DEUTSCHES ELEKTRONEN-SYNCHROTRON ( £ * ^ Production and Detection of Axion-Like Particles at the VUV-FEL: Letter of Intent

DE06FA427

DEUTSCHES ELEKTRONEN-SYNCHROTRON (£*^ in der HELMHOLTZ-GEMEINSCHAFT \X| DESY 06-098 hep-ex/0606058 June 2006

Production and Detection of Axion-Like Particles at the VUV-FEL: Letter of Intent

U. Kötz, A. Ringwald, T. Tschentscher Deutsches Elektronen-Synchrotron DESY, Hamburg

ISSN 0418-9833

NOTKESTRASSE 85 - 22607 HAMBURG DESY behält sich alle Rechte für den Fall der Schutzrechtserteilung und für die wirtschaftliche Verwertung der in diesem Bericht enthaltenen Informationen vor.

DESY reserves all rights for commercial use of information included in this report, especially in case of filing application for or grant of patents.

To be sure that your reports and preprints are promptly included in the HEP literature database send them to (if possible by air mail):

DESY DESY Zentralbibliothek Bibliothek Notkestraße 85 Platanenallee 6 22607 Hamburg 15738Zeuthen Germany Germany arXiv:hep-ex/0606058 v1 26 Jun 2006 ilto nsrn neatos uhainlk pseu- via axion-like photons Such two to of couple interactions. Peccei-Quinndoscalars absence strong the U(1) in explain a violation to of a introduced breaking [2], — par- the symmetry vacuum pseudoscalar from a the arising [1], axion in ticle the broken being the- example spontaneously the notable in is symmetry continuous that global a ory particles is light there Such if Model. arise Standard many the in predicted beyond are models matter ordinary to coupled weakly es.Tentbedffrnebtenapedsaa and pseudoscalar a between difference notable The versa. ih pnzr atceo ml mass small of particle frequency spin-zero mag- of external light photon an a of field, presence the netic in observable particular, similar In to effects. rise give interactions effective Both † ei ed,rsetvl.I h aeo clrparticle reads scalar interaction a the of photons, case two the to In coupling respectively. fields, netic h particle, the where teghtno,and tensor, strength orsodn author. Corresponding e eylgtsi-eoprilswihaevery are which particles spin-zero light very New NRDCINADMOTIVATION AND INTRODUCTION g L stecoupling, the is φγγ L φγγ rdcinadDtcino xo-iePrilsa h VUV the at Particles Axion-Like of Detection and Production F ln h rnvremgei edo ierarneeto arrangement linear Fr a Vacuum-UltraViolet of the field of magnetic setti transverse beam by the hypothesis photon along this pa the test spin-zero exploits to light propose which new we Intent, a of of of Letter production presence the the in through vacuum explained in generated light of polarization xeietoesawno fopruiyfrafimestablish firm experimen a other differen before for at anomaly running opportunity PVLAS by of the wit measured pseud of window be interpretation field The a can magnetic offers particle the period. experiment