In the Collider Detector at Fermilab, CDF, we have made the first observations of several “exclusive” processes in p¯p collisions, defined as p + ¯p → p(∗) ⊕ X ⊕ ¯p(∗), where the beam (anti)protons are diffractively scattered, with or

without dissociation, the “⊕” denotes a large rapidity gapΔη > 4.5 with no hadrons, and “X” is a simple state fully measured. The main part of the talk focuses on recent

X = π+π− data, through the double-pomeron exchange mechanism

Albrow, M.

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DOI 10.1393/ncc/i2015-11843-4Colloquia: LaThuile14IL NUOVO CIMENTO Vol. 37 C, N. 6 Novembre-Dicembre 2014Central exclusive production in CDFM. Albrow on behalf of the CDF CollaborationFermi National Accelerator Laboratory, Wilson Road, Batavia, IL 60510, USAricevuto il 31 Luglio 2014Summary. — In the Collider Detector at Fermilab, CDF, we have made the firstobservations of several “exclusive” processes in pp¯ collisions, defined as p + p¯ →p(∗) ⊕ X ⊕ p¯(∗), where the beam (anti)protons are diffractively scattered, with orwithout dissociation, the “⊕” denotes a large rapidity gap Δη > 4.5 with no hadrons,and “X” is a simple state fully measured. The main part of the talk focuses on recentX = π+π− data, through the double-pomeron exchange mechanism.PACS 13.85.Ni – Inclusive production with identified hadrons.PACS 13.75.Lb – Meson-meson interactions.PACS 14.40.Be – Light mesons (S = C = B = 0).At collider energies about 25% of the total pp(pp¯) cross section is elastic scattering, inwhich the (4-momentum-transfer)2 between the protons is carried by a pomeron. Despiteits name the pomeron IP (named after Pomeranchuk) is not a particle, as it only appearsin the t-channel, and although it should be part of QCD it involves low-Q2, very non-perturbative, interactions that we do not yet know how to calculate. In QCD languageit is at leading order a pair of gluons in a color singlet state with vacuum quantumnumbers; at higher orders loops of quarks, gluons, pions, etc. form a ladder. As we arein the regime where αS(Q2) ∼ 1 such loops are not suppressed, hence the theoreticaldifficulties. Phenomenologically (and pre-dating QCD) the pomeron is described as aRegge trajectory αIP(t) = α0 + α′t ∼ 1.08 + 0.2t, while mesons have Regge trajectories(reggeons) like αρ ∼ 0.55+0.9t [1]. The latter describe well the (s, t) behavior of reactionssuch as: π−+p→ π0+n, which QCD is not (yet) able to do. The value of α(0) determinesthe s-dependence of elastic scattering, when the reggeon (pomeron) is exchanged, andof total cross sections, which are related through the optical theorem. Meson exchangeswith α(0) < 1.0 decrease with increasing s, or equivalently increasing rapidity gap Δy,while pomeron exchange, with α(0) > 1.0, grows. For a rapidity gap Δy ∼ Δη  4c© Societa` Italiana di Fisica 8384 M. ALBROW on behalf of the CDF COLLABORATION(called here a “large” gap) the t-channel exchange is only the pomeron, or the photonwith spin J = 1 (Coulomb scattering, with a larger range and therefore usually smaller|t| than pomeron exchange). With a single gap we have elastic scattering and singleand double diffractive dissociation. At the Tevatron and the LHC the rapidity rangeΔytotal = 2×y(p) = 2×ln(√s/mp) = 15.3 and 18.1, respectively, allowing space for morethan one large rapidity gap. With two large gaps we have p+ p¯→ p(∗)⊕X⊕ p¯(∗), wherethe two t-channel exchanges across the gaps are either photons or pomerons, so there arethree classes: γ + γ, γ + IP, and IP + IP. In the Collider Detector at Fermilab, CDF, weobserved the first two for the first time in hadron-hadron collisions. The third reaction,double pomeron exchange or DIPE, was studied at the SPS (fixed target) and ISR [2](pp with√s = 23–63GeV), but without such large rapidity gaps, allowing some non-IPbackground. At the ISR the masses of the central state, M(X), were limited to about3GeV/c2, which is a good region for meson spectroscopy studies. The quantum numbersof the central state X are restricted to be IG JPC = 0+(even)++, which allows