Testing the Pauli Exclusion Principle for Electrons
J. Marton, S. Bartalucci, S. Bertolucci, C. Berucci, M. Bragadireanu, M. Cargnelli, C. Curceanu, S. Di Matteo, J.-P. Egger, C. Guaraldo, M. Iliescu, T. Ishiwatari, M. Laubenstein, E. Milotti, D. Pietreanu, K. Pisciccia, T. Ponta, A. Romero-Vidal, A. Scordo, D.L. Sirghi, F. Sirghi, L. Sperandio, O. Vazquez Doce, E. Widmann, J. Zmeskal
aa r X i v : . [ nu c l - e x ] M a y Testing the Pauli Exclusion Principle for Electrons
J. Marton , S. Bartalucci , S. Bertolucci , C. Berucci , ,M. Bragadireanu , , M. Cargnelli , C. Curceanu (Petrascu) , ,S. Di Matteo , J.-P. Egger , C. Guaraldo , M. Iliescu , T. Ishiwatari ,M. Laubenstein , E. Milotti , D. Pietreanu , K. Piscicchia , , T.Ponta , , A. Romero Vidal , A. Scordo , D.L. Sirghi , , F. Sirghi , ,L. Sperandio , O. Vazquez Doce , E. Widmann and J. Zmeskal The Stefan Meyer Institute for Subatomic Physics, Boltzmanngasse 3, A-1090 Vienna,Austria INFN, Laboratori Nazionali di Frascati, CP 13, Via E. Fermi 40, I-00044, Frascati (Roma),Italy ”Horia Hulubei” National Institute of Physics and Nuclear Engineering,Str. Atomistilor no. 407, P.O. Box MG-6, Bucharest - Magurele, Romania CERN, CH-1211, Geneva 23, Switzerland Institut de Physique UMR CNRS-UR1 6251, Universit´e de Rennes1, F-35042 Rennes, France Institut de Physique, Universit´e de Neuchˆatel, 1 rue A.-L. Breguet, CH-2000 Neuchˆatel,Switzerland Laboratori Nazionali del Gran Sasso, S.S. 17/bis, I-67010 Assergi (AQ), Italy Dipartimento di Fisica, Universit`a di Trieste and INFN– Sezione di Trieste, Via Valerio, 2,I-34127 Trieste, Italy CENTRO FERMI, Compendio del Viminale, Piazza del Viminale 1, I-00184 Roma, ItalyE-mail: [email protected]
Abstract.
One of the fundamental rules of nature and a pillar in the foundation of quantumtheory and thus of modern physics is represented by the Pauli Exclusion Principle. We knowthat this principle is extremely well fulfilled due to many observations. Numerous experimentswere performed to search for tiny violation of this rule in various systems. The experimentVIP at the Gran Sasso underground laboratory is searching for possible small violations of thePauli Exclusion Principle for electrons leading to forbidden X-ray transitions in copper atoms.VIP is aiming at a test of the Pauli Exclusion Principle for electrons with high accuracy, downto the level of 10 − - 10 − , thus improving the previous limit by 3-4 orders of magnitude.The experimental method, results obtained so far and new developments within VIP2 (follow-up experiment at Gran Sasso, in preparation) to further increase the precision by 2 orders ofmagnitude will be presented.