the of running of p direction short mass the the The a that anom varying find within PVLAS by We tested the determined power. be by similar can s implied of result increase lasers range visible VUV-FEL mass at the the expected at in available rate energies regeneration photon high The = µν eety h VA olbrto a eotdeiec fo evidence reported has collaboration PVLAS the Recently, = 4 1 ( etce lkrnnSnhornDS,Ntetae8,D 85, Notkestraße DESY, Elektronen-Synchrotron Deutsches F F φ g ˜ − µν 1 4 E ~ ste(ul lcrmgei field electromagnetic (dual) the is ) F φ g µν and F φ lihKötz,Ulrich µν µν stefil orsodn to corresponding field the is B ~ F = ˜ r h lcrcadmag- and electric the are µν φ g = ω φ g a silt noa into oscillate may E ~ m ∗ 2 E ~ φ − nra Ringwald Andreas · ω < B B, ~ ~ 2 n vice and , . etro Intent of Letter CP (2) (1) † , eeao 5.Fo h o-bevto fpoo re- photon of non-observation Brookhaven-Fermilab-Rochester-Trieste the the From generation, Ac- Brookhaven Beam Colliding in [5]. the out celerator for carried magnets prototype was pilot two type using A appar- this nothing. — of of out photons experiment transform photons can into these wall regenerating (pseudo-)scalars the ently these of magnetic side of other another some the finally on obstruction and located another into field absorbed, or turn then wall being will mag- a can without photons through a beam freely these across (pseudo-)scalar propagate shone of This is fraction 1. photons (pseudo-)scalars. a Fig. of field, see beam netic 4], a “light [3, if called experiments Namely, walls”) (sometimes through regeneration shining photon behind component perpendicular case. the latter is the the polar- in it in photon interacts whereas the that case, of field former component magnetic the the to is parallel it ization that is scalar a through detection sub- production final (left), and (pseudo-)scalar field wall, (right). regeneration magnetic a of photon through a view travel in sequent conversion Schematic photon through 1: FIG. ‡ h xliaino hsmcaimi h ai idea basic the is mechanism this of exploitation The n hmsTschentscher Thomas and γ gu htnrgnrto experiment regeneration photon a up ng rnvremgei ed hsmybe may This field. magnetic transverse a ioemgeso size of magnets dipole f tceculdt w htn.I this In photons. two to coupled rticle naoaosylrertto fthe of rotation large anomalously an r scncompete. can ts ril nepeaino h PVLAS the of interpretation article epc otelsrpolarization. laser the to respect h eEeto ae U-E,sent VUV-FEL, Laser ee-Electron γ eto xlso fteparticle the of exclusion or ment × btnilyteepce photon expected the ubstantially saa s clrntr a be can nature scalar vs. oscalar htneege.Teproposed The energies. photon t 267Hmug Germany Hamburg, -22607 ∗ l,i oprsnt h rate the to comparison in aly, B φ § L B ≈ φ -FEL: 0Tm. 30 B γ × EY06-098 DESY ∗ γ 2

