless photon and . We propose that the dijet resonance is the lightest pseudo-Goldstone isovector technipion () of the low-scale technicolor scenario. The immediate consequence of this hypothesis is a narrow technirho () resonance in the channel. This accounts for the large production rate.

In this Letter we show that a of mass decaying into plus of accounts for the CDF dijet excess. The signal sits near the peak of the distribution and will be less obvious than . We suggest ways to enhance this signal and tests of the ’s presence: (1) The ’s narrowness will be reflected in Eichten:1997yq ; Aaltonen:2009jb . The bins near will exhibit a sharp increase over background for . (2) The angular distribution in the frame will be approximately , indicative of the signal’s technicolor origin. We propose further tests of the technicolor hypothesis, including other resonantly produced states which can be discovered at the Tevatron and LHC.

Low-scale technicolor (LSTC) is a phenomenology based on walking
technicolor Holdom:1981rm ; Appelquist:1986an ; Yamawaki:1986zg ; Akiba:1986rr . The TC gauge coupling must run very slowly for 100s of TeV
above so that extended technicolor (ETC) can generate sizable quark
and lepton masses ^{1}^{1}1Except for the top quark mass, which requires additional dynamics such as topcolor Hill:1994hp . while suppressing flavor-changing neutral current
interactions Eichten:1979ah . This may be achieved if technifermions
belong to higher-dimensional representations of the TC gauge group. Then,
the constraints of Ref. Eichten:1979ah on the number of ETC-fermion
representations imply technifermions in the fundamental representation as
well. Thus, there are technifermions whose technipions’ decay constant Lane:1989ej . Bound states of these
technifermions will have masses well below a TeV — greater than the limit
Abazov:2006iq ; Aaltonen:2009jb and
probably less than the 600–700 GeV at which “low-scale” TC ceases to make
sense. Technifermions in complex TC representations imply a quarkonium-like
spectrum of mesons. The most accessible are the lightest technivectors, , and ;
these are produced as -channel resonances in the Drell-Yan process in
hadron colliders. Technipions are accessed in
decays. A central assumption of LSTC is that these technihadrons may be
treated in isolation, without significant mixing or other interference from
higher-mass states. Also, we expect that (1) the lightest technifermions are
-color singlets, (2) isospin violation is small for and ,
(3) , and (4) is not far above .
An extensive discussion of LSTC, including these points and precision
electroweak constraints, is given in Ref. Lane:2009ct .

Walking technicolor has another important consequence: it enhances relative to so that the all- decay channels of the likely are closed Lane:1989ej . Principal -decay modes are , , , a pair of EW bosons (including one photon), and fermion-antifermion pairs Lane:2002sm ; Eichten:2007sx ; Lane:2009ct . If allowed by isospin, parity and angular momentum, decays to one or more weak bosons involve longitudinally-polarized , the technipions absorbed via the Higgs mechanism. These nominally strong decays are suppressed by powers of . Decays to transversely-polarized are suppressed by . Thus, the are very narrow, . These decays provide striking signatures, visible above backgrounds within a limited mass range at the Tevatron and probably up to 600–700 GeV at the LHC Brooijmans:2008se ; Brooijmans:2010tn .

2. The new dijet resonance at the Tevatron Previous searches at the Tevatron focused on final states with and where one or both quarks was a tagged . This was advocated in Ref. Eichten:1997yq because couplings to standard-model fermions are induced by ETC interactions and are, naively, expected to be largest for the heaviest fermions. Thus, , and has been assumed, at least for . While reasonable for decays, it is questionable for because CKM-like angles may suppress . This is important because the inclusive at the Tevatron. If is turned off in the default model of decays used here Lane:2002sm , up to 40% of the signal is vetoed by a -tag . It is notable, therefore, that the CDF observation did not require -tagged jets Aaltonen:2011mk .

At first, it seems unlikely that could be found in untagged dijets because of the large background. However, Ref. Mrenna:1999xj studied without flavor-tagging and showed that a signal could be extracted. Recently, strong signals have been observed in production at the Tevatron Aaltonen:2009vh ; Aaltonen:2010rq . So, heavier dijet states resonantly produced with may indeed be discoverable at the Tevatron.

The CDF dijet excess was enhanced by requiring Aaltonen:2011mk . Such a cut was proposed in Ref. Eichten:1997yq . There it was emphasized that the small -value in and the fact that the is approximately at rest in the Tevatron lab frame cause the to be emitted with limited and its decay jets to be roughly back-to-back in .

3. Simulating Pythia 6.4 is
used throughout to generate the
signal Sjostrand:2006za . It employs the default -decay model of
Ref. Lane:2002sm in which is unhindered. The
input masses are . This
gives a peak in the simulated distribution near
^{2}^{2}2Other relevant LSTC masses are ;
; and which appear in
dimension-five operators for decays to transverse EW
boson Lane:2002sm ; Eichten:2007sx ; we take them equal to .
Other LSTC parameters are , , and .. This parameter choice is close to Case 2b of Contribution 8 in
Ref. Brooijmans:2010tn .