glueballformation as they are isoscalars and the pomerons are “glue-rich”. Quoting from thePDG [3] “The scalar (isoscalar) mesons are especially important to understand becausethey have the same quantum numbers as the vacuum. Therefore they can condense intothe vacuum and break a symmetry such as a global chiral U(Nf )× U(Nf ). The detailsof how this symmetry breaking is implemented in Nature is one of the most profoundproblem in particle physics”. So even without talking about glueballs, DIPE must beextremely interesting (see [4] for a review).Photon-photon collisions are “bread and butter” at e+e− colliders (no pomerons),but were observed for the first time in hadron-hadron collisions in CDF, both with X =e+e− [5] and X = μ+μ− [6,7]. The results agree with QED expectations, which providesan excellent control of the experiment. The μ+μ− spectrum shows very prominent J/ψand ψ(2S) signals; these JPC = 1−− states are photoproduced, γ + IP, and have beenstudied extensively in ep collisions at HERA but were seen here for the first time inhadron-hadron collisions. Unique to hadron-hadron collisions are IP + IP interactions,producing single mesons with the allowed quantum numbers, such as f0(600)/σ, f0(980),f2(1270), etc., already seen at lower energies [2], and the χc states [6]. Of course anyhadron states can be produced in pairs, including glueballs or states with exotic (non-qq¯)quantum numbers. Exclusive χb is expected at the LHC, and the same diagram but witha top-quark loop instead of a c, b loop can produce exclusive Higgs bosons at the LHC:p+ p→ p⊕H ⊕ p with no other produced particles. Seeing these events would be a veryinteresting, and very different, way of investigating the H(125), as the production is onlygg → H through loops (mainly top) and only CP = ++ is allowed. Thus seeing theH(125) this way proves that it has P = + without assumptions about CP conservationin the Higgs sector. When this was first proposed for the Tevatron [8] the cross sectionpredictions were spread over orders-of-magnitude. In CDF we then set out to observeX = γγ events which have the same mechanism, but with (mainly) a u-quark or c-quarkloop instead of a t-quark loop. This first (and so far, only) observation of p⊕ γγ ⊕ p(p¯)events [9] constrains the theory, such that we now expect σ(p⊕H(125)⊕p) ∼ 2×2÷2 fb. Thisis small, but similar to σ(H) → ZZ∗ → l+1 l−1 l+2 l−2 ∼ 1 fb; in both cases the continuumbackground is comparable to the signal, given the good mass resolution from the protonmeasurements.I now focus on IP + IP interactions in CDF. In the central region the detector hassilicon-strip and drift-chamber trackers in a solenoidal field, and a barrel of time-of-flight scintillators, surrounded by electromagnetic and hadronic calorimeters and muonchambers. In both forward directions there is an array of 48 gas Cherenkov counters,CENTRAL EXCLUSIVE PRODUCTION IN CDF 853.7 < |η| < 4.7, and Beam Shower Counters (BSC) 5.4 < |η| < 5.9. These had lead infront to detect photons. For the observation of exclusive γγ [9] there were more BSC outto |η| = 7.4 giving good efficiency for rejecting proton diffractive dissociation. These werenot available for the π+π− data presented below, so diffractive dissociation is includedin the data. At√s = 1960GeV CDF found 43 events with two γ-candidates withET > 2.5GeV, |η| < 1.0 and no tracks or unassociated calorimeter clusters in −7.4 <η < +7.4. The possible background of exclusive π0π0 events was consistent with zero.The cross section is 2.48+0.40−0.35(stat)+0.40−0.51(syst) pb, in agreement with a Durham Groupprediction [10]. The ingredients in that prediction are the cross section for g+ g → γ+γthrough quark loops, the unintegrated gluon distribution g(x, x′) which comes in as thefourth power, the Sudakov requirement of no gluon or hadron radiation, and no otherparton-parton collisions (the rapidity gap survival factor). Together these contributeto a theoretical uncertainty [11] of about ×2÷2 which feeds in to a similar uncertainty onσ(p ⊕ H ⊕ p) ∼ 2 fb at the LHC. At least this cross