HB stars

Galactic dark matter

FIG. 2: Two photon coupling g of the (pseudo-)scalar ver- FIG. 3: Exclusion region in mass mφ vs. coupling g for sus its mass mφ. The upper limits from BFRT data [6] on various current and future experiments. The laser experi- polarization (rotation and ellipticity data; 95 % confidence ments [6, 7] aim at (pseudo-)scalar production and detection level) and photon regeneration (95 % confidence level) are dis- in the laboratory. The galactic dark matter experiments [12] played as thick dots. The preferred values corresponding to exploit microwave cavities to detect pseudoscalars under the the anomalous rotation signal observed by PVLAS [7] are assumption that these pseudoscalars are the dominant con- shown as a thick solid line. The projected 95 % confidence stituents of our galactic halo, and the solar experiments search level upper limit which can be obtained with the proposed for axions from the sun [10]. The constraint from horizontal experiment (see text) is drawn as a dashed-dotted line. branch (HB) stars [9] arises from a consideration of stellar energy losses through (pseudo-)scalar production. The pre- dictions from two quite distinct QCD axion models, namely the KSVZ [13] (or hadronic) and the DFSZ [14] (or grand (BFRT) collaboration excluded values of the coupling unified) one, are also shown. −7 −1 −3 g < 6.7 × 10 GeV , for mφ ∼<10 eV [6] (cf. Fig. 2), at the 90 % confidence level. Recently, the PVLAS collaboration has reported an anomalous signal in measurements of the rotation of the may be hindered, for example, if the φγγ vertex is sup- polarization of photons in a magnetic field [7]. A possi- pressed at keV energies due to low scale compositeness of ble explanation of such an apparent vacuum magnetic φ, or if, in stellar interiors, φ acquires an effective mass ∼ dichroism is through the production of a light pseu- larger than the typical photon energy, keV, or if the doscalar or scalar, coupled to photons through Eq. (1) particles are trapped within stars [15, 16, 17]. or Eq. (2), respectively. Accordingly, photons polarized Clearly, an independent and decisive experimental test parallel (pseudoscalar) or perpendicular (scalar) to the of the pseudoscalar interpretation of the PVLAS obser- magnetic field disappear, leading to a rotation of the po- vation, without reference to axion production in stars larization plane [8]. The region quoted in Ref. [7] that (see [18, 19]), is urgently needed. In Ref. [20], one of us might explain the observed signal is (AR) was involved in the consideration of the possibility of exploiting powerful high-energy free-electron lasers 1 1 1.7 × 10−6 GeV− < g < 5.0 × 10−6 GeV− , (3) (FEL) in a photon regeneration experiment1 to probe the region where the PVLAS signal could be explained −3 −3 1.0 × 10 eV < mφ < 1.5 × 10 eV, (4) in terms of the production of a light spin-zero particle. In particular, it was emphasized that the free-electron laser obtained from a combination of previous limits on g vs. VUV-FEL [22] at DESY, which is designed to provide mφ from a similar, but less sensitive polarization exper- tunable radiation from the vacuum-ultraviolet (VUV; 10 iment performed by the BFRT collaboration [6] and the eV) to soft X-rays (200 eV), will offer a unique and timely g vs. mφ curve corresponding to the PVLAS signal (cf. opportunity to probe the PVLAS result. Notably, the Fig. 2). high photon energies available at the VUV-FEL increase A particle with these properties presents a theoreti- substantially the expected photon regeneration rate in cal challenge. It is hardly compatible with a genuine the mass range implied by the PVLAS anomaly, in com- QCD axion. Moreover, it must have very peculiar prop- parison to the one expected at visible (∼ 1 eV) lasers. In erties in order to evade the strong constraints on g from stellar energy loss considerations [9] and from its non- observation in helioscopes such as the CERN Axion So- lar Telescope [10, 11] (cf. Fig. 3). Its production in stars 1 This idea has been considered first in Ref. [21]. 3