1. Introduction
The Pauli Exclusion Principle (PEP) uncovered by Wolfgang Pauli in 1925 [1] is one of thecorner stones of quantum physics and thus it is at the basis of the foundation of modern physics.It is connected with spin statistics dividing the world in fermions and bosons. Therefore, PEPis one of the most important rules in physics. Based on it is our understanding of nature andthe consequences for the world of elementary particles up to compact objects (e.g. neutronstars) in the universe - but it is lacking a simple explanation as already stated by Pauli himself.olfgang Pauli showed later in 1940 the interconnection of the principle with the spin-statistics[2]. We know that the Pauli Principle is extremely well fulfilled but the the limit of validity- if any - is still an open question. Even a tiny violation of PEP would point to new physicswhich could show up at very high energies (at the Planck scale). In the last decades severalexperimental tests of PEP validity for different systems have been performed [3, 4, 5, 6, 7, 8].A method to experimentally test the PEP was developed by Ramberg-Snow [9]. PEP is testedfor fresh electrons, i.e. elementary particles having no interaction with the studied system thuscircumventing the Messiah-Greenberg super-selection rule[10]. These fresh electrons are providedby a strong electric current which is flowing through a solid metal conductor. Pauli-forbiddentransitions in this metal (e.g. K transitions from the 2p state to the 1s state already filled with 2electrons) exhibit an significant energy shift in the transition energy which is resolvable by x-rayspectroscopy. A search for these x-ray transition events can be performed with high sensitivitybut requires substantial background discrimination. It has to be noted that the normal
Ktransitions are present due to background radiation.In our previous VIP experiment at the underground laboratory LNGS (Laboratorio Nazionali diGran Sasso) we used an improved Ramberg-Snow experimental setup exploiting charge coupleddevices as x-ray detectors surrounding a solid cylindrical copper target. In this experiment wecould deduce an upper limit for the Pauli exclusion principle violation in the order of 10 − [11]. A strongly improved VIP2 experiment will be performed in the underground laboratoryLNGS in Gran Sasso taking again advantage of the excellent shielding against cosmic rays. Astrongly improved compact setup with passive and active shielding will be used. Silicon driftdetectors (SDDs) will serve as x-ray detectors providing a timing signal used in anti-coincidencewith scintillators (i.e. veto counters) to suppress actively background events.With the VIP2 experiment we want to improve the limit for PEP violation for electrons by 2orders of magnitude reaching the range of 10 − .
2. Experimental Method
The basic principle of the VIP experiment follows the method suggested by Greenberg andMohapatra [12] and is based on introducing fresh electrons into a copper strip. By applying anelectrical current on the strip and searching for the Pauli-forbidden radiative transitions to theground state that is already filled by two electrons. We look for 2p-1s transitions in copper whichare easily detectable with x-ray detectors. The energy of these non-Paulian transitions is shiftedfrom the normal transition energy by about 300 eV (7.729 keV instead of 8.040 keV) due to theadditional screening effect given by the two electrons in the 1s level [13]. The measurementswithout the current in the copper strip provide the X-ray background, where no PEP violatingtransitions are expected. Those with the current provide the forbidden X-ray transitions whenthe fresh electrons could lead to Pauli-forbidden transitions. Comparing the two energy spectra,with and without the currents, the limit of the probability of the violation of PEP is extracted.
The VIP experiment was installed in the underground laboratory of LNGS. This experiment useda copper cylinder as target and charge coupled devices (CCDs) as x-ray detectors. The array ofCCD detectors were used for studies of kaonic atoms [14, 15]. Details of the VIP experiment canbe found in ref.[16, 17]. The measurement by the VIP experiment improved very significantly thelimit previously obtained by Ramberg and Snow [9], thanks to the following features:(a) use ofCCD detectors instead of gaseous ones, having much better energy resolution (4-5 times better)and higher stability; (b) experimental setup located in the clean, low-background, environmentof the underground LNGS Laboratory; and (c) collection of much higher statistics (longer DAQperiods, thanks to the stability of CCDs). We made full use of these features to obtain animprovement of several orders of magnitude on the previous limit by Ramberg and Snow (see able 1.