section is no longer uncertain byorders of magnitude, as was the case earlier. To improve the prediction for σ(p⊕H ⊕ p)at the LHC the best procedure would probably be to measure σ(p + p→ p⊕ γγ ⊕ p) at√s = 13TeV. This requires some low-pileup running, with at most a few interactionsper bunch crossing, and preferably selecting single interactions. A CMS search at√s =7TeV [12] found no candidates, but in only 5 pb−1 of effective (no-pileup) luminosityLeff . Suppose we could get 250 hours (< 2 weeks) with mean pileup μ = 1. With 2800bunches one gets Leff ∼ 200 pb−1. Using the Durham Group’s central prediction onecan expect (with the right trigger, and assuming 60% efficiency) about 180 events with|η(γ)| < 3.0 and M(γγ) > 10GeV/c2, i.e. < 10% uncertainty. The backgrounds aresmall because pTγ1 ∼ pTγ2 and Δφ(γγ) ∼ 180◦. These latter conditions probably allowone to also use events with μ = 2 − 4. If one cannot measure both protons (e.g. withupgraded TOTEM detectors) one has to use Forward Shower Counters, FSC, and othervery forward “gap-detectors” and estimate the dissociation contribution. Independent ofpredicting the exclusive Higgs cross section, this measurement is an excellent test of theabove ingredients of the QCD calculation. IP + IP → γ + γ is so simple, what could benicer for studying the pomeron?Another CDF result is the observation of exclusive dijets [13]. A Roman pot spec-trometer was used, a set of three pots with scintillating fiber hodoscopes to measureantiproton tracks. These were behind quadrupole and dipole magnets and measuredthe momenta of diffractively scattered antiprotons. Events with p¯ track, at least twocentral jets with ET > 10GeV and a rapidity gap on the outgoing p-side (which didnot have Roman pots) were selected. The total mass M(X) of all calorimeter signals(|η| < 5.2) and the mass of the two leading jets M(JJ) were compared, as the ratioR(JJ) = M(JJ)/M(X) should be close to 1.0 for an exclusive dijet. An event generatorthat does not include exclusive dijets, pomwig [14], fails to describe the data (with jetET up to 35GeV) for R(JJ)  0.8, but the addition of exhume [15] gives agreement,taken as evidence for exclusive dijets. This is another important test of the “Durhammechanism”, and is in reasonable agreement. Nearly all of these exclusive dijets shouldbe gg, with a small admixture of bb¯; not only is this study important for QCD per se, ittells us about the background to exclusive H → bb¯. At the Tevatron D0 also measuredexclusive dijets [16].I now present preliminary results on p+p¯→ p(∗)⊕π+π−⊕p¯(∗) in CDF. It was possibleto trigger on two central (|η| < 1.3) calorimeter towers with a threshold as low as 0.5GeVby vetoing on signals in the BSC counters and the forward plug calorimeter. Therewas then very little pileup, easily removed by requiring exactly two tracks (and their86 M. ALBROW on behalf of the CDF COLLABORATIONFig. 1. – Differential cross section dσ/dM for two particles, assumed to be π+π−, in the statedkinematic region, between two forward rapidity gaps Δy > 4.6, at√s = 1960GeV.associated calorimeter signals) and no other activity in |η| < 5.9. This led to eventswith two rapidity gaps of Δη > 4.6. We recorded 90 (22) million events at√s =1960 (900)GeV, the lower energy data in a special 38-hour run we proposed mainly forthis purpose. To make the exclusivity cuts it was important to know the noise levels in allelements of the detector, for which we used 0-bias events, recording bunch crossings withno other requirements. The probability of the whole detector (|η| < 5.9) being emptyas a function of the individual bunch luminosity is an exponential with intercept 1.0,and the slope gives the inelastic cross section within that coverage, which at 1960GeVagrees well with the expectation: σ(vis) = (0.85 ± 0.05) × σ(inel)(61.0 ± 1.8mb). At√s = 900GeV the luminosity monitors were not calibrated and we used the relationσ(vis) = (0.90±0.05)×σ(inel)(52.7±1.6mb) to normalise the cross sections The σ(inel)values come from a global fit including totem values [17]. After