to the magnet and TABLE I: Achieved (2005) and expected (2007) VUV-FEL 2 parameters. 1 sin 2 qℓ 2005 2007 F (qℓ)= 1 (6) qℓ Bunch separation [ns] 1000 1000 " 2 # Bunches per train # 30 800 Repetition rate [1/s] 5 10 is a form factor which reduces to unity for small qℓ, cor- Photon wavelength [nm] 32 32 responding to large ω (cf. Fig. 4) or small mφ, Photon energy [eV] 38.7 38.7 Energy per pulse [µJ] 10 50 2 πω ω 6 m 12 12 −3 Photons per pulse # 1.6 × 10 8.1 × 10 mφ ≪ =3 × 10 eV . (7) 14 16 ℓ s 38.7 eV ℓ Average flux [1/s] 2.4 × 10 6.5 × 10 r For smaller ω or larger mφ, incoherence effects set in between the (pseudo-)scalar and the photon, the form this Letter of Intent, we propose a corresponding photon factor getting much smaller than unity, severely reducing regeneration experiment. the regenerated photon flux (5) (cf. Fig. 4). Therefore, a photon regeneration experiment exploiting the VUV- FEL beam has a unique advantage when compared to PHOTON REGENERATION AT THE VUV-FEL one using an ordinary laser operating near the visible (ω ∼ 1 eV): the sensitivity of the former extends to much 3 The proposed experiment is based on the assump- larger masses . In particular, the mass range (4) implied tion that the VUV-FEL can deliver photons with an by PVLAS is entirely covered, for ω = 38.7 eV photons energy ω = 38.7 eV and an average photon flux N˙ 0 = (cf. Eq. (7)). 6.5 × 1016 s−1 (cf. Table I). For the proposed photon In case a signal is found, one may use the possibility to regeneration experiment at the VUV-FEL, we study a tune the photon energy for a determination of the mass linear arrangement of 12 normal conducting dipole magnets which are freely available at DESY2. Each of these magnets has a magnetic field of 2.24 T and an integrated magnetic length of 1.029 m. The default arrangement consists of six plus six magnets, the beam absorber being placed between the first and second six. This arrangement corresponds to a magnetic field region of size BL = 2Bℓ = 27.66Tm. The proposed configuration is too large to fit into the VUV-FEL experimental hall. It has to be built on the ground before the entrance. Cor- respondingly, the FEL beam line has to be extended to the proposed experiment. The photons leave the VUV-FEL with horizontal linear polarization. In order to have a maximal coupling with a possible pseudoscalar/scalar, the magnetic field B~ of the magnets before the absorber should lie in the horizontal/perpendicular direction. We therefore foresee to exploit both possibilities of the magnetic field direction. FIG. 4: The second power of the form factor F , Eq. (6), For the proposed experiment, the expected flux of re- as a function of the laser frequency ω, for fixed length ℓ = generated photons is 6 m of the magnetic field region and different values of the pseudoscalar mass mφ, corresponding to the central value, m = 1.35 meV (solid), the lower value, m = 0.7 meV (short 4 1 2 N˙ 0 φ φ N˙ ≈ 1 × 10− s− F (qℓ) dashed), and the upper value, m = 2.0 meV (long dashed), f × 16 −1 φ 6.5 10 s ! of the range (4) suggested by PVLAS. 4 4 4 g B ℓ × 1 , (5) 10−6 GeV− 2.24T 6 m 2 ≪ 3 Running at conventional synchrotron radiation sources can ex- where q = mφ/(2ω) ( mφ) is the momentum transfer tend the accessible mass range by more than an order of mag- nitude. However, they do not bring an advantage in terms of average photon flux. But the continuous energy spectrum of the conventional synchrotron radiation reaching to high energies will 2 An option to use superconducting magnets is under study. result in a much larger background. 4 of the particle. This is done by lowering the energy of the rate is most sensitive to the hypothetical particle’s mass. FEL photons and observing the on-set of the incoherence Moreover, the well-defined beam of the VUV-FEL will ≈ 2 expected around ω mφℓ/2π. A determination of the not produce beam-related backgrounds. form factor F as a function of ω (cf. Fig. 4) will allow an The experiment should be done soon, before other ex- extraction of mφ. periments [23, 24, 25, 26] can compete. A first step to- wards this goal is the study of possible detectors and EXPECTED RESULTS their background rates. Finally, the proposed experiment could serve also as a test facility for an ambitious large scale photon regeneration experiment [27]. The flux prediction (5) uses the benchmark values for the VUV-FEL flux and for the proposed magnetic field Acknowledgments arrangement. For g and mφ in the parameter region pre- We would like to thank Markus Ahlers, Josef Feld- ferred by PVLAS, Eqs. (3) and (4), this results in a rate haus, Ioannis Giomataris, Hermann Herzog, Rolf-Dieter of regenerated photons ranging from about 1 mHz up to Heuer, Jörg Jäckel, Markus Körfer, Bernward Krause, 1 Hz. Markus Kuster, Axel Lindner, Hinrich Meyer, Norbert The very low predicted rates require therefore a detec- Meyners, Dieter Notz, Alexander Petrov, Christian Re- tor system with genfus, Horst Schulte-Schrepping, Albrecht Wagner, and Konstantin Zioutas for valuable input. • a large single photon efficiency at ∼ 40 eV,