Limits of the Pauli violation probability for electrons from different experiments.Experiment Target Upper limit of β /2 ref.Ramberg-Snow Copper 1.7x10 − [9]S.R. Elliott et al. Lead 1.5x10 − [18]VIP(2006) Copper 4.5x10 − [17]VIP(2012) Copper 4.7x10 − [11, 19]VIP2(goal) Copper 10 − tab.1). In order to achieve the signal/background increase which will allow a gain of two orders ofmagnitude for the probability of PEP violation for electrons, we are planning to build a newtarget, a new cryogenic system, use new detectors with timing capability and active veto sys-tem. As x-ray detectors we will use SDDs which were employed in der SIDDHARTA experimenton kaonic atoms at the DAFNE electron-positron collider of Laboratori Nazionali di Frascati.SDDs have an even better energy resolution than CCDs but additionally provide timing capa-bility [20]which allow to use anti-coincidence operation with scintillators and therefore activeshielding. The VIP2 system will provide • signal increase with a more compact system with higher acceptance and higher current flowin the new copper strip target • background reduction by decreasing the x-ray detector surface, more compact shielding(active veto system and passive), nitrogen filled box for radon radiation reductionIn the following table the numerical values for the improvements in VIP2 are given whichwill lead to an expected overall improvement of a factor higher than ∼ µ m thick, and is installed inthe center of the setup. The copper strip is cooled at ∼
90K by the use of an external cryogenicsystem using liquid argon as the cooling medium. The current connection lines made of copperwires with a cross-section area of 1.5 cm , allow a current flow of (at least) 100 A. The currentlines exhibit a temperature gradient from inside the vacuum chamber to the outside connectorsof about 180 K.Monte Carlo simulations were performed to study the effect of the active shielding in variousconfigurations of the setup and models of the background radiation. The background profilesmeasured at LNGS [22] were used as input parameters in the simulations.As the veto counters (see fig.3), 2 pieces of 10 cm thick plastic scintillators are found tobe optimal. We also simulated a setup with BGO inorganic crystals which yields even betterbackground suppression, but due to the high costs of this solution, we decided to propose plasticscintillators, e.g. Bicron 412. To summarize we are confident that the goal of VIP2 to reach anupper limit for possible PEP violation in the order of 10 − can be reached with the improvedsetup (see fig.4). able 2. List of numerical values of the changes in VIP2 in comparison to the VIP features(given in brackets)Changes in VIP2 value VIP2 (VIP) expected gainacceptance 12% 12increase current 100A (50A) 2reduced length 3 cm (8.8 cm) 1/3total linear factor 8energy resolution 170 eV (340 eV) 4reduced active area 6 cm (114 cm ) 20better shielding and veto 5-10higher SDD efficiency 1/2background reduction 200-400overall improvement > Figure 1.
An artist view of the VIP2 experimental setup. In the middle the copper conductorand the x-ray detectors are installed. Plastic scintillators with solid state photodetector readoutacting as active shielding (see fig.3) are surrounding this inner part. igure 2.
Photo of the VIP2 box assembled for first tests in the laboratory.
Figure 3.
Component from the active veto system of VIP2: Plastic scintillator bar with SiPMreadout electronic board.
3. Summary and Outlook
In the light of the importance of the Pauli Exclusion Principle high precision tests of this ruleof nature are well justified. Especially interesting are cases in which new fermions are used astest particles - in the case of VIP2 fresh electrons introduced by an electric current. In VIP2 thebackground will be reduced by a factor of 200-400 resulting in a gain of ∼ β /2. Together with the gain in signal the total gain improvement will excceed2 orders of magnitude.Therefore, in the VIP2 experiment the limit of the PEP violation can be reached to 10 − , whichis two orders of magnitude better than the previous limit of ∼ − . This limit is anticipatedto be obtained taking into account a data taking period similar to that of VIP1, i.e. about 3years). This incredible precision might help to prove or disprove tiny violations related to newphysics like e.g. superstring motivated effects [23]. Acknowledgement
The very important assistance of the INFN-LNGS laboratory staff during all phases ofpreparation, installation and data taking as well as the support from the HadronPhysics FP6 igure 4.
Results of PEP violation experiments for electrons.(506078), HadronPhysics2 FP7 (227431), HadronPhysics3 (283286) projects and the EU COST1006 Action is gratefully acknowledged. Especially we thank the Austrian Science Foundation(FWF) which supports the VIP2 project with the grant P25529-N20.
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