all exclusivity andquality cuts we have 350,223 (9,349) h+h− events. A study of the time-of-flight showsthat > 92% of the events are π+π−. Even though the collision time is not well known,the h+ and h− have different momenta and different path lengths, so identification ispossible for most tracks. The acceptance is a function of both pT (ππ) and M(ππ) and iscalculated for |y(ππ)| < 1.0, assuming isotropic decay of “X” → ππ. As the acceptanceis zero for low pT (ππ) below M(ππ) = 800MeV/c2, we only present the cross section,integrated over pT (ππ), for higher masses. Figure 1 shows the cross section (assumingpion masses) up to 5GeV/c2 at√s = 1960GeV. Features are the f0(980), a largebump probably both f2(1270) and f0(1370), a break at about 1550MeV/c2 followed bya smooth, almost-exponential fall off. A small peak at 3100MeV/c2 is consistent withphotoproduced J/ψ decaying to e+e− (events with muons were excluded). The data at√s = 900GeV look similar, but with much lower statistics; however in detail there aredifferences as shown by the ratio plot, fig. 2. As the acceptance is almost identical at thetwo energies the systematic uncertainties are small. Some clear structures in this ratiostill require explanation. One difference is that the rapidity gaps extend to |η| = 5.9 atCENTRAL EXCLUSIVE PRODUCTION IN CDF 87Fig. 2. – Ratio of dσ/dM(π+π−) at√s = 900:1960GeV in the stated kinematic region.both energies, while the beam (true) rapidity is ybeam = ln(√s/mp) = 6.87 and 7.64, sothat higher diffractive masses are included at√s = 1960GeV.In our previous observation of exclusive χc0(3415)→ J/ψ+γ [6] we could not resolvethe three χc states, and gave a cross section assuming only χc0. We can look for χc0 →π+π−(K+K−) which have the highest branching fractions for the χc0, and the massresolution easily resolves these states. Fitting the spectrum between 2.5GeV/c2 and4.5GeV/c2 with an empirical function excluding the χc0 region, we place upper limits onits exclusive production: dσ/dy(χc0)|y=0  20 nb (at 90% C.L.). This is only compatiblewith our previous observation if < 25% of the J/ψ + γ events were from χc0; note thatthe branching fractions of χc1(χc2)→ J/ψ + γ are 30× (17.4×) higher, so they are stillsuppressed in production, as expected [18].We also studied the angular distributions (dN/d cos θ∗) of the X → π+π−, and foundthem to agree with S-wave up to 1.5GeV/c2, above which they deviate, becoming increas-ingly forward-backward peaked. If the dominant resonance peak around 1.27GeV/c2 isthe f2(1270) it does not show up as a deviation from isotropy. The Legendre coefficientsshow a “wave” structure between 1.5 and 2.5GeV/c2, which is not understood.We have more data available for other channels. We are currently studying X = π0π0,η0η0, η0η′, and η′η′ production with four photon showers and 0, 1, or 2 pairs of chargedpions. The Durham Group predicts a hierachy, with η(′)η(′) > π0π0 at large M(X)because (a) the η and η′ are isoscalars, and (b) they probably have a high glue component.This would be very interesting; we have data but the analysis is ongoing.∗ ∗ ∗I thank the organizers of the La Thuile 2014 Workshop for inviting me to give thistalk, Fermilab and the DOE for support, and students Maria Zurek and Artur Swiechwho did most of the π+π− analysis “as a hobby”. I also thank Dino Goulianos, for 22years my collaborator in diffractive physics with CDF.88 M. ALBROW on behalf of the CDF COLLABORATIONREFERENCES[1] See e.g., Forshaw J. R. and Ross D. A., “Quantum Chromodynamics and the Pomeron”,in Cambridge Lect. Notes Phys. (Cambridge University Press, UK) 1997.[2] A˚kesson T. et al. (AFS Collaboration), Nucl. Phys. B, 264 (1986) 154.[3] Nakamura K. et al. (Particle Data Group), J. Phys. G, 37 (2010) 075021.[4] Albrow M. G., Coughlin T. D. and Forshaw J. R., Prog. Part. Nucl. 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Central exclusive production in CDF

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Central exclusive production in CDF

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