• a short response time,

• and a low noise rate. ∗ Electronic address: [email protected] ‡ Electronic address: [email protected] Three detector options are being considered: electron § multipliers, multi-channel plates, and avalanche photo Electronic address: [email protected] [1] S. Weinberg, “A New Light Boson?” Phys. Rev. Lett. diodes. One manufacturer quotes an efficiency of about 40, 223 (1978); F. Wilczek, “Problem of Strong P and 7 % for electron multipliers. For the two other options, T Invariance in the Presence of Instantons,” Phys. Rev. extrapolations point to a value of around 10 %. All three Lett. 40, 279 (1978). detectors show a response time in the 10 ns range. This [2] R. D. Peccei and H. R. Quinn, “CP Conservation in short response time allows a reduction of the noise rate the Presence of Instantons,” Phys. Rev. Lett. 38, 1440 by timing, exploiting the time structure of the photon (1977); R. D. Peccei and H. R. Quinn, “Constraints Im- beam. These detector performances, in particular the ef- posed by CP Conservation in the Presence of Instantons,” Phys. Rev. D 16, 1791 (1977). ficiencies and the response time, have to be studied at a [3] P. Sikivie, “Experimental Tests of the ‘Invisible’ Axion,” beamline of the VUV-FEL as soon as possible. At the Phys. Rev. Lett. 51, 1415 (1983) [Erratum-ibid. 52, 695 same time, the general background rates in the VUV- (1984)]; A. A. Anselm, “Arion <—> Photon Oscillations FEL environment have to be studied as well. in a Steady Magnetic Field. (In Russian),” Yad. Fiz. 42, In the case of a non-observation of photon regenera- 1480 (1985); M. Gasperini, “Axion Production by Elec- 59 tion, for an assumed running time of 12 × 12 h with an tromagnetic Fields,” Phys. Rev. Lett. , 396 (1987). 16 −1 [4] K. Van Bibber, N. R. Dagdeviren, S. E. Koonin, A. Ker- average photon flux of N˙ 0 =6.5×10 s at ω = 38.7 eV, man and H. N. Nelson, “An Experiment to Produce and a 7 % single photon efficiency and zero background, the Detect Light Pseudoscalars,” Phys. Rev. Lett. 59, 759 proposed experiment can establish a 95% confidence limit (1987). −7 −1 −3 of g< 8.8×10 GeV , for mφ ∼< 3×10 eV. In partic- [5] G. Ruoso et al., “Limits on light scalar and pseudoscalar ular, the experiment is expected to be able to firmly ex- particles from a photon regeneration experiment,” Z. clude the particle interpretation of the PVLAS anomaly Phys. C 56 (1992) 505. and to improve the current laboratory bound on g in the [6] R. Cameron et al., “Search for Nearly Massless, Weakly m >10−3 eV range (cf. Fig. 2). Coupled Particles by Optical Techniques,” Phys. Rev. D φ ∼ 47, 3707 (1993). [7] E. Zavattini et al. [PVLAS Collaboration], “Experimen- tal Observation of Optical Rotation Generated in Vac- CONCLUSIONS uum by a Magnetic Field,” Phys. Rev. Lett. 96, 110406 (2006) [arXiv:hep-ex/0507107]. The proposed experiment offers a window of opportu- [8] L. Maiani, R. Petronzio and E. Zavattini, “Effects of nity for a firm establishment or exclusion of the particle Nearly Massless, Spin Zero Particles on Light Propaga- tion in a Magnetic Field,” Phys. Lett. B 175, 359 (1986); interpretation of the PVLAS anomaly in the near future. G. Raffelt and L. Stodolsky, “Mixing of the Photon with It takes essential advantage of unique properties of the Low Mass Particles,” Phys. Rev. D 37, 1237 (1988). VUV-FEL beam. The available VUV-FEL photon ener- [9] G. G. Raffelt, “Astrophysical Axion Bounds Diminished gies are just in the range where the photon regeneration by Screening Effects,” Phys. Rev. D 33, 897 (1986); 5