The signal cross sections (including , , and ) are and ^{3}^{3}3No -factor has been used in any of our signal and
background calculations.. Only 20-30% of these cross sections come from
the . If , they
increase slightly to and . If is
suppressed, then , a decrease of 2/3, for a
total signal of .

Backgrounds come from standard model , including -jets, , , and multijet QCD. The last two amount to at the Tevatron and we neglect them. The others are generated at parton level with ALPGENv13 Mangano:2002ea and fed into Pythia for showering and hadronization. The Pythia particle-level output is distributed into calorimeter cells of size . After isolated leptons (and photons) are removed, all remaining cells with are used for jet-finding. Jets are defined using a midpoint cone algorithm with . For simplicity, we did not smear calorimeter energies; this does not significantly broaden our resolution near .

In extracting the and signals, we first adopted the cuts used
by CDF Aaltonen:2011mk ,^{4}^{4}4The CDF cuts are: exactly one
lepton, , with and ; exactly
two jets with and ; ; ; ; ; .. Our results are in Fig. 1. The data
correspond to . They reproduce the shape and
normalization of CDF’s Aaltonen:2011mk and
Cavaliere:2010zz distributions (except that not smearing
calorimeter energies does make our signal a narrow spike). We
obtain for the dijet signal in the five bins in
120–160 GeV. We find this agreement with CDF’s measurement remarkable. Our
model inputs are standard defaults, chosen only to match the dijet resonance
position and the small -value of . The resonance
is near the peak of the distribution ^{5}^{5}5The quadratic
ambiguity in the reconstruction was resolved by choosing the solution
with the smaller .. For the six bins in 240–300 GeV, we obtain
. ;

We then augmented the CDF cuts to enhance the signals. CDF required exactly two jets. We achieved greater acceptance and a modest sharpening of the dijet peak by combining a third jet with one of the two leading jets if it was within of either of them. We enhanced the and, especially, the signals by imposing topological cuts taking advantage of the kinematics Eichten:1997yq : (1) and (2) . The improvements seen in Fig. 2 are significant. We obtain for and for . Extracting the signal will require confidence in the background shape.

In addition to the and resonances, the -value and the -decay angular distribution are indicative of resonant production of . The resolution in is better than in and alone because jet measurement errors partially cancel. This is seen in Fig. 3 where we plot for vs. for six 16-GeV bins between 86 and . The sudden increase at in the three signal bins is clear.

The decay is dominated by . Therefore, the angular distribution of is approximately , where is the angle between the incoming quark and the outgoing in the frame Eichten:2007sx . The backgrounds are forward-backward peaked. We required , fit the background in with a quartic in , and subtracted it from the total. (In reality, of course, one would use sidebands.) The prediction in Fig. 3 matches the normalized well. Verification of this would strongly support the TC origin of the signal.

4. Other LSTC tests at the Tevatron and LHC

1) It is important to find the and states, expected to be close to , near . At the Tevatron, the largest production rates involve and . For our input parameters, these are and , respectively. Their existence, masses and production rates critically test the technifermions’ TC representation structure and the strength of the dimension-five operators inducing these decays. In addition, recent papers from DØ Abazov:2010ti and CDF Aaltonen:2011xp suggest that the channel is promising. The excess (signal) cross sections for our parameters are and .

2) Finding these LSTC signatures at the LHC is complicated by and other multijet backgrounds. The likely discovery and study channels at the LHC are the nonhadronic final states of ; , and Brooijmans:2008se ; Brooijmans:2010tn . The dilepton channel may well be the earliest target of opportunity.

3) The and -fractions of decays should be determined as well as possible. They probe the ETC couplings of quarks and leptons to technifermions, a key part of the flavor physics of dynamical electroweak symmetry breaking Eichten:1979ah .

If experiments at the Tevatron and LHC reveal a spectrum resembling these predictions, it could well be that low-scale technicolor is the “Rosetta Stone” of electroweak symmetry breaking. For it will then be possible to know its dynamical origin and discern the character of its basic constituents, the technifermions. The masses and quantum numbers of their bound states will provide stringent experimental benchmarks for the theoretical studies of the strong dynamics of walking technicolor just now getting started, see e.g. Appelquist:2010xv .

Acknowledgments We are grateful to K. Black, T. Bose, J. Butler, J. Campbell, K. Ellis, W. Giele, C. T. Hill, E. Pilon and J. Womersley for valuable conversations and advice. This work was supported by Fermilab operated by Fermi Research Alliance, LLC, U.S. Department of Energy Contract DE-AC02-07CH11359 (EE and AM) and in part by the U.S. Department of Energy under Grant DE-FG02-91ER40676 (KL). KL’s research was also supported in part by Laboratoire d’Annecy-le-Vieux de Physique Theorique (LAPTH) and he thanks LAPTH for its hospitality.

Note added in proof – An important corroboration of the signal is its isospin partner (suppressed by phase space and branching ratios) . We predict cross sections of at the Tevatron and at the 7-TeV LHC for and .

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