G. G. Raffelt and D. S. Dearborn, “Bounds on Hadronic (2005) [arXiv:astro-ph/0510324]. Axions from Stellar Evolution,” Phys. Rev. D 36, 2211 [19] M. Kleban and R. Rabadan, “Collider Bounds (1987); G. G. Raffelt, Stars As Laboratories For Fun- on Pseudoscalars Coupling to Gauge Bosons,” damental Physics: The Astrophysics of Neutrinos, Ax- arXiv:hep-ph/0510183. ions, and other Weakly Interacting Particles, University [20] R. Rabadan, A. Ringwald and K. Sigurdson, “Pho- of Chicago Press, Chicago, 1996; G. G. Raffelt, “Particle ton regeneration from pseudoscalars at X-ray laser physics from stars,” Ann. Rev. Nucl. Part. Sci. 49, 163 facilities,” Phys. Rev. Lett. 96, 110407 (2006) (1999). [arXiv:hep-ph/0511103]. [10] K. Zioutas et al. [CAST Collaboration], “First Results [21] A. Ringwald, “Fundamental Physics at an X-ray Free from the CERN Axion Solar Telescope (CAST),” Phys. Electron Laser,” in Proceedings of the Workshop Rev. Lett. 94, 121301 (2005). on “Electromagnetic Probes of Fundamental Physics”, [11] G. G. Raffelt, “Axions: Recent Searches and New edited by W. Marciano and S. White (World Scientific, Limits,” in Proceedings of the Eleventh International Singapore, 2003), pp. 63-74 [arXiv:hep-ph/0112254]. Workshop on “Neutrino Telescopes”, edited by Milla [22] Free-electron laser at TESLA Test Facility, Baldo Ceolin (Istituto Veneto di Scienze, Lettere http://www-hasylab.desy.de/facility/fel/main.htm ; ed Arti, Campo Santo Stefano, 2005), pp. 419-431 “SASE FEL at the TESLA facility, phase 2,” DESY- [arXiv:hep-ph/0504152]. TESLA-FEL-2002-01. [12] S. Eidelman et al. [Particle Data Group Collaboration], [23] Photon regeneration at PVLAS: “Review of particle physics,” Phys. Lett. B 592, 1 (2004). G. Cantatore [PVLAS Collabortion], “PVLAS: [13] J. E. Kim, “Weak Interaction Singlet And Strong CP recent results,” DESY seminar, Jan 17, 2006, Invariance,” Phys. Rev. Lett. 43, 103 (1979); M. A. Shif- http://www.desy.de/f/seminar/Cantatore.pdf man, A. I. Vainshtein and V. I. Zakharov, “Can Confine- [24] Photon regeneration at the Jefferson Laboratory FEL: ment Ensure Natural CP Invariance Of Strong Interac- K. Baker, private communication. tions?,” Nucl. Phys. B 166, 493 (1980). [25] Rémy Battesti [BMV Collaboration], “The Biréfringence [14] A. R. Zhitnitsky, “On Possible Suppression Of The Axion Magnétique du Vide project: axion search with Hadron Interactions. (In Russian),” Sov. J. Nucl. Phys. a pulsed magnet – Status of the experiment,” 31, 260 (1980) [Yad. Fiz. 31 (1980) 497]; M. Dine and presented at the Joint ILIAS-CAST-CERN Ax- W. Fischler, “The Not-So-Harmless Axion,” Phys. Lett. ion Training, November 30 - December 2, 2005, B 120, 137 (1983). http://cast.mppmu.mpg.de/axion-training-2005/axion-training.php [15] E. Masso and J. Redondo, “Evading Astrophysical Con- [26] Letter of Intent for Axion Laserexperiment at CERN: straints on Axion-Like Particles,” JCAP 0509, 015 L. Duvillaret, M. Finger Jr., M. Finger, M. Kral, (2005). K. A. Meissner, P. Pugnat, D. Romanini, A. Siemko, [16] P. Jain and S. Mandal, “Evading the astrophysical limits M. Sulc, J. Zicha, “QED Test and Axion Search by Means on light pseudoscalars,” arXiv:astro-ph/0512155. of Optical Techniques,” CERN-SPSC-2005-034. [17] J. Jaeckel, E. Masso, J. Redondo, A. Ringwald and [27] A. Ringwald, “Production and Detection of Very Light F. Takahashi, arXiv:hep-ph/0605313. Bosons in the HERA Tunnel,” Phys. Lett. B 569, 51 [18] A. Dupays, C. Rizzo, M. Roncadelli and G. F. Bignami, (2003). “Looking for light pseudoscalar bosons in the binary pul- sar system J0737-3039,” Phys. Rev. Lett. 95, 211302